Yuichi Mita

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.

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.

Multi-step constant-current charging method for electric vehicle, valve-regulated, lead/acid batteries during night time for load-levelling

For the popularization of electric vehicles (EVs), the conditions for charging EV batteries with available current patterns should allow complete charging in a short time, i.e., less than 5 to 8 h. Therefore, in this study, a new charging condition is investigated for the EV valve-regulated lead/acid battery system, which should allow complete charging of EV battery systems with multi-step constant currents in a much shorter time with longer cycle life and higher energy efficiency compared with two-step constant-current charging. Although a high magnitude of the first current in the two-step constant-current method prolongs cycle life by suppressing the softening of positive active material, too large a charging current magnitude degrades cells due to excess internal evolution of heat. A charging current magnitude of approximately 0.5 C is expected to prolong cycle life further. Three-step charging could also increase the magnitude of charging current in the first step without shortening cycle life. Four-or six-step constant-current methods could shorten the charging time to less than 5 h, as well as yield higher energy efficiency and enhanced cycle life of over 400 cycles compared with two-step charging with the first step current of 0.5 C. Investigation of the degradation mechanism of the batteries revealed that the conditions of multi-step constant-current charging suppressed softening of positive active material and sulfation of negative active material, but, unfortunately, advanced the corrosion of the grids in the positive plates. By adopting improved grids and cooling of the battery system, the multistep constant-current method may enhance the cycle life.

High-Performance Genuine Lithium Polymer Battery Obtained by Fine-Ceramic-Electrolyte Coating of LiCoO2

A lithium polymer battery with high-capacity (>180 mAh g - 1 ) LiCoO 2 was prepared by the fine-powder coating of highly lithium-ion-conductive ceramic electrolyte [Li 1 . 5 Al 0 . 5 Ge 1 . 5 (P04)3] on LiCoO 2 powder. The obtained cell (operated at 333 K) exhibited a good reversibility up to 4.4 V and a good sustainability of 105 mAh g - 1 at the 200th cycle. The oxidation of the polymer electrolyte was determined by alternating current (ac) impedance analysis, and it was observed that a suitable ceramic electrolyte coating is an effective oxidation barrier at the polymer/cathode interface.

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.

Improvement in High-Voltage Performance of All-Solid-State Lithium Polymer Secondary Batteries by Mixing Inorganic Electrolyte with Cathode Materials

By simply mixing an inorganic electrolyte (Li 3 PO 4 ) with LiCoO 2 cathode sheet materials in all-solid-state lithium polymer secondary batteries (LPBs) with a LiCoO 2 /lithium metal system, we markedly improved the charge-discbarge reversibility of such batteries in a high-voltage charge state of 4.4 V vs Li/Li + . Mixing Li 3 PO 4 with the cathode sheet materials suppressed the growing rate of the cathode/electrolyte interfacial resistance caused by the oxidation of the electrolyte during preservation tests of the LPBs at a high cathode potential under 2% of that for the case free of Li 3 PO 4 . This made the cycle performance and charge-state preservation characteristic of a Li 3 PO 4 -mixed LPB markedly improved. It is discussed based on X-ray diffraction analysis of the LPB that the generation of Co 3 O 4 from the LiCoO 2 cathode in the high-voltage state is suppressed by the Li 3 PO 4 mixing.