Ab initio simulations of liquid electrolytes for energy conversion and storage

Abstract

Understanding physicochemical properties of liquid electrolytes is essential for predicting and optimizing device performance for a wide variety of emerging energy technologies, including photoelectrochemical water splitting, supercapacitors, and batteries. In this work, we review recent progress and open challenges in predicting structural, dynamical, and electronic properties of the liquids using first-principles approaches. We briefly summarize the basic concepts of first-principles molecular dynamics (FPMD), and we discuss how FPMD methods have enriched our understanding of a number of liquids, including aqueous solutions, organic electrolytes and ionic liquids. We also discuss technical challenges in extending FPMD simulations to the study of liquid electrolytes in more complex environments, including the interface between electrolytes and electrodes, which is a key component in many energy storage and conversion systems. K E Y W O R D S energy conversion and storage, first-principles simulations, liquid electrolytes 1 | INTRODUCTIONLiquid electrolytes are essential components in a wide variety of emerging energy and environmental technologies, including hydrogen production through solar-water-splitting, [1,2] supercapacitors, [3][4][5] ion batteries, [6][7][8] and ion-selective membranes. [9][10][11] Within these devices, a large number of liquids have been explored, ranging from aqueous solutions and ionic liquids to organic, redox-type, and solid-state electrolytes. At the same time, continuing progress has been made during the past several decades in the development of novel liquids. For example, recent studies show that the use of highly concentrated aqueous electrolytes could open new opportunities for the design of high-voltage aqueous lithium-ion batteries while also significantly improving battery safety. [12,13] For several energy storage and conversion systems, the understanding of structure, dynamics, and electronic properties of liquid electrolytes is essential for predicting and optimizing device performance. For example, desolvation of ions in sub-nanometer carbon electrodes is known to lead to improved capacitive performance. [14][15][16] Controlling transport of the lithium ion in organic electrolytes and at the interface with the graphite anode is key to the development of next-generation lithium ion batteries. [17][18][19][20] Tailoring the electronic structures of aqueous solutions for facilitated charge transfer at semiconductor/water interfaces in photoelectrochemical cells is critical for improving the efficiency of water-splitting reactions for hydrogen production. [21,22] Last but not least, understanding the electronic properties of liquid electrolytes is one of the prerequisites for manipulating the electrochemical stability of electrode-electrolyte interfaces in ion batteries and supercapacitors. [5,23] Structural, dynamical, and electronic properties of liquid electrolytes are often intertwined. For example, it is now widely accepted that the solvation structure of ions in liquid...

Quantifying the hydration structure of sodium and potassium ions: taking additional steps on Jacob's Ladder

“…A promising and increasingly widely used approach is to use molecular dynamics simulations with a full quantum mechanical density functional theory treatment of the system. 25 The dispersion corrected generalized gradient approximation (GGA) functionals are by far the most widely used in the context of condensed phase simulation due to their low computational demand. Of these functionals the revised Perdew, Burke, and Ernzerhof functional with Grimme dispersion correction (revPBE-D3) [26][27][28] has been demonstrated to reasonably accurately reproduce the structure of bulk water.…”

A Computational Comparison of Ether and Ester Electrolyte Stability on a Ca Metal Anode

“…Ca 2+ also has a very low redox potential relative to SHE-nearly that of Li + -a feature that allows theoretical CIBs to have higher volumetric (and, with certain cathodes, gravimetric) energy density than LIBs [5]. Although multivalent ions can suffer from high diffusion barriers due to their increased charge density, the large ionic radius of Ca offsets these issues [4,6].…”

Intercalation of Lithium into Graphite: Effects of Surface Chemical Composition from First-Principles Simulations

“…2 For example, Li + yields a stable tetrahedrally coordinated solvation shell in the ethylene carbonate (EC) solvent used in the majority of LIBs. [10][11][12][13][14][15][16][17] Since this solvation shell must be shed in order for Li + to intercalate into graphite, it is expected that the energy cost associated with ion desolvation from solvent molecules is one of the factors governing the kinetics of interfacial ion transport. Several experimental studies support this view; in particular, the activation energy of Li + intercalation into anodes was found to depend significantly on electrolyte composition, which in turn determines ion solvation.…”