Understanding the High Voltage Behavior of LiNiO2 Through the Electrochemical Properties of the Surface Layer.

Nickel-rich layered oxides are adopted as electrode materials for EV's. They suffer from a capacity loss when the cells are charged above 4.15 V versus Li/Li+ . Doping and coating can lead to significant improvement in cycling. However, the mechanisms involved at high voltage are not clear. This work is focused on LiNiO2 to overcome the effect of M cations. Galvanostatic intermittent titration technique (GITT) and in situ X-ray diffraction (XRD) experiments are performed at very low rates in various voltage ranges (3.8-4.3 V,). On the "4.2-4.3 V" plateau the R2 phase is transformed simultaneously in R3, R3 with H4 stacking faults and H4. As the charge proceeds above 4.17 V cell polarization increases, hindering Li deintercalation. In discharge, such polarization decreases immediately. Upon cycling, the polarization increases at each charge above 4.17 V. In discharge, the capacity and dQ/dV features below 4.1 V remain constant and unaffected, suggesting that the bulk of the material do not undergo significant structural defect. This study shows that the change in polarization results from the electrochemical behavior of the grain surface having very low conductivity above 4.17 V and high conductivity below this threshold. This new approach can explain the behavior observed with dopants like tungsten.

In Honor of John Bannister Goodenough, an Outstanding Visionary.

John B [...].

Rational design of layered oxide materials for sodium-ion batteries.

Sodium-ion batteries have captured widespread attention for grid-scale energy storage owing to the natural abundance of sodium. The performance of such batteries is limited by available electrode materials, especially for sodium-ion layered oxides, motivating the exploration of high compositional diversity. How the composition determines the structural chemistry is decisive for the electrochemical performance but very challenging to predict, especially for complex compositions. We introduce the "cationic potential" that captures the key interactions of layered materials and makes it possible to predict the stacking structures. This is demonstrated through the rational design and preparation of layered electrode materials with improved performance. As the stacking structure determines the functional properties, this methodology offers a solution toward the design of alkali metal layered oxides.

Original Layered OP4-(Li,Na)xCoO2 Phase: Insights on Its Structure, Electronic Structure, and Dynamics from Solid State NMR.

The OP4-(Li/Na)xCoO2 phase is an unusual lamellar oxide with a 1:1 alternation between Li and Na interslab spaces. In order to probe the local structure, electronic structure, and dynamics, 7Li and 23Na magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy was performed in complementarity to X-ray diffraction and electronic and magnetic properties measurements. 7Li MAS NMR showed that NMR shifts result from two contributions: the Fermi contact and the Knight shifts due to the presence of both localized and delocalized electrons, which is really unusual. 7Li MAS NMR clearly shows several Li environments, indicating that, moreover, Co ions with different local electronic structures are formed, probably due to the arrangement of the Na+ ions in the next cationic layer. 23Na MAS NMR showed that some Na+ ions are located in the Li layer, which was not previously considered in the structural model. The Rietveld refinement of the synchrotron XRD led to the OP4-[Li0.42Na0.05]Na0.32CoO2 formula for the material. In addition, 7Li and 23Na MAS NMR spectroscopies provide information about the cationic mobility in the material: Whereas no exchange is observed for 7Li up to 450 K, the 23Na spectrum already reveals a single average signal at room temperature due to a much larger ionic mobility.

NaMoO2: a Layered Oxide with Molybdenum Clusters.

NaMoO2 was synthesized as a layered oxide from the reaction between the layered oxide Na2/3MoO2 and metal sodium. Its structure was determined from high-resolution powder X-ray diffraction, and it can be described as an α-NaFeO2 distorted structure in which sodium ions and molybdenum atoms occupy octahedral interstitial sites. Chains of "diamond-like" clusters of molybdenum were evidenced in the [MoO2] layers resulting from the Peierls distortion expected in a two-dimensional triangular lattice formed by transition metal atoms with a d3 electronic configuration. Molybdenum-molybdenum distances as short as 2.58 Å were found in these clusters. The magnetic moment recorded at low temperatures and at room temperature showed that NaMoO2 presents a very low magnetic susceptibility compatible with the localization of the 4d electrons in the Mo-Mo bonds. This localization was confirmed by DFT calculation that showed the NaMoO2 was diamagnetic at 0 K. A sodium battery was built using NaMoO2 as the positive electrode material, and we found that sodium ions can be reversibly deintercalated and intercalated in NaMoO2, indicating that this compound is one of the many phases existing in the NaxMoO2 system.

Probing Al Distribution in LiCo0.96Al0.04O2 Materials Using 7Li, 27Al, and 59Co MAS NMR Combined with Synchrotron X-ray Diffraction.

We prepared Al-doped LCO (LCA) powders with low Al content (4%) with a controlled Li/(Co + Al) stoichiometry by a solid-state reaction using Li2CO3 and two types of Co/Al precursors: simply mixed (Co3O4 and Al2O3) or heat-treated (Co3O4 and Al2O3). These samples were thereby used to propose a reliable protocol with the aim to discuss the homogeneity of the Al doping for LiCo1-yAlyO2 (LCA) prepared with low Al content by evidencing the distribution of Al within the powders, which clearly affects the electrochemical profiles of associated LCA//Li cells. For all samples we initially also characterized the Li/(Co + Al) stoichiometry by 7Li MAS NMR, to discard the possible effect of excess Li in the samples. Synchrotron XRD combined with 27Al and 59Co MAS NMR then provided a deep understanding of the doping homogeneity at the powder or particle scale. We showed that doping the Co3O4 spinel precursor by reacting it with Al2O3 may be avoided, as it most likely leads to an inhomogeneous mixture of Co3O4 and Co3-zAlzO4 as precursor, eventually reflecting in the final LiCo0.96Al0.04O2 powder, which shows a nonhomogeneous Al distribution. We believe that such a detailed characterization should be the first step toward a deeper understanding of the real beneficial effect(s) of Al doping on the high voltage performance of LCO.

Coulombic self-ordering upon charging a large-capacity layered cathode material for rechargeable batteries.

Lithium- and sodium-rich layered transition-metal oxides have recently been attracting significant interest because of their large capacity achieved by additional oxygen-redox reactions. However, layered transition-metal oxides exhibit structural degradation such as cation migration, layer exfoliation or cracks upon deep charge, which is a major obstacle to achieve higher energy-density batteries. Here we demonstrate a self-repairing phenomenon of stacking faults upon desodiation from an oxygen-redox layered oxide Na2RuO3, realizing much better reversibility of the electrode reaction. The phase transformations upon charging A2MO3 (A: alkali metal) can be dominated by three-dimensional Coulombic attractive interactions driven by the existence of ordered alkali-metal vacancies, leading to counterintuitive self-repairing of stacking faults and progressive ordering upon charging. The cooperatively ordered vacancy in lithium-/sodium-rich layered transition-metal oxides is shown to play an essential role, not only in generating the electro-active nonbonding 2p orbital of neighbouring oxygen but also in stabilizing the phase transformation for highly reversible oxygen-redox reactions.

Sodium Electrochemical Deintercalation and Intercalation in O3-NaRhO2 and P2-Na xRhO2 Layered Oxides.

Sodium transition metal layered oxides are a class of materials which exhibits fascinating properties, such as high thermoelectric power. Whereas most of the work conducted so far focused on 3d transition metals, mainly cobalt, compounds with 4d metals could be excellent materials to obtain new strongly correlated electron systems. This work is focused on Na xRhO2 compounds, with O3- and P2-type structures. The P2-type structure was obtained by ion exchange from the potassium phase P2-K0.62RhO2. This type of synthesis was conducted here for the first time on layered oxides with 4d transition metals. The phase diagram of both structures was explored by sodium electrochemical deintercalation/intercalation in a battery. The existence of single phases was shown with presumably different physical properties. As an example, the O'3-Na1/2RhO2 compound electrochemically obtained for the first time exhibits a metallic behavior, whereas the O3-NaRhO2 phase is a semiconductor. The synthesis of each single phase existing in both the O3- and P2-type systems should lead to new insights into the structure-properties relationships of this class of materials.

Influence of the Initial Li/Co Ratio in LiCoO2 on the High-Voltage Phase-Transitions Mechanisms.

The influence of the initial Li/Co stoichiometry in LiCoO2 (LCO) (1.00 ≤ Li/Co ≤ 1.05) on the phase-transition mechanisms occurring at high voltage during lithium deintercalation ( V > 4.5 vs Li+/Li) was investigated by in situ X-ray diffraction. Even if the excess Li+ in Li1.024Co0.976O1.976 does not hinder the formation of the H1-3 and O1 phases, the latter are obtained at higher voltages and exhibit larger c parameters compared with their analogues formed from Li1.00CoO2. We also showed that for the stoichiometric Li1.00CoO2 the deintercalation process is more complex than already reported, with the formation of an intermediate structure between H1-3 and O1.

P2-Na(x)Mn(1/2)Fe(1/2)O2 phase used as positive electrode in Na batteries: structural changes induced by the electrochemical (de)intercalation process.

The electrochemical properties of the P2-type NaxMn1/2Fe1/2O2 (x = 0.62) phase used as a positive electrode in Na batteries were tested in various voltage ranges at C/20. We show that, even if the highest capacity is obtained for the first cycles between 1.5 and 4.3 V, the best capacity after 50 cycles is obtained while cycling between 1.5 and 4.0 V (120 mAh g(-1)). The structural changes occurring in the material during the (de)intercalation were studied by operando in situ X-ray powder diffraction (XRPD) and ex situ synchrotron XRPD. We show that a phase with an orthorhombic P'2-type structure is formed for x ≈ 1, due to the cooperative Jahn-Teller effect of the Mn(3+) ions. P2 structure type stacking is observed for 0.35 < x < 0.82, while above 4.0 V, a new phase appears. A full indexation of the XRPD pattern of this latter phase was not possible because of the broadening of the diffraction peaks. However, a much shorter interslab distance was found that may imply a gliding of the MO2 slab occurring at high voltage. Raman spectroscopy was used as a local probe and showed that in this new phase the MO2 layers are maintained, but the phase exhibits a strong degree of disorder.

P2-Na(x)VO2 system as electrodes for batteries and electron-correlated materials.

Layered oxides are the subject of intense studies either for their properties as electrode materials for high-energy batteries or for their original physical properties due to the strong electronic correlations resulting from their unique structure. Here we present the detailed phase diagram of the layered P2-Na(x)VO(2) system determined from electrochemical intercalation/deintercalation in sodium batteries and in situ X-ray diffraction experiments. It shows that four main single-phase domains exist within the 0.5≤x≤0.9 range. During the sodium deintercalation (intercalation), they differ from one another in the sodium/vacancy ordering between the VO(2) slabs, which leads to commensurable or incommensurable superstructures. The electrochemical curve reveals that three peculiar compositions exhibit special structures for x = 1/2, 5/8 and 2/3. The detailed structural characterization of the P2-Na(1/2)VO(2) phase shows that the Na(+) ions are perfectly ordered to minimize Na(+)/Na(+) electrostatic repulsions. Within the VO(2) layers, the vanadium ions form pseudo-trimers with very short V-V distances (two at 2.581 Å and one at 2.687 Å). This original distribution leads to a peculiar magnetic behaviour with a low magnetic susceptibility and an unexpected low Curie constant. This phase also presents a first-order structural transition above room temperature accompanied by magnetic and electronic transitions. This work opens up a new research domain in the field of strongly electron-correlated materials. From the electrochemical point of view this system may be at the origin of an entire material family optimized by cationic substitutions.

O'3-Na(x)VO2 system: a superstructure for Na(1/2)VO2.

The electrochemical cycling in a sodium battery of the lamellar oxide NaVO(2) is reversible in the Na(x)VO(2) composition range 1/2 ≤ x ≤ 1. The complex electrochemical curve reveals the presence of several transitions taking place during deintercalation. With the help of in situ X-ray diffraction, we observed the structural transitions taking place between Na(2/3)VO(2) and Na(1/2)VO(2). The diffractograms show the presence of several monophasic domains separated by biphasic domains. All phases present a monoclinic distortion of the α-NaFeO(2) structure in the composition range 1/2 ≤ x ≤ 2/3. Moreover the presence of a superstructure is evidenced for Na(1/2)VO(2). It is the first time that an ordered structure is reported at the Na(1/2)MO(2) composition with an O'3 oxygen stacking. A thorough investigation of electrochemically obtained O'3-Na(1/2)VO(2) was performed. The structure refinement reveals the existence of a sodium/vacancy ordering, with a peculiar arrangement of the V-V distances hinting at a pairing of vanadium atoms. Our first measurements of the physical properties of O'3-Na(1/2)VO(2) show a semiconductor behavior and a complex thermal dependence of the magnetic susceptibility related to the pairing of the vanadium atoms.

Thermal stability of Li2MnO3: from localized defects to the spinel phase.

The thermal stability of Li(2)MnO(3) has been investigated by the means of coupled differential thermal analysis and thermogravimetric analysis associated with powder X ray diffraction. Various experiments performed in air and in argon allowed us to propose a mechanism of spinel-type defects formation in intergrowth with Li(2)MnO(3) when treated in air above 900 °C. The fidelity of the DIFFaX simulations performed led to the understanding of the influence of the existence of spinel type defects intergrowth on X ray and electron diffraction patterns. The formation of these defects occurs during cooling and is preceded by the formation of LiMnO(2) defects in heating. With sufficiently long thermal treatments, defects expand such that a spinel type phase can be observed after cooling.

Structural study of the Li(0.5)Na(0.5)MnFe2(PO4)3 and Li(0.75)Na(0.25)MnFe2(PO4)3 alluaudite phases and their electrochemical properties as positive electrodes in lithium batteries.

The alluaudite lithiated phases Li(0.5)Na(0.5)MnFe(2)(PO(4))(3) and Li(0.75)Na(0.25)MnFe(2)(PO(4))(3) were prepared via a sol-gel synthesis, leading to powders with spongy characteristics. The Rietveld refinement of the X-ray and neutron diffraction data coupled with ab initio calculations allowed us for the first time to accurately localize the lithium ions in the alluaudite structure. Actually, the lithium ions are localized in the A(1) and A(1)' sites of the tunnel. Mössbauer measurements showed the presence of some Fe(2+) that decreased with increasing Li content. Neutron diffraction revealed the presence of a partial Mn/Fe exchange between the two transition metal sites that shows clearly that the oxidation state of the element is fixed by the type of occupied site. The electrochemical properties of the two phases were studied as positive electrodes in lithium batteries in the 4.5-1.5 V potential window, but they exhibit smaller electrochemical reversible capacity compared with the non-lithiated NaMnFe(2)(PO(4))(3). The possibility of Na(+)/Li(+) ion deintercalation from (Na,Li)MnFe(2)(PO(4))(3) was also investigated by DFT+U calculations.

The structure of tavorite LiFePO4(OH) from diffraction and GGA + U studies and its preliminary electrochemical characterization.

Pure tavorite LiFePO4(OH) was synthesized through a hydrothermal route. A fine structural analysis was done by X-ray and neutron diffraction techniques. The structure consists of a three-dimensional network with iron(III) octahedra (FeO6) sharing corners, forming chains that run along the b direction. These chains are interconnected by PO4 tetrahedra, such as the resulting framework encloses tunnels of two different sizes running along the a and c axis. The lithium and hydrogen atoms were precisely localized in these tunnels. Theoretical (GGA + U) calculations performed for LiFePO4X materials (X = OH, F) confirmed our results and revealed that a unique lithium position is expected in LiFePO4(OH), as experimentally observed. For the first time, lithium intercalation was shown to occur in LiFePO4(OH) through the reduction of Fe3+ to Fe2+ at an average voltage of ~2.3 V (vs. Li(+)/Li) with a good cyclability.

Structure and properties of alkali cobalt double oxides A(0.6)CoO(2) (A = Li, Na, and K).

Lamellar A(x)CoO(2) cobalt double oxides with A = Li, Na, and K (x approximately 0.6) have been synthesized and their chemical (alkali content, oxidation state, and structure) and physical (resistivity, thermopower, magnetization, and specific heat) properties have been studied. All the three materials exhibit strong electron correlation emphasized by their behavior ranging from Fermi liquid to spin-polarized system. Our results show that both the dimensionality of the interactions and the nature of the alkali play a determining role on the properties.

Investigation of the new P'3-Na0.60VO2 phase: structural and physical properties.

A new layered phase Na(0.60)VO(2) was synthesized by chemical deintercalation of sodium from the pristine compound O3-NaVO(2). The Na(0.60)VO(2) compound exhibits a distorted P'3-type oxygen stacking (AABBCC) with an average monoclinic unit cell containing a = 4.9862(14) A, b = 2.8708(8) A, c = 5.917(2) A, and beta = 104.36(3) degrees. A modulated structure was observed by transmission electron microscopy and X-ray diffraction (XRD) measurements. Indexation of the XRD pattern was achieved by using the q vector equal to 0.44b*, and the 4D superspace group C2/m (0 beta 0) s0 was then deduced. The specific heat measurement showed a strong correlated system with a gamma value of around 20 mJ x mol(-1) x K(-2). The electrical conductivity shows a semiconductor-like behavior with an activation energy of 0.52 eV. A paramagnetic behavior of the susceptibility is observed below room temperature with a Curie constant equal to C = 0.076 emu x K(-1) x mol(-1) x Oe(-1). To explain this small value, a model of pseudotriangular clusters of vanadium with a random distribution of V(3+) and V(4+) was considered.

Sodium ion mobility in Na(x)CoO2 (0.6 < x < 0.75) cobaltites studied by 23Na MAS NMR.

Various P2 and P'3-Na(x)CoO(2) phases, with x ranging approximately from 0.6 to 0.75, have been studied by variable-temperature (23)Na magic angle spinning (MAS) NMR. Signal modification versus temperature plots clearly show that Na(+) ions are not totally mobile at room temperature on the NMR time scale. As the temperature increases, the line shape change of the (23)Na MAS NMR signal differs for the P2 and P'3 stackings and is interpreted by the differences of Na(+) ion sites and of sodium diffusion pathways in the two structures.

First experimental evidence of potassium ordering in layered k4co7o14.

The layered P2-K4Co7O14 oxide has been prepared and characterized by means of X-ray diffraction, electrical conductivity, thermopower, and magnetic measurements. The crystal structure of K4Co7O14 (P6(3)/m space group, Z=2, a=7.5171(1) A, and c=12.371(1) A) consists of a stacking of slabs of edge-shared CoO6 octahedra with K+ ions occupying ordered positions in the interslab space, leading to a a0 radical7xa0 radical7 supercell. Potential energy calculations at 0 K are in good agreement with the ordered distribution of potassium ions in the (ab) plane. This oxide is metallic, and the magnetic susceptibility is of Pauli-type, which contrasts with the Curie-Weiss behavior of the homologous NaxCoO2 (x approximately 0.6) oxide with close alkali content. The thermopower at room temperature is about one-third that of polycrystalline Na0.6CoO2.