High capacity Si/C nanocomposite anodes for Li-ion batteries

Nanocomposites of Si/C were synthesized from Si and polystyrene (PS) resin using high-energy mechanical milling (HEMM) followed by subsequent heat-treatment. The resultant nanocomposites are comprised of amorphous carbon and nanocrystalline silicon as verified by X-ray diffraction (XRD). The XRD results also indicate the presence of iron silicide (FeSi) arising as a contaminant during HEMM. The Si/C nanocomposite corresponding to Si:C = 1:2 composition obtained after milling in two stages of 12 h each for a total time period of 24 h shows a capacity as high as ∼850 mAh/g with reasonable capacity retention (∼1.1% loss/cycle). The increase in either heat-treatment temperature or milling time renders the nanocomposites more stable at the expense of capacity. Transmission electron microscopy (TEM) analysis shows that the HEMM derived Si nanocrystallites <50 nm in size are distributed homogeneously within the amorphous carbon matrix.     

Reversible high capacity nanocomposite anodes of Si/C/SWNTs for rechargeable Li-ion batteries

Nanocomposites comprising silicon (Si), graphite (C) and single-walled carbon nanotubes (SWNTs), denoted as Si/C/SWNTs, have been synthesized by dispersing SWNTs via high power ultrasonication into a pre-milled Si/C composite mixture, followed by subsequent thermal treatment. The Si/C composite powder was prepared by high-energy mechanical milling (HEMM) of elemental Si and graphite using polymethacrylonitrile (PMAN) as a diffusion barrier suppressing the possible mechanochemical reaction between silicon and graphite to form SiC, and further prevent the amorphization of graphite during extended milling. A nanocomposite with nominal composition of Si-35 wt.% SWNTs-37 wt.% exhibits a reversible discharge capacity of ∼900 mAh g⁻¹ with an excellent capacity retention of capacity loss of 0.3% per cycle up to 30 cycles. Functionalization of the SWNTs with LiOH significantly improves the cyclability of the nanocomposite containing Si-45 wt.% SWNTs-28 wt.% exhibiting a reversible capacity of 1066 mAh g⁻¹ and displaying almost no fade in capacity up to 30 cycles. The improved electrochemical performance is hypothesized to be attributed to the formation of a nanoscale conductive network by the dispersed SWNTs which leads to successful maintenance of good electrical contact between the electrochemically active particles during cycling.     

Magnesium silicide as a negative electrode material for lithium-ion batteries

Mg₂Si was synthesized by mechanically activated annealing and evaluated as a negative electrode material. A maximum discharge capacity of 830 mAh/g was observed by cycling over a wide voltage window of 5–650 mV versus Li, but capacity fade was rapid. Cycling over the range 50–225 mV versus Li produced a stable discharge capacity of approximately 100 mAh/g. X-ray diffraction (XRD) experiments showed that lithium insertion converts Mg₂Si into Li₂MgSi after lithium intercalation into Mg₂Si. Electrochemical evidence of Li–Si reactions indicated that the Li₂MgSi structure can be converted to binary lithium alloys with extensive charging.     

Silicon,graphite and resin based hard carbon nanocomposite anodes for lithium ion batteries

Nanocomposite based on graphite (C), silicon (Si) and poly[(o-cresyl glycidyl ether)-co-formaldehyde] resin based amorphous hard carbon (HC), denoted as Si/C/HC, have been synthesized by thermal treatment of mechanically milled graphite, silicon and resin of nominal composition C–18 wt.% Si–40 wt.% resin at 973 K, 1073 K and 1173 K in ultrahigh purity argon atmosphere. The formation of the electrochemically inactive SiC is bypassed as well as the amorphization kinetics of graphite is reduced during prolonged milling of graphite and Si in the presence of the resin. Microstructural analysis has confirmed that the Si nanoparticle gets embedded, and is homogeneously dispersed and distributed on the graphite matrix after mechanical milling as well as after thermal treatment. Electrochemical studies have revealed that the Si/C/HC based nanocomposite, tested as a lithium ion anode, synthesized after thermal treatment at 1173 K exhibits a stable capacity of ∼640 mAh g⁻¹ with an excellent capacity retention when cycled at a rate of ∼160 mA g⁻¹. The nanocomposite anode also shows a moderate rate capability when cycled at different discharge/charge rates. Scanning electron microscopy analysis indicates that the structural integrity and the microstructural stability of the nanocomposite during the alloying/dealloying process contribute to the good cyclability observed in the above nanocomposites.     

Silicon and carbon based composite anodes for lithium ion batteries

Composites comprising silicon (Si), graphite (C) and polyacrylonitrile-based disordered carbon (PAN-C), denoted as Si/C/PAN-C, have been synthesized by thermal treatment of mechanically milled silicon, graphite, and polyacrylonitrile (PAN) powder of nominal composition C–17.5 wt.% Si–30 wt.% PAN. PAN acts as a diffusion barrier to the interfacial diffusion reaction between graphite and Si to form the electrochemically inactive SiC during prolonged milling of graphite and Si, which could be easily formed in the absence of PAN. In addition, graphite, which contributes to the overall capacity of the composite and suppresses the irreversible loss, retains its graphitic structure during prolonged milling in the presence of PAN. The resultant Si/C/PAN-C based composites exhibit a reversible capacity of ∼660 mAh g⁻¹ with an excellent capacity retention displaying almost no fade in capacity when cycled at a rate of ∼C/4. Scanning electron microscopy (SEM) analysis indicates that the structural integrity and microstructural stability of the composite during the alloying/dealloying process appear to be the main reasons contributing to the good cyclability observed in the above composites.     

Nanosized silicon-based composite derived by in situ mechanochemical reduction for lithium ion batteries

Composite consisting of nanosized silicon, Li₄SiO₄ and other lithium-rich components was synthesized using high energy mechanical milling (HEMM) method. The reactive milling of SiO with lithium metal resulted in the oxidation of lithium and silicon, and reduction of SiO. X-ray diffraction (XRD) and high-resolution transmission electron microscope (HRTEM) were used to determine the phases present in the composite. In addition, cyclic voltammetry (CV), along with constant current discharge/charge tests, was used to characterize the electrochemical properties of the resultant material. Compared with pure SiO and pure silicon as anode materials, the as-prepared composite demonstrated larger capacity and superior cyclability even at high C-rate.     

Synthesis and characterization of electrochemically active graphite–silicon–tin composite anodes for Li-ion applications

Electrochemically active Si_0.66Sn_0.34 (SiSn) composite alloys dispersed in a carbon (graphite) matrix were synthesized using both wet and dry high-energy mechanical milling (HEMM). The resultant composites are comprised of amorphous carbon (in the case of dry HEMM) or crystalline carbon (in the case of wet HEMM), and crystalline silicon and tin (for both cases) as verified by X-ray diffraction (XRD). The XRD results also indicate the presence of iron–tin intermetallic (FeSn₂) arising as a contaminant during dry HEMM. The composite composition of 85C–15[Si_0.66Sn_0.34] (mol%) resulted in reversible discharge capacities as high as 800 mAh g⁻¹ with a reasonable capacity retention (1.36% loss/cycle). Scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) analyses were further conducted to examine the surface of the electrode and to determine the presence/absence of organic species resulting from reactions between the electrode, lithium ions and electrolyte, respectively.     

An investigation of the origin of the electrochemical hydrogen storage capacities of the ball-milled Co–Si composites

The Co–Si composites with a molar ratio of 2:1 are synthesized by ball-milling method and their potential as negative electrode materials of Ni–MH batteries is investigated. The microstructure, morphology and chemical state of the ball-milled Co–Si composites are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). XRD patterns show that the ball-milled samples for 10 and 20 h contain Co, Si and Co₂Si phases, and the ball-milled samples for 40 and 60 h are mainly amorphous Co₂Si alloys. In contrast to the high initial discharge capacity (1012 mAh/g) obtained for the sample ball-milled for 10 h, the discharge capacities of the samples ball-milled for 40 and 60 h are very low. It indicates that the hydrogen storage capacity of pure Co₂Si alloy is very low. It is found that the formation of active Co nanoparticles and Si oxidation are responsible for the high values of the initial discharge capacities of the ball-milled samples for 10 and 20 h. However, after the first cycle, the discharge capacities of the composites drop below 300 mAh/g. Based on XRD and cyclic voltammetric results, the remaining discharge capacity is mainly contributed by the conversion reaction of Co/Co(OH)₂.     

Enhanced electrochemical properties of nanostructured bismuth-based composites for rechargeable lithium batteries

Nanostructured Bi/C and Bi/Al₂O₃/C composites, prepared by high-energy mechanical milling (HEMM), are investigated as anode materials for Li-ion rechargeable batteries. X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) reveal that the Bi/C nanocomposite is composed of nano-sized Bi and amorphous C, while the Bi/Al₂O₃/C nanocomposite (obtained by the mechanochemical reduction of Bi₂O₃ and Al) is composed of nano-sized Bi, amorphous Al₂O₃, and amorphous C. The electrochemical reaction mechanism of the Bi/C nanocomposite electrode is identified by ex situ XRD analyses combined with a differential capacity plot. Electrochemical tests show that the Bi/C and Bi/Al₂O₃/C nanocomposites exhibit enhanced electrochemical performances compared with that of the pure Bi electrode.     

Electrochemical hydrogen storage property of Co–S alloy prepared by ball-milling method

A series of Co–S alloys were synthesized by means of ball milling of Co and S powders at different hours and investigated as the negative material for Ni/MH batteries. The structures and surface configuration of the alloys were characterized by XRD and TEM. The electrochemical measurements demonstrated that the Co–S particles showed excellent electrochemical reversibility and considerably high charge–discharge capacity. Among the alloys, the Co–S alloy milled 20 h showed relatively high discharge capacity and excellent cycling stability at discharge current density 25 mA/g. Its highest discharge capacity was about 350 mAh/g and remained 300 mAh/g after 100 cycles, the capacity retention rate was about 86%. The hydrogen storage mechanism was studied by XRD and TPD measurements.     

Preparation and lithium storage performances of g-C3N4/Si nanocomposites as anode materials for lithium-ion battery

As the anode material of lithium-ion battery, silicon-based materials have a high theoretical capacity, but their volume changes greatly in the charging and discharging process. To ameliorate the volume expansion issue of silicon-based anode materials, g-C₃N₄/Si nanocomposites are prepared by using the magnesium thermal reduction technique. It is well known that g-C₃N₄/Si nanocomposites can not only improve the electronic transmission ability, but also ameliorate the physical properties of the material for adapting the stress and strain caused by the volume expansion of silicon in the lithiation and delithiation process. When g-C₃N₄/Si electrode is evaluated, the initial discharge capacity of g-C₃N₄/Si nanocomposites is as high as 1033.3 mAh/g at 0.1 A/g, and its reversible capacity is maintained at 548 mAh/g after 400 cycles. Meanwhile, the improved rate capability is achieved with a relatively high reversible specific capacity of 218 mAh/g at 2.0 A/g. The superior lithium storage performances benefit from the unique g-C₃N₄/Si nanostructure, which improves electroconductivity, reduces volume expansion, and accelerates lithium-ion transmission compared to pure silicon.     

Superior catalytic effect of titania - porous carbon composite for the storage of hydrogen in MgH2 and lithium in a Li ion battery

A novel nanocomposite (0.2TiO₂ + AC) with two promising applications is demonstrated, (i) as an additive for promoting hydrogen storage in magnesium hydride, (ii) as an active electrode material for hosting lithium in Li ion batteries (surface area of activated carbon (AC): 491 m²/g, pore volume: 0.252 cc/g, size of TiO₂ particles: 20–30 nm). Transmission electron microscopy study provides evidence that well dispersed TiO₂ nanoparticles are enclosed by amorphous carbon nets. A thermogravimetry-differential scanning calorimetry (TG-DSC) study proves that the nanocomposite is thermally stable up to ∼400 °C. Volumetric hydrogen storage tests and DSC studies further prove that a 3 wt% of 0.2TiO₂+AC nanocomposite as additive not only lowers the dehydrogenation temperature of MgH₂ over 100 °C but also maintains the performance consistency. Moreover, as a working electrode for Li ion battery, 0.2TiO₂+AC offers a reversible capacity of 400 mAh/g at the charge/discharge rate of 0.1C and consistent stability up to 43 cycles with the capacity retention of 160 mAh/g at 0.4C. Such cost effective-high performance materials with applications in two promising areas of energy storage are highly desired for progressing towards sustainable energy development.     

Thin-film crystalline silicon mini-modules using porous Si for layer transfer

Layer transfer processes yield Si films of high electronic quality on low-cost non-Si carriers such as glass: a crystalline Si film grows on a Si–substrate wafer, is detached from that substrate and transferred to a low-cost non-Si device carrier. The substrate wafer is re-used for further growth cycles. Electrochemical etching of a porous Si (PSI) layer system into the substrate wafer enables homoepitaxial growth of monocrystalline Si films and facilitates the detachment of the film. We discuss the potential of crystalline thin-film cells from layer transfer and review the layer transfer work conducted at ZAE Bayern. The sevenfold use of a substrate wafer and the transfer of a 10 × 10 cm² epitaxial film from a 6″-wafer is demonstrated. A new module process that permits an integrated series connection by a single metallization step is demonstrated to yield a module efficiency of 10%.     

Electrochemical behavior of nanocrystalline TiNi doped by MWCNTs and Pd

Mechanical alloying process was introduced to produce nanocrystalline TiNi alloy. X-ray diffraction (XRD) analysis showed that, after 8 h of milling, the starting mixture of elements was decomposed into an amorphous phase. XRD confirmed formation of CsCl-type structure after annealing at 750 °C for 0.5 h. Atomic force microscopy observations revealed that 70% of grains had size below 100 nm. TiNi electrode alloy with and without palladium and/or multiwalled carbon nanotubes (MWCNTs) was prepared by ball co-milling. Scanning electron microscopy observations showed that after co-milling with 5 wt.% MWCNTs, particles size of TiNi alloy decreased. The TiNi + 5 wt.% Pd + 5 wt.% MWCNTs nanocomposite showed the highest discharge capacity (266 mAh/g at 3rd cycle). Addition of MWCNTs improved the electrode cycle stability.     

Enhancement of the electrochemical performance of free-standing graphene electrodes with manganese dioxide and ruthenium nanocatalysts for lithium-oxygen batteries

Hybrid ternary Graphene/Ruthenium/α-MnO₂ (rGO/Ru/α-MnO₂) flexible nanocomposite cathodes were fabricated via controlling both reduction and vacuum filtration processes without using a binder and conductive carbon additives for flexible Li-air battery system. To compare the electrochemical performance of the Graphene/Ruthenium/α-MnO₂ cathodes, bare rGO and rGO/Ru free-standing cathodes were also manufactured. rGO cathodes with well-dispersed α-MnO₂ nanowires and ruthenium nanoparticles were successfully synthesized and shown to dramatically increase (decrease) oxygen reduction (evolution) reactions. The enhancement on the electrochemical performance of the synthesized cathodes was attributed not only to catalysis effect of ruthenium and α-MnO₂ but also well-stacked morphology of the nanocomposite architecture which enables increased oxygen flow between the layers and, hence boosted reaction kinetics.Physical characterization of the cathodes was carried out using FESEM, EDS, TEM, XRD, XPS and Raman spectroscopy. The discharge product of the cathodes was also evaluated using TEM and XPS. Electrochemical performances of the cathodes were evaluated by means of CV, EIS, galvanostatic charge-discharge and electrochemical cycling tests. Thanks to the synergetic effect of Ruthenium and α-MnO₂ catalysts, our ternary rGO/Ru/α-MnO₂ cathodes were shown to serve full discharge capacity of 2225 mAh/g while rGO/Ru can deliver only 1670 mAh/g. Besides, the cycling stability of the ternary rGO/Ru/α-MnO2 cathodes was shown for 50 cycles at 650 mAh/g capacity limited tests in assembled Li–O2 batteries.     

Effect of synthesis method on the structure and electrochemical behaviour of Co–Si particles

Co–Si particles were prepared by means of three methods as sample A, B, and C. Structures of the samples were characterized by XRD and the electrochemical hydrogen storage properties were studied as negative electrodes in aqueous KOH solution. Results of cyclic voltammetry (CV) and cycling stability demonstrated that samples A and B showed excellent electrochemical reversibility and relatively high discharge capacities, whose maximum capacity are 245 and 214 mAh/g and kept 207.7 and 189.5 mAh/g after 80 cycles at current density 50 mAh/g. By comparing the XRD patterns of the electrodes on different charge–discharge states, the discharge capacity was attributed to hydrogenation of Co–Si particles.     

Developing lithium ion battery silicon/cobalt core-shell electrodes for enhanced electrochemical properties

In this work, core-shell Si/Co composite powders were produced using an electroless deposition process. The effect of different concentrations of CoSO₄ in the plating bath was studied to provide the Co deposition and to reveal the structure on the surface of silicon powders. The surface morphology and the composition of the produced Si/Co composite powders were characterized using scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). X-ray diffraction (XRD) analysis was performed to investigate the structure of the Si/Co composite powders. The discharge capacities of Si/Co composite electrodes were determined with cyclically testing, and resistivity of the produced Si/Co composite electrodes were studied using electrochemical impedance spectroscopy (EIS). The silicon/cobalt composite electrode produced using 40 g/L CoSO₄ exhibited the most stable capacity retention, and a discharge capacity of approximately 220 mAh/g was obtained after 15 cycles for this electrode. This study demonstrated that the conductivity of the electrodes was improved and the capacity retention of the Si/Co composite electrodes was increased by increasing the shell structured cobalt content on the surface of silicon powders due to the buffering effect of cobalt against huge volume changes during the lithiation and delithiation process.     

Effects of mechanical milling on hydrogen storage properties of alloy

The hydrogen storage alloy of Ti_0.32Cr_0.43V_0.25 was prepared by arc melting and high energy ball milling. Effects of ball milling were studied for various time periods (30–300 min) at 200 rpm. The hydrogen storage capacity of the alloy decreased with the increase in milling time. The reasons for the drop in the hydrogen storage capacity are twofold: surface contamination of the alloy powder and the microstructural changes. The latter includes the increase in lattice strain, the decrease in crystallite size and the consequent increase in subgrain boundaries. Despite the microstructural changes, the BCC phase of the alloy was maintained and its lattice constant remained nearly the same.     

A novel flower-like metal-based oxides with cross-linked networks for rapid lithium-ion storage

A cross-linked MnO₂ coated ZnFe₂O₄ hollow nanosphere composite is synthesized and controlled via a facial and handy route. The connected MnO₂ nanoplates form a cross-linked network, which is conducive to the rapid transfer of Li ions. The composite with unique architecture can not only release the strain and stress caused by the insertion and desertion of lithium ions but also greatly improve the electrical conductivity and lithium ion diffusivity. Consequently, when used as a lithium-ion battery anode material, the electrode shows an excellent initial reversible capacity of 933.5 mAh/g with an initial coulombic efficiency of 62.5%. After 100 cycles, the reversible capacity stabilized at 605.6 mAh/g with a high capacity retention of 91% from the 20th cycle to the 100th cycle. At a high current density of 3 A/g, an excellent capacity of 390.6 mAh/g can be retained. In this case, the electrode shows broad application prospects for novel power storage systems.     

Cu5Si–Si/C composites for lithium-ion battery anodes

Cu₅Si–Si/C composites with precursor atomic ratio of Si:Cu = 1, 2 and 4.5 have been produced by high-energy ball-milling of a mixture of copper–silicon alloy and graphite powder for anode materials of lithium-ion battery. X-ray diffraction and scanning electron microscope measurements show that Cu₅Si alloy is formed after the intensive ball milling and alloy particles along with low-crystallite Si are interspersed in graphite uniformly. Cu₅Si–Si/C composite electrodes deliver a larger reversible capacity than commercialized graphite and better cyclability than silicon. The increase of copper amount in the composites decreases reversible capacity but improves cycling performance. Cu₅Si–Si/C composite with Si:Cu = 1 demonstrates an initial reversible capacity of 612 mAh g⁻¹ at 0.2 mA cm⁻² in the voltage range from 0.02 to 1.5 V. The capacity retention is respectively 74.5 and 70.0% at the 40th cycle at the current density of 0.2 and 1 mA cm⁻².