Novel mobile and stationary applications require batteries with increasing energy density / specific energy, fast-charging capability,¹ and long life-time – while maintaining a high safety level. However, there is a dearth of knowledge on the interactions between these parameters. Additionally, the data sheets of commercial cells contain usually only a limited set of parameters, such as a wide temperature range and only one maximum charging C-rate. Therefore, it is highly interesting to correlate electrochemical results with material and electrode parameters.

In the present study, we investigated nine types of commercial Li-ion cells (including 18650 and 21700 cells) as well as cells built in our labs. The commercial cells were firstly subject to tests showing their suitability for further experiments. Our experiments reveal interesting specific influences and general trends of materials, electrode and cell properties on specific energy, fast-charging capability, cell resistance, and aging effects. Especially, Li deposition which becomes thermodynamically favorable at negative anode potentials vs. Li/Li⁺ is to be avoided due to its negative impact on safety and aging.²

In order to judge the fast-charging capability we defined three criteria which have to fulfilled:

1. Charged capacity after fast-charging in comparison to slow charging at 0.1C (>80%)
2. Temperature rise on cell surface (<60°C)
3. Occurrence of Li deposition, estimated from voltage relaxation curves.^3,4

Since voltage relaxation curves give only hints on Li deposition while averaging over the whole anode of the cell, it is compared with Post-Mortem analysis,⁵ measurements of the anode potential vs. Li/Li⁺ in 3-electrode full cells⁶ for exemplary cells, and with neutron diffraction data.⁴

The determination of fast-charging capability is applied to nine types of cylindrical cells for different ambient temperatures (0-40°C) and charging C-rates (0.1-3C), resulting in large data sets for further evaluation. Passing/failing of the three criteria as a function of C-rate and ambient temperature is visualized in fast-charging capability maps (Figure 1c,e).

Although the tested commercial cells are from different manufacturers and contain different cathode (NMCs, LFP, NCA, NMC/LMO blend) and anode materials (graphite with different amounts of Si <5%) it is possible to find interesting general trends. For example there is an interaction between the fulfillment of the three fast-charging criteria (Figure 1a) corresponding to either high-energy or high-power cells (Figure 1b,d). In contrast, the change from the 18650 to the 21700 format shows only minor effects on specific energy⁷ and fast-charging capability. However, the cell format has a significant effect on heating behavior, energy content per cell (~50% more energy in 21700 cells with same electrodes), and therefore most likely a positive impact on costs per produced Wh.⁷

The study reveals specific influences and general trends in Li-ion cells and gives a perspective for necessary material and electrode improvements to fulfill the requirements for future battery generations.

References:

1. S. Ahmed et al., J. Power Sources, 367, 250–262 (2017).
2. T. Waldmann, B.-I. Hogg, and M. Wohlfahrt-Mehrens, J. Power Sources, 384, 107–124 (2018).
3. C. Uhlmann, J. Illig, M. Ender, R. Schuster, and E. Ivers-Tiffée, J. Power Sources, 279, 428–438 (2015).
4. C. von Lüders et al., J. Power Sources, 342, 17–23 (2017).
5. T. Waldmann et al., J. Electrochem. Soc., 164, A3154–A3162 (2017).
6. T. Waldmann, M. Kasper, and M. Wohlfahrt-Mehrens, Electrochimica Acta, 178, 525–532 (2015).
7. J. B. Quinn, T. Waldmann, K. Richter, M. Kasper, and M. Wohlfahrt-Mehrens, J. Electrochem. Soc., 165, A3284–A3291 (2018).

The research leading to these results has been performed within the projects ReLiOn (BMBF, #03X4619C), MAT4BAT (EU/FP7, #608931), and FAB4LIB (BMBF, 03XP0142D). The authors would like to thank the German Federal Ministry of Education and Research (BMBF) and the European Commission for funding.

Figure 1 a) Correlation between criteria relevant for fast-charging. b),d) Aging model/performance model and c),e) fast-charging maps for high-power cells (b,c) and high-energy cells (d,e).