Elucidating the Interfacial Reactions for Conversion-Alloying Materials Towards the Realization of High-Performance Lithium-Ion Full-Cells

Abstract

The unique combination of high energy and power density made Li-ion batteries the state-of-the-art electrochemical energy storage technology for powering small consumer electronics as well as large-scale applications like electric vehicles (EVs).[1] This increasing diversity of potential applications, however, also results in a greater variety of required characteristics. For the use in EVs, in fact, two of the major issues of the current lithium-ion chemistry based on graphite anodes are the sluggish lithium transport across the solid electrolyte interphase (SEI) upon lithium intercalation into the graphite host structure and the very low lithiation potential – in combination intrinsically limiting the fast charging capability.[2] This intrinsic kinetic limitation triggered researchers to find alternative anodes, following, for instance, an alloying (e.g., Si) or a (multi-phase) conversion (e.g., Fe2O3) mechanism. Both alternatives commonly show higher capacities and frequently superior rate capabilities. Despite steady improvements, though, these alternatives still suffer from capacity fading due to extensive volume variations (especially alloying materials) and low energy efficiencies due to a significant voltage hysteresis (in particular for conversion materials) as well as an improvable coulombic efficiency. Conversion/alloying-materials (CAMs) are a rather new class of electrode compounds that combine the alloying and conversion mechanism in one single material.[3] Upon lithiation, nanograins of the non-alloying element are formed in situ and build a percolating electron conductive network. Besides allowing for the reversible formation of the simultaneously formed Li2O matrix, this metallic nano-network enables fast lithiation and delithiation kinetics, rendering CAMs a promising candidate for high-power applications. Nevertheless, there is still a lack of knowledge which parameters eventually determine the overall performance and cycling stability.Herein, we show that the reactions occurring at the electrode/electrolyte interface play a decisive role and that these reactions are largely dependent on the (surface) chemistry of the active material. The techniques used include amongst others ex situ synchrotron soft X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, and in situ isothermal microcalorimetry. It is shown that the stabilization of the interface is essential when targeting long-term stable performance – in both half-cells and full-cells. The latter provides a specific energy of 284 Wh kg-1 accompanied by an excellent energy efficiency of >93%.[1] N. Nitta, F. Wu, J. T. Lee, G. Yushin, Mater. Today 2015, 18, 252–264.[2] N. Loeffler, D. Bresser, S. Passerini, Johnson Matthey Technol. Rev. 2015, 59, 34–44.[3] D. Bresser, S. Passerini, B. Scrosati, Energy Environ. Sci. 2016, 9, 3348–3367.