A Timescale characterization of (NMC811)/SiOx-graphite 1Ah pouch cell: Improving cycling stability with carbonate-based electrolytes and triallyl phosphate (TAP) co-additive

Abstract

The role of 2 wt% triallyl phosphate as an electrolyte additive in (NMC811)/SiOx-graphite 1Ah pouch cells is studied using electrochemical impedance spectroscopy, long-term cycling, rate, timescale characterisation and X-ray photoelectron spectroscopy. Six kinetic processes are identified within the 100 kHz to 0.01 Hz frequency range. Anode processes consistently occur faster than the cathode and at higher frequencies. The triallyl phosphate (TAP) additive improves capacity retention from 70% to 85% after 500 cycles at 45 °C, exhibiting a higher and more stable coulombic efficiency, with ∼100% reduction in DCIR growth. TAP cells produce less gas during cycling. Timescale characterization suggests that the high impedance in the base formulation originates from cathodic charge transfer and cathode electrolyte interface (CEI) films in non-TAP-containing samples. The XPS data confirm the DRT findings, indicating the presence of high-impedance species associated with solvent oxidation and transition metal oxides (TMO) in the CEI of the TAP-free sample. The initial higher impedance of 14%–0.06 Ω observed in TAP cells is due to the formation of a thicker CEI in the TAP samples. Consequently, TAP samples show a ∼10 mAhg−1 lower rate capacity, at higher C-rates of 3C and 5C, while both samples exhibit consistent rate-capacity retention.