Title:	MD simulations of Li-battery electrolytes
DNr:	SNIC 2014/1-21
Project Type:	SNIC Medium Compute
Principal Investigator:	Daniel Brandell <Daniel.Brandell@kemi.uu.se>
Affiliation:	Uppsala universitet
Duration:	2014-02-28 – 2015-01-01
Classification:	10403 10406 10402
Keywords:

Abstract

Due to their high energy and power density, Li-ion batteries are envisioned to provide future energy storage solutions for of major importance for the global energy system. This is true not least for vehicles, where portable power is necessary. One of the most crucial components in the battery is the electrolyte, which constitutes the medium for ionic transport within the battery cell. Many possible electrolytes exist – ceramics, glasses, liquids, polymers, gel, ionic liquids, etc, all with different advantages and disadvantages. Generally, those providing fast ionic transport are often reactive, leading to loss of material and harmful side-reactions. The focus of the current study is therefore on exploring electrolyte materials with more appealing properties; not least polymer electrolytes and ionic liquids. Classical molecular dynamics simulations could be considered an ideal methodology for studying these transport processes in greater detail – the system size is appropriate, and the ionic transport can be accurately described using force field approximations. We are here focusing on different task, where novel and highly promising experimental work could benefit from insights from computational chemistry: (1) Ionic transport in bipolymeric or co-polymeric solid polymer electrolytes with very high Li transport numbers. Generally, the cation transport number Li electrolytes are in the range 0.1-0.2, at best. However, by covalently attaching the anion to a polymer backbone immobilizes the anion, leading to transport numbers close to 1. By mixing these polymers with poly(ethylene oxide) in polymer mixtures or by synthesizing block-copolymers, a flexible material with good ionic dissolution can be created. However, there exists many questions to answer regarding the nano-scale structure and its connection to the dynamic properties. It also needs to be tested computationally over an extended temperature range, since its operation between RT and 100 C is crucial for the applications. (2) Ionic liquid (room temperature molten salts)/polymer gel mixtures are highly promising, combining the mechanical robustness of the polymer with an inert solvent with low vapor pressure. The polymer is generally in-situ cross-linked, making it difficult to obtain detailed experimental information about its structure, and thereby how the ionic liquid penetrates the material. Finally, it is very much unclear how the polymer and ionic liquid contributes to the molecular-level transport processes. (3) Functionalization of electrode/electrolyte interfaces by self-assembly mono-layers. By adding silica groups onto electrode surfaces, an increased interaction with the electrolyte can be made feasible. However, the structural and dynamical properties of such layers are yet unknown, but could well be explored by MD simulations.