The propensity for lithium-ion batteries to catch fire has had major connotations in recent years, temporarily grounding Boeing’s fleet of 787 Dreamliners in 2015 and potentially spoiling Samsung's reputation for making high-quality consumer electronics after problems with fires in its Galaxy Note tablets.
The problem is caused by the liquid electrolyte used in most cells, which is flammable, especially at high temperatures. One way to get around this is to use a solid-state battery; but attempts to develop an efficient solid lithium-ion cell have stumbled because of difficulty in achieving electron flow between the electrodes inside the cell. Researchers at the University of Maryland Energy Research Centre and the A. James Clark School of Engineering now believe they have solved this problem.
The team, led by energy specialist Eric Wachsman and materials scientist Liangbing Hu, is working on batteries whose electrolyte is made from the mineral garnet, which are crystals of silicate associated generally with two or more metal ions.
Garnet has been used as a gemstone for millennia, and in more recent decades is also widely used as material in lasers. It conducts ions very well, and is very stable electrochemically, but its problem is that the impedance (electrical resistance) at the interface between a solid electrode and solid garnet is very high, so electrons cannot flow and complete the electrochemical circuit that releases the battery’s stored energy. The Maryland team has addressed this by depositing a thin film of aluminium oxide (Al203) onto the electrode surface.
In a paper in Nature Materials, the team explains that the ultrathin film reduced the impedance at the electrode-garnet interface from 1710 ohm/cm2 to only one ohm/cm2, while not stopping the flow of lithium ions. Moreover, because the garnet is so stable, the team could use electrodes made of metallic lithium at the cell anode – giving the highest possible theoretical energy density. The cell used high-capacity sulphur cathodes. The result is a battery that is easy to charge, discharges readily, is not flammable and retains low-cost; this trifecta of performance, safety and price will make them attractive to the market, Hu claims.
In a statement issued by the A. James Clark School of Engineering (a specialist department within the University of Maryland) Prof John Goodenough, a pioneer in lithium-ion cell development and now holder of a Chair in Engineering at the University of Texas, who was not involved with the research, pointed out two important innovations in the team's work: first, they changed the composition of the garnet to a dense polycrystalline solid with tight grain boundaries; and second, the aluminium oxide film prevents formation of whiskery substances known as dendrites on the surface of the anode during charging which block ion and electron transport. “"This [finding] is of considerable interest to those working to replace the flammable liquid electrolyte of the lithium-ion rechargeable battery with a solid electrolyte from which a lithium anode can be plated dendrite-free when a cell is being charged," Goodenough said.
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