Russian chemists have studied the high-voltage stability of concentrated solutions of lithium salts in a special solution. The new data will make it possible to advance in the development of the next generation of lithium-ion (and, in the future, sodium-ion) batteries, whose energy intensity will be increased due to increased operating voltage. About the study reports an article in the journal Electrochimica Acta. The work is supported by a grant from the Russian Science Foundation (RNF) within the framework of the Presidential Program of Research Projects.
Lithium-ion batteries have largely changed the face of our civilization. The rapid development of the technology of these batteries, which appeared in 1991, was due to the growing popularity of portable electronics: mobile phones, laptops, tablets and other gadgets. Now the market continues to develop thanks to the widespread introduction of electric transport, robotics, storage and distribution systems. One of the main advantages of lithium-ion batteries in front of their predecessors is their high operating voltage – an average of 3.3-3.8 volts, depending on the type of electrode material (cathode). The voltage of a single cell of the battery, along with its electrochemical capacity, determines its energy intensity, that is, the amount of energy stored. Many scientists and employees of companies are trying to solve the problem of further increasing the operating voltage of lithium-ion batteries to 5 volts or higher. The main obstacle is the absence of a suitable current conductor – electrolyte.
“High-voltage electrolytes are relevant for any type, including lithium-ion and sodium-ion batteries,” comments Oleg Drozhzhin, Ph.D. in chemistry and a senior research fellow at the Department of Electrochemistry, Moscow State University, Department of Chemistry, who directed this part of the work under the grant of the RNF. “If we have a battery with a working voltage of 3.7 volts, and we will make a battery with a working voltage of 5 volts, then it will almost 1.5 times increase energy intensity, and we will increase the amount of energy stored by 1.5 times.”
Lithium-ion batteries have been made for many years from liquid electrolytes – a special organic solvent with ions (charged particles) of lithium salts. The best current passes at a certain concentration of these salts in the solution, which has long been known – about 1 mole per liter. Therefore, all commercial lithium-ion batteries are made from electrolyte of this concentration. This electrolyte can withstand the voltage to 4.2-4.3 volts. However, when choosing a high-voltage system capable of storing a larger volume of energy, the quality of the current conductor no longer comes to the forefront, but its electrochemical resistance to oxidation with a higher potential, for example, 5 volts.
Since the positive high-voltage electrodes (cathodes) themselves are more resistant to voltage, the maximum battery voltage depends primarily on the stability of the electrolyte. This value is trying to approximate to 5 – 5.5 volts. For this purpose, various additives can be used, the solvent or salts in the electrolyte can be changed. In recent years, scientists around the world are studying solutions with an increased concentration of salts. Initially, the behavior of the negative electrode (anode) was considered, and only in 2016 it was possible to affect the cathode of the battery. Japanese chemists increased the stability of the lithium-ion cell at a voltage of 5 volts, increasing the concentration of the electrolyte solution from 1 mole per liter to 10 moles per liter. True, for this I had to use a fairly rare and expensive salt of LiFSA. Its use would greatly increase the cost of batteries.
Russian chemists decided to find a compromise between stability at a high voltage, high efficiency and cost of the electrolyte. They used a well-known, stable and cheap enough lithium salt and a high voltage resistance solutions studied on its base at concentrations of 1-3 moles per liter.
“Increasing the concentration of lithium salt LiBF4 to about three moles per liter significantly increases the high-voltage stability of the electrolyte. We showed that, first, the resistance of the electrolyte to high voltages increased, and secondly, it was possible to obtain a smaller spread of capacitance from cycle to cycle, which is often observed for dilute solutions, especially at high operating voltage, “says Oleg Drozhzhin on the results work.
In addition, scientists have tried to understand, because of what the properties of the resulting electrolyte change: because of the solution itself or formed on the surface of the positive electrode of the intermediate layer.
“In many ways, the properties of the electrolyte are determined by the properties of the layer – such a film on one of the electrodes, consisting of products of oxidation or reduction of electrolyte,” explains Oleg Drozhzhin. – If this is set exactly for the anode, then the cathode is more complicated. We made several experiments, examined the surface of the electrodes and found out that in the case of high-voltage concentrated electrolytes, this film is the same as in the case of “ordinary” dilute (1 mole per liter), or it is so thin that it has almost no effect, but it all depends on the properties of the solution itself. ”
The proposed approach can be used to create high-voltage lithium-ion batteries, as well as the next generation of electrochemical systems, for example, sodium-ionic or potassium-ionic batteries. These batteries are considered as a cheaper alternative to LIA for creating large-sized energy storage devices.
Source : Moscow State University