Although recent efforts have expanded the stability window of aqueous electrolytes from 1.23 V to >3 V, intrinsically safe aqueous batteries still deliver lower energy densities (200 Wh/Kg) when compared with state-of-the-art Li-ion batteries (~400 Wh/Kg). The essential origin for this gap comes from the location of their cathodic stability limit, which situates way above the hydrogen evolution potential at pH~7 (2.62 V vs. Li), thus excluding the use of the most ideal anode materials (graphite, Li metal).
In this work, we resolved this “cathodic challenge” by adopting an “inhomogeneous additive” approach, in which a fluorinated additive immiscible with aqueous electrolyte can be applied on anode surfaces as an interphase precursor coating. The strong hydrophobicity of the precursor minimizes the competitive water reduction during interphase formation, while its own reductive decomposition forms a unique composite interphase consisting of both organic and inorganic fluorides. The effective protection from such an interphase allows these high capacity/low-potential anode materials (graphite, Li metal) to couple with different cathode materials, leading to 4.0 V aqueous LIBs with high efficiency and reversibility. This new class of aqueous LIBs is expected to deliver energy densities approaching those of non-aqueous LIBs, but with extreme safety, environmental-friendliness and even the possibility of adopting flexible and open cell configurations, none of which is available from non-aqueous LIBs.