Nitrate-based ‘oversaturated gel electrolyte’ for high-voltage and high-stability aqueous lithium batteries

The energy density of an aqueous rechargeable battery is closely related to the working voltage which is limited by electrochemcial stability windows of aquoese electrolytes. In this study, an ‘oversaturated gel electrolyte’ (OSGE) is simply prepared with poly(vinyl alcohol) and cost-effective lithium nitrate saturated at 95 ºC. The excess salt is then crystallized at room temperature, which is dispersed equally by the continuous room-temperature saturated gel, forming the heterogeneous morphology. The electrochemical stability window is further extended through the transition of interaction between lithium ion and water molecular, solidifying the whole electrolyte, while the high ionic conductivity is basically retained. The electrochemical stability window is 3.2 V (-1.25 to 1.95 V vs. Ag/AgCl) for LiNO₃ OSGE with an ionic conductivity of 2.51×10⁻² S•cm⁻¹. To explain the wider electrochemical stability window of OSGE, Li ion solvation sheath is built through molecular dynamics (MD) simulation, of which OSGE owns less coordination number between Li ion and water molecular with a shortened distance between Li ion and NO₃⁻, which is consistent with experimental results. A coin cell with working voltage of 2.5 V has been assembled with VO₂ nanobelt anode, and LiNi_0.5Mn_1.5O₄ hollow sphere cathode. The LiNi_0.5Mn_1.5O₄ cathode was stabilized by LiNO₃ OSGE for 700 cycles with almost 99% coulombic efficiency and the highest energy density of 175.9 Wh•kg⁻¹ (based on the active matter of both anode and cathode). Moreover, the stability window of OSGE is still wide enough at elevated temperatures even at 80 ºC, ensuring the desired stability of the VO₂/LiMn₂O₄ battery with 2.0 V working voltage cycling at higher temperatures. Meanwhile, the universality and practicability are demonstrated by employing layered cathodes and thick electrodes in the OSGE-based system. This work provides a new route for utilizing the inexpensive inorganic salts with relatively low solubility in water into super-concentrated electrolyte systems via developing nitrated-based OSGEs for aqueous batteries with high voltage, high energy density and good stability.