Yasutaka Ohno

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.

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.

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.

Effects of alkyl chain in imidazolium-type room-temperature ionic liquids as lithium secondary battery electrolytes

Lithium secondary batteries that use a room-temperature ionic liquid as an electrolyte were investigated for the purpose of realizing high-safe batteries. For the improvement of stability under charge/discharge operation with electrodes, we focused attention on a series of l-alkyl-3-methyl-imidazolium bis(trifluoromethane sulfonyl)imide. The temperature dependence of ionic conductivity and battery charge-discharge performance were examined by changing the alkyl chain lengths: -methyl/-ethyl/-butyl/-hexyl/-octyl. According to the results, the effects of extending the alkyl chain were confirmed in, for example, the increase in carrier ion number, and the improvement of battery charge-discharge performance characteristics.

Quaternary ammonium room-temperature ionic liquid including an oxygen atom in side chain/lithium salt binary electrolytes: ab initio molecular orbital calculations of interactions between ions.

Interactions of the lithium bis(trifluoromethylsulfonyl)amide (LiTFSA) complex with N, N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium (DEME), 1-ethyl-3-methylimidazolium (EMIM) cations, neutral diethylether (DEE), and the DEMETFSA complex were studied by ab initio molecular orbital calculations. An interaction energy potential calculated for the DEME cation with the LiTFSA complex has a minimum when the Li atom has contact with the oxygen atom of DEME cation, while potentials for the EMIM cation with the LiTFSA complex are always repulsive. The MP2/6-311G**//HF/6-311G** level interaction energy calculated for the DEME cation with the LiTFSA complex was -18.4 kcal/mol. The interaction energy for the neutral DEE with the LiTFSA complex was larger (-21.1 kcal/mol). The interaction energy for the DEMETFSA complex with LiTFSA complex is greater (-23.2 kcal/mol). The electrostatic and induction interactions are the major source of the attraction in the two systems. The substantial attraction between the DEME cation and the LiTFSA complex suggests that the interaction between the Li cation and the oxygen atom of DEME cation plays important roles in determining the mobility of the Li cation in DEME-based room temperature ionic liquids.

Configurational Entropy of Lithium Manganese Oxide and Related Materials, LiCr y Mn2 − y O4 ( y = 0 , 0.3 )

The change in entropy of Li x Cr y Mn 2-y O 4 (y = 0, 0.3), AS, was determined by potentiometric and calorimetric approaches. The AS obtained by the two different methods showed similar trends. The peak in AS was explained by the partial Li-ion ordering of the Li(1)(0, 0, 0) and Li(2)(0.25, 0.25, 0.25) sites in the sublattice of the 8a site at x = 0.6. The following complete ordering at x = 0.5 was accompanied with the rearrangement of the partial ordering between Li(1) and Li(2). Although the AS peak was diminished by partial Cr-ion substitution because of the incomplete ordering due to the random dispersion of Cr ions, the peak position (x) of AS remained unchanged. This suggested that the voltage step between 0.6 > x > 0.5 in Li x Cr y Mn 2-y O 4 was not due to the structural change in the host spinel structure but mainly caused by the Li-ion partial ordering at x = 0.6, the rearrangement of the two domains (0.6 > x > 0.5), and the following perfect ordering (x = 0.5).

Phase transition and conductive acceleration of phosphonium-cation-based room-temperature ionic liquid

An unusual ionic conduction phenomenon related to the phase transition of a novel phosphonium-cation-based room-temperature ionic liquid (RTIL) is reported; we found that in the phase change upon cooling, a clear increase in ionic conductivity was seen as the temperature was lowered, which differs from widely known conventional RTILs; clearly, our finding of abnormality of the correlation between temperature change and ionic conduction is the first observation in the electrolyte field.