Current collectors are essential components in lithium-ion batteries, but are typically made of metal foils that do not contribute to the battery capacity. Now, a fire-extinguishing lightweight polymer-based current collector is developed that enhances both the energy density and safety of the battery.
Lithium-ion batteries are presently used in numerous applications including consumer electronics, electric vehicles and grid-storage systems. Ever since this type of battery was commercialized almost 30 years ago, it has been of prime importance to improve their energy density and safety. While the energy densities of contemporary lithium-ion batteries are more than double that of the first commercial battery1, improvements are still needed to increase, for example, the electric vehicle driving range to at least 500 km2 to ease range anxiety. However, increasing the energy stored in batteries could have a negative impact on the battery safety. So far, it has been challenging to simultaneously increase both the energy density and safety. Writing in Nature Energy, Yi Cui and colleagues from Stanford University now address this issue with a current collector based design approach3.
The energy density of a lithium-ion battery depends on the amount of charge that can be stored in the battery as well as its operating voltage. Therefore, a common approach to improve the battery is to develop new electrode materials with higher capacities and/or new electrode materials that can increase the battery-cell voltage. However, the energy density of the battery can also be increased by optimizing the battery design to make it more efficient. For example, batteries contain a number of inactive components such as current collectors, separators, electrolytes and the cell housings that do not actively contribute to the energy density, and whose weights and volumes should consequently be minimized1,4. However, as optimizations of the cell design (for example, using thinner separators) can introduce safety risks, there is a need for approaches that allow the energy density to be increased without jeopardizing the safety of the batteries.
In the work of Cui and team, the researchers replace the conventional metal current collector with a 9-μm-thick lightweight polyimide-based current collector. They load the current collector with a fire retardant (triphenyl phosphate), before coating the polyimide with 500-nm-thin copper layers (Fig. 1). The copper layers ensure a sufficiently high conductivity of the current collector. In addition to its low density, polyimide has comparable mechanical properties to that of copper foil (which is a typical current collector in commercial batteries) as well as high thermal stability. In this current collector design, the copper-coated polyimide also acts as a container for the flame retardant. In the case of short-circuiting and thermal runaway (which are the most common causes for battery fire and explosion), the different temperature coefficients of polyimide and the copper layers cause the composite current collector to fracture, releasing the fire retardant.
With this functionalized current collector approach, Cui and colleagues report an increase in the gravimetric energy density of between 8 to 26% (depending on the type of batteries they have tested) compared to a lithium-ion battery equipped with conventional current collectors. For example, the energy density of a conventional graphite/lithium cobalt oxide full cell is 226 Wh kg–1 after 200 cycles, whereas a corresponding value of 261 Wh kg–1 is obtained after replacing the conventional metal current collectors with polyimide-based current collectors. As the fire retardant is effectively sealed in the current collectors, the battery performance is not affected under normal operating conditions. This contrasts with the decreased battery performance observed when flame retardants are included in the electrolyte5. Cui and team also carry out extensive tests to demonstrate the ability of functionalized current collectors to extinguish fires caused by short circuits and thermal runaways.
Cui and colleagues’ results clearly demonstrate that it is possible to replace the conventional metal current collectors with lightweight polymer-based versions, hence minimizing these inactive components. Other types of functionalization approaches have been demonstrated to enhance the safety of lithium-ion batteries by, for example, embedding short-circuit detecting sensors in a separator5, but they do not necessarily increase the battery energy density. In this respect, Cui and team’s design concept could also be applied to the battery housing. As the weight of the battery housing corresponds to about 20% of the total weight of the battery6, an analogous development of new lightweight and functionalized housing materials could result in further significant increases in the energy density and safety of lithium-ion batteries. In addition, given the mechanical strength and flexibility of polyimide, the present polymer-based current collector approach could also facilitate the development of flexible lithium-ion batteries7.
Berg, E. J., Villevieille, C., Streich, D., Trabesinger, S. & Novak, P. J. Electrochem. Soc. 162, A2468–A2475 (2015).
Schmuch, R., Wagner, R., Hörpel, G., Placke, T. & Winter, M. Nat. Energy 3, 267–278 (2018).
Cui. et al. Nat. Energy https://doi.org/10.1038/s41560-020-00702-8 (2020).
Lu, Y.-C. et al. Energy Environ. Sci. 6, 750–768 (2013).
Liu, K., Liu, Y., Lin, D., Pei, A. & Cui, Y. Sci. Adv. 4, eaas9820 (2018).
Golubkov, A. W. et al. RSC Adv. 4, 3633–3642 (2014).
Hu, L., Wu, H., La Mantia, F., Yang, Y. & Cui, Y. ACS Nano 4, 5843–5848 (2010).
The author declares no competing interests.