Periodic Reporting for period 1 - 4SBATT (Sustainable Solid State Sodium Batteries)
Reporting period: 2022-09-01 to 2025-02-28
The Li-ion battery, developed in the last 30 years, is a very successful technology. However, it now faces the challenge of powering the e-mobility revolution, requiring a large increase of raw resources availability. At this point in history, given the roadmap of the European Green Deal and the need to reduce CO2 emissions, such a scale up should be seen as a unique opportunity to eliminate unsustainable elements from the batteries. Yet the Li-ion battery relies on a series of elements that are critical, most importantly Li, Co and natural graphite. Moreover, the safety of Li-ion batteries is often in question, and their energy content still needs to increase to satisfy the demand for extended driving ranges. In this context, in 4SBATT we aim to develop a solid-state battery based on Na, rather than Li, representing the best solution in terms of four key parameters: sustainability, energy density (specific and volumetric), readiness of adoption (i.e. compatibility with existing Li-ion production lines) and safety. Operating at the cross-section between inorganic chemistry, materials science and engineering, we develop a combined computational and experimental approach to effectively explore large chemical spaces and to identify successful synthesis routes to design and prepare novel Na-based inorganic compounds for positive electrodes, solid electrolytes and negative electrodes. Then the physical properties of materials and composite electrodes are characterized to understand, improve and engineer their performances. Finally we will assemble solid-state batteries, based on Na and sustainable elements, safe and targeting sufficiently high energy densities for commercial exploitation.
In our effort to establish sustainable sodium solid-state batteries (SSBs), we have been developing all the materials required in the electrochemical cells, i.e. both electrodes and the solid electrolyte (SE). The electrode materials are equally useful in developing Na-ion batteries with liquid electrolytes, which are used as a platform to test them prior to integration into solid-state cells.
Our advances related to positive electrode include the synthesis and investigation of a family of Li-substituted P2 layered oxides based on Mn and Ni. We explored the consequences of Li substitution, as well as the challenge of elevating the Na content in the pristine materials through solid-state synthesis. After identifying the optimal Li content for electrochemical performances, we employed advanced semi-simultaneous operando characterizations at large scale facilities to show that the excellent performance is attributed to the ideal combination of limited redox activity, absence of phase transitions and minimal volume change during charging. Identifying this optimal compositional range serves as a promising foundation for the further development of P2-layered oxides.
At the negative electrode side, first we focused on synthesizing and improving established carbon materials by increasing their gravimetric and volumetric capacity. We optimized a scalable synthesis process for hard carbon (HC) materials starting from cellulose. To increase the volumetric energy density, HC was then combined with Sn using spray drying synthesis, resulting in new HC-Sn composites. Our materials have been tested electrochemically in Na-ion half cells. The gravimetric and volumetric capacity of HC-Sn has been enhanced compared to values reported in the literature. Our positive and negative electrodes are now being also combined in full cells (with liquid electrolytes) to verify how they perform combined.
Beyond electrodes, we also aimed to discover and synthesize new solid-state electrolytes with high ionic conductivity and stable when in contact with the electrodes of our project. Sodium metal halides have drawn our attention due to their sustainability, processability and compatibility with high-voltage cathodes. We focused on the family of materials Na1+xZnxAl1-xCl4 (NZAC).
A computational setup was established to evaluate the feasibility of elemental substitutions based on density functional theory calculations and statistical thermodynamics. A range of potential substitution options for the solid electrolyte NaAlCl4 was investigated, and a thermodynamically stable substitution of Na2ZnCl4 with Al was identified, which induced the successful experimental synthesis of the NZAC compounds. The computational setup used is also being automated to be readily applicable to other compounds of interest, and it was tested on mixtures of sodium pnictides and chalcogenides.
Our best NZAC solid electrolyte exhibits a 5-fold increase in conductivity compared to pristine NAC. Even though this is not high enough to be applied as a SE separator, it is adequate for applying this material as catholyte. We also proved that the family of materials is composed by a NaAlCl4/Na2ZnCl4 2-phase mixture, with however Al3+ solubility on the Zn-rich side, hence vacancies can be introduced successfully into Na2ZnCl4, facilitating Na+ hopping. Further investigation of the ion conduction mechanism has been performed using NMR and MD Simulation.
Our advances related to positive electrode include the synthesis and investigation of a family of Li-substituted P2 layered oxides based on Mn and Ni. We explored the consequences of Li substitution, as well as the challenge of elevating the Na content in the pristine materials through solid-state synthesis. After identifying the optimal Li content for electrochemical performances, we employed advanced semi-simultaneous operando characterizations at large scale facilities to show that the excellent performance is attributed to the ideal combination of limited redox activity, absence of phase transitions and minimal volume change during charging. Identifying this optimal compositional range serves as a promising foundation for the further development of P2-layered oxides.
At the negative electrode side, first we focused on synthesizing and improving established carbon materials by increasing their gravimetric and volumetric capacity. We optimized a scalable synthesis process for hard carbon (HC) materials starting from cellulose. To increase the volumetric energy density, HC was then combined with Sn using spray drying synthesis, resulting in new HC-Sn composites. Our materials have been tested electrochemically in Na-ion half cells. The gravimetric and volumetric capacity of HC-Sn has been enhanced compared to values reported in the literature. Our positive and negative electrodes are now being also combined in full cells (with liquid electrolytes) to verify how they perform combined.
Beyond electrodes, we also aimed to discover and synthesize new solid-state electrolytes with high ionic conductivity and stable when in contact with the electrodes of our project. Sodium metal halides have drawn our attention due to their sustainability, processability and compatibility with high-voltage cathodes. We focused on the family of materials Na1+xZnxAl1-xCl4 (NZAC).
A computational setup was established to evaluate the feasibility of elemental substitutions based on density functional theory calculations and statistical thermodynamics. A range of potential substitution options for the solid electrolyte NaAlCl4 was investigated, and a thermodynamically stable substitution of Na2ZnCl4 with Al was identified, which induced the successful experimental synthesis of the NZAC compounds. The computational setup used is also being automated to be readily applicable to other compounds of interest, and it was tested on mixtures of sodium pnictides and chalcogenides.
Our best NZAC solid electrolyte exhibits a 5-fold increase in conductivity compared to pristine NAC. Even though this is not high enough to be applied as a SE separator, it is adequate for applying this material as catholyte. We also proved that the family of materials is composed by a NaAlCl4/Na2ZnCl4 2-phase mixture, with however Al3+ solubility on the Zn-rich side, hence vacancies can be introduced successfully into Na2ZnCl4, facilitating Na+ hopping. Further investigation of the ion conduction mechanism has been performed using NMR and MD Simulation.
The development of the above-described materials can be considered as a significant advance in all sectors of the battery. In some cases, these may not be considered breakthroughs from a pure performance standpoint; however, it should be kept in mind that we are aiming to develop materials based on sustainable or abundant elements, not necessarily just achieving breakthrough performances.
At the cathode side, we synthesized successfully P2 sodium layered oxides and demonstrated the optimal composition in terms of specific capacity and stability. With thorough investigations via x-ray based methods we verified that Li in the transition metal site eliminates phase transitions at high voltage, reduces volume change, and modifies the activation of oxygen redox. At the anode side, we synthesized materials based on hard carbons and were able to increase both the specific energy and, significantly, the volumetric energy density by realizing composites of HC with Sn. A comprehensive set of analytical tools is helping us understand the Sn distribution and how it operates to achieve improved performances. We expect this material will be a feasible anode also in full cell configurations with both liquid and solid electrolytes.
The successful combination of experiments and computation to explore, synthesize, and understand Na-based halide solid electrolytes has been shown to be a valid approach. Based on computed phase diagrams, we were able to identify promising substitutions to be realized experimentally. Our work on the NaAlCl4 electrolyte was the first to show the cation aliovalent substitution of NaAlCl4 and the enhanced ionic conductivity can be regarded as a significant advancement for this material family. This may also be regarded as somewhat unexpected, as we set out to investigate the doping of Zn into NaAlCl4, but we found there was no solubility, while on the contrary, we could substitute Al into Na2ZnCl4 and thereby obtained the improved conductivity. However, the ionic conductivity is still one order of magnitude lower than commonly used sulphide solid-state electrolytes, which indicates the importance of further investigation of novel materials, for example with mixed anions.
At the cathode side, we synthesized successfully P2 sodium layered oxides and demonstrated the optimal composition in terms of specific capacity and stability. With thorough investigations via x-ray based methods we verified that Li in the transition metal site eliminates phase transitions at high voltage, reduces volume change, and modifies the activation of oxygen redox. At the anode side, we synthesized materials based on hard carbons and were able to increase both the specific energy and, significantly, the volumetric energy density by realizing composites of HC with Sn. A comprehensive set of analytical tools is helping us understand the Sn distribution and how it operates to achieve improved performances. We expect this material will be a feasible anode also in full cell configurations with both liquid and solid electrolytes.
The successful combination of experiments and computation to explore, synthesize, and understand Na-based halide solid electrolytes has been shown to be a valid approach. Based on computed phase diagrams, we were able to identify promising substitutions to be realized experimentally. Our work on the NaAlCl4 electrolyte was the first to show the cation aliovalent substitution of NaAlCl4 and the enhanced ionic conductivity can be regarded as a significant advancement for this material family. This may also be regarded as somewhat unexpected, as we set out to investigate the doping of Zn into NaAlCl4, but we found there was no solubility, while on the contrary, we could substitute Al into Na2ZnCl4 and thereby obtained the improved conductivity. However, the ionic conductivity is still one order of magnitude lower than commonly used sulphide solid-state electrolytes, which indicates the importance of further investigation of novel materials, for example with mixed anions.