Tailoring Electrolytes For High-Power High-Load Li-Ion Extruded Electrode Batteries

This Work contains the collaborative efforts of nine German companies and institutions including EAS Batteries GmbH in Nordhausen, Thuringa and Asahi Kasei Europe GmbH in Düsseldorf, North-Rhine Westphalia, over a time period of 3 years within the BMBF-funded project HEADLINE (03XP0394D).

For lithium-ion battery (LIB) technology to be widely used as energy storage for electromobility applications, such as land vehicles, shipping and air transport, the energy density, fast charging capability and manufacturing costs of the cells must be further optimized.[1] The bottleneck of current LIB technology is the accessible battery energy density and power, since there is a limit of how much active material and thus capacity can an electrode be prepared with and at the same time be accessible during charge and discharge in a wide temperature range (-20 °C to 60 °C). Conventional carbonate-based electrolytes do not allow for sufficient lithiation and delithiation of high-load electrodes and thus, the power performance of high energy-density LIBs is limited. This limitation is exactly what the BMBF-funded HEADLINE (High-power high-load Li-Ion Cell-development for extruded electrodes) project is targeting at, in which a new concept of an energy-dense, fast-charging Li-ion cell with innovative, environmentally friendly manufacturing processes for high-capacity electrodes was developed and validated with high-power type and high-energy type demonstration cells. The innovation of the concept lies in the fact that both, durability and charge/discharge C-rate capability are significantly improved by using Asahi Kasei’s highly conductive Acetonitrile (AcN)-containing electrolyte in combination with high-load electrodes, manufactured by using a novel extrusion process, in which Ethylene carbonate (EC) is used instead of N-Methyl-2-pyrrolidon (NMP) as a solvent.

The primary goal for Asahi Kasei Europe in this project, was to improve the service life and cycling stability as well as power performance of LIB cylindrical cells. To achieve this goal, AcN was used as electrolyte co-solvent. AcN is a promising aprotic organic solvent for LIB electrolytes due to its high dielectric constant and low viscosity which potentially increases the electrolyte ionic conductivity.[2,3,4] The invention of Asahi Kasei, the moderately concentrated AcN-containing electrolytes exhibit high ionic conductivity and provide LIBs with outstanding power performance, low temperature cycling capability and high temperature stability.[2] By the right choice of additives and conducting salt and their amounts, Asahi Kasei succeeded to overcome AcN-solvent’s drawbacks which opens new opportunities for LIB development to overcome energy density limitations. To achieve high temperature stability and storage capability of the cell, specific electrolyte additives were added to the electrolyte formulation to suppress lithium salt degradation and HF generation. This unique approach of electrolyte development, by precisely tailoring the electrolyte formulation to the requirements of the cell, proved itself to be the key factor to the success of project HEADLINE. Asahi Kasei’s AcN-containing electrolyte was adapted to the special challenges displayed within the project, to overcome C-Rate limitations of especially tick and dense extruded electrodes. The novel extruded electrodes by EAS Batteries GmbH represent perfect candidates to show the benefit of the AcN-containing electrolyte. Due to the special extruded electrode structure an electrolyte with very high ionic conductivity was required to overcome C-Rate limitations at high electrode mass loading.

Constant current electrochemical experiments were performed to confirm the C-Rate and cycling performance of 22 Ah high power and 50 Ah high energy lithium iron phosphate (LFP)||Graphite cylindrical cells comprising conventional carbonate electrolyte and Asahi Kasei’s AcN-containing electrolyte. Moreover, in an attempt to verify the high temperature stability of these cells, storage tests were conducted at 60 °C. In order to satisfy the project criteria, the AcN-containing electrolyte formulation was tailored towards improved C-rate performance, cycle life and high temperature storage capabilities. Applying the adapted formulations to EAS’s Battery cells could demonstrate the benefits of the AcN-containing electrolyte, which fulfilled all project criteria and exceeded initial expectations.

EAS Batteries focused on the materials and process development for NMP-free electrode manufacturing. To replace the conventional NMP-based processing routes for cathode active materials, alternative binder polymers and environmentally friendly, non-toxic solvents such as EC were introduced. A key innovation in this area is the application of extrusion technology, which offers significant advantages due to its reduced electrode residence times. This, in turn, minimizes the exposure duration of the active materials to the solvent environment, leading to improved material integrity and sustainability. A primary objective of the project was to improve the efficiency and quality of high-viscosity extrusion-based coating techniques. Notably, the concept is expected to significantly overcome current limitations in coating width and processing speed. EAS Batteries contributed strongly to the success of the project, by optimizing electrode production and providing electrodes, cells and battery testing capabilities to the consortium.

Due to the united efforts of the project consortium, the translation of material and process innovations into high-performance extruded electrode cylindrical battery cells was a full success. Throughout the project, the developed material and process innovations are systematically implemented in high-energy density cylindrical battery cells. The chosen innovative 22 Ah and 50 Ah LFP||Graphite extruded electrode cylindrical battery cell format of EAS Batteries was used to demonstrate the improvements in cycling and C-rate constant current cycling experiments as well as battery storage tests, which show that Asahi Kasei’s tailored electrolyte formulations were able to improve the battery performance significantly in comparison to conventional carbonate electrolytes and enabling high-power and high-load extruded electrodes for the next generation of LFP||Graphite LIBs.

[1] A. Yoshino, Bull. Chem. Soc. Jpn. 2022, 95, 195–197
[2] N. Matsuoka, H. Kamine, Y. Natsume, A. Yoshino, ChemElectroChem, 2021,8, 3095–3104.
[3] C. H. Krause, P. Röring, H. Onishi, D. Diddens, J. H. Thienenkamp, G. Brunklaus, M. Winter, and I. Cekic-Laskovic, J. Chem. Phys. 2020, 152, 174701
[4] C. H. Krause, P. Röring, S. Röser, D. Diddens, J. H. Thienenkamp, I. Cekic-Laskovic, G. Brunklaus, and M. Winter, J. Power Sourc. 2020, 478, 229047