Jan Prochazka

Polycrystalline TiO2 Anatase with a Large Proportion of Crystal Facets (001): Lithium Insertion Electrochemistry

The electrochemical behavior of TiO anatase with a predominant (001) face (ANA001) was studied by cyclic voltammetry of Li insertion and chronoamperometry. Both voltammetric and chronoamperometric diffusion coefficients and rate constants proved the higher activity of ANA001 toward Li insertion compared to that of a reference anatase material (C240) with dominating (101) facets. The enhanced activity of the anatase (001) face for Li insertion stems from synergic contributions of a faster interfacial charge transfer at this surface and a facile Li transport within a more open structure of the anatase lattice in the direction parallel to the c-axis. Despite the larger particle size of ANA001, the values of integral charge capacity and Li-insertion coefficient further confirmed its improved Li-insertion properties. The results of this study further complete the analogous data published on single-crystal anatase electrodes and evidence their validity for nanocrystalline materials too.

Li Insertion into Li[sub 4]Ti[sub 5]O[sub 12] (Spinel)

Li 4 Ti 5 O 1 2 (spinel) materials were prepared with Brunauer-Emmett-Teller surface areas ranging from 1.3 to 196 m 2 /g. The corresponding average particle sizes varied from ca. 1 μm to ca. 9 nm. Twenty-five different materials were tested as Li insertion hosts in thin-film electrodes (2-4 μm) made from a pure spinel. Trace amounts of anatase in Li 4 Ti 5 O 1 2 were conveniently determined by cyclic voltammetry of Li insertion. Electrodes from nanocrystalline Li 4 Ti 5 O 1 2 exhibited excellent activity towards Li insertions even at charging rates as high as 250C. The charge capability at 50-250C was proportional to the logarithm of surface area for coarse particles (surface areas smaller than ca. 20 m 2 /g). With increasing charge/discharge rates, a narrowing plateau in performance was observed for materials with surface areas between ca. 20 to 100 m 2 /g. These materials can be charged/discharged nearly to the nominal capacity of L1 4 Ti 5 O 1 2 (175 mAh/g) within a wide range of the rates. Very small particles (surface areas > 100 m 2 /g) exhibit a growing decrease of charge capability at 50-250C. The Li-diffusion coefficients, calculated from chronoamperometry, decrease by orders of magnitude if the average particle size drops from ca. I μm to ca. 9 nm. However, the sluggish Li + transport in small particles is compensated by the increase in active electrode area. Materials having surface areas larger than ca. 100 m 2 /g also tend to show increased charge irreversibility. This could be caused by parasitic cathodic reactions, due to enhanced adsorption of reducible impurities (humidity) or the quality of the spinel crystalline lattice itself. The optimum performance of thin-film Li 4 Ti 5 O 1 2 electrodes is achieved, if the parent materials have surface areas between ca. 20 to 110 m 2 /g, with the maximum peak at 100 m 2 /g.Reference LPI-ARTICLE-2003-015doi:10.1149/1.1581262View record in Web of Science Record created on 2006-02-21, modified on 2017-05-12

Li Insertion into Li4Ti5 O 12 (Spinel) Charge Capability vs. Particle Size in Thin-Film Electrodes

Li 4 Ti 5 O 1 2 (spinel) materials were prepared with Brunauer-Emmett-Teller surface areas ranging from 1.3 to 196 m 2 /g. The corresponding average particle sizes varied from ca. 1 μm to ca. 9 nm. Twenty-five different materials were tested as Li insertion hosts in thin-film electrodes (2-4 μm) made from a pure spinel. Trace amounts of anatase in Li 4 Ti 5 O 1 2 were conveniently determined by cyclic voltammetry of Li insertion. Electrodes from nanocrystalline Li 4 Ti 5 O 1 2 exhibited excellent activity towards Li insertions even at charging rates as high as 250C. The charge capability at 50-250C was proportional to the logarithm of surface area for coarse particles (surface areas smaller than ca. 20 m 2 /g). With increasing charge/discharge rates, a narrowing plateau in performance was observed for materials with surface areas between ca. 20 to 100 m 2 /g. These materials can be charged/discharged nearly to the nominal capacity of L1 4 Ti 5 O 1 2 (175 mAh/g) within a wide range of the rates. Very small particles (surface areas > 100 m 2 /g) exhibit a growing decrease of charge capability at 50-250C. The Li-diffusion coefficients, calculated from chronoamperometry, decrease by orders of magnitude if the average particle size drops from ca. I μm to ca. 9 nm. However, the sluggish Li + transport in small particles is compensated by the increase in active electrode area. Materials having surface areas larger than ca. 100 m 2 /g also tend to show increased charge irreversibility. This could be caused by parasitic cathodic reactions, due to enhanced adsorption of reducible impurities (humidity) or the quality of the spinel crystalline lattice itself. The optimum performance of thin-film Li 4 Ti 5 O 1 2 electrodes is achieved, if the parent materials have surface areas between ca. 20 to 110 m 2 /g, with the maximum peak at 100 m 2 /g.Reference LPI-ARTICLE-2003-015doi:10.1149/1.1581262View record in Web of Science Record created on 2006-02-21, modified on 2017-05-12

Organized Mesoporous TiO2 Films Stabilized by Phosphorus: Application for Dye-Sensitized Solar Cells

A synthetic protocol was developed for the preparation of thin mesoporous TiO2 films with enhanced thermal stability. This objective was achieved by a modification of the procedure of supramolecular templating by a Pluronic P-123 copolymer by an addition of a small amount of phosphoric acid to the precursor solution. Thin mesoporous TiO2 films up to 2.3 mu m thick were prepared via layer-by-layer deposition. The roughness factor exceeding 1300 was achieved on these films. After annealing at 540 degrees C, well-developed 5-6 nm anatase nanocrystals were present in the pore walls of a still perfect mesoporous structure. The pore coalescence and structure collapse during heat-treatment were effectively hindered by the presence of phosphorus. Our P-modified mesoporous TiO2 films were sensitized with the N-945 dye and used as photoanodes of the dye-sensitized solar cells. They reached the conversion efficiency of 5.03 or 5.05% for the film thickness of 1.8 or 2.3 mu m, respectively. Whereas the roughness factor scaled linearly with the number of layers, the solar conversion efficiency reached a constant value for films consisting of eight or more layers

Heterostructures from Single-Wall Carbon Nanotubes and TiO2 Nanocrystals

Single-wall carbon nanotubes (SWCNTs) have been coated electrochemically with TiO 2 nanocrystals prepared by electrolytic oxidation of aqueous TiCl 3 solution. The nanocrystals correspond to anatase phase with a small amount of monoclinic TiO 2 (B). Raman measurements of the SWCNT-TiO 2 heterostructure at 2.54, 2.41, and 1.91 eV laser excitation energies indicate changes of the vibrational modes of both TiO 2 (anatase) and SWCNT. The high-resolution transmission microscopy study accompanied by selected area electron diffraction and energy-dispersive X-ray analysis proves that the SWCNT bundles are covered with TiO 2 (anatase) nanocrystals of 8 nm approximate size. The Li-insertion electrochemistry of the SWCNT-TiO 2 heterostructure has been studied with the aim to evaluate its possible application in Li-ion batteries. Electrochemical Li + insertion into TiO 2 gives Li x TiO 2 , where x = 0.42 (corresponding to charge capacity of 507 C g -1 ) with 0.98 insertion/extraction charge ratio. Furthermore, this technique exclusively confirms the presence of trace amounts of monoclinic TiO 2 (B) in addition to anatase.

Li Insertion into Li4Ti5O12 (Spinel)

Li 4 Ti 5 O 1 2 (spinel) materials were prepared with Brunauer-Emmett-Teller surface areas ranging from 1.3 to 196 m 2 /g. The corresponding average particle sizes varied from ca. 1 μm to ca. 9 nm. Twenty-five different materials were tested as Li insertion hosts in thin-film electrodes (2-4 μm) made from a pure spinel. Trace amounts of anatase in Li 4 Ti 5 O 1 2 were conveniently determined by cyclic voltammetry of Li insertion. Electrodes from nanocrystalline Li 4 Ti 5 O 1 2 exhibited excellent activity towards Li insertions even at charging rates as high as 250C. The charge capability at 50-250C was proportional to the logarithm of surface area for coarse particles (surface areas smaller than ca. 20 m 2 /g). With increasing charge/discharge rates, a narrowing plateau in performance was observed for materials with surface areas between ca. 20 to 100 m 2 /g. These materials can be charged/discharged nearly to the nominal capacity of L1 4 Ti 5 O 1 2 (175 mAh/g) within a wide range of the rates. Very small particles (surface areas > 100 m 2 /g) exhibit a growing decrease of charge capability at 50-250C. The Li-diffusion coefficients, calculated from chronoamperometry, decrease by orders of magnitude if the average particle size drops from ca. I μm to ca. 9 nm. However, the sluggish Li + transport in small particles is compensated by the increase in active electrode area. Materials having surface areas larger than ca. 100 m 2 /g also tend to show increased charge irreversibility. This could be caused by parasitic cathodic reactions, due to enhanced adsorption of reducible impurities (humidity) or the quality of the spinel crystalline lattice itself. The optimum performance of thin-film Li 4 Ti 5 O 1 2 electrodes is achieved, if the parent materials have surface areas between ca. 20 to 110 m 2 /g, with the maximum peak at 100 m 2 /g.Reference LPI-ARTICLE-2003-015doi:10.1149/1.1581262View record in Web of Science Record created on 2006-02-21, modified on 2017-05-12