Mn versus Al in Layered Oxide Cathodes in Lithium-Ion Batteries: A Comprehensive Evaluation on Long-Term Cyclability

摘要

Nickel-rich layered oxide cathodes with the composition LiNi1-x-yCoxMnyO2 (NCM, (1-x-y) = 0.6) are under intense scrutiny recently to contend with commercial LiNi0.8Co0.15Al0.05O2 (NCA) for high-energy-density batteries for electric vehicles. However, a comprehensive assessment of their electrochemical durability is currently lacking. Herein, two in-house cathodes, LiNi0.8Co0.15Al0.05O2 and LiNi0.7Co0.15Mn0.15O2, are investigated in a high-voltage graphite full cell over 1500 charge-discharge cycles (approximate to 5-10 year service life in vehicles). Despite a lower nickel content, NCM shows more performance deterioration than NCA. Critical underlying degradation processes, including chemical, structural, and mechanical aspects, are analyzed via an arsenal of characterization techniques. Overall, Mn substitution appears far less effective than Al in suppressing active mass dissolution and irreversible phase transitions of the layered oxide cathodes. The active mass dissolution (and crossover) accelerates capacity decline with sustained parasitic reactions on the graphite anode, while the phase transitions are primarily responsible for cell resistance increase and voltage fade. With Al doping, on the other hand, secondary particle pulverization is the more limiting factor for long-term cyclability compared to Mn. These results establish a fundamental guideline for designing high-performing Ni-rich NCM cathodes as a compelling alternative to NCA and other compositions for electric vehicle applications.