All-Solid-state litHium sulfUr Battery with a poLymer Electrolyte – SHUTTLE

The SHUTTLE project is an ANR JCJC project, divided into different main tasks, including a management task and three scientific tasks. SHUTTLE is a multidisciplinary project at the interface between polymer science (synthesis and characterization), electrochemistry (electrodes, electrolyte, interface), and materials science (characterization via X-ray and Neutron tomography). The methodology followed throughout the project is divided into several phases described below:

- Electrolyte synthesis: The first phase involved synthesizing block copolymers with binary conduction (once doped with lithium salt) and unipolar conduction (with the anion of the lithium salt grafted onto the copolymer backbone).

- Polymer electrolyte formulation: To simulate the diffusion of lithium polysulfides (Li₂Sₓ, 2 < x < 8) within the electrolyte when a lithium-sulfur battery is in operation, these polysulfides were added in controlled amounts according to their equivalent chain length (x = 4 or 8). A specific protocol was developed to formulate, under inert atmosphere, electrolytes composed of a mixture of the synthesized copolymers and lithium polysulfides.

- Physicochemical and electrochemical characterizations: The electrolyte materials (copolymer/Li₂Sₓ) were characterized using thermodynamic methods (DSC), gravimetric (TGA), spectroscopic (UV-Vis), physical (SAXS/WAXS), and electrochemical methods, including impedance spectroscopy.

- Interface analysis with Li metal via X-ray imaging (tomography): Symmetrical Li cells were cycled at different current densities, followed by post-cycling analysis to visualize, using X-ray tomography, the topological changes at the Li/electrolyte interfaces. This technique reveals defects such as lithium dendrites, delamination at interfaces, or the deposition of an insulating layer on the Li metal.

- Development of neutron imaging: By leveraging the contrast between Li and its isotope ⁶Li, it is possible to reveal the growth of Li dendrites within deuterated electrolyte. A specific methodology for designing electrochemical cells adapted to the Neutron and X-ray Tomography (NeXT) beamline at the Institut Laue-Langevin (ILL) in Grenoble was implemented.

The main results are listed following the structure of the Methodology section:

- Electrolyte synthesis: All target materials were produced in sufficient quantities to be studied as separators and functional binding agents within Li metal batteries with a sulfur-based composite positive electrode.

- Polymer electrolyte formulation: The methodology for reproducibly formulating electrolytes doped with lithium polysulfides was specifically developed and fully described in a scientific article published in Electrochimica Acta.

- Physicochemical and electrochemical characterizations: All electrolytes were thoroughly studied in terms of thermodynamic, gravimetric, and spectroscopic properties to account for the interaction between the polymer phase and the polysulfides, as well as physical aspects through local analysis of the block copolymer mesostructure and transport properties (ionic conductivity, diffusion coefficient, transport number). The correlation of all these characteristics enabled the precise mapping of the relationships between the structure of binary or unipolar electrolytes doped with polysulfides and their various transport properties. These insights allowed us to understand the effect of the proportion of the conductive phase within the electrolyte and the nature of the electrolyte in promoting or mitigating the transport of polysulfides according to their equivalent chain length (Li₂Sₓ with x = 4 or 8).

- Interface analysis with Li metal via X-ray imaging (tomography): The symmetrical Li cells that were cycled revealed a set of defects, such as delamination and a new layer preferentially forming at one Li/electrolyte interface. However, few lithium dendrites were observed.

- Neutron imaging development: An open-access article presents a proof of concept demonstrating the interest of neutron tomography for studying dendritic lithium electrodeposits. The challenge was to achieve a strong contrast between natural Li and ⁶Li within a deuterated electrolyte. To this end, a first-generation electrochemical cell was developed, cycled, and then taken to the imaging beamline at ILL (Institut Laue-Langevin, Grenoble). To validate our approach, this same cell was also imaged using X-ray tomography at the SIMAP laboratory in Grenoble and at the SOLEIL synchrotron (Gif-sur-Yvette).

Looking ahead, each task of the project will be further developed to gain more knowledge on the diffusion of polysulfides in polymer phases and within other types of solid electrolytes (e.g., ceramics). Indeed, the SHUTTLE project has enabled the development of a robust and reliable set of methodologies that should be generalized to other generations of electrolytes. Furthermore, the electrochemical cell developed for "all-solid" assemblies needs to be improved to ensure its compatibility with tomography measurements using both X-rays (in laboratory and synchrotron) and neutrons. These techniques are inherently non-invasive and allow the probing of hidden interfaces within electrochemical cells, enabling the study of their behavior during cycling. Finally, through this instrumental development, a dedicated test bench will also be established and made available to the scientific community for collaborative research in the field of energy materials.

1. L. Magnier et al., Frontiers in Energy Research 9 (2021) 266. (DOI:: 10.3389/fenrg.2021.657712, open access)

Energy production and storage are great challenges to ensure the energetic transition. High energy density, low cost, with extended cycle life batteries must be developed to promote renewable stationary applications (solar and wind farm) and electrified transport. Since their market introduction in 1991, lithium (Li)-ion batteries are the dominant solutions to power small electronic portable devices and are now used in most of the modern hybrid and full electric cars. However, for all of these applications this accumulator is not fully adequate because its energy density should be increased by factor two at minimum to answer the demand of the market whereas their energy levels off at about 250 Wh/kg due to their maturity. In addition, the presence of flammable liquid electrolyte is a strong safety issue (fire, explosion). To overcome these limitations a solution is to replace the unsafe liquid electrolyte by an inherently non-flammable solid polymer electrolyte. In addition to safety, the other advantage of polymer electrolytes resides in their chemical and electrochemical stability toward metallic Li. This material is ideally suited as negative electrode because of its high specific capacity (3860 mAh/g). At the positive electrode side, an interesting active material is sulfur (S8). The specific capacity of sulfur is important (1675 mAh/g) and permits to envision Li-S8 batteries with a specific energy density in the order of 500 Wh/kg, roughly twice that of conventional Li-ion accumulator. However, many hurdles remain to be solved to favor this battery technology such as the lithium polysulfides dissolution in to the electrolyte upon cycling (redox shuttle effect) which impairs the delivered capacity and the faradaic efficiency, and the prevention of dendrite growth at the negative electrode leading to short-cut issues. In this context, the project proposes to design an all-Solid-state litHium sulfUr baTTery with a poLymer Electrolyte (SHUTTLE). The goal is to develop a reliable device based on a new generation of sulfur based accumulator in order to increase in the energy density and cyclability. One of the originality of the project corresponds to the investigation of the functioning and failure modes by operando analysis of batteries in order to optimize the positive electrode texture and the polymer electrolyte properties, and to deeply understand the dendrite growth processes at the negative electrode. As a perspective, the project will develop a test bench of microstructural and topological analysis of electrochemical energy storage devices during cycling by X-ray and Neutron tomography.