The reduction of the internal resistance by the accelerations of Li⁺ transport and electron transfer promotes the high-speed charge/discharge of lithium ion batteries (LIBs). Recently, the improving charge/discharge has been attempted by exploiting model porous electrodes which have mesoporous structures such as anodic aluminum oxide (AAO) [1]. AAO is a suitable template for the preparation of the multi layered structures using its regular nanopore array. TiO₂ thin films have potential application in the negative electrodes of LIBs, so we prepared the SiO₂ or TiO₂/AAO nanocomposite by the liquid phase deposition (LPD) procedure in this work. The LPD method involves a metal oxide and hydroxide thin films synthetic procedure using the hydrolysis equilibria reactions of metal-fluoro complexes as follows;

 (Hydrolysis reaction)　MF_x^(x-2n)- + H₂O → MO_n + xF^- + 2nH⁺

(F^- scavenging reaction)　Al³⁺ + 6HF → H₃AlF₆ + 3/2H₂

 Previously, TiO₂ was deposited on the surface of AAO mesopores by the LPD within 250 nm, and in this study, the TiO₂ thin film onto AAO mesopore wall which has the extreme small diameter, i.e., ca. 20-40 nm, was prepared by the LPD. In this research, the achievement of the LPD reaction at 0 °C or less by the use of the H₂O/EtOH mixed solvent and the deposition of oxide thin film to mesopore inner wall owing to the decrease of the LPD reaction activity under low temperature were executed. Furthermore, the specific ion conductivity of the electrolyte solution in the mesopore channels in the prepared TiO₂/AAO nanocomposites was evaluated.

AAO membranes were prepared by two-step anodization as previous report [1,2]. AAO template whose thichness is ca. 500 m (i.e. the aspect ratio of a AAO membrane thichness to a mesopore diameter is ca. 5000) was dipped in (NH₄)₂SiF₆ or (NH₄)₂TiF₆ solution of various concentration for 0-60 h at -20-40 °C. Prepared SiO₂ or TiO₂/AAO nanocomposite was used as a separator of a four-electrode cell and an impedance measurement was carried out. LiClO₄/EC:DEC = 1:1 (v/v) of 0.5 to 2.5 mol L^-1was used as an electrolyte solution.

The results of LPD reaction using (NH₄)₂TiF₆ solution of 10 mol L^-1 in 1 : 1 (v/v) H₂O/EtOH at -20-5 °C (The LPD reaction solution was still liquid-state at -20 °C) for 10 h were shown in Fig. 1. The TiO₂ thin film deposition within the deep mesopores without collapse of the mesopore structure was achieved at -13.5 °C. The AAO mesopore structure collapsed because the LPD reaction reactivity was too high at 5 °C, and the TiO₂ thin film was not generated because the reactivity was too low at -20 °C. It is suggested that the moderate decrease in the LPD reaction activity by using an aqueous/organic solvent mixture was necessary to achieve oxide thin films deposition on the AAO mesopore wall, while retaining the AAO mesoporous structure. The Nyquist plots for four-electrode cells using SiO₂ or TiO₂/AAO nanocomposites show two semicircles for each nanocomposite. The lower frequency region of the semi-circle was assigned as ion transport resistance in the SiO₂ and TiO₂ modified mesopore walls, and the fitting of the semicircle in the low frequency region, the specific conductivity in the pore was calculated. For a LiClO₄ solution of 0.5-2.5 mol L^-1, the specific Li⁺ ion conductivities after SiO₂ or TiO₂ thin film coating by LPD in the mesopore of the AAO nanocomposites was compared with those without thin film coating (i.e. as-prepared AAO). The conductivity for the TiO₂/AAO nanocomposite is 3 times that for as-prepared AAO, whereas that the conductivity for the SiO₂/AAO nanocomposite is similar for as-prepared AAO. Even if the concentration was changed, the ratios of the specific Li⁺ ion conductivity in the SiO₂ or TiO₂ modified mesopore to the electrolyte solution and as-prepared AAO were almost constant. These activation energies of Li⁺ ion conduction in mesopores for each sample are 14.6 (as-prepared AAO), 13.8 (SiO₂/AAO), 13.2 kJ mol^-1 (TiO₂/AAO), respectively. It is suggested that the ionic conduction are influenced by surface electric potential of oxides.

This work was supported by CREST, JST. [1] T. Fukutsuka, et al., Electrochim. Acta., 199, 380 (2016)

[2] H. Masuda et al., Science, 268, 1466 (1995)