Systems-level investigation of aqueous batteries for understanding the benefit of water-in-salt electrolyte by synchrotron nanoimaging

Abstract

Water-in-salt (WIS) electrolytes provide a promising path toward aqueous battery systems with enlarged operating voltage windows for better safety and environmental sustainability. In this work, a new electrode couple, LiV3O8-LiMn2O4, for aqueous Li-ion batteries is investigated to understand the mechanism by which the WIS electrolyte improves the cycling stability at an extended voltage window. Operando synchrotron transmission x-ray microscopy on the LiMn2O4 cathode reveals that the WIS electrolyte suppresses the mechanical damage to the electrode network and dissolution of the electrode particles, in addition to delaying the water decomposition process. Because the viscosity of WIS is notably higher, the reaction heterogeneity of the electrodes is quantified with x-ray absorption spectroscopic imaging, visualizing the kinetic limitations of the WIS electrolyte. This work furthers the mechanistic understanding of electrode–WIS electrolyte interactions and paves the way to explore the strategy to mitigate their possible kinetic limitations in three-dimensional architectures.

Probing Kinetics of Water-in-Salt Aqueous Batteries with Thick Porous Electrodes

“…If the reaction is limited by other factors rather than by the long-range ionic diffusion in the electrolyte, such as solid-state diffusion in LMO or LVO, interfacial diffusion through SEI, or other interfacial reactions, the concentration of Li ions in the electrolyte should be homogeneous and thus lead to no chemical heterogeneity in LMO. Note that according to our previous study, 41 the Mn dissolution can be significantly suppressed when cycling in WIS electrolyte. Therefore, the inhomogeneity of the Mn valence state is primarily attributed to the reaction heterogeneity.…”

“…Figure 2 A shows the representative first full-cycle curves of the thin electrodes (40 μm) cycled under 0.1C and 0.5C rates, which are consistent with the previous report. 41 Figure 2 B shows the initial charging capacity of thin electrodes a range of varying thickness (10–60 μm) cycled under a 0.5C rate, as well as two boundary conditions of electrodes with the smallest thickness cycled at the slowest rate (10 μm at 0.1C) and with the largest thickness cycled at the fastest rate (60 μm at 1C). Figure 2 C,E and D,F show the rate-dependent cycling performance of high-porosity (HP) TPEs and low-porosity (LP) TPEs, respectively.…”

“… 15 The LVO–LMO couple provides a high capacity, moderate voltage range, and good X-ray imaging contrast. 41 Prior studies with dilute organic electrolyte indicated that the ionic diffusion in the porous electrode matrix, instead of the solid-phase diffusion in the active material, is the primary limitation in the spinel LiMn 2 O 4 cycled under the higher rate; 42 − 44 samples showed higher degrees of delithiation toward the surface of the electrode, leading to a gradient of delithiation into the depth of the electrode. 44 However, our prior work using spectroscopic imaging to map the chemical gradient in a thin LMO electrode in a LVO–LMO battery with WIS electrolyte did not exhibit such chemical gradients.…”

“… 44 However, our prior work using spectroscopic imaging to map the chemical gradient in a thin LMO electrode in a LVO–LMO battery with WIS electrolyte did not exhibit such chemical gradients. 41 It is thus of particular interest to investigate the chemical homogeneity of LMO within thick porous electrodes (TPEs) in ultrahighly concentrated aqueous electrolytes. By probing the spatial chemical heterogeneity, the multimodal results indicate that the ionic diffusion in the electrolyte is the primary rate-limiting factor.…”

Stable, high-performance, dendrite-free, seawater-based aqueous batteries

“…The distance from the edge of the top electrode’s projection to the center of the observation area was 200–400 μm which was significantly smaller than the separation of the electrodes (~2 mm), and the current densities in the observation area were similar to those inside the area of the electrodes. Comparing with other imaging techniques that have been utilized to study the metal deposition process, our in-situ optical system provide the direct top view that will clearly illustrate the deposition dynamics at different locations and is easy to use comparing with X-ray imaging which requires the synchrotron beamline 44 . To verify our assumption, a COMSOL model was developed to simulate the current density distribution across the bottom electrode (Supplementary Fig.…”