Lithium Vacancies Research Articles

First-Principles Study of Tritium Diffusion in the Li3TaO4 Crystal.

Li3TaO4 with a high melting point, good thermal stability, and higher lithium content has become a possible choice for breeder materials, which have potential applications in future fusion reactors. Perfect and defect crystal models of Li3TaO4 are set up, and all of the tritium-diffusion pathways have been studied by the first-principles method. The activation energy barriers of different diffusion pathways are calculated and analyzed considering the pathway length and tritium–oxygen interactions. The obtained minimum energy barrier for tritium diffusing in the perfect Li3TaO4 crystal is only 0.34 eV. The minimum energy barrier is less than 0.72 eV when tritium diffuses in the defect Li3TaO4 crystal in the presence of a lithium vacancy. Finally, the diffusion coefficients of tritium in the Li3TaO4 crystal are calculated, which further confirm that it is easy for tritium to escape from the trap of the lithium vacancy and diffuse in the crystal. Such a tritium-diffusion behavior is in favor of the tritium-release process of the Li3TaO4 crystal and could provide theoretical guidance for the future applications of Li3TaO4 materials.

The effect of lanthanoid defects on anionic solvation of Li in Li6.5La2+xZr1.5Ta0.5O12 from x = 0 to x = 1.2 garnet

The cubic phase of lithium lanthanum zirconium oxide (LLZO) is a solid electrolyte based on the garnet crystal structure which has shown promise in enabling batteries with significantly higher energy density compared to lithium-ion. Owing to its stability against lithium, LLZO is compatible with metallic lithium electrodes that, when paired with state-of-the-art cathodes, can achieve >1000 Wh/l. The preferred cubic LLZO phase is stabilized at room temperature by doping LLZO, which results in the formation of lithium vacancies (VLi′) that can achieve ionic conductivities in the 1 mS/cm range. While much attention has been paid to optimizing VLi′, discrepancies in lanthanum to zirconium (La:Zr) ratios provide an indication of the existence of vacancies at one or both of these sites. As La is the largest cation present, La vacancies should show the most dramatic effects on lattice parameter, which should affect lithium-ion transport within the LLZO sublattice. In turn, changes in lithium-ion site occupancy and solvation should affect activation energy and conductivity. However, the effects of the sub stoichiometric use of La on the crystal structure of LLZO have yet to be characterized.In this work, La content is varied in the LLZO structure Li6.5La2+xZr1.5Ta0.5O12 from x = 0 to x = 1.2. X-ray diffraction, SEM imaging, Raman Spectroscopy, electrochemical impedance measurements, and DC cycling were performed in order to correlate the effect of La deficiency on crystal structure and electrochemical properties of LLZO. Vacancy and decomposition mechanism are proposed. A linear increase in lattice parameter is observed over the range of La content measured. An increase in conductivity and CCD are also observed ranging from 0.649 mS/cm and 0.5 mA/cm2 at x = 0.2 to 0.789mS/cm and 0.80 mA/cm2 at x = 1.0.

New Solid Electrolyte Li8– xZr1 –xTaxO6 (x = 0–0.5) for Lithium Power Sources

In the work, new lithium-conducting solid electrolytes based on lithium zirconate are synthesized. They are obtained by doping Li8ZrO6 phase with isostructural Li7TaO6. It is shown that in the Li8– xZr1– xTaxO6 system, a series of solid solutions х = 0−0.5 based on Li8ZrO6 form. The conductivity of synthesized Li8 ‒ xZr1 – xTaxO6 solid solutions increases by 1–2 orders of magnitude as compared with undoped zirconate Li8ZrO6 due to the formation of lithium vacancies in the tetra- and octahedral layers of the structure. All-solid-phase electrochemical cells with Li7.85Zr0.85Ta0.15O6 electrolyte, 0.75Li2SnMo3O12 ∙ 0.25B2O3 glass-ceramic anode, and 0.2Li2O · 0.2LiF · 0.45V2O5 · 0.25B2O3 cathode are electrochemically tested. It is shown that the resistance of 0.75Li2SnMo3O12 · 0.25B2O3|Li7.85Zr0.85Ta0.15O6|0.2Li2O · 0.2LiF · 0.45V2O5 · 0.25B2O3 cell decreases after the charge—discharge cycling.

Maintaining structure and voltage stability of Li-rich cathode materials by green water-soluble binders containing Na+ ions

As high-capacity cathode materials for lithium-ion batteries, Li-rich materials still suffer from structure transformation during cycling, resulting in serious voltage fading. The improved properties achieved by traditional modification methods, such as surface coating and ion doping, are unsatisfactory. Here, two water-soluble binders, sodium carboxymethyl cellulose and sodium alginate, are employed to prepare electrodes in order to alleviate voltage fading of both Li1.2Ni0.2Mn0.6O2 and Li1.2Co0.13Ni0.13Mn0.54O2 during cycling. Besides the environmental friendliness, these two water-soluble binders have better bonding ability with active particles and conductive agent than that of conventional PVDF binder. Moreover, Na+ ions in water-soluble binders are speculated to occupy lithium vacancies in crystal structure during cycling and play an important role as a pillar in lithium layer, leading to good structure stability of Li-rich materials. Accordingly, the electrodes based on water-soluble binders exhibit improved electrochemical performances, including high reversible discharge specific capacity, excellent cycling stability, and good rate capability. Importantly, voltage fading as a stubborn issue of Li-rich materials is significantly alleviated by using these water-soluble binders to prepare electrodes. Exploring suitable binders for specific electrode materials is considered as an effective strategy to improve electrochemical properties of electrode materials for lithium-ion batteries.

LiFePO4-Based Composite Electrode Material: Synthetic Approaches, Peculiarities of the Structure, and Regularities of Ionic Transport Processes

This article discusses approaches to the synthesis of composite cathode materials based on lithium-intercalated transition metal phosphates (by the example of iron–lithium phosphate). Among various methods and approaches to the synthesis, solid-state synthesis with a preliminary mechanochemical treatment of a mixture of starting materials was chosen as a variant of the compromising of simplicity and processability. In a series of experiments, different factors were sequentially varied in order to identify their influence on the properties of the final product. The article further presents the results of a systematic study of the electrochemical properties of LiFePO4-based material using a set of electrochemical methods: galvanostatic and potentiostatic intermittent titration and cyclic voltammetry. An original approach to graphical presentation of the potential–lithium ion concentration dependence in the LiFePO4 electrode was developed: for LixFePO4 solid solutions, the dependence was plotted against the lithium ion concentration; in the case of Li1 – xFePO4 solid solutions, against the lithium vacancies’ concentration. This approach allowed correctly calculating and introducing the correction parameter z into the modified Randles–Sevcik equation, in order to determine the diffusion coefficient according to the cyclic voltammetry method. Analysis of the galvanostatic and potentiostatic intermittent titration transients was carried out in terms of the developed models that allow describing lithium transport in the diffusion layer with a permeable inner boundary (a mathematical model of heterogeneous system with two phases in equilibrium and an interphase boundary). A good agreement has been found between theoretically calculated and experimental current and potential transients. When varying the composition of the electrode with respect to lithium, kinetic parameters of lithium intercalation were determined: the lithium diffusion coefficient and the phase transition constant.

Formation and behaviors of helium bubbles in Li4SiO4: A molecular dynamics simulation

Molecular dynamics simulations were conducted to investigate the formation and behaviors of helium bubbles in Li4SiO4. We found that a single helium atom easily moved to a lithium vacancy with minimal energy, and helium dimers were also observed in Li4SiO4. The temperature directly influenced the formation and diffusion of helium, and helium atoms aggregated and formed helium bubbles at a temperature of 700 K. The release of helium bubbles at several temperatures was simulated to analyze the effects of helium bubble release on the Li4SiO4 surface. The temperature was also important factor influencing the recovery of defects. These results will be useful for establishing a physical model of irradiation-induced microstructure evolution inside tritium breeding material and provide a good reference for understanding the initial stage of helium bubble formation and growth in Li4SiO4.

Lithium Deficiencies Engineering in Li-Rich Layered Oxide Li1.098Mn0.533Ni0.113Co0.138O2 for High-Stability Cathode.

Li-rich layered oxides have been in focus because of their high specific capacity. However, they usually suffer from poor kinetics, severe voltage decay, and capacity fading. Herein, a long-neglected Li-deficient method is demonstrated to address these problems by simply reducing the lithium content. Appropriate lithium vacancies can improve dynamics features and induce in situ surface spinel coating and nickel doping in the bulk. Therefore, the elaborately designed Li1.098Mn0.533Ni0.113Co0.138O2 cathode possesses improved initial Coulombic efficiency, excellent rate capability, largely suppressed voltage decay, and outstanding long-term cycling stability. Specifically, it shows a superior capacity retention of 93.1% after 500 cycles at 1 C (250 mA g-1) with respect to the initial discharge capacity (193.9 mA h g-1), and the average voltage still exceeds 3.1 V. In addition, the discharge capacity at 10 C can be as high as 132.9 mA h g-1. More importantly, a Li-deficient cathode can also serve as a prototype for further performance enhancement, as there are plenty of vacancies.

Boosting Solid‐State Diffusivity and Conductivity in Lithium Superionic Argyrodites by Halide Substitution

Developing high-performance all-solid-state batteries is contingent on finding solid electrolyte materials with high ionic conductivity and ductility. Here we report new halide-rich solid solution phases in the argyrodite Li6 PS5 Cl family, Li6-x PS5-x Cl1+x , and combine electrochemical impedance spectroscopy, neutron diffraction, and 7 Li NMR MAS and PFG spectroscopy to show that increasing the Cl- /S2- ratio has a systematic, and remarkable impact on Li-ion diffusivity in the lattice. The phase at the limit of the solid solution regime, Li5.5 PS4.5 Cl1.5 , exhibits a cold-pressed conductivity of 9.4±0.1 mS cm-1 at 298 K (and 12.0±0.2 mS cm-1 on sintering)-almost four-fold greater than Li6 PS5 Cl under identical processing conditions and comparable to metastable superionic Li7 P3 S11 . Weakened interactions between the mobile Li-ions and surrounding framework anions incurred by substitution of divalent S2- for monovalent Cl- play a major role in enhancing Li+ -ion diffusivity, along with increased site disorder and a higher lithium vacancy population.

Boosting Solid‐State Diffusivity and Conductivity in Lithium Superionic Argyrodites by Halide Substitution

AbstractDeveloping high‐performance all‐solid‐state batteries is contingent on finding solid electrolyte materials with high ionic conductivity and ductility. Here we report new halide‐rich solid solution phases in the argyrodite Li6PS5Cl family, Li6−xPS5−xCl1+x, and combine electrochemical impedance spectroscopy, neutron diffraction, and 7Li NMR MAS and PFG spectroscopy to show that increasing the Cl−/S2− ratio has a systematic, and remarkable impact on Li‐ion diffusivity in the lattice. The phase at the limit of the solid solution regime, Li5.5PS4.5Cl1.5, exhibits a cold‐pressed conductivity of 9.4±0.1 mS cm−1 at 298 K (and 12.0±0.2 mS cm−1 on sintering)—almost four‐fold greater than Li6PS5Cl under identical processing conditions and comparable to metastable superionic Li7P3S11. Weakened interactions between the mobile Li‐ions and surrounding framework anions incurred by substitution of divalent S2− for monovalent Cl− play a major role in enhancing Li+‐ion diffusivity, along with increased site disorder and a higher lithium vacancy population.

Germanium as a donor dopant in garnet electrolytes

Cubic Li7La3Zr2O12 (LLZO) garnet electrolytes continue to be viewed as an enabler of all-solid-state lithium battery technologies, or as protective membranes for next-generation lithium battery systems. Supervalent dopants at the lithium sublattice are commonly used to stabilise the conductive cubic phase, through the creation of lithium vacancies. The use of germanium (Ge4+) as a higher valent dopant substituting for Li+ was studied here and shown to stabilise the cubic LLZO phase through substitution of x = 0.10 mol of Ge (at the tetrahedral 24d Li sites of the space group Ia-3d, based on the neutron powder diffraction result). This substitution preference follows that of Al3+ (having a similar ionic radius and reported to reside at 24d sites), but with a lower critical concentration for cubic phase stabilisation, in agreement with charge neutrality arguments to obtain the required Li content (ca. 6.4–6.6 per formula unit) for optimum conductivity. The x = 0.10 composition gave the highest bulk Li ion conductivity of 2.8 × 10−4 S cm−1 at 25 °C, on the order of reported values for Al-doped LLZO. Surface chemical analysis using time of flight secondary ion mass spectrometry showed Ge homogeneously distributed within the grains as well as some Li-, O- and Ge- enrichment along the grain boundaries. Cyclic voltammetry of the cells containing Ge-doped LLZO showed a redox stability up to +5 V.

Intrinsically Optimizing Charge Transfer via Tuning Charge/Discharge Mode for Lithium-Oxygen Batteries.

Lithium-oxygen batteries have an ultrahigh theoretical energy density, almost ten times higher than lithium-ion batteries. The poor conductivity of the discharge product Li2 O2 , however, severely raises the charge overpotential and pulls down the cyclability. Here, a simple and effective strategy is presented for regular formation of lithium vacancies in the discharge product via tuning charge/discharge mode, and their effects on the charge transfer behavior. The effects of the discharge current density on the lithium vacancies, ionic conductivity, and electronic conductivity of the discharge product Li2 O2 are systematically investigated via electron spin resonance, spin-alignment echo nuclear magnetic resonance, and tungsten nanomanipulators, respectively. The study by density functional theory indicates that the lithium vacancies in Li2 O2 generated during the discharge process are highly dependent on the current density. High current can induce a high vacancy density, which enhances the electronic conductivity and reduces the overpotential. Meanwhile, with increasing discharge current, the morphology of the Li2 O2 changes from microtoroids to thin nanoplatelets, effectively shortening the charge transfer distance and improving the cycling performance. The Li2 O2 grown in fast discharge mode is more easily decomposed in the following charging process. The lithium-oxygen battery cycling in fast-discharge/slow-charge mode exhibits low overpotential and long cycle life.

Easily Decomposed Discharge Products Induced by Cathode Construction for Highly Energy-Efficient Lithium-Oxygen Batteries.

The lithium-oxygen (Li-O2) battery is deemed as a promising candidate for the next-generation of energy storage system due to its ultrahigh theoretical energy density. However, low energy efficiency and inferior cycle stability induced by the sluggish kinetics of charge transfer in discharge products limit its further development in practical application. In this work, tin dioxide (SnO2) nanoparticles decorated carbon nanotubes (SnO2/CNTs) have been constructed as composite cathodes to manipulate the morphology and component of discharge products in Li-O2 batteries. Owing to the strong oxygen adsorption of SnO2, oxygen-reduction reactions tend to occur on composite cathode surfaces, resulting in the formation of flake-like discharge products of Li2- xO2 less than 10 nm in thickness rather than toroidal particles of several hundred nanometers. Such homogeneous nanosized discharge products with lithium vacancies markedly enhance the electrode kinetics and charge transfer in discharge products. Consequently, the Li-O2 batteries based on the SnO2/CNT cathodes show a small polarization voltage gap, which leads to superior energy efficiency (80%) compared with that based on pristine CNT cathodes. The results demonstrate that optimizing the discharge products with nanosized morphology and defective component by cathode construction is an effective strategy to realize the Li-O2 batteries with increased energy efficiency and improved cycle stability.

Synergy of Lithium, Cobalt, and Oxygen Vacancies in Lithium Cobalt Oxide for Airborne Benzene Oxidation: A Concept of Reusing Electronic Wastes for Air Pollutant Removal

In this study, a recovery strategy of turning cathode materials of waste lithium-ion batteries, lithium cobalt oxide (LiCoO2), into high-performance catalysts for the oxidation of airborne benzene is investigated. Part of the Co3+ in LiCoO2 was leached out from the [CoO6] octahedra by HNO3 solution via the disproportion reaction with the deintercalation of interlayer Li+ ions, resulting in the formations of Li, Co, and O vacancies. The presence of Li and O vacancies facilitated gaseous benzene adsorption and subsequent activation of adsorbed benzene, and the Co vacancies together with the oxygen vacancies induced the generation of plenty of active surface oxygen species, accordingly the catalytic activity of the acid-modified LiCoO2 was greatly improved with good resistance to the alternative “heating-cooling” operation and stability from 0.8 to 2.3 vol % of humidity. More importantly, the acid modification process is green since the HNO3 solution could be reused many times to produce effective catalysts,...

Tritium trapping and migration mechanisms in Li2O: a first-principles study.

The tritium recovery behaviors and related mechanisms in lithium-based breeding materials are the major concerns in fusion reactors. In the present work, the energetics of intrinsic point defects and H-related defects in Li2O has been investigated by the first-principles method. The results show that the formation energies, charge states and relative stability of the intrinsic point defects and H-related defects in Li2O under working conditions vary with the energy level. Based on the defect properties and migration process, a model of the tritium trapping and migration mechanisms in Li2O is proposed: (1) the bred tritium is first trapped by oxygen vacancies; (2) subsequently the tritium detrapped from oxygen vacancies is retrapped by lithium vacancies, forming T substituents; and (3) the T substituents hop along the Li lattice.

Effect of Defects on Spontaneous Polarization in Pure and Doped LiNbO₃: First-Principles Calculations.

Numerous studies have indicated that intrinsic defects in lithium niobate (LN) dominate its physical properties. In an Nb-rich environment, the structure that consists of a niobium anti-site with four lithium vacancies is considered the most stable structure. Based on the density functional theory (DFT), the specific configuration of the four lithium vacancies of LN were explored. The results indicated the most stable structure consisted of two lithium vacancies as the first neighbors and the other two as the second nearest neighbors of Nb anti-site in pure LN, and a similar stable structure was found in the doped LN. We found that the defects dipole moment has no direct contribution to the crystal polarization. Spontaneous polarization is more likely due to the lattice distortion of the crystal. This was verified in the defects structure of Mg2+, Sc3+, and Zr4+ doped LN. The conclusion provides a new understanding about the relationship between defect clusters and crystal polarization.

Electronic structure and properties of lithium-rich complex oxides.

Lithium-rich complex transition-metal oxides Li2MoO3, Li2RuO3, Li3RuO4, Li3NbO4, Li5FeO4, Li5MnO4 and their derivatives are of interest for high-capacity battery electrodes. Here, we report a first-principles density-functional theory study of the atomic and electronic structure of these materials using the Heyd-Scuseria-Ernzerhof (HSE) screened hybrid functional which treats all orbitals in the materials on equal footing. Dimerization of the transition-metal ions is found to occur in layered Li2MoO3, in both fully lithiated and partially delithiated compounds. The Ru-Ru dimerization does not occur in fully lithiated Li2RuO3, in contrast to what is commonly believed; Ru-Ru dimers are, however, found to occur in the presence of neutral lithium vacancies caused by lithium loss during synthesis and/or lithium removal during use. We also analyze the electronic structure of the complex oxides and discuss the delithiation mechanism in these battery electrode materials.

First Principles Study of Tritium Diffusion in Li₂TiO₃ Crystal with Lithium Vacancy.

Li2TiO3 is one of the most significant breeder materials and has potential applications in future fusion reactors. Defect models with three types of lithium vacancies were considered to study the diffusion behavior of tritium in Li2TiO3 by the density functional theory calculations. The possible tritium adsorption sites inside the lithium vacancy were examined and analyzed. The energy barrier of all diffusion paths between different adsorption sites was calculated and the minimum energy barrier is about 0.45 eV, which indicates that the tritium atom diffuses freely inside the lithium vacancy; when a tritium diffuses across the crystal in the typical three directions, our results reveal that the tritium atom prefers to move along the [010] direction. Furthermore, we found that the minimum energy barrier for the tritium atom to escape the trap of Li vacancy is 0.76 eV. After the tritium jumping out of the Li vacancy, the minimum energy barrier is 0.5 eV for the tritium atom diffusing in the crystal. Therefore, we predict that tritium can easily escape from the trap of the Li vacancy and then diffuse across the crystal. Such results are beneficial to the tritium release process in Li2TiO3 and could provide theoretical guidance for the future applications of the Li2TiO3 materials.

Lithium and gallium vacancies in LiGaO2 crystals

Lithium gallate (LiGaO2) is a wide-band-gap semiconductor with an optical gap greater than 5.3 eV. When alloyed with ZnO, this material offers broad functionality for optical devices that generate, detect, and process light across much of the ultraviolet spectral region. In the present paper, electron paramagnetic resonance (EPR) is used to identify and characterize neutral lithium vacancies (VLi0) and doubly ionized gallium vacancies (VGa2−) in LiGaO2 crystals. These S = 1/2 native defects are examples of acceptor-bound small polarons, where the unpaired spin (i.e., the hole) is localized on one oxygen ion adjacent to the vacancy. Singly ionized lithium vacancies (VLi−) are present in as-grown crystals and are converted to their paramagnetic state by above-band-gap photons (x rays are used in this study). Because there are very few gallium vacancies in as-grown crystals, a post-growth irradiation with high-energy electrons is used to produce the doubly ionized gallium vacancies (VGa2−). The EPR spectra allow us to establish detailed models for the two paramagnetic vacancies. Anisotropy in their g matrices is used to identify which of the oxygen ions adjacent to the vacancy has trapped the hole. Both spectra also have resolved structure due to hyperfine interactions with 69Ga and 71Ga nuclei. The VLi0 acceptor has nearly equal interactions with Ga nuclei at two Ga sites adjacent to the trapped hole, whereas the VGa2− acceptor has an interaction with Ga nuclei at only one adjacent Ga site.

Tritium adsorption in the lithium vacancy of Li2ZrO3: A first principles study

Tritium adsorption in the irradiation defects of the Li2ZrO3 is a fundamental problem to understand the tritium behavior during the release process. A comprehensive computational study of tritium/helium adsorption in the lithium vacancy of bulk Li2ZrO3 is presented by the density functional theory calculations. The most stable tritium adsorption position has been found and it is determined by the neighboring lithium–oxygen interactions. The results reveal that the intrinsic defect is the lithium vacancy with one electron and it transforms to be the neutral state after a tritium atom is adsorbed. Moreover, helium is adsorbed almost in the center of lithium vacancy without bonding with surrounding oxygen atoms, which could diffuse easily in the bulk Li2ZrO3. Therefore, we predict that the intrinsic Li vacancy tends to adsorb a positive ion T[Formula: see text] other than a neutral T atom. Our results provide theoretical support to understand the T behavior in the Li2ZrO3 crystal.

Polaron-Mediated Luminescence in Lithium Niobate and Lithium Tantalate and Its Domain Contrast

In this review article, we discuss photoluminescence phenomena mediated by polarons in lithium niobate (LNO). At first we present the fundamentals on polaron states in LNO and their energy levels, i.e., on free and bound electron polarons, on hole polarons as well as on bipolarons. We discuss the absorption measurements on reduced as well as on doped LNO that made the characterization of the formed polaron states possible by their absorption bands. Next, we proceed by reporting on the two polaron-mediated photoluminescence bands that have been observed in LNO: (1) A near-infrared luminescence band in the range of 1.5 eV shows a mono-exponential decay and a strong dependence on iron doping. This luminescence is emitted by bound polarons returning from an excited state to the ground state. (2) A luminescence band at visible wavelengths with a maximum at 2.6 eV shows a stretched-exponential decay and is strongly enhanced by optical damage resistant doping around the doping threshold. This luminescence stems from the recombination of free electron and hole polarons. The next major topic of this review are domain contrasts of the visible photoluminescence that have been observed after electrical poling of the substrate, as singly inverted domains show a slightly reduced and faster decaying luminescence. Subsequent annealing results in an exponential decrease of that domain contrast. We show that this contrast decay is strongly related to the mobility of lithium ions, thus confirming the role of polar defect complexes, including lithium vacancies, for these domain contrasts. Finally we discuss the extension of our investigations to lithium tantalate (LTO) samples. While the results on the domain contrast and its decay are similar to LNO, there are remarkable differences in their luminescence spectra.