A team of researchers led by a group from the University of Maryland has developed a halogen conversion–intercalation chemistry in graphite that produces composite electrodes with a capacity of 243 mAh g^-1 (for the total weight of the electrode) at an average potential of 4.2 volts versus Li/Li⁺. Combining this cathode with a passivated graphite anode, the team created a 4V-class aqueous Li-ion full cell with an energy density of 460 Wh kg^-1 of total composite electrode and about 100% Coulombic efficiency.

The cell is based on an anion conversion–intercalation mechanism that combines the high energy densities of conversion reactions, the excellent reversibility of intercalation and the improved safety of aqueous batteries. A paper on their work appears in the journal Nature.

Proposed conversion–intercalation chemistry. Schematic of the conversion–intercalation mechanism occurring in the LBC-G composite during its oxidation in WiBS aqueous-gel electrolyte. The two-stage reactions involve the oxidation of Br⁻ (about 4.0 V) and Cl⁻ (about 4.2 V) and their subsequent intercalation into the graphitic structure. The discharge is a complete reversal of the charge process. Yang et al.

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 anodes. However, the limited lithium intercalation capacities (less than 200 milliampere-hours per gram) of typical transition-metal-oxide cathodes preclude higher energy densities. Partial or exclusive anionic redox reactions may achieve higher capacity, but at the expense of reversibility.

… Using the anionic-redox reaction of halide anions (Br⁻ and Cl⁻) in graphite, a composite electrode containing equimolar lithium halide salts (LiBr)_0.5(LiCl)_0.5–graphite (hereafter denoted as LBC-G) was synthesized by mixing anhydrous LiBr and LiCl with graphite at an optimal mass ratio of 2:1:2 (corresponding to a molar ratio of (LiBr)_0.5(LiCl)_0.5C_~3.7). Herein, the highly concentrated water-in-bisalt (WIBS) electrolyte confined partially hydrated LiBr/LiCl within the solid cathode matrix, and upon oxidation, Br⁰ and Cl⁰ are stabilized by sequential intercalation into the graphite host as solid graphite intercalation compounds (GICs).

This new cathode chemistry inherits the high energy of the conversion reaction and the excellent reversibility of topotactic intercalation, and provides batteries that differ fundamentally from the ‘dual-ion’ batteries that reversibly intercalate complex anions (PF₆⁻, BF₄⁻, TFSI⁻; TFSI, bis(trifluoromethanesulfonyl)imide) into graphite at low packing density, where these stable anions experience no redox reactions, resulting in low capacities below 120 mAh g⁻¹.

—Yang et al.

Actual (red star) energy density of the LBC-G full cells (with LiBr/LiCl monohydrates), compared with various state-of-the-art commercial and experimental Li-ion chemistries using both non-aqueous (blue circles) and aqueous (green circles) electrolytes. For comparison, all energy densities were converted using the total weight of the positive and negative electrodes (not counting the electrolyte and cell packaging). Yang et al.

At 460 Wh kg^-1, the energy density of the full cell is greater than that of state-of-the-art non-aqueous LIBs. After considering the electrolyte mass, the full-cell energy density still reaches 304 Wh kg⁻¹.

Resources

• Chongyin Yang, Ji Chen, Xiao Ji, Travis P. Pollard, Xujie Lü, Cheng-Jun Sun, Singyuk Hou, Qi Liu, Cunming Liu, Tingting Qing, Yingqi Wang, Oleg Borodin, Yang Ren, Kang Xu & Chunsheng Wang (2019) “Aqueous Li-ion battery enabled by halogen conversion–intercalation chemistry in graphite” Nature volume 569, pages 245–250 doi: 10.1038/s41586-019-1175-6