Electrochemical and impedance investigation of the effect of lithium malonate on the performance of natural graphite electrodes in lithium-ion batteries

Lithium malonate (LM) was coated on the surface of a natural graphite (NG) electrode, which was then tested as the negative electrode in the electrolytes of 0.9 M LiPF6/EC–PC–DMC (1/1/3, w/w/w) and 1.0 M LiBF4/EC–PC–DMC (1/1/3, w/w/w) under a current density of 0.075 mA cm−2. LM was also used as an additive to the electrolyte of 1.0 M LiPF6/EC–DMC–DEC (1/1/1, v/v/v) and tested on a bare graphite electrode. It was found that both the surface coating and the additive approach were effective in improving first charge–discharge capacity and coulomb efficiency. Electrochemical impedance spectra showed that the decreased interfacial impedance was coupled with improved coulomb efficiency of the cells using coated graphite electrodes. Cyclic voltammograms (CVs) on fresh bare and coated natural graphite electrodes confirmed that all the improvement in the half-cell performance was due to the suppression of the solvent decomposition through the surface modification with LM. The CV data also showed that the carbonate electrolyte with LM as the additive was not stable against oxidation, which resulted in lower capacity of the full cell with commercial graphite and LiCoO2 electrodes.