Batteries with graphite anodes can lose 10% of their capacity during the initial charging cycle and each additional charge-discharge cycle reduces storage capacity further.
In order to understand the ageing mechanism and to uncover the reasons behind it, TUM scientists analysed the behaviour of batteries with graphite anodes and nickel-manganese-cobalt cathodes (NMC) at various temperatures.
Using a range of techniques, including X-rays and neutrons, they determined the significant loss in capacity during the formation step is caused by the build-up of a pacifying layer on the anode. This not only consumes active lithium, but also protects the electrolyte from decomposition at the anode. The researchers also found that active lithium in the cell is slowly used up in side reactions during operation and is thus no longer available. The process is also temperature dependent, becoming ‘quite strong’ at 60°C.
Battery manufacturers have determined through trial and error the optimal relationship between the electrode material and amount of lithium, but PhD student Irmgard Buchberger said: “Using our insights, individual processes can be improved. Possibilities include additives that improve the build-up of the pacifying layer, for example, or modification of the cathode surface.”
