The Synthesis,characterization And Modification Of Ni-rich Cathode Materials For Lithium Ion Batteries

Thanks to its high energy density advantage,LiNi_0.8Co_0.1Mn_0.1O₂?NCM811?has attracted attention in the field of LIB.But some serious problems must be well solved before its widely used.Firstly,the increase in nickel content puts stricter requirements on its preparation conditions.Secondly,serious capacity degradation and low thermal stability pose severe challenges to the cycle stability and safety of LIB.In addition,the high air sensitivity leads to the surface degradation of NCM811 materials,which further limits the commercial application.With comprehensive considerations above in mind,in the work,we have optimized the preparation process parameters and surface coating design of NCM811 to improve the lithium storage performance,cycle stability,thermal stability and storage stability in the air.The main research conclusions are presented as follows:?1?We optimize the cation mixing degree and the size of primary particles via controlling the calcination temperature,thereby improving the electrochemical performance of NCM811.Different calcination temperature will affect the crystallinity,cation mixing degree and primary particle size,and it is necessary to selected in combination with electrochemical performance.The NCM811 obtained via appropriate conditions,has moderate crystallinity,lower cation misalignment?3.66%?and primary particle size of 100-300 nm,and exhibits excellent cycle stability.In particular,it can still provide a specific discharge capacity of 136.9 m Ah g^-1 at a large current density of 10.0 C.?2?We propose an effective surface modification strategy to prepare NCM811 coated with solid electrolvte Li₃PO₄,and systematically study the effect of Li₃PO₄ coating on the electrochemical stability of NCM811.As a physical coating layer,Li₃PO₄ not only slows the corrosion of HF to the electrode material in the electrolyte,relieves the evolution of strain induced intergranular micro-cracks,but also maintains the characteristics of high Li⁺conductors with appropriate coating content.In the temperature range of-10-50 ^oC,the optimized cathode materials have produced excellent electrochemical performance in terms of lithium storage,capacity retention and rate performance.More competitively,the pouch-type devices deliver a cell-level energy density of?304.6 Wh kg^-1 after being combined with the C-Si composite anode material,and the reversible charge-discharge capacity remains 89.6%at a current density of 1.0 C for 250 cycles.?3?The Li₃PO₄ coating can reduce the sensitivity of NCM811 to air.We store the NCM811cathode before and after modification in a environment with 55 ^oC and relative humidity of80%.The test results show that Li₂CO₃ impurities are generated on the surface of the cathode material,and the content of impurities will increase with the increase of storage time.Electrochemical evaluation shows that the formation of Li₂CO₃ introduces energy barriers,causing some cathode materials to lose their electrochemical activity.However,these energy barriers are not fixed and can be reactivated through charge-discharge cycles.In addition,the result shows that the presence of Li₃PO₄ coating effectively blocks the contact of the electrode material with H₂O and CO₂ in the air,slows the growth rate of Li₂CO₃,and further enhances the workability of the material.