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Owing to the low cost and high abundance of sodium, sodium‐based batteries, especially those employing metallic sodium anodes, are considered for post‐lithium energy storage. In order to develop high‐performance and long‐lasting sodium‐metal batteries, however, the reversible Na‐metal stripping and plating challenge must be addressed. Most organic electrolytes suffer from non‐uniform and continuous formation of the solid electrolyte interphase as well as unfavorable dendritic growth. The use of sodium cyclopentadienide dissolved in tetrahydrofuran as the electrolyte reveals an improved reversibility of sodium dissolution and electrodeposition combined with an electrochemical stability window of around 2.2 V vs. Na/Na+ and an ionic conductivity of 1.36 mS cm−1 at 25 °C. Furthermore, the plated electrodes showed a remarkable morphology of the Na deposits, that is, no dendrite formation, whereby the above‐mentioned electrolyte could overcome the aforementioned cycling issues, thus suggesting suitability for further studies.

The cobalt-free spinel LiNi0.5Mn1.5O4 (LNMO) represents a promising candidate for more sustainable high-energy lithium-ion cathodes due to its high operating voltage (4.7 V vs. Li+/Li), low cost and environmental impact, especially when processed in aqueous suspension with water-soluble binders.[1,2] Although the intrinsic properties of the material are already studied extensively and understood quite well, the high working potential still poses challenges for LNMO-comprising lithium-ion battery cells, which need to be addressed before the material can be successfully commercialised. This specifically includes the decomposition of the electrolyte at high potentials and the dissolution of transition metal (TM) ions from the positive electrode and their migration to the negative electrode.[1,3] The latter leads to accelerated electrolyte decomposition also on the anode side and an overall increased gas formation inside the cell.[4] Besides the modification of the material and its surface itself, one of the most promising approaches is the use of electrolyte additives, since this is easy to implement and, therefore, also cost effective. Such additives can be designed to extend the electrochemical stability window, to stabilise the electrode|electrolyte interphases and/or to scavenge harmful species inside the electrolyte.[5–7] The ternary mixture introduced herein decomposes preferentially on both electrodes leading to improved interfacial stability, which is directly reflected in an increased cycling performance of true 5V LNMO‖graphite Li-ion cells – also at elevated temperatures. An in-depth XPS analysis unveiled the growth and composition of a stable interphase layer and showed that this limits the amount of TM deposits on the anode. Finally, in order to demonstrate the beneficial impact also in terms of commercial applicability, the gassing behaviour was investigated qualitatively by differential electrochemical mass spectrometry (DEMS) and quantitatively by exploiting Archimedes’ principle to determine the gas volume formed operando during the cycling of LNMO‖graphite pouch cells. [1] G. Liang, V. K. Peterson, K. W. See, Z. Guo, W. K. Pang, J. Mater. Chem. A 2020, 8, 15373–15398. [2] M. Kuenzel, D. Bresser, T. Diemant, D. V. Carvalho, G. T. Kim, R. J. Behm, S. Passerini, ChemSusChem 2018, 11, 562–573. [3] J. H. Kim, N. P. W. Pieczonka, L. Yang, ChemPhysChem 2014, 15, 1940–1954. [4] B. Michalak, B. B. Berkes, H. Sommer, T. Brezesinski, J. Janek, J. Phys. Chem. C 2017, 121, 211–216. [5] M. Y. Abeywardana, N. Laszczynski, M. Kuenzel, D. Bresser, S. Passerini, B. Lucht, Int. J. Electrochem. 2019, 2019, 1–7. [6] M. S. Milien, H. Beyer, W. Beichel, P. Klose, H. A. Gasteiger, B. L. Lucht, I. Krossing, J. Electrochem. Soc. 2018, 165, A2569–A2576. [7] A. Kazzazi, D. Bresser, M. Kuenzel, M. Hekmatfar, J. Schnaidt, Z. Jusys, T. Diemant, R. J. Behm, M. Copley, K. Maranski, J. Cookson, I. de Meatza, P. Axmann, M. Wohlfahrt-Mehrens, S. Passerini, J. Power Sources 2021, 482, 228975.

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