No abstract

Water-in-salt electrolytes (WiSE) are concentrated aqueous electrolytes recently developed that are of great interest because of their possible relevance for batteries. The origin for their promising application has been ascribed to the formation of percolating nanodomains in the bulk. However, the interfacial structure of WiSE still remains to be understood. In this paper, we characterize the potential-dependent double layer of a LiTFSIbased electrolyte on a charged electrode surface. Ultramicroelectrode (UME) measurements reveal a surface-confinement effect for a ferricyanide redox species at the electrode/WiSE interface. Potential-dependent atomic force microscopy (AFM) shows the presence of layers, the structure of which changes with the applied potential. Thicker layers (6.4 and 6.7 Å) are observed at positive potentials, associated with [Li(H 2 O) x ] + ([TFSI] − ) y ion pairs, while thinner layers (2.8 and 3.3 Å) are found at negative potentials and associated with [Li(H 2 O) x ] + alone. Vibrational spectroscopy shows that the composition of the double layer also changes with potential, where [TFSI] − is enriched at positive and [Li(H 2 O) x ] + enriched at negative potentials.

The insertion and removal of Li + ions into Li-ion battery electrodes can lead to severe mechanical fatigue because of the repeated expansion and compression of the host lattice during electrochemical cycling. In particular, the lithium manganese oxide spinel (LiMn 2 O 4 , LMO) experiences a significant surface stress contribution to electrode chemomechanics upon delithiation that is asynchronous with the potentials where bulk phase transitions occur. In this work, we probe the stress evolution and resulting mechanical fracture from LMO delithation using an integrated approach consisting of cyclic voltammetry, electron microscopy, and density functional theory (DFT) calculations. High-rate electrochemical cycling is used to exacerbate the mechanical deficiencies of the LMO electrode and demonstrates that mechanical degradation leads to slowing of delithiation and lithiation kinetics. These observations are further supported through the identification of significant fracturing in LMO using scanning electron microscopy. DFT calculations are used to model the mechanical response of LMO surfaces to electrochemical delithiation and suggest that particle fracture is unlikely in the [001] direction because of tensile stresses from delithiation near the (001) surface. Transmission electron microscopy and electron backscatter diffraction of the as-cycled LMO particles further support the computational analyses, indicating that particle fracture instead tends to preferentially occur along the {111} planes. This joint computational and experimental analysis provides molecularlevel details of the chemomechanical response of the LMO electrode to electrochemical delithiation and how surface stresses may lead to particle fracture in Li-ion battery electrodes.

at larger scales (electric vehicle or gridscale energy storage) has been hampered by insufficient energy densities, prohibitive material costs, and safety concerns. [2] While significant progress has been made in the development of high energy density battery materials, the implementation of these materials in practical systems without sacrificing cell longevity or safety remains a pressing scientific challenge. One of the best studied approaches to improve the energy density of LIBs is to replace existing graphitic anodes (372 mA h g −1 capacity) with metallic lithium (3860 mA h g −1 capacity). [3] Unfortunately, uneven lithium plating and stripping in conventional organic liquid electrolytes promotes the growth of lithium dendrites, accelerating the formation of internal short-circuits. Additionally, the low thermal stability of electrolyte solvents (often mixtures of cyclic and linear carbonates) incurs exothermic decomposition of the electrolyte in the event of cell failure, often resulting in catastrophic thermal runaway. [4-6] While many approaches have been explored to address these issues, one of the most promising options is the elimination of the liquid electrolyte in favor of a solid electrolyte. [7] Transitioning from conventional liquid electrolytes to Li +conducting solid electrolytes (SEs) presents two primary advantages. The high mechanical rigidity of inorganic SEs may act to suppress the formation of dendrites at lithium anodes, reducing the possibility of internal short circuits. [8] Additionally, the negligible flammability of most SEs dramatically lowers the risk of uncontrolled thermal runaway in the event of cell failure. [9,10] These potential benefits motivate the search for materials with sufficiently high ionic conductivities to serve as replacements for their liquid counterparts. Among the known SE formulations, the thiophosphate class of superionic conductors exhibit exceptionally fast lithium transport at room temperature. [11] Specifically, Li 10 GeP 2 S 12 (LGPS) demonstrates an ionic conductivity of ≈10 mS cm −1 , comparable to conductivities achievable in conventional liquid electrolytes. [12] Despite the favorable ionic conductivity of LGPS, its poor chemical and electrochemical stabilities remain key challenges for practical application. The narrow window of electrochemical stability of LGPS and many other thiophosphate

High-voltage lithium-ion cathode materials exhibit exceptional energy densities; however, rapid capacity fade during cell cycling prohibits their widespread utilization. Surface modification of cathode-active materials by organic self-assembled monolayers (SAMs) has emerged as an approach to improve the longevity of high-voltage electrodes; however, the surface chemistry at the electrode/electrolyte interphase and its dependence on monolayer structure remains unclear. Herein, we investigate the interplay between monolayer structure, electrochemical performance, and surface chemistry of high-voltage LiMn1.5Ni0.5O4 (LMNO) electrodes by the application of silane-based SAMs of variable length and chemical composition. We demonstrate that the application of both hydrophobic and hydrophilic monolayers results in improved galvanostatic capacity retention relative to unmodified LMNO. The extent of this improvement is tied to the structure of the monolayer with fluorinated alkyl-silanes exhibiting the greatest overall capacity retention, above 96% after 100 charge/discharge cycles. Postmortem surface analysis reveals that the presence of the monolayer enhances the deposition of LiF at the electrode surface during cell cycling and that the total surface concentration correlates with the overall improvements in capacity retention. We propose that the enhanced deposition of highly insulating LiF increases the anodic stability of the interphase, contributing to the improved galvanostatic performance of modified electrodes. Moreover, this work demonstrates that the modification of the electrode surface by the selection of an appropriate monolayer is an effective approach to tune the properties and behavior of the electrode/electrolyte interphase formed during battery operation.