LiMn2O4 as a Li-ion Battery Cathode. From Bulk to Electrolyte Interface

LiMn2O4 is ideal as a high-capacity Li-ion battery cathode material by virtue of its low toxicity, low cost, and the high natural abundance of Mn. Surface related reactions and bulk kinetics have been the major focus of this work. The main techniques exploited have been: electrochemical cycling, X-ray diffraction, X-ray photoelectron spectroscopy, infrared spectroscopy and thermal analysis. Interface formation between the LiMn2O4 cathode and carbonate-based electrolytes has been followed under different pre-treatment conditions. The variables have been: number of charge/discharge cycles, storage time, potential, electrolyte salt and temperature. The formation of the surface layer was found not to be governed by electrochemical cycling. The species precipitating on the surface of the cathodes at ambient temperature have been determined to comprise a mixture of organic and inorganic compounds: LiF, LixPFy (or LixBFy, depending on the electrolyte salt used), LixPOyFz (or LixBOyFz) and poly(oxyethylene). Additional compounds were found at elevated temperatures: phosphorous oxides (or boron oxides) and polycarbonates. A model has been presented for the formation of these surface species at elevated temperatures. The cathode surface structure was found to change towards a lithium-rich and Mn3+-rich compound under self-discharge. The reduction of LiMn2O4, in addition to the high operating potential, induces oxidation of the electrolyte at the cathode surface. A novel in situ electrochemical/structural set-up has facilitated a study of the kinetics in the LiMn2O4 electrode. The results eliminate solid-phase diffusion as the rate-limiting factor in electrochemical cycling. The electrode preparation method used results in good utilisation of the electrode, even at high discharge rates