Study of passive film formation on graphite surface lithiated in the polysiloxane based electrolyte for the application to lithium secondary battery

The solid electrolyte interface (SEI) which passivates the carbonaceous material in an organic electrolyte is at the forefront of battery research because the nature of the SEI strongly affects lithium ion battery performance. Aside from lithium ion cells, the lithium cell with polymer electrolyte has received considerable attention because of efforts to improve lithium cell safety. However, the nature of the SEI on the carbonaceous material in a polymer electrolyte is not understood in depth to the same extent as the SEI in organic electrolytes. In this project, siloxane-based electrolyte was studied to improve safety and performance of lithium secondary cells, and the SEI on the carbonaceous materials charged in the siloxane-based electrolyte was investigated. Two types of SEI films were observed to form on the highly oriented pyrolytic graphite (HOPG) lithiated in the siloxane-based electrolyte. These films were morphologically and compositionally distinct, and were described as island-like and gel-like. In addition, electrochemical impedance analysis was performed with an electrochemical cell containing a thin graphite electrode to clarify the electrical characteristics of the SEI. The value of the charge transfer resistance, Rct, for siloxane-based electrolyte was two orders of greater than conventional carbonate-based electrolytes. Electrolyte additives, such as vinyl ethylene carbonate (VEC), the type of electrolyte salt, and siloxane molecule structure reduced the value of Rct. Based on FT-IR spectra, the SEI was composed of the flexible groups -Si-O- and -C-O-. These flexible function groups are expected to absorb the volumetric changes of graphite particles during lithiating and delithiating in an electrochemical cell, which will prevent continuous decomposition of siloxane electrolyte on the graphite surface. In addition, the surface species on the lithium transition metal oxide (LiMeO2) delithiated in the polysiloxane-based electrolyte was investigated. The result indicated that the cathode potential should not exceed 4.1V vs. Li/Li+ to ensure long-term cell performance and to prevent electrolyte decomposition. Lastly, the charge and discharge performance of the electrochemical cell comprised of LiMeO2 cathode/polysiloxane-based electrolyte/graphite anode were evaluated for practical use, and the commercial feasibility of a lithium battery containing a polysiloxane-based electrolyte was demonstrated.