Research On Structural Modification And Electrochemical Performance Of LiCoO₂ Cathode Materials For Solid-State Lithium-Ion Battery

With the demand for energy upgrading,traditional lithium-ion batteries face severe challenges such as insufficient energy density and potential safety hazards.Solid-state lithium-ion batteries are expected to fundamentally solve the above problems.However,the commercial application of solid-state lithium-ion batteries still faces two major difficulties:Firstly,the problems caused by solid-state electrolytes:firstly,the ionic conductivity of solid-state electrolytes is low,resulting in low lithium-ion transfer rate of solid-state batteries;secondly,solid/solid interface is formed between the solid-state electrolytes and electrodes,resulting in high interface impedance.These two factors seriously affect the rate and cycling performance of solid-state batteries.Secondly,the development of suitable high-voltage cathode materials:compared with liquid electrolytes,solid-state electrolytes have a wider electrochemical window and are suitable for high-voltage cathode materials to improve the energy density of solid-state batteries.As one of the important factors to affect the performance of solid-state batteries,solid-state electrolytes have received extensive attention and research so far.In contrast,the research and development of high-voltage cathode materials suitable for solid-state batteries has not received much attention.Compared with other commercialized cathode materials,LiCoO₂ materials have higher working voltage and volumetric energy density,as well as higher levels of electronic conductance and ion diffusion,so it is the most promising cathode materials for solid-state lithium-ion batteries.Its application in solid-state lithium-ion batteries mainly faces the following challenges:the solid/solid interface contact between the cathode material and the solid electrolyte leads to low Li⁺transport efficiency;the application of cathode materials in high voltage systems exists structural collapse issues which lead to performance degradation.Based on these,this paper studies LiCoO₂ cathode materials for solid-state lithium-ion batteries.First,the appropriate particle size and doping method were screened out as the basic research object and modification method of LiCoO₂ materials for solid-state lithium-ion batteries.The experimental results show that among the LiCoO₂ materials with particle size（D50）of 4,6,8 and 10μm,the 6μm LiCoO₂ material with relatively small particle size has better rate capability and lithium ion transport rate.The research on Mg,Al doping and Mg Al co-doping modification of 6μm LiCoO₂ materials found that the co-doping of Mg and Al has a synergistic effect.On the one hand,the electronic conductivity of Mg Al co-doping LiCoO₂ is improved,which contribute to the rate performance of the material.On the other hand,Mg Al co-doping LiCoO₂can stabilize material’s structure and improve its cycling stability at high voltage.The electronic conductivity is increased from 10^-5 S/cm to 10^-2 S/cm,the discharge specific capacity at 5C rate is increased from 53.9 m Ah/g to 94.7 m Ah/g,the capacity retention after 40 cycles at 4.6V voltage increased from 42.3%to 67.8%.Further,the surface coating modification of the Mg Al-doped 6μm LiCoO2 material was carried out by using Ti,Nb,and Ti Nb elements,it was found that Ti and Nb element coating can improve the rate performance and cycle performance of the material under high voltage,and the improvement effect of Ti coating on the rate performance is better than that of Nb coating,and the improvement effect of Nb coating on the cycle performance is better than that of Ti coating.The discharge specific capacities of Ti,Nb and Ti Nb coatings at 5C high rate current are 122.1 m Ah/g,109.3 m Ah/g and 117.1 m Ah/g.The capacity retention after40 cycles at 4.6V high voltage of Ti,Nb and Ti Nb coatings are 89.8%,93.1%,and 92.2%.Ti or Nb coating is beneficial to reduce the surface resistance of the material and improve the lithium ion transport capacity,which leads to the improvement of cycle performance and rate performance under high voltage.Then solid-state batteries are assembled and tested,the Ti,Nb,Ti Nb coated materials all showed better discharge specific capacity and cycle performance than the uncoated materials,among which the Nb coating material performed better,with a specific discharge capacity of 160 m Ah/g at 0.1C and a capacity retention of 71.3%after 10 cycles.In this paper,the modification of 6μm LiCoO₂ material by Mg Al doping combined with Nb coating can significantly ameliorate the structural collapse and lithium ion transport difficulties that LiCoO₂ materials may face when using in solid-state lithium-ion batteries.The study has important significance for the application of LiCoO₂ materials under high voltage systems in solid-state lithium-ion batteries in the future.