Masaki Yoshio

Capacity Fading on Cycling of 4 V Li / LiMn2 O 4 Cells

The cycle-life behavior of a Li/1 M-LiPF 6 + EC/DMC(1:2 by volume)/LiMn 2 O 4 cell was investigated at various temperatures (0, 25, and 50°C). The capacity fades faster on cycling at high rather than low temperatures. The mechanisms responsible for the capacity fading of the spinel LiMn 2 O 4 during cycling were extensively investigated by chemical analysis of the dissolved Mn in combination with in situ x-ray diffraction, Rietveld analysis, and ac impedance techniques. Chemical analytical results indicated that the capacity loss caused by the simple dissolution of Mn 3+ accounted for only 23 and 34% of the overall capacity losses cycling at room temperature and 50°C, respectively. In situ x-ray diffraction results showed that the two-phase structure coexisting in the high-voltage region persists during lithium-ion insertion/extraction at low temperatures during cycling. By contrast, this two-phase structure was effectively transformed to a more stable, one-phase structure, accompanied by the dissolution of Mn and the loss of oxygen (e.g., Mn 2 O 3 .MnO) at the high temperature; this dominated the overall capacity-loss process. AC impedance spectra revealed that the capacity loss at the high temperature was also due in part to the decomposition of electrolyte solution at the electrode.

Carbon-coated silicon as anode material for lithium ion batteries: advantages and limitations

Abstract Carbon coating of silicon powder was studied as a means of preparation of silicon-based anode material for lithium ion batteries. Carbon-coated silicon has been investigated at various cycling modes vs. lithium metal. Ex situ X-ray data suggest that there is irreversible reduction of crystallinity of the silicon content. Since carbon layer preserving the integrity of the particle, the reversibility of the structural changes in the amorphous state Li–Si alloy provides the reversible capacity. The progressively decreased Coulomb efficiency with cycling indicates that more and more lithium ions are trapped in some form of Li–Si alloy and become unavailable for extraction. This is the main factor for the capacity fading during cycling. Qualitative studies of the impedance spectra of the electrode material at the first cycle for the fresh anode and at the last cycle after the anode capacity faded considerably and provide further support for this model of fading mechanism.

Carbon-Coated Si as a Lithium-Ion Battery Anode Material

Carbon-coated Si has been prepared by a thermal vapor decomposition method. Its electrochemical performance has been investigated by charge/discharge tests, cyclic voltammetric experiments, differential scanning calorimetry, and 7 Li-nuclear magnetic resonance, etc. This kind of material demonstrates good electrochemical performance as an anode material for lithium-ion batteries. The improvement in the electrochemical performance of Si is mainly attributed to the effect of carbon coating.

Effect of Carbon Coating on Electrochemical Performance of Treated Natural Graphite as Lithium‐Ion Battery Anode Material

Carbon-coated natural graphite has been prepared by thermal vapor decomposition treatment of natural graphite at 1,000 C. Natural graphite coated with carbon showed much better electrochemical performance as an anode material in both propylene carbonate-based and ethylene carbonate-based electrolytes than bare natural graphite. The effect of carbon coating on the electrochemical performance was investigated by solid-state {sup 7}Li-NMR in conjunction with standard electrochemical techniques.

Synthesis and Electrochemical Properties of ZnO-Coated LiNi0.5Mn1.5 O 4 Spinel as 5 V Cathode Material for Lithium Secondary Batteries

ZnO-coated LiNi 0 . 5 Mn 1 . 5 O 4 powders with excellent electrochemical cyclability and structural stability at elevated temperature have been synthesized by a sol-gel method. The structural degradation of the as-preparedLiNi 0 . 5 Mn 1 . 5 O 4 and ZnO-coated LiNi 0 . 5 Mn 1 . 5 O 4 electrodes before and after cycling in the 5 V region has been studied. The ZnO-coated LiNi 0 . 5 Mn 1 . 5 O 4 electrode showed almost no capacity loss and retained its original cubic spinel structure after 50 cycles. We found that ZnO played an important role in reducing the HF content in the electrolyte solution.

Correlating Capacity Fading and Structural Changes in Li1 + y Mn2 − y O 4 − δ Spinel Cathode Materials: A Systematic Study on the Effects of Li/Mn Ratio and Oxygen Deficiency

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.

Studies on an LiMnO spinel system (obtained by melt-impregnation) as a cathode for 4 V lithium batteries part 1. Synthesis and electrochemical behaviour of LixMn2O4

Abstract An Li x Mn 2 O 4 spinel phase is used as a cathode for 4 V lithium batteries and is prepared by the melt-impregnation method, in which melted LiOH or LiNO 3 is impregnated into MnO 2 pores, then reacted with MnO 2 at higher temperature. The effect of synthesis conditions on the electrochemical properties is investigated extensively. For optimum synthesis, the spinel Li x Mn 2 O 4 should be prepared in an N 2 atmosphere at a low temperature of less than 750 °C and for a short time of less than 48 h. The optimum LiMn 2 O 4 delivers an initial charge capacity of 135 mAh/g, and exhibits good rechargeability. The average specific capacity during first 50 cycles is about 120 mAh/g or more.

Preparation and characterization of nano-crystalline LiNi0.5Mn1.5O4 for 5 V cathode material by composite carbonate process

Abstract LiNi 0.5 Mn 1.5 O 4 has been synthesized using two different synthetic methods; a sol–gel method and a composite carbonate process. LiNi 0.5 Mn 1.5 O 4 obtained by the sol–gel method showed a nickel oxide impurity in the XRD diagram and two voltage plateaus at 4.1 and 4.7 V upon cycling. However, the LiNi 0.5 Mn 1.5 O 4 compound obtained by the composite carbonate process exhibited a pure cubic spinel structure ( Fd 3 m ) without any impurities and only one voltage plateau at 4.7 V in the charge/discharge curves. Furthermore, it showed an excellent cycling retention rate of over 96% in the high temperature test. The well-developed LiNi 0.5 Mn 1.5 O 4 obtained by the composite carbonate process contained many spherical particles of about 3–4 μm, made up of small nano-sized particles (50–100 nm). It was a unique powder characterization and these nano-sized particles improved the cycling performance of the LiNi 0.5 Mn 1.5 O 4 obtained by composite carbonate process.

Improvement of natural graphite as a lithium-ion battery anode material, from raw flake to carbon-coated sphere

Natural graphite is a promising candidate for the anode material in lithium-ion batteries. To enhance its electrochemical performance, raw natural graphite flakes have been rolled into spheres by impact milling and then coated with carbon by thermal vapor decomposition (TVD). The obtained spherical graphite samples show excellent performance in terms of high rate capacity, high reversible capacity, high coulombic efficiency and low irreversible capacity. The improvements in performance have been mainly correlated with the morphologies of carbon-coated spherical graphite.

Carbon-coated natural graphite prepared by thermal vapor decomposition process, a candidate anode material for lithium-ion battery

The electrochemical performance of thermal-vapor-decomposition carbon-coated natural graphite was studied in both propylene carbonate (PC)-based electrolytes and ethylene carbonate (EC)-based electrolytes. In the cyclic votammograms of carbon-coated natural graphite, the hump in the range of 1.3–0.7 V versus Li+/Li can be observed when the potential was swept at higher scan rates. This hump was investigated with reference of carbon black. The effect of electrode-fabrication pressure on the electrochemical performance of carbon-coated in the PC-based electrolyte has also been studied.