Researchers at Penn State and Pacific Northwest National Laboratory in the US have developed a new material that is made up of sodium, phosphorous, tin and sulfur, and which could be used as an alternative electrolyte to lithium in solid-state batteries. The discovery of the material’s structure could lead to cheaper batteries as sodium is more abundant than lithium as well as being safer to use, and the work also demonstrates a potential pathway to produce a new generation of advanced sodium-ion superionic conductors.
The electrolyte is one of three key constitutents of a solid-state battery, working to transfer charged ions. This creates an electrical current once the other parts, the anode and cathode, are connected in a circuit. Most rechargeable batteries in consumer electronics, from smart phones to computers, use a liquid, lithium-based electrolyte, so this could be an important breakthrough. As researcher Donghai Wang points out, “Liquid electrolytes have safety issues because they are flammable. That has been the driving force for us to find a good material for use in solid-state batteries.”
“Our approach that uses both computation and experiments allows us to analyze the reason why materials perform differently. That will make things faster for the next round of design because we know what we need to control in order to enhance ion transportation.”Zi-Kui Liu
As reported in Nano Energy [Yu et al. Nano Energy (2018) DOI: 10.1016/j.nanoen.2018.01.046], the sodium-ion electrolyte has a newly discovered structure – a tetragonal crystal shape. This means there is spaces where some sodium, tin and sulfur atoms would be that allow for it to transfer ions. The material has a wide voltage window and also high thermal stability. On heating liquid electrolytes to 1500C, they either catch fire or release a great deal of heat, which could cause damage to the other battery or electronic components. However, this material was found to perform well up to 4000C.
The electrolyte material has room temperature ionic conductivity of around a tenth of liquid electrolytes used in standard batteries, and it is crucial that the researchers found the particular configuration of defects within the crystal structure. The team developed and tested their new battery based on a collaborative design process, which helped them identify how different crystal formations, as well as inconsistencies in the material, can have an impact on performance.
As researcher Zi-Kui Liu said “Our approach that uses both computation and experiments allows us to analyze the reason why materials perform differently. That will make things faster for the next round of design because we know what we need to control in order to enhance ion transportation.”