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Nanoscaled Na3PS4 Solid Electrolyte for All-SolidState FeS2/Na Batteries with Ultra-High Initial Coulombic Efficiency of 95% and Excellent Cyclic Performances Hongli Wan, Jean Pierre Mwizerwa, Xingguo Qi, Xiaoxiong Xu, Hong Li, Qiang Zhang, Liangting Cai, Yong-Sheng Hu, and Xiayin Yao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01805 • Publication Date (Web): 02 Apr 2018 Downloaded from http://pubs.acs.org on April 2, 2018

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ACS Applied Materials & Interfaces

Nanoscaled Na3PS4 Solid Electrolyte for AllSolid-State FeS2/Na Batteries with Ultra-High Initial Coulombic efficiency of 95% and Excellent Cyclic Performances Hongli Wan†,ǁ, Jean Pierre Mwizerwa†,ǁ, Xingguo Qi‡,ǁ, Xiaoxiong Xu†, Hong Li‡, Qiang Zhang†,ǁ, Liangting Cai†, Yong-Sheng Hu*,‡, Xiayin Yao*,† †

Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences,

315201, Ningbo, P. R. China ‡

Key Laboratory for Renewable Energy, Beijing Key Laboratory for New Energy Materials and

Devices, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, 100190, Beijing, P.R. China ǁ

University of Chinese Academy of Science, 100049, Beijing, P.R. China

Corresponding Authors *E-mail: [email protected] (X.Y. Yao) and [email protected] (Y.-S. Hu)

ACS Paragon Plus Environment

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ABSTRACT. Nano-sized Na3PS4 solid electrolyte with an ionic conductivity of 8.44×10-5 S cm-1 at room temperature is synthesized by a liquid-phase reaction. The resultant all-solid-state FeS2/Na3PS4/Na batteries show an extraordinary high initial Coulombic efficiency of 95% and demonstrate high energy density of 611 Wh kg-1 at current density of 20 mA g-1 at room temperature. The outstanding performances of the battery can be ascribed to good interface compatibility and intimate solid-solid contact at FeS2 electrode/nanosized Na3PS4 solid electrolytes interface. Meanwhile, excellent cycling stability is achieved for the battery after cycling at 60 mA g-1 for 100 cycles, showing a high capacity of 287 mAh g-1 with the capacity retention of 80%.

Keywords: all-solid-state sodium battery, liquid-phase method, nanoscaled Na3PS4 electrolyte, Coulombic efficiency, cycling stability, interfacial contact


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All-solid-state sodium batteries employing inorganic electrolytes capture much attention, because of their high safety and the use of abundant sodium resource.1-4 With the pursuit of high energy density, continuous efforts are concentrated on synthesis of solid-state sodium electrolytes with high ionic conductivities. Especially, sulfide electrolytes possess excellent processability and low elastic modulus, leading to remarkable reduced grain-boundary resistance only through cold press.5 Up to now, the ionic conductivities of sulfide electrolytes3, 6-7 have reached 10-4 or 10-3 S cm-1. However, all-solid-state sodium batteries based on these electrolytes still show inferior electrochemical performances. The challenges lie in the insufficient solid-solid contact and poor interface compatibility between active materials and solid electrolytes.8 Employing liquid-phase synthesized electrolyte8 and electrolyte coated active material3 are efficient methods to enhance the physical contact at active materials/solid electrolytes interface. However, the particle size of the electrolyte synthesized with a liquid-phase reaction is still in the range of submicron level and needs to be further reduced. Jung et al. found that NaCrO2 electrodes coated with Na3SbS4 solid electrolyte show better electrochemical performances than that of mixed electrode with high ionic conductivity Na3SbS4 and NaCrO23 in all-solid-state NaCrO2/Na-Sn batteries. Nevertheless, solid electrolyte recrystallized from solvents shows much lower ionic conductivity than that of the solid-state synthesized one. In this work, ~ 200 nm sized Na3PS4 electrolyte is synthesized via a liquid-phase method and further employed as an electrolyte for room temperature all-solid-state sodium batteries. We select FeS2 as the cathode material and the resultant all-solid-state FeS2/Na3PS4/Na batteries possess outstanding electrochemical performances in terms of the high capacity, excellent rate capability and cycling stability.


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Figure 1. (a) XRD patterns of Na3PS4 samples. (b) Raman spectra of Na3PS4 samples and the precursor. Na3PS4 solid electrolyte was synthesized by a liquid phase reaction of Na2S and P2S5 with a fixed mole ratio of 3:1 in acetonitrile followed by an annealing procedure. As shown in Figure 1a, Na3PS4 precursor can totally transfer to pure cubic phase Na3PS4 at the optimized annealing temperature of 270 oC,9 which is identical to that of solid-state synthesized Na3PS4 electrolytes (Figure S1). Too high annealing temperatures, i.e. 290oC or 310oC, would form a tetragonal phase of Na3PS4, while a low annealing temperature of 250oC may lead to the residual of cocrystallized acetonitrile molecules. Figure 1b shows the Raman spectra of the obtained Na3PS4 electrolyte. For the Na3PS4 precursor, the Raman shift at 420cm-1 and 1436cm-1 are assigned to glass Na3PS49 and CH3 symmetric deformation10, suggesting that a trace of acetonitrile


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molecules are still present in the precursor. After annealing Na3PS4 at 270 oC, only one Raman peaks at 410cm-1, which attributes to the PS43- unit,9 is observed, implying that Na3PS4 precursor is crystallized to glass-ceramic phase and the co-crystallized acetonitrile molecules are removed, further confirming the purity of the Na3PS4 electrolyte. Figure 2a presents the scanning electron microscopy (SEM) of Na3PS4 electrolyte and the results show that the particle size is about 200nm, which is much smaller than that of electrolytes prepared by a mechanochemical technique (Figure S2)6, 9. The elemental mapping (EDX) results confirm that the Na3PS4 electrolyte contains Na, P, and S; besides, all of the component elements are distributed homogeneously throughout the electrolytes. The room temperature ionic conductivity and activation energy determined from the alternating current impedance method is 8.44×10-5 S cm-1 and 30.63kJ mol-1 for Na3PS4 electrolyte pellet prepared by the cold-press process (Figure 2b).

Figure 2. (a) SEM and EDX images of the Na3PS4 sample. (b) Temperature dependences of Na3PS4 electrolyte (the inset is impedance spectra of the Na3PS4 samples at different temperatures).


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Figure 3. (a) CV of Na/Na3PS4/C cell. (b) Cyclability of Na3PS4 in a symmetric Na/Na3PS4/Na cell. (c) CV of all-solid-state FeS2/Na3PS4/Na battery. (d) Galvanostatic discharge/charge profiles for FeS2 electrode at 20mA g-1. Cyclic voltammogram measurement (CV) was conducted to test the electrochemical stability of Na3PS4 against the metallic Na anode using Na/Na3PS4/C cell. As shown in Figure 3a, some irreversible reactions at 1.135V are occurred in the first cycle, which disappears in the following two cycles; while the peaks at around 0 V versus Na/Na+, associated with the reversible sodium deposition and dissolution, still remain. No significant oxidation peak is detected up to 5V, suggesting that the irreversible reaction in the first cycle is beneficial for the formation of a stable Na3PS4/ Na interface11, which was further evaluated with Na/Na3PS4/Na symmetric cells (Figure 3b). In the first few cycles, high voltage spike is observed, while during the subsequent


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cycles, a constant polarization exists, implying a stabilization of the interface. Herein, microsized FeS2 (Figure S3, S4) was employed as the cathode to demonstrate the superior electrochemical properties using the as-synthesized Na3PS4 electrolytes (Figure S5). The electrochemical reaction mechanism of FeS2 was investigated by CV. As shown in the first cathodic cycle (Figure 3c), a distinct peak at around 1.0V is observed, which attributes to the insertion of Na+. However, this peak disappears and shifts to about 1.7V in the second cycle, implying the phase change of FeS2 after the first discharge cycle, which can be confirmed by in situ XRD pattern of FeS2 in sodium ion battery based on liquid organic electrolyte (Figure S6). The results reveal that with the incessant insertion of Na+, the intensity of diffraction peaks related to FeS2 decrease, and disappear completed when discharge to 0.8V. This phenomenon is in consistent with the generally accepted reaction mechanism for FeS2-based sodium ion battery12 13, which suggests that the following Na+-insertion reaction occurs in FeS2 electrode at cut-off voltage of 0.8V-3.0V: xNa+ + xe- + FeS2 → NaxFeS2 (0﹤x﹤2). Thus, huge volume changes and large stress/strain for the active material caused by conversion reactions: Na2FeS2 + 2Na+ + 2e- → 2Na2S+Fe (0.8V-0V), which will lead to a structure collapse and capacity fade, can be avoided. In the first oxidation scan, anodic peaks at 2.3V and 2.6V are observed, which may be caused by the muti-step extraction of Na+ from the host material (NaxFeS2). In the following cycles, the CV curves are slightly different, which due to gradual phase transformation.13 Figure 3d shows the galvanostatic discharge/charge profiles for the FeS2 electrode in allsolid-state sodium batteries. An initial capacity of 437mAh g-1 is delivered, which correlates to a consumption of 1.96 moles (~ 2 moles) of sodium per mole of FeS2, confirming only insertion


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reaction occurs. Besides, it shows a Coulombic efficiency of 95%, which is much higher than that of the FeS2-based electrode in sodium ion batteries using liquid organic electrolytes (60% ~ 80%).13 However, the solid batteries employing solid-state synthesized Na3PS4 exhibit a lower Coulombic efficiency of 77% (Figure 3d), even though the employed electrolyte possesses a much higher ionic conductivity of 1.7×10-4 S cm-1 (Figure S7). The extraordinary Coulombic efficiency could be benefited from the intimate solid-solid interfacial contact and less side reaction between FeS2 and Na3PS4 electrolyte. The capacity loss might be derived from the reaction between Na3PS4 and Na to form a stable Na3PS4/Na interface during the first cycle (Figure S8), which will facilitate the cycling stability of the battery. After the first cycle, the cells show very stable discharge/charge capacities of 409 mAh g-1, exhibiting excellent electrochemical reversibility. Meanwhile, the discharge plateau at 1.2 V in the initial discharge curve shifts to 1.7 V in the second cycle onwards, indicating less charge-discharge polarization due to phase change of FeS2 after first cycle,13 matching well with the results revealed in the CV curves. The rate capability was further evaluated under different current densities (Figure 4a). The all-solid-state FeS2/Na3PS4/Na batteries can deliver discharge specific capacity of 383, 358, 305, 273, 165, and 84 mAh g-1 at current densities of 40, 60, 80, 100, 150, and 200 mA g-1 after the first aging cycle, respectively, which shows strongly contrasts to the cells using solid-state synthesized Na3PS4 electrolyte (Figure S9). The inset (Ragone plot) in Figure 4a shows that, the cells show energy density of 611 Wh kg-1 at 20 mA g-1, and power density of 234 W kg-1 at 200mA g-1. Furthermore, the discharge capacity can be maintained at 287 mAh g-1 with the capacity retention of 80% after cycling at a high current density of 60 mA g-1 for 100 cycles (Figure 4b), demonstrating excellent cycling stability, while the cell using solid-state synthesized


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electrolyte only shows a capacity of 21.9 mAh g-1 for 30 cycles (Figure S10). The gradual capacity fade of all-solid-state battery based on liquid-phase synthesized Na3PS4 electrolyte might be caused by the structural change of the FeS2, where the microspheres will crack into plates after cycling (Figure S11).

Figure 4. (a) Galvanostatic discharge/charge profiles for FeS2/Na cells at different current densities (the inset was Ragone plot ). (b) Cycling performance of FeS2/Na3PS4/Na cells. (c)


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Electrochemical performances of all-solid-state sodium ion/sodium batteries in the literature.

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1-3,

14-15

Clearly, the overall performances of all-solid-state FeS2/Na3PS4/Na batteries employing nano-sized solid electrolytes in our work are excellent among reported results in the literature (Figure 4c),

1-3, 14-15

which could be derived from the following respects. First, the nanosized

Na3PS4 electrolytes can enhance the contact area at electrode materials/electrolytes interface, leading to an intimate solid-solid interfacial contact. Second, FeS2 cathode and Na3PS4 exhibit good interface compatibility, thus maintaining high reversible capacity and rate capability. Third, the stable interfacial layer between Na3PS4 and Na is beneficial for the electrochemical kinetic process (Figure S8). Fourth, highly reversibility of FeS2 during charge-discharge processes contributes to the excellent cycling performance (Figure S12). Finally, the use of metallic sodium instead of sodium-based alloy as an anode could no doubt enhance the energy density of all-solid-state sodium battery. In summary, a cubic phase Na3PS4 solid electrolyte with the smallest particle size of around 200nm is successfully prepared through the reaction of Na2S and P2S5 in acetonitrile. All-solidstate FeS2/Na3PS4/Na batteries exhibit high reversible capacity (409 mA h g-1 at 20 mA g-1), excellent rate capability (383, 358, 305, 273, 165, and 84 mAh g-1 at 40, 60, 80, 100, 150, and 200 mA g-1, respectively) and high-rate cycling stability (stable reversible capacity of 287 mAh g-1 with the capacity retention of 80% under 60 mA g-1 for 100 cycles). The application of sulfide solid electrolyte with reduced particle size based on liquid-phase methods opens up the possibility for the development of high-energy room temperature all-solid-state sodium batteries. ASSOCIATED CONTENT


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Supporting Information. including experimental details; XRD and SEM of FeS2; XRD, SEM and EIS of the solid-state synthesized Na3PS4 electrolytes; Schematic illustration of battery configuration; In situ XRD patterns and the first discharge curve for FeS2 in sodium ion battery using liquid electrolyte; XRD of cathode, electrolyte layer and Na3PS4/Na interface after cycling; Rate capability and cyclic performance of FeS2/Na cells using the solid-state synthesized electrolytes; SEM of the cathode after cycling. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (X.Y. Yao) and [email protected] (Y.-S. Hu) Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The work was supported by funding from the National Key Technologies R&D Program (No. 2016YFB0901500), the Strategic Priority Program of the Chinese Academy of Sciences (Grant No. XDA09010201), Zhejiang Provincial Natural Science Foundation of China (Grant No. LD18E020004, LQ16E020003, LY18E020018, LY18E030011) and Youth Innovation Promotion Association CAS (2017342)

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