Synthesis Of LiNi_0.5Co_0.2Mn_0.3O₂ As Cathode Material For Lithium Ion Batteries

To meet the rapid development of electric vehicles,the pursuit of higher energy density lithium-ion battery is becoming increasingly strong,and ternary layered Li Ni_xCo_yMn_1-x-yO₂?NCM,0<x,y<1?cathode materials especially the Ni-rich?x?0.5?composition have been receiving intensive attention in recent years due to their higher capacity which can increase the energy density of batteries.With the increase of Ni content,the capacity will significantly improve,but the requirements of the synthesis conditions are getting higher,and Li Ni_0.5Co_0.2Mn_0.3O₂ has appropriate amount of the proportion of Ni,while ensuring the capacity and safety,the requirements of synthesis conditions are more relaxed.Li Ni_0.5Co_0.2Mn_0.3O₂is also the most widely used ternary material in the battery market,so it was selected for further research in this paper.Synthesis of Li Ni_0.5Co_0.2Mn_0.3O₂by coprecipitation is the most commonly used method,and the synthesized material has good dispersion,but the process is complex and time-consuming.While the flux synthesis method can provide a liquid-like synthesis environment which can accelerate ion diffusion and simplify the process,so we want to use a simpler flux synthesis to replace the complicated coprecipitation method to synthesize Li Ni_0.5Co_0.2Mn_0.3O₂ cathode material.In this paper,a flux synthesis method for generating Na Cl flux medium online was designed and Li Ni_0.5Co_0.2Mn_0.3O₂ cathode material was successfully prepared.We studied the effects of sintering temperature,annealing temperature and the excess lithium ratio on the properties of materials.The results showed that the synthesized materials exhibited good electrochemical performance after sintering at 750 ^oC for 5 h at 10%Li excess ratio,and annealing at 750 ^oC for 2 h after washing to remove soluble impurities,the highest discharge capacity at 0.2 C is 146 m Ah?g^-1.In addition,KOH was used to replace Na OH,and KCl was generated online as flux medium to synthesize Li Ni_0.5Co_0.2Mn_0.3O₂,meanwhile,Na OH and KOH were also used to generate Na Cl-KCl,a composite flux medium,to synthesize Li Ni_0.5Co_0.2Mn_0.3O₂.However,the electrochemical performances of these samples are not ideal.Through the analysis of the test results,we believe that the defects of the traditional flux synthesis itself cannot provide enough oxygen to ensure the partial oxidation of Ni²⁺to Ni³⁺in the synthesis process,resulting in serious Li⁺/Ni²⁺cation mixing,which ultimately affects the electrochemical performance of Li Ni_0.5Co_0.2Mn_0.3O₂.Based on the understanding the defects of the traditional flux synthesis,we proposed the in-situ self-oxygenation flux synthesis,and generated Na Cl-Na NO₃online in the preparation process of the precursor as the flux medium.The Na NO₃ in the precursor can release oxygen via thermal decomposition and thus an inner oxygen-rich environment within molten salt can be in-situ created and provide a sufficient oxygen environment that ensures sufficient oxidation of partial Ni²⁺to Ni³⁺during flux synthesis of Li Ni_0.5Co_0.2Mn_0.3O₂.Our comparative experiments demonstrate that this in-situ self-oxygenation can then reduce the Li⁺/Ni²⁺mixing due to the increased Ni³⁺/Ni²⁺ratio in the obtained Li Ni_0.5Co_0.2Mn_0.3O₂ and ultimately improved electrochemical performance.Compared with the traditional flux synthesis method,the sample synthesized by in-situ self-oxygenation flux synthesis have excellent rate capability with a reversible capacity of 164.5 m Ah g^-1at 0.1 C and128.3 m Ah g^-1 at 5 C,and also has good cycling stability at 1 C with a capacity retention ratio of 77.9%after 300 cycles between 3.0 and 4.3 V.