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Ni-rich positive electrodes for Li-ion batteries can provide enhanced initial discharge capacity yet suffer from significant capacity degradation upon cycling. Fluorination of Ni-rich NMC811 positive electrodes results in a capacity retention of more than 90% after 100 cycles upon cycling to 4.4 VLi. The increased cycling stability of F-modified NMC811 can be attributed to the modification of the oxide electronic structures, where density functional theory calculations shows that incorporating fluorine into the oxide lattice decrease the driving force of carbonate dissociation on the oxide surface. In situ infrared (IR) spectroscopy and ex situ X-ray photoelectron spectroscopy (XPS) further supports this argument by showing less carbonate oxidation for F-modified LiNi0.8Mn0.1Co0.1O2 (NMC811) than as-received NMC811 upon charging. The reduced carbonate oxidation is coupled with minimal salt decomposition on the electrode surface, as revealed by XPS. Further comparing in situ IR and XPS with that of the heat-treated NMC811 and Li2CO3 allows for decoupling the solvent decomposition products, where oxides are responsible for the vinylene carbonate formation with two hydrogens removed whereas surface metal carbonates promote dehydrogenated ethylene carbonate with just one hydrogen removed. This work points towards the importance of the anion engineering to increase the cycling stability of Ni-rich NMC positive electrodes. [ABSTRACT FROM AUTHOR]

The salicylate method is one of the ammonia quantification methods that has been extensively used in literature for quantifying ammonia in the emerging field of nitrogen (electro)fixation. The presence of iron in the sample causes a strong negative interference on the salicylate method. Today, the recommended method to deal with such interferences is the experimental correction method: the iron concentration in the sample is measured using an iron quantification method, and then the corresponding amount of iron is added to the calibration samples. The limitation of this method is that when a batch of samples presents a great iron concentration variability, a different calibration curve has to be obtained for each sample. In this work, the interference of iron III on the salicylate method was experimentally quantified, and a model was proposed to capture the effect of iron III interference on the ammonia quantification result. This model can be used to correct the iron III interferences on ammonia quantification. The great advantage of this correction method is that it only requires three experimental curves in order to correct the iron III interference in any sample provided the iron III concentration is below the total peak suppression concentration. [ABSTRACT FROM AUTHOR]

Understanding and controlling the (electro) chemical reactions between positive electrodes and electrolytes is essential to enhance the cycle life and safety of Li-ion batteries. Previous computational and experimental studies have shown that greater capacity loss of LiNixMnyCo1−x−yO2 (NMC) with increased Ni content can be attributed to the enhanced chemical oxidation of carbonate solvents by dehydrogenation and increased salt decomposition. In this study, we examine the role of a diphenyl carbonate (DPC) additive on the interfacial reactivity of LiNi1/3Mn1/3Co1/3O2, LiNi0.6Mn0.2Co0.2O2, LiNi0.8Mn0.1Co0.1O2 (NMC111, NMC622 and NMC811). Diffuse reflectance infrared Fourier Transform (DRIFT) spectroscopy on NMCs showed that adding DPC in the electrolyte suppressed signals associated with dehydrogenation of ethylene carbonate (EC) from LiNi1/3Mn1/3Ni1/3O2 to LiNi0.8Mn0.1Ni0.1O2 (NMC111 to NMC811). In addition, having DPC in the electrolyte was accompanied with less PF6− salt anion decomposition to form less-fluorine coordinated species such as lithium nickel oxyfluorides or PF3O-like species as revealed by combined infrared spectroscopy and X-ray Photoelectron Spectroscopy (XPS) for Ni-rich NMCs. Such observations are in agreement with previous work showing that DPC can increase the cycling performance of NMC811. The reduced reactivity between NMC such as NMC811 and electrolyte with DPC can be attributed to the formation of surface reaction products from the electrochemical oxidation of DPC occurring at lower voltages compared to the chemical oxidative dehydrogenation of carbonates. This hypothesis is supported by in situ infrared spectroscopy measurements, which revealed electrochemical oxidation of diphenyl carbonate upon charging at 3.9 VLi, accompanied by the detection of a feature around 1824 cm−1 attributed to organic oxidation products adsorbed on the oxide surface, and a stable electrode/electrolyte interface on NMC811 at higher voltages. [ABSTRACT FROM AUTHOR]

Replacing graphite with alloying Al negative electrodes would allow for the development of high energy density Li-ion batteries. However, large volume changes associated with the alloying/dealloying process often result in pulverization of the electrode and rapid capacity fade during cycling due to the continuous formation of solid electrolyte interphase (SEI) layers and loss of electronic contact. In this study, we report that increasing salt concentration in the electrolyte to > 5 mol dm-3 led to enhanced capacity retention during cycling of Li-Al half-cells, which was accompanied by nearly constant impedance for the Al electrode in lithium bis(fluorosulfonyl)imide (LiFSI)/dimethyl carbonate (DMC) 1:1.1 (mol/mol) superconcentrated electrolyte. X-ray photoelectron spectroscopy (XPS) revealed that a potential hold in the superconcentrated electrolyte formed an SEI layer with a greater LiF concentration than in standard 1 mol dm-3 solution. This was supported by Raman spectroscopy of LiFSI solutions in DMC, supplemented with density functional theory calculations, which showed an increased driving force for the reduction of FSI- anions to form LiF from Li+-coordinated DMC complexes with increasing salt concentration. Therefore, the enhanced capacity retention and stability can be attributed to the stability of LiF-rich SEI layers which limit carbonate reduction and charge transfer impedance growth. [ABSTRACT FROM AUTHOR]

Surface chemistry modification of positive electrodes has been used widely to decrease capacity loss during Li-ion battery cycling. Recent work shows that coupled LiPF6 decomposition and carbonate dehydrogenation is enhanced by increased metal-oxygen covalency associated with increasingNi and/or lithium de-intercalation inmetal oxide electrode, which can be responsible for capacity fading of Ni-rich oxide electrodes. Here we examined the reactivity of lithium nickel, manganese, cobalt oxide (LiNi0.6Mn0.2Co0.2O2, NMC622) modified by coating of Al2O3, Nb2O5 and TiO2 with a 1 M LiPF6 carbonate-based electrolyte. Cycling measurements revealed that Al2O3-coated NMC622 showed the least capacity loss during cycling to 4.6 VLi compared to Nb2O5-, TiO2-coated and uncoated NMC622, which was in agreement with smallest electrode impedance growth during cycling from electrochemical impedance spectroscopy (EIS). Ex-situ infrared spectroscopy of charged Nb2O5- and TiO2-coated NMC622 pellets (without carbon nor binder) revealed blue peak shifts of 10 cm-1, indicative of dehydrogenation of ethylene carbonate (EC), but not for Al2O3-coated NMC622. X-ray Photoelectron Spectroscopy (XPS) of charged TiO2-coated NMC622 electrodes (carbon-free and binder-free) showed greater salt decomposition with the formation of lithium-nickel-titanium oxyfluoride species, which was in agreement with ex-situ infrared spectroscopy showing greater blue shifts of P-F peaks with increased charged voltages, indicative of species with less F-coordination than salt PF6-anion on the electrode surface. Greater salt decomposition was coupled with the increasing dehydrogenation of EC with higher coating content on the surface. This work shows that Al2O3 coating on NMC622 is the most effective in reducing carbonate dehydrogenation and accompanied salt decomposition and rendering minimum capacity loss relative to TiO2 and Nb2O5 coating. [ABSTRACT FROM AUTHOR]

The lack of stable and economic supporting materials at high voltages hampers the development of electrocatalysts for oxygen evolution reaction (OER), which is the major source of energy loss in water splitting to produce hydrogen. In this work, we developed systematic methods to evaluate candidate compounds that can potentially replace traditional carbon support for OER catalysts. Stability, economic and conductivity criteria of the oxide support materials were studied and discussed. A nano-sized antimony-doped tin oxide was fabricated to support RuO2, which was shown to provide the highest stability and activity of OER in 0.5 M H2SO4 up to 2.5 VRHE and up to 55°C. [ABSTRACT FROM AUTHOR]

Fundamental understanding of the reactivity between electrode and electrolyte is key to design the safety and life of Li-ion batteries. Herein X-ray photoelectron spectroscopy was used to examine the electrode/electrolyte interface (EEI) on carbon-free, binder-free LiCoO2 powder and thin-film electrodes in LP57 electrolyte as function of potential. Upon charging of LiCoO2 a marked growth of oxygenated and carbonated species was observed on the surface, consistent with electrolyte oxidation at high potentials. We also demonstrated that LiCoO2 oxide surface was prone to decompose the salt starting at 4.1 VLi, as evidenced by the increase of LiF and LixPFyOz species upon charging. By DFT calculations we proposed a correlation between the interface composition and the thermodynamic tendency of the EC solvent for dissociative adsorption on the LixCoO2 surface, through the generation of reactive acidic OH groups on the oxide surface, which can have a role in the observed salt decomposition. This is consistent with the evidence of HF and PF2O2 - species at 4.6 VLi observed by solution 19F-NMR measurements. Finally we compared EEI composition between composite and model electrodes and discussed the changes and mechanisms induced by the electrode composition or the use of electrolyte additives.We showed that the addition of diphenyl carbonate (DPC) in the electrolyte has a strong impact on the formation of solvent and salt decomposition products at the EEI layer. [ABSTRACT FROM AUTHOR]

Layered LiNi0.8Mn0.1Co0.1O2 (NMC811) is one of the high-energy positive electrode (cathode) materials for next generation Li-ion batteries. However, compared to the structurally similar LiNi1/3Mn1/3Co1/3O2 (NMC111), it can suffer from a shorter lifetime due to its higher surface reactivity. This work studied and compared the formation of surface contaminations on NMC811 and NMC111 when stored under ambient conditions using electrochemical cycling, Raman spectroscopy and X-ray photoelectron spectroscopy. NMC811 was found to develop a surface layer of up to ∼10 nm thickness that was mostly composed of nickel carbonate species mixed with minor quantities of hydroxide and water after ambient storage for 1 year, while no significant changes were observed on the NMC111 surface. The amount of carbonate species was quantified by gas chromatographic (GC) detection of carbon dioxide generated when the NMC particles were dispersed in hydrochloric acid. Surface impurity species formed on NMC811 upon ambient storage not only lead to a significant delithiation voltage peak in the first charge, but also markedly reduce the cycling stability of NMC811-graphite cells due to significantly growing polarization of the NMC811 electrode. [ABSTRACT FROM AUTHOR]

Porous carbonaceous electrodes have the potential utility as electrodes for non-aqueous VRFBs due to being inexpensive, chemically stable, and electrochemically active. In this work, we provide insights on the electrochemical kinetics of [V(acac)3]0/[V(acac)3]+ and [V(acac)3]0/[V(acac)3]- redox reactions at freestanding sheets of multiwalled carbon nanotubes (MWCNTs), or buckypapers (BPs), and graphitized carbon fiber-based nonwovens papers (CPs). The use of freestanding sheets eliminates the need for a metal substrate; thus, the true electrochemical response at these electrodes is measured. Even though porous carbonaceous electrodes provide high surface area for the electrochemical reaction, their porous nature poses a fundamental challenge caused by thin-film-like diffusion process; leading to a marked ohmic resistance within the pores of the electrodes. Higher current densities are obtained with the BP electrode due to their larger surface area in comparison to the CP electrode; however, the BP electrode show less efficient kinetic facility for the investigated redox couples as evident by electrochemical impedance spectroscopy and polarization curves results. Charge-discharge cycling demonstrates that porous carbonaceous electrodes may require various treatments to modify the surface chemistries to obtain improved cycling efficiencies. [ABSTRACT FROM AUTHOR]

X-ray photoelectron spectroscopy (XPS) was used to investigate the surface chemistry of high voltage spinel, LiNi0.5Mn1.5O4 (LNMO) positive electrodes cycled 5 and 10 times in Li-cells with 1 M LiPF6 in (3:7) EC:DMC. The XPS spectra were collected using conventional Mg X-rays with energy of 1253.6 eV as well as synchrotron X-rays with energies of 2493.6 and 3498.4 eV in order to examine the depth distribution of various surface chemical species induced during cycling. The XPS spectra revealed a 5 - 10 nm surface layer of organic and LixPFyOz-type species formed as result of electrolyte decomposition, and a comparatively thinner layer composed of transition metal fluorides and LiF. These results suggest that electrolyte decomposition is a major contributor to parasitic reactions in LNMO battery electrochemistry. Limiting electrolyte decomposition with the use of solvents with wide electrochemical stability windows thus comprises a promising strategy for ensuring the practical feasibility of high voltage spinel materials in future Li-ion systems. [ABSTRACT FROM AUTHOR]

Charge storage mechanisms in oxygen-functionalized, multiwall carbon nanotube (MWNT) electrodes fabricated by the Layerby- Layer (LbL) self-assembly process were studied in two-electrode nonaqueous cells. In asymmetric cells with a Li metal or a lithium titanate (Li4Ti5O12) negative electrode, double-layer charging of MWNT surfaces is supplemented by surface faradaic reactions between Li+ and oxygen functional groups. The utilization of faradaic reactions leads to high gravimetric energies of 350 Wh/kgelectrode (with Li) and 140 Wh/kgelectrode (with LTO) at 5 kW/kgelectrode. Self-discharge studies of LbL-MWNT electrodes in asymmetric Li cells revealed that charge leakage comparable to traditional symmetric double-layer capacitors occurs at open circuit, and the presence of surface faradaic reactions accelerates the voltage recovery for electrodes polarized below open circuit voltage. Studies in different electrolytes reveal that both Li+ and TEA+ can participate in faradaic reactions with surface oxygen. The role of faradaic reactions is further demonstrated by tests in symmetric cells composed of two identical LbL-MWNT electrodes, where charge storage is limited to double layer charging owing to electrolyte charge neutrality requirements. As a result, the electrode energy decreased substantially (40 Wh/kgelectrode at 5 kW/kgelectrode). [ABSTRACT FROM AUTHOR]

Increasing oxygen permeation from oxide membranes can enable the development of next-generation power generation technologies with high efficiency and reduced CO2 mission. Here we introduce a new approach to design oxygen permeation membranes by covering the surfaces of pure oxygen-ion conductors with porous precious metal (Au and Ag) network or MIECs (LNO and LSCF). Oxygen permeation flux was enhanced by greatly by coating porous silver on the whole surface of pure oxygen ion-conducting membranes such as Zr0.85Y0.16O2-δ (YSZ), Bi1.5Er0.5O3 (BE25), and La0.8Sr0.2Ga0.8Mg0.115Co0.085O3-δ (LSGMC). Oxygen permeation flux of La2NiO4+δ (LNO)-sputtered LSGMC membranes was comparable to silver-coated LSGMC membranes which demonstrates that membranes devoid of the use of noble metal can be prepared using this geometry. In contrast, oxygen permeation fluxes of YSZ and BE25 membrane did not increase by sputtering LNO or LSCF film because of the reaction between the membrane and LNO or LSCF. We obtained an oxygen permeation flux of 2.2 × 10−8 cm−2 s−1 at 700°C by coating porous silver on the feed side of LNO-sputtered LSGMC membrane, which was comparable to that of LNO membrane. The rate-limiting process for oxygen permeation through these membranes will be discussed. [ABSTRACT FROM AUTHOR]

Films of La0.8Sr0.2CoO3-δ (LSC-82) with 45 and 90 nm thickness were grown epitaxially on (001) oriented single crystal yttria-stabilized zirconia with a thin gadolinium-doped ceria interlayer using pulsed laser deposition, and characterized using X-ray diffraction (XRD), depth-profile secondary mass spectrometry (DP-SIMS), and linear and nonlinear electrochemical impedance spectroscopy (EIS and NLEIS). The films were found to exhibit in-plane tensile strain and normal compressive strain (with overall increased lattice volume) relative to freestanding cubic LSC-82. The films also possess a compositional La/Sr gradient across their thickness, with enhanced Sr2+ composition (within the perovskite lattice) at the gas-exposed surface. The oxygen storage capacity of the films at 450-600°C (as measured using EIS capacitance) is greatly enhanced relative to freestanding cubic LSC-82, consistent with an apparent increase in oxygen-vacancy concentration at the gas-exposed surface of the films (as revealed by NLEIS). These results can be explained by a two-layer model in which a finite thickness of the perovskite lattice near the surface has an enhanced Sr dopant concentration, leading to a much higher concentration of oxygen vacancies than the underlying bulk material of nominal Sr composition. The nonlinear electrochemical response of the film is consistent with a dissociative adsorption rate law, provided the enhanced bulk vacancy concentration near the surface is included in the analysis. [ABSTRACT FROM AUTHOR]

Understanding the thermal stability of major reaction products, Li2O2 (space group P63/mmc) and Li2O (space group Fm...m) is critical to improve the safety characteristics of Li-air batteries. The changes in the crystal structure and surface chemistry of Li2O2 and Li2O were examined as a function of temperature via in situ X-ray diffraction (XRD) and in situ X-ray photoelectron spectroscopy (XPS). Significant decreases in the lattice parameters and the c/a ratio of Li2O2 were found at 280°C and higher. These structural changes can be attributed to the transformation of Li2O2 to Li2O2-δ, which is supported by density functional theory calculations. Upon further heating to 700°C, a lithium-deficient Li2-δO phase appeared at 300°C and gradually became stoichiometric upon further heating to -550°C. XPS measurements of Li2O2 revealed that Li2O appeared on the surface starting at 250°C, which is in agreement with the onset temperature of phase transformation as detected by XRD. In addition, the growth of Li2CO3 on the surface was found at 250°C, which can be attributed to chemical reactions between Li2O2/Li2O and carbon-containing species (e.g. hydrocarbons) present in the XPS chamber. This finding highlights the challenges of developing stable carbon-based oxygen electrode for Li-air batteries. [ABSTRACT FROM AUTHOR]

We have examined the electrochemical performance and the surface chemistry of quenched and annealed LiNi0.5Mn0.5O2 electrodes as a function of high voltage cutoff via X-ray photoelectron spectroscopy (XPS). After cycling for 20 cycles, the surface regions of quenched and annealed electrodes are significantly enriched in fluorine and lithium relative to pristine electrodes, with the degree of enrichment being greater for the annealed electrodes. Mn and Ni fluorides are observed on the surface of the cycled annealed electrodes while both Mn and Ni oxides and fluorides are observed on the surface of the cycled quenched electrodes. The XPS data indicate that the surface films formed on the annealed electrodes as a result of cycling are thicker than the films formed on the quenched electrodes. Significantly, the results indicate that Mn and Ni dissolution are suppressed by annealing, further supporting previous findings that showed an increase in the relative concentration of Mn4+ to Mn3+ and a decrease in the Ni/Li interlayer mixing for the annealed material relative to the quenched material. The formation of Mn and Ni fluorides and the suppression of highly resistive LiF upon cycling appear to play an important role toward the improved capacity retention of annealed LiNi0.5Mn0.5O2. [ABSTRACT FROM AUTHOR]

Hetero-structured interfaces of oxides, which can exhibit reactivity characteristics remarkably different from bulk oxides, are inter- esting systems to explore in search of highly active fuel cell catalysts for oxygen electrocatalysis. (001)-oriented La0.6Sr0.4CoO3-δ (LSC60-40113) thin films having thicknesses from tens to hundreds of nanometers were grown epitaxially on (001)-oriented yttria- stabilized zirconia (YSZ). Atomic force microscopy showed that all the film surfaces were of high quality with average surface roughness approximately one nanometer. The surface oxygen exchange kinetics of these films were determined by electrochemical impedance spectroscopy. While (001)-oriented LSC60-40113 thin films were found to have similar surface oxygen exchange coef- ficients to LSC60-40113 bulk, surface coverage of (001)-oriented La0.5Sr0.5CoO4±δ (LSC214) on the LSC60-40113 thin films led to significant enhancement in the surface oxygen kinetics up to ~3 orders of magnitude. Interestingly, LSC214-decorated LSC60-40113 films have comparable surface exchange kinetics (~110-5 cm at 550°C) to similarly prepared LSC214-decorated La0.8Sr0.2CoO3-δ films. Such high surface oxygen kinetics are among the most active SOFC cathode materials reported to date. [ABSTRACT FROM AUTHOR]

Size-dependent oxygen reduction reaction activity (ORR) and instability of Pt nanoparticles is of great importance in proton exchange membrane fuel cell applications. In this study, the size-dependence of ORR activity on Pt nanoparticles (NPs) was investigated on high-surface-area carbon supported Pt NPs below 5 nm in acidic electrolytes using rotating disk electrode method. The ORR activity was correlated to the estimated surface coverage by OH anion from cyclic voltammogram measurements and the surface composition and electronic structure of Pt NPs, which was studied using X-ray photoemission spectroscopy, and ultraviolet photoemission spectroscopy. The results revealed a size-independent ORR activity on Pt NPs below 5 nm, which was attributed to similar surface compositions and surface electronic structures of Pt NPs below 5 nm as well as comparable OH anion coverage at the potential where ORR was evaluated. In contrast, the instability of Pt NPs under accelerated potential cycling was found to be strongly dependent on the particle size. [ABSTRACT FROM AUTHOR]

The kinetics of the hydrogen oxidation reaction (HOR) and hydrogen evolution reaction (HER) on polycrystalline platinum [Pt(pc)] and high surface area carbon-supported platinum nanoparticles (Pt/C) were studied in 0.1 M KOH using rotating disk electrode (RDE) measurements. After corrections of noncompensated solution resistance from ac impedance spectroscopy and of hydrogen mass transport in the HOR branch, the kinetic current densities were fitted to the Butler-Volmer equation using a transfer coefficient of α = 0.5, from which HOR/HER exchange current densities on Pt (pc) and Pt/C were obtained, and the HOR/HER mechanisms in alkaline solution were discussed. Unlike the HOR/HER rates on Pt electrodes in alkaline solution, the HORIHER rates on a Pt electrode in 0. 1 M HCIO4 were limited entirely by hydrogen diffusion, which renders the quantification of the HOR/HER kinetics impossible by conventional RDE measurements. The simulation of the hydrogen anode performance based on the specific exchange current densities of the HORIHER at 80°C illustrates that in addition to the oxygen reduction reaction cell voltage loss on the cathode, the slow HOR kinetics are projected to cause significant anode potential losses in alkaline fuel cells for low platinum loadings (>130 mV at 0.05 mgpt/cmanode² and 1.5 A/cmanode²), contrary to what is reported for proton exchange membrane fuel cells. [ABSTRACT FROM AUTHOR]

Rechargeable lithium-air batteries have the potential to provide ≈3 times higher specific energy of fully packaged batteries than conventional lithium rechargeable batteries. However, very little is known about the oxygen reduction reaction (ORR) and oxygen evolution in the presence of lithium ions in aprotic electrolytes, which hinders the improvement of low round-trip efficiencies of current lithium-air batteries. We report the intrinsic ORR activity on glassy carbon (GC) as well as polycrystalline Au and Pt electrodes, where Au is the most active with an activity trend of Au ⪢ GC > Pt. Rotating disk electrode (RDE) measurements were used to obtain the kinetic current of the ORR and the reaction order with respect to oxygen partial pressure in 1 M LiClO4 propylene carbonate:1,2-dimethoxyethane (1:2 v/v). In addition, air electrodes with Vulcan carbon or Au or Pt nanoparticles supported on Vulcan were examined in Li-O2 single cells, where the observed discharge cell voltages follow the catalytic trend established by RDE measurements. The ORR mechanism and the rate-determining steps were discussed and contrasted with the ORR activity trend in acid and alkaline solutions. [ABSTRACT FROM AUTHOR]

Electrochemical measurements showed an ≈75% Pt surface area loss and an ≈40% specific activity loss for a membrane electrode assembly (MEA) cathode with acid-treated "Pt3Co" catalyst particles in a H2/N2 proton exchange membrane fuel cell after 24 h voltage cycling between 0.65 and 1.05 V vs reversible hydrogen electrode. Transmission electron microscopy, scanning transmission electron microscopy, associated X-ray energy dispersive spectroscopy, and high angle annular dark-field techniques were used to probe the microstructural changes of the MEA cathode and the compositional changes along the MEA cathode thickness and within individual PtxCo nanoparticles before and after voltage cycling. Further Co dissolution from acid-treated PtxCo particles that leads to an increased thickness of a Pt-enriched surface layer and the development of core/shell PtxCo particles was largely responsible for the reduction in the specific activity of PtxCo nanoparticle after potential cycling. The Pt weight loss associated with the formation of Pt crystallites near the cathode/membrane interface largely contributed to the measured electrochemical surface area loss, while particle growth of the PtxCo particles via Ostwald ripening played a lesser role. [ABSTRACT FROM AUTHOR]

Li/LiNi0.5Mn0.5O2 cells were cycled galvanostatically to 4.5 and 5.3 V at different current densities. Cycling to 5.3 V was shown to not only improve the reversible capacities and rate capability upon subsequent cycling in the voltage range of 4.5 and 2.0 V, but also to lead to the appearance of a reversible process at 4.3 V. LixNi0.5Mn0.5O2 electrodes exposed initially to 5.3 V were found to exhibit stable, reversible capacities of ∼190 mAh/g upon subsequent cycling to 4.5 V, which were considerably higher than those of electrodes (∼170 mAh/g) cycled only to 4.5 V for the identical number of cycles. The changes in the crystal structure of layered 03 LixNi0.5Mn0.5O2 were investigated by synchrotron X-ray powder diffraction and electron diffraction studies. Rietveld refinement analyses of X-ray diffraction data showed that octahedral nickel migrated from the lithium layer to transition metal layer, and to tetrahedral sites occurred upon cycling to 4.5 V. Increasing the charging voltage to 5.3 V appeared to remove all of the tetrahedral nickel and further increased the occupancy of octahedral nickel in the transition metal layer. The formation of O1 []Ni0.5Mn0.5O2 having hexagonal close-packed oxygen array was revealed in the electrode charged to 5.3 V by X-ray and electron diffraction. The changes in the cation arrangements of the LixNi0.5Mn0.5O2 structure are discussed with respect to lithium reactivity, reversible capacities, and lithium mobility. [ABSTRACT FROM AUTHOR]