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We describe the synthesis, crystal structure and lithium deinsertion–insertion electrochemistry of two new lithium-rich layered oxides, Li3MRuO5 (M = Mn, Fe), related to rock salt based Li2MnO3 and LiCoO2. The Li3MnRuO5 oxide adopts a structure related to Li2MnO3 (C2/m) where Li and (Li0.2Mn0.4Ru0.4) layers alternate along the c-axis, while the Li3FeRuO5 oxide adopts a near-perfect LiCoO2 (R3̅m) structure where Li and (Li0.2Fe0.4Ru0.4) layers are stacked alternately. Magnetic measurements indicate for Li3MnRuO5 the presence of Mn3+ and low spin configuration for Ru4+ where the itinerant electrons occupy a π*-band. The onset of a net maximum in the χ vs. T plot at 9.5 K and the negative value of the Weiss constant (ϑ) of −31.4 K indicate the presence of antiferromagnetic superexchange interactions according to different pathways. Lithium electrochemistry shows a similar behaviour for both oxides and related to the typical behaviour of Li-rich layered oxides where participation of oxide ions in the electrochemical processes is usually found. A long first charge process with capacities of 240 mA h g−1 (2.3 Li per f.u.) and 144 mA h g−1 (1.38 Li per f.u.) is observed for Li3MnRuO5 and Li3FeRuO5, respectively. An initial sloping region (OCV to ca. 4.1 V) is followed by a long plateau (ca. 4.3 V). Further discharge–charge cycling points to partial reversibility (ca. 160 mA h g−1 and 45 mA h g−1 for Mn and Fe, respectively). Nevertheless, just after a few cycles, cell failure is observed. X-ray photoelectron spectroscopy (XPS) characterisation of both pristine and electrochemically oxidized Li3MRuO5 reveals that in the Li3MnRuO5 oxide, Mn3+ and Ru4+ are partially oxidized to Mn4+ and Ru5+ in the sloping region at low voltage, while in the long plateau, O2− is also oxidized. Oxygen release likely occurs which may be the cause for failure of cells upon cycling. Interestingly, some other Li-rich layered oxides have been reported to cycle acceptably even with the participation of the O2− ligand in the reversible redox processes. In the Li3FeRuO5 oxide, the oxidation process appears to affect only Ru (4+ to 5+ in the sloping region) and O2− (plateau) while Fe seems to retain its 3+ state. [ABSTRACT FROM AUTHOR]

Aliovalent substitution of Nb5+ by Ti4+ in Sr2LuNbO6 is limited to 10% of Nb atoms. A full structural determination by NPD confirms this and reveals that the structure is better described as a superstructure of the simple cubic perovskite (as previously reported) with the monoclinic cell 21/2ap × 21/2ap × ²ap and ß ≈ 90° (S.G. P21/n). The substituted materials present both oxygen-vacancies induced by charge compensation and Sr-deficiency. Therefore, their formula should be given as Sr2-yLuNb1-xTixO6-δ. Electrical properties can be fully understood considering these compositional defects. The parent compound Sr2LuNbO6 presents low electrical conductivity in air, which improves by more than one order of magnitude upon Ti substitution. In any case, the title oxides show low electrical conductivity in a wide oxygen partial pressure (pO2) range (10-25 atm = pO2 = 10-1 atm). At high pO2 the conductivity increases with pO2 due to oxygen-vacancy annihilation and hole creation, according to a general p-type semiconducting mechanism; A-site substoichiometry and Tisubstitution are the origin of this behaviour. In the low pO2 region, the conductivity increases as the oxygen partial pressure decreases. Reduction of cations, Nb5+ or Ti4+, supports n-type conduction by electrons and oxygen vacancy creation. For the intermediate pO2 range a low ionic conduction contribution is observed. Although the estimated ionic conductivity is not high in the substituted compounds, the strategy seems to be valid since a significant enhancement of ionic conduction is observed upon aliovalent substitution. [ABSTRACT FROM AUTHOR]

The structural features of intercalated Li3AlxTi2−x(PO4)3 compounds, with x = 0 and 0.2, have been deduced by Rietveld analysis of neutron diffraction (ND) patterns recorded between 100 and 500 K. The Li insertion decreases the symmetry from R3̅c to R3̅ in analyzed compounds. In pristine Li1+xAlxTi2−x(PO4)3 samples, Li occupies mainly six-fold M1 sites at ternary axes; but in lithiated Li3AlxTi2−x(PO4)3 samples, Li is located near M2 positions at M3/M3′ four-fold coordinated sites. In both cases, Li arrangement minimizes electrostatic Li–Li repulsions. The insertion of lithium resulted in the reduction of Ti4+ to Ti3+ that shifts 7Li, 27Al and 31P MAS-NMR resonances towards more positive chemical shifts, improving the resolution of different sites. The detection of twelve components in 7Li MAS-NMR spectra recorded at room temperature suggests the location of Li+ ions at three-oxygen faces that define M2 cavities. From 7Li MAS-NMR spectra, the occupancy of sites and mobility of lithium were investigated in the temperature range 100–500 K. The correlation between structural information, deduced by neutron diffraction, and lithium mobility, deduced by NMR spectroscopy, provides new insights into structural factors that affect lithium mobility in materials with NASICON structure. [ABSTRACT FROM AUTHOR]

The electrochemical properties of TiO2 with the hollandite structure (TiO2(H)) as a negative electrode material for sodium-ion batteries are reported. TiO2(H) was obtained from hollandite K0.21TiO2 by an oxidation–ion extraction process. Na/TiO2(H) cells exhibit a large first discharge capacity of 280 mA h g−1 down to 0.2 V. After the first discharge the Na/TiO2(H) cells develop a reversible charge–discharge capacity of 85 mA h g−1 at C/8 rate in the 2.5–0.2 V voltage range; this corresponds to the reversible insertion of 0.25 Na per TiO2(H) formula unit. Chronoamperometry and potentiostatic intermittent titration techniques were used to further characterize the electrochemical reaction mechanism. Structural changes in the TiO2(H) electrode upon sodium insertion and extraction have been studied by ex situ XRD and high resolution in situ synchrotron diffraction techniques, for which appropriately modified coin-type cells were used. It is seen that sodium insertion into TiO2(H) is commenced with a single-phase solid solution followed by a structural transition from tetragonal I4/m to monoclinic I2/m symmetry, in which the skeleton framework is retained. The reversible transition includes few structural changes with a small volume change of only 1.1%. Fourier difference maps deduced from SXRD patterns revealed the location of Na ions in 4i sites in the tunnel space. The coordination arrangement around Na ions is distorted capped trigonal prisms formed by seven oxygen atoms. Although still far from the theoretical capacity (335 mA h g−1), the cycling properties at a low insertion potential together with the host framework stability indicate the feasibility of TiO2 with the hollandite structure as a negative electrode material for Na-ion batteries. [ABSTRACT FROM AUTHOR]

We describe the synthesis and crystal structure of Li3MRuO5 (M = Co and Ni), new rock salt related oxides. Both the oxides crystallize in the layered LiCoO2 (α-NaFeO2) structure, as revealed by powder XRD data. Magnetic susceptibility data suggest that the oxidation states of transition metals are Li3Co3+(ls)Ru4+(ls)O5 (ls = low spin) for the M = Co compound and Li3Ni2+Ru5+O5 for the M = Ni compound. Electrochemical investigations of lithium deintercalation–intercalation behaviour reveal that both Co and Ni phases exhibit attractive specific capacities of ca. 200 mA h g−1 at an average voltage of 4 V that has been interpreted as due to the oxidation of Co3+ and Ru4+ in Li3CoRuO5 and Ni2+ to Ni4+ in the case of Li3NiRuO5. Thus, a different role of Ru ions is played in the isostructural oxides. Finally, in both cases evidence of irreversible behaviour above 4.2 V is observed and interpreted as formation of high valent ions or alternatively oxidation of oxide ions. [ABSTRACT FROM AUTHOR]

The partial substitution of up to 5% Nd+3 by Ca+2 results in the oxide Nd1.90Ca0.10MgTiO5.94 that presents some remarkable structural features with a noticeable influence on its properties. In this oxide with a monoclinic perovskite-like structure and an octahedral tilting scheme (a-a-b+), both A- and B-ions are arranged in a rock-salt like manner, representing therefore the first example of a type of perovskite theoretically predicted. Besides this unprecedented arrangement of A- and B-ions, the oxygen vacancies created through doping with acceptor ions are trapped by association with the acceptor defects and hence the mobility of these vacancies is strongly limited. The oxygen conductivity of the substituted material is lower and the activation energy for oxygen motion is higher than those of the parent oxide, in which the concentration of anion vacancies is only due to intrinsic defects.

We report the rich magnetic behaviour of Sr2CoNb1-xTixO6 (0 ≤ x ≤ 0.5) oxides as a result of their complex microstructure. Although these oxides show an average simple-cubic perovskite structure, they present a flexible microstructure due to short-range ordering between Co/Ti and Nb cations in the perovskite B-sites. The microstructure consists of double-cubic perovskite domains grown in a simple-cubic perovskite matrix. The size and number of the double-cubic perovskite domains decrease as the Ti content increases. As a result of aliovalent substitution of Nb(5+) by Ti(4+) in the parent Sr2CoNbO6 mixed-valence Co(3+)/Co(4+) oxides are obtained. A spin glass-like state has been observed at low temperatures for all the series, with freezing temperatures increasing with the Ti-content in the range 22 to 33 K. Furthermore, the x = 0.3 and x = 0.5 samples show non-interacting superparamagnetic particle-like dynamics associated with relatively high amounts of Co(4+), with "blocking temperatures" of 13 and ∼16 K, respectively. The complex magnetic behaviour of the title oxides seems to be connected with the clustering of magnetic Co(3+) and the distribution of Co(4+) as a result of the microstructure.

We describe the synthesis, crystal structure and lithium deinsertion-insertion electrochemistry of two new lithium-rich layered oxides, Li3MRuO5 (M = Mn, Fe), related to rock salt based Li2MnO3 and LiCoO2. The Li3MnRuO5 oxide adopts a structure related to Li2MnO3 (C2/m) where Li and (Li0.2Mn0.4Ru0.4) layers alternate along the c-axis, while the Li3FeRuO5 oxide adopts a near-perfect LiCoO2 (R3[combining macron]m) structure where Li and (Li0.2Fe0.4Ru0.4) layers are stacked alternately. Magnetic measurements indicate for Li3MnRuO5 the presence of Mn(3+) and low spin configuration for Ru(4+) where the itinerant electrons occupy a π*-band. The onset of a net maximum in the χ vs. T plot at 9.5 K and the negative value of the Weiss constant (θ) of -31.4 K indicate the presence of antiferromagnetic superexchange interactions according to different pathways. Lithium electrochemistry shows a similar behaviour for both oxides and related to the typical behaviour of Li-rich layered oxides where participation of oxide ions in the electrochemical processes is usually found. A long first charge process with capacities of 240 mA h g(-1) (2.3 Li per f.u.) and 144 mA h g(-1) (1.38 Li per f.u.) is observed for Li3MnRuO5 and Li3FeRuO5, respectively. An initial sloping region (OCV to ca. 4.1 V) is followed by a long plateau (ca. 4.3 V). Further discharge-charge cycling points to partial reversibility (ca. 160 mA h g(-1) and 45 mA h g(-1) for Mn and Fe, respectively). Nevertheless, just after a few cycles, cell failure is observed. X-ray photoelectron spectroscopy (XPS) characterisation of both pristine and electrochemically oxidized Li3MRuO5 reveals that in the Li3MnRuO5 oxide, Mn(3+) and Ru(4+) are partially oxidized to Mn(4+) and Ru(5+) in the sloping region at low voltage, while in the long plateau, O(2-) is also oxidized. Oxygen release likely occurs which may be the cause for failure of cells upon cycling. Interestingly, some other Li-rich layered oxides have been reported to cycle acceptably even with the participation of the O(2-) ligand in the reversible redox processes. In the Li3FeRuO5 oxide, the oxidation process appears to affect only Ru (4+ to 5+ in the sloping region) and O(2-) (plateau) while Fe seems to retain its 3+ state.

Aliovalent substitution of Nb(5+) by Ti(4+) in Sr2LuNbO6 is limited to 10% of Nb atoms. A full structural determination by NPD confirms this and reveals that the structure is better described as a superstructure of the simple cubic perovskite (as previously reported) with the monoclinic cell 2(1/2)ap× 2(1/2)ap× 2ap and β≈ 90° (S.G. P21/n). The substituted materials present both oxygen-vacancies induced by charge compensation and Sr-deficiency. Therefore, their formula should be given as Sr2-yLuNb1-xTixO6-δ. Electrical properties can be fully understood considering these compositional defects. The parent compound Sr2LuNbO6 presents low electrical conductivity in air, which improves by more than one order of magnitude upon Ti substitution. In any case, the title oxides show low electrical conductivity in a wide oxygen partial pressure (pO2) range (10(-25) atm ≤pO2≤ 10(-1) atm). At high pO2 the conductivity increases with pO2 due to oxygen-vacancy annihilation and hole creation, according to a general p-type semiconducting mechanism; A-site substoichiometry and Ti-substitution are the origin of this behaviour. In the low pO2 region, the conductivity increases as the oxygen partial pressure decreases. Reduction of cations, Nb(5+) or Ti(4+), supports n-type conduction by electrons and oxygen vacancy creation. For the intermediate pO2 range a low ionic conduction contribution is observed. Although the estimated ionic conductivity is not high in the substituted compounds, the strategy seems to be valid since a significant enhancement of ionic conduction is observed upon aliovalent substitution.

The structural features of intercalated Li3AlxTi2-x(PO4)3 compounds, with x = 0 and 0.2, have been deduced by Rietveld analysis of neutron diffraction (ND) patterns recorded between 100 and 500 K. The Li insertion decreases the symmetry from R3̄c to R3̄ in analyzed compounds. In pristine Li1+xAlxTi2-x(PO4)3 samples, Li occupies mainly six-fold M1 sites at ternary axes; but in lithiated Li3AlxTi2-x(PO4)3 samples, Li is located near M2 positions at M3/M3' four-fold coordinated sites. In both cases, Li arrangement minimizes electrostatic Li-Li repulsions. The insertion of lithium resulted in the reduction of Ti(4+) to Ti(3+) that shifts (7)Li, (27)Al and (31)P MAS-NMR resonances towards more positive chemical shifts, improving the resolution of different sites. The detection of twelve components in (7)Li MAS-NMR spectra recorded at room temperature suggests the location of Li(+) ions at three-oxygen faces that define M2 cavities. From (7)Li MAS-NMR spectra, the occupancy of sites and mobility of lithium were investigated in the temperature range 100-500 K. The correlation between structural information, deduced by neutron diffraction, and lithium mobility, deduced by NMR spectroscopy, provides new insights into structural factors that affect lithium mobility in materials with NASICON structure.

Li(2)Ti(6)O(13) and H(2)Ti(6)O(13) were easily synthesized from Na(2)Ti(6)O(13) by successive Na(+)-Li(+)-H(+) ion exchange. The crystal structures of Na(2)Ti(6)O(13), Li(2)Ti(6)O(13) and H(2)Ti(6)O(13) were investigated using neutron powder diffraction. Monovalent A(+) cations (Na, Li and H) have been located using difference Fourier analysis. Although monoclinic lattice parameters (space group C2/m) of the three titanates remain almost unchanged with retention of the basic [Ti(6)O(13)(2-)] network, monovalent Na, Li and H cations occupy different sites in the tunnel space. By comparing the structural details concerning the A(+) oxygen coordination, i.e. NaO(8) square prismatic coordination, LiO(4) square planar coordination and covalently bond H atoms, with results from (23)Na, (7)Li and (1)H NMR spectroscopy we were able to obtain a more detailed insight into the respective local distortions and anharmonic motions. We were able to show that the site that the A(+) cation occupies in the hexatitanate channel structure strongly influences the lithium insertion properties of these compounds and therefore their usefulness as electrode materials for energy storage.

A detailed structural and electrochemical study of the ion exchanged Li(2)Ti(6)O(13) titanate as a new anode for Li-ion batteries is presented. Subtle structural differences between the parent Na(2)Ti(6)O(13), where Na is in an eightfold coordinated site, and the Li-derivative, where Li is fourfold coordinated, determine important differences in the electrochemical behaviour. While the Li insertion in Na(2)Ti(6)O(13) proceeds reversibly the reaction of lithium with Li(2)Ti(6)O(13) is accompanied by an irreversible phase transformation after the first discharge. Interestingly, this new phase undergoes reversible Li insertion reaction developing a capacity of 170 mAh g(-1) at an average voltage of 1.7 V vs. Li(+)/Li. Compared with other titanates this result is promising to develop a new anode material for lithium ion rechargeable batteries. Neutron powder diffraction revealed that Na in Na(2)Ti(6)O(13) and Li in Li(2)Ti(6)O(13) obtained by Na/Li ion exchange at 325 °C occupy different tunnel sites within the basically same (Ti(6)O(13))(2-) framework. On the other hand, electrochemical performance of Li(2)Ti(6)O(13) itself and the phase released after the first full discharge is strongly affected by the synthesis temperature. For example, heating Li(2)Ti(6)O(13) at 350 °C produces a drastic decrease of the reversible capacity of the phase obtained after full discharge, from 170 mAh g(-1) to ca. 90 mAh g(-1). This latter value has been reported for Li(2)Ti(6)O(13) prepared by ion exchange at higher temperature.

The half-metallic ferromagnet K(2)Cr(8)O(16) with the hollandite structure has been chemically modified using soft chemistry methods to increase the average oxidation state of chromium. The synthesis of the parent material has been performed under high pressure/high temperature conditions. Following this, different redox reactions have been carried out on K(2)Cr(8)O(16). Oxidation to obtain potassium-de-inserted derivatives, K(2-x)Cr(8)O(16) (0 ≤x≤ 1), has been investigated with electrochemical methods, while the synthesis of sizeable amounts was achieved chemically by using nitrosonium tetrafluoroborate as a highly oxidizing agent. The maximum amount of extracted K ions corresponds to x = 0.8. Upon oxidation the hollandite structure is maintained and the products keep high crystallinity. The de-insertion of potassium changes the Cr(3+)/Cr(4+) ratio, and therefore the magnetic properties. Interestingly, the Curie temperature increases from ca. 175 K to 250 K, getting therefore closer to room temperature.

Substitution of La(3+) by Sr(2+) in the double perovskite La(2)CoTiO(6) yields materials of the La(2-x)Sr(x)CoTiO(6) series showing a significant amount of trivalent cobalt ions when prepared at ambient atmosphere. The as-prepared compounds can be reduced in severe conditions retaining the perovskite structure while inducing the formation of a large amount of oxygen vacancies. The limit of aliovalent substitution in this series was found to extend up to x = 1. For substitution of La(3+) up to 15% cobalt and titanium are ordered, though the order is progressively lost as x increases; for x≥ 0.30 no ordering is observed as evidenced by magnetic measurements. The ability of these materials to present either cobalt ions in a mixed oxidation state or large amounts of anion vacancies depending on the atmosphere makes them interesting to be further investigated regarding their electrical and electrochemical properties, and hence, their usefulness in some electrochemical devices., (This journal is © The Royal Society of Chemistry 2011)