Cell designs and methods for characterisation of lithium protective membranes, solid electrolytes and beyond

Abstract

Battery systems employing a lithium metal negative electrode are attractive due to their high theoretical specific energy. However, it is necessary to protect the lithium electrode from crossover species such as polysulfides in Li-S systems, redox mediators in Li-O2 cells or dissolved cathode species in high voltage Li-ion batteries. Lithium conducting membranes and solid electrolytes can address this and other issues such as lithium dendrite growth. This work focuses on some of the key properties governing performance of such components: ion selectivity and transference number, interfacial resistance, and redox kinetics (mass transport of reactants and/or electron transfer). A number of novel cell designs and methods are developed for the evaluation of these properties. Firstly, a convenient, in-situ method is presented for evaluating the ability of membranes to block crossover species. The method employs a novel ‘Swagelok’ cell design equipped with a glassy carbon probe for voltammetric detection of crossover species, and is demonstrated using a model redox system. Building on this work, a cell design and method is presented for evaluating membrane selectivity and transference under operando conditions, that is, when a current is applied across the cell. By fitting of a simple analytical model to measurements with and without current, values are obtained for the diffusive permeability and transference number of crossover species in the membrane, and also for the lithium transference number in the case of highly selective membranes. Moving on to interfacial resistance, the rate of lithium ion transfer between a solid lithium conductor and liquid electrolytes is studied using 4- probe impedance measurements. An approach is illustrated for identifying the rate-limiting step, which may be desolvation of lithium ions or transport through a solid-liquid electrolyte interphase. Focusing next on the Li-S system, a Swagelok cell equipped with glassy carbon electrode is used to study solid polymer electrolytes for Li-S batteries, providing direct observation and measurement of polysulfide intermediates for the first time. Finally, a similar cell design is used to investigate the application of GITT to soluble redox reactants whose concentration is unknown, as is the case for polysulfides in Li-S batteries.

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Identifiers

ORCID for Nina Meddings: orcid.org/0000-0001-7242-6076

ORCID for Nuria Garcia-Araez: orcid.org/0000-0001-9095-2379

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