Characteristics of Anodic Aluminum Oxide As Separator for Li Ion Battery

To improve the safety of Li ion battery (LIB) in high energy applications, numerous efforts have been made in terms of choosing materials to be used.[1-2] However, these attempts have been limited by thermal stability and durability of separator including electrolyte. In this study, anodic aluminum oxide (AAO) film was used as a separator in Li ion battery for superior performance over commercialized polyolefin-based separator.

The AAO film has been prepared by two-step anodization method which is consisted of etching and anodizing processes. The as-prepared AAO film is around the 20 um thick and they showed perpendicular nanopore arrays having a diameter of 40nm.

The porosity of the AAO film is calculated about 58.79 %. [3] That value is higher than that of the commercial separator (55%, celgard 2500)

 The wettability of the AAO film was analyzed using contact angle measurement with liquid electrolyte (1M of LiPF6 in EC:DMC:DEC=1:1:1 (v/v/v)). The AAO film shows higher wettability with smaller contact angle as value of 0^o than commercial separator (36^o). Small contact angle means low cell resistance which is suitable for high power LIBs. [3]

The test for electrolyte absorbance property of the AAO film was conducted. The electrolyte uptakes of the AAO film and the commercial separator are determined to be 126.88 wt% and 264.28 wt% respectively. [3] The density of the AAO film (3.98g/cm3) is smaller than the commercial separator (14.23g/cm3), so they required less electrolyte to operate LIB. Although less amount of electrolyte is used, the AAO film shows higher ionic conductivity of 2.06 mS/cm with SUS electrode than the commercial separator (0.66 mS/cm). This can be explained by low contact angle, which is agreed with low bulk resistance and higher permittivity of AAO film, it contributes to ionic conductivity. (Dielectric constant of AAO film : 28.59, relative permittivity of commercial separator: 9.06 at 1KHz)

The interfacial compatibility of lithium metal with separator was investigated by AC impedance with a lithium metal anode. It could be observed that the interfacial resistance was 118.33 Ω for the AAO and 141.92 Ω for the commercial separator, respectively. It means that the AAO film provide better interfacial characteristics for LIB.

Capacity of the AAO explored with half-cell performance, while LiFePO₄was applied as cathode from 2V to 4.2V. The AAO film and commercial separator showed similar capacity at 0.2C, the highest values were 123.0 mAh/g for AAO film and 122.6 mAh/g for commercial separator. Capacity recovery test was evaluated to confirm the correspondence with high energy LIB. Commercial separator was degraded from 1C rate and did not recovered. However, in case of AAO film, the capacity maintained 110.9 mAh/g at 0.5C, 88.4 mAh/g at 1C rate and recovered 121.1 mAh/g after investigation. And the AAO film showed high columbic efficiency over 98% at 0.5C rate. These excellent electrochemical characteristics related to Li dendrite. The AAO film can mechanically suppress the formation of Li dendrites, and homogeneous current distribution derived from well-ordered nanopores of the AAO film helps prevent Li dendrite growth.

In conclusion, the AAO film is a promising separator for LIB, especially in high energy performance, and durability test is needed for further study in the near future.

Reference

1. Bruce, P. G.; Scrosati, B.; Tarascon, J. M., Angew. Chem. Int. ed., 47, 2930 (2008).
2. D. Bansal, B. Meyer, M. Salomon, J. Power Sources 178(2), 848 (2008).
3. J. Fang, A. Kelarakis, Y.-W. Lin, C.-Y. Kang, M.-H. Yang, C.-L. Cheng, Y. Wang, E.P. Giannelis, L.-D. Tsai, Phys. Chem. Chem. Phys. 13, 14457 (2011).