Insight into the intercalation mechanism of WSe2 onions toward metal ion capacitors: sodium rivals lithium

Abstract

WSe₂ onions were synthesized using a homemade chemical vapor deposition (CVD) method. The formation of a very fine particle size and spherical microstructure was attributed to the fast cooling rate and reduction of the surface energy of two-dimensional (2D) WSe₂ after nucleation and growth during the synthesis process. A specific capacity of 205 mA h g⁻¹ at 10 A g⁻¹vs. sodium ions can be reached, representing the best sodium ion rate capability ever reported for WSe₂. An exceptionally high capacitive contribution showed that fast intercalation kinetics were present, which were attributed to the intrinsic properties and the unique microstructure of WSe₂. Sodium ions could be stored faster than lithium ions throughout WSe₂ due to the different intercalation mechanisms, as concluded from electrochemical investigations and simulation. The sodium ion capacitors (SICs) delivered superior energy/power densities (the maximum energy density is 123.1 W h kg⁻¹ and the maximum power density is 14.1 kW kg⁻¹), demonstrating that WSe₂ is a promising alternative for SICs that could rival lithium ion capacitors (LICs) and shedding new light on novel 2D material design of SICs with high energy/power densities.