STREAM · Solid Through Rigid Electrolyte: Advanced Measurements

Solid Through Rigid Electrolyte: Advanced Measurements

The aim of the present research proposal is to study the influence of stack pressure on the degradation of Na-based solid-state batteries (i.e. dendritic growth) by coupling the electrochemical in-plane cell operation to on-line optical/electron microscopy characterization, in a selected class of Na-based ionic conductors exhibiting superior electrochemical performance. A correlation between the nature of the observed metal penetration through the solid electrolyte and the electrochemical signature will be obtained, representing a reliable dataset for further quality control tests. The investigation on the mechanical properties of SEs with a cutting-edge approach helps driving both syntheses and cell assembly of more reliable and long-life new-generation all solid-state batteries. The transition towards electrified vehicles encompasses both goals of reducing the GHG emission, as well as providing smart grid smartly management. Embarked batteries can buffer the electricity excess due to peak productions from renewables. EU has taken a huge commitment in terms of electrification of the car fleet, which is strictly dependent from its capacity to produce Li-ion batteries. Indeed, it has been estimated that in 2030, EU EV’s market will need an overall production of 600 GWh/year1. However, the mid-to-long-term sustainability of such policies rises some major ethics and economic concerns, related to the raw materials supply chain, such as in the case of cobalt, widely employed in commercial cathodes. Moreover, the theoretical limitations imposed by the available Li-ion technology push the scientific community to explore all-solid-state batteries (ASSBs) based on alternative-to-Li-ion chemistries, such as Na-based systems.2 ASSBs feature a solid separator which, being more (electro)chemically robust, opens the gate to high-operating voltage positive electrodes and, would also enable the use of a metallic negative electrode. The enhanced voltage window and the minimization of the dead mass at both electrolyte and anode side (no insertion electrode), result in high volumetric (>750 Wh/L) and gravimetric (>350 Wh/ kg) energy densities, with a theoretical 20% gain respect to conventional Li-ion batteries.3 However, to further improve the latter at least of 20-30% (4.5 – 5 V cells) and then meet the global market requests, solid electrolyte (SE) properties have to be drastically implemented. Among several class of SEs, complex hydride have recently drawn the attention of the researchers.4 In particular, hydroborates with large cluster anions, [BxHx]2- (x = 10,12), and their C-derivatives [CBx-1Hx]- being among the most stable molecules known, provide superior electrochemical robustness. Moreover, they exhibit low density, low toxicity and soft mechanical properties, high compatibility with metallic Na, and low area-specific resistance, confirming their possible use as electrolytes for next-generation ASSBs.5 Costs of the multistep syntheses of large-cage hydroborates has been considered, however, a major hurdle to their large-scale use as SEs. Nonetheless, several smart synthetic routes lowering the price per kg by a factor of ten have been recently reported, but in an optic of sustainability of raw materials, efforts should be devoted in seeking for environmentally benign and cost-effective recycling protocols. Nonetheless, ASSBs offering superior electrochemical performance (400 Wh kg−1, >1,000 Wh l−1 and >90% energy efficiency at 1C rate)6 are hindered by mechanical failure of the cell. Due to uneven plating at the negative electrode during electrochemical cycling, alkali metal progressively grows through the SE, eventually leading to a short circuit. Even though it was expected that SEs could hamper the dendrite penetration, thanks to sufficient shear modulus, several reports shows metallic growth through interconnected pores, grain boundaries and single crystals, with a variety of short circuit mechanism. The presence of voids between grains is also source of metal nucleation and growth, likely due to the local different electrochemical potential, as well as interfacial voids created during metal stripping that contributes to alter the impedance and promote metal filament formation. From a macroscopic point of view, pressure is one of the main parameters playing a role on dendrite penetration, both in the SSB fabrication as well as during its operation). A thorough investigation on such challenging phenomena is therefore crucial for further development of Na-based SSB’s performance, in particular for applications where elevate current densities are required. The development of operando optic imaging techniques coupled with electrochemical spectroscopy on Na-hydroborate SEs represents a cutting-edge study that overcomes the current state-of-the-art knowledge on the relation between mechanical and electrochemical properties of NSSBs.

Текст от CORDIS, на английски · Данни: CORDIS, © Европейски съюз