The spinel-structured LiNi0.5Mn1.5O4 (LNMO) is a promising high-voltage (~4.7 V vs. Li/Li+) positive electrode for lithium-ion batteries (LIBs). However, since the working voltage is higher than the electrochemical stability window of common electrolytes (~4.3 V vs. Li/Li+), electrolyte decomposes during cycling. This irreversible reaction causes low Coulombic efficiency. Moreover, hydrogen fluoride (HF) is generated as a result of electrolyte decomposition, which attacks the LNMO electrodes to cause electrode failure. Meanwhile, it is known that HF etches metal oxides, signifying that metal oxides can act as HF scavenger. In this work, copper oxide (CuO) is used as the HF scavenger for LNMO electrodes. The cycle performance of LNMO electrodes is significantly improved at elevated temperature due to the HF scavenging role of CuO.
2. Experimental
The composite LNMO electrode was prepared along with Super-P and PVdF binder, into which nano-sized CuO powder was added. The used electrolyte was LiPF6 dissolved in a mixture of ethylene carbonate (EC), ethyl methyl carbonate (EMC) and diethyl carbonate (DEC). The galvanostatic charge/discharge cycling was performed at 60oC. For X-ray photoelectron spectroscopy (XPS), the cycled LNMO electrodes were disassembled in an argon-filled glove box.
3. Results and Discussion
The CuO-free LNMO electrodes show poor performances. Due to severe electrolyte decomposition at 60oC, the Coulombic efficiency is poorer than that for the CuO-added electrodes. The CuO-free LNMO electrodes also show a larger electrode polarization and rapider capacity fading, all of which seem to be due to the electrode failure caused by HF attack. The outperformance of CuO-added electrodes over the CuO-free ones must be due to the beneficial role of CuO, which can act as a HF scavenger because of its basicity. The adverse effects of HF can be suppressed by removing HF in the electrolyte solution.
4. Reference
1) T. Yoon, et al., J. Power Sources, 215, 312 (2012).