Abstract
Aqueous electrochemical systems suffer from a low energy density due to a small voltage window of water (1.23 V). Using thicker electrodes to increase the energy density and highly concentrated “water-in-salt” (WIS) electrolytes to extend the voltage range can be a promising solution. However, thicker electrodes produce longer diffusion pathways across the electrode. The highly concentrated salts in WIS electrolytes alter the physicochemical properties which determine the transport behaviors of electrolytes. Understanding how these factors interplay to drive complex transport phenomena in WIS batteries with thick electrodes via deterministic analysis on the rate-limiting factors and kinetics is critical to enhance the rate-performance in these batteries. In this work, a multimodal approach—Raman tomography, operando X-ray diffraction refinement, and synchrotron X-ray 3D spectroscopic imaging—was used to investigate the chemical heterogeneity in LiV3O8–LiMn2O4 WIS batteries with thick porous electrodes cycled under different rates. The multimodal results indicate that the ionic diffusion in the electrolyte is the primary rate-limiting factor. This study highlights the importance of fundamentally understanding the electrochemically coupled transport phenomena in determining the rate-limiting factor of thick porous WIS batteries, thus leading to a design strategy for 3D morphology of thick electrodes for high-rate-performance aqueous batteries.
Highlights
Some recent works simulating the transport properties in WIS electrolytes showed that the anions in a WIS system can form a percolating network resulting in a larger Li+ transport or transference number than conventional electrolytes, while the overall diffusivities of all species decreased as electrolyte concentration increased.[19,21−23] Despite the contribution of Received: July 20, 2021
The rate-dependent behavior of thick porous electrodes (TPEs) indicates the ionic diffusion in the electrolyte remains as the chemical heterogeneities
The illustrated in Figure 1. 3D Raman spectroscopy maps the distribution of active material, conductive additive, and electrolyte in the electrodes to characterize the electrolyte bulk phase transformation and local chemical evolution were carried out to map the chemical heterogeneity of LixMn2O4 (0 ≤ x ≤ 1) during delithiation in both thin and thick electrodes
Summary
Extending applications of energy storage from electronic devices to electrical vehicles and grid storage pushes battery technologies, such as Li-ion and beyond-Li systems, to demand higher energy density, better safety, lower environmental impact, and lower cost.[1,2] Concerns of safety and environmental impacts mainly come from the usage of organic solvents as electrolytes which demonstrate an outstanding electrochemical performance in current Li-ion batteries but are highly flammable.[3−5] Li-ion batteries and beyond-Li systems based on aqueous electrolytes could be a solution to resolve these concerns by reducing the risk of fire, toxicity, and the cost to maintain a rigorous manufacturing environment.[5−7] Despite the benefits of aqueous Li-ion batteries, aqueous electrolytes exhibit lower electrolyte stability because of the limited voltage window of water (∼1.23 V), leading to unsatisfactory energy density and capacity retention.[8−10] Research efforts have been dedicated to increasing the operating voltage, energy density, and capacity retention; aqueous batteries with stable cycling at ∼1.5 V through arranging electrode materials and tuning the pH values of electrolytes have been reported.[10−12] In the family of aqueous electrolytes, water-in-salt (WIS) electrolytes, through dissolving ultrahigh concentrations of salts in water, effectively increase the voltage window of electrolyte (>3.0 V which depends on salt components and concentrations) and hold a promising direction for future aqueous batteries.[13−17]. The rate-dependent behavior in similar spatial distribution of chemical states from the surface the thick electrodes implies that there are kinetic limiting factors near the separator to the bottom near the current collector, in the system, which could be solid-state diffusion of Li ions which indicates that this chemical heterogeneity occurred within the electrodes, interfacial reactions of the lithiation/ consistently across the thickness of the electrode This is delithiation processes, and liquid-phase diffusion of ions in the consistent with the fact that the cells with thin electrodes do not WIS electrolyte. Future work may consider testing the systems with different separators, such as glass fiber separators, to study the effect of varying wettability in the transport phenomena
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