High Voltage Li-Ion Batteries: Identical Additives but Different Impacts in EC-Based vs. EC-Free Electrolytes
Authors: Kasnatscheew Johannes , Klein Sven , Placke Tobias , Stolz Lukas , Winter Martin 
Published: December 2024

Specific energy and energy density of conventional LiNixCoyMn
z
O2 (NCM) || graphite Li-ion batteries can be increased via further increase of the charge voltage, e.g., to 4.5 V, however is typically accompanied by a sudden and rapid capacity fade, literature-known as “rollover” failure. This cell failure is the result of plated lithium, e.g., Li dendrites, triggered in the course of electrode cross-talk, i.e., dissolution of transition metals (TMs) from the cathode and deposition on the anode.[1]

Interestingly, elimination of the apparently essential ethylene carbonate (EC) from a state-of-the-art electrolyte, i.e., from 1.0 M LiPF6 in a 3:7 mixture (by wt.) of EC and ethyl methyl carbonate (EMC) prevents this failure.[2] While the oxidative stability on the cathode side is similar in both electrolytes, as indicated by a decomposition plateau at 5.5 V vs. Li|Li+, the anode side in the EC-free electrolyte reveals significantly less TM deposits and Li metal dendrites compared to the EC-based electrolyte. The support of analytical and visual techniques demonstrates a pronounced increase in the amount of degraded LiPF6 species (Li
x
PO
y
F
z
), which are shown to effectively trap dissolved TMs and increase cycle life by suppressing the detrimental cross-talk (Figure 1). It is also validated in EC-based electrolytes simply via (1) electrolyte additive based on this species, i.e.., lithium difluorophosphate (LiDFP),[3] and via forced LiPF6 decomposition, either (2) at higher storage temperature[4] or (3) via reaction with coated separator[5]. Given its toxicity disadvantage,[6] other strategies, like electrolyte additives, cathode modifications, e.g., single crystal NCM,[7] cathode coatings,[8] or even anode modifications are discussed, as well.[9] The different impacts of respective additives in EC-based vs. EC-free electrolytes are discussed.

Reference
s
:

[1] S. Klein, P. Bärmann, T. Beuse, K. Borzutzki, J. E. Frerichs, J. Kasnatscheew, M. Winter, T. Placke ChemSusChem. 2021, 14, 595-613.
[2] S. Klein, S. van Wickeren, S. Röser, P. Bärmann, K. Borzutzki, B. Heidrich, M. Börner, M. Winter, T. Placke, J. Kasnatscheew Advanced Energy Materials. 2021, 11, 2003738.
[3] S. Klein, P. Harte, S. van Wickeren, K. Borzutzki, S. Roser, P. Barmann, S. Nowak, M. Winter, T. Placke, J. Kasnatscheew Cell Reports Physical Science. 2021, 2.
[4] S. Klein, P. Harte, J. Henschel, P. Barmann, K. Borzutzki, T. Beuse, S. van Wickeren, B. Heidrich, J. Kasnatscheew, S. Nowak, M. Winter, T. Placke Advanced Energy Materials. 2021, 11.
[5] S. Klein, J. M. Wrogemann, S. van Wickeren, P. Harte, P. Barmann, B. Heidrich, J. Hesper, K. Borzutzki, S. Nowak, M. Borner, M. Winter, J. Kasnatscheew, T. Placke Advanced Energy Materials. 2022, 12.
[6] M. Kubot, L. Frankenstein, E. Muschiol, S. Klein, M. Esselen, M. Winter, S. Nowak, J. Kasnatscheew ChemSusChem. 2023, 16, e202202189.
[7] S. Klein, P. Bärmann, O. Fromm, K. Borzutzki, J. Reiter, Q. Fan, M. Winter, T. Placke, J. Kasnatscheew J. Mater. Chem. A. 2021, 9, 7546-7555.
[8] S. Klein, L. Haneke, P. Harte, L. Stolz, S. van Wickeren, K. Borzutzki, S. Nowak, M. Winter, T. Placke, J. Kasnatscheew ChemElectroChem. 2022, 9, e202200469.
[9] S. Klein, P. Barmann, L. Stolz, K. Borzutzki, J. P. Schmiegel, M. Borner, M. Winter, T. Placke, J. Kasnatscheew ACS Applied Materials & Interfaces. 2021, 13, 57241-57251.

Figure 1