Investigation Of Synthesis And Electrochemical Properties Of Solid Electrolyte Li₁₀GeP₂S₁₂

With the wide application of lithium-ion batteries in the field of portable electronic devices and electric vehicles,people have put forward higher requirements for the safety performance and energy density of lithium-ion batteries.The utilization of liquid electrolytes in conventional lithium-ion batteries not only limits it’s energy density but also has potential risks.It is a trend to replace liquid electrolytes with solid electrolytes.However,all solid state batteries tend to have low capacity and poor cycling performance due to large impedance.Therefore,it is significant to figure out the actual electrochemical process and related kinetic parameters inside the battery when working.In this paper,We used Li₁₀GeP₂S₁₂ as solid electrolyte to fabricate all solid state batteries with LiCoO₂,LiNi_1/3Co_1/3Mn_1/3O₂ and LiNi_0.5Co_0.2Mn_0.3O₂ as positive materials respectively.And we tested the eletrochemical properties of the all solid state batteries through galvanostatic charge and discharge,cyclic voltammetry and eletrochemical impedance spectroscopy.Results and conclusions are as follows.（1）Li₁₀GeP₂S₁₂ was successfully synthesized,and its ionic conductivity at room temperature and the activation energy is 2.02×10^-3 S/cm and 29.8 kJ/mol respectively.A buffer layer of LiNbO3 was uniformly coated on the surface of LiCoO₂,LiNi_1/3Co_1/3Mn_1/3O₂ and LiNi_0.5Co_0.2Mn_0.3O₂.（2）The first-cycle discharge capacity of the all-solid-state battery with LiCoO₂,LiNi_1/3Co_1/3Mn_1/3O₂ and LiNi_0.5Co_0.2Mn_0.3O₂ as positive materails were 134.2 mAh/g,121.2 mAh/g and and 137.2 mAh/g respectively.The capacity retention rate of LiCoO₂ and LiNi_0.5Co_0.2Mn_0.3O₂ were respectively 69.4%and 74.9%after 50 cycles.However,the capacity retention rate of LiNi_1/3Co_1/3Mn_1/3O₂ was too much higher as93.7%.It can be seen that the battery with LiNi_0.5Co_0.2Mn_0.3O₂ as positive material has the highest discharge capacity in the first cycle while the battery with LiNi_1/3Co_1/3Mn_1/3O₂ as positive material has the best cycling performance.According to the cyclic voltammetry results,the polarization potential of the batteries with LiCoO₂,LiNi_1/3Co_1/3Mn_1/3O₂ and LiNi_0.5Co_0.2Mn_0.3O₂ as positive materails were 5,3and 4.5 times compared with conventional batteries repectively which should be the reason for the difference in cycling performance between them.（3）Through the results of the electrochemical impedance spectroscopy of the all solid state batteries,it is found that the typical nyquist plots of the batteries are composed of three parts,namely,the semi-circular in the high-frequency region,the semi-circle in the middle frequency region,and the line in the low-frequency region.Among them,the semi-circle in the high frequency region belongs to the impedance of electrolyte,the semi-circle in the middle frequency region belongs to the process of charge transfer,and the line in the low frequency region belongs to the process of solid state diffusion.The resistance of electrolyte of all the batteries start to increase when the potential is greater than 3.8 V,which means that Li₁₀GeP₂S₁₂ begins to decompose.The resistance of charge transfer of all the batteries is in accordance with the relevant formula.In addition,the total resistance of LiCoO₂,LiNi_1/3Co_1/3Mn_1/3O₂and LiNi_0.5Co_0.2Mn_0.3O₂ batteries were 452.0Ω,233.5Ωand 549.5Ωat 4.0 V,respectively,which should be the reason why LiNi_1/3Co_1/3Mn_1/3O₂ battery has the best cycling performance and smallest polarization potential.（4）After 100 cycles,the total resistance of LiNi_1/3Co_1/3Mn_1/3O₂ battery was979.1Ω.After 50 cycles,the total resistance of LiNi_0.5Co_0.2Mn_0.3O₂ battery was1036.6Ω,and the total resistance of LiCoO₂ battery reached 1954.4Ωwhich should be the root cause for the cycling performance of these batteries.In addition,the electrolyte resistance of the all solid state batteries changes similarly during the cycle which means the decomposition degree of the electrolyte is similar.However,the difference in charge transfer resistance is very large indicating that different active materials should be modified with different means to improve the interface compatibility between electrode and electrolyte.In this paper,the electrochemical performance of sulfide solid state lithium ion batteries was studied using three kind of cathode materials.It was found that improving the stability of sulfide solid electrolytes at high voltage is the most critical factor for improving the performance of all solid state batteries.Moreover,it is necessary to find a more suitable method to improve the electrode/electrolyte interface for different cathode materials.The research results in this paper is fundamental for the further development of all solid state lithium ion batteries.