Electrolytes For Lithium Batteries Research Articles

Thermodynamic and Kinetic Behaviors of Electrolytes Mediated by Intermolecular Interactions Enabling High-Performance Lithium-Ion Batteries.

Electrolyte solvation chemistry regulated by lithium salts, solvents, and additives has garnered significant attention since it is the most effective strategy for designing high-performance electrolytes in lithium-ion batteries (LIBs). However, achieving a delicate balance is a persistent challenge, given that excessively strong or weak Li+-solvent coordination markedly undermines electrolyte properties, including thermodynamic redox stability and Li+-desolvation kinetics, limiting the practical applications. Herein, we elucidate the crucial influence of solvent-solvent interactions in modulating the Li+-solvation structure to enhance electrolyte thermodynamic and kinetic properties. As a paradigm, by combining strongly coordinated propylene carbonate (PC) with weakly coordinated cyclopentylmethyl ether (CPME), we identified intermolecular interactions between PC and CPME using 1H-1H correlation spectroscopy. Experimental and computational findings underscore the crucial role of solvent-solvent interactions in regulating Li+-solvent/anion interactions, which can enhance both the thermodynamic (i.e., antireduction capability) and kinetic (i.e., Li+-desolvation process) aspects of electrolytes. Additionally, we introduced an interfacial model to reveal the intricate relationship between solvent-solvent interactions, electrolyte properties, and electrode interfacial behaviors at a molecular scale. This study provides valuable insights into the critical impact of solvent-solvent interactions on electrolyte properties, which are pivotal for guiding future efforts in functionalized electrolyte engineering for metal-ion batteries.

Investigation on the thermal runaway mechanism of electrolyte in lithium-ion batteries via ReaxFF molecular dynamics

Lithium-ion batteries have the advantages of high energy density and high cycle times, and have been widely used in portable electronic devices, electric vehicles, and large-scale energy storage. However, lithium-ion batteries are prone to thermal runaway. Subsequently, electrolyte decomposition and combustion occur, leading to an increase in battery temperature and even causing fires or explosions. In order to explore the reaction mechanism of thermal runaway in lithium-ion battery electrolytes, the volatile and combustible organic solvents are selected as the research object. Three different organic solvents were established, namely pure ethylene carbonate (C3H4O3) solvent and its mixture with dimethyl carbonate (C3H6O3) and diethyl carbonate (C5H10O3), respectively. The effect of heating on the thermal runaway of organic solvents was studied using reactive force field molecular dynamics (ReaxFF MD). All three organic solvents will produce a large amount of CH2 free groups and flammable organic substances such as C2H4 during pyrolysis, which is the main reason for thermal runaway of lithium-ion batteries. The research results may contribute to preventing and suppressing thermal runaway in lithium-ion batteries.

Synergistically Engineering Grains and Grain Boundaries toward Li Dendrite-Free Li7La3Zr2O12.

Cation-doped cubic Li7La3Zr2O12 is regarded as a promising solid electrolyte for safe and energy-dense solid-state lithium batteries. However, it suffers from the formation of Li2CO3 and high electronic conductivity, which give rise to an unconformable Li/Li7La3Zr2O12 interface and lithium dendrites. Herein, composite AlF3-Li6.4La3Zr1.4Ta0.6O12 solid electrolytes were created based on thermal AlF3 decomposition and F/O displacement reactions under a high-temperature sintering process. When the AlF3 is thermally decomposed, it leaves Al2O3/AlF3 meliorating the grain boundaries and F- ions partially displacing O2- ions in the grains. Due to the higher electronegativity of F- in the grains and the grain-boundary modification, these AlF3-Li6.4La3Zr1.4Ta0.6O12 deliver optimized electronic conduction and chemical stability against the formation of Li2CO3. The Li/AlF3-Li6.4La3Zr1.4Ta0.6O12/Li cell exhibits a low interfacial resistance of ∼16 Ω cm2 and an ultrastable long-term cycling behavior for 800 h under a current density of 200 μA/cm2, leading to Li//LiCoO2 solid-state batteries with good rate performance and cycling stability.

Interface engineering enabling non-corrosive sulfonimide salt for 4.4 V class lithium batteries

Lithium bis(trifluoromethanesulphonyl)imide (LiTFSI) with outstanding electrochemical and thermal stability has promising applications in electrolytes for lithium batteries. However, a low concentration of the sulfonimide salt electrolyte can corrode a commonly used aluminum (Al) current collector, posing a perennial conundrum that hinders its wide application. In this work, we find that pure H2O as an additive in the 1 M LiTFSI electrolyte effectively protects the Al current collector from corrosion by TFSI−. The interface engineering by specific adsorption of H2O plays a pivotal role in regulating the structure of the electrical double layer on the Al foil surface, blocking the interaction of TFSI− with Al. Meanwhile, hydrogen bonds are verified between H2O and TFSI−, weakening the attack of TFSI− on the Al collector and improving the Li+ conductivity. The mechanism has been confirmed by theoretical calculations and a variety of characterizations. Benefiting from the addition of H2O, the lithium batteries exhibit excellent cycling performance at a high cut-off voltage of 4.4 V even at −20 °C and a 3 Ah pouch full cell shows long cycling performance. The results demonstrate the high compatibility of the H2O-added electrolyte with high voltage cathode, lithium metal and graphite anodes. This work introduces a strategy for the utilization of sulfonimide salt at high voltage and wide temperature even at a low concentration.

A cyclic durable all-solid-state battery constructed with LiBH4-based electrolyte with high ionic conductivity at room temperature

LiBH4 has been considered as a promising solid electrolyte for lithium-ion batteries. However, the low ionic conductivity at room temperature has been the biggest challenge of LiBH4 solid-state electrolyte (SSE) toward practical solid-state batteries. In this work, a ternary electrolyte is designed and prepared by milling LiBH4 with LiNH2 and interfacially decorated with g-C3N4. It is revealed that the soft dispersed phase of two-dimensional g-C3N4 can build an effective transportation network in the composite electrolyte, thereby decreasing the interfacial impedance and reducing the energy barrier for lithium ion diffusion. The optimized SSE, LiBH4-2LiNH2-g-C3N4-10% (LBN-CN-10%), exhibits an ionic conductivity of 1.5×10−3 S cm−1, five orders of magnitude higher than that of the LiBH4 electrolyte at 30oC. The LiIn symmetric cell with LBN-CN-10% electrolyte delivers a low polarization of ~89mV over 10000hours. When assembling with Li4Ti5O12 cathode, the all-solid-state battery exhibits excellent cycling over 1000 cycles at 30 ℃, achieving a breakthrough in the lifetime of solid-state batteries with LiBH4 -based electrolyte. Moreover, the all-solid-state battery with LiNi0.8Co0.1Mn0.1O2 cathode also delivers a discharge plateau at 3.0V, revealing the high voltage withstanding of LBN-CN-10% electrolyte. This work provides new options for the development and practical application of room-temperature hydride all-solid-state batteries.

Concentration controlling of carboxylic ester-based electrolyte for low temperature lithium-ion batteries.

Low temperature has been a major challenge for lithium-ion batteries (LIBs) to maintain satisfied electrochemical performance, and the main reason is the deactivation of electrolyte with the decreasing temperature. To address this point, in present work, we develop a low-temperature resistant electrolyte which includes ethyl acetate (EA) and fluoroethylene carbonate (FEC) as solvent and lithium difluoro(oxalato)borate (LiDFOB) as the primary lithium salt. Due to the preferential decomposition of LiDFOB and FEC, a solid electrolyte interface rich in LiF is formed on the lithium metal anodes (LMAs) and lithium cobalt oxide (LCO) cathodes, contributing to higher stability and rapid desolvation of Li+ ions. The batteries with the optimized electrolyte can undergo cycling tests at -40 °C, with a capacity retention of 83.9 % after 200 cycles. Furthermore, the optimized electrolyte exhibits excellent compatibility with both LCO cathodes and graphite (Gr) anodes, enabling a Gr/LCO battery to maintain a capacity retention of 90.3 % after multiple cycles at -25 °C. This work proposes a cost-effective electrolyte that can activate potential LIBs in practical scenarios, especially in low-temperature environments.

Enhancing lithium-ion battery safety: Investigating the flame-retardant efficacy of bis(2,2,2-trifluoroethyl) carbonate during ethyl methyl carbonate combustion

Bis(2,2,2-trifluoroethyl) carbonate (BtFEC) is a candidate fire suppressant for lithium-ion batteries (LIBs). The high flammability of the electrolyte is the reason behind the frequent fire incidents reported with LIBs. These incidents are due to various factors, such as a default in the conception of the battery or operational abuse. The flame-retardant effectiveness of BtFEC was investigated by measuring its effect on the oxidation of ethyl methyl carbonate (EMC), a common LIB electrolyte component. Laminar flame speed (LFS) measurements for EMC-BtFEC in air mixtures were carried out at 403 K, 1 atm, for equivalence ratios (ϕ) between 0.8 and 1.4, and with 0.5 % vol. BtFEC. Additional measurements were made at ϕ = 1.1 with BtFEC addition ranging from 0.1 up to 1.4 % vol. To further understand the combustion chemistry of the EMC-BtFEC system, ignition delay times (time at the maximum peak production of the excited radical OH*) and CO time histories were measured in a shock tube with temperatures ranging from 1217 to 1732 K, at near-atmospheric pressure (1.24–1.42 atm), and for three equivalence ratios (ϕ = 0.5, 1.0, and 2.0). These new results constitute an experimental database that was used to evaluate the performance of a model assembled for both BtFEC and EMC using previous work from our group. The updated, detailed chemical kinetics mechanism presented in this study consists of 237 species and 1630 reactions. Sensitivity, rate-of-production, and reaction pathway analyses permitted the oxidation of the BtFEC-EMC system to be described. A flame sensitivity analysis exhibited the significant effect from the reactions H + CF2O ⇆ HF + CFO and CF3 + H ⇆ CF2 + HF, which consume H radicals that are involved in the branching reaction H + O2⇆ OH + O, ultimately reducing the LFS. This study shows that improvements in the base fluorine chemistry are still necessary to obtain better agreement with the experiments, especially at the speciation level.

Maleic Anhydride and (Ethoxy)pentafluorocyclotriphosphazene as Electrolyte Additives for High-Voltage LiCoO2/Si-Graphite Lithium-Ion Batteries.

Anhydride additives including maleic anhydride and succinic anhydride are initially selected as additives in the commercial electrolytes for high-voltage lithium-ion batteries with a Si-based anode. The introduction of (ethoxy)pentafluorocyclotriphosphazene as a flame retardant realizes the nonflammability of electrolytes, and the conductivity of electrolytes exceeds 10 mS cm-1 at 25 °C. Maleic anhydride and (ethoxy)pentafluorocyclotriphosphazene jointly contribute to the exceptional performances of 4.45 V LiCoO2/Si-graphite pouch cells at 25 °C. The capacity retention at 1C of 300 cycles reaches 78%, and the discharge capacity ratio of 6C/1C is approximately 83%. These results suggest that this nonflammable electrolyte has good application prospect. Scanning electron microscopy and X-ray photoelectron spectroscopy measurements are implemented to analyze the interface properties of electrodes.

Construction of Ion-Conducting Channels with Surface-Functionalized Carbon Nanotubes in Gel/Solid Polymer Electrolytes for Lithium-Ion Batteries

Construction of Ion-Conducting Channels with Surface-Functionalized Carbon Nanotubes in Gel/Solid Polymer Electrolytes for Lithium-Ion Batteries

Molecular-docking electrolytes enable high-voltage lithium battery chemistries.

Ideal rechargeable lithium battery electrolytes should promote the Faradaic reaction near the electrode surface while mitigating undesired side reactions. Yet, conventional electrolytes usually show sluggish kinetics and severe degradation due to their high desolvation energy and poor compatibility. Here we propose an electrolyte design strategy that overcomes the limitations associated with Li salt dissociation in non-coordinating solvents to enable fast, stable Li chemistries. The non-coordinating solvents are activated through favourable hydrogen bond interactions, specifically Fδ--Hδ+ or Hδ+-Oδ-, when blended with fluorinated benzenes or halide alkane compounds. These intermolecular interactions enable a dynamic Li+-solvent coordination process, thereby promoting the fast Li+ reaction kinetics and suppressing electrode side reactions. Utilizing this molecular-docking electrolyte design strategy, we have developed 25 electrolytes that demonstrate high Li plating/stripping Coulombic efficiencies and promising capacity retentions in both full cells and pouch cells. This work supports the use of the molecular-docking solvation mechanism for designing electrolytes with fast Li+ kinetics for high-voltage Li batteries.

Diagnostic Protocols for Evaluating the Degradation Mechanisms in Gel-Polymer Lithium Batteries.

Gel polymer electrolytes (GPEs) present a promising alternative to standard liquid electrolytes (LE) for Lithium-ion Batteries (LIBs) and Lithium Metal Batteries bridging the advantages of both liquid and solid polymer electrolytes. However, their cycle life still lags behind that of standard LIBs, and their degradation mechanisms remain poorly understood. A significant challenge is the need for specific diagnostic protocols to systematically study the degradation mechanisms of GPE-based cells. Challenges include the separation of cell components and effective washing, as well as the study of the solid electrolyte interfaces, all complicated by the semi-solid nature of GPEs. This paper provides a brief review of existing literature and proposes a comprehensive set of diagnostic tools for dismantling and evaluating the degradation of GPE-based LIBs. Finally, these methods and recommendations are applied to LiNi0.5Mn1.5O4 (LNMO)-graphite cells, revealing electrolyte oxidation as a major source of cell degradation.

Coordination of dissolved transition metals in pristine battery electrolyte solutions determined by NMR and EPR spectroscopy.

The solvation of dissolved transition metal ions in lithium-ion battery electrolytes is not well-characterised experimentally, although it is important for battery degradation mechanisms governed by metal dissolution, deposition, and reactivity in solution. This work identifies the coordinating species in the Mn2+ and Ni2+ solvation spheres in LiPF6/LiTFSI-carbonate electrolyte solutions by examining the electron-nuclear spin interactions, which are probed by pulsed EPR and paramagnetic NMR spectroscopy. These techniques investigate solvation in frozen electrolytes and in the liquid state at ambient temperature, respectively, also probing the bound states and dynamics of the complexes involving the ions. Mn2+ and Ni2+ are shown to primarily coordinate to ethylene carbonate (EC) in the first coordination sphere, while PF6- is found primarily in the second coordination sphere, although a degree of contact ion pairing does appear to occur, particularly in electrolytes with low EC concentrations. NMR results suggest that Mn2+ coordinates more strongly to PF6- than to TFSI-, while the opposite is true for Ni2+. This work provides a framework to experimentally determine the coordination spheres of paramagnetic metals in battery electrolyte solutions.

Design of High-Performance Formyl-Functionalized COF Aerogels as Quasi-Solid Lithium Battery Electrolyte by a Solvent Substitution Strategy.

Covalent organic framework (COF) aerogels with functional groups offer exceptional processability and functionality for various applications. These hierarchical porous materials combine the advantages of COFs with the benefits of aerogels, overcoming the limitations of conventional insoluble and nonfusible COF powders. However, achieving both high crystallinity and shape retention remains a challenge for functionalized COF aerogels. In this work, we develop a novel and general solvent substitution method for the one-step synthesis of formyl-functionalized COF aerogels without harsh vacuum conditions. These aerogels exhibit excellent processing capabilities, superior mechanical strength, and enhanced functionality. As a proof-of-concept, they were used in adsorption and lithium metal battery applications, significantly maximizing the structural advantages of COFs, e.g.: (i) the hierarchical porous structure is fully wetted by the electrolyte to form continuous transport channels; (ii) the polar groups, which are easier to be acquired, help in desolvation and transfer of Li+; (iii) the regular pore structures stabilize deposition of Li+ and inhibit the growth of lithium dendrites. These combined benefits contribute to a lighter battery with improved energy density and enhanced safety.

Fluorinated Linkers Enable High-Voltage Pyrrolidinium-based Dicationic Ionic Liquid Electrolytes.

Novel fluorinated, pyrrolidinium-based dicationic ionic liquids (FDILs) as high-performance electrolytes in energy storage devices have been prepared, displaying unprecedented electrochemical stabilities (up to 7 V); thermal stability (up to 370 °C) and ion transport (up to 1.45 mS cm‑1). FDILs were designed with a fluorinated ether linker and paired with TFSI/FSI counterions. To comprehensively asess the impact of the fluorinated spacer on their electrochemical, thermal, and physico-chemical properties, a comparison with their non-fluorinated counterparts was conducted. With a specific focus on their application as electrolytes in next-generation high-voltage lithium-ion batteries, the impact of the Li-salt on the characteristics of dicationic ILs was systematically evaluated. The incorporation of a fluorinated linker demonstrates significantly superior properties compared to their non-fluorinated counterparts, presenting a promising alternative towards next-generation high-voltage energy storage systems.

Polymer-based solid/semi-solid electrolytes in lithium ion batteries

Abstract The inception of lithium-ion batteries with liquid electrolytes can be traced back to the early 1980s. Nevertheless, the utilization of liquid electrolytes has many drawbacks, including its susceptibility to combustion, limited energy density, and very brief operational lifespan. Consequently, there is currently a concerted effort to substitute liquid electrolytes with solid compounds. This study investigates the electrochemical and mechanical features of solid/semi-solid electrolytes used in lithium ion batteries (LIBs) as well as their performance relative to conventional liquid electrolytes, with LIBs having unique challenges related to high flammability, electrochemical instability, and low mechanical stability posed by conventional liquid electrolytes versus solid polymer electrolytes (SPEs) which provide greater safety, mechanical stability but lower ionic conductivity than liquid counterparts. SPEs offer better safety but lack sufficient ionic conductivity which limits their potential. In order to overcome these obstacles, the implementation of gel-based and composite solid and semi-solid electrolytes is proposed as a means to improve ionic conductivity, electrochemical stability, and mechanical stability. The study suggests that a focus should be placed on solid composite electrolytes as they possess higher mechanical stability, which contributes to improved safety. Additionally, these electrolytes exhibit enhanced ionic conductivity within the range of 10−4 to 10−2 S/cm, hence enhancing the performance of LIBs.

A green and innovative approach to separate hexafluorophosphate from wastewater

Effluent derived from spent lithium-ion batteries (LIBs) electrolyte or discarded ionic liquids (ILs) is a potentially resource of hexafluorophosphate (PF6−) which can be recovered through solvent extraction. In this study, an efficient extraction approach was proposed to recover the hexafluorophosphate from solutions. A basic investigation on the interactions between extractants and hexafluorophosphate was firstly carried out using density functional theory (DFT), which demonstrated the stability of the extracted complex structure. The effects of extractant (Alamine336) and modifier, common anions and metallic ions in waste-streams, initial pH, and temperature on the extraction efficiency were explored. The results showed that 96.16 % of hexafluorophosphate could be extracted and transferred into organic phase using Alamine336/modifier as extractant under optimal conditions (0.15 mol/L of Alamine336 and 0.25 mol/L of modifier). The organic phase exhibited an excellent extraction capability of more than 95 % under optimal condition. Moreover, it was found that the use of sodium hydroxide as stripping reagent presented a superb stripping and cycling properties. Thermodynamic analysis and spectral data indicated that PF6− was extracted into the organic phase in the form of R3NH⋅PF6, which is a spontaneous exothermic process. The novel extraction method provides a broad application perspective for the removal and recovery of valuable hexafluorophosphate from aqueous system.

Computational simulation-assisted research on chloride solid electrolytes for lithium-ion batteries

Computational simulation-assisted research on chloride solid electrolytes for lithium-ion batteries

Application of Li6.4La3Zr1.45Ta0.5Mo0.05O12/PEO Composite Solid Electrolyte in High-Performance Lithium Batteries.

Replacing the flammable liquid electrolytes with solid ones has been considered to be the most effective way to improve the safety of the lithium batteries. However, the solid electrolytes often suffer from low ionic conductivity and poor rate capability due to their relatively stable molecular/atomic architectures. In this study, we report a composite solid electrolyte, in which polyethylene oxide (PEO) is the matrix and Li6.4La3Zr1.45Ta0.5Mo0.05O12 (LLZTMO) and Li6.4La3Zr1.4Ta0.6O12 (LLZTO) are the fillers. Ta/Mo co-doping can further promote the ion transport capacity in the electrolyte. The synthesized composite electrolytes exhibit high thermal stability (up to 413 °C) and good ionic conductivity (LLZTMO-PEO 2.00 × 10-4 S·cm-1, LLZTO-PEO 1.53 × 10-4 S·cm-1) at 35 °C. Compared with a pure PEO electrolyte, whose ionic conductivity is in the range of 10-7~10-6 S·cm-1, the ionic conductivity of composite solid electrolytes is greatly improved. The full cell assembled with LiFePO4 as the positive electrode exhibits excellent rate performance and good cycling stability, indicating that prepared solid electrolytes have great potential applications in lithium batteries.

Local charge redistribution enables single ionic conductor for fast charge solid Li battery

Solid polymer electrolytes (SPEs) have been regarded as hopeful candidate electrolyte for solid-state lithium battery. However, the low Li+ transference number and poor interface stability pose great challenges for the high rate capability of SPE. Herein, inspired from the density functional theory (DFT) calculations that local positive charge distribution can be regulated by introducing strong electron-withdrawing groups, which can selectively anchor TFSI−, largely enhance the Li+ transference number. As predicted, the obtained CPE-NO2 deliver a remarkable Li+ transference number of 0.91, equal to a single-ion conductor for Li+, largely high than that of the SPE (0.31), according well with the molecular dynamics (MD) simulations and pyridine complexation experiments. Furthermore, the PEO||Li interfacial stability, flame retardant ability, and mass transfer of Li+ in interface can also be largely enhanced by interfacial by-product, which are fast Li+ conductors according to TOF-SIMS results. Impressively, the Li|CPE-NO2|Li cell exhibit superior cyclability for 2200 h at 0.1 mA cm−2, and the solid LFP|CPE-NO2|Li battery delivers prominent capacity of 127.4 mAh g−1 at a high rate 5 C (12 mins). This work breaks the fast charge limitation of solid lithium battery, and provides a feasible approach for the construction of advanced solid electrolyte.

Ultraviolet-cured heat-resistant and stretchable gel polymer electrolytes for flexible and safe semi-solid lithium-ion batteries

Lithium-ion (Li-ion) batteries assembled with gel polymer electrolytes (GPEs) are predicted to increase battery energy density while maintaining battery safety. In this work, poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) is successfully used as a polymer matrix to prepare a new kind of GPE (PHHNT) by ultraviolet (UV) curing technology. The polymer network structure formed by photopolymerization technology significantly enhances the tensile properties of the electrolyte. Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) improves the concentration and transport efficiency of lithium ions in the electrolyte as well as the ionic conductivity (1.22 × 10−3 S cm−1) of the GPEs. In addition, the unique tubular structure of the halloysite nanotube (HNT) and the opposite charges carried by the inner and outer surfaces provide additional channels for the lithium salt and facilitate its dissociation. This enhances the expedited movement of Li+ inside the electrolyte, while simultaneously improving the electrochemical and thermal durability of the electrolyte. The assembled LiFePO4 (LFP)/PHHNT/Li battery has an initial discharge-specific capacity of 140.5 mAhg−1 at a current density of 0.5C, showing a high capacity retention rate of 89 % after 120 cycles. The design of this composite material significantly improves the overall performance of the electrolyte and is more suitable for flexible and safe solid-state Li-ion batteries.