Improvement of the Thermal Stability of Li-Ion Batteries by Polymer Coating of LiMn2O4

A new approach has been used to minimize surface degradation of the LiMn2O4 cathode in lithium-ion batteries by using surface modification. LiMn2O4 particles used as the active material in cathode fabrication were modified by surface adsorption of poly(diallyldimethylammonium chloride) (PDDA). The adsorption and electrochemical performance of the modified cathode material were characterized and compared with that of the untreated LiMn2O4-based cathode. Transmission electron microscopy (TEM), scanning electron microscopy (SEM), and energy-dispersive X-ray analysis (EDAX) confirmed the formation of a thin polymer film on the surface of LiMn2O4 particles. The modified LiMn2O4-based cathode showed improved stability during charge/discharge cycling in an organic electrolyte at room temperature. Further, the measured capacity fading after storage at elevated temperature decreased. Capacity fading measured on cathodes made of PDDA-coated LiMn2O4 powder was lowest for cathodes obtained from powder coated in solutions containing between 30 and 50 mM PDDA. In situ atomic force microscopy (AFM) observation of the cathodes at room temperature showed minor changes in surface topography during a potential cycle. Our hypothesis is that the adsorbed polymer layer inhibits surface reactions that cause degradation. The present method for preparing surface-modified LiMn2O4 (SM-LMO) particles extends the lifetime of the lithium-ion battery by arresting the Mn+ dissolution, thereby increasing battery stability.