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SHORT COMMUNICATION Materials Today d Volume 44 d April 2021 Contrary to modified polymers [27–30], pure PEO-based SPE is shown to be mechanically prone to both, Li dendrite penetration and compression in the cell [23]. The practical cells (e.g. coin cells) can lead to a pressure-induced shrinkage of the distance between the electrodes, which renders the short-circuits more likely, as it significantly facilitates Li dendrite penetration, partic- ularly at higher temperatures (e.g. 60 C) [22,23]. For example, a proper spacer between the electrodes and/or application at lower temperatures (e.g. 40 C) can minimize this issue and realize a benchmark, i.e. suitable reference system for R&D [31], for linear PEO-based SPE in NMC622||Li cells. Given these recent findings, a thorough characterization and evaluation of this benchmark is pending, particularly the correla- tion of well-characterized physicochemical aspects with the, up to now, rather unexplored cell performance. In this work, the salt concentrations and ionic conductivities are linked with the per- formance behavior in NMC622|SPE|Li cells. With the support of basic theoretical physicochemical relations, i.e. the Sand equation [32–39], precious relations with practical cell application can be subsequently derived, including the determination of a Li+ diffu- sion constant (D+ ) of the SPE. These insights, obtained only on NMP followed by the addition of carbon black and NMC622. The mixture was homogenized using a dissolver. The slurry was casted on aluminium foil using a doctor blade with a wet coating thickness of 50 mm. The electrode sheets were dried at 80 C under vacuum for 3 hours, punched into circular electrode and dried again at 120 C over night before use. The average active mass loading of NMC622 electrodes was %2 mg cm2. The cells were prepared in two electrode setup (coin cell) using a NMC622 based positive electrode [40], the PEO-based SPE as polymer membranes and lithium metal as negative electrode. The cells were assembled using the polymer membranes (12 mm diameter) inside rings of mylar foil (outer diameter: 16 mm, inner diameter: 12 mm) sandwiched between lithium metal (overall 16 mm diameter, due to the SPE, only 12 mm active diameter) and NMC622 electrodes (12mm diameter) and/or between a further lithium metal in case of symmetrical cell configuration. d) Electrochemical measurements All constant current cycling experiments were conducted on a Maccor Series 4000 battery cell test system at 60 C or 40 C in a climate chamber (Binder KB400). The used C-rates and corre- sponding specific currents are mentioned within the text and/ or in the figure captions. e) Ionic conductivity measurements Electrochemical impedance spectroscopy (EIS) was conducted utilizing an Autolab PGSTAT302N with FRA32M high frequency analyzer and MUX.SCNR16 16-fold multiplexer. The prepared SPE samples were sandwiched between stainless steel (SS) block- ing electrodes and a PTFE spacer disc was used to keep the sample dimensions of 100 mm height and 12 mm diameter constant in the coin cell (CR2032) housing. The sample cells were pre- heated at 70 C for 2 h prior to the measurement to improve the surface wetting of the SS electrodes with the considered poly- mer samples. The EIS measurements were performed in the fre- quency range of 1 MHz to 1 Hz with an applied voltage amplitude of 10mV in the temperature range of 0–80C in 5C steps. The temperature was controlled using a Binder MK53 climate chamber. Results and discussion The ionic conductivities for varied lithium bis(trifluoromethane sulfonyl)imide (LiTFSI) salt concentrations in poly(ethylene oxide)-based solid polymer electrolyte (PEO-based SPE) are depicted in Fig. 1(a). Considering 60 C as the typical application temperature for PEO-based SPEs, the ionic conductivities are rel- atively similar, except for 50:1 (EO:Li) ratio. The initial charge profiles of LiNi0.6Mn0.2Co0.2O2 (NMC622)||Li cells are shown in Fig. 1(b). In line with the relations in ionic conductivity, the cells with SPEs with varied salt concentrations behave similar and reveal equal specific charge capacities (195 mAh g1), again, except for the 50:1 (EO:Li) concentration, where the capacity is lowered by 10 mAh g1. Given the similar performance for the salt concentrations up to 20:1 (EO:Li) for these conditions, it is reasonable to use the lowest possible salt concentration as starting point for a practical Li electrochemical tests, allow prognosis of practical threshold cur- rents for given salt concentrations and temperatures and finally can successfully correlate physicochemical, theoretical and prac- tical aspects. Experimental Sigma-Aldrich, Germany. Lithium bis(trifluoromethanesulfo nyl)imide (LiTFSI, 99.9%) and polyvinylidene difluoride (PVdF, Solef 5130) were purchased from Solvay, France. Super C65 car- bon black was received from Imerys, France. Mylar foil (100 mm thickness) was purchased from DuPont, USA. The active materi- als LiNi0.6Mn0.2Co0.2O2 (NMC 622) were purchased from Tar- gray, Canada. Lithium metal (Albemarle) was used as counter and reference electrode. Material storage and sample prepara- tions was performed in a dryroom (dew point 65 C). PEO was dried under vacuum (107 mbar) at 45 C and LiTFSI at 110 C for 2 days before use. All other chemicals were used as received. b) PEO-based SPE membrane preparation PEO-based SPE polymer membranes were prepared by dry mixing PEO and LiTFSI with varied salt concentrations using a mortar. The obtained mixture was stored in a pouch bag under vacuum overnight (60 C). The resulting gum-like material was sandwiched between Mylar foil sheets and pressed at 100 C with an applied pressure of 15 bar for 10 min. The thickness of the resulting membrane in the range of 100 ± 5 mm was controlled by the usage of a spacer. c) Electrode preparation and cell assembly NMC622 electrodes consisting of 91wt% NMC622, 4wt% Car- bon Black and 5wt% PVdF were prepared by dissolving PVdF in a) Materials Poly(ethylene oxide) (PEO, MW pyrrolidinone (NMP, anhydrous, 99.5%) were purchased from 300.000 Da), 1-methyl-2- 10 RESEARCH: Short CommunicationPDF Image | Sand equation and its enormous practical relevance
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