Kuniaki Tatsumi

The Influence of the Graphitic Structure on the Electrochemical Characteristics for the Anode of Secondary Lithium Batteries

Carbon is one of the best candidate materials for the negative electrode of rechargeable lithium batteries; however, the electrochemical characteristics are not fully understood in terms of the structure of the materials. The relationship linking the volume ration of the graphitic structure (P{sub 1}) of mesocarbon microbeads (MCMBS) and the electrochemical characteristics has been examined, and it was found that the capacity in the range between 0 to 0.25 V (vs. Li/Li{sup +}) in 1 mol/dm{sup 3} LiClO{sub 4}/ethylene carbonate (EC) + 1,2-diethoxyethane (DEE) electrolyte increased with an increase of the P{sub 1} of the MCMBs. This result shows that the lithium storage mechanism in this potential range is the lithium-intercalation reaction into the graphitic layers with the AB or ABC stacking. On the other hand, MCMB heat-treatment temperature (HTT) 1,000 C showed much larger capacity in the range between 0.25 to 1.3 V than higher HTT MCMBs, and it is suggested the interaction among each graphite layer is weaker in nongraphitized carbon than that in well-graphitized ones.

Changes in the structure and physical properties of the solid solution LiNi1−xMnxO2 with variation in its composition

The layered oxides LiNi1−xMnxO2 (x = 0.1–0.5) were synthesized and characterized using synchrotron X-ray diffraction, TOF neutron diffraction, SQUID magnetometry, ICP spectroscopy, XAFS, and electrochemical measurements. All the samples were single-phase and adopted the α-NaFeO2 structure; LiNi1−xMnxO2 can be represented as Li(Ni2+xNi3+1−2xMn4+x)O2. Structural analysis using synchrotron and neutron diffraction data demonstrated that the lattice parameters of LiNi0.5Mn0.5O2 are a = 2.892 A and c = 14.302 A and that the chemical composition can be expressed by referring to the Wyckoff positions 3a and 3b as [Li0.91Ni0.09]3a[Li0.09Mn0.5Ni0.41]3bO2. The lattice parameters a and c and the fraction of Ni at the 3a site of LiNi1−xMnxO2 increased with Mn content up to the x = 0.4 composition and then showed little change between x = 0.4 and 0.5. An increase in the Ni–O distance was observed with increasing x. The appearance of ferromagnetism was clearly observed at x = 0.4–0.5 as the Ni2+ and Mn4+ content increased. The discharge capacity of the Li/LiNi1−xMnxO2 cell decreased from 190 mAh g−1 (x = 0.1) to 140 mAh g−1 (x = 0.5).

All-Solid-State Lithium Secondary Battery with Li2S – C Composite Positive Electrode Prepared by Spark-Plasma-Sintering Process

Electrochemically active lithium sulfide-carbon (Li 2 S-C) composite positive electrodes, prepared by the spark plasma sintering process, were applied to all-solid-state lithium secondary batteries with a Li 3 PO 4 -Li 2 S-SiS 2 glass electrolyte. The electrochemical tests demonstrated that In/Li 2 S-C cells showed the initial charge and discharge capacities of ca. 1010 and 920 mAh g -1 -Li 2 S, respectively, which showed higher discharge capacity and coulombic efficiency (ca. 91%) than the Li/Li 2 S-C cells with nonaqueous liquid electrolytes (ca. 200-380 mAh g -1 -Li 2 S and ca. 27%, respectively). The ex situ S K-edge X-ray absorption fine structure measurements suggested the appearance and disappearance of elemental sulfur in the positive electrodes after charging and discharging, respectively, indicating that the ideal electrochemical reaction Li 2 S ⇔ 2Li + S proceeded in the present all-solid-state cells. Such ideal electrochemical reaction, due probably to the suppression of the dissolution of Li 2 S in the form of polysulfides into the electrolytes, would result in higher coulombic efficiency and discharge capacity as compared with those of the liquid-electrolyte cells.

Studies on PVdF-based gel polymer electrolytes

Abstract A complex of polymer, plasticizer and lithium salts can be used as a solid gel polymer electrolyte in lightweight and rechargeable lithium batteries. Considerable research has been directed towards the development of a gel polymer with high conductivity at room temperature. In this work, a gel polymer electrolyte using polyvinylidene fluoride (PVdF)-1000 (KF), a plasticizer of 1:1 ethylene carbonate (EC) and propylene carbonate (PC), and LiBF 4 salt is optimized. Gel electrolytes have high ionic conductivity, good mechanical stability, a wide electrochemical stable window, and a stable lithium interface. The results of preliminary charge–discharge of cells are discussed in detail.

Solid‐State Lithium‐Polymer Batteries Using Lithiated MnO2 Cathodes

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.

Anode Performance of Vapor‐Grown Carbon Fibers in Secondary Lithium‐Ion Batteries

Chopped vapor-grown carbon fibers (VGCFS) were studied as anodes for secondary lithium ion batteries using a 1 mol/dm{sup 3} LiClO{sub 4} in a 1:1 (by volume) mixture of ethylene carbonate (EC) and diethylcarbonate (DEC) electrolyte. VGCFs were prepared from hydrocarbons by a vapor-grown method and chopped to ca. 10 {mu}m length. Three different diameters of the VGCFS, 1, 2, and 3 {mu}m (1GWH, 2GWH, and 3GWH, respectively) were used. The VGCFs chopped after graphitization (the 2A method-VGCFs) displayed a higher capacity than those chopped before (the 1A method-VGCFs). In particular, 2GWH-2A gave a capacity of 363 mAh/g carbon, 1.6-fold higher than the capacity of 2GWH-1A; this is almost equal to the theoretical intercalation capacity of an ideal graphite (LiC{sub 6}). The cyclic voltammogram of 2GWH-2A showed the most significantly different profile from that of natural graphite among all of the VGCFs. It is suggested that a new structural change is induced in the well-graphitized VGCFs during the chopping process that affects the lithium storage reaction.

Participation of Oxygen in Charge/Discharge Reactions in Li1.2Mn0.4Fe0.4O2: Evidence of Removal/Reinsertion of Oxide Ions

We have investigated the charge-discharge mechanism in the first cycle and the origin of its high charge―discharge capacity for Li 1.2 Mn 0.4 Fe 0.4 O 2 (0.5Li 2 MnO 3 ·0.5LiFeO 2 ) positive electrode material of lithium ion batteries. Results reveal that oxygen loss occurs in the entire region of the Li 1.2 Mn 0.4 Fe 0.4 O 2 particles composed of Mn-rich (Fe-substituted Li 2 MnO 3 ) and Fe-rich (Mn-substituted LiFeO 2 ) nanodomains during the first charge. Nanodomains of Mn-Li ferrites with a spinel structure start to be formed along the particle surfaces. During the first discharge, the extracted oxygen is partially reinserted preferentially into the Fe-rich nanodomains as oxide ions rather than in the Mn-rich nanodomains, and the proportion of the spinel nanodomains decreases. The origin of the high charge―discharge capacity might be ascribed to the participation of the oxide ions and neutral oxygen species in charge compensation by incorporation of the LiFeO 2 component into Li 2 MnO 3 . Irreversible capacity at the first cycle can be caused by the irreversible loss of oxygen during the charge and irreversible structural changes throughout the cycle: the movements of transition metal ions inducing random cation-site occupation throughout the cycle, associated with the formation and incomplete disappearance of the spinel ferrite nanodomains which is almost electrochemically-inactive under the applied voltage range.

Fe-rich and Mn-rich nanodomains in Li1.2Mn0.4Fe0.4O2 positive electrode materials for lithium-ion batteries

The authors investigated the distribution and local valence states of transition metal ions in a positive electrode material for lithium-ion batteries, Li1.2Mn0.4Fe0.4O2 nanoparticles, by electron energy-loss spectroscopy combined with scanning transmission electron microscopy. The experiments clarified the coexistence of Mn-rich and Fe-rich nanodomains in each single particle, and it is found that Fe-rich nanodomains contain Mn3+ ions which should be active in a redox reaction in spite of previous views of inactive Mn4+ ions in this material. The authors discuss a redox mechanism associated with the nanodomains.

Anode characteristics of non-graphitizable carbon fibers for rechargeable lithium-ion batteries

Abstract Non-graphitizable carbon fibers heat-treated between 1000 and 1200 °C gave capacity higher than the capacity of LiC6 (372 mAh g−1) with a significant capacity below 0.1 V during oxidation. 7Li nuclear magnetic resonance (7Li-NMR) observation on lithium insertion into the carbon fibers suggested that lithium in the carbons are classified into two species. One of the lithium species was the same as that in graphitizable carbons. However, the other lithium species was quite different from that in graphitizable carbons, because the line shifts in the 7Li-NMR spectra of the carbon fibers fully lithiated to 0 V were between 80–110 ppm (versus LiCl); these shifts are larger than the maximum shift of lithium in graphitizable carbons (∼45 ppm). In particular, a significant capacity below 0.1 V corresponded to the formation of a new lithium species.

Fine Li(4 −x)/3Ti(2 − 2x)/3FexO2(0.18 ≤x≤ 0.67) powder with cubic rock-salt structure as a positive electrode material for rechargeable lithium batteries

Li4/3Ti2/3O2–LiFeO2 solid solution, Li(4 − x)/3Ti(2 − 2x)/3FexO2 (0.18 ≤ x ≤ 0.67), which has the cubic rock-salt structure (Fmm, average particle size less than 100 nm), was synthesized from Fe-Ti co-precipitates by hydrothermal reaction with excess LiOH and KClO3 at 220 °C. Calcination of the products with lithium hydroxide in an oxidative atmosphere leads to the oxidation of trivalent iron to a 3+/4+ mixed valence state. Hydrothermally-obtained Li1.2Ti0.4Fe0.4O2 gave maximum initial charge (266 mA h g−1) and discharge capacities (153 mA h g−1 around 3 V) between 2.5 and 4.8 V. Calcination enabled us not only to improve the crystallinity, but also suppress the discharge capacity fading with cycle number. Two plateaus around 3 and 4 V were observed on discharging by decreasing the amount of Li extraction (0.4 Li per chemical formula). Although the cubic rock-salt structure was retained during both charge and discharge processes, a partial 3d-cation displacement from octahedral 4a to tetrahedral 8c sites and some oxygen loss were observed after electrochemical delithiation. In-situ57Fe Mossbauer spectroscopy showed evidence of the Fe3+/Fe4+ redox only around the 4 V region.