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HINDERING DENDRITE GROWTH IN LITHIUM METAL BATTERIES

Periodic Reporting for period 1 - HIDDEN (HINDERING DENDRITE GROWTH IN LITHIUM METAL BATTERIES)

Reporting period: 2020-09-01 to 2022-02-28

The HIDDEN project develops self-healing methods to enhance the lifetime of Li-metal batteries by 50 %, and to increase the energy density of batteries 50 % above the current level achievable with current Li-ion batteries. The materials and their processes for functional battery layers are targeted to be scalable and industry compatible. To achieve these goals, HIDDEN develops novel self-healing thermotropic liquid crystalline electrolytes and piezoelectric separator technologies to prevent dendrite growth. The consortium applies multiscale modelling for electrolyte design to speed up the development of new materials. Dendrite growth is monitored with algorithms, which allows following the effect of the selected self-healing methods and triggering the self-healing at a correct time by increasing the cell temperature. During the first period, the consortium has focused on developing the self-healing methods and materials. During the second period, the consortium will test, modify, and validate the methods, and scale up the production.

The project brings together a strong interdisciplinary consortium of seven partners, industry and research balanced, with state-of-the-art background in battery chemistry and physics, materials modelling and analysis, upscaling of novel technologies by printing and coating, as well as in industrial assembling of battery cells. This is complemented by external advisory board with representation of key industry end-users.
The HIDDEN project started by defining specifications for the batteries that will be developed during the project. This work was completed during the first 6 months. As first steps, we have also synthesized a model liquid crystal electrolyte, which has been used for developing processing methods for the electrolyte, as well as to study its properties to generate data e.g. for the modelling work, which will guide the next steps in the synthesis work. We have also tested materials and layouts for a printed heating element, which will be used to trigger the self-healing reaction on demand. In addition, the first piezoelectric separators have been prepared and their properties and processing methods are currently under investigation. In order to trigger the self-healing methods at the right time, we have also studied non-invasive methods to detect the dendrite growth, which will be finally integrated with the battery management system to control the self-healing. Data handling is also an important part of the project. Thus, we have created a joint HIDDEN database for the results, which will help to share data, and to guarantee reliable handling and storage of the results and the process parameters. Finally, attracting more people in the battery field is one of our targets. We try to reach this goal by sharing information about the project and next generation batteries through many routes. The project coordinator has e.g. been a visiting lecturer, giving the students a view on the current battery research and talking about the potential and the need for new people to work in this field, in order to reach our common goal of climate neutrality.

One of the topics where we have recently focused on is processing of the novel self-healing batteries. The liquid crystalline electrolyte and the piezoelectric separator require special processing methods, as the standard Li-ion manufacturing processes are not optimal as such for the HIDDEN materials. Our goal is to be able to manufacture self-healing batteries in a way, which is scalable and safe, and which does not require too complicated processing steps. This would enable adoption of the self-healing functionalities in commercial cells in future, even though the project, as well as the whole Battery 2030+ initiative, operates at low TRL. The results look promising so far, as we have been able to coat our model liquid crystalline electrolyte on top of the NMC electrode and infiltrate it into the cathode as well. We are now proceeding from coin cell test to pouch cells, which requires also good practices for cutting of the electrodes. Laser cutting of the combined NMC and solid electrolyte layer, as well as the Li metal anode, seems to be working for these materials. Regarding the piezoelectric separator, we have found ways to control the porosity of the layer and upscaling of the coating process is planned.
Thermotropic Liquid Crystals (TLCs) form an ordered fluid-like structure under a certain temperature range. Usually, liquid crystals are not inherently ionically conducting. If used as an electrolyte, TLCs need to be engineered to encode ionic transport features to generate Thermotropic Ionic Liquid Crystals (TILCs). TILCs have been shown to guide Li+ transport and lead to smooth Li deposition onto the Li-metal anode, preventing dendrite formation. HIDDEN used this innovative approach as a starting point and developed it a step further by synthesizing a series of model TILCs. These TILC-Gen.1 have been used to develop a scalable process to coat the electrolyte on/in the cathode, and to generate data for modelling. The first laser cutting tests with cathode-TILC composites are also done. Next, novel TILCs with different (i.e. fine-adjusted) chemical structures were synthetized: TILC-Gen.2 materials. The first TILC-Gen.2 electrolytes have been designed, synthesized, and evaluated, and the data will be used to design TILCs with optimal performances. We have also developed a printed heating element, aimed to trigger on demand self-healing.

The piezoelectric effect is the ability of certain materials to accumulate electric charge in response to a mechanical stress. HIDDEN will use this phenomenon in a separator. When growing dendrites will eventually reach the separator, it will bend, and generate a local electric field. This will guide the Li+ cations to deposit smoothly between the growing dendrites, and not on top of them, increasing the cell cycle life. HIDDEN has tried two methods for enabling piezoelectric separator – casting a porous self-standing poly(vinylidene fluoride) (PVDF) separator, and coating the porous PVDF layer on a commercial polypropylene (PP) separator. Both were successful, but the later cannot be efficiently poled with an external field. So, the porous PVDF separator is taken forward for battery cycling.

Several cell characterization techniques were screened out and some selected to be suitable for detecting dendrite growth. The validation results show that specific parameters allow detecting degradation in the tested Li-metal batteries. The results have been shared in a public report, found from the project website. The tested detection techniques can be implemented in embedded systems to sense degradation, activate self-healing methods, and assess the overall benefits. Some techniques are easier than others, but in general all the non-invasive techniques proposed can be implemented in battery management systems (BMSs), except (till date) for coulombic efficiency. The consortium sees also potential in the use of sensors developed by Spartacus, Sensibat or Instabat, which could be ultimately integrated with the BMS.

The HIDDEN project will increase the quality, reliability and lifetime of Li-metal batteries, paving the way for electrification of transportation. In addition, we expect to generate industrial opportunities for the next generation battery industry in Europe.
The HIDDEN consortium at the M12 General Assembly meeting at VTT