Jun-ichi Yamaki

Synthesis, redox potential evaluation and electrochemical characteristics of NASICON-related-3D framework compounds

The framework compounds M2(SO4)3 with M = (Ti Fe), (V Fe), Fe and LixM2(PO4)3 with M = Ti, (V Fe), Fe, were synthesized and electrochemically characterized by the coin-cell method. Use of larger (XO4)n− polyanions not only allows fast Li+-ion conduction in an open three-dimensional framework that is selective for the working alkali ion on discharge; it also stabilizes operative redox potentials Fe3+Fe2+, Ti4+Ti3+ and V3+V2+ that give open-circuit voltages Voc > 2.5 V as well as access to V4+V3+, Ti3+Ti2+ and Fe2+Fe+ couples. Separation of the V4+V3+ and V3+V2+ couples were found to be 2.0 V. Fe2(SO4)3 has both monoclinic and rhombohedral modifications that give a flat open-circuit voltage Voc = 3.6 V versus Li and a reversible capacity for ~ 1.8 lithium atoms per formula unit. LixFe2(SO4)3 shows an abrupt voltage drop occurring for x > 2 that can be held in check by the addition of buffers such as Li3Fe2(PO4)3, FeV(SO4)3 and LiTi2(PO4)3. Changing the polyanion group from (SO4)2− to (PO4)3− in these framework compounds decreases the redox potentials from 3.2 to 2.5 V for the Ti4+Ti3+ couple, 2.5 to 1.7 V for the V3+V2+ couple and 3.6 to 2.8 V for the Fe3+Fe2+ couple. Comparative advantages and disadvantages of framework cathodes for Li rechargeable battery applications are discussed.

Characterization and cathode performance of Li1 − xNi1 + xO2 prepared with the excess lithium method

Samples of Li1 − zNi1 + xO2 with various x values were synthesized and their electrochemical properties, phase transitions, and ordering phenomena were investigated comparatively. In order to synthesize samples with a small x value, an excess lithium was used as a starting material to compensate for lithium loss during the calcination process. A stoichiometric sample with a large reversible capacity of more than 200 mAh g−1 is also described.

Electrochemical Properties of NaTi2(PO4)3 Anode for Rechargeable Aqueous Sodium-Ion Batteries

The charge/discharge characteristics of NaTi2(PO4)3 as an anode active material for aqueous sodium-ion battery containing 2 M Na2SO4 aqueous electrolyte were examined. Cyclic voltammograms, galvanostatic discharge/charge and XRD data of the material indicated that sodium can be reversibly intercalated into NASICON-type NaTi2(PO4)3 without serious degradation of the host structure. The best reversible capacity at rate of 2.0 mA cm−2 was 93% of the theoretical capacity of 133 mAh g−1 and the plateau voltage was 2.1 V versus Na/Na+.

Thermal behavior of Li1−yNiO2 and the decomposition mechanism

Abstract We report the thermal behavior of Li 1− y NiO 2 up to 300°C which we investigated using thermogravimetry (TG) and differential scanning calorimetry (DSC). The decomposition mechanism was studied using the X-ray diffraction data obtained for the thermally decomposed products. At about 200°C Li 1− y NiO 2 turned into Li (1− y )/(2− y ) Ni 1/(2− y ) O which has a rock-salt structure. This was accompanied by oxygen evolution. A significant exothermic process was observed for compounds with y ≥0.7 and this resulted in products with disordered rock-salt structures. Using these results, we propose a mechanism in which the exothermic behavior is caused by random cation mixing.

A consideration of lithium cell safety

The safety characteristics of commercial lithium ion cells are examined in relation to their use as batteries for cellular phones. This report describes a theoretical approach to an understanding of cell safety, example results of safety tests that we performed on lithium ion cells, and also presents our views regarding cell safety.

Thermal stability of LixCoO2 cathode for lithium ion battery

It is well known that charged LixCoO2 (x<1) is metastable, and that oxygen evolution has been observed at temperatures above 200 °C. LixCoO2, delithiated by a chemical method using H2SO4, was investigated by means of differential scanning calorimetry (DSC) with/without an electrolyte (1 M LiPF6/ethylene carbonate (EC)+dimethyl carbonate (DMC)). The lithium content x in the delithiated LixCoO2 was determined by atomic absorption spectroscopy. The DSC profile of Li0.49CoO2 showed two exothermic peaks, one beginning at 190 °C and the other beginning at 290 °C. From high-temperature X-ray diffraction (XRD), it was found that the first peak, from 190 °C, was the phase transition from a monoclinic (R3m) to a spinel structure (Fd3m). The DSC measurements of Li0.49CoO2 with the electrolyte at various mixing ratios showed two exothermic peaks, one beginning at 190 °C and the other at 230 °C. The exothermic heat of each peak was proportional to the amount of Li0.49CoO2. The peak starting at 190 °C probably resulted from the decomposition of solvent due to an active cathode surface, and the peak starting at 230 °C was electrolyte oxidation caused by released oxygen from Li0.49CoO2. The exothermic heat from 190 to 230 °C based on cathode weight was 420±120 J/g, and that from 230 to 300 °C was 1000±250 J/g.

Cathode performance and voltage estimation of metal trihalides

Compounds with highly ionic metal-ligand bonds are attractive for use as high voltage cathodes. Therefore, we investigated metal trihalide cathodes, in particular trifluorides (MF3). FeF3 shows a mean discharge voltage and quasi open-circuit voltage (QOCV) of 3.0 and 3.4 V, respectively. X-ray analysis shows that the lithiation reaction proceeds in a topotactic manner. TiF3 and VF3 have mean discharge voltages of 2.5 and 2.2 V, respectively. These three compounds have the same layer structures and almost the same rechargeable capacity of 80 mAh g−1. MnF3, which has a monoclinic lattice, exhibits an initial voltage of 4.2 V, however, lithiation does not proceed due to a high overvoltage. These high voltages indicate the highly ionic nature of MF3 cathodes, and they correspond to values estimated using the standard electrode potentials of the ‘naked’ ions.

Electrochemical and Thermal Behavior of LiNi1 − z M z O 2 ( M = Co , Mn , Ti )

The authors report the synthesis and electrochemical properties of highly stoichiometric LiNi{sub 1{minus}z}M{sub z}O{sub 2} (M = Co, Mn, Ti, z {le} 0.3) samples. With the excess lithium method, samples with a well-defined layered structure can be prepared in air. A large rechargeable capacity of about 200 mAh/g is obtained for 10% substitutives. Structural changes during charging and lithium ordering phenomena are discussed. The authors describe the thermal behavior of the substitutives and report the enhanced thermal stability and large rechargeable capacity of the manganese substitutives.

The cathodic decomposition of propylene carbonate in lithium batteries

Abstract Cathodic decomposition of propylene carbonate (PC) on the graphite electrode in lithium batteries is investigated galvanostatically and potentiostatically. A new reaction process of PC decomposition is proposed, in which the PC decomposition reaction completes with the formation of the lithium intercalated compound (GIC). The process is also argued theoretically and experimentally. Many experimental results support this process.

Thermal stability of graphite anode with electrolyte in lithium-ion cells

Abstract Thermal stability of electrochemically lithiated graphite with 1 M LiPF6/EC+DMC and PVdF-binder has been investigated. DSC measurements using an airtight sample case reveal a mild heat generation started from 130 °C with a small peak at 140 °C. The mild heat generation continued until a sharp exothermic peak appeared at 280 °C. The heat evolved in the small peak at 140 °C decreased by storage of the lithiated graphite with PVdF and the electrolyte at 50 °C for 3 days before the DSC measurements. The lithiated graphite with the electrolyte without PVdF-binder did not show the small peak at 140 °C. The peak at 140 °C seems to be caused by the reaction (the Solid Electrolyte Interphase (SEI) formation) of the electrolyte and lithiated graphite, whose surface is covered by poly(vinylidene flouride) (PVdF)-binder without formation of SEI at a lower temperature. The mild heat generation from 140 to 280 °C is the reaction of the lithiated graphite and the electrolyte through SEI (SEI formation), because there was no such mild heat generation when non-lithiated graphite was used. The peak at 280 °C is probably a direct reaction of lithiated graphite and electrolyte by a breakdown of SEI.