Study on multifunctional phosphite-based additives to improve electrochemical properties of LiNi0.5Mn1.5O4 cathodes in lithium ion batteries

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Study on multifunctional phosphite-based additives to improve electrochemical properties of LiNi0.5Mn1.5O4 cathodes in lithium ion batteries
Song, Young-Min
Choi, Nam-Soon
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Graduate School of UNIST
Nowadays, lithium ion batteries (LIBs) are one of the fastest growing fields as substitute resources due to relative long term cycle and higher energy density than other batteries. Although LIBs have been commercialized in our life, desire values for extensive applications, which require higher energy/power density such as electric vehicle (EV), hybrid electric vehicle (HEV), plug-in hybrid electric vehicle (PHEV), military and aerospace allications, stationary energy storage still remained challenging. To overcome the limited energy density of LIBs, LiNi0.5Mn1.5O4, which operates in the vicinity of 4.7 V vs. Li/Li+, has been considered as a promising cathode material for LIBs due to high energy densities, low cost, eco-friendly and its high specific capacity. However, there are some defects regarding poor cycle performance in full cell at elevated temperature, severe decomposition of solvent and salt, transition metal dissolution, weakness of storage capacity (self-discharge), and so on. Herein, to resolve the problems of high-voltage LiNi0.5Mn1.5O4 cathode, various organo phosphorus-based additives are investigated as a functional additive forming the solid electrolyte interphase(SEI) layer on the cathode surface. Because this protective later has less resistive character, it can remain to high capacity at high current rate, moreover, indicate superior cycling performance at elevated temperatures in Li/LiNi0.5Mn1.5O4 half cell and graphite/LiNi0.5Mn1.5O4 full cell. To understand the effect of TMSP on the transition metal dissolution, inductively coupled plasma-mass spectrometer (ICP/MS) and energy-dispersive X-ray spectroscopy (EDS) are used as instruments. The results show TMSP-added electrolytes reduce dissolution of Mn and Ni from LiNi0.5Mn1.5O4 cathode. To check structure stability and ability to protect self-discharge at high temperatures, the fully charged cells are stored at 60 oC, and then the samples are transported to ex-situ X-Ray diffraction (XRD), and the data reveals TMSP-derived SEI layer keeps their nature charged structure which means protect self-discharge during storing at 60 oC. Another functional effect is to diminish HF produced by hydrolysis of LiPF6 due to water trace in the cell via 19F and 31P NMR spectra of the electrolyte with and without 0.5wt% TMSP after hydrolysis tests at room temperature. By reducing HF, transition metal dissolution from LiNi0.5Mn1.5O4 cathode by directly attack can be mitigated and protect persistent decomposition of salts. Also, to confirm the critical impact and additional function of organo phosphorus-based additives, we introduce progressive study comparing TMSP with other additives commonly retaining phosphite core. In addition, from an analysis of surface chemistry of SEI layers on the high voltage cathode, we can find similar components of SEI layers formed by various phosphite-added electrolytes via ex-situ X-ray photoelectron spectroscopy (XPS), and propose its common functions which eliminate HF and alleviate decomposition of LiPF6 by hydrolysis via nuclear magnetic resonance (NMR). To understand the effect of various phosphite-types additives regarding self-discharge, open circuit voltage (OCV) of the cells with and without additives is measured at high temperatures during 6 days and capacity retention is conducted after storage. To sum up, there are lots of additives to improve performance of lithium ion battery, TMSP is one of promising additive to make organic and inorganic based SEI layer on the LiNi0.5Mn1.5O4 cathode and show superior electrochemical performance.
Department of Energy Engineering(Battery Science and Technology)
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