Battery aging mechanism discovery helps extend storage capacities
Lithium ion batteries with graphite anodes suffer their first significant loss of capacity during the initial charging cycle, the formation step. A battery loses up to ten percent of its capacity in the process. Each additional charge-discharge cycle reduces storage capacity further, if only insignificantly. Capacity is also lost through the mere storage of batteries – especially above room temperature.
TUM scientists at the Department of Technical Electrochemistry and from the FRM II claim to have come a closer to narrowing the knowledge gap in their latest experiments. In order to understand the aging mechanism and to uncover the reasons behind them, TUM scientists combined electrochemical investigations with measurement methodologies as diverse as X-ray diffraction, impedance measurements and prompt gamma activation analysis (PGAA).
The researchers deployed the methodologies to analyze the behavior of batteries with graphite anodes and nickel-manganese-cobalt cathodes, so-called NMC cells, at various temperatures. NMC cells are popular in electromobility because they have a large capacity and can theoretically handle charging voltages up to under five volts. However, above 4.4 volts aging effects increase strongly.
Using X-ray diffraction, the scientists investigated the loss of active lithium during multiple charging cycles. Impedance measurements were used to register the
increasing resistance in the battery cells. Neutron activation analysis ultimately facilitated the accurate determination of minute quantities of transition metals on the graphite electrodes.
The capacity loss in the formation step is caused by the build-up of a pacifying layer on the anode which 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 no longer available. The process is temperature dependent: At 25°C the effect is relatively weak but 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.
By way of trial and error, battery manufacturers have determined the optimal relationship between the electrode material and amount of lithium. “Using our insights, now individual processes can be improved,” said 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.”
Reference:
I. Buchberger, S. Seidlmayer, A. Pokharel, M. Piana, J. Hattendorff, P. Kudejova, R. Gilles, and H. A. Gasteiger; Aging Analysis of Graphite/LiNi1/3Mn1/3Co1/3O2 Cells
Using XRD, PGAA, and AC Impedance; Journal of The Electrochemical Society, 162, A2737 (2015); DOI: 10.1149/2.0721514jes
Related articles and links:
https://jes.ecsdl.org/content/162/14/A2737.abstract
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