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Influence of Li substitution on the structure and electrochemical performance of P2-type Na0.67Ni0.2Fe0.15Mn0.65O2 cathode materials for sodium ion batteries
Journal of Power Sources 396 (2018) 639–647
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Influence of Li substitution on the structure and electrochemical performance of P2-type Na0.67Ni0.2Fe0.15Mn0.65O2 cathode materials for sodium ion batteries
T
Yong Wang, Guorong Hu, Zhongdong Peng, Yanbing Cao, Xiangwan Lai, Xianyue Qi, Zhanggen Gan, Wei Li, Zhongyuan Luo, Ke Du∗ School of Metallurgy and Environment, Central South University, Changsha, 410083, PR China
H I GH L IG H T S
investigate the effect of Li content on P2-type oxide. • We phase and Li MnO grow with increasing lithium content. • O3 Li (Ni Fe Mn ) O shows the optimal electrochemical properties. • Na • Li-doping can reduce the resistance and enhance the high-voltage structure stability. 2
0.67
0.2
0.2
3
0.15
0.65 0.8
2+δ
A R T I C LE I N FO
A B S T R A C T
Keywords: Sodium ion battery Cathode materials Lithium substitution Multiple phases
A series of layered Na0.67Lix(Ni0.2Fe0.15Mn0.65)1-xO2+δ (x = 0, 0.1, 0.2 and 0.3) compounds is prepared via a facile oxalate coprecipitation method and a solid-state reaction process, and investigated as the promising positive electrode materials for Na-ion batteries. As the Li content increases, O3 phase and Li2MnO3 gradually grow at the expense of the P2 phase and the particles become smaller and more agglomerated based on X-ray diffraction, scanning electron microscopy and transmission electron microscope results. The optimal Na0.67Li0.2(Ni0.2Fe0.15Mn0.65)0.8O2+δ shows the improved electrochemical performance with a high specific capacity of 151 mAh g−1 and 78% capacity retention over 50 cycles at 0.1 C (15 mA g−1). When tested at 5 C (750 mA g−1) rate, the electrode exhibits a discharge capacity of 68 mAh g−1. The results of electrochemical impedance spectroscopic and ex-situ X-ray diffraction measurements demonstrate that the improved cyclability and rate capability of the Li-substituted cathode can be ascribed to the decreased resistance and the enhanced structure stability in the high voltage of 4.3 V.
1. Introduction
Thus far, various types of cathode materials for SIBs have been extensively investigated, including layered oxides [4–11], polyanionic compounds [10,12–17], metal hexacyanometalates [18–20], and organic compounds [21]. Among them, layered oxides (NaxTMO2, TM = 3 d transition metals), which were categorized into P-type and Otype by Demals [22], have attracted much attention for their high theoretical capacity. For instance, P2-Na2/3[Fe1/2Mn1/2]O2 layered oxide containing earth-rich elements was found to give a considerable discharge capacity of 190 mAh g−1, comparable to that of nickel-rich lithium oxide, but unfortunately, it underwent fast capacity fade only in 30 cycles [23]. Partial Ni substitution for Fe was used to reduce the content of Mn3+ which causes the well-known Jahn-Teller effect and improve the cycling stabilities with the capacity retention increased
Lithium ion batteries (LIBs) have already been successfully commercialized in portable electronics and expanding into electric vehicles and large scale power energy storage devices in virtue of their high specific energy and excellent cycling stability. However, the limited geological reserves and increasing cost of lithium may lay the hurdle in the way for the further development. It's therefore essential to develop alternative battery systems that are cost-effective and sustainable [1]. Recently, sodium ion batteries (SIBs), which have the similar energy storage mechanism to lithium counterpart, have made a resurgence of research interest due to the inexpensive and earth abundant Na resources [2,3].
∗
Corresponding author. E-mail addresses: [email protected], [email protected] (K. Du).
https://doi.org/10.1016/j.jpowsour.2018.06.058 Received 11 April 2018; Received in revised form 9 June 2018; Accepted 11 June 2018 0378-7753/ © 2018 Elsevier B.V. All rights reserved.
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2.2. Material characterization
from 67% to 80% at 30 t h cycle and could also promote the energy density for the Ni2+/4+ high redox potential [24]. However, these P2type Na-Ni-Fe-Mn-O oxides still suffer from the undesired phase transformation in a low or high concentration of sodium contents that seriously deteriorate the structural integrity and thus lead to poor longterm cycling performance [25]. On the other side, lithium ion substitution to sodium layered oxides has been demonstrated as an effective method to stabilize the crystalline structure and enhance the electrochemistry performance [26–28]. Christopher S. Johnson et al. firstly introduced Li into Na1.0Li0.2Ni0.25Mn0.75Oδ cathode [29]. The whole structure is maintained even charged to a high voltage of 4.4 V and shows very smooth charging/discharging curves owing to the stabilized TM layer after Li introduction. Jing Xu and co-works further investigated the role of the lithium cations in P2-Na0.80[Li0.12Ni0.22Mn0.66]O2 [28]. They found that more Na ions tend to remain in the prismatic positions due to the existence of most univalent Li ions occupying the TM layers, which could also help to maintain the P2-structure in the area of high voltage. Zhou et al. successfully synthesized a P2/O3Na0.66Li0.18Mn0.71Ni0.21Co0.08O2+δ composite with outstanding electrochemical performance by incorporating minor O3 phase into Lisubstituted P2-structure [30]. Multiphase Na0.7Li0.3Ni0.5Mn0.5O2+d material, also proposed by Christopher S. Johnson's group, showed superior rate capability due to the beneficial synergistic effect from the multiple phases including P2 and O3 structures [26]. According to the latest findings, the structure of other sodium manganese-based layered oxides, such as P2-Na0.6[Li0.2Mn0.8]O2 [31], P2/O3-Na2/ 3Li0.18Mn0.8Fe0.2O2 [32], could be also stabilized by doping of lithium. All of these efforts further strengthen the evidence of the positive effect of Li substitution on layered oxides. Inspired by the previous works, we made the first attempt to introduce Li ions into P2-Na0.67Ni0.2Fe0.15Mn0.65O2 base material. A comprehensive investigation on Li-substituted Na0.67Lix(Ni0.2Fe0.15Mn0.65)1-xO2+δ (x = 0, 0.1, 0.2, 0.3) was carried out to check the effect of Li substitution on the structure, morphology and electrochemical performance of the P2-type base material. Based on the analysis results of structure, morphology and electrochemical tests, we found multiple phases co-exist and the lessened particle size after lithium was introduced into the P2-Na0.67Ni0.2Fe0.15Mn0.65O2. The optimum Na0.67Li0.2(Ni0.2Fe0.15Mn0.65)0.8O2+δ cathode exhibited outstanding performance with reduced internal resistance and more stable structure even charged up to 4.3 V compared to the P2-type base material. We proposed that some new insights presented here are of benefit to design high-property layered positive materials for SIBs.
The chemical composition of all the prepared samples was investigated by inductively coupled plasma atomic emission spectroscopy (ICP-AES, Optima 4300DV, PE Ltd.). The crystalline structures of the synthesized Na0.67Lix(Ni0.2Fe0.15Mn0.65)1-xO2+δ (x = 0, 0.1, 0.2, 0.3) samples were analyzed via X-ray diffraction (XRD, RigakuD/maxb) using Cu Kα radiation in a 2θ range of 10°–80° at the scan speed of 2° min−1. The surface morphology and particle size were observed via scanning electron microscopy (SEM, JEOL JSM-6360 L V, Japan). Transmission electron microscope (TEM) was conducted on Philips CM200 microscope system. X-ray photoelectron spectroscopy (XPS, VG Multilab, 2000) measurements were applied to analyze the surface oxidation states of transition metals (Ni, Fe and Mn) for the various ratio particles. 2.3. Electrochemical measurements The electrochemical properties of the samples were evaluated by using a CR2025 coin cell assembled with a Na metal anode in an Arfilled glove box with H2O and O2 contents controlled under 1 ppm. Cathodes were fabricated by blending the as-synthesized powders (80%) with acetylene black (10%) and polyvinylidene fluoride (10%). 1 M NaClO4 solution in 95 vol % propylene carbonate (PC) and 5 vol % fluorinated ethylene carbonate (FEC) and the glass fiber GF/D (Whatman) were employed as the electrolyte and separator, respectively. The constant current charge-discharge experiments were conducted on a coin cell testing system (Land CT, 2001A) purchased from Land Co., China in the voltage window of 1.5–4.3 V vs Na/Na+ where 1 C = 150 mAg−1 at 25 °C. Cyclic voltammetry (CV) and Electrochemical impedance spectroscopic (EIS) tests of all the cells were typically performed at a CHI660D Electrochemical Workstation (Shanghai Chen Hua). 3. Results and discussion We successfully synthesized Ni0.2Fe0.15Mn0.65C2O4·2H2O precursor from co-precipitation process. As shown in Fig. S1, the as-prepared oxalate precursor has a similar crystalline structure to MnC2O4·2H2O, which suggests a solid solution of Ni0.2Fe0.15Mn0.65C2O4·2H2O. After calcined with Na2CO3 and Li2CO3, the element contents of the as-synthesized Na0.67Lix(Ni0.2Fe0.15Mn0.65)1-xO2+δ (x = 0, 0.1, 0.2, 0.3) were analyzed via ICP-AES, as described in Table S1, and the test results are very close to the targeted stoichiometry. The powder XRD patterns of all the prepared samples are presented in Fig. 1. It can be seen that the crystal structure sensitively depends on the Li/TM ratio. The diffraction peaks of the pristine sample can agree well with a hexagonal P2-type structure with space group P63/mmc without other phases. However, with increasing the amount of lithium, extra weak diffraction peaks at 18.7° and 41.8° appear in L01, which are further strengthened in L02 and L03. At the same time, a peak at 16.3° arises in L02, and maintains intensity with lithium content incenses. These new peaks can be assigned to the integrated monoclinic Li2MnO3 component (C2/m) and O3-type structure (R-3m), respectively. In comparison with the P2 structure in L02, the XRD patterns of L03 show obvious differences. The 012, 103, 104 reflections regarding the diffraction signatures of P2 phase are almost invisible, while the diffraction peaks of Li2MnO3 and O3 structures grow in intensity, particularly for the Li2MnO3 phase. This suggests the excess of lithium (≥20%) probably prefers to form the layered oxides (Li2MnO3). Also, a relatively high proportion of Li (≥20%) leads to a rather broad diffraction peaks that means low crystallinity in the structure at the fixed temperature. Morphologies of all the samples were measured by SEM and TEM, as illustrated in Fig. 2. The pristine Na0.67Ni0.2Fe0.15Mn0.65O2 shows flakelike primary particles with the size of around 1–3 μm (Fig. 2a and b). This morphology consists with that typically observed for P2-type
2. Experimental section 2.1. Material synthesis The Na0.67Lix(Ni0.2Fe0.15Mn0.65)1-xO2+δ (x = 0, 0.1, 0.2, 0.3) samples (here after denoted as L00, L01, L02, L03) were synthesized via the solid-state reaction of Na2CO3, Li2CO3 and Ni0.2Fe0.15Mn0.65C2O4·2H2O. The oxalate precursor of Ni0.2Fe0.15Mn0.65C2O4·2H2O was synthesized by a coprecipitation method. Appropriate amounts of NiSO4·6H2O, MnSO4·H2O, and FeSO4·7H2O were dissolved in deionized water with a concentration of 0.5 mol L−1 and pumped slowly into a continuous stirred reactor. Meanwhile, 0.5 mol L−1 of (NH4)2C2O4·H2O and H2SO4 solution with a desired amount were also separately pumped into the reactor. The pH value was adjusted at 3.0 ± 0.1 with the temperature at 30 °C. After the reaction, the light-yellow mixed oxalate was filtered, washed, and then dried in a vacuum box at 120 °C for 24 h. The dried powders were firstly analyzed via ICP-AES to determine the atomic compositions and then thoroughly mixed with Na2CO3 and Li2CO3 by the stoichiometric ratio and sintered at 900 °C for 24 h in air, slowly cooled to room temperature in furnace and finally stored under Ar atmosphere until use. 640
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no electrochemical activity above 2.5 V in layered oxide [24,42–44]. For the 2.7–4.3 V voltage region shown in Fig. 4a, three pairs of redox peaks at 3.43/3.3 V, 3.76/3.49 V and 4.24/3.8 V should be related to the contribution of the Ni2+/Ni3+, Ni3+/Ni4+ and Fe3+/Fe4+ redox pair respectively [24,42,44,45]. The two double peaks related to the Ni2+/Ni4+ redox couple integrate into a pair of broad ones and the polarizations of redox pairs change to smaller by introducing Li in Fig. 4b, thus leading to smooth profiles and enhanced structure stability. As lithium content increased to 0.2, a new reduction peak appears at about 2.7 V and the intensity of the oxidation peak at 3.25 V becomes stronger in comparison with that of the L01 sample. It is obvious that the emerging reduction peak at 2.7 V should be relevant to the increasing part of the oxidation peak at 3.25 V, which is attributed to the phase transformation of the O3-P3 couple due to the rising proportion of O3 phase. This phase transition will be further proved by ex-situ XRD tests in the next section. Electrochemical properties of the Na0.67Lix(Ni0.2Fe0.15Mn0.65)1xO2+δ (x = 0, 0.1, 0.2, 0.3) samples are further evaluated by galvanostatic charge/discharge measurement. Fig. 5 illustrates the voltage curves and the corresponding differential capacity (dQ/dV) profiles of the as-prepared materials. The charge/discharge profiles of the pristine sample seem quite rough for the existence of several potential plateaus, which can be confirmed by a lot of sharp peaks in the derivative plots in Fig. 5b. With the increase of Li content, the charge/discharge curves become much smoother, associated with less and broader peaks in the derivative curves, in good accordance with the CV results. This phenomenon indicates the positive impact of Li on smoothing the curves by suppressing Na/vacancy ordering, phase transition during sodium (de) intercalation [31,46,47]. When x = 0.2 or 0.3, the initial derivative plots show a strong oxidation peak around 4.2 V vs Na/Na+ and then the peak disappears in the following charge/discharge process. The feature is relevant to the irreversible potential plateau at 4.2 V in the first charging curve in Fig. 5e and g, which is commonly observed in Lirich compositions (4.5 V vs Li/Li+). This process is believed to be caused by the lithium extraction and oxygen evolution from Li2MnO3 structures described by previous works [48,49]. The similar charge plateau at 4.2 V of Li2MnO3 compounds in Na-based O3-type cathode materials has been recently reported by Jing Xu and co-workers [50]. After the initial charging process, single smooth voltage profiles are observed above 3.0 V, which indicates a solid-solution (de)intercalation reaction [29]. Additional voltage plateau appears at about 3.0 V, suggesting the co-existence of multiple phases. This process probably includes the O3-P3 phase transition, which agrees well with the CV test results in Fig. 4c and d. The initial discharge capacity of the L00 sample is 207 mAh g−1, but unfortunately it suffers a rapid capacity decline during the following cycles [24]. Upon long-term cycling, the peak at 4.2 V of differential capacity plots for the pristine sample in Fig. 5b gradually diminished and almost vanished after 30 cycles. In addition, the intensity of the other redox peaks within the voltage range of 1.5–4.3 V is largely weakened, associated with the increase of the electric potential difference of these redox pairs. As shown in Fig. 5c, e and g, the 1 s t discharge capacity of L01, L02 and L03 sample deceases to 172, 151 and 136 mAh g−1 respectively, probably due to the reduced proportion of electrochemical active elements of TM (TM = Ni, Fe, Mn). Different to the pristine sample, the potential shifts of the redox peaks caused by the electrode polarization are gradually relieved in L01, L02 and L03 sample, which can be explained by the enhanced structure stability after the introduction of Li ions. Fig. 6a exhibits the cycling behavior of all the samples. The discharge capacity of the pristine L00 declines from 207 mAh g−1 at the 1 s t cycle to 65 mAh g−1 at the 50 t h cycle, corresponding a poor capacity retention of 31%. For lithium substituted samples, the capacity retention is largely improved to 70%, 78% and 77% for L01, L02 and L03, respectively. Fig. 6b displays the rate capability tests at different current densities which are ranged from 0.1 to 5 C rate. The specific discharge capacities delivered by the various compounds at different C-
Fig. 1. XRD patterns of Na0.67Lix(Ni0.2Fe0.15Mn0.65)1-xO2+δ (x = 0, 0.1, 0.2, 0.3) samples.
layered material [33]. Particle size and shape are affected by the content of lithium. The samples with a small amount of Li, such as L01 with main crystal of P2 phase, exhibit almost the same shape with clear facets and sharp edges (Fig. 2c and d). By further introducing Li, the smaller particles (around 0.5–1 μm) like grains gradually appear at the expense of the flakes and become more agglomerated (Fig. 2e–h). This phenomenon was also observed in P3/P2/O3Na0.76Mn0.5Ni0.3Fe0.1Mg0.1O2 composite [34]. EDS mapping was employed to investigate the elemental distributions of Na, Ni, Fe and Mn on the surface of the L02 cathodes material. As shown in Fig. S2, it can be observed that all elements are uniformly distributed throughout the particles, confirming the successful preparation of the solid solution of Ni0.2Fe0.15Mn0.65C2O4·2H2O precursor and the even element distribution after calcined with alkali carbonate. The effect of lithium introduction on the surface element oxidation state was evaluated by XPS. Fig. 3 illustrates the XPS spectra of Mn 2p, Fe 2p and Ni 2p of all the samples. It can be seen that the Mn 2P3/2 main peak centers at 641.9 eV, demonstrating the existence of Mn ion at +4 state. The resulting binding energy of Fe 2P3/2 is around 710.4 eV, which is in quite good agreement with that of Fe3+ in Fe2O3, indicating that the valence state of the iron is +3. The main peak of Ni 2P3/2 located at 854.5 eV can be ascribed to Ni2+. Although the main peaks of Mn 2p3/2, Fe 2p3/2 and Ni 2p3/2 decline in intensity, the location and width almost keep the same, probably resulting from the reduced proportions of the TM ions with increasing lithium content. This indicates the unchanged oxidation state of three transition metal ions in all the samples. Based on the results of XPS analysis, the valence states of manganese, iron and nickel are +4, +3 and + 2, respectively, matching well with the previous studies [24,33,35–41]. Therefore, it can be concluded that the oxidation states of Mn, Fe and Ni ions do not change after Li substitution. The electrochemical (de)intercalation reaction of the various samples was initially characterized via cyclic voltammetry (CV), applied to explore the influence of lithium contents on the electrochemical properties of all samples. Fig. 4 exhibits the first five CV plots of the cathodes at a scanning speed of 0.1 mV s−1. At low voltage range, for all samples, the reversible oxidation/reduction peaks located at around 2.0 V corresponds to Mn4+/Mn3+ redox process because Mn4+ shows 641
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Fig. 2. (a-h) SEM and the corresponding TEM images of Na0.67Lix(Ni0.2Fe0.15Mn0.65)1-xO2+δ samples with x = 0, 0.1, 0.2 and 0.3, respectively.
with considerations of capacity retention, reversible capacity, rate capability and initial coulombic efficiency, which is more suitable as a potential candidate for the cathodic materials of SIBs. Electrochemical impedance spectroscopic (EIS) tests were applied to further evaluate the effect of lithium introduction on the stability of the as-prepared material. Fig. 7 shows the Nyquist plots of the as-prepared sample after 5 and 50 cycles at 4.3 V. These plots are comprised of the typical compositions, i.e. two semicircle connected with a slope line, observed in AC impedance spectra. The intercept of Nyquist curve with Z′ axis is assigned to the solution resistance (Rs). The semicircle in the high frequency region is usually associated with the resistance of Na ions transfer through the surface films (Rf) and another one in the medium-high frequency area is related to the charge-transfer resistance (Rct) of the electrodes. All EIS parameters were acquired by Z-view
rates are shown in Table 1. The Li-free L00 electrode can give a capacity of 205, 160, 121, 90, 56, 32 and 4 mAh g−1 at 0.1, 0.2, 0.5, 1, 2, 3 and 5 C rates, respectively. However, for the Li-substituted electrodes, the rate performances are significantly enhanced. The L02 exhibits the discharge capacity of 151, 135, 125, 107, 90, 79 and 68 mAh g−1 at the same rates. The capacity of 68 mAh g−1 for L02 at the high rate of 5 C (750 mA g−1) is comparable to that of P2/O3Na0.66Li0.18Mn0.71Ni0.21Co0.08O2+δ (69 mAh g−1 at 5 C equal to 500 mA g−1) [30]. After the current density is reduced to 0.1 C, the L00 sample can recover only 70.2% of the reversible capacity whereas the L02 sample can recover 99% and still retain 150 mAh g−1, providing further evidence of the sample stability. A comparison of L02 with other multiphase materials is given in Table S2 [27,34,35,51–53]. It can be seen that the L02 sample shows excellent electrochemical properties 642
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Fig. 3. XPS spectra of (a) Mn 2p, (b) Fe 2p, (c) Ni 2p of the as-prepared powder samples.
over 50 cycles. This is due to the changes of the layer structure and the gradual difficulty of the Na ion insertion/deinsertion on the electrode during cycling, which leads to the cycle performance deterioration of the L00 cathode (Fig. 6a). But conversely, the Li-doped sample exhibits considerably decreased Rct value, only showing 3799 Ω from 134.8 Ω over 50 cycles for the L02 electrode. The reduced Rct value demonstrates that lithium ion substitution can effectively inhibit structure
software. As shown in Table 2, the values of Rf almost remain unchanged for all samples. But the value of Rf for the lithium substituted sample is smaller than that of the pristine one, especially for the L02 sample, which suggests that the substitution of lithium improves the conductivity of the solid electrolyte interface. It is evident that the Rct of the L00 sample increases dramatically from 315.4 Ω at the 5 t h cycle to 8771 Ω at the 50 t h cycle, consistent with its poor capacity retention
Fig. 4. Cyclic voltammogram profiles of various Na0.67Lix(Ni0.2Fe0.15Mn0.65)1-xO2+δ samples at a scan rate of 0.1 mV s−1: (a) x = 0, (b) x = 0.1, (c) x = 0.2 and (d) x = 0.3. 643
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Fig. 5. Representative charge/discharge profiles and the corresponding differential capacity curves of the as-prepared cathodes cycled in the voltage range of 1.5–4.3 V at a rate of 15 mA g−1.
charged up to 4 V, but only a slight shift of (00l) peaks toward lower angle, suggesting the enlarged spacing of NaO2 layers owing to the increased electrostatic repulsion force between adjacent oxygen anions. However, when further charged to 4.3 V, the (00l) peaks almost vanished and a new peak located at about 17° was observed, which is assigned to the characteristic peak of a low-crystallinity phase (referred to as the “Z” phase) [54]. This phase transformation can be named as P2-“Z” (there was no commonly accepted definition about the high-voltage phase (“Z”)). Upon discharge, the “Z” phase gradually transformed back to P2 phase. In
change after cycling. Also, the decreased resistance can facilitate the Na ion diffusion and improve the electrochemical properties, particularly for rate performance. To further analyze the interrelation between the enhanced electrochemical cycling performance and the intergrowth structure of P2/O3/ Li2MnO3 composite, ex-situ XRD measurements at various states were carried out to study the structural evolution of the pure Na-P2 phase and P2/ O3/Li2MnO3 composite during the first charge/discharge process, as shown in Fig. 8a and b. Obviously, the pure Na-P2 structure is retained upon
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Fig. 6. Cycling performance (a) and rate capability (b) of the as-prepared Na0.67Lix(Ni0.2Fe0.15Mn0.65)1-xO2+δ (x = 0, 0.1, 0.2, 0.3) samples. Table 1 Specific discharge capacities delivered by Na0.67Lix(Ni0.2Fe0.15Mn0.65)1-xO2+δ (x = 0, 0.1, 0.2, 0.3) compounds at different C-rates. Sample
0.1C [mAh g−1]
0.2C [mAh g−1]
0.5C [mAh g−1]
1C [mAh g−1]
1C [mAh g−1]
3C [mAh g−1]
5C [mAh g−1]
0.1C (recover) [mAh g−1]
L00 L01 L02 L03
205 176 151 135
160 156 135 123
121 137 125 102
90 106 107 87
56 83 90 72
32 60 79 65
4 46 68 52
144 142 150 131
and SEM after different cycling tests, as shown in Fig. 8c–h. For the L00 sample, the (002) peak of P2 structure is shifted to lower angle for the extensively cycled cathodes (Fig. 8c), indicating an irreversible loss of Na ions from the crystal structure due to the structural instability of the pristine material. In contrast, for the modified electrodes, a slight (003) peak shift of O3 phase can be observed after cycling probably resulting from the O3-P3 phase transition, but the shift of the (002) peak of P2 structure is much smaller than the pristine one (Fig. 8d), suggesting the enhanced structural stability after Li substitution due to the inhibited phase transformation under high voltage. The SEM images of the two samples after different cycles are shown in Fig. 8e–h. It can be seen that cracks appeared in the plate-like particles for the L00 sample after extensive cycling tests, while the integrity of the particles for the Li-substituted materials was well preserved. The severe crack is probably the result of the large mechanical stress generated from the dramatic change (P2-“Z” phase transition) of the P2Na0.67Ni0.2Fe0.15Mn0.65O2 crystal structure in the high voltage range, which is largely relieved in L02 sample due to the enhanced structure stability. To summarise, the structure stability especially in the high voltage range can significantly influence the cycling behavior of the cathodes and thus lithium substitution can be used as an effective way to improve the electrochemical performance of P2-type Na-Ni-Fe-Mn-O oxides.
addition, the (100) Bragg peak shifted to higher angel during charge due to the shortened TM-O (TM = Ni, Fe, Mn) bond length and recovered on discharge. For the L02 sample, upon initial charging process, the (00l) peaks of the P2, O3 and Li2MnO3 phase gradually shifted to lower angle, which is evidence of sodium (lithium) ions extraction. However, the (00l) peaks for O3 phase disappeared when charged to 3.3 V, which can correspond to the O3-P3 phase transition. Actually, the (00l) peaks of P3 phase integrated into the (00l) peaks of P2 phase to form asymmetrical ones which means the coexistence of both phases. This phase transition is inevitable in O3 phase via Li substitution, consistent with the previous report on Li-containing Na [Li0.05(Ni0.25Fe0.25Mn0.5)0.95]O2 layered oxides [55]. Remarkably different from the pristine sample, the (00l) peaks of P2 and P3 phase in the L02 sample were well preserved even charged to a high voltage of 4.3 V, indicating that the undesired phase transformations during desodiation, such as P2-“Z”, P3-O3′, are effectively suppressed [56]. During the subsequent discharge process, all peaks in the XRD patterns were fully recovered to the original state with the exception of (001) peak of Li2MnO3 structure. The irreversible behavior indicates that the extracted Li ions do not intercalate back into the electrode in the discharging state. It can be explained that the deintercalated Li ions will be diluted to a very low concentration in the electrolyte, and unlikely to transform back reversibly [26]. To further check out the robustness of L00 and L02, both electrodes were analyzed by XRD
Fig. 7. Electrochemical impedance spectroscopy (EIS) of electrodes in sodium half-cells. Nyquist plot of the as-prepared sample at 4.3 V after 5 t h (a) and 50 t h (b) cycles. 645
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XRD analysis showed that the additional O3 and Li2MnO3 phases formed and developed with the increasing Li amount. Higher Li content also resulted in smaller and agglomerated particles observed by SEM and TEM. Electrochemical measurements demonstrated that the Li substitution could largely improve the cycling stability and rate capability. Especially, the Na0.67Li0.2(Ni0.2Fe0.15Mn0.65)0.8O2+δ cathode material showed the best electrochemical performances compared to the other Li-containing samples. Reversible capacities of 151 mAh g−1 were delivered at 0.1 C and a relatively high capacity retention of 78% was obtained with 50 cycles. Even at a high rate of 5 C, Na0.67Li0.2(Ni0.2Fe0.15Mn0.65)0.8O2+δ could still give a reversible specific capacity of 68 mAh g−1. According to the EIS and ex-situ XRD results, the improvement in electrochemical properties for the Li-substituted samples should be ascribed to the reduced resistance and the increased structure stability in the high voltage of 4.3 V compared to the Li-free sample. Therefore, the new finding in this work may stimulate the new design for high-performance cathode materials for SIBs.
Table 2 EIS parameters of the equivalent circuits after different cycles. Sample
L00 L01 L02 L03
After 5 cycles
After 50 cycles
Rs/Ω
Rf/Ω
Rct/Ω
Rs/Ω
Rf/Ω
Rct/Ω
9.169 9.193 9.465 9.801
315.4 246.4 221.7 235.8
679.4 585.1 134.8 513.4
9.801 9.919 9.141 9.425
319.5 250.6 225.4 239.6
8771 6959 3799 5231
4. Conclusion In summary, a series of Na0.67Lix(Ni0.2Fe0.15Mn0.65)1-xO2+δ (x = 0, 0.1, 0.2, 0.3) cathode materials were successfully synthesized by using a two-stage synthesis process. The introduction of lithium into P2-type Na0.67Ni0.2Fe0.15Mn0.65O2 has a significant influence on the crystal structure, morphology and electrochemical performance. The result of
Fig. 8. Ex-situ XRD patterns collected during the first charge/discharge process of the (a) L00 and (b) L02 at different states. XRD patterns of the (c) L00 and (d) L02 after different cycles. SEM images of (e and f) the L00 sample and (g and h) the L02 sample after 1 and 50 cycles.
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Acknowledgements
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The authors gratefully acknowledge the Nature Science Foundation of China (Grant 51772333 and 51602352), the Nature Science Foundation of Hunan province (Grant No.2015JJ3152) and Central South University Research Foundation of Teacher (2014JSJJ005). Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx. doi.org/10.1016/j.jpowsour.2018.06.058. References [1] D. Larcher, J.M. Tarascon, Nat. Commun. 7 (2015) 19–29. [2] N. Yabuuchi, K. Kubota, M. Dahbi, S. Komaba, Chem. Rev. 114 (2014) 11636–11682. [3] L.P. Wang, J. Mater. Chem. 3 (2015) 9353–9378. [4] P.E. Vassilaras, X. Ma, X. Li, G. Ceder, J. Electrochem. Soc. 160 (2012) 45–48. [5] N. Yabuuchi, H. Yoshida, S. Komaba, Electrochemistry 80 (2012) 716–719. [6] R. Berthelot, D. Carlier, C. Delmas, Nat. Mater. 10 (2011) 74–80. [7] M. Guignard, C. Didier, J. Darriet, P. Bordet, E. Elkaïm, C. Delmas, Nat. Mater. 12 (2013) 74–80. [8] K. Kubota, I. Ikeuchi, T. Nakayama, C. Takei, N. Yabuuchi, H. Shiiba, M. Nakayama, S. Komaba, J. Phys. Chem. C 119 (2015) 166–175. [9] Y. Nanba, T. Iwao, B.M.D. Boisse, W. Zhao, E. Hosono, D. Asakura, H. Niwa, H. Kiuchi, J. Miyawaki, Y. Harada, Chem. Mater. 28 (2016) 1058–1065. [10] Z. Jian, L. Zhao, H. Pan, Y.S. Hu, H. Li, W. Chen, L. Chen, Electrochem. Commun. 14 (2013) 86–89. [11] B. J, C. RJ, A. AR, C. J, R. P, G. CP, B. PG, J. Am. Chem. Soc. 136 (2014) 17243–17248. [12] H. Kim, R.A. Shakoor, C. Park, S.Y. Lim, J.S. Kim, N.J. Yong, W. Cho, K. Miyasaka, R. Kahraman, Y. Jung, Adv. Funct. Mater. 23 (2013) 1147–1155. [13] X. Wu, J. Zheng, Z. Gong, Y. Yang, J. Mater. Chem. 21 (2011) 18630–18637. [14] P. Barpanda, G. Oyama, S.I. Nishimura, S.C. Chung, A. Yamada, Nat. Commun. 5 (2014) 4358. [15] N. Hassanzadeh, S.K. Sadrnezhaad, G. Chen, Electrochim. Acta 220 (2016) 683–689. [16] H. Moriwake, A. Kuwabara, C.A.J. Fisher, M. Nose, H. Nakayama, S. Nakanishi, H. Iba, Y. Ikuhara, J. Power Sources 326 (2016) 220–225. [17] S. Li, J. Guo, Z. Ye, X. Zhao, S. Wu, J.X. Mi, C.Z. Wang, Z. Gong, M.J. Mcdonald, Z. Zhu, ACS Appl. Mater. Interfaces 8 (2016) 17233–17238. [18] C.D. Wessells, R.A. Huggins, Y. Cui, Nat. Commun. 2 (2011) 193–198. [19] H. Lee, Y.I. Kim, J.K. Park, J.W. Choi, Chem. Commun. 48 (2012) 8416–8418. [20] L. Wang, Y. Lu, J. Liu, M. Xu, J. Cheng, D. Zhang, J.B. Goodenough, Angew. Chem. Int. Ed. 52 (2013) 1964–1967. [21] W. Luo, M. Allen, V. Raju, X. Ji, Adv. Energy Mater. 4 (2014) 1400554. [22] C. Delmas, C. Fouassier, P. Hagenmuller, Phys. B+C 99 (1980) 81–85. [23] N. Yabuuchi, M. Kajiyama, J. Iwatate, H. Nishikawa, S. Hitomi, R. Okuyama, R. Usui, Y. Yamada, S. Komaba, Nat. Mater. 11 (2012) 512–517. [24] D. Yuan, X. Hu, J. Qian, P. Feng, F. Wu, R. Mao, X. Ai, H. Yang, Y. Cao, Electrochim. Acta 116 (2014) 300–305.
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Influence of Li substitution on the structure and electrochemical performance of P2-type Na0.67Ni0.2Fe0.15Mn0.65O2 cathode materials for sodium ion batteries
T
Yong Wang, Guorong Hu, Zhongdong Peng, Yanbing Cao, Xiangwan Lai, Xianyue Qi, Zhanggen Gan, Wei Li, Zhongyuan Luo, Ke Du∗ School of Metallurgy and Environment, Central South University, Changsha, 410083, PR China
H I GH L IG H T S
investigate the effect of Li content on P2-type oxide. • We phase and Li MnO grow with increasing lithium content. • O3 Li (Ni Fe Mn ) O shows the optimal electrochemical properties. • Na • Li-doping can reduce the resistance and enhance the high-voltage structure stability. 2
0.67
0.2
0.2
3
0.15
0.65 0.8
2+δ
A R T I C LE I N FO
A B S T R A C T
Keywords: Sodium ion battery Cathode materials Lithium substitution Multiple phases
A series of layered Na0.67Lix(Ni0.2Fe0.15Mn0.65)1-xO2+δ (x = 0, 0.1, 0.2 and 0.3) compounds is prepared via a facile oxalate coprecipitation method and a solid-state reaction process, and investigated as the promising positive electrode materials for Na-ion batteries. As the Li content increases, O3 phase and Li2MnO3 gradually grow at the expense of the P2 phase and the particles become smaller and more agglomerated based on X-ray diffraction, scanning electron microscopy and transmission electron microscope results. The optimal Na0.67Li0.2(Ni0.2Fe0.15Mn0.65)0.8O2+δ shows the improved electrochemical performance with a high specific capacity of 151 mAh g−1 and 78% capacity retention over 50 cycles at 0.1 C (15 mA g−1). When tested at 5 C (750 mA g−1) rate, the electrode exhibits a discharge capacity of 68 mAh g−1. The results of electrochemical impedance spectroscopic and ex-situ X-ray diffraction measurements demonstrate that the improved cyclability and rate capability of the Li-substituted cathode can be ascribed to the decreased resistance and the enhanced structure stability in the high voltage of 4.3 V.
1. Introduction
Thus far, various types of cathode materials for SIBs have been extensively investigated, including layered oxides [4–11], polyanionic compounds [10,12–17], metal hexacyanometalates [18–20], and organic compounds [21]. Among them, layered oxides (NaxTMO2, TM = 3 d transition metals), which were categorized into P-type and Otype by Demals [22], have attracted much attention for their high theoretical capacity. For instance, P2-Na2/3[Fe1/2Mn1/2]O2 layered oxide containing earth-rich elements was found to give a considerable discharge capacity of 190 mAh g−1, comparable to that of nickel-rich lithium oxide, but unfortunately, it underwent fast capacity fade only in 30 cycles [23]. Partial Ni substitution for Fe was used to reduce the content of Mn3+ which causes the well-known Jahn-Teller effect and improve the cycling stabilities with the capacity retention increased
Lithium ion batteries (LIBs) have already been successfully commercialized in portable electronics and expanding into electric vehicles and large scale power energy storage devices in virtue of their high specific energy and excellent cycling stability. However, the limited geological reserves and increasing cost of lithium may lay the hurdle in the way for the further development. It's therefore essential to develop alternative battery systems that are cost-effective and sustainable [1]. Recently, sodium ion batteries (SIBs), which have the similar energy storage mechanism to lithium counterpart, have made a resurgence of research interest due to the inexpensive and earth abundant Na resources [2,3].
∗
Corresponding author. E-mail addresses: [email protected], [email protected] (K. Du).
https://doi.org/10.1016/j.jpowsour.2018.06.058 Received 11 April 2018; Received in revised form 9 June 2018; Accepted 11 June 2018 0378-7753/ © 2018 Elsevier B.V. All rights reserved.
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2.2. Material characterization
from 67% to 80% at 30 t h cycle and could also promote the energy density for the Ni2+/4+ high redox potential [24]. However, these P2type Na-Ni-Fe-Mn-O oxides still suffer from the undesired phase transformation in a low or high concentration of sodium contents that seriously deteriorate the structural integrity and thus lead to poor longterm cycling performance [25]. On the other side, lithium ion substitution to sodium layered oxides has been demonstrated as an effective method to stabilize the crystalline structure and enhance the electrochemistry performance [26–28]. Christopher S. Johnson et al. firstly introduced Li into Na1.0Li0.2Ni0.25Mn0.75Oδ cathode [29]. The whole structure is maintained even charged to a high voltage of 4.4 V and shows very smooth charging/discharging curves owing to the stabilized TM layer after Li introduction. Jing Xu and co-works further investigated the role of the lithium cations in P2-Na0.80[Li0.12Ni0.22Mn0.66]O2 [28]. They found that more Na ions tend to remain in the prismatic positions due to the existence of most univalent Li ions occupying the TM layers, which could also help to maintain the P2-structure in the area of high voltage. Zhou et al. successfully synthesized a P2/O3Na0.66Li0.18Mn0.71Ni0.21Co0.08O2+δ composite with outstanding electrochemical performance by incorporating minor O3 phase into Lisubstituted P2-structure [30]. Multiphase Na0.7Li0.3Ni0.5Mn0.5O2+d material, also proposed by Christopher S. Johnson's group, showed superior rate capability due to the beneficial synergistic effect from the multiple phases including P2 and O3 structures [26]. According to the latest findings, the structure of other sodium manganese-based layered oxides, such as P2-Na0.6[Li0.2Mn0.8]O2 [31], P2/O3-Na2/ 3Li0.18Mn0.8Fe0.2O2 [32], could be also stabilized by doping of lithium. All of these efforts further strengthen the evidence of the positive effect of Li substitution on layered oxides. Inspired by the previous works, we made the first attempt to introduce Li ions into P2-Na0.67Ni0.2Fe0.15Mn0.65O2 base material. A comprehensive investigation on Li-substituted Na0.67Lix(Ni0.2Fe0.15Mn0.65)1-xO2+δ (x = 0, 0.1, 0.2, 0.3) was carried out to check the effect of Li substitution on the structure, morphology and electrochemical performance of the P2-type base material. Based on the analysis results of structure, morphology and electrochemical tests, we found multiple phases co-exist and the lessened particle size after lithium was introduced into the P2-Na0.67Ni0.2Fe0.15Mn0.65O2. The optimum Na0.67Li0.2(Ni0.2Fe0.15Mn0.65)0.8O2+δ cathode exhibited outstanding performance with reduced internal resistance and more stable structure even charged up to 4.3 V compared to the P2-type base material. We proposed that some new insights presented here are of benefit to design high-property layered positive materials for SIBs.
The chemical composition of all the prepared samples was investigated by inductively coupled plasma atomic emission spectroscopy (ICP-AES, Optima 4300DV, PE Ltd.). The crystalline structures of the synthesized Na0.67Lix(Ni0.2Fe0.15Mn0.65)1-xO2+δ (x = 0, 0.1, 0.2, 0.3) samples were analyzed via X-ray diffraction (XRD, RigakuD/maxb) using Cu Kα radiation in a 2θ range of 10°–80° at the scan speed of 2° min−1. The surface morphology and particle size were observed via scanning electron microscopy (SEM, JEOL JSM-6360 L V, Japan). Transmission electron microscope (TEM) was conducted on Philips CM200 microscope system. X-ray photoelectron spectroscopy (XPS, VG Multilab, 2000) measurements were applied to analyze the surface oxidation states of transition metals (Ni, Fe and Mn) for the various ratio particles. 2.3. Electrochemical measurements The electrochemical properties of the samples were evaluated by using a CR2025 coin cell assembled with a Na metal anode in an Arfilled glove box with H2O and O2 contents controlled under 1 ppm. Cathodes were fabricated by blending the as-synthesized powders (80%) with acetylene black (10%) and polyvinylidene fluoride (10%). 1 M NaClO4 solution in 95 vol % propylene carbonate (PC) and 5 vol % fluorinated ethylene carbonate (FEC) and the glass fiber GF/D (Whatman) were employed as the electrolyte and separator, respectively. The constant current charge-discharge experiments were conducted on a coin cell testing system (Land CT, 2001A) purchased from Land Co., China in the voltage window of 1.5–4.3 V vs Na/Na+ where 1 C = 150 mAg−1 at 25 °C. Cyclic voltammetry (CV) and Electrochemical impedance spectroscopic (EIS) tests of all the cells were typically performed at a CHI660D Electrochemical Workstation (Shanghai Chen Hua). 3. Results and discussion We successfully synthesized Ni0.2Fe0.15Mn0.65C2O4·2H2O precursor from co-precipitation process. As shown in Fig. S1, the as-prepared oxalate precursor has a similar crystalline structure to MnC2O4·2H2O, which suggests a solid solution of Ni0.2Fe0.15Mn0.65C2O4·2H2O. After calcined with Na2CO3 and Li2CO3, the element contents of the as-synthesized Na0.67Lix(Ni0.2Fe0.15Mn0.65)1-xO2+δ (x = 0, 0.1, 0.2, 0.3) were analyzed via ICP-AES, as described in Table S1, and the test results are very close to the targeted stoichiometry. The powder XRD patterns of all the prepared samples are presented in Fig. 1. It can be seen that the crystal structure sensitively depends on the Li/TM ratio. The diffraction peaks of the pristine sample can agree well with a hexagonal P2-type structure with space group P63/mmc without other phases. However, with increasing the amount of lithium, extra weak diffraction peaks at 18.7° and 41.8° appear in L01, which are further strengthened in L02 and L03. At the same time, a peak at 16.3° arises in L02, and maintains intensity with lithium content incenses. These new peaks can be assigned to the integrated monoclinic Li2MnO3 component (C2/m) and O3-type structure (R-3m), respectively. In comparison with the P2 structure in L02, the XRD patterns of L03 show obvious differences. The 012, 103, 104 reflections regarding the diffraction signatures of P2 phase are almost invisible, while the diffraction peaks of Li2MnO3 and O3 structures grow in intensity, particularly for the Li2MnO3 phase. This suggests the excess of lithium (≥20%) probably prefers to form the layered oxides (Li2MnO3). Also, a relatively high proportion of Li (≥20%) leads to a rather broad diffraction peaks that means low crystallinity in the structure at the fixed temperature. Morphologies of all the samples were measured by SEM and TEM, as illustrated in Fig. 2. The pristine Na0.67Ni0.2Fe0.15Mn0.65O2 shows flakelike primary particles with the size of around 1–3 μm (Fig. 2a and b). This morphology consists with that typically observed for P2-type
2. Experimental section 2.1. Material synthesis The Na0.67Lix(Ni0.2Fe0.15Mn0.65)1-xO2+δ (x = 0, 0.1, 0.2, 0.3) samples (here after denoted as L00, L01, L02, L03) were synthesized via the solid-state reaction of Na2CO3, Li2CO3 and Ni0.2Fe0.15Mn0.65C2O4·2H2O. The oxalate precursor of Ni0.2Fe0.15Mn0.65C2O4·2H2O was synthesized by a coprecipitation method. Appropriate amounts of NiSO4·6H2O, MnSO4·H2O, and FeSO4·7H2O were dissolved in deionized water with a concentration of 0.5 mol L−1 and pumped slowly into a continuous stirred reactor. Meanwhile, 0.5 mol L−1 of (NH4)2C2O4·H2O and H2SO4 solution with a desired amount were also separately pumped into the reactor. The pH value was adjusted at 3.0 ± 0.1 with the temperature at 30 °C. After the reaction, the light-yellow mixed oxalate was filtered, washed, and then dried in a vacuum box at 120 °C for 24 h. The dried powders were firstly analyzed via ICP-AES to determine the atomic compositions and then thoroughly mixed with Na2CO3 and Li2CO3 by the stoichiometric ratio and sintered at 900 °C for 24 h in air, slowly cooled to room temperature in furnace and finally stored under Ar atmosphere until use. 640
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no electrochemical activity above 2.5 V in layered oxide [24,42–44]. For the 2.7–4.3 V voltage region shown in Fig. 4a, three pairs of redox peaks at 3.43/3.3 V, 3.76/3.49 V and 4.24/3.8 V should be related to the contribution of the Ni2+/Ni3+, Ni3+/Ni4+ and Fe3+/Fe4+ redox pair respectively [24,42,44,45]. The two double peaks related to the Ni2+/Ni4+ redox couple integrate into a pair of broad ones and the polarizations of redox pairs change to smaller by introducing Li in Fig. 4b, thus leading to smooth profiles and enhanced structure stability. As lithium content increased to 0.2, a new reduction peak appears at about 2.7 V and the intensity of the oxidation peak at 3.25 V becomes stronger in comparison with that of the L01 sample. It is obvious that the emerging reduction peak at 2.7 V should be relevant to the increasing part of the oxidation peak at 3.25 V, which is attributed to the phase transformation of the O3-P3 couple due to the rising proportion of O3 phase. This phase transition will be further proved by ex-situ XRD tests in the next section. Electrochemical properties of the Na0.67Lix(Ni0.2Fe0.15Mn0.65)1xO2+δ (x = 0, 0.1, 0.2, 0.3) samples are further evaluated by galvanostatic charge/discharge measurement. Fig. 5 illustrates the voltage curves and the corresponding differential capacity (dQ/dV) profiles of the as-prepared materials. The charge/discharge profiles of the pristine sample seem quite rough for the existence of several potential plateaus, which can be confirmed by a lot of sharp peaks in the derivative plots in Fig. 5b. With the increase of Li content, the charge/discharge curves become much smoother, associated with less and broader peaks in the derivative curves, in good accordance with the CV results. This phenomenon indicates the positive impact of Li on smoothing the curves by suppressing Na/vacancy ordering, phase transition during sodium (de) intercalation [31,46,47]. When x = 0.2 or 0.3, the initial derivative plots show a strong oxidation peak around 4.2 V vs Na/Na+ and then the peak disappears in the following charge/discharge process. The feature is relevant to the irreversible potential plateau at 4.2 V in the first charging curve in Fig. 5e and g, which is commonly observed in Lirich compositions (4.5 V vs Li/Li+). This process is believed to be caused by the lithium extraction and oxygen evolution from Li2MnO3 structures described by previous works [48,49]. The similar charge plateau at 4.2 V of Li2MnO3 compounds in Na-based O3-type cathode materials has been recently reported by Jing Xu and co-workers [50]. After the initial charging process, single smooth voltage profiles are observed above 3.0 V, which indicates a solid-solution (de)intercalation reaction [29]. Additional voltage plateau appears at about 3.0 V, suggesting the co-existence of multiple phases. This process probably includes the O3-P3 phase transition, which agrees well with the CV test results in Fig. 4c and d. The initial discharge capacity of the L00 sample is 207 mAh g−1, but unfortunately it suffers a rapid capacity decline during the following cycles [24]. Upon long-term cycling, the peak at 4.2 V of differential capacity plots for the pristine sample in Fig. 5b gradually diminished and almost vanished after 30 cycles. In addition, the intensity of the other redox peaks within the voltage range of 1.5–4.3 V is largely weakened, associated with the increase of the electric potential difference of these redox pairs. As shown in Fig. 5c, e and g, the 1 s t discharge capacity of L01, L02 and L03 sample deceases to 172, 151 and 136 mAh g−1 respectively, probably due to the reduced proportion of electrochemical active elements of TM (TM = Ni, Fe, Mn). Different to the pristine sample, the potential shifts of the redox peaks caused by the electrode polarization are gradually relieved in L01, L02 and L03 sample, which can be explained by the enhanced structure stability after the introduction of Li ions. Fig. 6a exhibits the cycling behavior of all the samples. The discharge capacity of the pristine L00 declines from 207 mAh g−1 at the 1 s t cycle to 65 mAh g−1 at the 50 t h cycle, corresponding a poor capacity retention of 31%. For lithium substituted samples, the capacity retention is largely improved to 70%, 78% and 77% for L01, L02 and L03, respectively. Fig. 6b displays the rate capability tests at different current densities which are ranged from 0.1 to 5 C rate. The specific discharge capacities delivered by the various compounds at different C-
Fig. 1. XRD patterns of Na0.67Lix(Ni0.2Fe0.15Mn0.65)1-xO2+δ (x = 0, 0.1, 0.2, 0.3) samples.
layered material [33]. Particle size and shape are affected by the content of lithium. The samples with a small amount of Li, such as L01 with main crystal of P2 phase, exhibit almost the same shape with clear facets and sharp edges (Fig. 2c and d). By further introducing Li, the smaller particles (around 0.5–1 μm) like grains gradually appear at the expense of the flakes and become more agglomerated (Fig. 2e–h). This phenomenon was also observed in P3/P2/O3Na0.76Mn0.5Ni0.3Fe0.1Mg0.1O2 composite [34]. EDS mapping was employed to investigate the elemental distributions of Na, Ni, Fe and Mn on the surface of the L02 cathodes material. As shown in Fig. S2, it can be observed that all elements are uniformly distributed throughout the particles, confirming the successful preparation of the solid solution of Ni0.2Fe0.15Mn0.65C2O4·2H2O precursor and the even element distribution after calcined with alkali carbonate. The effect of lithium introduction on the surface element oxidation state was evaluated by XPS. Fig. 3 illustrates the XPS spectra of Mn 2p, Fe 2p and Ni 2p of all the samples. It can be seen that the Mn 2P3/2 main peak centers at 641.9 eV, demonstrating the existence of Mn ion at +4 state. The resulting binding energy of Fe 2P3/2 is around 710.4 eV, which is in quite good agreement with that of Fe3+ in Fe2O3, indicating that the valence state of the iron is +3. The main peak of Ni 2P3/2 located at 854.5 eV can be ascribed to Ni2+. Although the main peaks of Mn 2p3/2, Fe 2p3/2 and Ni 2p3/2 decline in intensity, the location and width almost keep the same, probably resulting from the reduced proportions of the TM ions with increasing lithium content. This indicates the unchanged oxidation state of three transition metal ions in all the samples. Based on the results of XPS analysis, the valence states of manganese, iron and nickel are +4, +3 and + 2, respectively, matching well with the previous studies [24,33,35–41]. Therefore, it can be concluded that the oxidation states of Mn, Fe and Ni ions do not change after Li substitution. The electrochemical (de)intercalation reaction of the various samples was initially characterized via cyclic voltammetry (CV), applied to explore the influence of lithium contents on the electrochemical properties of all samples. Fig. 4 exhibits the first five CV plots of the cathodes at a scanning speed of 0.1 mV s−1. At low voltage range, for all samples, the reversible oxidation/reduction peaks located at around 2.0 V corresponds to Mn4+/Mn3+ redox process because Mn4+ shows 641
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Fig. 2. (a-h) SEM and the corresponding TEM images of Na0.67Lix(Ni0.2Fe0.15Mn0.65)1-xO2+δ samples with x = 0, 0.1, 0.2 and 0.3, respectively.
with considerations of capacity retention, reversible capacity, rate capability and initial coulombic efficiency, which is more suitable as a potential candidate for the cathodic materials of SIBs. Electrochemical impedance spectroscopic (EIS) tests were applied to further evaluate the effect of lithium introduction on the stability of the as-prepared material. Fig. 7 shows the Nyquist plots of the as-prepared sample after 5 and 50 cycles at 4.3 V. These plots are comprised of the typical compositions, i.e. two semicircle connected with a slope line, observed in AC impedance spectra. The intercept of Nyquist curve with Z′ axis is assigned to the solution resistance (Rs). The semicircle in the high frequency region is usually associated with the resistance of Na ions transfer through the surface films (Rf) and another one in the medium-high frequency area is related to the charge-transfer resistance (Rct) of the electrodes. All EIS parameters were acquired by Z-view
rates are shown in Table 1. The Li-free L00 electrode can give a capacity of 205, 160, 121, 90, 56, 32 and 4 mAh g−1 at 0.1, 0.2, 0.5, 1, 2, 3 and 5 C rates, respectively. However, for the Li-substituted electrodes, the rate performances are significantly enhanced. The L02 exhibits the discharge capacity of 151, 135, 125, 107, 90, 79 and 68 mAh g−1 at the same rates. The capacity of 68 mAh g−1 for L02 at the high rate of 5 C (750 mA g−1) is comparable to that of P2/O3Na0.66Li0.18Mn0.71Ni0.21Co0.08O2+δ (69 mAh g−1 at 5 C equal to 500 mA g−1) [30]. After the current density is reduced to 0.1 C, the L00 sample can recover only 70.2% of the reversible capacity whereas the L02 sample can recover 99% and still retain 150 mAh g−1, providing further evidence of the sample stability. A comparison of L02 with other multiphase materials is given in Table S2 [27,34,35,51–53]. It can be seen that the L02 sample shows excellent electrochemical properties 642
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Fig. 3. XPS spectra of (a) Mn 2p, (b) Fe 2p, (c) Ni 2p of the as-prepared powder samples.
over 50 cycles. This is due to the changes of the layer structure and the gradual difficulty of the Na ion insertion/deinsertion on the electrode during cycling, which leads to the cycle performance deterioration of the L00 cathode (Fig. 6a). But conversely, the Li-doped sample exhibits considerably decreased Rct value, only showing 3799 Ω from 134.8 Ω over 50 cycles for the L02 electrode. The reduced Rct value demonstrates that lithium ion substitution can effectively inhibit structure
software. As shown in Table 2, the values of Rf almost remain unchanged for all samples. But the value of Rf for the lithium substituted sample is smaller than that of the pristine one, especially for the L02 sample, which suggests that the substitution of lithium improves the conductivity of the solid electrolyte interface. It is evident that the Rct of the L00 sample increases dramatically from 315.4 Ω at the 5 t h cycle to 8771 Ω at the 50 t h cycle, consistent with its poor capacity retention
Fig. 4. Cyclic voltammogram profiles of various Na0.67Lix(Ni0.2Fe0.15Mn0.65)1-xO2+δ samples at a scan rate of 0.1 mV s−1: (a) x = 0, (b) x = 0.1, (c) x = 0.2 and (d) x = 0.3. 643
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Fig. 5. Representative charge/discharge profiles and the corresponding differential capacity curves of the as-prepared cathodes cycled in the voltage range of 1.5–4.3 V at a rate of 15 mA g−1.
charged up to 4 V, but only a slight shift of (00l) peaks toward lower angle, suggesting the enlarged spacing of NaO2 layers owing to the increased electrostatic repulsion force between adjacent oxygen anions. However, when further charged to 4.3 V, the (00l) peaks almost vanished and a new peak located at about 17° was observed, which is assigned to the characteristic peak of a low-crystallinity phase (referred to as the “Z” phase) [54]. This phase transformation can be named as P2-“Z” (there was no commonly accepted definition about the high-voltage phase (“Z”)). Upon discharge, the “Z” phase gradually transformed back to P2 phase. In
change after cycling. Also, the decreased resistance can facilitate the Na ion diffusion and improve the electrochemical properties, particularly for rate performance. To further analyze the interrelation between the enhanced electrochemical cycling performance and the intergrowth structure of P2/O3/ Li2MnO3 composite, ex-situ XRD measurements at various states were carried out to study the structural evolution of the pure Na-P2 phase and P2/ O3/Li2MnO3 composite during the first charge/discharge process, as shown in Fig. 8a and b. Obviously, the pure Na-P2 structure is retained upon
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Fig. 6. Cycling performance (a) and rate capability (b) of the as-prepared Na0.67Lix(Ni0.2Fe0.15Mn0.65)1-xO2+δ (x = 0, 0.1, 0.2, 0.3) samples. Table 1 Specific discharge capacities delivered by Na0.67Lix(Ni0.2Fe0.15Mn0.65)1-xO2+δ (x = 0, 0.1, 0.2, 0.3) compounds at different C-rates. Sample
0.1C [mAh g−1]
0.2C [mAh g−1]
0.5C [mAh g−1]
1C [mAh g−1]
1C [mAh g−1]
3C [mAh g−1]
5C [mAh g−1]
0.1C (recover) [mAh g−1]
L00 L01 L02 L03
205 176 151 135
160 156 135 123
121 137 125 102
90 106 107 87
56 83 90 72
32 60 79 65
4 46 68 52
144 142 150 131
and SEM after different cycling tests, as shown in Fig. 8c–h. For the L00 sample, the (002) peak of P2 structure is shifted to lower angle for the extensively cycled cathodes (Fig. 8c), indicating an irreversible loss of Na ions from the crystal structure due to the structural instability of the pristine material. In contrast, for the modified electrodes, a slight (003) peak shift of O3 phase can be observed after cycling probably resulting from the O3-P3 phase transition, but the shift of the (002) peak of P2 structure is much smaller than the pristine one (Fig. 8d), suggesting the enhanced structural stability after Li substitution due to the inhibited phase transformation under high voltage. The SEM images of the two samples after different cycles are shown in Fig. 8e–h. It can be seen that cracks appeared in the plate-like particles for the L00 sample after extensive cycling tests, while the integrity of the particles for the Li-substituted materials was well preserved. The severe crack is probably the result of the large mechanical stress generated from the dramatic change (P2-“Z” phase transition) of the P2Na0.67Ni0.2Fe0.15Mn0.65O2 crystal structure in the high voltage range, which is largely relieved in L02 sample due to the enhanced structure stability. To summarise, the structure stability especially in the high voltage range can significantly influence the cycling behavior of the cathodes and thus lithium substitution can be used as an effective way to improve the electrochemical performance of P2-type Na-Ni-Fe-Mn-O oxides.
addition, the (100) Bragg peak shifted to higher angel during charge due to the shortened TM-O (TM = Ni, Fe, Mn) bond length and recovered on discharge. For the L02 sample, upon initial charging process, the (00l) peaks of the P2, O3 and Li2MnO3 phase gradually shifted to lower angle, which is evidence of sodium (lithium) ions extraction. However, the (00l) peaks for O3 phase disappeared when charged to 3.3 V, which can correspond to the O3-P3 phase transition. Actually, the (00l) peaks of P3 phase integrated into the (00l) peaks of P2 phase to form asymmetrical ones which means the coexistence of both phases. This phase transition is inevitable in O3 phase via Li substitution, consistent with the previous report on Li-containing Na [Li0.05(Ni0.25Fe0.25Mn0.5)0.95]O2 layered oxides [55]. Remarkably different from the pristine sample, the (00l) peaks of P2 and P3 phase in the L02 sample were well preserved even charged to a high voltage of 4.3 V, indicating that the undesired phase transformations during desodiation, such as P2-“Z”, P3-O3′, are effectively suppressed [56]. During the subsequent discharge process, all peaks in the XRD patterns were fully recovered to the original state with the exception of (001) peak of Li2MnO3 structure. The irreversible behavior indicates that the extracted Li ions do not intercalate back into the electrode in the discharging state. It can be explained that the deintercalated Li ions will be diluted to a very low concentration in the electrolyte, and unlikely to transform back reversibly [26]. To further check out the robustness of L00 and L02, both electrodes were analyzed by XRD
Fig. 7. Electrochemical impedance spectroscopy (EIS) of electrodes in sodium half-cells. Nyquist plot of the as-prepared sample at 4.3 V after 5 t h (a) and 50 t h (b) cycles. 645
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XRD analysis showed that the additional O3 and Li2MnO3 phases formed and developed with the increasing Li amount. Higher Li content also resulted in smaller and agglomerated particles observed by SEM and TEM. Electrochemical measurements demonstrated that the Li substitution could largely improve the cycling stability and rate capability. Especially, the Na0.67Li0.2(Ni0.2Fe0.15Mn0.65)0.8O2+δ cathode material showed the best electrochemical performances compared to the other Li-containing samples. Reversible capacities of 151 mAh g−1 were delivered at 0.1 C and a relatively high capacity retention of 78% was obtained with 50 cycles. Even at a high rate of 5 C, Na0.67Li0.2(Ni0.2Fe0.15Mn0.65)0.8O2+δ could still give a reversible specific capacity of 68 mAh g−1. According to the EIS and ex-situ XRD results, the improvement in electrochemical properties for the Li-substituted samples should be ascribed to the reduced resistance and the increased structure stability in the high voltage of 4.3 V compared to the Li-free sample. Therefore, the new finding in this work may stimulate the new design for high-performance cathode materials for SIBs.
Table 2 EIS parameters of the equivalent circuits after different cycles. Sample
L00 L01 L02 L03
After 5 cycles
After 50 cycles
Rs/Ω
Rf/Ω
Rct/Ω
Rs/Ω
Rf/Ω
Rct/Ω
9.169 9.193 9.465 9.801
315.4 246.4 221.7 235.8
679.4 585.1 134.8 513.4
9.801 9.919 9.141 9.425
319.5 250.6 225.4 239.6
8771 6959 3799 5231
4. Conclusion In summary, a series of Na0.67Lix(Ni0.2Fe0.15Mn0.65)1-xO2+δ (x = 0, 0.1, 0.2, 0.3) cathode materials were successfully synthesized by using a two-stage synthesis process. The introduction of lithium into P2-type Na0.67Ni0.2Fe0.15Mn0.65O2 has a significant influence on the crystal structure, morphology and electrochemical performance. The result of
Fig. 8. Ex-situ XRD patterns collected during the first charge/discharge process of the (a) L00 and (b) L02 at different states. XRD patterns of the (c) L00 and (d) L02 after different cycles. SEM images of (e and f) the L00 sample and (g and h) the L02 sample after 1 and 50 cycles.
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Acknowledgements
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