Aqueous supercapacitors are attracting increasing attention owing to their high power density, cyclability and environmental friendliness. However, thermodynamic limit of water electrolysis limits the cell voltage and window of common aqueous supercapacitors, restricting the energy density. We have previously demonstrated that advanced hybrid supercapacitors that employ aqueous electrolyte with a multi-layered water-stable protected negative electrode consisting of laminated metallic lithium or lithiated graphite, polymer electrolyte and water-stable lithium ion conducting solid electrolyte Li_1+x+yTi_2-xAl_xSi_yP_3-yO₁₂ (LTAP), can extend the negative electrode potential close to 0 V vs Li/Li⁺, thus providing a cell voltage close to 4 V with aqueous catholytes. In this work this hybrid supercapacitor concept is further extended to high concentration electrolytes or “water-in-salt (WIS)” electrolytes. We will demonstrate that 4 V-class cell voltage with wide voltage window can be delivered for advanced hybrid supercapacitors with capacitive or pesudocapacitive positive electrodes in WIS electrolytes combined with a protected Li anode. A typical advanced hybrid supercapacitor with MnO₂ electrode shows a 4.4 V maximum cell voltage with a 1.5 V window, an energy density of 405 Wh/(kg-MnO₂) at a power density of 0.88 kW kg/(kg-MnO₂). A high energy density of 163 Wh kg/(kg-MnO₂) is maintained at a power density of 16.7 kW kg/(kg-MnO₂). These advanced hybrid supercapacitors show acceptable cycle stability and good energy retentions (around 90% within 3000 cycles).

Fig. Galvanostatic charge/discharge curves for advanced hybrid supercapacitor with MnO₂ in 21m LiTFSI and protected lithium negative electrode at (a) a current density of 0.1 mA/cm² and U_cell varized from 1.0 to 1.5 V (U_min = 2.9 V and U_max = 3.9→4.4 V) and (b) a fixed U_cell = U_max – U_min = 4.4 – 2.9 V = 1.5 V) and various current densities (Left axis: cell voltage; Right axis: electrode potentials of MnO₂ electrode and multi-layered water-stable protected lithium anode). (c) Specific capacity (1) and capacitance (2) extracted from the discharge curves of MnO₂ electrode from advanced hybrid supercapacitor with MnO₂ in 1 M Li₂SO₄ at U_cell = U_max – U_min = 4.3 – 3.3 V = 1.0 V. Specific capacity (3) and capacitance (4) extracted from the discharge curves of MnO₂ electrode from advanced hybrid supercapacitor with MnO₂ in 21 m LiTFSI at the U_cell = U_max – U_min = 4.4 – 2.9 V = 1.5 V.