Development of high-halogen argyrodites for all-solid all-sulfide battery
All-solid-state batteries have been enjoying renewed interest in recent years, as this technology offers the prospect of higher energy densities due to the use of lithium as a negative electrode, as well as increased battery safety compared with Li-ion technology. The use of sulfides as positive electrode materials coupled with argyrodite as solid electrolyte are interesting systems to develop. The argyrodites achieve ionic conductivities close to those of liquid electrolytes. Moreover, the electrochemical stability window of sulfides is close to that of argyrodite, making all-sulfide technology a promising one for the development of all-solid batteries.
In order to improve the conduction properties of argyrodites, recent studies have shown that ionic conductivity is highly dependent on their local structure. Solid-state NMR thus appears to be a promising technique for probing the local environments of the nuclei mentioned, and in particular for quantifying the variety of different local environments favoring an increase in ionic conductivity. Some compositions enriched in halides appear to promote ionic conduction, and the synthesis of corresponding materials and their structure will be studied.
The thesis will focus on two main areas: the study of all-sulfide batteries and the fine characterization of argyrodite with controlled local structures. Halogen-rich argyrodites will be developed and studied to determine the influence of different local environments on conduction properties.
Development of thin film negative electrodes for Li-free all-solid-state batteries
The aim of this work is to develop 'Li-free' negative electrodes for new generations of high energy density all-solid-state lithium batteries. The function of this type of electrode is to provide a significant gain in energy density in the battery, to facilitate its manufacture by eliminating the need to handle lithium metal and, most importantly, to enable the formation of a homogeneous, dendrite-free lithium film when the battery is charged.
These electrodes will be based on the functionalisation of a metal collector with thin-film materials comprising at least one lithiophilic material (typically a compound that can be alloyed with lithium) and an inorganic ionic conductor. These electrodes are prepared by physical vacuum deposition processes such as sputtering or thermal evaporation. It will therefore be necessary to study the influence of the composition and structure of the lithiophilic layer on the nucleation and growth mechanism of the lithium film and on the evolution of the electrode during charge/discharge cycles. The role of chemical/mechanical interactions with the ionic conducting layer will also be investigated.
This work, which is part of a national CEA/CNRS joint project, will be carried out at the CEA Tech site in Pessac, which has a full range of vacuum deposition and thin film characterisation equipment, in close collaboration with ICMCB CNRS in Bordeaux. It will benefit from the many characterisation resources (confocal optical microscopy, SEM/cryo FIB, ToF-SIMS, SS-NMR, µ-XRD, AFM,...) available in the various partner laboratories involved in the project.
New silicon-based alloys and composites for all-solid-state batteries: from combinatorial synthesis by magnetron sputtering to mechanosynthesis
All-solid state lithium-ion batteries using sulphide-based electrolytes are among the most studied at present in order to improve energy density, safety and fast charging. Although lithium metal was initially the preferred choice for the anode, the difficulties encountered in its implementation and the performance achieved suggest that alternatives should be proposed. Silicon offers an interesting compromise in terms of energy density and lifetime. However, it is necessary to look at anode materials developed for all-solid state batteries. To this end, we propose to collaborate with CEA Tech Nouvelle-Aquitaine, which has set up a combinatorial synthesis methodology using magnetron sputtering, in order to accelerate the identification of new compositions of silicon-based materials. Libraries of materials with compositions gradient in thin films will be prepared at CEA Tech Nouvelle-Aquitaine and then studied at CEA Grenoble. The most promising compositions will then be prepared by mechanosynthesis and characterised at CEA Grenoble. Significant work will be carried out on milling processes in order to optimise particle size and homogeneity, as well as structure and microstructure. Attention will also be paid to integration in all-solid state cells, drawing on the laboratory's expertise.
Simulation of heterogeneities in battery cells using materials with lower environmental impact
The electrification of vehicles to decarbonize our activities faces a dilemma concerning batteries, their environmental impact and the supply of materials needed to manufacture them. The low-environmental-impact materials being considered today to meet these needs (LF(M)P, sodium-ion technology, etc.) have specific electrochemical characteristics that need to be anticipated before they can be used in large-capacity batteries. These two- or multi-phase materials have an electrical potential that is only slightly dependent on the state of charge. This characteristic favors the appearance of a highly heterogeneous state of charge in the cell. The complex mechanism is linked in particular to fast charging, which is very important for vehicles, and which creates significant heating at the heart of the cells. These heterogeneities limit battery performance and shorten their lifespan. In addition, the flat voltage profile and heterogeneities make it extremely difficult to diagnose the cell's state of charge and state of health. Yet this information is crucial for battery management that maximizes battery life.
Our laboratory is developing advanced modeling tools that enable us to simulate these phenomena. Using a highly detailed numerical model of a large cell, applied to realistic cycling conditions, the candidate will highlight the internal state of cells, which is difficult to access experimentally, and show how cycling, thermal management or diagnostic strategies need to be adapted for the more sustainable chemistries envisaged today. To do this, he will use CEA's software platforms and supercomputers, and draw on CEA/LITEN's expertise covering all technological stages, from materials to real-life cell testing.