Working jointly with scientists from the Institute for Problems of Chemical Physics of RAS and the Ural Federal University, researchers from Skoltech have demonstrated that it is possible to develop high-capacity, high-power batteries from organic materials without the use of lithium or other rare elements.
Furthermore, they exhibited the remarkable stability of cathode materials and an unprecedented level of energy density in fast charge/discharge potassium-based batteries. The study outcomes have been reported in the Journal of Materials Chemistry A, The Journal of Physical Chemistry Letters, and Chemical Communications.
Lithium-ion batteries are unavoidable in the everyday life of humans: they are extensively used for energy storage, specifically in portable electronics. The need for batteries has been increasing owing to the rapid development of electric vehicles, which are attracting ever-growing investment.
For instance, Volvo has planned to increase the share of electric vehicles to 50% of its cumulative sales by 2025. Daimler has declared its intent to discontinue internal combustion engines totally, switching its focus toward electric vehicles.
But the large-scale use of lithium-ion batteries leads to the acute shortfall in the resources required for their production. Transition metals typically used in cathodes—like nickel, cobalt, and manganese—are quite rare and costly, as well as toxic.
Although a major portion of the less abundant lithium is produced by a few countries, the worldwide lithium supply is too little for all traditional automobiles to be substituted by electric vehicles driven by lithium batteries.
As predicted by the German Research Center for Energy Economics (FFE), in the decades to come, the shortage of lithium reserves might turn out to be a major problem. Scientists have recently proposed considering other alternatives, such as potassium and sodium, which have chemical properties similar to that of lithium.
Led by Professor Pavel Troshin, researchers from Skoltech have achieved considerable advances in developing potassium and sodium batteries based on organic cathode materials. The outcomes of the research have been described in three publications in top international scientific journals.
The first paper describes a polymer that consists of hexaazatriphenylene fragments. The new material was found to be equally appropriate for lithium, potassium, and sodium batteries which charge in 30 to 60 seconds, while preserving their energy storage capacity even after thousands of charge-discharge cycles.
Versatility is one of the key advantages of organic materials. Their redox mechanisms are much less specific to the nature of the counter-ion, which makes it easier to find an alternative to lithium-ion batteries. With lithium prices going up, it makes sense to replace it with cheaper sodium or potassium that will never run out. As for inorganic materials, things are a lot more complicated.
Roman Kapaev, PhD student, Skoltech
Kapaev is the first author of the first paper. The disadvantage in this case is that the operating potential of the hexaazatriphenylene-based polymer cathode is low (nearly 1.6 V with respect to K+/K potential), which leads to decreased energy storage capacity.
In the second study, the researchers put forward another material, a dihydrophenazine-based polymer that overcomes this disadvantage and guarantees an increase in the average operating voltage of the battery, of up to 3.6 V.
Aromatic polymer amines can make excellent high-voltage organic cathodes for metal-ion batteries. In our study, we used poly-N-phenyl-5,10-dihydrophenazine in the potassium battery cathode for the first time. By thoroughly optimizing the electrolyte, we obtained a specific energy of 593 W×h/kg, a record-high value for all the currently known K-ion battery cathodes.
Philipp Obrezkov, PhD student, Skoltech
Obrezkov is the first author of the second paper. One main issue with the metal-ion batteries, specifically those including a metal anode, are metal dendrites that grow into the cell and cause short circuit. These are usually accompanied by fire and even explosion.
This can be prevented by replacing pure alkali metals with their alloys, which are in liquid state at the battery operating temperature. Recently Professor John B. Goodenough, a 2019 Nobel Prize winner, proposed this. It is well-known that the low-melting potassium and sodium alloy (NaK) contains nearly 22% of sodium by weight, with a melting point of −12.7 °C.
In the third study, an analogous potassium-sodium alloy applied on carbon paper was used as an anode, and the redox-active polymers derived earlier were used as cathodes. It was found that such batteries can be charged-discharged within 10 seconds.
Fascinatingly, one of the polymer cathodes presented the highest energy capacity for potassium batteries, whereas the other one exhibited outstanding stability, with just 11% of capacity lost after 10,000 charge/discharge cycles. Moreover, the batteries based on these two materials exhibited unmatched power characteristics of about 100,000 W/kg—a level characteristic to supercapacitors.
Currently, metal-ion batteries and supercapacitors are the most common energy storage solutions. The former store a lot of energy per unit mass, but charge slowly and lose capacity rather quickly after a number of cycles, whereas the latter charge fast and withstand tens of thousands of cycles, but have poor storage capacity.
Pavel Troshin, Team Leader, Skoltech
Troshin added, “We showed that electroactive organic materials can pave the way for a new generation of electrochemical energy storage devices combining the advantages of metal-ion batteries and supercapacitors, thus eliminating the need for costly transition metal compounds and lithium.”