Behind the Stability of Graphite Anode in PC-Containing Electrolytes with a New Additive

Lithium-ion battery (LIB) is one of the most promising power sources for electronic devices and electric vehicles because of their high energy densities and relatively long lifetime. Graphite is widely used as the anode electrode material in the state-of-the-art LIBs. However, a 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 higher low-temperature conductivity, the graphite anode experiences significant exfoliation problem during the initial lithium intercalation step. Thus a PC-containing electrolyte cannot be used in LIBs with graphite as the anode unless some solid electrolyte interphase (SEI) film-formation additives are included in the electrolytes. Usually these additives are electrochemically reduced predominantly on the graphite anode surface before PC is intercalated and reduced. Such additives normally contain unsaturated bonds or are unstable cyclic compounds, for instance, vinylene carbonate (VC) and fluoroethylene carbonate (FEC). They play an important role in the protection of the graphitic anode structure from destruction by PC reduction. However, these additives also have certain shortcomings. Usually these additives result in a thick SEI layer, which significantly reduces the rate capability and limits low-temperature performance and cycling stability at elevated temperatures through additional impedance and poor thermal stability.

Recently, we developed a novel electrolyte additive that can suppress the PC reductive decomposition on the graphite anode without forming a thick and resistant SEI film. As shown in Figure 1, this additive can not only significantly improve the compatibility between graphite anode and PC-containing electrolyte, but also exhibit superior rate capability and excellent cycling stability at elevated temperatures. This new electrolyte additive can be applied to LIBs and other electrochemical systems using PC-containing electrolytes and graphite anodes.

In this work, we systematically investigated the mechanism of the SEI formation by this additive and the protection of graphite anode in PC-containing electrolytes. Several techniques, including computational calculations and simulations, ¹⁷O nuclear magnetic resonance (NMR), “soft” electrospray ionization mass spectrometry (ESI-MS), time of flight second ion mass spectrometry (ToF-SIMS), etc. have been used to reveal the reasons behind the compatibility and stability of graphite anode in PC-containing electrolytes with this new additive. 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 Technology of the U.S. Department of Energy (DOE). The ¹⁷O NMR and ToF-SIMS 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.