(Invited) CsPF6 and Propylene Carbonate in Conventional LiPF6/Carbonate Electrolytes for Enhanced Lithium-Ion Battery Performances in Wide Temperature Range

Abstract

Lithium-ion batteries (LIBs) have achieved great successes in portable electronic devices and have now started to enter the large-scale energy storage applications in transportations (such as pure electric vehicles and plug-in hybrid electric vehicles) and smart grid. Graphite is widely used as the anode material in the state-of-the-art LIBs. However, the graphite anode is usually only compatible with an ethylene carbonate (EC)-based electrolyte. In a propylene carbonate (PC)-containing electrolyte, which is advantageous over the EC-based counterpart due to its wider temperature range and lower cost, the graphite anode experiences significant exfoliation problem by the PC intercalation during the Li+ intercalation at charging steps. Thus a PC-containing electrolyte cannot be used in LIBs with graphite anode unless some solid electrolyte interphase (SEI) film-formation additives are included in the electrolytes. Such SEI formation additives normally contain unsaturated bonds or are unstable cyclic compounds, e.g. vinylene carbonate (VC), fluoroethylene carbonate (FEC), and lithium bis(oxalato)borate (LiBOB). They play an important role in the protection of the graphite structure from destruction by PC intercalation. However, these additives usually result in a thick SEI layer, which greatly reduces rate capability, limits low-temperature performance and hampers cycling stability at elevated temperatures through additional impedance and poor thermal stability.Recently, we have demonstrated that cesium cation (Cs+) as a novel electrolyte additive can direct the formation of a uniform, ultrathin and compact SEI layer on graphite electrode surface in EC-PC-containing electrolytes.[1] This SEI can not only significantly improve the compatibility between graphite anode and PC-containing electrolyte, but also lead to superior rate capability, enhanced low-temperature discharge performance, and excellent cycling stability at elevated temperatures.We also foundthat PC content in the electrolytes has great effect on the performances of graphite anode in half cells and full cells, and an optimal PC content of 20% by weight in solvent mixtures is identified.[2] The synergistic effects of Cs+ additive and appropriate amount of PC enable the formation of a robust, ultrathin and compact SEI layer on the surface of graphite electrode, which is only permeable for de-solvated Li+ ions and allows fast Li+ transport through it. In the continuation study, we further optimized the electrolyte formulations with CsPF6 additive by adjusting the EC and PC contents in electrolytes and evaluated the performances of graphite||LiNi0.80Co0.15Al0.05O2 full cells in terms of cycling at room temperature and elevated temperature, rate capacity and low-temperature discharge. One optimized electrolyte formulation is found to give about 70% discharge capacity retention at -40°C when compared to the room temperature discharge of the full cells, and simultaneously maintains other excellent performances. It is indicated that the optimal electrolyte consisting of CsPF6 and PC in conventional LiPF6/EC-based electrolyte is of great potential use in the state-of-the-art LIBs. The details of the results will be reported at the meeting. Acknowledgements This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies, the Advanced Battery Materials Research Programs of the U.S. Department of Energy (DOE), and the Laboratory Directed Research and Development (LDRD) Project under the Technology Investment Program at Pacific Northwest National Laboratory (PNNL). The microscopy and spectroscopy measurements were performed at the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the DOE’s Office of Biological and Environmental Research and located at PNNL.