Lithium iron sulphide as a positive electrode material for rechargeable lithium batteries

Abstract

Lithium iron sulphide has been investigated as a low-cost, high energy density and relatively safe positive electrode material for secondary lithium batteries. Lithium iron sulphide was synthesised, characterised and compared with natural pyrite samples and was shown to have a capacity of 350 mAh.g-1 upon cycling between 1.45 and 2.80 V vs. Li. The capacity was attributed to the Fe2+/Fe3+ redox couple at potentials up to 2.55 V, and oxidation of sulphur sites from Fe3+(S2-)2 to Fe3+S2-(S2)2-0.5 up to 2.80 V. The cycle life performance of lithium iron sulphide is poor when the cell is cycled between 1.45 and 2.80 V, with the cell loosing approximately 1.4 mAh.g-1 per cycle, although this performance is superior to comparable pyrite electrodes. Calcium doped samples of lithium iron sulphide were synthesised. Calcium doping was shown to impact upon lithium transport properties of the bulk lithium iron sulphide, improving the rate performance of the material. Improvements in cycle life performance of the calcium doped samples were offset by decreased specific capacity due to lithium substitution. The poor cycle life performance of lithium iron sulphide cells was attributed to the utilisation of the high voltage plateau corresponding to sulphur site oxidation/reduction. Experiments utilising a variety of negative electrode materials has identified the formation of soluble polysulphide species upon cycling of the cell, which reduce irreversibly at the negative electrode, contributing to active mass loss and poor cycle life performance. In-situ XRD studies have highlighted the structural decomposition that occurs upon utilisation of the sulphide, which results in irreversible amorphisation of the lithium iron sulphide crystal structure. Lithium iron sulphide was treated via coating with lithium boron oxide glass and a novel carbon coating method via thermal decomposition of butyl-methyl-pyrrolydinium-dicyanimide. Both treatments were shown to increase the cycle life performance of lithium iron sulphide, due to decreased dissolution of polysulphide upon cycling. The choice of binder, electrode formulation and electrolyte was also shown to impact upon the cycle life performance of lithium iron sulphide cells.

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ORCID for John R. Owen: orcid.org/0000-0002-4938-3693

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