Functional electrolytes: Novel type additives for cathode materials, providing high cycleability performance

We present the results of novel type additives to improve the cathode cycleability performance. Benzene derivatives (biphenyl and o-terphenyl) and heterocyclic compounds (furan, thiophene, N-methylpyrrole and 3,4-ethylenedioxythiophene), which have lower oxidation potentials than those of electrolyte solvents are picked up. We use MO calculations in the selection of additives and prove that the calculated HOMO values agree well with the measured oxidation potentials. To clarify the additive performance, electrochemical properties and cycleability of the additive are investigated. The additives are found to be decomposed on cathode to form very thin film. We have named this resulting novel-type thin surface film as electro-conducting membrane (ECM) since it is different from solid electrolyte interphase (SEI) by the point of its electro-conductivity. The nature and the component of ECM are studied with X-ray photoelectron spectroscopy (XPS) and auger electron spectroscopy (AES) in details. It is concluded that these additives, which were formerly known as overcharge protecting proofs, contribute to improve cathode cycleability by forming very thin cathode surface layer in the case of slight amount of addition.