A V$_4$C$_3$T$_z$//K$_{0.04}$MnO$_2$ Supercapacitor Cell in a 5 M LiNO$_3$ Electrolyte

MXenes are excellent candidates for supercapacitor applications in aqueous electrolytes. Due to their two-dimensionality, which results in a high packing density, MXenes deliver a high energy per unit volume. However, as most MXenes perform in negative electrochemical windows, the challenge in building a full cell device lies on finding a suitable positive electrode that matches the high performance per unit volume of the MXene while being stable in the same electrolyte and enabling a suitable enhancement of the voltage range of the resulting cell. In the case of aqueous electrolytes, activated carbon electrodes have been traditionally used as the positive electrode counterpart of MXenes. However, its low density undermines the performance per unit volume. In this work, we build a full cell using a V$_4$C$_3$T$_z$ MXene as negative electrode and cryptomelane-type K$_{0.04}$MnO$_2$ as positive electrode and evaluate it in a 5 M LiNO$_3$ electrolyte. The full cell performed in a 1.8 V voltage range and showed a superior volumetric energy as compared to a V$_4$C$_3$T$_z$//activated carbon cell in a 3 M H$_2$SO$_4$ electrolyte at low volumetric power (previously reported by us). The V$_4$C$_3$T$_z$//K$_{0.04}$MnO$_2$ cell also outperformed in volumetric energy other previously reported Ti$_3$C$_2$T$_z$//MnO$_2$ and Ti$_3$C$_2$T$_z$//activated carbon cells evaluated in a water-in-salt electrolyte at a medium-to-high power per volume range. An aqueous and neutral electrolyte at standard concentration, such as 5 M LiNO$_3$, brings environmental advantages versus acidic electrolytes and circumvents power performance problems brought in by highly concentrated electrolytes.