Lithium-ion batteries have played an important role in the commercialisation of portable consumer electronics. It is small wonder therefore that their production is assumed to increase significantly in the next few decades, whereas battery prices are expected to decrease. Lithium-ion batteries are used for electric vehicles as well as large-scale energy storage applications. As the demand for energy density is ever increasing and myriads of technically-advanced portable electronics are released onto the market every year, new methods of improving battery operation have to be found. Separators are an important component in any battery that greatly influence the life and safety of a battery. Generally, these separators are usually passive and cannot reversibly switch their properties in order to optimise battery performance.
Now (2021), scientists have looked into methods to improve battery control and safety and created an iongate separator which makes use of the switchable ionic conductivity of the conducting polymer polypyrrole (PPy). For this reason, they deposited a polypyrrole membrane on a conventional polyoleﬁn separator and created a separator that showed low ionic resistance during its oxidized active state, and high ionic resistance in the reduced passive state. This was because of the rapid and reversible redox state transition of PPy where Li+ ions travel via mobile anion dopants along the PPy backbone while in the active state, but cease motion in the passive state as the ions are expelled in the reduction state. The scientists were able to transform the oxidized polypyrrole membrane from a polycationic exchange membrane to a more neutral state upon reduction, which could prevent ion crossover if the density was sufficient. This switching of states was achieved either externally by using a potentiostat connected to a third iongate electrode or by directly shorting the iongate material to the anode. Although previous reports had shown that a PPy iongate electrode could successfully prevent transient ion crossover in aqueous solutions, the method had never been used for organic electrolytes or for batteries.
Also, the scientists optimised the membrane switching ratio versus thickness and chose 300nm as the ﬁnal thickness. They found that when the layer became thinner, more pin holes formed in the iongate due to the surface porosity of the Celgard substrate. This enabled the liquid electrolyte to penetrate an optimally-dense iongate layer and decreased the reduced passive-state resistance. When the iongate layer was too thick, the resistance in both the oxidized and the reduced state became much greater than that of a conventional cell without the iongate separator.
Scientists have long been trying to improve the performance of lithium-ion batteries. In 2014, a membrane embedded with synthetic chiral receptors and different terminal structures was developed, consisting of either hydrophobic triisopropylsilyl (TIPS) groups or hydrophilic hydroxy groups. The receptors had ligand-binding units that interacted with cationic amphiphiles such as 2-phenethylamine (PA). A conductivity study showed that the receptors hardly showed ion transportation at the ligand-free state. After ligand binding, the receptor consisting of TIPS termini showed improved current due to ion transportation. The receptor became conductive again by the second addition of PA. In contrast, the other receptor having the hydroxy terminal groups hardly showed ion transportation.
In 2017, scientists created tension-responsive transmembrane multiblock amphiphiles. There, a single-transmembrane amphiphile reacted to both expanding and contracting tensions and ion transportation was stipulated by expanding tension to form a supramolecular channel, while little transportation was witnessed if there was no tension. In contrast, a three-transmembrane amphiphile showed little spectroscopic response to tensions, probably because of weaker stacking of membrane-spanning units than in the single-transmembrane amphiphile. Nevertheless, the three-transmembrane amphiphile was capable of ion transportation by creating a unimolecular channel even in a tensionless environment, and the ion-transporting activity decreased with expanding tension.
The separator iongate has many advantages: it is flexible and displays improved wettability. It is a novel approach to battery safety and control through its dynamic control of the separator ionic conductivity and ion ﬂux. When storing a battery with the iongate separator in the reduced passive state, the ion ﬂux can be suppressed. The iongate battery shows a 37% reduction in capacity loss as compared to a normal cell and nearly completely eliminated transition metal crossover when stored at 55°C for 2 weeks. The iongate battery also displays a cycling performance similar to a normal battery while in the active state, but effectively shuts-off the cell when the iongate is reduced to the passive state. Furthermore, the iongate can be turned off by directly shorting it to the lithium anode, which might serve as a safeguard in the event of an internal short.
Despite these promising results, several parameters still have to be improved before batteries with ion gates can be commercialised. In any case, there is a great amount of conducting polymer materials with similar properties that look promising for the future development of iongate separators for battery and other electrochemical storage applications.