Interfacial Li-storage phenomena in nanoporous anatase TiO2 electrodes

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 [1]. 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 [2]. 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.

Figure 1: (a) TEM and FESEM images of the calcined microporous TiO2 anode material. The SAED pattern in the TEM micrograph clearly confirms pure anatase phase. Primary particle size is below 10 nm. The HRTEM micrograph shows the agglomerates formed from the particles, with an average size of  ~100 nm. (b ) battery performance data: charge-discharge curves and rate capability of the calcined anatase material, and for comparison rate capabilities of the non-calcined anatase and of commercial 10 nm anatase particles. Reprinted with permission from [1]. Copyright 2011 WILEY-VCH Verlag GmbH & Co. KGaA.

Figure 2: Charge/discharge capacities versus number of cycles for the calcined np-TiO2 (dis)charged directly at a rate a) 1C (charge/discharge to theoretical capacity within 1 h), and b) 5C (charge/discharge to theoretical capacity within 12 min). c) Charge/discharge capacities when the material (dis)charged with rates ascending stepwise from C/5 to 60C.
Reprinted with permission from [1]. Copyright 2011 WILEY-VCH Verlag GmbH & Co. KGaA.

Publications:

  1. J.-Y. Shin, D. Samuelis, J. Maier Sustained lithium storage performance of hierarchical nanoporous anatase TiO2 at high rates: emphasis on interfacial storage phenomena, Adv. Func. Mater. 2011, 3464 ,DOI: 10.1002/adfm.201002527
  2. J. Maier, Angew. Chemie Int. Ed. 2013,DOI: 10.1002/anie.201205569
  3. D. Samuelis, J.-Y. Shin, J.Maier, MPI-FKF annual report 2012, p.76
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