Search

Nanosized lithium titanium oxide (Li 4 Ti 5 O 12 ) powder has been prepared using LiCl as a high-temperature flux. X-ray diffraction, scanning electron microscope, and transmission electron microscope measurements indicate that the obtained Li 4 Ti 5 O 12 particles are uniform and the particle size of Li 4 Ti 5 O 12 powder can be controlled by flux content and heating time. Under the optimal synthetic condition, the particle-size distribution is narrow (∼100 nm). The hybrid electrochemical supercapacitors using a nanosized Li 4 Ti 5 O 12 negative electrode in combination with an activated carbon positive electrode show much better rate capability than those based on the conventional Li 4 Ti 5 O 12 prepared by solid-state reaction.

The electrochemical profiles of three kinds of Li-ion intercalated compounds, LiMn 2 O 4 , LiCo 1/3 Ni 1/3 Mn 1/3 O 2 , and LiCoO 2 , used as positive electrodes for hybrid aqueous electrochemical supercapacitors in combination with activated carbon (AC) negative electrode were studied in a 1 M Li 2 SO 4 solution. The effects of pH in the electrolyte solution on the stability of Li-ion intercalation reaction, the evolution potential of oxygen, and the specific capacity of both the Li-ion intercalated compound and AC electrode materials were intensively investigated by cycling voltammetry and charge/discharge tests in 1 M Li 2 SO 4 solutions with various pH values from pH 7 to 13. LiMn 2 O 4 shows stable Li-ion intercalation in the solution over pH 7, LiCo 1/3 Ni 1/3 Mn 1/3 O 2 over pH 11, and LiCoO 2 over pH 9. Under optimal conditions, the specific energy, rate capability, and cycling performance of hybrid cells based on the three kinds of positive electrodes and AC negative electrode were compared. There was no significant difference in the specific energy among the three hybrid cells, but the hybrid cell based on the LiMn 2 O 4 showed good cycling life and rate capability. LiCoO 2 also had good rate capability but with poor cycling performance, and LiCo 1/3 Ni 1/3 Mn 1/3 O 2 showed good cycling life but with poor rate capability.

Supercapacitors coupled with batteries and fuel cells are considered promising midterm and long-term solutions for low- and zeroemission transport vehicles by providing the power peaks for startstop, acceleration, and recovering the breaking energy. Nowadays, much research on the electrochemical capacitors aim to increase power and energy density as well as lower fabrication costs and at the same time focus on making environmentally friendly materials. Compared with the traditional electrochemical double-layer capacitor EDLC, the hybrid electrochemical supercapacitor which relies on two different electrode materials shows a higher power density and a corresponding cycle life. This was recently reported by Telcordia Technologies with a new device named nonaquous asymmetric hybrid electrochemical supercapacitor HBEC with an intercalation compound Li4Ti5O12 which was used as the negative material and active carbon which was used as the positive material. 1 The new

This paper describes a vapor-phase, precursor method that generates MgO nanowires at relatively low temperatures (800–900 °C) as compared to previous approaches (≥ 1200 °C) that directly use MgO as the source material. Magnesium diboride (MgB2) powders were used as the precursor, which slowly decomposed into Mg and MgB4 under a constant flow of argon gas at temperatures as low as 700 °C. The Mg vapor subsequently reacted with trace O2 contained in the reaction system and formed MgO vapor. Under appropriate conditions, the MgO vapor could reach supersaturation, condense onto the surface of a solid substrate that was placed on top of the precursor powders, and grow into highly anisotropic nanostructures. Depending on the distance between the MgB2 source and the location on the solid substrate, a gradient in the MgO concentration existed. As a result, MgO nanostructures with different morphologies were formed on the solid support, including whiskers, tapered nanowires whose diameters were continuously reduced from ∼200 to 20–30 nm over a distance of ∼50 μm, and nanowires having uniform diameters (15–20 nm) and lengths up to ∼30 μm. When a mixture of dihydrogen and argon gases was used as the reaction atmosphere, MgO nanowires with a uniform cross-section of ∼150 nm in diameter could grow several millimeters long without branching.

The electrochemical lithium-ion extraction/insertion properties of two spinel lithium manganese oxides, stoichiometric Li 0.99 Mn 2 O 4 and lithium-rich Li 1.04 Mn 1.93 O 4 , were studied. Their voltage profile and cyclic voltammetry all show two-step reaction for lithium-ion extraction/insertion at their 4 V plateau. The high-resolution in situ X-ray diffraction (XRD) studies show both materials experience three cubic phases, cubic I, II, and III, during cycling at 4 V plateaus. Two two-phase coexistence regions were observed. These results show that phase transitions between cubic I and II and between cubic II and III are first-order transitions. The in situ XRD data also reveal that lithium-rich composition does not suppress the phase transitions of spinel between their three cubic phases, but leads to the increase of variation range of the lattice parameters of the cubic phases. This in turn reduces the jumping step of the lattice parameter between adjacent phases and increases the overlap of the adjacent phase. This study shows conclusively that the three-phase process is the intrinsic feature of spinel Li 1+y Mn 2 O 4 during lithium-ion extraction/insertion on the 4 V plateau.

All solid-state lithium/polymer battery ~LPB! using a metallic lithium anode and solvent-free polymer electrolyte has been demonstrated to be the most promising secondary battery for electric vehicle applications because of its low risk of liquid electrolyte leakage, higher energy density, and shape flexibility compared to other systems. The main problems that are still to be solved are related to a low conductivity at ambient temperature. Many efforts have been devoted to the development and improvement of the electrolyte’s ionic conductivity for such batteries. 1-6 One of the most successful approaches is to add a small-particle size ceramic powder in the polymer electrolyte. 7-10 Most studies have focused primarily on investigating the influence of the addition of ceramic powders on the ionic conductivity and interfacial stability with lithium. Indeed, these new types of composite polymer electrolytes result in an increase in the ionic conductivity and enhanced lithium/polymerelectrolyte interface stability. However, very few studies have been devoted to measure its electrochemical profile in a real battery system, specially the electrochemical stability of the polymer electrolyte on a cathode. The high ionic conductivity and interface stability with lithium is of primary importance when it was applied into a lithium/polymer battery in combination with a composite cathode, and the electrochemical stability against a high voltage is also important. Until now, the evaluation of the electrochemical stability was normally characterized by linear sweep voltammetry ~LSV! or cyclic voltammetry ~CV! of the polymer electrolyte on a smooth stainless steel blocking electrode. The polymer electrolytes with or without the ceramic additive have been reported to be stable at least above 4 V vs. Li/Li 1 . 4,6 However, when a composite electrolyte works as an electrolyte separator combined with a positive electrode in a real battery system, it contacts the porous composite electrode containing active material and conductive material, e.g., carbon black, acetylene black, graphite, etc., rather than a smooth inert electrode. In the present study, we are interested in investigating the battery performance of the Li/LixMnO2 cell based on the ceramicadditive polymer electrolyte, rather than the ion conductivity evaluation. We found that some of the ceramic modified composite polymer electrolytes behave in a manner similar to that of the ceramicfree polymer electrolyte, showing a poor charge/discharge efficiency due to poly~ethylene oxide !~ PEO! decomposition during cycling even they showed increased ionic conductivity, whereas the PEO decomposition suppression was observed on these doped components which form a stable interreaction between the ceramic surface and the PEO segment. The possible PEO decomposition mechanism was discussed.

We have prepared the intermetallic compound Cu 6 Sn 5 using mechanical-alloying, gas-atomizing, and melt-spinning techniques, The electrochemical performance of the compound is critically dependent on its morphology due to different preparation methods. The Cu 6 Sn 5 alloy created by mechanical alloying, consisting of < I μm thick flake powder has the best battery performance of all compounds. It delivers a rechargeable capacity of 200 mAh/g (2000 Ah/L) over 50 cycles when the cycled voltage range is restricted to 0.2-1.5 V. The effect of the mechanical-alloying lime and Cu/Sn ratio on its battery performance was further investigated. The presence of excess Cu in alloy, relative to Cu 6 Sn 5 , showed improved cyclability at the expense of capacity, whereas an excess of Sn resulted in poor cyclability. A lithium-ion cell based on a flaked Cu-Sn mierocomposite alloy negative electrode and a 5 V LiNi x Mn 2-x O 4 positive electrode was assembled. The cell showed an average working voltage at 4.0 V and cycled well with a reversible capacity of ca. 200 mAh/g based on the pure Cu-Sn alloy when a cell was cycled between 3.5 and 4.6 V.

We have used a self-reaction process, called the JMC (Japanese Metal and Chemical Co., Ltd.) method, to prepare a series of defect spinels Li 8n/n+4 Mn 8 / n+4 O 4 , Li 2 Mn 5 O 12-δ (n = 0.8), Li 2 Mn 3 O 7-δ (n = 0.65), and Li 2 Mn 4 O 9-δ (n = 0.5). We found that it is easy to oxidize a defect spinel with a higher lithium content (Li/Mn ratio of 0.8) during synthesis. At the same time, however, the defect spinel easily becomes oxygen deficient. By contrast, a defect spinel with a smaller lithium content, especially Li/Mn of 0.5, is difficult to fully oxidize. The defect spinels deliver at initial capacity of 160 mAh/g both in the liquid-electrolyte and solid-state polymer-electrolyte-based cells. Li 2 Mn 3 O 7-δ shows the best battery performance; the capacity loss rate is 0.18% per cycle for a lithium-polymer cell during the first 100 cycles at 65°C, and the cell gives a specific energy of 360 Wh/kg based on the pure oxide. All compounds are thermally stable up to 200°C when they are in contact with polymer electrolytes, but undergo thermal runaway over 200°C. The exothermic reaction proceeds via a redox reaction among Mn 4+ , Mn 3+ , and the polymer electrolyte.

Several series of Li 1-x Mn 2 O 4+δ samples with the spinel structure were synthesized. These samples had different Li/Mn ratios (by varying the Li/Mn ratio used in starting materials) and various oxygen contents (by controlling synthesis conditions, including temperature, heat-treatment time, and purging gas during both the solid-state reaction and annealing). In systematic studies of charge-discharge cycling behavior and in situ X-ray diffraction (XRD) at room temperature, it was found that both the charge/ discharge profile and the structural changes during cycling are closely related to the degree of oxygen deficiency created in the synthesis process. Their effects on the capacity fading are much more important than the Li/Mn ratio or other factors. A higher degree of oxygen deficiency is accompanied with a faster fading of capacity during cycling. In cells using spinet cathodes with an oxygen deficiency, the capacity lading during cycling occurs on both the 4.2 and 4 0 V plateaus. This behavior is quite different from that found in cathodes without an oxygen deficiency, where most of the capacity fading occurs on the 4.2 V plateau region only. Our in situ XRD results indicate clearly that the capacity fading on the 4.2 V plateau is related to the phase transition between the cubic II and cubic III (λ-MnO 2 ) structure, while the capacity fading on the 4.0 V plateau is related to the phase transition between the cubic I and cubic II spinel structures. The effects of oxygen deficiency on the structural phase transition of Li 1±y Mn 2 O 4±δ -type materials at temperatures around 10°C were also studied. It was found that this phase transition is closely related to the degree of oxygen deficiency of the material. In samples with no oxygen deficiency, this phase transition disappeared.

We have used a lithiated MnO 2 , Li 0 33 MnO 2 , with ordered alternating one-dimensional [1 × 2] and [1 × 1] channels as a cathode material in solid-state lithium/polymer cells. An optimized cell can operate at moderate temperatures (40-80°C). Li 0.33 MnO 2 delivers a rechargeable capacity of 160 mAh/g with a flat potential plateau at ca. 3.0 V vs. Li/Li + at the C/3 rate and 65°C, corresponding to a specific energy of 450 Wh/kg of the pure oxide. Cells show good rate capability and excellent cyclability when cycled between 2.7 and 3.5 V at 80% depth of discharge, whereas a capacity decline was observed when cycled between 2.0 and 3.5 V. Capacity fading upon cycling is believed to be due to the formation of a thin layer of spinel phase (transformation to Li 0.5 MnO 2 from Li 0.33 MnO 2 ) on the particle surfaces, as well as to increased cell resistance during charge/discharge cycling. The cell self-discharge at high temperature and the thermal stability of Li 0.33 MnO 2 in contact with the polymer electrolyte are also discussed.