Effect of fluoroethylene carbonate in the electrolyte for Lini_0.5Mn_1.5O₄ cathode in lithium-ion batteries

Fluoroethylene carbonate (FEC) was studied as an additive for the electrolyte in lithium ion batteries with the LiNi_0.5Mn_1.5O₄ (LNMO) spinel cathode operating at a high potential beyond 4.7 V (vs. Li/Li⁺). It was found that the FEC additive was electrochemically active for the 1^st charge cycle on the LNMO cathode. The presence of a large amount of FEC (more than 40 vol%) in the electrolyte caused severe side reactions with abnormally long voltage plateaus. In contrast, when the electrolyte contained less than 30 vol% FEC, the surface of the LNMO cathode was stabilized by the formation of the solid-electrolyte interphase (SEI), leading to improved cyclability. However, the resistance from the SEI limited the rate capability because of sluggish lithium transportation through the SEI and electronic insulation between the particles in the electrode.