Binder-Free Thick-Film Anodes of Si Coated with Rutile-Type TiO₂

Abstract

As an anode material for high-performance Li-ion battery, we have synthesized composites of rutile-type TiO2 and Si by a facile sol-gel method because we focus that a fast Li-ion diffusion in TiO2 can improve a slow kinetics of Li-ion transfer in Si anode. We have investigated anode performance of binder-free thick-film electrodes prepared by a gas-deposition method using the TiO2/Si composites obtained. This method has a unique advantage: essential electrochemical reactions of only active materials can be evaluated because the thick-film electrodes do not include any binder and conductive additive. Electrode performance as Li-ion battery anode was evaluated in beaker-type there-electrode cells using the TiO2/Si electrodes as working electrodes and Li sheets as counter and reference electrodes. The electrolyte was LiClO4dissolved in propylene carbonate at a concentration of 1 M. Galvanostatic charge–discharge tests were carried out using an electrochemical measurement system at 303K. X-ray diffraction analysis and field-emission scanning electron microscopic observations revealed that the Si surface of the composites was uniformly covered by rutile TiO2 nanoparticles with 10–50 nm in size. Charge–discharge curves were measured for the TiO2/Si composite electrodes with weight ratios of 43/57 wt.% and 66/34 wt.%. For every electrode, we clearly observed potential plateaus in the charge (lithiation) and discharge (delithiation) processes at 0.05 V and 0.45 V vs. Li/Li+ at the first cycles. These potential plateaus are attributed to the alloying/dealloying reactions of Li–Si. After the second cycles, the charge plateaus inclined and rose to 0.1–0.2 V vs. Li/Li+, which has been explained as resulting from amorphization of Si at the first cycle and its single-phase reaction with Li+in the subsequent cycles. The electrodes behaved very alike until the 100th cycle. Figure 1 shows cycling performances of the TiO2/Si composite electrodes. The initial discharge capacities of 1500 and 790 mA h g−1 were obtained for the TiO2/Si electrodes of 43/57 wt.% and 66/34 wt.%. These capacities correspond to capacities per Si of 2500 and 2100 mA h g(Si)−1. The composite electrode of TiO2/Si (43/57 wt.%) exhibited a remarkably improved cyclability at a current rate of 1.6C: the discharge capacity of 710 mA h g−1 (1170 mA h g(Si)−1) could be achieved even at the 900th cycle. In addition, the electrode showed a specific capacity per Si weight was as large as 1870 mA h g(Si)−1 even at a high current rate of 4.8C, whereas an Si electrode showed the comparable capacity at a low rate of 1.6C. It is considered that a fast Li-ion diffusion in TiO2 provides smooth insertion/extraction of Li-ion into/from the composite electrodes. The results offer a utility of rutile TiO2 as a Li-ion conductor in Si-based electrodes for the next-generation Li-ion battery.