Rapid microwave-assisted synthesis of graphene nanosheet/Co3O4 composite for supercapacitors

Graphene nanosheet (GNS)/Co₃O₄ composite has been rapidly synthesized by microwave-assisted method. Field emission scanning electron microscopy and transmission electron microscopy observation reveals the homogeneous distribution of Co₃O₄ nanoparticles (3-5 nm in size) on graphene sheets. Electrochemical properties are characterized by cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance spectroscopy. A maximum specific capacitance of 243.2 F g⁻¹ has been obtained at a scan rate of 10 mV s⁻¹ in 6 M KOH aqueous solution for GNS/Co₃O₄ composite. Furthermore, the composite exhibits excellent long cycle life along with ∼95.6% specific capacitance retained after 2000 cycle tests.     

Preparation and electrochemical capacitance of cobalt oxide (Co3O4) nanotubes as supercapacitor material

Cobalt oxide (Co₃O₄) nanotubes have been successfully synthesized by chemically depositing cobalt hydroxide in anodic aluminum oxide (AAO) templates and thermally annealing at 500 °C. The synthesized nanotubes have been characterized by scanning electron microscope (SEM), transmission electron microscope (TEM) and X-ray diffraction (XRD). The electrochemical capacitance behavior of the Co₃O₄ nanotubes electrode was investigated by cyclic voltammetry, galvanostatic charge-discharge studies and electrochemical impedance spectroscopy in 6 mol L⁻¹ KOH solution. The electrochemical data demonstrate that the Co₃O₄ nanotubes display good capacitive behavior with a specific capacitance of 574 F g⁻¹ at a current density of 0.1 A g⁻¹ and a good specific capacitance retention of ca. 95% after 1000 continuous charge-discharge cycles, indicating that the Co₃O₄ nanotubes can be promising electroactive materials for supercapacitor.     

Synthesis of LiNi0.9Co0.1O2 from Li2CO3, NiO or NiCO3, and CoCO3 or Co3O4 and their electrochemical properties

Cathode active materials with a composition of LiNi_0.9Co_0.1O₂ were synthesized by a solid-state reaction method at 850 °C using Li₂CO₃, NiO or NiCO₃, and CoCO₃ or Co₃O₄, as the sources of Li, Ni, and Co, respectively. Electrochemical properties, structure, and microstructure of the synthesized LiNi_0.9Co_0.1O₂ samples were analyzed. The curves of voltage vs. x in Li_xNi_0.9Co_0.1O₂ for the first charge–discharge and the intercalated and deintercalated Li quantity Δx were studied. The destruction of unstable 3b sites and phase transitions were discussed from the first and second charge–discharge curves of voltage vs. x in Li_xNi_0.9Co_0.1O₂. The LiNi_0.9Co_0.1O₂ sample synthesized from Li₂CO₃, NiO, and Co₃O₄ had the largest first discharge capacity (151 mA h/g), with a discharge capacity deterioration rate of −0.8 mA h/g/cycle (that is, a discharge capacity increasing 0.8 mA h/g per cycle).     

A delithiated LiNi0.65Co0.25Mn0.10O2 electrode material: A structural, magnetic and electrochemical study

A crystalline LiNi_0.65Co_0.25Mn_0.10O₂ electrode material was synthesized by the combustion method at 900 °C for 1 h. Rietveld refinement shows less than 3% of Li/Ni disorder in the structure. Lithium extraction involves only the Ni²⁺/Ni⁴⁺ redox couple while Co³⁺ and Mn⁴⁺ remain electrochemically inactive. No structural transition was detected during cycling in the whole composition range 0 < x < 1.0. Furthermore, the hexagonal cell volume changes by only 3% when all lithium was removed indicating a good mechanical stability of the studied compound. LiNi_0.65Co_0.25Mn_0.10O₂ has a discharge capacity of 150 mAh/g in the voltage range 2.5-4.5 V, but the best electrochemical performance was obtained with an upper cut-off potential of 4.3 V. Magnetic measurements reveal competing antiferromagnetic and ferromagnetic interactions - varying in strength as a function of lithium content - yielding a low temperature magnetically frustrated state. The evolution of the magnetic properties with lithium content confirms the preferential oxidation of Ni ions compared to Co³⁺ and Mn⁴⁺ during the delithiation process.     

Soft template synthesis of mesoporous Co3O4/RuO2·xH2O composites for electrochemical capacitors

Co₃O₄/RuO₂·xH₂O composites with various Ru content (molar content of Ru = 5%, 10%, 20%, 50%) were synthesized by one-step co-precipitation method. The precursors were prepared via adjusting pH of the mixed aqueous solutions of Co(NO₃)₂·6H₂O and RuCl₃·0.5H₂O by using Pluronic123 as a soft template. For the composite with molar ratio of Co:Ru = 1:1 annealed at 200 °C, Brunauer-Emmet-Teller (BET) results indicated that the composite showed mesoporous structure, and the specific surface area of the composite was as high as 107 m² g⁻¹. The electrochemical performances of these composites were measured in 1 M KOH electrolyte. Compared with the composite prepared without template, the composite with P123 exhibited a higher specific capacitance. When the molar content of Ru was rising, the specific capacitance of the composites increased significantly. It was also observed that the crystalline structures as well as the electrochemical activities were strongly dependent on the annealing temperature. A capacitance of 642 F/g was obtained for the composite (Co:Ru = 1:1) annealed at 150 °C. Meanwhile, the composites also exhibited good cycle stability. Besides, the morphologies and textural characteristic of the samples were also investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM).     

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.     

Influence of the tetravalent cation on the high-voltage electrochemical activity of LiNi0.5M1.5O4 spinel cathode materials

The electrochemical properties of substituted LiNi_0.5Mn_1.5−xM_xO₄ spinels at high potential (>4 V vs Li⁺/Li) have been investigated for M = Ti and Ru, in order to determine the role of the tetravalent cation in such systems where nickel is a priori the only electroactive species. These systems are found to form extended solid solutions (up to x = 1.3 and x = 1.0 for Ti and Ru, respectively) that were characterized by X-ray diffraction and Raman spectroscopy. Titanium substitution induces a drastic decrease in high potential electrochemical capacity, whereas the capacity is maintained and the kinetics are even improved in the presence of ruthenium. These results are completed by new results on the Li_4−2xNi_3xTi_5−xO₁₂ spinel system, which shows not any high potential activity in spite of the presence of up to 0.5 Ni²⁺ per spinel formula unit on the octahedral site. Taking into account previous data on LiNi_0.5Ge_1.5O₄, we clearly show that even if the tetravalent cation does not participate in the overall redox reaction, electrochemical activity is only possible when nickel is surrounded by tetravalent cations able to accept a local variation of valence (Mn, Ru), whereas full-shell cations such as Ti⁴⁺ and Ge⁴⁺ block the necessary electron transfer pathways in the spinel oxide electrode.     

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)     

Comparison of structure and electrochemistry of Al- and Fe-doped LiNi1/3Co1/3Mn1/3O2

LiNi_1/3Co_1/3−xM_xMn_1/3O₂ (M = Fe and Al; x = 0, 1/20, 1/9 and 1/6) have been synthesized by firing the co-precipitates of metal hydroxides. The impacts of Fe and Al doping on the structure and electrochemical performances of LiNi_1/3Co_1/3Mn_1/3O₂ are compared by means of powder X-ray diffraction (XRD), scanning electron microscopy (SEM), and galvanostatic charge/discharge test as cathode materials for lithium ion batteries. These materials keep the same layered structure as the LiNi_1/3Co_1/3Mn_1/3O₂ host. It is found that Fe- and Al-doped LiNi_1/3Co_1/3Mn_1/3O₂ show different characteristics in lattice parameter and cycling voltage plateau with increasing dopant dose. More interestingly, low Al doping (x < 1/20) improves the structural stability while Fe doping does not have such effect even at low Fe content.     

Enhanced thermoelectric properties induced by chemical pressure in Ca3Co4O9

We report the investigation of boron substitution on structural, electrical, thermal, and thermoelectric properties of Ca_3−xB_xCo₄O₉ (x=0, 0.5, 0.75, and 1) in the temperature range between 300 K and 5 K. X-ray diffraction studies show that the Ca₃Co₄O₉ phase is successfully preserved as the majority phase in the x=0.5 sample despite the small size of boron ions. Electrical transport measurements confirm that B³⁺ substitution for Ca²⁺ causes an increase in resistivity due to the decrease in carrier concentration. x=0.5 sample is found to have a Seebeck coefficient of 181 μV/K at room temperature which is ~1.5 times higher than that of the pure Ca₃Co₄O₉. Our results indicate that the chemical pressure due to the large ionic radii difference between B³⁺ (0.27 Å) and Ca²⁺ (1 Å) enhances the thermoelectric properties as long as the unique crystal structure of Ca₃Co₄O₉ is preserved.     

LiNi0.1Mn0.1Co0.8O2 electrode material: Structural changes upon lithium electrochemical extraction

We reported here on the synthesis, the crystal structure and the study of the structural changes during the electrochemical cycling of layered LiNi_0.1Mn_0.1Co_0.8O₂ positive electrode material. Rietveld refinement analysis shows that this material exhibits almost an ideal α-NaFeO₂ structure with practically no lithium-nickel disorder. The SQUID measurements confirm this structural result and evidenced that this material consists of Ni²⁺, Mn⁴⁺ and Co³⁺ ions.Unlike LiNiO₂ and LiCoO₂ conventional electrode materials, there was no structural modification upon lithium removal in the whole 0.42 ≤ x ≤1.0 studied composition range. The peaks revealed in the incremental capacity curve were attributed to the successive oxidation of Ni²⁺ and Co³⁺ while Mn⁴⁺ remains electrochemically inactive.     

Hydrothermal synthesis of Co3O4 microspheres as anode material for lithium-ion batteries

Co₃O₄ microspheres were synthesized in mass production by a simple hydrothermal treatment. One micrometer-sized spherical particles with well-crystallization could be obtained by XRD and SEM. Higher specific surface area (93.4 m² g⁻¹) and larger pore volume (78.4 cm³ g⁻¹) by BET measurements offered more interfacial bondings for extra sites of Li⁺ insertion, which resulted in the anomalous large initial irreversible capacity and capacity cycling loss due to SEI film formation. The capacity retention of Co₃O₄ microspheres involved first forming acted as Li-ion anode material is almost above 90% from 12th cycle and it retain lithium storage capacity of 550.2 mAh g⁻¹ after 25 cycles, which show good long-life stability. The electrochemical impedance spectroscopy (EIS) tests before and after cyclic voltammetry measurements and charge-discharge experiments were carried out and the corresponding D_Li values were also calculated. The relationship of the ac impedance spectra and the cycling behaviors was discussed. It is found that the decrease of capacity results from the larger Li⁺ charge-transfer impedance and the lower lithium-diffusion processes on cycling, which is in very good agreement with the electrochemical behaviors of Co₃O₄ electrode.     

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.     

Insignificant impact of designed oxygen release from high capacity Li[(Ni1/2Mn1/2)xCoy(Li1/3Mn2/3)1/3]O2 (x + y = 2/3) positive electrodes during the cycling of Li-ion cells

The properties of graphite/Li[(Ni_0.5Mn_0.5)_xCo_y(Li_1/3Mn_2/3)_1/3]O₂ (x + y = 2/3, y = 1/12 and 1/6) Li-ion cells are reported. There is an extended plateau near 4.5 V during the first charging of the cells that corresponds to the simultaneous removal of Li and oxygen from the Li[(Ni_0.5Mn_0.5)_xCo_y(Li_1/3Mn_2/3)_1/3]O₂ (x + y = 2/3, y = 1/12 and 1/6) electrodes. The release of this oxygen directly within a Li-ion cell has been a cause for concern. However, it was found that subsequent to O₂ release, Li-ion cells delivered a high reversible positive electrode specific capacity near 250 mAh/g at C/30 between 2.5 and 4.8 V, the cells did not display increased irreversible capacity relative to counterparts having Li metal negative electrodes and the cells retained 85% of their initial capacity after 70 cycles at C/6 between 2.5 and 4.6 V. Therefore, the O₂ released during the first charge does not significantly impact the electrochemical properties of graphite/Li[(Ni_0.5Mn_0.5)_xCo_y(Li_1/3Mn_2/3)_1/3]O₂ (x + y = 2/3) lithium-ion cells.     

Characterization of yttrium substituted LiNi0.33Mn0.33Co0.33O2 cathode material for lithium secondary cells

LiNi_0.33−xMn_0.33Co_0.33Y_xO₂ materials are synthesized by Y³⁺ substitute of Ni²⁺ to improve the cycling performance and rate capability. The influence of the Y³⁺ doping on the structure and electrochemical properties are investigated by means of X-ray diffraction (XRD), scanning electron microscope (SEM), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS) and galvanostatic charge/discharge tests. LiNi_0.33Mn_0.33Co_0.33O₂ exhibits the capacity retentions of 89.9 and 87.8% at 2.0 and 4.0 C after 40 cycles, respectively. After doping, the capacity retentions of LiNi_0.305Mn_0.33Co_0.33Y_0.025O₂ are increased to 97.2 and 95.9% at 2.0 and 4.0 C, respectively. The discharge capacity of LiNi_0.305Mn_0.33Co_0.33Y_0.025O₂ at 5.0 C remains 75.7% of the discharge capacity at 0.2 C, while that of LiNi_0.33Mn_0.33Co_0.33O₂ is only 47.5%. EIS measurement indicates that LiNi_0.305Mn_0.33Co_0.33Y_0.025O₂ electrode has the lower impedance value during cycling. It is considered that the higher capacity retention and superior rate capability of Y-doped samples can be ascribed to the reduced surface film resistance and charge transfer resistance of the electrode during cycling.     

New NiFe2−xCrxO4 spinel films for O2 evolution in alkaline solutions

Spinel-type ternary ferrites with composition NiFe_2−xCr_xO₄ (0 ≤ x ≤ 1) were synthesized by a precipitation method and their physicochemical and electrocatalytic properties have been investigated using IR, XRD, BET surface area, XPS, impedance and Tafel polarization techniques. The study indicated that substitution of Cr from 0.2 to 1.0 mol in the spinel matrix increased the apparent electrocatalytic activity of the base oxide towards the O₂ evolution reaction in 1 M KOH at 25 °C. The apparent electrocatalytic activity of the oxide with 0.8-1.0 mol Cr was found to be the greatest among the present series of oxides investigated. It is noteworthy that the electrocatalytic activity of the oxide with x = 0.8-1.0 was also greater than those of other spinel/perovskite O₂ evolving electrocatalysts reported in literature.     

On the spinel formation in Co1 − xO/Co2TiO4 composites via reactive sintering, exsolution and oxidation

The formation mechanism and microstructural development of the spinel phases in the Co_1 − xO/Co₂TiO₄ composites upon reactive sintering the Co_1 − xO and TiO₂ powders (9:1 molar ratio) at 1450 °C and during subsequent cooling in air were studied by X-ray diffraction and analytical electron microscopy. The Co₂TiO₄ spinel occurred as inter- and intragranular particles in the matrix of Ti-doped Co_1 − xO grains with a rock salt-type structure during reactive sintering. The submicron sized Co₂TiO₄ particles were able to detach from grain boundaries in order to reach an energetically favorable parallel orientation with respect to the host Co_1 − xO grains via a Brownian-type rotation/coalescence process. Upon cooling in air, secondary Co₂TiO₄ nanoparticles were precipitated and the Ti-doped Co_1 − xO host was partially oxidized as Co_3 − δO₄ spinel by rapid diffusion along the {1 1 1} and {1 0 0}-decorated interphase interface and the free surface of the composites.     

Enhanced electrochemical stability of Al-doped LiMn2O4 synthesized by a polymer-pyrolysis method

LiAl_xMn_2−xO₄ samples (x = 0, 0.02, 0.05, 0.08) were synthesized by a polymer-pyrolysis method. The structure and morphology of the LiAl_xMn_2−xO₄ samples calcined at 800 °C for 6 h were investigated by powder X-ray diffraction and scanning electron microscopy. The results show that all samples have high crystallinity, regular octahedral morphology and uniform particle size of 100-300 nm. The electrochemical performances were tested by galvanostatic charge-discharge and cyclic voltammetry. The results demonstrate that the Al-doped LiMn₂O₄ can be very well cycled at an elevated temperature of 55 °C without severe capacity degradation. In particular, the LiAl_0.08Mn_1.92O₄ sample demonstrates excellent capacity retention of 99.3% after 50 cycles at 55 °C, confirming the greatly enhanced electrochemical stability of LiMn₂O₄ by a small quantity of Al-doping.     

The negative and positive structural effects of Ga doping in the electrochemical performance of LiCoO2

The electrochemical performance of LiGa_yCo_1−yO₂ electrodes_,y = 0.005 and 0.1, was studied. Charge/discharge curves in galvanostatic condition were obtained for a series of voltages ranging from 4.35 to 4.7 V. Samples were previously characterized using X-ray diffraction (XRD), Fourier transform infrared (FTIR), X-ray absorption spectroscopy (XAS) and conductivity measurements. Comparison of data obtained for LiGa_yCo_1−yO₂ and the pristine oxide LiCoO₂ is provided. Ga doping at 0.5 mol% drastically reduces the electrode capacity fading when charges at 4.35 or 4.5 V are applied, while Ga doping at 10 mol% spoils the Ga-doped oxides electrochemical response, even at a cut-off limit of 4.35 V. These positive and negative effects were attributed to changes in both the electronic conductivity and local structure of LiCoO₂, induced by substitution of Co for Ga and cationic disorder. For y = 0.005, a slight distortion in the LiCoO₂ local structure occurs, and formation of states in the band gap that the raise oxide electronic conductivity is observed, explaining the electrochemical improvement in LiGa_yCo_1−yO₂. In contrast, hybridization with oxygen occurs when y = 0.1, opening LiCoO₂ band gap and making LiGa_yCo_1−yO₂ an insulator and a cationic disorder material that leads to poor electrochemical performance. Finally, XAS results show that Ga doping does not change the LiCoO₂ charge transfer mechanisms, where oxygen is reduced during lithium extraction.     

Impact of cobalt substitution for manganese on the structural and electrochemical properties of LiNi0.5Mn0.5O2

A series of LiNi_0.5Mn_0.5−xCo_xO₂ (0 ≤ x ≤ 0.5) compounds was prepared by a solid state reaction, and their structure, surface state and electrochemical characteristics were also investigated by XRD, XPS, EIS and charge-discharge cycling. The non-equivalent substitution of cobalt for manganese induced an increase in the average valence of nickel, thereby shrinking in the lattice volume. Moreover, Co non-equivalent substitution could not only reduce the impurity content but also significantly decreased the charge transfer resistance, thereby improving the rate capabilities.