D. Samuelis, J.-Y. Shin, J. Maier
A hierarchical nanoporous TiO2 (anatase) with sufficiently high surface area is synthesized and its remarkable electrode performance is studied . The material is made of extremely small particles with diameters of only around 8 nm. Since nanopowders of such small particles typically cannot be packed very efficiently (volumetric energy density is a key performance parameter for nowadays batteries), it is important for battery application that the particles form larger agglomerates, as shown here in Figure 1a. Still, there is enough void space in the fairly dense agglomerates so that the liquid electrolyte can ionically contact each primary particle. Hence, only the last 4 nm to the middle of the primary particle have to be bridged by slow solid state chemical diffusion of lithium. The battery performance of this anatase material is exceptionally good, even at charge/discharge rates of 60 C (full charge/discharge in one minute), capacities of more than 50 mAh g-1 can be achieved (Figure 1b).
Electrode materials with such high surface area are relevant in conjunction with capacitive interfacial storage mechanism . For electrodes made from this anatase material, we were able to show that cycling at high rates furthermore increases the proportion of interfacial storage and to a certain extent manages to suppress sluggish bulk lithiation, which leads to material degradation for Li contents above Li0.5TiO2.