Revealing cycling rate-dependent capacity decay in LiNi0.6Co0.2Mn0.2O2 at 4.6 V

Layered oxides LiNi_xCo_yMn_1-x-yO₂ (NCM, or NCMxy(1-x-y)) are regarded as promising cathode candidates for high-energy lithium-ion batteries (LIBs) owing to their combined strengths in capacity, operating potential and manufacturing cost. However, NCM materials suffer from several electrochemical cycling problems, such as severe capacity fade and voltage decay, especially at high C rates and high voltages. Herein, using LiNi_0.6Co_0.2Mn_0.2O₂ as a representative, we demonstrate that the asynchronous reaction among active particles is the core reason for the accelerated capacity fade of NCM622 at high cycling rates. In detail, the inhomogeneity between particles is aggravated with increasing current density and accumulates with cycling, resulting in some pseudo “inactive” particles and reversible rapid capacity decay. At low cycling rates, the large lattice stresses on the active particles (4.6 V, vs. Li⁺/Li) and the formation of disordered rock salt structures result in the irreversible capacity fade of NCM622. This work provides a new understanding of the correlation between the cycling rate-non-synchronous reaction-capacity degradation for LiNi_0.6Co_0.2Mn_0.2O₂, and new insights may be employed to guide the design of high-rate and long-life batteries.