Lithium-ion (Li-ion) batteries are used in everything from smart-phones to electric vehicles. At the same time, Li-ion batteries also have a number of serious disadvantages. Among them are possible fire risk and loss of capacity in the cold as well as posing a significant threat to the environment when disposed of.
Chemists are considering redox-active nitroxyl-containing polymers as materials that could become the basis for new batteries. They are characterized by a high energy density (amount of energy per unit volume) and speed of charging and discharging as a result of redox reactions. The use of such polymers is hampered by their insufficient electrical conductivity, which prevents the accumulation of charge even when using additives with high conductivity, such as coal.
To solve this problem, a team of researchers synthesized a polymer based on the nickel-salen complex (NiSalen). The molecules of this polymer act as a molecular wire on which energy-intensive nitroxyl fragments are attached. The molecular architecture of the material makes it possible to simultaneously achieve high power, high capacitance, and low-temperature characteristics.
When the team investigated the mechanism of charge transfer in this class of compounds, it turned out that they have two directions of development: First, they can be used as protective layers in conjunction with traditional Li-ion battery materials. Second, they can become an active component of electrical storage materials.
In the first phase of development, the team tested the concept of a new material, mixing individual components that simulate the conducting chain with redox-active nitroxyl-containing polymers. It was important to ensure that all parts of the structure worked together and reinforced each other. After that, the stage of substance synthesis began, which involved sensitive components that can easily collapse at the slightest mistake.
Of the several polymers obtained, only one was found to be stable and efficient. The main chain of the new material is formed by complexes of nickel with ligands called “salen.” A stable free radical was attached to it through covalent bonds capable of rapid oxidation and reduction (charge and discharge).
The new battery could excel in situations where very fast charging is required or when operating at low temperatures. It contains fewer metals that can cause environmental harm; nickel is present in a small amount in the polymer but there is much less of it than in Li-ion batteries.