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.     

Low operational temperature Li–CFx batteries using cathodes containing sub-fluorinated graphitic materials

Commercial lithium/polycarbon monofluoride batteries [Li–(CF)_n] are typically current-limited and are therefore not implemented in high-rate or low-temperature applications. Recent results suggest, however, that CF-based cathodes that use sub-fluorinated CF_x (SFCF_x) active materials in a thin electrode form factor are able to support very high currents (up to 5 C) while still providing a significant fraction of their specific capacity. In this work, the low temperature efficacy of these materials is examined in a −40 °C environment. CF_0.54 and CF_0.65 powders were characterized using X-ray diffraction, scanning electron microscopy, and X-ray energy dispersive spectroscopy. These materials were then implemented in a spray-deposited electrode using a 1-mil (∼25 μm) aluminum foil current collector and PVDF as a binder. Electrochemical tests showed that these materials were able to deliver specific capacity values up to five times greater than commercial CF_1.08 powder inserted into identically fabricated test cells tested at −40 °C. Testing also indicated that a room-temperature pre-discharge step was necessary to condition the electrode materials before exposure to the low-temperature test environment.     

Crystallographic and electrochemical characteristics of La0.7Mg0.3Ni3.5 − x(Al0.5Mo0.5)x (x = 0–0.8) hydrogen storage alloys

The crystal structure, hydrogen storage property and electrochemical characteristics of the La_0.7Mg_0.3Ni_3.5 − x(Al_0.5Mo_0.5)_x (x = 0–0.8) alloys have been investigated systematically. It can be found that with X-ray powder diffraction and Rietveld analysis the alloys are of multiphase alloy and consisted of impurity LaNi phase and two main crystallographic phases, namely the La(La, Mg)₂Ni₉ phase and the LaNi₅ phase, and the lattice parameter and the cell volume of both the La(La, Mg)₂Ni₉ phase and the LaNi₅ phase increases with increasing Al and Mo content in the alloys. The P–C isotherms curves indicate that the hydrogen storage capacity of the alloy first increases and then decreases with increasing x, and the equilibrium pressure decreases with increasing x. The electrochemical measurements show that the maximum discharge capacity first increases from 354.2 (x = 0) to 397.6 mAh g⁻¹ (x = 0.6) and then decreases to 370.4 mAh g⁻¹ (x = 0.8). The high-rate dischargeability of the alloy electrode increases lineally from 55.7% (x = 0) to 73.8% (x = 0.8) at the discharge current density of 1200 mA g⁻¹. Moreover, the exchange current density of the alloy electrodes also increases monotonously with increasing x. The hydrogen diffusion coefficient in the alloy bulk increases with increasing Al and Mo content and thus enhances the low-temperature dischargeability of the alloy electrode.     

Crystallographic and electrochemical characteristics of icosahedral quasicrysyalline Ti45−xZr35−xNi17+2xCu3 (x = 0–8) powders

Ti_45−xZr_35−xNi_17+2xCu₃ (x = 0, 2, 4, 6 and 8) icosahedral quasicrystalline phase (I-phase) alloy powders are synthesized by mechanical alloying and subsequent annealing techniques, and the crystallographic and electrochemical characteristics are investigated. The alloy powders are I-phase, and the quasi-lattice constant decreases with increasing x value. The maximum discharge capacity of the I-phase alloy electrodes first increases and then decreases with increasing x value, and the Ti₃₉Zr₂₆Ni₂₉Cu₃ I-phase electrode exhibits the highest discharge capacity of 274 mAh g⁻¹. The high-rate dischargeability at the discharge current density of 240 mA g⁻¹ increases from 55.31% (x = 0) to 74.24% (x = 8). Cycling stability also increases with increasing x value. The improvement in electrochemical characteristics may be ascribed to the added nickel, which not only improves the electrochemical activity, but also makes the alloy more resistant to oxidation.     

Effect of titanium substitution in layered LiNiO2 cathode material prepared by molten-salt synthesis

LiNi_1−xTi_xO₂ (0 ≤ x ≤ 0.1) compounds have been synthesized by a direct molten-salt method that uses a eutectic mixture of LiNO₃ and LiOH salts. According to X-ray diffraction analysis, these materials have a well-developed layered structure (R3-m) and are an isostructure of LiNiO₂. The LiNi_1−xTi_xO₂ (0 ≤ x ≤ 0.1) compounds have average particle sizes of 1–5 μm depending on the amount of Ti salt. Charge–discharge tests show that a LiNi_1−xTi_xO₂ (0 ≤ x ≤ 0.1) cathode prepared at 700 °C has an initial discharge capacity as high as 171 mA h g⁻¹ and excellent capacity retention in the range 4.3–2.8 V at a current density of 0.2 mA cm⁻².     

Effect of the Sm content on the structure and electrochemical properties of La1.3 − xSmxCaMg0.7Ni9 (x = 0–0.3) hydrogen storage alloys

La_1.3 − xSm_xCaMg_0.7Ni₉ (x = 0–0.3) hydrogen storage alloys were prepared by inductive melting and the effect of the Sm content on the structure and electrochemical properties was investigated in the paper. The Sm substitution for La in La_1.3 − xSm_xCaMg_0.7Ni₉ (x = 0–0.3) alloys does not change the main phase structure (the rhombohedral PuNi₃-type structure), but leads to a shrinkage of unit cell and a decrease of hydrogen storage capacity. With the increase of the Sm content in the alloys, the maximum discharge capacity of electrode decreases from 400.2 (x = 0) to 346.6 mAh g⁻¹ (x = 0.3), but the high-rate dischargeability and cycling stability is improved. After 100 cycles, the capacity retention rate increases from 75 (x = 0) to 85% (x = 0.3).     

Effect of particle size on lithium intercalation rates in natural graphite

The intercalation rate of Li⁺-ions in flake natural graphite with particle size that ranged from 2 to 40 μm was investigated. The amount of Li⁺-ions that intercalate at different rates was determined from measurement of the reversible capacity during deintercalation in 1 M LiClO₄/1:1 (volume ratio) ethylene carbonate–dimethyl carbonate. The key issues in this study are the role of particle size and fraction of edge sites on the rate of intercalation and deintercalation of Li⁺-ions. At low specific current (15.5 mA/g carbon), the composition of lithiated graphite approaches the theoretical value, x=1 in Li_xC₆, except for the natural graphite with the largest particle size. However, x decreases with an increase in specific current for all particle sizes. This trend suggests that slow solid-state diffusion of Li⁺-ions limits the intercalation capacity in graphite. The flake natural graphite with a particle size of 12 μm may provide the optimum combination of reversible capacity and irreversible capacity loss in the electrolyte and discharge rates used in this study.     

Oligo(ethylene glycol)-functionalized disiloxanes as electrolytes for lithium-ion batteries

Functionalized disiloxane compounds were synthesized by attaching oligo(ethylene glycol) chains, –(CH₂CH₂O)–_n, n = 2–7, via hydrosilation, dehydrocoupling, and nucleophilic substitution reactions and were examined as non-aqueous electrolyte solvents in lithium-ion cells. The compounds were fully characterized by ¹H, ¹³C, and ²⁹Si nuclear magnetic resonance (NMR) spectroscopy. Upon doping with lithium bis(oxalato)borate (LiBOB) or LiPF₆, the disiloxane electrolytes showed conductivities up to 6.2 × 10^?4 S cm^?1 at room temperature. The thermal behavior of the electrolytes was studied by differential scanning calorimetry, which revealed very low glass transition temperatures before and after LiBOB doping and much higher thermal stability compared to organic carbonate electrolytes. Cyclic voltammetry measurements showed that disiloxane-based electrolytes with 0.8 M LiBOB salt concentration are stable to 4.7 V. The LiBOB/disiloxane combinations were found to be good electrolytes for lithium-ion cells; unlike LiPF₆, LiBOB can provide a good passivation film on the graphite anode. The LiPF₆/disiloxane electrolyte was enabled in lithium-ion cells by adding 1 wt% vinyl ethylene carbonate (VEC). Full cell performance tests with LiNi_0.80Co_0.15Al_0.05O₂ as the cathode and mesocarbon microbead (MCMB) graphite as the anode show stable cyclability. The results demonstrate that disiloxane-based electrolytes have considerable potential as electrolytes for use in lithium-ion batteries.     

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.     

Synthesis of LiCo1−xNixO2 from a low temperature solution combustion route and characterization

Nickel substituted lithium–cobalt oxides, LiCo_1−xNi_xO₂ (0<x<0.4), have been synthesized in a very short time by a solution combustion method at 350 °C using diformyl hydrazine as a fuel. Pure phases with hexagonal lattice structure have been obtained. These compounds facilitate reversible insertion/extraction of Li⁺ ions with good discharge capacity between 3.0 and 4.4 V versus Li/Li⁺. Results of the studies by powder X-ray diffraction, scanning electron microscopy, cyclic voltammetry, galvanostatic charge–discharge cycling and ac impedance measurements are presented.     

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.     

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.     

Characterization of Zn- and Fe-substituted LiMnO2 as cathode materials in Li-ion cells

Layered LiMn_1−xM_xO₂ (M = Zn or Fe) (0 ≤ x ≤ 0.3) samples are synthesized from the corresponding sodium analogues by an ion-exchange method using LiBr in n-hexanol at 160 °C. The samples are subjected to physicochemical and electrochemical characterization. X-ray diffraction data indicate the formation of layered structures for the LiMn_1−xZn_xO₂ samples up to x = 0.3 and for LiMn_1−xFe_xO₂ samples up to x = 0.2. Among these, LiMn_0.95Zn_0.05O₂ and LiMn_0.95Fe_0.05O₂ provide the highest capacity values of 180 and 193 mAh g⁻¹, respectively. Both Zn- and Fe-substituted samples display good capacity retention up to 30 charge–discharge cycles. Electrochemical impedance spectroscopy and galvanostatic intermittent titration data corroborate the results obtained from cyclic volatmmetry and charge–discharge cycling.     

Effects of Pd substitution on the electrochemical properties of Mg0.9−xTi0.1PdxNi (x = 0.04–0.1) hydrogen storage alloys

Amorphous Mg_0.9−xTi_0.1Pd_xNi (x = 0.04–0.1) hydrogen storage alloys were prepared by mechanical alloying (MA). The effects of Pd substitution on the electrochemical properties of the Mg_0.9−xTi_0.1Pd_xNi (x = 0.04–0.1) electrode alloys were studied by cyclic charge–discharge, linear polarization, anodic polarization, electrochemical impedance spectroscopy (EIS), and hydrogen diffusion coefficient experiments. It was found that the cyclic capacity retention rate C₅₀/C₁ of the quaternary alloys was greatly improved due to the substitution of Pd for Mg. Mg_0.8Ti_0.1Pd_0.1Ni electrode alloy retained the discharge capacity above 200 mAh g⁻¹ even after 80 charge–discharge cycles, possessing the longest cycle life in the studied quaternary alloys. The improvement of cycle life was ascribed to the formation of passive film on the surface of these electrode alloys. X-ray photoelectron spectroscopy (XPS) analysis proved that the passive film was composed of Mg(OH)₂, TiO₂, NiO, and PdO, which synergistically protected the alloy from further oxidation. The Auger Electron Spectroscopy (AES) study revealed that the thickness of passive film increased with augmentation of the Pd content. The electrochemical impedance study of electrode alloys after different cycles demonstrated that the passive film became thicker during cycles and its thickness also increased with Pd content augmentation. It was also found that the augmentation of Pd content resulted in the decrease of exchange current density I₀ and the increase of the charge-transfer resistance R_ct. With increasing the Pd amount in the Mg_0.9−xTi_0.1Pd_xNi (x = 0.04–0.1) electrode alloys, hydrogen diffusion coefficient D was gradually enhanced at first. Then, it decreased with augmentation of cycle due to the growth of passive film on the surface of the alloys.     

Electrochemical insertion of lithium and sodium into (MoO2)2P2O7

Insertion of lithium and sodium into phosphate (MoO₂)₂P₂O₇ was investigated electrochemically to determine the usefulness as a possible cathode for ion-transfer secondary batteries. Specific charges of up to 250 mA h g⁻¹ were obtained for A/(MoO₂)₂P₂O₇ (A: Li, Na) cells with liquid organic electrolytes in the first reduction half-cycle at room temperature. Intercalation processes under constant current densities of 0.2 mA cm⁻² were reversible within the range of composition 0.85<x<4.0 for lithium and 0.5<x<3.1 for sodium in A_x(MoO₂)₂P₂O₇ (A: Li, Na), respectively. Structural changes induced by lithium or sodium intercalation were followed by ex situ X-ray diffraction measurements, and the phase change from the crystal to the amorphous was observed in both cases.     

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.     

The effects of partial substitution of Cr for Ni on the electrochemical properties of Mg1.75Al0.25Ni1−xCrx (0 ≤ x ≤ 0.3) electrode alloys

In this paper, the substitution of different amounts of Cr for Ni in the hydrogen storage electrode alloy of Mg_1.75Al_0.25Ni has been carried out to form quaternary Mg_1.75Al_0.25Ni_1−xCr_x (0 ≤ x ≤ 0.3) alloys by means of solid diffusion method (DM). The XRD profiles exhibited that the quaternary alloys still kept the same main phase of Mg₃AlNi₂ (S.G. Fd3m) as that of ternary Mg_1.75Al_0.25Ni alloy. The electrochemical studies found that Cr substituted quaternary alloy reached its maximum discharge capacity (165 mAh g⁻¹) after 2 cycles, which was larger than that of the Mg_1.75Al_0.25Ni alloy (154 mAh g⁻¹). Among these quaternary alloys, the Mg_1.75Al_0.25Ni_0.9Cr_0.1 electrode alloy was found possessing the highest cycling capacity retention rate. Cyclic voltammetry (CV) results and anodic polarization curves demonstrated that appropriate content (x lower than 0.1) of Cr effectively improved the reaction activity of electrode and inhibited the cycling capacity degradation to some degree. Electrochemical impedance spectroscopy (EIS) analyses indicated that the increase of Cr content would raise the polarization resistance R_p on the particle surface of these quaternary alloys.     

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.     

Effect of (Al,Mg) substitution in LiNiO2 electrode for lithium batteries

Stabilized lithium nickelate is receiving increased attention as a low-cost alternative to the LiCoO₂ cathode now used in rechargeable lithium batteries. Layered LiNi_1−x−yM_xM_yO₂ samples (M_x = Al³⁺ and M_y = Mg²⁺, where x = 0.05, 0.10 and y = 0.02, 0.05) are prepared by the refluxing method using acetic acid at 750 °C under an oxygen stream, and are subsequently subjected to powder X-ray diffraction analysis and coin-cell tests. The co-doped LiNi_1−x−yAl_xMg_yO₂ samples show good structural stability and electrochemical performance. The LiNiAl_0.05Mg_0.05O₂, cathode material exhibits a reversible capacity of 180 mA h g⁻¹ after extended cycling. These results suggest that the threshold concentration for aluminum and magnesium substitution is of the order of 5%. The co-substitution of magnesium and aluminium into lithium nickelate is considered to yield a promising cathode material.     

Metal oxyfluorides TiOF2 and NbO2F as anodes for Li-ion batteries

Lithium insertion and extraction in to/from the oxyfluorides TiOF₂ and NbO₂F is investigated by galvanostatic cycling, cyclic voltammetry and impedance spectroscopy in cells using Li-metal as a counter electrode at ambient temperature. The host compounds are prepared by low-temperature reaction and characterized by powder X-ray diffraction (XRD), Rietveld refinement and Brunauer, Emmett and Teller (BET) surface area. Crystal structure destruction occurs during the first-discharge reaction with Li at voltages below 0.8–0.9 V for Li_xTiOF₂ as shown by ex situ XRD and at ≤1.4 V for Li_xNbO₂F to form amorphous composites, ‘Li_xTi/NbO_y–LiF’. Galvanostatic discharge–charge cycling of ‘Li_xTiO_y’ in the range 0.005–3.0 V at a current density of 65 mA g⁻¹ gives a capacity of 400 (±5) mAh g⁻¹ during 5–100 cycles with no noticeable capacity fading. This value corresponds to 1.52 mol of recycleable Li/Ti. The coulombic efficiency (η) is >98%. Results on ‘Li_xNbO_y’ show good reversibility of the electrode and a η >98% is achieved only after 10 cycles (range 0.005–3.0 V and at 30 mA g⁻¹) and a capacity of 180 (±5) mAh g⁻¹ (0.97 mol of Li/Nb) was stable up to 40 cycles. In both ‘Li_xTiO_y’ and ‘Li_xNbO_y’, the average discharge and charge voltages are 1.2–1.4 and 1.7–1.8 V, respectively. The impedance spectral data measured during the first cycle and after selected numbers of cycles are fitted to an equivalent circuit and the roles played by the relevant parameters as a function of cycle number are discussed.