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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 Mo1.33CTz 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 Mo1.33CTz, 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 Ti3C2Tz, 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 Ti3C2Tz 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.