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Battery Separators: How can the plastics industry meet current challenges?

Mark DeMeuse & Mark DeMeuse – May 25, 2017

Li-ion BatteryIn 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 prevent 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, main ones are:

  • An increase in its wettability, and
  • An improvement in its high temperature performance

This article will help you understand these needs in detail and highlight ways that the plastics industry can help meet these current challenges.

Let's start by understanding the general structure of a lithium-ion battery. And the position of the separator in the battery through the figure shown below:

Lithium-Ion Battery
Typical Lithium-Ion Battery Construction


Now, let's turn our attention towards the challenges that hinder performance of the separator.


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 growing 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.

Challenges with Battery Separator


2. Thermal Stability of Separator


The other 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. These cells are used in consumer electronics batteries and can achieve a high level of thermal stability.

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.


Proposed Materials to Modify Separator


Hydrophilic Monomer Coating to Improve Wettability


Proposed Solutions to Modify SeparatorThere 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.

Ceramic Coating to Improve Thermal Performance


Proposed Solutions to Modify SeparatorThe 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.

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, preventing 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. Thus, there remains interest in other approaches to increase the high temperature performance of battery separators.


Developments in Battery Separators using Polymers


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

Increasing Wettability with Surfactants based on Polyoxyalkylene


  • Novel and unique wetting agents have been developed and used. Surfactants that are based on polyoxyethylene and polyoxypropylene triblock copolymers have been utilized to enhance the separator wettability toward the electrolyte. The use of these novel triblock copolymers has been reported to improve the battery discharge capacity. The rate discharge is also improved along with providing excellent cycling stability.

  • Another approach that is being explored as a way to enhance the wettability of separators is to blend additives into the separator formulation. Those additives should favorably interact with the electrolyte material. Typically, such additives need to have a polar chemical nature. Also, due to that fact 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. That too, without negatively impacting the other separator properties such as:

    • Strength, and
    • Pore structure

Increasing Polarity of Electrolyte by Crosslinking PEO onto PP Separator


  • The other approach that is being developed to increase the electrolyte affinity of polyolefin separators is named hydrogen induced crosslinking or HHC. This technique involves 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.

Increasing Thermal Performance with Polyimides


  • There have also been several recent innovative attempts to improve the high temperature performance of separators and batteries in general. Researchers at Duke University have developed a composite material. It is 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.

  • The other development that has been focused on increasing the high temperature performance of battery separators has involved the utilization of polymers other than polyolefins as the separator material. Polymers such as polyimides are being evaluated in this approach. While such polymers can indeed provide high temperature performance, it is not always easy to obtain sufficiently porous membranes to use as battery separators. Often times, the use of high temperature polymers as battery separator materials require the development of unique processing scenarios.


Conclusion


There continue to exist opportunities for the plastics industry to make significant contributions to the current challenges for battery separators. 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.



1 Comments on "Battery Separators: How can the plastics industry meet current challenges?"
Joseph W Jun 6, 2017
The problem of changing surface energies of polyolefins to enhance wettability and thermo-oxidative stabiltiy of polyolefins are no longer challenges. The challenge appears in finding those with the solutions. The talent drain in the industry and the down sizing has led to a new talent migration elsewhere other than the United States. We have the solutions to these problems ! However, most companies are finding solutions to problems that do not exist while others hold the answers and provide solutions to problems that truly exist.

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