Understanding of Polyacrylic Acid as Binder for High Silicon Containing Anodes and Ni-Rich Cathodes

Abstract

With the increased demand on electric vehicles, portable devices and energy storage systems, a great attention is being drawn to improve the energy density of lithium-ion batteries (LiBs) [1]. The commercial LiBs anodes are usually consisted of graphite-based material, which limits the energy density due to its relatively low theoretical capacities. Silicon (Si) has been recognized as an alternative anode material with its high theoretical capacity, natural abundance, and low discharge potential. Despite the above-mentioned advantages, Si undergoes a substantial volume expansion (300% approximately) during the battery operation [2]. There are some approaches to mitigate the adverse of expansion, whereas the cost and scalability of these techniques are the bottlenecks to solve this issue [3]. In this work, we focus on maintaining the structural integrity, along with the coordination of Li as functional binder which could be an effective solution to the abovementioned challenges.In addition to the energy density and capacity targets, considering more environmentally friendly approaches is also one of the key topics for battery manufacturing. Aqueous processing of the anodes is already state-of-the-art whilst, the cathode has its own challenges to be overcome. Ni-rich transition oxide active materials, such as LiNi0.8MN0.1Co0.1 (NMC811), are attractive for high-capacity target applications. Due to the high reactive surface of the active material, during the slurry processing, NMC811 reacts with water, which leads to Li+/H+ exchange [4]. Thus, the slurry results in high pH which can corrode the aluminum foil current collector and cause poor capacity due to Li loss. The advantage of using polyacrylic acid (PAA) is to employ COOH, in which the leached Li+ can bond to the backbone. Hence, the pH of slurry is maintained on desirable coating range and the capacity loss can be reduced due to the proposed aqueous processing.In this study, we explore the utilization of a well-known polymer, polyacrylic acid, as binder for both negative and positive electrodes. On the anode, PAA is functionalized with Li+ and used LiPAA prior to electrode production. It is highly crucial to observe how the dynamics of Si and LiPAA impacts the anode performance during the cycling. To comprehend this, post mortem, scanning electron microscopy (SEM) was performed to detect the positioning and concentration of Si on pristine and cycled anodes. On the cathode side, the focus is to utilize the leached Li+ during the slurry processing by having PAA as binder. This technique has been already successfully implemented by our group on the research pilot scale [5]. The idea is to make pH stable slurry without using any additional acid, in conjunction with creating in situ functional binder (LiPAA). Deep understanding of functional binders is to serve as a guide for the rational design of Si-electrode and aqueous NMC811 cathodes for LiBs.