Mechanistic Understanding of the Role of Evaporation in Electrode Processing

Abstract

Electrode processing based on the state-of-the-art materials represents a scientific opportunity toward a cost-effective measure for improving the lithium-ion battery performance. In this regard, perhaps the most important is the drying step in a typical non-aqueous based slurry processing which can profoundly impact the electrode microstructure and hence performance. Solvent evaporation during drying plays a critical role in the redistribution of the particulate phases consisting of active particle, conductive additive and binder. In this work, we attempt to provide a mechanistic understanding of the role of solvent evaporation on the electrode characteristics and performance via a combined experimental and theoretical analysis. This study elucidates that a non-uniform distribution of the constituent phases, especially the relatively mobile conductive additive and binder, can develop which depends on the solvent evaporation, particle diffusion and sedimentation attributes. Experimental results and theoretical analysis reveal the impact of evaporation rate on the conductive additive and binder distribution in the electrode microstructure and resulting electrochemical performance. Our analysis has shown that a slower two-stage drying, as opposed to a high-rate single-stage drying, allows for a favorable distribution of binder and conductive additive, thus reducing internal cell resistance and improving electrochemical performance. Increasing concerns about depleting fossil fuel reserves, energy security, and climate change have given rise to interest in the adoption of renewable energy in place of traditional, petroleum-based fuels. The implementation and usage of renewable energies have been limited due to lack of efficient storage and transportation infrastructure. Recent improvements in the energy density and durability of lithiumion batteries (LIBs) have made them an increasingly attractive means of energy storage. [1][2][3][4] Further improvements in lithium-ion technology would increase the viability of widespread adoption of electric vehicles and renewable energy integration into the electric grid. For example, improvements in the capacity of LIBs would not only improve the effective range of electric vehicles, 5,6 but also improve their cycle life by reducing the depth of discharge, which in turn increases the viability of LIBs for use in grid energy storage applications. 7The performance of Li-ion batteries depends on the electrode materials, the choice of electrolyte, and the cell architecture.4,8 A typical LIB positive electrode (cathode) is composed of a combination of Li-containing active material, conductive additive, polymeric binder, and pore space that is filled with an electrolyte. Typically, these are created by casting out and drying a thin film of slurry containing these multi-phase components. 19 but little attention is paid to the physical understanding of the electrode processing. The importance of this electrode preparation step cannot be overemphasized. In addition to determining the cel...

Binder migration during drying of lithium-ion battery electrodes: Modelling and comparison to experiment

“…Other common techniques found in the literature include air drying [26], exposure to infrared radiation [11] and the use of a vacuum oven [21,22]. While some of these can result in drastically different drying times the vacuum oven technique used in Stein et al [22] led to a film shrinkage time of around 1 minute (3 minutes) when a temperature of 120 o C (70 o C) was used which is in line with our estimates. The PVDF diffusion coefficient D = 1.14 × 10 −10 m 2 /s is obtained, as described in the supplementary information, by qualitatively matching the model solutions to the experimental results presented in [6].…”

“…In both [6] and [12] it was found that rapid removal of the solvent causes an enrichment of binder in the upper regions that cannot be compensated by diffusion at high drying rates. We note also the work of Stein et al [22], on very slow drying of cathode films, which shows drying rate dependence of the binder distribution. Theoretical models detailing the physical mechanisms governing the drying of single-component and two-component colloidal suspensions are studied in [24,25] and [26,27], respectively, while drying of polymer solutions is investigated in [28,29].…”

“…Electrode drying has been the subject of intense experimental research in recent years [5,6,9,18,19,12,20,21,22] and a consensus has developed that changes to the drying process parameters (temperature, air-flow, pressure and radiation intensity) significantly affect the final electrode microstructure and hence the electrochemical and mechanical properties of the electrode. High temperatures and drying rates have been observed to lead to accumulation of binder at the evaporation surface and corresponding depletion close to the current collector [5,9,19,20].…”

“…High temperatures and drying rates have been observed to lead to accumulation of binder at the evaporation surface and corresponding depletion close to the current collector [5,9,19,20]. The consequences of a non-uniform binder distribution include lower adhesion of the electrode to the current collector [5,6,9,20], increased electrical resistivity [5,22] and decreased cell capacity [6]. Chou et al [23] conclude that even though binder makes up only a small fraction of the electrode composition, it plays a very important role in the cycling stability and rate capability of the electrode.…”

Production Research as Key Factor for Successful Establishment of Battery Production on the Example of Large‐Scale Automotive Cells Containing Nickel‐Rich LiNi0.8Mn0.1Co0.1O2 Electrodes

“…The less the solvent that has to be removed, the more time and energy can be saved, which leads to cost reduction. In addition, solvent removal has been associated with particle redistribution in the electrode and can especially provoke the migration of the relatively mobile binder and conductive additives to the coating surface . However, from an industrial point of view, also the one‐pot approach is advantageous due to reduced mixing times.…”