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

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

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

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

Lithium Bis(fluorosulfonyl)imide/Poly(ethylene oxide) Polymer Electrolyte for All Solid-State Li–S Cell

Solid polymer electrolytes (SPEs) comprising lithium bis­(fluorosulfonyl)­imide (Li­[N­(SO₂F)₂], LiFSI) and poly­(ethylene oxide) (PEO) have been studied as electrolyte material and binder for the Li–S polymer cell. The LiFSI-based Li–S all solid polymer cell can deliver high specific discharge capacity of 800 mAh g_sulfur^–1 (i.e., 320 mAh g_cathode^–1), high areal capacity of 0.5 mAh cm^–2, and relatively good rate capability. The cycling performances of Li–S polymer cell with LiFSI are significantly improved compared with those with conventional LiTFSI (Li­[N­(SO₂CF₃)₂]) salt in the polymer membrane due to the improved stability of the Li anode/electrolyte interphases formed in the LiFSI-based SPEs. These results suggest that the LiFSI-based SPEs are attractive electrolyte materials for solid-state Li–S batteries

Formulation and Characterization of PS-Poly(ionic liquid) Triblock Electrolytes for Sodium Batteries

Solvent-free solid polymer electrolytes (SPE) are gaining more attention to develop postlithium battery technologies due to the safety and performance benefits of solid-state batteries. In this work, we present a new SPE for a sodium metal battery based on high salt concentration polymer electrolyte membranes comprising mixed anions, polymerized ionic liquid (PIL), block copolymer (BCP) polystyrene-b-poly­(diallydimethylammonium)­bis­(trifluoromethanesulfonyl)­imide-b-polystyrene (PS-b-PDADMATFSI-b-PS) and NaFSI salt. The maximum salt concentration incorporated was up to 1:2 mol ratio (PIL block: NaFSI). The ionic conductivity was 10–3 S cm–1 at 70 °C for 1:2 composition, and the anion diffusion as measured by 19F NMR decreased. FTIR measurement indicates that the ion coordination in the polymer–salt mixtures changes with composition. The storage modulus as measured by dynamic mechanical analysis (DMA) was observed in the range 300 MPa at −40 °C to 35.8 MPa at 70 °C. The optimized electrolyte (1:2 mol ratio) membrane was investigated for its long-term stability against Na metal cycling with Na/Na symmetrical cells demonstrating stable Na plating/stripping behavior at 0.2 mA cm–2 at 70 °C. Finally, an Na|NaFePO4 cell cycled with a specific capacity of 118 mAh g–1 at C-rate C/20 at 70 °C and a good Coulombic efficiency (98%), showing the promising potential of these solvent-free triblock copolymer electrolytes in Na metal batteries

Aprotic Li–O₂ Battery: Influence of Complexing Agents on Oxygen Reduction in an Aprotic Solvent

Several problems arise at the O₂ (positive) electrode in the Li-air battery, including solvent/electrode decomposition and electrode passivation by insulating Li₂O₂. Progress partially depends on exploring the basic electrochemistry of O₂ reduction. Here we describe the effect of complexing-cations on the electrochemical reduction of O₂ in DMSO in the presence and absence of a Li salt. The solubility of alkaline peroxides in DMSO is enhanced by the complexing-cations, consistent with their strong interaction with reduced O₂. The complexing-cations also increase the rate of the 1-electron O₂ reduction to O₂^•– by up to six-fold (<i>k</i>° = 2.4 ×10^–3 to 1.5 × 10^–2 cm s^–1) whether or not Li⁺ ions are present. In the absence of Li⁺, the complexing-cations also promote the reduction of O₂^•– to O₂^2–. In the presence of Li⁺ and complexing-cations, and despite the interaction of the reduced O₂ with the latter, SERS confirms that the product is still Li₂O₂

Polymer-Rich Composite Electrolytes for All-Solid-State Li–S Cells

Polymer-rich composite electrolytes with lithium bis­(fluorosulfonyl)­imide/poly­(ethylene oxide) (LiFSI/PEO) containing either Li-ion conducting glass ceramic (LICGC) or inorganic Al₂O₃ fillers are investigated in all-solid-state Li–S cells. In the presence of the fillers, the ionic conductivity of the composite polymer electrolytes (CPEs) does not increase compared to the plain LiFSI/PEO electrolyte at various tested temperatures. The CPE with Al₂O₃ fillers improves the stability of the Li/electrolyte interface, while the Li–S cell with a LICGC-based CPE delivers high sulfur utilization of 1111 mAh g^–1 and areal capacity of 1.14 mAh cm^–2. In particular, the cell performance gets further enhanced when combining these two CPEs (Li | Al₂O₃–CPE/LICGC–CPE | S), reaching a capacity of 518 mAh g^–1 and 0.53 mAh cm^–2 with Coulombic efficiency higher than 99% at the end of 50 cycles at 70 °C. This study shows that the CPEs can be promising electrolyte candidates to develop safe and high-performance all-solid-state Li–S batteries

High-Performance P2-Phase Na_2/3Mn_0.8Fe_0.1Ti_0.1O₂ Cathode Material for Ambient-Temperature Sodium-Ion Batteries

High-performance Mn-rich P2-phase Na_2/3Mn_0.8Fe_0.1Ti_0.1O₂ is synthesized by a ceramic method, and its stable electrochemical performance is demonstrated. ²³Na solid-state NMR confirms the substitution of Ti⁴⁺ ions in the transition metal oxide layer and very fast Na⁺ mobility in the interlayer space. The pristine electrode delivers a second charge/discharge capacity of 146.57/144.16 mA·h·g^–1 and retains 95.09% of discharge capacity at the 50th cycle within the voltage range 4.0–2.0 V at C/10. At 1C, the reversible specific capacity still reaches 99.40 mA·h·g^–1, and capacity retention of 87.70% is achieved from second to 300th cycle. In addition, the moisture-exposed electrode reaches reversible capacities of more than 130 and 80 mA·h·g^–1 for C/10 and 1C, respectively, with excellent capacity retention. The correlation between overall electrochemical performance of both electrodes and crystal structural characteristics are investigated by neutron powder diffraction. The stability of pristine electrode’s crystallographic structure during the charge/discharge process has been investigated by in situ X-ray diffraction, where only a solid solution reaction occurs within the given voltage range except for a small biphasic mechanism occurring at or below 2.2 V during the discharge process. The relatively small substitution (20%) at the transition metal site leads to stable electrochemical performance, which is in part derived from the structural stability during electrochemical cycling. Therefore, the small cosubstitution (e.g., with Ti and Fe) route suggests a possible new scope for the design of sodium-ion battery electrodes that are suitable for long-term cycling

Domino Reactions Enabling Sulfur-Mediated Gradient Interphases for High-Energy Lithium Batteries

Silicon (Si)-based anodes are currently considered a feasible solution to improve the energy density of lithium-ion batteries owing to their sufficient specific capacity and natural abundance. However, Si-based anodes exhibit low electric conductivities and large volume changes during cycling, which could easily trigger continuous breakdown/reparation of the as-formed solid-electrolyte-interphase (SEI) layer, seriously hampering their practical application in current battery technology. To control the chemoelectrochemical instability of the conventional SEI layer, we herein propose the introduction of elemental sulfur into nonaqueous electrolytes, aiming to build a sulfur-mediated gradient interphase (SMGI) layer on Si-based anodes. The SMGI layer is generated through the domino reactions (i.e., electrochemical cascade reactions) involving the electrochemical reductions of elemental sulfur followed by nucleophilic substitutions of fluoroethylene carbonate, which endows the corresponding SEI layer with strong elasticity and chemomechanical stability and enables rapid transportation of Li+ ions. Consequently, the prototype Si||LiNi0.8Co0.1Mn0.1O2 cells attain a high-energy density of 622.2 W h kg–1 and a capacity retention of 88.8% after 100 cycles. Unlike previous attempts based on sophisticated chemical modifications of electrolyte components, this study opens a new avenue in interphase design for long-lived and high-energy rechargeable batteries