The promising features of charge storage within a niobium oxide
While intercalation pseudocapacitance is a fast and high level charge storage mechanism, it often suffers from some diffusion limitations – ion transport – that limit their charge storage capability. Charge storage is also often restricted to the surface or near-surface, limiting its use to thin electrodes. In this work, nano-sized Nb2O5 material with fast ion diffusion in the bulk of the particle was prepared. Thick electrodes (up to 40 µm thick) prepared with T-Nb2O5 offer the promise of exploiting intercalation pseudocapacitance to obtain high-rate charge-storage devices.
Following publications in 2010 and 2012 that reported a pseudocapacitive behavior when lithium ions are electrochemically intercalated in a niobium oxide (an allotropic form of the niobium pentoxide: T-Nb2O5), a new study has been carried out on this material by a French-American team. Led by Pr. Bruce Dunn (University of California, USA) and Pr. Patrice Simon (Associate Director of the RS2E, University of Toulouse, France), the research project was a first-time collaboration between teams from three universities (California, Cornell and Toulouse).
To begin with, scientists performed a cyclic voltammetry (briefly, they applied a varying potential to the electrode and measured the resulting currents) on a T-Nb2O5 microelectrode of 30 µm width. They found that the reaction kinetics – the speed at which charges move and are stored – are fast for charging times as quick as 60 seconds; thus kinetics appear far less limited than in other intercalation materials.
To determine if these improvements are restricted to thin electrodes, electrochemical properties of thick films (40 µm) were benchmarked with a high power LTO (Li4Ti5O12) Li-ion battery anode. While initially higher (175 mAh/g vs.140 mAh/g), the gravimetric capacity of LTO fades and collapses for discharge rates beyond 60C (see figure below, click to zoom-in). In contrast, T-Nb2O5 electrodes were able to keep 40% of their capacity at 1,000C (fig. 1). It is then clear that T-Nb2O5 shows a significant improvement over conventional electrode materials at very high discharge rates.
The teams also investigated the way charges are stored within T-Nb2O5 through a crystallographic analysis (see figure below, click to zoom-in). It appears that the storage takes place in the bulk of the material, through natural tunnels allowing fast ionic transport (fig. 2). It also confirmed that the reaction between lithium ions and T-Nb2O5 is of redox nature.
While the next step in the study will be to examine the behavior of T-Nb2O5 through cycles of charges/discharges in hybrid supercapacitor systems, these results already offer the promise of exploiting intercalation pseudocapacitance in other materials to obtain high-rate charge-storage devices.
High-rate electrochemical energy storage through Li+ intercalation pseudocapacitance. Veronica Augustyn, Jérémy Come, Michael A. Lowe, Jong Woung Kim, Pierre-Louis Taberna, Sarah H. Tolbert, Hector D. Abruña, Patrice Simon & Bruce Dunn.
Nature Materials, Advanced Online Publication, 14 avril 2013, DOI: 10.1038/NMAT3601.