Correlation between transition metal ion migration and the voltage ranges of electrochemical process for lithium-rich manganese-based material

Abstract

Voltage decay of Li-rich cathode material is caused by migration of transition metal (TM) ions and layered phase transformation to spinel phase. Here electrochemical studies demonstrate that charge/discharge voltage ranges have a strong impact on the voltage decay. For cells cycling in the charge/discharge condition of voltage range at 2.0–4.6 V, 3.2–4.6 V and 4.2–4.6 V, voltage decay phenomena occur to the same extent. While for the serial conditions of 2.0–4.2 V, 2.0–4.4 V, 2.0–4.6 V, 2.0–4.8 V and 2.0–4.6V + hold at 4.6 V for 5 h, voltage decay starts to occur at a typical voltage when the potential is high enough, and then phase transformation becomes severer with increased potentials, or prolonged time holding at high voltages. TM ions at high voltages have high enough energy, which can be denoted as “activation energy”, to stride across the transition state and migrate to Li vacancy, which results in spinel formation and voltage decay.