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Three dimensional frameworks of super ionic conductor for thermodynamically and dynamically favorable sodium metal anode
Journal Pre-proof Three Dimensional Frameworks of Super Ionic Conductor for Thermodynamically and Dynamically Favorable Sodium Metal Anode Min Guo, Huanglin Dou, Wanyu Zhao, Xiaoli Zhao, Bingxin Wan, Jiahe Wang, Yuantao Yan, Xiaomin Wang, Zi-Feng Ma, Xiaowei Yang PII:
S2211-2855(20)30036-7
DOI:
https://doi.org/10.1016/j.nanoen.2020.104479
Reference:
NANOEN 104479
To appear in:
Nano Energy
Received Date: 3 December 2019 Revised Date:
9 January 2020
Accepted Date: 9 January 2020
Please cite this article as: M. Guo, H. Dou, W. Zhao, X. Zhao, B. Wan, J. Wang, Y. Yan, X. Wang, Z.-F. Ma, X. Yang, Three Dimensional Frameworks of Super Ionic Conductor for Thermodynamically and Dynamically Favorable Sodium Metal Anode, Nano Energy, https://doi.org/10.1016/ j.nanoen.2020.104479. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
Graphical abstract
Stable plating/stripping behaviors of sodium with dendrite-free morphology and improved cycling stabilities is realized by sodium super ionic conductor.
Three
Dimensional
Frameworks
of
Super
Ionic
Conductor
for
Thermodynamically and Dynamically Favorable Sodium Metal Anode
Min Guoa, Huanglin Doua, b, Wanyu Zhaoa, Xiaoli Zhaoa*, Bingxin Wana, Jiahe Wanga, Yuantao Yana, Xiaomin Wangb, Zi-Feng Mac, Xiaowei Yanga*
a
School of Materials Science and Engineering, Interdisciplinary Materials Research
Center, Tongji University, Shanghai 201804, China b
College of Materials Science and Engineering, Taiyuan University of Technology,
Taiyuan 030024, China c
Shanghai Electrochemical Energy Devices Research Center, School of Chemistry and
Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
Corresponding Author *Email: [email protected]; [email protected]
Abstract Metallic sodium anodes are highly attractive due to the high specific capacity and natural abundance, but the uncontrollable Na dendrite issues impede its practical implementation. Nucleation, growth and dissolution of Na are an inseparable sequential process, so the variations in these reaction thermodynamics and kinetics could lead to dendrite proliferation and low utilization of Na anode. It appears to be particularly important to optimize the whole continuous process by introducing a super ionic conductor material, Na3V2(PO4)3, as the modulation medium of continuous nucleation, growth and dissolution to suppress Na dendrite in this work. The initial intercalation reaction of Na+ to NVP thermodynamically improves the affinity of Na and NVP, leading to a low-barrier nucleation and homogenous Na-ion flux. Importantly, the super ionic conductor dynamically provides ultrafast migration channels to promote the interfacial Na-ion transport, contributing to a reduced electrochemical polarization and uniform Na-ion distribution. As a result, the cell
displays superior rate performance and improved cycling stabilities in both half and full cells. This work proposes a new strategy for a thermodynamically and dynamically favorable nucleation and reversible plating/stripping of Na, which can be extend to other metal anodes.
Keywords: sodium metal anode, sodium super ionic conductor, steady nucleation, dendrite-free, reaction kinetics
Introduction Significant research interest has been devoted to sodium metal anode due to the high capacity (1160 mAh g-1), low electrode potential (-2.71 V vs. the standard hydrogen electrode) and natural abundance of sodium [1, 2]. Nevertheless, the safe and stable operation of Na metal batteries has been hindered by the uncontrollable Na dendrite problems. To tackle the problem, various strategies have been put forward, including electrolyte additives [3, 4], artificial SEI [5, 6], solid-state electrolyte [7, 8] and three-dimensional (3D) current conductors [9, 10]. However, few strategies focus on the inseparable sequential process of nucleation, growth and dissolution of Na in term of both thermodynamics and kinetics. According to classical nucleation mechanism, the initial nucleating stage plays a crucial role to the subsequent deposition behavior. The nucleation of a new phase needs to surmount a free energy barrier thermodynamically, the larger the energy barrier, the more difficult to deposition [11-14]. Commonly, the nucleation barrier can be modulated through regulating the affinity between Li/Na and substrate. If the substrate and Li/Na metal have poor affinity, this weak interaction may lead to remarkable nucleation barriers (nucleation overpotentials), thus the large electric field driving force will be inevitable to promote Li/Na metal to grow from the root and form whisker shape [15]. Inversely, the strong affinity between the substrate and Li/Na metal is conductive to a low barrier nucleation, in favor of a surface growth pattern with dendrite-suppressed morphology [16, 17]. Targeting the nucleation problem, numerous efforts have been made to reduce Li/Na nucleation barriers by the introduction of nucleation seeds (such
as Au, Ag and Sn2+) [13, 14, 18]. However, it is worth noting that alloying nucleation reactions generate huge volume change in these seed particles, unfavorable to steady nucleation. Concerning the nucleation and following deposition/dissolution are an inseparable sequential process, stable plating/stripping is equally important, which are dynamically affected by interfacial ion migration. The bottleneck in the ion mobility to respond to the electrochemical reactions in time may cause mass electron aggregation, resulting in large electrochemical polarization on the electrode surface. The resultant polarization will promote selective electrodeposition of Na on local sites where dendrites and inhomogeneous deposition of sodium metal can proliferate. To address this issue, the tactics with enhanced interfacial mass/charge transfer by introducing ionic conducting paths can help to redistribute the ion flux at the metal-electrolyte interphase and promote the interfacial Na+ transport kinetics [19-21]. Therefore, it is urgent and meaningful to explore new strategies to solve the nucleation and interfacial issue in terms of thermodynamics and dynamics. Herein, we design a stable host by involving the three-dimensional frameworks of super ionic conductor and find that it could effectively guide the nucleation and simultaneously accelerate interfacial ion transport. In this work, the natrium super ionic conductor Na3V2(PO4)3 (NVP) microflowers composed of nanoflakes could afford plentiful nucleation sites and ion transport pathways. During the Na plating process, Na ions are prioritized intercalating into Na3V2(PO4)3 crystal to form sodiated Na3+xV2(PO4)3 phase. The initial sodiation reaction improves the affinity of Na and NVP, conducive to a low barrier nucleation. After intercalation of Na, the increased electronegativity of NVP can further enhance the ability of NVP to absorb Na+ in the electrolyte, which helps to uniform the Na+ flux and realize the homogeneous deposition of Na. Furthermore, compared with the conventional alloying nucleation reaction with huge volume change, the intercalation reactions of NVP accompanied with ignorable volume fluctuation are responsible for constructing more stable Na ions nucleation condition. More importantly, as a typical super ionic conductor, the cross-linked distributed fast ion network can homogenize Na-ion flux and provide
additional ion carrier paths for Na to deposit and dissolve. These collaborative strategies lead to more stabilized Na plating/stripping with a low overpotential and greatly prolonged lifespan. Compared to bare Cu foil, the Na@3D-NVP electrode presents higher coulombic efficiencies and lower polarizations. The lifetime of Na plating/stripping for symmetric Na@3D-NVP cell maintains up to 400 h with a stabilized overpotential of 119.3 mV at 1 mA cm-2 for 2 mAh cm-2. The results demonstrate that the super ionic conductor in regulating the nucleation barrier and promoting the reversible plating/stripping of sodium is very effective.
Figure 1. Schematic illustration of Na plating/stripping behaviors on (a) Na foil and (b) 3D NVP matrix.
Results and discussion Figure 1 illustrates the schematic diagrams of Na plating and stripping processes on bare Na foil and 3D NVP matrix. Though Na foil could be used as the anode, its rough surface and “hostless” nature trigger the inhomogeneous nucleation and dendrite generation (Figure 1a). As depicted in Figure 1b, 3D NVP scaffolds integrating the merits of super ionic conductor and polar 3D framework are proposed to restrain dendritic growth. The initiatory intercalation reaction of NVP shows negligible volume change, and the stable structure helps to establish steady Na ions nucleation
condition and navigate the subsequent growth to form dendrite-free plating morphology. Importantly, benefiting from the super ionic conductive property, the abundant fast ionic conducting channels in the deposited Na@3D-NVP electrode are favorable to facilitate rapid ion migration and low surface transfer resistance. Besides, the polar functional groups (Si-O, O-H) in glass fiber (GF) matrix are able to adsorb considerable Na+ to alleviate the electrostatic interactions between Na+ and protuberances, to avoid the accumulation of Na ions around protuberances [22, 23]. And the dispersed MXene nanosheets can enhance the electronic conductivity of the entire scaffold [24, 25]. All these characteristics are conductive to a reversible plating and stripping of Na metal anode.
Figure 2. (a) XRD patterns. (b) High resolution spectra of V 2p; (c-d) SEM images; (e) elemental mapping of 3D NVP scaffold.
The structure and phase characteristic of the 3D NVP scaffold are detected by X-ray diffraction (XRD). As shown in Figure 2a, the diffraction peaks marked with triangles and clubs are indexed to GF framework and NASICON structured Na3V2(PO4)3 (JCPDS card No. 053-0018) [26], respectively, which is further confirmed by X-ray photoelectron spectrum (XPS) measurement (Figure 2b and S1). As shown in Figure 2c-d and S2, 3D interconnected structure with pores is revealed for the pristine GF framework, and a mixed ion and electron conducting scaffold is well formed after infusing with the mixture of Na3V2(PO4)3 and MXene. It can be observed that the super ionic conductor Na3V2(PO4)3 shows microflowered morphology with a size distribution from a few to tens of microns. The corresponding elemental mapping with homogenous distribution of V, P and Na elements in Figure 2e further confirms the existence of NVP.
Figure 3. The deposition behaviors of Cu foil, GF and 3D NVP. Plating profile and nucleation overpotentials at the current density of (a-b) 0.1 mA cm-2 and (c-d) 2 mA cm-2. (e) Tip voltages at the current densities from 0.1 to 2 mA cm-2. (f) Nyquist plots.
To investigate the nucleation behaviors of sodium in NVP scaffold, we measure the plating curves on different substrates. Here, Cu foil and pristine GF matrix are chosen as the control groups. Figure 3a-d show the plating profiles of Cu, GF and 3D NVP at 0.1 and 2 mA cm-2. As shown, the 3D NVP display an obvious sodiation reaction process before Na plating. The potential plateau at ~1.5 and ~0.3 V is ascribed to the
intercalation of Na+ into Na3V2(PO4)3 crystal, and the slope region is attributed to the formation of the solid electrolyte interphase (SEI) at such low anode potential below the stability limit of the electrolyte [27, 28], which can be observed on the galvanostatic discharge curves and cyclic voltammetry (CV) profile when using Na3V2(PO4)3 as the anode material (Figure S3 and 4). The preformed SEI layer on the surface of 3D NVP framework may be helpful to produce more stable SEI in the following deposition process and finally induce excellent structural stability of Na anode [13, 29-31]. The initial pre-sodiation reaction facilitates a low barrier nucleation because Na plating tends to proceed preferentially in the pre-sodiated region, rather than surmounting extra-large energy to create new plating sites [32]. After intercalation, an electron deviation from Na to NVP will occur [30, 33], which can increase the electronegativity of NVP and enhance the ability of NVP to absorb Na+ in the electrolyte, to uniform the Na+ flux and realize the uniform deposition of Na. Na ions preferentially pass through the fast ion conductor channels with a low overpotential rather than break the highly resistive solid electrolyte interface layers. This property could rapidly compensate for sodium ion depletion near the electrode surface during plating/stripping process. All these advantages may lead to a stable nucleation and can be verified by the plating curves. The voltage dips at the beginning of plating curves are determined by nucleation energy barriers and the following plateaus are controlled by mass transfer [31]. From Figure 3a-d, the 3D NVP shows the lowest voltage dips among all the matrixes, suggesting a low nucleation barrier. And Figure 3e compares the voltage dips (tip voltages) at different current densities from 0.1 to 2 mA cm-2. Unsurprisingly, the 3D NVP shows the lowest tip voltages in these frameworks, further confirming the lower sodium nucleation barriers. Concretely, the nucleation overpotential is defined as the difference between the tip voltage and plateau voltage [31]. As depicted in Figure 3b and d, the overpotential of bare Cu foil is 183.9 mV at 0.1 mA cm-2, and rapidly increases to 513.1 mV at 2 mA cm-2, illustrating the energy used to overcome the heterogeneous nucleation barrier is very large, while the nucleation overpotentials of 3D NVP are small, which delivers the overpotentials of 16.8 and 78.5 mV at 0.1 and 2 mA cm-2, respectively, indicating lower nucleation barrier and a more
stable mass transfer process. Electrochemical impedance spectroscopy (EIS) measurement is carried out to measure the interfacial and charge transfer resistances after deposition. The corresponding equivalent circuit diagram is shown in Figure S5 [31, 34] and the fitted impedance parameters are listed in Table S1. The fitted results reveal that the interfacial resistance as well as the charge transfer resistance of 3D NVP framework are considerably reduced compared with that of the bare Cu foil.
Figure 4. (a-d) Morphology evolution of 3D NVP at different deposition sites. (e) Top-view and (f) bottom-view images of 3D NVP after deposition 10 mAh cm-2. SEM images of Cu foil after plating (g) 2, (h) 7, and (i) 10 mAh cm-2. (j) The corresponding sites of 3D NVP with different deposition capacities.
To study the deposition behaviors, the morphology evolution of 3D NVP is further studied at the current density of 0.1 mA cm-2 for different areal capacities. Initially, the
NVP shows microflowers shape (Figure 4a and S 6a). Then at the beginning of electrochemical plating, it still maintains microflower structure after reacting with Na3V2(PO4)3 to form sodiated phase Na3+xV2(PO4)3 (Figure 4b and S 6b). The X-ray diffraction results shows the sodiated NVP scaffold keeps good NASICON structure (Figure S7), indicating the excellent structure stability [28]. As further increasing the deposition capacity, Na metal continues to deposit along the surface of NVP, and then NVP microflowers are gradually covered and wrapped by the plated sodium (Figure 4c-d and S 6c-d). Further enhancing the plating capacity to 10 mAh cm-2, the pores in the NVP scaffold are filled with Na meal and the isolated Na-wrapping NVP particles are joint together. As shown in Figure 4e-f and and S 6e-f, the NVP matrix exhibits a flat surface and bottom morphology without dendritic sodium formation. In contrast, the branch-like structured Na metal is observed at the initial plating for bare Cu foil (Figure 4g and S 6g), and large amounts of columnar Na dendrites and long filaments shaped “dead Na” are observed on the surface of Cu foil when increasing the deposition capacities (Figure 4h-i and S 6h-i). The large dendrites and dead Na not only increase interface resistance, but also may trigger internal short circuit. As shown in Figure S8, the bare GF framework with small deposition capacity exhibits no obvious change in the top-view, but some large protuberances in the bottom. And the protuberances become larger with the increasing of deposition capacity (Figure S8 e-f). These results suggest the 3D NVP can effectively guide the growth of sodium metal.
Figure 5. (a) Schematic diagram of symmetric cell. (b) Tafel plot in exchange current density test. (c) Voltage profile of GITT in NaǁNa and Na@3D-NVPǁNa@3D-NVP symmetric cells. (d) Enlargement of the green box in GITT profile.
To further explore the reaction kinetics and interfacial properties of the obtained anode during the plating/stripping cycling, galvanostatic intermittent titration technique (GITT) method and exchange current density experiments are conducted. Herein, the corresponding voltage-time curves are monitored via NaǁNa and Na@3D-NVPǁNa@3D-NVP symmetric cells (Figure 5a). As shown from the Tafel plot in Figure 5b, in high overpotential region, the current density is dominated by mass transfer from electrolyte to the electrode surface. The rapid Na+ transport channels in Na@3D-NVP electrode will facilitate Na ions to transfer from electrolyte to the Na surface. As a result, the Na@3D-NVP electrode has higher exchange current density than the bare Na electrode, demonstrating a faster mass transfer process. In the low overpotential part, the current density is dominated by charge transfer between electrode and the redox species. The higher current density of Na@3D-NVP
electrode than the bare Na electrode demonstrates the faster Na deposition/dissolution kinetics [19]. As shown in Figure 5c, the GITT curves of Na@3D-NVPǁNa@3D-NVP cell has a significantly lower overpotential compared with the NaǁNa cell, indicating the Na@3D-NVP electrode possesses faster mass transfer kinetics. Notably, there is a large spike at the beginning of each step for NaǁNa cell, followed by a plateau and an abrupt increased bump polarization. The spike at the beginning corresponds to the initial Na nucleation overpotential [35, 36] and the subsequent abruptly increased polarization originates from the electro-dissolution of bulk Na at the stripping steps [37, 38]. The apparent nucleation and bulk dissolution overpotential of NaǁNa cell indicate the large nucleation barrier and sluggish interfacial ion transport. Conversely, the Na@3D-NVPǁNa@3D-NVP cell shows almost no nucleation and bulk dissolution overpotentials (Figure 5d). The stabilized plating and stripping plateaus indicate a lower energy barrier for both nucleation and plating/stripping process.
Figure 6. Voltage-time curves at 1 mA cm-2 for (a) 1 mAh cm-2 and (b) 2 mAh cm-2. EIS analysis of (c) NaǁNa and (d) Na@3D-NVPǁNa@3D-NVP symmetric cells at different cycles at 1 mA cm-2 for 1 mAh cm-2. (e) Rate performances from 0.5 to 10 mA
cm-2 with a fixed capacity of 1 mAh cm-2 of NaǁNa and Na@3D-NVPǁNa@3D-NVP symmetric cells.
Generally, many works evaluate the cycling stability of cells using Li or Na metal as the counter electrode [39-41]. Here, we also measure the long cycling performance based on Na metal anode. NaǁNa@Cu and NaǁNa@3D-NVP cells are tested at the current densities from 0.5 to 2 mA cm-2 with a fixed areal capacity of 2 mAh cm-2. As displayed in Figure S9a-b, the Na@Cu electrode only maintains dozens of hours and suddenly become short circuit, while the voltage hysteresis of Na@3D-NVP electrode maintain at ~62.2 and ~137.6 mV over 770 and 400 hours at 0.5 and 1 mA cm-2. Even at 2 mA cm-2, the Na@3D-NVP electrode still displays a stable polarization (199.3 mV) over 205 h (Figure S9c). Furthermore, the overpotential and long-term cycling stabilities of the symmetric cells are analyzed in Figure 6 and S10. Noticeably, the Na@3D-NVPǁNa@3D-NVP cell offers a long lifespan up to 300 h with stable and smooth voltage profiles (voltage hysteresis 124.2 mV) at 1 mA cm-2 (Figure 6a). The corresponding voltage profiles exhibit regular and flat plateau without spike and bump, indicating a steady nucleation and fast reaction kinetics. And longer galvanostatic time for 2 h is further measured at 1 mA cm-2, the lifespan of Na plating/stripping in Na@3D-NVP symmetric cell still maintains 400 h and the overpotential almost stabilizes at 119.3 mV (Figure 6b), while the bare NaǁNa cell fails rapidly under the same test conditions. More importantly, the Na@3D-NVP symmetric cell still shows superior stability more than 200 h at higher current density of 2 mA cm-2 (Figure S10). The excellent behaviors of Na@3D-NVP electrode should be attributed to the low nucleation barrier and plentiful ion transport channels. EIS analysis is performed to reveal the interfacial stability in symmetric cell and the equivalent circuit model is shown in Figure S5 [42]. Figure 6c-d depict the interfacial resistance of NaǁNa and Na@3D-NVPǁNa@3D-NVP cells at different cycles and the fitted results are listed in Table S2. Before cycling, the NaǁNa cell exhibits a large resistance due to the spontaneously generated thick SEI layer, while the Na@3D-NVPǁNa@3D-NVP cell shows a low resistance. After 90th cycles, NaǁNa cell exhibits drastically increased
resistance than that of the cell after 50th cycles due to the thick SEI and constantly accumulated “dead Na” [43]. In contrast, the low and stable interfacial resistances of Na@3D-NVPǁNa@3D-NVP cell in the 50th and 90th cycles indicate more stable electrode interface and superior interfacial mass/charge transfer dynamics [44]. Furthermore, upon continuous cycling under step-increased current densities from 0.5 to 10 mA cm-2, the Na@3D-NVPǁNa@3D-NVP cell exhibits stable voltage curves with the low overpotential, while the NaǁNa cell shows a gradual increased polarization, suggesting large nucleation and electro-dissolution polarization (Figure 6e). To better evaluate the electrochemical performance of Na@3D-NVP anode, the cycling performance of symmetrical cells using different optimization strategies in carbonate electrolyte are summarized in Table S3. The superior cyclic stabilities in carbonate electrolyte in our work demonstrate the strategy that simultaneously realizes steady nucleation and rapid ion transport via natrium super ionic conductor is effective.
Figure 7. (a) Comparison of the coulombic efficiencies of Cu foil, GF and 3D NVP at 1 and 2 mA cm-2 for the capacity of 1 mAh cm-2. The plating/striping curves of (b) 3D NVP at 1 mA cm-2 in different cycles and (c) Cu foil, GF and 3D NVP at 2 mA cm-2.
Since metal anodes always suffer from low coulombic efficiency (CE), CE is a crucial index to evaluate the galvanostatic cycling of Na deposition and stripping. CEs in this work are calculated from the ratio of the capacity of Na stripped to that of Na deposited in each cycle [22]. The initial CE of the cell with 3D NVP is 77.3% at 1 mA cm-2 with the deposition capacity of 1 mAh cm-2 in Figure 7a, possibly attributed to the sodiation reactions and the formation of SEI layer. After that, the CEs quickly increase and maintain at around 98%. The voltage hysteresis is defined as the difference between the voltage of Na depositing and stripping, which is determined by the reaction dynamics and interfacial properties [45]. As explicated in Figure 7b, the corresponding voltage curves of NVP scaffold at 1 mA cm-2 exhibit a stable polarization voltage (~195.5 mV) in 184 cycles, and it also shows the lowest electrochemical hysteresis at the current density of 2 mA cm-2 (Figure 7c), suggesting fast ion and electron transport kinetics. When increasing the deposition capacity to 2 mAh cm-2, the NVP framework still shows the highest CE (Figure S11), about 97% and 96% at 0.5 and 1 mA cm-2, respectively. In contrast, the GF matrix shows inferior electrochemical service life, and Cu foil electrode delivers the shortest cycling performance with fluctuant CEs. The same phenomenon has been observed at 1 mA cm-2 with a capacity of 5 mAh cm-2 (Figure S12). Compared with large dendrites on Cu foil, the flatter surface with dendrites-free morphology of NVP electrode also benefits the stabilization of SEI layer during cycling and renders prolonged lifespan. In order to explore the effect of the loading amount of Na3V2(PO4)3 on the performance and practical capacity, a proper loading of Na3V2(PO4)3 that balance of performance and practicability is discussed. As shown in Figure S13a, the cycle life of 3D framework become longer as the increase of NVP loading at 1 mA cm-2 with capacity of 0.5 mAh cm-2. When the loading of NVP is further increased, the electrochemical performance improves little. Moreover, the initial coulombic efficiencies for 3D frameworks with different NVP loading amount are further investigated at 1 mA cm-2 with the capacity of 5 mAh cm-2. As revealed in Figure S13b, though the initial coulombic efficiencies of different frameworks display slightly decrease as the increase of NVP loading, the initial coulombic efficiencies for
all frameworks are above 80%. Therefore, considering the cycle stability and initial coulombic efficiency, the proper loading of NVP in this work is chosen as adding 0.1g NVP in 1 milliliter MXene solution (3D NVP framework).
Figure 8. (a) Illustration of full cell. Performance comparison of NVPǁNa@3D-NVP and NVPǁNa cells: (b) cycling stabilities at 10C. (c) rates comparison, (d) charge-discharge curves at 10 and 50C.
To further demonstrate the exciting plating/stripping performance of Na@3D-NVP anode, a full cell with Na3V2(PO4)3 cathode is carried out (Figure 8a). As explicated in Figure 8b, the full cell with Na@3D-NVP anode displays a good stability up to 500 cycles with a capacity of 93.6 mAh g-1 at 10C, higher than that of the cell using Na foil anode (86.6 mAh g-1). Even at 100C, the capacity retention of the cell with Na@3D-NVP anode almost remains 100%, better than that of the cell with bare Na anode (Figure S14). Besides, the full cell with Na@3D-NVP anode displays superior rates performance (Figure 8c) and lower polarization (81.5 mV at 10C, 278.4 mV at 50C) than the cell with Na anode (179.1 mV at 10C, 583.4 mV at 50C) (Figure 8d). The detailed polarization values are defined as the differences between the potentials of 50%
full charge and 50% full discharge plateau [46]. Based on the above results, we propose that the fast ion conductor can effectively guide Na nucleation, restrain the dendrite growth and facilitate ion migration, therefore enhancing the electrochemical performance.
Conclusions In summary, an effective approach based on super ionic conductor material was proposed to guide Na nucleation and simultaneously enhance ion transportation kinetics. During the plating process, the 3D NVP facilitates a steady nucleation at the initial deposition stage and navigates the subsequent Na plating to form dendrite-free morphology. Besides, the electrode equipped with the 3D NVP also delivers a low voltage hysteresis and high coulombic efficiency (98% for 190 cycles at 1 mA cm-2 for 1 mAh cm-2). Benefiting from the dendrite-free morphology with stable SEI layer and abundant ion transport channels, the obtained Na@3D-NVP electrode shows low overpotential and long lifespan (up to 400 h with 119.3 mV voltage hysteresis at 1 mA cm-2 for 2 mAh cm-2). Moreover, with Na3V2(PO4)3 as the cathode, the full cell exhibits superior electrochemical performance.
Supporting information Supporting information is available from the online version.
Acknowledgements This work was supported by the National Natural Science Foundation of China (21676165, 21938005 and 21905206) and Shanghai Sail Program (19YF1450800).
Conflict of interest The authors declare no conflict of interest.
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Research Highlights
>Natrium super ionic conductor is introduced to prepare dendrite-free Na metal anode. >3D NVP framework could thermodynamically guide the nucleation and dynamically accelerate interfacial ion transport >The final obtained electrode delivers prolonged cycling stabilities and small overpotential.
Personal portrait photo and biosketch Min Guo
Huanglin Dou
Wanyu Zhao
Xiaoli Zhao
BingXin Wan
Jiahe Wang
Min Guo received her master’s degree (2016) from South China University of Technology and then is a Ph.D candidate in School of Materials Science and Engineering in Tongji University. Her research interests mainly focuses on the electrode materials for sodium ion batteries. Huanglin Dou received his master’s degree (2017) from Taiyuan University of Technology and then is a Ph.D candidate in the college of Materials Science and Engineering in Taiyuan University of Technology. His research interests mainly focuses on the electrode materials for lithium ion batteries. Wanyu Zhao received her undergraduate degree from Central South University in 2018. After that she studied for a doctor degree under the supervision of Prof. Xiaowei Yang in Tongji University. Her research interests focus on the metal anodes of batteries. Xiaoli Zhao received her Ph. D. degree from Zhejiang University, China, in 2016 under the supervision of Prof. Chao Gao. She joined Prof. Xiaowei Yang’s group in Tongji University, China, as an assistant professor in the same year. Her current research interests are 2D-nanomaterial-based nanofluidics and energy conversion/storage materials. BingXin Wan received his undergraduate degree from Yanshan University in 2017. After that the studied for a master degree under the supervision of Prof.Xiaowei Yang in Tongji University. Her researches mainly focus on metal anode design and magnesium batteries Jiahe Wang received his undergraduate degree from Tongji University in 2017. After that he studied for a master degree under the supervision of Prof. Xiaowei Yang in Tongji University. His research interests focus on metal anode protection and magnesium batteries.
Yuantao Yan
Xiaomin Wang
Zi-Feng Ma
Xiaowei Yang
Yuantao Yan received his MS at Shanghai University in 2016 after completing his BS at Shanghai University of Engineering Science in 2014. He is currently a Ph.D. student under the supervision of Prof. Xiaowei Yang at Tongji University. His research focuses on Si-based anode materials for next-generation high-energy-density Li-ion batteries. Xiaomin Wang received her Ph. D. (2005) in Materials Processing from Taiyuan University of Technology, China. She is currently a professor in the College of Materials Science and Engineering, Taiyuan University of Technology. Her research interest is new carbon materials and their application for supercapacitor, fuel cells, lithium/sodium ion battery and lithium sulfur batteries. Zi-Feng Ma is currently Distinguished Professor of Shanghai Jiao Tong University and the founding director of Shanghai Electrochemical Energy Devices (SEED) Research Center. He received his BSc and MSc Degrees in Chemical Engineering from Zhejiang University, and received his Ph.D. in Chemical Engineering from South China University of Technology in 1995. He was appointed to be the Chief Scientist of the 973 program of China in 2007 and 2013, respectively. He is also Chairman of China Associate of Energy Storage Engineering. His research interests are rechargeable secondary battery, fuel cell and electrochemical energy system engineering. Xiaowei Yang received his Ph.D. degree from Shanghai Jiao Tong University in 2011 with Prof. Zi-Feng Ma. He carried out researches on tunable layered graphene gel in Prof. Dan Li’s group at Monash University (2009-2014) and is currently a professor at Tongji University and chair professor at Chang’an University. His current research interests are centered on the synthesis and properties of 2D soft materials and their applications in energy
storage and conversion, nanofluidics and biomedicines.
Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S2211-2855(20)30036-7
DOI:
https://doi.org/10.1016/j.nanoen.2020.104479
Reference:
NANOEN 104479
To appear in:
Nano Energy
Received Date: 3 December 2019 Revised Date:
9 January 2020
Accepted Date: 9 January 2020
Please cite this article as: M. Guo, H. Dou, W. Zhao, X. Zhao, B. Wan, J. Wang, Y. Yan, X. Wang, Z.-F. Ma, X. Yang, Three Dimensional Frameworks of Super Ionic Conductor for Thermodynamically and Dynamically Favorable Sodium Metal Anode, Nano Energy, https://doi.org/10.1016/ j.nanoen.2020.104479. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
Graphical abstract
Stable plating/stripping behaviors of sodium with dendrite-free morphology and improved cycling stabilities is realized by sodium super ionic conductor.
Three
Dimensional
Frameworks
of
Super
Ionic
Conductor
for
Thermodynamically and Dynamically Favorable Sodium Metal Anode
Min Guoa, Huanglin Doua, b, Wanyu Zhaoa, Xiaoli Zhaoa*, Bingxin Wana, Jiahe Wanga, Yuantao Yana, Xiaomin Wangb, Zi-Feng Mac, Xiaowei Yanga*
a
School of Materials Science and Engineering, Interdisciplinary Materials Research
Center, Tongji University, Shanghai 201804, China b
College of Materials Science and Engineering, Taiyuan University of Technology,
Taiyuan 030024, China c
Shanghai Electrochemical Energy Devices Research Center, School of Chemistry and
Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
Corresponding Author *Email: [email protected]; [email protected]
Abstract Metallic sodium anodes are highly attractive due to the high specific capacity and natural abundance, but the uncontrollable Na dendrite issues impede its practical implementation. Nucleation, growth and dissolution of Na are an inseparable sequential process, so the variations in these reaction thermodynamics and kinetics could lead to dendrite proliferation and low utilization of Na anode. It appears to be particularly important to optimize the whole continuous process by introducing a super ionic conductor material, Na3V2(PO4)3, as the modulation medium of continuous nucleation, growth and dissolution to suppress Na dendrite in this work. The initial intercalation reaction of Na+ to NVP thermodynamically improves the affinity of Na and NVP, leading to a low-barrier nucleation and homogenous Na-ion flux. Importantly, the super ionic conductor dynamically provides ultrafast migration channels to promote the interfacial Na-ion transport, contributing to a reduced electrochemical polarization and uniform Na-ion distribution. As a result, the cell
displays superior rate performance and improved cycling stabilities in both half and full cells. This work proposes a new strategy for a thermodynamically and dynamically favorable nucleation and reversible plating/stripping of Na, which can be extend to other metal anodes.
Keywords: sodium metal anode, sodium super ionic conductor, steady nucleation, dendrite-free, reaction kinetics
Introduction Significant research interest has been devoted to sodium metal anode due to the high capacity (1160 mAh g-1), low electrode potential (-2.71 V vs. the standard hydrogen electrode) and natural abundance of sodium [1, 2]. Nevertheless, the safe and stable operation of Na metal batteries has been hindered by the uncontrollable Na dendrite problems. To tackle the problem, various strategies have been put forward, including electrolyte additives [3, 4], artificial SEI [5, 6], solid-state electrolyte [7, 8] and three-dimensional (3D) current conductors [9, 10]. However, few strategies focus on the inseparable sequential process of nucleation, growth and dissolution of Na in term of both thermodynamics and kinetics. According to classical nucleation mechanism, the initial nucleating stage plays a crucial role to the subsequent deposition behavior. The nucleation of a new phase needs to surmount a free energy barrier thermodynamically, the larger the energy barrier, the more difficult to deposition [11-14]. Commonly, the nucleation barrier can be modulated through regulating the affinity between Li/Na and substrate. If the substrate and Li/Na metal have poor affinity, this weak interaction may lead to remarkable nucleation barriers (nucleation overpotentials), thus the large electric field driving force will be inevitable to promote Li/Na metal to grow from the root and form whisker shape [15]. Inversely, the strong affinity between the substrate and Li/Na metal is conductive to a low barrier nucleation, in favor of a surface growth pattern with dendrite-suppressed morphology [16, 17]. Targeting the nucleation problem, numerous efforts have been made to reduce Li/Na nucleation barriers by the introduction of nucleation seeds (such
as Au, Ag and Sn2+) [13, 14, 18]. However, it is worth noting that alloying nucleation reactions generate huge volume change in these seed particles, unfavorable to steady nucleation. Concerning the nucleation and following deposition/dissolution are an inseparable sequential process, stable plating/stripping is equally important, which are dynamically affected by interfacial ion migration. The bottleneck in the ion mobility to respond to the electrochemical reactions in time may cause mass electron aggregation, resulting in large electrochemical polarization on the electrode surface. The resultant polarization will promote selective electrodeposition of Na on local sites where dendrites and inhomogeneous deposition of sodium metal can proliferate. To address this issue, the tactics with enhanced interfacial mass/charge transfer by introducing ionic conducting paths can help to redistribute the ion flux at the metal-electrolyte interphase and promote the interfacial Na+ transport kinetics [19-21]. Therefore, it is urgent and meaningful to explore new strategies to solve the nucleation and interfacial issue in terms of thermodynamics and dynamics. Herein, we design a stable host by involving the three-dimensional frameworks of super ionic conductor and find that it could effectively guide the nucleation and simultaneously accelerate interfacial ion transport. In this work, the natrium super ionic conductor Na3V2(PO4)3 (NVP) microflowers composed of nanoflakes could afford plentiful nucleation sites and ion transport pathways. During the Na plating process, Na ions are prioritized intercalating into Na3V2(PO4)3 crystal to form sodiated Na3+xV2(PO4)3 phase. The initial sodiation reaction improves the affinity of Na and NVP, conducive to a low barrier nucleation. After intercalation of Na, the increased electronegativity of NVP can further enhance the ability of NVP to absorb Na+ in the electrolyte, which helps to uniform the Na+ flux and realize the homogeneous deposition of Na. Furthermore, compared with the conventional alloying nucleation reaction with huge volume change, the intercalation reactions of NVP accompanied with ignorable volume fluctuation are responsible for constructing more stable Na ions nucleation condition. More importantly, as a typical super ionic conductor, the cross-linked distributed fast ion network can homogenize Na-ion flux and provide
additional ion carrier paths for Na to deposit and dissolve. These collaborative strategies lead to more stabilized Na plating/stripping with a low overpotential and greatly prolonged lifespan. Compared to bare Cu foil, the Na@3D-NVP electrode presents higher coulombic efficiencies and lower polarizations. The lifetime of Na plating/stripping for symmetric Na@3D-NVP cell maintains up to 400 h with a stabilized overpotential of 119.3 mV at 1 mA cm-2 for 2 mAh cm-2. The results demonstrate that the super ionic conductor in regulating the nucleation barrier and promoting the reversible plating/stripping of sodium is very effective.
Figure 1. Schematic illustration of Na plating/stripping behaviors on (a) Na foil and (b) 3D NVP matrix.
Results and discussion Figure 1 illustrates the schematic diagrams of Na plating and stripping processes on bare Na foil and 3D NVP matrix. Though Na foil could be used as the anode, its rough surface and “hostless” nature trigger the inhomogeneous nucleation and dendrite generation (Figure 1a). As depicted in Figure 1b, 3D NVP scaffolds integrating the merits of super ionic conductor and polar 3D framework are proposed to restrain dendritic growth. The initiatory intercalation reaction of NVP shows negligible volume change, and the stable structure helps to establish steady Na ions nucleation
condition and navigate the subsequent growth to form dendrite-free plating morphology. Importantly, benefiting from the super ionic conductive property, the abundant fast ionic conducting channels in the deposited Na@3D-NVP electrode are favorable to facilitate rapid ion migration and low surface transfer resistance. Besides, the polar functional groups (Si-O, O-H) in glass fiber (GF) matrix are able to adsorb considerable Na+ to alleviate the electrostatic interactions between Na+ and protuberances, to avoid the accumulation of Na ions around protuberances [22, 23]. And the dispersed MXene nanosheets can enhance the electronic conductivity of the entire scaffold [24, 25]. All these characteristics are conductive to a reversible plating and stripping of Na metal anode.
Figure 2. (a) XRD patterns. (b) High resolution spectra of V 2p; (c-d) SEM images; (e) elemental mapping of 3D NVP scaffold.
The structure and phase characteristic of the 3D NVP scaffold are detected by X-ray diffraction (XRD). As shown in Figure 2a, the diffraction peaks marked with triangles and clubs are indexed to GF framework and NASICON structured Na3V2(PO4)3 (JCPDS card No. 053-0018) [26], respectively, which is further confirmed by X-ray photoelectron spectrum (XPS) measurement (Figure 2b and S1). As shown in Figure 2c-d and S2, 3D interconnected structure with pores is revealed for the pristine GF framework, and a mixed ion and electron conducting scaffold is well formed after infusing with the mixture of Na3V2(PO4)3 and MXene. It can be observed that the super ionic conductor Na3V2(PO4)3 shows microflowered morphology with a size distribution from a few to tens of microns. The corresponding elemental mapping with homogenous distribution of V, P and Na elements in Figure 2e further confirms the existence of NVP.
Figure 3. The deposition behaviors of Cu foil, GF and 3D NVP. Plating profile and nucleation overpotentials at the current density of (a-b) 0.1 mA cm-2 and (c-d) 2 mA cm-2. (e) Tip voltages at the current densities from 0.1 to 2 mA cm-2. (f) Nyquist plots.
To investigate the nucleation behaviors of sodium in NVP scaffold, we measure the plating curves on different substrates. Here, Cu foil and pristine GF matrix are chosen as the control groups. Figure 3a-d show the plating profiles of Cu, GF and 3D NVP at 0.1 and 2 mA cm-2. As shown, the 3D NVP display an obvious sodiation reaction process before Na plating. The potential plateau at ~1.5 and ~0.3 V is ascribed to the
intercalation of Na+ into Na3V2(PO4)3 crystal, and the slope region is attributed to the formation of the solid electrolyte interphase (SEI) at such low anode potential below the stability limit of the electrolyte [27, 28], which can be observed on the galvanostatic discharge curves and cyclic voltammetry (CV) profile when using Na3V2(PO4)3 as the anode material (Figure S3 and 4). The preformed SEI layer on the surface of 3D NVP framework may be helpful to produce more stable SEI in the following deposition process and finally induce excellent structural stability of Na anode [13, 29-31]. The initial pre-sodiation reaction facilitates a low barrier nucleation because Na plating tends to proceed preferentially in the pre-sodiated region, rather than surmounting extra-large energy to create new plating sites [32]. After intercalation, an electron deviation from Na to NVP will occur [30, 33], which can increase the electronegativity of NVP and enhance the ability of NVP to absorb Na+ in the electrolyte, to uniform the Na+ flux and realize the uniform deposition of Na. Na ions preferentially pass through the fast ion conductor channels with a low overpotential rather than break the highly resistive solid electrolyte interface layers. This property could rapidly compensate for sodium ion depletion near the electrode surface during plating/stripping process. All these advantages may lead to a stable nucleation and can be verified by the plating curves. The voltage dips at the beginning of plating curves are determined by nucleation energy barriers and the following plateaus are controlled by mass transfer [31]. From Figure 3a-d, the 3D NVP shows the lowest voltage dips among all the matrixes, suggesting a low nucleation barrier. And Figure 3e compares the voltage dips (tip voltages) at different current densities from 0.1 to 2 mA cm-2. Unsurprisingly, the 3D NVP shows the lowest tip voltages in these frameworks, further confirming the lower sodium nucleation barriers. Concretely, the nucleation overpotential is defined as the difference between the tip voltage and plateau voltage [31]. As depicted in Figure 3b and d, the overpotential of bare Cu foil is 183.9 mV at 0.1 mA cm-2, and rapidly increases to 513.1 mV at 2 mA cm-2, illustrating the energy used to overcome the heterogeneous nucleation barrier is very large, while the nucleation overpotentials of 3D NVP are small, which delivers the overpotentials of 16.8 and 78.5 mV at 0.1 and 2 mA cm-2, respectively, indicating lower nucleation barrier and a more
stable mass transfer process. Electrochemical impedance spectroscopy (EIS) measurement is carried out to measure the interfacial and charge transfer resistances after deposition. The corresponding equivalent circuit diagram is shown in Figure S5 [31, 34] and the fitted impedance parameters are listed in Table S1. The fitted results reveal that the interfacial resistance as well as the charge transfer resistance of 3D NVP framework are considerably reduced compared with that of the bare Cu foil.
Figure 4. (a-d) Morphology evolution of 3D NVP at different deposition sites. (e) Top-view and (f) bottom-view images of 3D NVP after deposition 10 mAh cm-2. SEM images of Cu foil after plating (g) 2, (h) 7, and (i) 10 mAh cm-2. (j) The corresponding sites of 3D NVP with different deposition capacities.
To study the deposition behaviors, the morphology evolution of 3D NVP is further studied at the current density of 0.1 mA cm-2 for different areal capacities. Initially, the
NVP shows microflowers shape (Figure 4a and S 6a). Then at the beginning of electrochemical plating, it still maintains microflower structure after reacting with Na3V2(PO4)3 to form sodiated phase Na3+xV2(PO4)3 (Figure 4b and S 6b). The X-ray diffraction results shows the sodiated NVP scaffold keeps good NASICON structure (Figure S7), indicating the excellent structure stability [28]. As further increasing the deposition capacity, Na metal continues to deposit along the surface of NVP, and then NVP microflowers are gradually covered and wrapped by the plated sodium (Figure 4c-d and S 6c-d). Further enhancing the plating capacity to 10 mAh cm-2, the pores in the NVP scaffold are filled with Na meal and the isolated Na-wrapping NVP particles are joint together. As shown in Figure 4e-f and and S 6e-f, the NVP matrix exhibits a flat surface and bottom morphology without dendritic sodium formation. In contrast, the branch-like structured Na metal is observed at the initial plating for bare Cu foil (Figure 4g and S 6g), and large amounts of columnar Na dendrites and long filaments shaped “dead Na” are observed on the surface of Cu foil when increasing the deposition capacities (Figure 4h-i and S 6h-i). The large dendrites and dead Na not only increase interface resistance, but also may trigger internal short circuit. As shown in Figure S8, the bare GF framework with small deposition capacity exhibits no obvious change in the top-view, but some large protuberances in the bottom. And the protuberances become larger with the increasing of deposition capacity (Figure S8 e-f). These results suggest the 3D NVP can effectively guide the growth of sodium metal.
Figure 5. (a) Schematic diagram of symmetric cell. (b) Tafel plot in exchange current density test. (c) Voltage profile of GITT in NaǁNa and Na@3D-NVPǁNa@3D-NVP symmetric cells. (d) Enlargement of the green box in GITT profile.
To further explore the reaction kinetics and interfacial properties of the obtained anode during the plating/stripping cycling, galvanostatic intermittent titration technique (GITT) method and exchange current density experiments are conducted. Herein, the corresponding voltage-time curves are monitored via NaǁNa and Na@3D-NVPǁNa@3D-NVP symmetric cells (Figure 5a). As shown from the Tafel plot in Figure 5b, in high overpotential region, the current density is dominated by mass transfer from electrolyte to the electrode surface. The rapid Na+ transport channels in Na@3D-NVP electrode will facilitate Na ions to transfer from electrolyte to the Na surface. As a result, the Na@3D-NVP electrode has higher exchange current density than the bare Na electrode, demonstrating a faster mass transfer process. In the low overpotential part, the current density is dominated by charge transfer between electrode and the redox species. The higher current density of Na@3D-NVP
electrode than the bare Na electrode demonstrates the faster Na deposition/dissolution kinetics [19]. As shown in Figure 5c, the GITT curves of Na@3D-NVPǁNa@3D-NVP cell has a significantly lower overpotential compared with the NaǁNa cell, indicating the Na@3D-NVP electrode possesses faster mass transfer kinetics. Notably, there is a large spike at the beginning of each step for NaǁNa cell, followed by a plateau and an abrupt increased bump polarization. The spike at the beginning corresponds to the initial Na nucleation overpotential [35, 36] and the subsequent abruptly increased polarization originates from the electro-dissolution of bulk Na at the stripping steps [37, 38]. The apparent nucleation and bulk dissolution overpotential of NaǁNa cell indicate the large nucleation barrier and sluggish interfacial ion transport. Conversely, the Na@3D-NVPǁNa@3D-NVP cell shows almost no nucleation and bulk dissolution overpotentials (Figure 5d). The stabilized plating and stripping plateaus indicate a lower energy barrier for both nucleation and plating/stripping process.
Figure 6. Voltage-time curves at 1 mA cm-2 for (a) 1 mAh cm-2 and (b) 2 mAh cm-2. EIS analysis of (c) NaǁNa and (d) Na@3D-NVPǁNa@3D-NVP symmetric cells at different cycles at 1 mA cm-2 for 1 mAh cm-2. (e) Rate performances from 0.5 to 10 mA
cm-2 with a fixed capacity of 1 mAh cm-2 of NaǁNa and Na@3D-NVPǁNa@3D-NVP symmetric cells.
Generally, many works evaluate the cycling stability of cells using Li or Na metal as the counter electrode [39-41]. Here, we also measure the long cycling performance based on Na metal anode. NaǁNa@Cu and NaǁNa@3D-NVP cells are tested at the current densities from 0.5 to 2 mA cm-2 with a fixed areal capacity of 2 mAh cm-2. As displayed in Figure S9a-b, the Na@Cu electrode only maintains dozens of hours and suddenly become short circuit, while the voltage hysteresis of Na@3D-NVP electrode maintain at ~62.2 and ~137.6 mV over 770 and 400 hours at 0.5 and 1 mA cm-2. Even at 2 mA cm-2, the Na@3D-NVP electrode still displays a stable polarization (199.3 mV) over 205 h (Figure S9c). Furthermore, the overpotential and long-term cycling stabilities of the symmetric cells are analyzed in Figure 6 and S10. Noticeably, the Na@3D-NVPǁNa@3D-NVP cell offers a long lifespan up to 300 h with stable and smooth voltage profiles (voltage hysteresis 124.2 mV) at 1 mA cm-2 (Figure 6a). The corresponding voltage profiles exhibit regular and flat plateau without spike and bump, indicating a steady nucleation and fast reaction kinetics. And longer galvanostatic time for 2 h is further measured at 1 mA cm-2, the lifespan of Na plating/stripping in Na@3D-NVP symmetric cell still maintains 400 h and the overpotential almost stabilizes at 119.3 mV (Figure 6b), while the bare NaǁNa cell fails rapidly under the same test conditions. More importantly, the Na@3D-NVP symmetric cell still shows superior stability more than 200 h at higher current density of 2 mA cm-2 (Figure S10). The excellent behaviors of Na@3D-NVP electrode should be attributed to the low nucleation barrier and plentiful ion transport channels. EIS analysis is performed to reveal the interfacial stability in symmetric cell and the equivalent circuit model is shown in Figure S5 [42]. Figure 6c-d depict the interfacial resistance of NaǁNa and Na@3D-NVPǁNa@3D-NVP cells at different cycles and the fitted results are listed in Table S2. Before cycling, the NaǁNa cell exhibits a large resistance due to the spontaneously generated thick SEI layer, while the Na@3D-NVPǁNa@3D-NVP cell shows a low resistance. After 90th cycles, NaǁNa cell exhibits drastically increased
resistance than that of the cell after 50th cycles due to the thick SEI and constantly accumulated “dead Na” [43]. In contrast, the low and stable interfacial resistances of Na@3D-NVPǁNa@3D-NVP cell in the 50th and 90th cycles indicate more stable electrode interface and superior interfacial mass/charge transfer dynamics [44]. Furthermore, upon continuous cycling under step-increased current densities from 0.5 to 10 mA cm-2, the Na@3D-NVPǁNa@3D-NVP cell exhibits stable voltage curves with the low overpotential, while the NaǁNa cell shows a gradual increased polarization, suggesting large nucleation and electro-dissolution polarization (Figure 6e). To better evaluate the electrochemical performance of Na@3D-NVP anode, the cycling performance of symmetrical cells using different optimization strategies in carbonate electrolyte are summarized in Table S3. The superior cyclic stabilities in carbonate electrolyte in our work demonstrate the strategy that simultaneously realizes steady nucleation and rapid ion transport via natrium super ionic conductor is effective.
Figure 7. (a) Comparison of the coulombic efficiencies of Cu foil, GF and 3D NVP at 1 and 2 mA cm-2 for the capacity of 1 mAh cm-2. The plating/striping curves of (b) 3D NVP at 1 mA cm-2 in different cycles and (c) Cu foil, GF and 3D NVP at 2 mA cm-2.
Since metal anodes always suffer from low coulombic efficiency (CE), CE is a crucial index to evaluate the galvanostatic cycling of Na deposition and stripping. CEs in this work are calculated from the ratio of the capacity of Na stripped to that of Na deposited in each cycle [22]. The initial CE of the cell with 3D NVP is 77.3% at 1 mA cm-2 with the deposition capacity of 1 mAh cm-2 in Figure 7a, possibly attributed to the sodiation reactions and the formation of SEI layer. After that, the CEs quickly increase and maintain at around 98%. The voltage hysteresis is defined as the difference between the voltage of Na depositing and stripping, which is determined by the reaction dynamics and interfacial properties [45]. As explicated in Figure 7b, the corresponding voltage curves of NVP scaffold at 1 mA cm-2 exhibit a stable polarization voltage (~195.5 mV) in 184 cycles, and it also shows the lowest electrochemical hysteresis at the current density of 2 mA cm-2 (Figure 7c), suggesting fast ion and electron transport kinetics. When increasing the deposition capacity to 2 mAh cm-2, the NVP framework still shows the highest CE (Figure S11), about 97% and 96% at 0.5 and 1 mA cm-2, respectively. In contrast, the GF matrix shows inferior electrochemical service life, and Cu foil electrode delivers the shortest cycling performance with fluctuant CEs. The same phenomenon has been observed at 1 mA cm-2 with a capacity of 5 mAh cm-2 (Figure S12). Compared with large dendrites on Cu foil, the flatter surface with dendrites-free morphology of NVP electrode also benefits the stabilization of SEI layer during cycling and renders prolonged lifespan. In order to explore the effect of the loading amount of Na3V2(PO4)3 on the performance and practical capacity, a proper loading of Na3V2(PO4)3 that balance of performance and practicability is discussed. As shown in Figure S13a, the cycle life of 3D framework become longer as the increase of NVP loading at 1 mA cm-2 with capacity of 0.5 mAh cm-2. When the loading of NVP is further increased, the electrochemical performance improves little. Moreover, the initial coulombic efficiencies for 3D frameworks with different NVP loading amount are further investigated at 1 mA cm-2 with the capacity of 5 mAh cm-2. As revealed in Figure S13b, though the initial coulombic efficiencies of different frameworks display slightly decrease as the increase of NVP loading, the initial coulombic efficiencies for
all frameworks are above 80%. Therefore, considering the cycle stability and initial coulombic efficiency, the proper loading of NVP in this work is chosen as adding 0.1g NVP in 1 milliliter MXene solution (3D NVP framework).
Figure 8. (a) Illustration of full cell. Performance comparison of NVPǁNa@3D-NVP and NVPǁNa cells: (b) cycling stabilities at 10C. (c) rates comparison, (d) charge-discharge curves at 10 and 50C.
To further demonstrate the exciting plating/stripping performance of Na@3D-NVP anode, a full cell with Na3V2(PO4)3 cathode is carried out (Figure 8a). As explicated in Figure 8b, the full cell with Na@3D-NVP anode displays a good stability up to 500 cycles with a capacity of 93.6 mAh g-1 at 10C, higher than that of the cell using Na foil anode (86.6 mAh g-1). Even at 100C, the capacity retention of the cell with Na@3D-NVP anode almost remains 100%, better than that of the cell with bare Na anode (Figure S14). Besides, the full cell with Na@3D-NVP anode displays superior rates performance (Figure 8c) and lower polarization (81.5 mV at 10C, 278.4 mV at 50C) than the cell with Na anode (179.1 mV at 10C, 583.4 mV at 50C) (Figure 8d). The detailed polarization values are defined as the differences between the potentials of 50%
full charge and 50% full discharge plateau [46]. Based on the above results, we propose that the fast ion conductor can effectively guide Na nucleation, restrain the dendrite growth and facilitate ion migration, therefore enhancing the electrochemical performance.
Conclusions In summary, an effective approach based on super ionic conductor material was proposed to guide Na nucleation and simultaneously enhance ion transportation kinetics. During the plating process, the 3D NVP facilitates a steady nucleation at the initial deposition stage and navigates the subsequent Na plating to form dendrite-free morphology. Besides, the electrode equipped with the 3D NVP also delivers a low voltage hysteresis and high coulombic efficiency (98% for 190 cycles at 1 mA cm-2 for 1 mAh cm-2). Benefiting from the dendrite-free morphology with stable SEI layer and abundant ion transport channels, the obtained Na@3D-NVP electrode shows low overpotential and long lifespan (up to 400 h with 119.3 mV voltage hysteresis at 1 mA cm-2 for 2 mAh cm-2). Moreover, with Na3V2(PO4)3 as the cathode, the full cell exhibits superior electrochemical performance.
Supporting information Supporting information is available from the online version.
Acknowledgements This work was supported by the National Natural Science Foundation of China (21676165, 21938005 and 21905206) and Shanghai Sail Program (19YF1450800).
Conflict of interest The authors declare no conflict of interest.
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Research Highlights
>Natrium super ionic conductor is introduced to prepare dendrite-free Na metal anode. >3D NVP framework could thermodynamically guide the nucleation and dynamically accelerate interfacial ion transport >The final obtained electrode delivers prolonged cycling stabilities and small overpotential.
Personal portrait photo and biosketch Min Guo
Huanglin Dou
Wanyu Zhao
Xiaoli Zhao
BingXin Wan
Jiahe Wang
Min Guo received her master’s degree (2016) from South China University of Technology and then is a Ph.D candidate in School of Materials Science and Engineering in Tongji University. Her research interests mainly focuses on the electrode materials for sodium ion batteries. Huanglin Dou received his master’s degree (2017) from Taiyuan University of Technology and then is a Ph.D candidate in the college of Materials Science and Engineering in Taiyuan University of Technology. His research interests mainly focuses on the electrode materials for lithium ion batteries. Wanyu Zhao received her undergraduate degree from Central South University in 2018. After that she studied for a doctor degree under the supervision of Prof. Xiaowei Yang in Tongji University. Her research interests focus on the metal anodes of batteries. Xiaoli Zhao received her Ph. D. degree from Zhejiang University, China, in 2016 under the supervision of Prof. Chao Gao. She joined Prof. Xiaowei Yang’s group in Tongji University, China, as an assistant professor in the same year. Her current research interests are 2D-nanomaterial-based nanofluidics and energy conversion/storage materials. BingXin Wan received his undergraduate degree from Yanshan University in 2017. After that the studied for a master degree under the supervision of Prof.Xiaowei Yang in Tongji University. Her researches mainly focus on metal anode design and magnesium batteries Jiahe Wang received his undergraduate degree from Tongji University in 2017. After that he studied for a master degree under the supervision of Prof. Xiaowei Yang in Tongji University. His research interests focus on metal anode protection and magnesium batteries.
Yuantao Yan
Xiaomin Wang
Zi-Feng Ma
Xiaowei Yang
Yuantao Yan received his MS at Shanghai University in 2016 after completing his BS at Shanghai University of Engineering Science in 2014. He is currently a Ph.D. student under the supervision of Prof. Xiaowei Yang at Tongji University. His research focuses on Si-based anode materials for next-generation high-energy-density Li-ion batteries. Xiaomin Wang received her Ph. D. (2005) in Materials Processing from Taiyuan University of Technology, China. She is currently a professor in the College of Materials Science and Engineering, Taiyuan University of Technology. Her research interest is new carbon materials and their application for supercapacitor, fuel cells, lithium/sodium ion battery and lithium sulfur batteries. Zi-Feng Ma is currently Distinguished Professor of Shanghai Jiao Tong University and the founding director of Shanghai Electrochemical Energy Devices (SEED) Research Center. He received his BSc and MSc Degrees in Chemical Engineering from Zhejiang University, and received his Ph.D. in Chemical Engineering from South China University of Technology in 1995. He was appointed to be the Chief Scientist of the 973 program of China in 2007 and 2013, respectively. He is also Chairman of China Associate of Energy Storage Engineering. His research interests are rechargeable secondary battery, fuel cell and electrochemical energy system engineering. Xiaowei Yang received his Ph.D. degree from Shanghai Jiao Tong University in 2011 with Prof. Zi-Feng Ma. He carried out researches on tunable layered graphene gel in Prof. Dan Li’s group at Monash University (2009-2014) and is currently a professor at Tongji University and chair professor at Chang’an University. His current research interests are centered on the synthesis and properties of 2D soft materials and their applications in energy
storage and conversion, nanofluidics and biomedicines.
Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: