Periodic Reporting for period 1 - STREAM (Solid Through Rigid Electrolyte: Advanced Measurements)
Reporting period: 2023-09-01 to 2025-08-31
Summary of the context and overall objectives of the project
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.
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.
Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far
The core of the project STREAM has been the development of solid ionic conductors, based on closoborate salts and their derivatives, as possible candidates for next-generation post-Li solid-state batteries (SSBs) and their further assessment of the relationship between mechanical properties (tensile strength, compression, etc.) and electrochemical and physical degradation. Particular attention has been devoted to the understanding of the processes leading to sudden cell short circuits, which are driven by the uneven cationic plating on electrode and further growth thorough the grains of the solid electrolyte. To this regard, a set of chemical compounds, based on the peculiar boron chemistry, have been selected. Indeed, these boron salts, possessing very chemically stable large-cage anions, showcase weak electrostatic interaction with mobile cations, as well as a coordinative motion when the thermal energy is sufficient to trigger anion dynamics. This eventually pushes the long-range cationic conductivity, known as paddle-wheel effect. According to the most recent literature in the field, the fast anionic reorientation can also be promoted by enhancing configurational entropy for the available coordination sites for mobile cations, and this can be achieved by synthetizing solid solutions of salts, whose anions can be different in terms of size, charge distribution and moment of inertia. A series of thorough physicochemical an electrochemical characterization has been performed throughout the WP, as described more in detail below, to select the best candidates in terms of ionic conductivity and electrochemical stability. In order to establish a correlation from chemical composition of the selected ionic conductors, and their resistance to dendrite penetration as function of thermodynamic parameters, such as pressure and temperature, in situ pressure-dependent and temperature-dependent electrochemical impedance spectroscopy (EIS) tests have been carried out, in particular for the systems NaCB11H12 and Na2B12H12, which showcase the best performance in terms of electrochemical stability, resulting among the studied compounds as the least prone to oxidation. The tests have been conducted by the use and development of particularly tuned electrochemical cells, allowing for pressure control and adjustment. From data treatment of the EIS measurements it has been possible to separate the contribution to the impedance arising from the areal surface resistance (ASR) and that from the bulk material, providing a feedback on the pelletizing pressure adopted to cold press the ionic conductors. Due to the difficulties faced in setting up the optical operando measurement configuration, also related to serious flaws on the available argon-filled glovebox, It has been decided, in agreement with research group and collaborators to pursue investigation on the relationship between ionic motion and anionic disorder, by testing other compounds of interest, in the class of the amide-hydride systems, in particular Mg-based one, which can be useful for solid-state Mg-based energy storage devices. In particular, it was tackled the system 6Mg(NH2)2+9LiH+xLiBH4, where x can be 1, 2, 6 and 12. Non-ambient EIS measurements have been carried out, in order to establish a correlation between component ratio and ionic conductivity and activation energy. Moreover, to get more insights into surface properties and chemistry of the closoborate-based salts, a series of X-ray photoelectron spectroscopy measurements have been undertaken, in collaboration with the Université de Pau et Pays de L’Adour. Preliminary results indicate a correlation between milling action and increased number of reactive sites at the surface, thus correlating the progressive conductivity loss to the formation of possible opening anion cage reactions and/or anion dimerization. Lastly, in collaboration with the Utrecht University, it has been explored the role of inert additives, such as silica and magnesia, to form nanocomposites with superior room temperature ionic conductivity. In the final part of the project, the development of a synthetic protocol for positive electrodes compatible with the closoborate-based salts has been implemented. NaCrO2 has been reported as one of the positive electrodes that keeps its electrochemical performance, in terms of voltage operation, specific capacity and capacity retention when coupled with solid electrolytes. However, its synthesis requires a high temperature treatment (ca. 900 °C) at controlled atmosphere for 24 hours and implementation. During this WP, it has developed a synthetic route based on the use of a solid-state microwave assisted protocol which allows for the rapid preparation of NaCrO2. By using acetylene-based carbon as susceptor (i.e. element able to efficiently adsorb the radiation in the range of MW and releasing it under heat to the sample), it has been possible to achieve the full transformation of the precursor within minutes and without the need of atmosphere control. Further XRD characterization demonstrates that the synthesis route (conventional vs. MW) does not have an impact on the structure, whereas FEG-SEM images revealed a light difference in the morphology and particle size, mostly due to the rapid thermal gradient, which can have an effect on the electrochemical performance. Classical galvanostatic cycling, cyclic voltammetry have been performed, showing a slight decrease in the coulombic efficiency in the case of MW-NaCrO2, which have been investigated by more advanced techniques, such as the galvanostatic intermittent titration techniques (GITT), in order to retrieve differences in the Na+ diffusion coefficients for the materials prepared by conventional synthesis and that obtained by microwave-assisted treatment. The protocol has been extended to the synthesis of other layered transition metal oxides, based on Cr and Fe, and featuring different cations.
Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)
Achieved correlation between mechanical features and critical current density for the class of compound of interest, development of the setup for operando optical measurements. Development of a protocol for XPS, and XRS analyses of closoborate-based salts and their carbon derivatives; exploring the role of anion mixing and additive to promote fast ionic mobility in hydride-based materials. Development of an alternative synthetic protocol for transition-metal layered oxides of interest for solid-state batteries. Study of the difference between conventional and alternative solid-state syntheses.