Lithium Bis(trifluoromethylsulfonyl)imide Research Articles

A General Strategy toward Enhanced Electrochemical and Mechanical Performance of Solid‐State Lithium Batteries through Constructing Covalently Bonded Electrode Materials/Electrolyte Interfaces

AbstractSolid‐state lithium batteries (SSLBs) offer inherent safety and high energy density for next‐generation energy storage, but the large interfacial resistance and poor physical connection between electrode materials and the solid electrolyte (SE) severely impede their practical applications. This work reports a general strategy to introduce covalent bonds between electrode materials and SE, not only reducing interfacial resistance but also enhancing electrochemical stability and mechanical robustness of SSLBs. The covalent bonding is accomplished by functionalizing electrode surfaces with C═C groups, enabling in situ copolymerization with telechelic polymers in the SE. This approach is applicable to various cathode/anode materials and SEs. Particularly, the SSLBs comprising LiFePO4 cathode, Li6.75La3Zr1.75Ta0.25O12/(polyethylene oxide (PEO)/lithium bis(trifluoromethylsulfonyl)imide (LiTFSI)/silk composite SE and metallic lithium anode exhibit a specific capacity of 158.4 mAh g−1 and can be cycled at 2 C for 2200 times with >80% retention. Additionally, the SSLBs containing the high nickel‐content LiNi0.96Co0.03Mn0.01O2 cathode can afford a high specific capacity of 201.8 mAh g−1. Comprehensive experimental examination and theoretical simulations confirm that the lowered interfacial resistance and intimately contacted electrode materials/electrolyte interfaces facilitate Li+ transport at different stages of charge. Furthermore, the covalently bonded electrode/electrolyte interfaces also endow SSLBs with outstanding mechanical stability, enabling flexible SSLB pouch cells to operate under various bending states without performance decay.

Optimize Electrolytes and Additives for Lithium-Ion Batteries Using C/SiO2 Synthesized from Rice Husk As an Anode Material

Silicon-based anodes derived from agricultural waste, particularly rice husks, have recently been focused on replacing conventional graphite due to its abundance, low cost, fast charge ability, and remarkable theoretical capacity (~ 3600 mAh.g-1). SiO2 has several drawbacks, including low electrical conductivity and short cycling life associated with high-volume expansion (~ 300%) during the charge-discharge process. Hence, a hybrid composite C/SiO2 was an ideal solution to overcome these challenges.To strengthen the electrochemical properties of LIBs, the compatibility between electrodes and electrolytes plays a crucial role. Lithium hexafluorophosphate (LiPF6) and Lithium bis(trifluoromethylsulfonyl) imide (LiTFSI) are promising candidates for investigation due to their effectiveness cost. Regarding LiPF6, widely used in LIBs, offers high solubility and stability, while LiTFSI provides enhanced ionic mobility. However, LiTFSI is corrosive to aluminum current collectors, and LiPF6 unexpectedly dissociates to form HF acid and PF5, leading to cycling degradation. To mitigate these issues, additives such as Fluoroethylene-carbonate (FEC) and Vinylene Carbonate (VC) are introduced to enhance battery performance. This study employed FEC and VC, adding in the optimal electrolyte to stabilize the SEI layer, prevent lithium loss during cycling due to the instability of the interfacial layer, as well as to enhance high-rate performance. In detail, FEC could effectively prevent SEI degradation by enhancing the F-based component of this layer, stabilizing the SEI layers, and protecting electrodes. Simultaneously, VC offers a synergistic effect on the interphase modulation by reinforcing the inorganic/organic components. Thus, we investigate various electrolyte systems to evaluate half-cell performance of C/SiO2 synthesized from rice husk as the anode. Then, the optimal-chosen electrolytes were used to assemble full-cell LFP||C/SiO2 and investigated at high-rate performance.According to the experimental sections, there are two main lithium-salts comprised of LiPF6 and LiTFSI, which dissolved in three solvent systems, including EC: DEC (1: 1-v/v); EC: DMC (1:1-v/v), and EC: DMC: DEC (1:1:1 – v/v/v). After optimizing the solvents, the system of EC: DEC (1: 1-v/v) shows the highest performance, which was chosen to investigate the impact of various ratios of FEC and VC additive. The 1M LiPF6 in EC: DEC (1:1-v/v) + 5 wt% FEC has the highest specific capacity compared to the LiPF6 samples, reaching the value of 220.0 mAh.g-1. Meanwhile, the 1M LiTFSI in EC: DEC (1:1-v/v) 2.5 wt% FEC + 2.5 wt% VC is the best electrolyte to offer a stable capacity of 223.4. mAh.g-1 after 50 cycles in comparison to LiTFSI samples. This phenomenon is plausible with the conductivity of 6.16 and 7.45 mS.cm-1, respectively. Furthermore, they undergo remarkable oxidation at 4.05 and 4.23 V at a current density of 0.002 mA, demonstrating they have a good stability potential window. Since there is no significant difference in half-cell performance, they were used to evaluate full-cell properties. According to battery performance, the full-cell LFP||C/SiO2 using LiPF6 + 5 wt% FEC shows that the initial capacity reaches 111.6 mAh.g-1 and is maintained 40% after 50 cycles with a stable CE above 96%. Full cell LiTFSI +2.5 wt% FEC + 2.5 wt% VC shows an initial discharge capacity of 159.9 mAh.g-1. However, this capacity value is significantly reduced to only 28.5 mAh.g-1 after 50 cycles and CE ~ 94%. Regarding rate capability results, full cell 1M LiPF6 in EC: DEC (1:1-v/v) + 5 wt% FEC illustrates better performance in all discharged rates than full cells using LiTFSI salt. Moreover, this full-cell shows a discharge capacity of ~ 50 mAh.g-1 at high-rate performance while full-cell using LiTFSI-based electrolyte displays a negligible capacity value. To explain, the total resistance of 25 Ohm is obtained for full cell LiPF6 + 5 wt%. FEC is three times lower than full-cell 1M LiTFSI + 2.5 wt% FEC + 2.5wt% VC. Furthermore, the HR-TEM ex-situ indicates that the stability of SEI layer formation is consistent in thickness but exhibits uneven distribution when using different electrolytes. Besides, the SEI components analyzed by XPS data indicate the presence of carbonate groups and LixPFyOz-based compounds due to the irreversible reactions on the SEI layer after the initial discharge cycle. This research brings some remarkable information to study the electrolyte compatibility for full-cell performance using C/SiO2 as an anode material. Figure 1

Organic Ionic Plastic Crystal/Polymer Composite Electrolyte Prepared By Solution Casting Method for Solid-State Lithium Batteries

Solid-state electrolytes have been considered as a promising candidate to address the safety issues for next-generation lithium batteries. Organic ionic plastic crystals (OIPCs) are attracting increasing interests as solid electrolyte materials due to their unique advantages, such as plasticity, nonflammability, good thermal and electrochemical stability. In this study, an OIPC-based composite electrolyte consisting of the OIPC 1-ethyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (Pyr12TFSI), lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) and the polymer polyvinylidene fluoride (PVDF) has been developed by a facile solution casting strategy. A free-standing and flexible OIPC/polymer composite membrane was fabricated by the solution casting method, which not only provides flexibility and better electrode/electrolyte contact, but also is more compatible with current battery processing methods. The composite membrane used in Li/Li symmetric cell was cycled stably over 900 h at a current density of 0.1 mA cm−2 at 50 °C, demonstrating that the OIPC/polymer composite electrolyte enabled the reversible and stable lithium plating and stripping behaviors. Further tests of the OIPC/polymer composite as solid electrolyte in LiFePO4/Li cell presented a high specific capacity of 149 mAh g−1 at 0.1 C and a long cycle life of over 440 cycles with capacity retention of 89% at 0.5 C at 50 °C, which showed improved rate capability and cycling stability in comparison with the composites with similar compositions but obtained by powder pressing method. This study demonstrated the potential of the OIPC/polymer composite solid electrolyte prepared by solution casting method and will promote the development of high-performance OIPC-based composite electrolytes for solid-state batteries.

Conformation-assisted solid-solid phase transition of LiTFSI electrolyte salt and the lithium ion coordination

Single crystal growth and characterization of the lithium bis(trifluoromethyl sulfonyl)imide (LiTFSI), the most common electrolyte salt for lithium-ion batteries, have been performed and succeeded in unraveling the atomic structures of its different crystalline phases. The structures of two crystalline phases (phase I: orthorhombic, Pccn; phase II: monoclinic, P21/c) have been determined through temperature-dependent X-ray crystallography of the LiTFSI single crystal on heating, and the solid-solid phase transformation between phase I and phase II has been dictated. Interestingly, a conformational change of TFSI⁻ from transoid to cisoid has been discovered during the transition from phase I to phase II, which has been further confirmed by the temperature-dependent Raman spectroscopy. The coordination of Li⁺ with the TFSI⁻ ions of different conformations has been also elucidated in the polymorphic crystalline structures. The solid-solid phase transformation of the first-order leads to the cracking of the LiTFSI crystal, probably along the lithium-ion or the fluorine-rich layer in phase II. In the molten state, the coexistence of the transoid conformation and the cisoid conformation is found in the TFSI⁻ ions, affirming the recent observation in the concentrated non-crystalline state. This work is anticipated to shed light on the (de)solvation and the transport of lithium ions in complex fluids encompassing LiTFSI electrolyte solutions from the structural aspects.

Cation valency in water-in-salt electrolytes alters the short- and long-range structure of the electrical double layer

Highly concentrated aqueous electrolytes (termed water-in-salt electrolytes, WiSEs) at solid-liquid interfaces are ubiquitous in myriad applications including biological signaling, electrosynthesis, and energy storage. This interface, known as the electrical double layer (EDL), has a different structure in WiSEs than in dilute electrolytes. Here, we investigate how divalent salts [zinc bis(trifluoromethylsulfonyl)imide, Zn(TFSI)2], as well as mixtures of mono- and divalent salts [lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) mixed with Zn(TFSI)2], affect the short- and long-range structure of the EDL under confinement using a multimodal combination of scattering, spectroscopy, and surface forces measurements. Raman spectroscopy of bulk electrolytes suggests that the cation is closely associated with the anion regardless of valency. Wide-angle X-ray scattering reveals that all bulk electrolytes form ion clusters; however, the clusters are suppressed with increasing concentration of the divalent ion. To probe the EDL under confinement, we use a Surface Forces Apparatus and demonstrate that the thickness of the adsorbed layer of ions at the interface grows with increasing divalent ion concentration. Multiple interfacial layers form following this adlayer; their thicknesses appear dependent on anion size, rather than cation. Importantly, all electrolytes exhibit very long electrostatic decay lengths that are insensitive to valency. It is likely that in the WiSE regime, electrostatic screening is mediated by the formation of ion clusters rather than individual well-solvated ions. This work contributes to understanding the structure and charge-neutralization mechanism in this class of electrolytes and the interfacial behavior of mixed-electrolyte systems encountered in electrochemistry and biology.

Engineering the Hole Transport Layer with a Conductive Donor-Acceptor Covalent Organic Framework for Stable and Efficient Perovskite Solar Cells.

Spiro-OMeTAD doped with lithium-bis(trifluoromethylsulfonyl)-imide (Li-TFSI) and tertbutyl-pyridine (t-BP) is widely used as a hole transport layer (HTL) in n-i-p perovskite solar cells (PSCs). Spiro-OMeTAD based PSCs typically show poor stability owing to the agglomeration of Li-TFSI, the migration of lithium ions (Li+), and the existence of potential mobile defects originating from the perovskite layer. Thus, it is necessary to search for a strategy that suppresses the degradation of PSCs and overcomes the Shockley Queisser efficiency limit via harvesting excess energy from hot charge carrier. Herein, two covalent organic frameworks (COFs) including BPTA-TAPD-COF and a well-defined donor-acceptor COF (BPTA-TAPD-COF@TCNQ) were developed and incorporated into Spiro-OMeTAD HTL. BPTA-TAPD-COF and BPTA-TAPD-COF@TCNQ could act as multifunctional additives of Spiro-OMeTAD HTL, which improve the photovoltaic performance and stability of the PSC device by accelerating charge-carrier extraction, suppressing the Li+ migration and Li-TFSI agglomeration, and capturing mobile defects. Benefiting from the increased conductivity, the addition of BPTA-TAPD-COF@TCNQ in the device led to the highest power conversion efficiency of 24.68% with long-term stability in harsh conditions. This work provides an example of using COFs as additives of HTL to enable improvements of both efficiency and stability for PSCs.

Ion-Exchange Polymer Network Enhanced Interfacial Compatibility for Stable and Efficient Inverted Perovskite Solar Cells.

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as a low-cost and water-processable hole transport material has been widely used in various optoelectronic devices. Although the incorporation of anionic polyelectrolyte PSS in PEDOT contributes to superior water solubility, the trade-off between efficiency and stability remains a challenging issue, limiting its reliable application in perovskite solar cells (PSCs). Herein, we proposed an ion-exchange (IE) strategy to effectively control the doping degree, interfacial charge dynamics, and reliability of PEDOT:PSS in PSCs. This IE approach based on hard cation-soft anion rules enabled effective anion exchange between PEDOT:PSS and lithium bis(trifluoromethylsulfonyl)imide (LiTFSI), which favored enhancing the film conductivity, regulating the perovskite crystallization, and reducing the carrier losses at the interfaces. Consequently, a notable increase of the open-circuit voltage from 0.88 to 1.02 V was realized, resulting in a champion efficiency of 18.7% compared to the control (15.4%) in inverted PSCs. More encouragingly, this IE strategy significantly promoted the thermal and environmental stability of unsealed devices by maintaining over 80% of initial efficiencies after 2000 h. This work provides an effective way to regulate the doping state of the PEDOT-based hole transport material and guides the development of robust polymeric conducting materials for efficient perovskite photovoltaics.

Functionalization of Cathode–Electrolyte Interface with Ionic Liquids for High-Performance Quasi-Solid-State Lithium–Sulfur Batteries: A Low-Sulfur Loading Study

We introduce a quasi-solid-state electrolyte lithium-sulfur (Li–S) battery (QSSEB) based on a novel Li-argyrodite solid-state electrolyte (SSE), Super P–Sulfur cathode, and Li-anode. The cathode was prepared using a water-based carboxymethyl cellulose (CMC) solution and styrene butadiene rubber (SBR) as the binder while Li6PS5F0.5Cl0.5 SSE was synthesized using a solvent-based process, via the introduction of LiF into the argyrodite crystal structure, which enhances both the ionic conductivity and interface-stabilizing properties of the SSE. Ionic liquids (IL) were prepared using lithium bis(trifluoromethyl sulfonyl)imide (LiTFSI) as the salt, with pre-mixed pyrrolidinium bis(trifluoromethyl sulfonyl)imide (PYR) as solvent and 1,3-dioxolane (DOL) as diluent, and they were used to wet the SSE–electrode interfaces. The effect of IL dilution, the co-solvent amount, the LiTFSI concentration, the C rate at which the batteries are tested and the effect of the introduction of SSE in the cathode, were systematically studied and optimized to develop a QSSEB with higher capacity retention and cyclability. Interfacial reactions occurring at the cathode–SSE interface during cycling were also investigated using electrochemical impedance spectroscopy, cyclic voltammetry, and X-ray photoelectron spectroscopy supported by ab initio molecular dynamics simulations. This work offers a new insight into the intimate interfacial contacts between the SSE and carbon–sulfur cathodes, which are critical for improving the electrochemical performance of quasi-solid-state lithium–sulfur batteries.

High toughness, ultra-sensitive, and extreme environment resistant fabric temperature sensors based on supramolecular-polymer dual network ionic conductive elastomers

Liquid-free ionic conductive elastomers (ICEs) show great potential for next-gen wearable electronics, free from issues like solvent evaporation and freezing. However, crafting ICEs with both strength and temperature sensitivity remains a significant challenge. In this study, a supramolecular gel based on 1,3:2,4-bis (3,4-dimethylbenzene methylene) sorbitol (DMBS) is selected as the first network, and poly(ethylene glycol) methyl ether methacrylate (PEGMA) copolymerized with diallyl dimethyl bis (trifluoromethyl sulfonyl)imide (DADMATFSI) is added as the second network, with the addition of lithium bis(trifluoromethyl sulfonyl)imide (LiTFSI). The supramolecular-polymer dual network ion-conductive elastomer SP-DN-LiTFSI is created. The elastomer exhibits exceptional mechanical characteristics, including a maximum tensile stress of 1591.1 kPa and an impressive strain of 2361 %. Additionally, it demonstrates an extraordinarily high toughness of 33.121 MJ m−3. Moreover, this elastomer-based temperature sensor is responsive to external stimuli, such as blowing, and boasts an outstanding temperature sensing coefficient (S) of 0.114 °C−1 within the range of 30–60 °C. Remarkably, it can discern even the slightest temperature variations as low as 0.5 °C within the typical human body temperature range. This work represents an innovative integration of a dual network structure and two strategies for elastomer fabrication. It provides a novel approach for developing highly sensitive wearable temperature measurement devices that exhibit exceptional resistance to mechanical wear.

Correlation between lithium-ion diffusion and coordination environment in solid polymer electrolytes: a molecular dynamics study

Lithium-ion diffusion in solid polymer electrolytes (SPEs) is a pivotal characteristic that significantly influences overall lithium-ion battery performance. This characteristic can be affected by the coordination environment of lithium ions within the polymer matrix. However, the correlation between lithium-ion diffusion and its coordination environment in biopolymer-based SPEs such as carboxymethyl chitosan (CMCS) remains understudied. In this study, we used molecular dynamics (MD) simulations to investigate this correlation. Lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) was used as the lithium salt in the simulated systems. All MD simulations were conducted using the GROMACS package with the general AMBER force field (GAFF). The coordination structures around Li+ were successfully estimated using the radial distribution function obtained from the MD simulations. These results indicate a preference for Li+ coordination with oxygen atoms, both from the CMCS polymer chains (OCMCS) and TFSI− ions (OTFSI-). The coordination number between Li+ and OCMCS decreases as the concentration of LiTFSI increases. The diffusion coefficients of Li+ varied depending on the concentration of LiTFSI and demonstrated a sensitivity to the coordination structure of Li+. A high diffusion coefficient of Li+ ions was observed at low LiTFSI concentrations, where Li+ was primarily coordinated with oxygen atoms from the CMCS polymer chains.

Ab Initio Molecular Dynamics Simulations of Aqueous LiTFSI Solutions─Structure, Hydrogen Bonding, and IR Spectra.

Simulations of Density Functional Theory-based ab initio molecular dynamics (AIMD) have been performed for a series of aqueous lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) solutions with concentrations ranging from salt-in-water to water-in-salt systems. Analysis of the structure of electrolytes has revealed a preference of Li+ cations to interact with water molecules. In concentrated LiTFSI solutions, water molecules form small associates. The total number of hydrogen bonds (HBs) in the system decreases with salt concentrations, with bonds to water acceptors being only partially replaced by interactions with TFSI anions. Infrared (IR) spectra in the region of the O-H stretching frequency calculated from AIMD trajectories are in good agreement with experimental data. Statistics of oscillations of individual O-H bonds have shown correlations between vibrational frequencies and the structure of HBs formed by water. The changes in the IR spectrum have been related to the varying contributions of different local environments of the water molecules. The abundances of the three spectral components calculated from the simulations agree well with the decomposition of the experimental IR spectra reported in the literature.

Investigating the Molecular Orientation and Thermal Stability of Spiro-OMeTAD and Its Dopants By Near Edge X-Ray Absorption Fine Structure

This work describes the utilization of near edge X-ray absorption fine structure (NEXAFS) to investigate the hole transporting material (HTM) 2,2ʹ,7,7ʹ-tetrakis(N,N-di-p-methoxyphenylamine)- 9,9ʹ-spirobifluorene (Spiro-OMeTAD) and its most common dopants, lithium bis-(trifluoromethylsulfonyl) imide (LiTFSI), 4-tert-butylpiridine (tBP) and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ). Utilizing angle resolved NEXAFS, the orientation of the molecules on the surface can be observed. The data suggests it is difficult to determine any orientational preference for Spiro-OMeTAD deposited on a surface due to the 3D propeller-like geometry of this molecule. Both doped and undoped samples show thermal stability beyond the glass transition temperature of the molecules. Significant changes to the Spiro-OMeTAD spectra were observed with the addition of the dopants, in particular the C K-edge. Differences are also observed in the valence band spectra when dopants are added. It is also demonstrated how the doping combination of LiFTSI with tBP and, F4-TCNQ act as p-type dopants by altering the position of the HOMO levels. The F4-TCNQ induces a larger change in the HOMO levels when compared to the LiTFSI and tBP. These results are important to increase our understanding of Spiro-OMeTAD and the effect dopants have on this material for next generation solar cells.

Nanostructure of Locally Concentrated Ionic Liquids in the Bulk and at Graphite and Gold Electrodes.

The physical properties of ionic liquids (ILs) have led to intense research interest, but for many applications, high viscosity is problematic. Mixing the IL with a diluent that lowers viscosity offers a solution if the favorable IL physical properties are not compromised. Here we show that mixing an IL or IL electrolyte (ILE, an IL with dissolved metal ions) with a nonsolvating fluorous diluent produces a low viscosity mixture in which the local ion arrangements, and therefore key physical properties, are retained or enhanced. The locally concentrated ionic liquids (LCILs) examined are 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (HMIM TFSI), 1-hexyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate (HMIM FAP), or 1-butyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate (BMIM FAP) mixed with 1,1,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether (TFTFE) at 2:1, 1:1, and 1:2 (w/w) IL:TFTFE, as well as the locally concentrated ILEs (LCILEs) formed from 2:1 (w/w) HMIM TFSI-TFTFE with 0.25, 0.5, and 0.75 m lithium bis(trifluoromethylsulfonyl)imide (LiTFSI). Rheology and conductivity measurements reveal that the added TFTFE significantly reduces viscosity and increases ionic conductivity, and cyclic voltammetry (CV) reveals minimal reductions in electrochemical windows on gold and carbon electrodes. This is explained by the small- and wide-angle X-ray scattering (S/WAXS) and atomic force microscopy (AFM) data, which show that the local ion nanostructures are largely retained in LCILs and LCILEs in bulk and at gold and graphite electrodes for all potentials investigated.

Lithium Self-Diffusion in a Polymer Electrolyte for Solid-State Batteries: ToF-SIMS/ssNMR Correlative Characterization and Modeling Based on Lithium Isotopic Labeling.

Manufacturers aim to commercialize efficient and safe batteries by finding new strategies. Solid-state electrolytes can be seen as an opportunity to develop batteries with a high energy density. They allow the use of lithium foil as the anode, increasing the energy density. Also, they are composed of nonflammable materials making them safer than liquid electrolytes. However, to enhance the electrochemical performances of forthcoming solid-state lithium metal batteries, phenomena governing ionic conductivity have yet to be mastered in such devices. Lithium isotopic tracing was successfully used in previous works to further understand lithium ion transport mechanisms in batteries. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) and 6/7Li high-resolution solid-state nuclear magnetic resonance (ssNMR) spectroscopy are two complementary techniques probing local and global scale, respectively. Both techniques can distinguish lithium isotopes. Here, four polymer membranes were elaborated with the same lithium concentration, but with various isotopic enrichments from 7.6 to 95.4% of 6Li. The selected material was a poly(ethylene oxide) (PEO) membrane containing lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) as lithium salt. They are widely studied in the lithium battery field. First, reliable ToF-SIMS and ssNMR methodologies were validated in light of the converging results. They led to accurate determination of lithium isotopic abundance of polymer membranes with a 1 or a 2% uncertainty, respectively. Then, the developed methodologies were applied to characterize lithium self-diffusion in a polymer membrane. Furthermore, numerical simulations based on a two-dimensional diffusion model compared with ToF-SIMS analyses allowed us to extract a lithium self-diffusion coefficient of 1.6 × 10-12 m2·s-1 at 60 °C, which complements other published values. The robust methodologies described in this work can be extended to various applications and materials. They stand as powerful strategies to better understand lithium ionic transport, especially in multiphase materials, for example, in hybrid solid-state electrolytes.

Lithium nitrate mediated dynamic formation of solid electrolyte interphase revealed by in situ Fourier transform infrared spectroscopy

A stable solid electrolyte interphase (SEI) plays a vital role in the cyclic stability and Coulombic efficiency (CE) of high-performance lithium-sulfur (Li-S) batteries. It is recognized that the LiNO3 additive can stabilize the SEI of the lithium electrode. However, the exact mechanism of the LiNO3 additive on the SEI of the lithium electrode remains unclear. In this work, we first revealed the mediation mechanism of LiNO3 additive on the dynamic evolution of the SEI on a lithium anode surface through in situ Fourier transform infrared (FTIR) spectroscopy and ab initio molecular dynamics (AIMD) methods. The FTIR and AIMD results directly proved that LiNO3 can accelerate the reduction of lithium bis(trifluoromethyl sulfonyl)imide (LiTFSI) into small molecules rich in Li2O on lithium anode, thus forming a compact and stable SEI after immersing in the LiNO3-containing electrolyte. Moreover, the decomposition of LiTFSI and the solvent is hindered in the subsequent lithium deposition stripping process due to the stable SEI, thus leading to higher Coulombic efficiency and long-term cyclic stability. In addition, an ROSO2Li-like intermediate is also observed during the lithium deposition process while decomposing or diffusing away during the lithium stripping process, maintaining a dynamic formation/dissolution equilibrium of the SEI. This research provides a new insight into understanding the role of LiNO3 in stabilizing lithium electrode.

Enhanced LiMn2O4 Thin-Film Electrode Stability in Ionic Liquid Electrolyte: A Pathway to Suppress Mn Dissolution.

Spinel-type lithium manganese oxide (LiMn2O4) cathodes suffer from severe manganese dissolution in the electrolyte, compromising the cyclic stability of LMO-based Li-ion batteries (LIBs). In addition to causing structural and morphological deterioration to the cathode, dissolved Mn ions can migrate through the electrolyte to deposit on the anode, accelerating capacity fade. Here, we examine single-crystal epitaxial LiMn2O4 (111) thin-films using synchrotron in situ X-ray diffraction and reflectivity to study the structural and interfacial evolution during cycling. Cyclic voltammetry is performed in a wide range (2.5-4.3 V vs Li/Li+) to promote Mn3+ formation, which enhances dissolution, for two different electrolyte systems: an imidazolium ionic liquid containing lithium bis-(trifluoromethylsulfonyl)imide (LiTFSI) and a conventional carbonate liquid electrolyte containing lithium hexafluorophosphate (LiPF6). We find exceptional stability in this voltage range for the ionic liquid electrolyte compared to the conventional electrolyte, which is attributed to the absence of Mn dissolution in the ionic liquid. X-ray reflectivity shows a negligible loss of cathode material for the films cycled in the ionic liquid electrolyte, further confirmed by inductively coupled plasma mass spectrometry and transmission electron microscopy. Conversely, a substantial loss of Mn is found when the film is cycled in the conventional electrolyte. These findings show the significant advantages of ionic liquids in suppressing Mn dissolution in LiMn2O4 LIB cathodes.

Achieving Practical High-Energy-Density Lithium-Metal Batteries by a Dual-Anion Regulated Electrolyte.

Lithium-metal batteries (LMBs) using lithium-metal anodes and high-voltage cathodes have been deemed as one of the most promising high-energy-density battery technology. However, its practical application is largely hindered by the notorious dendrite growth of lithium-metal anodes, the fast structure degradation of the cathode, and insufficient electrode-electrolyte interphase kinetics. Here, a dual-anion regulated electrolyte is developed for LMBs using lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) and lithium difluoro(bisoxalato)phosphate (LiDFBOP) as anion regulators. The incorporation of TFSI- in the solvation sheath reduces the desolvation energy of Li+ , and DFBOP- promotes the formation of highly ion-conductive and sustainable inorganic-rich interphases on the electrodes. Significantly enhanced performance is demonstrated on Li||LiNi0.83 Co0.11 Mn0.06 O2 pouch cells, with 84.6% capacity retention after 150 cycles in 6.0 Ah pouch cells and an ultrahigh rate capability up to 5 C in 2.0 Ah pouch cells. Furthermore, a pouch cell with an ultralarge capacity of 39.0 Ah is fabricated and achieves an ultrahigh energy density of 521.3Wh kg-1 . The findings provide a facile electrolyte design strategy for promoting the practical utilization of high-energy-density LMBs.

Investigating the Molecular Orientation and Thermal Stability of Spiro‐OMeTAD and its Dopants by Near Edge X‐Ray Absorption Fine Structure

AbstractThis study describes the utilization of near edge X‐ray absorption fine structure (NEXAFS) to investigate the hole transporting material (HTM) 2,2ʹ,7,7ʹ‐tetrakis(N,N‐di‐p‐methoxyphenylamine)‐ 9,9ʹ‐spirobifluorene (Spiro‐OMeTAD) and its most common dopants, lithium bis‐(trifluoromethylsulfonyl) imide (LiTFSI), 4‐tert‐butylpiridine (tBP), and 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4‐TCNQ). By changing the angle of the sample with respect to the beam, the orientation of the molecules on the surface can be observed. The data suggest that it is difficult to determine any orientational preference for Spiro‐OMeTAD deposited on a surface due to the 3D propeller‐like geometry of this molecule. Both doped and undoped samples show thermal stability beyond the glass transition temperature of the molecules. Significant changes to the Spiro‐OMeTAD spectra are observed with the addition of the dopants, in particular the C K‐edge. Differences are also observed in the valence band spectra when dopants are added. It is also demonstrated how the doping combination of LiFTSI with tBP and, F4‐TCNQ act as p‐type dopants by altering the position of the HOMO levels. The F4‐TCNQ induces a larger change in the HOMO levels when compared to the LiTFSI and tBP. These results are important to increase the understanding of Spiro‐OMeTAD and the effect dopants have on this material for next generation solar cells.

Highly conductive polyacrylonitrile-based hybrid aqueous/ionic liquid solid polymer electrolytes with tunable passivation for Li-ion batteries

The rapid growth in demand for lithium-ion batteries that can deliver more energy and power has generated concerns over safety. Aqueous electrolytes are a strong candidate to alleviate this apprehension, however their ability to overcome the “cathodic challenge” is limited due to anion-dominated passivation at the anode. In this work, the recently developed “hybrid aqueous/nonaqueous” electrolyte (HANE) strategy was employed to tune the degree of passivation at the anode in solid polymer electrolytes (SPEs) by using various ionic liquids as the nonaqueous component. Whereas common HANE systems sacrifice ionic conductivity to create a more robust passivation layer at the cathodic limit, the “hybrid aqueous/ionic liquid” SPEs (HAILSPEs) investigated in this work do not. Two HAILSPE systems (H1, H2.5) were fabricated from a blend of polyacrylonitrile (PAN), water, lithium bis(trifluoromethylsulfonyl)imide (LiTFSI), and either triethylsulfonium-TFSI (S2,2,2) or N-methyl-N-propylpyrrolidinium-TFSI (Pyr1,3). These HAILSPE systems demonstrated a remarkable improvement in transport properties compared to their predecessors, achieving room temperature ionic conductivities of up to 5.39 mS/cm. A reduction in apparent activation energy and nearly complete decoupling of ionic transport from polymer chain mobility were found to contribute to this increase. Stable and complete growth of a passivating layer at 2 V vs. Li/Li+ was also observed, which was tuned by changing the ionic liquid. The work presented here provides a potential route for overcoming the “cathodic challenge” in aqueous SPEs.

Ionogel Electrolytes Supported by Zwitterionic Copolymers Featuring Lithium Ion-Mediated, Noncovalent Cross-Links

A series of ionogel electrolytes, composed of an ionic liquid/lithium salt solution supported by a linear zwitterionic (ZI) copolymer scaffold containing solely noncovalent cross-links, were created in order to study the effects of varying the ZI fraction within the copolymer as well as the lithium ion concentration on gel mechanical properties and ion transport. The copolymer scaffold was synthesized via in situ UV photopolymerization of the ZI monomer 2-methacryloyloxyethyl phosphorylcholine (MPC) together with the non-ZI comonomer 2,2,2-trifluoroethyl methacrylate (TFEMA) in varying ratios, using a fixed fraction of 25 mol % total monomers. Ionic liquid electrolyte (ILE) solutions of 0.1, 0.3, 0.5, or 1 M lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) dissolved in the ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMP TFSI) were employed. For ionogels created using the 1 M LiTFSI ILE, varying the MPC content from 6.3 mol % to 17 mol % in the precursor solution resulted in a substantial increase in gel elastic modulus, from 20 kPa to 11 MPa. In contrast, room temperature ionic conductivity values for the same samples remained nearly constant, varying from 0.57–0.70 mS cm–1. While the in situ photopolymerization approach remains highly versatile for ionogel formation, an alternative method is introduced here for the first time that can produce ionogels “on-demand” using a pre-made ZI copolymer/ionic liquid solution to which a Li+ ILE is subsequently added.