Lithium-ion Battery Separators and their Role in Safety

--

This story is contributed by Ashish Gogia.

• Separators are electrochemically inactive thin porous membranes that physically separate the cathode from the anode, while allowing ion transport to occur.
• Separator shutdown above the melting point seeks to prevent thermal runaway by stopping ion flow between the cathode and anode, but separator breakdown at even higher temperatures can lead to thermal runaway.
• Ceramic-coated separators and high melting point polymer materials are promising candidates due to their improved thermal stability and tolerance for abuse, but further development is still needed for increased safety and reliability.

Introduction

The safety of Li-ion batteries (LIBs) is of paramount importance, especially for aviation and automobile applications. LIBs have been widely used in electric vehicles, portable devices, and grid energy storage during the past several decades due to their high specific energy density and stable cycling performance. Since the commercialization of LIBs in 1991 by Sony Inc., the energy density of LIBs has been aggressively increased. However, if a lithium-ion cell is accidentally overcharged or abused, the stored chemical energy can be abruptly released during a thermal runaway in the form of fire or explosions. Thermal runaway refers to the mechanism of exothermic chemical reactions between the electrodes and electrolyte that occurs once a cell is heated above a certain temperature; the increase in the internal temperature causes the reaction to become spontaneous and self-sustaining. The threshold temperature for thermal runaway reactions depends on the chemistry of the cell, state of charge and cell design. Therefore, the key to ensuring battery safety is to control the processes leading up to thermal runaway.

Desired Characteristics of a Battery Separator

One of the critical battery components for ensuring safety is the separator. Separators (shown in Figure 1) are thin porous membranes that physically separate the cathode and anode, while allowing ion transport. Most micro-porous membrane separators are made of polyethylene (PE), polypropylene (PP), and layered combinations such as PE/PP and PP/PE/PP. They must be electrochemically, thermally, mechanically, and dimensionally stable to endure battery fabrication and operation. It is highly desirable that separators possess high permeability to the electrolyte for high power operation. Separator design is further complicated by additional considerations such as tolerance for electrical abuse, thermal conductivity, chemical inertness to other cell materials, and benign failure modes in the event of an internal short or thermal runaway [1].

Separator Shutdown and Breakdown

Separators in most commercial LIBs have a built-in shutdown mechanism. As the temperature of a cell increases, the polymeric separators melt and the pores close, stopping further ion transport and current flow in a mechanism known as separator shutdown. Above a certain temperature, however, the integrity of the separator is lost, allowing for direct contact between the cathode and the anode in a mechanism known as separator breakdown. The internal short resulting from separator breakdown leads directly to thermal runaway. Therefore, it is desired for a particular separator to have a shutdown temperature and breakdown temperature as far apart as possible in order to avoid or delay the thermal runaway process. The ability of a separator to shut down a battery depends on parameters such as molecular weight, percent crystallinity (density), and processing history.

Commercial Separators for Enhanced Safety

• Tri-layer Separators

Most batteries used in cell phones and tablets use a single layer of polyethylene (PE) as a separator, with a typical pore size of 200 nm-1 𝜇m, and a thickness of 10–30 𝜇m [2]. Since the 2000s, larger industrial batteries have started using tri-layer separators with polypropylene (PP) to improve the reliability of thermal shutdown when there is a temperature rise in multi-cell configurations. Commercial tri-layer PP/PE/PP separators take advantage of the difference in the melting point of PP (165°C) and PE (135°C), using PE as the shutdown layer and PP to protect structural integrity. During an abuse event, the PE layer will melt at a temperature of 135°C and close the pores in the separator to stop the current flow while the PP layer, which has a higher melting temperature than PE, remains solid. However, such protection is only effective below the melting point of PP.

The use of ultrathin separators to increase energy and power density, and reduce internal resistance also raises safety concerns. While it is common to have a separator thickness of 25.4 μm, many go down to thicknesses of 20 μm, 16 μm and now even 12 μm without significantly compromising the cell’s properties. However, thin separators may have adverse effects on the mechanical strength, which is especially important during cell assembly, and safety. In modern LIBs, the separator makes up just 2–3 percent of the cell’s weight.

• Ceramic-coated Separators

Commercially available ceramic-coated separators have also gained popularity, in which ceramic particles, such as alumina, silica, or zirconia with binders, are slurry-coated on polymer membranes [3]. The addition of this thin ceramic coating provides better thermal and mechanical stability as well as excellent wettability due to their high hydrophilicity and high surface area. Although the micron-thick ceramic coating with suitable binders has been effective in improving the thermal stability of separators, it adds weight, volume, and processing time. In addition, the detachment or delamination of the ceramic coating from the polymer membrane due to poor adhesion could also lead to battery failure during operation. Current research in ceramic-coated separator design includes the search for binder-free, scalable, fast, and cost-effective techniques that can be used to deposit a nanometer layer of ceramic coating on the polymer membrane [4]. This thin coating reduces shrinkage of the separator at the shutdown temperature, essential for improving battery safety. More abuse and safety testing is needed on this class of separators to determine their value to improving cell and battery safety.

Conclusion

Although separators in a lithium-ion cell are electrochemically inactive, they play a very active role in cell safety. For electrochemical cell chemistries, the separator should be as thin as possible to maximize power and capacity, but possess the physical strength and thermal stability to maintain the mechanical and electrical separation between the electrodes, even under high temperature abuse conditions. In addition, it should have larger electrolyte uptake for lowering the cell resistance, and have a highly porous structure. Ceramic-coated separators and high melting point polymer materials offer some improvement in thermal stability and abuse tolerance for lithium-ion cell separators but, in general, more evaluation is needed to quantify the safety impact of these new separators. Simulations to improve the understanding of the separator microstructure would also be beneficial for improving the safety and reliability of separator design.

References

[1] Arora, P., Zhang, Z.Battery Separators. Chem. Rev. 2004, 104, 4419–4462. Available from: https://doi.org/10.1021/cr020738u.

[2] Lee, H., Yanilmaz, M., Toprakci, O., Fu, K., Zhang, X.A Review of Recent Developments in Membrane Separators for Rechargeable Lithium-Ion Batteries. Energy & Environmental Science 2014, 7, 3857–3886. Available from: http://dx.doi.org/10.1039/C4EE01432D.

[3] Fu, D., Luan, B., Argue, S., Bureau, M.N., Davidson, I.J.Nano Sio2 Particle Formation and Deposition on Polypropylene Separators for Lithium-Ion Batteries. J. Power Sources 2012, 206, 325–333. Available from: http://www.sciencedirect.com/science/article/pii/S0378775311021859.

[4] Gogia, A., Wang, Y., Rai, A.K., Bhattacharya, R., Subramanyam, G., Kumar, J.Binder-Free, Thin-Film Ceramic-Coated Separators for Improved Safety of Lithium-Ion Batteries. ACS Omega 2021, Available from: https://pubs.acs.org/doi/abs/10.1021/acsomega.0c05037.

Acknowledgments

Many thanks to Tejal Sawant, Katherine He and Timothy Suen for reviewing this article and providing helpful suggestions.

Ashish Gogia is a Ph.D. candidate at the University of Dayton, Ohio in Electrical & Computer Engineering. His research involves fundamental and applied studies on solid-state Li-ion battery systems, specifically targeting the safety and efficiency of next generation batteries. His research also includes work on battery separators (liquid electrolyte-based batteries) and modeling of polymer nanocomposites using Dissipative Particle Dynamics (DPD) simulations.