Hitoshi Ota

Analysis of Vinylene Carbonate Derived SEI Layers on Graphite Anode

The solid electrolyte interface (SEI) formation on composite graphite and highly oriented pyrolytic graphite in a vinylene carbonate (VC)-containing electrolyte was analyzed using evolved gas analysis, Fourier transform infrared analysis, two-dimensional nuclear magnetic resonance, X-ray photoelectron spectroscopy, time of flight-secondary-ion mass spectrometry, and scanning electron microscopy. We found that the SEI layers derived from VC-containing electrolytes consist of polymer species such as poly (vinylene carbonate) (poly(VC)), an oligomer of VC, a ring-opening polymer of VC, and polyacetylene. Moreover, lithium vinylene dicarbonate, (CHOCO 2 Li) 2 , lithium divinylene dicarbonate, (CH=CHOCO 2 Li) 2 , lithium divinylene dialkoxide, (CH=CHOLi) 2 , and lithium carboxylate, RCOOLi, were formed on graphite as VC reduction products. The presence of VC in the ethylene carbonate (EC)-based electrolyte caused a decrease in the reductive gases of the EC dimethyl carbonate solvent such as C 2 H 4 , CH 4 , and CO. The VC-derived SEI layer was formed at a potential more positive than 1.0 V vs. Li/Li + . Effective SEI formation by reduction of VC progresses before that of EC. The thermal decomposition temperature of the SEI layer derived from VC shifted to a higher temperature compared to that derived from the VC-free electrolytes. We concluded that the thermal stability of the VC-derived SEI layer has a close relation to high-temperature storage characteristics at elevated temperatures.

XAFS and TOF–SIMS analysis of SEI layers on electrodes

We have used X-ray absorption fine structure (XAFS) in order to analyze the solid electrolyte interface (SEI) layer on the graphite anode and the LiCoO2 cathode in a lithium-ion battery. The SEI layers on the electrodes in the propylene carbonate (PC)-based electrolyte containing an ethylene sulfite (ES) additive were analyzed based on the different sulfur oxidation states with sulfur K-edge X-ray absorption near-edge structure spectroscopy (S K-edge XANES), X-ray photoelectron spectroscopy (XPS) and time-of-flight–secondary ion mass spectrometry (TOF–SIMS). The SEI layer on the graphite anode was mainly consisted of a sulfite-type compound with an inorganic film like Li2SO3 and an organic films like ROSO2Li. Furthermore, it was proven that the SEI layer on the graphite anode contained alkyl sulfide species. We also found that the SEI layer on the LiCoO2 cathode also contained alkyl sulfide species.

Structural and Functional Analysis of Surface Film on Li Anode in Vinylene Carbonate-Containing Electrolyte

The lithium cycling efficiencies of the lithium anode in the ethylene carbonate (EC)-based electrolytes were improved by adding vinylene carbonate (VC) to the electrolyte. We analyzed the surface films of deposited lithium on a nickel substrate in a VC-containing electrolyte with scanning electron microscopy, Fourier transform infrared spectroscopy, two-dimensional nuclear magnetic resonance, gel permeation chromatography, and X-ray photoelectron spectroscopy. The corresponding surface films comprise various polymeric species including poly-(vinylene carbonate) [poly-(VC)], oligomeric VC, and a ring-opened polymer of VC. Furthermore, the surface film of carbon double bonds (C = C-O) and lithium carboxylate (RCOOLi) as reduction products of VC were formed on deposited lithium. These structures of the surface film on the lithium anode were similar to those on the graphite anode. At elevated temperatures, the VC-containing electrolyte led to the formation of surface films comprising poly(VC). The VC-derived polymeric surface film, which exhibited gel-like morphology, could prevent the deleterious reaction which occurs between deposited lithium and the electrolyte, resulting in an enhanced lithium cycling efficiency.

Characterization of Lithium Electrode in Lithium Imides/Ethylene Carbonate, and Cyclic Ether Electrolytes I. Surface Morphology and Lithium Cycling Efficiency

The surface and cross-sectional morphologies of deposited lithium on a nickel substrate in LiN(SO 2 C 2 F 5 ) 2 (LiBETI) electrolytes with ethylene carbonate (EC) + tetrahydropyran (THP), dimethoxyethane, dimethylcarbonate (DMC), propylenecarbonate (PC), γ-butyrolactone (GBL) (1:1) binary solvents were investigated by scanning electron microscopy observation. Dendritic morphology of deposited lithium was observed in DMC-, PC-, and GBL-containing solvents. However, deposited lithium in a EC + THP electrolyte exhibited a fine particle-like morphology and had a thinner surface film. The EC + THP electrolyte provided excellent performance based on the results of cycling characteristics using a Li/LiCoO 2 cell and Li/Ni cell. The morphology of deposited lithium in an EC + tetrahydrofuran electrolyte was strongly influenced by the kind of solutes. In LiBETI electrolyte, freshly deposited lithium exhibited uniform and fine particle-like morphology. In contrast, dendritic morphology was observed in LiN(SO 2 C 2 F 5 ) 2 (LiTFSI) electrolyte, resulting in a subsequent decrease in the lithium cycling efficiency. Electrolyte temperature was also an important factor influencing the lithium surface and efficiency. The use of LiTFSI electrolyte at elevated temperatures could suppress dendritic formation, resulting in excellent efficiency and cycling characteristics. We confirmed that the lithium surface morphology correlated well with the lithium cycling efficiency and the efficiency was strongly influenced by combinations of solvents and solutes.

Characterization of Lithium Electrode in Lithium Imides/Ethylene Carbonate and Cyclic Ether Electrolytes II. Surface Chemistry

Chemical components of surface films of deposited lithium on nickel substrates in electrolytes with LiN (SO 2 CF 3 ) 2 ) (LiTFSI), LiN (SO 2 C 2 F 5 ) 2 (LiBETI), LiPF 6 solutes, and tetrahydrofuran solvents were characterized by Fourier-transform infrared, two-dimensional nuclear magnetic resonance (2D NMR), X-ray photoelectron spectroscopy, evolved gas analysis, and ion chromatograph in order to understand the electrochemical performance of lithium imide/cyclic ether-based electrolytes. The top layers of the surface film were ROCO 2 Li, Li 2 CO 3 , polymer constituents, and LiF. The inner layers of the surface film consisted of Li 2 O and carbide species. In imide/cyclic ether-based electrolytes, Li 2 S 2 O 4 and Li 2 SO 3 as outer layers, and Li 2 S as the inner layer were formed on a nickel substrate as reductive constituents of imide solute. We found that organic surface layers consisted of lithium etoxides, lithium ethylene dicarbonate (CH 2 OCO 2 Li) 2 , polyethylene oxide, and lithium ethylene dicarbonate containing an oxyethylene unit by 1 H, 13 C, and 2D NMR. Li cycling efficiency affects not only the deposited lithium morphology but also chemical components.

Assembling process for microscopic components using magnetic force

Current methods of assembling micromachines, for example handling components using a manipulator, require accurate component positioning. In addition, it is difficult to establish electrical connections between the components with these methods. To solve these problems, this paper investigates a process for assembling microscopic components using magnetic polarity and attraction. The aim is to conduct basic research into the processes of assembling multiple components in a short time. First, the components are positioned tentatively by generating torque in the components due to magnetic polarity. After being positioned accurately by means of the tapers, the components can be connected mechanically and electrically. To study the validity of this concept, an assembly test was conducted. Bonding elements and assembly test equipment were fabricated for this test in order to examine the positioning accuracy achieved when the components were joined.

Development of coil winding process for radial gap type electromagnetic micro-rotating machine

This paper presents a manufacturing process for forming the coil winding on a cylindrical core to make a stator for a micro-generator. A cylindrical stator 1 mm in diameter and 0.5 mm in length was fabricated. This stator consists of a cylindrical core made of permalloy and six coils of copper wire with a cross section of 7 M m X 15 ,U m which were wound 20 turns per coil and with 2 pm thick polyimide insulation layers. The stator was fabricated by securing the coil onto the core die, and electroplating the remaining area with permalloy. A micro-generator 1.2 mm in diameter was fabricated incorporating this stator, and the voltage induced by the generator was measured. The examination revealed that the newly developed process is applicable to the manufacture of electromagnetic microdevices.

Micro-optical switch with uni-directional I/O fibers

The authors have developed a new 2/spl times/2 micro-optical switch. The switch features a uni-directional input/output and a drive mechanism with latch functions. The newly developed structure combines two moving mirrors and a fixed V mirror to implement a 2/spl times/2 micro-optical switch with uni-directional input/output. Furthermore, the moving mirrors have been reduced in size by integrating the drive and latch functions by the use of magnetic shaft mirrors and driving coils. The micro-optical switch has been implemented as a module mounted in a ceramic package. The modular switch is 23.5/spl times/9.86/spl times/6.76 mm in size and provides a switching time of 1.3 ms for 15 V (469 mA) and an input pulse width of 1 ms. The reflectivity of the moving mirror is 98.6% (reflection loss: 0.06 dB) and the connection loss between the fixed mirror surfaces is 5.4 dB.

Fabrication and surface modification process for micro gas bearing

A hydrodynamic gas bearing able to reduce power losses is selected, and the possibility of reducing its size is examined. The first consideration is how laser-assisted etching can be employed to form spiral grooves on the cylindrical shaft. In a hydrodynamic gas bearing, the shaft and bearing come into contact with each other when the actuator is started and stopped. It is thus necessary to reduce the frictional torque during starting and to ensure a satisfactory level of wear resistance. Next, the grooved shaft surface is modified by ion mixing. Its sliding characteristics are investigated to assess the reduction in friction and improvement in wear resistance. This reveals that it is possible to form a spiral grove 6 /spl mu/m in width and 2 /spl mu/m in depth on a 0.5-mm-diameter shaft, It is also established that, by forming a CrN film on the shaft, it is possible to achieve a friction coefficient of approx. 0.2, representing a satisfactory wear resistance. It is thus concluded that the hydrodynamic gas bearing selected can be used as a micro-actuator component.

Torque measurement method using air turbine for micro devices

The authors have developed a general-purpose system that can measure very low torques in the order of 10/sup -7/ Nm. The new method proposed here uses wind pressure to apply a load to a turbine attached to the output shaft of the device. It can therefore be used for all rotating micro-devices. The use of wind pressure reduces the loss during measurement, and makes it possible to measure low levels of torque easily by simply attaching the turbine to the device. In the present study, the measuring principle of the new system was verified. In addition, a prototype micromotor 1.6 mm in diameter was fabricated and used to demonstrate that the new system was able to measure torques in the order of 10/sup -7/ Nm while the motor was in operation.