New halogen conversion intercalation chemistry used to create high-energy lithium-ion water-based batteries

by:CTECHi     2021-08-05

According to foreign media reports, a team of researchers from the University of Maryland introduced halogen conversion intercalation chemistry into graphite for the first time, and innovatively developed a composite electrode with a capacity of 243 mAh/g (in terms of total electrode weight) and an average potential of 4.2 V, vs Li/Li+. The team members combined this cathode with a passivated graphite anode to create a lithium-ion water-based full battery that can reach 4V, with an energy density of 460 Wh/kg and a coulombic efficiency of about 100%.

More and more researchers are beginning to use 'water-in-salt' electrolytes, which can greatly expand the electrochemical window of aqueous lithium-ion batteries to 3-4 volts , It is possible to couple the high-voltage cathode with the low-potential graphite anode. However, due to the limited lithium insertion capacity of typical transition metal oxide cathodes, less than 200 mA h/g, higher energy density cannot be obtained. The redox reaction of part or all of the anions can increase the capacity, but it is not reversible.

The battery is based on anion conversion-intercalation mechanism, combined with a high energy density conversion reaction, has excellent intercalation reversibility, and improves the safety of water-based batteries. The new cathode chemical method has the high energy of the conversion reaction and the excellent reversibility of topologically directed insertion, so it is called the conversion-intercalation chemical mechanism. Through the oxidation-reduction reaction of halogen anions (Br? and Cl?) in graphite, anhydrous LiBr, LiCl and graphite are mixed in an optimal mass ratio of 2:1:2 to synthesize an equimolar lithium halide salt (LiBr) 0.5 (LiCl) 0.5-graphite (hereinafter referred to as LBC-G) composite electrode. Among them, the high-concentration 'water-in-bisalt' (WIBS) electrolyte limits part of the hydrated LiBr/LiCl in the solid cathode matrix. After oxidation, Br0 and Cl0 are sequentially inserted into the graphite matrix and stabilized as solid graphite intercalation compounds (GICs).

This kind of battery is fundamentally different from the 'dual ion' battery. The dual-ion battery inserts complex anions (PF6?, BF4? and TFSI? etc.) into graphite reversibly at low packing density. These stable anions do not undergo redox reactions, resulting in a capacity of less than 120mAh/g. The energy density of the LBC-G full battery is about 460 Wh/kg, which exceeds the most advanced non-aqueous liquid lithium-ion battery. Taking into account the quality of the electrolyte, its energy density can still reach 304 Wh/kg.

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