Cationic polymer-in-salt electrolytes for fast metal ion conduction and solid-state battery applications

Cationic polymer-in-salt electrolytes for fast metal ion conduction and solid-state battery application

Molecular Dynamics Simulation and Pulsed-Field Gradient NMR Studies of Bis(fluorosulfonyl)imide (FSI) and Bis[(trifluoromethyl)sulfonyl]imide (TFSI)-Based Ionic Liquids

The pulsed-field-gradient spin−echo NMR measurements have been performed on 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide ([emim][FSI]) and 1-ethyl-3-methylimidazolium [bis[(trifluoromethyl)sulfonyl]imide] ([emim][TFSI]) over a wide temperature range from 233 to 400 K. Molecular dynamics (MD) simulations have been performed on [emim][FSI], [emim][TFSI], [N-methyl-N-propylpyrrolidinium][FSI] ([pyr13][FSI]), and [pyr13][TFSI] utilizing a many-body polarizable force field. An excellent agreement between the ion self-diffusion coefficients from MD simulations and pfg-NMR experiments has been observed for [emim][FSI] and [emim][TFSI] ILs. The structure factor of [pyr13][FSI], [pyr14][TFSI], and [emim][TFSI] agreed well with the previously reported X-ray diffraction data performed by Umebayashi group. Ion packing in the liquid state is compared with packing in the corresponding ionic crystal. Faster transport found in the FSI-based ILs compared to that in TFSI-based ILs is associated with the smaller size of FSI− anion and lower cation−anion binding energies. A significant artificial increase of the barriers (by 3 kcal/mol) for the FSI− anion conformational transitions did not result in slowing down of ion transport, indicating that the ion dynamics is insensitive to the FSI− anion torsional energetic, while the same increase of the TFSI− anion barriers in [emim][TFSI] and [pyr13][TFSI] ILs resulted in slowing down of the cation and anion transport by 40−50%. Details of ion rotational and translational motion, coupling of the rotational and translational relaxation are also discussed

Lithium Bis(fluorosulfonyl)imide for Stabilized Interphases on Conjugated Dicarboxylate Electrode

Carbonyl-based negative electrodes have received considerable interest in the domain of rechargeable lithium batteries, owing to their superior feasibility in structural design, enhanced energy density, and good environmental sustainability. Among which, lithium terephthalate (LiTPA) has been intensively investigated as a negative electrode material in the past years, in light of its relatively stable discharge plateau at low potentials (ca. 1.0 V vs Li/Li+) and high specific capacity (ca. 290 mAh g–1). However, its cell performances are severely limited owing to the poor quality of the solid-electrolyte-interphase (SEI) layer generated therein. Here, we report the utilization of lithium bis(fluorosulfonyl)imide (LiFSI) as an electrolyte salt for forming a Li-ion permeable SEI layer on the LiTPA electrode and subsequently improving the cyclability and rate performance of the LiTPA-based cells. Our results show that, differing from the reference electrolyte containing the lithium hexafluorophosphate (LiPF6) salt, the electrochemical reductions of the FSI– anions occur prior to the lithiation processes of LiTPA electrode, which is capable of building an inorganic-rich SEI layer containing lithium fluoride (LiF) and lithium sulfate (Li2SO4). Consequently, the lithium metal (Li°)||LiTPA cell shows significantly improved cycling performance than the LiPF6-based reference cell. This work provides useful insight into the reductive processes of the FSI– anions on negative electrodes, which could spur the deployment of highly sustainable and high-energy rechargeable lithium batteries

Lithium-pyrazole-3,4,5-tricarbonitrile: Ion Pairing and Lithium Ion Affinity Studies

Negative charge stabilized by ring delocalization on five-membered rings is a practical and theoretically interesting alternative to conventional fluorine-based anions. Coordination of the lithium cation to the pyrazole-3,4,5-tricarbonitrile (PATC) anion was studied using vibrational spectroscopy (Raman and IR) and ab initio SCF-MO Hartree−Fock (HF) calculations. Four 1:1 ion pair geometries were found, one being energetically more stable. By comparing theoretical spectra with both IR and Raman spectra of salt solutions, it was found that the lithium ion favors bidentate coordination to the ring nitrogen atoms, as suggested by the binding energies. Finally, comparisons were made with previously calculated coordination strengths for other similar lithium anion 1:1 systems

PEDOT Radical Polymer with Synergetic Redox and Electrical Properties

The development of new redox polymers is being boosted by the increasing interest in the area of energy and health. The development of new polymers is needed to further advance new applications or improve the performance of actual devices such as batteries, supercapacitors, or drug delivery systems. Here we show the synthesis and characterization of a new polymer which combines the present most successful conjugated polymer backbone and the most successful redox active side group, i.e., poly­(3,4-ethylenedioxythiophene) (PEDOT), and a nitroxide stable radical. First, a derivative of the 3,4-ethylenedioxythiophene (EDOT) molecule with side nitroxide stable radical group (TEMPO) was synthesized. The electrochemical polymerization of the PEDOT-TEMPO monomer was investigated in detail using cyclic voltammetry, potential step, and constant current methods. Monomer and polymer were characterized by NMR, FTIR, matrix-assisted laser desorption ionization time of flight mass spectrometry (MALDI-TOF MS), electron spin resonance (ESR) spectroscopy, elemental analysis, cyclic voltammetry, and four-point probe conductivity. The new PEDOT-TEMPO radical polymer combines the electronic conductivity of the conjugated polythiophene backbone and redox properties of the nitroxide group. As an example of application, this redox active polymer was used as a conductive binder in lithium ion batteries. Good cycling stability with high Coulombic efficiency and increased cyclability at different rates were obtained using this polymer as a replacement of two ingredients: conductive carbon additive and polymeric binders

Cost-Effective Synthesis of <i>Triphylite</i>-NaFePO₄ Cathode: A Zero-Waste Process

Triphylite-NaFePO4 attracts considerable attention as a cathode material for sodium-ion batteries due to its theoretical capacity (154 mAh/g), sharing also the excellent properties of the analogous triphylite-LiFePO4 used in commercial lithium-ion batteries. In this work, triphylite-NaFePO4 is synthesized from triphylite-LiFePO4 by a low-cost, eco-friendly method, enabling the recovery and subsequent reuse of lithium. NaFePO4 was evaluated as a cathode material in half-cells, exhibiting an initial discharge capacity of 132 mAh/g and good capacity retention (115 mAh/g and ∼100% of Coulombic efficiency after 50 cycles; 101 mAh/g and ∼100% of Coulombic efficiency after 200 cycles). This research confirms that the triphylite-NaFePO4 cathode material is an attractive candidate for sodium-ion batteries, with potential for future commercialization

Structural, Transport, and Electrochemical Investigation of Novel AMSO₄F (A = Na, Li; M = Fe, Co, Ni, Mn) Metal Fluorosulphates Prepared Using Low Temperature Synthesis Routes

We have recently reported a promising 3.6 V metal fluorosulphate (LiFeSO4F) electrode, capable of high capacity, rate capability, and cycling stability. In the current work, we extend the fluorosulphate chemistry from lithium to sodium-based systems. In this venture, we have reported the synthesis and crystal structure of NaMSO4F candidates for the first time. As opposed to the triclinic-based LiMSO4F phases, the NaMSO4F phases adopt a monoclinic structure. We further report the degree and possibility of forming Na(Fe1−xMx)SO4F and (Na1−xLix)MSO4F (M = Fe, Co, Ni) solid-solution phases for the first time. Relying on the underlying topochemical reaction, we have successfully synthesized the NaMSO4F, Na(Fe1−xMx)SO4F, and (Na1−xLix)MSO4F products at a low temperature of 300 °C using both ionothermal and solid-state syntheses. The crystal structure, thermal stability, ionic conductivity, and reactivity of these new phases toward Li and Na have been investigated. Among them, NaFeSO4F is the only one to present some redox activity (Fe2+/Fe3+) toward Li at 3.6 V. Additionally, this phase shows a pressed-pellet ionic conductivity of 10−7 S·cm−1. These findings further illustrate the richness of the fluorosulphate crystal chemistry, which has just been recently unveiled

Diffusion Coefficients from ¹³C PGSE NMR MeasurementsFluorine-Free Ionic Liquids with the DCTA^– Anion

Pulsed-field gradient spin–echo (PGSE) NMR is a widely used method for the determination of molecular and ionic self-diffusion coefficients. The analysis has thus far been limited largely to ¹H, ⁷Li, ¹⁹F, and ³¹P nuclei. This limitation handicaps the analysis of materials without these nuclei or for which these nuclei are insufficient for complete characterization. This is demonstrated with a class of ionic liquids (or ILs) based on the nonfluorinated anion 4,5-dicarbonitrile-1,2,3-triazole (DCTA^–). It is demonstrated here that ¹³C-PGSE NMR can be used to both verify the diffusion coefficients obtained from other nuclei, as well as characterize materials that lack commonly scrutinized nuclei  all without the need for specialized NMR methods

Self-Healing Janus Interfaces for High-Performance LAGP-Based Lithium Metal Batteries

The application of NASICON-type Li1.5Al0.5Ge1.5P3O12 (LAGP) solid electrolyte in lithium (Li) metal batteries has been retarded by its instability toward metallic Li and the poor interfacial compatibility with cathodes. Here we report a durable LAGP-based Li metal battery by employing self-healing polymer electrolytes (SHEs) as Janus interfaces. The SHEs were constructed on both sides of LAGP pellets by in situ polymerizing a functional monomer and a cross-linker in ionic liquid-based (anodic side in contact with Li metal) or adiponitrile (AN)-based (cathodic side) electrolytes. The as-developed SHEs show flame-retardant, high ionic conductivity (>10–3 S cm–1 at 25 °C), excellent interfacial compatibility with electrodes, and effective inhibition of Li dendrite formation. The LAGP-based Li metal||LiMn2O4 batteries with the SHE interfaces deliver a high reversible capacity with a long cycle

Novel Na⁺ Ion Diffusion Mechanism in Mixed Organic–Inorganic Ionic Liquid Electrolyte Leading to High Na⁺ Transference Number and Stable, High Rate Electrochemical Cycling of Sodium Cells.

Ambient temperature sodium batteries hold the promise of a new generation of high energy density, low-cost energy storage technologies. Particularly challenging in sodium electrochemistry is achieving high stability at high charge/discharge rates. We report here mixtures of inorganic/organic cation fluorosulfonamide (FSI) ionic liquids that exhibit unexpectedly high Na⁺ transference numbers due to a structural diffusion mechanism not previously observed in this type of electrolyte. The electrolyte can therefore support high current density cycling of sodium. We investigate the effect of NaFSI salt concentration in methylpropylpyrrolidinium (C₃mpyr) FSI ionic liquid (IL) on the reversible plating and dissolution of sodium metal, both on a copper electrode and in a symmetric Na/Na metal cell. NaFSI is highly soluble in the IL allowing the preparation of mixtures that contain very high Na contents, greater than 3.2 mol/kg (50 mol %) at room temperature. Despite the fact that overall ion diffusivity decreases substantially with increasing alkali salt concentration, we have found that these high Na⁺ content electrolytes can support higher current densities (1 mA/cm²) and greater stability upon continued cycling. EIS measurements indicate that the interfacial impedance is decreased in the high concentration systems, which provides for a particularly low-resistance solid-electrolyte interphase (SEI), resulting in faster charge transfer at the interface. Na⁺ transference numbers determined by the Bruce–Vincent method increased substantially with increasing NaFSI content, approaching >0.3 at the saturation concentration limit which may explain the improved performance. NMR spectroscopy, PFG diffusion measurements, and molecular dynamics simulations reveal a changeover to a facile structural diffusion mechanism for sodium ion transport at high concentrations in these electrolytes