Neutrons explain ageing process in Li-ion batteries

Researchers have now come a step closer to identifying the cause of lithium-ion battery ageing. 

Scientists from the Department of Technical Electrochemistry and the Research Neutron Source FRM II at the Technical University of Munich (TUM) recently combined electrochemical investigations with measurement methodologies as diverse as X-ray diffraction, impedance measurements and prompt gamma activation analysis (PGAA) in order to understand the ageing mechanism and to uncover the reasons behind them.

They deployed these methodologies to analyse the behaviour of batteries with graphite anodes and nickel-manganese-cobalt cathodes, so-called NMC cells, at various temperatures. NMC cells are popular in electromobility since they have a large capacity and can theoretically handle charging voltages up to just under 5 V. However, above 4.4 V ageing effects increase strongly.

Using X-ray diffraction, the scientists investigated the loss of active lithium over multiple charging cycles. They used impedance measurements to register the increasing resistance in the battery cells. Neutron activation analysis ultimately facilitated the accurate determination of extremely minute quantities of transition metals on the graphite electrodes.

Mechanisms of capacity reduction

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. 

The research group determined two key mechanisms for the loss of capacity during operation — the active lithium in the cell is slowly used up in various side reactions and is thus no longer available. The process is very temperature-dependent: at 25°C the effect is relatively weak, but it becomes quite strong at 60°C.

When charging and discharging cells with a higher upper cut-off potential (4.6 V), cell resistance increases rapidly. The transition metals deposited on the anode may increase the conductivity of the pacifying layer and thereby speed up the decomposition of the electrolyte.

On the road to better lithium-ion batteries

By way of trial and error, battery manufacturers have determined the optimal relationship between the electrode material and lithium. “Using our insights, now individual processes can be improved,” said Irmgard Buchberger, PhD student at the Department of Electrochemistry at TUM. “Possibilities include additives that improve the build-up of the pacifying layer, for example, or modifications of the cathode surface.”

The research was funded by the German Federal Ministry of Education and Research (BMBF) in the context of the ExZellTUM project. The prompt gamma activation analysis was done in cooperation with the Heinz Maier-Leibnitz Center (MLZ) at the PGAA instrument of the Research Neutron Source FRM II at TUM.