Preparation And Electrochemical Performance Of LiNi_0.8Co_0.15Al_0.05O₂ Nickel-rich Cathode Material

The nickel-rich LiNi_0.8Co_0.15Al_0.05O₂?NCA?material has high energy density and can meet the requirements of long-lasting power batteries.NCA materials have attracted attention from domestic and foreign battery industries as it has been successfully applied to Tesla pure electric vehicles.However,in the application of NCA materials,the electrochemical performance of the materials is degraded due to the phase transition of the material surface from the layered structure to the rock salt phase structure and the serious interface reaction between the material surface and the electrolyte.Therefore,in this paper,the NCA material is modified by in situ film formation of electrolyte and the construction of shell concentration gradient structure.The main research contents and results are as follows:?1?The NCA material is prepared by sol-gel method,the calcination time of NCA material is optimized and the morphology of the material is characterized.The results show that the cation mixing degree of NCA material decreases first and then increases with the extension of the calcination time,and the material has a smaller cation mixing degree when the calcination time is 18 h.The NCA material morphology is particles of tens to hundreds of nanometers.?2?The electrochemical performance,morphology,interfacial composition,and metal ion dissolution content of NCA electrode materials in different electrolyte systems are studied,and the mechanism of synergy between lithium difluorooxalate borate?LiDFOB?salt and sulfolane?SL?solvent is analyzed.The research results show that,LiDFOB-based electrolyte has good film-forming properties compared with LiPF₆-based electrolyte.LiDFOB decomposes on the surface of the NCA electrode material to form a smooth,dense protective film during the battery charge and discharge process.The protective film prevents the direct contact between the electrolyte and the material surface,which inhibits the interface reaction between the electrode material surface and the electrolyte,the dissolution of transition metal elements,and the phase transition of the material surface from the layered structure to the rock salt phase structure.However,the LiDFOB salt continuously decomposes in a carbonate solvent,which makes the film thicker,resulting in a decrease in cycle performance.Therefore,the synergistic decomposition of SL solvent and LiDFOB salt produces more stable LiF and S-containing compounds with good conductivity.They suppress the increase in film thickness and improve the interfacial transmission performance of the material,thereby further improving the cycling performance and rate performance of the material.?3?NCA precursor with constant element distribution?CC-NCAOH?and NCA precursor with shell concentration gradient structure?CSG-NCAOH?are synthesized by adjusting the preparation process with coprecipitation method.The results show that the NCAOH precursors and NCA materials with constant element distribution and shell concentration gradient structure are successfully synthesized.?4?By performing electrochemical performance tests on CC-NCA and CSG-NCA materials and correlating them with the characterization of the surface composition of the material before and after cycling,the mechanism of the shell concentration gradient structure protecting the material interface is analyzed.The results show that CSG-NCA materials have better cycle performance at room temperature and high temperature compared with CC-NCA materials.The reason can be attributed to two aspects:on the one hand,adjusting the distribution of Ni on the surface of the NCA material can reduce the LiOH/Li₂CO₃ content on the surface of the material,inhibiting the interface reaction between the surface of the material and the electrolyte,thereby reducing the consumption of active Li⁺and the interface film impedance of the material;on the other hand,the less Ni content on the surface of the material is helpful to reduce the reduction of Ni⁴⁺to Ni²⁺on the surface of the material during charging,inhibiting the phase change of the surface of the material from the layered structure to the rock salt phase structure,thereby reducing the interface transfer impedance.