超高镍正极材料空气稳定性研究及其电化学性能

Air-exposure influences on structural stability and electrochemical performance of ultrahigh Ni cathodes

超高镍氧化物被认为是锂离子动力电池的首选正极材料,但是其在空气中的存储稳定性较差,严重影响了实际应用。本文研究了LiNi_(0.95)Co_(0.025)Mn_(0.025)O_(2)(NCM)材料暴露在空气中的稳定性及其对电化学储锂性能的影响,进一步探索了超高镍正极材料颗粒表面残留碱性物质(残碱)随时间变化的形成机制。材料表征结果表明,超高镍材料在短时间内暴露于空气时,其表面形成孤岛状的残碱,且这些残碱颗粒的尺寸随着暴露时间的延长而增大。电化学交流阻抗谱(EIS)和微分容量曲线(dQ/dV)测试结果表明,残碱的形成显著增加了NCM正极材料的电化学阻抗,加剧了超高镍正极材料储锂循环中的不可逆相变和结构退化,从而影响了放电容量和循环寿命。在2.7~4.3 V(vs.Li^(+)/Li)的工作电压和0.5 C电流密度下,未暴露空气的初始NCM正极材料的首圈放电比容量为208.1 mAh/g,循环200圈后容量保持率为70.7%,而在空气中暴露12 h和14天后的NCM材料的首圈放电比容量分别为202.9 mAh/g和171.8 mAh/g,循环200圈后的容量保持率仅有60.1%和53.1%。

Ultrahigh-Ni oxides have been widely considered as promising cathode materials for highenergy and high-power lithium-ion batteries,but they suffer from poor air storage stability,and thus seriously affect their practical applications.This article investigates the stability of LiNi_(0.95)Co_(0.025)Mn_(0.025)O_2(NCM) exposed to air environment and its influences on electrochemical lithium storage performance,further exploring the formation mechanism of residual alkali substances on the surface of ultrahigh-Ni cathode material particles over time.The material characterization results indicate that the ultrahigh-Ni material forms isolated residual alkali substances on its surface within a short period of exposure to air,and the size of residual alkali particles increases with prolonged exposure time.Electrochemical impedance spectroscopy(EIS) and differential capacity curves(d Q/d V) indicate that the formation of residual alkali significantly increases the electrochemical impedance of NCM cathodes,and exacerbates irreversible phase transitions and structural degradation during the lithium storage cycling,leading to decreased discharge capacity and worse cycling stability.Under the working voltage of 2.7~4.3 V(vs.Li~+/Li)and current density of 0.5 C,the initial NCM cathode material without exposure to air delivers an initial discharge capacity of 208.1 mAh/g and reaches a 70.7%capacity retention after 200 cycles.However,after 12 hours and 14 days of exposure to air,initial discharge capacities of cathodes decrease to 202.9 mAh/g and 171.8 mAh/g,respectively,with capacity retentions of only 60.1%and 53.1%after 200 cycles.