Using a new class of materials that can transport lithium ions much faster than typical electrodes, researchers at the University of Cambridge have hit upon a discovery that could be used to make higher-power, faster-charging batteries.
Batteries are based upon three components: a positive electrode, a negative electrode and an electrolyte. Charging a lithium-ion battery extracts its lithium ions from the positive electrode and moves them through the crystal structure and electrolyte into the negative electrode, where they are stored. The faster this process can occur, the faster the battery can be charged.
Researchers typically address the challenge of improving this process by attempting to shrink the battery. “But it's difficult to make a practical battery with nanoparticles,” explained Kent Griffith, a postdoctoral researcher in the chemistry department at Cambridge. “You get a lot more unwanted chemical reactions with the electrolyte, so the battery doesn't last as long, plus it's expensive to make."
“Nanoparticles are also challenging to work with on a practical level, as they tend to be quite 'fluffy,’” added Griffith’s colleague Clare Grey, a Cambridge chemistry professor. “It's difficult to pack them tightly together, which is key for a battery's volumetric energy density."
The researchers turned instead to niobium tungsten oxides, which have a complex mixed-metal composition and larger particle sizes than many other electrode materials. Their open, rigid crystalline structure allows lithium ions to move through them in three dimensions — and at rates that are several orders of magnitude higher than what is typical.
The materials also represent a safer alternative to graphite, which is often used to make negative electrodes for lithium-ion batteries. When charged at high rates, graphite tends to form dendrites — spindly lithium metal fibers that can create a short-circuit and cause the batteries to catch fire, or even explode.
Moreover, a lot of nanoparticle structures take multiple steps to synthesize through processes that yield only a tiny amount of material. The niobium tungsten oxides, by contrast, are simple to make — giving promise to scalability. And even though the oxides offer a lower cell voltage than some electrode materials, this is actually a positive — both for safety, and keeping the practical energy density high during fast cycles.
"Fields stagnate if you don't keep looking for new compounds," Clare added. "These interesting materials give us a good insight into how we might design higher rate electrode materials."
The research appears in the journal Nature.
