S.-P. Szu

Effect of precursors on the structure of phosphosilicate gels 29Si and 31P MAS-NMR study

Gels were prepared using phosphoric acid, triethyl phosphate and trimethyl phosphite as the precursors of phosphorus. The upper limit for P 2 O 5 for gel formation is 90 mol% for gels using phosphoric acid and trimethyl phosphite, and 70 mol% for gels using triethyl phosphate. Gels prepared from phosphoric acid with more than 10 mol% of P 2 O 5 partially crystallize to Si 5 O(PO 4 ) 6 after heat treatment at 200°C for 10 h. Under the same heat treatment condition, all the gels prepared from trimethyl phosphite crystallize. Gels prepared from triethyl phosphate are amorphous even after heat treatment at 800°C for 10 h. However, chemical analysis indicates that most of the phosphorus in the triethyl-phosphate-prepared gels is driven off during the heating process; no more than 8 mol% of the initial amount of P 2 O 5 is retained in the gels. The effects of phosphorus precursors on the phosphosilicate gels were studied by X-ray diffraction, and 29 Si and 31 P magic angle spinning (MAS)-nuclear magnetic resonance (NMR). 29 Si and 31 P spectra show that Si O P and P O P linkages do not form in the xerogels (i.e. gels heat treated to ∼ 60°C). The gels heat treated at 200°C or above (those that did not crystallize) show evidence of Si O P and P O P network formation.

Structure and ionic conductivity in lithium—lead—borate glasses

Abstract Li 2 OPbOB 2 O 3 and LiClLi 2 OPbOB 2 O 3 glasses with high lithium content were prepared using Li 2 CO 3 and LiCl as the lithium starting material respectively. Although Cl does not substitute directly into the BO network as indicated by FTIR and 11 B NMR, it does serve to modify the network structure indirectly by affecting the O/(B + Pb) ratio. The conductivities of the LiCl-containing glasses were found to be at least two orders of magnitude higher than those of the Li 2 Co 3 -prepared glasses. The highest conductivity measured by ac impedance was 2.8 × 10 −3 (Ω cm ) −1 at 35°C with E a = 0.6 eV in the glass with composition 0.27 (LiCl) 2 0.29Li 2 O0.09PbO0.34B 2 O 3 . Temperature-dependent 7 Li NMR study shows two different activated processes in the Li 2 CO 3 -prepared glasses: (1) a short-range motion with low activation energy at low temperature, which is ascribed to the motion of Li + ions on the surfaces of PbO 4 pyramids and BO 4 tetrahedra; and (2) in the high temperature range with higher activation energy motion of Li + ions bonded to non-bridging oxygens of BO 3 units. In glasses with high LiCl content, the Li + ions associated with the Cl − ions dominate the Li + ion motion in the temperature range measured (−100–200°C).

X-ray diffraction and29Si NMR study of polymerized and infiltrated lithium silicate gels

Abstract X-ray diffraction and 29 Si NMR were used to monitor structural changes in lithium silicate gels following heat treatments at temperatures from 200 to 850 °C. The gels were prepared in two ways: polymerized gels were prepared by hydrolysis of tetraethylorthosilicate (TEOS) with a methanol-water-LiNO 3 or LiOH solution and infiltrated gels were prepared by soaking colloidal silica gels in methanolic LiOH or aqueous LiNO 3 solution. All of the gels have approximately 15 mol% Li 2 O. X-ray diffraction analysis indicated that the polymerized gel with LiNO 3 as a precursor had the highest crystallization temperature (∼ 800 °C), while the infiltrated gel with LiOH as a precursor had the lowest crystallization temperature (∼ 200 °C). It was also found that Li 2 SiO 3 formed preferentially at lower temperatures while Li 2 Si 2 O 5 crystallized at higher temperatures. The change of local structure in heat treated samples was probed by 29 Si NMR. The sequence of formation of various non-bridging silicate and silanol groups is consistent with the results from X-ray diffraction.

Effect of precursors on lithium containing silicate gels studied by 7Li nuclear magnetic resonance

Lithium containing silica gels were prepared by hydrolysis of tetraethylorthosilicate (TEOS) with a methanol-water-LiNO3 or LiOH solution and by infiltration of silica gel with LiOH or LiNO3 solution. Gels prepared in this way, dried and heat treated between 200 and 850°C were studied by solid state 7Li static and magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. There are dramatic differences in the 7Li NMR spectra of the various gels. The lineshape of the NMR spectra for gels prepared from LiOH (both via hydrolysis of TEOS and infiltration of prepared silica gel) is similar to that of crystalline Li2SiO3 where the lithium ions are associated with the silica network. The spectra do not change significantly with increasing temperature up to 850°C. For gels prepared from LiNO3, the 7Li NMR spectra indicate the presence of two different Li species; one of the Li species is attributed to hydrated lithium ions trapped in the pores of the gels, the other Li species is due to crystalline LiNO3. When the LiNO3 gels are heat treated above 650°C, the lineshape of the NMR spectra becomes similar to that of Li2SiO3.

The effect of precursors on the ionic conductivity in lithium silicate gels

Abstract ac impedance measurements were carried out on lithium silicate gels prepared in four different ways. The first two referred to as polymerized gels are prepared by hydrolysis of tetraethylorthosilicate (TEOS) with either a methanol-water-LiNO3 solution, or a methanol-LiOH solution. The second two referred to as infiltrated gels are prepared by soaking previously formed colloidal silica gels in either LiNO3 or LiOH solutions. The ionic conductivity results indicate different conducting mechanisms in the gels prepared with LiNO3 versus those with LiOH. In the LiNO3 gels, the conductivity mechanism is associated with the xerogel/crystalline-LiNO3 composite, while in the LiOH gels the lithium is found to be associated with non-bridging oxygens. Furthermore, the conductivity in the LiNO3 gels is higher in the cooling cycle than in the heating cycle, corresponding to a supercooling of the LiNO3 melt below its crystallization temperature of 254°C. In general, polymerized gels have higher conductivities than infiltrated gels.

Preparation, characterization and ionic conductivity of (LiCl)2-B2O3-SiO2 xerogels

Abstract (LiCl) 2 -B 2 O 3 -SiO 2 xerogels were prepared by a sol-gel technique. Fourier transform infrared (FTIR) spectra indicate the incorporation of boron into the silica network to form Si-O-B links. 11 B, 7 Li and 29 Si nuclear magnetic resonance (NMR) spectroscopies were used to probe the structure of the xerogels. For a fixed lithium content (10 mol% (LiCl) 2 ), the ionic conductivity of xerogels with B 2 O 3 /SiO 2 ratios of 1/7 to 1/5 is similar to that of lithium chloride, while the conductivity behavior of xerogels with B 2 O 3 /SiO 2 ratios of 1/10 to 1/8 is similar to those of ionically conducting glasses. For samples with the same lithium content, the conductivity increases with decreasing B 2 O 3 /SiO 2 ratio and increasing fraction of BO 4 units (N 4 values). This is attributed to the increasing concentration of lithium ions associated with tetrahedral BO 4 and non-bridging oxygens in the xerogel.

Lithium ion conductivity of sol-gel synthesized LiCl containing ZnOSiO2 xerogels☆

Abstract LiClZnOSiO2 xerogels synthesized by the sol-gel method are shown by powder X-ray diffraction, Fourier transform infrared and 7Li and 29Si NMR spectra to be composites of microcrystalline LiCl and ZnOSiO2 xerogel. Ionic conductivity measurements of the xerogels show extrinsic and intrinsic behavior. Below 280°C, Li+ motion is dominated by a space charge mechanism (significant for 0.40LiCl-0.12ZnO-0.48SiO2), which is indicative of composite character. Above 280°C the conductivity is attributed to the intrinsic conductivity of LiCl in the pores of the xerogel. The increased ionic conductivity with increasing Zn/Si ratio (up to 1 4 ) in the high temperature regime (> 280°C) is ascribed to the decrease of activation energy due to the dissolution of LiCl in the xerogel. The highest Li+ ion conductivity is found to be ∼ 10−3 S/cm at 450°C in the composition 0.40LiCl-0.12ZnO-0.48SiO2, which is half an order of magnitude higher than that of the same composition melt-quenched glass.

Effect of lithium salts on the ionic conductivity of lithium silicate gels

Abstract Lithium silicate gels were prepared by hydrolyzing tetraethyl orthosilicate (TEOS) with various monovalent and divalent lithium salts. In addition to lithium nitrate, which has been used before, lithium chloride, lithium sulfate, lithium acetate and mixtures were used. All gels had the nominal composition 15 mol% Li 2 O·85 mol% SiO 2 . AC conductivity measurements were carried out using the complex impedance method. Measurements were made on dried gels heated from room temperature to 600°C, with blocking electrodes and frequencies from 0.1 Hz to 65 kHz. Heat treated gels were checked with X-ray diffraction for the presence of crystals. Over the temperature range where conductivities were measurable, the lithium nitrate gels had the highest conductivities (> 10 −3 (Ω cm) −1 at 300°C), lithium chloride gels had conductivities two orders of magnitude lower, and lithium sulfate and lithium acetate gels had the lowest values and the highest activation energies. Only the lithium nitrate gels appeared to have salt crystals which assisted in ionic conductivity.

Ac complex impedance, dc resistivity, 7Li and 23Na NMR studies of the layered Li(Na)xMo2O4 system

Abstract Layered lithium molybdates of formula LixMo2O4 have been shown earlier to be promising cathode materials for secondary batteries. Nevertheless, a better understanding of the evolution of their transport properties as a function of Li composition and water intercalation is still required. This paper reviews the recent progress towards this understanding. ac and dc measurements have been used to describe the transport properties of the following compositions: Li1.3Mo2O4, Li2Mo2O4, Li0.5(H2O) 1.3Mo2O4. A combined analysis of the Arrhenius plots of conductivity and NMR spectra for the above samples suggests only one type of atomic site, between the MoO2 layers, for the Li ions, in agreement with the results obtained by X-ray crystallographic studies. A similar study of the corresponding sodium phases: Na0.5mo2O4, Na0.9Mo2O4, Na1.3Mo2O4 and Na0.4(H2O)1.2 Mo2O4, is also presented.

Applications of AC Complex Impedance Spectroscopy to Fast Ion Conducting Lithium Silicate Gels

Alkali silicate gels, and in particular, lithium silicate gels, have been studied over the last ten years because they represent a new approach to preparing fast ion conductors.1Initially, the idea was that gel-prepared alkali silicates and conventional melted alkali silicates were chemically and structurally identical.2, 3 This turns out not to be the case.4The fact that gel-prepared glasses and conventional glasses are not identical in all ways has set off an explosion of experimental studies on gels.5, 6 At the same time, this fact has created an expectation that physical properties, especially transport properties, will not be the same in gel-prepared and conventional silicates of similar composition. Based on this expectation, the interest in transport properties has increased and many fast ion conductors from gels have been synthesized and characterized.7