Apart from particle-type inorganic solid electrolytes, organic, i.e. solid polymer electrolytes (SPEs), are of high potential interest for the realization of next generation Li metal batteries, given their abundance, low cost, electrochemical stability and wetting ability.(1, 2)

Nevertheless, the poor ion transport in SPEs limits the battery operation to elevated temperature and/or lower rates only and remains main focus of R&D.(3) Besides the internal-resistance induced polarizations (overpotentials), it is the onset of concentration polarization, which determines the operation limit in terms of e.g. temperature and current density.

This work aims to practically unravel the mystery of concentration polarization by means of simple electrochemical experiments in Li||Li cells and mathematical descriptions via the well-known Sand and diffusion equations.(4, 5) The conformity of theory and experiments allows valuable mathematical determinations and predictions of parameter and operation limits. For example, these equations can predict the practical onset of concentration polarization. Also, parameter can be obtained, e.g. diffusion coefficients, based on the experimentally observed polarization onsets.

The relevance of concentration polarization including its impact on the cell performance even in high voltage LiNi_0.6Mn_0.2Co_0.2O₂ (NMC622)||Li cells is demonstrated by experimentally varying the applied current, the salt concentration, the temperature as well as the cell set-up (e.g. electrolyte thickness and electrode area-oversizing).(6) The validity of these relations is additionally confirmed in state-of-the-art liquid, i.e. LiPF₆/carbonate-based, electrolytes and the special case of single-ion conducting electrolytes is discussed.

1. L. Stolz, S. Röser, G. Homann, M. Winter and J. Kasnatscheew, The Journal of Physical Chemistry C, 125, 18089 (2021).
2. J. Mindemark, M. J. Lacey, T. Bowden and D. Brandell, Prog. Polym. Sci., 81, 114 (2018).
3. J. Janek and W. G. Zeier, Nature Energy, 1, 16141 (2016).
4. L. Stolz, G. Homann, M. Winter and J. Kasnatscheew, Data in Brief, 34, 106688 (2021).
5. L. Stolz, G. Homann, M. Winter and J. Kasnatscheew, Materials Today, 44, 9 (2021).
6. L. Stolz, G. Homann, M. Winter and J. Kasnatscheew, ChemSusChem, 14, 2163 (2021).