MXene-based symmetric supercapacitors with high voltage and high energy density

MXene-based aqueous symmetric supercapacitors (SSCs) are attractive due to their good rate performances and green nature. However, it remains a challenge to reach voltages much over 1.2 V, which significantly diminishes their energy density. Herein, we report on Mo_1.33CT_z MXene-based SSCs possessing high voltages in a 19.5 M LiCl electrolyte. Benefiting from the vacancy-rich structure and high stable potential window of Mo_1.33CT_z, the obtained SSCs deliver a maximum energy density of >38.2 mWh cm⁻³ at a power density of 196.6 mW cm⁻³ under an operating voltage of 1.4 V, along with excellent rate performance and impressive cycling stability. This highly concentrated LiCl electrolyte is also applicable to Ti₃C₂T_z, the most widely studied MXene, achieving a maximum energy density of >41.3 mWh cm⁻³ at a power density of 165.2 mW cm⁻³ with an operating voltage of 1.8 V. The drop in energy density with increasing power in the Ti₃C₂T_z cells was steeper than for the Mo-based cells. This work provides a roadmap to develop superior SSCs with high voltages and high energy densities.