Aromatic isocyanate as a new type of electrolyte additive for the improved performance of Li-ion batteries

Aromatic isocyanate, 4-fluorophenyl isocyanate and phenyl isocyanate, were first used to reduce the initial irreversible capacities during the formation of the solid electrolyte interface (SEI) on a graphite surface. Results showed that the addition of 1–5 wt.% isocyanate to propylene carbonate-containing electrolytes could effectively reduce the initial irreversible capacities in the SEI formation and increase the cycleability of Li-ion batteries. The improvement is attributed to the high reactivity of isocyanate with chemisorbed oxygen groups, such as carboxyl and phenol, which are inevitably present in the prismatic (edge) sites of graphite and are known among the sources to cause the initial irreversible capacities of a graphite anode. It is proposed that the isocyanate reacts with carboxyl and phenol groups to form more stable products, and that the resulting products have a better affinity to the subsequently formed SEI. In addition, the presence of isocyanate assists in scavenging water and acidic HF impurities from the electrolyte.