London > Scientists have identified a new class of materials which could be used to make smartphone batteries that charge faster. The researchers, from the University of Cambridge in the UK, used materials with a complex crystalline structure and found that lithium ions move through them at rates that far exceed those of typical electrode materials, which equates to a much faster-charging battery.
Although these materials, known as niobium tungsten oxides, do not result in higher energy densities when used under typical cycling rates, they come into their own for fast charging applications, according to the research published in the journal Nature. Additionally, their physical structure and chemical behaviour give researchers a valuable insight into how a safe, super-fast charging battery could be constructed, and suggest that the solution to next-generation batteries may come from unconventional materials.
"We are always looking for materials with high-rate battery performance, which would result in a much faster charge and could also deliver high power output," said Kent Griffith, a postdoctoral researcher in Cambridge's Department of Chemistry. In their simplest form, batteries are made of three components: a positive electrode, a negative electrode and an electrolyte.
When a battery is charging, lithium ions are extracted from the positive electrode and move through the crystal structure and electrolyte to the negative electrode, where they are stored. The faster this process occurs, the faster the battery can be charged, researchers said.
The niobium tungsten oxides used in the current work have a rigid, open structure that does not trap the inserted lithium, and have larger particle sizes than many other electrode materials. Griffith suggests that the structural complexity and mixed-metal composition are the very reasons the materials exhibit unique transport properties.
"Many battery materials are based on the same two or three crystal structures, but these niobium tungsten oxides are fundamentally different," said Griffith. The oxides are held open by 'pillars' of oxygen, which enables lithium ions to move through them in three dimensions.
"The oxygen pillars, or shear planes, make these materials more rigid than other battery compounds, so that, plus their open structures means that more lithium ions can move through them, and far more quickly," Griffith said. Using a technique called pulsed field gradient (PFG) nuclear magnetic resonance (NMR) spectroscopy, which is not readily applied to battery electrode materials, the researchers measured the movement of lithium ions through the oxides, and found that they moved at rates several orders of magnitude higher than typical electrode materials.
Most negative electrodes in current lithium-ion batteries are made of graphite, which has a high energy density. However, when charged at high rates, it tends to form spindly lithium metal fibres known as dendrites, which can create a short-circuit and cause the batteries to catch fire and possibly explode.
"In high-rate applications, safety is a bigger concern than under any other operating circumstances," said Professor Clare Grey, also from the Department of Chemistry at Cambridge. "These materials, and potentially others like them, would definitely be worth looking at for fast-charging applications where you need a safer alternative to graphite," Grey said.
