(Battery Division Student Research Award Sponsored by Mercedes-Benz Research & Development) Sustainable High-Performance Lithium-Ion Batteries: Aqueous Processing of Cobalt-Free High-Energy Cathodes

Abstract

Increasing the environmental benignity of the lithium-ion battery (LIB) technology is one of the greatest challenges associated with their continuously increasing demand. At the same time, the performance metrics in terms of energy density, power capability and reliability have to be at least maintained to meet the requirements of the rapidly growing electric vehicle market.[1] Besides introducing cobalt-free high-performance active materials, such as the high-voltage spinel LiNi0.5Mn1.5O4 (LNMO),[2] the implementation of aqueous electrode processing strategies for lithium-ion cathodes, is a key step to realize environmentally benign battery production. To achieve this ambitious goal, the deployment of nature-derived, inexpensive polymers in combination with the use of water as dispersion agent provides the great possibility to spare energy-intensive drying procedures and costly dry-room conditions, and thus, will turn the whole battery fabrication more eco-efficient and abolish the need of harmful and rather costly N-methyl-2-pyrrolidone (NMP) as processing solvent for mutagenic and teratogenic fluorine-containing binders.[3,4] What could easily be established for graphite anodes, appears more complicated for transition metal oxide-based and imperatively cobalt-free cathode materials, which suffer from lithium leaching and transition metal dissolution when getting in contact with water, which has, so far, rendered the use of aqueous binders very challenging.[5,6] The strategies developed in our group enable the achievement of this desirable goal: The first one, i.e., the addition of phosphoric acid during electrode preparation, targets the prevention of the aluminum current collector corrosion and simultaneously stabilizes the LNMO particle surface, hence, avoiding the active material degradation in contact with water. The second one, i.e., the crosslinking of the binder by means of citric acid, provides an enhanced cycling stability of the aqueous processed electrodes.[ 6,7] Remarkably, these two complementary approaches can be easily adapted in the electrode preparation process without requiring any additional processing step. To achieve positive electrode tapes, readily available for large-scale high-energy lithium-ion batteries, a carbon-coated current collector is used. It provides functional groups to crosslink the positive electrode coating layer to the underlying aluminum foil. The resulting electrodes demonstrate significant improvements concerning the electrode to current collector adhesion and their capacity retention.[ 8] Through the further optimization of the initial formation cycles, these electrodes offer an electrochemical performance exceeding that of LNMO-based reference electrodes, comprising state-of-the-art poly(vinylidene difluoride) (PVdF) as binder. Transferring this knowledge to other modified biopolymers, such as chitosan and guar gum in combination with relations derived between structure and performance of electrodes made from these cheap, abundant and naturally available binders finally leads to the realization of entirely water-processed high-performance LIBs employing sustainable cobalt-free high-voltage LNMO cathodes and graphite anodes.[ 9,10] References D. Larcher and J. M. Tarascon, Nat. Chem., 7, 19–29 (2015).M. Kuenzel et al., Mater. Today, In Press (2020) https://doi.org/10.1016/j.mattod.2020.04.003.D. Bresser, D. Buchholz, A. Moretti, A. Varzi, and S. Passerini, Energy Environ. Sci., 11, 3096–3127 (2018).A. Kwade et al., Nat. Energy, 3, 290–300 (2018).Michael M. Thackeray, J. Am. Ceram. Soc., 82, 3347–3354 (1993).N. Loeffler et al., ChemSusChem, 9, 1112–1117 (2016).M. Kuenzel et al., ChemSusChem, 11, 562–573 (2018).M. Kuenzel et al., ACS Appl. Energy Mater., 3, 218–230 (2020).M. Kuenzel et al., Batter. Supercaps, 3, 155–164 (2020).M. Kuenzel et al., ChemSusChem, 13 (2020) https://doi.org/10.1002/cssc.201903483.