Manufacturing Processes of Microporous Polyolefin Separators for Lithium-Ion Batteries and Correlations between Mechanical and Physical Properties

1. Introduction

2 and graphite as a cathode and anode, respectively, as described in

Cathode: LiCoO2 ↔ LinCoO2 +

n

Li+ +

n

e−

(1)

Anode: 6C +

n

Li+ +

n

e− ↔ LinC6

(2)

With the rapidly growing demand for power consumption, lithium-ion batteries (LIBs) have emerged as a sustainable energy source for portable electronic devices and energy storage systems owing to their high specific energy and power ( Figure 1 a), flexible and lightweight design, and long lifespan compared with other battery technologies [ 1 ]. LIBs operate at voltages >4 V via reactions involving lithium ions at electrodes during charge and discharge. The following are examples of the reactions occurring in the operation of LIBs with LiCoOand graphite as a cathode and anode, respectively, as described in Figure 1 b [ 2 ]:

+ ions formed or consumed by the aforementioned reactions. In various types of commercial LIBs, the main function of the separator is to prevent short circuits caused by physical contact of the two electrodes (

A separator is a porous permeable membrane that can transport Liions formed or consumed by the aforementioned reactions. In various types of commercial LIBs, the main function of the separator is to prevent short circuits caused by physical contact of the two electrodes ( Figure 2 ) [ 3 ]. Thus, the chemical and electrochemical resistance of the separator as well as its mechanical durability are critical to battery safety. The separator should not be dissolved by or react with the electrolyte solution, which is mainly composed of organic carbonates and esters mixed with Li salts, such as lithium hexafluorophosphate [ 4 ]. In addition, it must be electrochemically stable during cell operation and mechanically strong enough to withstand the high tension in the course of battery assembly [ 5 ]. The mechanical strength is also required to avoid cell short circuiting and thermal runaway via the penetration of lithium dendrites through the separator as a result of the plating of metallic lithium on the surface of graphite anode during cycling [ 6 ]. The microstructure of the separator should be carefully designed. A separator with large pores is more susceptible to shorts and self-discharge, especially during high-temperature storage, as well as failure during the high-potential (hi-pot) testing. At the same time, a small pore size can lead to higher resistance and poor cycle life during high-temperature cycling and storage [ 7 ]. Higher porosity is also preferable as more liquid electrolyte can be stored to achieve higher ionic conductivity, but it will also result in mechanical properties inferior to those of less porous separators.

Separators are typically classified into six types: microporous membranes, nonwoven membranes, electrospun membranes, membranes with external surface modification, composite membranes, and polymer blends [ 8 ]. Despite the poor thermal stability and wettability of liquid electrolytes, the microporous membranes have dominated the overall market in the LIB industry based on low cost and simplicity of fabrication over other types. In particular, those with ceramic coating have been regarded as the best currently available option. The advantages and disadvantages of each type of separator are documented in a comprehensive review [ 8 ].

+/Li [

As base films for such safety-enhanced ceramic-coated separators, microporous polyolefin membranes have the merits of high porosity and uniform pore-size distribution, electrochemical stability at 4.2 V or higher vs. Li/Li [ 9 10 ], high mechanical strength, and inexpensive materials and manufacturing. In recent years, commercial microporous polyolefin separators have been made thinner, decreasing to <20 µm, as a means of maximizing the energy density of portable electronics and electric vehicles. However, this lowers the maximum endurable mechanical load and dimensional stability at elevated temperatures, thereby making the assembled battery more vulnerable to external damage. The risks associated with thin separators have been demonstrated by thermal ramp tests, overcharge tests, and induced internal and external short circuit tests [ 11 ]. In addition, a larger mean pore size and higher porosity generally result in lower ionic resistance and thus high specific battery power [ 12 ]. In this case, the poorer mechanical strength is expected to increase the possibility of inner-battery electrical short circuiting. It is thus necessary to compensate for the side effects the separators may encounter and to preserve their safety.

The present review will focus on microporous polyolefin separators for LIBs working based on liquid electrolytes. We will introduce both conventional and new manufacturing processes for commercial separators based on the underlying physics and associated theory. This review aims to compare the mechanical and physical properties of different types of separators and understand the correlations between them. This may allow for the proper design of next-generation separators and the tailoring of their mechanical and physical properties.