Evaluating electrolyte additives in dual-ion batteries: Overcoming common pitfalls

Electrolyte additives that form a protective cathode-electrolyte interface (CEI) layer on graphite are sought-after in dual-ion batteries (DIBs). Identifying suitable candidates remains, however, challenging due to lack of universal testing protocols. In this study, specified amounts of vinylene carbonate, fluoroethylene carbonate, lithium bis(oxalato)borate and lithium difluoro(oxalato)borate were added to a 4 M lithium bis(fluorosulfonyl)imide in dimethyl carbonate electrolyte used in Li-graphite and Li4Ti5O12-graphite DIBs. Galvanostatic cycling at 10 mA g−1 resulted in coulombic efficiencies < 90% for all additives and both cell designs, revealing significant irreversibility at the cathode. Self-discharge tests and electrochemical impedance measurements in a three-electrode setup further showed that side-reactions at the graphite electrode induced Li-trapping in Li4Ti5O12. Increasing the specific current to 100-1000 mA g-1 seemingly enhanced the coulombic efficiency (> 98%) and discharge capacity (90-100 mAh g-1), owing to kinetically-suppressed side-reactions. This was hence highlighted as an inappropriate condition to evaluate the additives' ability to form a passivating CEI. In addition, running such measurements with Li metal as the counter electrode was demonstrated to be problematic, as most additives significantly affected the Li metal plating/stripping, especially for the higher cycling rates.