A team of Russian scientists including HSE MIEM researchers have used superconcentrated salt solutions to produce effective water-based electrolytes that demonstrate high conductivity and electrochemical stability and require lower amounts of non-toxic salts, making the batteries safer and less expensive than classical non-aqueous ones. The study is published in The Journal of Physical Chemistry C.
Electrolytes are chemicals that conduct electricity when melted or dissolved. They are used in batteries to convert energy from a chemical reaction into electrical energy. Electrolytes used for metal-ion batteries often consist of alkali metal salts in organic solvents (dimethyl carbonate, ethylene carbonate, and others). But in the last decade, water-based electrolytes have become increasingly popular, as their assembly does not require a strict humidity-controlled environment, unlike typical organic electrolytes.
From a practical perspective, replacing an organic solvent with water can reduce production costs and improve a battery's safety.
The problem of aqueous electrolytes is their low electrochemical stability. Chemical reactions on electrodes resulting in oxygen and hydrogen evolution can lead to rapid degradation of electrode material and battery failure. This limits the choice of suitable electrode materials as well as the capacity of batteries using aqueous electrolytes.
The use of saturated salt solutions has been proposed as a way around the problem of low battery capacity. The study authors suggest two options for saturated solutions: those with sodium propionate and those using a mixture of lithium and potassium formates (formic acid salts) . These are simple salts without fluoride or heavy elements, making them more environmentally friendly than typical electrolytes. Lower amounts of such salts are required to achieve high conductivity. This would be the first use of such solutions as electrolytes for aqueous batteries or supercapacitors.
Saturated salt solutions increase the electrochemical potential window, which is the most important characteristic that determines electrolyte stability. In turn, a wider electrochemical potential window makes it possible to choose electrode materials with the greatest difference of potentials, leading to increased battery power and charge capacity. By conservative estimates, a potential window of 1.5 V to 2.5 V and modern electrode materials it is possible to achieve an energy capacity of a lead battery.
The researchers tested the electrolytes on several commercial electrode materials used in batteries today and demonstrated good charge performance.
We hope that our research will encourage the scientific community to explore new solutions for electrolytes. Our findings can contribute to green chemistry and green power projects, improve battery utilisation, and support the creation of new sustainable energy technologies.
Text author: Ekaterina Korchagina