A. Kuhn

Hollandite-type TiO2: a new negative electrode material for sodium-ion batteries

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.

Structural Factors That Enhance Lithium Mobility in Fast-Ion Li1+xTi2–xAlx(PO4)3 (0 ≤ x ≤ 0.4) Conductors Investigated by Neutron Diffraction in the Temperature Range 100–500 K

Structural features responsible for lithium conductivity in Li(1+x)Ti(2-x)Al(x)(PO4)3 (x = 0, 0.2, and 0.4) samples have been investigated by Rietveld analysis of high-resolution neutron diffraction (ND) patterns. From structural analysis, variation of the Li site occupancies and atomic thermal factors have been deduced as a function of aluminum doping in the temperature range 100-500 K. Fourier map differences deduced from ND patterns revealed that Li ions occupy M1 sites and, to a lower extent, M3 sites, disposed around ternary axes. The occupation of M1 sites by Li ions is responsible for the preferential expansion of the rhombohedral R3c unit cell along the c axis with temperature. The occupation of less symmetric M3 sites decreases electrostatic repulsions among Li cations, favoring ion conductivity in Li(1+x)Ti(2-x)Al(x)(PO4)3 compounds. The variations detected on long-range lithium motions have been related to variations of the oxygen thermal factors with temperature. The information deduced by ND explains two lithium motion regimes deduced previously by (7)Li NMR and impedance spectroscopy.

Mechanical grinding of Si3N4 to be used as an electrode in lithium batteries

Abstract The effect of mechanical grinding (MG) on the electrochemical properties of Si 3 N 4 is studied as the processing produces different particle size. The evolution of the amorphisation stage has been monitored by X-ray diffraction (XRD), although particle size has also been studied through transmission electron microscopy experiments. Processed Si 3 N 4 has been used as the positive electrode in electrochemical cells, where the negative electrode is lithium. We do not expect an intercalation process during the discharge step of such a type of cell, but a behaviour similar to that named as “convertible compounds”. As the main result, the specific capacity of these lithium cells is increased up to 34% in relation to the starting, non-milled material. However, even though the size of the Si 3 N 4 particles has been reduced, from micrometer to nanometer scale, no appreciable changes in the cycling behaviour is observed when compared with the pristine material.

H2Ti6O13, a new protonated titanate prepared by Li+/H+ ion exchange: synthesis, crystal structure and electrochemical Li insertion properties

The hexatitanate H2Ti6O13 is obtained by a simple successive Na+/Li+/H+ ion exchange of Na2Ti6O13. The crystal structure of H2Ti6O13 was solved from both synchrotron and neutron powder diffraction. H2Ti6O13 crystallizes in the monoclinic space group C2/m, with a = 14.6702(3) A; b = 3.7447(1) A; c = 9.2594(2) A; β = 96.941(2)°. The monoclinic symmetry of the [Ti6O13]2− framework is preserved during the exchange reaction. When compared to the positions of Na and Li in Na2Ti6O13 and Li2Ti6O13, the position of the proton is shifted towards the O3 atomic position, where it forms a covalent O–H bond. The vicinity of the proton to the O5 atom across the tunnel allows for the formation of a classical (asymmetric) hydrogen bond. H2Ti6O13 has been tested as a Li insertion material to assess its use as an electrode in lithium rechargeable batteries. It reacted irreversibly with ca. 6 Li ions per formula unit at an average voltage of 1.5 V vs. Li+/Li, with a specific discharge capacity of 315 mA h g−1. However, after first discharge, a reversible specific capacity of 170 mA h g−1 was developed. H2Ti6O13 then yielded a higher reversible specific capacity than Na2Ti6O13 and comparable to Li2Ti6O13. Besides structural details, IR spectroscopy has been used to further assess possible reaction mechanisms pointing to the transformation of H2Ti6O13 to Li2Ti6O13 when reacting with the very first two lithium ions.

Li3MRuO5 (M = Co, Ni), new lithium-rich layered oxides related to LiCoO2: promising electrochemical performance for possible application as cathode materials in lithium ion batteries

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 (alpha-NaFeO2) structure, as revealed by powder XRD data. Magnetic susceptibility data suggest that the oxidation states of transition metals are Li3Co3+(ls)Ru4+(ls) O-5 (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.

Synthesis and Electrochemical Behaviour of Ramsdellite LiCrTiO 4

A ramsdellite with composition LiCrTiO 4 has been obtained by heating the spinel of same composition to high temperature. The new ramsdellite has been investigated in view of its possible use as an electrode material in lithium rechargeable batteries. Lithium can be partially extracted from ramsdellite LiCrTiO 4 and further intercalated into, by contrast to the spinel of same composition. The average operating voltage during lithium extraction is 4 Volts vs. lithium, and the process produces a specific capacity of 90 mAh/g at 0.1 mA/cm 2 . On the other hand, upon reduction from open circuit voltage, lithium can be reversibly intercalated into the ramsdellite polymorph at ca. 1.5 V vs. lithium yielding a rechargeable capacity of 110 mAh/g at 0.1 mA/cm 2 .