Traditional Organic Electrolyte Research Articles

Tough and temperature-resistant mask-based separator realized by K-β”-Al2O3 protective layer for stabilizing NaK liquid metal batteries

The development of low-cost, high-temperature-resistant separators is crucial for optimizing the reaction interface and improving the safety performance of liquid NaK metal batteries. This paper presents a novel separator with a solid electrolyte layer, Mask@40 %β-Al2O3, primarily composed of K-β”-Al2O3 known for its exceptional ionic conductivity and high-temperature resistance. Specifically designed for room-temperature liquid NaK metal batteries, the Mask@40 %β-Al2O3 separator effectively transforms the unstable liquid–liquid interface between liquid NaK metal electrodes and traditional organic electrolytes into a stable solid–liquid interface. This transformation prevents short-circuiting caused by area collapse under high-temperature conditions. Moreover, the Bare Mask component can adsorb a conventional organic electrolyte to construct a stable solid–liquid interface on the cathode side, thereby reducing the typically observed high interfacial resistance associated with conventional solid electrolytes. The cost-effective nature and simple preparation process of the Mask@40 %β-Al2O3 separator address cost concerns while presenting an encouraging new direction for applying room-temperature liquid NaK metal batteries in extreme conditions.

(Invited) Engineering Local Chemical Environments in Electrolytes and at Electrode-Electrolyte Interface for Efficient Aqueous Batteries

Electrolytes are an essential component of energy storage devices. Electrolyte composition has a significant impact on the environmental footprint, safety, price and performance of a battery. Aqueous electrolytes are non-flammable and can offer safer battery operation and lower toxicity. However, they suffer from a smaller electrochemical stability window compared to traditional organic electrolytes, which leads to lower energy density. To circumvent the small electrochemical stability window, highly concentrated “water-in-salt” (WIS) electrolytes, which demonstrate significantly wider stability window, were proposed and successfully applied to various aqueous energy storage systems including Li-ion and Zn metal batteries that are highly attractive for grid energy storage. However, since WIS electrolytes require large amounts of salt (often toxic) and have dramatically increased viscosity, it in turn limits their transport properties, charge-discharge rates, and usability in batteries.In this talk, the use of highly concentrated "water-in-salt" (WIS) electrolytes as a means of achieving increased charging efficiency and a wider stability window will be discussed in the context of aqueous Li-ion and Zn metal batteries. Further, we elaborate the role of cation solvation environment, electrolyte structure and hydrogen bonding on the physicochemical properties and electrochemical response of the system. We show that electrolyte composition affects the interfacial kinetics of Zn plating/stripping as well as the kinetics of the parasitic processes of electrolyte decomposition. Finally, we discuss how relatively dilute electrolytes can be engineered for efficient aqueous batteries. References Suo, O. Borodin, T. Gao, M. Olguin, J. Ho, X. Fan, C. Luo, C. Wang and K. Xu, Science, 2015, 350, 938Yamada, K. Usui, K. Sodeyama, S. Ko, Y. Tateyama and A. Yamada, Nat. Energy, 2016, 1, 16129R. Lukatskaya, J.I. Feldblyum, D.G. Mackanic, F. Lissel, D.L. Michels, Y. Cui, Z. Bao, Energy Environ. Sci., 2018, 11, 2876-2883 D.G. Vazquez, T. P. Pollard, J. Mars, J.M. Yoo, H.G. Steinrück, S. E. Bone, O. V. Safonova, M. F. Toney, O. Borodin, M.R. Lukatskaya, Energy Environ. Sci., 2023, DOI: 10.1039/D3EE00205E

Tight-Binding Modelling of Deep Eutectic Solvent Based Electrolytes

Deep eutectic solvents (DES) have in recent years gained momentum in the development of new electrolytes for lithium-ion batteries (LiBs)1. Due to their easy creation, tuning possibilities, wide electrochemical stability windows, and low vapor pressures, they are not only able to create safer LiB electrolytes as compared to traditional organic electrolytes, but more importantly, they may enable high voltage LIB cells2. One aspect not yet fully understood is the connection between the symmetry and entropy of the DES local structure and their macroscopic properties. Exploring this connection could assist in rational design of more performant electrolytes, including controlled electrochemical properties, by symmetry and entropy tuning.Herein, we have studied in detail a few simple DES electrolytes created using the hydrogen bond donor N-methyl-acetamide (MAc) combined with each of the three different lithium salts: LiBF4, LiDFOB, and LiBOB, in 1:4 molar ratios – as previously shown advantageous 3. We chose these salts as they exhibit pseudospherical (BF4-), asymmetrical linear (DFOB), and symmetric linear (BOB) anion geometries and thus are plausible to introduce local symmetry differences. Molecular dynamic simulations were run using DFTB+ 4 and the xTB method 5, and as a first level geometrical analysis we look at the (partial) coordination and solvation numbers of the lithium ions.

Azole derived deep eutectic electrolyte for high performance lithium metal-free full batteries

Electrolytes are considered the lifeblood of lithium batteries, facilitating ion transfer between the cathode and anode. However, the flammability, volatility and corrosiveness of organic electrolytes impede the sustainable development of lithium batteries. Deep eutectic electrolytes (DEEs) have emerged as a promising alternative, exhibiting high stability, non-flammability and wide electrochemical window. Herein, a series of DEEs with varying molar ratios of azoles and LiTFSI were facilely prepared through the interaction of Li⋯N. These DEEs demonstrate unique advantages, including non-flammability, high conductivity and a favourable Li+ transference number. In particular, the DEEs ImL-4:1 and PyL-4:1 (Imidazole:LiTFSI = 4:1 and Pyrazole:LiTFSI = 4:1, respectively) achieve high ionic conductivities of 4.5 × 10−3 and 5.8 × 10−3 S cm−1 at 80 °C, respectively. Notably, the electrochemical window of PyL-4:1 is stable, making it a suitable electrolyte for the lithium metal-free Li4Ti5O12/LiFePO4 full battery. This system exhibits an initial specific capacity of 147 mAh g−1 and maintains stable cycling performance over 200 cycles. This study offers valuable insights into the design of electrolytes, addressing the challenges posed by traditional organic electrolytes and paving the way for sustainable lithium battery development.

An Ionic Liquid Electrolyte Additive for High-Performance Lithium–Sulfur Batteries

With the development of mobile electronic devices, there are more and more requirements for high-energy storage equipment. Traditional lithium-ion batteries, like lithium–iron phosphate batteries, are limited by their theoretical specific capacities and might not meet the requirements for high energy density in the future. Lithium–sulfur batteries (LSBs) might be ideal next-generation energy storage devices because they have nearly 10 times the theoretical specific capacities of lithium-ion batteries. However, the severe capacity decay of LSBs limits their application, especially at high currents. In this study, an ionic liquid (IL) electrolyte additive, TDA+TFSI, was reported. When 5% of the TDA+TFSI additive was added to a traditional ether-based organic electrolyte, the cycling performance of the LSBs was significantly improved compared with that of the LSBs with the pure traditional organic electrolyte. At a rate of 0.5 C, the discharge specific capacity in the first cycle of the LSBs with the 5% TDA+TFSI electrolyte additive was 1167 mAh g−1; the residual specific capacities after 100 cycles and 300 cycles were 579 mAh g−1 and 523 mAh g−1, respectively; and the average capacity decay rate per cycle was only 0.18% in 300 cycles. Moreover, the electrolyte with the TDA+TFSI additive had more obvious advantages than the pure organic ether-based electrolyte at high charge and discharge currents of 1.0 C. The residual discharge specific capacities were 428 mAh g−1 after 100 cycles and 399 mAh g−1 after 250 cycles, which were 13% higher than those of the LSBs without the TDA+TFSI additive. At the same time, the Coulombic efficiencies of the LSBs using the TDA+TFSI electrolyte additive were more stable than those of the LSBs using the traditional organic ether-based electrolyte. The results showed that the LSBs with the TDA+TFSI electrolyte additive formed a denser and more uniform solid electrolyte interface (SEI) film during cycling, which improved the stability of the electrochemical reaction.

The Rational Design of Low‐Barrier Fluorinated Aluminum Substrates for Anode‑Free Sodium Metal Battery

AbstractAnode‐free Na metal batteries are acclaimed for their high energy densities achieved through current collectors in situ plated with Na metal in the absence of active negative materials. The advancement of these devices hinges on the development of affordable current collectors for effective Na deposition and the design of advanced electrolytes with suppressed Na metal loss as measured against the poor cycling performance and safety issues associated with traditional organic electrolytes and convectional current collectors. Herein, the authors report a novel strategy for fabricating an Al current collector through annealing and fluorination using hydrofluoric acid to optimize its crystal orientation and surface properties in a bid to establish highly reversible Na deposition/dissolution processes. Through a series of characterization, electrochemical, and computational analyses, it is ascertained that the F‐rich surface of the (100)‐oriented Al substrate provides high‐affinity nucleation sites that initiate and sustain a uniform Na metal deposition and form a propitious solid electrolyte interphase layer. The present anode‐free Na metal battery prototype comprised of the treated Al substrate, a high mass loading Na3V2(PO4)3 positive electrode, and an ionic liquid electrolyte achieves a high average Coulombic efficiency (≈98%) over 50 cycles: indeed, groundbreaking performance for uncoated current collectors that do not require activation cycles.

Practical level of low-N/P ratio sodium metal batteries: On the basis of deposition/dissolution efficiency in the aspects of electrolytes and temperature

Rechargeable sodium metal batteries have drawn immense interest as next-generation batteries, owing to their high theoretical energy density and the natural abundance of Na resources. However, Na metal batteries with excess amounts of Na metal which typically employ traditional organic electrolytes, are plagued by a myriad of electrochemical deficiencies that cause low Coulombic efficiencies, severe dendrite formation, thermal runaways, and fire hazards during operations. To establish a pathway for high energy density Na metal batteries, this work focuses on the practicability of the Na lean-metal battery and presents an in-depth perspective into the electrochemical mechanisms of utilizing a bis(fluorosulfonyl)amide ionic liquid electrolyte at both room- and intermediate temperatures. Here, the ionic liquid electrolyte is confirmed to yield a higher Na metal deposition/dissolution efficiency than common organic electrolytes. Electrochemical and computational investigations entailing the ionic liquid performance reveal that elevating the operating temperature to 90 °C increases the number of anions coordinated with Na+ and leads to the formation of a robust anion-based interfacial layer that facilitates effective and smooth Na metal deposition/dissolution. Furthermore, full cells comprising a Na metal negative with restricted loading mass and a Na3V2(PO4)3 positive electrode (low negative/positive electrode capacity ratio of 1.97, weight ratio of 0.19) demonstrates outstanding cycleability with high Coulombic efficiency (∼100%) and practical energy density (280 Wh kg–1).

An intrinsically non-flammable organic electrolyte for wide temperature range supercapacitors

Non-aqueous supercapacitors have received considerable attention owing to their durability, high energy and power densities. However, the traditional organic electrolytes used are a combination of ionic liquids (ILs) and highly flammable organic solvents, causing severe safety concerns due to the potential catastrophic fires or explosions of the electrolytes. Herein, we report a non-flammable triethyl phosphate (TEP)-based binary electrolyte. The interaction between TEP and 1-butyl-3-methylimidazolium tetrafluoroborate (EMIMBF4) was investigated by the Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), and Nuclear Magnetic Resonance (NMR). The result demonstrated that the hydrogen bonding in EMIMBF4 ion pairs were effectively broken/weakened by TEP, endowing improved ionic conductivity and reduced viscosity. By coupling carbon nanosheet electrodes, the symmetric supercapacitor fabricated with the binary electrolyte can operate well in a wide temperature range (from −20 °C to 80 °C), delivering a highest energy and power density of 52.1 Wh kg−1 and 19.7 kW kg−1, an outstanding capacitance retention of 86.5 % after 10,000 cycles and a self-discharge as low as 44 %. This work opens up new frontiers to develop safe and superior supercapacitors at wide temperature range.

Gel Polymer Electrolytes Design for Na-Ion Batteries.

Na-ion battery has the potential to be one of the best types of next-generation energy storage devices by virtue of their cost and sustainability advantages. With the demand for high safety, the replacement of traditional organic electrolytes with polymer electrolytes can avoid electrolyte leakage and thermal instability. Polymer electrolytes, however, suffer from low ionic conductivity and large interfacial impedance. Gel polymer electrolytes (GPEs) represent an excellent balance that combines the advantages of high ionic conductivity, low interfacial impedance, high thermal stability, and flexibility. This short review summarizes the recent progress on gel polymer Na-ion batteries, focusing on different preparation approaches and the resultant physical and electrochemical properties. Reasons for the differences in ionic conductivity, mechanical properties, interfacial properties, and thermal stability are discussed at the molecular level. This Review may offer a deep understanding of sodium-ion GPEs and may guide the design of intermolecular interactions for high-performance gel polymer Na-ion batteries.

Physical and Electrochemical Properties of Pyrrolidinium-Based Ionic Liquid and Methyl Propionate Co-Solvent Electrolyte

Advancements in modern technology has led to increasing demand for high-capacity batteries. Lithium metal batteries have high specific capacity and are a promising candidate for post lithium-ion batteries. Traditional organic electrolytes have poor compatibility with lithium metal batteries. Ionic liquids (IL) with the addition of co-solvents have shown promised in lithium metal battery systems. In this work, 1-butyl-1-methyl-pyrrolidinium bis(trifluoromethanesulfonyl)imide ([Pyr14][TFSI]) was selected for its large electrochemical window and methyl propionate (MP) for its low viscosity and melting point. The IL/MP mixture with 0.8m LiTFSI maintained a large electrochemical window (>5V), extended liquid range of the IL and noticeably improved conductivity (from 0.56 mS/cm for 0.8m LiTFSI in [Pyr14][TFSI] to 11 mS/cm for 0.8m LiTFSI in a mixture of 1 to 7 mole ratio of [Pyr14][TFSI] to MP at 25°C). Also, we have identified that the addition of cyclic carbonate is important to increase the columbic efficiency for lithium deposition and stripping.

A design of Nafion-coated bilayered quasi-solid electrolyte for lithium-O2 batteries with high performance

In this paper, a novel bilayer-structured quasi-solid polymer electrolyte (NSPE) has been designed with separate catholyte and anolyte, which compose of P(VDF-HFP)/Li-Nafion solid polymer electrolyte and TEMPO as cathodic additives. The SEM images shows that the surface morphology of NSPE is smoother and more uniform than that of SPE. The Li-O2 battery using NSPE showed high ionic conductivity, low interface impedance, good rate performance, and higher security and good cycling stability while reducing polarization. Lithium-air (also known as lithium-oxygen) batteries have attracted considerable global attention in recent years due to their extremely high energy density (11,140 W·h·kg −1 ). The electrolyte is a key element in lithium-air batteries and the traditional organic electrolyte has great safety risk due to leakage. On the contrary, the polymer electrolyte has the advantages of high safety, high stability and easy processing comparing with the organic liquid electrolytes. In this paper, a new idea was proposed to coat the Nafion membrane on a layer of polymer for blocking the oxidation reduction electric (RM) and Li based on the selective permeability on lithium ion of the Nafion membrane. Self-made thickness-controllable Nafion membrane, polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP) and 2,2,6,6-tetramethylpiperidinooxy (TEMPO) were used to prepare a quasi solid polymer electrolyte (NSPE). Electrochemical workstation and LAND battery testing system were used to perform a galvanostatic charge/discharge test on Li-O 2 . The ionic conductivity of NSPE was 4.3 × 10 −4 S·cm −1 at room temperature and the discharge platform was 2.6 V and the charging voltage was 3.7 V after 50 cycles with the cut-off capacity of 500 mA·h·g −1 .

Upgrading Traditional Organic Electrolytes toward Future Lithium Metal Batteries: A Hierarchical Nano-SiO2-Supported Gel Polymer Electrolyte

Developing safe and high-energy-density lithium metal batteries (LMBs) are considered as the focus of next-generation rechargeable batteries. However, the inevitable lithium reacting with the liqui...

High performance lithium-ion batteries with pillar[5]quinone/ion-liquid system

Abstract The capacity of cathode is a short slab issue of lithium-ion batteries (LIBs). Organic quinone cathode materials with high theoretical capacities could effectively solve the problem. Here, pillar[5]quinone (P5Q) with high theoretical capacity of 446 mAh g−1 is chosen as the cathode, while it is puzzled over fast dissolution rate in traditional organic electrolytes. Recently, ionic liquids have been reported the weak dissolution towards organic compounds. At the same time, their peculiarities of non-volatilization and non-combustibility could reduce the safety risk of using traditional organic electrolytes in LIBs. In this work, LIBs combined with LiPF6/[PY13][TFSI] (Lithium Hexafluorophosphate/N-methyl-N-propylpyrrolidiniumbis(trifluoromethanesulfonyl)amide) electrolyte and P5Q cathode show excellent electrochemical performance, principally reflected in stable cycling performance (maintain at 284 mAh g−1 at 0.2 C current density after 200 cycles with capacity retention of 70%) and high rate property (reach 220 mAh g−1 at 2 C). This strategy would further realize the practical application of organic LIBs.

Constructing High-Energy-Density Aqueous Supercapacitors with Potassium Iodide-Doped Electrolytes by a Precharging Method

Aqueous supercapacitors (SCs) are safe and environmentally friendly with a high power density while having low-voltage windows and a low energy density. To address these challenges, poly(vinyl alcohol) potassium borate (PVAPB) gel polymer electrolytes (GPEs), which have demonstrated good electrochemical stability and wettability with electrodes, were prepared by in situ electrodeposition and then doped with potassium iodide (KI). SCs based on commercial active carbon electrodes and KI-doped PVAPB GPEs could work stably at 1.6 V and achieve a specific capacitance of 164.7 F g–1 and an energy density of 14.38 Wh kg–1 at a current density of 1 A g–1, which are both significantly higher than for SCs without KI of 69.9 F g–1 and 4.38 Wh kg–1. Meanwhile, the SC prepared with KI-doped PVAPB GPEs has a high ionic conductivity of 1.7 mS cm–1 (more than 200% higher than for the traditional organic electrolyte) with a power density of 3.55 kW kg–1. Subsequently, we propose a precharging method to further boost the c...

Preformed Anodes for High-Voltage Lithium-Ion Battery Performance: Fluorinated Electrolytes, Crosstalk, and the Origins of Impedance Rise

Preformation of graphite electrodes, in a highly fluorinated electrolyte, show exemplary performance when incorporated into LiNi0.5Mn0.3Co0.2O2//graphite cells (NMC//Gr) containing a traditional organic electrolyte. NMC//Gr cells, using preformed graphite electrodes, showed enhanced capacity and power retention as well as improved coulombic efficiencies. The increased performance was only observed with the use of specific electrolytes during the preforming step, where graphite electrodes, when preformed with the baseline organic carbonate electrolyte, did not show the same benefits. The identity of the preforming electrolyte was also observed to influence electrode crosstalk, where compounds generated at one electrode can affect the opposite electrode. The work herein presents both physical and electrochemical evidence of electrode crosstalk and reveals the beneficial effect of the preforming procedure in limiting the associated degradation mechanisms thereof. The insights gained may lead to new methodologies for the design of electrochemically robust interfaces that can enable high-voltage, lithium-ion batteries.

Rate limiting activity of charge transfer during lithiation from ionic liquids

Abstract Given the increased use of room temperature ionic liquid electrolytes in Li-ion batteries, due to their non-flammability and negligible volatility, this study evaluates the lithiation kinetics to understand and improve the rate performance of Li-ion batteries. Lithium titanate spinel is used as a model electrode and the electrolyte is composed of LiTFSI and TFSI-coordinated alkoxy-modified phosphonium ionic liquid. Based on the analysis of activation energies for each process, we report that the charge-transfer reaction at the electrode/electrolyte interface is the rate-limiting step for cell operation. This finding is further supported by the observation that a 50-fold decrease in charge-transfer resistance at higher temperatures leads to a significant performance improvement over that of a traditional organic electrolyte at room temperature. Charge-transfer resistance and electrolyte wetting on the electrode surface are critical processes for optimal battery performance, and such processes need to be included when designing new ionic liquids in order to exceed the power density obtained with the use of current carbonate-based electrolytes.

Cathode Design for High Energy Molten Salt Lithium-Oxygen Batteries

State of the art commercial lithium ion batteries use cathodes such as lithium cobalt oxide which rely on insertion and removal of lithium ions from a host material. However, insertion cathode materials are limited in their capacity, and replacing them with a cathode that employs growth and dissolution of new phases could significantly increase a battery’s energy density. For example, oxygen and sulfur cathodes have been widely researched to this end, with both cases involving the growth of a lithium-rich compound on a current collector/catalyst support. While the lithium-oxygen battery is promising with regard to its energy density, there are many practical challenges that remain to be solved. For instance, traditional organic electrolytes decompose in the presence of superoxide anions, intermediates in the growth of the lithium peroxide discharge product. However, by replacing the organic electrolyte with a molten salt, we can inherently avoid this organic electrolyte decomposition. In addition, the use of a molten salt electrolyte results in a system operating at elevated temperature with a large concentration of lithium ions, encouraging faster diffusion and kinetics. The morphology of the lithium peroxide grown in this molten salt lithium-oxygen battery is notably different from that previously observed in literature. While previous works have observed thin films, platelets, and “toroids” on the order of several hundred nanometers, we observe much larger (several micron) structures which appear to be stacks of hexagonal layers. We believe these stacks to be a new morphology of lithium peroxide growth. These new lithium peroxide morphologies are characterized with SEM, EDS, and XRD. In addition to the lithium-oxygen system described above, we introduce another phase-forming chemistry, whereby a molten nitrate salt serves as both an active material and the electrolyte. Molten nitrate salts have been previously studied as an active material in a primary lithium battery where lithium oxide irreversibly forms as nitrate converts to nitrite. We will describe how the use of a nanoparticle heterogeneous catalyst allows the reversible growth and dissolution of large (several micron) lithium oxide crystals in this system, as substantiated by SEM, XRD, and TEM. After introducing these molten salt lithium batteries, we address the effect of cathode geometry on the discharge capacity. In particular, we note that the growth of such large, solid phase species on the surface of the catalyst support imposes new design restrictions when optimizing a cathode for energy density. For instance, it is not just the surface area of the catalyst support that determines the discharge capacity, but also the amount of usable pore volume to allow this solid phase discharge product to form. As a proof of concept, we design and implement an architected electrode with large pore volume and relatively small surface area. Such cathode design principles could be extended to other phase-forming chemistries.

Improvement in Electrochemical Properties of Ionic Liquid PYR14tfsi By Mixing with BMImBF4 As High Safety Electrolyte

As is known, the characteristics of electrolyte play a significant role in lithium-ion batteries in terms of high conductivity, good compatibility with electrode material and safety. Because of the volatility and flammability of the traditional organic electrolytes, the safety risk has been a big concern for the batteries, especially when it is used in hybrid electric vehicles (HEVs) and electric vehicles (EVs). On the other hand, the room temperature ionic liquids (RTILs) usually are low volatile, non-flammable liquids with wide electrochemical window that benefit to use as electrolytes in high-safety lithium battery. Since the chemical and physical properties are strongly related to combination of the cations and anions composed, unique ionic liquids may be designed for the particular application. Recently, the RTILs formed by imidazolium, pyrrolidinium, piperidinium cations and hexafluorophosphate (PF6 -), tetrafluoroborate (BF4 -), bis(trifluoromethanesulfonyl)imide (TFSI-) anions have been frequently used as the electrolytes for lithium-ion batteries. However, as reported, the organic solvents i.e. ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC) and diethyl carbonate (DEC), which are used for the traditional liquid electrolytes, have often been mixed with the RTIL for reducing the viscosity to increase the conductivity. Yet, the thermal stability can’t be improved essentially. In this study, a solvent-free ternary RTIL-based electrolyte system has been constructed by the RTILs, 1-butyl-3-methylimidazolium tetrafluoroborate (BMImBF4), 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR14TFSI) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). Three different electrolytes with various compositions are prepared and characterized by FTIR, thermogravimetric analysis, differential scanning calorimetry, linear sweep voltammetry, conductivity meter and rheometer. The results show that all three electrolytes are thermally stable up to about 350℃ and have wide electrochemical windows (~5V). Besides, the lowest viscosity and the highest conductivity obtained were 66.2 cP and 2.76 mS, respectively. The highest discharge capacity of the LiFePO4/electrolytes/Li half-cell obtained in the voltage range of 2.5–4.3 V at 0.2C and 55℃ is 158 mAh g-1. This study reveals that the RTIL-based ternary electrolyte is a promising electrolyte candidate for the high-safety lithium-ion battery. Acknowledgements The authors gratefully acknowledge the Ministry of Science and Technology, Taiwan, R. O. C. and Chung Yuan Christian University for supporting this research work.

Insights into the Performance Limits of the Li7P3S11 Superionic Conductor: A Combined First-Principles and Experimental Study.

The Li7P3S11 glass-ceramic is a promising superionic conductor electrolyte (SCE) with an extremely high Li(+) conductivity that exceeds that of even traditional organic electrolytes. In this work, we present a combined computational and experimental investigation of the material performance limitations in terms of its phase and electrochemical stability, and Li(+) conductivity. We find that Li7P3S11 is metastable at 0 K but becomes stable at above 630 K (∼360 °C) when vibrational entropy contributions are accounted for, in agreement with differential scanning calorimetry measurements. Both scanning electron microscopy and the calculated Wulff shape show that Li7P3S11 tends to form relatively isotropic crystals. In terms of electrochemical stability, first-principles calculations predict that, unlike the LiCoO2 cathode, the olivine LiFePO4 and spinel LiMn2O4 cathodes are likely to form stable passivation interfaces with the Li7P3S11 SCE. This finding underscores the importance of considering multicomponent integration in developing an all-solid-state architecture. To probe the fundamental limit of its bulk Li(+) conductivity, a comparison of conventional cold-press sintered versus spark-plasma sintering (SPS) Li7P3S11 was done in conjunction with ab initio molecular dynamics (AIMD) simulations. Though the measured diffusion activation barriers are in excellent agreement, the AIMD-predicted room-temperature Li(+) conductivity of 57 mS cm(-1) is much higher than the experimental values. The optimized SPS sample exhibits a room-temperature Li(+) conductivity of 11.6 mS cm(-1), significantly higher than that of the cold-pressed sample (1.3 mS cm(-1)) due to the reduction of grain boundary resistance by densification. We conclude that grain boundary conductivity is limiting the overall Li(+) conductivity in Li7P3S11, and further optimization of overall conductivities should be possible. Finally, we show that Li(+) motions in this material are highly collective, and the flexing of the P2S7 ditetrahedra facilitates fast Li(+) diffusion.

Electrochemical performance of Na/NaFePO4 sodium-ion batteries with ionic liquid electrolytes

Rechargeable Na/NaFePO4 cells with a sodium bis(trifluoromethanesulfonyl)imide (NaTFSI)-incorporated butylmethylpyrrolidinium (BMP)–TFSI ionic liquid (IL) electrolyte are demonstrated with an operation voltage of ∼3 V. High-performance NaFePO4 cathode powder with an olivine crystal structure is prepared by chemical delithiation of LiFePO4 powder followed by electrochemical sodiation of FePO4. This IL electrolyte shows high thermal stability (>400 °C) and non-flammability, and is thus ideal for high-safety applications. The effects of NaTFSI concentration (0.1–1.0 M) on cell performance at 25 °C and 50 °C are studied. At 50 °C, an optimal capacity of 125 mA h g−1 (at 0.05 C) is found for NaFePO4 in a 0.5 M NaTFSI-incorporated IL electrolyte; moreover, 65% of this capacity can be retained when the charge–discharge rate increases to 1 C. This ratio (reflecting the rate capability) is higher than that found in a traditional organic electrolyte. With a 1 M NaTFSI-incorporated IL electrolyte, a 13% cell capacity loss after 100 charge–discharge cycles is measured at 50 °C, compared to the 38% observed in an organic electrolyte under the same conditions.