Phosphate-based Electrolyte Research Articles

Effects of reagent purity on plasma electrolytic oxidation of titanium in an aluminate–phosphate electrolyte

The effect of reagent purity on AC plasma electrolytic oxidation of titanium in aluminate–phosphate-based electrolytes has been studied under constant rms current conditions. The resultant coatings were examined by high-resolution electron microscopy and X-ray diffraction, revealing rutile, Al2TiO5, α-Al2O3 and γ-Al2O3, with an aluminium-enriched outer region and phosphorus-enriched band near the coating base. A lower purity of the electrolyte led to an erratic voltage response and delamination of coating material in scratch tests. A thin chlorine-enriched region near the substrate, attributed to the inward migration of chloride ions during coating growth, is suggested to be the cause of the delamination. Deliberate additions of chloride to the electrolyte resulted in highly porous coatings.

A study on microstructure and corrosion resistance of ZrO2-containing PEO coatings formed on AZ31 Mg alloy in phosphate-based electrolyte

ZrO2-containing ceramic coatings formed on the AZ31 Mg alloy were fabricated in an alkaline electrolyte containing sodium phosphate and potassium fluorozirconate (K2ZrF6) by plasma electrolytic oxidation (PEO). X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) techniques were used to study the phase structure and composition of the coatings. It is indicated that the coatings formed in the K2ZrF6-containing electrolyte were composed of MgO, MgF2 and t-ZrO2. Morphological investigation carried out by scanning electron microscopy (SEM) and stereoscopic microscopy, revealed that the uniformity of coatings increased and roughness of coatings decreased after the addition of K2ZrF6. Electrochemical investigation was achieved by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) test. The results showed that the PEO coating formed in K2ZrF6-containing electrolyte exhibited an improved corrosion resistance than that of the coating formed in K2ZrF6-free electrolyte. In addition, the polarization and EIS tests results both showed that the suitable concentration (2.5g/l) of K2ZrF6 is of significant ability to improve the corrosion resistance of coatings. However, 5g/l and 10g/l K2ZrF6 has a negative effect on improving the corrosion resistance of PEO coatings compared with the coating formed in 2.5g/l K2ZrF6-containing electrolyte.

(Invited) Challenges with Quantum Chemistry-Based Screening of Electrochemical Stability of Lithium Battery Electrolytes

Challenges with the quantum chemistry based-screening of electrochemical stability of solvents and salts with potential applications in lithium batteries are discussed. Initial high throughput screening of carbonate and phosphate-based electrolyte solvents provided insight into first and second reduction and oxidation potentials and reorganization energies of these solvents. It has been found that it was important to include a lithium cation in the screening of semifluorinated solvents. Two reduction pathways has been found for lithium complexed with semifluorinated solvents and salts such as lithium bis(fluorosulfonyl)imide (LiFSI): the low rate defluorination reaction occurring at high potentials and fast solvent or anion reduction occurring at significantly lower potentials. A spontaneous deprotonation of carbonate solvents at the surface of the completely de-lithiated LiNi0.5Mn1.5O4 cathode has been found.

Effect of nanocrystalline surface of substrate on microstructure and wetting of PEO coatings

In this research, effect of surface mechanical attrition treatment (SMAT) was studied on characteristics of the coatings formed using plasma electrolyte oxidation (PEO) on AA1230 aluminium. To do so, first, the samples were coated with and without SMAT pre-process. Coating by PEO method was carried out in two phosphate-based and silicate-based suspensions of silicon nitride nanoparticles for 10 min. Next, to study the distribution of the nanoparticles and elements inside the coating, surface morphology of the samples was examined using scanning electron microscopy and energy dispersive spectroscopy. In addition, wettability test was performed on all samples using the deionized water. Also, droplet expansion manner on the porous oxide coating surface and its relation with fine structure of the coating was investigated. The results indicated that samples coated in silicate-based electrolyte have a relatively rougher microstructure as compared to samples coated in phosphate-based electrolyte. The average surface pores area percentage of the samples coated in silicate-based electrolytes was 13.9% greater than that of samples coated in phosphate-based electrolyte. Moreover, the average height to diameter (h/d) value obtained from wettability test for samples coated in silicate-based electrolyte was 13.3% greater than that of phosphate-based electrolyte.

Specific Electrical Conductivity in Molten Potassium Dihydrogen Phosphate KH2PO4 Electrolyte at ~300 °C

The conductivity of puremolten KH2PO4 salt and two mixtures with more and less water (in the KH2PO4–H2O and KH2PO4–KPO3 systems) were measured at temperatures of 265–320 °C and under their own water vapor pressures. Molten KH2PO4 is a promising electrolyte for an intermediate temperature pressurized water electrolyzer because of its high conductivity of ~0.30 Ohm-1 cm-1 at 300 °C. The conductivity data are given as polynomial functions of temperature and composition. The KH2PO4 (KDP) salt is one of the most used nonlinear crystals for laser radiation conversion required in laser fusion systems [1], and recently also the application of the melt as the electrolyte for high temperature water electrolysis was suggested [2]. The purpose of the present work was to investigate the level of specific electrical conductivity of the melt, of which there was no previous data available. Also it was an objective to determine analytical expressions for the conductivity versus temperature and water content. Such information is essential in the optimization of the high temperature water electrolysis application of the electrolyte. Upon heating, the salt melts and becomes a useful electrolyte [2]. During the melting process, the following reaction (1) is known to take place: 2 KH2PO4 <--> K2H2P2O7 + H2O (1) whereby the melt loses water during evaporation and the melting point drops considerably, from the true value ~280 °C perhaps down to ~262 °C. In an enclosed ampoule, the water vapor pressure over the molten salt is considerable, around 8 bars at 300 ºC, according to our recent results, obtained by Raman spectroscopy [3]. The obtained data are reproduced in Fig. 1 as points along 3 curves, obtained by plotting the logarithm of the conductivity σ (ordinate scale) as a function of reciprocal absolute Kelvin temperature T (against 1000/T): Log10σ = A - B x 1000/T (2) As expected, the curves are approximately linear; i. e. the σ data can be reproduced by (2). The conductivity (2) decreases with addition of KPO3 (or the loss of water in open cells) and increases with temperature in the range between ~180 and ~300 °C. The studied compositions and temperature ranges were limited by the crystallization points and the vapor pressure (note the risk of explosion) that depended markedly on the temperature [3]. During standing, the viscosity and surface tension increased significantly, judging from visual observations when shaking the melts (dissolution of SiO2?). [1] W. Cai and A. Katrusiak, “Structure of high-pressure phase KH2PO4”, Dalton Trans., vol. 42, pp. 863-866, 2013. [2] C. B. Prag, “Intermediate Temperature Steam Electrolysis with Phosphate-Based Electrolytes.” Ph.D. thesis, supervisors: N. J. Bjerrum, E. Christensen, Li Qingfeng, I. M. Petrushina, Kgs. Lyngby: DTU Energy, 1- 154, 2014. [3] R. W. Berg, A.V. Nikiforov, I. M. Petrushina and N. J.Bjerrum, “Determination of water vapor pressure over and demonstration of water electrolysis in a molten potassium dihydrogen phosphate KH2PO4 at ~300 °C by means of Raman spectroscopy”, EEST2015, extended abstract, 16-22 August 2015, Vancouver, Canada. Fig. 1. Conductivity results as explained in the text. Insert shows cell of fused quartz glass, sealed under vacuum. (A) Pressure equilibration tube, (B) Cell compartment, (C) Capillary tube for conduction and (D) Electrode feed throughs adapted from standard halogen lamps. Figure 1

A two-step anodic method to fabricate self-organised nanopore arrays on stainless steel

We report a simple two-step anodic method to fabricate self-organized nanopore arrays on 316L stainless steel surface, with the initial anodization in perchloric acid-based electrolyte followed by the second anodization in sodium dihydrogen phosphate-based electrolyte. The morphology and chemical analysis of anodic overlayers after the first and second anodizations were explored by AFM, SEM and XPS. The influence of various applied voltages, current density, various anodization times and temperatures on nanoscaled morphology was investigated in the process of the second anodizing step. As a result, the dimension of the nanopores depends linearly on the anodization time and was controlled by the applied voltage and current density in a certain range. At the premise of no appreciable expansion in the mean pore size after the second step in contrast to the initial pores, the depth of nanopores could reach seven times of the previous depth of 10nm. The analysis of XPS spectrum performed on the nanostructure, the energy shifts of Cr 2p3/2, Fe 2p3/2, O 1s, and Ni 2p3/2 levels showed that principal constituents of the anodic overlayer are chromium and iron components. No metallic Fe, Cr, and Ni peaks were detected on the surface after the second anodization. The depth of nanopores was roughly in agreement with the thickness estimations obtained from depth profiling data and different chemical compositions that exist at different depths of even a thin anodic overlayer.

A study on photocatalytic activity of micro-arc oxidation TiO2 films and Ag+/MAO-TiO2 composite films

First, micro-arc oxidation (MAO) TiO2 films have been prepared on pure titanium in a phosphate-based electrolyte, and then the Ag+/MAO-TiO2 composite films have been fabricated by Ag+ impregnation in this paper. The microstructure and composition of MAO-TiO2 films and Ag+/MAO-TiO2 composite films have been studied by means of scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy-dispersive X-ray spectroscopy (EDS). The photocatalytic activity of both films was evaluated by photocatalytic decolorization of methylene blue (MB) in aqueous solution as a model pollutant under sunlight irradiation simulation with homemade ultraviolet–visible spectroscopy (UV–vis). The results showed that the photocatalytic activity of MAO-TiO2 films increased with increasing the applied voltage and concentration in a certain scope. The morphology of Ag+/MAO-TiO2 composite films were of significantly difference and superior photocatalytic activity compared to the MAO-TiO2 film. Also, Ag+ impregnation was able to enhance the photocatalytic efficiency of MAO-TiO2 film.

Laser Melting Effects on Microstructure and Corrosion Behavior of Plasma Electrolytic Oxidation Nanocomposite Coatings on Pure Titanium

In this study, pure titanium was treated using hybrid techniques of plasma electrolytic oxidation (PEO) in phosphate-based electrolyte including alumina nanoparticles and laser surface treatment (LST) process. The aim is to control laser parameters, such that heat input is adjusted to only melt PEO coating and to preserve the interface between the coating and substrate in order to create a dense layer across the thickness of PEO coating. To accomplish this purpose, after the PEO process, surface of samples was exposed to LST by an 80W pulsed laser using the voltage of 230V with the pulse durations of 4, 6 and 8ms. X-ray diffraction was used to study the composition of phases and Scanning Electron Microscope (SEM) was used to study the morphology and porosity of surface and cross-sectional area and also the distribution of elements in both coating and substrate. The corrosion behaviour of coatings was examined by cyclic potentiodynamic polarization test. Results showed that in all samples, in the range of selected laser parameters, the interface between coating and substrate remains after the LST. The density of porosity on surface and cross-sectional area is reduced by about 75% in hybrid coatings of laser treated PEO (L-PEO) compared to PEO coatings. By an increase in pulse duration, deep cracks appear and transport electrolytes to substrates; therefore, the corrosion resistance of coatings are decreased.

High Voltage Li-Ion Battery Development

The approach of replacing the state-of-the-art Li-ion battery cathodes such as 4.2 V lithium cobalt oxide LiCoO2 and lithium nickel cobalt manganese oxide LiNi1/3Co1/3Mn1/3O2 with high voltage cathodes such as 4.7 V lithium nickel manganese spinel LiMn1.5Ni0.5O4 (LNMO) and 4.8 V lithium cobalt phosphate LiCoPO4 (LCP) [1], versus Li/Li+, for the increase of energy density has encountered problems including lower Coulombic efficiency (CE) [2-4], faster capacity fading [5], and therefore, shorter cycle life than that of the state-of-the-art Li-ion batteries have achieved. At elevated temperatures, e.g. 55 oC, the LNMO/graphite or LCP/graphite cells when cycled in the state-of-the-art Li-ion electrolytes, e.g. 1.2 M LiPF6 in EC:EMC (3:7 w/o), shows CE values can be as low as below 90% [4]. What are the issues? Electrolyte degradation at high voltages is one. The instability of the cathode at high voltage is another one, which may include the instability of cathode structure itself and the high cathode reactivity with the electrolytes. Additionally, the degradation at the cathode side also plays a role in degrading the anode resulting in poor cell performance. The understanding of electrolyte oxidative stability at high voltages through computation [6] and surface analysis of reaction products on the surfaces of both cathode and anode [7] indicates that very much improved electrolytes are urgently needed. The modification of the electronic structures of LNMO and LCP through substitutions [4,5] also improves capacity retention. This suggests that the stability of the high voltage cathodes can be improved.The developments of improved electrolytes including the use of additives and fluorinated solvents for improved electrolyte stability and improved LNMO and LCP cathodes using substitution for stabilizing the cathodes will be reviewed. Recent advances coupling the improvements in high voltage cathode materials and electrolytes together for better performance will also be presented. ACKNOWLEDGEMENT The authors wish to express their gratitude to the DOE ABR program for partial financial support. REFERENCES A. Manthiram, Materials Challenges and Opportunities of Lithium Ion Batteries, Phys. Chem. Lett., 2011, 2, 176-184.H. Duncan, D. Duguay, Y. Abu-Lebdeh, I. J. Davidson, “Study of the LiMn1.5Ni0.5O4/Electrolyte Interface at Room Temperature and 60 oC,” J. Electrochem. Soc., 2011, 158 (5), A537-A545.J. Wolfenstine, U. Lee, B. Poese and J. L. Allen, “Effect of oxygen partial pressure on the discharge capacity of LiCoPO4,”J. Power Sources, 2005 144 226.D. W. Shin, C. A. Bridges, A. Huq, M. P. Paranthaman, A. Manthiram, Role of Cation Ordering and Surface Segregation in High-Voltage Spinel LiMn1.5Ni0.5−xMxO4 (M = Cr, Fe, and Ga) Cathodes for Lithium-Ion Batteries, Chem. Mater., 2012, 24, 3720-3731.J. L. Allen, T. R. Jow, J. Wolfenstine, “Improved Cycle Life of Fe-substituted LiCoPO4,” J. Power Sources, 2011, 196, 8656.O. Borodin, W. Behl, and T. R. Jow, “Oxidative Stability and Initial Decomposition Reactions of Carbonate, Sulfone and Alkyl Phosphate-Based Electrolytes,” J. Phys. Chem. C, 2013, 117, 8661-8682.D. Lu, M. Xu, L. Zhou, A. Garsuch, B. L. Lucht, “Failure Mechanism of Graphite/LiNi0.5Mn1.5O4 Cells at High Voltage and Elevated Temperature,” J. Electrochem. Soc., 2013, 160 (5), A3138-A3143.

Oxidative Stability and Initial Decomposition Reactions of Carbonate, Sulfone, and Alkyl Phosphate-Based Electrolytes

The oxidative stability and initial oxidation-induced decomposition reactions of common electrolyte solvents for batteries and electrical double layer capacitors were investigated using quantum chemistry (QC) calculations. The investigated electrolytes consisted of linear (DMC, EMC) and cyclic carbonate (EC, PC, VC), sulfone (TMS), sulfonate, and alkyl phosphate solvents paired with BF4–, PF6–, bis(fluorosulfonyl)imide (FSI–), difluoro-(oxalato)borate (DFOB–), dicyanotriazolate (DCTA–), and B(CN)4– anions. Most QC calculations were performed using the M05-2X, LC-ωPBE density functional and compared with the G4MP2 results where feasible. The calculated oxidation potentials were compared with previous and new experimental data. The intrinsic oxidation potential of most solvent molecules was found to be higher than experimental values for electrolytes even after the solvation contribution was included in the QC calculations via a polarized continuum model. The presence of BF4–, PF6–, B(CN)4–, and FSI– anions near the solvents was found to significantly decrease the oxidative stability of many solvents due to the spontaneous or low barrier (for FSI–) H- and F-abstraction reaction that followed the initial electron removal step. Such spontaneous H-abstraction reactions were not observed for the solvent complexes with DCTA– or DFOB– anions or for VC/anion, TMP/PF6– complexes. Spontaneous H-transfer reactions were also found for dimers of the oxidized carbonates (EC, DMC), alkyl phosphates (TMP), while low barrier H-transfer was found for dimers of sulfones (TMS and EMS). These reactions resulted in a significant decrease of the dimer oxidation potential compared to the oxidation potential of the isolated solvent molecules. The presence of anions or an explicitly included solvent molecule next to the oxidized solvent molecules also reduced the barriers for the oxidation-induced decomposition reaction and often changed the decomposition products. When a Li+ cation polarized the solvent in the ECn/LiBF4 and ECn/LiPF6 complexes, the complex oxidation potential was 0.3–0.6 eV higher than the oxidation potential of ECn/BF4– and ECn/PF6–.

Synergistic Effect of 2-Amino-5-(Ethylthio)-1,3,4-Thiadiazole and Chloride Ion on Cu Anodic Dissolution in a Phosphate-Based Electrolyte

Synergistic Effect of 2-Amino-5-(Ethylthio)-1,3,4-Thiadiazole and Chloride Ion on Cu Anodic Dissolution in a Phosphate-Based Electrolyte

Investigation of 5-aminotetrazole as an Corrosion Inhibitor in Phosphate-Based Electrolyte

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Synthesis and Lithium Ion Conductivities of Zirconium Phosphate-Based Solid Electrolytes

The lithium ion conducting properties of lithium- and MⅣ (M = Al, In, Y)-doped zirconium pyrophosphates synthesized via solid state reaction were investigated. The ionic conductivity of the compounds increased with increasing Li content. The activation energy of LixMxZr1-xP2O7 decreased as the lattice parameter increased owing to the enlargement of the size of bottleneck between the cavities. Li0.55(Li0.15Y0.1Zr0.75)P2O7 with a high Li content and a large lattice parameter exhibited a conductivity of 1.7×10-3 S cm-1 at 350 °C, which is sufficient for its application as a solid electrolyte for sensors.

Adsorption–desorption study of benzotriazole in a phosphate-based electrolyte for Cu electrochemical mechanical planarization

In this article, the adsorption–desorption behavior of benzotriazole (BTA) in a phosphate-based electrolyte developed for Cu electrochemical mechanical planarization (ECMP) is studied. The formation of a continuous BTA passive film adsorbed on the Cu surface has been characterized by atomic force microscopy (AFM). Additionally, the adsorption behavior of BTA was found to be mass-transfer-controlled at lower operating potentials (≤0.7 V vs. Ag/AgCl). Using a microfluidic electrochemical device and electrochemical impedance spectrum (EIS), it was also observed that at low BTA concentrations (≤0.002 M), more time was necessary to form an effective passive film on the Cu surface. Furthermore, the desorption time obtained from a potentiometric response to the removal of BTA from the electrolyte increases with increasing BTA concentration or decreasing applied potential. It is critical to expand the operating potential window and to reduce the usage of inhibitors in the proposed ECMP process to enhance the removal rate and the reduction of organic residues. Therefore, the combined microfluidic and electrochemical methodology is proven useful in finding suitable BTA concentrations and a wider potential window.

Characterization of phosphate electrolytes for use in Cu electrochemical mechanical planarization

Electrochemical mechanical planarization (ECMP) is a potential replacement or complement to conventional chemical mechanical planarization (CMP) techniques. ECMP can operate at very low downforces (<1.0 psi), potentially without slurry particles or oxidizers, and can also be tailored to achieve specific dissolution rates via applied potential. Through the modification of a phosphate-based electrolyte by the use of benzotriazole (BTA), a possible ECMP electrolyte was developed. Utilizing a rotating disk electrode (RDE), a broad range of electrolyte characteristics were screened. The most important parameters investigated were pH, salt concentration, and BTA concentration. The optimal electrolyte composition was found to be around 1 M potassium phosphate with a pH value of 2. Removal rates varied from 750 to 2500 nm min −1 within an operating potential window from 0.5 to 1.0 V vs. Ag/AgCl reference electrode.

Trivalent Sc3+ Ion Conduction in Sc1/3Zr2(PO4)3 Solids with the NASICON-Type Structure

A new solid electrolyte, Sc1/3Zr2(PO4)3, with a trivalent ion conductivity similar to that of the Sc2(WO4)3-type tungstate series is investigated. It is demonstrated that, unlike the tungstate-based electrolyte, the phosphate-based electrolyte can be used in a variety of atmospheres, including a reducing one. The Figure is an SEM image of Sc deposited at the cathodic surface after electrolysis.

Lithium scandium phosphate-based electrolytes for solid state lithium rechargeable microbatteries

Li 3Sc 2(PO 4) 3 is a promising candidate for use as an electrolyte in solid state lithium rechargeable microbatteries due to its stability in air, ease of preparation, and resistance to dielectric breakdown. The room temperature ionic conductivity was optimized resulting in an increase of over two orders of magnitude to 3×10 −6 S/ cm. The formation of Li 3(Sc 2− x M x )(PO 4) 3, where M= Al 3+ or Y 3+, resulted in the decrease of porosity, greater sinterability, and considerable enhancement of the ionic conductivity. Yttrium substitutions enhanced the conductivity slightly while aluminum increased the room temperature ionic conductivity to 1.5×10 −5 S/ cm for x=0.4. Preliminary electron beam evaporation of Li 3Sc 2(PO 4) 3 yielded amorphous thin films with ion ic conductivity as high as 5×10 −5 S/ cm and a composition of Li 4.8Sc 1.4(PO 4) 3.

Sodium phosphate-based solid electrolytes

Na 3 PO 4 is a flexible host material for making ionic substitutions that can lead to enhanced conductivity. Two doping systems (4Na→M and Na+P→Y) are reviewed. Both systems exhibit extensive ranges of solid solution in the high temperature, cubic γ form of Na 3 PO 4 . In the hexavalent, Y systems, solid solution formation is accompanied by significant increases in lattice parameter (≤0.1 A); in the tetravalent, M systems, small increases or decreases occur on doping (≤0.02 A). In both sets of systems, conductivity increases markedly with sodium ion vacancy concentration; maximum conductivities are in excess of 0.01 ohm −1 cm −1 at 300°C. The factors affecting conductivities and the reasons for the sigmoidal Arrhenius conductivity plots are discussed.