The stability and reliability of electricity grids requires a continuous balance between energy supply and demand. Renewable generation, such as solar and wind, is intermittent, which causes sudden and unanticipated changes to power output and contributes to electricity grid instability. The aqueous lithium-ion battery (LIB) or sodium-ion battery (SIB) may solve both the safety problem associated with lithium-ion batteries that use highly toxic and flammable organic solvents, and the poor cycling life associated with commercialized aqueous rechargeable batteries including lead-acid and nickel-metal hydride systems. Aqueous LIB and SIB have been demonstrated to be one of the most promising stationary power sources for the sustainable energy such as wind and solar power.

As early as in 2005, we developed a new concept hybrid electrochemical supercapacitor in which a Li-ion intercalated compound was used as a positive electrode in combination with an activated carbon negative electrode in a Li₂SO₄ aqueous electrolyte (ZL. 200510025269.6). The negative electrode stores charge through a reversible nonfaradaic reaction of Li-ion on the surface of an activated carbon. The positive electrode utilizes a reversible faradic reaction of Li-ion insertion/extraction in the lithium-ion intercalated compounds. A hybrid cell consisting of a spinel LiMn₂O₄ positive and an AC negative electrode shows excellent reversibility with a sloping voltage profile from 0.8 to 1.8 V at an average voltage near 1.3 V, and delivers an 8 Wh/kg of 60000 F practical cell. In order to improve the specific energy, we employed LiTi₂(PO₄)₃, but the cell shows poorer cycling stability compared with the pure AC anode. We analyzed the stability of electrode materials in aqueous electrolytes extensively. We found that, in the presence of oxygen, the discharged state of lithium-ion intercalated compounds (LIC) of all negative electrode materials suitable for aqueous lithium-ion batteries reacts with water and O₂, with no dependence on the pH value of the electrolyte, which is mainly responsible for the capacity fading of aqueous lithium-ion batteries during charge/discharge cycling. By eliminating the O₂ (using a sealed cell), adjusting the pH values of the electrolyte, and using carbon-coated electrode materials; the LIB shows much cycling stability. A commercialized hybrid LIB shows a specific energy of ca. 30 Wh/kg, and exhibits a good cycling stability over 3500 cycles.

References:

1. Xia et al., TiP₂O₇ and Expanded Graphite Nanocomposite as Anode Material for Aqueous Lithium-Ion Batteries. ACS Appl. Mater. Interfaces, 9, 8075-8082 (2017).
2. Xia et al., Environmentally-friendly aqueous Li (or Na)-ion battery with fast electrode kinetics and super-long life, Adv., 2: e1501038 (2016).
3. Xia et al., Raising the Cycling Stability of Aqueous Lithium-Ion Batteries by Eliminating O₂ in the Electrolyte ”, Nat. Chem., 2, 760-765 (2010).
4. Xia et al., Hybrid aqueous energy storage cells using activated carbon and lithium-intercalated compounds I. The C/LiMn₂O₄ system, Electrochem. Soc., 153(2): A450-A454 (2006).