Katsuhito Takei

Gas generation mechanism due to electrolyte decomposition in commercial lithium-ion cell

Abstract To elucidate the gas generation mechanism due to electrolyte decomposition in commercial lithium-ion cells after long cycling, we developed a device which can accurately determine the volume of generated gas in the cell. Experiments on LixC6/Li1−xCoO2 cells using electrolytes such as 1 M LiPF6 in propylene carbonate (PC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) are presented and discussed. In the nominal voltage range (4.2–2.5 V), compositional change due mainly to ester exchange reaction occurs, and gaseous products in the cell are little. Generated gas volume and compositional change in the electrolyte are detected largely in overcharged cells, and we discussed that gas generation due to electrolyte decomposition involves different decomposition reactions in overcharged and overdischarged cells.

5 V Class All-Solid-State Composite Lithium Battery with Li3 PO 4 Coated LiNi0.5Mn1.5 O 4

A 5 V class ceramic/polymer composite all-solid-state lithium battery was prepared. The cell configuration was [Li 3 PO 4 coated LiNi 0.5 Mn 1.5 O 4 |solid polymer electrolyte| Li]. The total cell impedance was 4 kΩ at 333 K and the discharge capacity was 100 mAh g -1 with a discharge voltage plateau in both 4.7 and 4.1 V regions. X-ray absorption near-edge structure results indicated that both transition metal ions, Ni and Mn, involved in the oxidation/reduction processes. The cell without Li 3 PO 4 showed a lower discharge voltage plateau (<3.5 V) than the composite one. Although the Li 3 PO 4 film was so thin that it could be nearly removed with only 2 min of Ar etching in X-ray photoelectron spectroscopy, Li 3 PO 4 is thought to have a function as a solid electrolyte interface between LiNi 0.5 Mn 1.5 O 4 and SPE to prevent the degradation of solid polymer electrolyte.

Precise Electrochemical Calorimetry of LiCoO2/Graphite Lithium-Ion Cell Understanding Thermal Behavior and Estimation of Degradation Mechanism

The thermal behavior of a lithium-ion cell during charge and discharge was determined using an isothermal calorimeter. In order to assign the thermal characteristics of the lithium-ion cell to the cathode (LiCoO 2 ) and the anode (graphite) material, a LiCoO 2 /Li cell and a graphite/Li cell were prepared. The thermal behaviors were compared with that of a lithium-ion (LiCoO 2 /graphite) cell. The notable thermal characteristics could be attributed to the individual electrode materials. In particular, the discontinuous thermal profiles showed good agreement with the phase change of the host structure of each electrode material. The degradation factor of commercially available lithium-ion cells was determined using these discontinuous thermal profiles as an indicator of the electrode reactions. We found that the decrease in the effective active material of the graphite is the main cause of capacity fading after cycling.

Physicochemical and Electrochemical Properties of Glyme-LiN(SO2F)2 Complex for Safe Lithium-ion Secondary Battery Electrolyte

Electrolyte performance of CH 3 -(OC 2 H 4 ) 3 -CH 3 (TG)/LiN(SO 2 F) 2 (LiFSI) 1:1 molar mixture was investigated for high-safety lithium-ion secondary batteries by various physicochemical and electrochemical measurements. The TG-LiFSI electrolyte formed 1:1 complex, and showed relatively high thermal stability. Stable charge/discharge (intercalation/deintercalation) of lithium ions with both LiFePO 4 positive electrode and graphite negative electrode were enabled with high coulombic efficiency and long cycles. Moreover, long cycle charge/discharge operations of [LiFePO 4 positive electrode|TG-LiFSI electrolyte|graphite negative electrode] cell was achieved with 82% of capacity retention at 100 cycles.

Electrochemical and calorimetric approach to spinel lithium manganese oxide

Abstract A simultaneous electrochemical and calorimetric measurement was conducted on a spinel lithium manganese oxide (LiMn 2 O 4 )-cathode, carbon-anode prismatic lithium ion cell. From the comparison of thermal behavior at room temperature (25°C) and at an elevated temperature (40°C), a pronounced exothermic thermal behavior was observed during charging in the high voltage region (0.2 x x Mn 2 O 4 ) at elevated temperatures. The degraded cell that was operated at 40°C indicated that the capacity fading occurred mainly in the same high voltage region in advance of lower region. The irreversible thermal behavior observed by calorimetry clearly indicates the degradation of LiMn 2 O 4 at elevated temperatures.

An X-ray photoelectron spectroscopy study on the surface film on carbon black anode in lithium secondary cells

Abstract The property of the surface film formed on the carbon black anode of a lithium cell at the initial reduction stage was investigated. About 90% of the carbon black surface was covered with a smooth film about 10–15 A in thickness after five cyclic voltammetry cycles. The X-ray photoelectron spectroscopy analysis revealed that this film contained oxygen atom bonding directly to the carbon atoms and was formed by decomposition of the solvent. The inhibiting effect of the surface film against further solvent decomposition reaction disappeared after washing with ethanol or heat treatment above 200 °C. The surface film was confirmed to form also by chemical reduction using lithium naphthalide, where lithium insertion in the carbon black took place. The inhibiting effect of a chemically formed surface film against the solvent decomposition was less marked than that of an electrochemically formed film.

Effects of the macroscopic structure of carbon black on its behaviour as the anode in a lithium secondary cell

Abstract The anodic behaviour of various carbon black (CB) samples prepared by the oil furnace method has been investigated in 1 M LiPF6 solution in ethylene carbonate-1,2-dimethoxyethane 50 50 (by volume). These CB samples had different macroscopic structures, though their microscopic parameters such as d002 and Lc remained constant substantially. The irreversible capacity at the initial reduction stage increased almost linearly with the BET surface area. The reversible capacity, on the other hand, showed no correlation with the BET area and was large for a highly aggregated coarse CB sample. The neck position connecting two primary CB particles was considered to be mainly active for the electrochemical insertion/extraction of lithium including effective electrolyte penetration on to the carbon surface.

AC Impedance Study of High-Power Lithium-Ion Secondary Batteries—Effect of Battery Size

We measured the internal resistance of high-power lithium-ion secondary batteries for next-generation electric vehicles. We succeeded in separating the two elements of the electrode (positive and negative)/electrolyte interfacial resistances (R int ) by measuring the ac impedance spectrum at a low state of charge and low temperature. In addition, R int took a value close to the capacity ratio between small- (280 mAh) and large-capacity (7 Ah) cells when the internal resistances of the two cells calculated from the ac impedance spectrum were compared. These results indicate that interfacial degradation, which is one of the main factors causing battery degradation, can be estimated by analyzing R int using a smaller cell.

Static and Transport Properties of Alkyltrimethylammonium Cation-Based Room-Temperature Ionic Liquids

We have measured physicochemical properties of five alkyltrimethylammonium cation-based room-temperature ionic liquids and compared them with those obtained from computational methods. We have found that static properties (density and refractive index) and transport properties (ionic conductivity, self-diffusion coefficient, and viscosity) of these ionic liquids show close relations with the length of the alkyl chain. In particular, static properties obtained by experimental methods exhibit a trend complementary to that by computational methods (refractive index ∝ [polarizability/molar volume]). Moreover, the self-diffusion coefficient obtained by molecular dynamics (MD) simulation was consistent with the data obtained by the pulsed-gradient spin-echo nuclear magnetic resonance technique, which suggests that computational methods can be supplemental tools to predict physicochemical properties of room-temperature ionic liquids.

The 200 V 2 kWh energy storage multicell system with 25 Wh Li/LiV3O8 single cells

Abstract A 200 V 2 kWh Li/LiV 3 O 8 multicell system was constructed by seventy-seven 25 Wh single component cells in series connection and was operated under connection with a commercial power line. The discharge power of 1.94 kWh and the energy efficiency of over 93% were demonstrated almost as designed. The rate capability of this multicell system was poor compared with that of a single cell. The internal resistance of the system resulted in the iR loss, and the safely set operating voltage limits for the system reduced the output energy of individual cells. The non-uniformity of the operating voltage and of the state-of-charge for all component cells were also analyzed in addition to the local temperature rise in the stack.