A Novel High-Power Battery-Pseudocapacitor Hybrid Based on Fast Lithium Reactions in Silicon Anode and Titanium Dioxide Cathode Coated on Vertically Aligned Carbon Nanofibers

Abstract

An electrochemical cell representing a battery-supercapacitor hybrid is demonstrated with a Si anode and a TiO2 cathode based on Lithium chemistry. Both materials are fabricated as coaxial shells with an oblique nanocolumnar structure anchored on vertical aligned carbon nanofiber arrays. The Li+ ion transport and electrical connection is greatly enhanced with such nanoporous core-shell architectures, leading to optimal Li storage properties. The full theoretical capacity of the shell materials has been obtained at normal C-rates (C/1 to C/2) for Si (∼3,000 to 3500mAhg−1) and TiO2 (∼170mAhg−1) half-cells, respectively, with excellent cycling stability. More importantly, much higher rates (up to 4.7CSi for Si and 76CTiO2 for TiO2) can be applied at relatively small capacity loss, approaching the properties of supercapacitors. The charge-discharge profiles show battery-supercapacitor hybrid features, which are attributed to the short Li+ diffusion path across the solid materials and the large pseudocapacitive contribution from fast surface reactions. A full cell containing similar volume of Si and TiO2 shows a high specific energy (103Whkg−1) at low current rates, comparable to a decent battery, and a remarkable specific power (56,000Wkg−1) at high current rates, matching the state-of-the-art supercapacitors.