One-step Preparation of Nanoarchitectured TiO2 on Porous Al as Integrated Anode for High-performance Lithium-ion Batteries

Titanium dioxide (TiO2) is an attractive anode material for energy storage devices due to its low-volume-change and high safety. However, TiO2 anodes usually suffer from poor electrical and ionic conductivity, thus causing dramatic degradation of electrochemical performance at rapid charge/discharge rates, which has hindered its use in energy storage devices. Here, we present a novel strategy to address this main obstacle via using nanoarchitectured TiO2 anode consisting of mesoporous TiO2 wrapped in carbon on a tunnel-like etched aluminum substrate prepared by a simple one-step approach. As a result of this nanoarchitecture arrangement, the anode exhibits excellent rate performance and superior cyclability. A rate up to 100 C is achieved with a high specific capacity of about 95 mA h g−1, and without apparent decay after 8,000 cycles.

. TG-DSC curves of Al (a) , TO/Al (b) , and TO gel (c) measured from 50 to 600 o C in air flow with a heating rate of 10 o C min -1 .
The as-deposited TiO 2 film is amorphous and contains a huge amount of organic groups and residues. Therefore, a thermal treatment is employed to remove these organic matters and improve the crystallinity. The heat-treatment process is investigated via thermogravimetry-differential scanning calorimetry (TG-DSC) and the results are shown in Figure S2. For the blank Al substrate ( Figure S2a), its weight hardly changes until 550 o C, after which, caused by the oxidation of Al, the weight increases sharply and a vast heat releases. While, for the TiO 2 gel, there are one endothermic and three exothermic peaks located at 50-110, 240-350, 375-450 and 475-600 o C, respectively. The endothermic peak corresponds to the evaporation of organics. The first exothermic peak is caused by the combustion and pyrolysis of organics. The second exothermic peak may be due to the crystallization of TiO 2 . And the last exothermic peak attributes to the combustion of pyrolitic carbon. When TiO 2 precursor is covered on Al substrate, the evolution of TiO 2 during the annealing process ( Figure S2b) is similar to that of TiO 2 gel ( Figure S2c).    impedance spectra of as-assembled cells were conducted in the frequency range of 100 kHz to 10 mHz, and their Nyquist plots are depicted in Figure S6. It can be seen that all plots are composed of a semicircle at high frequencies which are related to the Ohmic resistance and charge transfer resistance, and a short inclined line in low frequency regions which is due to the ion diffusion within the anode.
An equivalent circuit fitting the experimental impedance spectra is proposed to interpret the impedance results, as shown in Figure S6. This equivalent circuit model contains Rs related to Ohmic resistance in circuit, Rct and a constant phase element (CPE) corresponding to the charge transfer resistance through the electrode-electrolyte interface, and a Warburg impedance (Wo) associated with Li + diffusion in TiO 2 films. The fitted impedance parameters are listed in Table S1.
The results of fitting analysis indicate that the Rs values of three samples change little with the annealing temperature and are less than 3 Ω, which indicates Al hardly reacts with O 2 from air below 500 o C (as mentioned above) and keeps high electronic conductivity. However, Rct show a little difference after heat-treated at different temperature. The charge transfer resistance value of TO/Al-450 is 28.22 Ω, which is lower than that of TO/Al-400 (28.79 Ω) and TO/Al-500 (32.76 Ω), respectively. As the impedance is in inverse proportional to electrical conductivity, this suggests a higher charge transfer rate for TO/Al-450 electrode, which is mainly caused by the higher content of pyrolytic carbon and better crystallinity in TO/Al-450 as analyzed in main text. It should be pointed out that the Rct of three sample are rather low, which favors the high-rate performance.

Equation S6
where I p , A, n, C 0 , D and υ are the peak current density, the electrode electroactive area, the number of electron transferred, the maximum concentration of Li-ions in the electrode (C 0 =0.048 mol cm -3 ), 2,3 the apparent chemical diffusion coefficient, and the scan rate, respectively.
According to the work of Lindstrom and Lindquist, 4,5 at lower scan rates and for thinner films, the electroactive area can be considered as the entire inner area, which was determined by the following equation. 4-6

Equation S7
where A i is the entire inner area of nanoporous electrode, V t is the entire film volume, V p is the pore volume, P is the porosity, and r is the average radius of the active material particles in film. In our case, the scan rate is low (≤ 10 mV S-1), the film is thin (~ 100 nm) and mesoporous. Thus the inner surface area can be used as electroactive area. From Figure S7d, the value of I p /υ 1/2 can be calculated. Therefore, according to Equation S6 and Equation S7, the Li + apparent chemical diffusion coefficient of TO/Al-400, TO/Al-450, and TO/Al-500 are 1.47 × 10 -13 , 4.45 × 10 -13 , and 2.61 × 10 -13 cm 2 s -1 , respectively. It shows that the Li + apparent chemical diffusion coefficient increases in order of TO/Al-400 < TO/Al-500 < TO/Al-450.