Voltage decay for lithium-excess material of Li[Li1/5Co2/5Mn2/5]O2 during cycling analyzed via backstitch method

Lithium-excess (LEX) materials of Li2MnO3∙LiMO2 (M = Co, Ni, Fe, and so on) with large reversible capacities are promising positive electrode materials for use in lithium-ion batteries. However, the application of LEX materials as a positive electrode is hindered by the voltage decay phenomenon, i.e., significant changes in voltage profile during cycling. To date, there is no efficient approach to alleviate this constraint due to insufficient knowledge about the nature of voltage decay. In this study, the Li[Li1/5Co2/5Mn2/5]O2 (Co-LEX) material was subject of the backstitch method, which facilitated simultaneous measurement of overvoltage and reversible electrode potential. The contribution of overvoltage and voltage hysteresis to the voltage decay was evaluated, and it was attributed to the deviation of reversible electrode potentials during charge and discharge. Backstitch charge–discharge curve analyses revealed that the reversible electrode potentials of both the charge and discharge reactions decreased with increasing number of cycles. Over 100 cycles, the overvoltage increased, whereas the voltage hysteresis remained nearly constant. Hence, lowering the reversible electrode potentials and increasing the overvoltage led to the voltage decay for the Co-LEX material. Overall, the energy loss due to accumulation of voltage hysteresis during 100 cycles (~ 1,800 kJ mol−1) was too large to explain the voltage decay induced by the structural transformation from a layered structure to a spinel structure. In this paper, we evaluated the contribution of overvoltage and voltage hysteresis resulting from the deviation of reversible electrode potentials between charge and discharge to the voltage decay for the first time.