Naoaki Kumagai

Influence of manganese(II), cobalt(II), and nickel(II) additives in electrolyte on performance of graphite anode for lithium-ion batteries

Manganese dissolution into an electrolyte from the spinel LiMn2O4 in the lithium-ion cell has been recently investigated. In order to study the influence of the dissolved manganese species on the lithium intercalation/deintercalation into a natural graphite electrode, the electrochemical behavior of graphite was investigated in 1 mol dm−3 LiClO4 electrolyte solution containing a small amount of Mn(II) by the addition of manganese(II) perchlorate. During the charging process, Mn(II) ions were firstly electroreduced on the electrode around 1.0 V versus Li/Li+ followed by irreversible decomposition of the electrolyte and lithium intercalation into the graphite. By microscopic observation of the graphite surface, manganese deposition was confirmed after the charge/discharge test. Due to the manganese deposition, the reversible capacity of the graphite electrode was drastically decreased. Furthermore, the cyclability of the anode was degraded with the amount of the manganese additive increasing. We compared these results with those of the cobalt(II) and nickel(II) additives by dissolving the corresponding perchlorates. Furthermore, we discussed the influence in practical cells based on the consideration of electrochemistry of the deposited metals.

Effects of Al doping on the microstructure of LiCoO2 cathode materials

Abstract LiCo 1− x Al x O 2 (0≤ x ≤0.3, R 3 m ) powders by the emulsion drying method and their microstructure studies were carried out in order to understand Al-doping effect on the electrochemical properties. Through X-ray diffraction experiment, we confirmed that Co was well substituted by Al while maintaining the α-NaFeO 2 layer structure type. The substitution of Al was effective on suppressing cobalt dissolution at 4.5 V versus Li/Li + and on diminishing changes in a - and c -axes during lithium intercalation. For x =0.1, the open-circuit voltage was slightly higher than that of undoped LiCoO 2 . Chemical diffusion coefficient of lithium in LiCo 1− x Al x O 2 was higher than that of LiCoO 2 because of a smaller variation in the longer c -axis.

Enhanced structural stability and cyclability of Al-doped LiMn2O4 spinel synthesized by the emulsion drying method

Spinel LiMn 2 O 4 oxide is of great interest as a low cost and environmentally kind intercalation cathode material for rechargeable lithium batteries, but has suffered from structural unstability during intercalation/deintercalation and severe capacity fading with cycling. Here, we successfully synthesized the spinel LiAl x Mn 2-x O 4 compounds by calcination of the emulsion-dried precursor at 850°C in air. The Al doping range in the spinel phase was much wider. Characteristic X-ray mapping for the LiAl 0.3 Mn 1-7 O 4 confirmed that Al and Mn were uniformly distributed throughout the crystalline oxide particles. The interaction energy between lithium positive-positive ions for lithium insertion in the host structure remarkably increased with Al doping, and the x value between 0.2 and 0.3 in LiAl x Mn 2-x O 4 was a critical point for showing a simple potential variation. Al-doped samples exhibited excellent cyclablility with showing relatively higher discharge capacity retention due mainly to the stabilized structure as well as the reduced strain during repeated intercalation/deintercalation of lithium by Al substitution.

Structural changes of Nb2O5 and V2O5 as rechargeable cathodes for lithium battery

Abstract The structural changes of T-Nb2O5 and V2O5 cathodes with discharge and recharge were investigated by X-ray photoelectron spectroscopy (ESCA) and X-ray diffractometory etc. ESCA spectra of the discharge products shows that M5+ is reduced to a lower valence state such as M4+ on the discharge, and the chemical bond between Li+ inserted into the oxide and O2− of the oxide exhibits a higher ionic character than that of Li2O. X-ray diffraction measurement shows that T-Nb2O5 gives the reversible structural change accompanying the disorder and order of the atomic arrangement on the charge-discharge cycling. On the other hand, V2O5 takes two discharge steps, within the 1st step of which it gives the reversible lattice change along b-axis caused by the intercalation of Li+ into the oxide layer, whereas within the 2nd step it is considered that the reaction in the charge-discharge takes place in the vicinity of the surface of the oxide particle. In both oxides, ternary phases such as LixM2O5 are produced as discharge products, where x is at least 2 for Nb2O5 and x is at least 3 for V2O5.

Electrochemical and In Situ XAFS-XRD Investigation of Nb2O5 for Rechargeable Lithium Batteries

Nb 2 O 5 exhibits various crystal systems, such as orthorhombic (o), tetragonal (t), and monoclinic (m), among which Nb 2 O 5 synthesized at 900-1000°C is commercially used as a cathode material of the 2-V lithium ion battery. The battery performances depended on the structure of Nb 2 O 5 , and the t-Nb 2 O 5 synthesized at 1000°C exhibited an excellent cycling performance with a large discharge capacity of 190 mAh (g oxide) -1 . The structural variations of Nb 2 O 5 during electrochemical reaction were examined. The in situ synchrotron radiation-X-ray diffraction (XRD) measurement indicated that o- and t-Nb 2 O 5 maintain their original crystal lattices, accompanying a small change in the cell volume even after the Li intercalation. The in situ X-ray absorption fine structure (XAFS) analysis of o- and t-Nb 2 O 5 revealed that the continuous variation from Nb 5+ to Nb 4+ took place during the intercalation process. A significant rearrangement of the Nb-O octahedra accompanied by the change of Nb-O and Nb-Nb interactions occurred in both structures with Li intercalation. XRD and XAFS data suggests that the two-dimensional layer structure of t-Nb 2 O 5 seems to be more flexible regarding the Li intercalation compared with the three-dimensional structure of o-Nb 2 O 5 . This may account for the better cyclic performance of the former material as the electrode material.

Effect of cobalt addition to nickel hydroxide as a positive material for rechargeable alkaline batteries

Nickel hydroxide powder is used widely as an active material in pasted-type nickel electrodes. The effect on the electrochemical behaviour of cobalt addition (0.19–9.9 wt.%) to the nickel hydroxide powder is investigated. The physical properties of several nickel hydroxide powders containing different amounts of cobalt compound are examined by inductively coupled spectroscopy, laser diffraction, BET, X-ray diffraction and scanning electron microscopy measurements. The interlayer distance of the layered nickel hydroxide diminishes to a small value with increase in cobalt content. Moreover, the charge and discharge potentials of nickel hydroxide samples decrease with an increase in cobalt content. The chemical diffusion coefficients of the proton (D) in the nickel hydroxide samples with different amounts of cobalt are measured by a current-pulse relaxation technique. The D and D0 values increase with an increase in cobalt content, and the activation energy for proton diffusion is in the 0.20–0.33 eV range.

Ultrasonically Treated LiV[sub 3]O[sub 8] as a Cathode Material for Secondary Lithium Batteries

A new treatment of LiV{sub 3}O{sub 6} has been proposed for improving its electrochemical behavior as a cathode material for secondary lithium batteries. The ultrasonically treated products in water were characterized by x-ray diffractometry, thermogravimetric analysis, scanning electron microscopy, and specific surface area measurements. These measurements showed that the ultrasonic treatment process of crystalline LiV{sub 3}O{sub 8} causes a decrease in crystallinity and considerable increases in specific surface area and interlayer spacing. The product, ultrasonically treated in water for 3 h and then heat-treated at 250 C for 6 h, showed a high initial discharge capacity of 283 mAh/g active material, which was 62% higher than that of the starting material, and was charge-discharge cycled without a large capacity loss during 40 cycles. The ultrasonic treatment of LiV{sub 3}O{sub 8} can improve not only the specific capacity but also the cycling behavior.

Synthesis of layered MnO2 by calcination of KMnO4 for rechargeable lithium battery cathode

Layered manganese oxide (KxMnO2) powders were synthesized by simple decomposition of KMnO4 at 300–800°C in air. These manganese oxides were characterized by powder X-ray diffractometry, atomic absorption spectrometry, chemical redox titration, and X-ray photoelectron spectroscopy. Their electrochemical performances for secondary lithium batteries were investigated in a LiClO4–propylene carbonate (PC) solution. After removing side products, which were water soluble, the KxMnO2 (x≤0.484) cathode unerwent reversible electrochemical intercalation/deintercalation of lithium. When the KxMnO2 powders were synthesized at 300°C and then washed with water or acidic solution after the synthesis, they exhibited satisfactory behavior with a specific capacity of 130–140 mAh g−1 in the region of 3 V vs. Li/Li+ for more than 20 cycles.

Capacity fading of LiMn2O4 electrode synthesized by the emulsion drying method

We adapted the emulsion drying method to obtain highly crystalline spinel LiMn2O4 phase using LiNO3 and Mn(NO3)2·6H2O as starting materials. The emulsion-dried powders were calcined at various temperatures for 24 h in air, and their crystalline phases were identified as a cubic spinel structure with the space group Fd3m by X-ray diffraction study. The initial discharge capacity of the samples calcined at 650°C, 750°C and 850°C were approximately 120 mA h/g, irrespective of calcination temperature. However, their capacity fadings were significantly dependent on the calcination temperature. To investigate the structural changes in the oxide cathode, XRD experiment was carried out as functions of lithium content and charge–discharge cycling number. Through the SEM observation, it was found that particles were disrupted, and the degree of disruption increased with lithium content and cycling.

Neutron powder diffraction studies of LiMn2-yAlyO4 synthesized by the emulsion drying method

Abstract The aluminum-doped spinel-type lithium manganese oxides, LiMn2−yAlyO4, have been successfully synthesized through the emulsion drying method, and their crystal structures were studied by neutron powder diffraction. Rietveld refinements of their neutron diffraction data revealed that Al was substituted selectively for Mn at the 16d site and that Li occupied only the 8a site. Thus, the Al-doped compound can be expressed as [Li]8a[Mn2−yAly]16d[O4]32e (y=0–0.452). The lattice constant a decreased linearly with increasing the occupation factor g(Al). The length of (Mn, Al)O bonds in (Mn, Al)O6 octahedra changed in a manner expected from average ionic radii for Mn1−g(Al)Alg(Al).