Electrolytes For Lithium Metal Batteries Research Articles

Highly liquid retentive, ordered ion transport quasi-solid polymer electrolytes for lithium metal batteries

Solid-state lithium metal batteries (SSLMB) offer a compelling solution to address the low energy density issue present in current liquid lithium-ion batteries (LIB). Nevertheless, SSLMB electrolytes continue to face specific challenges that impede their commercial viability. These challenges encompass low ionic conductivity, high production cost, instability of interfacial contact, and other hurdles. In this context, a cost-effective quasi-solid polymer electrolyte (QPE) based on Polyvinylidene fluoride (PVDF) was developed to reshape the transport of lithium-ion (Li+) within the electrolyte, resulting in accelerated Li+ transport, with an achieved ionic conductivity of 0.71 mS cm−1 at 25 °C. The assembled LiFePO4|PVDF-OH|Li cell demonstrates exceptional battery performance, boasting an initial capacity of 145.9 mAh/g at 30 °C and maintaining an impressive capacity retention of 85.4 % even after 1000 cycles. The experimental findings underscore the critical role played by the immobilized solvent within the innovative PVDF-based QPE in facilitating the migration of Li+, consequently enhancing electrochemical performance. This study showcases a pioneering approach aimed at enhancing ionic conductivity, representing a significant stride towards the development of low-cost, high-performance, and easily processable solid polymer electrolytes, with potential implications for broader research in the field of energy storage systems.

Perfluorosulfonic Acid Membranes as Gel-Polymer Electrolytes for Lithium Metal Batteries

Perfluorosulfonic Acid Membranes as Gel-Polymer Electrolytes for Lithium Metal Batteries

Construction of high-performance solid-state electrolytes for lithium metal batteries by UV-curing technology

With high energy density and high safety, all-solid-state lithium metal batteries (ASSLMBs) are considered the most competitive next-generation energy storage batteries. In this work, different formulations of PEGDA-based solid-state electrolytes were prepared by solution flow casting and UV-irradiated in situ polymerization. Polyethylene oxide (PEO) was introduced to solve the problem of improving the flexibility of the substrate, and a 1:5 mass ratio between them was determined. Inorganic ceramic filler Li1.25Al0.25Zr1.75(PO4)3 (LAZP) and plasticizer butanedinitrile (SCN) were introduced to improve the ionic conductivity (σ) of the electrolyte. The best performing PP15-30-SCN sample showed a 40-fold improvement in room temperature ionic conductivity over PEGDA and lithium ion mobility number of 0.43. The ASSLMBs were assembled with composite electrolytes and tested at 60 °C. The polarization of PP15-30-SCN was significantly reduced, and the battery cycled stably at current densities from 0.1 to 2 C, with an average discharge specific capacity of 111.78 mAh g−1 at 1C. The average specific capacity at 1C was 111.78 mAh g−1. Meanwhile, the capacity retention rate was more than 99% after more than 70 long cycles at 0.5C.

Directing Fluorinated Solid Electrolyte Interphase by Solubilizing Crystal Lithium Fluoride in Aprotic Electrolyte for Lithium Metal Batteries

AbstractLithium fluoride (LiF) facilitates robust and fast‐ion‐transport solid electrolyte interphase (SEI) in lithium metal batteries. Fluorinated solvents/salts are ubiquitously employed to introduce LiF into SEI through electrochemical decomposition, but this approach is usually at the expense of their continuous consumption. A direct approach to fluorinate SEI that employs crystal LiF is limited by its poor solubility in the current battery electrolyte formulation. Dissolving crystal LiF in high‐dielectric‐constant solvents, like ethylene carbonate (EC) is nearly neglected. Herein, the feasibility of directly fluorinating SEI by the addition of crystal LiF in aprotic electrolyte with the assistance of EC is verified, and its mechanisms in fluorination of SEI and anti‐acidification of electrolyte are explored. The dissolved LiF is encapsulated by solvent‐/salt‐derived organic skins to promote the fluorinated SEI. Meanwhile, the presence of LiF in electrolyte alters hazardous thermodynamic equilibrium, suppressing the production of acid species to mitigate electrolyte acidification and SEI degradation. Such collective benefits yield a capacity retention ratio of ≈88% after 150 cycles at a high areal capacity (4.5 mAh cm−2) in Li||NCM622 cells. This facile and effective fluorination of SEI contributes to an in‐depth understanding of SEI formation and rational design of well‐performing lithium metal batteries.

Host–Guest Interactions and Transport Mechanism in Poly(vinylidene fluoride)-Based Quasi-Solid Electrolytes for Lithium Metal Batteries

Host–Guest Interactions and Transport Mechanism in Poly(vinylidene fluoride)-Based Quasi-Solid Electrolytes for Lithium Metal Batteries

All-Polymer Nanocomposite as Salt-Free Solid Electrolyte for Lithium Metal Batteries.

Solid polymer electrolytes that combine both a high lithium-ion transference number and mechanical properties at high temperatures are searched for improving the performance of batteries. Here, we show a salt-free all-polymer nanocomposite solid electrolyte for lithium metal batteries that improves the mechanical properties and shows a high lithium-ion transference number. For this purpose, lithium sulfonamide-functionalized poly(methyl methacrylate) nanoparticles (LiNPs) of very small size (20-30 nm) were mixed with poly(ethylene oxide) (PEO). The morphology of all-polymer nanocomposites was first investigated by transmission electron microscopy (TEM), showing a good distribution of nanoparticles (NPs) even at high contents (50 LiNP wt %). The crystallinity of PEO was investigated in detail and decreased with the increasing concentration of LiNPs. The highest ionic conductivity value for the PEO 50 wt % LiNP nanocomposite at 80 °C is 1.1 × 10-5 S cm-1, showing a lithium-ion transference number of 0.68. Using dynamic mechanic thermal analysis (DMTA), it was shown that LiNPs strengthen PEO, and a modulus of ≈108 Pa was obtained at 80 °C for the polymer nanocomposite. The nanocomposite solid electrolyte was stable with respect to lithium in a Li||Li symmetrical cell for 1000 h. In addition, in a full solid-state battery using LiFePO4 as the cathode and lithium metal as the anode, a specific capacity of 150 mAhg-1 with a current density of 0.05 mA cm-2 was achieved.

SnCl4 initiated formation of polymerized solid polymer electrolytes for lithium metal batteries with fast ion transport interfaces

Solid-polymer-electrolytes (SPEs) are under active consideration for their high-energy and safe electrochemical energy storage. However, conventional polymer electrolytes are facing poor electrolyte/electrode interfacial contact and undesired lithium dendrite problems. Here, we report a polymerized-dioxolane (P-DOL) electrolytes created by in situ polymerization of DOL in the presence of tin tetrachloride (SnCl4), which enables ultra-conformal interfacial contact with all cell components. More importantly, SnCl4 plays a dual function by initiating ring-opening polymerization of DOL and simultaneously forming a LiCl/LixSn-based alloy composite protective layer on Li metal anodes. The robust Li-rich ion conductive alloys coupled with an electronically insulating surface component form fast ion transport channels. Using the SnCl4-initiated P-DOL SPEs exhibits a high ionic conductivity (>2 mS cm−1) and a wide electrochemical window (4.7 V) at room temperature. Applications of P-DOL SPEs in Li/Li symmetric and Li/LiFePO4 batteries further demonstrate low polarization and high capacity retention. This work provides a facile and practical approach to designing P-DOL SPEs for high-performance lithium metal batteries.

Unveiling decaying mechanism of non-flammable all-fluorinated carbonate electrolytes in lithium metal batteries with 4.6-V LiCoO2 cathodes at elevated temperatures

Highly fluorinated electrolytes have attracted much attention in lithium metal batteries (LMBs) because of their unique properties, such as increased anodic stability and altered coordination state of Li+ ions. However, the complex decaying mechanism of fluorinated electrolytes at elevated temperatures is not well understood, which is essential to assess their suitability for practical applications in harsh environments. Herein the influence of elevated temperatures on the stability of fluorinated electrolytes and the formation of interphases between the 4.6-V LiCoO2 cathode and electrolyte are systematically investigated. The results reveal that elevated temperatures significantly affect the stability of the electrolyte and the formation of interphases. In particular, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropylether undergoes decomposition catalyzed by reactive oxygen from the delithiated cathode at high temperatures and 4.6 V. Meanwhile, the degradation of fluorinated electrolytes is primarily caused by the temperature-induced decomposition of fluoroethylene carbonate. Although methyl (2,2,2-trifluoroethyl) carbonate has the highest stability among the solvents in this fluorinated system, it still experiences degradation in harsh environments. These findings highlight the importance of considering highly fluorinated ethers and the synergistic effects between fluorinated carbonate solvents, lithium salts, and solvation structures when designing fluorinated electrolytes, especially for applications at elevated temperatures. This work sheds light on the design and optimization of highly fluorinated electrolytes, enabling the development of 4.6-V LiCoO2-based LMBs capable of withstanding challenging environments and operating conditions.

Interconnected Three-Dimensional Porous Alginate-Based Gel Electrolytes for Lithium Metal Batteries.

Lithium batteries have been widely used in our daily lives for their high energy density and long-term stability. However, their safety problems are of paramount concern for consumers, which restricts their scale applications. Gel polymer electrolytes (GPEs) compensate for the defects of liquid leakage and lower ionic conductivity of solid electrolytes, which have attracted a lot of attention. Herein, a 3D interconnected highly porous structural gel electrolyte was prepared with alginate dressing as a host material, poly(ethylene oxide) (PEO), and a commercial liquid electrolyte. With rich polar functional groups and (CH2-CH2-O) segments on the polymer matrix, the transportation of Li+ is faster and uniform; thus, the formations of lithium dendrite were significantly inhibited. The cycle stability of symmetrical Li||Li batteries with modified composite electrolytes (SAA) is greatly improved, and the overpotential remains stable after more than 1000 h. Meanwhile, under the same conditions, the cycle performance of batteries with unmodified electrolytes is inferior and overpotentials are nearly 1 V after 100 h. Additionally, the capacity retention of Li||LiFePO4 with SAA is more than 95% after 200 cycles, while those of the others declined sharply. The alginate dressing-based GPEs can greatly enhance the mechanical and thermal stability of PEO-based GPEs, which provides an environmentally friendly avenue for gel electrolytes' applications in lithium batteries.

Additive‐guided Solvation‐regulated Flame‐retardant Electrolyte Enables High‐voltage Lithium Metal Batteries with Robust Electrode Electrolyte Interphases

AbstractWidening the voltage window of the nickel‐rich layered oxide cathode‐based lithium metal batteries (LMBs) can effectively improve the energy density of rechargeable batteries. However, serious safety issues associated with the high reactivity between LiNi0.8Co0.1Mn0.1O2 (NCM811) and electrolyte at high cut‐off voltage remains challenging. Herein, a flame‐retardant electrolyte with the ability to form a robust armor‐like electrode electrolyte interphase (EEI) with LiF and LixByOz compounds for stabilizing Li||NCM811 batteries is proposed. Such electrolyte exhibits the high thermal stability with flame‐retardant effect for ensuring the battery safety at high voltage. The armor‐like EEI can effectively protect both NCM811 and lithium (Li) for improving the battery cycling performance. As a result, the capacity retention rate of the NCM811 cathode with such electrolyte reached 68% after 150 cycles at 4.6 V. This work provides an effective reference for the reasonable design of high‐voltage, flame‐retardant electrolytes for LMBs.

Decoupling of Li+ Conductivity and Mechanical Stability in a Thermally Reversible Concentrated Sulfone-based Gel Electrolyte for Lithium Metal Batteries

Decoupling of Li+ Conductivity and Mechanical Stability in a Thermally Reversible Concentrated Sulfone-based Gel Electrolyte for Lithium Metal Batteries

In situ polymerization of solid-state polymer electrolytes for lithium metal batteries: a review

The practical application of commercialized lithium-ion batteries (LIBs) currently faces challenges due to using liquid electrolytes (LEs), including limited energy density and insufficient safety performance.

Self-Standing Single-Ion Borate Salt-Based Polymer Electrolyte for Lithium Metal Batteries.

Polymer electrolytes (PEs) with excellent flexibility and superior compatibility toward lithium (Li) metal anodes have been deemed as one of the most promising alternatives to liquid electrolytes. However, conventional lithium bis(trifluoromethanesulfonyl)imide (LiTFSI)-based dual-ion PEs suffer from a low Li ion transference number and notorious Li dendrite growth. Here, a single-ion conducting polyborate salt without any fluorinated groups, polymeric lithium dihydroxyterephthalic acid borate (PLDPB), is presented for addressing the issues of Li metal batteries. Owing to a nearly immovable bulky anion and the presence of a rigid benzene structure, the PLDPB@poly(ethylene oxide) (PEO) PE exhibits an ultrahigh Li ion transference number (0.94) and excellent mechanical strength, which could significantly restrict the growth of Li dendrites. Postmortem analysis reveals that a fluorine-free solid electrolyte interphase (SEI) enriched with B-O and benzene-containing species is formed on the surface of the Li metal anode, thereby facilitating elimination of excessive parasitic reactions and simultaneously suppressing the formation of Li dendrites. Consequently, the LiFePO4/Li cells with PLDPB@PEO PEs show an improved long-term cycling performance and high capacity retention (90.0%) and Coulombic efficiency (99.9%) after 500 cycles. This work may inspire new ideas to boost the development of single-ion conducting salts for dendrite-free Li metal batteries.

Metal-Organic Framework Glass as a Functional Filler Enables Enhanced Performance of Solid-State Polymer Electrolytes for Lithium Metal Batteries.

Polymers are promising candidates as solid-state electrolytes due to their performance and processability, but fillers play a critical role in adjusting the polymer network structure and electrochemical, thermal, and mechanical properties. Most fillers studied so far are anisotropic, limiting the possibility of homogeneous ion transport. Here, applying metal-organic framework (MOF) glass as an isotropic functional filler, solid-state polyethylene oxide (PEO) electrolytes are prepared. Calorimetric and diffusion kinetics tests show that the MOF glass addition reduces the glass transition temperature of the polymer phase, improving the mobility of the polymer chains, and thereby facilitating lithium (Li) ion transport. By also incorporating the lithium salt and ionic liquid (IL), Li-Li symmetric cell tests of the PEO-lithium salt-MOF glass-IL electrolyte reveal low overpotential, indicating low interfacial impedance. Simulations show that the isotropic structure of the MOF glass facilitates the wettability of the IL by enhancing interfacial interactions, leading to a less confined IL structure that promotes Li-ion mobility. Finally, the obtainedelectrolyte is used to construct Li-lithium iron phosphate full batteries that feature high cycle stability and rate capability. This work therefore demonstrates how an isotropic functional filler can be used to enhance the electrochemical performance of solid-state polymer electrolytes.

Synthesis and Characterization of Vitrimer-like Self-Healing Polymer Electrolytes for Lithium Metal Batteries

Solid-polymer electrolytes have become crucially significant with the fast-expanding energy requirements in both the academic and industrial spheres due to their better safety and energy density compared to liquid electrolytes. Despite these encouraging possibilities, a few obstacles are associated with these electrolytes, chief among them being the limited charge transport via the solid electrode-electrolyte interface. Due to the difference in electrochemical potential between the electrode and the electrolyte, this obstruction initiates electrode-electrolyte interactions through electrode volumetric changes, interface reactions, and space charge layers.One possible way to enhance the LIB performance could be by integrating a new class of covalent adaptable networks called vitrimers.The self-healing, shape memory, recyclability, and reprocessibility properties of vitrimers, a class of covalent adaptable networks (CANs), are derived from a covalent molecular network that can change its topology through molecular rearrangements while maintaining the overall number of network bonds. In this manner, Li dendrites that might develop during battery cycling could be eliminated and the electrolyte-electrode contact could be restored by vitrimers. Figure 1

Nanocrystalline cellulose reinforced poly(ethylene oxide) electrolytes for lithium-metal batteries with excellent cycling stability

Polyethylene oxide (PEO) based polymer electrolytes are still the state of the art for commercial lithium-metal batteries (LMBs) despite their remaining challenges such as the limited ionic conductivity at ambient temperature. Accordingly, the realization of thin electrolyte membranes and, thus, higher conductance is even more important, but this requires a sufficiently high mechanical strength. Herein, the incorporation of nanocrystalline cellulose into PEO-based electrolyte membranes is investigated with a specific focus on the electrochemical properties and the compatibility with lithium-metal and LiFePO4-based electrodes. The excellent cycling stability of symmetric Li||Li cells, including the complete stripping of lithium from one electrode to the other, and Li||LiFePO4 cells renders this approach very promising for eventually yielding thin high-performance electrolyte membranes for LMBs.

Revealing the Anion-Solvent Interaction for Ultralow Temperature Lithium Metal Batteries.

Anion solvation in electrolytes can largely change the electrochemical performance of the electrolytes, yet has been rarely investigated. Herein, three anions of bis(trifluoromethanesulfonyl)imide (TFSI), bis(fluorosulfonyl)imide (FSI), and derived asymmetric (fluorosulfonyl)(trifluoro-methanesulfonyl)imide (FTFSI) are systematically examined in a weakly Li+ cation solvating solvent of bis(3-fluoropropyl)ether (BFPE). In-situ liquid secondary ion mass spectrometry demonstrates that FTFSI- and FSI- anions are associated with BFPE solvent, while weak TFSI- /BFPE cluster signals are detected. Molecular modeling further reveals that the anion-solvent interaction is accompanied by the formation of H-bonding-like interactions. Anion solvation enhances the Li+ cation transfer number and reduces the organic component in solid electrolyte interphase, which enhances the Li plating/stripping Coulombic efficiency at a low temperature of -30°C from 42.4% in TFSI-based electrolytes to 98.7% in 1.5m LiFTFSI and 97.9% in LiFSI-BFPE electrolytes. The anion-solvent interactions, especially asymmetric anion solvation also accelerate the Li+ desolvation kinetics. The 1.5m LiFTFSI-BFPE electrolyte with strong anion-solvent interaction enables LiNi0.8 Mn0.1 Co0.1 O2 (NMC811)||Li (20µm) full cell with stable cyclability even under -40°C, retaining over 92% of initial capacity (115 mAhg-1 , after 100 cycles). The anion-solvent interactions insights allow to rational design the electrolyte for lithium metal batteries and beyond to achieve high performance.

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.

Confining Ionic Liquids in Developing Quasi-Solid-State Electrolytes for Lithium Metal Batteries.

The concept of confining ionic liquids (ILs) in developing quasi-solid-state electrolytes (QSSEs) has been proposed, where ILs are dispersed in polymer networks/backbones and/or filler/host pores, forming the so-called confinement, and great research progress and promising research results have been achieved. In this review, the progress and achievement in developing QSSEs using IL-confinement for lithium metal batteries (LMBs), together with advanced characterizations and simulations, were surveyed, summarized, and analyzed, where the influence of specific parameters, such as IL (type, content, etc.), substrate (type, structure, surface properties, etc.), confinement methods, and so on, was discussed. The confinement concept was further compared with the conventional one in other research areas. It indicates that the IL-confinement in QSSEs improves the performance of electrolytes, for example, increasing the ionic conductivity, widening the electrochemical window, and enhancing the cycle performance of the assembled cells, and being different from those in other areas, that is, the IL-confinement concept in the battery area is in a broad extent. Finally, insights into developing QSSEs in LMBs with the confinement strategy were provided to promote the development and application of QSSE LMBs.

Nafion-212 Membrane Solvated by Ethylene and Propylene Carbonates as Electrolyte for Lithium Metal Batteries.

The use of cation-exchange membranes as electrolytes for lithium metal batteries can prevent the formation of lithium dendrites during extended cycling and guarantee safe battery operation. In our study, the Nafion-212 membrane in lithium form solvated by a mixture of ethylene carbonate and propylene carbonate (EC-PC) was used as an electrolyte in a lithium metal battery with the LiFePO4 cathode. The Nafion-212-EC-PC electrolyte is electrochemically stable up to 6 V, indicating its suitability for high-energy density batteries. It has an ionic conductivity of 1.9 × 10-4 S/cm at 25 °C and a high lithium transference number. The symmetric Li|Nafion-212-EC-PC|Li cell shows a very low overvoltage of ~0.3 V at a current density of ±0.1 mA/cm2. At 25 °C, the LiFePO4|Nafion-212-EC-PC|Li battery exhibits a capacity of 141, 136, 125, and 100 mAh/g at 0.1, 0.2, 0.5, and 1C rates, respectively. It maintains a capacity of 120 mAh/g at 0 °C and 0.1C with stable performance for 50 charge/discharge cycles. The mechanism of conductivity and capacity retention at low temperatures is discussed.