Toward Practical Li Metal Batteries: Importance of Separator Compatibility Using Ionic Liquid Electrolytes

Long-term cycling studies of high capacity Li-metal|lithium iron phosphate (LFP, 3.5 mAh/cm2) cells were carried out using two highly concentrated ionic liquid electrolytes (ILEs). Cells incorporating N-propyl-N-methylpyrrolidinium bis­(fluorosulfonyl)­imide (C3mpyrFSI) or triethylmethyl­phosphonium bis­(fluorosulfonyl)­imide (P1222FSI), with 50 mol % lithium bis­(fluorosulfonyl)­imide (LiFSI) electrolytes were shown to operate for over 180 cycles at 50 °C at a rate of C/2 (1.75 mA/cm2). The choice of separator was identified as a critical factor to enable high areal capacity cycling, with the occurrence of cell failure through a short-circuiting mechanism being particularly sensitive to separator characteristics. Several commercial separators were characterized and tested, and their performance was related to membrane properties such as the MacMullin number, pore size, and contact angle. Celgard 3000 series separators were found to support long-term cycling due to their combination of desirable nanoporosity and wettability. The most compatible cell components were assembled into a pouch cell to further demonstrate the feasibility of ILE incorporation into high-capacity lithium metal batteries for commercial purposes

Supported Ionic Liquid Gel Membrane Electrolytes for a Safe and Flexible Sodium Metal Battery

Concerns about the sustainability of lithium supplies has stimulated interest in alternative energy storage chemistries including in sodium metal and sodium ion batteries. Gel ionic liquid electrolytes are investigated here as an important option for secondary sodium batteries due to their leakage-free and superior safety when compared to standard flammable electrolytes. Supported ionic liquid gel membranes (SILGMs) were prepared as both electrolyte and separator for a sodium metal battery using a carbon-coated sodium vanadium phosphate material (Na3V2(PO4)3@C or NVP@C) as cathode. SILGM-based coin cells exhibit a specific capacity retention of 92% after 150 charge–discharge cycles with a Coulombic efficiency of 99.9%. We also demonstrate the operation of SILGMs in a laminated flexible sodium battery. The SILGM-based flexible battery exhibits a good flexibility and shows a remarkably stable operation even when opening the device or cutting into pieces. It is expected that SILGMs will become promising separator/electrolyte materials in practical application and thus will promote the development of nonflammable and flexible sodium batteries

High-Performance Cycling of Na Metal Anodes in Phosphonium and Pyrrolidinium Fluoro(sulfonyl)imide Based Ionic Liquid Electrolytes

We have investigated the sodium electrochemistry and the evolution and chemistry of the solid–electrolyte interphase (SEI) upon cycling Na metal electrodes in two ionic liquid (IL) electrolytes. The effect of the IL cation chemistry was determined by examining the behavior of a phosphonium IL (P111i4FSI) in comparison to its pyrrolidinium-based counterpart (C3mpyrFSI) at near-saturated NaFSI salt concentrations (superconcentrated ILs) in their dry state and with water additive. The differences in their physical properties are reported, with the P111i4FSI system having a lower viscosity, higher conductivity, and higher ionicity in comparison to the C3mpyrFSI-based electrolyte, although the addition of 1000 ppm (0.1 wt %) of water had a more dramatic effect on these properties in the latter case. Despite these differences, there was little effect in the ability to sustain stable cycling at moderate current densities and capacities (being nearly identical at 1 mA cm–2 and 1 mAh cm–2). However, the IL based on the phosphonium cation is shown to support more demanding cycling with high stability (up to 4 mAh cm–2 at 1, 2, and 4 mA cm–2 current density), whereas C3mpyrFSI rapidly failed (at 1 mA cm–2 /4 mAh cm–2). The SEI was characterized ex situ using solid-state 23Na NMR, XPS, and SEM and showed that the presence of a Na complex, identified in our previous work on C3mpyrFSI to correlate with stable, dendrite-free Na metal cycling, was also more prominent and coexisted with a NaF-rich surface. The results here represent a significant breakthrough in the development of high-capacity Na metal anodes, clearly demonstrating the superior performance and stability of the P111i4FSI electrolyte, even after the addition of water (up to 1000 ppm (0.1 wt %)), and show great promise to enable future higher-temperature (50 °C) Na-metal-based batteries

Redox Chemistry of the Superoxide Ion in a Phosphonium-Based Ionic Liquid in the Presence of Water

Stable electrogenerated superoxide ion has been observed for the first time in a phosphonium-based ionic liquid in the presence of water, leading to a chemically reversible O₂/O₂^•– redox couple instead of the disproportionation reaction that is usually observed. It appears that the cation solvates the superoxide anion, stabilizing it against the disproportionation reaction. The electrogeneration is studied at various levels of water or other diluents including toluene to explore the limits of stability of the superoxide ion under these conditions

Ionic Liquid Adsorption and Nanotribology at the Silica–Oil Interface: Hundred-Fold Dilution in Oil Lubricates as Effectively as the Pure Ionic Liquid

The remarkable physical properties of ionic liquids (ILs) make them potentially excellent lubricants. One of the challenges for using ILs as lubricants is their high cost. In this article, atomic force microscopy (AFM) nanotribology measurements reveal that a 1 mol % solution of IL dissolved in an oil lubricates the silica surface as effectively as the pure IL. The adsorption isotherm shows that the IL surface excess need only be approximately half of the saturation value to prevent surface contact and effectively lubricate the sliding surfaces. Using ILs in this way makes them viable for large-scale applications

Polar Organic Cations at Electrified Metal with Superconcentrated Ionic Liquid Electrolyte and Implications for Sodium Metal Batteries

Understanding the potential-induced changes across the electrode/electrolyte interface, the so-called electric double layer (EDL), is essential to adjust the working properties of energy-storage devices. Electrolytes with a high molar ratio of metal salt to solvent (1:1 salt:solvent), e.g., superconcentrated ionic liquids (ILs), enable uniform metal deposition, formation of stable solid-electrolyte interphase (SEI), and higher redox stability, which make them attractive for battery applications. However, the presence of an organic IL cation and its interactions with metal salt complexes can significantly impact the mechanism of charge transfer at an electrode compared with conventional ether/ester-based electrolytes. The competition between IL and metal cations to enter the electrified interface affects interfacial chemistry, a key determinant of metal deposition potential and the nature of the SEI. This, in turn, is also affected by IL cation and anion chemistries, which are not yet fully understood. This letter demonstrates that the polarity of an organic IL cation, which is expressed through its dipole moment (μ), and its redox stability can serve as a predictive descriptor for EDL structure in superconcentrated IL electrolytes and the implications for charge transfer. We showed that, in the family of pyrrolidinium cations, a less polar organic cation with a small μ, e.g. N-methyl-N-ethylpyrrolidinium [C2mpyr]+, packs tighter and in a greater number at a negatively charged electrode/electrolyte interface in comparison to more polar IL cations with greater μ, e.g. N-methyl-N-propylpyrrolidinium [C3mpyr]+ and N-methyl-N-methoxymethylpyrrolidinium [C2O1mpyr]+. This IL cation-rich interface results in a greater overpotential for Na deposition, whereas the nature of the SEI and sodium anode cycling behavior correlate with both the dipole moment and the reductive stability of the IL cation

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

Solid State Li Metal/LMO Batteries based on Ternary Triblock Copolymers and Ionic Binders

Triblock copolymers containing an ionophilic polymerized ionic liquid block, sandwiched between two ionophobic polystyrene blocks, were investigated as solid polymer electrolytes (SPE) to simultaneously provide mechanically robust, free-standing membranes with high lithium conductivity and an optimized electrolyte composition. The conductivity reached 8 × 10–5 S cm–1 and 6.5 × 10–4 S cm–1 at 30 and 80 °C, respectively, with an anodic stability above 4.5 V. Highly stable Li metal symmetric cycling was demonstrated, with an overpotential of 130 mV for over 300 h at 50 °C at a current density of 0.5 mA cm–2/0.5 mAh cm–2. Attempts were also made to incorporate the SPE as the binder in an LMO cathode formulation. The best cell performance, however, was obtained when substituting the SPE in the LMO cathode formulation with a PMA solid-state gel electrolyte, resulting in a high-performance solid-state Li|polymer eletrolyte|LMO device with stable cycling at C/5, and an impressive capacity retention (i.e., 105 mAh g–1 after 150 cycles at 0.1 mA cm–2) with a Coulombic efficiency around 99.4%

Ball milling: A green mechanochemical approach for synthesis of nitrogen doped carbon nanoparticles

Technological and scientific challenges coupled with environmental considerations have attracted a search for robust, green and energy-efficient synthesis and processing routes for advanced functional nanomaterials. In this article, we demonstrate a high-energy ball milling technique for large-scale synthesis of nitrogen doped carbon nanoparticles, which can be used as an electro-catalyst for oxygen reduction reactions after a structural refinement with controlled thermal annealing. The resulting carbon nanoparticles exhibited competitive catalytic activity (5.2 mA cm-2 kinetic-limiting current density compared with 7.6 mA cm-2 on Pt/C reference) and excellent methanol tolerance compared to a commercial Pt/C catalyst. The proposed synthesis route by ball milling and annealing is an effective process for carbon nanoparticle production and efficient nitrogen doping, providing a large-scale production method for the development of highly efficient and practical electrocatalysts