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Improvement of Electrochemical Performances of All-Solid-State Argyrodite-Based Lithium Batteries
Several sulfide glasses, glass-ceramics or ceramics have been developed as promising electrolytes for all-solid-state Li-ion battery. Ionic conductivity values ranging, at 298 K, from 10-4 to 10-2 S/cm (i.e. close and even higher than organic electrolytes) have been reported with electrochemical stability windows up to 10 V (V vs. Li0). These inorganic solid electrolytes have been demonstrated as suitable to fabricate all-solid-state batteries with different electrode materials (LiCoO2, Li4Ti5O12). The so-called Li-Argyrodite Li6PS5X (X= Cl, Br, I) family presents ionic conductivity values up to 7·10-3 S/cm at 298K [1] and the Br-containing Li-Argyrodite has been used to fabricate all-solid-state batteries with carbon coated Li4Ti5O12 (c-LTO) and Li-Al alloy as positive and negative electrode materials, respectively [2].
We have recently developed a new and straightforward route for the synthesis of the Li6PS5Cl phase, electrochemically more stable than Br-phase, by mechanical milling [3].
Experiments
The synthesis of the Li6PS5Cl solid electrolyte has already been described [3].
All-solid-state half-cells were assembled as follows. A mixture of LiCoO2 (LCO) (or Li4Ti5O12 (LTO)) and Li6PS5Cl (SE) was first mixed by mechanical-milling; the obtained mixture and VGCF was then ground in an agate mortar and used as the composite electrode. Li6PS5Cl was used for the separation between the working and counter electrodes. 10 mg of composite electrode and 80 mg of electrolyte powder were set in a polycarbonate tube and then pressed under 375 MPa. Then a lithium foil was placed on the electrolyte as the counter and reference electrode. Finally two stainless steel rods were used on both sides of the pellet as current collectors; a constant pressure was maintained during all the process on the three-layered pellet. Full-cells of 10 mg LCO/80 mg Li6PS5Cl/10 mg LTO were also assembled in one step.
The galvanostatic electrochemical investigation was performed at room temperature, using a Multichannel Potentiostat device (VMP 2, Bio-logic, France) with a current density of 5 mA/g of composite electrode (64 µA/cm² in this configuration), between 2.5 and 4.1 V for LiCoO2 and between 1 and 2.2 V for Li4Ti5O12 electrodes (V vs. Li+/Li0). The C/x rates mentioned in this study correspond to the extraction/insertion of 0.5 Li+ (for LiCoO2) or 3 Li+ (for Li4Ti5O12) in x hours.
Results and Discussion
All-solid-state LCO (or LTO) half-cells have been realized and their compositions optimized. A maximum of 125 mAh/g has been reached for a composite containing 60 wt% of LCO, compared to 75 mAh/g obtained by a classical milling [4]. The preliminary mechanical milling of the active material and the electrolyte tends to increase the homogeneity of the composite and so the delivering capacity. A huge enhancement is obtained for LTO-based composite, from 10 mAh/g (classical milling) to 125 mAh/g, for 60 wt% of active material.
Based on the two half-cells results, we fabricated a all-solid-state full-cell. The battery delivers a stable capacity of 100 mAh/g during the first 10 cycles with a very small polarization of 70 mV at a C/15 rate.
We improved drastically the all-solid-state battery performances by cycling at high temperatures. A full-cell of 20 mg LCO/80 mg Li6PS5Cl/20 mg LTO delivered a capacity of 100 mAh/g of LiCoO2, at 150°C at a rate of 10C, over 70 cycles and with a coulombic efficiency as high as 99,8 %.
Conclusions
LCO and LTO half-cells using Li6PS5Cl as solid electrolyte exhibit a capacity of 125 mAh/g for both materials. This electrolyte was also suitable to assemble all-solid state full-cells capable of operating at room and high temperature.
References
[1] H.J. Deiseroth, et al., Angewandte Chemie, 2008. 47(4): p. 755-758.
[2] F. Stadler, et al., ECS Trans., 2010. 25(36): p.177-183.
[3] S. Boulineau, et al., Solid State Ionics, 2012. 221: p. 1-5.
[4] S. Boulineau, et al., Solid State Ionics, 2013. 242 : p.45–48.