Further elevating the energy density of aqueous zinc-ion hybrid capacitors toward batteries through voltage-window-expansion engineering

Although increasing the voltage window should be more effective by square rewarding than the specific capacitance for enhancing the energy density of zinc-ion hybrid capacitors (ZIC), the lack of an ideal cathode has hindered its realization. Herein, Ti3C2Tx-PPy/Bi2S3 composite was fabricated through voltage-window-expansion engineering, where Ti3C2Tx layers were covered by a tremella-like network of PPy through hydrogen bonds (-NH···O- and -NH···F-), and then compounded with pseudocapacitive Bi2S3. Due to the redox reactions of Bi2S3 and PPy at low and high potentials, the working voltage window of AZIC-TPB//ZnSO4//Zn was elevated up to 2.1 V with low-cost electrolyte ZnSO4, exhibiting a high energy density comparable to that of batteries (269.09 Wh Kg−1 with a power density of 1564.73 W Kg−1). Even at a power density of 12947.15 W Kg−1, the energy density of AZIC kept as high as 107 Wh Kg−1, far exceeding the common level. Density functional theory (DFT) calculation demonstrated the obvious electron transfer between Ti3C2Tx and PPy (or Bi2S3) and advanced conductivity, which promoted the redox reaction and facilitate the charge transfer in the electrochemical process. The strategy of voltage-window-expansion engineering here can be further extended to aqueous energy storage devices, offering a viable path to enhancing energy density.