Hajime Miyashiro

Imidazolium-Based Room-Temperature Ionic Liquid for Lithium Secondary Batteries Effects of Lithium Salt Concentration

To understand the basic properties of lithium secondary batteries which consist of nonflammable and nonvolatile room-temperature ionic liquid electrolytes, we examined the ionic conductivity, electrolyte/electrode interfacial resistance, and charge-discharge rate characteristics by varying the lithium salt concentration in the room-temperature ionic liquid, lithium salt binary electrolytes. By using a modified imidazolium cation-based room-temperature ionic liquid as an electrolyte, the lithium secondary batteries achieved a stable charge-discharge operation of more than 100 cycles (cathode LiCoO 2 , anode lithium metal, voltage region 3.0-4.2 V, current density 1/8 C). Moreover, we found that an optimal lithium salt concentration exists for obtaining an excellent battery rate performance, which depends on delicate balances in several factors, such as ionic conductivity (viscosity), interfacial resistances at the LiCoO 2 cathode/electrolyte interface, and the lithium metal anode/electrolyte interface.

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.

Reversibility of Lithium Secondary Batteries Using a Room-Temperature Ionic Liquid Mixture and Lithium Metal

Lithium secondary batteries that use a room-temperature ionic liquid containing a lithium salt as an electrolyte are prepared (cathode: , anode: lithium metal). The prepared batteries showed values near the theoretical charge-discharge capacity in the first cycle and excellent reversibility (initial discharge capacity: , 100th discharge capacity: vs , C/8) at room temperature.

Highly reversible lithium metal secondary battery using a room temperature ionic liquid/lithium salt mixture and a surface-coated cathode active material

For the purpose of realizing high-voltage, high-capacity, long-life and safe rechargeable batteries, a lithium secondary battery that uses high-voltage stable ZrO2-coated LiCoO2 cathode powder and a nonvolatile high-safety room temperature ionic liquid was fabricated.

All-solid-state lithium secondary battery with ceramic/polymer composite electrolyte

Abstract Dense lithium lanthanum titanate, (Li,La)TiO 3 , pellets were prepared using a spark-plasma sintering (SPS) method. The obtained (Li,La)TiO 3 pellets showed relatively high lithium ion conductivity, typically 10 −3 S cm −1 at 22 °C, with an activation energy of 30.1 kJ mol −1 . Lithium manganese oxide, LiMn 2 O 4 , was deposited on (Li,La)TiO 3 pellets by an electrostatic spray deposition (ESD) method at 400 °C without significant formation of by-products at the interface (Li,La)TiO 3 /LiMn 2 O 4 . An all-solid-state battery system, LiMn 2 O 4 /(Li,La)TiO 3 /SPE/Li, where a solid polymer electrolyte (SPE) was sandwiched between (Li,La)TiO 3 and Li, showed good charge/discharge characteristics over 100 cycles at 60 °C.

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.

Quaternary Ammonium Room-Temperature Ionic Liquid/Lithium Salt Binary Electrolytes: Electrochemical Study

To determine the properties of the quaternary ammonium cation room-temperature ionic liquid [N,N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium bis (trifluoromethylsulfonyl) amide (DEMETFSA)] used in lithium secondary battery electrolytes, the lithium ionic transport properties of electrolytes, the characteristics of the interface of a LiCoO 2 cathode and a metallic lithium anode, and battery performance were widely investigated. A DEMETFSA-LiTFSA binary electrolyte showed high chemical stability with lithium metal electrode and a relatively high lithium cationic transport number (0.13), as determined by electrochemical measurements. The prepared [LiCoO 2 cathode|DEMETFSA-LiTFSA binary electrolyte|lithium metal anode] cell showed sufficient charge/discharge reversibility over 100 cycles (voltage range, 4.2-3.0 V). Moreover, the reversibility of capacities and coulombic efficiencies degraded with increasing upper cutoff voltage owing to cathode/electrolyte interfacial degradation, which were analyzed in detail by impedance measurements.

Quaternary ammonium room-temperature ionic liquid including an oxygen atom in side chain/lithium salt binary electrolytes: ionic conductivity and 1H, 7Li, and 19F NMR studies on diffusion coefficients and local motions.

A room-temperature ionic liquid (RTIL) of a quaternary ammonium cation having an ether chain, N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)amide (DEME-TFSA), is a candidate for use as an electrolyte of lithium secondary batteries. In this study, the electrochemical ionic conductivity, sigma, of the neat DEME-TFSA and DEME-TFSA-Li doped with five different concentrations of lithium salt (LiTFSA) was measured and correlated with NMR measurements of the diffusion coefficients D and the spin-lattice relaxation times T1 of the individual components DEME (1H), TFSA (19F), and lithium ion (7Li). The ion conduction of charged ions can be activated with less thermal energy than ion diffusion which contains a contribution from paired ions in DEME-TFSA. In the doped DEME-TFSA-Li samples, the sigma and D values decreased with increasing salt concentration, and within the same sample generally DLi<DTFSA<DDEME except for the sample having the lowest salt concentration at low temperatures. Since plots of the temperature dependence of T1 of the 1H and 7Li resonances showed T1 minima, the correlation times tauc(H) and tauc(Li) were calculated for reorientational motions of DEME and the lithium jump, respectively. At the same temperature, tauc(Li) is longer than tauc(H), suggesting that the molecular motion of DEME occurs more rapidly than the lithium jump. Combining the DLi and tauc(Li), averaged distances for the lithium jump were estimated.

Improvement of Degradation at Elevated Temperature and at High State-of-Charge Storage by ZrO2 Coating on LiCoO2

A uniform ZrO 2 coating on LiCoO 2 cathode materials for rechargeable lithium batteries was applied by a spray coating technique. The cells showed improved cycle performance and better durability of storing the cell (calendar life) under a high-voltage charging condition (4.2 V-313 K). X-ray diffraction and calorimetric study revealed that no marked change was observed in the bulk properties, such as crystal structure and phase transition, in the cathode during charge and discharge. The suppression of the increase of cathode/electrolyte interfacial impedance was observed by ZrO 2 coating. Thus, the improved electrochemical performance in the higher voltage region (>4.2 V) is ascribed to the stabilization of the interface between the cathode and electrolyte materials.

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.