J.D. Robertson

Fabrication and characterization of amorphous lithium electrolyte thin films and rechargeable thin-film batteries

Amorphous oxide and oxynitride lithium electrolyte thin films were synthesized by r.f. magnetron sputtering of lithium silicates and lithium phosphates in Ar, Ar + O2, Ar + N2, or N2. The composition, structure, and electrical properties of the films were characterized using ion and electron beam, X-ray, optical, photoelectron, and a.c. impedance techniques. For the lithium phosphosilicate films, lithium ion conductivities as high as 1.4 × 10−6 S/cm at 25 °C were observed, but none of these films selected for extended testing were stable in contact with lithium. On the other hand, a new thin-film lithium phosphorus oxynitride electrolyte, synthesized by sputtering Li3PO4 in pure N2, was found to have a conductivity of 2 × 10-6 S/cm at 25 °C and excellent long-term stability in contact with lithium. Thin-films cells consisting of a 1 μm thick amorphous V2O5 cathode, a 1 μm thick oxynitride electrolyte film, and a 5 μm thick lithium anode were cycled between 3.7 and 1.5 V using discharge rates of up to 100 μA/cm2 and charge rates of up to 20 μA/cm2. The open-circuit voltage of 3.6 to 3.7 V of fully-charged cells remained virtually unchanged after months of storage.

Electrical properties of amorphous lithium electrolyte thin films

The impedance of xLi2O·ySiO2·zP2O5 thin films deposited by RF-magnetron sputtering was analyzed using two models in which the frequency dependence of the bulk response was represented by: (1) a Cole-Cole dielectric function and (2) a constant phase angle element. Increases in the conductivity with Li2O concentration and with addition of SiO2 to Li2O-P2O5 compositions are attributed to an increase in Li+ mobility caused by changes in the film structure. A new amorphous oxynitride electrolyte, Li3.3PO3.9N0.17, prepared by sputtering Li3PO4 in N2, has a conductivity at 25°C of 2×10−6S/cm and is stable in conta with lithium.

Nanocrystalline Li{sub x}Mn{sub 2{minus}y}O{sub 4} cathodes for solid-state thin-film rechargeable lithium batteries

Thin-film cathodes of lithium manganese oxide, 0.3-3 μm thick, were deposited by rf magnetron sputtering of a LiMn 2 O 4 ceramic target onto unheated substrates. The resulting films were dense, ∼4.2 g/cm 3 , with a ∼50 A nanocrystalline spinel structure. The film composition was typically Li x Mn 2-y O 4 with y ∼ 0.3 and 1.2 < x < 2.2. When cycled in a thin-film rechargeable lithium battery, specific cathode capacities of 145 ± 23 and ∼270 mAh/g were realized for discharge from 4.5 V to either 2.5 or 1.5 V, respectively. The discharge and charge current densities were limited by the resistivity of lithium transport into and through the cathode. After thousands of cycles at 25°C, there was a small increase in cell resistance. After several hundred cycles at 100°C, the discharge curves developed a stable knee at ∼4 V characteristic of crystalline LiMn 2 O 4 cathodes. The polarization of the discharge/charge cycles were interpreted in terms of free energy of mixing curves.

Lithium manganese nickel oxides Li{sub x}(Mn{sub y}Ni{sub 1{minus}y}){sub 2{minus}x}O{sub 2}. 1: Synthesis and characterization of thin films and bulk phases

The series Li x (Mn y Ni 1-y ) 2-x O 2 for x ≤ 1.33 and 0.38 ≤ y ≤ 0.50 shows a very close relationship to its parent series Li x Ni 2-x O 2 . The refine lattice parameters for at least 0.93 ≤ x ≤ 1.26 are a linear function of the concentration ratio Li/(Mn + Ni) which in turn is proportional to the averaged valence state of the transition metals. Li x (Mn y Ni 1-y ) 2-x O 2 is able to reversibly coprecipitate/reinsert Li 2 O and release/absorb O 2 . This self-regulation mechanism seems to always adjust the number of cations to an undisturbed oxygen sublattice according to the rule cations/anions = 1, which holds true at least for temperatures up to 800°C and oxygen partial pressures above 10 -5 atm. Samples prepared in air and under O 2 did not show nucleation of Li 2 O, not even for x > 1.0. The series Li x (Mn y Ni 1-y ) 2-x O 2 where 0.38 ≤ y ≤ 0.50 crystallizes in a rhombohedral unit cell (space group R3m) for x 1.25. The similarity between Li x Ni 2-x O 2 and Li x (Mn y Ni 1-y ) 2-x O 2 strongly suggests a rhombohedral → cubic transition at x 0.6 for the latter series. Derived from the linear dependence of the X-ray density on the stoichiometric parameter x, an equation was found with which the lithium concentration of Li x (Mn y Ni 1-y ) 2-x O 2 thin film phases over the entire range 0 ≤ x ≤ 1.33 can be determined accurately without extensive ion-beam analysis. XPS measurements on a film with the bulk stoichiometry Li 1.10 Mn 0.39 Ni 0.51 O 2 gave evidence for Mn 4+ and Mn 3+ , but no indication was found for nickel valence states other than Ni 2+ In order to meet the above-given stoichiometry, the averaged nickel valence state had to increase with film depth

Lithium Manganese Nickel Oxides Li x ( Mn y Ni1 − y ) 2 − x O 2 II. Electrochemical Studies on Thin‐Film Batteries

In a lithium thin-film battery, the reversible discharge capacity of a Li 1.12 Mn 0.44 Ni 0.44 O 2 cathode deposited by rf magnetron sputtering and postannealed at 750°C under O 2 could be increased by 80% to 136 μAh/mg when cycled between 4.8-2.5 V instead of 4.2-2.5 V. An 82 atom % Li 1.03 Mn 0.42 Ni 0.55 O 2 /18 atom % Li 2 O (14 vol %) cathode prepared from a rf magnetron sputter-deposited film that was annealed at 700°C under N 2 supplied a reversible discharge capacity of 146 μAh/mg between 5.3-1.5 V. For a given lithium concentration in the cathode during cycling, the magnitude of the chemical potential of sites on the lithium layers (3a sites) in both rhombohedral cathode phases decreased whenever the charge cutoff voltage was raised. This thermodynamic change is attributed to the migration of transition metal ions from the 3b layer sites to vacancies on the lithium layers at high potentials. These transition metal ions also explain the kinetic limitations the cathodes exhibited at higher current densities. Only one rhombohedral phase could be detected by ex situ X-ray diffraction (XRD) measurements over the voltage range 4.6-1.5 V. At 1.5 V, however, possible additional phases might have been present but not detectable due to their low concentration and/or their X-ray amorphousness. The maximum valence state of the transition metal ions of +4 was reached in rhombohedral Li x Mn 0.44 Ni 0.44 O 2 at 4.6 V where about 0.4 Li + per formula units remained on the lithium layers. Such a high lithium concentration between the MO 2 slabs (M = metal ion on 3b layer sites) prevented the phase from developing a unit cell with the extremely small c axis parameter found for NiO 2 and CoO 2 and is believed to be an important prerequisite for the good cycle stability of Li x Mn 0.44 Ni 0.44 O 2 between 4.8-2.5 V

Sputtering of lithium compounds for preparation of electrolyte thin films

Abstract Several lithium compounds have been sputtered in a planar rf magnetron source. Lithium orthophosphate was observed to sputter normally and nearly stoichiometric films were obtained. Sputtering of lithium orthosilicate, however, resulted in extremely lithium deficient films. This was due to the decomposition of the target and the segregation of large amounts of Li 2 O to the unsputtered areas of the target.

Radio‐frequency magnetron sputtering of pure and mixed targets of Li4SiO4, Li3PO4, and Li2O

Pure and mixed targets of Li4SiO4, Li3PO4, and Li2O were sputtered in both argon and argon/oxygen process gases using a 1‐in. diam radio‐frequency planar magnetron source. The appearance of the sputter target, the self‐bias voltage of the target, the film deposition rate, and the optical emission of the plasma were monitored during film growth. The films were analyzed by energy dispersive x‐ray spectrometry, proton induced gamma emission, and atomic spectrometry. Films grown from targets of Li3PO4 were found to be near stoichiometric. In contrast, films grown from targets of, or containing, Li4SiO4 were lithium deficient due to the decomposition and segregation of Li2O away from the sputtered area on the surface of the target. A similar redistribution of material was observed for Li2O targets as well. Reproducible and homogeneous film compositions with stoichiometric lithium concentrations can best be achieved by codeposition with independently controlled sputter sources onto a rotating substrate.

Ion beam analysis of the bone tissue of Alzheimer's disease patients

Abstract It has been hypothesized that perturbations in element metabolism play a role in the etiology and/or pathogenesis of Alzheimers disease (AD). No conclusion regarding this hypothesis has been reached, however, as results for central nervous system tissues from different research groups are contradictory. We are currently utilizing external-beam thick-target FIXE and PIGE analyses to investigate the elemental concentrations in the bone tissue of AD patients. Because bone acts as a “repository” for many trace elements, these measurements should provide information on the long-term trace-element status of AD patients. With the simultaneous PIXE/PIGE measurements, we are able to instrumentally determine the concentrations of oxygen, phosphorus, calcium, and 12–15 minor and trace elements in a single 30 min irradiation. Initial results obtained from the IBA measurements of both cortical and trabecular bone autopsy samples from four AD patients and twelve age-matched controls indicate a possible imbalance in Zn, Br and Rb.

Ion beam analysis of lithium-ion conducting amorphous electrolyte films

The Li/Si and Li/P ratios in 1–2 νm thick xLi2O: ySiO2: zP2O5 amorphous electrolyte films were determined by proton-ind gamma-ray emission analysis (PIGE) using a 4 MeV proton beam. These measurements were performed with the new IBA facility which has been installed at the University of Kentucky 7.5 MV, Model CN Van de Graaff accelerator. The nondestructive PIGE determination of the Li content of the films played an integral role in understanding this three component system as the PIGE measurements proved to be more reliable than either SIMS or ICP-AE analyses. The conductivity of the films was strongly dependent on the Li2O content; reduction of the Li2O content of the films resulted in large decreases in the mobility of the lithium ions.

A novel preconcentration technique for the PIXE analysis of water

Abstract The potential of using dried algae as a novel preconcentration technique for the analysis of water samples by PIXE was examined. The algae cells were found to contain significant levels of P and S, indicative of phosphorous- and sulfur-containing groups on the cell wall or inside the algae cells which may serve as potential binding sites for metal ions. When C. vulgaris was used on mixed metal solutions, linear responses were observed for Ag+, Ba2+, and Cd2+ in the concentration range from 10 ng/g to 1 μg/g; for Cu2+ and Pb2+ from 10 ng/g to 5 μg/g; and for Hg2+ from 10 ng/g to 10 μg/g. When S. bacillaris was used, linear responses were observed from 10 ng/g up to 10 μg/g for all of the metal cations investigated. The PIXE results demonstrated that metal binding at low concentrations involves replacement of sodium on the cell wall and that at high concentrations magnesium was also replaced. Competitive binding studies indicate that the metal ions, Ag+, Ba2+, Cd2+, Cu2+, and Pb2+, share common binding sites with binding efficiencies varying in the sequence of Pb2+ > Cu2+ > Ag2+ > Cd2+ > Ba2+. The binding of Hg2+ involved a different binding site with an increase in binding efficiency in the presence of Ag+.