The Composite of Aluminum Oxide/Pvdf-HFP Membrane for Advanced Li-S Batteries
Authors: Ahn Yong-keon , Yu Ji-Sang , Woo Sang-Gil , Yoo Jeeyoung , Cho Sanghun , You Duck-Jae , Kim Youn Sang , Kim Ki Jae 
Published: February 2020

The potential candidates for post lithium ion batteries, lithium-air (Li-O2) and lithium-sulfur (Li-S) batteries, have been drawing keen attention for large-scale energy storage systems such as automotive and smart-grid applications. Current lithium ion technology is facing to the challenging limitations in high specific capacities and energy densities. Even though the distinctive achievement in insertion-based electrode materials brought the increase of charge-storage capacities, it is still remaining as a huge task of delivering the higher capacity. To overcome these major drawbacks, it is necessary to discern the limitation of electrochemical reactions and devise a promising option.
Sulfur is, the most abundant element in the earth’s crust, one of the attracting alternatives for offering the high specific capacity and energy density. The utilization of sulfur as a cathode material undergoes electrochemical conversion reactions with lithium ions in which it accommodates more ions and electrons relative to the lithium-ion batteries. Its theoretical capacity and energy density are as high as 1672 mA h g-1 and 2600 W h kg-1, respectively. There is no doubt the Li-S cell is the possible solution to meet the demand, however, there are some critical prerequisites to commercialization. For example, the conversion reactions accompanying volume expansion, safety issues in use of metallic lithium anode, and polysulfides dissolution by shuttle mechanism have been not yet solved satisfactorily for the past decades. Herein, we introduce the composite of anodic aluminum oxide (AAO) and poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) polymer as lithium ion transport media in Li-S battery system for prevention of polysulfide shuttle reaction, which leads to capacity fading due to loss of active material from the cathode, and utilization of metallic lithium as anode. With the perspectives, our composite material can act as a separator to avoid electrical short-circuiting between anode and cathode, also, as a solid-state gel polymer electrolyte facilitating lithium ion transportation. Moreover, both AAO and PVdF-HFP have relatively high dielectric constant (ε) (about 9 and 8.4, respectively), which can help effective dissolution of lithium salts and maintain high concentration of charge carriers [1]. The configuration of AAO/PVdF-HFP composite is noteworthy that the poreless PVdF-HFP layer can effectively impede the lithium polysulfides dissolution and travelling to the surface of metallic lithium anode, where insoluble phase of Li2S is formed resulting in low coulombic efficiency (CE) and long-term cyclability [2]. Also, the hexagonal nanopore-arrays of AAO induces the uniform current pathways and high rate of liquid electrolyte uptake with infiltration as reported in the previous study. In terms of the mechanical properties, the composite of gel polymer electrolyte and ceramic separator exhibits excellent mechanical reinforcement to inhibit the short-circuit risk by lithium dendrite formation [3]. In conclusion, the model study of composite material with anodic aluminum oxide (AAO) and poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) polymer is expected that structurally ideal construction of separator-electrolyte system for Lithium-sulfur batteries (Li-S) can solve the challenging issues of capacity loss by polysulfides blocking via poreless character of PVdF-HFP layer. Furthermore, the composite can bring the enhancement of electrochemical performance, the part of gel polymer electrolyte decreases the interfacial resistance, and high mechanical moduli of nanoporous ceramic membrane leads to stable operation with metallic lithium anode. Thus, the model study of AAO/PVdF-HFP composite provides the possible solution to safety concerns on Li metal anode and an advanced Li-S cell configuration.
References:
[1] Zhu, Y. S. et al., Cheap Glass fiber mats as a matrix of gel polymer electrolytes for lithium ion batteries. Sci. Rep. 3, 3187 (2013)
[2] Kim, J. et al., Poreless Separator and Electrolyte Additive for Lithium-Sulfur Batteries with High Areal Energy Densities. ChemNanoMat 1, 240-245 (2015)
[3] Tu, Z. et al., Nanoporous Polymer-Ceramic Composite Electrolytes for Lithium Metal Batteries. Adv. Energy Mater. 4, 1300654 (2014)