Development of Pseudocapacitive Behavior in Sodium-Ion Electrode Materials

Pseudocapacitive responses generally occur when reversible redox reactions take place at or near the surface of an electrode material and are sufficient rapid so that the material’s electrochemical features resemble those of a carbon-based electrochemical capacitor. Such behavior suggests the prospect of developing energy storage materials with the energy density of batteries and the power density and cycle life of electrical double-layer capacitors. Although a number of Li⁺ electrode materials exhibit pseudocapacitive properties, there have been only a few reports of pseudocapacitive Na⁺ electrode materials. This presentation reviews our recent findings in which non-crystalline TiO₂ and VO₂ display pseudocapacitive signatures. Both systems exhibit excellent Na⁺ capacities at relatively high rates when prepared as nanoscale materials. In the case of TiO₂, Na⁺ insertion induces the formation of amorphous layers of 3 to 5 nm thick in the initial anatase material. When anatase particles are reduced in size to 10 nm, the entire particle becomes amorphous and exhibits pseudocapacitive behavior as redox reactions occur throughout the particle. Specific capacities in excess of 200 mAh/g are achieved at rates of 3C. In comparison, disordered VO₂ occurs in the as-prepared material. Its linear voltage profile under galvanostatic conditions occurs over a wide range, from 1 to 4 V vs. Na/Na⁺, and resembles the response one obtains for an electrical double layer capacitor. The difference here, however, is that the specific capacitance is significantly greater, up to 400 mAh/g, because of redox reactions. Taken together, these results underscore the prospect that short range order may be an important consideration in the design of high performance. Na⁺ electrode materials.