Li insertion into Li4Ti5O12 p(Spinel) -: Charge capability vs. particle size in thin-film electrodes

Li4Ti5O12 (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 mum to ca. 9 nm. Twenty-five different materials were tested as Li insertion hosts in thin-film electrodes (2-4 mum) made from a pure spinel. Trace amounts of anatase in Li4Ti5O12 were conveniently determined by cyclic voltammetry of Li insertion. Electrodes from nanocrystalline Li4Ti5O12 exhibited excellent activity towards Li insertion, 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 Li4Ti5O12 (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. 1 mm 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 Li4Ti5O12 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. (C) 2003 The Electrochemical Society.