William Jack Casteel

Electrochemical and Spectroscopic Investigations of the Overcharge Behavior of StabiLife Electrolyte Salts in Lithium-Ion Batteries

Electrochemical and spectroelectrochemical studies were conducted to understand the fundamental processes that occur during the overcharge of lithium-ion batteries with electrolyte solutions containing the StabiLife electrolyte salt (Li 2 B 12 F 12 )- Cyclic voltammetric experiments on these electrolyte solutions show well defined, quasi-reversible oxidation and reduction peaks with an onset potential of 4.6-4.9 V, depending upon the dielectric constant of the electrolyte. When oxidized in organic electrolytes, commonly used in lithium-ion batteries, Li 2 B 12 F 12 forms the stable, pale yellow B 12 F 1- * 12 radical anion. This is confirmed in solution by in situ electron paramagnetic resonance measurements. The overcharge chemistry of the StabiLife electrolyte salts was further probed in full cell configurations with mesocarbon microbeads as an anode and LiCoO 2 as a cathode. A Li metal reference electrode was incorporated to monitor the voltage changes associated with both the anode and the cathode during overcharge. Overcharge in cells based on LiPF 6 electrolytes very rapidly leads to cell potentials >5.5 V, while similar cells containing Li 2 B 12 F 12 based electrolytes exhibit stable voltage holding at ~4.7 V. X-ray photoelectron spectroscopy analysis confirms that massive lithium plating has occurred in the LiPF 6 -based cells. Evidence of only trace lithium plating is observed in Li 2 B 12 F 12 -based cells. A plausible mechanism for the reductive completion of the redox shuttle is proposed.