Electrolyte Engineering for Long-Life Li-SPAN Batteries

Sulfurized polyacrylonitrile, or SPAN, has been studied as an alternative to elemental sulfur as a cathode in lithium–sulfur batteries. Unlike elemental S, the material features a solid-phase conversion reaction during charge and discharge, which shows promise in providing long cycle life under lean electrolyte conditions. However, this altered mechanism also imposes a unique set of electrolyte design requirements. In this Review, we outline the key advancements in electrolyte engineering and discuss the design principles of these electrolytes with a focus on the solvation structures and their ability to control the interphasial chemistry on both the Li and the SPAN surfaces. We then argue for the need to develop electrolytes with improved transport properties while preserving their high stabilities in order to realize Li-SPAN batteries with practical energy densities

Predicting the Ion Desolvation Pathway of Lithium Electrolytes and Their Dependence on Chemistry and Temperature

To better understand the influence of electrolyte chemistry on the ion-desolvation portion of charge-transfer beyond the commonly applied techniques, we apply free-energy sampling to simulations involving diethyl ether (DEE) and 1,3-dioxoloane/1,2-dimethoxyethane (DOL/DME) electrolytes, which display bulk solvation structures dominated by ion-pairing and solvent coordination, respectively. This analysis was conducted at a pristine electrode with and without applied bias at 298 and 213 K to provide insights into the low-temperature charge-transfer behavior, where it has been proposed that desolvation dominates performance. We find that, to reach the inner Helmholtz layer, ion-paired structures are advantageous and that the Li+ ion must reach a total coordination number of 3, which requires the shedding of 1 species in the DEE electrolyte or 2–3 species in DOL/DME. This work represents an effort to predict the distinct thermodynamic states as well as the most probable kinetic pathways of ion desolvation relevant for the charge transfer at electrochemical interphases

Effect of Electrolyte Chemistry and Sulfur Content in Li||Sulfurized Polyacrylonitrile (SPAN) Batteries

Sulfurized polyacrylonitrile (SPAN) is considered as a high-value cathode material, which leverages the high energy of S redox while mitigating the negative externalities that limit elemental S cycling. As such, the sulfur content in Li-SPAN batteries plays a critical role. In this work, we demonstrate that high-S loading SPAN cathodes, where the PAN backbone approaches the saturation point without signs of elemental S, are highly dependent on the electrolyte chemistry for long-term reversibility. Specifically, we find that a localized-high-concentration electrolyte (LHCE) further enhances the reversible capacity and cycling stability of SPAN cathode with optimized S content relative to a carbonate control, largely due to the formation of a compatible interphase. With this LHCE as the electrolyte and 43% sulfur ratio of SPAN as the cathode, a full cell applying N/P ratio = 1.82, a cathode loading of 6 mAh cm–2 (9.2 mg cm–2), and an electrolyte loading of 7 μL mg–1 SPAN can be cycled for 100 cycles with 433 mAh g–1 retained capacity and retains much of this reversibility even at 60 °C. This work reveals the molecular origin of optimized sulfur ratio in SPAN cathodes while providing guidance in electrolyte design for Li||SPAN cells with high capacity and cyclability

Thin Solid Electrolyte Layers Enabled by Nanoscopic Polymer Binding

To achieve high-energy all-solid-state batteries (ASSBs), solid-state electrolytes (SE) must be thin, mechanically robust, and possess the ability to form low resistance interfaces with electrode materials. Embedding an inorganic SE into an organic polymer combines the merits of high conductivity and flexibility. However, the performance of such an SE-in-polymer matrix (SEPM) is highly dependent on the microstructure and interactions between the organic and inorganic components. We report on the synthesis of a free-standing, ultrathin (60 μm) SEPM from a solution of lithium polysulfide, phosphorus sulfide, and ethylene sulfide (ES), where the polysulfide triggers the in situ polymerization of ES and the formation of Li3PS4. Reactant ratios were optimized to achieve a room-temperature conductivity of 2 × 10–5 S cm–1. Cryogenic electron microscopy confirmed a uniform nanoscopic distribution of β-Li3PS4 and PES (polyethylene sulfide). This work presents a facile route to the scalable fabrication of ASSBs with promising cycling performance and low electrolyte loading

Thin Solid Electrolyte Layers Enabled by Nanoscopic Polymer Binding

To achieve high-energy all-solid-state batteries (ASSBs), solid-state electrolytes (SE) must be thin, mechanically robust, and possess the ability to form low resistance interfaces with electrode materials. Embedding an inorganic SE into an organic polymer combines the merits of high conductivity and flexibility. However, the performance of such an SE-in-polymer matrix (SEPM) is highly dependent on the microstructure and interactions between the organic and inorganic components. We report on the synthesis of a free-standing, ultrathin (60 μm) SEPM from a solution of lithium polysulfide, phosphorus sulfide, and ethylene sulfide (ES), where the polysulfide triggers the in situ polymerization of ES and the formation of Li3PS4. Reactant ratios were optimized to achieve a room-temperature conductivity of 2 × 10–5 S cm–1. Cryogenic electron microscopy confirmed a uniform nanoscopic distribution of β-Li3PS4 and PES (polyethylene sulfide). This work presents a facile route to the scalable fabrication of ASSBs with promising cycling performance and low electrolyte loading

The Role of Ion Transport in the Failure of High Areal Capacity Li Metal Batteries

Recent advancements in electrolyte research have substantially improved the cycle life of Li metal batteries (LMBs) but often under moderate areal capacity. The design principles overwhelmingly emphasize the reduction of electrolyte reactivity toward Li. In this work, we find that high areal capacity (>6 mAh cm–2) Li||sulfurized polyacrylonitrile (SPAN) batteries fail primarily due to shorting events when paired with four types of localized high-concentration electrolytes (LHCEs), which is correlated with electrolyte transport properties, including ionic conductivity and Sand’s capacity. These LHCE systems, despite their high Coulombic efficiencies for Li metal cycling, produce macroscopically non-uniform Li deposits when operating under transport limitation. This deficiency leads to short circuit over repeated cycling, as evidenced by a quantitative, statistical analysis of SEM images. Based on these insights, we fabricated a 2 Ah pouch cell, which demonstrates a cell energy density of >260 Wh kg–1 for more than 70 cycles. Our findings emphasize the significance of the bulk transport properties of electrolytes and the statistical morphological information on cycled Li for long-life LMBs

Oxidative Stabilization of Dilute Ether Electrolytes via Anion Modification

State-of-the-art lithium metal batteries typically rely on ether electrolytes with high salt concentration and/or fluorinated solvents to enable stable cycling. Their high manufacturing costs at scale have motivated us to consider dilute, nonfluorinated ether electrolytes. However, their poor oxidative stability has precluded their application in cells employing transition-metal oxide cathodes, which operate at >4 V vs Li/Li+. Herein, we present a possible route forward for the oxidative stabilization of these electrolytes, which enabled the reversible cycling of LiNi0.8Mn0.1Co0.1O2 at a cutoff of 4.4 V in electrolytes composed only of 1 M salt and 1,2-dimethoxyethane. Through computational and experimental material characterization, it was determined that this behavior was driven by a passivating interphase composed largely of perfluoro alkane species. This work provides a method for the oxidative stabilization of ether electrolytes with a low base materials cost

Locally Saturated Ether-Based Electrolytes With Oxidative Stability For Li Metal Batteries Based on Li-Rich Cathodes

Li metal batteries applying Li-rich, Mn-rich (LMR) layered oxide cathodes present an opportunity to achieve high-energy density at reduced cell cost. However, the intense oxidizing and reducing potentials associated with LMR cathodes and Li anodes present considerable design challenges for prospective electrolytes. Herein, we demonstrate that, somewhat surprisingly, a properly designed localized-high-concentration electrolyte (LHCE) based on ether solvents is capable of providing reversible performance for Li||LMR cells. Specifically, the oxidative stability of the LHCE was found to heavily rely on the ratio between salt and solvating solvent, where local-saturation was necessary to stabilize performance. Through molecular dynamics (MD) simulations, this behavior was found to be a result of aggregated solvation structures of Li+/anion pairs. This LHCE system was found to produce significantly improved LMR cycling (95.8% capacity retention after 100 cycles) relative to a carbonate control as a result of improved cathode-electrolyte interphase (CEI) chemistry from X-ray photoelectron spectroscopy (XPS), and cryogenic transmission electron microscopy (cryo-TEM). Leveraging this stability, 4 mAh cm–2 LMR||2× Li full cells were demonstrated, retaining 87% capacity after 80 cycles in LHCE, whereas the control electrolyte produced rapid failure. This work uncovers the benefits, design requirements, and performance origins of LHCE electrolytes for high-voltage Li||LMR batteries

Unravelling Ultrafast Li Ion Transport in Functionalized Metal–Organic Framework-Based Battery Electrolytes

Nonaqueous fluidic transport and ion solvation properties under nanoscale confinement are poorly understood, especially in ion conduction for energy storage and conversion systems. Herein, metal–organic frameworks (MOFs) and aprotic electrolytes are studied as a robust platform for molecular-level insights into electrolyte behaviors in confined spaces. By employing computer simulations, along with spectroscopic and electrochemical measurements, we demonstrate several phenomena that deviate from the bulk, including modulated solvent molecular configurations, aggregated solvation structures, and tunable transport mechanisms from quasi-solid to quasi-liquid in functionalized MOFs. Technologically, taking advantage of confinement effects may prove useful for addressing stability concerns associated with volatile organic electrolytes while simultaneously endowing ultrafast transport of solvates, resulting in improved battery performance, even at extreme temperatures. The molecular-level insights presented here further our understanding of structure–property relationships of complex fluids at the nanoscale, information that can be exploited for the predictive design of more efficient electrochemical systems

Unravelling Ultrafast Li Ion Transport in Functionalized Metal–Organic Framework-Based Battery Electrolytes

Nonaqueous fluidic transport and ion solvation properties under nanoscale confinement are poorly understood, especially in ion conduction for energy storage and conversion systems. Herein, metal–organic frameworks (MOFs) and aprotic electrolytes are studied as a robust platform for molecular-level insights into electrolyte behaviors in confined spaces. By employing computer simulations, along with spectroscopic and electrochemical measurements, we demonstrate several phenomena that deviate from the bulk, including modulated solvent molecular configurations, aggregated solvation structures, and tunable transport mechanisms from quasi-solid to quasi-liquid in functionalized MOFs. Technologically, taking advantage of confinement effects may prove useful for addressing stability concerns associated with volatile organic electrolytes while simultaneously endowing ultrafast transport of solvates, resulting in improved battery performance, even at extreme temperatures. The molecular-level insights presented here further our understanding of structure–property relationships of complex fluids at the nanoscale, information that can be exploited for the predictive design of more efficient electrochemical systems