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Abstract
Sulphur, boron and phosphorous containing electrolyte additives were evaluated in cells containing pristine electrodes from a commercial EV lithium ion cell against a standard baseline electrolyte. Following formation and a full cell ageing step, cycling performance and impedance spectroscopy were used to elucidate the most effective additives. The additive tris trimethyl silyl phosphite (TTSPi) showed the most promise; with improved cell capacities and reduced impedances observed after formation. X-ray photoelectron spectroscopy (XPS) measurements on anode elemental surface profiles were correlated with the electrochemical performance. It was observed that increased lithium fluoride content on the surface of the anodes typically produced cells with lower impedance. Sulphur containing additives also showed improved cell behaviours; and the decomposition and chemical reactions of these compounds at the anode surface is discussed in detail. The main influence of TTSPi was to reduce the amount of oxygen (C=O) and sulphur in the electrolyte interphase (SEI) layer; to be replaced with hydrocarbons.
Highlights
Improved battery performance is still required for the large scale uptake of electric vehicles (EV), and in the emerging energy storage market
The decomposition voltages of the electrolyte additives were investigated in half cells, with lithium metal counter electrodes
There were prominent peaks with LiDFOB (1.6 V vs. Li/Li+), TFMB (1.4 V vs. Li/Li+), PES (1.2 V vs. Li/Li+), and several others at 1.1 V vs. Li/Li+. All these voltages were in the solid electrolyte interphase (SEI) formation range, and at a higher voltage than the control electrolyte (0.7 V vs. Li/Li+)
Summary
Improved battery performance is still required for the large scale uptake of electric vehicles (EV), and in the emerging energy storage market. Lithium ion batteries have the most to offer in terms of power, energy and lifetime, and costs have reduced dramatically over the last five years [1]. One of the key components of lithium ion batteries is the electrolyte, which is traditionally comprised of a lithium salt dissolved in a mixture of organic solvents [2,3,4]. The solvent is composed of two (or more) organic carbonates, often one cyclic and one linear. This gives an electrolyte with relatively high conductivity, and a wide operating temperature range. During the initial (formation) charge, the solvent and salt components of the electrolyte are reduced on the anode to produce a layer called the solid electrolyte interphase (SEI). The SEI protects the lithiated graphite from reacting with the electrolytes, and by stabilising the SEI, reduces the risk of thermal runaway [7,8,9]
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