Localized High-concentration Electrolyte Research Articles

Understanding steric hindrance effect of solvent molecule in localized high-concentration electrolyte for lithium metal batteries

High energy density lithium metal batteries (LMBs) have garnered significant research interests in the past decades. However, the growth of lithium dendrites and the low Coulombic efficiency (CE) of Li metal anode pose significant challenges for the development of LMBs. Herein, we report a triethyl orthoformate (TEOF)-based localized high-concentration electrolyte (LHCE) that facilitates a highly reversible Li metal anode with dendrite-free deposition morphologies and an average Coulombic efficiency of 99.1% for 450 cycles. Mechanistic study reveal that the steric hindrance caused by the terminal ethyl groups in the TEOF solvent molecule results in a weak solvating ability, leading to the formation of anion-dominant solvation structures. The anion-dominant solvation sheaths play an essential role in the formation of a LiF-rich solid-electrolyte interphase (SEI), which effectively suppresses the growth of Li dendrites. Furthermore, the TEOF-based electrolyte demonstrates the stable cycling of high-voltage Li||NMC811 cells. These results provide insights into understanding of steric hindrance effect on electrolyte solvation structure and offer valuable guidance for the design of electrolyte solvents in the development of lithium metal batteries.

Retuning Solvating Ability of Ether Solvent by Anion Chemistry toward 4.5 V Class Li Metal Battery

AbstractEther‐based electrolytes have good compatibility for lithium metal, which is widely used in lithium metal batteries (LMBs). However, the electrolyte is not suitable for high voltage (>4.0 V) due to its poor anti‐oxidation behavior. Herein, an anion chemistry strategy is designed for a typical 1,2‐dimethoxyethane (DME)‐based electrolyte with 1 m lithium bis(fluorosulfonyl)imide (LiFSI), by using nitrate anion (NO3−) with a high donor number (DN) value to regulate the coordination of DME and FSI–. It is evidenced that the interaction between them is weakened due to the competitive advantage of NO3−, therefore a weakly solvating electrolyte is achieved. It promotes the formation of an inorganic‐rich passivation layer on the cathode and facilitates the de‐solvation process, thus it is suitable for high voltage even at 4.5 V. The anion‐regulated weakly solvating electrolyte (ARWSE) designed here is different from the high‐concentration electrolyte (HCE), localized HCE, and WSE which are of high cost and complexity. The ARWSE is demonstrated to be an adjustable electrolyte system, effectively improving the interfacial chemistry of LMBs.

Demarcating the Impact of Electrolytes on High‐Nickel Cathodes and Lithium‐Metal Anode

AbstractThe ever‐growing demand for low‐cost, high‐energy‐density lithium‐ion batteries (LIBs) makes high‐nickel layered oxide cathodes, especially LiNiO2 (LNO), one of the most appealing candidates. However, poor structural and surface instability that leads to a short cycle life remains a formidable challenge. Herein, a systematic investigation of LNO performance in two different electrolytes (a conventional carbonate‐based LP57 electrolyte and an ether‐based localized high‐concentration electrolyte (LHCE)) with different charge cut‐off voltages is presented. These findings show that the cathode‐electrolyte reactivity is the main factor dictating the performance degradation of LNO at high voltages rather than bulk integrity. While LHCE can provide good stability beyond 4.2 V with a robust, uniform solid–electrolyte interphase (SEI) layer on the Li‐metal anode, there is no significant difference in cyclability at 4.15 V (96% capacity retention after 200 cycles) for both LP57 and LHCE. From LNO symmetric cells, carbonate‐based electrolyte is found to be good for LNO stability while ether‐based electrolyte is beneficial toward Li‐metal anode. Thus, a suitable electrolyte or a low cut‐off voltage is necessary to maintain a decent cycle life. Altogether, this work highlights the impact of electrolyte and cut‐off voltage on LNO and Li‐metal, which can help guide the development of cells based on LNO.

Interfacial pressure improves calendar aging of lithium metal anodes

Lithium (Li) metal is a promising anode because its theoretical specific capacity is approximately ten times larger than graphite. However, Li anodes suffer from long-term capacity fade due to Li stranding (becoming electronically disconnected) and electrolyte decomposition. Applied interfacial pressure has been shown to improve Li anode cycling, likely due to reincorporating stranded or “dead” Li into the anode. Calendar aging can also lead to Li capacity loss due to electrolyte decomposition/Li corrosion and the formation of stranded Li. Some research suggests that calendar aging during cycling results in reversible capacity losses due to Li stranding and reconnection. We here investigate the effect of applied interfacial pressure on Li anode calendar aging during cycling with incorporated rest steps in a localized high-concentration electrolyte (LHCE) to understand if pressure can mitigate stranded Li formation during rest by manipulating the Li morphology. Pouch cells exhibit more stable cycling and denser Li deposits between 10 kPa and 1,000 kPa of applied pressure compared to no applied pressure. Despite drops in CE during periodic rest cycles, the average cumulative lost capacity and average coulombic efficiency (CE) of cells over 50 cycles show that cells aged with incorporated rest steps perform similarly to cells cycled without added rests. This similar average CE suggests that dead Li is largely responsible for drops in CE during rest rather than irreversible Li corrosion and that the dead Li can be reconnected in subsequent cycling. The addition of a lithiophilic ZnO coating to the Cu working electrode increases the adhesion and coverage of Li deposits at low pressures and improves CE during the first cycle.

Improving dual electrodes compatibility through tailoring solvation structures enabling high-performance and low-temperature Li||LiFePO4 batteries

Li||LiFePO4 (LFP) batteries have good stability and high energy density. However, they exhibit unsatisfactory low-temperature electrochemical performance. Due to the fragile interfacial passivation layers and sluggish kinetics, commercial electrolytes fail to simultaneously achieve acceptable stabilization with dual electrodes in low-temperature Li||LFP batteries. Herein, a novel localized high-concentration electrolyte (LHCE) with great dual-electrodes compatibility is proposed to match with the low-temperature Li||LFP batteries. With increasing local concentration, the FSI- sequentially replaces the solvent molecules and enters the first solvation sheath, forming the anion-dominated solvation structures. This effectively suppresses free solvents decomposition and constructs the anion-derived passivation layers with inorganic-rich components, further contributing to the rapid transport kinetics and endowing the LHCE with great dual electrodes compatibility. These dual-electrodes co-stabilization effects of the LHCE are originally clarified in the low-temperature Li||LFP batteries. The designed LHCE also delivers low freezing point (-99.8 ℃), high ionic conductivity (2.4 mS cm−1 at −40 ℃), and superior stability (>4.7 V vs. Li/Li+). Hence, the Li||LFP batteries with LHCE possess superb cyclic stability at low temperatures, delivering a high discharge capacity of 120 mAh g−1 over 300 cycles at −20 ℃. Moreover, compared to commercial electrolytes, LHCE endows the Li||LFP batteries with superior low-temperature performances under practical conditions, including limited Li anode (3 mAh cm−2) and a wide temperature range (30 ℃ to −40 ℃).

Ultrastable Dendrite-Free Potassium Metal Batteries Enabled by Weakly-Solvated Electrolyte.

Potassium (K) metal is considered one of the most promising anodes for potassium metal batteries (PMBs) because of its abundant and low-cost advantages but suffers from serious dendritic growth and parasitic reactions, resulting in poor cyclability, low Coulombic efficiency (CE), and safety concerns. In this work, we report a localized high-concentration electrolyte (LHCE) consisting of potassium bis(fluorosulfonyl)imide (KFSI) in a cosolvent of 1,2-dimethoxyethane (DME) and 1,1,2,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) to solve the problems of PMBs. TTE as a diluent not only endows LHCE with advantages of low viscosity, good wettability, and improved conductivity but also solves the dendrite problem pertaining to K metal anodes. Using the formulation of LHCE, a CE of 98% during 800 cycles in the K||Cu cell and extremely stable cycling of over 2000 h in the K||K symmetric cell are achieved at a current density of 0.1 mA cm-2. In addition, the LHCE shows good compatibility with a Prussian Blue cathode, allowing almost 99% CE for the K||KFeIIFeIII(CN)6 full cell during 100 cycles. This promising electrolyte design realizes high-safety and energy-dense PMBs.

Unique double-layer solid electrolyte interphase formed with fluorinated ether-based electrolytes for high-voltage lithium metal batteries

Li metal batteries using high-voltage layered oxides cathodes are of particular interest due to their high energy density. However, they suffer from short lifespan and extreme safety concerns, which are attributed to the degradation of layered oxides and the decomposition of electrolyte at high voltage, as well as the high reactivity of metallic Li. The key is the development of stable electrolytes against both high-voltage cathodes and Li with the formation of robust interphase films on the surfaces. Herein, we report a highly fluorinated ether, 1,1,1-trifluoro-2-[(2,2,2-trifluoroethoxy) methoxy] ethane (TTME), as a co-solvent, which not only functions as a diluent forming a localized high concentration electrolyte (LHCE), but also participates in the construction of the inner solvation structure. The TTME-based electrolyte is stable itself at high voltage and induces the formation of a unique double-layer solid electrolyte interphase (SEI) film, which is embodied as one layer rich in crystalline structural components for enhanced mechanical strength and another amorphous layer with a higher concentration of organic components for enhanced flexibility. The Li||Cu cells display a noticeably high Coulombic efficiency of 99.28% after 300 cycles and Li symmetric cells maintain stable cycling more than 3200 h at 0.5 mA/cm2 and 1.0 mAh/cm2. In addition, lithium metal cells using LiNi0.8Co0.1Mn0.1O2 and LiCoO2 cathodes (both loadings ∼3.0 mAh/cm2) realize capacity retentions of >85% over 240 cycles with a charge cut-off voltage of 4.4 V and 90% for 170 cycles with a charge cut-off voltage of 4.5 V, respectively. This study offers a bifunctional ether-based electrolyte solvent beneficial for high-voltage Li metal batteries.

Stable Harsh-Temperature Lithium Metal Batteries Enabled by Tailoring Solvation Structure in Ether Electrolytes.

For lithium metal batteries (LMBs), the elevated operating temperature results in severe capacity fading and safety issues due to unstable electrode-electrolyte interphases and electrolyte solvation structures. Therefore, it is crucial to construct advanced electrolytes capable of tolerating harsh environments to ensure stable LMBs. Here, we proposed a stable localized high-concentration electrolyte (LHCE) by introducing the highly solvating power solvent diethylene glycol dimethyl ether (DGDME). Computational and experimental evidence discloses that the original DGDME-LHCE shows favorable features for high-temperature LMBs, including high Li+-binding stability, electro-oxidation resistance, thermal stability, and nonflammability. The tailored solvated sheath structure achieves the preferred decomposition of anions, inducing the stable (cathode and Li anode)/interphases simultaneously, which enables a homogeneous Li plating-stripping behavior on the anode side and a high-voltage tolerance on the cathode side. For the Li||Li cells coupled with DGDME-LHCE, they showcase outstanding reversibility (a long lifespan of exceeding 1900 h). We demonstrate exceptional cyclic stability (∼95.59%, 250 cycles), high Coulombic efficiency (>99.88%), and impressive high-voltage (4.5 V) and high-temperature (60 °C) performances in Li||NCM523 cells using DGDME-LHCE. Our advances shed light on an encouraging ether electrolyte tactic for the Li-metal batteries confronted with stringent high-temperature challenges.

Interplay of molten salt and dopants in tuning the performance of single-crystalline LiNiO2

Single-crystalline layered oxide cathodes have seen great research interest in recent years. Adoption of single-crystal morphology improves mechanical robustness and eliminates intergranular boundaries, improving the chemical stability of the cathode. However, single-crystal cathodes also suffer from worsened lithium-diffusion kinetics stemming from the significantly increased diffusion path lengths. Sluggish lithiation kinetics induce concentration gradients within the cathode particle during cell operation, resulting in the accumulation of lattice strain, which deteriorates cell performance. Foreign dopants have been shown to greatly improve ion diffusion in many high-Ni layered oxide cathode systems. In particular, we identify Ti and Te as promising dopants to ameliorate the slow diffusion kinetics. We find that doping 1 mol.% Te into single crystal LiNiO2 greatly improves Li + diffusivity, as determined through galvanostatic intermittent titration technique. Further incorporating 2 mol.% Al and pairing with lithium-metal anode in a localized high concentration electrolyte (LHCE) delivers 206 mA h g−1 at C/3 rate and retains 94% capacity after 100 cycles in Li metal half cells. By addressing kinetically mediated degradation with Te doping, and surface-based degradation with Al and LHCE, the cathode experiences very little polarization and overpotential growth, enabling stable cycling with high accessible capacity.

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.

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.

Designing Localized High Concentration Electrolytes Based on Fluorinated Solvents for Lithium-Ion Batteries

Reduction at the anode can affect electrolyte decomposition, solid electrolyte interphase (SEI) formation and growth, and thus the lithium solvation/de-solvation near the SEI, and ultimately lead to various perilous side reactions such as inactive lithium formation. Lithium ions solvated in the electrolyte solution along with salt anions diffuse towards the surface of the electrode. At the charged surface, these solvated ions can undertake different pathways leading to various reductive decomposition products to subsequently form the SEI. The transfer of electrons from the electrode to the salt anions form inorganic SEI products. The SEI layer gradually thickens during repeated charge/discharge cycles due to electron exposure to electrolyte or electrolyte diffusion to the anode surface. This gradual thickening of SEI layer decreases active lithium ions, solvents, and salts and increases cell resistance and lowers the cell capacity and Coulombic efficiency. Essentially, the choice of electrolytes has a significant influence on the formation of an SEI and its underlying chemical and mechanical properties. Optimizing the electrolytes is crucial for an SEI formation since the properties of the SEI significantly affect the lithium-ion batteries’ cyclability, life time, capacity retention, high power density, rate capability, and safety. One approach of stabilizing the SEI is to utilize an electrolyte with high concentration of salt, also known as High-Concentration Electrolytes (HCEs). This approach modifies the Li+ solvation structure to form contact ion pairs (CIP) and aggregates (AGG) while decreasing solvent-separated ion pairs (SSIPs) so that the salt anion, such as FSI-, is preferentially decomposed to form a robust LiF-rich SEI. LiF is considered a beneficial SEI component to block electron transport. By introducing a diluent (a non-solvating solvent) in the HCE to form Localized High Concentration Electrolyte (LHCE), the disadvantages of the HCE, such as low ionic conductivity, high viscosity and high cost, can be minimized while retaining the highly concentrated salt-solvent clusters as they are in the HCE. LHCEs based on fluorinated solvents and diluents can further stabilize the electrode-electrolyte interface. Recent studies showed that the presence of fluorine in the SEI, either in the form of simple inorganic fluorides (LiF) or organofluoro-moieties, brought positive impacts such as expanded electrochemical stability window and high ionic transport. Fluorinated solvents can shift the oxidation stability to a higher voltage compared to their nonfluorinated counterparts. Fluorinated electrolytes enable a high lithium plating Coulombic efficiency and suppresses lithium dendrite formation to a greater extent. In this work we focused on identifying the selection rules for the diluent for designing LHCEs to preserve or improve the local high salt concentration clusters to facilitate the formation of an inorganic rich anion derivative film on the anode as well as to enhance ionic conductivity to enable fast charging. Some of the important properties to consider while selecting a diluent are: - diluent molecules must offer little or no solubility to the salt so that they have minimal participation in the solvation clusters, they must be readily miscible with the solvating solvent so that they dissolve and remove some solvent molecules from the clusters; effectively increasing the salt concentration in the solvation clusters, diluents should be distributed on the periphery of salt-solvent clusters, diluents should have low viscosity, to reduce the overall viscosity of the formulated electrolyte, which in turn improves the ionic conductivity since the low viscosity of diluents allow for higher mobility of the ionic clusters. We analyzed LHCEs consisting of different diluents and diluent molar ratios in a comparative fashion to understand their properties in retaining or improving the structures of the high concentration salt-solvent clusters and improving ionic conductivity. We varied the diluent molar ratio to understand its relationship to increasing salt concentration gradients in the center of the solvent-salt clusters. We also analyzed the relationship between diluent molar ratio and ionic conductivity and found that an optimum diluent molar ratio exists for which the ionic conductivity can be maximized. Our findings serve as design guidelines for practical applications of LHCEs.

Understanding Solvation Structures of Localized High-Concentration Electrolytes Used for Li-Metal Batteries

Liquid electrolytes in batteries are typically treated as macroscopically homogeneous ionic transport media, despite having complex chemical composition and atomistic solvation structures, thus leaving a knowledge gap of microstructural characteristics. A promising electrolyte system design has recently been developed with the title of a localized high-concentration electrolyte (LHCE). These electrolytes are a summation of three components – a charge-carrying salt, a solvent that solvates the salt, and a diluent that is highly immiscible with the salt. This creates the perception of a high-concentration electrolyte (HCE) with high anion-cation interactions that supports a salt-derived, stable solid-electrolyte interphase, while an LHCE improves bulk electrolyte properties, such as ionic conductivity and viscosity, when compared to an HCE. Much trial by error design of this LHCE concept has been supported by simulations to confirm ionic interactions, cycling data to confirm cell stability, and post-mortem analysis to provide evidence of mechanisms. However, a systematic design to extend the capabilities of LHCEs is still to be desired. Here, we analyzed a specific LHCE mixture with the use of a ternary phase diagram to dictate the best viable mixture to extend the localization of solvation structures while maintaining viable cell-level capabilities. It was seen that the electrolyte mixture has complex mechanisms from individual ion solvation up to the solution network scale, both of which are dictated by the interactions between the three electrolyte components. With the support of both experimental and computational analysis of different electrolyte features, we have begun a methodology of formulating the most ideal LHCE mixtures for improving the cyclability of high-capacity battery chemistries.

Ultrathin, Smooth Li-Metal Anode Via Electrodeposition for Stable Cycling of Li-Betal Batteries Under Practical Conditions

Lithium (Li) metal anodes (LMAs) with high specific capacity (3860 mAh g–1) and lowest redox voltage (–3.04 V vs SHE) are considered superlative candidates for high energy batteries. Ultra-thin, large-area LMA is crucial for building Li-metal batteries (LMBs) that can deliver high volumetric and gravimetric energy density in practice. As the charging and discharging continue, the growth of Li dendrite is regarded as one of the significant challenges. The surface nature of LMAs is the main aspect of Li dendrite formation and interface reaction. However, most studies overlooked the initial condition of LMA surface. Native passivation layer (NPL) of commercial Li foils is inherently uneven composed of Li2CO3, LiOH, and Li2O, and its chemical nature is mostly determined under manufacturing conditions. Incomplete passivation at an initial stage can trigger sporadic Li dendrite growth and chemical/structural deterioration of LMAs, adversely affecting the cycling stability of LMBs.This work presents the electrochemical pre-passivation to build an electrolyte-derived native layer (electro-native layer, E-NPL) through Li electrodeposition method. Electrodeposited LMA (ed-LMA) was prepared by pre-deposition at the Cu foil using localized high-concentration electrolyte (LHCE) and post calendaring for smooth LMA surface. As-prepared ed-LMA has 4.13 mAh cm-2 of areal capacity with ~22 µm, and reduced roughness, which is much lower than that of manufactured LMA (m-LMA). Particularly, the E-NPL derived by LHCE can tailor the LMA surface by enriching the inorganic SEI compounds such as LiF, and N-SOx. In addition, decreased amount of passivated components such as Li2CO3, LiOH, and hydroxide were found in XPS analysis of the anodes. Compared to manufactured LMA (m-LMA), the ed-LMA exhibited lower overpotential for Li plating/stripping cycles and enhanced capacity retention. Benefiting from better compatibility with newly plated Li, it showed higher plating density without electric isolation from beneath Li surface, mitigating the anode swelling during subsequent cycling. We successfully demonstrated battery cycling stability of Li||NMC622 cells exploiting ed-LMA (195 cycle at 80% retention), superior to m-LMA (110 cycle at 80% retention) under stringent conditions.

Pre-Treatment of Li-Metal Anode with Hydrohalic-Acid for Stable Cycling of Li-Metal Batteries Under Practical Conditions

Lithium (Li) metal anodes (LMAs) are considered as an ultimate choice for beyond Li-ion batteries because of their high capacity (3860 mAh g–1) and low working potential (–3.04 V vs. SHE). However, due to extremely high surface reactivity, uncontrollable dendritic plating, and infinite volume expansion, making Li-metal batteries (LMBs) has long encountered fatal challenges. Heterogeneous solid-electrolyte interphase (SEI) can be the main reason for Li dendrite growth, stemming from the electrolyte byproducts and native LMA surface. In this regard, uniform passivation of LMAs and homogenous SEI at the initial stage is necessary which can occur uniform Li+ ion flux. Furthermore, halogenated SEI is considered as promising component for achieving superior mechanical strength, insulating properties and thermodynamic stability.In this study, we present a simple LMA treatment using hydrohalic acids (HXs, X = F, Cl, Br, and I) to regenerate native passivation layer (NPL) of as-manufactured LMAs and evenly nourish halogenated compounds. The water molecules of aqueous HXs can dissolve the existing inorganic Li-compounds in the NPL, e.g., Li2O, LiOH, and Li2CO3, rendering the chemical reactions of HXs with pure Li, enriching the LiX compounds within the NPL. A comparative study was done to reveal which LiX compounds is beneficial to stabilize the LMA surface and to enhance the cycling stability of LMBs. Furthermore, it is elucidated that the pivotal role of LiX compounds was proved in the interfacial reaction when using localized high concentration electrolyte (LHCE) as an advanced electrolyte in LMBs. Through comparative study, building LiCl-rich SEI at the initial stage effectively reduces the energy barrier of Li nucleation and interfacial resistances, thereby enhancing the cycling performances in LMBs even under practical conditions.

(Invited) Improving Calendar Life of Si-Based Lithium-Ion Batteries By FEC/VC-Free Electrolytes

Silicon (Si) has been regarded as one of the most promising anode materials for the next generation lithium-ion batteries (LIBs) with high energy density because it has 10 times higher theoretical specific capacity (4200 mAh/g) than that of graphite. However, severe volume change (~300%) of Si during lithiation and delithiation hinders the practical application of Si anode. In the last decade, significant progresses have been made on the specific energy and cycle life of Si based LIBs. However, calendar-life of these batteries is still far less than 10-year target required by DOE’s electrical vehicle (EV) project.In the current work, the role of FEC/VC in conventional carbonate electrolytes has been investigated. It has been found that FEC/VC causes large impedance growth on both Si and graphite anodes at 55 oC. The cut-off voltage of NMC622 has been found to significantly affect its impedance. NMC622 at 4.2V shows no impedance growth at 55 °C for 8 days, while severe impedance growth is observed at 4.4V. To address the instability of both SEI and CEI at elevated temperature, a new Localized High Concentration Electrolyte (LHCE) has been developed to improve the calendar life of Si based LIBs by effectively limiting impedance growth of Si/NMC622 full cells. In addition, the cycle life of the full cell is improved. The mechanism of Si full cells with improved calendar life will also be discussed.

(Invited) Important Factors Affecting Calendar Life of Lithium Metal Batteries

With the growing maturity of lithium (Li) ion battery technologies, Li metal battery (LMB) is being revitalized in recent years to achieve high energy density. Great progress has been made in understanding the Li utilization and extending the cycle life of LMBs. However, the study of calendar life of LMBs is still limited, although the calendar life of the battery is also critical for application of LMBs. Therefore, there is an urgent need to investigate the calendar life of LMBs systematically and extend their calendar life. In this work, we will report our recent works on investigation of the calendar life of LMBs based on Li||NMC811 cells using advanced localized high concentration electrolyte, which has already demonstrated excellent cycling stability. Two key criteria, self-discharge rate (percentage of capacity loss per day at open circuit) and recoverable capacity after long-term storage, are used to quantify the calendar aging at different states of charge (SOCs) and temperatures. Together with the postern mortem analysis of Li metal anode and NMC811 cathode retrieved from the long-term stored cells, this work provided an in depth understanding on the important factors affecting calendar aging of LMBs. These findings can guide the storage of LMBs and shed light on the further development of LMBs. Figure 1

Solvent modulation strategy for Sb-based anode to achieve stable potassium storage

High-concentration electrolyte (HCE) and localized high-concentration electrolyte (LHCE) strategies have been successfully developed for potassium-ion batteries (PIBs) to address the interface incompatibility issue, especially for the high-theoretical-capacity antimony (Sb)-based alloy-typed anode suffering from the drastic volume expansion problem. However, the high costs along with special solvents for these electrolytes significantly limits their widespread applications in PIBs. Herein, a new dilute electrolyte of 1 M KFSI in a mixed solvent of ethylene carbonate (EC) and 1, 2-dimethoxyethane (DME) (1:1 v/v) is developed to stabilize the K-storage kinetics of Sb anodes. The mixed solvent of common ester and ether promotes the formation of stable K+-EC solvent pair and thus generates a robust solid-electrolyte-interphase (SEI) for stable K-storage. As a result, the fabricated Sb@NC anode exhibits a stable capacity of 376 mAh/g after 100 cycles at 100 mA g−1, which is comparable to that in the HCE of 5 M KFSI/DME. These results demonstrated that low-cost and high-compatibility electrolytes for alloy-typed anodes can be achieved via rational design of the solvation structures for dilute electrolytes, resulting into stable K-storage properties and huge prospects of commercialized applications.

Tunning solvation structure in non-flammable, localized high-concentration electrolytes with enhanced stability towards all aluminum substrate-based K batteries

Although metallic potassium or graphite anode with low redox potentials are desirable for potassium-ion batteries (PIBs), the severe capacity decay caused by consecutive reduction reactions on the aggressively reactive anode surface is inevitable because of the scarcity of an effective passivation layer. Additionally, the development of PIBs faces potential safety hazards due to the usage of reactive anodes with flammable electrolytes. Herein, a fire-retardant localized high-concentration electrolyte (LHCE) is formulated by adding a low-polarity co-solvent of highly fluorinated ether into the concentrated potassium bis(fluorosulfonyl)imide/trimethyl phosphate. A unique electrolyte solvation environment with the enhanced anion-cation interaction in an LHCE facilitates a durable anion-derived solid electrolyte interphase (SEI) with self-accelerated interfacial kinetics and minimizes the SEI dissolution compared to the conventional carbonate-based electrolyte. Consequently, the unmodified Al substrate could sustain stable potassium metal cycling above 3400 h (over 800 cycles) with high reversibility (98%), and the graphite anode retains prolonged cyclic stability (over 300 cycles) with a high reversible capacity (257.6 mAh g−1) and capacity retention (92.6 % at 0.2 C). Meanwhile, Al corrosion at high voltage is significantly suppressed, and the electrochemical window is extended due to less free solvents in an LHCE, enabling good electrolyte compatibility with high-voltage cathodes. Coupling non-flammable solvents and the concept of LHCE could facilitate a better understanding of electrolyte chemistry and offer more opportunities to use K or graphite anode for all Al substrate-based PIBs highlighted with high performance and superb safety.

Bifunctional Localized High-Concentration Electrolyte for the Fast Kinetics of Lithium Batteries at Low Temperatures.

Traditional lithium batteries cannot work well at low temperatures due to the sluggish desolvation process, which limits their applications in low-temperature fields. Among various previously reported approaches, solvation regulation of electrolytes is of great importance to overcome this obstacle. In this work, a tetrahydrofuran (THF)-based localized high-concentration electrolyte is reported, which possesses the advantages of a unique solvation structure and improved mobility, enabling a Li/lithium manganate (LMO) battery to cycle stably at room temperature (retains 85.9% after 300 cycles) and to work at a high rate (retains 69.0% at a 10C rate). Apart from that, this electrolyte demonstrates superior low-temperature performance, delivering over 70% capacity at -70 °C and maintaining 72.5 mAh g-1 (≈77.1%) capacity for 200 cycles at a 1C rate at -40 °C. Also, even when the rate increases to 5C, the battery could still operate well at -40 °C. This work demonstrates that solvation regulation has a significant impact on the kinetics of cells at low temperatures and provides a design method for future electrolyte design.