Synthesis and electrochemical properties of layered Li[Li(1/3−x/3)CrxMn(2/3−2x/3)]O2 prepared by sol-gel method

The Li[Li_(1/3−x/3)Cr_xMn_(2/3−2x/3)]O₂ (0.15 ≤ x ≤ 0.3) cathode materials were synthesized by sol-gel process using aqueous solutions of metal acetates and citric acid as the chelating agent. The precipitate of metal citrate was dried in a vacuum oven for 10 h at 100 °C. After drying, the gel precursor was calcined at 300 °C for about 10 h. The resulted powder was ground and heated at 900 °C. The structural characterization was carried out by fitting the XRD data with Rietveld program. The samples exhibited a well defined layered structure and the unit cell parameters linearly increased with increasing chromium contents in Li[Li_(1/3−x/3)Cr_xMn_(2/3−2x/3)]O₂ Surface morphology was determined by SEM and HRTEM and it is found that the cathode material consisted of highly ordered single crystalline particles with layered-hexagonal structure. Test cells were assembled and cycled in the voltage range of 2.0-4.9 V with a current density of 7.947 mA/g. Electrode with (x = 0.2) delivered a high reversible capacity of around 280 mA h/g in cycling.     

Electrochemical performance of LiV3−xNixO8 cathode materials synthesized by a novel low-temperature solid-state method

A series of cathode materials for lithium ion batteries with the formula LiV_3−xNi_xO₈ (x = 0.000, 0.025, 0.050 and 0.100) have been synthesized by a novel low-temperature solid-state method. The synthesized cathode materials have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), discharge-charge test, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). These results indicate that LiV_2.95Ni_0.050O₈ shows much better electrochemical performances than LiV₃O₈. This is due to better electrochemical reversibility and lower particle-to-particle resistance after Ni²⁺ doping.     

Isothermal calorimetry investigation of Li1+xMn2−yAlzO4 spinel

The heat generation of LiMn₂O₄, Li_1.156Mn_1.844O₄, and Li_1.06Mn_1.89Al_0.05O₄ spinel cathode materials in a half-cell system was investigated by isothermal micro-calorimetry (IMC). The heat variations of the Li/LiMn₂O₄ cell during charging were attributed to the LiMn₂O₄ phase transition and order/disorder changes. This heat variation was largely suppressed when the stoichiometric spinel was doped with excess lithium or lithium and aluminum. The calculated entropy change (dE/dT) from the IMC confirmed that the order/disorder change of LiMn₂O₄, which occurs in the middle of the charge, was largely suppressed with lithium or lithium and aluminum doping. The dE/dT values obtained did not agree between the charge and the discharge at room temperature (25 °C), which was attributed to cell self-discharge. This discrepancy was not observed at low temperature (10 °C). Differential scanning calorimeter (DSC) results showed that the fully charged spinel with lithium doping has better thermal stability.     

Optimum synthesis of Li2Fe1−xMnxSiO4/C cathode for lithium ion batteries

Li₂Fe_1−xMn_xSi0₄/C cathode materials were synthesized by mechanical activation-solid-state reaction. The effects of Mn-doping content, roasting temperature, soaking time and Li/Si molar ratio on the physical properties and electrochemical performance of the Li₂Fe_1−xMn_xSi0₄/C composites were investigated. The materials were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM), charge-discharge tests and AC impedance measurements. SEM images suggest that the morphology of the Li₂Fe_1−xMn_xSi0₄/C composite is sensitive to the reaction temperature. Samples synthesized at different temperatures have different extent of agglomeration. Being charged-discharged at C/32 between 1.5 and 4.8 V, the Li₂Fe_0.9Mn_0.1Si0₄/C synthesized at the optimum conditions shows good electrochemical performances with an initial discharge capacity of 158.1 mAh g⁻¹ and a capacity retention ratio of 94.3% after 30 cycles. AC impendence investigation shows Li₂Fe_0.9Mn_0.1SiO₄/C have much lower resistance of electrode/electrolyte interface than Li₂FeSiO₄/C.     

Optimizing preparation conditions for 5 V electrode performance, and structural changes in Li1−xNi0.5Mn1.5O4 spinel

A new lithium-excess method is used for the synthesis of LiNi_0.5Mn_1.5O₄ electrode materials at temperatures in the 600-800 °C range. Higher average manganese oxidation state and lower impurity contents are detected from X-ray diffraction in the spinel samples, as compared with stoichiometric synthesis. These properties cause the virtual elimination of the 4 V capacity, thus allowing a higher performance of these 5 V materials. A two-phase model of lithium extraction-insertion is detected by X-ray diffraction of electrodes prepared at different extensions of charge.     

Zr-doped Li[Ni0.5−xMn0.5−xZr2x]O2 (x = 0, 0.025) as cathode material for lithium ion batteries

Layered Li[Ni_0.5−xMn_0.5−xZr_2x]O₂ (x = 0, 0.025) have been prepared by the mixed hydroxide and molten-salt synthesis method. The individual particles of synthesized materials have a sub-microsize range of 200-500 nm, and LiNi_0.475Mn_0.475Zr_0.05O₂ has a rougher surface than that of LiNi_0.5Mn_0.5O₂. The Li/Li[Ni_0.5−xMn_0.5−xZr_2x]O₂ (x = 0, 0.025) electrodes were cycled between 4.5 and 2.0 V at a current density of 15 mA/g, the discharge capacity of both cells increased during the first ten cycles. The discharge capacity of the Li/LiNi_0.475Mn_0.475Zr_0.05O₂ cell increased from 150 to 220 mAh/g, which is 50 mAh/g larger than that of the Li/LiNi_0.5Mn_0.5O₂ cell. We found that the oxidation of oxygen and the Mn³⁺ ion concerned this phenomenon from the cyclic voltammetry (CV). Thermal stability of the charged Li[Ni_0.5−xMn_0.5−xZr_2x]O₂ (x = 0, 0.025) cathode was improved by Zr doping.     

In situ XAFS analysis of the LixNi0.8Co0.2O2 cathode during cycling in lithium batteries

The layered LiNi_0.8Co_0.2O₂ system has drawn interest as a cathode material for lithium battery high-power applications. In order to determine the charge compensation mechanism and structural perturbations occurring in the system during cycling, in situ battery X-ray absorption fine-structure spectroscopy (XAFS) measurements were conducted on a cell cycled at a moderate rate and typical Li-ion battery operating voltages (3.0-4.1 V). The XAFS data collected at the Ni and Co edges approximately every 30 min during cycling revealed details about the response of the cathode to Li insertion and extraction. These measurements on the Li_xNi_0.8Co_0.2O₂ cathode (0.29<x<0.78) demonstrated that the material retains excellent structural short-range order leading to superior cycling. Interestingly, the Co and Ni atoms behaved differently in response to Li insertion/extraction. This study corroborates previous work that explains the XAFS of the Ni atoms in terms of a Ni³⁺ Jahn-Teller ion. An analysis of the metal-metal distances suggests, contrary to a qualitative analysis of the X-ray absorption near-edge structure (XANES), that Co³⁺ is oxidized to the maximum extent possible (within the Li content range of this experiment) at x=0.47±0.04, while Ni³⁺ is oxidized in equal and linear increments proportional to the battery's state-of-charge. XAFS results on discharge show an almost completely reversible process with charge compensation through Co⁴⁺ and Ni⁴⁺ site reduction and a return to the original structural state.     

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.     

Parallel oxygen and chlorine evolution on Ru1−xNixO2−y nanostructured electrodes

Nanocrystalline materials with chemical composition corresponding to formula Ru_1−xNi_xO_2−y (0.02 < x < 0.30) were prepared by sol-gel approach. Substitution of Ru by Ni has a minor effect on the structural characteristics extractable from X-ray diffraction patterns. The electrocatalytic behavior of Ru_1−xNi_xO_2−y with respect to parallel oxygen (oxygen evolution reaction, OER) and chlorine (chlorine evolution reaction, CER) evolution in acidic media was studied by voltammetry combined with differential electrochemical mass spectrometry (DEMS). The DEMS data indicate a significant decrease of the over-voltage for chlorine evolution with respect to that of pure RuO₂. The oxygen evolution is slightly hindered. The increasing Ni content affects the electrode material activity and selectivity. The overall material's activity increases with increasing Ni content. The activity of the Ru-Ni-O oxides towards Cl₂ evolution shows a distinguished maximum for material containing 10% of Ni. Further increase of Ni content results in suppression of Cl₂ evolution in favor of O₂ evolution. A model reflecting the cation-cation interactions resulting from Ni-doping is proposed to explain the observed trends in electrocatalytic behavior.     

Structural and electrochemical studies of (SnxCo1−x)60C40 alloys prepared by mechanical attriting

Using a vertical-axis attritor, samples of (Sn_xCo_1−x)₆₀C₄₀ for 0 ≤ x ≤ 0.7 have been prepared in increments of Δx = 0.1. The effect of Sn content on the structure and performance of the Sn-Co-C nanocomposites was examined by X-ray diffraction (XRD), ¹¹⁹Sn Mössbauer effect spectroscopy and electrochemical methods. XRD shows a diffraction pattern characteristic of a nanostructured material having amorphous CoSn grains in a carbon matrix for x = 0.5. Broad Bragg peaks of Co₃C, SnCo₃C_0.7 and CoSn₂ were observed for samples with 0 ≤ x ≤ 0.1, 0.1 ≤ x ≤ 0.4 and 0.5 < x ≤ 0.7, respectively. ¹¹⁹Sn Mössbauer effect spectroscopy shows the presence of an amorphous CoSn component in samples with 0.3 ≤ x ≤ 0.6. Samples with a large amount of the SnCo₃C_0.7 phase showed small capacity for lithium. Therefore, SnCo₃C_0.7 is best described as an inactive phase. The sample with x = 0.7 shows the highest specific capacity (about 600 mAh/g) of all samples prepared but exhibited poor capacity retention after cycle 45. Excellent charge-discharge capacity retention, reasonable specific capacity (>450 mAh/g) along with stable differential capacities were observed for samples near x = 0.5.     

Synthesis and electrochemical properties of lithium nickel oxysulfide (LiNiSyO2−y) material for lithium secondary batteries

The layered oxysulfide LiNiS_yO_2−y compounds were synthesized and characterized to investigate the effect of sulfur on the electrode performance of LiNiO₂. LiNiO₂ precursors were first synthesized by a sol-gel method using adipic acid as a chelating agent and then doped with sulfur powders by a solid-state reaction under the flow of oxygen to prepare LiNiS_yO_2−y compounds. Pure LiNiO₂ electrode showed a gradual decrease of discharge capacity with cycle number, whereas the capacity retention rate of LiNiS_yO_2−y electrodes significantly improved. The initial discharge capacity of the LiNiS_yO_2−y cells was lower than that of LiNiO₂ cell and decreased with the increasing content of sulfur substituted in LiNiS_yO_2−y.     

Improved cycling performance of oxygen-stoichiometric spinel Li1+xAlyMn2−x−yO4+δ at elevated temperature

Li_1+xAl_yMn_2−x−yO_4±δ spinel cathode materials for lithium-ion batteries have been prepared by two methods, a specific two-step and the conventional one-step solid-state calcination methods. Compared with the conventional method, the new two-step method can guarantee the oxygen stoichiometry in spinel samples as well as reduced surface area. These characters lead to the improvement in cycling performance of spinel cathode even at elevated temperature. Moreover, the increase in doping amount of Al into Mn-spinel contributes to smearing the oxygen deficiency at high calcination temperature (1000 °C). The oxygen stoichiometric spinel samples exhibited greatly improved cycling performance. Further, Mn dissolution from spinel cathodes into the electrolyte was sufficiently suppressed even at elevated temperature of 60 °C. This beneficial influence would be reflected more remarkably in the cycles of lithium-ion full cells (spinel/C).     

Synthesis and characterization of abundant Ni-doped LiNixMn2−xO4 (x = 0.1-0.5) powders by spray-drying method

The structure and electrochemical properties of LiNi_xMn_2−xO₄ cathode materials for lithium ion batteries were studied by the means of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscope (TEM), cyclic voltammetry, and galvanostatic charge-discharge tests. The cathodes with different Ni contents (LiNi_xMn_2−xO₄, x = 0.1, 0.2, 0.3, 0.4, and 0.5) were synthesized by a spray-drying method and showed a single-phase spinel structure without any impurity. The amount of Ni has a large effect on the electrochemical characteristics. Capacity values of different voltage ranges (4- and 5-V ranges) change obviously with amount of Ni-doped. Also, the total discharge capacities increase with the Ni content, and all of them have good cycle stability.     

MnxCu1−xCo2O4 used as bifunctional electrocatalyst in alkaline medium

Composite film electrodes containing mechanically mixed Mn_xCu_1−xCo₂O₄ (0 ≤ x ≤ 1) particles, carbon black Vulcan XC72R and poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) were formed on the glassy carbon disk surface of a rotating ring-disk electrode (RRDE) and studied for the oxygen reduction and evolution reactions (ORR and OER, respectively) in 1 M KOH solution. The electrocatalytic activities for both reactions were observed to depend strongly on the Mn content in CuCo₂O₄. An opposite trend was observed for the apparent and intrinsic electrocatalytic activities for the ORR; the simultaneous presence of Cu and Mn was found to be detrimental to the intrinsic charge density, but beneficial to the geometric charge density with a maximum for Mn_0.6Cu_0.4Co₂O₄. The latter was characterized by the highest total number of electrons exchanged per O₂ molecule, n, close to 4, greater k₁ (4e⁻ process)/k₂ (2e⁻ process) ratios, and by a unique and low Tafel slope (−41 mV dec⁻¹). The results obtained for the OER showed that the intrinsic electrocatalytic activity is determined by the number of active sites (Co⁴⁺) electrochemically formed at the oxide surface prior to the OER, from Co³⁺ cations. The partial substitution of Cu by Mn in CuCo₂O₄ was found to decrease the OER activity.     

Uncommon potential hysteresis in the Li/Li2xVO(H2−xPO4)2 (0 ≤ x ≤ 2) system

Physical and electrochemical investigations of vanadium phosphates, Li_2xVO(H_2−xPO₄)₂ (0 < x < 2), have been undertaken. H⁺/Li⁺ ionic exchange from VO(H₂PO₄)₂ to Li₂VO(HPO₄)₂ leads to grain decrepitation. Further ionic exchange toward formation of Li₄VO(PO₄)₂ lowers the symmetry. As inferred from potentiodynamic cycling correlated to ex situ and in situ X-ray diffraction (XRD), the system Li/Li₄VO(PO₄)₂ shows several phase transformations that are associated with thermodynamical potential hysteresis that span from roughly 15 mV to more than 1.8 V. Small hysteresis are associated with topotactic reactions and with V^V/V^IV and V^III/V^II redox couples. Large potential hysteresis values (>1 V) were observed when oxidation of V^III to V^IV is involved.     

Aluminothermal synthesis and characterization of Li3V2−xAlx(PO4)3 cathode materials for lithium ion batteries

Monoclinic Li₃V_2−xAl_x(PO₄)₃ with different Al³⁺ doping contents (x = 0, 0.05, 0.08, 0.10 and 0.12) have been prepared by a facile aluminothermal reaction. Aluminum nanoparticles have been used as source for Al³⁺ and nucleus for Li₃V_2−xAl_x(PO₄)₃ nucleation as well as reducing agent in the aluminothermal strategy. The products were investigated by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) and electrochemical methods. The XRD results show that the as-obtained Li₃V_2−xAl_x(PO₄)₃ has a phase-pure monoclinic structure, irrespective of the Al³⁺ doping concentration. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) results reveal that the charge-transfer resistance of the Li₃V₂(PO₄)₃ is reduced and the reversibility is enhanced after V³⁺ substituted by Al³⁺. In addition, The Li₃V_2−xAl_x(PO₄)₃ phases exhibit better cycling stability than the pristine Li₃V₂(PO₄)₃.     

Improvement of electrochemical properties of Li[Ni0.4Co0.2Mn(0.4−x)Mgx]O2−yFy cathode materials at high voltage region

Spherical Li[Ni_0.4Co_0.2Mn_(0.4−x)Mg_x]O_2−yF_y (x = 0, 0.04, y = 0, 0.08) with phase-pure and well-ordered layered structure have been synthesized by heat-treatment of spherical [Ni_0.4Co_0.2Mn_0.4−xMg_x]₃O₄ precursors with LiOH·H₂O and LiF salts. The average particle size of the powders was about 10-15 μm and the size distribution was quite narrow due to the homogeneity of the metal carbonate, [Ni_0.4Co_0.2Mn_(0.4−x)Mg_x]CO₃ (x = 0, 0.04) precursors. Although the Li[Ni_0.4Co_0.2Mn_0.36Mg_0.04]O_1.92F_0.08 delivered somewhat slightly lower initial discharge capacity, however, the capacity retention, interfacial resistance, and thermal stability were greatly enhanced comparing to the Li[Ni_0.4Co_0.2Mn_0.4]O₂ and Li[Ni_0.4Co_0.2Mn_0.36Mg_0.04]O₂.     

Structure and electrochemical characteristics of TiV1.1Mn0.9Nix (x = 0.1-0.7) alloys

The structure and electrochemical properties of TiV_1.1Mn_0.9Ni_x (x = 0.1-0.7) solid solution electrode alloys have been investigated. It is found that these alloys mainly consist of a solid solution phase with body centered cubic (bcc) structure and a C14 Laves secondary phase. The solid solution alloys show easy activation behavior, high temperature dischargeability, high discharge capacity and favorable high-rate dischargeability as a negative electrode material in Ni-MH battery. The maximum discharge capacity is 502 mAh g⁻¹ at 303 K when x = 0.4. Electrochemical impedance spectroscopy (EIS) test shows that the charge-transfer resistance at the surface of the alloy electrodes decreases obviously with increasing Ni content.     

Preparation and characterization of novel spinel Li4Ti5O12−xBrx anode materials

Br-doped Li₄Ti₅O₁₂ in the form of Li₄Ti₅O_12−xBr_x (0 ≤ x ≤ 0.3) compounds were successfully synthesized via solid state reaction. The structure and electrochemical properties of the spinel Li₄Ti₅O_12−xBr_x (0 ≤ x ≤ 0.3) materials were investigated. The Li₄Ti₅O_12−xBr_x (x = 0.2) presents the best discharge capacity among all the samples, and shows better reversibility and higher cyclic stability compared with pristine Li₄Ti₅O₁₂, especially at high current rates. When the discharge rate was 0.5 C, the Li₄Ti₅O_12−xBr_x (x = 0.2) sample presented the excellent discharge capacity of 172 mAh g⁻¹, which was very close to its theoretical capacity (175 mAh g⁻¹), while that of the pristine Li₄Ti₅O₁₂ was 123.2 mAh g⁻¹ only.     

Influence of structure and composition upon performance of tin phosphate based negative electrodes for lithium batteries

Tin oxide and amorphous tin borophosphates have recently received significant attention as possible new negative electrode materials for lithium batteries. In this study, we have carefully investigated a number of different well-characterised tin phosphates as electrodes in Li-ion cells, in order to better understand the mode of operation of these materials and how their performance is related to structure and composition. The materials that were investigated were crystalline cubic and layered SnP₂O₇, LiSn₂(PO₄)₃, Sn₂P₂O₇, and Sn₃(PO₄)_2, and amorphous Sn₂BPO₆. Cubic SnP₂O₇ showed the best performance with a reversible specific charge capacity of >360 mA h g⁻¹ and a capacity retention of 96% over 50 cycles when cycled between 0.02 and 1.2 V versus Li_m. The three Sn(IV) materials showed lower initial reversible capacity but better capacity retention than the three Sn(II) materials in the study. Their higher proportion of inert matrix material can partly explain this. However, cubic SnP₂O₇ cycled significantly better than its layered polymorph, which shows that the structure of the starting material is also of great importance. Another important conclusion drawn from the results is that it is not necessary for the starting material to be amorphous, or if crystalline, to have small grain size, to cycle well. The three pyrophosphates all show an initial reduction capacity that corresponds to around 2 Li per P₂O₇⁴⁻ unit more than is predicted by theory. This might be explained by reductive break-up of the POP bond.