Sodium battery nanomaterials

C H A P T E R

4 Sodium battery nanomaterials Lizhuang Chen and Xueying Li School of Environmental and Chemical Engineering, Jiangsu University of Science and Technology, Zhenjiang, P.R. China

The Li-ion batteries (LIBs) as clean energy with high-energy density and strong stability in cycling performance have grown up to the secondary battery industry market since their early commercialization in the 1990s. The high demand for lithium has outpaced the capability of the supply of raw materials due to the increased growth of lithium-based systems over the past few years. With the continually mining of lithium, its availability has been decreased to unsustainably low level [1,2]. This implies that LIBs are hard to keep up with the marketing needs of customers in the future. It is necessary, thus, to develop a new energy storage technology to release the pressure of LIBs. The Na-ion battery (NIB) technology was recently recognized as a promising system in the future to replace the LIBs due to the lower price and natural abundance of sodium, similar intercalation chemistry to lithium as well. The mechanism sodium storage in NIBs is similar to that in LIBs. The cathode materials, anode materials, electrolytes, and separators have influence on the electrochemical performance of NIBs.

4.1 Cathode nanomaterials for sodium storage 4.1.1 Overview of cathode materials Many NIB cathode materials still exhibit poor performance and significant degradation over electrochemical cycling. The understanding of electrochemical sodiation and desodiation processes in NIB cathode materials is crucial for achieving superior sodium storage performance [3]. Now, we will introduce layered oxides, polyanion compounds, and organic compounds as NIB cathode materials for understanding material mechanisms regarding the structure, Na ordering, Na1 diffusion, and phase transformation.

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4.1.2 Layered oxides type cathode materials The layered Na transition metal oxides NaxMO2 and Na-based alloys could exhibit stable phases at their sodium intermediates. Therefore the cyclic voltammetry curves present multiple voltage plateaus during the electrochemical cycling. Each pair equilibrium voltage could identify the intermediate stable phases as a function of Na concentration. Warburg shows the Na1 conductivity in Thu¨ringe glasses in 1884 [4]. The discovery of high Na1 ion conductivity in β-Al2O3 at intermediate temperatures by Kummer and Yao opened new perspectives for applications. After a large number of works performed on the structure and the Na1 diffusion process, the new materials with high ionic conductivity were discovered. All material families with tunnels structure (hollandite), layer structures oxides, and chalcogenides or tunnels structures were considered. In all of these materials, the cations must avoid electronic conductivity and not reducible to avoid reaction with the alkali metal (negative electrode). At the end of the 1970s, many research groups opened the way to reversible lithium batteries with aprotic liquid electrolytes, the very first research work concerned the intercalation of lithium in layered dichalcogenides. Only intercalation in TiS2 and WO3 was considered as Na battery electrodes [5,6]. The layered oxides, either LiMO2 or NaxMO2 (M 5 3d element) were all discovered toward the end of the 1960s. The story about the sodium deficient phase NaxMO2 started in Bordeaux between 1970 and 1972, when Fouassier published the phase diagram of the NaxMnO2 and NaxCoO2 systems [7,8]. The structural classification is now currently used: the combination of a letter (O or P), which denotes the environment of the alkali ions (octahedral or prismatic), and a number (1, 2, 3, 4, 6, 9), which identifies the number of MO2 slabs within the hexagonal cell. In 1978, many researchers initiated research on the NaxMO2 layered (M 5 Co, Ni, Cr, Mn, Ti, Nb) oxides, especially the relationship between electrochemical intercalation and structural modifications [9
13]. A typical layered structure of NaxMO2 consisted of edge-sharing MO6 octahedral layers and Na ions layers with alternately stacking pattern. Delmas et al. first specified these layered structure into two main groups: P2-type or O3-type according to the amount of oxide layer stacking and Na1 environment. Symbol of “P” or “O” represents a prismatic or octahedral coordination environment of Na ions, and the “2” or “3” suggests the number of metal oxide layers with different kinds of “O” stacking in each cell unit [14]. Schematic illustration of crystal structures of P2-type and O3-type phases is descripted in Fig. 4.1 [15]. P2-type NaxMO2 consisted of two kinds of TMO6 layers (i.e., AB and BA layers) with Na1 located at trigonal “prismatic” (P) sites. Na1 could occupy two different types of trigonal prismatic sites: Naf (Na1) contacts the two TMO6 octahedra of the adjacent slabs along its face, whereas Nae (Na2) contacts the six surrounding TMO6 octahedra along its edges. The large Coulombic repulsion between Naf and Nae hinder the two adjacent Na ions site be occupied simultaneously. Thereby, P2-NaxMO2 is classified as a 2 H phase with a space group ˚ ) and other metal ions. of P63/mmc. Owing to the difference ionic radius of Na ions (1.02 A 1 The Na and metal ions are accommodated at distinct octahedral sites with a cubic closepacked (ccp) oxygen array. The edge-shared NaO6 and TMO6 octahedra order into alternate layers perpendicular to [111], forming the NaO2 and TMO2 slabs, respectively. As a layered structure, NaMO2 is composed of crystallographically three kinds of AB, CA, and BC MO2 layers with different O stacking (Fig. 4.1C) to describe the unit cell, and Na ions

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FIGURE 4.1 A schematic of the crystal structures for layered Na-containing oxides (NaxMO2) for (A) P2-type, (B) O2-type, (C) O3-type, and (D) P3-type stackings. The blue and yellow balls represent the metal and Na ions in the O-type frameworks, respectively. Source: Reproduced with permission N. Yabuuchi, K. Kubota, M. Dahbi, S. Komaba, Chem. Rev. 114 (2014) 11636. WileyVCH.

are accommodated at the “octahedral” (O) sites between MO2 layers forming a typical O3type layered structure. O3-NaxMO2 exhibit cation-ordered rock-salt superstructure oxides. Both P2-type and O3-type phases commonly suffer from a series of phase transitions involving different stacking sequences of the oxide layers during the electrochemical cycling. The transformation from P2-type phase to O2-type phase could result in a significant crystal structure’s contraction and decreased interlayer distance (Fig. 4.1A and B). The large prismatic sites are stabilized by large Na1 in the P2-type phase, metal oxides slabs glide formed octahedral sites after Na1 extraction, thereby, a new O2-type phase with a unique AB AC AB oxygen stacking (Fig. 4.1B). This O2-type phase is consisted of two different MO2 slabs with AB and AC oxygen arrangements, where vacancies are left between AB and AC layers. The O2-type layered phase with ccp array, sharing edges of NaO2 layers and MO2 layers on both sides. According to the oxygen packing, the O2-type phase can be identified as an intergrowth structure between ccp and hcp arrays. As shown in Fig. 4.1C, Na1 are originally stabilized at edge-shared octahedral sites with MO6 octahedra in the O3-type phase. Na1 at prismatic sites become energetically stable, associated with the formation of vacancies, similar to the P2-type phase. Thereby, the typical oxygen stacking changes from “AB CA BC” to “AB BC CA,” and this new phase is classified as a P3-type phase (Fig. 4.1D). The phase transition from P3/O3- to P2-type phase is impossible by means of electrochemical desodiation because these phase transitions require breakage or reformation of M
O bonds via heat treatment. The O3-type phase often undergoes more complex phase transition than P2 structure, whereas the P2-type phase commonly shows one P2-O2 type phase transition due to the structural evolution during the sodication/desodication processes. The O3-NaMO2 often deliver a high capacity because of its

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high initial Na content, whereas P2-NaMO2 exhibit a good cycling stability and rate capabilities due to the good structure integrity and low-diffusion barrier. Large Na1 ions coupled with ordering arrangement between Na1 and vacancies at different Na contents could result in the couples phase transitions during the extraction of Na from the layers. These irreversible phase transitions lead to structural collapse and a rapid capacity degradation. Many research works devote to suppress or reduce the irreversible phase transitions for enhancing the sodium storage performance of layered oxides. Layered cathode materials facilitate to incur poor performance and increased transportation cost after exposure to air. The design of air-stable cathode materials to improve their storage stability is also another major challenge. Although the cyclability of these electrode materials has been obviously improved in the recent research works, there are still other issues to be addressed that may have an influence on the long-term cycling stability, such as the Jahn
Teller effects of Mn31 and Fe31 migration.

4.1.3 Ployanion-type cathode materials In addition to layered oxide materials, polyanion compounds have been widely investi42 22 gated as cathode materials for NIBs. Although heavy polyanion groups ðPO32 4 ; P2 O7 ; SO4 Þ limit the specific capacity of these materials, these multiple polyanions offer a diverse channel of open-framework crystal structures, and the strong covalent bonding of the polyanions provides robust structural framework during electrochemical cycling, such as Na3V2(PO4)3, Na2FeP2O7, Na4M3(PO4)2P2O7 often as main polyanionic insertion materials for NIBs [16]. The strong covalent bonding in the polyanion units increased the safety and voltage. Due to the inductive effect of polyanion groups, which was first demonstrated by Manthiram and Goodenough in 1989, polyanionic cathode materials exhibit higher operating potentials than oxide materials [16]. In this section, we will discuss the phosphates, fluorophosphates, pyrophosphates, sulfates, and carbonophosphates referring the structure, ordering, diffusion, and phase transformation. 4.1.3.1 Phosphates NaMPO4 (M 5 Fe, Mn) The NaMPO4 compounds have olive (Fig. 4.2A) and maricite structures (Fig. 4.2B). Multiple computational studies indicated that these two structures have similar energies in NaMPO4 and experimental studies indicated that maricite is the thermodynamically stable phase [17,18]. However, the maricite phase lacks a good Na1 diffusion path in the structure (Fig. 4.2B). The electrochemically active olivine NaMPO4 has a Pnma space group and comprises vertex-sharing MO6 octahedra and PO4 tetrahedra that share one edge and all vertices with MO6 octahedra (Fig. 4.2A). The Na sites in olivine NaMPO4 from linear chains parallel to the b-axis providing open channels for Na-ion diffusion along the b-axis (Fig. 4.2A). The Na1 could migrate along several possible pathways in olivine NaMPO4, such as [010], [001], and [101]. Computations were performed the Na1 transport along [010] channels during desodiating and sodiating process in NaxFePO4 need lowest migration energy of 0.28 or 0.38 eV, respectively. The diffusion mechanism in the olivine NaxFePO4 is similar to LixFePO4. The facile diffusion of Na1 along [010] channels indicates olivine NaxFePO4 is mainly a one-dimensional (1D) Na-ion conductor (Fig. 4.2C) [19].

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FIGURE 4.2 Crystal structures of (A) olivine and (B) maricite NaFePO4. (C) Calculated Na1 migration pathway along [010] channels in olivine NaFePO4. (D) Calculated Na1 diffusion barriers in olivine NaxFePO4 (blue) and LixFePO4 (red) at x 5 0 and 1. Source: Reproduced with permission from (A and B) Q. Bai, L. F. Yang, H. L. Chen, Y. F. Mo, Adv. Energy. Mater. 8 (2018) 1702998. Copyright 2018, Wiley-VCH; (C) M. Nakayama, S. Yamada, R. Jalem, T. Kasuga, Solid State Ionics 286 (2016) 40. Copyright 2016, Elsevier; (D) S.P. Ong, V.L. Chevrier, G. Hautier, A. Jain, C. Moore, S. Kim, et al., Energy Environ. Sci. 4 (2011) 3680. Copyright 2011, Royal Society of Chemistry.

Compared to LixFePO4, Na1 diffusion in NaxFePO4 has a higher migration energy barrier (Fig. 4.2D) [17]. The exchange of Na1 with M cation on neighboring sites in olivine NaxFePO4 resulted in blocking the facile 1D channel along the b-axis. The olivine NaxFePO4 often exhibits a decay capacity. If Na is completely exchanged with M in olive NaxMPO4, the olive structure would transform into the maricite structure, which has no obvious open channels for low-barrier alkali-ion diffusion. 4.1.3.2 Na1 superionic conductor structured NaxM2(PO4)3 (M 5 V, Ti; x 5 1,2,3) Owing to the PO4-based polyanionic materials structural similarity with Na11xZr2P3-xSixO12 having very fast Na1 conductivity, they are named Na1 superionic conductor (NASICON). In variable valence metal ions in the NASICON structure, NaxM2(PO4)3 (M 5 V, Ti; x 5 1, 2, 3) with open three-dimensional (3D) ion transport channels and high ion diffusion rates was first reported by Goodenough and his co-workers, who research the potential variation of V41/V31, V31/V21, Fe31/Fe21, Nb51/Nb41, and Nb41/Nb31 in the process of lithium insertion/extraction [20,21]. For example, the composition NaxV2(PO4)3 is demonstrated with good electrochemical performance with respect to capacity, cyclability, rate capability, and stability. Chen and his group first reported the fabrication of carbon-coated Na3V2(PO4)3 as a novel electrode material for NIBs by a one-step solid-state reaction [22]. In order to determine the mechanism of sodium insertion/extraction into/out of the

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FIGURE 4.3 (A) Ex-situ XPS studies of NVP/C electrodes: (1) pristine sample, (2) charged, (3) discharged. (B) In-situ XRD patterns of the Na3V2(PO4)3/Na cell cycled between 3.7 and 2.7 V at a current rate of C/10, V Na3V2 (PO4)3, T NaV2 (PO4)3. (C) Schematic representation of the Na3V2(PO4)3 structure. (D) Possible Na-ion migration paths in Na3V2 (PO4)3 along x, y, and curved z directions. Source: Reproduced with permission from (A) K. Saravanan, C.W. Mason, A. Rudola, K.H. Wong, P. Balaya, Adv. Energy Mater. 3 (2013) 444. Copyright 2013, WileyVCH; (B) Z. Jian, W. Han, X. Lu, H. Yang, Y.-S. Hu, J. Zhou, et al., Adv. Energy Mater. 3 (2013) 156. Copyright 2013, Wiley-VCH; (D) W. Song, X. Ji, Z. Wu, Y. Zhu, Y. Yang, J. Chen, et al., J. Mater. Chem. A 2 (2014) 5358. Copyright 2014, Royal Society of Chemistry.

Na3V2(PO4)3 lattice, both ex-situ X-ray photoelectron spectroscopy (XPS) (Fig. 4.3A) and in-situ X-ray diffusion (XRD) (Fig. 4.3B) were carried out [23,24]. The results indicated that the mechanism of sodium insertion/extraction can be ascribed to two equilibrium electrochemical reactions involving V41/V31 plateau at 3.4 V and V 31/V21 plateau at 1.6 V, which enables the construction of a symmetric cell. The 3.4 V plateau involves a twophase reaction mechanism between Na3V2(PO4)3 and NaV2(PO4)3 involving preferential Na1 extraction from a particular site. In an effort to understand the 3D characteristics of the internal ion transportation paths of Na3V2(PO4)3, first-principle calculations combined with experiments were conducted by evaluating the activation energies toward Na3V2(PO4)3. It was proven that two pathways along the x and y directions and one possible curved route for ion migration were favored with 3D transport characteristics (Fig. 4.3C and D), providing ample evidence for the theoretical capacity of 117 mA h g21 [25]. 4.1.3.3 Pyrophosphates NIBs have been researched extensively over the past few years, along with Na-based pyrophosphates such as NaMP2O7 (M 5 Ti, V, Fe), Na2MP2O7 (M 5 Fe, Mn, Co), and

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Na4M3(PO4)2P2O7 (M 5 Fe, Co, Mn). Na2MP2O7 (M 5 Fe, Mn, Co) has different structural configurations, that is, triclinic (space group P1), monoclinic (space group P21/C), and tetragonal (space group P42/mnm). These three different structures of Na2MP2O7 (M 5 Fe, Mn, Co) all contribute to the Na1 transport direction channel. NaFeP2O7 consists of two different structures due to irreversible phase transitions at different temperatures. It was reported that the corner-sharing P2O4 and FeO3 octahedral units constituted the main skeleton of the FeP2O7 structure. The PO4 tetrahedron shared its corners with the other three octahedral and one tetrahedron, whereas the FeO6 octahedron shared all its apices with the PO4 tetrahedra. Furthermore, such a special structure, comprising FeO3 octahedral layers and P2O4 tetrahedral layers, allows Na1 ions to migrate in the plane parallel to (001). Similar compounds such as NaTiP2O7 and NaVP2O7 have also been reported [26]. Overall, the pyrophosphate systems form a fundamentally interesting cathode family with rich structural diversity and (de)sodiation behavior. Nevertheless, similar to the Li22xMP2O7 systems, it is difficult to realize two-electron reaction leading to complete desodiation. Thus the realizable capacity in Na22xMP2O7 is low, making them impractical for real-life usages. Research should be geared to find ways to extract two-electron reactions, controlling (off-) stoichiometry and to discover novel Na2MP2O7 (M 5 Ti, Cu, Cr) pyrophosphate systems for fundamental studies. Pyrophosphates offer rich structural diversity and polymorphism owing to the arrangement of corner-sharing P2O7 (PO4-PO4) units in 42 linear or twisted manner. When isolated PO32 4 units are present along with P2 O7 units, mixed phosphates are formed as first reported by Sanz et al. [27]. With a general formula of Na4M3(PO4)2P2O7 (M 5 3d metals like Co, Mn, Ni), it forms a 3D orthorhombic framework (space group: Pn21a) consisting of corner/edge-sharing MO6 octahedra and PO4 tetrahedra. The PO4 units share one edge and two corners with neighboring MO6 octahedra so as to develop a pseudolayered structure along the bc-plane. These layers are in turn bridged by P2O7 pyrophosphate units along the a-axis creating large tunnels hosting Na1 cations in four distinct sites. Following, in an effort to improve the energy density, isostructural Na4Co3(PO4)2P2O7 was probed for Na insertion. Reversible (de)sodiation with a capacity exceeding 95 mA h g21 was observed with a 4.5 V Co31/Co21 redox potential, benchmarking the highest ever redox potential for Na battery materials. These mixed phosphates have 3D Na1 diffusion pathways with low-migration barriers (0.2
0.24 eV) and high diffusion coefficients (DNa 5 10210
10211 cm2 s21) that favor efficient and high rate (de)sodiation activity [28]. Similar to layered transition metal oxides, it depicts multiple distinct voltage plateaus indicating a progressive phase transition involving Na-ion structural ordering. Mixed phosphates are rare examples of cathode systems with multiple electron activity having a good combination of electrochemical performance, operational safety, and thereby possible commercial prospects. 4.1.3.4 Fluorophosphates Due to the incorporation of electronegative F ions with high iconicity into phosphatebased materials, the fluorophosphates can be considered as a promising materials for high-voltage cathodes in NIBs [28]. For example, Na2FePO4F, NaVPO4F, Na3V2(PO4)3F3, and Na3V2O2x(PO4)2F3-x Na2FePO4F were first reported by Ellis and his group [29]. It crystallizes in Pbcn orthorhombic space group, and the bioctahedral [Fe2O7F2] units and [PO4] tetrahedral units share their corners to constitute a 2D framework. The two Na1 ions are

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located in both the [FePO4F] interlayers and near the sheets. To better understand how the atomic-scale features influence the electrochemical properties of Na2FePO4F, a computational study quantified that the volume expansion of Na2FePO4F or Na2MnPO4F during sodiation is less than 20%, conforming the robustness of the crystal framework during electrochemical cycling. Similar to phosphate cathodes, the robust 2D diffusion network consisting of interconnected FeO4F2-PO4 polyhedra in Na2FePO4F provides facile Na-ion diffusion and good intercalation reversibility. Among those reported cathodes, vanadium (V) based fluorophosphates cathode materials have attracted more attention due to their high operating voltages and high theoretical capacities, as shown in Fig. 4.4 [30]. The distinguished features are derived from the inductive effect of the PO32 4 polyanion and strong P
O bond, ensuring the high working potential versus Na1/Na and the stable structure during the charge/discharge process. NaVPO4F has two types of phase that exhibit good sodium storage performances: tetragonal symmetry structure (space group I4/mmm) and monoclinic structure (space group C2/c). The tetragonal NaVPO4F phase structure is in an agreement with its related counterpart, Na3Al2(PO4)3F2 structure in which [Al2O8F3] bioctahedra and [PO4] tetrahedral are connected to each other by a corner-sharing O vertex. Na1 distributes through the produced cavities. The structure is also similar to Na3V2(PO4)2F3, possessing a tetragonal crystal structure with a space group of P42/mnm [31]. As shown in Fig. 4.5A and B, Na3V2(PO4)2F3 is composed of [V2O8F3] bioctahedral and [PO4] tetrahedral units connected by sharing oxygen atoms. Na1 occupies the channel cavities along the a- and b-axis, which are surrounded by four P and three O. There are two types of Na sites (fully occupied site of Na1 and half occupied site of Na2) present in the Na3V2(PO4)2F3 structure, in which Na1 at Na2 sites have higher chemical potential than that at the Na1 site. As the cathode for NIBs, two potential plateaus at 3.7 and 4.2 V were observed in the voltage range of 2.0
4.5 V, corresponding to the two-step redox reaction of V31/V41 [32]. In the voltage range of 1.6
4.6 V, two of three Na1 can be reversibly extracted from/inserted into NaxV2(PO4)2F3 (1 # x # 3) with a theoretical capacity of 128 mA h g21, corresponding to an energy density of B507 W h kg21. In order to better understand the sodium exaction FIGURE 4.4 Average discharge potential (V vs Na1/Na) as a function of theoretical capacity (mA h g21) and energy density (W h kg21) of representative V-based materials as cathodes for NIBs. Source: Reproduced with permission Q.H. Wang, J.T. Xu, W.C. Zhang, M.L. Mao, Z.X. Wei, L. Wang, et al., J. Mater. Chem. A 6 (2018) 8815. Copyright 2018, Royal Society of Chemistry.

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FIGURE 4.5 Schematic representation of the Na3V2(PO4)2F3 crystal structure viewed along the (A) a-axis and (B) c-axis. Na1 and Na2 in (B) are fully occupied Na sites and half occupied Na sites, respectively. (C) Electrochemical curves of potentialcomposition and (D) sodium distribution within NaxV2(PO4)2F3 (1 , x , 3). The red/black circles [extreme right of (D)] indicate that only two positions out of four are expected to be occupied. The orange dotted circle [bottom-left of (D)] shows that the sodium site in NaV2(PO4)2F3 is empty. Source: Reproduced with permission from (A and B) R.A. Shakoor, D.H. Seo, H. Kim, Y.U. Park, J. Kim, S.W. Kim, et al., J. Mater. Chem. 22 (2012) 20535. Copyright 2012, Royal Society of Chemistry; (C and D) M. Bianchini, F. Fauth, N. Brisset, F. Weill, E. Suard, C. Masquelier, et al., Chem. Mater., 27 (2015) 3009. Copyright 2015, American Chemical Society.

mechanism of Na3V2(PO4)2F3, high-resolution synchrotron XRD was carried out. It was revealed that four intermediate phase transformations occurred from Na3V2(PO4)2F3 to NaV2(PO4)2F3, as shown in Fig. 4.5C. The detailed route of sodium rearrangement inside the unit cell is illustrated in Fig. 4.5D. 4.1.3.5 Sulfates While the alkali metal fluorosulfates have high redox potential stemming from the 2 2 electronegativity of both SO22 4 and F species, their presence (particularly F ) make them

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prone to moisture absorption and chemical degradation. The stability of those materials can be partly circumvented by omitting F2 species altogether. Using this clue, an entirely new alluaudite-type sulfate framework, Na2Fe2(SO4)3 has been explored. Unlike typical NASICON-related structures with corner-sharing FeO6 octahedra, Na2Fe2(SO4)3 forms a unique alluaudite-type framework, with edge-sharing FeO6 octahedra. Then, the edgesharing FeO6 octahedra units bridge together by SO4 units, forming a 3D framework with large tunnels along the c-axis. Thanks to the special structure, a very suitable operating potential of 3.8 V was observed based on the Fe31/Fe21 redox couple. To date, this is the highest potential among all Fe-based NIB cathode materials. The researchers also explored the bisulfates and hydroxysulfates as cathodes for NIBs. The iron-based bisulfates and hydroxysulfates are paid more attention for their open channels for Na1 migration and electrochemically active. When tested in Na half-cell architecture, reversible uptake of 2 Na1 (per f.u.) was observed good cycling stability, which is due to the reversible topotactic transformation of crystalline jarosite to its amorphous sodiated derivative. SO4-based polyanionic insertion compounds deliver high redox potential stemming from the electronegative SO4 units. They can be prepared by low-temperature routes (T , 350 C), making the synthesis sustainable. However, they are inherently hygroscopic and should be handled in inert atmosphere. Sulfates are economic precursors and often found as byproducts of industrial/agricultural processes. So, tapping SO42 species with abundant Na and Fe sources can in theory realize economic Na
Fe
S
O cathode materials for high-energy density Na battery for large-scale grid storage. 4.1.3.6 Fluorosulfates Na metal fluorosulfates [NaMSO4F, M 5 3d metals] was unraveled by topotactic reaction between NaF and MSO4 H2O monohydrate precursors by low-temperature (T , 300 C) routes like ionothermal, solid-state, and dissolution
precipitation methods. Due to the large cationic size of Na1 (116 pm: Na1CN56), deviating from the LiMSO4F tavorite structure (triclinic, P-1), the NaMSO4F sulfates adopt a max-wellite framework (monoclinic, C2/c) with higher symmetry. [28]. The NaMSO4F has poor conductivity. Despite several optimization efforts, the activity of NaFeSO4F remained abysmal. While pristine NaFeSO4F was found to be inactive, it was possible to realize Na1 (de)insertion in KFeSO4F derivatives. Further exploration should focus on unraveling unknown fluorosulfates such as (1) vanadyl analogs [A(VO)SO4F, A 5 Li/Na/K], and (2) ammonium analogs [(NH4)MSO4F].



4.1.3.7 Silicates Owing to their low cost and large abundance, Li-based silicates have been intensively studied as cathode materials. Compounds such as Li2FeSiO4 display a large variety of polymorphs and attractive electrochemical performances [26,28]. Therefore in the quest for new cathode materials for NIBs, their Na counterparts became of increasing interest and recent research efforts have been dedicated to the exploration of new thermodynamically stable Na-based silicates. The redox potential of silicates is lower than the ones for sulfates or phosphates due to the weaker inductive effect. However, given the low stability window of current electrolytes, this drawback might be an advantage. Furthermore, the lower molecular weight of the SiO42 4 group is beneficial in terms of capacity.

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4.1.3.8 Borates 32 One of the major drawbacks of the above-described SO22 4 and PO4 polyanions is their large molecular weight, which is detrimental to the capacity of the cathode. Borates, however, being the lightest polyanion, can drastically reduce the dead weight of the cathode material, and thus increase the gravimetric capacity. Another advantage of borate-based compounds is the different oxygen coordination states that boron can occur in, which offer a large variety of structural frameworks that might be susceptible for cation intercalation [26,28]. Indeed, over 200 various borate-based minerals have been reported, which offer a wide choice of potentially attractive cathode materials. However, only few compounds (borophosphates, pentaborates) have been reported so far.

4.1.4 Prussian blue-type cathode materials Prussian blue (PB) and its analogs (PBAs, Na2M[Fe(CN)6], M 5 Fe, Co, Mn, Ni, Cu, etc.) are a large family of transition-metal hexacyanoferrates with open-framework structure, abundant redox-active sites, strong structural stability, low cost, and environmental friendliness [33]. Particularly, due to their large ionic channels and interstices in the lattice, PBs are one of the few host materials that can accommodate larger alkali cations, such as Na1 and K1 ions, for facile and reversible insertion reactions. Benefiting from this structural feature, PB compounds have been intensively investigated as a new alternative and lowcost Na-insertion cathode during past five years [34
37]. Hence, various PBAs such as Na1.4Cu1.3Fe(CN)6, Na1.94Ni1.03Fe(CN)6, and Na0.61Fe[Fe(CN)6]0.94 have been reported as cathode materials for NIBs. However, due to the higher oxidation state of transition metals in frameworks, many PBAs can only offer one electron per formula unit, which results in relatively low capacity and also suffer from a sodium-deficiency problem. To overcome these problems, Na-rich sodium ion hexacyanoferrate compounds, also known as Prussian white, with the chemical formula Na2-xFe[Fe(CN)6]1-yWynH2O (where W denotes a vacancy occupied by coordinating water; x, y, and n are small) are viewed as promising cathode materials [38]. Since each molecular formula of PBs contains two redox centers M12/M13 and Fe12/Fe13 suggesting that PBs can reach a two-electron redox capacity, corresponding to a reversible 2 Na1 storage per molecular unit. As demonstrated recently, Na2FeFe-PB can release a considerably high capacity of  160 mA h g21 at average high potential of 3.1 V and Na2MnMn-PBA can reach a high reversible capacity of 209 mA h g21 at high potential of 3.50 V (vs Na/Na1), corresponding to high-energy densities of 496 and 730 W h kg21, respectively. Such high-energy densities realized by the PB cathodes even exceed those of spinel LiMn2O4 (  430 W h kg21) and olivine LiFePO4 (  530 W h kg21) currently used for commercial Li-ion power batteries, possibly offering a promising alternative to their LIBs. In addition, except for a wide selection of M metals, Fe element in the PBs can also be substituted by many other transition metals with variable valence such as Co, Ni, Mn, Cu, and Zn, forming a series of structurally similar but electrochemically tunable cathode host materials [39,40]. Benefiting from their low materials cost, structural stability, and electrochemical tenability, PBs, particularly Fe- and Mn-based PBs, may serve as a high-performance cathode for commercial development of NIBs.

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4.2 Anode nanomaterial for sodium storage 4.2.1 Overview of anode material As an important component of batteries, negative electrode materials with high specific capacity and long-life cycling property are crucial to increase the overall energy storage density of cells [15]. Based on electrochemical reaction mechanisms, there are mainly three mechanisms involved in sodium storage for anode materials: the intercalation/deintercalation reaction, the conversion reaction, and the alloying/dealloying reaction. Carbon materials and Ti-based oxides are developed as NIB negative electrode materials on the basis of intercalation reaction mechanism.

4.2.2 Intercalation-based anode materials 4.2.2.1 Carbon materials Extensive research shows that carbon-based materials not only have a low sodiumembedded platform, high capacity, and good cycle stability, but also have the advantages of abundant resources and simple preparation. Therefore carbon materials have become the most important anode material for the industrialization of NIBs. The carbon materials mainly involved in graphite and amorphous carbon. 4.2.2.2 Graphite Graphite is the most common negative electrode material for commercial LIBs, but its theoretical capacity is only 372 mA h g21 when used as a negative electrode material for NIBs. Because the ionic radius of sodium is 55% larger than the ionic radius of lithium, and the carbon and sodium ions in graphite cannot form a stable Na
C bond, sodium ions cannot enter the graphite sheet through the intercalation reaction to form a stable intercalation compound [41]. Therefore graphite cannot be directly applied to a anode electrode material of an NIB. In order to achieve Na1 storage of graphite, the effective method is to increase the graphite layer spacing. Carbon has a “card house” structure containing graphite-like crystallites and amorphous regions [42]. The crystallites are stacked together with a small number of approximately parallel graphene sheets with a large d-spacing (0.36
0.4 nm). Three different Na storage environments and three corresponding modes of interaction with hard carbon have been reported in the literature: adsorption at surface active sites, adsorption-like nanopore filling, and interlayers between graphene layers with appropriate d-spacing. By simulating the interaction of the graphite-like layer with Na ions with d-spacing varying from 0.37 nm, a minimum spacing for Na intercalation, to 2.0 nm [43]. The results of Fig. 4.6 predict that the mode of interaction of Na with the graphite layer depends on the spacing between the layers. Especially for the interlayer spacing of 0.37
0.47 nm, the minimum value of the interaction curve is between the graphite sheets. The interaction intensity increases with increasing interlayer spacing and reaches 1.38 eV at a layer spacing of 0.47 nm. This layer spacing range supports true sodium ion insertion. When the layer spacing increases, the shape of the interaction curve changes: the minimum value gradually

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FIGURE 4.6 Interaction of Na ion with two graphitic surfaces as a function of the distance to the surfaces normalized by the intersurface spacings. The corresponding spacings are shown in the legend. The characteristic energy regions corresponding to three mechanisms for Na1 interactions with graphitic carbon electrode are shown as shaded rectangles. Source: Reproduced with permission S. Qiu, L.F. Xiao, M.L. Sushko, K.S. Han, Y.Y. Shao, M.Y. Yan, et al., Adv. Energy Mater. 7 (2017) 1700403. Copyright 2017, Wiley-VCH.

flattens, and the maximum interaction energy decreases as the layer spacing increases. In this regime, Na will still intercalate into graphitic regions. However, Na will be more fluid and will occupy a wider area between the graphite sheets. The interaction energy function between Na and graphite surface is calculated by Morse form of potential energy:
2 EðrÞ 5 Ead 12e2aðr2reqÞ where a 5 (k/2Ead)0.5, Ead is the adsorption energy and req is the equilibrium distance. The ˚ calculated using DFT simulations for the adsorption values Ead 5 0.69 eV and req 5 2.42 A of Na on graphite have been used [44]. The bond force constant k was determined using the empirical Badger’s rule [45], which was extensively tested with quantum mechanical ˚ , and simulations [46]. According to this rule, k 5 A(req 2 B)23 with constant A 5 1.86 eV A constant B depends on the interacting atom row numbers in the periodic table. The values ˚ was used for the interactions of Na with graphite [47]. The interaction energy of B 5 0.74 A for Na ion with two graphite sheets separated by the distance d can be then calculated as E(r) 1 E(d 2 r), where r is the distance between Na ion and one graphite surface. When the interlayer spacing is 1.0 nm, the energy curve has two distinct minimum values, indicating a transition to the surface adsorption state. The maximum interaction energy is independent of the layer spacing and is equal to 0.69 eV. The method commonly used by researchers in Na1 is to preoxidize graphite with sulfuric acid and potassium permanganate solution, and introduce some oxygen-containing functional groups (such as nitro, hydroxyl, sulfonic acid groups, etc.) on the graphite surface to form oxidation and expansion. Since the presence of oxygen-containing functional groups on the surface of the graphite increases the distance between the sheets, the graphite oxide, and the expanded graphite can efficiently store Na1 by the intercalation reaction [48]. In addition to increasing the interlayer spacing, several recent studies have shown that when a suitable electrolyte solvent is selected, Na1 can be inserted into the graphite interlayer by co-intercalation of Na1 and solvent molecules. With the help of some specific solvents, graphite can reversibly store Na1 and has good cycle performance.

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FIGURE 4.7 The co-intercalation mechanism of solvent and the performance after 8000 cycles. Source: Reproduced with permission J.J. Braconnier, C. Delmas, P. Hagenmuller, Mater. Res. Bull. 17 (1982) 993. Copyright 2015 American Chemical Society.

In addition to increasing the interlayer spacing, several recent studies have shown that when a suitable electrolyte solvent is selected, Na1 can be inserted into the graphite interlayer by co-intercalation of Na1 and solvent molecules. With the favor of some specific solvents, graphite can reversibly store Na1 with good cycle performance. Cohn et al. [49] coated sodium ions with diglyme as a solvent shell. As shown in Fig. 4.7, this nontacky coating facilitates rapid insertion and removal of sodium ions between the multilayer graphene and reduces solvation, which maintains a reversible capacity of approximately 100 mA h g21at a current density of 30 mA g21, and no attenuation after 8000 cycles. 4.2.2.3 Amorphous carbon Amorphous carbon can be classified into soft carbon and hard carbon according to the degree of difficulty in graphitization. Soft carbon, also known as graphitizable carbon, is a transitional carbon that can be converted to graphitized carbon by heat treatment at temperatures above 2000 C. Soft carbon is mainly derived from pyrolysis of organic polymers and petroleum asphalt. Compared with graphitized carbon, the degree of graphitization of soft carbon is low, the grain size is small, and the interplanar spacing is large, which also facilitates the insertion and removal of sodium ions during charge and discharge, and is favorable for compatibility with the electrolyte. Despite this, some of the shortcomings of soft carbon itself limit its use in batteries, mainly due to lower specific capacity and severe voltage hysteresis. Hard carbon is difficult to graphitize even at temperatures above 3000 C. The precursor is the thermal decomposition of hot melt resins, such as some phenolic resins and cellulose present in plants, mainly pyrolytic carbon, resin carbon, and carbon black. Hard carbon has a single carbon atom layer, which is more spaced than the soft carbon layer, which is more conducive to the diffusion of sodium ions. In addition, abundance of lattice defects in the atomic layer provides more active sites for sodium ions, so hard carbon has a larger specific capacity. However, lattice defects also bring some disadvantages while increasing the capacity. Sodium ions are difficult to escape after being embedded in the lattice defects of the atomic layer, which brings about a problem that the first charge and discharge reversible specific capacity loss is large, and the first coulomb

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efficiency is low. Heteroatoms in hard carbon also cause more severe voltage hysteresis than soft carbon. At the same time, hard carbon has no obvious charging and discharging platform, which also makes the output voltage of the battery unstable. 4.2.2.4 Heteroatom-doped carbon materials The introduction of heteroatoms (such as N, S, B, P) into the carbon lattice is one of the key factors for improving electrochemical performance [50]. Due to the electronegativity difference between heteroatoms and carbon atoms, the introduction of an appropriate number of dopant atoms in the carbon matrix can provide an active site for immobilization of Na1, thereby improving the electrochemical performance of the cell. For example, a nitrogen heteroatom is introduced into the graphene crystal lattice, which is mainly in the form of electrochemically active pyridinium nitrogen and pyrrole nitrogen, and is applied to the anode material of the NIB, which can effectively improve the cycle performance and rate performance of the battery. At present, there are mainly two synthetic routes for introducing electrochemically active heteroatoms into materials, the first being pretreatment, such as the preparation of nitrogen-doped graphene and carbon nanotubes by chemical vapor deposition (CVD). The precursor containing a heteroatom and carbon (pyridine, polyacrylonitrile, polyaniline, etc.) can also be pyrolyzed at a high temperature to directly bond the heteroatom to the carbon skeleton. In 2009, Liu et al. [51] first used NH3 as a nitrogen source, CH4 as a carbon source, and copper as a catalyst. N-doped graphite thin was synthesized by the CVD method (Fig. 4.8). By controlling the ratio and flow rate of NH3 and CH4, the doping degree of N was high 8.9%. Guo et al. [52] created a unique N, O dual-doped biocarbon nanosheet with fractional porosity through direct pyrolysis of low-cost cuttlefish and simple air oxidation activation (AOA) technology. As the AOA time increases, the thickness of the carbon sheet can be effectively reduced (from 35 to 5 nm), which may lead to tunable preparation of carbon nanosheets with a certain thickness. Furthermore, an increase in the N-doping amount from 7.5 to 13.9 at.% was observed after AOA, demonstrating the unique role of AOA in adjusting the doping heteroatoms of the carbon matrix. It is worth noting that by adjusting the thickness of the carbon sheet and the heteroatom doping by AOA, an excellent sodium capacity cycle retention capacity combination is achieved. Specifically, a current state-of-the-art Na1 storage capacity of 640 mA h g21 was obtained, which was comparable with the lithium-ion storage in carbon materials. Even after charging/discharging at large current densities (2 and 10 A g21) for 10,000 cycles, the as-obtained samples still retained the capacities of 270 and 138 mA h g21, respectively, with more than 90% retention. The other is a postreaction method in which a heteroatom-containing precursor (triphenylphosphine, urea, melamine, borane, boron, and ammonia) is reacted with various graphene porous carbons or the like to introduce a heteroatom. 4.2.2.5 Titanium-based oxides As a classic metal oxide, titanium dioxide (TiO2) has been widely studied in NIBs due to its structural stability, safe working voltage, and low consumption as an anode material [53]. However, the original sodium TiO2 has low sodium ion diffusion ability and poor conductivity, which seriously affects its sodium ion storage performance [54]. One strategy is to reduce the migration of sodium ions by reducing the size of TiO2 to the nanoscale,

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FIGURE 4.8 (A) CV curves of CBCS-A3, (B) capacitive (red) contribution to charge storage of CBCS-A3 anode in NIBs at 0.4 mV s21, (C) the ratios of capacitive charge storage against the total charge storage in CBCS, CBCS-A3, and CBCS-A8, (D) the charge
discharge curves of CBCS-A3, (E) the rate performance of the CBCS and the samples treated by AOA for various time, (F) the cycle performance of CBCS-A3 at 2 and 10 A g21, respectively, and (G) the schematic illustration of the evolution of the porosity and the pseudocapacitance in carbon matrix with prolonging AOA treatment. Source: Reproduced with permission Y.Q. Guo, W. Liu, R.T. Wu, L.J. Sun, Y. Zhang, Y.P. Cui, et al., ACS Appl. Mater. Interfaces 10 (2018) 38376. Copyright 2018, American Chemical Society.

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thereby achieving fast rate performance. Another strategy is to combine TiO2 with a carbon material to prepare a nanostructured composite. It can effectively enhance the electron transport capacity of TiO2, inhibit the dissolution of intermediate products, reduce the agglomeration of nanostructured TiO2 materials, and improve the cycle stability of TiO2 materials [55]. So, the most important thing at present is to construct a new TiO2/carbon nanostructured material to improve its sodium storage capacity [56]. In 2011, Johnson and Rajh of the Argonne National Laboratory in the United States first studied the performance of TiO2 as an anode material for NIBs [57]. Studies have shown that when the diameter of amorphous TiO2 nanotubes is larger than 80 nm, the material is at the 15th week of the cycle. The reversible specific capacity can still reach 150 mA h g21. Since then, TiO2 has gradually attracted people’s attention as a cathode material for NIBs. TiO2 exhibits a variety of polymorphisms due to the different spatial packing arrangement of TiO6 octahedrons, and the coordination of titanium ions with oxygen ions is six times. TiO2 polymorphisms reported as anode materials for NIBs include anatase, rutile, TiO2-B (bronze), TiO2-H (hollandite), and amorphous TiO2. Anatase TiO2 has a quadrilateral structure (Fig. 4.9A), and the space group I41/amd consists of four sides of TiO6 octahedron and adjacent octahedron. The twisted cubic dense oxygen crystal lattice forms a 3D skeleton structure [58]. Rutile has four aspects of P42/mnm structure (Fig. 4.9B), which consists of angular shared octahedrons and is considered to be the most stable thermodynamic form of TiO2 [59]. TiO2-B exhibits a monoclinic C2/m structure, and an edge-sharing corner-sharing TiO6 regular octahedron forms a more open structure (Fig. 4.9C), resulting in a low density of TiO2-B (3.73 g cm23) more than anatase (3.89 g cm23) and rutile (4.25 g cm23) stage [59]. The open channel is parallel to the b-axis. The TiO2-H exhibits a FIGURE 4.9 Structures of (A) anatase, (B) rutile, (C) TiO2-B, and (D) TiO2-H. Source: Reproduced with permission W.G. Wang, X. Wu, J. Wang, L.J. Fu, Y.S. Zhu, Y.P. Wu, X. Liu, Adv. Mater. Technol, 3 (2018) 1800004. Copyright 2018, WILEY-VCH.

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FIGURE 4.10 (A) High-resolution TEM image of TiO2/graphene composite, clear lattices with spacings of 0.62 and 0.35 nm are assigned to the (001) planes of TiO2-B and (101) planes of anatase, respectively. (B) Highmagnification SEM image of G-TiO2, revealing the structural detail of an individual microsheet. (C) Contribution ratios of the capacitive and diffusion-controlled charge from CV curves at various scan rates. (D) Rate performance at various current densities from 50 to 12,000 mA g21. Source: Reproduced with permission C. Chen, Y. Wen, X. Hu, X. Ji, M. Yan, L. Mai, et al., Nat. Commun.6 (2015) 6929. Copyright 2015, Nature Publish Group.

tetragonal I4/m structure (Fig. 4.9D), which is a cross-linked double-stranded chain in the [001] direction run by a double-stranded TiO6 regular octahedron, edge, and shared form (202) channels. The density of TiO2-H is even lower (3.46 g cm23) [60]. Among such polymorphic forms of TiO2, anatase TiO2 is most widely studied as an NIB anode material [61,62]. As shown in Fig. 4.10, Chen et al. reported that Na1 insertion pseudocapacitors (76.9% TiO2-B and 23.1% anatase TiO2)/graphene nanocomposites in TiO2 make NIBs have high rate capability and long cycle life [63]. This cross-linking of graphene with TiO2 nanocrystals provides a more feasible sodium intercalation/deintercalation channel at the graphene
TiO2 interface with lower energy barrier, enabling fast charge storage and long-term recyclability. This superior performance can be attributed to the robust structure of the nanocomposite. This naturally rich, nontoxic, polymorphic TiO2 is considered an interesting and promising candidate for NIB anode materials. But at the same time, it will encounter many challenges with poor cycle and rate performance. Therefore continuous efforts are still needed to realize the commercial application of high-performance TiO2 and its composite materials in the near future.

4.2.3 Conversion-based anode materials Although the intercalation-based electrode materials exhibit long-term cycling performance, they deliver limited specific capacity due to intrinsically low theoretical capacities. Owing to the oxides, sulfides, and phosphides with high theoretical capacity and low cost, those materials have been investigated recently.

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4.2.3.1 Oxides Metal oxides have been extensively studied as anodes for LIBs and have high reversible capacities, even higher than theoretical capacity. It is promising to develop new oxides electrode material with a higher theoretical capacity to further increase the energy density of the batteries. Metal oxides have been extensively studied for their low-cost, high theoretical specific capacity, and are used in high-capacity anode materials for NIBs [64]. Metal oxides as anode materials for NIBs mainly include Fe2O3, Fe3O4, Co3O4, NiO2, MnO2, MoO2, SnO2, and SbO2, etc. The mechanism of sodium storage can be divided into two categories: (1) when M in MxOy is an electrochemically inactive element (such as Fe, Co, Ni, Cu, etc.), it reacts with sodium to form M and Na2O; (2) when M is an electrochemically active element (such as Sn, Sb, etc.), the first reaction occurs to form the metal element M and Na2O, and then, the metal M is alloyed with Na to form NanM. The corresponding reaction formula is shown below: 1. MxOy 1 2y Na1 1 2y 4e2 -x M 1 y Na2 O; 2. M 1 n Na1 1 n e2 -NanM: Metal oxides exhibit high capacity due to a multistep reaction process, but such materials have poor cycle stability as anode materials for NIBs. This is mainly due to the poor conductivity of the metal oxide and the large volume expansion during charging and discharging [65]. The metal oxide material may deteriorate the integrity of the electrode structure due to poor self-conductivity and large volume expansion during the cycling, resulting in poor cycle performance and rate performance. Iron oxide is a common transition metal oxide. It has the advantages of high theoretical capacity (1007 mA h g21), stable chemical properties, environmental friendliness, high safety performance, abundant raw material sources, and low cost. It is considered to be a promising NIB anode material [66]. Nanostructured iron oxide was first reported as a potentially viable NIB anode, providing a capacity of 350 mA h g21 after 30 cycles of 40 mA g21 [65]. In addition, most of the currently reported Fe2O3 electrodes for NIBs have inferior cycling performance and poor rate capability during charge and discharge, due to its low electron conductivity and large specific volume change. To address these drawbacks, the general strategy is to combine the designed Fe2O3 nanostructures with carbonbased conductive substrates such as thin carbon shells, carbon nanotubes (CNTs), graphene oxide (GO), or reduced graphene oxide (r-GO) to form a composite. The material maintains structural integrity and improves the electrical conductivity of the Fe2O3-based anode material. In order to increase the utilization of active materials, there still some work devote to design the hierarchical structure of composites. Such as 3D porous γ-Fe2O3@C nanocomposite [67], nanoflakes assembled hydrangea-like Fe2O3@C@MoS2@C nanocomposite [68], in-situ grown Fe2O3 single crystallites on reduced graphene oxide nanosheets [69] have exhibit superior specific capacity and excellent stability. Among various transition metal oxides, spinel Co3O4 is one of the most promising anode materials for NIBs, with a theoretical specific capacity of approximately 890 mA h g21 [70]. However, transition metal oxides generally have disadvantages such as large volume change during conductive soidation/desoidation, resulting in low specific capacity, poor rate capability, and fast capacity decay of Co3O4 electrodes [71]. In general,

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there are two effective strategies to alleviate the above problems and improve the Na storage capacity of the Co3O4-based electrode. One strategy is to design robust nanostructures for Co3O4, such as nanowires and nanosheets. Reducing the particle size can effectively shorten the Na1 ion diffusion path and promote the effective adjustment of volume change. Another strategy is to prepare Co3O4-based composites from highly conductive carbon materials such as carbon nanotubes and graphene. Carbon materials not only provide a fast electron transfer medium, but also have a buffer matrix, reduced volume change, and electrode pulverization. However, the rate performance and cycle performance of the Co3O4-based electrode are still unsatisfactory, and the specific capacity obtained is still much lower than the theoretical capacity of Co3O4. Therefore there is an urgent need for a reasonable architecture design, integrating and optimizing these two strategies to further improve the Na storage performance of the Co3O4-based electrode. These are also key factors in improving the overall insertion/extraction kinetics of the electrode stage and maintaining their integrity over multiple charge and discharge cycles [72]. For example, Xu et al. [73] synthesized rambutan-like hybrid hollow spheres of carbon confined Co3O4 nanoparticles by a simple one-pot hydrothermal method. The layered hollow structure of ultrafine Co3O4 nanoparticles is embedded in a continuous carbon matrix, which greatly improves the stability of the structure and the rapid dynamics of the electrode. The hollow structure can be well adapted to the strain in the sodiation/desodiation process. While the ultrafine particle size and continuous carbon matrix of Co3O4 ensure high electrochemical reactivity and rapid charge transport, making this layered mixed hollow structure storage for Na is very efficient. Fig. 4.11A displays the cycle performances of the three Co3O4 electrodes cycled at a current density of 0.2 A g21 for 60 cycles. It can be seen that the cycle performance of the R-Co3O4/C hybrid hollow sphere electrode is significantly better than that of the other two structures. This shows that the structure of the Co3O4 has a relatively large influence on the cycle performance. Fig. 4.11B shows the advantages of the charge and discharge process of the composite. It can be seen that the rational design of the nanostructure can greatly improve the electrochemical performance of the material.

FIGURE 4.11 (A) Cycle performances of the three electrodes at a current density of 0.2 A g21. (B) The schematic illustration of the advantages of the R-Co3O4/C hybrid hollow sphere during sodiation and desodiation. Source: Reproduced with permission M. Xu, Q.Y. Xia, J.L. Yue, X.H. Zhu, Q.B. Guo, J.W. Zhu, et al. Adv. Funct. Mater. (2018) 6, 1807377. Copyright 2018, WILEY-VCH.

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Tirado et al. [74] reported for the first time that the transition metal oxide spinel NiCo2O4 was obtained as an NIB anode material by annealing the oxalate precursor. The material is completely reduced during the electrochemical reaction and produces Na2O with a reversible capacity of only 200 mA h g21, well below its theoretical capacity (890 mA h g21). There are two main reasons for the irreversible capacity loss of the first cycle: First, the first step of the battery reaction is irreversible to some extent; second, the formation of the SEI film also consumes a part of the sodium ion. Therefore it is generally improved by electrochemical metal properties by synthesizing metal oxides to have novel micro/nanostructures or by recombining with conductive materials to inhibit volume expansion and promote the transport of ions and electrons. SnO2 also has a high theoretical capacity as an NIB anode material. The reaction between SnO2 and sodium ions is divided into two steps of transformation and alloying. The reaction formula is as follows: 1. SnO2 1 4Na1 1 4e2 -Sn12Na2 O; 2. Sn 1 3:75Na1 1 3:75e2 -Na3:75 SnðNa15 Sn4 Þ: Due to the poor kinetic performance of the first step to generate Na2O, it is difficult to achieve a complete reverse reaction in this step. In addition, during the conversion from Sn to Na15Sn4, the volume undergoes a large expansion, resulting in flaking of the particles and rapid decay of capacity [75]. Therefore the SnO2 material has poor performance at large rate, and how to overcome the problem of slow conversion kinetics becomes the key. Another problem with SnO2 is still the large volume change during the cycle, resulting in poor cycle performance [76]. At present, there are two main modification methods: (1) to prepare nanomaterials with loose structure; (2) to combine with carbon-based or other matrix materials to alleviate volume changes. Therefore there is a demand for a support to buffer volume changes and to immobilize comminuted particles. The composite material prepared by compounding with the carbon material effectively alleviates the large volume change of the electrode material during the sodium deintercalation process. At the same time, the presence of the conductive carbon material also contributes to the improvement of the electrical conductivity of the composite material, thereby increasing the specific capacity. Graphene has a typical two-dimensional (2D) structure and is an excellent base material for the growth of metal oxides [77]. The excellent electron conduction properties and stable structure of graphene are beneficial to improve the conductivity and buffer volume change of the composite. Therefore the preparation of SnO2/ graphene composites is one of the effective ways to improve the electrochemical performance of SnO2. In summary, metal oxides have a high theoretical capacity by conversion with sodium and are one of the candidates for excellent NIB anode materials. However, there is still a huge challenge in the electrochemical properties of the electrode material. Although the oxide as an electrode material has a high theoretical capacity, the disadvantages such as low coulomb efficiency limit its development. Despite these challenges, with the success of graphene, the discovery and application of metal oxide nanosheets has opened up a new field for sustainable energy applications. It is believed that the development of new oxide hybrid materials will further enhance the performance of sustainable energy devices and contribute to solving the current environmental and energy crisis.

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4.2.3.2 Sulfides In recent years, metal sulfides have drawn great attention in energy storage [78]. Due to the high theoretical capacity and easily-controlled morphology of transition metal sulfides, it is considered to be one of the ideal anode materials for NIBs. In fact, the sodium storage mechanism of most metal sulfide electrode materials in NIBs is similar to that in LIBs, and is still mainly embedded, alloy, and conversion reactions. However, since the radius of the Na1 is larger than the radius of the Li1, the process of insertion and removal and diffusion in the electrode material of the same structure is relatively difficult, and the structure of the electrode material after embedding changes greatly. Therefore the large volume changes, sluggish Na1 diffusion kinetics, and poor electrical conductivity have limited its commercial applications. Compared to related metals and oxides, MSs have smaller volume changes and higher first cycle efficiencies due to their greater reversibility and are therefore expected to exhibit better mechanical stability [2]. In addition, the discharge product of the metal sulfides (Na2S) is superior to the discharge product of the metal oxide (Na2O); the M
S bond is in the metal oxide, and the MSs are weaker than the M
O bond, which is kinetically favorable for the conversion reaction. Therefore metal sulfides have attracted attention in potential applications in NIBs [79]. The 2D-layered structure MoS2 consists of a single layer or a plurality of layers of sulfide. The upper and lower layers of the single-layer MoS2 are sulfur atom, and the middle layer is molybdenum atoms. This type of structure has strong covalent bonds between the layers, and there is a weak van der Waals force between the layers. The multilayer MoS2 consists of several single. The layer consists of MoS2 with a layer spacing of approximately 0.65 nm. The 2D-layered structure facilitates the intercalation of sodium ions, and provides a larger space for ion intercalation, avoiding volume expansion when sodium is embedded, thereby maintaining structural stability [80]. The layered structure MoS2 has a high specific capacity as a negative electrode material, and the sodium storage capacity is 500
800 mA h g21 [81]. But the 2D structured MoS2 material is due to layer to layer, the van der Waals force acts to easily agglomerate during electrochemical charging and discharging, resulting in a decrease in effective contact between the electrolyte and the active material, and a rapid decay of the reversible capacity of the electrode. Therefore improving the structural stability of 2D-layered MoS2 and improving the electronic conductivity are the key scientific problems to be solved in the application of MoS2 materials as anode materials for NIBs. For instance, Xu et al. utilized a simple solvothermal method to synthesize a series of MoS2 nanosheets@nitrogen-doped graphene composites [82]. The MoS2 nanosheets exhibit tunable size and number of layers as well as interplanar spacing by reasonably changing the source of the material and the solvent. The synthetic route including the MoS2 nanosheets morphology is shown in Fig. 4.12. The solvothermal approach is introduced to construct a series of MoS2 nanosheets (dozens layered MoS2 nanosheets: DLMoS2; few layered MoS2 nanosheets: FL-MoS2; ultrasmall MoS2 nanosheets: US-MoS2) anchored on the NG to form the final hybrid anode materials. The US-MoS2@NG is enough to achieve a highly stable cycling performance, illustrating a specific capacity of 198 mA h g21 at 1 A g21 after 1000 cycles and has a high cycle efficiency. SnS2-based nanomaterials are attractive alternatives to graphite due to their attractive properties, including low cost, abundant yield, environmental friendliness, and high

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FIGURE 4.12 Schematic illustration of the synthesis procedures and morphologies of the DLMoS2@NG, FL-MoS2@NG, and USMoS2@NG (scale bars: 100 nm). Source: Reproduced with permission X. Xu, R.S. Zhao, W. Ai, B Chen, H.F. Du, L.S. Wu, et al. Adv. Mater. (2018) 27, 1800658. Copyright 2018, WILEY-VCH.

theoretical specific capacity (645 mA h g 21 ), which is nearly twice that of graphite. The product has received much attention. SnS2 is a 2D-layered hexagonal CdI2-type-layered structure [83]. In each unit cell, a layer of tin atoms is interposed between the upper and lower layers of sulfur atoms, and a covalent bond is formed between sulfur and tin. Therefore it acts as a constraint and is limited to the structure, and the van der Waals force between the layers is weak. Thereby, SnS2 has poor conductivity and is prone to occur in the process of the sodiation/desodiation process. Severe volume expansion causes the electrode material to pulverize and crack, destroying the structure of the electrode [84]. Na2S and polysulfides are easily dissolved in the electrolyte, contaminate the electrolyte, and are also partially deposited on the anode material, resulting in a decrease in the utilization of the electrode active material and also affecting the electron transport. Cui et al. [85] directly grown SnS2 nanosheets on SnO2/C composites, and synthesized SnS2/graphene-carbon nanotube aerogels (SnS2/GCA) by pressure sulfidation on the basis of maintaining the original morphology of the carbon skeleton (Fig. 4.13). A high capacity of 600.3 mA h g21 was achieved after 100 cycles at 0.2 A g21 and 304.8 mA h g21 at an ultrahigh current density of 10 A g21 because of the large surface area mitigation of strain in the novel structure. The rate performance of the SnS2/GCA and SnS2 electrodes is also shown in Fig. 4.13. SnS2 is a layered crystal with large interlayer spacing at high current density. Since the GCA conductive network promotes Na1 diffusion kinetics, Na1 diffuses rapidly in the SnS2 interlayer and exhibits impressive electrochemical performance. FeS2 is a typical nonlamellar metal sulfide. FeS2 has the advantages of abundant reserves, low price, nontoxicity, stable physical and chemical properties, and high theoretical capacity (894 mA h g21), and it is expected to be used as anode material for NIBs. However, FeS2 has poor cycle stability when used as the anode of an NIB. At present, the

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FIGURE 4.13 (A) Schematic representation of fabrication of SnS2/GCA; (B) CV curves of SnS2/GCA electrode at a scan rate of 0.1 mV s21; (C) cyclic capacities at 0.2 A g21; (D) rate performance of SnS2/GCA and SnS2 electrodes; (E) long-term cyclic performance of SnS2/GCA at a high current density of 3 A g21. Source: Reproduced with permission J. Cui, S.S. Yao, Z.H. Lu, J. Qiu, W.G. Chong, F. Ciucci, et al. Adv. Energy. Mater. (2017) 10, 1702488. Copyright 2017, WILEY-VCH.

main methods to improve the cycle stability of FeS2 as the negative electrode of NIB include: (1) optimizing the cutoff voltage, avoiding volume expansion by not converting reaction; (2) optimizing electrolyte to reduce polysulfide [86]; (3) preparing nanosized FeS2 particles to shorten the diffusion distance of Na1; (4) doping other metal elements to form MxFe1-xS2 material, improving the rate performance of material in the cutoff voltage range; [87] (5) compounding with graphene to improve the conductivity of the material while buffering the volume expansion during charge and discharge [88]. Carbon materials have the advantages of good electrical conductivity, small volume change during charge and discharge process, and stable chemical properties, so they are very suitable for forming composite materials with FeS2 [89]. Chen et al. [90] report a simple one-pot solvothermal method of a FeS2/CNT neural network nanostructure composite

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FIGURE 4.14 Cycling performances and coulombic efficiency of all the FeS2-based electrodes and the CNTs-only control sample at current density of 200 mA g21. Source: Reproduced with permission Y.Y. Chen, X.D. Hu B. Evanko, X.H. Sun, X. Li, T.Y. Hou, et al. Nano Energy 46 (2018) 117. Copyright 2018, Elsevier.

(FeS2/CNT-NN) that exhibits outstanding electrochemical performance as an NIB anode. Among these composite materials, uniform microspheres assembled from FeS2 nanoparticles act as “body” and CNTs act as “neuritis.” The weight ratio of FeS2 to carbon nanotubes not only affects the morphology of the composite, but also affects the storage properties of sodium ions. As shown in Fig. 4.14, a stable cycle capacity of 394 mA h g21 can be achieved by optimizing this ratio for 400 cycles at 200 mA g21. This excellent electrochemical energy is at two points. On the one hand, the phase transition of FeS2 to the conductive layered NaxFeS2 occurs in the first few cycles, making the intercalation reaction highly reversible. In addition, its redox reaction is dominated by pseudocapacitive behavior, which also explains the unexpected high-rate capability and long-lasting recyclability of sodium storage. In summary, transition metal sulfides have great potential as anodes for NIBs, but they still have the disadvantage of poor cycle stability. In the future, more work needs to analyze its causes and design a reasonable structure to overcome this problem. It is hoped that the stability of the high energy and power density will be improved by designing an electrode material of a reasonable structure. 4.2.3.3 Phosphides Phosphorus element as an electrode material: its theoretical specific capacity is up to 2596 mA h g21 [91]. However, its own insulating properties and the large volume expansion during the reaction make the phosphor electrode material exhibit poor electrochemical performance. By studying the electrochemical reaction process of transition metal phosphide (MPx), the researchers found that similar to MOx and MSx, MPx reacts with alkali metals (such as Na) to form M and Na3P. The metal element M is uniformly dispersed in the Na3P phase, which accelerates the kinetics of the Na3P oxidation reaction, and thus exhibits excellent electrochemical performance when the MPx is used as an electrode material. Among all MP-based NIB anodes, iron phosphide has attracted particular attention due to its low cost and theoretical capacity of up to 926 mA h g21. However, the use of FeP nanomaterials as anode materials for NIBs will face some inevitable challenges in addition to the advantages of high theoretical capacity and strong electrochemical activity. One of the main challenges is the dramatic volume change that will result in significant

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mechanical strain with the onset of Na1 conversion. This strain destroys the electrode integrity, resulting in a loss of electrical contact between the electron channel and the active particles, causing severe capacity degradation over several cycles. Another challenge in the practical application of FeP-based anode materials is their relatively poor electron conductivity, which hinders the rapid transfer of electrons within the anode material, thereby limiting their electrochemical response. Therefore the key to using FeP nanomaterials as anode materials for NIBs is to reduce the mechanical strain due to volume change and improve the kinetics of Na1 insertion/extraction, thus achieving an efficient conversion reaction process. In order to solve these problems, it is a promising method to reduce the phosphorus content in MPs, which usually produces metal-rich phosphides (e.g., Cu3P, Ni2P, etc.). Another key strategy is to design special microstructures of electrodes with MP, such as core
shell or porous structures, nanowires, and nanotubes, which can effectively buffer volume changes and also improve contact between the interface electrodes/electrolytes. Furthermore, the introduction of the carbonaceous material into the electrode can alleviate the aggregation of the phosphide particles and improve the electrical conductivity. Ma et al. [92] designed a new type of electrode, the strategy is to coat carbon-coated FeP particles on a conductive carbon nanotube network (CNT@FeP-C) for excellent sodium ion storage. Such a unique structure demonstrated excellent long-life cycling stability (a 95% capacity retention for more than 1200 cycles at 3 A g21 ) and rate capability delivered 272 mA h g21 at 8 A g21) (Fig. 4.15).

FIGURE 4.15 (A) The charge
discharge profiles, (B) the rate capability of the CNT@FeP-C, (C) the long-term cycling performance of the CNT@FeP-C electrode at the current rate of 3 A g21, and (D) Nyquist plots of the electrodes (Inset: equivalent circuit model used for fitting). Source: Reproduced with permission C.R. Ma, Z.G. Fu, C.J. Deng, X.Z. Liao, Y.S. He, Z.F. Ma, et al., Chem. Commun.54 (2018) 11348. Copyright 2018, Royal Society of Chemistry.

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CoP with high theoretical capacity (894 mA h g21) and relatively low charge and discharge voltages (0.9 and 0.6 V) is also considered to be an ideal material for secondary batteries [93]. However, due to the large volume change of CoP during charge and discharge, the specific capacity decreases and the cycle life decreases rapidly. In view of the above problems, this paper can reduce the volume effect of sodium deintercalation process by controlling the morphology of cobalt phosphide and its distribution in carbon materials, thereby improving its electrochemical performance. Because of the inactivity of coagulating energy of cobalt phosphide, Co2P, CoP, and CoP3 can exist in stable properties. However, the stability of cobalt phosphide gradually decreases as the content of P element increases. Therefore the orthorhombic phase structure of Co2P is the most stable and most conductive of all phosphides. Although the cobalt phosphide based material has a high theoretical specific capacity, the application of the cobalt phosphide based material in the negative electrode material of the NIB still faces problems such as poor conductivity and large volume expansion. An effective way to overcome these two problems is to prepare nanostructured anode materials. Li et al. [94] prepared cobalt phosphide/reduced graphene oxide (CoP/RGO) nanohybrid materials by simple chemical precipitation method and low-temperature phosphating method. These electrode materials demonstrate a large capacity of about 490 mA h g21, at a current density of 100 mA g21, after 100 cycles. They also deliver good rate performance, demonstrating about 381 mA h g21 specific capacity when the current density reaches 2.0 A g21. Among the nickel phosphide, Ni3P, Ni12P5, Ni5P4, NiP2, and Ni2P are the most widely studied, and the phosphorus-rich phase nickel phosphide has a higher capacity. The metalrich phase phosphide has better metal properties than the phosphorus-rich phase phosphide because its Ni
Ni bond accounts for a higher proportion of Ni
P and P
P bonds and exhibits a lower reaction voltage [22]. However, during the continuous cycle, both the phosphorus-rich phase and the reversible capacity of the metal-rich phase phosphide exhibit a tendency to deteriorate rapidly due to changes in volume and causes of detachment from the copper foil. A large amount of research is also devoted to solving the above problems, and the main methods include the construction of nanostructures and the improvement of their electrical conductivity. Nanostructured materials can accept larger volume changes and shorten the diffusion path of sodium ions during cycling. Wang et al. [95] synthesized a mixture of submicrospheres having a glucose-derived carbon shell assembled based on nickel phosphide nanoparticles by a Ni-glycerate precursor by a carbon coating route and a subsequent calcination-phosphating method. These electrode materials demonstrate a large capacity of about 296 mA h g21, at a current density of 50 mA g21, after 100 cycles. Due to its unique structure, copper phosphide exposes more active sites on the surface. As a negative electrode material of an LIB, the advantage is that the volume is small and the theoretical specific capacity is large, but the volume of the copper phosphide changes greatly during charging and discharging, resulting in rapid decay of capacity [96]. At the same time, during the insertion and deintercalation of Na1, the phosphide copper nanoparticles are easily agglomerated, which makes the specific surface area of the particles smaller, which greatly reduces the service life of the battery. The electrical properties of the material are strongly related to its shape and size. In order to improve the cycle performance of LIBs, it is particularly important to synthesize copper phosphide of specific morphology and size. At the same time, the anchoring of phosphating copper on materials

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with outstanding electrical properties and large specific surface area can also improve its electrical properties, which is becoming a research hotspot. Zhao et al. [97] used a simple high-energy ball-milling method to prepare nanostructured CuP2/C composites. The final product consisted of submicron-sized CuP2 particles coated with Super-P carbon black on the surface. The presence of the carbon black coating not only enhances the electrical conductivity of the composite, but also buffers the volume change of the active material. As a result, the composite electrode material exhibited a large specific capacity of .500 mA h g21 and good rate performance. In addition, the composite electrode material exhibits good short-term cycle stability, but still lacks long-term cycle stability (Fig. 4.16). Tin phosphide has a special layered structure with a theoretical capacity of 1230 mA h g21 and very high electrochemical activity. However, tin phosphide also has some disadvantages as a negative electrode material. First, the reversible capacity is not high. During the process of charge and discharge, the intercalation and extraction process of sodium ions makes the volume effect of tin phosphide larger, causing the phosphide morphology to be destroyed and seriously degrading its electrochemical performance. Second, for tin phosphide, the material itself is not highly conductive, which means that

FIGURE 4.16

(A) A schematic illustration of the general synthetic procedure of nanostructured CuP2/C composites and their structure evolution during electrochemical charge and discharge; (B) rate capability of CuP2/C electrode materials under varying current densities as indicated; (C) cycling performance and coulombic efficiency of CuP2/C at 150 mA h g21. Source: Reproduced with permission F.P. Zhao, N. Han, W.J. Huang, J.J. Li, H.L. Ye, F.J. Chen et al., J. Mater. Chem. A3 (2015) 21754. Copyright 2015, Royal Society of Chemistry.

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the material has a large internal resistance, which is extremely disadvantageous for the electrochemical performance of tin phosphide. Therefore in order to reflect the superiority of tin phosphide, it is necessary to modify the tin phosphide. As a modifying material, carbon material has many advantages, such as good electrical conductivity, which improves the conductivity of the battery, facilitates rapid charging, and has a large specific surface area, which increases the reaction site and increases the sodium ion for the electrochemical reaction. The combination of phosphide and carbon-based materials can not only promote strength and avoid weakness, but also fully exert synergy between carbonaceous materials and phosphide nanomaterials, improve cycle life, electron transport capability, and rate of electrode materials. The carbon shell layer can provide a buffer space for the expansion of the tin phosphide and serve as a physical support when the volume changes, thereby maintaining the overall structural stability of the electrode material. Liu et al. [98] designed and fabricated an yolk
shell structure for high capacity and electrochemically stable Sn4P3 electrodes. The yolk
shell arrangement consists of Sn4P3 nanoparticles completely protected by a thin, conformal, and self-supporting carbon shell. The rationally designed void space in between the shell and nanoparticles allows for the expansion of Sn4P3, without deforming the carbon shell or disrupting the SEI on the outside surface. Due to these unique structural features, Sn4P3@C nanospheres showed attractive electrochemical properties as a potential anode candidate, such as a high reversible capacity (790 mA h g21) and excellent rate capability and stable cycle performance. Germanium phosphide is a potential negative electrode material because of its high theoretical capacity and incredible rate performance for NIBs. However, the large volume change that occurs when phosphine is directly converted to Na3P results in poor cycleability, which limits its further application on NIBs. Tseng et al. [99] proposed the preparation of mesoporous phosphide(MGePx) microspheres having a diameter of 0.5 to 1.5 μm by a one-step and no template method (Fig. 4.17). The MGePx microspheres herein consist of nanoparticles of about 10 nm and also have a narrow pore size distribution of about 4 nm. During charge and discharge, MGePx can shorten the transmission distance of the energy carrier and provide buffer space for the transmission of Na ions. MGePx effectively prevents drastic changes in volume and maintains the structural integrity of the electrode material. Due to these unique structural features, the MGePx exhibit very high reversible capacity (704 mA h g21 after 100 cycles at 0.2 C) and stable cycling performance (200 cycles at 0.6 C) in NIBs. FIGURE 4.17 Schematic illustrations of possible growth routes of SGePx and MGePx. Source: Reproduced with permission K.W. Tseng, S.B. Huang, W.C. Chang, H.Y Tuan, Chem. Mater. 30 (2018) 4440. Copyright 2018, American Chemical Society.

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4.2.4 Alloying/dealloying-based anode material 4.2.4.1 Tin anode material Tin (Sn) can be alloyed with Na to form Na15Si4 in a multistep alloy, releasing a theoretical specific capacity of 847 mA h g21, but the Sn negative electrode is one of the most negative anode materials subjected to volume expansion. Huang et al. [100] first reported that Sn can form Na0.5Sn and Na15Sn4 alloy materials with Na. They used the in-situ TEM characterization technique to measure the corresponding volume expansion values of 60% and 420%, respectively, in order to alleviate the volume expansion effect of Sn. The main problems are the following methods (Fig. 4.18). First, the electrode material is coated. In addition to the common carbon coating, Han et al. [101] deposited a layer of Al2O3 on the surface of the Sn anode by atomic layer deposition (ALD). This protective film can make the Sn particles still good when the charge and discharge are subjected to bulk expansion. Adhered to the substrate, and if this protective layer is absent, the Sn nanoparticles will leave the current collector. Second, a polyacrylic acid (PAA) adhesive is used. Experiments have shown that the use of this adhesive instead of the traditional polyvinylidene fluoride (PVdF) adhesive can effectively improve the electrochemical performance of the Sn electrode material. The reason is that the long chain polymer segment of PAA can encapsulate the Sn electrode and inhibit its volume expands. Third, using a functionalized fiber collector, Zhu et al. [102] also developed a novel wood fiber substrate for Sn loading that not only mitigates the stress changes caused by the volume change of Sn, but also the porous structure. The functionalized fiber collector can act as a container for the electrolyte to improve the wettability of the material and the diffusion rate of ions. 4.2.4.2 Antimony anode material The antimony-based material is one of the representatives of the alloy mechanism anode material. Each antimony atom accommodates up to three sodium ions in the process of de-sodium/deposition to form the Na3Sb alloy phase, so the theoretical specific FIGURE 4.18 Schematic of the hybrid anode nanostructures and their corresponding sodiation processes. (A) The hybrid nanostructure SnNPs@CNF. (B) During sodiation/desodiation, a bare SnNP debonds from the carbon nanofiber substrate. (C) For the ALD-Al2O3-coated SnNPs@CNF, the ALD-Al2O3 coating is first uniformly converted to a Na 2 Al 2 O layer, followed by the reversible sodiation/desodiation of the SnNP core. Note that the Na 2 Al 2 O layer deforms to accommodate the swelling and shrinking of the SnNP core, maintaining the anode structural integrity. Source: Reproduced with permission X.G. Han, Y. Liu, Z. Jia, Y.C. Chen, J.Y. Wan, N. Weadock, et al., Nano Lett. 14 (2014) 139. Copyright 2018, American Chemical Society.

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capacity can reach 660 mA h g21 and strontium in sodium. In this process, the potential is about 0.52 V, and it has a suitable potential platform [103]. Therefore the bismuth-based material is also a hot spot for researchers. However, the volume of the bismuth-based material will expand by approximately 390% during the de-sodium/deadline process, resulting in rapid decay of capacity and slow electrochemical behavior. In order to alleviate the volume expansion and improve the dynamic behavior, researchers have adopted different methods to design materials. For example, the material is designed into a nanostructure, compounded with a carbon material, and alloyed with other metals. Structurally, nanoporous structures are the primary choice for researchers in designing materials. Because of its unique internal space, it can greatly buffer volume expansion, and can shorten the electron transport path to improve electrochemical performance. For example, Ji et al. [104] prepared a Sb nano hollow sphere using a simple electrochemical displacement method, and this method was first reported. Applying this material to the negative electrode material of NIB, at a current density of 50 mA g21, the cycle of 50 cycles shows a reversible specific capacity of 622 mA h g21, which is close to the theoretical specific capacity. The material is very good even at 1600 mA g21. The reversible specific capacity of 315 mA h g21 is still available at large current densities. In 2015, Ji et al. [105] used the displacement reaction to replace the elemental Sb with Zn and Sb31 and then etch away the excess Zn with hydrochloric acid to leave the pores to form nanoporous hollow microspheres as shown in Fig. 4.19. The first application of this material to an NIB anode exhibited excellent electrochemical performance. At a current density of 100 mA g21, 100 cycles of cycling exhibited a high reversible specific capacity of 617 mA g21 and maintained a capacity retention of up to 97.2%. Charging and discharging at a current density of 3200 mA g21, the reversible specific capacity can still reach 312.9 mA h g21. In terms of composition, compounding with carbon materials is one of the methods studied by most research workers, because as the number of cycles increases, the nanoparticles of Sb may be agglomerated, resulting in lower utilization of materials, affecting the cycle performance. Due to its good electrical conductivity and special 1D or 2D structure, FIGURE 4.19 A schematic of the formation of Sb PHMSs: (A) The original Zn MS template, (B) the formation of Zn@Sb MSs during the replacement reaction, and (C) Sb PHMSs after etching the Zn. (D
F) The corresponding SEM images of (A)
(C). Source: Reproduced with permission H.S. Hou, M.J. Jing, Y.C. Yang, Y. Zhang, Y.R. Zhu, W.X. Song, et al., J. Mater. Chem. A 3 (2015) 2971. Copyright 2015, Royal Society of Chemistry.

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FIGURE 4.20 Schematic illustration of the formation mechanism of Cu2Sb nanoparticles integrated on Cu foil. Source: Reproduced with permission L.B. Wang, C.C. Wang, N. Zhang, F.J. Li, F.Y. Cheng, J. Chen, ACS Energy Lett. 2 (2017) 256. Copyright 2016, American Chemical Society.

carbon materials can increase the conductivity of the electrode material, buffer volume expansion, prevent particle agglomeration, and achieve uniform distribution of nanoparticles, and the electrochemical properties of the material are obvious improvement. For example, Sb embedded in carbon fiber/carbon nanospheres is considered to be one of the methods to effectively buffer volume expansion and increase the rate of ion diffusion and electron transfer [106]. Another method of modification is to form alloys or intermetallic compounds with inert or active metals. The structure is very stable, the volume change during the process of deintercalating lithium or sodium is small, and the conductivity of the electrode material is also greatly improved, which is beneficial to the performance of electrochemical performance. Another method of modification is to form alloys or intermetallic compounds with inert or active metals. The structure is very stable, the volume change during the sodium removal/insertion process is small, and the conductivity of the electrode material is also greatly improved, which is beneficial to the performance of the electrochemical performance. Chen et al. [107] used a one-pot displacement reaction method to replace Cu with Sb31 in an ethanol solution and copper foil to form a composite material of Cu2Sb alloy nanoparticles and Cu (Cu2Sb/Cu), as shown in the figure. Due to the complete nanostructure of the material, the anode material applied to the NIB has a good sodium ion diffusion rate and a rapid electrolyte permeation rate. In addition, the nanopore can effectively buffer the volume expansion and shorten the electron transmission distance to exhibit excellent cycle and rate performance (Fig. 4.20). 4.2.4.3 Red phosphorus Among many kinds of anode materials, phosphorus gradually enters people’s sight with its higher theoretical capacity (2596 mA h g21) and its relatively low oxidation
reduction potential (0.4 V vs Na/Na1) [108]. Phosphorus has three allotropes, white phosphorus, red phosphorus (RP), and black phosphorus. The chemical instability of white phosphorus can ignite at normal temperature and is toxic to human body. Therefore candidates for lithium storage anode materials are RP and black phosphorus. The structure of black phosphorus is similar to that of graphite, but black phosphorus is difficult to produce and has poor air stability and is difficult to store. In contrast, RP is expected to be one of the candidates for the next generation of negative electrode materials due to its good chemical stability, large earth storage, environmental friendliness, and low cost.

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Despite these advantages, the poor conductivity (  1 3 10214 S cm21) and volume change (  400%) of pure RP results in rapid capacity decay and low coulombic efficiency, which hinders the application of RP [109]. In order to overcome the problem of poor conductivity and volume expansion of RP, the researchers designed RP-based composite materials with various frameworks. In one aspect, various conductive matrices are introduced to improve electron conductivity and inhibit volumetric expansion of RP during electrochemical processes. On the other hand, it is desirable to minimize the RP particle size and even form RP-carbon bonds to control P volume changes and to shorten ion and electron diffusion lengths during cycling [110]. Ball milling is one of the main methods for preparing RP/C composites because of its ease of operation and magnification experiments. In 2013, Oh and co-workers reported on the ball-milling processes to obtain amorphous RP/C composites as NIB anodes [111]. The images from Oh’s work, as shown by scanning and transmission electron microscopy (i.e., SEM and TEM) (Fig. 4.21A and B), the final product has a nonuniform particle size ranging from a few hundred nanometers to a few

FIGURE 4.21 (A) TEM and (B) SEM images of amorphous RP/C composite. (C) Charge
discharge curves and (D) cycling performance of RP/C composite. Source: Reproduced with permission Y. Kim, Y. Park, A. Choi, N.S. Choi, J. Kim, J. Lee, et al., Adv. Mater. 25 (2013) 3045. Copyright 2013, Wiley-VCH.

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FIGURE 4.22 (A) TEM image of RP/SWCNT composite. (B) Cycling performance and Coulombic efficiency of the RP/SWCNT composite under 2 A g21. Source: Reproduced with permission Y. Zhu, Y. Wen, X. Fan, T. Gao, F. Han, C. Luo, et al., ACS Nano 9 (2015) 3254. Copyright 2015, American Chemical Society.

microns. The composite provided a reversible capacity of 1890 mA h g21 at 143 mA g21 and a capacity attenuation of less than 7% in 30 cycles (Fig. 4.21C and D). Despite some advance, there are still some problems with the ball-milling strategy: (1) ball milling is a process of high-energy consumption and structural damage; and (2) relatively large RP particle size and incomplete coverage of the conductive matrix will hinder the NIB RP / carbon composite improvements in carbon composites. A simple handmilling method has also been employed to fabricate an RP/multiwalled carbon nanotube (MWCNT) composite [112]. Compared to ball-milled composites, simple mixing of RP/ MWCNTs with microscopic RP and uneven coverage of MWCNTs resulted in faster capacity decay (capacity retention of 76.6% after only 10 cycles). The vaporization/adsorption strategy is a nondestructive process compared to ball milling, ensuring that the RP is uniformly dispersed in the integrated carbon matrix [113]. Generally, the mechanism of this technique is: (1) heating RP at a temperature above the RP sublimation temperature to form P4 vapor; sublimation is driven by a pressure differential, P4 vapor diffuses into the void/frame of the carbon, and then adsorbs and deposits on the inner surface; (3) after condensation, P4 is converted back to RP to produce a nanostructured RP/carbon composite. Wang and co-workers reported an improved vaporization
condensation method for the preparation of RP/single-walled carbon nanotube (SWCNT) composites for NIB [114]. The synthesis is carried out at a higher temperature (600 C) and under vacuum, which can generate a strong driving force to make the P4 vapor easily adsorbed into the SWCNT and facilitate the uniform dispersion of the RP in the SWCNT. The RP / SWCNT composite exhibited a capacity of  700 mA h g21 at 50 mA g21 (based on the weight of the composite) and a high capacity retention of 80% after 2000 cycles (Fig. 4.22A and B).

4.3 Electrolyte A typical liquid electrolyte for a sodium secondary battery is a sodium salt dissolved in an organic solvent. Although there are many types of organic solvents and liquid salts, not

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all types are suitable for sodium secondary batteries. In order to use liquid electrolytes for sodium secondary batteries, they should have the following characteristics: (1) The electrolyte should have high ionic conductivity. Electrolytes with higher ionic conductivity have superior battery performance. The movement of sodium ions at the electrodes and diffusion within the electrolyte are especially important when sodium secondary batteries are rapidly charged or discharged. At room temperature, liquid electrolytes of sodium secondary batteries should have an ionic conductivity higher than 1023 S/cm. (2) The electrolyte should exhibit high chemical and electrochemical stability toward electrodes. Since NIBs engage in electrochemical reactions at the cathode and anode, the electrolyte should be electrochemically stable within the potential range of redox reactions at the two electrodes. In addition, the electrolyte should be chemically stable toward various metals and polymers constituting the cathode, anode, and battery. (3) The electrolyte should be used over a wide temperature range. NIBs with liquid electrolytes are usually employed in mobile devices and must satisfy the above requirements in the temperature ranging from
20 C to 60 C. At higher temperatures, electrochemical stability drops, whereas ionic conductivity increases. (4) The electrolyte should be highly safe. Organic solvents used in electrolytes are flammable and may cause fires or explosions when heated to high temperatures during short circuits. Higher ignition points or flash points are favored, and nonflammable materials should be used if possible. The electrolyte should have a low toxicity in case of leakage or disposal. (5) The electrolyte should be low cost. High-performance electrolytes may be difficult to commercialize if they come at a high cost. Given the fierce market competition for NIBs, expensive materials are unlikely to be adopted. As mentioned above, electrolytes in sodium secondary batteries should exhibit high ionic conductivity over a wide temperature range, and remain stable over a wide potential with a working voltage higher than the battery. Electrolyte characteristics are determined by properties of the solvent and sodium salt, and vary according to combinations. The electrolyte is divided into liquid electrolyte, solid electrolyte, and gel polymer electrolyte. Among them, liquid and solid electrolytes in sodium battery have become an important development direction of future energy storage [115,116], whereas gel polymer electrolyte cannot be mass produced, it remains to be investigated [117]. Therefore the solid and liquid electrolytes are studied below, and their electrical properties, electrochemical properties and thermal stability are summarized.

4.3.1 Liquid electrolyte The liquid electrolyte is classified into an organic solvent electrolyte, an aqueous solution electrolyte, and an ionic liquid electrolyte. Among them, the organic solvent electrolyte is widely used due to its good comprehensive performance; the aqueous solution electrolyte is concerned because of its environmental friendliness, low cost, and low corrosivity; the ionic liquid electrolyte is wide, nonflammable, and nonvolatile due to its electrochemical window. Advantages cause concern. 4.3.1.1 Organic solvent electrolyte The organic solvent electrolyte is widely used due to its high ionic conductivity and good solubility. Among them, the commonly used organic solvent: ethylene carbonate,

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TABLE 4.1 Effect of FEC addition on PC decomposition and retention capacity of 50 cycles. FEC addition amount (%)

PC decomposition degree

Cycle 50 lap capacity retention

0

High

Low

2

Low

High

10

Lower

Higher

Adapted from D.K. Zhang, Y. Liu, Y.K. Xi, S.L. Jin, R. Zhang, M.L. Jin, J. Tech. 2018, 18, 324. Copyright 2018, Journal of Technology.

propylene carbonate, dimethyl carbonate, diethyl carbonate, etc. The sodium salt commonly used is sodium hexafluorophosphate, sodium perchlorate, sodium bistrifluoromethanesulfonimide, etc. Komaba et al. [118] studied the hard carbon negative electrode with NaClO4 as the electrolyte salt, and when using PC as the organic solvent, adding 2% (by volume) of fluoroethylene carbonate can significantly improve the capacity retention of the battery. Even in the case of PC:DMC, which is easily decomposed by sodium metal, with the addition of FEC, the deintercalating ability of sodium ions in it is also significantly improved, as shown in Table 4.1 [119]. The authors also mentioned in other articles that the addition of FEC can effectively prevent the decomposition of PC, so it can significantly improve the capacity retention rate and charge and discharge efficiency of the battery, thereby maintaining the coulombic efficiency of the battery to 98% to 99%. Wang et al. [120] studied the effects of 1 mol L21 NaClO4 (EC/DMC) and 1 mol L21 NaClO4 (FEC/DMC) electrolyte on the morphology and structure of the solid electrolyte interface membrane. The additive FEC plays a large role in the sodium battery electrolyte in DMC. Lee et al. [121] used Na4Fe3(PO4)2(P2O7) as the positive electrode and sodium ClO4 as the sodium salt, which was compared with the organic solvents of different ratios. The reference electrolyte EC: PC 5 5:5 (the same percentage by volume) has the highest viscosity. After adding DEC, EMC, and DMC, the viscosity gradually decreases, and the ionic conductivity is inversely proportional to the viscosity. After adding 5% by weight of FEC to the electrolyte additive, a protective film NaF can be formed between the positive electrode material and the metallic sodium, and this protective film can effectively prevent decomposition of the linear carbonate. Moreover, the addition of FEC (0.5 mol L21 NaClO4, EC:PC:DEC 5 5:3:2) increased the first charge and discharge effect rate of the battery from 16% to 90%; in addition, at 30 C. The battery was charged and discharged at 0.5 C for 300 times, the discharge capacity still reached 90.5 mA h g21, the capacity retention rate reached 97.5%, and the charge and discharge efficiency reached 99.6%. Piernas-muoz et al. [122] used Na0.75Fe2.08(CN)6 4H2O as the positive electrode material, and used NaClO4 and NaPF6 as the sodium salt. By using different organic solvents, it was found that: 1 mol L21 of the sodium salt of NaPF6 has the best performance exhibited by an electrolyte having an organic solvent ratio of EC: PC: FEC 5 49:49:2. The stable voltage platform is 2.4 to 4.2 V, and the charging capacity is 130 mA h g21 after charging and discharging for 10 times at 0.1 C, maintaining 99.5% charge and discharge stability, and 87% capacity retention. Moreover, in practical applications, an organic electrolyte having a sodium salt of NaClO4 has a risk of explosion. Therefore an organic solvent electrolyte containing NaPF6 as a sodium salt is relatively suitable.



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At present, the most commonly used sodium battery electrolyte uses carbonate as solvent, NaPF6 or NaClO4 as sodium salt. With the addition of appropriate FEC, SEI film is formed on the surface of the electrode to effectively promote Na1 insertion and inhibit side reactions.

4.4 Aqueous solution electrolyte The aqueous solution electrolyte contains no combustibles inside, has good safety performance, and can adapt to higher working temperatures. It also has high ionic conductivity due to its small internal resistance. The materials used are all inorganic and low in cost. Park et al. [123] use NaTi2(PO4)3 as the negative pole, respectively. 2 mol L21 Na2SO4 and 4 mol L21 NaOH were used as aqueous solution electrolytes, and the charge and discharge characteristics were measured by using zinc flakes as positive electrode. The results show that when the solution environment is alkaline, the capacity of the battery decays quickly. Therefore the NaOH aqueous solution does not cooperate with the electrolyte. The discharge capacity was measured at a discharge rate of 2 mA cm21 in an organic solvent electrolyte [1 mol L21 NaClO4/EC: DMC 1:1 (volume percent)] and an aqueous solution electrolyte (2 mol L21 Na2SO4). The results showed that the two reached 120 and 123 mA h g21, respectively. From the above results, the difference between the two electrolytes is not obvious, and the polarization of the electrode during charging and discharging in the aqueous solution is smaller than that in the organic solvent electrolyte. This phenomenon occurs during high current charging and discharging. More clearly, this indicates that the internal resistance and viscosity of the aqueous solution electrolyte are much smaller than those of the organic solvent electrolyte.

4.5 Ionic liquid electrolyte The ionic liquid electrolyte is characterized by nonflammable and substantially nonvolatile. In recent years, attention has been paid. The synthesis of Bagno et al. [124] [Hmim] [IBr2] (Hmim is 3-methylimidazole), the conductivity is up to 4 S m21, and the viscosity is only 17 mPa s. The synthesized [Bmim][IBr2] has a conductivity of 0.62 S m21 and a viscosity of 57.3 mPa s, and starts to decompose at 196 C. As an important indicator of the electrolyte, decomposition temperature and melting point are equally important, and a higher decomposition temperature means more reliable cycle stability and service life.

4.5.1 Solid electrolyte The solid electrolyte is classified into a solid polymer electrolyte (SPE) and an inorganic solid electrolyte. Liquid electrolytes have potential safety hazards such as leakage, burning, and corrosiveness. In order to develop battery safety and high-energy storage performance, solid electrolytes have become a new direction to improve the above problems. The solid electrolyte has the advantages of good thermal stability, long cycle life, low cost, and low leakage.

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4.6 Solid polymer electrolyte The SPE has the advantages of light volume, low liquid leakage, low cost, good flexibility, and does not cause battery volume expansion during charging and discharging, and can provide new solutions for solving battery leakage. The development path also provides corresponding conditions for mass production and application in the future. The SPE is usually composed of an organic polymer matrix and a salt dissolved in the polymer group, and further contains an inorganic functional material or the like. And currently used polymer matrix is polyvinyl alcohol (PVA), polyacrylonitrile (PAN), polyethylene oxide (PEO). Moreover, the current research methods and mechanisms of sodium ion SPE are derived from the research methods of lithium-ion SPE.

4.7 Inorganic solid electrolyte Inorganic solid electrolytes include Na-β-Al2O3, NASICON solid electrolytes, etc. Naβ-Al2O3 is solid electrolyte material. β-Al2O3 is an important component in the application of solid electrolytes in sodium
sulfur batteries, and is represented by the general formula of M2OxAl2O3 (M 5 Na1, K1, Rb1, Ag1, etc., x 5 5
11). From the microstructure, β-Al2O3 has two different crystal structures: β-Al2O3 and β00 -Al2O3. And the phase of the oxygen ion stack between the chemical composition and the ion conductive layer is different between the two phases. The β phase is Na2O(8-11)Al2O3, and the β00 phase is Na2O(5-7)Al2O3, and since the β00 phase contains more Na1, and the conductivity of the β phase is higher, the β phase is more It is suitable for improving the reversible capacity of the battery and the retention rate during the insertion of sodium ions. Zhu et al. [125] found that the doping elements in the β00 phase have a great influence, and it is found that it is easier to make Na-β-Al2O3 by doping Mg21. Na2O is formed in the middle, and its Na2O increases the conductivity of the Beta-Al2O3 solid electrolyte. Wei et al. [125] assembled a battery with a Na-β-Al2O3 solid electrolyte, which circulated well at 350 C. Kim et al. [126] used Na-β’’-Al2O3 as the solid electrolyte and separator, S/C composite as the positive electrode, and sodium metal as the negative electrode. At room temperature, the solid electrolyte membrane can prevent the dendrites from passing through the solid, thus achieving a coulombic efficiency of 98% to 99%. Yu et al. [127] found a liquid flow battery of a liquid biphenyl sodium negative electrode and used Na-β"-Al2O3 as a solid electrolyte and a separator, and the battery exhibited good cycle performance at room temperature. The above results demonstrate the excellent performance of the Na-β"-Al2O3 solid electrolyte for NIBs, which limits the application on a large scale when in the high temperature range.

4.8 Na1 superionic conductor type solid electrolyte The structure of NASICON solid electrolyte material is as follows: Na11xZr2Si2-xPxO12 (0 # x # 3). Yue et al. [128] proposed to replace p by Si, and introduce Na here to balance the

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NASICON-type solid electrolyte material. When x 5 2, the conductivity reaches the optimum value, so the room temperature ionic conductivity of the relatively pure NASICON is about 67 mS m21. In addition to the high ionic conductivity, the NASICON-type solid electrolyte has a lower value thermal conductivity. Zhang et al. [129] found a new type of interface wetting agent, which can effectively improve the utilization rate of NASICON solid electrolyte by dripping on the positive side of solid battery, greatly improving the rate and cycle stability of the battery at room temperature. There is almost no significant change after 10,000 cycles, and the performance is better in the current solid-state battery. Based on the research of current NASICON solid electrolyte materials, it has high ionic conductivity and excellent electrochemical performance, and its low thermal expansion means that it can be used in medium and high temperature environments, increasing the use of sodium batteries. At present, there are few reports on inorganic solid electrolytes, mainly because ion migration in electrolytes is difficult, resulting in lower conductivity, which limits the use in that battery. Increasing the conductivity is the main direction for the development of inorganic solid electrolytes in the future. The NIB electrolyte has an important influence on the electrochemical performance and safety performance of the NIB. This section describes NIB fluids, solid electrolytes, including organic solvent electrolytes, aqueous electrolytes, ionic liquid electrolytes, SPEs, and inorganic solid electrolytes. NIBs are currently commonly used with NaPF6 or an organic solvent electrolyte combined with NaClO4 and carbonate. These electrolytes have been successfully used in NIBs and exhibit good electrochemical performance. However, longterm use tends to corrode the electrode, affecting the stability of the SEI film, and posing safety risks such as leakage and burning. Therefore for the stability of the SEI film, it is necessary to pay more attention to the interface problem of the electrode process and to improve the compatibility of the electrolyte with the electrode. For solid electrolytes, finding a consistent sodium salt and increasing the conductivity of the electrolyte are major problems. Therefore NIBs still have many challenges in basic science and key technologies, and we are constantly trying to explore them.

4.8.1 Separators Separators are nonactive materials that do not participate in electrochemical reactions. They provide a pathway for ion transport that is essential for battery operation and separate physical contact between the anode and the cathode. Together with anode, cathode, and electrolyte, separators play an important role in determining battery performance and safety. In order to obtain a better understanding of electrolyte characteristics, we should examine separator functions, structure, and characteristics.

4.8.2 Separator functions Similar to LIBs, the separator is a key component in NIBs to prevent contact between the cathode and anode. Although there are no reactions in cells, its structure and properties considerably affect the electrochemical performance of NIBs, including internal

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resistance, cycle ability, safety, energy density, and power density. Separators in NIBs are microporous polymer films with pores ranging from nanometers to micrometers. Though electrode materials provide significant contribution to the energy density of the battery, the separator plays a vital role in deciding the safety, duration, and performance of batteries. The glass fiber membrane is considered as the most compatible separator for NIB due to its high ionic conductivity and reasonable performance. Polyethene (PE), polypropylene (PP), and PVdF were also widely used as separator in NIBs. Commercialized separators have pores that are 0.03
1 mm large, a porosity of 30%
50%, and low thermal shutdown temperature. If the temperature rises during internal short circuits, the melted separator blocks pores and restricts ion movement, thus improving battery safety by delaying thermal reactions. When making batteries, thin separators are used to maximize battery capacity. In particular, separators with a thickness of 16 mm are used in high-capacity cylindrical batteries.

4.8.3 Basic requirements of separators A number of factors must be considered in selecting the best separator for a particular battery and application. The characteristics of each available separator must be weighed against the requirements and one selected that best fulfills these needs. A wide variety of properties are required of separators used in batteries. The considerations that are important and influence the selection of the separator include the following. 1. Thickness: Since the ionic conductivity of organic liquid electrolytes is 100 times lower than that of aqueous electrolytes, it is important to maximize electrode surface area while reducing the distance between electrodes to achieve high output and energy density. As such, film thickness should be no more than 25 mm. The most commonly used separators have a film thickness of 20, 16, or 10 mm. Thin separators increase the discharge capacity of electrodes by increasing the concentration of surrounding liquid electrolyte and facilitating the movement of substances. However, thin separators may cause pinholes and are prone to tearing. Battery safety is also reduced from the increased risk of short circuits between electrodes. 2. MacMullin number: The MacMullin number is the resistance of the separator filed with an electrolyte divided by the resistance of the electrolyte alone, and is usually as high as 10
12. 3. Electrical resistance: The separator serves as an insulator and should have a low electrical resistance when filled with electrolyte. A high electrical resistance negatively affects battery characteristics including discharge capacity. 4. Permeability: Permeability, expressed in Gurley units, measures the time taken for air to flow through under uniform conditions (uniform pressure, uniform marea, etc.). It is one of the characteristics of separators that affect battery performance. 5. Pore size and porosity: Porosity is usually at around 40%. Pore size should be below tens of micrometers and smaller than particle size to prevent internal short circuits from dendritic growth and impurities. 6. Puncture strength: Internal short circuits may be caused by impurities released from electrodes, the surface state of the anode and cathode, and dendritic growth of

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7. 8.

9.

10.

11. 12. 13.

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sodium. Puncture strength represents the resistance of the separator against such threats and is measured by compressing the separator with a probe. A higher value lowers the risk of internal short circuits caused by separators. Thermal shrinkage: While thermal shrinkage differs according to manufacturer, 1 h of drying at 90 oC in vacuum should result in less than 5% shrinkage. Tensile strength: Like winding, tensile strength is a property that has a significant effect on the manufacturing process. Separators have high tensile strength in the direction of elongation. A separator with a thickness of 25 mm has a tensile strength above 1000 kgf/cm2 in the machine direction (MD). In the case of uniaxial elongation, tensile strength in the transverse direction is as low as 1/10 of the machine direction. For two-axis elongation, tensile strength in the transverse direction is roughly the same as that in the machine direction. Shutdown: Shutdown is a safety function that cuts off the circuit by blocking micropores during excess current caused by internal or external short circuits. PE separators are commonly used in sodium secondary batteries as micropores are shutdown to prevent the temperature from rising in case of early short-circuiting. Melt integrity: Melt integrity is a characteristic that maintains the form of the separator for long periods above the meltdown temperature. Along with shutdown, it is an important factor in acquiring battery safety. Wettability: A fast wetting rate and sufficient wettability are required. Chemical stability: Chemical stability refers to stability under redox conditions. Separators should exhibit corrosion resistance to electrolytes at high temperatures. Average molecular weight and MW distribution: This is an important factor that determines thermal and mechanical characteristics of polyolefin substances. Outstanding mechanical properties and narrow melting range can be achieved.

The sequence of importance of the various criteria depends on the applications of batteries. The characteristics provide a large spectrum of requirements for separators in batteries. In many cases, a compromise in requirements for the separator must generally be made to optimize performance, safety, cost, etc. For example, batteries that are characterized by small internal resistance and consume little power require separators that are highly porous and thin, but the need for adequate physical strength may require that they are thick.

4.8.4 Wettability and wetting speed Wettability and wetting speed are two important physical properties of separators, which are important to the operating characteristics of a battery, are electrolyte absorption and electrolyte retention. High-quality separators should be able to absorb a significant amount of electrolyte and also reserve the absorbed electrolyte when the cell is under operation. In order to achieve minimal battery internal resistance, it is necessary to add a maximum amount of electrolyte to the separator. The wettability of the diaphragm can limit the performance of the battery by increasing the diaphragm and battery resistance. In an actual battery, the wetting speed of the separator is related to the filling time of the electrolyte. The wetting rate depends on the type of polymer (surface energy), pore size,

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porosity, and the degree of curvature of the separator. There is currently no known separator wettability test. However, simply dropping a drop of electrolyte onto the separator and observing whether the droplets slide quickly into the separator is a good indicator of wettability. Contact angle is also a good way to measure wettability. The absorption of electrolyte by many hydrophobic polymer separators can be enhanced by wetting agents or ionic functional groups such as ion exchange membranes.

4.8.5 High-temperature stability When the high-temperature caused by an external or internal short circuit exceeds the current, the reaction between the electrode and the decomposition of the electrolyte or electrolyte may trigger the release of gas or liquid and cause ignition. Here, the separator helps to improve the safety of the battery. As the temperature of the battery increases, the diaphragm melts, blocks the micropores, and limits the ionic conductivity. Even if the battery stops reacting at the shutdown temperature, the ignition is delayed as the thermal diffusion time increases. Since the internal temperature of the battery continues to rise, the separator should have a higher melting temperature. In addition to basic characteristics such as porosity and permeability, shutdown temperature and melting temperature are important factors affecting battery safety. Since short-circuit and melting characteristics are not considered, attempts have been made to increase the melting characteristics by appropriately distributing the molecular weight and the polyethylene, thereby suppressing the flow above the low temperature. Another methodist composite material with different melting temperatures, such as PE and PP, has a lower shutdown temperature and higher melting characteristics.

4.8.6 Materials of separators 4.8.6.1 Microporous polyolefin film In the early days, the separators were made of polyethylene having micropores. When the melting temperature exceeds 120 C, the movement of ions and organic solvents through the small holes is limited, and the battery loses its activity. Since polyethylene is still mobile at high temperatures, it is difficult to melt it during ignition. To overcome this weakness, polyethylene is mixed or stacked with PP, and the melting temperature of the PP is at least 40 C higher. However, PE separators have been more actively developed due to the complexity of their multilayer structure and higher manufacturing costs. A study is currently underway to address the pressure resistance properties of ultrahigh molecular weight polyethylene that occurs above the melting temperature of PE. 4.8.6.2 Porous Na1 superionic conductor film PVdF has been used as a binder for NIB electrodes. Compared with polyolefins, fluorinated polymers contain a fluorine atom with high electronegativity in the main chain, and strong interaction with polar solvents gives them a stronger affinity with liquid electrolytes. The crystallization temperature and crystallinity of PVdF are reduced by

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References

157

copolymerization of VdF and HFP. Although PVdF has high electrolyte uptake and ionic conductivity [130]. It is not used in NIBs because its mechanical strength is weaker than that of polyolefins. Other developments include synthetic separators that combine the high mechanical strength of polyolefins with the superior quality of PVdF polymers.

4.8.7 Separator manufacturing process The film technology consists of two parts, extrusion and stretching. Although the extrusion is usually carried out by a twin-screw extruder, a single-screw extruder can be used if the mixing of the polymer and the solvent is not involved in the production process. The sheet extruded from the t-die is stretched in the machine direction by uniaxial stretching or mechanical stretching and biaxial stretching. The two-axis stretch film is more suitable as a separator due to its high strength and isotropy. Another method is to first extrude with a cylindrical die and then elongate with a tubular shape. As NIBs become more compact while achieving higher energy density and power density, the separator should be thin, high-strength, and less pronto shrinkage. Especially when a PE-laminated aluminum film is used as a packaging material, dimensional stability is critical because external force may cause bending or twisting of the battery. A thinner separator facilitates impregnation of the electrolyte, but may reduce the amount of impregnation or retention of the electrolyte. Therefore it is necessary to improve the miscibility between the separator and the electrolyte. In order to meet high-energy density and high-power characteristics, it is necessary to further improve the closed cell and melting properties of the separator.

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]

J.J. Kim, K. Yoon, I. Park, K. Kang, Small Methods 1 (2017) 1700219. Y. Liang, W.H. Lai, Z.C. Miao, S.L. Chou, Small 14 (2018) 1702514. A. Eftekhari, D.W. Kim, J. Power Sources 395 (2018) 336. C. Delmas, Adv. Energy Mater. 17 (2018) 1703137. B.G. Silbernagel, M.S. Whittingham, Mater. Res. Bull. 11 (1976) 29. M.S. Whittingham, J. Electrochem, Soc. Solid State Sci. 122 (1975) 713. J.P. Parant, R. Olazcuaga, M. Devalette, C. Fouassier, P. Hagenmuller, J. Solid State Chem. 3 (1971) 1. C. Fouassier, G. Matejka, J.-M. Reau, P. Hagenmuller, J. Solid State Chem. 6 (1973) 532. C. Delmas, J.J. Braconnier, C. Fouassier, P. Hagenmuller, Solid State Ionics 3/4 (1981) 165. J.J. Braconnier, C. Delmas, P. Hagenmuller, Mater. Res. Bull. 17 (1982) 993. C. Delmas, J.J. Braconnier, A. Mazaaz, P. Hagenmuller, Rev. Chim.Miner. 19 (1982) 343. A. Maazaz, C. Delmas, P. Hagenmuller, J. Inclusion Phenom. 1 (1983) 45. A. Mendiboure, C. Delmas, P. Hagenmuller, J. Solid State Chem. 57 (1985) 323. P.F. Wang, Y. You, Y.X. Yin, Y.G. Guo, Adv. Energy Mater. 8 (2017) 1701912. N. Yabuuchi, K. Kubota, M. Dahbi, S. Komaba, Chem. Rev. 114 (2014) 11636. Y.J. Fang, J.X. Zhang, L.F. Xiao, X.P. Ai, Y.L. Cao, H.X. Yang, Adv. Sci. 5 (2017) 1600392. S.P. Ong, V.L. Chevrier, G. Hautier, A. Jain, C. Moore, S. Kim, et al., Energy Environ. Sci. 4 (2011) 3680. A. Saracibar, J. Carrasco, D. Saurel, M. Galceran, B. Acebedo, H. Anne, et al., Phys. Chem. Chem. Phys. 18 (2016) 13045. [19] M. Nakayama, S. Yamada, R. Jalem, T. Kasuga, Solid State Ionics 286 (2016) 40. [20] A.K. Padhi, C. Masquelier, K.S. Nanjundaswamy, J.B. Goodenough, J. Electrochem. Soc. 144 (1997) 2581. [21] J.B. Goodenough, H.Y.P. Hong, J.A. Kafalas, Mater. Res. Bull. 11 (1976) 203.

Advanced Nanomaterials for Electrochemical Energy Conversion and Storage

158 [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52] [53] [54] [55] [56] [57] [58] [59] [60] [61] [62] [63] [64] [65] [66] [67] [68] [69] [70] [71] [72]

4. Sodium battery nanomaterials

Z. Jian, L. Zhao, H. Pan, Y.S. Hu, H. Li, W. Chen, et al., Electrochem. Commun. 14 (2012) 86. K. Saravanan, C.W. Mason, A. Rudola, K.H. Wong, P. Balaya, Adv. Energy Mater. 3 (2013) 444. Z. Jian, W. Han, X. Lu, H. Yang, Y.-S. Hu, J. Zhou, et al., Adv. Energy Mater. 3 (2013) 156. W. Song, X. Ji, Z. Wu, Y. Zhu, Y. Yang, J. Chen, et al., J. Mater. Chem. A 2 (2014) 5358. Q. Ni, Y. Bai, F. Wu, C. Wu, Adv. Sci. 3 (2017) 1600275. F. Sanz, C. Parada, J.M. Rojo, C. Ruı´z-Valero, Chem. Mater. 13 (2001) 1334. P. Barpanda, L. Lander, S.I. Nishimura, A. Yamada, Adv. Energy Mater. 17 (2018) 1703055. B.L. Ellis, W.R. Makahnouk, Y. Makimura, K. Toghill, L.F. Nazar, Nat. Mater. 6 (2007) 749. Q.H. Wang, J.T. Xu, W.C. Zhang, M.L. Mao, Z.X. Wei, L. Wang, et al., J. Mater. Chem. A 6 (2018) 8815. R.A. Shakoor, D.H. Seo, H. Kim, Y.U. Park, J. Kim, S.W. Kim, et al., J. Mater. Chem. 22 (2012) 20535. M. Bianchini, F. Fauth, N. Brisset, F. Weill, E. Suard, C. Masquelier, et al., Chem. Mater. 27 (2015) 3009. K. Itaya, I. Uchida, V.D. Neff, Acc. Chem. Res. 19 (1986) 162. C.D. Wessells, S.V. Peddada, R.A. Huggins, Y. Cui, Nano Lett. 11 (2011) 5421. L. Wang, Y. Lu, J. Liu, M. Xu, J. Cheng, D. Zhang, et al., Angew. Chem. Int. Ed. 52 (2013) 1964. C. Wu, J. Qian, H. Yang, Sci. Chi. Chem. 5 (2017) 11. F. Ma, Q. Li, T. Wang, H. Zhang, G. Wu, Sci. Bull. 62 (2017) 358. X. Tang, H. Liu, D. Su, P.H.L. Notten, G.X. Wang, Nano Res. 8 (2018) 3979. Y.H. Lu, L. Wang, J.G. Cheng, J.B. Goodenough, Chem. Commun. 48 (2012) 6544. J. Qian, M. Zhou, Y. Cao, H. Yang, J. Electrochem. (Chin. Ed.) 18 (2012) 108. Z.H. Wang, S.M. Selbach, T. Grande, RSC Adv. 4 (2014) 4069. Y. Wen, K. He, Y.J. Zhu, F.D. Han, Y.H. Xu, I. Matsuda, et al., Nat. Commun. 5 (2014) 4033. S. Qiu, L.F. Xiao, M.L. Sushko, K.S. Han, Y.Y. Shao, M.Y. Yan, et al., Adv. Energy Mater. 7 (2017) 1700403. F.G. Galilei, U.S. Padova, V.F. Marzolo, J. Phys. Chem. B. 110 (2006) 14832. R.M. Badger, J. Chem. Phys. 2 (1934) 128. J. Cioslowski, G.H. Liu, R.A.M. Castro, Chem. Phys. Lett. 331 (2000) 497. D. Herschbach, V.W. Laurie, J. Chem. Phys. 35 (1961) 458. Y. Cao, L. Xiao, M.L. Sushko, W. Wang, B. Schwenzer, J. Xiao, et al., Nano Lett. 12 (2012) 3783. A.P. Cohn, K. Share, R. Carter, L. Oakes, C.L. Pint, Nano Lett. 16 (2016) 543. C.Z. Zhang, N. Mahmood, H. Yin, F. Liu, Y.L. Hou, Adv. Mater. 25 (2013) 4932. D.C. Wei, Y.Q. Liu, Y. Wang, H.L. Zhang, L.P. Huang, G. Yu, Nano Lett. 9 (2009) 1752. Y.Q. Guo, W. Liu, R.T. Wu, L.J. Sun, Y. Zhang, Y.P. Cui, et al., ACS Appl. Mater. Interfaces 10 (2018) 38376. Y. Mei, Y. Huang, X. Hu, J. Mater. Chem. A 4 (2016) 12001. B. Wang, F. Zhao, G. Du, S. Porter, Y. Liu, P. Zhang, et al., ACS Appl. Mater. Interfaces 8 (2016) 16009. C. Chu, J. Yang, Q. Zhang, N. Wang, F. Niu, X. Xu, et al., ACS Appl. Mater. Interfaces 9 (2017) 43648. M. Ge, C. Cao, J. Huang, S. Li, Z. Chen, K.Q. Zhang, et al., J. Mater. Chem. A 4 (2016) 6772. H. Xiong, M.D. Slater, M. Balasubramanian, C.S. Johnson, T. Rajh, J. Phys. Chem. Lett. 2 (2011) 2560. Z. Yang, D. Choi, S. Kerisit, K.M. Rosso, D. Wang, J. Zhang, et al., J. Power Sources 192 (2009) 588. Y. Zhang, Z. Jiang, J. Huang, L.Y. Lim, W. Li, J. Deng, et al., RSC Adv. 5 (2015) 79479. W.G. Wang, Y. Liu, X. Wu, J. Wang, L.J. Fu, Y.S. Zhu, et al., Adv. Mater. Technol. 3 (2018) 1800004. L.M. Wu, D. Bresser, D. Buchholz, G.A. Giffin, C.R. Castro, A. Ochel, et al., Adv. Energy Mater. 5 (2015) 1401142. S.M. Oh, J.Y. Hwang, C.S. Yoon, J. Lu, K. Amine, I. Belharouak, et al., ACS Appl. Mater. Interfaces 6 (2014) 11295. C. Chen, Y. Wen, X. Hu, X. Ji, M. Yan, L. Mai, et al., Nat. Commun. 6 (2015) 6929. J.E. Elshof, H. Yuan, P.G. Rodriguez, Adv. Energy Mater. 6 (2016) 1600355. M. Valvo, F. Lindgren, U. Lafont, F. Bjorefors, K. Edstrom, J. Power Sources 245 (2014) 967. L. Zhang, H.B. Wu, X.W. Lou, Adv. Energy Mater. 4 (2013) 1300958. N. Zhang, X.P. Han, Y.C. Liu, X.F. Hu, Q. Zhao, J. Chen, Adv. Energy Mater. 5 (2014) 1401123. D.H. Xu, W.Y. Chen, M.T. Zheng, X.Y. Huang, Y.P. Fang, X.Y. Yu, Electrochimica Acta. 265 (2018) 419. T. Li, A.Q. Qin, L.L. Yang, J. Chen, Q.F. Wang, D.H. Zhang, et al., ACS Appl. Mater. Interfaces 9 (2017) 19900. H. Kim, H. Kim, H. Kim, J. Kim, G. Yoon, K. Lim, et al., Adv. Funct. Mater. 26 (2016) 5042. G.L. Xu, T. Sheng, L. Chong, T. Ma, C.J. Sun, X. Zuo, et al., Nano Lett. 17 (2017) 953. H.H. Li, Z.Y. Li, X.L. Wu, L.L. Zhang, C.Y. Fan, H.F. Wang, et al., J. Mater. Chem. A 4 (2016) 8242.

Advanced Nanomaterials for Electrochemical Energy Conversion and Storage

References

[73] [74] [75] [76] [77] [78] [79] [80] [81] [82] [83] [84] [85] [86] [87] [88] [89] [90] [91] [92] [93] [94] [95] [96] [97] [98] [99] [100] [101] [102] [103] [104] [105] [106] [107] [108] [109] [110] [111] [112] [113] [114] [115] [116] [117] [118] [119] [120] [121] [122] [123] [124]

159

M. Xu, Q.Y. Xia, J.L. Yue, X.H. Zhu, Q.B. Guo, J.W. Zhu, et al., Adv. Funct. Mater. 6 (2018) 1807377. R. Alcantara, M. Jaraba, P. Lavela, J.L. Tirado, Chem. Mater. 14 (2002) 2847. J.S. Chen, X.W. Lou, Small 9 (2013) 1877. M. Gu, A. Kushima, Y.Y. Shao, J.G. Zhang, J. Liu, N.D. Browning, et al., Nano Lett 13 (2013) 5203. J. Zhu, D. Deng, Chem. Eng. Sci 154 (2016) 54. X.Y. Yu, L. Yu, X.W. Lou, Adv. Energy Mater. 6 (2016) 1501333. Y. Xiao, S.H. Lee, Y.K. Sun, Adv. Energy Mater. 7 (2017) 1601329. Y. Teng, H. Zhao, Z. Zhang, Z. Li, Q. Xia, Y. Zhang, et al., ACS Nano 10 (2016) 8526. D.W. Su, S.X. Dou, G.X. Wang, Adv. Energy. Mater. 5 (2015) 1401205. X. Xu, R.S. Zhao, W. Ai, B. Chen, H.F. Du, L.S. Wu, et al., Adv. Mater. 27 (2018) 1800658. S.H. Choi, Y.N. Ko, J.K. Lee, Y.C. Kang, Adv. Funct. Mater. 25 (2015) 1780. P. Zhou, X. Wang, W.H. Guan, D. Zhang, L.B. Fang, Y.Z. Jiang, ACS Appl. Mater. Interfaces 9 (2017) 6979. J. Cui, S.S. Yao, Z.H. Lu, J. Qiu, W.G. Chong, F. Ciucci, et al., Adv. Energy. Mater. 10 (2017) 1702488. Z. Hu, Z.Q. Zhu, F.Y. Cheng, K. Zhang, J.B. Wang, C.C. Chen, et al., Energy. Environ. Sci 8 (2015) 1309. K. Zhang, M. Park, L. Zhou, Angew. Chem. Int. Ed. 55 (2016) 12822. W. Chen, S. Qi, L. Guan, J Mater. Chem A 5 (2017) 5332. W. Chen, S. Qi, M.M. Yu, X.M. Feng, S.Z. Cui, J.M. Zhang, et al., Electrochim. Acta 230 (2017) 1. Y.Y. Chen, X.D. Hu, B. Evanko, X.H. Sun, X. Li, T.Y. Hou, et al., Nano Energy 46 (2018) 117. Z.L. Li, H.L. Zhao, J. Mater. Chem. A 6 (2018) 24013. C.R. Ma, Z.G. Fu, C.J. Deng, X.Z. Liao, Y.S. He, Z.F. Ma, et al., Chem. Commun. 54 (2018) 11348. J. Zhang, K. Zhang, J.H. Yang, G.H. Lee, J. Shin, V.W.H. Lau, et al., Adv. Energy Mater. 8 (2018) 1800283. Q. Li, S.H. Dong, Y.N. Zhang, S. Feng, Q.Z. Wang, J.J. Yuan, Eur. J. Inorg. Chem. 29 (2018) 3433. J.M. Wang, B.B. Wang, X.J. Liu, G. Wang, H. Wang, J.T. Bai, J. Colloid Interface Sci. 538 (2019) 187. M.P. Fan, Y. Chen, Y.H. Xie, T.Z. Yang, X.W. Shen, N. Xu, et al., Adv. Funct. Mater. 26 (2016) 5019. F.P. Zhao, N. Han, W.J. Huang, J.J. Li, H.L. Ye, F.J. Chen, et al., J. Mater. Chem. A 3 (2015) 21754. J. Liu, P. Kopold, C. Wu, P.A. Aken, J. Maier, Y. Yu, Energy Environ. Sci. 8 (2015) 3531. K.W. Tseng, S.B. Huang, W.C. Chang, H.Y. Tuan, Chem. Mater. 30 (2018) 4440. Y.H. Liu, Y.H. Xu, Y.J. Zhu, J.N. Culver, C.A. Lundgren, K. Xu, et al., ACS Nano 7 (2013) 3627. X.G. Han, Y. Liu, Z. Jia, Y.C. Chen, J.Y. Wan, N. Weadock, et al., Nano Lett. 14 (2014) 139. H.L. Zhu, Z. Jia, Y.C. Chen, N. Weadock, J.Y. Wan, O. Vaaland, et al., Nano Lett. 13 (2013) 3093. J. He, Q. Wei, T. Zhai, Chem. Front. 2 (2018) 437. H.S. Hou, M.J. Jing, Y.C. Yang, Y.R. Zhu, L.B. Fang, W.X. Song, et al., ACS Appl. Mater. Interfaces 6 (2014) 16189. H.S. Hou, M.J. Jing, Y.C. Yang, Y. Zhang, Y.R. Zhu, W.X. Song, et al., J. Mater. Chem. A 3 (2015) 2971. Y.J. Zhu, X.G. Han, Y.H. Xu, Y.H. Liu, S.Y. Zheng, K. Xu, et al., ACS Nano 7 (2013) 6378. L.B. Wang, C.C. Wang, N. Zhang, F.J. Li, F.Y. Cheng, J. Chen, ACS Energy Lett. 2 (2017) 256. Y. Yu, W. Li, Z. Yang, Y. Jiang, Z. Yu, L. Gu, Carbon 78 (2014) 455. W. Li, Z. Yang, M. Li, Y. Jiang, X. Wei, X. Zhong, et al., Nano Lett. 16 (2016) 1546. B. Ruan, J. Wang, D. Shi, Y. Xu, S. Chou, H. Liu, et al., J. Mater. Chem. A 3 (2015) 19011. Y. Kim, Y. Park, A. Choi, N.S. Choi, J. Kim, J. Lee, et al., Adv. Mater. 25 (2013) 3045. W.J. Li, S.L. Chou, J.Z. Wang, H.K. Liu, S.X. Dou, Nano Lett. 13 (2013) 5480. C. Marino, A. Debenedetti, B. Fraisse, F. Favier, L. Monconduit, Electrochem. Commun. 13 (2011) 346. Y. Zhu, Y. Wen, X. Fan, T. Gao, F. Han, C. Luo, et al., ACS Nano 9 (2015) 3254. X. Li, P. Yan, M. Engelhard, A. Crawford, V. Viswanathan, C. Wang, et al., Nano Energy 27 (2016) 664. L. Bodenes, A. Drawiche, L. Monconduit, H. Matinez, J. Power Sources. 273 (2015) 14. S. Komaba, W. Murata, T. Ishikawa, N. Yabuuchi, T. Ozeki, T. Nakayama, et al., Adv Funct Mater. 21 (2011) 3859. S. Komaba, T. Ishikawa, N. Yabuuchi, W. Murata, A. Ito, Y. Ohsawa, ACS Appl. Mater. Interfaces. 3 (2011) 4165. D.K. Zhang, Y. Liu, Y.K. Xi, S.L. Jin, R. Zhang, M.L. Jin, J. Tech. 18 (2018) 324. S.W. Wang, H.F. Hu, D.Y. Wang, C. Shen, J Inorg Mater. 32 (2017) 596. Y. Lee, J. Lee, H. Kim, K. Kang, N. Choi, J. Power Sources. 320 (2016) 49. M. Jose´Piernas-Mun˜oz, E. Castillo-Martı´nez, J.L. Go´mez-Ca´mer, T. Rojo, Electrochim. Acta. 200 (2016) 123. S. Park, I. Gocheva, S. Okada, J. Electrochim. Soc. 158 (2011) A1067. A. Bagno, C. Butts, C. Chiappe, F. D’Amico, J.C.D. Lord, D. Pieraccini, et al., Org. Biomol. Chem. 3 (2005) 1624.

Advanced Nanomaterials for Electrochemical Energy Conversion and Storage

160 [125] [126] [127] [128] [129] [130]

4. Sodium battery nanomaterials

C. Zhu, J. Xue, J. Alloy Compd. 517 (2012) 182. I. Kim, J.Y. Park, C.H. Kim, J.W. Park, J.P. Ahn, J.H. Ahn, et al., J. Power Sources. 301 (2016) 332. J. Yu, Y.S. Hu, F. Pan, Z. Zhang, Q. Wang, H. Li, et al., Nat. Commun. 8 (2017) 14629. Y. Yue, F. Zhou, W. Pang, J. Lu, Chin. J. Inorg. Chem. 11 (1995) 19. Q. Zhang, Z. Wen, Y. Liu, S. Song, X. Wu, J. Alloy Compd. 479 (2009) 494. S. Suriyakumar, M. Raja, N. Angulakshmi, K.S. Nahmb, A.M. Stephan, RSC Adv. 6 (2016) 92020.

Advanced Nanomaterials for Electrochemical Energy Conversion and Storage