Air-exposure influences on structural stability and electrochemical performance of ultrahigh Ni cathodes
Ultrahigh-Ni oxides have been widely considered as promising cathode materials for high-energy 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 LiNi0.95Co0.025Mn0.025 O2(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(dQ/dV)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.
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