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

   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.