A German research group has discovered the two key mechanisms for the loss of capacity in lithium ion batteries and hope the findings will lead to more efficient battery manufacture.
The scientists from the Technical University of Munich found that the active lithium in the cell was slowly used up in various side reactions in a very temperature dependent process – At 25 °C the effect is relatively weak but becomes quite strong at 60 °C.
Lithium ion batteries with graphite anodes were patented only in 1989 but have become important for the operation of everything from small electrical devices to cars and locomotives.
But the have a key weakness – a tendency to age that reduces their potential storage capacity through a process that has not until now been completely understood.
Batteries with graphite anodes suffer their first significant loss of capacity during the initial charging cycle when they can lose up to 10% of capacity. Each additional charge-discharge cycle reduces storage capacity further, albeit by a tiny amount each time. Capacity is also lost through the mere storage of batteries – especially above room temperature.
The TUM scientists found that the significant capacity loss in the formation step is caused by the build-up of a pacifying layer on the anode. This consumes active lithium, but also protects the electrolyte from decomposition at the anode.
Battery manufacturers have determined the optimal relationship between the electrode material and lithium, largely through trial and error, the TUM researchers say.
“Using our insights, now individual processes can be improved,” says Irmgard Buchberger, PhD student at the Department of Electrochemistry at TU Munich. “Possibilities include additives that improve the build-up of the pacifying layer, for example, or modifications of the cathode surface.”
Bill Condie is a science journalist based in Adelaide, Australia.
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