Dissolution of Transition Metals in Lithium Ion Batteries with NMC-Based Electrodes
Authors: Warnecke Alexander , Graff Rita , Sauer Dirk Uwe 
Published: February 2020

2016 the lithium-ion battery is commercially available for 25 years and has improved since the marked entrance drastically. This shows that a lot of effort was put into research finding new materials for longer lifetime, higher energy density and power capability. This results in a high diversity of electrode materials so there is not only one combination of anode and cathode active materials that is used. Since a few years the combination of graphite anodes and lithium nickel manganese cobalt oxide (NMC) is seen as a promising candidate for electric vehicle batteries [1]. There was put a lot of effort into research on ageing effects of graphite electrodes and the processes are basically understood. For NMC cathode, the basic ageing mechanisms are known, but the research goal is rather on increasing the energy density, than on a longer lifetime. This can be explained by the lower influence of the cathode on the ageing of the cell, on first view.
Vetter et al. [2] describe the basic aging mechanisms. They list them in general as aging of active material, degradation of inactive components, oxidation of electrolyte components and interaction of aging products with the anode. The scope of this work is on an effect that belongs to the interaction of aging products with the anode, the dissolution of the transition metals. The dissolved transition metal of the cathode is deposited on the anode, which leads to an increased SEI formation and thus a higher capacity loss [3]. This effect is studied within the framework of the European project Batteries2020 [4], where the main objective is, to develop better batteries for the next generation of electric vehicles. This will be done by using improved NMC cathode materials.
The base line of batteries in the project is a commercial available 20 Ah pouch cell from EIG with a 4:4:2 NMC, which is aged under different conditions to investigate the influence factors for aging. In total 161 cells are aged under different calendric and cyclic conditions. Selected cells are taken out of test to study them in detail. Most of the cells are taken out of test at the end of life, which is defined by 80% remaining capacity, but at some conditions cells are analyzed over the aging process to see a development of degrading parameters. After taking the cells out of test, a detailed post mortem analysis under argon atmosphere will be done. Part of the post mortem analysis is the metal content determination of the anode by ICP-OES measurement. With this technique the deposited transition metals on the anode can be measured and compared with a fresh cell. As can be seen in Figure 1 the concentration of the NMC on the anode of the aged cell is up to five times higher compared to a new cell, depending on the ageing condition.
To study the influence of different stress factors for this effect a method that is also described in [5] is used, where active material samples are stored in electrolyte and the concentration of transition metals in the electrolyte are measured over time. For the dependency of the state of charge, samples with different amount of intercalated lithium are tested. The temperature dependency is studied storing the active material samples in different ovens. This leads to a function which can describe the dissolution of the transition metals for the calendric storage of NMC cells with respect to temperature. This function will be validated by examining cells from the ageing matrix and cells from real driving profiles.
As an outlook, additional NMCs are studied to investigate the influence of the composition. For this a 1:1:1 NMC is used, which was introduced as the first NMCs on the marked and a 6:2:2 NMC from the Batteries2020 project, which shows the current development trend to high nickel content, for increasing energy densities.
The research leading to these results has received funding from the European Union’s Seventh Framework Programme (FP7/2007-2013) under grant agreement no. 608936.

References

[1] Z. Liu et al., Journal of Power Sources, 81–82 (1999), 416-419
[2] J. Vetter et al., Journal of Power Sources, 147 (2005), 269-281
[3] Y. Dai et al, Journal of The Electrochemical Society, 160 (2013), A182-A190
[4] www.batteries2020.eu
[5] M. Börner et al., Journal of The Electrochemical Society, 163 (2016), A831-A837

Figure 1