Functional electrolyte: Design of corrosion inhibition additives to protect Cu current collectors in over-discharged state

Lithium-ion secondary batteries are increasingly being used not only for ICT devices but also for large power sources such as EVs. Typically, the battery module discharge termination voltage is fixed in the device/power supply, but the sequentially connected single cells in the assembled battery may become over-discharged, thus there is a concern that the battery performance is degraded by the dissolution of the Cu current collector. Cu ions diffuse into the electrolyte due to the dissolution of the Cu current collector and precipitate on the anode during charging. This causes blackening of the Cu surface and an increase in cell resistance, and if it progresses further, it may lead to a short circuit. Therefore, we searched for an additive that would improve the battery performance by suppressing the dissolution of Cu, which causes a short circuit during over-discharge. Since 1999, we have proposed a "functional electrolyte" in which a small amount of an additive with a new function is added to the electrolyte. In this work, we focused on additives with carbodiimide groups (-N=C=N-) that prevent dissolution of the Cu collectors during over-discharge and improve the battery performance, and report the finding that N,N′-dicyclohexylcarbodiimide (DCC) exhibits such a unique behavior. •N,N′-dicyclohexylcarbodiimide (DCC) is a new type of Cu corrosion inhibition additive.•DCC had a high recovery rate of laminated cells after over-discharge.•Electrolyte with DCC exhibited good discharge capacity and cycling performance.•DCC complexes were not easily deposited due to their high solubility.•The analysis suggests that DCC complexes form a thin layer on the Cu surface.