Alois Kuhn

New electrode materials for lithium rechargeable batteries

Abstract In this paper, a contribution to the search of new electrode materials for lithium batteries is reported. First, we will present the electrochemical behavior of an Aurivillius-type phase with the composition Bi 4 V 2 O 11 . This oxide may be used as cathode material (390 A h/kg) if the voltage is not so low (average voltage 1.6 V). On the other hand, if Bi 4 V 2 O 11 is reduced down to 0.5 V, it reacts with 28 Li ions per formula unit. Considering only the low voltage region, Li 28 Bi 4 V 2 O 11 could be a candidate to anode material (360 A h/kg at 0.7 V). Our research has also been directed towards the applications of several types of titanium oxides in lithium batteries. Among these compounds, we present the results for K x Ti 8 O 16 and Li 2 Ti 3 O 7 . The best results are those obtained from the ramsdellite Li 2 Ti 3 O 7 : the large reversibility, low polarization and relatively high capacity (235 A h/kg) make this compound a promising material as negative electrode for lithium ion cells. However, the relatively high average potential, close to 1.4 V, would reduce considerably the performance of a rocking-chair battery using this ramsdellite instead of carbon.

Lithium intercalation in KxTi8O16 compounds

Several bronzes KxTi8O16 have been prepared from K1.64(1)Ti8O16 by means of oxidative reactions. These bronzes have been intercalated with lithium by electrochemical methods in order to test their performances as electrode materials in lithium rechargeable batteries. The main result is that although the maximum capacity (260 Ah kg−1) is reached in the compound with the lowest potassium content, a low reversibility and high polarisation are obtained. Bronzes with intermediate potassium content are more attractive for applications in rechargeable lithium batteries. These compounds maintain an acceptable capacity (200 Ah kg−1) while having high cyclability and low polarisation. The application should be addressed to anode materials rather than to cathode due to the low voltage (1.75 V) obtained during the intercalation reaction.

VO2F: a new transition metal oxyfluoride with high specific capacity for Li ion batteries

Hitherto unreported vanadium oxyfluoride VO2F has been synthesized using a solid state reaction at a pressure of 4 GPa and 800 °C. This long awaited vanadium oxyfluoride fills the existing gap of ReO3-type MO2F compounds of Group 5 elements, from which only NbO2F and TaO2F have been known to exist to date. VO2F crystallizes with the VF3-type structure, space group Rc, with a = 5.1226(1) A and c = 13.0686(3) A as determined by powder X-ray diffraction. Highly structured diffuse streaking observed in electron diffraction patterns evidences local O/F ordering. VO2F exhibits two regions upon discharge in a lithium cell, an upper sloped region in the range of 3.9–2.2 V and a lower plateau at 2.15 V. Discharge of VO2F to 1 V provides a gravimetric capacity of 450 mA h g−1. VO2F can reversibly insert up to 1 Li+ per vanadium above 2.15 V without destruction of the host structure, delivering a gravimetric capacity as high as 250 mA h g−1 and pointing to VO2F as a promising intercalation electrode.

Defect and dopant properties of the α- and β-polymorphs of the Li3FeF6 lithium battery material

Li3FeF6 has attracted recent interest as a promising positive electrode for rechargeable lithium batteries. The defect chemistry and cation doping behaviour of the α- and β-polymorphs of Li3FeF6 have been investigated by advanced atomistic modelling techniques. Our simulations show good reproduction of both monoclinic (α) and orthorhombic (β) experimental structures, which are related to the cryolite crystal structure. The most favourable defect types are found to be the Li Frenkel and off-stoichiometry (Li-excess) disorder, suggesting that interstitial lithium is possible in the cryolite structure, and is important in rationalizing the lithium intercalation chemistry. Monovalent dopant substitution for Li and divalent substitution for Fe are energetically favourable, and could be a synthesis strategy to optimise the electrochemical behaviour of Li3FeF6.

The role of the Eu3+/Eu2+ redox-pair in the electrical properties of Sr2EuNb1−xTixO6−δ oxides

In the search for new materials potentially useful as SOFC components perovskite-like oxides of the Sr2EuNb1−xTixO6−δ series have been obtained, the solubility limit being ca. x = 0.15. Rietveld refinements of XRD data and SAED and HRTEM have demonstrated that these compounds are monoclinic with cell parameters a ≈ b = ap√2 ≈ 5.88 A, c = 2ap ≈ 8.28 A, and S.G. P21/n. As required for SOFC materials, these oxides are stable under a wide range of oxygen partial pressures from the ambient condition to pO2 ≈ 10−30 atm. Aliovalent substitution of Nb5+ by Ti4+ improves the electrical conductivity in air by two orders of magnitude for the end-member of the series (x = 0.15) compared with the parent material. Magnetic measurements, pO2 dependence of conductivity and ion-blocking measurements demonstrate that the predominant conduction mechanism depends on the oxygen partial pressure. In the high pO2 region (from 10−5 to 0.21 atm) p-type conduction is dominant due to the presence of oxygen vacancies which are being annihilated as pO2 increases. Under severe reducing conditions (pO2 below 10−22 atm), n-type conduction dominates. Magnetic measurements demonstrate reduction of Eu3+ occurs whereas the rest of the elements remain in their highest oxidation states. Thus, the Eu3+/Eu2+ redox pair participates in the equilibrium defect responsible for n-type conduction. For intermediate pO2 (10−20 to 10−8 atm) a significant pO2-independent ionic conduction, due to the motion of anionic vacancies, is the dominant conduction mechanism, though a minor electronic n-type contribution is also observed, associated with the reduction of Eu3+ (this occurs at pO2 as high as 10−10 atm). Oxides in the Sr2EuNb1−xTixO6−x/2 series constitute an interesting example of rare-earth perovskites in which the rare-earth ions play a role not only in the structural but also in the electrical behaviour.

Comprehensive investigation of the lithium insertion mechanism of the Na2Ti6O13 anode material for Li-ion batteries

Sodium hexatitanate Na2Ti6O13 with a tunnel structure has been proposed to be an attractive anode material for lithium ion batteries because of its low insertion voltage, structural stability and good reversibility. In order to obtain a full understanding of the properties of this titanate, a combination of in situ synchrotron X-ray diffraction, neutron diffraction and 7Li MAS solid-state NMR spectroscopy is used in the present work. During the first insertion stage (centered at 1.3 V), lithium is allocated in square planar LiO4 2c (Li1) sites, minimizing electrostatic repulsion with Na ions. During the second lithium uptake (centered at 1.1 V), Li ions pass from 2c to empty 4i (Li2) sites of y/b = 0.5 planes, near Ti3+ cations. Distribution of Li+ and Na+ ions with respect to Ti3+ cations was deduced from Fourier map differences (Rietveld technique) and NMR quantitative analyses in Li0.8Na2Ti6O13 and Li1.7Na2Ti6O13 samples. 7Li MAS-NMR analysis showed that Li ions occupy three fourfold coordinated sites with reasonable Li+–O–Ti3+ bond distances, while Na cations remain at eightfold coordinated positions near Ti4+ cations as deduced from 23Na MAS-NMR spectroscopy. 7Li MAS-NMR recorded at increasing temperatures suggests that Li ions move along sinusoidal paths to reduce Li–Na electrostatic interactions. Li mobility along the b-axis is favored by partial occupation of interstitial 4i sites (Li3) located at both sides of Na cations in y/b = 0 planes. In lithium inserted samples the most probable – Li1(2c) → Li3(4i) → Li1(2c) – conduction paths were deduced. However, formation of Li pairs at y/b = 0 planes (Li2 sites), where Li ions are located near Ti3+ cations, reduces the amount of mobile Li ions that participate in conduction processes. Proximity of lithium to Li and Na ions limits insertion to ca. 2 Li ions per structural formula.

Synthesis and characterization of NaNiF3·3H2O: An unusual ordered variant of the ReO3 type

A new hydrated sodium nickel fluoride with nominal composition NaNiF3·3H2O was synthesized using an aqueous solution route. Its structure was solved by means of ab initio methods from powder X-ray diffraction and neutron diffraction data. NaNiF3·3H2O crystallizes in the cubic crystal system, space group Pn3̅ with a = 7.91968(4) Å. The framework, derived from the ReO3 structure type, is built from NaX6 and NiX6 (X = O, F) corner-shared octahedra, in which F and O atoms are randomly distributed on a single anion site. The 2a × 2a × 2a superstructure arises from the strict alternate three-dimensional linking of NaX6 and NiX6 octahedra together with the simultaneous tilts of the octahedra from the cube axis (φ = 31.1°), with a significant participation of hydrogen bonding. NaNiF3·3H2O corresponds to a fully cation-ordered variant of the In(OH)3 structure, easily recognizable when formulated as NaNi(XH)6 (X = O, F). It constitutes one of the rare examples for the a(+)a(+)a(+) tilting scheme with 1:1 cation ordering in perovskite-related compounds. The Curie-like magnetic behavior well-reflects the isolated paramagnetic Ni(2+) centers without worth mentioning interactions. While X-ray and neutron diffraction data evidence Na/Ni order in combination with O/F disorder as a main feature of this fluoride, results from Raman and magic-angle spinning NMR spectroscopies support the existence of specific anion arrangements in isolated square windows identified in structural refinements. In particular, formation of water molecules derives from unfavorable FH bond formation.

Synthesis, Characterization and Electrochemical Study of the Solid Solution LixNa0.875−δFe0.875Ti1.125O4 (0 ≤ δ ≤ 0.4, x ≥ 0)

The calcium ferrite type structure of Na 0.875 Fe 0.875 Ti 1.125 O 4 [1] has double tunnels which are occupied by double rows of sodium atoms running along the b-axis. We have partially removed sodium from this compound at moderate temperatures with different oxidizing agents. Electrochemical studies show that the resulting materials, Na 0.875 Fe 0.875 Ti 1.125 O 4 (0 ≤ δ ≤ 0.4), can reversibly intercalate lithium at potentials between 3.8 and 1 volt. At the lowest voltages, a compound containing ∼ 0.5 lithium per formula is formed. By chemical reaction of Na 0.875 Fe 0.875 Ti 1.125 O 4 with n-butyllithium, the maximum lithium content also corresponds to Li 0.5 Na 0.875 Fe 0.875 Ti 1.125 O 4 . This suggests that many more than the one eighth of the empty sodium sites, per unit cell, of the parent phase are now being occupied by lithium.

Structural and Electrochemical Study of Vanadium-Doped TiO2 Ramsdellite with Superior Lithium Storage Properties for Lithium-Ion Batteries.

Titanium-oxide-based materials are considered attractive and safe alternatives to carbonaceous anodes in Li-ion batteries. In particular, the ramsdellite form TiO2 (R) is known for its superior lithium-storage ability as the bulk material when compared with other titanates. In this work, we prepared V-doped lithium titanate ramsdellites with the formula Li0.5 Ti1-x Vx O2 (0≤x≤0.5) by a conventional solid-state reaction. The lithium-free Ti1-x Vx O2 compounds, in which the ramsdellite framework remains virtually unaltered, are easily obtained by a simple aqueous oxidation/ion-extraction process. Neutron powder diffraction is used to locate the Li channel site in Li0.5 Ti1-x Vx O2 compounds and to follow the lithium extraction by difference-Fourier maps. Previously delithiated Ti1-x Vx O2 ramsdellites are able to insert up to 0.8 Li(+) per transition-metal atom. The initial gravimetric capacities of 270 mAh g(-1) with good cycle stability under constant current discharge conditions are among the highest reported for bulk TiO2 -related intercalation compounds for the threshold of one e(-) per formula unit.

Driving Electrons to Anti-Bonding States: On the Synthesis of New Niobium Cluster Chlorides by Electrochemical Lithium Intercalation

The lithium intercalation chemistry of LiNb 6 Cl 15 , a 16 e − Nb-cluster, has been explored in order to obtain new Nb-cluster compounds. As a result, three different phases have been detected. Full de-intercalation of lithium produces Nb 6 Cl 15 , a new 15 e − Nb-cluster. The oxidation reaction is reversible since lithium can be intercalated again to produce the parent LiNb 6 Cl 15 . On the other hand, intercalation of lithium into LiNb 6 Cl 15 seems to proceed through two single phases with the following stoichiometries: Li 1.5 Nb 6 Cl 15 and Li 3 Nb 6 Cl 15 . For these two compositions the extra electrons (0.5 and 2 respectively/formula) should enter the e g * molecular orbitals arising from Nb-Nb interactions inside the cluster. The reductions of LiNb 6 Cl 15 leading to these two new electron-rich Nb-cluster are reversible as detected by chronopotentiometry.