Effects of rough interface on impedance of solid LiPON in MIM cells

The influences of the rough interface on the impedance of solid lithium phosphorus oxynitride (LiPON) in its metal/insulator/metal (MIM) cells over a wide frequency are studied. Using magnetron sputtering method, Al/LiPON/Al cells with different rough interfaces are fabricated. With increasing the Al films electrode roughness, the deposited LiPON films on these electrodes are correspondingly roughened, which result the interface in MIM cell roughened. Rough LiPON films make their ionic resistance decreasing and the real contact area and the bulk capacitance of MIM cells increasing. These lead to the ionic conductivity of LiPON films improved from 1.09 to 2.70 μS/cm. Meanwhile, the bulk capacitance and the electrolyte resistance in MIM cells could be separated by changing the interface roughness. Rough interfaces in MIM cells contribute to the differences in the morphology of LiPON thin films, which result in the decrease of the activation energy of ionic conductivity E _g from 0.45 to 0.38 eV. Without changing LiPON components ratio and sputtering conditions for the thin films deposition, or using any heat treatment, we demonstrate an effective way to improve the ionic conductivity by increasing the roughness of the contact electrode in MIM cells, corresponding roughening the LiPON films and decreasing its ionic impedance.     

Investigation of the solid-state electrolyte/cathode LiPON/LiCoO<Subscript>2</Subscript> interface by photoelectron spectroscopy

The electronic structure of a solid electrolyte/solid electrode interface (SESEI) of an all-solid-state thin film battery was investigated. The thin film battery consisted of a LiPON solid electrolyte and a LiCoO₂ cathode. The lithium phosphorus oxynitride (LiPON) electrolyte was RF sputtered in a step-by-step procedure onto the cathode and investigated by photoelectron X-ray-induced spectroscopy after each deposition step. An intermediate layer was found—composed of some new species—that differs in its chemical composition from the cathode as well as the LiPON solid electrolyte material and changes with growing layer thickness. In contrast, the electronic structure of the underlying cathode material remained predominantly unchanged.     

Polymer electrolytes plasticized with hyperbranched polymer for lithium polymer batteries

Hyperbranched polymers (HBPs) with different terminal groups and different ethylene oxide (EO) chain lengths were prepared, and the influence of the HBP structures including molecular weights and molecular weight distribution on the ionic conductivity and the mechanical property of the composite polymer electrolytes composed of poly (ethylene oxide) (PEO), HBP, BaTiO₃ as a ceramic filler, and LiN(CF₃SO₂)₂ as a lithium salt were investigated. It was found that the molecular weights of the HBP do not affect significantly the ionic conductivity, but the molecular weight distribution might affect it, and also further branching at the terminals of the HBP led to a decrease in the ionic conductivity. The HBP with longer EO chain length was effective for enhancement of the ionic conductivity in comparison with the HBP with shorter one. The increase in cross-linkable groups (acryloyl group) at the terminals of the HBP improved the tensile strength, but caused the ionic conductivity to decrease. Loosely cross-linked composite polymer electrolyte showed higher ionic conductivity and higher tensile strength than no cross-linked one. Paper presented at the Patras Conference on Solid State Ionics — Transport Properties, Patras, Greece, Sept. 14 – 18, 2004.     

Organic-inorganic nanocomposite materials prepared by the sol-gel route as new ionic conductors in quasi solid state electrolytes

Nanocomposite organic/inorganic materials made through sol-gel method exhibit high values of ionic conductivity when they were impregnated with the redox couple I ₃ ⁻ /I⁻ Two different kinds of nanocomposite materials, depending on the different interactions between silica and poly(ethylene)oxide or poly(propylene)oxide blends, were prepared by the sol-gel technique in room temperature. Gels, for both nanocomposite materials, were obtained by acetic acid catalyzed solvolysis and were regulated by formation of intermediate products, such as silicon ester and -Si-O-Si-oligomers. Time-resolved fluorescence techniques and conductivity measurements were performed in order to define the parameters which allow maximum probe mobility and minimum confinement conditions with the aim to apply these materials in quasi solid state electrolytes. Paper presented at the Patras Conference on Solid State Ionics — Transport Properties, Patras, Greece, Sept. 14 – 18, 2004.     

Interface design in nanosystems of Advanced Superionic Conductors

First attempt of practical realization of new interface engineering approach “from advanced materials to advanced devices” for nanosystems of Advanced Superionic Conductors (ASICs), based on AgI (CuI) compounds is presented. Crystallochemical method of symmetry perfect ASIC//electrode interface searching is developed. Some new theoretical results of ASIC//indifferent electrode conjugated commensurate heteropairs with coherent interfaces and preliminary experimental results of the creation of thin-film supercapacitor — prototype based on the lattice matched heterojunction — are given. Future perspectives of the ASIC//electrode interface design suited for micro(nano)electronics and microsystem technology (MST) are discussed. Paper presented at the Patras Conference on Solid State Ionics — Transport Properties, Patras, Greece, Sept. 14–18, 2004.     

Thermally activated ionic conduction and local structure in solid LiPON thin films

Compact lithium phosphorous oxynitride (LiPON) thin films, as a solid-state electrolyte for all solid-state Li batteries and electrochromic (EC) devices, with the high ratio of the triply coordinated –N< (N_t) over the doubly coordinated –N= (N_d) structural units was deposited by a conventional reactive RF magnetron sputtering of a Li₃PO₄ target at a low pure N₂ pressure. The effect of heat treatment from 200 to 500 °C on the ionic conductivity and local structure of LiPON thin films were investigated by scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS) core level analysis. A dramatical improvement of ionic conductivity from 1.1 to 3.28 μS/cm and microstructure changes were happened on the LiPON thin films while annealed for 1 h at 300 °C, which was linked to structural differences with a highest ratio of –N< over –N= structural units. The work proves that a proper heat treatment on LiPON thin film can effectively improve its ionic conductivity and change its microstructure.     

Hole Injection Enhancement of MoO_3/NPB/Al Composite Anode

An ultra-thin molybdenum(VI) oxide(MoO_3) modification layer can significantly improve hole injection from an electrode even though the MoO_3 layer does not contact the electrode. We find that as the thickness of the organic layer between MoO_3 and the electrode increases, the hole injection first increases and it then decreases.The optimum thickness of 5 nm corresponds to the best current improvement 70%, higher than that in the device where MoO_3 directly contacts the Al electrode. According to the 4,4-bis[N-(1-naphthyl)-N-phenyl-amino] biphenyl(NPB)/MoO_3 interface charge transfer mechanism and the present experimental results, we propose a mechanism that mobile carriers generated at the interface and accumulated inside the device change the distribution of electric field inside the device, resulting in an increase of the probability of hole tunneling through the injection barrier from the electrode, which also explains the phenomenon of hole injection enhanced by MoO_3/NPB/Al composite anode. Based on this mechanism, different organic materials other than NPB were applied to form the composite electrode with MoO_3. Similar current enhancement effects are also observed.     

Mixed conductivity of glasses in the P2O5-V2O5-Na2O system

The conductivity of glasses in the P₂O₅−[(1−x) V₂O₅−x Na₂O] system is studied as a function of temperature and composition. For all compositions, the conductivity variations as a function of temperature follow an Arrhenius type relationship: . The activation energies and pre-exponential factors corresponding to the V₂O₅ richest compositions are lower than that corresponding to the ionic ones. Isothermal variations of the conductivity as a function of composition show a deep minimum for a molar ratio x near 0.65. On either side of this minimum, the conductivity is mainly electronic (x<0.7) or ionic (x>0.8). The variations are interpreted assuming a prevailing diluting effect of the non predominantly present oxide without any interactions between the electronic and ionic charge carriers. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998.     

Computational screening of doping schemes for LiTi2(PO4)3 as cathode coating materials

In all-solid-state lithium batteries,the impedance at the cathode/electrolyte interface shows close relationship with the cycle performance.Cathode coatings are helpful to reduce the impedance and increase the stability at the interface effectively.LiTi2(PO4)3(LTP),a fast ion conductor with high ionic conductivity approaching 10^-3S·cm^-1,is adopted as the coating materials in this study.The crystal and electronic structures,as well as the Li^+ion migration properties are evaluated for LTP and its doped derivatives based on density functional theory(DFT)and bond valence(BV)method.Substituting part of Ti sites with element Mn,Fe,or Mg in LTP can improve the electronic conductivity of LTP while does not decrease its high ionic conductivity.In this way,the coating materials with both high ionic conductivities and electronic conductivities can be prepared for all-solid-state lithium batteries to improve the ion and electron transport properties at the interface.     

Composite doped emeraldine–polyethylene oxide-bonded lithium-ion nano-tin anodes with electronic–ionic mixed conduction

Mixed-conducting lithium-ion doped emeraldine polyaniline (PAni)–polyethylene oxide (PEO) blends have been developed to achieve an optimal electronic–ionic conductivity balance in nano-tin composite anodes. Electrochemical evaluation was performed on the anodes with differing electrode preparation procedures, doping methods and PEO contents. Results indicate that both good electronic and ionic conductivity in the binder are required for rapid lithium insertion/extraction and low polarization. This doped PAni–PEO polymer blend is an attractive binder for high capacity composite anodes with low polarization.     

P-Type electronic conduction in CeO2- and LaGaO3-based solid electrolytes

Modifications of the e.m.f. and faradaic efficiency techniques, taking into account electrode polarization in the measuring cells, in combination with the use of electrodes having sufficiently high polarization resistances enable a precise determination of minor electronic contributions to the conductivity of solid electrolytes. These methods were used to determine the p-type conductivity of compositions based on La(Sr)Ga(Mg)O_3-δ (LSGM) and Ce(Gd)O_2-δ (CGO) at 900–1270 K. The oxygen ion transference numbers of these materials under oxygen/air gradient vary in the range 0.999–0.970, increasing with decreasing temperature. Substitution of 2 % gadolinium in Ce_0.80Gd_0.20O_2-δ with praseodymium was found to increase the electron-hole conduction by 2.5 – 4 times. At temperatures above 700 K, both the partial oxygen ionic and p-type electronic conductivities of LaGaO₃-based phases are higher than those in CGO. The electron-hole transport in LSGM tends to increase with the magnesium concentration, while the activation energy is essentially independent of composition. Electronic conduction in CGO and LSGM electrolytes was also found to be influenced by the ceramic microstructure. Paper presented at the 8th EuroConference on Ionics, Carvoeiro, Algarve, Portugal, Sept. 16–22, 2001.     

Mixed conductive electrode materials for sensors and SOFC

Modified active electrode materials based upon rare earth manganites were developed for different solid electrolyte electrochemical cells. The preparation, structure, thermal expansion, the state of oxygen on the surface, the electronic and ionic conductivity of the perovskites Ln_1−xCa(Sr)_xMn_1−y(Co, Ni)_yO_3−δ with various compositions and electrode kinetics on the manganite electrode/solid electrolyte interfaces were investigated. The value of the bulk conductivity was larger than 150 S/cm (at 1100 K) and increased significantly with increasing contents of Ni or Co. The thermal expansion coefficients of rare earth manganites were close to those of ZrO₂ based solid electrolytes. The expansion coefficients of Co or Ni subsituted lanthanum manganites increase with Co or Ni substitution and are over 12•10⁻⁶K⁻¹. The ionic conductivities were determined using encapsulated zirconia microelectrodes based on a Hebb-Wagner analysis of the currentvoltage curves. The relatively high oxide ion conductivity of 10⁻⁵ S/cm at 900...1000 K was found by Ni or Co doped manganites. Studies of the electrode kinetics using complex impedance spectroscopy show that Co and Ni doped manganites have advantages if used as electrodes as compared with these for noble metals. Paper presented at the 1st Euroconference on Solid State Ionics, Zakynthos, Greece, 11–18 Sept. 1994     

Oxygen ion transport numbers: assessment of combined measurement methods

Several modifications of the faradaic efficiency and electromagnetic field (EMF) methods, taking electrode polarisation resistance into account, were considered based on the analysis of ion transport numbers and p-type electronic conductivity of ceramics at 973–1,223 K. In air, the activation energies for p-type electronic and oxygen ionic transport are 115 ± 9 and 71 ± 5 kJ/mol, respectively. The oxygen ion transference numbers vary in the range 0.992–0.999, increasing when oxygen pressure or temperature decreases. The apparent electronic contribution to the total conductivity, estimated from the classical faradaic efficiency and EMF techniques was considerably higher than true transference numbers due to a non-negligible role of interfacial exchange processes. The modified measurement routes give reliable and similar results when p(O₂) values at the electrodes are high enough, whilst decreasing the oxygen pressure leads to a systematic error for all techniques associated with measurements of concentration cell EMF. This effect, presumably due to diffusion polarisation, increases with decreasing temperature. The most reliable results in the studied p(O₂) range were provided by the modified faradaic efficiency method.     

Electrochemical characterization of a composite polymer electrolyte with improved lithium metal electrode interfacial properties

In the development of rechargeable lithium polymer batteries it is of paramount importance to control the passivation phenomena occurring at the lithium electrode interface. It is well estabilished that the type and the growth of the lithium passivation layer is unpredictably influenced by the presence of liquid components and/or impurities in the electrolyte. Therefore, one approach to improve the stability of the lithium interface is the use of liquid-free, highly pure electrolytes. The electrochemical properties of a composite polymer electrolyte obtained by hot pressing a mixture of polyethylene oxide (PEO), a lithium salt (lithium tetrafluoroborate, LiBF₄) and a powdered ceramic additive (γ-LiAlO₂), will be presented and discussed. The electrochemical characterization included the determination of the ionic conductivity, the anodic break-down voltage and, most importantly, the stability of the lithium metal electrode interface and the lithium stripping-plating process efficiency. The main feature of this dry, true solid-state electrolyte is a very good compatibility with the lithium metal electrode, demonstrated by a very high lithium cycling efficiency, which approaches a value of 99%. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998.     

Proton and native-ion conductivities in Y2O3 at high temperatures

The ionic conductivity of hot-pressed samples of undoped Y₂O₃ has been studied by the emf-method in atmospheres of controlled oxygen and water-vapor pressures. The variation in the ionic conductivity was studied as a function of time (7 months at 1200°C), temperature (600–1300°C), water-vapor pressure (3–1400 Pa), and oxygen pressure (10⁻¹⁰ −10⁵ Pa). The overall conductivity can be divided into contributions from electronic carriers (mainly electron holes), native ionic defects, and hydrogen defects. The transport of charged hydrogen species is dominated by migration of “free” protons. The hydrogen-ion conductivity is detectable under all conditions and becomes the dominant ionic-conductivity contribution at high water-vapor pressures and low temperatures. The ionic contributions are discussed in terms of grain-boundary and bulk transport properties. Native-ion and proton-diffusion coefficients in yttria are estimated. Equations for the emf of oxide specimens containing charged hydrogen defects have been derived.     

Ionic conduction and chemical bonds of Li ion in La4/3−yLi3yTi2O6

The electronic state of La_4/3−yLi_3yTi₂O₆ (y=0.21) was studied by the DV-Xα cluster method. Four model clusters were used to calculate the density of state (DOS), the bond overlap population (BOP) and the net charge (NC). A Li ion in the model cluster was moved from 1b site to another 1b site along the x axis, and the BOP and the NC calculated were discussed. Furthermore, we calculated the potential energy with the movement of the Li ion along the x axis. Paper presented at the Patras Conference on Solid State Ionics - Transport Properties, Patras, Greece, Sept. 14 – 18, 2004.     

The nature of conduction pathways in mixed alkali phosphate glasses

In this paper we discuss the nature of the ion conduction pathways in Li_xRb_1−xPO₃ glasses. Our investigations are based on a bond valence analysis of reverse Monte Carlo (RMC) produced structural models in quantitative agreement with neutron and X-ray diffraction data. In a previous letter [11] we have shown that this approach enables us to reproduce and understand the mixed alkali effect (MAE) directly from the structural models. The results have shown that the drastic drop of the conductivity for an intermediate composition (x ≈ 0.5) is mainly caused due to the blocking by immobile unlike cations, which is highly effective since the two types of alkali ions are randomly mixed and have distinctly different conduction pathways of low dimensionality. Here, we explore the local dimensionality of the pathways and discuss its implications for the network of pathways and the related ionic conductivity. Paper presented at the Patras Conference on Solid State Ionics — Transport Properties, Patras, Greece, Sept. 14 – 18, 2004.     

Effect of melt compounding temperature on dielectric relaxation and ionic conduction in PEO–NaClO<Subscript>4</Subscript>–MMT nanocomposite electrolytes

Polymer nanocomposite electrolytes (PNCEs) of poly(ethylene oxide) and sodium perchlorate monohydrate complexes with montmorillonite (MMT) clay up to 20 wt.% MMT concentration of poly(ethylene oxide) (PEO) are synthesized by melt compounding technique at melting temperature of PEO (∼70 °C) and NaClO₄ monohydrate (∼140 °C). Complex dielectric function, electric modulus, alternating current (ac) electrical conductivity, and impedance properties of these PNCEs films are investigated in the frequency range 20 Hz to 1 MHz at ambient temperature. The direct current conductivity of these materials was determined by fitting the frequency-dependent ac conductivity spectra to the Jonscher power law. The PNCEs films synthesized at melting temperature of NaClO₄ monohydrate have conductivity values lower than that of synthesized at PEO melting temperature. The complex impedance plane plots of these PNCEs films have a semicircular arc in upper frequency region corresponding to the bulk material properties and are followed by a spike in the lower frequency range owing to the electrode polarization phenomena. Relaxation times of electrode polarization and ionic conduction relaxation processes are determined from the frequency values corresponding to peaks in loss tangent and electric modulus loss spectra, respectively. A correlation is observed between the ionic conductivity and dielectric relaxation processes in the investigated PNCEs materials of varying MMT clay concentration. The scaled ac conductivity spectra of these PNCEs materials also obey the ac universality law.     

Ionic conductivity in hydrogels for contact lens applications

Biphasic hydrogel polymers are in the forefront of new extended wear contact lens development. In the biphasic hydrogel the objective is to produce co-continuous domains of siloxane units for high oxygen permeability, coupled with hydrophilic units forming aqueous channels for hydraulic and ion mobility. These are distributed in phase separated nano-scale regions such that the material is optically clear while achieving the required properties to maintain corneal health and lens movement. This paper describes how Impedance Spectroscopy permits a rapid measurement of ion conductivity in a range of silicone and non-silicone hydrogel materials with water contents ranging from 18% to 75% equilibrium water content. For non-silicone hydrogels relative sodium ion conductivity follows a typical percolation curve. However, for silicone hydrogels ion mobility is three orders of magnitude higher than conventional hydrogels of the same equilibrium water content. The influence of electrolyte concentration, interfacial electrode sample contact pressure and temperature are also reported. Paper presented at the Patras Conference on Solid State Ionics — Transport Properties, Patras, Greece, Sept. 14 – 18, 2004.