Lithium dynamics in LiMn2O4 probed directly by two-dimensional (7)Li NMR

LiMn2O4 has been studied using magic-angle-spinning nuclear magnetic resonance (MAS NMR). 1D MAS NMR shows three Li resonances assigned to different crystallographic sites. At low temperatures an extra peak appears, indicating charge ordering of Mn3+ and Mn4+. Direct observation of the lithium dynamics was possible using rotor-synchronized 2D exchange NMR. A millisecond time scale exchange of lithium starts around 285 K between the 8a and the 16c site. At 380 K lithium even starts to hop between more than two sites. The activation energies and Li jump rates are derived and are in agreement with those determined macroscopically.     

Ionic conductivity and interfacial resistance of electrospun poly(acrylonitrile)/poly(methyl methacrylate) fibrous membrane-based polymer electrolytes for lithium ion batteries

Electrospun poly(acrylonitrile) fibrous membrane (PAN-EFM) is prepared and enhanced by adding poly(methyl methacrylate)(PMMA) and subsequently minimizing the average diameter of the PAN/PMMA blend fibers. Electrospinning of the 50/50 wt% PAN/PMMA solution is carried out with the aim of the simultaneous presence of both polymers on the fiber surface. Their presence in exterior surface is confirmed using the Fourier transform infrared spectroscopy-attenuated total reflectance (FTIR-ATR) technique next to the leaching of PMMA with acetone. The process parameters are optimized in four stable modes with the average diameter decreasing from 445 to 150 nm. Mechanical strength of the membrane is measured and reported. Comparing the sample electrochemical properties of the EFMs reveals that the addition of PMMA increases ionic conductivity from 1.02 to 3.31 mS cm^?1 and reduces interfacial resistance from ~1000 to ~400?Ω. It is also demonstrated that the ~300-nm reduction in average diameter of the blend fibers increases ionic conductivity from 3.31 to 5.81 mS cm^?1 and reduces interfacial resistance from ~400 to ~200?Ω.     

Applications of (7)Li NMR in biomedicine

The applications of (7)Li NMR spectroscopy and imaging in biology and experimental medicine have been progressing steadily. The interest derives primarily from the clinical use of Li salts to treat mania and manic-depressive illness. One area of investigation is ionic transport across the cellular membrane and compartmentation, so as to elucidate the mechanism(s) of therapeutic action and toxicity in clinical practice. The second is the development of a noninvasive, in vivo analytical tool to measure brain Li concentrations in humans, both as an adjunct to treatment and as a mechanistic probe. Here we review progress to date in this area.     

Mixed solid electrolytes based on poly(ethylene oxide)

Polymer solid electrolytes from a PEO-NaI system were mixed with Nasicon and Al₂O₃ powders. As a result an increase of ionic conductivity exceeding 10^–1 S/cm at room temperature was observed for both cases. This increase was due to a higher concentration of amorphous phase which resulted apparently from a higher nucleation rate during the solidification process. The samples were studied using impedance spectroscopy, X-ray diffraction, electron microscopy, NMR, and other techniques.     

Ionic Pathways in Li13Si4 investigated by 6Li and 7Li solid state NMR experiments

Local environments and dynamics of lithium ions in the binary lithium silicide Li₁₃Si₄ have been studied by ⁶Li MAS-NMR, ⁷Li spin-lattice relaxation time and site-resolved ⁷Li 2D exchange NMR measurements as a function of mixing time. Variable temperature experiments result in distinct differences in activation energies characterizing the transfer rates between the different lithium sites. Based on this information, a comprehensive picture of the preferred ionic transfer pathways in this silicide has been developed. With respect to local mobility, the results of the present study suggests the ordering Li6/Li7>Li5>Li1>Li4 >Li2/Li3. Mobility within the z=0.5 plane is distinctly higher than within the z=0 plane, and the ionic transfer between the planes is most facile via Li1/Li5 exchange. The lithium ionic mobility can be rationalized on the basis of the type of the coordinating silicide anions and the lithium-lithium distances within the structure. Lithium ions strongly interacting with the isolated Si⁴⁻ anions have distinctly lower mobility than those the coordination of which is dominated by Si₂⁶⁻ dumbbells.     

Raman study of ion-molecules interaction in poly(methylmetacrylate)-based gel electrolytes

Gel electrolytes have been obtained, containing LiX (X=ClO₄, N(CF₃SO)₂, AsF₆) dissolved in a ethylene carbonate-propylene carbonate mixture and PMMA as polymeric matrix. Ionic conductivity has been measured, for two different lithium salts. The changes in the Raman spectra have been studied as a function of the polymer content, lithium salt concentrations and for different anions. Two satellite bands of the internal bending and stretching modes of ethylene carbonate appear in the spectrum of lithium containing samples, because of the cation-solvent molecule interaction. Paper presented at the 1st Euroconference on Solid State Ionics held in Zakynthos, Greece, Sept. 11–18, 1994     

Dipole excitation of alpha clusters in (6)Li and (7)Li

Dipole excitations in highly excited energy regions of (6)He and (7)He nuclei were investigated via the ((7)Li,(7)Be) reaction with an incident energy of 65A MeV at forward scattering angles. The resonances at Q approximately equal to -30 MeV observed commonly for both (6)Li and (7)Li targets were found to be excited via both spin-flip and spin-nonflip transitions with DeltaL = 1. Based on the observed excitation energy, width, and cross section of each resonance, the relevant resonances are inferred to be analogs of the dipole resonances of alpha clusters in the (6)Li and (7)Li nuclei.     

Raising the softening temperature of poly(vinylidene fluoride) gel electrolytes

Factors affecting the softening temperature of polymer gel electrolytes (PGEs) made from poly(vinylidene fluoride) (PVDF) have been investigated. The melting temperature transition has been found to rise with increased polymer concentration and salt concentration but reduced by solvent dielectric constant. The solvent dielectric constant was reduced by mixing propylene carbonate (PC) with the non-solvent phenyl propanol (PhP). The use of lithium salt bis(oxalate)borate (LiBOB) in place of lithium tetrafluroborote (LiBF₄) gives further enhancement to the softening temperature of PGEs. In all of those cases, there is an eventual trade-off between increased softening temperature and reduced ionic conductivity, in this fabricated gel electrolyte. Here, a variety of ways to tailor the properties of PGEs for different applications has been shown.     

Properties of gel polymer electrolytes based on poly(butyl acrylate) semi-interpenetrating polymeric networks toward Li-ion batteries

A new type of gel polymer electrolyte (GPE) based on poly(butyl acrylate) (PBA) semi-interpenetrating polymer networks (IPNs) and polyvinylidene fluoride (PVDF) was prepared in different molar ratios ranging from 1:0.5 to 1:1. A series of structure characterizations of PBA/PVDF had been measured using FTIR, XRD, and SEM. The electrolyte uptake test revealed that when the semi-IPNs were swollen with the commercial liquid electrolyte solutions, they showed an outstanding electrolyte uptake of 120% with a chemically cross-linked structure. All results indicated that the GPE exhibited the best performance when the molar ratio of BA and PVDF was 1:0.5. The prototype cell assembled with LiFePO₄ as cathode, lithium metal as anode, and GPE as the electrolyte as well as separator retained 94% of its initial specific capacity after 100 charge-discharge cycles, showing an excellent cycling stability and a high electrochemical window (up to 4.5 V against Li⁺/Li) at room temperature. Compared with the liquid electrolyte, the GPE exhibited a similar stable cycling performance and was suitable for practical application in Li-ion batteries.     

NMR studies of cation transport in the crystalline ion conductors MCF3SO3 (M = Li,Na) and Li7TaO6

In this contribution we present studies on the mechanism of ion transport in crystalline solid electrolytes employing a range of different solid state nuclear magnetic resonance (NMR) experiments. The first part is devoted to the elucidation of a possible correlation of cation transport and anion reorientation in the dynamically disordered rotor phases of alkali trifluoromethane sulfonates MCF₃SO₃ (M = Li, Na) employing ⁷Li, ¹³C, ¹⁷O and ²³Na NMR line shape analysis, whereas the second part focuses on the tracking of cation diffusion pathways in the hexaoxometalate Li₇TaO₆ utilizing ⁶Li 1D and 2D exchange MAS NMR approaches.     

Conductivity, dielectric behaviour and thermal stability studies of lithium ion dissociation in poly(methyl methacrylate)-based gel polymer electrolytes

Thin films of poly(methyl methacrylate) (PMMA) with lithium triflate (LiCF₃SO₃) were prepared by using the solution-casting method with PMMA as the host polymer. Ionic conductivity and dielectric measurements were carried out on these films. The highest conductivity for polymer electrolyte with a ratio of 65:35 was found to be 9.88 × 10⁻⁵ S cm⁻¹, which is suitable for the production of mobile phone battery. Thermal gravimetric analysis was carried out to evaluate the thermal stability of the polymer electrolyte. The addition of salts will increase thermal stability of the polymer electrolyte.     

Solid polymer electrolytes based on poly(lithium carboxylate) salts

The ionic conductivity, lithium ion transference number, electrochemical stability, and thermal property of solid polymer electrolytes composed of poly(ethylene oxide) (PEO) and poly(lithium carboxylate)s, (poly(lithium acrylate) (Poly(Li-A)) or poly(lithium fumarate) (Poly(Li-F)), with and without BF₃·OEt₂ were investigated. The ionic conductivities of all solid polymer electrolytes were enhanced by one to two orders of magnitude with addition of BF₃·OEt₂ because the dissociation of lithium ion and carboxylate anion was promoted by the complexation with BF₃. The lithium ion transference number in the solid polymer electrolytes based on poly(lithium carboxylate)s showed relatively high values of 0.41–0.70, due to the suppression of the transport of counter anion by the use of a polymeric anion. The solid polymer electrolytes with addition of BF₃·OEt₂ showed good electrochemical stability.     

A phosphate additive for poly(ethylene oxide)-based gel polymer electrolytes

The influence of an organophosphosphate additive on poly(ethylene oxide) lithium bis(trifluoromethylsulfonyl)imide-based gel polymer electrolytes for secondary lithium battery applications is described. Tris(2-(2-methoxyethoxy)ethyl)phosphate, is compared to the well known gel-battery component, propylene carbonate, through a study of complex impedance analysis, differential scanning calorimetry, and limiting oxygen index combustion analysis. The conductivities of the gels at low concentrations of tris(2-(2-methoxyethoxy)ethyl)phosphate (1.9–4.2 mol%) are higher to those of propylene carbonate-based systems with the same concentration. Despite micro-phase separation at high concentrations of tris(2-(2-methoxyethoxy)ethyl)phosphate (7.0–14.9 mol%), the conductivities remain comparable to systems that use propylene carbonate. The addition of tris(2-(2-methoxyethoxy)ethyl)phosphate to poly(ethylene oxide) gives increased fire retardance, while the addition of propylene carbonate to poly(ethylene oxide) results in increased flammability.     

Electrical and electrochemical properties of poly (methyl methacrylate) based nanocomposite gel electrolytes dispersed with dedoped (insulating) polyaniline nanofibers

In this work, we have investigated the effect of dedoped (insulating) polyaniline (PAni) nanofibers on the electrical and electrochemical properties of poly(methyl methacrylate) (PMMA)-based gel electrolytes. PAni nanofibers have been synthesized using interfacial polymerization technique. By analysis of X-ray diffraction (XRD) and impedance spectroscopy results, it has been demonstrated that the incorporation of dedoped PAni nanofibers up to a moderate concentration (4 wt%) to PMMA–(PC?+?DEC)–LiClO₄ gel polymer electrolyte system significantly enhances the ionic conductivity of the electrolyte system, which can be attributed to the inhibition of polymer chain reorganization upon dispersion of high aspect ratio nanofibers in PMMA matrix resulting in reduction in polymer crystallinity, which gives rise to an increase in ionic conductivity. At higher concentration, dedoped nanofibers appear to get phase separated and form insulating clusters, which impede ionic transport. The phase separation phenomena at higher fraction of nanofibers are confirmed by XRD. Studies on electrochemical behavior reveal that electrochemical potential window increases with the increase of nanofibers loading.     

6Li and 7Li MAS NMR spectra of complex lyonsite-type Li2Zn2(MoO4)3 and LiRb3Hf2(MoO4)6 lithium molybdates

⁶Li and ⁷Li MAS NMR spectra of complex Li₂Zn₂(MoO₄)₃ and LiRb₃Hf₂(MoO₄)₆ molybdates of lyonsite-type structure are studied at temperatures ranging from 25 to 100°C. NMR signals are attributed to their sources. It is shown that the presence of a transition metal such as Hf in the +4 oxidation state in the structure of lyonsite contributes to the increased mobility of lithium ions along the channels of the crystal structure.     

Experimental investigations on poly(ethylene oxide) based sodium ion conducting composite polymer electrolytes dispersed with SnO2

New Na⁺ ion conducting composite polymer electrolytes comprising of polyethylene oxide (PEO)-NaClO₄ and PEO-NaI complexes dispersed with SnO₂ are reported. The results of the studies based on optical microscopy, X-ray diffraction (XRD), differential scanning calorimetry (DSC), Fourier transform infra-red (FTIR) spectroscopy, impedance analysis and mechanical testing are presented and discussed. The electrical conductivity of ≈5·10⁻⁵ S·cm⁻¹ at 40 °C was achieved for the dispersion of ≈10 wt.% of SnO₂ in both systems. The composition dependence of the conductivity has been well correlated with the variation in glass transition temperature and degree of crystallinity. A substantial enhancement in the mechanical properties of the composite films was observed at the cost of slight decrease in the conductivity at higher concentration of SnO₂. The temperature dependence of the conductivity follows apparently the Arrhenius type thermally activated process below and above the melting temperature of PEO. The conductivity of the materials has been found to be strongly humidity dependent. The materials are shown to be ionic with t_ion>0.9. The electrochemical stability of the materials has been observed to be up to ≈3.2 V for (PEO)₂₅NaClO₄+x% SnO₂ and is limited to ≈1.9 V for (PEO)₂₅NaI+x% SnO₂.     

Lithium superionic conductors Li3InBr6 and LiInBr4 studied by 7Li, 115In NMR

Li₃InBr₆ undergoes a phase transition to a superionic phase at 314 K associated with a steep increase of the conductivity (σ = 4 × 10^− 3 Scm^− 1 at 330 K). This superionic phase is isomorphous with Li₃InCl₆ in which a positional disorder at the In³⁺ site is introduced. A pseudo cubic-close-packing of the bromide ions is formed in this phase. On the other hand, a new superionic phase of LiInBr₄ was found above ca 315 K and its structure was confirmed to be a defect spinel. The dynamic properties of the cations in these two superionic phases were investigated by ⁷Li and ¹¹⁵In NMR spectroscopy.     

Polymer electrolytes based on poly(vinylidene fluoride-co-hexafluoropropylene) with crosslinked poly(ethylene glycol) for lithium batteries

The combination of a poly(ethylene glycol) (PEG) network and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) copolymer chains is one of the most efficient means for modifying PVDF-HFP gel electrolytes. Previous preparations tend to introduce contamination into the polymer gel electrolyte because of irradiation, high temperature or the initiator needed for crosslinking which might result in the electrochemical degradation. In order to overcome the above disadvantages, a new method has been developed to successfully prepare the semi-interpenetrating polymer networks of PVDF-HFP based electrolytes with crosslinked diepoxy polyethylene glycol (DIEPEG). In this process, impurities are avoided because of a moderate reaction temperature at 50 °C and poly(ethylenimine) (PEI) as the crosslinking agent. Microporous films with various compositions are prepared and characterized. Thermal, mechanical, swelling and electrochemical properties, as well as microstructures of the prepared polymer electrolytes have been investigated using thermogravimetric analysis, electrochemical impedance spectroscopy, linear sweep voltammetry, and scanning electron microscopy. The results show that the blend polymer electrolyte with PVDF-HFP/PEI + DIEPEG (60:40 w/w) has an ionic conductivity of 2.3 mS cm^? 1 at room temperature in the presence of 1 M LiPF₆ in EC and DMC (1:1 w/w). All the blend electrolytes are electrochemically stable up to 4.8 V versus Li/Li⁺. The results reveal that this new method may be very promising for improving PVDF-HFP based electrolytes.