Imide Salt Research Articles

Formulation and Recycling of a Novel Electrolyte Based on Bio-Derived γ-Valerolactone and Lithium Bis(trifluoromethanesulfonyl)imide for Lithium-Ion Batteries.

This work introduces a novel electrolyte comprising lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt dissolved in bio-based γ-valerolactone (GVL) for lithium-ion batteries (LIBs). Moreover, a simple and sustainable aqueous-based recycling approach for recovering the imide-based lithium salt is proposed. Beyond the sustainable origin of the GVL solvent, this electrolyte exhibits reduced flammability risk, characterized by a flash point of 136°C, along with favorable transport properties (conductivity of 6.2mScm-1 at 20°C) and good electrochemical stability (5.0V vs Li+/Li). Its good compatibility with graphite and lithium iron phosphate electrodes ensures remarkable cycling stability in LIB full-cells after 200 galvanostatic cycles at 1C. Furthermore, the proposed liquid-liquid phase electrolyte recycling method allows for a nearly complete recovery of the LiTFSI salt (97-99%) and the GVL solvent (78%). The feasibility of the recycling process is validated by the reutilization of the recovered LiTFSI salt in electric double-layer capacitors, achieving performances similar to that of the pristine salt with exceptional long-term stability.

A Comprehensive Study on the Parameters Affecting Magnesium Plating/Stripping Kinetics in Rechargeable Mg Batteries

AbstractMg metal anode‐based battery is a more sustainable, lower cost, and higher energy density alternative to Li‐ion. However, this battery chemistry also faces several challenges associated with the high charge density of Mg2+, including achieving high reversibility and low voltage hysteresis for Mg metal plating/stripping. While significant improvements are achieved in last decades, they involve rather complex electrolyte formulations and/or Mg salts difficult to produce, and the use of unpractical substrates such as platinum. Here, significant improvement in terms of Mg plating kinetics is achieved in electrolytes containing commercial magnesium bis(trifluoromethanesulfonyl)imide salt (Mg(TFSI)2) by using titanium substrate with similar crystal structure and lattice parameter as Mg leading to lower nucleation overpotential. Low salt concentration electrolyte and addition of dibutyl magnesium (Mg(butyl)2) also enable the formation of thinner interphase, richer in solvent based decomposition products, further improving Mg plating kinetics. This work highlights the complex role of Mg(butyl)2, often considered as a simple drying agent, and how it impacts ion solvation favoring the mobility of electroactive cationic species, paving the way toward better electrolyte design with improved cation transference number.

A novel multifunctional electrolyte system for high performance Li-ion batteries

Lithium-ion batteries have advantages such as environmental protection and sustainable use, but the widespread application of lithium-ion battery-powered vehicles is limited by the inability to quickly charge and extreme ambient temperatures, and reasonable design of electrolyte components can effectively solve these problems. Here, a novel fluoroethylene carbonate (FEC)/ethyl acetate (EA) based electrolyte containing lithium difluorophosphate (LiPO2F2, 2 wt%) and lithium bis(fluorosulfonyl)imide salt (LiFSI, 1.0 M) is first reported. The electrolyte containing these components has the characteristics of low freezing point, low viscosity and high ionic conductivity. Especially, due to the preferential decomposition of FEC, LiFSI and LiPO2F2, uniform multiphase inorganic LiF and Li3N species are formed on the electrode surface with enhanced robustness and ion transfer kinetics. So, the graphite||LiNi0.5Co0.2Mn0.3O2&LiMn2O4 (Gr||LMO&NCM523) batteries in the designed electrolyte maintain high capacity and coulombic efficiency (100 %) after 200 times at 1C charge and discharge current density at 25 °C and 45 °C. At −20 °C, the batteries have a coulombic efficiency of 50 % and capacity of 38.0 mA h g−1 after 200 charging and discharging cycles, showing excellent low-temperature fast dynamics. Our work supplies an efficient way to accelerate the engineering application of Li-ion batteries.

A 3D Printed Pumpless Nonaqueous Organic Redox Flow Cell

Redox flow batteries (RFBs) have considerable promise in addressing the ever-increasing demands for robust energy storage devices. The nature of these devices makes it possible to create storage with potential applicability at multiple size scales1. However, conventional RFBs require energy to pump the liquid electrolyte, reducing the overall system efficiency, and limiting its viability to mid- and large-scale operations only2. Eliminating the pumping load can improve overall efficiency and create opportunities for mobile applications. In this work, we demonstrate a 3D printed prototype of a pumpless non-aqueous organic RFB that induces flow in the electrolyte from sinusoidal rocking motion. The body of the flow cell was created using 3D printing, allowing us to modify and adapt geometry easily during testing.First, the compatibility of the 3D-printed material was studied by testing the chemical stability of widely available additive manufacturing materials under RFB conditions. Then, we evaluated the performance of this pumpless system using an easily scalable, soluble, and stable one electron donor phenothiazine derivative, N-(2-(2-methoxyethoxy)ethyl)phenothiazine (MEEPT) which has been widely studied for RFB applications3. We used 0.25 M MEEPT and its bis(trifluoromethanesulfonyl)imide radical cation salt (MEEPT-TFSI) as a redox-active couple in 0.5 M tetraethylammonium bis(trifluoromethanesulfonyl)imide (TEATFSI)/acetonitrile (MeCN). Our results suggest an inversely proportional empirical relationship between limiting current density and the frequency of sinusoidal motion due to varying mass transfer rates. After 100 cycles in this symmetric cell with an area specific resistance of 2.62 Ωcm2, we achieved a capacity retention of 63% and an average Coulombic efficiency of 96%, with minimal sign of decomposition of the redox couple when analyzed post-cell cycling.The results of this study can be used for 3D printing material selection in numerous electrochemical devices that use organic solvents, including flow cells, supercapacitors, and lithium-ion batteries. Using sinusoidal motion to drive electrolytes instead of a pump can increase the theoretical efficiency and decrease the overall size of a flow cell. This makes it attractive in wearable applications where biomechanical motion can be harnessed.1 Wang, W. et al. Recent Progress in Redox Flow Battery Research and Development. Advanced Functional Materials 23, 970-986 (2013). https://doi.org:10.1002/adfm.2012006942 Tian, C. H., Chein, R., Hsueh, K. L., Wu, C. H. & Tsau, F. H. Design and modeling of electrolyte pumping power reduction in redox flow cells. Rare Metals 30, 16-21 (2011). https://doi.org:10.1007/s12598-011-0229-13 Milshtein, J. D. et al. High current density, long duration cycling of soluble organic active species for non-aqueous redox flow batteries. Energy & Environmental Science 9, 3531-3543 (2016). https://doi.org:10.1039/C6EE02027E

Highly Concentrated Asymmetric KTFSI for Aqueous Potassium Ion Batteries

AbstractPotassium imide salt (KCTFSI), featuring cyano (C≡N) and trifluoromethanesulfonyl (TFSI) branches, is reported for the first time as a promising electrolyte for aqueous potassium‐ion batteries (KIBs). In contrast to the limited solubility of symmetric KTFSI (1.5m), asymmetric KCTFSI achieves a high saturation concentration of 32.2m within a “water‐in‐salt” region. Experimental and theoretical studies reveal that the remarkable enhancement of solubility is associated with the asymmetry of CTFSI anions and likely the preferential formation of contact‐ion pairs through C≡N coordination. The 32.2m KCTFSI solution not only extends the electrochemical stability window (3.80 V on Al) but also maintains a liquid state at low temperatures (−15 °C) for an extended period, in sharp contrast to the immediate solidification of potassium bis(fluorosulfonyl)imide (KFSI, 30m) and potassium trifluoromethanesulfonate (KOTF, 22m) solutions. Thanks to its wide voltage window and supercooling behavior, KCTFSI demonstrates excellent compatibility with perylene‐3,4,9,10‐tetracarboxylic diimide (PTCDI) anode and KVPO4F (or K2Fe[Fe(CN)6]·2H2O) cathode over a wide temperature range. For instance, full cells of PTCDI/KCTFSI/KVPO4F exhibit reasonable cyclability with an average discharge voltage of 1.7 V over 300 charge/discharge cycles at −15 °C. it is anticipated that this work will inspire new designs of hybrid electrolytes based on CTFSI to advance the realization of viable aqueous KIBs.

Boosting Li-ion transport for graphite electrodes with lithium bis(fluorosulfonyl)imide salt and methyl acetate additive for fast-charging Li-ion batteries

Lithium-ion battery (LIB) is now widely used in the world. However, its unsatisfactory fast-charging performance is limiting its applications. The fast-charging capability of LIB is highly affected by the surface composition of graphite electrode, which can be modified through electrolyte design. Herein we demonstrate that the combination of lithium bis(fluorosulfonyl) imide (LiFSI) salt with methyl acetate (MA) additive in carbonate electrolyte enables fast-charging Li-ion battery. Results from X-ray photoelectron spectroscopy and transmission electron microscopy indicate that the LiFSI-based electrolyte with MA forms a stable and less-resistive solid electrolyte interphase on graphite anode compared to electrolyte with LiPF6, facilitating the transport of Li-ion and achieving a superior fast-charging performance with a capacity of 230 mAh g−1 at 2 C. An electrolyte swapping experiment also indicates that the LiFSI salt forms a more stable SEI on graphite than LiPF6. This work provides a conducive electrolyte design for fast charging Li-ion battery with graphite anode.

Enhancing Cycling Stability of Lithium Metal Batteries by a Bifunctional Fluorinated Ether

AbstractThe lifespan of lithium (Li) metal batteries (LMBs) can be greatly improved by the formation of inorganic‐rich electrode‐electrolyte interphases (EEIs) (including solid‐electrolyte interphase on anode and cathode‐electrolyte interphase on cathode). In this work, a localized high‐concentration electrolyte containing lithium bis(fluorosulfonyl)imide (LiFSI) salt, 1,2‐dimethoxyethane (DME) solvent and 1,2‐bis(1,1,2,2‐tetrafluoroethoxy)ethane (BTFEE) diluent is optimized. BTFEE is a fluorinated ether with weakly‐solvating ability for LiFSI so it also acts as a co‐solvent in this electrolyte. It can facilitate anion decomposition at electrode surfaces and promote the formation of more inorganic‐rich EEI layers. With an optimized molar ratio of LiFSI:DME:BTFEE = 1:1.15:3, LMBs with a high loading (4 mAh cm−2) lithium nickel manganese cobalt oxide (LiNi0.8 Mn0.1 Co0.1) cathode can retain 80% capacity in 470 cycles when cycled in a voltage range of 2.8–4.4 V. The fundamental understanding on the functionality of BTFEE revealed in this work provides new perspectives on the design of practical high‐energy density battery systems.

Harnessing Cold Sintering to Fabricate Composite Polymer Electrolytes - A Paradigm Shift in Organic-Inorganic Material Assembly

The development of composite electrolytes for all-solid-state batteries is an emerging field, but the creation of predominantly inorganic electrolytes remains challenging. In this study, Li6.25Al0.25La3Zr2O12 (Al-LLZO), a ceramic material selected for its high ionic conductivity (1 × 10−4 S.cm−1 at ambient temperature) was shaped by the cold-sintering process (CSP). The organic phase was synthesized by free-radical polymerization of two poly(ethylene oxide) methacrylate derivatives in the presence of lithium bis(trifluoromethanesulfonyl)imide salts (LiTFSI). The polymethacrylate network with dangling poly(ethylene oxide) (PEO) chains was thus obtained. This in situ polymerization allows the one-pot synthesis of the composite electrolyte during CSP. Remarkably, the ionic conductivity of the CSP pellet varied with the nature of the organic phase, ranging from 1 × 10−4 to 1 × 10−5 S.cm−1 for non-grafted and grafted TFSI anion on the PEO-based network, respectively. Additionally, the transport of Li+ remained unaffected by the inorganic material’s nature as long as it contained Li species. Furthermore, a significant enhancement of the ionic conductivity was observed in the composite pellet compared to the TFSI grafted network (10−5 to 10−7 S.cm−1, respectively). Electrochemical impedance spectroscopy measurements revealed changes in the Al-LLZO||PEO-based polymer interface during CSP with the formation of an interphase, confirmed by a low activation energy value (0.1 eV).

Solution-processed poly(vinylidene difluoride)/cellulose acetate/Li1+xAlxTi2-x(PO4)3 composite solid electrolyte for improving electrochemical performance of solid-state lithium-ion batteries at room temperature

To enhance energy density and secure the safety of lithium-ion batteries, developing solid-state electrolytes is a promising strategy. In this study, a composite solid-state electrolyte (CSE) composed of poly(vinylidene difluoride) (PVDF)/cellulose acetate (CA) matrix, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt, and Li1.3Al0.3Ti1.7(PO4)3 (LATP) fillers is developed via a facile solution-casting method. The PVDF/CA ratio, LiTFSI, and LATP fractions affect the crystallinity, structural porosity, and thermal and electrochemical stability of the PVDF/CA/LATP CSE. The optimized CSE (4P1C-40LT/20F) presents a high ionic conductivity of 4.9 × 10-4 S cm−1 and a wide electrochemical window up to 5.0 V vs. Li/Li+. A lithium iron phosphate-based cell containing the CSE delivers a high discharge capacity of over 160 mAh g−1 at 25 °C, outperforming its counterpart containing PVDF/CA polymer electrolyte. It also exhibits satisfactory cycling stability at 1C with approximately 90 % capacity retention at the 200th cycle. Additionally, its rate performance is promising, demonstrating a capacity retention of approximately 80 % under varied rates (2C/0.1C). The increased amorphous region, Li+ transportation pathways, and Li+ concentration of the 4P1C-40LT/20F CSE membrane facilitate Li+ migration within the CSE, thus improving the battery performance.

An Investigation of Li-Ion Cell Degradation Caused by Simulated Autoclave Cycles

Li[Ni0.6Mn0.4Co0.0]O2/graphite (NMC640, balanced for 4.1 V cut-off) and Li[Ni0.83Mn0.06Co0.11]O2/graphite (Ni83, balanced for 4.06 V cut-off) pouch cells were tested using lab-simulated autoclave conditions. After every cycle, the cells at either 3.4, 3.7, or 3.9 V were placed in a 120°C oven for 40 min to undergo an “autoclave” run, then continued for another cycle. Electrolyte blends using lithium bis(fluorosulfonyl)imide (LiFSI) salt were used to improve the cycle-life of autoclaved cells. The lab autoclave protocol was also performed on LiFePO4/graphite (LFP) and NMC commercial cylindrical cells, which were advertised for use in or found in autoclaved medical devices. LFP cells performed poorly in the simulated autoclave tests, while commercial high-temperature-tolerant NMC cylindrical cells and the pouch cells performed similarly. In continuous testing at 85°C, the pouch cells had better capacity retention than both cylindrical cell types. However, the pouch cells suffered from electrolyte permeation through the polymer seals. The pouch cell chemistries incorporated in cylindrical cell format would probably give superior performance to the commercial cells in the autoclave tests. Cell lifetimes were improved when cells were placed into the 120°C oven at a lower voltage suggesting that hospitals should charge Li-ion cells after the autoclaving process instead of standard practice of before.

Osteoporosis Failure of Aluminum Current Collector Induced Crosstalk Degradation at the Imide-Type Lithium Salt Comprised Practical-Level Lithium-Ion Batteries

The atypical failure mechanism caused by the inclusion of lithium bis(fluorosulfonyl)imide (LiFSI) salt in lithium-ion batteries (LIB) is elucidated. When subjected to elevated temperature cycling, the LiFSI salt triggers the degradation of the aluminum current collector, leading to the dissolution of Al ions into the electrolyte. These dissolved Al ions then migrate toward the negative electrode surface where they spontaneously reduce and form Al deposits due to the low electrode potential. This Al deposition further catalyzes the cathodic decomposition of the electrolyte, impacting the interphasial resistance of the negative electrode and consuming both Li ions and electrolyte components. Upon extended cycling with LiFSI-containing electrolytes, a notable decline in the reversible capacity of LIB becomes evident due to cross-talk failure resulting from Al current collector corrosion. Consequently, to enhance the cycling performance of LIBs using LiFSI-based electrolytes, it is necessary to simultaneously prevent Al corrosion and subsequent deposition on the surface of the negative electrode.

Polycarbonate-based composite polymer electrolytes with Al2O3 enhanced by in situ polymerized electrolyte Interlayers for all-solid-state lithium-metal batteries

Lithium-metal batteries (LMBs) have attracted significant attention as promising energy storage devices due to their high energy density. However, the practical application of LMBs is hindered by issues such as dendrite growth and electrolyte decomposition, which lead to poor cycling stability. To address these challenges, this study focuses on investigating the performance of poly(ethylene carbonate) (PEC)-based composite polymer electrolytes (CPEs) with an in situ polymerized poly(vinyl ethylene carbonate) (PVEC)-based electrolyte interlayer. The electrolytes were optimized by incorporating lithium bis(fluorosulfonyl)imide (LiFSI) salt, lithium bis(oxalato)borate (LiBOB) and lithium nitrate (LiNO3) additives, as well as Al2O3 filler. The objective was to improve the compatibility and cycling stability of LMBs by enhancing the electrolyte/electrode interfacial properties. Remarkably, the Li||Li symmetric cell utilizing the optimized CPE exhibited outstanding stability, operating for 700 h without short-circuiting at 0.1 mA cm−2, indicating excellent interfacial compatibility. Additionally, the Li||LiFePO4(LFP) cells using the same electrolyte demonstrated impressive performance, delivering initial discharge capacity of 159.32 mAh g−1 at 0.1 C with retention and coulombic efficiency close to 100 % after 100 cycles at 60 °C. This paper highlights the effectiveness of utilizing polymer electrolyte interlayers and additives like LiNO3 in CPEs to enhance the interfacial compatibility and cycling stability of LMBs.

Stable room-temperature sodium-sulfur battery enabled by pre-sodium activated carbon cloth anode and nonflammable localized high-concentration electrolyte

Room-temperature (RT) sodium-sulfur (Na–S) battery is a promising energy storage technology with low-cost, high-energy-density and environmental-friendliness. However, the current RT Na–S battery suffers from various problems, such as poor cycling stability and poor electrolyte-electrode compatibility caused by polysulfide shuttling and active Na-metal anode. Here, a nonflammable localized high-concentration electrolyte (LHCE) (sodium bis(trifluoromethanesulfonyl)imide salt, sulfolane solvent and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether diluent in a molar ratio of 1:2:2), and a pre-sodium activated carbon cloth (NaACC) anode, are combined to enable stable and high-safety RT Na–S battery. In LHCE, the shuttle effect of polysulfide is greatly alleviated, a stable anions-derived NaF-rich cathode electrolyte interphase is formed, and a solid-solid conversion mechanism of a solid S8-solid Na2Sn (1 ≤ n ≤ 3) is achieved. Meanwhile, the Na dendrites and side reactions between electrolyte and anode are effectively inhibited by using NaACC anode. Therefore, under the synergistic effect of electrolyte regulation and negative electrode structure optimization, although using a low N/P ratio of 1.7, the RT Na–S battery also delivers a high initial capacity of 1220.8 mAh g−1, good rate capability and stable cycling performance with a high average Coulombic efficiency of 99.1 %. The capacity still remains at 704.5 mAh g−1 after 100 cycles. This innovative combination of LHCE and NaACC anode in Na–S batteries provides a new idea for further achieving stable RT Na–S batteries with long-cycle-life and high-safety.

Elucidating the role of polar functional groups in fluorinated polymer artificial interphase for stable lithium anodes

The properties of solid electrolyte interphase (SEI) are pivotal for stable lithium (Li) metal anodes. However, the process of formation and ion transport behavior of lithium fluoride (LiF) through the polar groups of organic fluorinated artificial SEI has not been clearly elucidated. Herein we demonstrate that the key role of polar functional groups in fluorinated polymer artificial SEI (FPASEI) is elucidated by using 2,2,2-trifluoroethylacrylate (TFEA) through radical polymerization. Density functional theory and ab initio molecular dynamics calculations demonstrate that the abundant CO polar groups in TFEA contribute a large number of electrons, thereby facilitating the degradation kinetics of CF bond cleavage in lithium bis(trifluoromethanesulfonyl)imide salt to yield LiF. Moreover, the presence of CF, COC, and CO groups in TFEA facilitate the migration of Li+ towards interchain regions through cation-dipole interaction, leading to dendrite-free Li deposition. The implementation of FPASEI in a Li||Li cell results in 500 h of Li plating/stripping cycling at 1 mA cm−2 and retains a capacity retention rate of 80 % after 265 cycles in Li||LiNi0.8Co0.1Mn0.1O2 cell. This study not only emphasizes the critical role of polar groups in artificial SEI but also presents a promising strategy for achieving stable Li metal batteries.

Understanding Na+ Diffusion, Physicochemical Behavior, and Electrochemical Performance of a Gel Polymer Electrolyte.

Gel polymer electrolytes (GPEs) represent a credible alternative to organic liquid electrolytes (LEs) for safer sodium metal batteries. As a compromise between solid polymer electrolytes and LEs, GPEs ensure a good ionic conductivity, improve the electrolyte/electrode interface, and prevent solvent leaks. Herein, a GPE based on acrylate-bifunctionalized polyethylene glycol chains mixed with an ether solvent (TEGDME) and a polyethylene glycol diacrylate (PEG600DA) in a 50/50 wt % ratio was prepared by ultraviolet photopolymerization. Sodium bis(fluorosulfonyl)imide salt (NaFSI) was added at different concentrations to study its interactions with the solvent and/or the cross-linked polymer. Infrared spectroscopy, thermogravimetric analysis, differential scanning calorimetry, and swelling ratio characterizations were combined to determine the physicochemical properties of the GPE. Complementary characterizations including electrochemical impedance spectroscopy, chronopotentiometry, and cyclic voltammetry allowed correlating the physicochemical properties of the GPE to its electrochemical performance. Then, improvements were obtained by careful combination of its components. The cross-linking agent allowed us to obtain a polymer matrix that traps the organic solvent and prevents leakage. Such a solvent inclusion reduces the rigidity of the membrane and lowers its viscosity, offering a room temperature ionic conductivity of 4.8 × 10-4 Ω-1 cm-1. The control of polymer's tortuosity leads to a stable cycling vs sodium metal over several hundred hours without increase of the polarization. Finally, optimization of the salt loading plays a major role in electrostatic cross-linking, leading to an improvement of the mechanical properties of the GPE without reducing its conductivity.

A Comparative Study on Electrochemical Performance of Single versus Dual Networks in Lithium Metal/Polysulfide-Polyoxide Co-Network/Lithium Titanium Oxide Cathode

The present article introduces a strategy for controlling oxidation and reduction reactions within polymer electrolyte membrane (PEM) networks as a means of enhancing storage capacity through the complexation of dissociated lithium cations with multifunctional groups of the polymer network. Specifically, co-polymer networks based on polysulfide (PS) and polyoxide (PO) precursors, photo-cured in the presence of succinonitrile (SCN) and lithium bis(trifluoro methane sulfonyl imide) (LiTFSI) salt, exhibited ionic conductivity on the order of mid 10−4 S/cm at ambient temperature in the 30/35/35 (weight %) composition. Lithium titanate (LTO, Li4Ti5O12) electrode was chosen as an anode (i.e., a potential source of Li ions) against lithium iron phosphate (LFP, LiFePO4) cathode in conjunction with polysulfide-co-polyoxide dual polyelectrolyte networks to control viscosity for 3D printability on conformal surfaces of drone and aeronautic vehicles. It was found that the PS-co-PO dual network-based polymer electrolyte containing SCN plasticizer and LiTFSI salt exhibited extra storage capacity (i.e., specific capacity of 44 mAh/g) with the overall specific capacity of 170 mAh/g (i.e., for the combined LTO electrode and PEM) initially that stabilized at 153 mAh/g after 50th cycles with a reasonable capacity retention of over 90% and Coulombic efficiency of over 99%. Of particular interest is the observation of the improved electrochemical performance of the polysulfide-co-polyoxide electrolyte dual-network relative to that of the polyoxide electrolyte single-network.

Multiple Na+ transport pathways and interfacial compatibility enable high-capacity, room-temperature quasi-solid sodium batteries

Sodium-metal batteries (SMBs) are ideal for large-scale energy storage due to their stable operation and high capacity. However, they have safety issues caused by severe dendrite growth and side reactions, particularly when using liquid electrolytes. Therefore, it is critically important to develop electrolytes with high ionic conductivity and improved safety that are non-flammable and resistant to dendrites. Here, we developed polymerized polyethylene glycol diacrylate (PEGDA)-modified poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) electrolytes (PPEs) with highly conductive sodium bis(trifluoromethanesulfonyl)imide and corrosion-inhibitive sodium bis(oxalato)borate salts for SMBs. Well-complexed PEGDA not only increases the amorphicity of the PVDF matrix, but also offers numerous Lewis basic sites through the polar groups of carbonyl and ether groups (i.e., electron donors). The presence of the Lewis basic sites facilitates the dissociation of sodium salt and transportation of Na+ within the PVDF matrix. This results in the generation of additional Na+ transport pathways, which can enhance the performance of the battery. Among PPEs, the optimized PPE-50 exhibits a high ionic conductivity of 3.42 × 10–4 S cm−1 and a mechanical strength of 14.0 MPa. A Na||Na symmetric cell with PPE-50 displays high stability at 0.2 mA cm−2 for 800 h. PPE-50 further displays high capacity, e.g., a Na3V2(PO4)3|PPE-50|Na battery delivers a decent discharge capacity of 101.5 mAh g−1 at 1.0C after 650 cycles. Our work demonstrates the development of high-performance quasi-solid polymer electrolytes with multiple transport pathways suitable for room-temperature SMBs.

Easily accessible linear and hyperbranched polyesters as solid polymer electrolytes

Herein we report synthesis, characterization, thermal properties, ionic conductivity and oxidative stability of novel linear and hyperbranched polyesters. Tri-acrylate ester of commercially available 1,1,1-Tris(hydroxymethyl)propane (M1) was used as the primary building block for the synthesis of linear and hyperbranched polymers. A3 + B2 type polycondensation between M1 and dithiols of two different chain lengths (C3, C6) by 100 % atom efficient thiol-acrylate Michael addition reaction, followed by consumption of the unreacted acrylate esters (in the linear or terminal units) with 1-butane thiol produced two hyperbranched polymer namely HB-P1 and HB-P2. For synthesis of linear analogues, one of the equivalent acrylate esters was reacted first with 1-butane thiol and the resulting di-acrylate monomer was polymerized with the C3 and C6-dithiols, producing linear polymers L-P1 and L-P2. All the polymers showed molecular weight (Mn) in the range of 5000–6000 gmol−1 with low dispersity. TGA analysis revealed sufficient thermal stability of the polymers for the application as solid polymer electrolyte. All the polymers are amorphous, showing only a glass transition in the range of ∼ -45 °C (C3 spacer) to −50 °C (C6 spacer) and no crystallization peak. In presence of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt the Tg increased by ∼ 10 °C. Solid polymer electrolytes were prepared by incorporating LiTFSI at a molar ratio of polymer carbonyl units to lithium of C=O/Li of 10 and ionic conductivity (σ) was evaluated by electrochemical impedance spectroscopy (EIS). Moderate values in the range of 10-7 S cm−1 were obtained at 25 °C with slightly higher values for the linear polymers. The ionic conductivity increased up to ∼ 2 orders of magnitude at higher temperatures. The oxidative stability of the polymer electrolytes against lithium-metal electrodes revealed onset potential for the first degradation in the range of 3.8 V, indicating moderate stability that might be suitable for testing applications in all-solid-state lithium-metal batteries using sulfur or lithium iron phosphate cathodes.

Solid Electrolyte Interphase Formation at the Ionic Liquid Electrolyte—Lithium-Metal Interface Using an Ab Initio Molecular Dynamics Approach

An interfacial study is performed using ab initio molecular dynamics (AIMD) simulations to elucidate the electrochemical interfacial stability of an ionic liquid (IL) as component of an electrolyte in contact with a Li-metal anode when an additive, vinylene carbonate (VC), is added to the electrolyte. The IL electrolyte is composed of 1-butyl-1-methylpyrrolidinium bis(fluoro-sulfonyl)-imide (PYR14+FSI−) with lithium bis(trifluoromethane-sulfonyl)imide salt (Li+TFSI−) in a 8:2 concentration ratio. Reactions observed at the interface electrolyte/Li-metal during the AIMD simulations are verified by electronic structure calculations using density functional theory calculations considering a continuum solvent model. We found that the addition of VC to the IL prevents further reduction of the counter-ion TFSI− and retards reduction of FSI− in comparison to the case when the additive VC is absent. The PYR14+ reduction is never observed with or without the additive. The 1st, 2nd, and 3rd electron affinities of the PYR14+ are −0.08, −0.36 and −3.65 eV, indicating little to large repulsive behavior against electrons. The opposite is observed for the two negative ions, FSI− and TFSI−, which yield 1st electron affinities of 1.73 and 1.83 eV, respectively. Reactions obtained under open circuit conditions. Dissociation mechanisms are presented for all IL components reduced at the IL/Li-metal interface.

Weakly Polar Ether‐Aided Ionic Liquid Electrolyte Enables High‐Performance Sodium Metal Batteries over Wide Temperature Range

AbstractSodium (Na) metal batteries (SMBs) have attracted many attentions because of high specific capacity and low redox potential. However, SMBs encounter several crucial challenges including Na dendrites growth, severe interfacial side reactions and safe issues from liquid organic electrolyte. In this study, a non‐flammable 1,2‐bis (1,1,2,2‐tetrafluoroethoxy) ethane (TFEE) aided ionic liquid (IL) electrolyte (TILE) is reported with sodium bis(trifluoromethylsulfonyl)imide (NaTFSI) salt and N‐methyl‐N‐propylpyrrolidinium bis(fluorosulfonyl)imide ([Py13][FSI]) IL solvent. The IL solvent contains plentiful organic cations and anions, which helps to provide a cationic shielding effect by [Py13]+ cations and regulate the Na+ solvation structure with more FSI− and TFSI− anions. The unique solvation structure of Na+ facilitates a stable inorganic‐rich solid electrolyte interphase (SEI), effectively inhibiting unstable Na deposition and suppressing side reactions between the electrolyte and Na‐metal in SMBs. Therefore, the Na3V2(PO4)3 (NVP)||Na cells with TILE can deliver good rate capability and superior cycling performance with capacity retention of 95.5% and Coulombic efficiency of ≈99.9% after 2000 cycles at 5C rate and 25 °C. The high‐voltage NaNi0.33Fe0.33Mn0.33O2(NFM)||Na cells with TILE can also deliver good cycling performance with capacity retention of 92% after 150 cycles in the voltage range of 2.0–4.1 V at 0.5C rate and 25 °C.