Tailoring Binder Systems for LiNi0.6Mn0.2Co0.2O2-Based Positive Electrode by Investigating Various Polyvinylidene Difluorides for Different Active Material Morphologies

Abstract

Polyvinylidene difluoride (PVdF) is the state-of-the-art binding agent for positive electrodes (cathodes) consisting of Ni-rich materials such as LiNi0.6Mn0.2Co0.2O2 (NMC622) and a conductive additive, which are processed with the organic solvent N-methyl-pyrrolidone. Processing and composition of the positive electrode strongly influence the specific energy and electrochemical performance of the corresponding lithium ion batteries (LIBs). Various binder systems were studied for state-of-the-art positive electrodes, however, a deeper understanding of the impact on the electrode properties is yet to be acquired.1,2 Besides the electronical and mechanical properties of the positive electrode, the binding agent additionally affects the charge/discharge cycling stability and rate capability of the battery cell. An important factor is the ratio of binder content to surface area of the solid components.3 For instance, using a low binder coverage (ratio of binding agent to solid components) can lead to weak adhesion between composite electrode and current collector, resulting in a decrease in cycling stability. This is due to mechanical degradation upon prolonged charge/discharge cycling, as altering volume extension and reduction leads to mechanical stress and therefore in particle cracking and contact loss. On the contrary, extensive binder coverage leads to high internal resistance, due to hindered lithium mobility and potentially reduced electronical conductivity of the composite electrode.The properties of binding agents vary depending on chain length and the degree of polymerization, hence affecting intermolecular stability, swelling rate, and adhesion to the current collector. While the morphologies of active materials are predetermined (primary/secondary particles) and the variation of the conductive additive can influence the pore structure and conductivity of the electrode, the binding agent is essential for the distribution of the conductive additive and the adhesion to the current collector. Binding agents make up a comparably small part in the electrode composition, however, it is the electrode component that can essentially enhance the lifetime of a battery.Thus, it is crucial to understand the influence of the binder characteristics on the electronical, mechanical, and electrochemical properties of the cathode. To understand how binder coverage can enhance or impair these properties, primary particles (high surface area) and secondary particles (comparably low surface area) of NMC622 were processed with various polyvinylidene difluoride binding agents differing in molecular weight. Experiments regarding the physical electrode properties such as adhesion and electronic through-plane conductivity, as well as the electrochemical properties concerning rate capability and long-term charge/discharge cycling were compared and evaluated for the two active material variants. Additionally, aging studies with one of the binding agents were carried out to investigate the influence of the active material surface on the aging products found in the electrolyte and by that gaining a deeper understanding for the aging mechanisms in different NMC622 cell set ups. The combination of the mentioned experimental techniques enables the determination of a tailored binder system for a specific active material morphology and the associated electrode composition for the intended LIB application.[1] Gordon, R.; Kassar, M.; Willenbacher, N., ACS omega 2020, 5 (20), 11455–11465.[2] Chen, H.; Ling, M.; Hencz, L.; Ling, H. Y.; Li, G.; Lin, Z.; Liu, G.; Zhang, S., Chemical reviews 2018, 118 (18), 8936–8982.[3] Weichert, A.; Göken, V.; Fromm, O.; Beuse, T.; Winter, M.; Börner, M., Journal of Power Sources 2022, 551, 232179.