In Situ Conductive Coating Strategies for Nanocrystal-Based Li-Ion Battery Electrodes Beyond Carbonization

Abstract

One of the critical issues in Li-ion battery systems is overcoming the inherent poor electronic conductivity of the electrodes and the associated slow electrode/electrolyte interfacial electrochemical kinetics because of the formation of solid electrolyte interphase (SEI). This issue is particularly challenging when high-safety electrode materials like lithium iron phosphate (LiFePO4-LFP) and lithium titanate (Li4Ti5O12-LTO) are employed. Also inevitable material degradation occurs at such interfaces that, for example, was recently discussed in connection to 2D LTO nanocrystals (1) and the spontaneous reaction between Li2FeSiO4 (LFS) and LiPF6-based electrolyte (2). Michel Armand’s pioneering work (3) on carbon coating provides an elegant and economic strategy to overcome aforementioned issues, leading to their successful commercialization by Hydro-Québec. With the advent of different nanocrystals as active electrode materials for high energy or power applications new opportunities arise for innovative engineering the electrode/electrolyte interfaces at the nanoscale. Herein two modification processes are tailored to fabrication of 2D nano-Li4Ti5O12 as anodes and to mechanochemically-nanosized orthorhombic Li2FeSiO4 (Pmn21 LFS) as cathodes. With the goal on one hand replacing the expensive NMP-PVDF coating process and on the other hand tackling the low nominal capacity issue of LTO electrodes, we are working on an alternative sustainable fabrication method involving in situ coating of reduced graphene oxide (redGO) onto 2D nano-LTO. This process involves electrophoretic co-deposition of hydrated lithium titanate (Li2−xHx)Ti2O5·yH2O) and GO that is followed by controlled thermal conversion into redGO-LTO composite anodes. Meanwhile, the exploration of Pmn21 Li2FeSiO4 as high-capacity cathode has been impeded because of lacking suitable surface modification techniques. Traditional carbon coating at elevated temperature is not suitable here as it would induce a phase transformation of LFS from orthorhombic Pmn21 into monoclinic P21/n. Therefore, in another development we have successfully coated Pmn21 LFS with a conductive polymer via in situ polymerization at room temperature. In developing this method we discovered significant differences between LFP and LFS as substrates reflecting differences in surface redox reactivity between the orthophosphate and orthosilicate polyanionic frameworks. Thanks to this low-T non-C coating method the nanometric Pmn21 LFS cathode reveals itself as very promising high-energy density and stable cathode material. H. C. Chiu, X. Lu, J. Zhou, L. Gu, J. Reid, R. Gauvin, K. Zaghib and G. P. Demopoulos, Adv. Energy Mater., 7, 1601825 (2017).Z. Arthur, H. C. Chiu, X. Lu, N. Chen, V. Emond, K. Zaghib, D. T. Jiang and G. P. Demopoulos, ChemCommun, 52, 190 (2016).M. Armand et al., Method for synthesis of carbon-coated redox materials with controlled size, US Patent No. 2004/0033360A1, Feb. 19, 2004.