Volume 229, Issue 9 - Themed Issue: Lithium Ions in Solids – Between Basics and Better Batteries / Paul Heitjans

Lithium diffusion in the spinel type cubic titanium disulphide ( c -Li x TiS 2 , x = 0.69 and 0.75) is investigated theoretically with periodic density-functional theory (DFT) method. The calculated unit cell length and bond distances for a series of Li x TiS 2 (0 < x < 1) compounds are in agreement with the experimental data, with the maximum deviation of +0.06% for the lattice parameter and −1.2% for the bond length. In agreement with Vegard's law, the unit cell length a is an almost linear function of x . The calculated average intercalation potentials for the series Li x TiS 2 (0 < x < 1) range between 1.6 to 1.9 V which is in the range of experimental findings. Competing pathways for Li diffusion in c -Li x TiS 2 (x = 0.69 and 0.75) are investigated using the climbing-image Nudged-Elastic-Band (cNEB) approach. Li + ions can migrate along the 〈100〉 and 〈110〉 directions suggesting that Li + diffusion in c -Li x TiS 2 is three-dimensional (3D). The calculated activation energy values for the considered migration pathways show that Li + diffusion along the 〈100〉 directions is more preferable that that along the 〈110〉 directions.

Layered titanium disulfide is used as lithium-ion intercalating electrode material in batteries. The room-temperature stable trigonal 1T polymorphs of the intercalates Li x TiS 2 ( x  ≤ 1) are widely-investigated. However, the rombohedral 3R polymorphs, being stable at higher temperatures for large x , are less well known. In this study, we report on the synthesis of phase-pure 1T-Li x TiS 2 ( x  = 0.7,   0.9) and its transformation to the 3R phase between 673 and 873 K as monitored using high-temperature neutron powder diffractometry. For the 3R polymorph, full Rietveld refinements show lithium ions to be statistically distributed over octahedral voids at the fractional coordinates 0, 0, 1/2 , exclusively. The comparison of Madelung energies with results of periodic quantum-chemical calculations reveals that the evolution of lattice parameters and the room-temperature stability of the 1T phase are not governed by electrostatics, but by correlation and polarization. The insights gained do not only elucidate the structure of 3R-Li x TiS 2 , but also help to understand and control polymorphism in layered transition-metal sulfides.

Chemical intercalation of lithium in single crystals of TiX 2 (X = S, Se, Te) was studied at pressures of 10–500 MPa and temperatures of 293–363 K. Crystals were synthesized by chemical vapor synthesis and cut with a LASER into squares (2.2 × 2.2 mm). Samples and 1.6 mol · L – 1 n-butyl lithium solutions in n-hexane (BuLi) were sealed in gold capsules. Experiments were carried out in cold seal pressure vessels for 5–20 days using Ar as pressure medium. A large fraction of samples showed degradation after the experiments, i.e., at low pressures and at high temperatures. On samples with minor degradation, lithium profiles were measured parallel to the crystallographic ab-plane , using femtosecond-LASER Ablation-Inductively Coupled Plasma-Optical Emission Spectroscopy and -Mass Spectrometry (fs-LA-ICP-OES and -MS). Most of the profiles could be well fitted by the specific solution of Fick's second law for the given boundary conditions. Lithium diffusivities ( D Li ) and lithium contents at the crystal edges ( x Li,S ) were determined from the fits. At 500 MPa and room temperature lithium diffusivities are similar in TiS 2 ((1.07 to 2.41) × 10 – 15  m 2  · s – 1 ) and TiSe 2 ((1.48 to 2.10) × 10 – 15  m 2  · s – 1 ) but significantly lower in TiTe 2 ((0.07 to 0.55) × 10 – 15  m 2  · s – 1 ). Moreover, diffusivity of Li in TiTe 2 decreases significantly with increasing pressure whereas for the other chalcogenides a pressure dependence cannot be evidenced. Lithium surface contents are lower for TiS 2 ( x   =  0.15 to 0.36) than for TiSe 2 (0.66 to 0.71) and TiTe 2 (0.59 to 0.99), slightly elevated compared to ambient pressure experiments. These findings indicate, that the ionicity of the Ti–X bonding as well as the size of the interlayer space (van der Waals gap) play a crucial role for the degree of lithium insertion and for the pressure dependence of lithium diffusivity.

A new cell was developed to study the behavior of powders during axial compaction at temperatures up to 950 K and pressures up to 1 GPa. Electrical conductivity of the sample is measured by impedance spectroscopy. Change of sample volume can be simultaneously monitored using linear variable differential transducers. Flushing with gases allows pre-conditioning of material surfaces before compaction. The cell was tested with two different grain size fractions (60–150 μm and 200–250 μm) of lithium trisilicate glass in the brittle deformation range (315 K) and in the partially plastic deformation range (ca. 660 K). The DC conductivity increases linearly with pressure up to 750 MPa at about 660 K. Lower conductivity for the smaller grain size fraction under these conditions is attributed to higher shares of pores between the grains, acting as resistors.

In this work nuclear magnetic resonance (NMR) and impedance spectroscopy (IS) studies on Li ion dynamics in microcrystalline γ -LiAlO 2 are presented. The sample was prepared by solid state synthesis between Li 2 CO 3 and Al 2 O 3 in air, followed by a quenching procedure. The presence of phase-pure γ -LiAlO 2 was confirmed by X-ray powder diffraction including Rietveld refinement. Further structural characterization was done with 6 Li, 7 Li and 27 Al NMR. Several NMR techniques such as spin-lattice relaxation measurements, motional narrowing experiments, as well as spin-alignment echo were employed for the investigation of Li ion diffusion. The measurements were carried out at high temperatures (up to 970 K) in order to access the regime of Li ion motion being very slow. The dc conductivities measured by IS in the temperature range from 680 K to 870 K were converted to diffusion coefficients being compatible with those obtained by NMR.

We investigated lithium self-diffusion in amorphous lithium aluminate (LiAlO 2 ) layers between room temperature and 473 K. For the experiments, amorphous 6 LiAlO 2 (30 nm)/ 7 LiAlO 2 (1200 nm) isotope hetero-structures were deposited by ion-beam sputtering on sapphire substrates. Diffusion profiles were analysed by secondary ion mass spectrometry (SIMS). The results show that the diffusivities obey the Arrhenius law with an activation enthalpy of (0.94 ± 0.02) eV. This is not much different to the activation enthalpy of 1.14 eV found for LiAlO 2 single crystals by impedance spectroscopy. It rationalizes the only modest enhancement of diffusivities in amorphous lithium aluminate compared to single crystals of three to five orders of magnitude in the temperature range studied, when compared with, e.g., lithium niobate.

Li x Na 2− x Ti 6 O 13 has received attention as 3 d -metal oxide based anode material for possible battery application. Generally, titanium oxides represent excellent Li hosts due to their zero-strain behavior, cycling stability and high operating voltage. New developments choose Na as charge carrier, but less effort is put in the investigation of mixed cation conductors. Owing to the synthesis route of Li x Na 2− x Ti 6 O 13 (0 ≤  x  ≤ 2) the coordination of Na and Li in the channels is different which had been investigated by means of X-ray and neutron diffraction. Up to now, no Raman spectroscopy has been applied. This oxide is highly Raman-active, thus the local structure can also be analyzed in terms of vibrational spectroscopy. Micro-Raman spectroscopy carried out at room temperature with different cation contents ( x  = 0, 0.33, 1, 2) shows the continuous change from Na to Li by a superposition of the modes for Na 2 Ti 6 O 13 and Li 2 Ti 6 O 13 . The only exceptions are two distinct modes. They appear either for Li (118 cm – 1 ) or Na (135 cm – 1 ). The results confirm the high-temperature phase stability of Na 2 Ti 6 O 13 as well as the anisotropic thermal expansion of the unit cell seen by in situ X-ray powder diffraction under two different gas atmospheres. Combining these results, we suppose that the anisotropic thermal expansion of the lattice parameters is affected by the normal vectors of the corresponding bond vibrations in Na 2 Ti 6 O 13 and Li 2 Ti 6 O 13 . Crystalline-orbital calculations of the Raman shifts of Li x Na 2− x Ti 6 O 13 were carried out for the cation contents x  = 0, 1, 2 and Raman modes were assigned to specific bond vibrations supported by theory. Besides, this gives additionally information about the Li/Na ratio in a new and simple way.

The monoclinic polymorph of Li 2 TiO 3 ( β -form) is known to be a relatively poor Li ion conductor. Up to now, no information is available on how the ion transport properties change when going from well-ordered crystalline Li 2 TiO 3 to a structurally disordered form with the same chemical composition. Here, we used high-energy ball milling to prepare nanocrystalline, defect-rich Li 2 TiO 3 ; ion dynamics have been studied via impedance spectroscopy. It turned out that ball milling offers the possibility to enhance long-range ion transport in the oxide by approximately 3 orders of magnitude. Its effect on the oxide ceramic is two-fold: besides the introduction of a large number of defects, the originally μm-sized crystallites are decreased to crystallites with a mean diameter of less than 50 nm. This process is accompanied by a mechanically induced phase transformation towards the α -form of Li 2 TiO 3 ; besides that, a significant amount of amorphous materials is produced during milling. Structural disorder in nanocrystalline as well as amorphous Li 2 TiO 3 is anticipated to play the capital role in governing Li ion dynamics of the sample finally obtained.

Electrodes made of amorphous silicon are a promising alternative to graphite as anode material for the next generation of high-capacity Li-ion batteries. In order to optimize such batteries the mechanism of lithium incorporation into the electrode during charging has to be elucidated. In the present study we measured the modification of lithium distribution taking place during galvanostatic lithiation of about 600 nm thick amorphous silicon film electrodes at low current densities of about 30 μA/cm 2 (∼ C/14) by Secondary Ion Mass Spectrometry. The results were confirmed by X-ray Photoelectron Spectroscopy. The results indicate a two-step lithiation procedure where the electrode is transformed first into a homogeneously lithiated Li x Si phase with a low Li content of x  ≈ 0.3. During later stages of the lithiation process, the results are in agreement with the penetration of a highly lithiated phase via a moving phase boundary, as also observed for crystalline silicon.

Thin film technology is applied in different fields of Li-ion battery research and development, such as the fabrication of thin film cells and model electrodes. Data obtained by surface and interface analysis of thin films provides important insights into fundamental processes such as charge compensation mechanism or interface formation. In this overview, we present the analysis of LiCoO 2 and LiPON thin films using photoemission. It includes data obtained directly after preparation, after exposure to molecular species and/or after electrode-electrolyte interface formation. Information both on electronic structure and surface chemistry of the films is deduced from core level spectra, valence band spectra, Fermi level position and work function. Next to PES analysis of our films, we address composition, structure and functionality in more general terms in relation to literature.

Graphitic materials are currently the state-of-the-art anode materials for lithium ion secondary batteries. By chemical modification, the electrochemical performance of the pristine materials can be improved. In this paper we report on the preparation of nanostructured copper particles on graphite by thermal decomposition of copper formate. With this technique a novel, simple and low cost method for a homogeneous deposition of nanostructured copper particles on graphite was established. Different amounts of copper were realized and their influence on the electrochemical behavior of the active material was investigated. The copper particles had a size distribution between 50 nm and 300 nm. Electrochemical measurements displayed an improved performance of the synthesized composite material compared to the pristine material. Cyclic voltammetry showed a suppressed cointercalation of solvated lithium and an increased formation of the solid electrolyte interphase (SEI). Battery cycling demonstrated an increased discharge capacity and cycling stability.

Crystalline low agglomerated tungsten diselenide (WSe 2 ) nanoparticles were successfully produced by the aerosol synthesis method. The particle size and morphology were changed by process parameters. The products were characterized by X-ray diffraction and high-resolution transmission electron microscopy. The WSe 2 nanoparticle morphology varies from a spherical shape to flake-like, layered structures. Usually, as-prepared particles consist of 5–7 single layers of WSe 2 . These as-synthesized WSe 2 nanoparticles were used as anodes to assemble lithium-ion batteries. A high reversible capacity and very stable cycle performance can be obtained, implying that these WSe 2 nanostructures could provide new types of electrode materials for rechargeable Li-ion batteries.

Spinel-type Li 4 Ti 5 O 12 is an attractive anode material for lithium-ion battery applications owing to its high specific energy and almost vanishing mechanical strain upon Li-(de)intercalation. The impact of oxygen non-stoichiometry and aliovalent Mg-doping on the defect chemistry of Li 4 Ti 5 O 12 has been characterized by quantitative electron paramagnetic resonance (EPR) spectroscopy. The results obtained demonstrate that the Mg 2+ -ions are amphoterically incorporated at the here studied concentrations on both lithium (Mg Li • ) and titanium (Mg′′ Ti ) sites. Mg 2+ -doping thus simultaneously results in the formation of donor (Mg Li • ) and acceptor (Mg′′ Ti ) centers. Furthermore, upon both oxygen deficiency and Mg 2+ -doping, the concentration of `polaronic'-type Ti 3+ -states is increased, accounting for the enhanced electronic conductivity.

The formation of the solid electrolyte interphase (SEI) and the corresponding irreversible capacity ( C irr ) of graphite negative electrodes in lithium-ion batteries strongly depend on the surface morphology and surface chemistry of the respective graphite material. In particular, not only the BET specific surface area but also the absolute and relative extent of the basal plane and the “non-basal plane” surface area play a crucial role in SEI formation, as the transport of lithium ions into/from the anode during the charge/discharge operation mainly takes place via the prismatic surfaces. In this work, we report on the assessment of the graphite surface heterogeneity and the correlation between the amount of the different graphitic surfaces and the irreversible capacity. Based on the analysis of gas sorption data, we will show the influence of graphite surface groups on the adsorptive potential distribution of graphitic materials, in particular on the high energy surface sites (above 60 K), i.e. the so-called “defect” surfaces. These are related to polar carbon surface groups, such as carboxylic groups, alcohol groups or lactone groups. In contrast, less polar surface groups like quinone groups seem not to be related to the “defect” surface area, but belong to the prismatic surface area (below 50 K). Furthermore, we will demonstrate that the irreversible capacity of the first charge/discharge cycle is nearly proportional to the “non-basal plane” surface area, independent from the type of graphitic material.