LITHIUM-ION batteries that are widely used in modern day electronic gadgets, long-range electric cars and for storage of energy from renewable sources, the development of which fetched this year’s Nobel prize in chemistry to the trio of scientists, use graphite or petroleum coke as the anode material that gives up the intercalated lithium ions in its bulk during electrolysis to be readily taken up by the cobalt oxide cathode.
Scientists are still far from realising the original idea of the Laureate Stanley Whittingham to use lithium metal itself as the anode, which has a much higher energy density potential than today’s lithium-ion batteries, because of the highly reactive nature of lithium metal. If realised, these promise lightweight and long-lasting next generation lithium batteries that could, for instance, potentially double the range of electric cars.
But the problem of bringing these batteries from the laboratory to the market has been their high failure rate. The major issue is what is known as low Coulombic efficiency, meaning they undergo a limited number of (charge-discharge) cycles before they stop working. That is because as the battery goes through the cycles, its stores of active lithium and electrolyte get depleted.
Now, a research team led by scientists at the University of California at San Diego (UCSD) has identified the root cause of the failure. Bits of lithium metal deposits break off from the surface of the anode during discharging and are trapped as “dead” or inactive lithium that the battery cannot access.
The work, which has been published in a recent issue of “Nature”, challenges the conventional belief that lithium metal batteries fail because of the growth of a layer, called the solid electrolyte interface (SEI), between the lithium anode and the electrolyte. Although researchers developed various ways to control and stabilise the SEI layer, the problem remained unresolved. “The cells fail because a lot of inactive lithium is forming in these batteries. So there was another important aspect that was being overlooked,” said Y. Shirley Meng, a nano engineering professor at UCSD.
What the researchers found was that because of the lithium deposits in the SEI layer, they lose their electric connection to the anode, becoming inactive lithium that can no longer be cycled through the battery. The trapped lithium is largely responsible for lowering the Coulombic efficiency of the cell.
The present team made the discovery by developing a technique to measure the amounts of inactive (lithium) species on the anode—for the first time in battery research—and study their micro and nanostructures.