Li Solid State Research Articles

Noveltert-Butyl-tris(3-hydrocarbylpyrazol-1-yl)borate Ligands: Synthesis, Spectroscopic Studies, and Coordination Chemistry

The lithium (1) and thallium (2) salts of five new tert-butyl-tris(3-hydrocarbylpyrazol-1-yl)borate ligands [t-BuTp(R)]- (R = H, a; Me, b; i-Pr, c; t-Bu, d; Ph, e) have been synthesized and characterized. Because of steric congestion at B, the reaction between t-BuBH3Li x 0.5 Et2O and excess 2,5-dimethylpyrazole Hpz(Me2) afforded the bis-pz(Me2) derivative, Tl[t-BuBH(3,5-Me2pz)2] (3) after metathesis with TlNO3. The compounds were characterized by elemental analysis and NMR spectroscopy. The Li salts 1a and 1c exhibit fluxional behavior on the NMR time scale in solution at room temperature. The solid-state 7Li and 11B NMR spectra of 1c suggest that this salt exists as a mixture of axial and equatorial isomers. The partial hydrolysis of 1d afforded the dimeric Li complex {Li[t-BuB(pz(t-Bu))2(mu-OH)]}2 (4). The crystal structure of 4 shows two Li cations encapsulated by the heteroscorpionate [t-BuB(OH)(3-t-Bupz)2]- ligands. A salt elimination reaction between FeCl2(THF)1.5 and 2 equiv of Li[t-BuTp(R)] (R = H, Me) followed by an in situ one-electron oxidation produced good yields of the homoleptic, paramagnetic low-spin iron(III) complexes [Fe(t-BuTp)2]PF6 (5) and [Fe(t-BuTp(Me))2]PF6 (6) that were characterized by elemental analyses, magnetic susceptibility measurements in solution and the solid phase, 1H NMR, high-resolution mass spectrometry, Mössbauer spectroscopy, and single-crystal X-ray diffraction. The crystals are composed of discrete molecular units with the central Fe(III) ion in an almost perfectly octahedral coordination to six nitrogen atoms. Compound 5 has the shortest Fe-N bond lengths ever reported for [Fe(RTp(R)')2]+-type compounds.

Neutron diffraction and electrochemical studies on Li 1− xAg 2Sn

A large sample of single phase, polycrystalline Li 1− x Ag 2Sn has been synthesized by melting the elements in sealed tantalum tubes and subsequent annealing. Li 1− x Ag 2Sn crystallizes with a ternary ordered version of the BiF 3 structure. The lithium atoms were clearly localized from a neutron powder diffraction study: λ = 109.59 pm , a = 659.37 ( 7 ) pm ; space group Fm 3 ¯ m ; R P = 12.2 % and R F = 5.1 % . Time-dependent electrochemical polarisation techniques, i.e., coulometric titration, chronopotentiometry, chronoamperometry and cyclic voltammetry were used to study the kinetics of lithium ion diffusion in this stannide. Prior investigations on the lithium mobility in Li 1− x Ag 2Sn using temperature dependent solid state 7Li NMR showed positive indications of Li 1− x Ag 2Sn being an anode material. Li 1− x Ag 2Sn has been characterized using electrochemical measurements and the chemical diffusion coefficient, D Li has been determined. The chemical diffusion coefficient values of ∼ 10 −6 – 10 −7 cm 2 s −1 and a nearly reversible insertion/deinsertion in the voltage range between 1.6 and 2.1 V point out that Li 1− x Ag 2Sn is another example of a ternary intermetallic compound with good lithium and electron mobility.

Solid-State 23Na and 7Li NMR Investigations of Sodium- and Lithium-Reduced Mesoporous Titanium Oxides

Mesoporous titanium oxide synthesized using a dodecylamine template was treated with 0.2, 0.6, and 1.0 equiv of Li- or Na-naphthalene. The composite materials were characterized by nitrogen adsorption, powder X-ray diffraction, X-ray photoelectron spectroscopy, elemental analysis, thermogravimetric analysis, and solid-state 23Na and 7Li NMR spectroscopy. In all cases the wormhole mesoporosity was retained as evidenced by BET surface areas from 400 to 700 m(2)/g, Horvath-Kawazoe pore sizes in the 20 Angstroms range, and a lack of hysteresis in the nitrogen adsorption isotherms. Variable-temperature conductivity studies show that the Li-reduced materials are semiconductors, with conductivity values 3 orders of magnitude higher than those of the Na-reduced materials. Electrochemical measurements demonstrate reversible intercalation/deintercalation of Li+ ions into pristine mesoporous Ti oxides with good cycling capacity. Solid-state 23Na NMR reveals two distinct Na environments: one corresponding to sodium ions in the mesoporous channels and the other corresponding to sodium ions intercalated into the metal framework. 23Na NMR spectra also indicate that the relative population of the framework site increases with increased reduction levels. Solid-state 7Li NMR spectra display a single broad resonance, which increases in breadth with increased reduction levels, though individual resonances inferring the presence of channel and framework Li species are not resolved. Comparisons of the lithium chemical shifts with published values suggests an "anatase-like structure" with no long-range order in the least-reduced samples but a "lithium titanate-like structure" with no long-range order in the higher reduced materials.

7Li MAS NMR studies of layered Li xCoO 2 prepared using eutectic self-mixing technique for a cathode material

Layered Li x CoO 2 ( x=1.0, 1.05, and 1.1) samples prepared by eutectic self-mixing technique were characterized by solid-state 7Li MAS NMR. Simulations of 7Li NMR spectra were performed to identify the lithium sites. For Li 1.0CoO 2, a unique lithium site was observed at about 0 ppm, which was assigned as octahedral site having C Q=0.92 MHz and η=0. By increasing content of lithium in Li x CoO 2, additional lithium sites having larger quadrupolar interaction were observed. For Li 1.05CoO 2, an additional site including octahedral site was observed and determined as C Q=3.0 MHz and η=0. A resonance at about −15 ppm was also detected. For Li 1.1CoO 2, a lithium site having C Q=3.77 MHz and η=0.1 was additionally observed. The electrochemical property of Li x CoO 2 was extremely related with local environment of lithium sites.

Solid-state NMR Studies of Al-doped and Al 2O 3-coated LiCoO 2

As a cathode material for commercial lithium rechargeable battery, Al-doped and Al 2O 3-coated LiCoO 2 were structurally characterized and compared by using solid-state 7Li and 27Al magic angle spinning (MAS) NMR. The structural states of lithium and aluminum in those samples were successfully identified by calculation of NMR spectra. 7Li MAS NMR spectra of samples had shown similar features having more than three lithium sites, which were determined as quadrupole coupling constant with the same asymmetric parameter ( η = 0.1); C Q = 1.10 MHz for octahedral site, and C Q = 2.97 MHz and C Q = 3.83 MHz for shoulders. 27Al MAS NMR spectra of samples, however, showed significant difference in two observed aluminum sites. In the simulation of NMR spectrum for Al 2O 3-coated LiCoO 2, the values of C Q and η were obtained; 4.45 MHz and η = 0.86 for tetrahedral site and 4.31 MHz and η = 0.81 for octahedral.

Spectroscopic investigation of the gel polymer electrolytes based on methy methacrylate-styrene copolymers

Gel polymer electrolytes composed of methyl methacrylate-styrene copolymers (PMS) and electrolyte solution (LiTFSI in EC/DMC) were prepared and characterized. Depending on the molar composition of the copolymer, these gel polymer electrolytes exhibited different electrochemical and mechanical properties. In order to investigate the physical interactions among organic solvents, polymer, and lithium ions occurred in the gel polymer electrolyte, Raman spectroscopy and solid state 7Li NMR spin–spin relaxation measurements were performed.

Synthesis and Electrochemistry of Li- and Na-Fulleride Doped Mesoporous Ta Oxides

A mesoporous tantalum oxide lithium fulleride composite was synthesized by solution intercalation of C60 into a prefabricated Li-TaTMS material and characterized by elemental analysis, X-ray diffraction (XRD), Raman spectroscopy, nitrogen adsorption−desorption, X-ray photoelectron spectroscopy (XPS), superconducting quantum interference device (SQUID) magnetometry, and solid-state 13C and 7Li NMR. The room conductivity measurements of Li fulleride material showed that this material was insulating, as opposed to previously synthesized K and Na based mesoporous Ta oxide fulleride composites with similar composition, which were semiconducting or metallic. Solid-state 7Li NMR of this lithium composite exhibited a single Li environment. XPS measurements indicated an oxidation of the tantalum oxide walls had occurred upon intercalation of the fullerene 13C NMR, and Raman measurements were consistent with one fulleride species in the pores. Electrochemical measurements revealed largely irreversible behavior upon intercalation/deintercalation of Li+ into this material. This was attributed to the insulating nature of this composite impeding charge transport through the channels. In contrast, the corresponding Na fulleride intercalate showed reversible Li insertion, possibly due to enhanced charge transport through the semiconducting structure.

Electrochemical and solid state NMR characterization of composite PEO-based polymer electrolytes

A comprehensive matrix of composite poly(ethyleneoxide) (PEO)-based solid-state electrolytes was developed in order to systematically study a number of variables and their impact upon the electrochemical properties of the resulting materials. The different parameters studied in the fabrication of these materials include: (i) the lithium electrolyte salt type, (ii) the ether oxygen to lithium ratio, (iii) the molecular weight of PEO, (iv) the type of ceramic additive used, either aluminum oxide (Al 2O 3), silicon oxide (SiO 2), or titanium oxide (TiO 2), (v) the particle size of the additives used, and (vi) the concentration of additive (wt.%). The standard lithium salt used for the preparation of these electrolytes was lithium trifluoromethanesulfonate (lithium triflate or LiSO 3CF 3), which served as the baseline electrolyte salt. Other lithium salts investigated include: lithium perchlorate (LiClO 4) and lithium bis-trifluoromethanesulfonimide (LiN(SO 2CF 3) 2). Conductivity measurements were performed for each electrolyte membrane over a wide temperature range (23–100 °C). In addition, cyclic voltammetry measurements were performed on selected PEO membranes as a function of temperature to determine the impact of various parameters upon the electrochemical stability. It was observed that the parameter that displayed the most significant effect upon the PEO-base polymer conductivity was the lithium salt type employed. The lithium triflate salt-containing PEO polymers demonstrated the best mechanical properties before and after heat treatment. Ceramic fillers also appear to enhance the mechanical properties of PEO polymer electrolytes at temperatures above the melting point of PEO (60–70 °C). In addition to investigating the electrochemical characteristics of the composite membrane, solid state 7Li NMR characterization was performed to study ionic mobility by measuring spectral line widths and lithium self-diffusion coefficients. It was determined that ceramic nanoparticle additives can enhance the Li + diffusivity without a corresponding increase in polymer segmental mobility.

Preparation of new polymer electrolytes based on poly(acrylonitrile- co-vinylimidazoline) matrix and improvement of polarization behavior of lithium electrode in the electrolyte by using cascade nitrile compound

New polymer electrolytes based on poly(acrylonitrile- co-vinylimidazoline), P(AN-VI) were prepared and their chemical and physical properties were characterized by conductance, DSC, and solid state 7Li- and 13C NMR measurements. Conductivity for the P(AN-VI)-based electrolytes was about 10 −4 S cm −1 at 30 °C and 10 −3 S cm −1 at 60 °C. Addition of the cascade nitrile compound, ED4CN {–[–CH 2N(CH 2CH 2CN) 2] 2} which acts as a plasticizer for polyacrylonitrile-based electrolytes, to P(AN-VI)-based electrolytes reduced their conductivities. Polarization behavior of a lithium electrode in the P(AN-VI)-based electrolytes was improved by the addition of ED4CN. Exchange current density of the lithium electrode in the P(AN-VI)-based electrolyte with ED4CN was one order of magnitude higher than that in the ED4CN-free P(AN-VI) electrolyte. Addition of ED4CN to the P(AN-VI)-based electrolyte changed the environment of the lithium ions in the electrolyte. Dominant coordination of the ED4CN molecules to the lithium ions was confirmed by 7Li NMR measurements and increased the effective concentration of the lithium ions in the electrolytes.

Solid-state NMR and XANES studies of lithium and silver silicate gels synthesized by the sol–gel route

The objective of this study is to understand the effect of low temperature sol–gel synthesis on the microstructural properties of lithium [ xLi 2O–(1− x)SiO 2; x=0.1–0.8 in steps of 0.1] and silver [ xAg 2O–(1− x)SiO 2; x=0.1–0.8 in steps of 0.1] silicate xerogels via solid state nuclear magnetic resonance (NMR) and X-ray absorption near edge structure (XANES) techniques. The Li silicate xerogels were analyzed with solid-state 7Li and 29Si NMR and the Ag silicate xerogels were studied with Ag XANES. At high Li loading, 7Li NMR shows quadrupolar satellite transitions attributed to LiNO 3, a phase also found with X-ray diffraction (XRD). At low Li loading, both NMR and XRD results show an amorphous xerogel. The silicate network is monitored with 29Si NMR and shows evidence of Li incorporation. For the Ag silicate xerogels, Ag–L-III XANES spectral studies show a local environment similar to AgNO 3 for low Ag loading levels, and an increased Ag oxidation for higher Ag loading levels. Si K edge spectra show only an amorphous phase, with no evidence of a crystalline quartz phase. The electrical conductivity of the lithium silicates was estimated from impedance data and the highest conductivity is exhibited by the 0.3Li 2O–0.7SiO 2 composition xerogel. The conductivity dependence on loading level strongly suggests that the observed conductivity is due to Li + mobility. However, further experimental studies are needed to rule out the possibility that the conductivity is, at least in part, due to H + mobility. Variation in conductivity is explained qualitatively using existing theoretical models.

Novel composite polymer electrolyte comprising mesoporous structured SiO 2 and PEO/Li

Novel composite polymer electrolytes comprise of hexagonal array of mesoporous structured MCM-41, and poly(ethylene oxide) (PEO)/Li show that the conductivity and mechanical properties are improved simultaneously. The comparison of small angle X-ray diffraction (XRD) of mesoporous MCM-41 and blended films of PEO:Li/MCM-41 shows that the nano-porous SiO 2 channels were not destroyed in PEO/Li. Solid-state 7Li NMR spectra identified two major lithium species attributed to the Li + ions associated with PEO, and intercalation or penetration of polymer and Li + ions both within and outside the channels of mesoporous MCM-41. The scanning electron microscopy (SEM) photographs indicated that the electrolytes are miscible and homogeneous up to 8 wt.% of MCM-41, and an optimal conductivity is reached at this composition. However, at higher weight ratios (>10 wt.%), the Li/MCM-41-rich domain developed, and the conductivity decreased with increasing mesoporous material. Apart from the fundamental random diffusion within the amorphous PEO, additional conducting mechanism is established by replacing the nearby vacancy (“hole”) with lithium ion on MCM-41 surface, which bears lower activation energy E a. As a result, enhancement of conductivity is observed when the polymer and oxide are well miscible. This additional mechanism is absent in the case of spherical fillers such as TiO 2, Al 2O 3 or SiO 2 nano-particles in PEO-based electrolytes.

Structural analysis of lithium-excess lithium manganate cathode materials by 7Li magic-angle spinning nuclear magnetic resonance spectroscopy

The local structures of lithium-excess lithium manganese spinel oxides were studied by high-resolution solid-state 7Li magic-angle spinning (MAS) NMR spectroscopy. Two resonance lines at ∼500 and ∼555 ppm were observed for the spinels in 7Li MAS NMR spectra. Spinel stability tests in which spinel powder was stored in electrolyte solution were performed to analyze the changes in the lithium local structure after manganese dissolution. After the spinel stability test, the intensity of the resonance at ∼500 ppm decreased, whereas new resonance line at 0 ppm was observed. The lithium content of the 0 ppm peak increases with the storage time in electrolyte. SEM and chemical analysis suggested a surface coating of non-spinel lithium compounds, the presence of defects on particles surface and fluorine incorporation into the aged spinel. In addition, about 60–70% of lithium remains in the spinel framework after the storage.

Rhombohedral Form of Li3V2(PO4)3 as a Cathode in Li-Ion Batteries

Ion exchange from the monoclinic NASICON phase of Na3V2(PO4)3 results in the new lithiated form of Li3V2(PO4)3, with unit cell parameters a = 8.316(1) A and c = 22.484(1) A in the space group R3. It exhibits a reversible V3+/V4+ redox couple at an attractive average potential of ∼3.8V vs Li corresponding to the extraction of 2 Li in the voltage window 3.0−4.1 V (125 mA·h/g). Oxidation is characterized by a two-phase process with an equilibrium potential of 3.77 V, which results in an 8% contraction of the c axis of the original rhombohedral unit cell as a result of partial population of the Li sites along the c axis. This is supported by solid-state 7Li NMR data, which shows that the transformation is accompanied by conversion of the site in the R3 cell at 85 ppm to one Li site at 62 ppm in LiV2(PO4)3. At least 1.3Li can be reversibly reinserted into the framework on discharge.

Solid-state 7Li NMR studies of inverse spinel LiNi xCo 1− xVO 4 cathode materials

Solid-state 7Li NMR spectroscopy revealed different Ni/Co distributions in an inverse spinel structure of LiNi x Co 1− x VO 4 cathode materials prepared by either a high-temperature solid-state reaction method (abbreviated as the HT method) or a low-temperature solution co-precipitation method (abbreviated as the LT method). Solid-state 7Li NMR measurements confirmed that the lithium nuclear spin was dominated by a chemical shift anisotropy (CSA) interaction compounded with a small second-order quadrupolar interaction. Ni/Co distribution and inhomogeneity of chemical shift tensors ( δ CSA and η CSA constants) associated with crystalline defects are accountable for the variation in spread of the MAS spinning side-band manifolds between nickel uptake and preparation methods. The NMR study also revealed that the HT method yielded broader Ni/Co distributions with greater lithium defects, while the LT method gave much more homogeneous Ni/Co distributions with smaller defects. The NMR results were consistent with XRD data that showed a gradual expansion in the unit-cell lattice with increasing Co content. All cells suffered a large irreversible loss in overall capacity in the first cycle and became stable in terms of cycle efficiency during later cycling. We have found that LiNi x Co 1− x VO 4 cathode materials prepared by the LT method with more homogeneous Ni/Co distribution and smaller crystalline defects offer a small advantage in capacity over the HT method.

Alkali Metal Oxides, Peroxides, and Superoxides: A Multinuclear MAS NMR Study

The many roles of alkali metal−oxygen phases as catalyst components and promoters motivated a survey of the NMR properties of these compounds. Solid state 7Li, 23Na, 39K, 87Rb, and 133Cs magic angle spinning NMR spectra are reported for the following alkali metal oxides, peroxides and superoxides: Li2O, Li2O2, Na2O, Na2O2, NaO2, KO2, Rb2O2, RbO2, Cs2O2, and CsO2. Commercial samples labeled as “Cs2O” proved to be mixtures of Cs2O2 and CsO2. The superoxide (O2-) anion is paramagnetic, and this property leads to highly temperature-dependent chemical shifts for the alkali metal nuclei in all of the superoxides. All four superoxides also show phase transitions between polymorphic forms of cubic and lower symmetry, and these transitions profoundly affect the alkali metal NMR spectra. For CsO2 and RbO2, purely ionic bonding restricts the paramagnetic shift to the through-space dipolar mechanism, and large temperature-dependent shifts in the tetragonal phases vanish upon transition to the cubic phases. 13C in si...

Lithium-7 NMR investigation of electrochemical reaction of lithium with SnO

We report on solid state 7Li NMR measurements of electrochemically lithiated SnO. At low Li contents (Li/SnO≤2), the results are consistent with the formation of amorphous Li 2O. For Li/SnO ratio of 4.3, in addition to the amorphous Li 2O phase, there are at least two distinct Sn/Li alloy environments which are similar to those observed in a reference Li 2.3Sn alloy. However the Li + environments in a sample with Li/SnO ratio equal to 6.4 and an Li 4.4Sn reference alloy differ markedly. The NMR data are thus consistent with the simple model of Li insertion proposed by other groups, involving the formation of Li 2O and Li x Sn alloys (SnO+ xLi→Li 2O+Li x−2 Sn), at low and intermediate Li contents. At high Li content, however, the Li x−2 Sn alloy structure is significantly different in the electrochemically lithiated SnO material, compared to that in the reference alloy.

7Li solid-state NMR of lithium amides: structural information from7Li quadrupolar coupling constants

Lithium-7 quadrupolar coupling constants, χ(7Li), were measured for a number of microcrystalline lithium amides by solid-state 7Li magic angle spinning (MAS) NMR. A linear correlation between χ(7Li) and the structural N—Li—N angle known from x-ray diffraction work was found: χ(7Li)=(4.1±0.5)∢N—Li—N -(110.8±69.0). On this basis, structural information for new systems becomes available. For the polymer material [H2C(CH2)3NLi]n a ladder structure is proposed. The π-flip of the five-membered metallacycle formed by the coordination of tetramethylethylenediamine (TMEDA) with lithium in [(H2C(CH2)3NLi)2·TMEDA]2 was detected by 13C cross-polarization/MAS DNMR spectroscopy. From the analysis of the temperature-dependent lineshape of the methyl signals, a barrier of ΔG≠(293)=66 kJ mol-1 was derived for this rotational process in the solid, which compares favourably with data derived for related compounds. © 1998 John Wiley & Sons, Ltd.

7Li solid-state NMR spectroscopic study of cyclopentadienyllithium complexes

Eighteen cyclopentadienyllithium complexes were studied by solid-state 7Li NMR spectroscopy. It is shown that the chemical shift gives information on the type of complex, i.e. whether the complex is a contact ion pair, a solvent-separated ion pair, a sandwich structure or a polymeric material. These shift differences are due to the ring current of the aromatic anion. In order to obtain information about the solvation of the lithium cation, the quadrupolar coupling constant χ has to be determined. The χ value depends on the number and type of donor atoms in the ligand, where the nitrogen-containing ligands give rise to higher χ values than the oxygen analogues. © 1998 John Wiley & Sons, Ltd.

Electrical, thermal and NMR investigation of composite solid electrolytes based on PEO, LiI and high surface area inorganic oxides

High salt concentration polymer composite electrolytes, containing poly(ethylene oxide) (PEO), LiI and nanoscale Al 2O 3 or MgO and, in some cases, ethylene carbonate (EC) and poly(methyl methacrylate) (PMMA), were investigate by electrical conductivity, differential scanning calorimetry (DSC) and both wide-line and high resolution solid state 7Li NMR. The high resolution NMR spectrum of a sample with a PEO:Li ratio of 3:2 is consistent with at least two Li environments, one solvated by the polymer and one in small ionic clusters. A correlation between the observed change in Li environment at elevated temperature and a sudden increase in conductivity is suggested. All results demonstrate that the conduction mechanism in concentrated composite electrolytes is significantly different than in more dilute polymer electrolytes, in which the ion transport process is associated with polymer segmental motion.

Hydrothermal synthesis and crystal structure ofα-LiZnAsO4

Single crystals of lithium zinc arsenate,α-LiZnAsO4 , suitable for structure determination were prepared from an aqueous solution of H3AsO4 , Zn(NO3)2 and LiOH under hydrothermal conditions, at 240 °C. α-LiZnAsO4 has a fully ordered phenacite structure built from a three-dimensional framework of cornersharing AsO4 , ZnO4 and LiO4 tetrahedra. The tetrahedra form nets of six-rings surrounded by six four-rings in the direction of the c-axis. α-LiZnAsO4 crystallises in the trigonal space group R3 (no. 146), a=14.009(2), c=9.386(2) Å, Z=18 and Dc=3.958 g cm–3 . The structure was refined to: wR(F2)=0.130, R(F)=0.050 andS=0.982, for F2>2sigma;(F2). Synchrotron radiation, λ=0.9540 Å, was used for single crystal diffraction giving 917 unique reflections. Solid state 7Li MAS NMR showed a broadened Li signal implying more than one inequivalent Li (δ 0.21, reference: 1.00 mol dm–3 LiCl). Linear thermal expansion coefficients in the temperature range 25–800 °C were determined using time resolved in situ synchrotron X-ray powder diffraction. A weak reversible, possibly second order, thermal event was observed at 682(1) °C using differential scanning calorimetry.