Microwave-enhanced electrochemical cycling performance of the LiNi0.2Mn1.8O4 spinel cathode material at elevated temperature

Abstract

The well-established poor electrochemical cycling performance of the LiMn₂O₄ (LMO) spinel cathode material for lithium-ion batteries at elevated temperature stems from the instability of the Mn³⁺ concentration. In this work, a microwave-assisted solid-state reaction has been used to dope LMO with a very low amount of nickel (i.e., LiNi_0.2Mn_1.8O₄, herein abbreviated as LMNO) for lithium-ion batteries from Mn₃O₄ which is prepared from electrolytic manganese oxide (EMD, γ-MnO₂). To establish the impact of microwave irradiation on the electrochemical cycling performance at an elevated temperature (60 °C), the Mn³⁺ concentration in the pristine and microwave-treated LMNO samples was independently confirmed by XRD, XPS, ⁶LiMAS-NMR and electrochemical studies including electrochemical impedance spectroscopy (EIS). The microwave-treated sample (LMNO_mic) allowed for the clear exposure of the {111} facets of the spinel, optimized the Mn³⁺ content, promoting structural and cycle stability at elevated temperature. At room temperature, both the pristine (LMNO) and microwave-treated (LMNO_mic) samples gave comparable cycling performance (>96% capacity retention and ca. 100% coulombic efficiency after 100 consecutive cycling). However, at an elevated temperature (60 °C), the LMNO_mic gave an improved cycling stability (>80% capacity retention and ca. 90% coulombic efficiency after 100 consecutive cycling) compared to the LMNO. For the first time, the impact of microwave irradiation on tuning the average manganese redox state of the spinel material to enhance the cycling performance of the LiNi_0.2Mn_1.8O₄ at elevated temperature and lithium-ion diffusion kinetics has been clearly demonstrated.