Electrolytes for lithium-ion batteries with wide operating temperature range

Abstract

Lithium-ion batteries have been extensively investigated as power sources for electric vehicles due to their high energy density. Electric vehicle applications require a superior retention of performance (10-15 years of calendar life) over a wide range of temperatures (-30 to 60 °C). The electrolyte used in commercial lithium-ion batteries are composed of LiPF6 dissolved in organic carbonates or esters, among which ethylene carbonate (EC) is a required component due to the importance of EC in the formation of the anode solid electrolyte interphase (SEI). However, LiPF6 is very sensitive to environmental moisture and has poor thermal stability. Furthermore, EC leads to poor performance at low temperature due to its high melting point. Lithium difluoro(oxalato)borate (LiDFOB) is a promising alternative lithium salt for lithium-ion battery electrolytes. The ligand exchange reaction of LiDFOB to generate Lithium tetrafluoroborate (LiBF4) and Lithium bis(oxalato)borate (LiBOB) was investigated by Nuclear Magnetic Resonance (NMR) spectroscopy. A thermally induced equilibrium exists between LiDFOB, LiBF4 and LiBOB and the equilibrium favors LiDFOB. A novel salt, lithium tetrafluorooxalatophosphate [LiPF4(C 2O4), LiFOP] was developed. It has much better thermal and hydrolytic stability and performance retention upon accelerated aging. This unique combination of properties makes LiFOP an interesting alternative to LiPF6. The cycling performance of LiFOP electrolyte is compared with LiPF6 electrolyte in the presence of several different electrode materials. In order to develop a better understanding of the sources of performance differences between LiFOP and LiPF6, the surfaces of the electrodes have been analyzed after cycling. Propylene carbonate (PC) is a good candidate for improving low temperature performance due to a low melting point, and comparable dielectric constant compared to EC. Unfortunately, replacing EC for PC in a LiPF6 battery, results in continuous electrolyte reduction and exfoliation of the graphite anode. This problem can be solved by using LiFOP with PC. To confirm reversible cycling behavior of LiFOP/PC electrolytes, the cycling properties were investigated with a natural graphite anode and either LiNi1/3Co1/3Mn 1/3O2 or LiFePO4 cathodes. A comparison of the low temperature cycling performance of LiPF6/EC electrolyte and LiFOP/PC electrolyte was also conducted. Methyl butyrate (MB) is also a good low-temperature cosolvent when it is used with EC and EMC. The cycling performance of cells with LiFOP/MB electrolyte was investigated with a natural graphite anode and a LiNi1/3Co 1/3Mn1/3O2 cathode. Ex-situ surface analysis was also conducted by Electrochemical Impedance Spectroscopy (EIS), Scanning Electron Microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS) and Fourier transfer infrared spectroscopy (FT-IR) to diagnose the issues after accelerated aging.