Performance improvement and failure mechanism of LiNi0.5Mn1.5O4/graphite cells with biphenyl additive

Abstract

The cycling performance of the LiNi_0.5Mn_1.5O₄/graphite cells cycled to 4.8 V versus Li/Li⁺ at room temperature has been investigated. The control electrolyte is 1.0 M LiPF₆ ethylene carbonate (EC)/ethylmethyl carbonate (EMC)/fluoroethylene carbonate (FEC) (10/70/20, v/v) with 2 wt% lithium bis(oxalate)borate (LiBOB). Biphenyl (BP) (0.5 wt% and 1 wt%) is employed as the in situ electrochemical coating additive for LiNi_0.5Mn_1.5O₄ cathode. The results indicate the potential window of electrolyte without BP is up to 5.5 V versus Li/Li⁺ on Pt electrode, but still decomposed seriously on both cathode and anode surfaces during prolonged cycles. The improved cycling performance with added 0.5 wt% BP in the control electrolyte can be attributed to the in situ electrochemical coating film on LiNi_0.5Mn_1.5O₄ surface derived from BP electro-polymerization, which decreases the Mn dissolution and the lattice changes of LiNi_0.5Mn_1.5O₄ cathode, and further inhibits Mn deposition and additional SEI formation on graphite surface. However, the thicker and compact BP electro-polymerization film using 1BP electrolyte on the cathode surface might inhibit the lithium intercalation/deintercalation and increase the polarization, then accelerates the capacity fading.