Interfacial Control at Lithium-Ion Electrodes Using Ultrathin Polymer Coatings Generated By Initiated Chemical Vapor Deposition

Abstract

Interfacial processes at the surfaces of charge-storing materials in Li-ion batteries have a great impact on such critical operational characteristics as rate capabilities, cycle and calendar life, and safety. Solid-electrolyte interfaces (SEIs) that form spontaneously in situ serve to control surface reactivity in most Li-ion systems, yet may not yield optimized electrode and battery performance. Recent advancements in deposition protocols for nanoscale coatings open new opportunities to pursue deliberately designed & applied surface coatings at Li-ion electrodes. Initiated chemical vapor deposition (iCVD) has emerged as a promising route to generate ultrathin polymer coatings with thickness control at the nanoscale [1], even on complex, porous substrates [2], while also being readily adaptable to scaled manufacturing. As a further benefit for electrochemical applications, the siloxane-based polymers most commonly generated by iCVD contain ether-like functionalities that support the solvation and transport of Li+ salts, transforming them into Li-ion conductors [3]. We are exploring iCVD protocols as a means to apply nanometers-thick polysiloxanes at the surfaces of commercially available powder-composite electrodes, including graphite-based anodes and metal oxide/phosphate-containing cathodes. The performance of polymer-modified electrodes is assessed in conventional Li-ion coin cells, in which we assess such properties as specific power, irreversible capacity (e.g., due to SEI formation), coulombic efficiency and cycle life, as we vary the thickness of the polymer coating from 5–50 nm. Test cells containing these electrodes are also subjected to more extreme operating conditions (overcharge and overdischarge; high-rate charging at low temperatures) to assess the benefits of the nanoscale polymer coating for suppressing undesirable reactions at the electrode/electrolyte interface. Tenhaeff, W.E.; Gleason, K.K. Adv. Func. Mater. 2010, 18, 979.Sassin, M.B.; Long, J.W.; Wallace, J.M.; Rolison, D.R. Mater. Horizons 2015, 2, 502.B. Reeja-Jayan, N. Chen, J. Lau, J. A. Kattirtzi, P. Moni, A. Liu, I. G. Miller, R. Kayser, A. P. Williard, B. Dunn, and K. Gleason, Macromolecules 48, 5222 (2015)