Trivalent Nickel Research Articles

Polyaniline Induced Trivalent Ni in Laser-Fabricated Nickel Oxides for Efficient Oxygen Evolution Reaction.

Although it is generally acknowledged that transition metals at high oxidation states represent superior oxygen evolution reaction (OER) activity, the preparation and stability of such a high-valence state are still a challenge, which requires relatively harsh reaction conditions and is unstable under ambient conditions. Herein, we report the formation of trivalent nickel (Ni3+) in laser-fabricated nickel oxides induced by polyaniline (PANI) under electrochemical activation via a significant charge transfer between Ni and N, as confirmed by X-ray photoelectron spectroscopy and density functional theory calculations. Thereafter, the presence of Ni3+ and the improved conductivity by PANI effectively increase the electrochemical OER activity of the samples together with excellent long-term stability. This work provides new insights for the rational manufacture of high-valence metal for electrochemical reactions.

Trivalent Nickel-Catalyzing Electroconversion of Alcohols to Carboxylic Acids.

The comprehension of activity and selectivity origins of the electrooxidation of organics is a crucial knot for the development of a highly efficient energy conversion system that can produce value-added chemicals on both the anode and cathode. Here, we find that the potential-retaining trivalent nickel in NiOOH (Fermi level, -7.4 eV) is capable of selectively oxidizing various primary alcohols to carboxylic acids through a nucleophilic attack and nonredox electron transfer process. This nonredox trivalent nickel is highly efficient in oxidizing primary alcohols (methanol, ethanol, propanol, butanol, and benzyl alcohol) that are equipped with the appropriate highest occupied molecular orbital (HOMO) levels (-7.1 to -6.5 eV vs vacuum level) and the negative dual local softness values (Δsk, -0.50 to -0.19) of nucleophilic atoms in nucleophilic hydroxyl functional groups. However, the carboxylic acid products exhibit a deeper HOMO level (<-7.4 eV) or a positive Δsk, suggesting that they are highly stable and weakly nucleophilic on NiOOH. The combination (HOMO, Δsk) is useful in explaining the activity and selectivity origins of electrochemically oxidizing alcohols to carboxylic acid. Our findings are valuable in creating efficient energy conversions to generate value-added chemicals on dual electrodes.

Nonredox trivalent nickel catalyzing nucleophilic electrooxidation of organics

A thorough comprehension of the mechanism behind organic electrooxidation is crucial for the development of efficient energy conversion technology. Here, we find that trivalent nickel is capable of oxidizing organics through a nucleophilic attack and electron transfer via a nonredox process. This nonredox trivalent nickel exhibits exceptional kinetic efficiency in oxidizing organics that possess the highest occupied molecular orbital energy levels ranging from −7.4 to −6 eV (vs. Vacuum level) and the dual local softness values of nucleophilic atoms in nucleophilic functional groups, such as hydroxyls (methanol, ethanol, benzyl alcohol), carbonyls (formamide, urea, formaldehyde, glucose, and N-acetyl glucosamine), and aminos (benzylamine), ranging from −0.65 to −0.15. The rapid electrooxidation kinetics can be attributed to the isoenergetic channels created by the nucleophilic attack and the nonredox electron transfer via the unoccupied eg orbitals of trivalent nickel (t2g6eg1). Our findings are valuable in identifying kinetically fast organic electrooxidation on nonredox catalysts for efficient energy conversions.

Three-in-one strategy for boosted charge transport in Ag/Co-Ni(OH)2 @nickel foam capacitive electrodes toward membrane-free electrocatalytic H2 production

The cost-effective and scalable provision of H2 energy necessitates the development of a membrane-less electrolyzed water technique. In this study, we implemented a comprehensive three-in-one strategy to fabricate Ag/Co-Ni(OH)2 @nickel foam charge storage media, which were successfully employed to decouple the tightly coupled hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) steps, leading to efficient and membrane-less two-step water electrolysis. During the cathodic HER, we observed the concurrent oxidation of the Ni(OH)2 capacitive electrode to NiOOH while maintaining a consistent HER at a sustained current of 100 mA for 1800 s, together with a favorable operating potential of 1.43 V. Subsequently, the anodic OER encompassed the regenerations of Ni(OH)2 component, wherein NiOOH was specifically transformed back to Ni(OH)2, facilitating the O2-generation under a predetermined operating voltage of 0.31 V. In-situ Raman spectroscopy verified the reversible interconversion between the divalent nickel and trivalent nickel during step-1 of HER and step-2 of OER. Moreover, a Ni-Zn battery was conceived and constructed through the amalgamation of NiOOH and Zn sheets, effectively supplanting step-2 of the OER. This configuration enabled the uninterrupted integration of the H2-production and battery discharging step, presenting a novel and self-sustained approach for H2 production without the requirement of an external power supply.

Improving electric-energy transmission performance of LixNi1-xO linear ceramic resistors by Li+/Ni3+ ion occupation regulation for pulsed power technology

Linear ceramic resistor receives extensive attention for its great advantages of fast energy transmission in pulsed power technology. However, the research about improving the linear current-voltage characteristics and pulsed charge-discharge speed of LixNi1-xO solid solution is rarely reported. Herein, we fabricated the LixNi1-xO linear ceramic resistors through a solid-phase reaction method, and the effects of the Li+ ion on the microstructure, current-voltage characteristics and pulsed charge-discharge properties were studied. Specifically, the electron holes exchange between divalent and trivalent nickel ions is enhanced by Li substitution, which contributed to the significantly decreasing in nonlinear coefficient (α) and resistivity. On a separate note, the rising edge of pulse charge-discharge is influenced by the linearity of LixNi1-xO ceramic resistors. The closer the nonlinear coefficient is to 1, the shorter the pulsed charge-discharge time is. Meanwhile, the defect dipoles induced by Li substitution may also contribute to the reduction of discharge time of LixNi1-xO linear ceramic resistors. As a result, the Li0.03Ni0.97O linear ceramic resistor with an excellent nonlinear coefficient of 1.03 exhibits extraordinary fast discharge speed (t0.9 = 94.8 ns) at 1.0 kV. The investigation on LixNi1-xO linear ceramic resistors suggests the potential of this material to be applied in the pulsed power system.

Impact of mixed anion ordered state on the magnetic ground states of S=1/2 square-lattice quantum spin antiferromagnets, Sr2NiO3Cl and Sr2NiO3F

The magnetic properties of the $S=1/2$ two-dimensional square-lattice antiferromagnets ${\mathrm{Sr}}_{2}{\mathrm{NiO}}_{3}X$ $(X=\mathrm{Cl}, \mathrm{F})$ with the trivalent nickel ions in a low-spin state were studied by magnetic susceptibility, heat capacity, neutron powder diffraction, high-field electron spin resonance (ESR), muon spin rotation and relaxation $({\ensuremath{\mu}}^{+}\mathrm{SR})$ measurements, and density functional theory (DFT) calculations. Both oxyhalides are isostructural to an ideal quantum square-lattice antiferromagnet ${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{2}{\mathrm{Cl}}_{2}$, but the chlorine/fluorine anion exclusively occupies an apical site in an ordered/disordered manner with an oxygen anion, resulting in the formation of highly distorted ${\mathrm{NiO}}_{5}X$ octahedra with an off-center nickel ion. Magnetic susceptibility measurements revealed a remarkable difference between these two compounds: the magnetic susceptibility of ${\mathrm{Sr}}_{2}{\mathrm{NiO}}_{3}\mathrm{Cl}$ exhibited a broad maximum at approximately 35 K, which is typical of low-dimensional antiferromagnetic behavior. In contrast, the magnetic susceptibility of ${\mathrm{Sr}}_{2}{\mathrm{NiO}}_{3}\mathrm{F}$ exhibited spin-glass-like behavior below 12 K. No anomaly associated with long-range magnetic ordering was observed in the heat capacity, ESR, and neutron powder diffraction experiments. However, ${\ensuremath{\mu}}^{+}\mathrm{SR}$ measurements revealed the emergence of a static magnetic ordered state below ${T}_{\mathrm{N}}=28\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ in ${\mathrm{Sr}}_{2}{\mathrm{NiO}}_{3}\mathrm{Cl}$ and a short-range magnetic state below ${T}_{\mathrm{N}}=18\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ in ${\mathrm{Sr}}_{2}{\mathrm{NiO}}_{3}\mathrm{F}$. The DFT calculations suggested that the unpaired electron occupied a ${d}_{3{z}^{2}\ensuremath{-}{r}^{2}}$ orbital, and ferromagnetic couplings between the nearest-neighbor nickel spins were energetically favored. The mechanism of ferromagnetic superexchange interactions and the reason for the difference between the magnetic ground states in these nickel oxyhalides are discussed.

Increasing of the corrosion resistance by preparing the trivalent nickel complex

This paper is intended to investigate the preparation of a complex ion Ni chloride (II) and with ligand Schiff base (N, N) dimethyl of the amino Benzaldhyde and characterized ligand prepared and complexion Ni chloride(II). These metallic organic chemical compounds have been checked using Spectroscopic Techniques Spectrums of infrared and spectra of UV. The study included a test of the Corrosion Rate of free matter and nickel complex in tap water solution in a linear polarization method By the linear polarization curves resulting from Tafel device readings. The results showed that the nickel complex has an effective inhibitory effect due to the low current value, which increases its corrosion resistance.

An Overall Reaction Integrated with Highly Selective Oxidation of 5‐Hydroxymethylfurfural and Efficient Hydrogen Evolution

AbstractThermodynamically favorable electrooxidation reactions of biomass derivatives integrated with hydrogen evolution reaction (HER) can simultaneously provide value‐added chemicals and hydrogen, and eventually meeting the need for clean and sustainable energy development. Herein, the integration of a six‐electron involved anodic half‐reaction‐selective electrooxidation of 5‐hydroxymethylfurfural into 2,5‐furandicarboxylic acid (FDCA) on a hierarchically layered double hydroxide (CoFe@NiFe) with a cathodic HER in a two‐chamber system is reported. The overall reaction reaches 38 mA cm−2 at 1.40 V and exhibits 100% selectivity to yield FDCA and a nearly 100% Faraday efficiency with hydrogen production of 901 µmol cm−2. Several operando techniques confirm that the trivalent nickel species in the CoFe@NiFe catalyst are mainly responsible for this 100%‐selective oxidation reaction. This integrated overall reaction is thus a new strategy to utilize cheap catalysts and biomass derivatives to simultaneously produce value‐added chemicals and sustainable energy materials, and eventually to solve current challenges of energy depletion and environmental pollution.

Synthesis, Characterization, and Catalytic Property of a Hybrid Nanoscale Polyoxoniobate

A nanoscale polyoxoniobate based on trivalent nickel complex and Lindqvist-type cluster, [Ni(en)3]2[H2Nb6O19]·8H2O (abbreviated as NEN) (en = ethylenediamine, C2H8N2) has been synthesized and structurally characterized by elemental analyses, IR spectrum, UV–visible spectra, X-ray photoelectron spectrum (XPS), thermogravimetric analysis (TGA) and single-crystal X-ray diffraction. Structural analysis shows that NEN is an ionic compound, in which the cations are two trivalent nickel complexes [Ni(en)3]3+ and the anion is a Lindqvist-type polyoxoniobate [Nb6O19]8− cluster. Notably, NEN can catalyze the production of reactive oxygen species efficiently in aqueous solution system.

Self-doping synthesis of trivalent Ni2O3 as a hole transport layer for high fill factor and efficient inverted perovskite solar cells.

Nickel oxide (NiOx) as a hole transport layer has been vastly investigated in perovskite solar cells (PSCs) due to the nature of p-type doping, highly transparent materials, and deep-lying valence bands. In this paper, a new phase based on trivalent Ni2O3 is synthesized by low temperature solution processing of mixed nickel (acetate/nitrate). In comparison, high-temperature solution-processing of divalent NiOx resulted in novel Ni2O3 thin films that display better consistency and superior energy compatibility with perovskite thin films. In this respect, high-performance perovskite solar cells are efficiently produced utilizing MA0.85FA0.15PbI0.9Cl0.1 perovskite with a power conversion efficiency (PCE) reaching 17.89% and negligible hysteresis comparable to 14.37% for NiOx. The Ni2O3-based PSCs reported the highest fill factor (FF) (82.66%) compared to that of divalent NiOx (67.53%). Different characterization studies and analyses supply proof of improved film quality, increased transport and extraction of charges, and suppressed charge recombination. Meanwhile, the device exhibits low hysteresis compared to sol-gel-processed NiOx.

C-O and C-N Functionalization of Cationic, NCN-Type Pincer Complexes of Trivalent Nickel: Mechanism, Selectivity, and Kinetic Isotope Effect.

This report presents the synthesis of new mono- and dicationic NCN-NiIII complexes and describes their reactivities with protic substrates. (NCN is the pincer-type ligand κ N, κ C, κ N-2,6-(CH2NMe2)2-C6H3.) Treating van Koten's trivalent complex (NCN)NiIIIBr2 with AgSbF6 in acetonitrile gives the dicationic complex [(NCN)NiIII(MeCN)3]2+, whereas the latter complex undergoes a ligand-exchange reaction with (NCN)NiIIIBr2 to furnish the related monocationic complex [(NCN)NiIII(Br)(MeCN)]+. These trivalent complexes have been characterized by X-ray diffraction analysis and EPR spectroscopy. Treating these trivalent complexes with methanol and methylamine led, respectively, to C-OCH3 or C-NH(CH3) functionalization of the Ni-aryl moiety in these complexes, C-heteroatom bond formation taking place at the ipso-C. These reactions also generate the cationic divalent complex [(NCN)NiII(NCMe)]+, which was prepared independently and characterized fully. The unanticipated formation of the latter divalent species suggested a comproportionation side reaction between the cationic trivalent precursors and a monovalent species generated at the C-O and C-N bond formation steps; this scenario was supported by direct reaction of the trivalent complexes with the monovalent compound (PPh3)3NiICl. Kinetic measurements and density functional theory analysis have been used to investigate the mechanism of these C-O and C-N functionalization reactions and to rationalize the observed inverse kinetic isotope effect in the reaction of [(NCN)NiIII(Br)(MeCN)]+ with CH3OH/CD3OD.

Comparative Study on Removal of NOx and Soot with A-site Substituted La2NiO4 Perovskite-like by Different Valence Cation

La1.8M0.2NiO4 (M = Na+, Sr2+, Ce3+) perovskite-like catalysts were prepared by citric acid complexation method. XRD, BET, FT-IR, SEM, XPS, H2-TPR, O2-TPD, MS-Soot-TPR, MS-NO-TPD and catalytic activity measurements were carried out to investigate the effect of A-site substitution on structure and catalytic performance for simultaneous removal soot and NOx. The characterization results show that La1.8M0.2NiO4 catalyst has high concentration of oxygen vacancies, more surface active oxygen, more trivalent nickel ions and better reducibility, which determines its better catalytic performance. The introduction of low valence cations at the A site significantly reduces the characteristic combustion temperature of soot and effectively promotes the reduction of NOx by soot. La1.8Sr0.2NiO4 catalyst exhibited the best soot removal performance with Ti 331 °C and Tm 473 °C, while La1.8Na0.2NiO4 catalyst showed the highest NOx conversion of 90%. Based on in situ DRIFTS and other characterization results, a possible mechanism for simultaneous removal of NOx and soot was proposed.

Peony petal-like 3D graphene-nickel oxide nanocomposite decorated nickel foam as high-performance electrocatalyst for direct glucose alkaline fuel cell

Nickel foam modified by various electron mediators is an ideal non-noble metal anode for direct glucose alkaline fuel cell. However, high cost, low stability, and toxicity of electron mediators largely hamper their practical application. Herein, we demonstrated a one-step electrodeposition method to produce high-performance nickel foam electrode decorated with reduced graphene oxides and nickel oxides. The structure and morphology of the resulting 3D graphene-nickel oxide nanocomposites were characterized by UV–Vis, Raman spectroscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy. Furthermore, a composite anode was prepared by rolling an activated carbon layer on the fabricated nickel foam electrode. At ambient temperature, the fuel cell equipped with the composite anode exhibited excellent performance with a high peak power density of 13.48 W m−2 under the condition of 1 M glucose, 3 M KOH, which was 39.30% higher than that of the bare activated carbon anode cell. In particular, electrochemical measurements demonstrated the high performance of the nanocomposite modified activated carbon anode was likely attributed to the synergistic effect of high conductivity of graphene and the catalytic activity of trivalent nickel towards glucose oxidation.

Restoration of Degraded Nickel‐Rich Cathode Materials for Long‐Life Lithium‐Ion Batteries

AbstractThe sensitive surface of nickel‐rich layered oxides often suffers from the erosion of CO2 and H2O in air. The results of erosion, including formation of impurities such as Li2CO3 and LiOH, degradation of layered structures and disproportionation of trivalent nickel, would cause cation mixing and structural instability. In order to overcome these inherent drawbacks, we present a targeted solvothermal followed by re‐oxidation approach to restore the structural stability and electrochemical performances of degraded Ni‐rich layered cathode materials. Owing to this unique approach, the resultant LiNi0.8Co0.15Al0.05O2 (NCA) demonstrate improved layered structure, a stable nanoscale coating shell and enhanced surface stability. The revived NCA enables higher reversible capacity and longer cycle life than that of degraded NCA. Specifically, the restored NCA can retain 82 % initial capacity after 200 cycles under 1 C at room temperature, and 87 % and 55 % initial capacity after 100 and 500 cycles under 1 C at 55 °C, respectively. This approach could be applied to improve the structural stability and electrochemical properties of degraded Ni‐rich layered oxides and suggests great potential in industrial applications.

Hierarchical Fe-doped NiOx nanotubes assembled from ultrathin nanosheets containing trivalent nickel for oxygen evolution reaction

To design highly efficient and low-cost electrocatalysts for oxygen evolution reaction (OER) is a great challenge for electrochemical water splitting. Herein, we report the preparation of hierarchical Fe-doped NiOx nanotubes assembled from ultrathin nanosheets with a thickness of 2.4nm. X-ray photoelectron spectroscopy (XPS) spectra, X-ray absorption near-edge structure (XANES) spectroscopy and extended X-ray absorption fine structure (EXAFS) spectroscopy investigations have been suggested that the incorporation of Fe results in abundant trivalent nickel and nickel vacancies. Benefitting from their unique geometric and electronic structure, the as-prepared hierarchical Fe-doped NiOx nanotubes exhibit outstanding OER activity and stability, with overpotential of 310mV and a small Tafel slope of ~ 49mVdec−1 at 10mAcm−2.

Layered LiNi0.8Co0.1Mn0.1O2 Prepared through Calcination in Air with Preoxidized Precursor

The layered Ni-rich oxide LiNi0.8Co0.1Mn0.1O2 is very attractive as a cathode material owing to its large reversible capacity. However, the inherent poor cycling stability of Ni-rich cathode materials has always been a serious issue limiting their commercialization. Moreover, LiNi0.8Co0.1Mn0.1O2 should be calcined under oxygen flow to promote the oxidation of Ni2+ to Ni3+, which makes it costly. In this study, we introduce a technique for preoxidation of the LiNi0.8Co0.1Mn0.1O2 precursor by KMnO4 to partially oxidize Ni2+ in the precursor (KMnO4-treatment) and eliminate the need for calcination in pure oxygen atmosphere. X-ray photoelectron spectroscopy analysis of the KMnO4-treated precursor and LiNi0.8Co0.1Mn0.1O2 obtained from the KMnO4-treated precursor in air (KMnO4-LiNi0.8Co0.1Mn0.1O2) indicates that the valence states of nickel ions are higher than those in the untreated (pristine) precursor and LiNi0.8Co0.1Mn0.1O2 obtained from the pristine precursor in air (Air-LiNi0.8Co0.1Mn0.1O2). The structure of the KMnO4-LiNi0.8Co0.1Mn0.1O2 shows no distinct differences except for a slight expansion along the c-axis and lower cation disorder. Electrochemical measurements demonstrate that KMnO4-LiNi0.8Co0.1Mn0.1O2 has better rate capability and cycling stability at cutoff voltages of 4.3 and 4.5 V than Air-LiNi0.8Co0.1Mn0.1O2. The improved cycling performance of KMnO4-LiNi0.8Co0.1Mn0.1O2 can be attributed to the favorable structural change due to the more oxidized trivalent nickel ion.

Synthesis, Characterization, and Oxidation of New POCNimine-Type Pincer Complexes of Nickel

This report describes the synthesis and characterization of new pincer-type complexes of divalent and trivalent nickel. Refluxing toluene mixtures of NiBr2(CH3CN)n and the unsymmetrical pincer-type ligands Ph-POCHNimine-R gave (Ph-POCNimine-R)Ni(II)Br in good yields (Ph-POCHNimine-R = 1-(Ph2PO),3-(CHNR)C6H4; R = Bn, 1; Ph, 2; t-Bu, 3; Cy, 4). The C–H nickelation step involved in these syntheses is facilitated in the presence of NEt3. Treating compounds 1–4 with Br2, I2, N-bromosuccinimide (NBS) and N-chlorosuccinimide appears to bring about the desired one-electron oxidation of the Ni(II) center in these complexes, but the putative trivalent species decompose over time and could not be isolated. One of the decomposition products obtained in the Br2 reaction was identified as the zwitterionic compound [Br3Ni(II){κO-(E)-N-(3-((diphenylphosphoryl)oxy)benzylidene)benzenaminium}], 5. To identify the factors that are important for isolation of stable trivalent derivatives, we prepared the divalent precursors (i...

Preparation and Structural Stability of LiNiO2 in Separate Synthesis Methods at Different Temperatures

The LiNiO2 has been the most widely used cathode material in commercial lithium ion batteries because of its high discharge capacity, low cost and environmentally acceptable properties. However, it is difficult to obtain stoichiometric LiNiO2 by solid-state reaction method at high temperature in air atmosphere. This is due to the instability of trivalent nickel species and disordering of cationic distribution at lithium sites. In this paper, we have synthesized the LiNiO2 cathode material by sol-gel and solid state reaction methods at different temperatures (between 750 oC and 800 oC) for obtaining the sample in good phase formation. The sol-gel synthesis method is mainly used because the chemical reaction is easy and also it requires only a low reaction temperature. Further it yields a high degree of homogeneity compared to the conventional solid state synthesis. The crystal structure, bonding nature and vibrational features of the compound are investigated. The crystalline powders are characterized for their phase identification using x-ray diffraction analysis (XRD). All the samples synthesized by these two methods possessed the α-NaFeO2 structure of the rhombohedral system (space group, 3 R m ) with no evidence of any impurities. The morphological features of the powders are characterized by field effect scanning electron microscopy (FESEM). The grain sizes of the samples are found to be approximately 1.5-2 μm. The FT-IR spectroscopic data of LiNiO2 reveal the structure of the oxide lattice constituted by LiO6 and NiO6 octahedra. From this study we conclude that the LiNiO2 synthesized by sol-gel method is structurally stable and it can be used as cathode in lithium ion batteries.

A Study on Structural and Electrochemical Properties of Co Fixed LiNi0.5+XCo0.2Mn0.3-XO2 (x=0, 0.1, 0.2) Using Synchrotron Based X-Ray Techniques

A recent attention in lithium ion battery research is focused on increasing its energy density and structural stability. As a result, the layered Li[NixCoyMnz]O2 (x+y+z=1) electrode materials have attracted a great deal of attention as an alternative of LiCoO2, due to its remarkable electrochemical properties. This three-component system shows high capacity of LiNiO2, layered character of LiCoO2, low cost and thermal stability of manganese in LiMnO2.[1,2] Recently, a lot of research is being carried out to improve the energy density by increasing the Ni content. However, there is always a trade-off between the benefits of large amount of Ni atoms and the risk of side effects, such as poor thermal stability and lower cyclability. In this study, the structural and electrochemical properties of Co fixed LiNi0.5+xCo0.2Mn0.3-xO2 (x=0, 0.1, 0.2) cathode material were investigated by using synchrotron based X-ray diffraction and absorption techniques to understand the effects of Ni and Mn components. The Rietveld refinement of diffraction data was performed by using R-3m model. Refinement results show that lattice parameters of LiNi0.5Co0.2Mn0.3O2 (x=0) are a=2.8697Å, c=14.2378Å, and by increasing the nickel content in LiNi0.5+xCo0.2Mn0.3-xO2 (x=0, 0.1, 0.2)lattice parameters “a” and “c” shrinkas shown in Table 1. Generally, it is believed that the Ni2+ content in the Li layer increase when the total amount of Ni component increase in the three component system. However, in the Co fixed LiNi0.5+xCo0.2Mn0.3-xO2 cathode material system, interestingly cation mixing decreases by increasing the nickel content. The NCM721 (x=0.2) electrode has lower Ni2+ ions in the lithium layer compared to NCM622 (x=0.1) which has even lower quantity of Ni2+ ions than that of NCM523 (x=0). In pristine electrode material, the theoretical value of average Ni oxidation state is Ni2.4+, Ni2.67+, Ni2.86+ for x=0, 0.1, 0.2 respectively and this trend is confirmed by Ni K-edge XANES spectra. The Ni K-edge XANES spectra show shift in energy position with change of nickel content in the electrode material. In NCM523, Ni K-edge position is close to that of Ni2+, while that of NCM721 is at higher energy position, indicating higher Ni valence (close to Ni3+) as shown in Figure 1. The increase of average Ni oxidation state in NCM721 pristine material suggests that Ni3+ component is predominant in its transition metal layer. Therefore, shrinking of lattice parameters by increasing the the nickel quantity can be explained on the basis of Ni3+ content in the transition metal layer. In the Co fixed system (rCo3 +=0.545Å), the unit cell dimensions in a hexagonal setting shrink by increasing the Ni content because of the difference in size between divalent and trivalent nickel ions (rNi2+=0.69Å and rNi3+=0.56Å). When the Ni3+ increases, the Ni2+ and the Mn4+ (rMn4 + =0.53Å) content decrease, net result is decrease in lattice parameters because the increment is small (rNi3+-rMn4+=0.03Å) compare to the decrement (rNi3+-rNi2+=-0.13Å). For the Ni2+ content in Li layer, we can also say that there is low opportunity for Ni2+ to go into the Li layer because the total amount of Ni2+ content decreases with increase of the Ni content. Consequently, these effects of nickel content in the layered structure would affect electrochemical properties such as capacity, cyclability and rate capability; hence their study can assist in designing safe and high performing electrode materials. The more detailed structural and electrochemical properties of Co fixed LiNi0.5+xCo0.2Mn0.3-xO2 cathode material will be presented at the time of meeting.[1] M.S. Whittingham, Chem. Rev. 104 (2004) 4271-4301[2] K.-W. Nam et al., Journal of Power Sources 189 (2009) 515-518

Crystallographic Characteristics and p-Type to n-Type Transition in Epitaxial NiO Thin Film

We were able to systematically control crystallographic characteristics and electrical properties of nickel oxide epitaxial thin films integrated with cubic yttria-stabilized zirconia (c-YSZ)-buffered silicon(001) substrates. The NiO epilayers were grown under several oxygen partial pressures by pulsed laser deposition. The out-of-plane orientation of the NiO layers showed an interesting behavior where it changed from ⟨111⟩ at lower pressures (7 × 10–6 Torr) to ⟨100⟩ at higher pressures (5 × 10–2 Torr). This observation was attributed to the nature of surface termination and templating effect of the c-YSZ{100} platform at different pressures. With the use of θ–2θ and φ scans of X-ray diffraction, the epitaxial alignment across the NiO/c-YSZ interface was determined to be {111}NiO|||{100}c-YSZ and ⟨110⟩NiO||⟨100⟩c-YSZ for the heterostructure grown under a low pressure and {100}NiO||{100}c-YSZ and ⟨100⟩NiO||⟨100⟩c-YSZ for the heterostructure grown under a high oxygen pressure. Our high-resolution TEM studies revealed formation of atomically sharp interfaces with no evidence of interfacial reaction and confirmed the established epitaxial relationships. n-Type electrical conductivity was observed in the NiO epilayers grown under lower pressures, which was turned into p-type in the films deposited under higher oxygen partial pressures. Besides, the electrical resistivity increased with the growth pressure. These observations were correlated to the nature of point defects in the NiO crystal. The formation of oxygen vacancies and metallic nickel at lower pressures and excess oxygen and trivalent nickel at higher pressures was revealed by XPS. We established a structure–property correlation in NiO/c-YSZ{100}/Si{100} thin film epitaxial heterostructures with special emphasis on the stoichiometry and crystallographic characteristics.