High energy cells with SiO/graphite composite anode tend to demonstrate fast capacity fade and gas generation during long term cycling test, due primarily to large volume expansion of Si particles and continuous SEI formation which consumes electrolyte components quickly. As a result, loss of accessible anode material and depletion of electrolyte are common failure mechanisms. Overcoming those challenges requires careful electrolyte formulation to create a stable anode SEI layer with the goal of extending cycle life and reducing cell gas generation.

In this study, a commercial grade small-format pouch cell with SiO/graphite and NCM 523 was selected as an electrochemical test vehicle to screen various electrolyte formulations. A baseline was obtained with commercial electrolytes, which revealed fluoroethylene carbonate (FEC) as a key enabling additive for cycle life. Unfortunately severe cell gassing is often observed with cells that exhibited superior cycle life. A rigorous electrolyte formation matrix was then constructed by varying FEC content, solvent type and ratio, introducing anti-gassing additives and mixed salts. Subsequent testing of these formulations demonstrated that up to 2% 1,3-propane sultone (PS) could effectively reduce cell gassing, but the associated higher cell impedance also adversely impacted cycle life.  Formulations with lithium bis(oxalato)borate (LiBOB) as a secondary salt had little influence on cell cyclability but significantly increased cell gassing.  The culpability of the additive was confirmed via gas composition analysis. Other variables such as solvent ratio and additives commonly used in graphite based cells appeared to be much less effective for Si containing anodes. Through composition modification, an optimal electrolyte formulation was identified with long cycle life and low gas generation.