In the search for the perfect battery, scientists have two main goals: to create a device that can store a large amount of energy and that is also safe. Many batteries contain liquid electrolytes that are potentially flammable.
Solid-state lithium-ion batteries, made up of all-solid components, are becoming increasingly attractive to scientists as they offer an enticing combination of higher safety and increased energy density.
Researchers at the University of Waterloo (Canada) have discovered a new solid electrolyte that offers several important benefits.
This electrolyte, composed of lithium, scandium, indium, and chlorine, is a good conductor of lithium ions, but a poor conductor of electrons. This combination is necessary to create an all-solid-state battery that will last more than a hundred cycles at high voltage (above 4 volts) and thousands of cycles at intermediate voltage without significant capacity loss. The chloride nature of the electrolyte is key to its stability at operating conditions above 4 volts, which means it is suitable for the typical cathode materials that form the basis of today’s lithium-ion cells.
“The main attraction of the solid state electrolyte is that it cannot catch fire and allows it to be placed efficiently in a battery cell. We were pleased to demonstrate stable operation at high voltage,” said Linda Nazar, Distinguished Professor of Chemistry at the University of Waterloo.
Current versions of solid state electrolytes focus heavily on sulfides, which oxidize and decompose at voltages above 2.5 volts. Therefore, they require the inclusion of an insulating coating around the cathode material that operates above 4 volts, which impairs the ability of electrons and lithium ions to move from the electrolyte to the cathode.
“With sulfide electrolytes, you have a kind of puzzle—you want to isolate the electrolyte from the cathode electronically so that it doesn’t oxidize, but you still need electronic conductivity in the cathode material,” Nazar added.
Although the Nazar group was not the first to invent the chloride electrolyte, the decision to replace half of the indium with scandium, based on their previous work, proved to be advantageous in terms of lower electronic and higher ionic conductivity.
“Chloride electrolytes are becoming more attractive because they only oxidize at high voltages, and some of them are chemically compatible with the best cathodes we have,” she said.
One of the chemical keys to ionic conduction lies in the criss-crossing three-dimensional structure of the spinel material. The researchers had to balance two competing goals – to load the spinel with as many charge-carrying ions as possible, but also to leave areas open to the movement of ions.
“Imagine you’re throwing a party – you want people to come, but you don’t want too many of them,” Nazar said.
According to her, in an ideal situation, half of the places in the spinel structure should be occupied by lithium, while the other half would remain open.