You may have read about the issues related to lithium-ion batteries lately. Situations where the batteries have swelled or even caused a fire or an explosion. The question is, could cellulose fibrils be used to prevent these issues? Or would there be other functions in the batteries where the fibrils would be useful or even open new opportunities?
MFC as a strengthening aid in separators
We have previously described how the large surface area of Microfibrillated Cellulose (MFC) can be exploited in different applications. The microfibrils, when uniformly dispersed, can strengthen films and membranes, or MFC can even be used as a film as such.
But what about separators? The primary purpose of a separator in the battery is to act as an insulator between the two electrodes and prevent short circuit. Although the separator is an insulator, it still needs to allow the transportation of the ionic species between the electrodes. Ideally, a separator would be thermally, chemically and mechanically stable. At the same time, it should have resistance to organic solvents, good wettability, and high puncture resistance as well as being thin.
Typically, separators are high strength polyolefin-based porous membranes such as polyethylene (PE) and polypropylene (PP). The challenges with these thermoplastic polymers are related to their temperature stability, as they soften at 130 °C and 165 °C, respectively. The softening, however, is also utilized as a safety feature since the collapse of the pores leads to a shutdown of the battery, as the movement of the ions is prevented. In some rare cases, the shrinkage of the separator may result in the exposure of the electrodes, leading to an internal short circuit, and in a worst case, into a fire or explosion of the battery.
When we think about the properties of cellulose fibrils, such as low conductivity; temperature stability; chemical stability; insolubility in common organic solvents; pH stability and great film-strength, one can say that there is potential for cellulose fibrils in separators, together with other polymers or as such. You can find more information on this topic on Chenga's et al. article published in Journal of Bioresources and Bioproducts.
MFC as a binder for electrodes
The negative electrode (anode) of a conventional lithium-ion cell is usually made from carbon whereas the positive electrode (cathode) is a metal oxide, such as LiCoO2 (LCO-lithium-cobalt), LiMn2O4 (LMO-lithium-manganese), LiFePO4 (LFP-lithium-phosphate), and Li(NiMnCo)O2 (NMC-nickel manganese cobalt).
Polyvinylidene fluoride (PVdF) is a common binder used for the manufacturing of cathodes. It is normally dissolved in a polar solvent, such as N-methyl-2-pyrrolidone (NMP). Due to the processing requirements, cost and environmental issues, binder manufacturers move gradually from using PVdF and instead use aqueous base materials such as SBR copolymers (Styrene Butadiene Rubber).
--> SpecialChem have created a comprehensive guide on the topic of SBR: Read more here.
The high surface of MFC, tolerance against ions, combined with a water based system, opens new, environmentally friendly ways to produce electrodes for lithium ion batteries. Lately, Jabbour et al. used finely refined cellulose as a binder for producing paper cathodes whereas Wang et al. produced carbon aerogel anodes from bacterial cellulose. These examples show us that cellulose fibrils could be used not only as a binder but also as a source of carbon for the anodes.
As we see, cellulose fibrils could be used to increase the safety of lithium-ion batteries as well as to decrease the environmental burden related to the manufacturing of them. Maybe the fibrils will even take us closer to flexible, light weighted batteries and devices.
* This blog post was originally posted in 2016, but we have updated it to give you the latest news and ideas on the topic.
Otto Soidinsalo works as a technical application manager at Borregaard. He has a Ph.D. in organic synthesis from the University of Helsinki and his working experience ranges from organic synthesis, cellulose ethers and its applications to nanocrystalline cellulose and microfibrillated cellulose.
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