Advanced separators for lithium-ion batteries

Abstract

The separator technology is a major area of interest in lithium-ion batteries (LIBs) for high-energy and high-power applications such as portable electronics, electric vehicles and energy storage for power grids. Separators play an essential part that physically prevents direct contact between positive and negative electrodes while acting as an electrolyte reservoir to transport lithium ions. The characteristics of different separators would directly affect the performance under cell abuse; hence separators are crucial for battery safety. This paper introduces the characteristics of separators, means to improve traditional commercial polymeric separators and novel materials for separators. Other novel high-performance separators are also briefly discussed in this paper. Insights from this paper illustrate that various strategies could enhance the performance of separators, and better performance and safety can be achieved in separators in high-energy lithium-ion batteries.

A Comparative Review of Lead-Acid, Lithium-Ion and Ultra-Capacitor Technologies and Their Degradation Mechanisms

“…This degradation will be discussed under the appropriate section later in this article. The properties that are focussed on when choosing a selector are, amongst others, pore size, porosity, permeability, electrolyte wettability, mechanical properties, chemical stability, ionic conductivity, ion migration and storage, thickness, dimensional stability, thermal stability and shrinkage, shutdown effect and cost [139].…”

“…Porosity determines electrolyte storage capacity and rate of ion conductivity of the separator [142]. Increased porosity leads to better ion conductivity (ideally 40-60% [139]) due to lower internal resistance and thus increased charge/discharge rate, more uniform distribution of the current and a lower chance of a short circuit. However, the higher the porosity, the lower the thermal and mechanical stability [143].…”

“…However, tensile strength is inversely proportional to porosity and ionic conductivity-thus, higher values increase internal resistance. Lower values will lead to the possibility of short circuiting between the electrodes, which can lead to or aggravate thermal runaway [139].…”

“…Ideally, the separator should meet all of the specifications [102]; however, the best value for all of the properties cannot be achieved at the same time, as many are inversely proportionate to each other [150]. Therefore, necessary performance parameters are augmented in lieu of the appropriate parameters for the specific application (some applications prefer higher charge/discharge rate, whereas others prefer robustness and thermal stability) [139,142].…”

“…They are the most commercially used separators due to the low production cost, higher mechanical strength and electrochemical stability [109], but they have low electrolyte absorption, are hydrophobic, have poor wettability, low porosity, poor thermal stability and a low thermal deformation temperature (80-85 • C and 100 • C, respectively), leading to thermal shrinkage and short circuits [157][158][159]-all of these decrease battery cycle life [139]. They are usually used in a PE/PP or PP/PE/PP configuration due to their individual characteristics; PE has good flexibility but a low melting point (130 • C), whereas PP has good mechanical properties and a high melting point (165 • C) [139,160]. The combination of the two therefore leads to low closed cell temperature and high fusing temperature, improved cyclability and safety performance of the battery [102].…”