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

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.