Physics

Electrode

Organic chemistry

Graphite oxide

Halogen

Anode

Endocrinology

Engineering

Faraday efficiency

Intercalation (chemistry)

Physical chemistry

Power (physics)

Inorganic chemistry

Aqueous solution

Chemistry

Materials science

Lithium (medication)

Graphite

Alkyl

Cathode

Battery (electricity)

Quantum mechanics

Electrolyte

Chemical engineering

Electrochemistry

Medicine

Nature

The use of ‘water-in-salt’ electrolytes has considerably expanded the electrochemical window of aqueous lithium-ion batteries to 3 to 4 volts, making it possible to couple high-voltage cathodes with low-potential graphite anodes1–4. However, the limited lithium intercalation capacities (less than 200 milliampere-hours per gram) of typical transition-metal-oxide cathodes5,6 preclude higher energy densities. Partial7,8 or exclusive9 anionic redox reactions may achieve higher capacity, but at the expense of reversibility. Here we report a halogen conversion–intercalation chemistry in graphite that produces composite electrodes with a capacity of 243 milliampere-hours per gram (for the total weight of the electrode) at an average potential of 4.2 volts versus Li/Li+. Experimental characterization and modelling attribute this high specific capacity to a densely packed stage-I graphite intercalation compound, C3.5[Br0.5Cl0.5], which can form reversibly in water-in-bisalt electrolyte. By coupling this cathode with a passivated graphite anode, we create a 4-volt-class aqueous Li-ion full cell with an energy density of 460 watt-hours per kilogram of total composite electrode and about 100 per cent Coulombic efficiency. This anion conversion–intercalation mechanism combines the high energy densities of the conversion reactions, the excellent reversibility of the intercalation mechanism and the improved safety of aqueous batteries. Composite cathodes created by anionic redox reactions of bromine and chlorine intercalated into graphite, combined with water-in-salt electrolyte and graphite anodes, provide aqueous lithium-ion batteries with improved energy density.

Aqueous Li-ion battery enabled by halogen conversion–intercalation chemistry in graphite