New electrolytes for lithium ion batteries using LiF salt and boron based anion receptors

The thermal and electrochemical stability, as well as compatibility with various bench mark cathode and anode materials of two new lithium fluoride salt (LiF) based electrolytes have been studied. These two new electrolytes are formed by using boron-based anion receptors, tris(pentafluorophenyl) borane (TPFPB), or tris(2H-hexafluoroisopropyl) borate (THFPB) as additives, which were designed and synthesized at Brookhaven National Laboratory (BNL), to dissolve the LiF salt in carbonate solvents. The transference number of Li⁺ for these electrolytes is as high as 0.7 and the room-temperature conductivity is around 2 × 10⁻³ S cm⁻¹. The electrolytes containing propylene carbonate (PC) show superior low-temperature conductivity properties. The electrochemical window is approaching 5.0 V. It was also found that the new electrolytes work well with LiCoO₂ or LiMn₂O₄ cathodes. However, when PC containing electrolytes were used, PC co-intercalation is still a problem for graphite anodes. The formation of a stable solid electrolyte interface layer on the surface of anode in this type of electrolyte needs to be studied further.     

Carbonate-modified siloxanes as solvents of electrolyte solutions for rechargeable lithium cells

Influence of mixing carbonate-modified siloxanes into LiPF₆-ethylene carbonate (EC)/ethylmethyl carbonate (EMC) (mixing volume ratio = 3:7) mixed solvent electrolytes on charge-discharge cycling properties of lithium was examined. As the solute, 1 M (M: mol L⁻¹) LiPF₆ was used. As siloxanes, 4-(2-trimethylsilyloxydimethylsilylethyl)-1,3-dioxolan-2-one and 4-(2-bis(trimethylsilyloxy)methylsilylethyl)-1,3-dioxolan-2-one were investigated. These siloxanes are derivatives of butylene cyclic carbonate or vinyl ethylene carbonate. Charge-discharge cycling efficiencies of lithium metal anodes improved and an impedance of anode/electrolyte interface decreased by mixing siloxanes, compared with those in 1 M LiPF₆-EC/MEC alone. Slightly better cycling behavior of natural graphite anode was obtained by adding siloxanes. Si-C/LiCoO₂ cells exhibited better anode utilization and good cycling performance by using 1 M LiPF₆-EC/MEC + siloxane electrolytes. Thermal behavior of electrolyte solutions toward graphite-lithium anodes was evaluated with a differential scanning calorimeter. By adding siloxanes, temperature starting the large heat-output of graphite-lithium anodes with 1 M LiPF₆-EC/MEC electrolyte solutions shifted to higher temperature about 100 °C. However, amount of heat-output did not decrease by adding siloxanes.     

Improvement of cyclability of Si as anode for Li-ion batteries

Silicon working as anode for Li-ion batteries has attracted much attention due to its high capacity (∼4200 mAh g⁻¹). However, due to the large volume expansion during lithiation, the capacity of silicon fades very fast. In this systematic study, we focus on the issue to fight the capacity fading. Results show that Si with sodium carboxymethyl cellulose (Na-CMC) as a polymer binder exhibits a better cyclability than that with poly(vinylidene fluoride) (PVDF). Yet differing from the system used in PVDF, the addition of vinylene carbonate (VC) does not improve or even worsens the performance of the system using Na-CMC. In addition, the small particle size of Si, a large amount of carbon black (CB), the good choice of electrolyte/conducting salt and charge-discharge window also play important roles to enhance the cyclability of Si. It is found that electrode consisting of 40 wt.% nano-Si, 40 wt.% carbon black and 20 wt.% Na-CMC (pH 3.5) displays the best cyclability, and in the voltage range from 0 to 0.8 V, after 200 cycles, its capacity can still keep 738 mAh g⁻¹ (C/2, in 1 M LiPF₆ ethylene carbonate/diethyl carbonate electrolyte, with VC-free), almost twice as that of graphite.     

Surface chemistry and morphology of the solid electrolyte interphase on silicon nanowire lithium-ion battery anodes

Silicon nanowires (SiNWs) have the potential to perform as anodes for lithium-ion batteries with a much higher energy density than graphite. However, there has been little work in understanding the surface chemistry of the solid electrolyte interphase (SEI) formed on silicon due to the reduction of the electrolyte. Given that a good, passivating SEI layer plays such a crucial role in graphite anodes, we have characterized the surface composition and morphology of the SEI formed on the SiNWs using X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). We have found that the SEI is composed of reduction products similar to that found on graphite electrodes, with Li₂CO₃ as an important component. Combined with electrochemical impedance spectroscopy, the results were used to determine the optimal cycling parameters for good cycling. The role of the native SiO₂ as well as the effect of the surface area of the SiNWs on reactivity with the electrolyte were also addressed.     

Fabrication and electrochemical characterization of a vertical array of MnO2 nanowires grown on silicon substrates as a cathode material for lithium rechargeable batteries

A complementary metal-oxide-semiconductor (CMOS) compatible process for fabricating on-chip microbatteries based on nanostructures has been developed by growing manganese dioxide nanowires on silicon dioxide (SiO₂)/silicon (Si) substrate as a cathode material for lithium rechargeable batteries. High aspect-ratio anodized aluminum oxide (AAO) template integrated on SiO₂/Si substrates can be exploited for fabrication of a vertical array of nanowires having high surface area. The electrolytic manganese dioxide (EMD) nanowires are galvanostatically synthesized by direct current (dc) electrodeposition. The microstructure of these nanowire arrays is investigated by scanning electron microscopy and X-ray diffraction. Their electrochemical tests show that the discharge capacity of about 150 mAh g⁻¹ is maintained during a few cycles at the high discharge/charge rate of 300 mA g⁻¹.     

A transmission electron microscopy study of crack formation and propagation in electrochemically cycled graphite electrode in lithium-ion cells

Subsurface defects and local compositional changes that occurred in graphite anodes subjected to cyclic voltammetry tests (vs. Li/Li⁺, using an electrolyte consisting of 1 M LiClO₄ in a 1:1 volumetric mixture of ethylene carbonate and 1,2-dimethoxy ethane) were investigated using high-resolution transmission electron microscopy (HR-TEM). Cross-sections of anodes prepared by focused ion beam (FIB) milling indicated that graphite layers adjacent to solid electrolyte (SEI)/graphite interface exhibited partial delamination due to the formation of interlayer cracks. The SEI layer formed on the graphite surface consisted of Li₂CO₃ that was identified by {1 1 0} and {0 0 2} crystallographic planes. Lithium compounds, LiC₆, Li₂CO₃ and Li₂O, were observed on the surfaces of separated graphite layers. Deposition of these co-intercalation compounds near the crack tip caused partial closure of propagating graphite cracks during electrochemical cycling, and possibly reduced the crack growth rate. Graphite fibres that were observed to bridge crack faces likely provided an additional mechanism for the retardation of crack propagation.     

Structural and electrochemical study of the reaction of lithium with silicon nanowires

The structural transformations of silicon nanowires when cycled against lithium were evaluated using electrochemical potential spectroscopy and galvanostatic cycling. During the charge, the nanowires alloy with lithium to form an amorphous Li_xSi compound. At potentials <50 mV, a structural transformation occurs. In studies on micron-sized particles previously reported in the literature, this transformation is a crystallization to a metastable Li₁₅Si₄ phase. X-ray diffraction measurements on the Si nanowires, however, show that they are amorphous, suggesting that a different amorphous phase (Li_ySi) is formed. Lithium is removed from this phase in the discharge to form amorphous silicon. We have found that limiting the voltage in the charge to 70 mV results in improved efficiency and cyclability compared to charging to 10 mV. This improvement is due to the suppression of the transformation at low potentials, which alloys for reversible cycling of amorphous silicon nanowires.     

C-rate performance of silicon oxycarbide anodes for Li+ batteries enhanced by carbon nanotubes

We show that employing single wall carbon nanotubes (CNTs) as the conducting agent significantly increases the capacity of silicon oxycarbide anodes at high C-rates. In these anodes 515 mAh g⁻¹ can be extracted in just over 3 min. The capacity decreases to 300 mAh g⁻¹ at the same extraction rate when carbon black is used as the conducting agent. The CNT anodes have good cyclic stability, retaining 89.2% of initial capacity after 40 cycles. The coulombic efficiency ranges from 95% to 100%.     

Vapor-induced solid-liquid-solid process for silicon-based nanowire growth

Silicon-based nanowires have been grown from commercial silicon powders under conditions with different oxygen and carbon activities. Nanowires grown in the presence of carbon sources consisted of a crystalline SiC core with an amorphous SiO_x shell. The thickness of the SiO_x shell decreased as the oxygen concentration in the precursor gases decreased. Nanowires grown in a carbon-free environment consisted of amorphous silicon oxide with a typical composition of SiO_1.8. The growth rate of nanowires decreased with decreasing oxygen content in the precursor gases. SiO_1.8 nanowires exhibited an initial discharge capacity of ∼1300 mAh g⁻¹ and better stability than those of silicon powders. A vapor-induced solid-liquid-solid (VI-SLS) mechanism is proposed to explain the nanowire growth (including silicon and other metal-based nanowires) from powder sources. In this approach, both a gas source and a solid-powder source are required for nanowire growth. This mechanism is consistent with experimental observations and also can be used to guide the design and growth of other nanowires.     

Influence of surface modification of LiCoO2 by organic compounds on electrochemical and thermal properties of Li/LiCoO2 rechargeable cells

LiCoO₂ is the most famous positive electrode (cathode) for lithium ion cells. When LiCoO₂ is charged at high charge voltages far from 4.2 V, cycleability of LiCoO₂ becomes worse. Causes for this deterioration are instability of pure LiCoO₂ crystalline structure and an oxidation of electrolyte solutions LiCoO₂ at higher charge voltages. This electrolyte oxidation accompanies with the partial reduction of LiCoO₂. We think more important factor is the oxidation of electrolyte solutions. In this work, influence of 10 organic compounds on electrochemical and thermal properties of LiCoO₂ cells was examined as electrolyte additives. As a base electrolyte solution, 1 M (M: mol L⁻¹) LiPF₆-ethylene carbonate (EC)/ethylmethyl carbonate (EMC) (mixing volume ratio = 3:7) was used. These compounds are o-terphenyl (o-TP), Ph-X (CH₃)_n (n = 1 or 2, X = N, O or S) compounds, adamantyl toluene compounds, furans and thiophenes. These additives had the oxidation potentials (Eox) between 3.4 and 4.7 V vs. Li/Li⁺. These Eox values were lower than that (6.30 V vs. Li/Li⁺) of the base electrolyte. These additives are oxidized on LiCoO₂ during charge of the LiCoO₂ cells. Oxidation products suppress the excess oxidation of electrolyte solutions on LiCoO₂. As a typical example of these organic compounds, o-TP (Eox: 4.52 V) was used to check the fundamental properties of these organic additives. Charge-discharge cycling tests were carried out for the Li/LiCoO₂ cells with and without o-TP. Constant current charge at 4.5 V is mainly used as a charge method. Cells with 0.1 wt.% o-TP exhibited slightly better cycling performance and lower polarization than those without additives. Lower polarization arises from a decrease in a resistance of interface between electrolyte solutions and LiCoO₂ by surface film formation resulted from oxidation of o-TP. Oxidation products were found by mass spectroscopy analysis to be mixture of several polycondensation compounds made from two to four terphenly monomers. Thermal stability of LiCoO₂ with electrolyte solutions did not improve by addition of o-TP. Slightly better charge-discharge cycling properties were obtained by using organic modifiers. However, when industrial applications were considered, drastic improvements have not been obtained yet. One of reasons may be too large influence of the deterioration of stability of pure LiCoO₂ structure at high voltage charging for industrial use. We hope to realize the tremendous improvements of high energy, long cycle life and safe lithium cells by the combination of both LiCoO₂ with more stable structure such as LiCoO₂ treated with MgO and new organic additives with molecular structure more carefully designed.     

Surface structural disordering in graphite upon lithium intercalation/deintercalation

We report on the origin of the surface structural disordering in graphite anodes induced by lithium intercalation and deintercalation processes. Average Raman spectra of graphitic anodes reveal that cycling at potentials that correspond to low lithium concentrations in Li_xC (0 ≤ x < 0.16) is responsible for most of the structural damage observed at the graphite surface. The extent of surface structural disorder in graphite is significantly reduced for the anodes that were cycled at potentials where stage-1 and stage-2 compounds (x > 0.33) are present. Electrochemical impedance spectra show larger interfacial impedance for the electrodes that were fully delithiated during cycling as compared to electrodes that were cycled at lower potentials (U < 0.15 V vs. Li/Li⁺). Steep Li⁺ surface-bulk concentration gradients at the surface of graphite during early stages of intercalation processes, and the inherent increase of the Li_xC d-spacing tend to induce local stresses at the edges of graphene layers, and lead to the breakage of C-C bonds. The exposed graphite edge sites react with the electrolyte to (re)form the SEI layer, which leads to gradual degradation of the graphite anode, and causes reversible capacity loss in a lithium-ion battery.     

Photocurrent and photoelectrochemical hydrogen production with tin porphyrin and platinum nanowires immobilized with nafion on glassy carbon electrode

Platinum nanowires mixed with Tin meso-tetra (4-pyridyl) porphine dichloride and nafion solution was used to modify the surface of glassy carbon electrode for photocurrent generation and photo-electrochemical hydrogen production. Different concentrations of porphyrin (50 μM, 100 μM, 300 μM and 500 μM) and platinum loading (200 μg/cm², 400 μg/cm², 600 μg/cm² and 800 μg/cm²) were tested at −150 mV Vs Ag/AgCl in reaction cell containing the modified glassy carbon electrode as working electrode, platinum wire as counter electrode and Ag/AgCl as reference electrode, under illumination to determine the optimum, based on photocurrent production in 50 mM potassium hydrogen phthalate buffer (pH 3) containing 0.1Na₂SO₄ as supporting electrolyte. Optimum photocurrent was obtained at 100 μM tin porphyrin and 600 μg/cm² platinum loading. Detectable amount of hydrogen was produced at −350 mV Vs Ag/AgCl under irradiation with visible light.     

Dimethyl methyl phosphate: A new nonflammable electrolyte solvent for lithium-ion batteries

A new fire retardant-dimethyl methyl phosphate (DMMP) was tested as a nonflammable electrolyte solvent for Li-ion batteries. It is found that in the addition of chloro-ethylene carbonate (Cl-EC) as an electrolyte additive, the electrochemical reduction of DMMP molecules can be completely suppressed and the graphite anode can be cycled very well with high initial columbic efficiency (∼84%) and excellent cycling stability in the DMMP electrolyte. The prismatic C/LiCoO₂ batteries using 1.0 mol L⁻¹ LiClO₄ + 10% Cl-EC + DMMP electrolyte exhibited almost the same charge and discharge performances as those using conventional carbonate electrolytes, suggesting a feasible use of this new electrolyte for constructing nonflammable Li⁺-ion batteries.     

Development of a novel portable-size PEMFC short stack with electrodeposited Pt hydrogen diffusion anodes

This paper presents, for the first time, a five-cell polymer electrolyte membrane fuel cell (PEMFC) short stack with electrodeposited hydrogen diffusion anodes. The anodes were manufactured by means of galvanostatic pulse electrodeposition and the cathodes by air-brushing. Nafion^® 212 was employed as a solid polymer electrolyte membrane in all cases. The short stack, whose cells had an active geometric area of 14 cm², was assembled and tested under different operating conditions. A peak power of about 11 W was obtained at 50 °C and atmospheric pressure using hydrogen and air feed, whereas a smaller value of 8.6 W was obtained from a five-cell short PEMFC stack with conventional hydrogen diffusion anodes under the same operating conditions. The better performance of the cells described in this paper has been assigned to the higher utilization of the platinum in the electrodeposited anodes compared to the conventional ones.     

Performance improvement of direct carbon fuel cell by introducing catalytic gasification process

In this paper, the effects of catalytic gasification on the solid oxide electrolyte DCFC (direct carbon fuel cell) performance are experimentally investigated and analyzed using K, Ca, Ni as catalyst in carbon black and controlling the temperatures of cell and carbon black at 750 °C and 700-1000 °C, respectively. The average power densities are 976, 1473 and 1543 W m⁻² respectively for 900, 950 and 1000 °C pure carbon black gasification. Catalytic gasification improves the DCFC performance significantly. For the same performance of pure carbon black, the gasification temperatures decrease about 200, 130 and 150 °C with K, Ca and Ni additives, respectively. The catalytic effects for carbon black gasification with CO₂ are: K > Ni > Ca. For typical identical temperature DCFC operating at 750 °C, the power densities of 0.7 V discharging are 1477, 1034 and 1123 W m⁻² for the carbon black with K, Ca and Ni additives, respectively. It is possible to reduce the operation temperature of DCFC to the medium temperature range of solid oxide electrolyte (600-800 °C) by introducing catalytic gasification process.     

Effect of ethanol-assisted electrode fabrication on the performance of silicon anodes

The performance of high-silicon-content anodes was tested as a function of silicon particle-size (44 μm, 1.8 μm and 70-100 nm), surface pre-treatment and solvent chosen for anode preparation. Two simple procedures, leading to significant improvements in electrode performance are reported. First, pre-treatment of nano-Si in ethanol which unexpectedly yields functionalised surfaces improving cycling stability. Second, the use of a 30:70 solution of ethanol and water to dissolve the CMC-binder for the electrode preparation boosts specific battery capacity. Ethanol pre-treatment of nano-Si also resulted in improved adhesion of the electrode to the current collector as well as in de-agglomeration of nano-Si powder. All these treatments improved capacity stability during cycling. Changes in surface chemistry of nano-Si before and after ethanol treatment have been analysed by XPS. A stable capacity of about 1630 mAh g⁻¹ was obtained after 25 cycles for an electrode containing 80% silicon using ethanol during electrode coating preparation.     

Fabrication and performance of membrane solid oxide fuel cells with La0.75Sr0.25Cr0.5Mn0.5O3−δ impregnated anodes

La_0.75Sr_0.25Cr_0.5Mn_0.5O_3−δ (LSCrM)-impregnated anodes have been fabricated by infiltrating 70% porous yttria-stabilized zirconia (YSZ) matrixes with an LSCrM solution. In these anodes, LSCrM is a primary electronic conducive phase while the well-sintered YSZ provides an ionic-conducting pathway throughout the electrode. The maximum power densities of a single cell with YSZ + 35 wt.% LSCrM composite anode reach 567 and 561 mW cm⁻² at 850 °C in dry H₂ and CH₄, respectively. Further, Ag and Ni are added via nitrate impregnating method for improving electronic conductivity and catalytic activity. With the addition of 6 wt.% Ni and 2 wt.% Ag to the YSZ + 32 wt.% LSCrM composite anode, the maximum power densities at 850 °C increase to 1302 mW cm⁻² in dry H₂ and 769 mW cm⁻² in dry CH₄. No carbon deposition is detected in the tested anodes, even with the presence of Ni.     

Preparation and performances of porous polyacrylonitrile–methyl methacrylate membrane for lithium-ion batteries

A copolymer, polyacrylonitrile–methyl methacrylate P(AN–MMA), was synthesized by suspension polymerization with acrylonitrile (AN) and methyl methacrylate (MMA) as monomers. With this copolymer, polymer membrane was prepared by phase inversion. The performances of the polymer were characterized by FTIR, SEM, DSC/TG, EIS and LSV. The copolymer contains CH₂, CN and CO bonds, and shows its thermal stability up to 300 °C. The polymer membrane has a porous structure with an average pore diameter of 0.5 μm. The conductivity of the polymer electrolyte is 1.25 mS cm⁻¹ at room temperature, and it is electrochemically stable up to 5 V (vs. Li). Using the polymer electrolyte as the gel polymer electrolyte (GPE), the cell Li/GPE/LiCoO₂ shows its cyclic stability as good as the cell with liquid electrolyte.     

Low-temperature solid oxide fuel cells with La1−xSrxMnO3 as the cathodes

La_1−xSr_xMnO₃ (LSM) has been widely developed as the cathode material for high-temperature solid oxide fuel cells (SOFCs) due to its chemical and mechanical compatibilities with the electrolyte materials. However, its application to low-temperature SOFCs is limited since its electrochemical activity decreases substantially when the temperature is reduced. In this work, low-temperature SOFCs based on LSM cathodes are developed by coating nanoscale samaria-doped ceria (SDC) onto the porous electrodes to significantly increase the electrode activity of both cathodes and anodes. A peak power density of 0.46 W cm⁻² and area specific interfacial polarization resistance of 0.36 Ω cm² are achieved at 600 °C for single cells consisting of Ni-SDC anodes, LSM cathodes, and SDC electrolytes. The cell performances are comparable with those obtained with cobalt-based cathodes such as Sm_0.5Sr_0.5CoO₃, and therefore encouraging in the development of low-temperature SOFCs with high reliability and durability.     

Ionic liquid electrolyte based on S-propyltetrahydrothiophenium iodide for dye-sensitized solar cells

A new ionic liquid S-propyltetrahydrothiophenium iodide (T₃I) was developed as the solvent and iodide ion source in electrolyte for dye-sensitized solar cells. The electrochemical behavior of the /I⁻ redox couple and effect of additives in this ionic liquid system was tested and the results showed that this ionic liquid electrolyte revealed good conducting abilities and potential application for solar devices. The effects of LiI and dark-current inhibitors were investigated. The dye-sensitized solar cell with the electrolyte (0.1 mol L⁻¹ LiI, 0.35 mol L⁻¹ I₂, 0.5 mol L⁻¹ NMBI in pure T₃I) gave short-circuit photocurrent density (J_sc) of 11.22 mA cm², open-circuit voltage (V_oc) of 0.61 V and fill factor (FF) of 0.51, corresponding to the photoelectric conversion efficiency (η) of 3.51% under one Sun (AM1.5).