High-Capacitance Pseudocapacitors from Li⁺ Ion Intercalation in Nonporous, Electrically Conductive 2D Coordination Polymers

Electrochemical capacitors (ECs) have emerged as reliable and fast-charging electrochemical energy storage devices that offer high power densities. Their use is still limited, nevertheless, by their relatively low energy density. Because high specific surface area and electrical conductivity are widely seen as key metrics for improving the energy density and overall performance of ECs, materials that have excellent electrical conductivities but are otherwise nonporous, such as coordination polymers (CPs), are often overlooked. Here, we report a new nonporous CP, Ni₃(benzenehexathiolate) (Ni₃BHT), which exhibits high electrical conductivity of over 500 S/m. When used as an electrode, Ni₃BHT delivers excellent specific capacitances of 245 F/g and 426 F/cm³ in nonaqueous electrolytes. Structural and electrochemical studies relate the favorable performance to pseudocapacitive intercalation of Li⁺ ions between the 2D layers of Ni₃BHT, a charge-storage mechanism that has thus far been documented only in inorganic materials such as TiO₂, Nb₂O₅, and MXenes. This first demonstration of pseudocapacitive ion intercalation in nonporous CPs, a class of materials comprising thousands of members with distinct structures and compositions, provides important motivation for exploring this vast family of materials for nontraditional, high-energy pseudocapacitors.