Rechargeable lithium ion batteries (LIBs), one of the most important energy storage technologies, have become an indispensable part in our daily life. However, safety issue is one of the most urgent concerns associated with further advances in next-generation high-energy batteries. Safety issues such as poor thermal stability, flammable reaction products, leakage of electrolyte and internal short circuits for the use of organic electrolytes in lithium ion rechargeable batteries remain unresolved. As an alternative, the use of a solid electrolyte (SE, based on a polymer or a ceramic) instead of organic electrolytes has been suggested as a preferred solution to these safety issues. All-solid-state lithium batteries employing SEs can possess high energy density, no-leakage of electrolytes, flame resistant and flexible geometry. Generally, as a SE for all-solid-state lithium batteries, it needs to maintain electrical isolation between electrodes of opposite polarity while allowing free ionic transport. Therefore, the physical and chemical properties of the electrolyte directly determine the performance of all-solid-state lithium batteries.
Recently, Prof. Xiaoxiong Xu and his group in Ningbo Institute of Materials Technology and Engineering (NIMTE, CAS) have successfully developed a series of novel SE materials. A novel sulfide electrolyte 70Li2S?29P2S5?1Li3PO4 (mol%) glass-ceramic was prepared by a high-energy ball milling technique and subsequent heat-treatment process. The Li+ conductivity is enhanced by the substitution of Li3PO4 for P2S5, and the 70Li2S?29P2S5?1Li3PO4 glass-ceramics exhibit the highest total conductivity of 1.87×10-3 S cm-1 at 25 oC and the lowest activation energy of 18 kJ mol-1. The all-solid-state cell In-Li/70Li2S?29P2S5?1Li3PO4/LiCoO2 exhibits a discharge capacity of 108 mAh g-1, which is 20% higher compared to the In-Li/70Li2S?30P2S5/LiCoO2 cell (Figure 1a). The higher capacity of cell assembled by 70Li2S?29P2S5?1Li3PO4 is attributed to the high Li+ conductivity and low interface resistance of electrode-electrolyte.
For a high-energy battery using Li metal as a negative electrode, the electrolyte is one of the most critical factors that significantly affect the cell performances. Unfortunately, rechargeable batteries based on Li negative electrodes have not been commercialized ascribed to the unstable reactions between Li metal and electrolytes. 75Li2S?25P2S5 (mol%) (Li3PS4) crystal has been reported and demonstrated as one of the most promising electrolyte materials for high-energy battery with metallic Li negative electrode, whereas its lithium ionic conductivity is still too low to meet the demand for all-solid-state cells with high capacity. New 75Li2S?(25-x)P2S5?xP2O5 (mol%) solid state electrolytes were synthesized. The electrolyte substituted with 1 mol% P2O5 presents the highest conductivity of 8 × 10-4 S cm-1 at room temperature, which increases up to 56% compared to that of the pristine sample and the discharge capacity after 30 cycles of the cell using the 75Li2S?24P2S5?1P2O5 is 85.2% of the initial capacity, whereas that of the cell using the 75Li2S?25P2S5 is 76.2% as shown in Figures 1b and 1c. Besides, the as-prepared 75Li2S?24P2S5?1P2O5 electrolyte exhibits good electrochemical stability and compatibility against metallic lithium electrode (Figure 1d).
The above results have been published on Journal of Power Sources (2015) and Journal of the Electrochemical Society, （2016）. Also, two Chinese patents have been filed (201310535524.6; 201510585679.X).
Figure 1 Properties of novel binary sulfide electrolytes: (a) Charge-discharge curves of the all-solid-state cell In-Li/70Li2S·(30-x)P2S5·xLi3PO4/LiCoO2, x = 0 and 1 (b) Cycling life of the all-solid-state cell Li/75Li2S · (25-x)P2S5 · xP2O5/LiCoO2 (x = 0% and 1%) at 25◦C. (c) Temperature dependence of Li+-ion conductivities for the 75Li2S·(25-x)P2S5·xP2O5 glass-ceramics.(d) Cyclability of the 75Li2S·24P2S5·1P2O5 glass-ceramics.
For Li2S-GeS2-P2S5 sulfides electrolytes, Li3.25Ge0.25P0.75S4 and Li10GeP2S12 were successfully prepared. The ionic conductivities of Li10GeP2S12 and Li3.25Ge0.25P0.75S4 at room temperature are 8.27×10-3 and 2.03×10-3 S cm-1, respectively (Figure 2Ⅰ). The effect of solid electrolytes, i.e. Li10GeP2S12 and Li3.25Ge0.25P0.75S4, on the rate and low temperature performances of LiNi0.8Co0.15Al0.05O2 (NCA) cathode in all-solid-state LIBs is investigated. The results in Figure 2Ⅱshows that all-solid-state lithium batteries based on Li10GeP2S12 exhibit superior rate performances (72.3 mAh g-1 at 1C and room temperature) and low temperature performances (79.2 mAh g-1 at 0.1C and -10 oC), which can be ascribed to its higher ionic conductivity and lower interfacial resistance between Li10GeP2S12 and NCA cathode. It is indicated that all-solid-state lithium batteries consisting of NCA cathode and Li10GeP2S12 solid electrolyte can realize its potential application in low temperature environment, as shown in Figure 2Ⅲ.
These results have been published on Solid State Ionics,（2015）. A Chinese patent has been filed (CN201510009660.0).
Figure 2 Properties of ternary sulfides: (Ⅰ) Temperature dependence of Li+-ion conductivities for the Li10GeP2S12 and Li3.25Ge0.25P0.75S4.(Ⅱ) Initial charge and discharge voltage profiles for NCA/Li10GeP2S12/Li-In (a), and NCA/Li3.25Ge0.25P0.75S4/Li-In (b) at different C-rate between 2.0 V and 3.7 V at room temperature. (Ⅲ) The capacity retentions of cell NCA/Li10GeP2S12/Li-In (a), and NCA/Li3.25Ge0.25P0.75S4/Li-In (b) at 0.1 C.
Dry electrolyte membranes based on PEO have been widely studied due to its ability to solvate a wide variety of salts through interaction of its ether oxygens with cations, which can be used in all-solid-state lithium battery designs in a free standing form without modifying current battery fabrication processes. However, PEO-based solid polymer electrolytes (SPEs) have not been widely used in commercial LIBs ascribed to their low ionic conductivities (10-6 - 10-8 S cm-1) at room temperature and poor electrochemical stability. Here, Li10GeP2S12 (LGPS) was incorporated into PEO matrix to fabricate a novel SPE composite. The lithium ion conductivities of as-prepared composite membranes were evaluated, and the optimal composite membrane exhibited a maximum ionic conductivity of 1.21 × 10-3 S cm-1 at 80 oC (Figure 3a) and an electrochemical window of 0-5.7 V. The LGPS microparticles, acting as active fillers incorporation into the PEO matrix, have a positive effect on the ionic conductivity, lithium ion transference number and electrochemical stability. The LiFePO4/Li battery using such SPE exhibits fascinating electrochemical performances in term of high capacity retention (92.5% after 50 cycles at 60 oC), as shown Figure 3b, demonstrating it as a promising electrolyte applied in solid state batteries based on lithium metal electrode.The results have been published on Journal of Power Sources,（2016）.
Figure 3. Properties of composite SPE: (a) Arrhenius plots for the ionic conductivities of the membranes with various LGPS contents. (b) The capacity retentions of cell LiFePO4/PEO18-LiTFSI-1%LGPS/Li and cell LiFePO4/PEO18-LiTFSI/Li at 0.5 C.
Prof. Xiaoxiong Xu: xuxx@nimte.ac.cn Dr. Shaojie Chen: chenshaojie@nimte.ac.cn
Research Group Url:
http://english.nimte.cas.cn/pe/fas/201509/t20150911_152262.html
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