Sand equation and its enormous practical relevance

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Materials Today d Volume 44 d April 2021 SHORT COMMUNICATION The Sand equation and its enormous practical relevance for solid-state lithium metal batteries Lukas Stolz 1, Gerrit Homann 1, Martin Winter 1,2,⇑, Johannes Kasnatscheew 1,⇑ 1 Helmholtz-Institute Münster, IEK-12, Forschungszentrum Jülich GmbH, Corrensstraße 46, 48149 Münster, Germany 2 MEET Battery Research Center, Institute of Physical Chemistry, University of Münster, Corrensstraße 46, 48149 Münster, Germany In this work, different Li salt concentrations and ionic conductivities of poly(ethylene oxide)-based solid polymer electrolytes (PEO-based SPEs) are correlated with the performance of LiNi0.6Mn0.2Co0.2O2 (NMC622)||Li full cells. While the SPEs with different salt concentrations behave similarly in NMC622|| Li cells at 60 C, their influence on the specific capacities is significant at 40 C. Below a distinct salt concentration, i.e. > 20:1 (EO:Li), a sudden blocking-type polarization appears, indicatable by an almost vertical voltage profile, both in full and in Li||Li symmetric cells. The corresponding time and current density for this polarization-type is shown to mathematically fit with the Sand equation, which subsequently allows calculation of D+ . According this relation, lack of Li+ in the electrolyte close to the Li electrode surface can be concluded to be the origin of this polarization, but is shown to appear only for “kinetically limiting” conditions e.g. above a threshold current density, above a threshold SPE thickness and/or below a threshold salt concentration (ionic conductivity), i.e. at mass transfer limiting conditions. With the support of this relation, maximal applicable current densities and/or SPE thicknesses can be calculated and predicted for SPEs. Introduction Lithium metal batteries (LMBs) promise higher energy densities and specific energies compared to the state-of-the-art (SOTA) Li ion batteries (LIBs) [1–4]. However, a suitable solid electrolyte or liquid electrolyte/separator system for high-performance and safe cell (-stack) operation remains the key for application and is the predominant actual focus of research and development (R&D) [5–11]. The organic-, i.e. solid polymer-based electrolytes (SPEs) are beneficial with respect to costs due to well-developed processing aspects and rather low amount of Li+ utilization compared to inorganic-based electrolytes (e.g. ceramics) [12]. Moreover, the flexible and particle-free nature of SPEs allows superior wettabil- ity and ionic conduction with the electrodes, thus an essential kinetic requirement for cell application [13]. The SOTA bench- ⇑ Corresponding authors. E-mail addresses: Winter, M. (m.winter@fz-juelich.de), Kasnatscheew, J. (j.kasnatscheew@fz. juelich.de). mark polymer for SPEs is based on poly(ethylene oxide) (PEO) and is intensively investigated, however, overwhelmingly in a physicochemical manner only [14–16]. The lack of literature reports for cell application, particularly for high voltage, is related with the literature believed instability of PEO-based SPEs with high voltage/energy electrodes, e.g. LiNi0.6Mn0.2Co0.2O2 (NMC622) [17–19]. However, a systematic recent study questions this assumption for short-term cycling and could counterintuitively assign the typical “voltage noise” failure to the Li|SPE interface, i.e. short-circuits via Li dendrite growth [20,21] penetrating to the cathode [22–24]; while the NMC622|SPE interface is shown to be more oxidatively stable than believed [24], though its impact on capacity fade [25], e.g. in cumulative manner [26], remains unsolved. Incorporation of PEO in a robust PEO-based network proves this principle, as the suppression of dendrite penetration indeed prevents this fail- ure and enables charge/discharge cycling performance for numerous cycles even at 40 C [22,23]. 1369-7021/! 2020 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). 9 https://doi.org/10.1016/j.mattod.2020.11.025 RESEARCH: Short Communication

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