Mechanism for improving the cycle performance of LiNi0.5Mn1.5O4 by RuO2 surface modification and increasing discharge cut-off potentials

Abstract

The cycle performance of LiNi_0.5Mn_1.5O₄ (LNMO) is improved greatly by a surface modification with discrete RuO₂ particles in combination with setting discharge cut-off potentials to 4.5 V. A specific capacity value as high as 113.8 mA h g⁻¹ after 1000 cycles is attained. In order to not only clarify the mechanism for the improvement by the RuO₂ particles deposited, but also to elucidate which chemical properties of surface-modifying materials play a critical role in the evolution of the solid electrolyte interphase (SEI) layer with cycling, the electrochemical impedance spectra and X-ray photoelectron spectra (XPS) of a bare LNMO electrode, discrete RuO₂ particle modified-LNMO electrode and discrete Al₂O₃ particle modified-LNMO electrode were compared. The XPS spectra of the SEI layers of these electrodes cycled between 4.5 and 5.2 V indicate that a stable SEI layer on the RuO₂-modified LNMO electrode has the characteristics of higher content of LiF and longer poly-ethylenecarbonate chains with extremely low content of –CF₂O– groups. The crucial point for the formation of the stable SEI layer is the sustainable consumption of the F radicals generated by the electrochemical decomposition of LiPF₆ at high potentials. We conclude that the hydrolysable property of ruthenium fluorides, intermediate products obtained during the formation of the SEI layer on the RuO₂-modified LNMO electrode, and the high catalytic activity of RuO₂ itself for electrochemical oxygen evolution guarantee the sustainable consumption of the F radicals. Al₂O₃ lacks these special chemical properties to attain the sustainable consumption, thus leading to no improvement by the modification with discrete Al₂O₃ particles.