Development of Electrolyte for Negative-Limited LiNi0.5Mn1.5O4 / Li4Ti5O12 Li Ion Cells

Abstract

Spinel Li4Ti5O12 (LTO) is an attractive candidate for negative electrode materials of lithium ion battery because the LTO anode shows the high rate capability, long cycle life, outstanding safety characteristics, and a flat insertion/extraction potential of at 1.5 V vs. Li+/Li. The 4V-class cathode (such as LiCoO2, LiNi1/3Mn1/3Co1/3O2, and LiMn2O4) / LTO battery has a low working voltage of about 2.5 V, leading to its low energy density. Some researchers have studied the LiNi0.5Mn1.5O4 (LNMO) / LTO battery system, because LNMO cathode offers a high operating potential at 4.7 V, which increases the working voltage of the battery using the LTO anode to 3.1 V [1] [2] [3]. Amine et al. reported that the negative limited LNMO / LTO cell showed a high rate capability and long cycle life at room temperature [1]. However, we found that the negative-limited LNMO /LTO battery showed a poor cycle characteristic at 50 ℃ (figure1-a). In this presentation, we aim to clarify the degradation mechanism of the negative-limited LNMO / LTO battery in order to improve the cycle life at a high temperature. For this purpose, we disassembled the LNMO / LTO cells after the cycle test at 50 ℃ and extracted the electrodes and electrolyte. The extracted LNMO / Li-metal half-cell and the extracted LTO / Li-metal half-cell were assembled and tested. It was found that the capacity of the LNMO electrode did not decrease. On the other hand, the capacity of the LTO electrode decreased. It is revealed that the degradation of the negative-limited LNMO/LTO battery was caused by the decline of the LTO anode capacity. The XPS spectra of the LTO electrode indicated that LiF covered around the LTO materials and caused its capacity degradation. To reveal the generation pathway of LiF, the extracted electrolyte was analyzed by GC-MS, LC-MS/MS and 19F-NMR. The results revealed that the reduction reaction of the chain-like carbonate such as dimethyl carbonate and the ring-opening reaction of the cyclic carbonate such as ethylene carbonate caused the LiF generation. Especially, the lithium alkoxide generated by the ring-opening reaction of the cyclic carbonate accelerated the LiF generation reaction. The use of the electrolyte containing only chain-like carbonate as a solvent significantly improved the cycle characteristics at 50 ℃ (figure1-b).