Interface Investigations of a Commercial Lithium Ion Battery Graphite Anode Material by Sputter Depth Profile X‑ray Photoelectron Spectroscopy

Here we provide a detailed X-ray photoelectron spectroscopy (XPS) study of the electrode/electrolyte interface of a graphite anode from commercial NMC/graphite cells by intense sputter depth profiling using a polyatomic ion gun. The uniqueness of this method lies in the approach using 13-step sputter depth profiling (SDP) to obtain a detailed model of the film structure, which forms at the electrode/electrolyte interface often noted as the solid electrolyte interphase (SEI). In addition to the 13-step SDP, several reference experiments of the untreated anode before formation with and without electrolyte were carried out to support the interpretation. Within this work, it is shown that through charging effects during X-ray beam exposure chemical components cannot be determined by the binding energy (BE) values only, and in addition, that quantification by sputter rates is complicated for composite electrodes. A rough estimation of the SEI thickness was carried out by using the LiF and graphite signals as internal references

Aqueous/Nonaqueous Hybrid Electrolyte for Sodium-Ion Batteries

Here, we report an aqueous/nonaqueous hybrid electrolyte based on sodium trifluoromethanesulfonate with an expanded electrochemical window up to 2.8 V and high conductivity (∼25 mS cm^–1 at 20 °C). The hybrid electrolyte inherits the safety characteristic of aqueous electrolytes and the electrochemical stability of nonaqueous systems, enabling stable and reversible operation of the Na₃V₂(PO₄)₃/NaTi₂(PO₄)₃ sodium-ion battery

Complex Nature of Ionic Coordination in Magnesium Ionic Liquid-Based Electrolytes: Solvates with Mobile Mg²⁺ Cations

The Raman shifts of the TFSI⁻ expansion-contraction mode in <i>N</i>-butyl-<i>N</i>-methylpyrrolidinium bis­(trifluoromethanesulfonyl)­imide ionic liquid (IL) electrolytes were analyzed to compare the ionic coordination of magnesium with lithium and sodium. In the Mg²⁺-IL electrolytes, the TFSI^– anions are found in three different potential energy environments, while only two populations of TFSI^– are evident in the Na⁺- and Li⁺-IL electrolytes. For Mg²⁺, the high frequency peak component is associated with a TFSI^– that is in a bidentate coordination with a single metal cation and can therefore be considered a contact ion pair (CIP) solvate. The mid frequency component is attributed primarily to bridging aggregate (AGG) TFSI^– solvate or a weakly bound monodentate CIP TFSI^–. The low frequency peak is well-known to be associated with “free” TFSI^– anions. The average number of TFSI^– per Mg²⁺ cation (<i>n</i>) is 3 to 4. In comparison, the value of <i>n</i> is 4 at very low concentrations and decreases with increasing salt mole fraction to 2 for Li⁺ and Na⁺, where <i>n</i> of Na⁺ is larger than that of Li⁺ at any given concentration. The results imply the existence of anionic magnesium solvates of varying sizes. The identity of the Mg²⁺ charge-carrying species is complex due to the presence of bridging AGG solvates in solution. It is likely that there is a combination of single Mg²⁺ solvate species and larger complexes containing two or more cations. In comparison, the primary Li⁺ and Na⁺ charge-carrying species are likely [Li­(TFSI)₂]⁻ and [Na­(TFSI)₃]^2– in the concentration range successfully implemented in IL-based electrolyte batteries. These solvates result in Mg²⁺ cations that are mobile in the IL-based electrolytes as demonstrated by the reversible magnesiation/demagnesiation in V₂O₅ aerogel electrodes

Two-Dimensional Titanium Carbide/RGO Composite for High-Performance Supercapacitors

Ti₃C₂T_<i>x</i>, a 2D titanium carbide in the MXenes family, is obtained from Ti₃AlC₂ through selective etching of the Al layer. Due to its good conductivity and high volumetric capacitance, Ti₃C₂T_<i>x</i> is regarded as a promising candidate for supercapacitors. In this paper, the fabrication of Ti₃C₂T_<i>x</i>/RGO composites with different proportions of Ti₃C₂T_<i>x</i> and RGO is reported, in which RGO acts as a conductive “bridge” to connect different Ti₃C₂T_<i>x</i> blocks and a matrix to alleviate the volume change during charge/discharge process. In addition, RGO nanosheets can serve as a second nanoscale current collector and support as well for the electrode. The electrochemical performance of the as-fabricated Ti₃C₂T_<i>x</i>/RGO electrodes, characterized by CV, GCD, and EIS, are also reported. A highest specific capacitance (<i>C</i>_s) of 154.3 F/g at 2 A/g is obtained at the Ti₃C₂T_<i>x</i>: RGO weight ratio of 7:1 combined with an outstanding capacity retention (124.7 F/g) after 6000 cycles at 4 A/g

Conformations and Vibrational Assignments of the (Fluorosulfonyl)(trifluoromethanesulfonyl)imide Anion in Ionic Liquids

Investigations of the (fluorosulfonyl)­(trifluoromethanesulfonyl)­imide (FTFSI) anion, incorporated in various ionic liquids, by means of density functional theory (DFT) methods and differential scanning calorimetry (DSC), X-ray diffraction (XRD), and Raman techniques are reported in this work. Theoretical studies using DFT methods (B3LYP/6-31G**) show that there are three likely anion geometries (syn, gauche, and anti) separated by less than 3 kJ·mol^–1. The energy barrier to conversion between the anti and syn/gauche conformers is between 10 and 14 kJ·mol^–1 and lower than 10 kJ·mol^–1 for rotations around the SNSF and SNSC dihedral angles, respectively. The FTFSI anion has a characteristic vibration at 730 cm^–1 assigned to the expansion and contraction of the entire anion that is sensitive to ionic interactions with metal cations. DSC, XRD, and Raman studies indicate that an alkali metal salt containing the FTFSI anion, KFTFSI, exists in two crystalline forms. Form II converts to form I via a solid–solid phase transition at 96.9 °C. The FTFSI expansion–contraction mode at 745 cm^–1 in KFTFSI form I shifts to 741 cm^–1 in form II. It can be hypothesized that this shift is due to the presence of different anion geometries or varying ionic interactions in the two crystalline forms

Mechanisms of Magnesium Ion Transport in Pyrrolidinium Bis(trifluoromethanesulfonyl)imide-Based Ionic Liquid Electrolytes

Inert polar aprotic electrolytes based on pyrrolidinium bis­(trifluoromethanesulfonyl)­imide ionic liquids were investigated for Mg battery applications. On a molecular scale, there are two TFSI^– populations coordinating Mg²⁺ ions: one in a bidentate coordination to a single Mg²⁺ and one in a bridging geometry between two Mg²⁺ ions. On average, each Mg²⁺ cation is surrounded by three to four TFSI^– anions. The electrolytes, in general, remain amorphous far below ambient conditions, which results in a wide useable temperature range in practical devices. There is a change in the ratio of bidentate:bridging TFSI^– and in the conductivity, viscosity, and diffusion behavior at a salt mole fraction of 0.12–0.16. At concentrations above this threshold, there is a more dramatic decrease of the diffusion coefficients and the conductivity with increasing salt concentration due to slower exchange of the more strongly coordinated bidentate TFSI^–. The mechanism of ion transport likely proceeds via structural diffusion through exchange of the bridging and “free” TFSI^– anions within adjacent [Mg_<i>n</i>(TFSI)_<i>m</i>]^{(<i>m</i>−2<i>n</i>)–} clusters and exchange of bidentate anions via a bidentate to bridging mechanism. The vehicular mechanism likely makes only a small contribution. At concentrations above approximately 0.16 mole fraction, the structural diffusion is more closely related to the tightly bound bidentate anions

Crystalline Complexes of Pyr_12O1TFSI-Based Ionic Liquid Electrolytes

This study examines the formation of previously unreported crystalline phases of <i>N</i>-methoxyethyl-<i>N</i>-methylpyrrolidinium bis­(trifluoromethanesulfonyl)­imide (Pyr_12O1TFSI). The melting point of pristine Pyr_12O1TFSI, determined by conductivity measurements, is between −20 and −17.5 °C. Formation of this crystalline phase is difficult and only occurs under specific conditions. Pyr_12O1TFSI readily forms 1:1 phases with both NaTFSI and Mg­(TFSI)_2. The results of single crystal structure determinations are presented. The Na⁺ crystalline phase provides clear evidence that the Pyr_12O1⁺ cation can coordinate some metal ions, but this coordinative interaction does not occur with all metal cations, e.g., Mg²⁺, and in all states of matter, e.g., Na⁺-IL solutions. The TFSI^– ions are found in two different aggregate solvates in the Pyr_12O1TFSI:NaTFSI 1:1 phase and in contact ion pair and aggregate solvates in the Pyr_12O1TFSI:Mg­(TFSI)₂ 1:1 phase. The Pyr_12O1TFSI:Mg­(TFSI)₂ crystalline phase gives insight into the local structure of the liquid electrolyte, where it is likely that a maximum of approximately 30% of the total TFSI^– can likely be coordinated in a bridging geometry, and the rest are in a bidentate coordination geometry. This ratio is determined from both the crystal structure and the Raman spectroscopy results

Insights into the Effect of Iron and Cobalt Doping on the Structure of Nanosized ZnO

Here we report an in-depth structural characterization of transition metal-doped zinc oxide nanoparticles that have recently been used as anode materials for Li-ion batteries. Structural refinement of powder X-ray diffraction (XRD) data allowed the determination of small though reproducible changes in the unit cell dimensions of four ZnO samples (wurtzite structure) prepared with different dopants or different synthesis conditions. Moreover, large variations of the full width at half-maximum of the XRD reflections indicate that the crystallinity of the samples decreases in the order ZnO, Zn_0.9Co_0.1O, Zn_0.9Fe_0.1O/C, and Zn_0.9Fe_0.1O (the crystallite sizes as determined by Williamson–Hall plots are 42, 29, 15, and 13 nm, respectively). X-ray absorption spectroscopy data indicate that Co is divalent, whereas Fe is purely trivalent in Zn_0.9Fe_0.1O and 95% trivalent (Fe³⁺/(Fe³⁺ + Fe²⁺) ratio = 0.95) in Zn_0.9Fe_0.1O/C. The aliovalent substitution of Fe³⁺ for Zn²⁺ implies the formation of local defects around Fe³⁺ such as cationic vacancies or interstitial oxygen for charge balance. The EXAFS (extended X-ray absorption fine structure) data, besides providing local Fe–O and Co–O bond distances, are consistent with a large amount of charge-compensating defects. The Co-doped sample displays similar EXAFS features to those of pure ZnO, suggesting the absence of a large concentration of defects as found in the Fe-doped samples. These results are of substantial importance for understanding and elucidating the modified electrochemical lithiation mechanism by introducing transition metal dopants into the ZnO structure for the application as lithium-ion anode material

From Nanoscale to Microscale: Crossover in the Diffusion Dynamics within Two Pyrrolidinium-Based Ionic Liquids

Knowledge of the ion motion in room temperature ionic liquids (RTILs) is critical for their applications in a number of fields, from lithium batteries to dye-sensitized solar cells. Experiments on a limited number of RTILs have shown that on macroscopic time scales the ions typically undergo conventional, Gaussian diffusion. On shorter time scales, however, non-Gaussian behavior has been observed, similar to supercooled fluids, concentrated colloidal suspensions, and more complex systems. Here we characterize the diffusive motion of ionic liquids based on the <i>N</i>-butyl-<i>N</i>-methylpyrrolidinium (PYR₁₄) cation and bis­(trifluoro methanesulfonyl)­imide (TFSI) or bis­(fluorosulfonyl)­imide (FSI) anions. A combination of pulsed gradient spin–echo (PGSE) NMR experiments and molecular dynamics (MD) simulations demonstrates a crossover from subdiffusive behavior to conventional Gaussian diffusion at ∼10 ns. The deconvolution of molecular displacements into a continuous spectrum of diffusivities shows that the short-time behavior is related to the effects of molecular caging. For PYR₁₄FSI, we identify the change of short-range ion–counterion associations as one possible mechanism triggering long-range displacements

A Combined Theoretical and Experimental Study of the Influence of Different Anion Ratios on Lithium Ion Dynamics in Ionic Liquids

In this paper, we investigate via experimental and simulation techniques the transport properties, in terms of total ionic conductivity and ion diffusion coefficients, of ionic liquids doped with lithium salts. They are composed of two anions, bis­(fluorosulfonyl)­imide (FSI) and bis­(trifluoromethanesulfonyl)­imide (TFSI), and two cations, <i>N</i>-ethyl-<i>N</i>-methylimidazolium (emim) and lithium ions. The comparison of the experimental results with the simulations shows very good agreement over a wide temperature range and a broad range of compositions. The addition of TFSI gives rise to the formation of lithium dimers (Li⁺–TFSI^––Li⁺). A closer analysis of such dimers shows that involved lithium ions move nearly as fast as single lithium ions, although they have a different coordination and much slower TFSI exchange rates