Improving the performance of sulphur doped LiMn2O4 by carbon coating

Abstract

Sulphur-doped spinel LiMn2O3.99S0.01 is a promising cathode material for lithium-ion batteries. It can be obtained in a form of nanocrystals by a facile, eco-friendly and water-based sol-gel synthesis. In this work, LMOS was successfully coated with a 2–3 nm conductive carbon layer fabricated from pyrolysis of modified poly(N-vinylformamide) - water-soluble polymer precursor. Carbon coated material exhibits greatly increased performance under high currents and improved cyclability in long-term charge-discharge tests. Pristine and carbon-coated samples are characterized by X-ray diffraction, thermogravimetric analysis, elemental analysis, transmission electron microscopy and X-ray photoelectron spectroscopy. Electrochemical performance of the materials is examined by galvanostatic charge-discharge tests, cyclic voltammetry and electrochemical impedance spectroscopy. Carbon coated sample present a highly enhanced rate capability, withstanding up to 100C rate and maintaining up to 67.5% of initial 1C discharge capacity at 50C rate (1C - 148 mA g−1 – current required to fully charge a cell in time of 1 h). Li+ ion apparent diffusion coefficient in carbon coated sample is approximately twice increased. Electrochemical impedance spectroscopy has shown, that carbon coating decreases internal resistances of the cell caused by material passivation and charge-transfer reactions.