There is significant interest in the development of higher energy lithium ion batteries (LIB) for electric vehicles. One method of improving the energy density of lithium ion batteries is to increase the operating voltage of the cells by increasing the working potentials of positive electrode employing for example lithium nickel manganese spinel LiNi
0.5Mn
1.5O
4 or layered mixed metal oxide Li
2MnO
3-LiMO
2 as the active material. However, cycling of lithium-ion cells to high voltages (~5.0 V vs lithium reference electrode, LRE) proceeds with relatively low (99% and less) coulombic efficiency. Among the primary contributors to the poor cycling efficiency are the electrochemical oxidation reactions of the electrolytes at the high positive potentials of positive electrode. We have been conducting a two part investigation of electrolytes for high voltage LiNi
0.5Mn
1.5O
4 and Li
2MnO
3-LiMO
2 cathodes. First, we have been studying the reaction of standard electrolyte (LiPF
6 in EC/EMC) with the surface of the LiNi
0.5Mn
1.5O
4 and Li
2MnO
3-LiMO
2 particles. Second, we have been developing novel additives designed to sacrificially react on the surface of LNMS cathode materials to generate a passivation layer which inhibits further electrolyte oxidation. Our investigation of the reactions of electrolyte with the surface of LiNi
0.5Mn
1.5O
4 cathode materials include electrodes cycled to 4.9 V vs Li, cycled under accelerated aging conditions, and cathode particles stored at elevated temperature (85 °C) in the presence of electrolytes. This will allow us to develop a better understanding of the detrimental electrochemical and thermal reactions of the electrolyte with the cathode surface. Two classes of additives, inorganic and organic, have been investigated which are preferentially oxidized to form a cathode SEI which inhibits the oxidative reactions of the cathode with the electrolyte the cathode particles. Incorporation of the additives has an effect on the capacity retention of the cells when cycled to 4.9 V especially under accelerated aging and improves the cycling efficiency. After cycling and storage experiments an ex-situ analysis of the electrodes or cathode particles was conducted via a combination of SEM, XPS, and FT-IR spectroscopy to determine the relationship between the cathode surface films and of performance differences for different electrolytes.
AcknowledgementsWe thank the Batteries for Advanced Transportation Technologies (BATT) Program supported by the U.S. Department of Energy Office of Vehicles Technologies, DOE EPSCoR, and BASF for funding.
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