Electrochemical and quantum chemical studies of the reactions of transition metals M (M = Co, Fe and Ni) with LiF and Li2O

It has been a puzzle that transition metals can unexpectedly react with lithium-based matrixes of LiF and Li₂O in the potential versus Li/Li⁺ range from 0.01 to 3.5 V at room temperature. The electrochemical and theoretical investigations on the reactions of transition metals M (M = Co, Fe and Ni) with LiF and Li₂O were carried out. The electrochemical reactivity of metal cobalt with LiF and Li₂O has been examined by the discharge and charge, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) measurements. Density functional theoretical calculation results suggested that the stable compounds MLiF and MLi₂O could be formed by the insertion of transition metal (M) in lithium-based matrixes with exothermic as an intermediate. The theoretical calculations provide an understanding in chemical reaction of M with LiF and Li₂O. The small molecular or clusters reaction may play an important role in the electrochemical reaction of metal with transition Li₂O or LiF, which could be used to explain for the unexpectedly reaction of transition metal with LiF and Li₂O.     

Structural and optical properties of magnesium nitride formed by a novel electrochemical process

The electrochemical formation of magnesium nitride (Mg₃N₂) films in LiCl-KCl containing Li₃N at 723 K was investigated. From a thermodynamic point of view, a potential-pN³⁻ diagram was constructed for the Mg-N system in an analogous fashion to Pourbaix diagrams for aqueous solutions. As a result, the thermodynamically stable region of Mg₃N₂ in LiCl-KCl-Li₃N was identified. XRD analysis revealed that Mg₃N₂ film was obtained by potentiostatic electrolysis of a magnesium electrode between 0.4 and 0.8 V (versus Li⁺/Li), and the structure of obtained Mg₃N₂ was anti-bixbyite (a = 1.001 nm). Reflectance measurements clarified that assuming direct transition, the bandgap energy was 3.15 eV and assuming indirect transition, the bandgap energy was 2.85 eV.     

Improved electrochemical properties of Li1+x(Ni0.3Co0.4Mn0.3)O2−δ (x = 0, 0.03 and 0.06) with lithium excess composition prepared by a spray drying method

Layered Li_1+x(Ni_0.3Co_0.4Mn_0.3)O_2−δ (x = 0, 0.03 and 0.06) materials were synthesized through the different calcination times using the spray-dried precursor with the molar ratio of Li/Me = 1.25 (Me = transition metals). The physical and electrochemical properties of the lithium excess and the stoichiometric materials were examined using XRD, AAS, BET and galvanostatic electrochemical method. As results, the lithium excess Li_1.06(Ni_0.3Co_0.4Mn_0.3)O_2−δ could show better electrochemical properties, such as discharge capacity, capacity retention and C rate ability, than those of the stoichiometric Li_1.00(Ni_0.3Co_0.4Mn_0.3)O_2−δ. In this paper, the effect of excess lithium on the electrochemical properties of Li_1+x(Ni_0.3Co_0.4Mn_0.3)O_2−δ materials will be discussed based on the experimental results of ex situ X-ray diffraction, transmission electron microscopy (TEM) and galvanostatic intermittent titration technique (GITT)     

Electrochemical performance of the carbon coated Li3V2(PO4)3 cathode material synthesized by a sol-gel method

A carbon coated Li₃V₂(PO₄)₃ cathode material for lithium ion batteries was synthesized by a sol-gel method using V₂O₅, H₂O₂, NH₄H₂PO₄, LiOH and citric acid as starting materials, and its physicochemical properties were investigated using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) spectroscopy, scanning electron microscopy (SEM), energy dispersive analysis of X-ray (EDAX), transmission electron microscope (TEM), and electrochemical methods. The sample prepared displays a monoclinic structure with a space group of P2₁/n, and its surface is covered with a rough and porous carbon layer. In the voltage range of 3.0-4.3 V, the Li₃V₂(PO₄)₃ electrode displays a large reversible capacity, good rate capability and excellent cyclic stability at both 25 and 55 °C. The largest reversible capacity of 130 mAh g⁻¹ was obtained at 0.1C and 55 °C, nearly equivalent to the reversible cycling of two lithium ions per Li₃V₂(PO₄)₃ formula unit (133 mAh g⁻¹). It was found that the increase in total carbon content can improve the discharge performance of the Li₃V₂(PO₄)₃ electrode. In the voltage range of 3.0-4.8 V, the extraction and reinsertion of the third lithium ion in the carbon coated Li₃V₂(PO₄)₃ host are almost reversible, exhibiting a reversible capacity of 177 mAh g⁻¹ and good cyclic performance. The reasons for the excellent electrochemical performance of the carbon coated Li₃V₂(PO₄)₃ cathode material were also discussed.     

The LixMPn4 phases (M/Pn = Ti/P, V/As): new negative electrode materials for lithium ion rechargeable batteries

Li_xTiP₄ ternary phases (x=7 and 9) show very close electrochemical behaviors versus lithium compared to the previously reported Li₉VAs₄ and Li₇VP₄. Up to seven lithium ions reversibly react with Li₉TiP₄, leading to specific and volumetric capacities of 970 mAh/g and 1650 mAh/cm³, respectively, at average potentials close to 1 V. Galvanostatic and potentiodynamic experiments reveal that lithium extraction/insertion follow different mechanisms: a two-phase process is evidenced on charge whereas a more complex process is achieved on discharge. Besides, in situ X-ray diffraction (XRD) patterns show that both Li₉TiP₄ and Li₉VAs₄ undergo a reversible crystalline to amorphous structural phase transition upon cycling.     

A macaroni-like Li1.2V3O8 nanomaterial with high capacity for aqueous rechargeable lithium batteries

A macaroni-like Li_1.2V₃O₈ nanomaterial was directly prepared through a facile solution route using β-cyclodextrin (β-CD) as a template reagent. Its crystal structure was determined by the X-ray diffraction (XRD) pattern. From the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) micrographs, we observed that the as-prepared Li_1.2V₃O₈ material consisted of the aggregated macaroni-like nanoparticles and showed a porous structure. The electrochemical properties of the as-prepared Li_1.2V₃O₈ in 1.0 M Li₂SO₄ aqueous electrolyte were studied through cyclic voltammograms and charge-discharge measurements. The results revealed that the as-prepared Li_1.2V₃O₈ could deliver the initial specific capacities of 189, 140, and 101 mAh g⁻¹ at 0.1, 0.5, and 1.0 C, respectively. It suggests that the as-prepared Li_1.2V₃O₈ should have an attractive future to be applied in aqueous rechargeable lithium battery (ARLB).     

Synthesis of LiFePO4/C by solid-liquid reaction milling method

LiFePO₄/C was synthesized by the method of solid-liquid reaction milling, using FeCl₃·6H₂O, Li₂CO₃ and (NH₄)₂HPO₄ and glucose, which was used as reductant (carbon source). The samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), TG-DTA analysis, infrared absorption carbon-sulfur analysis and electrochemical performance test. The sample synthesized at 680 °C for 8 h showed, at initial discharge, a capacity of 155.8, 153.2, 148.5, 132.7 mAh g^− 1 at 0.2 °C, 0.5 °C, 1 °C and 3 °C rate respectively. The sample also showed an excellent capacity retention as there was no significant capacity fade after 10 cycles.     

Layered lithium transition metal nitrides as novel anodes for lithium secondary batteries

We report the approach to overcome the deterrents of the hexagonal Li_2.6Co_0.4N as potential insertion anode for lithium ion batteries: the rapid capacity fading upon long cycles and the fully Li-rich state before cycling. Research reveals that the appropriate amount of Co substituted by Cu can greatly improve the cycling performance of Li_2.6Co_0.4N. It is attributed to the enhanced electrochemical stability and interfacial comparability. However, doped Cu leads to a slightly decreased capacity. High energy mechanical milling (HEMM) was found to effectively improve the reversible capacity associated with the electrochemical kinetics by modifying the active hosts’ morphology characteristics. Moreover, the composite based on mesocarbon microbead (MCMB) and Li_2.6Co_0.4N was developed under HEMM. The composite demonstrates a high first cycle efficiency at 100% and a large reversible capacity of ca. 450 mAh g⁻¹, as well as a stable cycling performance. This work may contribute to a development of the lithium transition metal nitrides as novel anodes for lithium ion batteries.     

Relationship between electrochemical behavior and Li/vacancy arrangement in ramsdellite type Li2+xTi3O7

The changes of Li⁺/vacancy arrangement in Li_2+xTi₃O₇ with a ramsdellite-type structure upon topo-electrochemical Li⁺ insertion were investigated by the entropy measurement of reaction combined with the Monte Carlo simulation. The experimental entropy measurement was conducted by potentiometric and calorimetrical methods. The obtained experimental data were in good accordance with simulated results.The results indicated that the ordered Li⁺/vacancy arrangement appeared at the compositions of x ∼ 0.45 and ∼1.20, where the observed entropy of reaction humped. The ordering of Li/vacancy were also indicated at the composition x ∼ 0.24 and 1.16 in Li_2+xTi₃O₇ by the Monte Carlo simulation which considers the most stable Li/vacancy arrangement in terms of Coulombic interaction. This substantial agreement between electrochemical behaviors and computational results confirmed that the formation of superstructure arising from Li/vacancy arrangement during the electrochemical reaction deeply related to the atomic level Coulombic interactions.     

Spray-drying synthesized LiNi0.6Co0.2Mn0.2O2 and its electrochemical performance as cathode materials for lithium ion batteries

Layered LiNi_0.6Co_0.2Mn_0.2O₂ materials were synthesized at different sintering temperatures using spray-drying precursor with molar ratio of Li/Me = 1.04 (Me = transition metals). The influences of sintering temperature on crystal structure, morphology and electrochemical performance of LiNi_0.6Co_0.2Mn_0.2O₂ materials have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and charge-discharge test. As a result, material synthesized at 850 °C has excellent electrochemical performance, delivering an initial discharge capacity of 173.1 mAh g^− 1 between 2.8 and 4.3 V at a current density of 16 mA g^− 1 and exhibiting good cycling performance.     

The first cycle characteristics of Li[Ni1/3Co1/3Mn1/3]O2 charged up to 4.7 V

In this research, we studied the first cycle characteristics of Li[Ni_1/3Co_1/3Mn_1/3]O₂ charged up to 4.7 V. Properties, such as valence state of the transition metals and crystallographic features, were analyzed by X-ray absorption spectroscopy and X-ray and neutron diffractions. Especially, two plateaus observed around 3.75 and 4.54 V were investigated by ex situ X-ray absorption spectroscopy. XANES studies showed that the oxidation states of transition metals in Li[Ni_1/3Co_1/3Mn_1/3]O₂ are mostly Ni²⁺, Co³⁺ and Mn⁴⁺. Based on neutron diffraction Rietveld analysis, there is about 6% of all nickel divalent (Ni²⁺) ions mixed with lithium ions (cation mixing). Meanwhile, it was found that the oxidation reaction of Ni²⁺/Ni⁴⁺ is related to the lower plateau around 3.75 V, but that of Co³⁺/Co⁴⁺ seems to occur entire range of x in Li_1−x[Ni_1/3Co_1/3Mn_1/3]O₂. Small volume change during cycling was attributed to the opposite variation of lattice parameter “c” and “a” with charging-discharging.     

Electrochemical lithium insertion in (PO2)4(WO3)2m (2 ≤ m ≤ 10): Relation among the electrochemical insertion process and structural features

In this work it is presented a review of the main results obtained during the electrochemical lithium insertion in the family of monophosphate tungsten bronzes (PO₂)₄(WO₃)_2m (2 ≤ m ≤ 10). This family of oxides is a good system in order to study the relation among the electrochemical processes observed in the course of lithium insertion and the changes of bronzes structures. By means of X-ray diffraction experiments, the nature of Li_x(PO₂)₄(WO₃)_2m phases has been elucidated and a correlation with the reversible/irreversible processes observed during the electrochemical insertion has been established. The electrical properties of the inserted Li_x(PO₂)₄(WO₃)_2m phases were measured and a relation with the amount of lithium inserted and m was also found.     

Investigation of lithium diffusion in nano-sized rutile TiO2 by impedance spectroscopy

The kinetics of the electrochemical lithium insertion reaction in nano-sized rutile TiO₂ has been investigated using ac impedance spectroscopy. The experimental data are obtained for a rutile compound synthesized via a solution technique and characterized by a morphology corresponding to spherical particles made of a large number of very thin nanorods 20 nm thick. The results are discussed as a function of the Li content x for 0 < x ≤ 0.8 in Li_xTiO₂, the temperature over the range 10-50 °C and the number of discharge-charge cycles. The significant linear decrease of the chemical diffusion coefficient D_Livs. the lithium content and the high values of D_Li found in the composition range 0 < x ≤ 0.5 are discussed and related with the electrochemical behaviour of the nano-sized material. From comparison with the bulk material, a promoting effect of the morphology on the kinetic characteristics is evidenced. For the first time an experimental evaluation of the activation energy for Li diffusion in nano-sized rutile TiO₂ is obtained; the value of 0.35 eV being much lower than that reported from computational experiments for the micro-sized oxide. This work also demonstrates a new system takes place from the second cycle, characterized by a significant improvement of Li diffusion by a factor five and allowing high rates to be used.     

A promising sol-gel route based on citric acid to synthesize Li3V2(PO4)3/carbon composite material for lithium ion batteries

Li₃V₂(PO₄)₃/carbon composite material was synthesized by a promising sol-gel route based on citric acid using V₂O₅ powder as a vanadium source. Citric acid acts not only as a chelating reagent but also as a carbon source, which enhance the conductivity of the composite material and hinder the growth of Li₃V₂(PO₄)₃ particles. The structure and morphology of the sample were characterized by TG, XRD and TEM measurements. XRD results reveal that Li₃V₂(PO₄)₃/carbon was successfully synthesized and has a monoclinic structure with space group P2₁/n. TEM images show Li₃V₂(PO₄)₃ particles are about 45 nm in diameter embeded in carbon networks. Galvanostatic charge/discharge and cyclic voltammetry measurements were used to study its electrochemical behaviors which indicate the reversibility of the lithium extraction/insertion processes. Li₃V₂(PO₄)₃/carbon performed in a voltage window (3.0-4.8 V) exhibits higher discharge capacity, better cycling stability and its discharge capacity maintains about 167.6 mAh/g at a current density of 28 mA/g after 50 cycles.     

Synthesis and electrochemical performance of Li2FeSiO4/carbon/carbon nano-tubes for lithium ion battery

Li₂FeSiO₄/carbon/carbon nano-tubes (Li₂FeSiO₄/C/CNTs) and Li₂FeSiO₄/carbon (Li₂FeSiO₄/C) composites were synthesized by a traditional solid-state reaction method and characterized comparatively by X-ray diffraction, scanning electron microscopy, BET surface area measurement, galvanostatic charge-discharge and AC impedance spectroscopy, respectively. The results revealed that the Li₂FeSiO₄/C/CNT composite exhibited much better rate performance in comparison with the Li₂FeSiO₄/C composite. At 0.2 C, 5 C and 10 C, the former composite electrode delivered a discharge capacity of 142 mAh g⁻¹, 95 mAh g⁻¹, 80 mAh g⁻¹, respectively, and after 100 cycles at 1 C, the discharge capacity remained 95.1% of its initial value.     

Preparation of micro-dot electrodes of LiCoO2 and Li4Ti5O12 for lithium micro-batteries

Fabrications of micro-dot electrodes of LiCoO₂ and Li₄Ti₅O₁₂ on Au substrates were demonstrated using a sol-gel process combined with a micro-injection technology. A typical size of prepared dots was about 100 μm in diameter, and the dot population on the substrate was 2400 dots cm⁻². The prepared LiCoO₂ and Li₄Ti₅O₁₂ micro-dot electrodes were characterized with scanning electron microscopy, X-ray diffraction, micro-Raman spectroscopy, and cyclic voltammetry. The prepared LiCoO₂ and Li₄Ti₅O₁₂ micro-dot electrodes were evaluated in an organic electrolyte as cathode and anode for lithium micro-battery, respectively. The LiCoO₂ micro-dot electrode exhibited reversible electrochemical behavior in a potential range from 3.8 to 4.2 V versus Li/Li⁺, and the Li₄Ti₅O₁₂ micro-dot electrode showed sharp redox peaks at 1.5 V.     

Nanocomposite Fe2O3-Se as a new lithium storage material

The electrochemical properties of nanocomposite Fe₂O₃-Se thin film prepared by pulsed laser deposition (PLD) method have been investigated by cyclic voltammetry and charge/discharge measurements. A large reversible capacity of nanocomposite Fe₂O₃-Se thin film was found to be around 650 mAh g⁻¹. A new couple of reduction and oxidation peaks at 1.4 and 1.8 V were observed from cyclic voltammogram for the first time. Our data demonstrated that nanocomposite Fe₂O₃-Se exhibit larger capacity and better cycle performance than pure Fe₂O₃. The electrochemical reaction mechanisms of Fe₂O₃-Se with lithium were examined by X-ray photoelectron spectroscopy (XPS), high resolution transmission electron microscopy (HRTEM) and selected-area electron diffraction (SAED). The reversible conversions reaction of nanosized metal Fe with Li₂Se and Li₂O formed after initial discharge process into FeSe and Fe₂O₃ respectively were revealed.     

Niobium doped lithium titanate as a high rate anode material for Li-ion batteries

Niobium doped lithium titanate with the composition of Li₄Ti_4.95Nb_0.05O₁₂ has been prepared by a sol-gel method. X-ray diffraction (XRD) and scanning electron microscope (SEM) are employed to characterize the structure and morphology of Li₄Ti_4.95Nb_0.05O₁₂. The Li₄Ti_4.95Nb_0.05O₁₂ electrode presents a higher specific capacity and better cycling performance than the Li₄Ti₅O₁₂ electrode prepared by the similar process. The Li₄Ti_4.95Nb_0.05O₁₂ exhibits an excellent rate capability with a reversible capacity of 135 mAh g⁻¹ at 10 C, 127 mAh g⁻¹ at 20 C and even 80 mAh g⁻¹ at 40 C. Electrical resistance measurement and electrochemical impedance spectra (EIS) reveal that the Li₄Ti_4.95Nb_0.05O₁₂ exhibits a higher electronic conductivity and faster lithium-ion diffusivity than the Li₄Ti₅O₁₂, which indicates that niobium doped lithium titanate (Li₄Ti_4.95Nb_0.05O₁₂) is promising as a high rate anode for the lithium-ion batteries.     

The structure and electrochemical performance of LiFeBO3 as a novel Li-battery cathode material

LiFeBO₃ cathode material has been synthesized successfully by solid-state reaction using Li₂CO₃, H₃BO₃ and FeC₂O₄·2H₂O as starting materials. The crystal structure has been determined by the X-ray diffraction. Electrochemical tests show that an initial discharge capacity of about 125.8 mAh/g can be obtained at the discharge current density of 5 mA/g. When the discharge current density is increased to 50 mA/g, the specific capacity of 88.6 mAh/g can still be held. In order to further improve the electrochemical properties, the carbon-coated LiFeBO₃, C-LiFeBO₃, are also prepared. The amount of carbon coated on LiFeBO₃ particles was determined to be around 5% by TG analysis. In comparison with the pure LiFeBO₃, a higher discharge capacity, 158.3 mAh/g at 5 mA/g and 122.9 mAh/g at 50 mA/g, was obtained for C-LiFeBO₃. Based on its low cost and reasonable electrochemical properties obtained in this work, LiFeBO₃ may be an attractive cathode for lithium-ion batteries.     

Synthesis of LiNi1/3Co1/3Mn1/3O2 cathode materials by using a supercritical water method in a batch reactor

LiNi_1/3Co_1/3Mn_1/3O₂ and LiCoO₂ cathode materials were synthesized by using a supercritical water (SCW) method with a metal salt solution in a batch reactor. Stoichiometric LiNi_1/3Co_1/3Mn_1/3O₂ was successfully synthesized in a 10-min reaction without calcination, while overlithiated LiCoO₂ (Li_1.15CoO₂) was synthesized using the batch SCW method. The physical properties and electrochemical performances of LiNi_1/3Co_1/3Mn_1/3O₂ were compared to those of Li_1.15CoO₂ by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), and charge/discharge cycling tests. The XRD pattern of LiNi_1/3Co_1/3Mn_1/3O₂ was found to be similar to that of Li_1.15CoO₂, showing clear splitting of the (0 0 6)/(1 0 2) and (1 0 8)/(1 1 0) peak pairs as particular characteristics of the layered structure. In addition, both cathode powders showed good crystallinity and phase purity, even though a short reaction time without calcination was applied to the SCW method. The initial specific discharge capacities of the Li_1.15CoO₂ and LiNi_1/3Co_1/3Mn_1/3O₂ powders at a current density of 0.24 mA/cm² in 2.5-4.5 V were 149 and 180 mAh/g, and their irreversible capacity loss was 20 and 17 mAh/g, respectively. The discharge capacities of the Li_1.15CoO₂ and LiNi_1/3Co_1/3Mn_1/3O₂ powders decreased with cycling and remained at 108 and 154 mAh/g after 30 cycles, which are 79% and 89% of the initial capacities. Compared to the overlithiated LiCoO₂ cathode powders, the LiNi_1/3Co_1/3Mn_1/3O₂ cathode powders synthesized by SCW method had better electrochemical performances.