Recent Advances in Polyolefin-Based Separators for Li-ion Battery Applications: A Review

In recent years, extensive studies have been conducted on secondary lithium-ion (Li-ion) batteries for innumerable applications, including power source for the portable electronics, electric vehicle, and electric storage reservoir. It is well-known that Li-ion batteries are made up of four different components, i.e., anode, cathode, electrolyte and separator. Even though separators play no roles in the electrochemical reactions, they physically separate the anode and cathode to avoid electrical short circuits when the free flow of Li ions transfers through the liquid electrolyte. According to the above-given statements, a separator takes a key part in the safety and the power capability of Li-ion batteries. Unil today, various separators have been introduced, among which polyolefin membrane separators have proved to be the most promising separators for commercial Li-ion batteries, arising from their outstanding properties, e.g., electrochemical stability, good mechanical strength, cost-efficiency, and thermal shutdown properties. Nevertheless, these membrane separators have faced several key drawbacks for vehicular storage, including non-polarity, low surface energy and poor thermal stability. Accordingly, the aim of this study is to review the types of polyolefin microporous separators which are widely employed in Li-ion batteries and all the intricate approaches taken for their surface modification. Several approaches including grafting by different methods, mussel-inspired technique and functionalization by inorganic nanostructures have been widely utilized to overcome the above- problems. It has been proven in the literature that outstanding properties can be provided through mono- and multilayer polyolefin separators if they are modified by proper strategies and materials, by which stat-of-the-art Li-ion batteries can be introduced..