Ultrathin Carbon Deficient Molybdenum Carbide (α-MoC_1-) Enables High-Rate Mg-Ion-based Energy Storage

Dual-electron transfer with Mg²⁺-ion intercalation outperforms typical alkali metal-ion (Li⁺, Na⁺, K⁺) systems with superior charge storage efficiency while the neutral electrolytes can achieve a working voltage beyond the hydrolysis window of 1.23 V. Hence, aqueous Mg-ion electrolytes are promising for electrochemical energy storage devices to boost the energy density and solve the safety challenges synchronously. However, the Mg-based electrochemical energy storage (EES) devices are generally confined by poor rate performance due to the slow Mg²⁺ diffusion in the electrode materials. In this paper, we demonstrate that carbon-deficient carbide could function as a promising electrode material in Mg²⁺-ion-based EES. An electrode made of such carbide can operate over an extended window up to 2.4 V in 1 M magnesium acetate, showing superior performance of high capacitance (125.2 F/g), high energy density (25.1 Wh/kg), and high power density (3934.8 W/kg). Ab initio simulation reveals migration energy of Mg²⁺ being lower than that of Li⁺ diffusing from one carbon defect to another in the α-MoC_1- lattice, supporting the experimental results that a symmetric supercapacitor made of α-MoC_1- in an electrolyte based on Mg²⁺ outperforms electrolytes based on Li⁺.