The synthesis and properties of nanocrystalline electrode materials by mechanical alloying

In this work, we have experimentally studied the structure and electrochemical properties of nanocrystalline TiFe- and LaNi₅-type alloys. These materials were prepared by mechanical alloying (MA) followed by annealing. The properties of hydrogen host materials can be modified substantially by alloying to obtain the desired storage characteristics. It was found that the respective replacement of Fe in TiFe by Ni and/or by Cr, Co, Mo, Zr improved not only the discharge capacity but also the cycle life of these electrodes. In the nanocrystalline TiFe_0.25Ni_0.75, powder discharge capacity up to 155 mA h g⁻¹ was measured (at 40 mA g⁻¹ discharge current). On the other hand, a partial substitution of Ni by Al or Mn in LaNi_5−xM_x alloy leads to an increase in discharge capacity. The alloying elements such as Al, Mn and Co greatly improved the cycle life of LaNi₅ material. For example, in the nanocrystalline LaNi_3.75Mn_0.75Al_0.25Co_0.25 powder, discharge capacity up to 258 mA h g⁻¹ was measured (at 40 mA g⁻¹ discharge current). The studies show, that electrochemical properties of Ni–MH batteries are the function of the microstructure and the chemical composition of used electrode materials.     

Electrochemical characterization on layered lithium ruthenate for electrochemical supercapacitors

Electrochemical characteristics of lithium ruthenate (Li_xRuO_2+0.5x·nH₂O) for electrochemical capacitors' electrode material were first examined in this paper by cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic charge–discharge tests. Results show that Li_xRuO_2+0.5x·nH₂O has electrochemical capacitive characteristic within the potential range of − 0.2–0.9 V (vs. SCE) in 1 M Li₂SO₄ solution. The capacitance mainly arises from pseudo-capacitance caused by lithium ions' insertion/extraction into/out of the Li_xRuO_2+0.5x·nH₂O electrode. The specific capacitance of 391 F g^− 1 can be delivered at 1 mA charge–discharge current for Li_xRuO_2+0.5x·nH₂O electrode with an energy density of 65.7 W h kg^− 1. This material also exhibits an excellent cycling performance and there is no attenuation of capacitance over 600 cycles.     

Silver-coated LiVPO4F composite with improved electrochemical performance as cathode material for lithium-ion batteries

Nano-structured LiVPO₄F/Ag composite cathode material has been successfully synthesized via a sol–gel route. The structural and physical properties, as well as the electrochemical performance of the material are compared with those of the pristine LiVPO₄F. X-ray diffraction (XRD) and scanning electron microscopy (SEM) reveal that Ag particles are uniformly dispersed on the surface of LiVPO₄F without destroying the crystal structure of the bulk material. An analysis of the electrochemical measurements show that the Ag-modified LiVPO₄F material exhibits high discharge capacity, good cycle performance (108.5 mAh g⁻¹ after 50th cycles at 0.1 C, 93% of initial discharge capacity) and excellent rate behavior (81.8 mAh g⁻¹ for initial discharge capacity at 5 C). The electrochemical impedance spectroscopy (EIS) results reveal that the adding of Ag decreases the charge-transfer resistance (R_ct) of LiVPO₄F cathode. This study demonstrates that Ag-coating is a promising way to improve the electrochemical performance of the pristine LiVPO₄F for lithium-ion batteries cathode material.     

Rapid synthesis of LiCr0.15Mn1.85O4 by glycine–nitrate method

LiCr_0.15Mn_1.85O₄ spinel has been successfully synthesized by glycine–nitrate method (GNM). The presence of pure spinel phase was confirmed by long term XRPD measurements and the Rietveld structural refinement. Lattice parameter was estimated to be 8.2338 Å. Average particle size of prepared powder material is below 500 nm. The BET surface area is 9.6 m² g^− 1. As a cathode material for lithium batteries LiCr_0.15Mn_1.85O₄ shows initial discharge capacity of 110 mA h g^− 1 and capacity retention of 83% after 50 cycles.     

Structural and electrochemical properties of LiNi0.5Mn0.5 − xAlxO2 (x = 0, 0.02, 0.05, 0.08, and 0.1) cathode materials for lithium-ion batteries

Layered LiNi_0.5Mn_0.5 ? xAl_xO₂ (x = 0, 0.02, 0.05, 0.08, and 0.1) series cathode materials for lithium-ion batteries were synthesized by a combination technique of co-precipitation and solid-state reaction, and the structural, morphological, and electrochemical properties were examined by XRD, FT-IR, XPS, SEM, CV, EIS, and charge–discharge tests. It is proven that the aliovalent substitution of Al for Mn promoted the formation of LiNi_0.5Mn_0.5 ? xAl_xO₂ structures and induced an increase in the average oxidation number of Ni, thereby leading to the shrinkage of the lattice volume. Among the LiNi_0.5Mn_0.5 ? xAl_xO₂ materials, the material with x = 0.05 shows the best cyclability and rate ability, with discharge capacities of 219, 169, 155, and 129 mAh g^? 1 at 10, 100, 200, and 400 mA g^? 1 current density respectively. Cycled under 40 mA g^? 1 in 2.8–4.6 V, LiNi_{0. 5}Mn_0.45Al_0.05O₂ shows the highest discharge capacity of about 199 mAh g^? 1 for the first cycle, and 179 mAh g^? 1 after 40 cycles, with a capacity retention of 90%. EIS analyses of the electrode materials at pristine state and state after first charge to 4.6 V indicate that the observed higher current rate capability of LiNi_{0. 5}Mn_0.45Al_0.05O₂ can be understood due to the better charge transfer kinetics.     

Controllable synthesis of layered Co–Ni hydroxide hierarchical structures for high-performance hybrid supercapacitors

A facile solvothermal method is developed for synthesizing layered Co–Ni hydroxide hierarchical structures by using hexamethylenetetramine (HMT) as alkaline reagent. The electrochemical measurements reveal that the specific capacitances of layered bimetallic (Co–Ni) hydroxides are generally superior to those of layered monometallic (Co, Ni) hydroxides. The as-prepared Co_0.5Ni_0.5 hydroxide hierarchical structures possesses the highest specific capacitance of 1767 F g⁻¹ at a galvanic current density of 1 A g⁻¹ and an outstanding specific capacitance retention of 87% after 1000 cycles. In comparison with the dispersed nanosheets of Co–Ni hydroxide, layered hydroxide hierarchical structures show much superior electrochemical performance. This study provides a promising method to construct hierarchical structures with controllable transition-metal compositions for enhancing the electrochemical performance in hybrid supercapacitors.     

Inorganic polymer phosphazene disulfide as cathode material for rechargeable lithium batteries

Novel inorganic network polymer phosphazene disulfide [(NPS₂)₃]_n was synthesized by a solution cross-link method. IR and element content analysis confirmed the polymer's molecular structure. The polymer has an average particle size of d_0.5 = 7.7 μm and the specific surface area is 57.4 m² g^− 1. TG/DTA analysis showed that [(NPS₂)₃]_n underwent a decomposition reaction from 200 to 300 °C. When used as cathode material in lithium batteries, its initial discharge capacity was 459.1 mAh g^− 1, which is almost 93.5% of theoretical specific capacity (490.9 mAh g^− 1). After 30 charge–discharge cycles, the discharge capacity of [(NPS₂)₃]_n stabilized at approximately 400.1 mAh g^− 1 which revealed an excellent cyclic ability. Therefore [(NPS₂)₃]_n is of great potential as cathode material for secondary lithium batteries.     

SnO2-decorated multiwalled carbon nanotubes and Vulcan carbon through a sonochemical approach for supercapacitor applications

Multiwalled carbon nanotubes (MWCNTs) and Vulcan carbon (VC) decorated with SnO₂ nanoparticles were synthesized using a facile and versatile sonochemical procedure. The as-prepared nanocomposites were characterized by means of transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and Fourier transform infra red spectroscopy. It was evidenced that SnO₂ nanoparticles were uniformly distributed on both carbon surfaces, tightly decorating the MWCNTs and VC. The electrochemical performance of the nanocomposites was evaluated by cyclic voltammetry and galvanostatic charge/discharge cycling. The as-synthesized SnO₂/MWCNTs nanocomposites show a higher capacity than the SnO₂/VC nanocomposites. Concretely, the SnO₂/MWCNTs electrodes exhibit a specific capacitance of 133.33 F g⁻¹, whereas SnO₂/VC electrodes exhibit a specific capacitance of 112.14 F g⁻¹ measured at 0.5 mA cm⁻² in 1 M Na₂SO₄.     

Rechargeability improvement of δ-type chemical manganese dioxide with the co-doping of Bi and Ni in alkaline electrolyte

δ-MnO₂ with the doping of Ni and Bi was prepared through a simple chemical precipitation/oxidation method. Its structure was confirmed by the X-ray diffraction tests. The results of cyclic voltammetry and galvanostatic charge–discharge tests showed that both the doping of Bi and Ni benefited the electrochemical activity of the MnO₂ electrode. Compared to the un-doped electrode, the Bi-doped one showed larger discharge capacity and the Ni-doped one showed higher discharge potential and better cycleability. With the co-doping of 5 wt% Bi and 10 wt% Ni, the discharge capacity of the MnO₂ electrode reached 252 mA h g^?1 at a 0.2C rate and 116 mA h g^?1 at a 1C rate, respectively. Its capacity remained in 105 mA h g^?1 after 50 cycles at a 1C rate, but the capacity of a commercial electrolytic MnO₂ electrode was only 37 mA h g^?1.     

Sonochemical synthesis of LiNi0.5Mn1.5O4 and its electrochemical performance as a cathode material for 5 V Li-ion batteries

LiNi_0.5Mn_1.5O₄ was synthesized as a cathode material for Li-ion batteries by a sonochemical reaction followed by annealing, and was characterized by XRD, SEM, HRTEM and Raman spectroscopy in conjunction with electrochemical measurements. Two samples were prepared by a sonochemical process, one without using glucose (sample-S1) and another with glucose (sample-S2). An initial discharge specific capacity of 130 mA h g⁻¹ is obtained for LiNi_0.5Mn_1.5O₄ at a relatively slow rate of C/10 in galvanostatic charge–discharge cycling. The capacity retention upon 50 cycles at this rate was around 95.4% and 98.9% for sample-S1 and sample-S2, respectively, at 30 °C.     

Preparation and electrochemical properties of mesoporous Co3O4 crater-like microspheres as supercapacitor electrode materials

Mesoporous Co₃O₄ microspheres with unique crater-like morphology were obtained by utilizing the mesoporous silica material MCM-41 as a template. The analysis results of N₂ adsorption–desorption measurement indicate that the product has a large Brunauer–Emmett–Teller (BET) surface area of 60 m² g⁻¹ and a narrow pore size distribution centering around 3.7 nm. Its electrochemical properties were investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements. The findings reveal that this novel morphology material has a smaller inner resistance of about 0.4 Ω and a higher onset frequency of 550 Hz. This material can provide a high specific capacitance of 102 F g⁻¹ and a large capacity retention of 74% in 500 continuous cycles test at a sweep rate of 3 mV s⁻¹. More significantly, the mass loading of electroactive species can reach as large as 2 mg cm⁻², which is one order of magnitude larger than common amount used.     

Effects of Ca doping on the electrochemical properties of LiNi0.8Co0.2O2 cathode material

LiNi_0.8Co_0.2O₂ and Ca-doped LiNi_0.8Co_0.2O₂ cathode materials were synthesized via a rheological phase reaction method. It is found that the Ca doping significantly improves reversible capacity, cycling performance, thermal stability and rate capability. The Ca-doped LiNi_0.8Co_0.2O₂ cathode material maintains nearly its initial discharge capacity up to 100 cycles at room temperature. It also delivers an initial discharge capacity of 183 mA h g^− 1 and still keeps 131 mA h g^− 1 even after 120 cycles at 60 °C. These results, together with the X-ray diffraction and electrochemical impedance spectroscopy analysis, reveal that Ca²⁺ ions occupy Li⁺ ion sites to form Ca_Li defects and lithium vacancies (V_Li′), which reduce the resistance and increases conductivity of LiNi_0.8Co_0.2O₂.     

Preparation of carbon nanoparticles from electrolysis of molten carbonates and use as anode materials in lithium-ion batteries

The electrochemical reduction of molten Li–Na–K carbonates at 450 °C provides “quasi-spherical” carbon nanoparticles with size comprised between 40 and 80 nm (deduced from AFM measurements). XRD analyses performed after washing and heat-treatment at various temperatures have revealed the presence of graphitised and amorphous phases. The d₀₀₂ values were close to the ideal one obtained for pure graphite. Raman spectroscopy has pointed out surface disordering which increases with increasing temperature of the heat-treatment. The presence of Na and Li on the surface of the carbon powder has been evidenced by SIMS. The maximum Na and Li contents were observed for carbon samples heat-treated at 400 °C. Their electrochemical performances vs. the insertion/deinsertion of lithium cations were studied in 1 M LiPF₆–EC : DEC : DMC (2 : 1 : 2). The first charge–discharge cycle is characterised by a high irreversible capacity as in the case of hard-disordered carbon materials. However, the potential profile in galvanostatic mode is intermediate between that usually observed for graphite and amorphous carbon: rather continuous charge–discharge curves sloping between 1.5 and 0.3 V vs. Li / Li⁺, and successive phase transformations between 0.3 and 0.02 V vs. Li / Li⁺. The best electrochemical performances were obtained with carbon powders heat-treated at 400 °C which exhibits a reversible capacity value of 1080 mAh g^− 1 (composition of Li_2.9C₆). This sample has also both the lowest surface disordering (deduced from Raman spectroscopy), and the highest Na and Li surface contents (deduced from SIMS).     

High surface area monodispersed Fe3O4 nanoparticles alone and on physical exfoliated graphite for improved supercapacitors

Highly conductive, unsophisticated and easy to be obtained physical exfoliated graphite (PHG) supporting well dispersed magnetite, Fe₃O₄/PHG nanocomposite, has been prepared by a one-step chemical strategy and physico-chemical characterized. The nanocomposite, favoured by the a-polar nanoparticles (NPs) capping, results in a self-assembled monolayer of monodispersed Fe₃O₄, covering perfectly the hydrophobic surfaces of PHG. The nanocomposite as an electrode material was fabricated into a supercapacitor and characterized by cyclic voltammetry (CV) and galvanostatic charge–discharge measurements. It shows, after a suitable annealing, significant electrochemical properties (capacitance value of 787 F/g at 0.5 A g⁻¹ and a Fe₃O₄/PHG weight ratio of 0.31) and good cycling stability (retention 91% after 30,000 cycles). Highly monodispersed very fine Fe₃O₄ NPs, covered by organic chains, have been also synthesized. The high surface area Fe₃O₄ NPs, after washing to leave a low content of organic chains able to avoid aggregation without excessively affecting the electrical properties of the material, exhibit remarkable pseudocapacitive activities, including the highest specific capacitance over reported for Fe₃O₄ (300 F/g at 0.5 A g⁻¹).     

Local electronic structure of LiMn2O4 probed by solid state 7Li-NMR

Crystalline LiMn₂O₄ materials were synthesized through a low temperature reflux (LTR) process. The LTR material exhibits an essential electrochemical behavior to present only one transition during the charge/discharge process and gives promises for the future application. The X-ray data hardly distinguishes either conventional high temperature (HT) or LTR material since both materials possess similar molecular structure. However, solid-state NMR results suggest substantial difference for their electronic structure. The ⁷Li-NMR of HT material exhibits the isotropic peak at 529 ppm and spreads for 160 kHz wide indicating the interaction between the lithium nucleus and the conducting electrons around the nucleus. The LTR material presents the main isotropic peak at 217 ppm but spreads for almost 400 kHz wide asymmetrically implying the existence of multiple electronic environments.     

Spray pyrolyzed NiO–C nanocomposite as an anode material for the lithium-ion battery with enhanced capacity retention

NiO–C nanocomposite was prepared by a spray pyrolysis method using a mixture of Ni(NO₃)₂ and citric acid solution at 600 °C. The microstructure and morphology of the NiO–C composite were characterized by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS) mapping, and thermogravimetric analysis (TGA). The results showed that the NiO nanoparticles were surrounded by amorphous carbon. Electrochemical tests demonstrated that the NiO–C nanocomposites exhibited better capacity retention (382 mAh g^− 1 for 50 cycles) than that of pure NiO (141 mAh g^− 1 for 50 cycles), which was also prepared by spray pyrolysis using only Ni(NO₃)₂ as precursor. The enhanced capacity retention can be mainly attributed to the NiO–C composite structure, composed of NiO nanoparticles surrounded by carbon, which can accommodate the volume changes during charge–discharge and improve the electrical conductivity between the NiO nanoparticles.     

Rapid preparation of high electrochemical performance LiFePO4/C composite cathode material with an ultrasonic-intensified micro-impinging jetting reactor

A joint chemical reactor system referred to as an ultrasonic-intensified micro-impinging jetting reactor (UIJR), which possesses the feature of fast micro-mixing, was proposed and has been employed for rapid preparation of FePO₄ particles that are amalgamated by nanoscale primary crystals. As one of the important precursors for the fabrication of lithium iron phosphate cathode, the properties of FePO₄ nano particles significantly affect the performance of the lithium iron phosphate cathode. Thus, the effects of joint use of impinging stream and ultrasonic irradiation on the formation of mesoporous structure of FePO₄ nano precursor particles and the electrochemical properties of amalgamated LiFePO₄/C have been investigated. Additionally, the effects of the reactant concentration (C = 0.5, 1.0 and 1.5 mol L⁻¹), and volumetric flow rate (V = 17.15, 51.44, and 85.74 mL min⁻¹) on synthesis of FePO₄·2H₂O nucleus have been studied when the impinging jetting reactor (IJR) and UIJR are to operate in nonsubmerged mode. It was affirmed from the experiments that the FePO₄ nano precursor particles prepared using UIJR have well-formed mesoporous structures with the primary crystal size of 44.6 nm, an average pore size of 15.2 nm, and a specific surface area of 134.54 m² g⁻¹ when the reactant concentration and volumetric flow rate are 1.0 mol L⁻¹ and 85.74 mL min⁻¹ respectively. The amalgamated LiFePO₄/C composites can deliver good electrochemical performance with discharge capacities of 156.7 mA h g⁻¹ at 0.1 C, and exhibit 138.0 mA h g⁻¹ after 100 cycles at 0.5 C, which is 95.3% of the initial discharge capacity.     

Structural and electrochemical properties of layered lithium nitridocuprates Li3 − xCuxN

We report a systematic study of the layered lithium nitridocuprates Li_3 ? xCu_xN with 0.1 ≤ x ≤ 0.39. The structural data obtained from experimental XRD patterns, Rietveld refinements and unit cell parameters calculation vs x, indicate that copper (I) substitute interlayer lithium ions in the parent nitride Li₃N to form the Li_3 ? xCu_xN compound without any Li vacancy in the Li₂N^? layer. Electrochemical results report Li insertion into the corresponding layered structures cannot take place in the 1.2/0.02 V voltage range as in the case of lithium into nitridonickelates and nitridocobaltates. However, in the initial charge process of Li_3 ? xCu_xN at 1.4 V leading to a specific capacity higher than 1000 mA h/g, the oxidation of copper and nitride ions is probably involved inducing a strong structural disordering process. As a consequence a new rechargeable electrochemical system characterized by discharge–charge potential of ≈ 0.3 V/1.2 V appears from the second cycle. Cycling experiments 0.02 V voltage/0.02 V range induce a complete destruction of the layered host lattice and the presence of Cu₃N in the charge state suggests a conversion reaction. The capacity recovered in the 1.4/0.02 V range practically stabilizes around 500 mA h/g after 20 cycles.     

Preparation and electrochemical investigation of the cobalt hydroxide carbonate/activated carbon nanocomposite for supercapacitor applications

Cobalt hydroxide carbonate/activated carbon (AC) composite was successfully synthesized by hydrothermal method. Morphological characterizations of cobalt hydroxide carbonate/AC composite were carried out by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and the results show that the cobalt hydroxide carbonate nanorods are well dispersed on the AC. Due to the synergistic effects arising from cobalt hydroxide carbonate nanorods and AC, the electrochemical performances of pure cobalt hydroxide carbonate material is significantly improved by the addition of AC. The composite shows a specific capacitance of 301.44 F g⁻¹ at a current density of 1 A g⁻¹ in 6 M KOH electrolyte and exhibits good cycling stability. Based on the above results, the cobalt hydroxide carbonate/AC composite shows a considerable promise as electrode for electrochemical applications.     

Device linearity improvement and current enhancement utilizing high-to-low doping-channel FETs

We report device linearity improvement and current enhancement in both a heterostructure FET (HFET) and a camel-gate FET (CAMFET) using InGaAs/GaAs high-low and GaAs high-medium-low doped channels, respectively. In an HFET, a low doped GaAs layer was employed to build an excellent Schottky contact. In a GaAs CAMFET, a low doped layer together withn⁺andp⁺layers formed a high-performance majority camel-diode gate. Both exhibit high effective potential barriers of >1.0 V and gate-to-drain breakdown voltages of >20.0 V (atI_g=1.0 mA mm⁻¹). A thin, high doped channel was used to enhance current drivability and to improve the transconductance linearity. A 2×100 μm²HFET had a peak transconductance of 230 mS mm⁻¹and a current density greater than 800 mA mm⁻¹. The device had a transconductance of more than 80 percent of the peak value over a wide drain current range of 200 to 800 mA mm⁻¹. A 1.5×100 μm²CAMFET had a peak transconductance of 220 mS mm⁻¹and a current density greater than 800 mA mm⁻¹. Similarly, the device had a transconductance of more than 80 percent of the peak value over a wide drain current range of 160 to 800 mA mm⁻¹. The improvement of device linearity and the enhancement of current density suggest that high-to-low doped-channel devices for both an HFET and a CAMFET are suitable for high-power large signal circuit applications.