Electrolytes For Lithium Metal Batteries Research Articles

Investigation of properties and performance of three novel highly concentrated ether-functionalised ionic liquid electrolytes for lithium metal batteries

Exploring highly concentrated (>50 mol% Li-salt) ionic liquids as electrolytes for lithium metal batteries can lead to the development of safer, high-performance batteries with increased energy density and improved cycling stability. However, at high concentrations of Li-salt, ionic liquid electrolytes can have high viscosity and slow ion transport kinetics. Here, we investigate the use of novel ether-functionalised cations as a promising approach for increasing fluidity and weakening Li+-anion interactions in highly concentrated ionic liquid electrolytes to facilitate faster transport of Li+. Three small ether-functionalised cations, namely 1-methoxymethyl-1,1,1-trimethylammonium ([N111,1O1]+), N-methoxymethyl-N-methylpiperidinium ([C1O1mpip]+) and N-methoxymethyl-N-methylmorpholinium ([C1O1mmor]+) are compared when paired with bis(fluorosulfonyl)imide ([FSI]−) anion at a 1:1 mol ratio with LiFSI. The study provides insights into the relationship between the structure of the ether-functionalised cations and the properties of the resulting electrolytes, including thermal behaviour, ionic conductivity, viscosity, ion diffusivity, Li plating/stripping behaviour and Li+-transference number. The performance of the electrolytes was tested in Li|LiFePO4 (LFP) cells (1.14 mAh cm−2) at 0.57 mA cm−2 (C/2) for 100 cycles at 50 °C. (LiFSI)0.5([C1O1mpip][FSI])0.5 and (LiFSI)0.5([N111,1O1][FSI])0.5, have high Li-transference numbers of 0.47 ± 0.02 and 0.37 ± 0.02, respectively, and both display 0.85 mAh cm−2 areal discharge capacity after 100 cycles, with capacity retention over 98.5% (in comparison to the first cycle after preconditioning cycles), demonstrating the feasibility of these electrolytes in Li-metal cells.

Deep eutectic solvent with film-forming fluoroethylene carbonate as a nonflammable electrolyte for lithium metal batteries

Nonflammable deep eutectic solvents (DESs) are potential electrolytes for Li metal batteries. However, their practical application is prevented by their degradation with Li metal at low voltages. Here, we use fluoroethylene carbonate (FEC), a film-forming compound, as a cosolvent in DES-based electrolytes, demonstrating that compatibility with Li metal can be improved due to the formation of a robust and compact solid electrolyte interphase layer on the anode surface. The optimal electrolyte (50%DES) exhibits the Li+ conductivity of 1.58 mS cm−1 at room temperature. After the introduction of 50%FEC, the DES electrolyte shows a considerable reduction in viscosity from 1206.3 mPa s−1 to 20.4 mPa s−1 while retaining high flame-retardant efficiency. The Li/LiFePO4 cell with 50%DES electrolyte exhibits a reversible discharge capacity of 140.7 mAh g−1 at 50 mA g−1 and a capacity retention of 94.3 % after 200 cycles. The good cycling stability of the cell in 50%DES electrolyte can be attributed to the formation of a robust SEI layer containing inorganic-rich compounds (LiF and Li3N) with rapid Li+ transport on the Li metal anode that can improve interfacial stability and suppress Li dendrite growth.

Plasma Modification of Interfaces in Ceramic Nanofiber–Polymer Electrolytes for Lithium Metal Batteries

Ceramic–polymer composite electrolytes hold a great promise as next-generation electrolytes because of strong mechanical properties and wide electrochemical stability window. However, it remains challenges in improving ionic conductivity and lithium ion transference numbers. In this work, plasma treatment has been performed on Li0.33La0.557Ti0.995Al0.005O3 (LLATO) nanofibers. The treated nanofibers are then incorporated with a polymer matrix to form a composite electrolyte. Plasma treatment has modulated oxygen vacancies, functional groups, polarity, and interfacial interaction of LLATO-polymer. This has improved lithium ion transport at interface of LLATO-polymer electrolyte. As a result, the LLATO-polymer electrolyte shows enhanced ionic conductivity and mechanical performance. This work has implication in design of ceramic-polymer composite electrolytes for solid state lithium metal batteries.

Lithiated Phosphoryl Cellulose Nanocrystals Enhance Cycling Stability and Safety of Quasi-Solid-State Lithium Metal Batteries.

Cycling stability and safety are two of the main challenges facing lithium metal batteries with metallic lithium as anodes. Quasi-solid-state lithium metal batteries based on gel polymer electrolytes are one of the important development directions for lithium metal batteries addressing those challenges. Herein, we prepare lithiated phosphoryl cellulose nanocrystals (PCNC-Li) as a modification material for poly(vinylidene fluoride) (PVDF) gel polymer electrolyte to improve cycling stability and safety of quasi-solid-state lithium metal batteries. The synthesized PCNC-Li tends to form a uniform network structure on the surface of the PVDF membrane, in which the phosphoryl groups grafted regularly on celluloses can regulate the transport of lithium ions. As a result, a more uniform ion flux and more stable lithium anode interface support an obviously improved cycling stability for lithium metal batteries. Moreover, the introduction of the PCNC-Li coating layer makes the modified PVDF membranes have a better thermal stability and an enhanced mechanical strength, which is beneficial for improvement of safety of lithium metal batteries. This work provides a new alternative to fabricating a better composite gel polymer electrolyte for lithium metal batteries.

Optimizing Li-ion Solvation in Gel Polymer Electrolytes to Stabilize Li-Metal Anode.

Gel polymer electrolytes (GPEs) have potential as substitutes for liquid electrolytes in lithium-metal batteries (LMBs). Their semi-solid state also makes GPEs suitable for various applications, including wearables and flexible electronics. Here, we report the initiation of ring-opening polymerization of 1,3-dioxolane (DOL) by Lewis acid and the introduction of diluent 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (TTE) to regulate electrolyte structure for a more stable interface. This diluent-blended GPE exhibits enhanced electrochemical stability and ion transport properties compared to a blank version without it. FTIR and NMR proved the effectiveness of monomer polymerization and further determined the molecular weight distribution of polymerization by gel permeation chromatography (GPC). Experimental and simulation results show that the addition of TTE enhances ion association and tends to distribute on the anode surface to construct a robust and low-impedance SEI. Thus, the polymer battery achieves 5 C charge-discharge at room temperature and 200 cycles at low temperature -20 °C. The study presents an effective approach for regulating solvation structures in GPEs, promoting advancements in the future design of GPE-based LMBs.

Research Progress of Liquid Electrolytes for Lithium Metal Batteries at High Temperatures.

Lithium metal batteries (LMBs) are the most promising high energy density energy storage technologies for electric vehicles, military, and aerospace applications. LMBs require further improvement to operate efficiently when chronically or routinely exposed to high temperatures. Electrolyte engineering with high temperature tolerance and electrode compatibility has been essential to the development of LMBs. In this review, the primary obstacles to achieving high-temperature LMBs are first explored. Subsequently, electrolyte tailoring options, such as lithium salt optimization, solvation structure modification, and the addition of additives are reviewed in detail. In addition, the feasibility of utilizing LMBs at high temperatures has been investigated. In conclusion, this study provides insights and perspectives for future research on electrolyte design at high temperatures.

Boron nitride nanofibers enhanced composite PEO-based solid-state polymer electrolytes for lithium metal batteries

Polyethylene oxide (PEO)-based solid-state polymer electrolytes (SPEs) are limited by their poor cyclic stability and inferior ionic conductivity for applicating in high-safety, long-cycling and high-energy-density lithium metal batteries. Herein, porous boron nitride nanofibers (BNNFs) are filled into PEO-based SPE, which significantly suppresses Li dendrites growth and enhances the electrochemical performance of Li metal battery. BNNFs with high porosity have more active sites to connect with PEO, which can effectively reduce the crystallinity of the PEO matrix and enhance its ionic conductivity. Moreover, owing to the hardness and good stability of BNNFs, BNNFs/PEO/LiTFSI electrolyte exhibits a wider electrochemical window, better mechanical property and higher thermal stability compared with PEO/LiTFSI electrolyte. Consequently, the Li symmetric cell composed of 1% BNNFs/PEO/LiTFSI performs good cyclic stability (>1800 h), and the Li||1% BNNFs/PEO/LiTFSI||LFP full battery shows obviously improved performances in charge-discharge polarization voltage, discharge specific capacity, rate performance and cyclic stability than the Li||PEO/LiTFSI||LFP battery.

Synergistic effect and enhanced safety of double dynamic solid polymer electrolytes for lithium metal batteries

A solid polymer electrolyte with low mechanical strength and poor flexibility will cause mechanical degradation, short circuits, or functional failures in lithium ion batteries (LIBs). Therefore, introducing an double dynamic solid polymer electrolytes with high ionic conductivity, good mechanical strength, and excellent self-healing ability for stabilizing the solid electrolyte interface, preventing battery fire, and suppressing Li dendrite growth will further improve the reliability, security, and lifetime of the LIBs. Herein, a series of dual dynamic self-healing solid polymer electrolytes (DD-shSPEs) are developed via the Schiff base reaction between different molar ratios of polyethylene glycol derivative (tat-PEG) and terephthalaldehyde. The hydrogen bond and the highly reversible imine bonds endow DD-shSPEs with rapid self-repairing capacity to cure mechanical damage within 5 min at normal temperature. The self-healing polymer electrolytes also exhibit excellent stretchability, which can satisfy the external deformation of the battery. Meanwhile, after self-healing, the ionic conductivities of the electrolytes can hold the original values. Also, the initial discharge capacity (IDC) of the battery based on DD-shSPEs reach to 149.3 mAh g-1 under a current rate of 0.1 C, and still maintains 144.2 mAh g−1 after 100 cycles. Besides, the polymer electrolyte (PE) also exhibits good interfacial stability and have a cycle life of more than 650 h at a current density of 0.2 mA cm-2. Accordingly, this work shows that the high-performance PE based on self-healing materials is promising for constructing high stable solid-state energy storage device.

Enabling the Operation of Highly Compatible LiI-3-Hydroxypropionitrile Small-Molecule Solid-State Electrolytes in Lithium Metal Batteries via Stepped-Amorphization Strategy.

Integrating the advantages of both inorganic ceramic and organic polymer solid-state electrolytes, small-molecule solid-state electrolytes represented by LiI-3-hydroxypropionitrile (LiI-HPN) inorganic-organic hybrid systems possess good interfacial compatibility and high modulus. However, their lack of intrinsic Li+ conduction ability hinders potential application in lithium metal batteries until now, despite containing LiI phase composition. Herein, inspired by evolution tendency of ionic conduction behaviors together with first-principles molecular dynamics simulations, we propose a stepped-amorphization strategy to break the Li+ conduction bottleneck of LiI-HPN. It involves three progressive steps of composition (LiI-content increasing), time (long-time standing), and temperature (high-temperature melting) regulations, to essentially construct a small-molecule-based composite solid-state electrolyte with intensified amorphous degree, which realizes efficient conversion from an I- to Li+ conductor and improved conductivity. As a proof, the stepped-optimized LiI-HPN is successfully operated in lithium metal batteries cooperated with Li4 Ti5 O12 cathode to deliver considerable compatibility and stability over 250 cycles. This work not only clarifies the ionic conduction mechanisms of LiI-HPN inorganic-organic hybrid systems, but also provides a reasonable strategy to broaden the application scenarios of highly compatible small-molecule solid-state electrolytes.

Highly Conductive Iodine and Fluorine Dual-Doped Argyrodite Solid Electrolyte for Lithium Metal Batteries

Sulfide-type argyrodite solid electrolytes (SEs) with halide doping have attracted serious interest. Although other halides such as Cl and Br have been found to enhance Li-ion transport in argyrodites, the direct synthesis of efficiently conductive Li6PS5I without postprocessing has rarely been investigated. In this work, we report the one-step synthesis of highly conductive Li6PS5I with an impressive ionic conductivity of 2.5 × 10–4 S cm–1 at room temperature through a solvent-based method. Moreover, by introducing F– to partially replace I–, hybrid-doped argyrodites Li6PS5FxI1–x (x = 0.25, 0.5, 0.75) have been synthesized. Li6PS5F0.25I0.75 achieves the highest conductivity of 3.5 × 10–4 S cm–1 due to the energetic preference for anion-disordering among F–/I– and S2–, which facilitate faster Li transport as supported by density functional theory (DFT) calculations. With higher F content in argyrodites, Li6PS5F0.75I0.25 displays the best electrochemical stability toward Li metal, as evidenced by long-term stable cycling in Li symmetric cells up to 1100 h. Solid-state Li metal batteries with an active cathode of Li4Ti5O12 (LTO) display an initial specific capacity of 140 mAh g–1 and remain at 105 mAh g–1 after 200 cycles, suggesting great battery cycling performance. This research has developed new compositions in the argyrodite SE family which could lead to advancements in the development of solid-state Li metal batteries.

Homogeneous and mechanically stable solid–electrolyte interphase enabled by trioxane-modulated electrolytes for lithium metal batteries

Homogeneous and mechanically stable solid–electrolyte interphase enabled by trioxane-modulated electrolytes for lithium metal batteries

Liquid crystals as additives in solid polymer electrolytes for lithium metal batteries

The forefront of research for new electrolyte materials for tomorrow’s electrochemical energy storage technologies is based on thoroughly designed molecular architectures. Here, liquid crystals (LCs) mixed with poly(ethylene-oxide) (PEO) and lithium salts are proposed as solid polymer electrolytes (SPEs) for application in lithium-metal batteries. The PEO matrix comprises the LC, which gives orientational properties, whereas the PEO chains with lithium salts are responsible for the ionic conductivity. N-(4-Methoxybenzylidene)-4-butylaniline (MBBA) is used as proof-of-concept and new LC molecules are developed for further research. Ionic conductivities (7·10–4 S·cm−1 at 40 °C) up to three times higher than the reference without LC are achieved. In addition, the SPEs are characterized electrochemically, thermally and by polarized optical microscope demonstrating the preservation of the liquid crystalline behavior even after mixing with PEO and lithium salts.Graphical abstract

Facile access to CO2-sourced polythiocarbonate dynamic networks and their potential as solid-state electrolytes for lithium metal batteries.

Poly(monothiocarbonate)s are an interesting class of sulfur-containing materials whose application as solid polymer electrolytes was barely studied, certainly due to the elusive production of diversified polymer architectures. Herein, a new liquid CO2-sourced bis(⍺-alkylidene cyclic carbonate) monomer was designed at high yield to allow its one-step and solvent-free copolymerization with thiols to produce linear and cross-linked polymers in mild conditions. The influence of the monomer structure on the thermal properties and the ionic conductivity of linear polymers was assessed. The polymer network showed to be thermally re-processable owing to the dynamic nature of the monothiocarbonate bonds. A solid polymer electrolyte was easily obtained from the cross-linked material when combined with LiTFSI salt. The solid polymer electrolyte was characterized by an ionic conductivity reaching 6×10-6 S.cm-1 at room temperature with a lithium transference number of 0.37 and a wide electrochemical stability window (4.0 V vs Li0/Li+) valid for lithium cycling. This work thus reports an attractive valorizing approach for carbon dioxide to deliver under mild operating conditions poly(monothiocarbonate)-containing novel covalent adaptable network CAN materials of high potential for energy applications, especially as solid electrolytes for batteries.

High-performance localized high-concentration electrolytes by diluent design for long-cycling lithium metal batteries

Electrolyte design is essential for stabilizing lithium metal anodes and localized high-concentration electrolyte (LHCE) is a promising one. However, the state-of-the-art LHCE remains insufficient to ensure long-cycling lithium metal anodes. Herein, regulating the solvation structure of lithium ions in LHCE by weakening the solvating power of diluents is proposed for improving LHCE performance. A diluent, 1,1,2,2,3,3,4,4-octafluoro-5-(1,1,2,2-tetrafluoroethoxy) pentane (OFE), with weaker solvating power is introduced to increase the proportion of aggregates (an anion interacts with more than two lithium ions, AGG-n) in electrolyte compared with the commonly used 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE). The decomposition of AGG-n in OFE-based LHCE intensifies the formation of anion-derived solid electrolyte interphase and improves the uniformity of lithium deposition. Lithium metal batteries with OFE-based LHCE deliver a superior lifespan of 190 cycles compared with 90 cycles of TTE-based LHCE under demanding conditions. Furthermore, a pouch cell with OFE-based LHCE delivers a specific energy of 417 Wh/kg and undergoes 49 cycles. This work provides guidance for designing high-performance electrolytes for lithium metal batteries.

Recent Advances and Future Perspectives of PVDF-Based Composite Polymer Electrolytes for Lithium Metal Batteries: A Review

Lithium metal batteries (LMBs) using solid-state electrolytes (SSEs) can achieve dual enhancement of energy density and safety performance and have attracted considerable attention. In spite of their excellent flexibility and processability, poly(vinylidene fluoride) (PVDF)-based electrolytes, regarded as a prominent member of SSEs in LMBs, suffer from low ionic conductivity and high internal resistance. The structure of PVDF-based composite polymer electrolytes (CPEs) allows them to combine the advantages of inorganic components and PVDF substrates. The use of modification and advanced technology can achieve higher mechanical strength and improved electrochemical properties of PVDF-based CPEs, but their practical application still needs to be explored. This paper reviews recent research progress of PVDF-based CPEs in LMBs, focuses on the methods to improve the electrochemical and safety performance of PVDF-based CPEs, and specifically discusses five aspects of properties upgrading as the main line, namely, the ionic conductivity, ion migration properties, mechanical properties, thermal stability, and electrochemical stability windows. Finally, the challenges and application prospects of high-performance PVDF-based CPEs in future development are presented.

Understanding Lithium-Ion Dynamics in Single-Ion and Salt-in-Polymer Perfluoropolyethers and Polyethyleneglycol Electrolytes Using Solid-State NMR

Lithium metal batteries with high energy densities can enable a revolution in energy storage and accelerate shifts in electric transportation and electricity generation. However, several morphological and electro-chemo-mechanical challenges impede their development. Solid-state electrolytes such as those based on polymers show great promise in replacing liquid electrolytes in lithium metal batteries. Polyether-based polymer electrolytes are the most investigated but are plagued by low room-temperature ionic conductivity and poor oxidative stability. Hence, there is great need for the development and understanding of ion transport in new classes of polymer electrolytes. Perfluoropolyether (PFPE)-based electrolytes have shown improved oxidative stability, but little is understood about their lithium solvation and transport mechanism. In this work, we use multinuclear solid-state magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy to investigate the lithium cation environment and mobility in crosslinked single-ion and salt-in-polymer PFPE electrolytes and compare it directly to that of well-known polyethylene glycol (PEG) electrolytes. We show that the interaction of the lithium cation with the polymer backbone is weaker in PFPE systems compared to PEG, likely resulting in stronger ion pairing in the PFPE systems. Line shape analyses show lower lithium mobility in PFPE electrolytes despite lower activation energies being derived from spin-lattice relaxation (T1) measurements as compared to those for the PEG systems. The rapid relaxation is instead ascribed to the local fluctuations caused by polymer backbone mobility. By studying different modes of ion binding (single-ion vs salt-in-polymer), we show that differences across polymer backbones (PFPE vs PEG) have a greater effect on mobility than differences in ion binding modes within each polymer class (especially when single-ion conducting site density is not high). Our ability to use MAS NMR to study polymer electrolytes in their native state opens new opportunities to develop and understand novel polymer or hybrid solid-state electrolytes for next-generation lithium metal batteries.

Stable Non‐flammable Phosphate Electrolyte for Lithium Metal Batteries via Solvation Regulation by the Additive

AbstractThe application of lithium metal batteries (LMBs) is impeded by safety concerns. Employing non‐flammable electrolytes can improve battery reliability while the cost and performance deterioration limit their popularization. Herein, a high‐performance non‐flammable electrolyte is designed, 1.5 m LiTFSI in propylene carbonate (PC)/triethyl phosphate (TEP) (4:1 by vol.) with 4‐nitrophenyl trifluoroacetate (TFANP) as the additive, which can facilitate the construction of LiF‐rich solid electrolyte interphase (SEI) on Li anode surface and cathode electrolyte interphase (CEI) on cathode surface through its prioritized decomposition. In TFANP‐containing electrolyte, the decreased TEP coordination number in the solvation sheath relieves the adverse effect of active TEP on both the SEI and CEI for suppressing the growth of Li dendrites and reducing the continuous electrolyte consumption. Thus, the Li||LiNi0.6Co0.2Mn0.2O2 battery with such an electrolyte can deliver 132 mAh g−1 after 150 cycles with high coulombic efficiency (99.5%) and superior rate performance (110 mAh g−1 at 5 C, 1 C = 200 mA g−1). This work provides a new additive insight on non‐flammable electrolyte for reliable LMBs.

Salt-Concentrated Siloxane-Based Electrolytes for Lithium Metal Batteries: Physical Properties, Electrochemical Properties, and Cell Performance

Salt-Concentrated Siloxane-Based Electrolytes for Lithium Metal Batteries: Physical Properties, Electrochemical Properties, and Cell Performance

Highly Oxidation‐Resistant Ether Gel Electrolytes for 4.7 V High‐Safety Lithium Metal Batteries

AbstractEther electrolytes are considered to be one of the most promising candidates for lithium metal batteries (LMBs) since they help to form a stable solid electrolyte interface (SEI) on lithium anodes. However, conventional liquid ether electrolytes are not suitable for the high‐voltage LMBs over 4.5 V due to their narrow potential windows. Herein, a gel electrolyte strategy is demonstrated to improve the high voltage stability of ether electrolytes in LMBs with Li‐rich layered oxide (LLO) cathodes that usually operate at 4.6 V and above. Compared with a liquid electrolyte, the electrochemical window of the gel electrolyte with cross‐linked amide framework is increased by 1 V. The strong cross‐linked framework not only regulates the uniform growth of cathode electrolyte interface (CEI)/SEI on the cathode/anode sides, but also greatly increases the Li+ transference number and inhibits the generation of harmful substances by anchoring PF6− anions, thus providing a long cycle for LMBs at a high voltage up to 4.7 V. Simultaneously, the flame retardancy and flexibility of the gel electrolyte ensures the safe operation of high‐voltage LMBs. Therefore, the LLO||Li pouch cell using gel electrolyte realizes stable cycling at a high voltage 4.6 V, and can pass the acupuncture test.

Localized high-concentration electrolytes for lithium metal batteries: progress and prospect

Localized high-concentration electrolytes for lithium metal batteries: progress and prospect