An analysis of rechargeable lithium-ion batteries after prolonged cycling

We report herein on extensive post-mortem analysis of rechargeable Li-ion batteries after prolonged cycling at 25 and 40 °C. The 18650-type batteries produced by standard procedure at LG Chem. Inc. were comprised of graphitic carbon anodes, LiCoO2 cathodes, and a commercial, Li-battery grade ethylene carbonate, ethyl–methyl carbonate (EC-EMC)/LiPF6 solution. The batteries were dismantled under highly pure inert atmosphere after prolonged cycling by a specially designed system, followed by analysis of both electrodes by SEM, XRD, FTIR, and electrochemical methods (voltammetry, impedance spectroscopy). The residual solution was analyzed by gas chromatography–high-resolution mass spectrometry (GCMS), and by Karl Fischer titration (for trace H2O content). It was found that the structure of both electrodes is basically retained during prolonged cycling. However, they developed surface films that considerably increase their impedance, thus badly affecting the battery performance after a few hundred charge–discharge cycles. It also appears that part of the cathode's active mass became electronically isolated from the current collector. Hence, the impedance of both electrodes increases considerably upon cycling. The residual solutions contained 10–15 times more water than the pristine solution, as well as products of trans-esterification of the alkyl carbonates, and some fluorinated and phosphorous-containing organic species. The surface chemistry of both electrodes seems to be dominated primarily by LiF formation. Capacity fading mechanisms of Li-ion batteries are discussed.