The improved physical and electrochemical performance of LiNi0.35Co0.3−xCrxMn0.35O2 cathode materials by the Cr doping for lithium ion batteries

We have successfully prepared the layered structure LiNi_0.35Co_0.3−xCr_xMn_0.35O₂ with various Cr contents by a co-precipitation method. Many measurement methods have been applied to characterize the physical and electrochemical properties of LiNi_0.35Co_0.3−xCr_xMn_0.35O₂, such as XRD, SEM, BET and electrochemical test. SEM showed that the addition of Cr has obviously changed the morphologies of their particles and increased the size of grains. The specific surface area of LiNi_0.35Co_0.3−xCr_xMn_0.35O₂ decreases lineally from 4.9 m² g⁻¹ (x = 0) to 1.8 m² g⁻¹ (x = 0.1) with the increasing of Cr contents. Moreover, we have found that the Cr doping can greatly improve the density of the powder, which is beneficial to solve the problem of lower electrode density for these layered LiNi_0.35Co_0.3−xCr_xMn_0.35O₂ cathode materials. Electrochemical test indicated that the cycling performance of LiNi_0.35Co_0.3−xCr_xMn_0.35O₂ can be significantly improved with the increasing of Cr contents, although the initial discharge capacity of the sample has a little decrease.     

Synthesis and characterizations of Li[CrxLi(1−x)/3Mn2(1−x)/3]O2 (0.15 ≤ x ≤ 0.3) cathode materials in rechargeable lithium batteries

A series of Li[Cr_xLi_(1−x)/3Mn_2(1−x)/3]O₂ (0.15 ≤ x ≤ 0.3) cathode materials was prepared by citric acid-assisted, sol–gel process. Sub-micron sized particles were obtained and the X-ray diffraction (XRD) results showed that the crystal structure was similar to layered lithium transition metal oxides (R-3m space group). The electrochemical performance of the cathodes was evaluated over the voltage range 2.0–4.9 V at a current density of 7.947 mA g⁻¹. The Li_1.27Cr_0.2Mn_0.53O₂ electrode delivered a high reversible capacity of up to 280 mAh g⁻¹ during cycling. Li[Cr_xLi_(1−x)/3Mn_2(1−x)/3]O₂ yielded a promising cathode material.     

Transport and electrochemical properties of the LiyCrxMn2−xO4 (0 < x < 0.5) cathode material

In this paper structural, electrical, electrochemical and thermal (DSC) characterization of series of manganese spinel samples with manganese substituted to different degree (x = 0–0.5) with chromium are presented. The conductivity and thermoelectric power measurements were performed in wide temperature range also versus oxygen partial pressure and for deintercalated samples. Electrochemical studies of these cathode materials were conducted in Li/Li⁺/Li_yCr_xMn_2−xO₄ type cells. Substitution of manganese with chromium causes disappearance of the phase transition characteristic of LiMn₂O₄ spinel. Studies of electrical properties reveal that Cr ions do not participate in charge transport at low temperatures. In the charge curves of Li/Li⁺/Li_yCr_xMn_2−xO₄ cells there are two visible plateaux, separated with distinct potential jump (∼0.5 V), which position on Li content perfectly matches the Mn³⁺ content in the doped cathode material. The lower plateau is related to the Mn³⁺ → Mn⁴⁺ oxidation, while the next of higher voltage, of the dopant Cr³⁺ → Cr⁴⁺ oxidation. The schematic diagrams of relative Mn–Cr electronic levels alignment are proposed.     

Electrochemical properties of iodine-containing lithium manganese oxide spinel

Iodine-containing, cation-deficient, lithium manganese oxides (ICCD-LMO) are prepared by reaction of MnO₂ with LiI. The MnO₂ is completely transformed into spinel-structured compounds with a nominal composition of Li_1−δMn_2−2δO₄I_x. A sample prepared at 800 °C, viz. Li_0.99Mn_1.98O₄I_0.02, exhibits an initial discharge capacity of 113 mA h g⁻¹ with good cycleability and rate capability in the 4-V region. Iodine-containing, lithium-rich lithium manganese oxides (ICLR-LMO) are also prepared by reaction of LiMn₂O₄ with LiI, which results in a nominal composition of Li_1+xMn_2−xO₄I_x. Li_1.01Mn_1.99O₄I_0.02 shows a discharge capacity of 124 mA h g⁻¹ on the first cycle and 119 mA h g⁻¹ a on the 20th cycle. Both results indicate that a small amount of iodine species helps to maintain cycle performance.     

Synthesis and characterization of lithium aluminum-doped spinel (LiAlxMn2−xO4) for lithium secondary battery

LiAl_xMn_2−xO₄ has been synthesized using various aluminum starting materials, such as Al(NO₃)₃, Al(OH)₃, AlF₃ and Al₂O₃ at 600–800°C for 20 h in air or oxygen atmosphere. A melt-impregnation method was used to synthesize Al-doped spinel with good battery performance in this research. The Al-doped content and the intensity ratio of (3 1 1)/(4 0 0) peaks can be important parameters in synthesizing Al-doped spinel which satisfies the requirements of high discharge capacity and good cycleability at the same time. The decrease in Mn³⁺ ion by Al substitution induces a high average oxidation state of Mn ion in the LiAl_xMn_2−xO₄ material. The electrochemical behavior of all samples was studied in Li/LiPF₆-EC/DMC (1:2 by volume)/LiAl_xMn_2−xO₄ cells. Especially, the initial and last discharge capacity of LiAl_0.09Mn_1.97O₄ using LiOH, Mn₃O₄ and Al(OH)₃ complex were 128.7 and 115.5 mAh/g after 100 cycles. The Al substitution in LiMn₂O₄ was an excellent method of enhancing the cycleability of stoichiometric spinel during electrochemical cycling.     

Synthesis and electrochemical properties of Li[Li(1−2x)/3NixMn(2−x)/3]O2 as cathode materials for lithium secondary batteries

Layered Li[Li_(1−2x)/3Ni_xMn_(2−x)/3]O₂ materials with x=0.41, 0.35, 0.275 and 0.2 are synthesized by means of a sol–gel method. The layered structure is stabilized by a solid solution between LiNiO₂ and Li₂MnO₃. The discharge capacity increases with increasing lithium content at the 3a sites in the Li[Li_(1−2x)/3Ni_xMn_(2−x)/3]O₂. A Li[Li_0.2Ni_0.2Mn_0.6]O₂ electrode delivers discharge capacities of 200 and 240 mAh g⁻¹ with excellent cycleability at 30 and 55 °C, respectively.     

Synthesis and electrochemical properties of Li[Ni0.8Co0.1Mn0.1]O2 and Li[Ni0.8Co0.2]O2 via co-precipitation

Spherical Li[Ni_0.8Co_0.2−xMn_x]O₂ (x = 0, 0.1) with phase-pure and well-ordered layered structure have been synthesized by heat-treatment of spherical [Ni_0.8Co_0.2−xMn_x](OH)₂ and LiOH·H₂O precursors. The structure, morphology, electrochemical properties, and thermal stability of Li[Ni_0.8Co_0.2−xMn_x]O₂ (x = 0, 0.1) were studied. The average particle size of the powders was about 10–15 μm and the size distribution was narrow due to the homogeneity of the metal hydroxide [Ni_0.8Co_0.2−xMn_x](OH)₂ (x = 0, 0.1). The Li[Ni_0.8Co_0.2−xMn_x]O₂ (x = 0, 0.1) delivered a discharge capacity of 197–202 mAh g⁻¹ and showed excellent cycling performance. Compared to Li[Ni_0.8Co_0.2]O₂, Li[Ni_0.8Co_0.1Mn_0.1]O₂ exhibited greater thermal stability resulting from improved structural stability due to Mn substitution.     

Chemical and structural instabilities of lithium ion battery cathodes

The chemical and structural stabilities of various layered Li_1−xNi_1−y−zMn_yCo_zO₂ cathodes are compared by characterizing the samples obtained by chemically extracting lithium from the parent Li_1−xNi_1−y−zMn_yCo_zO₂ with NO₂BF₄ in an acetonitrile medium. The nickel- and manganese-rich compositions such as Li_1−xNi_1/3Mn_1/3Co_1/3O₂ and Li_1−xNi_0.5Mn_0.5O₂ exhibit better chemical stability than the LiCoO₂ cathode. While the chemically delithiated Li_1−xCoO₂ tends to form a P3 type phase for (1 − x) < 0.5, Li_1−xNi_0.5Mn_0.5O₂ maintains the original O3 type phase for the entire 0 ≤ (1 − x) ≤ 1 and Li_1−xNi_1/3Mn_1/3Co_1/3O₂ forms an O1 type phase for (1 − x) < 0.23. The variations in the type of phases formed are explained on the basis of the differences in the chemical lithium extraction rate caused by the differences in the degree of cation disorder and electrostatic repulsions. Additionally, the observed rate capability of the Li_1−xNi_1−y−zMn_yCo_zO₂ cathodes bears a clear relationship to cation disorder and lithium extraction rate.     

Effect of sulfur and nickel doping on morphology and electrochemical performance of LiNi0.5Mn1.5O4−xSx spinel material in 3-V region

The effect of nickel and sulfur substitution for manganese and oxygen on the structure and electrochemical properties of the LiNi_0.5Mn_1.5O_4−xS_x is examined. The LiNi_0.5Mn_1.5O_4−xS_x (x = 0 and 0.05) compounds are successfully synthesized at 500 and 800 °C by co-precipitation using the metal carbonate (Ni_0.5Mn_1.5)CO₃ as a precursor. The resulting powder with sulfur doping exhibits different morphology from a Ni-only doped spinel in terms of particle size and surface texture. The LiNi_0.5Mn_1.5O_4−xS_x (x = 0 and 0.05) powders are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and galvanostatic charge–discharge cycling. The nickel- and sulfur-doped spinel displays excellent capacity retention and rate capability in the 3-V region, compared with Ni-only doped spinel material.     

Synthesis and electrochemical characteristics of LiCrxNi0.5−xMn1.5O4 spinel as 5 V cathode materials for lithium secondary batteries

A series of electrochemical spinel compounds, LiCr_xNi_0.5−xMn_1.5O₄ (x=0, 0.1, 0.3), are synthesized by a sol–gel method and their electrochemical properties are characterized in the voltage range of 3.5–5.2 V. Electrochemical data for LiCr_xNi_0.5−xMn_1.5O₄ electrodes show two reversible plateaus at 4.9 and 4.7 V. The 4.9 V plateau is related to the oxidation of chromium while the 4.7 V plateau is ascribed to the oxidation of nickel. The LiCr_0.1Ni_0.4Mn_1.5O₄ electrode delivers a high initial capacity of 152 mAh g⁻¹ with excellent cycleability. The excellent capacity retention of the LiCr_0.1Ni_0.4Mn_1.5O₄ electrode is largely attributed to structural stabilization which results from co-doping (chromium and nickel) and increased theoretical capacity due to substitution of chromium.     

Layered Li(Ni0.5−xMn0.5−xM2x′)O2 (M′=Co,Al, Ti; x=0, 0.025) cathode materials for Li-ion rechargeable batteries

Layered Li(Ni_0.5−xMn_0.5−xM_2x′)O₂ materials (M′=Co, Al, Ti; x=0, 0.025) were synthesized using a manganese-nickel hydroxide precursor, and the effect of dopants on the electrochemical properties was investigated. Li(Ni_0.5Mn_0.5)O₂ exhibited a discharge capacity of 120 mAh/g in the voltage range of 2.8–4.3 V with a slight capacity fade up to 40 cycles (0.09% per cycle); by doping of 5 mol% Co, Al, and Ti, the discharge capacities increased to 140, 142, and 132 mAh/g, respectively, and almost no capacity fading was observed. The cathode material containing 5 mol% Co had the lowest impedance, 47 Ω cm², while undoped, Ti-doped, and Al-doped materials had impedance of 64, 62, and 99 Ω cm², respectively. Unlike the other dopants, cobalt was found to improve the electronic conductivity of the material. Further improvement in the impedance of these materials is needed to meet the requirement for powering hybrid electric vehicle (HEV, <35 Ω cm²). In all materials, structural transformation from a layered to a spinel structure was not observed during electrochemical cycling. Cyclic voltammetry and X-ray photoelectron spectroscopy (XPS) data suggested that Ni and Mn exist as Ni²⁺ and Mn⁴⁺ in the layered structure. Differential scanning calorimetry (DSC) data showed that exothermic peaks of fully charged Li_1−y(Ni_0.5−xMn_0.5−xM_2x′)O₂ appeared at higher temperature (270–290 °C) than LiNiO₂-based cathode materials, which indicates that the thermal stability of Li(Ni_0.5−xMn_0.5−xM_2x′)O₂ is better than those of LiNiO₂-based cathode materials.     

High-temperature sulfuric acid decomposition over complex metal oxide catalysts

Activity and stability of FeTiO₃, MnTiO₃, NiFe₂O₄, CuFe₂O₄, NiCr₂O₄, 2CuO·Cr₂O₃, CuO and Fe₂O₃ for the atmospheric decomposition of concentrated sulfuric acid in sulfur-based thermochemical water splitting cycles are presented. Catalyst activity was determined at temperatures from 725 to 900 °C. Catalytic stability was examined at 850 °C for up to 1 week of continuous operation. The results were compared to a 1.0 wt% Pt/TiO₂ catalyst. Surface area by nitrogen physisorption, X-ray diffraction analyses, and temperature programmed desorption and oxidation were used to characterize fresh and spent catalyst samples.Over the temperature range, the catalyst activity of the complex oxides followed the general trend: 2CuO·Cr₂O₃ > CuFe₂O₄ > NiCr₂O₄ ≈ NiFe₂O₄ > MnTiO₃ ≈ FeTiO₃. At temperatures less than 800 °C, the 1.0 wt% Pt/TiO₂ catalyst had higher activity than the complex oxides, but at temperatures above 850 °C, the 2CuO·Cr₂O₃ and CuFe₂O₄ samples had the highest activity.Surface area was found to decrease for all of the metal oxides after exposure to reaction conditions. In addition, the two complex metal oxides that contained chromium were not stable in the reaction environment; both leached chromium into the acid stream and decomposed into their individual oxides. The FeTiO₃ sample also produced a discoloration of the reactor due to minor leaching and converted to Fe₂TiO₅. Fe₂O₃, MnTiO₃ and NiFe₂O₄ were relatively stable in the reaction environment. In addition, CuFe₂O₄ catalyst appeared relatively promising due to its high activity and lack of any leaching issues; however it deactivated in week-long stability experiments.Complex metal oxides may provide an attractive alternative to platinum-based catalyst for the decomposition of sulfuric acid; however, the materials examined in this study all displayed shortcomings including material sintering, phase changes, low activity at moderated temperatures due to sulfate formation, and decomposition to their individual oxides. More effort is needed in this area to discover metal oxide materials that are less expensive, more active and more stable than platinum catalysts.     

Oxidation kinetics and phase evolution of a Fe–16Cr alloy in simulated SOFC cathode atmosphere

The oxidation behavior of a Fe–16Cr alloy containing a small amount of Mn oxidized in air for up to 500 h within the temperature range of 650–850 °C was examined. Two consecutive oxidation stages were found and these obeyed the parabolic rate law with various rate constants. Formation and growth of Cr₂O₃, MnCr₂O₄, surface nodules and oxide spallation were found to be responsible in the oxidation stages accordingly to various situations. The thin film X-ray diffraction, SEM and EDX confirmed the duplex oxide microstructure with MnCr₂O₄ on top of Cr₂O₃, Cr and Mn diffusion in Cr₂O₃ is considered to be responsible for the formation of each layer, respectively. The estimated area specific resistance (ASR) suggests the possibility of using this alloy as the interconnect material in reduced temperature SOFCs, however, surface modification to enhance its oxidation and spallation resistances is desired.     

Improvement of electrochemical properties of LiNi0.5Mn1.5O4 spinel material by fluorine substitution

LiNi_0.5Mn_1.5O_4−xF_x (0 ≤ x ≤0.1) cathodes, synthesized by ultrasonic spray pyrolysis at 900 °C, exhibit superior structural and electrochemical properties. The samples are characterized by X-ray diffraction, scanning electron microscopy, differential scanning calorimetry, and electrochemical measurements. During Li⁺ extraction, LiNi_0.5Mn_1.5O_4−xF_x has a smaller lattice variation and area-specific impedance than LiNi_0.5Mn_1.5O₄. This enhances the rate capability, especially at high C-rates. LiNi_0.5Mn_1.5O_4−xF_x also exhibits better resistance than LiNi_0.5Mn_1.5O₄ to attack by HF.     

Stannum doping of layered LiNi3/8Co2/8Mn3/8O2 cathode materials with high rate capability for Li-ion batteries

Sn doped lithium nickel cobalt manganese composite oxide of LiNi_3/8Co_2/8Mn_3/8−xSn_xO₂ (0 ≤ x ≤ 0.10) was synthesized by stannum substitute of manganese to enhance its rate capability at first time. Its structure and electrochemical properties were characterized by X-ray diffraction (XRD), SEM, cyclic voltammetry (CV), galvanostatic intermittent titration technique (GITT) and charge/discharge tests. LiNi_3/8Co_2/8Mn_3/8−xSn_xO₂ had stable layered structure with α-NaFeO₂ type as x up to 0.05, meanwhile, its chemical diffusion coefficient D_Li of Li-ion was enhanced by almost one order of magnitude, leading to notable improvement of the rate capability of LiNi_3/8Co_2/8Mn_3/8O₂. The compound of x = 0.10 showed the best rate capability among Sn doped samples, but its discharge capacity reduced markedly due to secondary phase Li₂SnO₃ and increase of cation-disorder. The compound with x = 0.05 showed high rate capability with initial discharge capacity in excess of 156 mAh g⁻¹. It is a promising alternative cathode material for EV application of Li-ion batteries.     

Synthesis and electrochemical properties of Mo-doped Li[Ni1/3Mn1/3Co1/3]O2 cathode materials for Li-ion battery

The layered Li[Ni_(1−x)/3Mn_(1−x)/3Co_(1−x)/3Mo_x]O₂ cathode materials (x = 0, 0.005, 0.01, and 0.02) were prepared by a solid-state pyrolysis method (700, 800, 850, and 900 °C). Its structure and electrochemical properties were characterized by XRD, SEM, XPS, cyclic voltammetry, and charge/discharge tests. It can be learned that the doped sample of x = 0.01 calcined at 800 °C shows the highest first discharge capacity of 221.6 mAh g⁻¹ at a current density of 20 mA g⁻¹ in the voltage range of 2.3–4.6 V, and the Mo-doped samples exhibit higher discharge capacity and better cycle-ability than the undoped one at room temperature.     

RuxCrySez electrocatalyst loading and stability effects on the electrochemical performance in a PEMFC

The present paper presents a study of the Ru_xCr_ySe_z chalcogenide electrocatalyst based on physical–chemical characterization through scanning electron (SEM), atomic force (AFM) microscopy and energy dispersion elemental analysis (EDS), thermal stability using differential scanning calorimeter (DSC), electrochemical kinetics towards the oxygen reduction reaction (ORR) in acid media by rotating ring-disk electrode (RRDE) and single and three-stack membrane-electrode assembly (MEA) performance as a function of catalyst loading (10%, 20% and 40% W from 0.2 to 2 mg cm⁻²). Results indicate an electrocatalyst with chemical composition of Ru₆Cr₄Se₅. AFM images showed 80–160 nm nanoparticle agglomerates. Good thermal stability of the cathode Ru₆Cr₄Se₅ was established after 100 h of continuous operation. The electrochemical kinetics study (RRDE) resulted in a electrocatalyst with high activity towards the ORR, preferentially proceeding via 4e⁻ charge transfer pathway towards water formation (i.e., O₂+4H⁺+4e⁻→2H₂O), with a maximum of 2.8% H₂O₂ formation at 25 °C. Finally, MEA tests revealed a maximum power density of 220 mW cm⁻² with a catalyst loading of 20 wt% at 1.6 mg cm⁻².     

Accelerated synthesis and electrochemical performance of Li1+x(Ni0.5Mn0.5)O2+δ cathode materials

Synthesis of Li_1+xNi_0.5Mn_0.5O_2+δ (x = 0, 0.3) cathode materials for secondary lithium batteries was performed by freeze-drying method using various chemical precursors. Electrochemical performance of materials demonstrates considerable dependence on their chemical prehistory and Li stoichiometry. Increase in duration of the final annealing at 900 °C since 1–12 h results in decreasing capacity values for all x and chemical prehistories. Observed capacity fall is explained by smaller grain size and, therefore, shorter Li diffusion pathways in the samples obtained using short time annealing.     

Exploration of high capacity LiNi0.5Mn1.5O4 synthesized by solid-state reaction

LiNi_0.5Mn_1.5O₄ was prepared by an improved solid-state reaction at high heating and cooling rates, the mixed precursors were initially heated up to 900 °C, then directly cooled down to 600 °C and heated for 24 h in air. X-ray diffraction (XRD) pattern shows that LiNi_0.5Mn_1.5O₄ has cubic spinel structure; scanning electron microscopic (SEM) image shows that the particle size is about 0.2 μm together with homogenous distribution. Electrochemical measurements show that LiNi_0.5Mn_1.5O₄ powders delivered up to 143 mAh g⁻¹ with superior cycling performance at the rate of 5/7C.     

Electrochemical performance behavior of combustion-synthesized LiNi0.5Mn0.5O2 lithium-intercalation cathodes

LiNiO₂, partially substituted with manganese in the form of a LiNi_0.5Mn_0.5O₂ compound, has been synthesized by a gelatin assisted combustion method [GAC] method. Highly crystalline LiNi_0.5Mn_0.5O₂ powders with R3m symmetry have been obtained at an optimum temperature of 850 °C, as confirmed by PXRD studies. The presence of cathodic and anodic CV peaks exhibited by the LiNi_0.5Mn_0.5O₂ cathode at 4.4 and 4.3 V revealed the existence of Ni and Mn in their 2+ and 4+ oxidation states, respectively. The synthesized LiNi_0.5Mn_0.5O₂ cathode has been subjected to systematic electrochemical performance evaluation, via capacity tapping at different cut-off voltage limits (3.0–4.2, 3.0–4.4 and 3.0–4.6 V) and the possible extraction of deliverable capacity under different current drains (0.1C, 0.5C, 0.75C and 1C rates). The LiNi_0.5Mn_0.5O₂ cathode exhibited a maximum discharge capacity of 174 mAh g⁻¹ at the 0.1C rate between 3.0 and 4.6 V. However, a slightly decreased capacity of 138 mAh g⁻¹ has been obtained in the 3.0–4.4 V range, when discharged at the 1C rate. On the other hand, extended cycling at the 0.1C rate encountered an acceptable capacity fade in the 3.0–4.4 V range (<10%) for up to 50 cycles.