Elsevier

Journal of Power Sources

Volume 164, Issue 1, 10 January 2007, Pages 351-364
Journal of Power Sources

Review
A review on the separators of liquid electrolyte Li-ion batteries

https://doi.org/10.1016/j.jpowsour.2006.10.065Get rights and content

Abstract

This paper reviews the separators used in liquid electrolyte Li-ion batteries. According to the structure and composition of the membranes, the battery separators can be broadly divided as three groups: (1) microporous polymer membranes, (2) non-woven fabric mats and (3) inorganic composite membranes. The microporous polymer membranes are characterised by their thinness and thermal shutdown properties. The non-woven mats have high porosity and a low cost, while the composite membranes have excellent wettability and exceptional thermal stability. The manufacture, characteristics, performance and modifications of these separators are introduced and discussed. Among numerous battery separators, the thermal shutdown and ceramic separators are of special importance in enhancing the safety of Li-ion batteries. The former consists of either a polyethylene (PE)–polypropylene (PP) multilayer structure or a PE–PP blend which increases safety by allowing meltdown of the PE to close the ionic conduction pathways at a temperature below that at which thermal runway occurs. Whereas the latter comprises nano-size ceramic materials coated on two sides of a flexible and highly porous non-woven matrix which enhances the safety by retaining extremely stable dimensions even at very high temperatures to prevent the direct contact of the electrodes.

Introduction

The separator is a critical component in liquid electrolyte batteries, and is placed between the positive electrode and the negative electrode to prevent physical contact of the electrodes while enabling free ionic transport and isolating electronic flow. It mostly is a microporous layer consisting of either a polymeric membrane or a non-woven fabric mat. Essentially, it must be chemically and electrochemically stable towards the electrolyte and electrode materials, and must be mechanically strong to withstand the high tension during the battery assembly operation. Structurally, the separator should have sufficient porosity to absorb liquid electrolyte for the high ionic conductivity. However, the presence of the separator adds electrical resistance and takes up limited space inside the battery, which adversely affects battery performance. Therefore, selection of an appropriate separator is critical to the battery performance, including energy density, power density, cycle life and safety. For high energy and power densities, the separator is required to be very thin and highly porous while still remaining mechanically strong. For battery safety, the separator should be able to shut the battery down when overheating occurs, such as the occasional short circuit, so that thermal runaway can be avoided. The shutdown function can be obtained through a multilayer design of the separator, in which at least one layer melts to close the pores below the thermal runaway temperature and the other layer provides mechanical strength to prevent physical contact of the electrodes. In this paper, the separators used in non-aqueous liquid electrolyte Li and Li-ion batteries will be reviewed in terms of the separator type, manufacture, function and structural modification.

Section snippets

Requirements and characterization

The essential function of a separator is to prevent physical contact of the positive and negative electrodes while permitting free ion flow. The separator itself does not participate in any cell reactions, however, its structure and properties considerably affect the battery performance, including the energy and power densities, cycle life and safety. The requirements for use in rechargeable Li and Li-ion batteries are generally listed below.

Type and manufacture of the separators

According to the structure and composition, battery separators can be broadly divided into three types: (1) microporous polymer membranes, (2) non-woven fabric mats and (3) inorganic composite membranes. These three types of separators are, respectively, featured by the thinness, high porosity and excellent thermal stability. Among them, the microporous polyolefin membranes have been most widely used in liquid electrolyte batteries due to their comprehensive advantages of performance, safety

Surface modification

Due to the inherent hydrophobic properties of non-polar polyolefin separators, the electrolytes containing a high content of polar solvents, such as EC, PC, GBL, etc., exhibit a poor wettability and electrolyte retention. In this case, surface modification is necessary to make the polyolefin separators hydrophilic [83]. The modification either treats the separator with a wetting agent (mostly a surfactant) [84], [85] or grafts hydrophilic functional groups onto the surface and pore wells of the

Concluding remarks

The separator is a critical component of a battery, its main function is to prevent physical contact of the electrodes while permitting ions to flow freely. The separator itself does not participate in any cell reactions, however, its properties significantly determine the performance and safety of the batteries. For high energy and power densities, the separator is required to be very thin and highly porous, while it adversely affects the safety and cycle life of the battery as a result of the

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