(632b) Ultrathin Polymer Coatings As Artificial Solid Electrolyte Interphases for Lithium Ion Battery Anodes

Stable, reversible cycling of conventional lithium ion batteries is enabled by the passivation of graphitic anodes in alkyl carbonate electrolytes. As the anode is lithiated in the first charging step, electrochemical reduction of the electrolyte results in the formation of a self-limited solid electrolyte interphase (SEI) that largely limits further reaction with the electrolyte. However, in situ formation of the SEI places severe restrictions on liquid electrolyte compositions, requiring high concentrations of ehtylene cabonate (EC), which increases electrolyte viscosity, reduces oxidative stability, and limits the temperature window in which the cell can opeorate. Furthermore, for next-generation anode materials (e.g. Li metal and Li-Si alloy), in situ formation of a chemically and physically stable SEI does not occur. To address these issues, we are developing artificial SEIs based on ultrathin polymeric coatings to prepassivate lithium ion battery anodes. Initiated chemical vapor deposition (iCVD) is used to deposit conformal polymer coatings in a non-line-of-sight fashion on the complex three-dimensional architecture of conventionally cast composite electrodes (storage material + binder and conductive additive). In this study, ultrathin crosslinked poly(methacrylic acid) (PMAA) polymer is deposited on graphite electrode by iCVD. The film is then deprotonated in basic solution to introduce Li⁺ as the counter-ion. The ability of iCVD to simultaneously crosslink the PMAA during deposition through copolymerization is essential, as it prevents dissolution in liquid media and is used to modulate infiltration of liquid electrolyte into the polymer film. Characterization by scanning electron microscopy confirms that the polymer layers conformally coat the electrodes, and x-ray photolectron spectroscopy indicates that coating coverage is complete – the characteristic chemical composition of the electrode is completely occluded by the polymer coatings. The first cycle irreversibilty of 10.0% for conventional graphite anodes is reduced to 6.1% in graphite coated with poly(MAA). Also, electrolyte consumption is considerably mitigated during cycling at elevated temperatures, resulting in a significant improvement of the cycling life. Polymer thin film coatings have also been applied to model thin film anode materials prepared using RF magnetron sputtering. Both surface initiated atom transfer radical polymerization and iCVD were employed to synthesize the polymer thin films. On Si electrodes, a 75nm-thick poly(methyl methacrylate) layer results in an improved first cycle coulombic efficiency of 76.3% relative to 62.4% for the untreated Si electrode and improved the capacity retention during galvanostatic cycling. Electrochemical impedance spectroscopy conducted as a function of cycling showed that the polymer coatings slowed the growth in resistance of the passive layer. Based on these studies of conventional composite electrodes and model thin film anodes, we will discuss the key material properties of these ultrathin polymer films that are required for effective prepassivation.