Battery Separators: How Can the Plastics Industry Meet the Challenges?

TAGS:  Electrical & Electronics      New Energy Solutions    

In recent years, there have been intensive efforts to develop advanced battery separators for rechargeable lithium-ion batteries for different applications such as:

• Portable electronics
• Electric vehicles, and
• Energy storage for power grids

In these developments, the separator is a critical component of the batteries. This is because; it provides a physical barrier between the positive and negative electrodes. Doing so, it prevents electrical short circuits. In addition, the separator must be porous to allow for the effective transport of the lithium ions in the battery.

The performance of the lithium-ion batteries is greatly affected by the materials and structure of the separator.

Despite the advances that have been made in the development of separator materials, there are still several challenges that currently exist. These challenges are primarily due to new and emerging applications of Li-ion batteries. Among the existing challenges of the separator, the main ones are:

• An increase in its wettability,
• An improvement in its high temperature performance, and
• The ability to be able to produce increasingly thinner separators

Let's begin by understanding the general structure of a lithium-ion battery and the position of the separator in the battery (see figure below) before we discuss above-mentioned challenges in detail and how the plastics industry can help meet these challenges:

Main Challenges with Battery Separators

1. Wettability of the Separator

The wettability of the separator toward non-aqueous electrolytes can significantly impact the performance of a lithium-ion battery. The wettability issue is a continuing concern. This is because larger batteries are being developed for applications such as electric vehicles.

Any un-wetted active material will cause for:

• An under-utilization of the electrode capacity, and
• An increase in the electrolyte resistance

The wettability concern becomes more critical for large capacity electrodes. These are developed for vehicle applications where the entrance area is limited and the transport distance increases for the electrolyte.

Currently, most commercial separators for lithium-ion batteries are typically porous polyolefin films, both polyethylene and polypropylene. These polymer separators are generally not compatible with some conventional electrolytes that include solvents of high dielectric constants, such as:

• Ethylene carbonate
• Propylene carbonate, and
• Gamma-butyrolactone

This is due to the low surface energies of the polyolefins. Also, this incompatibility can lead to incomplete filling or extended manufacturing times. In addition, the distribution of the electrolyte in the cell is uneven and this leads to poor long-term stability of the battery as well as uneven current distributions.

2. Thermal Stability of Separator

Another current continuing need for battery separators is an increased thermal stability compared to current polyolefin materials. A growing demand for large format cells that have more energy than usual small format cells is driving a need for the development of separators that have an increased thermal stability compared to polyolefin materials. These larger format cells are being used in both electric vehicle and power grid applications.

This is because the larger batteries have a greater potential to have thermal events. These events can lead to rapid large temperature increases and ultimately fires and even explosions.

3. Thinner Separators

As batteries that possess higher power and energy densities continue to be developed, there is a need to include more active electrode material. This means that there is less space in the batteries for the separator and the separator must be thinner than current products. However, at the same time, it is desirable to retain similar mechanical properties with the thinner separators.

This is necessary because it is the mechanical properties of the separators that are one of the factors that are responsible for providing safety to the battery. The puncture strength of the separator is particularly important in this regard and it is well established that the puncture strength is inversely related to the thickness for a particular type of separator.

Proposed Methods to Modify Separator

Hydrophilic Monomer Coating to Improve Wettability

There are several materials solutions that have been proposed to improve the wettability of battery separators. All of these approaches have focused on a modification of the separator to affect its hydrophilic nature. That change is expected to improve the compatibility with the common electrolyte materials. The improved compatibility will lead to an enhanced wettability of the separator by the electrolyte.

Coating with a gel polymer electrolyte through the use of a wetting agent has been attempted to address the wettability concern. However, that approach involves a complex, multi-step process and the relatively expensive modification with adequate hydrophilic monomers. They increase the surface energy enough to absorb the electrolyte solutions. The use of a wetting agent generally improves wettability but it is unable to increase the electrolyte retention.

In addition, the separators are hydrophilic only temporarily. This is because the wetting agents are subject to washing away by the liquid electrolytes upon battery cycling or storage. Due to these issues, there continues to be a great deal of interest in alternative materials solutions to improve the wettability of battery separators.

Process Approaches for PP Membrane Wettability

Along with material approaches to improve the wettability, there have also been several process approaches to increase the wettability of separators. An approach that attempts to improve the wettability of PP membranes is to treat them with corona discharges. Such a treatment process introduces chemical functionalities onto the surface of the separator and, thus, the surface energy increases. However, the heat that is generated during the corona treatment process can have the effect of closing some of the pores in the separator and, thus, the usefulness of the separator is compromised. Thus, very tight control of the process parameters must be maintained during this operation.

A second technology that has been explored to improve the wettability of PP separators is to increase the porosity level through an adjustment of the processing parameters. For example, the stretching conditions that are used to produce the separators can be modified to yield higher porosity products. This means that the overall porosity of the separator and the average pore size will be higher and this can lead to an improvement in the wettability features of the separator. The drawback to this approach is that highly porous separators are quite weak and they can be very difficult to handle during battery fabrication processes.

Ceramic Coating to Improve Thermal Performance

The majority of efforts to increase the high temperature performance of battery separators have focused on the use of a thin, ceramic coating layer on the polyolefin-based membrane. Typically, the ceramic coating contains a material like aluminum oxide. It is applied at thicknesses that range from 2 to 4 microns and can be applied to one or both sides of the separator.

The idea behind the use of the ceramic material is that at the temperature at which the polyolefin begins to melt and shrink the ceramic remains intact and provides physical integrity. Its presence prevents the electrodes in the battery from contacting each other. Thus, the presence of the ceramic material prevents dangerous short circuits from taking place.

While the ceramic material does indeed provide higher temperature performance to the battery separator, the actual improvement in the overall battery safety is still to be quantified. The optimum ceramic formulation is still to be identified. In addition, potential issues with the removal of the ceramic coating exist. This is because the ceramic consists of an inorganic material that is being applied to an organic polymer substrate.

That issue is usually addressed through the use of a binder material. That material enhances the adhesion between the ceramic coating and the separator substrate. However, the binder materials have not been optimized for the environment that exists in a battery.

Typical binder materials such as PVDF do not have sufficient thermal stability to withstand temperatures that can be encountered in larger electric vehicle batteries. On the other hand, polymers such as polyimides and polysiloxanes have been shown to function effectively as binder polymers at elevated temperatures. However, these materials add significant cost to the raw material feature of the separator and, thus alternative solutions continue to be explored.

Thus, there remains interest in other approaches for the development of other binder materials to increase the high temperature performance of battery separators.

Also, the replacement of the ceramic coating with a coating that is based on a high temperature polymer, like polyimides, has been proposed as an alternative approach. That method will limit the need for a binder material because the adhesion between two polymers is likely to be better than the adhesion between dissimilar inorganic and organic materials. However, due to the organic nature of the high temperature polymer, the temperature performance that is provided by this approach is lower than is possible with the use of a ceramic coating.

Stronger Materials for Thinner Separators

As the thickness of separator is decreased, some of the mechanical properties, such as the puncture strength also decrease. This presents a safety issue for the batteries as it increases the potential for holes in the separator that can lead to short circuits. This issue is being addressed through an examination of different polymers other than polyethylene and polypropylene in the separators.

For example, poly (4-methyl 1-pentene) and polyesters have been studied in battery separators. Also, special polyolefin grades, such as ones produced using metallocene catalysts are being studied. Examples of several of the special polyolefin grades that have very high molecular weights are:

• High crystallinity polypropylene
• UHMWPE

All of these suggested materials provide separators that have higher modulus and strength values than standard polyethylene and polypropylene materials. Due to that fact, the thickness of the separators can be decreased and the strength can still be maintained. However, often the use of these new materials means that the processing scenarios need to be modified and this is an area that requires additional investigations.

In general, there is a trade-off between the enhanced mechanical properties and the ease of processing of materials and the advantages that are obtained need to be weighed against the issues, like lower product yields, that are encountered.

Developments in Battery Separators using Polymers

There have been several innovative recent developments to improve the wettability and high temperature performance of battery separators.

• Silica nanoparticles have been attached to pores and pore walls of a poly(m-phenylene isophthalamide) separator. The separator that is produced is heat-resistant and has improved wettability toward the electrolyte. The polymer provides the high temperature resistance while the silica particles are useful for affecting the wettability feature of the separator. The nano-silica-decorated separator is a potential candidate in lithium-ion batteries with high safety features.
  
  
  
  Schematic of the Preparation of the PMIA@SiO₂ Separator (Source: Springer)


• Sponge-like polyimide separators have been prepared by a non-solvent phase separation process. A separator with high thermal stability, high porosity and excellent wettability in the liquid electrolyte was produced. The results suggest that the process technology that was used for the separator production can be an effective approach for preparing high performance separators.


• Novel polyetherimides (PEI’s) that are based on the use of para-phenylenediamine (pPD) and bisphenol-acetondiphthalicanhydride (BPADA) have been used to produce separators through a phase inversion process. The separators cover a range of morphologies and properties that lead to a wide range of conductivities. They also showed low degrees of swelling in electrolyte solvents and fast electrolyte wicking. In addition, dynamic mechanical analysis demonstrated that the separators display dimensional stability up to a temperature of 220°C.


• Surfactants that are based on polyoxyethylene and polyoxypropylene triblock copolymers have been utilized to:
  
  • Enhance the separator wettability toward the electrolyte
  • Improve the battery discharge capacity
  • Improve the discharge rate along with providing excellent cycling stability.


• Another approach to enhance the wettability of separators is to blend additives into the separator formulation. These additives:
  
  • Should favorably interact with the electrolyte material.
  • Need to have a polar chemical nature.
  Due to their chemical nature there will likely be a natural incompatibility with the polymer that is used to produce the separator. This limits the amount of the additive that can be used without negatively impacting the other separator properties, such as strength and pore structure.


• Hydrogen Induced Crosslinking (HHC): This technique increases the electrolyte affinity of polyolefin separators by covalently crosslinking polyethylene oxide onto PP separators. The polar functionalities of the PEO are preserved through the selective cleavage of C-H bonds. Also, through the subsequent crosslinking of the resulting radicals that are generated on the PEO and PP polymer chains. Lithium-ion batteries with the modified separator have a low internal resistance and high capacity retention.
  
  
  
  Hydrogen Induced Crosslinking (Source: Research Gate)


• Researchers at Duke University have developed a composite material (a combination of hexagonal boron nitride and an ionic liquid). The resultant material can act as both a separator and an electrolyte in the battery. Its use allows for higher operating temperatures than are possible with current separator materials.


• Other approach focuses on increasing the high temperature performance of battery separators which involves the utilization of polymers such as polyimides as the separator material.

Conclusion

There continue to exist opportunities for the plastics industry to make significant contributions to the current challenges for battery separators. Many of those opportunities are based on the realization that separators that are used in consumer electronic batteries may not be the best materials for larger batteries that are used in other applications such as electric vehicles.

Solutions that are being utilized presently suffer from certain deficiencies. Those deficiencies need to be addressed through the development of unique and novel materials that satisfy the needs of the battery separator market. Some advances have been made on this topic, but additional work is required to completely optimize the performance of this critical component of batteries.


This article was originally published on May 25, 2017 and updated on Aug 18, 2020.