DFT modelling of explicit solid–solid interfaces in batteries: methods and challenges

Abstract

Density Functional Theory (DFT) calculations of electrode material properties in high energy density storage devices like lithium batteries have been standard practice for decades. In contrast, DFT modelling of explicit interfaces in batteries arguably lacks universally adopted methodology and needs further conceptual development. In this paper, we focus on solid-solid interfaces, which are ubiquitous not just in all-solid state batteries; liquid-electrolyte-based batteries often rely on thin, solid passivating films on electrode surfaces to function. We use metal anode calculations to illustrate that explicit interface models are critical for elucidating contact potentials, electric fields at interfaces, and kinetic stability with respect to parasitic reactions. The examples emphasize three key challenges: (1) the "dirty" nature of most battery electrode surfaces; (2) voltage calibration and control; and (3) the fact that interfacial structures are governed by kinetics, not thermodynamics.To meet these challenges, developing new computational techniques and importing insights from other electrochemical disciplines will be beneficial.High energy density batteries are instrumental to vehicle electrification and to grid storage which helps alleviate intermittency in solar and wind energy generation. Deeper understanding of existing materials and the search of new electrode and electrolyte materials are crucial for developing higher capacity, faster-charging, safer, and longer-lasting batteries. 1 Battery interfaces are also universally acknowledged to be critical for governing battery rate capability and stability. 2 Charge/discharge processes, as well as many side reactions that lead to degradation, self-discharge, and thermal runaway, initiate at interfaces.Electronic structure Density Functional Theory (DFT) modelling of the crystalline interior of electrodes, for the purpose of predictng phase stability, equilibrium voltages, and lithium diffusion kinetics, has been widely practised for decades. 3-7 It provides important guidance to experiments. In contrast, DFT modelling of explicit battery interfaces arguably needs further conceptual development and systematization. Here we distinguish explicit interfaces where two phases are in contact in the same simulation cell, from single-phase DFT calculations used to infer interfacial properties indirectly. 8 The present work focuses on modelling methods, offers somewhat pedagogical discussions on the rationale behind the DFT approaches used in our group, and examines future directions and improvements.Simple material interfaces, mainly involving lithium metal, are used as illustrations, but the principles involved should be broadly applicable to other electrodes. Even simple materials exhibit complex interfaces that require substantial approximations; the nature and consequences of some key implicit approximations are highlighted. We restrict ourselves to vanishing current densities. 9 This paper is intended to be a topical, critical overview, not a comprehensive revi...

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