Influence of Doping, Surface Area, and Porosity of Nanostructure Electrodes in Lithium-Ion Batteries

Lithium-ion batteries store electrical energy in the form of chemical potential, the same as that in primary batteries; however, the charge-discharge process in lithium-ion batteries is more complex as it involves not only Faradaic reactions at the interface between electrode and electrolyte, but also mass and charge transport and volume change of electrodes that commonly possess low electrical conductivity.  One strategy for improving battery performance is the use electrodes away from thermodynamic equilibrium.  These include nanostructures with high surface energy, poor-crystalline materials, and materials with significant surface or bulk defects. Such materials are in a higher energy state and more readily undergo phase transformation and nucleation.  Their less closely packed structures enhance mass transport, lithium-ion accommodation, and tolerance to volume change.

As an example, Figure 1 shows cycling performance of V₂O₅ electrodes in pure form (a), after Sn doping (b), and in the nano-belt configuration (c) [1,2].  Both the pure and Sn-doped electrodes exhibit an initial drop in specific capacity followed by an activation period that is somewhat longer for the Sn-doped electrode than for the pure electrode.  The specific capacity of the pure electrode decays to about 180 mAh g^–1 within 50 cycles.  Notably, the Sn-doped electrode retains nearly its maximum capacity of 320 mAh g^–1 throughout 50 cycles.  Given the similar morphologies of these two electrodes, shown in Fig. 2(a) and (b), the change in capacity is most likely due to a chemical or electronic structure effect brought about by Sn-doping.

In contrast, the Na400 nano-belt electrode, prepared by reaction of V₂O₅ with H₂C₂O₄ and NaNO₃ and subsequent calcining to 400 °C, exhibits a steady yet slightly increasing capacity with cycling.  The capacities of the pure V₂O₅ electrode (a) and the nano-belt electrode (c) become similar at about 40 cycles.  This similarity may reflect changes in surface area of the pure electrode.  Initially, the nano-belt electrode has, due to its well-formed belt structure [Fig. 2(c)], lower surface area than does the pure electrode.  With cycling time the pure electrode eventually loses surface area, reaching a point similar to that of the nano-belt.

The extent of other factors, such as surface and bulk defects and volume change, are currently under investigation.  This presentation will consider V₂O₅ electrodes and a new lithium titanate electrode as two model materials to illustrate the influences of doping, surface defects and carbon coating, and nanostructures on the lithium-ion intercalation properties. 

1. Y. Li, J. Yao, E. Uchaker, M. Zhang, J. Tian, X. Liu, and G. Cao, J. Phys. Chem. C, 117, 23507 (2013).

2. S. Liang, T. Chen, A. Pan, D. Liu, Q. Zhu, and G. Cao, ACS Appl. Mater. Interfaces, 5, 11913 (2013).