Function Of Li Content Research Articles

Substantial oxygen loss and chemical expansion in lithium-rich layered oxides at moderate delithiation.

Delithiation of layered oxide electrodes triggers irreversible oxygen loss, one of the primary degradation modes in lithium-ion batteries. However, the delithiation-dependent mechanisms of oxygen loss remain poorly understood. Here we investigate the oxygen non-stoichiometry in Li1.18-xNi0.21Mn0.53Co0.08O2-δ electrodes as a function of Li content by using cycling protocols with long open-circuit voltage steps at varying states of charge. Surprisingly, we observe substantial oxygen loss even at moderate delithiation, corresponding to 2.5, 4.0 and 7.6 ml O2 per gram of Li1.18-xNi0.21Mn0.53Co0.08O2-δ after resting at upper capacity cut-offs of 135, 200 and 265 mAh g-1 for 100 h. Our observations suggest an intrinsic oxygen instability consistent with predictions of high oxygen activity at intermediate potentials versus Li/Li+. In addition, we observe a large chemical expansion coefficient with respect to oxygen non-stoichiometry, which is about three times greater than those of classical oxygen-deficient materials such as fluorite and perovskite oxides. Our work challenges the conventional wisdom that deep delithiation is a necessary condition for oxygen loss in layered oxide electrodes and highlights the importance of calendar ageing for investigating oxygen stability.

Planar Gliding and Vacancy Condensation: The Role of Dislocations in the Chemomechanical Degradation of Layered Transition-Metal Oxides

Stacking faults driven by dislocations have been observed in layered transition-metal oxide cathodes both in cycled and uncycled materials. The reversibility of stacking-sequence changes directly impacts the material performance. Irreversible glide due to lattice invariance or local compositional changes can initiate a catastrophic sequence of degradation mechanisms. In this study we compare the chemomechanical properties of LiCoO2 and LiNiO2 by combining density functional theory and anisotropic linear elasticity theory. We calculate stacking fault energies as a function of Li content and quantify the extent to which excess Ni hinders stacking-sequence changes. We then characterize screw dislocations, which mediate stacking-sequence changes, and find a peculiarly compliant behavior of LiNiO2 due to the interaction of Jahn–Teller distortions with the dislocation strain field. Finally, we analyze the tendency of vacancies to segregate along dislocation lines. This study represents the first instance of explicit ab initio atomistic dislocation models in layered oxides and paves the way for the understanding and optimization of the chemomechanical behavior of cathode active materials during battery operation.

Metal Release Mechanism and Electrochemical Properties of Lix(Ni1/3Mn1/3Co1/3)O2

Complex metal oxides (CMOs) are used broadly in applications including electroreactive forms found in lithium-ion battery technology. Computational chemistry can provide unique information about how the properties of CMO cathode materials change in response to changes in stoichiometry, for example, changes of the lithium (Li) content during the charge–discharge cycle of the battery. However, this is difficult to measure experimentally due to the small cross-sectional area of the cations. Outside of operational conditions, the Li content can influence the transformations of the CMO when exposed to the environment. For example, metal release from CMOs in aqueous settings has been identified as a cross-cutting mechanism important to CMO degradation. Computational studies investigating metal release from CMOs show that the thermodynamics depend on the oxidation states of lattice cations, which is expected to vary with the lithium content. In this work, computational studies track changes in metal release trends as a function of Li content in Lix(Ni1/3Mn1/3Co1/3)O2 (NMC). The resulting dataset is used to construct a random forest tree (RFT) machine learning (ML) model. A modeling challenge in delithiation studies is the large configurational space to sample. Through investigating multiple configurations at each lithium fraction, we find structural features associated with favorable energies to chemically guide the identification of relevant structures and adequately predict voltage values.

High Pressure Effect on Structural and Electrochemical Properties of Anionic Redox-Based Lithium Transition Metal Oxides

Summary The richness of anionic redox chemistry in the solid state offers new opportunities and a possible paradigm shift in energy storage. The excess capacity that goes beyond conventional theoretical values is attributed to the anionic redox in Li-rich transition metal oxide cathodes for Li-ion batteries. Their electrochemical behavior is thermodynamically determined by structural evolution. To better understand the electrochemical dependence on structural factors, we have induced structural modifications in pristine and electrochemically activated Li1.144Ni0.136Co0.136Mn0.544O2 through high-pressure treatment. A unique cyclical change of structural reordering is observed in the anionic redox-activated material during operando pressure sweep, characterized by a periodic evolution of superlattice peak intensity in synchrotron X-ray diffraction patterns. During the structural reordering period, the bulk compressibility of the material decreases, even becoming negative. These insights elucidate the structural flexibility and metastability of anionic redox-based materials, which can undergo large compressions and structural modifications while delivering good electrochemical properties.

Impact of Li doping on the photophysical properties of perovskite absorber layer FAPbI3

It has been demonstrated that ions used in the contact layers in a perovskite based solar device can migrate through the perovskite absorber and have a significant impact on the device performance. In this work, we investigate the direct effect of Li on the structural and optical properties of formamidinium lead triiodide perovskites thin films (FAPbI3). The ratio between the desirable α-phase black perovskite and the undesirable δ yellow phase is studied as a function of Li content in the film. Our results show that the presence of a small amount of Li ions can have a huge impact on the structural, optical and photophysical properties of the FAPbI3 layer by enhancing the formation of δ-FAPbI3 instead of the black α-FAPbI3 phase. The structural analyses show that the FAPbI3 doped by Li atoms leads to the distortion of the mean α-FAPbI3 perovskite structure due the appearance of non-perovskite structure based δ-FAPbI3. The optical measurements indicate a clear decrease of the absorbance with increasing the Li content. The photoluminescence (PL) intensity of the Li doped FAPbI3 decreases considerably indicating a significant quenching and enhanced carrier extraction after Li doping. Li ions have also been shown to reduce non-radiative recombination due to free electrons produced by n-type doping. This work provides insights into the effect of the Li ions on the photophysical properties of the FAPbI3 absorber layer and the recombination mechanism, and helps to unravel new ways for development of optoelectronic devices, more particularly thin film perovskite solar cells with high-stability and high-performance.

Crystallographic disorders depending on monovalent cations addition and their effects on ZnO's characteristics

Pure and lithium (Li) doped ZnO films have been grown on soda lime glass substrates by chemical spray pyrolysis technique. Zinc chloride (ZnCl2) and lithium chloride (LiCl), resoluble in deionized water, were used in the process. The growth mechanism of the films as a function of Li content, resulting texture and topography were discussed in detail by X-ray diffraction method (XRD) and scanning electron microscopy (SEM). Obtained films exhibited single phase with polycyrstalline nature. At the same time, changes in crystal quality due to lithium addition were observed and these changes were explained as the presence of lithium atoms in the ZnO's crystal structure as interstitial atoms and the replacement of Li+ and Zn2+ions. Topographical features of the films were confirmed by SEM images. In SEM photographs, it was observed that the crystallite size of the films varied as a result of an increase in the lithium doping concentration. Thus, a significant correlation was found between the structural and morphological properties of the films due to Li contribution. Optical properties of the films were evaluated by UV-VIS-NIR spectroscopy and found that the optical band gap of ZnO films exhibit increasing tendency up to 4 at.%Li content and decrease for further Li content. Elemental composition and presence of Li were approved by X-ray photoelectron spectroscopy (XPS) and Li 1s peak were observed in 55.6 eV in 5 at.% Li doped ZnO films.

Unraveling the structural mechanism of Li insertion in γ′-V2O5 and its effect on cycling properties

We report the first electrochemical and structural study of the γ′-V2O5 polymorph toward Li insertion, with cycling properties evaluated in the 4 V-2.4 V, 3.6 V-2.4 V and 4 V-2.15 V potential ranges. This cathode material is synthesized through topotactic lithium removal from the ternary γ-LiV2O5 bronze, using a strong oxidizing agent. It exhibits two pairs of well-defined reversible steps at 3.58/3.47 V and 2.42/2.36 V separated by a sharp potential drop of about 1 V. A high specific capacity of 285 mAh g−1 is involved in the 4.00 V-2.15 V voltage window corresponding to the insertion of nearly 2 Li/mole of oxide. The γ′-V2O5 material can deliver stable specific capacities of 120–185 mAh g−1 over 45 cycles at C/10 when the lower cut-off voltage is limited to 2.4 V. Cycling experiments in the widest 4.00 V-2.15 V potential range induce however a significant capacity decline. A detailed XRD and Raman spectroscopy study delivers a detailed picture of the structural changes occurring in γ′-V2O5 as a function of Li content. A complete phase diagram of the γ′-V2O5/Li system is provided during the first discharge-charge cycle in the extended 4.00 V–2.15 V voltage range, i.e. for 0 ≤ x ≤ 1.94 in LixV2O5. It is demonstrated the existence of a wide solid solution domain in the 0.4 ≤ x < 1.4 composition range (3.6 V-2.4 V), leading to a remarkable enhancement of the cycle performance in the corresponding controlled voltage window. The Raman fingerprint of the fully lithiated ζ-Li1.94V2O5 phase is newly identified, indicating that deep structural rearrangements at the atomic scale take place during the γ-Li1.4V2O5 → ζ-Li1.94V2O5 transition. The resulting phase diagram accounts for the potential-composition profile and sheds light on the nature of the cycling properties in the different voltage windows.

Present understanding of the stability of Li-stuffed garnets with moisture, carbon dioxide, and metallic lithium

Fast lithium-ion conducting garnet-type metal oxides are promising membranes for next-generation all-solid-state Li batteries and beyond Li-ion batteries, including Li-air and Li-S batteries, due to their high total Li-ion conductivity and excellent chemical stability against reaction with elemental Li. Several studies have been reported on structure-chemical composition-ionic conductivity property in Li-stuffed garnet-type metal oxides. Here, an overview of the chemical and electrochemical stability of lithium-based garnets against moisture/humidity, aqueous solutions, carbon dioxide, sulfur, and metallic lithium are analyzed. Moisture and aqueous stability studies focus on understanding the crystal structure stability, the proton exchange capacity as a function of Li content in Li-stuffed garnets, and how the protonated species affect the crystal structure and mass transport properties. H+/Li+ exchange was found to be in the range of 2–100%. Stability concerning Li-ion conductivity and morphology under carbon dioxide are discussed. Interfacial chemical stability with lithium metal characterized by electrochemical stability window, Li dendrite formation and area specific resistance (ASR) for the reaction Li⇌Li++e− are presented. Recent attempts to suppress dendrite formation and to reduce ASR via surface modification are also highlighted. Li and Li-stuffed garnet interface ASR values are shown to be as high as >2000 Ω cm2 and as low as 1 Ω cm2 at room temperature for surface modified Li-stuffed samples. Furthermore, recent studies on Li-S battery utilizing chemically stable Li – garnet electrolyte are also discussed.

Phase transformation mechanism in lithium manganese nickel oxide revealed by single-crystal hard X-ray microscopy

Understanding the reaction pathway and kinetics of solid-state phase transformation is critical in designing advanced electrode materials with better performance and stability. Despite the first-order phase transition with a large lattice mismatch between the involved phases, spinel LiMn1.5Ni0.5O4 is capable of fast rate even at large particle size, presenting an enigma yet to be understood. The present study uses advanced two-dimensional and three-dimensional nano-tomography on a series of well-formed LixMn1.5Ni0.5O4 (0≤x≤1) crystals to visualize the mesoscale phase distribution, as a function of Li content at the sub-particle level. Inhomogeneity along with the coexistence of Li-rich and Li-poor phases are broadly observed on partially delithiated crystals, providing direct evidence for a concurrent nucleation and growth process instead of a shrinking-core or a particle-by-particle process. Superior kinetics of (100) facets at the vertices of truncated octahedral particles promote preferential delithiation, whereas the observation of strain-induced cracking suggests mechanical degradation in the material.

Strong Li-Content Dependence of Li Diffusivity in TiO2-B

The lithiation process in TiO2-B as a function of Li content x is studied using density functional theory. Considering a variety of possible pathways, we find that stepwise insertion of Li in TiO2-B layers, followed by insertion of Li in O layers, is kinetically the most favorable scenario. Diffusion coefficient (D) varies only by 2 orders of magnitude with x for Li contents up to x = 0.75 (3.03 × 10–10 cm2/s ≤ D ≤ 7.94 × 10–8 cm2/s), but it becomes almost 6 and 26 orders of magnitude smaller for 0.75 < x ≤ 1.0 and 1.00 < x < 1.25, respectively. To make a direct comparison to experimental measurements, the maximum diffusion length was estimated for different values of C-rate. It is found that, for any reasonable value of C-rate between 0.1C and 10C, bulk TiO2-B cannot attain capacities larger than 251 mAh g–1. However, larger capacities up to 335 mAh g–1 can be obtained by nanosized TiO2-B. In addition, we find that a capacity higher than 335 mAh g–1 cannot be achieved. Our results suggest that the small ...

OH defects in quartz in granitic systems doped with spodumene, tourmaline and/or apatite: experimental investigations at 5–20 kbar

The incorporation of OH defects in quartz as a function of Li content in the bulk system and pressures was investigated. Quartz crystals were grown in water-saturated granitic systems, containing various amounts Li, B and P, supplied as accessory phases such as spodumene, tourmaline or apatite in the starting mixtures. High pressure experiments were performed at temperatures between 900 and 1100 °C, and pressures between 5 and 20 kbar with a piston cylinder apparatus, and the synthesized quartz crystals were analyzed by IR spectroscopy, electron microprobe and LA-ICP-MS spectroscopy. All IR absorption spectra revealed absorption features that can be assigned to AlOH (3313, 3379 and 3431 cm−1) and (4H)Si defects (3585 cm−1), whereas quartz grown in the Li and B systems exhibited two additional bands related, respectively, to LiOH (3483 cm−1) and BOH defects (3596 cm−1). It was further observed that LiOH incorporation increases with higher spodumene content in the starting material and decreases with pressure, until no LiOH defects are observed at pressure higher than 15 kbar. Specifically, the most pronounced reduction of LiOH defects occurs in a rather narrow pressure interval (10–15 kbar) close to the high-quartz/low-quartz transition. However, the link between the transition and the defect incorporation remains unclear. Li total concentrations always exceed the Li-coupled LiOH defects, suggesting the simultaneous presence of dry AlLi defects. Results of this study suggest that LiOH defects are detectable only in quartz crystals grown from middle and upper crustal sections (such as hydrothermal quartz) and not in quartz from deep roots of orogenic granitoids.

First-Principles Studies of Delithiation Mechanisms of Hybrid Li-Ion/Li-O2 Battery Materials

Lithium metal oxides with high Li2O contents, such as Li6MnO4, Li6CoO4 and Li5FeO4 , areof particular interest as novel, high capacity hybrid cathode materials due to their potential dual Li-ion/Li-O2 activity [1, 2]. Previous experiments on the charge process of Li5FeO4 have demonstrated a significant electrochemical capacity of 475 mAh/g in a Li-O2 cell[3]. However, the geometrical and electronic structure changes during the charge process remain elusive, and whether and how oxygen is evolved is uncertain. In this study, we employed first-principles density functional theory (DFT) to monitor the structural and energetic changes of hybrid batteries along various delithiation routes. Two delithiation routes of Li6MnO4, Li6CoO4 and Li5FeO4systems, namely Li removal and 2Li+O concerted removal, are investigated by DFT calculations. Step-by-step removals have been carried out and the stabilities of the Li and Li/O deficient systems are investigated based on DFT energies of each extraction step. The energetic competition between Li and oxygen removal is determined as a function of Li content and compared among the three systems. Bader charge analysis is used to compare charge compensation tendencies, upon delithiation, among Fe, Co and Mn. The implications of these results for other hybrid battery systems will be discussed. Acknowledgements This work was supported as a part of the Center for Electrochemical Energy Science, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Basic Energy Sciences under award number DE-AC02–06CH11. Use of the Advanced Photon Source, a US DOE Office of Science User Facility operated by Argonne National Laboratory, was supported by DOE under Contract No. DE-AC02-06CH11357. Use of the Center for Nanoscale Materials, an Office of Science user facility, was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. Reference: M. M. Thackeray, M. K. Y. Chan, L. Trahey, S. Kirklin, and C. Wolverton, “Vision for Designing High-Energy, Hybrid Li Ion/Li-O2 Cells,” Journal of Physical Chemistry Letters 4, 3607 (2013).S. Kirklin, M. K. Y. Chan, L. Trahey, M. M. Thackeray, and C. Wolverton, “High-throughput screening of high-capacity electrodes for hybrid Li-ion/Li-O2 cells,” Physical Chemistry Chemical Physics 16, 22073 (2014).L. Trahey, C. S. Johnson, J. T. Vaughey, S.-H. Kang, L. J. Hardwick, S. A. Freunberger, P. G. Bruce, and M. M. Thackeray, “Activated Lithium-Metal-Oxides as Catalytic Electrodes for Li–O2 Cells,” Electrochemical and Solid-State Letters 14, A64-A66 (2011). The submitted manuscript has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.

Characterization of Electronic and Ionic Transport in Li1-xNi0.33Mn0.33Co0.33O2 (NMC333) and Li1-xNi0.50Mn0.20Co0.30O2 (NMC523) as a Function of Li Content

Despite the extensive commercial use of Li1-xNi1-y-zMnzCoyO2 (NMC) as the positive electrode in Li-ion batteries, and its long research history, its fundamental transport properties are poorly understood. These properties are crucial for designing high energy density and high power Li-ion batteries. Here, the transport properties of NMC333 and NMC523 are investigated using impedance spectroscopy and DC polarization and depolarization techniques. The electronic conductivity is found to increase with decreasing Li-content (increasing state-of-charge) from ∼10−7 Scm−1 to ∼10−2 Scm−1 over Li concentrations x = 0.00 to 0.75, corresponding to an upper charge voltage of 4.8 V with respect to Li/Li+. The lithium ion diffusivity is at least one order of magnitude lower, and decreases with increasing x to at x = ∼0.5. The ionic conductivity and diffusivity obtained from the two measurements techniques (EIS and DC) are in good agreement, and chemical diffusion is limited by lithium transport over a wide state-of-charge range.

Erratum: Characterization of Electronic and Ionic Transport in Li1-xNi0.33Mn0.33Co0.33O2 (NMC333) and Li1-xNi0.50Mn0.20Co0.30O2 (NMC523) as a Function of Li Content [J. Electrochem. Soc., 163, A1512 (2016)

Erratum: Characterization of Electronic and Ionic Transport in Li1-xNi0.33Mn0.33Co0.33O2 (NMC333) and Li1-xNi0.50Mn0.20Co0.30O2 (NMC523) as a Function of Li Content [J. Electrochem. Soc., 163, A1512 (2016)

Li ion dynamics along the inner surfaces of layer-structured 2H-LixNbS2.

Layer-structured materials, such as graphite (LiCy) or Lix(Co,Ni,Mn)O2, are important electrode materials in current battery research that still relies on insertion materials. This is due to their excellent ability to reversibly accommodate small alkali ions such as Li(+) and Na(+). Despite of these applications, microscopic information on Li ion self-diffusion in transition metal sulfides are relatively rare. Here, we used (7)Li nuclear magnetic resonance (NMR) spectroscopy to study translational Li ion diffusion in hexagonal (2H) LixNbS2 (x = 0.3, 0.7, and 1) by means of variable-temperature NMR relaxometry. (7)Li spin-lattice relaxation rates and (7)Li NMR spectra were used to determine Li jump rates and activation barriers as a function of Li content. Hereby, NMR spin-lattice relaxation rates recorded with the spin-lock technique offered the possibility to study Li ion dynamics on both the short-range and long-range length scale. Information was extracted from complete diffusion-induced rate peaks that are obtained when the relaxation rate is plotted vs inverse temperature. The peak maximum of the three samples studied shifts toward higher temperatures with increasing Li content x in 2H-LixNbS2. Information on the dimensionality of the diffusion process was experimentally obtained by frequency dependent Rρ measurements carried out at T = 444 K, that is in the high-temperature regime of the rate peaks. A slight, but measurable frequency-dependence within this limit is found for all samples; it is in good agreement with predictions from relaxation models developed to approximate low-dimensional (2D) jump diffusion.

Ferroelectric and dielectric properties in Li-doped ZnO nanorods

In the present work, Li-doped ZnO nanorods have been deposited onto SiO2 glass substrates by the simple and economical Chemical Bath Deposition method. These layers were further characterized in terms of structural, morphological, dielectric and ferroelectric properties as a function of Li content. X-ray diffraction patterns revealed the highly c-oriented hexagonal wurtzite structure of the films. Both dielectric measurements and ferroelectric investigations at nano- and micro-scale pointed out the existence of a ferroelectric behavior in such doped ZnO nanostructures. This ferroelectric activity can be further enhanced by increasing the dopant content in the material.

Structural and Chemical Transformations in Li2MnO3

Li2MnO3is a critical component in the family of the so-called `Li-excess' materials, which are attracting attention as advanced cathode materials for Li-ion batteries. We present first-principle calculations to investigate the electrochemical activity and structural stability of stoichiometric LixMnO3(0 < x < 2) as a function of Li content. We find that the Li2MnO3 structure is electrochemically activated above 4.5 V on delithiation and that charge neutrality in the bulk of the material is mainly maintained by anion oxidization. While oxygen vacancy formation is found to be thermodynamically favorable for x < 1, the activation barriers for O migration remain high throughout the Li composition range, impeding any significant oxygen release from the bulk of the compound. Furthermore, we show that, defect layered structures, where some Mn resides in the Li layer, become thermodynamically favorable at lower Li content (x < 1), indicating a strong tendency towards local spinel-like domain transformation. Concurrently, the calculated energy barriers for Mn migration from the Mn-layer into the Li-layer suggests a Li2MnO3 structural instability for x < 0:5. Based on our observations, we suggest a critical phase transformation path for forming nuclei of spinel-like domains within the matrix of the original layered structure. We also show that formation of defect layered structures during the first charge manifests in a significant depression of the voltage profile on the first discharge, providing a possible background for the observed `voltage fade' of the Li-excess materials.

Structural and Chemical Evolution of the Layered Li‐Excess LixMnO3 as a Function of Li Content from First‐Principles Calculations

Li2MnO3 is a critical component in the family of “Li‐excess” materials, which are attracting attention as advanced cathode materials for Li‐ion batteries. Here, first‐principle calculations are presented to investigate the electrochemical activity and structural stability of stoichiometric LixMnO3 (0 ≤ x ≤ 2) as a function of Li content. The Li2MnO3 structure is electrochemically activated above 4.5 V on delithiation and charge neutrality in the bulk of the material is mainly maintained by the oxidization of a portion of the oxygen ions from O2− to O1−. While oxygen vacancy formation is found to be thermodynamically favorable for x &lt; 1, the activation barriers for O2− and O1− migration remain high throughout the Li com­position range, impeding oxygen release from the bulk of the compound. Defect layered structures become thermodynamically favorable at lower Li content (x &lt; 1), indicating a tendency towards the spinel‐like structure transformation. A critical phase transformation path for forming nuclei of spinel‐like domains within the matrix of the original layered structure is proposed. Formation of defect layered structures during the first charge is shown to manifest in a depression of the voltage profile on the first discharge, providing one possible explanation for the observed voltage fade of the Li‐excess materials.

First-principles study of the nano-scaling effect on the electrochemical behavior in LiNi0.5Mn1.5O4

Nano-scaling of electrode materials is often used in battery applications to enhance performance, particularly relating to rate capability. However, for the high-voltage spinel LiNi0.5Mn1.5O4 conflicting results have been reported on the benefits of nano-scaling. In this study, we present first-principles calculations to investigate the effect of nano-scaling on LiNi0.5Mn1.5O4, specifically focusing on the roles and coupling between surface stability, cation ordering and phase behavior. We calculate and compare the surface energy for the low index facets (100), (110), and (111), and find that the most stable facet is dependent on the cation ordering at the surface layer. In this context, we predict a spontaneous surface reconstruction in the cation-ordered structure which leads to a deviation from the perfect surface cation ordering and results in an enhanced accessibility to solid solution behavior as a function of Li content. Our results imply that nano-scaling will be more beneficial for the cation-ordered structure, as compared to the disordered structure where the solid solution region is already intrinsically accessible for a broad range of Li concentrations.

Variation of Ti Valence with Lithium Content in Lithiated/Delithiated Li4Ti5O12 Studied by X-Ray Absorption near the Edge Structure

Synchrotron-based x-ray absorption near-edge structure (XANES) spectroscopy is applied to investigate the variation of Ti valence in the discharge and charge processes of Li4Ti5O12. Through analysis of the XANES data, the average valence of Ti can be calculated, which allows quantitative determination of the ratios of Ti3+/Ti4+ as a function of Li content in the electrodes. It is found that the ratios in the composites of Li4Ti5O12/acetylene black (AB)-carbon are larger than those in Li4Ti5O12/Ag/AB-carbon after the discharge to 1.1 V, indicating a larger amount of Li inserted into the former than that into the latter. This finding provides a good explanation for the fact that the Li4Ti5O12/AB-carbon samples exhibit larger storage capacities than the Li4Ti5O12/Ag/AB-carbon ones prepared in this work, concerning the larger Ti3+/Ti4+ ratio and the larger amount of Li+ inserted in the electrode for satisfying the charge neutralization requirement.