Artificial solid electrolyte interphase for aqueous lithium energy storage systems

Aqueous lithium energy storage systems address environmental sustainability and safety issues. However, significant capacity fading after repeated cycles of charge-discharge and during float charge limit their practical application compared to their nonaqueous counterparts. We introduce an artificial solid electrolyte interphase (SEI) to the aqueous systems and report the use of graphene films as an artificial SEI (G-SEI) that substantially enhance the overall performance of an aqueous lithium battery and a supercapacitor. The thickness (1 to 50 nm) and the surface area (1 cm² to 1 m²) of the G-SEI are precisely controlled on the LiMn₂O₄-based cathode using the Langmuir trough–based techniques. The aqueous battery with a 10-nm-thick G-SEI exhibits a discharge capacity as high as 104 mA·hour g^–1 after 600 cycles and a float charge current density as low as 1.03 mA g^–1 after 1 day, 26% higher (74 mA·hour g^–1) and 54% lower (1.88 mA g^–1) than the battery without the G-SEI, respectively. We propose that the G-SEI on the cathode surface simultaneously suppress the structural distortion of the LiMn₂O₄ (the Jahn-Teller distortion) and the oxidation of conductive carbon through controlled diffusion of Li⁺ and restricted permeation of gases (O₂ and CO_x), respectively. The G-SEI on both small (~1 cm² in 1.15 mA·hour cell) and large (~9 cm² in 7 mA·hour cell) cathodes exhibit similar property enhancement, demonstrating excellent potential for scale-up and manufacturing.