The concept relies on understanding the way vibrations move through the crystal lattice of lithium ion conductors and correlating that with the way they inhibit ion migration. This provides a way to discover new materials with enhanced ion mobility, allowing rapid charging and discharging. At the same time, the researchers claim the method can be used to reduce the material’s reactivity with the battery’s electrodes, which can shorten its useful life. These two characteristics — better ion mobility and low reactivity — have tended to be mutually exclusive.
Professor Yang Shao-Horn explains the idea was inspired by work she and her team were doing which looked at understanding and controlling catalysts for water splitting and applying it to ion conduction.
While electrons flow from one pole of the battery to the other, positive ions flow the other way, through an electrolyte, or ion conductor, sandwiched between those poles, to complete the flow.
Typically, that electrolyte is a liquid. A lithium salt dissolved in an organic liquid is a common electrolyte in today’s lithium-ion batteries. But that substance is flammable and has sometimes caused these batteries to catch fire. Consequently, the search for a solid material to replace it, which would eliminate that issue, has ensued.
A variety of promising solid ion conductors exist, but none are stable when in contact with both the positive and negative electrodes in lithium-ion batteries, Shao-Horn adds. So, seeking solid ion conductors that have both high ion conductivity and stability is critical.
However, sorting through the different structural families and compositions is tricky. To overcome this, Shao-Horn proposes to find materials that have ion conductivity comparable to that of liquids, but with the long-term stability of solids. The team’s research explored questions such as: ‘What is the fundamental principle?’ and ‘What are the design principles on a general structural level that govern the desired properties?’. A combination of theoretical analysis and experimental measurements has now yielded some answers, the researchers say.
“We realised that there are a lot of materials that could be discovered, but no understanding or common principle that allows us to rationalise the discovery process,” says Sokseiha Muy, the paper’s lead author. “We came up with an idea that could encapsulate our understanding and predict which materials would be among the best.”
According to MIT, the key was to examine the lattice properties of these solid materials’ crystalline structures. This governs how vibrations, such as waves of heat and sound (known as phonons) pass through materials.
This novice way of looking at the structures allows accurate predictions of the materials’ actual properties, the researchers explain.
“Once you know [the vibrational frequency of a given material], you can use it to predict new chemistry or to explain experimental results,” Shao-Horn says.
The researchers say they observed a good correlation between the lattice properties, determined using the model and the lithium ion conductor material’s conductivity. They found, in particular, that the vibrational frequency of lithium itself can be fine-tuned by tweaking its lattice structure, using chemical substitution or dopants to subtly change the structural arrangement of atoms.
The team believe this concept can provide a powerful tool for developing better-performing materials that could lead to dramatic improvements in the amount of power that could be stored in a battery of a given size or weight, as well as improved safety.
The team adds that it has already used the method to find some ‘promising candidates’ and that the techniques could also be adapted to analyse materials for other electrochemical processes, such as solid-oxide fuel cells; membrane based desalination systems; or oxygen-generating reactions.
