We report herein on the mechanism for surface film formation on graphite negative electrodes of lithium-ion batteries in several ethylene carbonate (EC)- and propylene carbonate (PC)-based electrolyte solutions. This review is based on extensive studies of surface morphology changes of graphite during charging and discharging and ion-solvent interactions in various non-aqueous electrolyte solutions. In situ electrochemical scanning probe microscopy (SPM) was used as a main analytical tool, and effects of co-solvents on surface film formation in EC-based solutions, effects of film-forming additives on surface film formation in PC-based solutions, and growth of surface film are discussed and reviewed in this article.

Large capacity Li-ion cells with 100 Ah had been developed to meet the requirements of space applications. These cells are organic-electrolyte Li-ion cells of LiCoO₂/Graphite systems. Calendar and cycle life characteristics of the 100 Ah Space Li-ion cells have been evaluated under various test conditions with wide range of temperature (0°C-60°C), depth of discharge (3%-80% DOD), and state of charge (0%-100% SOC). The test was started in June 1999, and the data have been accumulated for around four years. From these results, based on our capacity loss mechanism, we have confirmed that the calendar and cycle capacity loss of the Li-ion cells can be calculated by using simple formulas. To calculate the calendar capacity loss, multiply the rate constant (kf) by the square root of storage time. The rate constant kf is function of temperature and SOC, and we have confirmed that the kf is affected by electrical potential of a negative electrode. To calculate the cycle capacity loss, multiply the rate constant (kc) by the square root of cycle number. The rate constant kc is function of DOD, and we have confirmed that the kc is not affected by temperature in the range of 15°C to 60°C. Our life estimation results shows that our Space Li-ion cells have sufficient capability to achieve the mission life requirements for several kinds of artificial satellite such as geostationary orbit satellites (GEOs) and low earth orbit satellites (LEOs).

The changes of O K-edge X-ray absorption spectroscopy (XAS) have been investigated using multiple scattering theory for before and after Li inserted perovskite, Li_xLa_1/3NbO₃. Qualitative accordance of spectral change was obtained between experimental and theoretical spectra. It is concluded that the absorption peaks at 532 eV which appeared after Li insertion were due to an interaction between Li and oxygen.

Novel solid polymer electrolytes containing group 13/III metal alkoxides having Lewis acidity have been synthesized by hybridization of polymer from methoxy poly(ethylene glycol) monomethacrylate, B {(OC₂H₄)₁₂OCH₃}₃ or Al(OC₂H₅)₃ or Ga(OC₂H₅)₃, and LiCl ; the resulting solid polymer electrolyte containing Ga(OC₂H₅)₃ exhibited a conductivity of one order higher than that of alkoxide-free electrolyte.

In this work, we present a set of thermal characterization experiments of charged prismatic polymer lithium-ion battery (PLB). Cells at different state of charge (SOC) were tested inside an accelerated rate calorimeter (ARC) to determine the onset-of-thermal runaway (OTR) temperatures, and the thermally activated components were followed by monitoring the impedance (at 1 kHz) and the open circuit voltage (OCV) as a function of temperatures. An increase in the impedance was observed at around 133°C corresponding to the polyethylene separator shutdown. Above 140°C, the OCV dropped to zero indicating an internal short-circuit due to the separator meltdown, suggesting that the pinholes created in the separator at meltdown were large enough to create an internal short circuit. In addition, electrochemical impedance spectroscopy (EIS) at some SOCs was performed in the frequency range of 10 kHz-0.1 Hz at various temperatures (25-130°C). Results were modeled using an appropriate equivalent circuit, and parameters that affected the battery behavior at high temperatures would be identified.

Electrodeposition of metallic lithium has been investigated on a tungsten electrode in 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfone)imide (BMPTFSI) room-temperature molten salt. It was suggested that a passivation film forms on the electrode surface as a result of the reductive decomposition of the organic cation, BMP⁺. The film obtained in the absence of LiTFSI seems to have no conductivity and reduced further at more negative potential. On the other hand, the film obtained in BMPTFSI containing 1 M LiTFSI seems to have Li⁺-conductivity and enable the deposition and dissolution of metallic lithium electrochemically. The addition of ethylene carbonate reduced the amount of electric charge required for the formation of the passivation film.

Amorphous 66.7V₂O₅ • 33.3FeOOH (mol%) powders were synthesized by mechanical milling treatments of the mixture of crystalline V₂O₅ and FeOOH at room temperature in air. The mechanochemically synthesized amorphous powders worked as a cathode in a cell with lithium metal as an anode. The amorphous cathode materials maintained the high capacities over 200 mAh g⁻¹ at least up to the 50th cycle at an apparent current density of 0.5 mA cm⁻² at room temperature. Amorphous 66.7V₂O₅ • 33.3FeOOH powders obtained by mechanical milling is one of the promising candidates as cathode materials for lithium secondary batteries.

A new type of lithium ion conducting solid electrolyte, LiNO₃ with (Gd_0.9La_0.1)₂O₃, was successfully developed. The highest lithium ion conductivity of 2.02 × 10⁻¹ S·cm⁻¹ was obtained at 773 K for the 0.325(Gd_0.9La_0.1)₂O₃-0.35LiNO₃ solid whose composition is the solubility limit of LiNO₃ into (Gd_0.9La_0.1)₂O₃ crystal lattice. Since the Li⁺ ion conductivity was comparable to that of the highest one of LISICON series, the present 0.325(Gd_0.9La_0.1)₂O₃-0.35LiNO₃ solid was expected to be one of suitable candidates for practical Li⁺ ion conducting solids.

Lithium iron oxides (Li_xFe_yO_z) were synthesized using LiOH and α-FeOOH by the solid-state reaction at various temperatures (200-800°C). The lithium iron oxide obtained at 200°C showed an amorphous nano-crystalline phase in the XRD diagram. It presented not only a high initial discharge capacity (215 mAh g⁻¹), but also excellent cycle retention from the 11th to the 50th cycle (95%) between 1.5 V and 4.5 V. However, the powder obtained at 800°C showed a single α-LiFeO₂ phase and exhibited very poor cycle performance below 5 mAh g⁻¹ under the same test condition.

Li metal was charged on a substrate in binary electrolytes, propylene carbonate (PC) with dimethyl carbonate (PC-DMC) or ethylene carbonate with DMC (EC-DMC), containing Li bis(perfluoroethylsulfonyl)imide [Li(C₂F₅SO₂)₂N] in the presence of metal iodide additives, magnesium iodide (MgI₂) or aluminum iodide (AlI₃). This electrodeposited Li with the additives showed a high cycleability even after it was transferred to the corresponding electrolyte without any additives; its cycleability was nearly equal to or, in some cases, higher than that of Li metal cycled all along with the corresponding additive. This means that the primary Li charge in the presence of the additives can provide a Li metal anode with an enhanced cycleability no longer with the need for the additives during the subsequent cycles. Ac impedance analysis revealed that the interfacial resistance of Li anode prepared by the leadoff charge with the additives remains fairly low even in the subsequent cycles.

Polymeric gel electrolytes have been prepared by a thermal casting method, which consist of poly(vinylidene fluoride-co-hexafluoropropylene) [P(VdF-HFP)] as a host polymer, propylene carbonate (PC) or a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) as a plasticizer, and LiX (X = ClO₄ or (C₂F₅SO₂)₂N) as a carrier salt. The ionic conductivity and the mechanical properties of the polymeric gel electrolyte were varied with the combination of the lithium salt and the plasticizer solvent. The electrode performance of polyacene (PAS) in the gel electrolyte was briefly examined to discuss the applicability of the gel electrolyte to lithium ion battery systems. A half cell assembled with the gel film containing 1.0 mol dm⁻³ Li(C₂F₅SO₂)₂N/(EC + DEC) showed discharge capacity of about 600 Ah kg⁻¹ (with respect to mass of PAS), which was equivalent to that observed for PAS in conventional liquid electrolytes.

The individual self-diffusion coefficients of the lithium ion, anion and solvent in the electrolytes for lithium batteries have been measured by the pulsed-gradient spin-echo (PGSE) ⁷Li, ¹⁹F, and ¹H NMR, respectively. Based on the classical Stokes-Einstein equation for self-diffusion coefficient, bulk viscosity and Stokes radius of the diffusing species, the solvation effects around the lithium ion in 17 different solvents are discussed from the hydrodynamic diffusing radius of the lithium ion. The normalized anion hydrodynamic sizes of N(SO₂CF₃)₂ (TFSI) and BF₄ in GBL and PC are also calculated by using the diffusion coefficients and the van del Waals radius. The self-association of BF₄ at concentrated region is discussed.

Dibenzo[c,e][1,2]dithiin (1), phenanthro[4,5-cde][1,2]dithiin (2), triphenyleno[l,12-cde][l,2]dithiin (3), and 6,7-dithiodibenzo[c,e][1,2]dithiin (4) were synthesized and the electrochemical properties of their compounds were investigated as cathode active materials in lithium secondary batteries. When solid polymer electrolyte (SPE) was used for making cells, the initial discharge capacity of the cell made of 2 (abbreviated as the cell 2 and so on) and the cell 3 were 177.4 mAh/g and 122.3 mAh/g at 60°C respectively. Furthermore, the cell 4, which has two disulfides in a molecule, showed the good initial discharge capacity of 264.1 mAh/g and the large rechargeable capacity, and the best electrochemical behaviors among 1, 2, 3, and 4.

The effect of the addition of inorganic salts on the cathodic potential of a room temperature ionic liquid (RTIL) containing l-ethyl-3-methylimidazolium (EMI) and bis(trifluoromethylsulfonyl)amide (TFSI) ions with high purity ([H₂O] <50 ppm, [K⁺] from KTFSI <25 ppm, [Br⁻] from EMIBr <25 ppm) was investigated by linear sweep voltammmetry of Pt electrode. The added salts in this study were 0.01 mol dm⁻³ of various alkali and ammonium salts. As a result, only lithium salts shifted the cathodic limit of potential of EMITFSI toward negative direction. Other salts containing alkali cations and the ammonium cation exhibited no significant effects. The effect of lithium salt addition on the cathodic limit was lost in the presence of 460 ppm of water.

By means of an electrostatic spray deposition technique, thin-films (0.5 µm thick) of LiM_xMn_2−xO₄ (M = Ni, Co) were fabricated on gold substrates. The prepared thin-films had a spinel-related structure. Cyclic voltammetry (CV) measurements were performed in a potential range of 3.4-5.5 V vs. Li/Li⁺. Both LiNi_xMn_2−xO₄ and LiCo_xMn_2−xO₄ exhibited reversible CV peaks at higher than 4.5 V. LiNi_xMn_2−xO₄ showed two peaks at 4.7 V, and those corresponded to the redox of Ni^2+/4+ in the spinel structure, while the redox of Mn^3+/4+ took place at 4 V. Similarly, LiCo_xMn_2−xO₄ displayed current responses at 5 V due to the redox of Co^3+/4+.

The Ag-Sn system has been studied as a promising alternative anode material for advanced Li-ion batteries. Some electrodes with suitable composition and nanostructure exhibit excellent cycleability, being capable of maintaining a reversible capacity of ∼200 mAh/g over 300 cycles. The study reveals that the substantially enhanced cycling performance of these anode materials can be attributed to nanosized microstructure, uniform distribution of structurally stable compound Ag₃Sn in Sn matrix as well as suitable Ag₃Sn/Sn ratio.

Ethyl nonafluorobutyl ether (EFE) has been used as a co-solvent of nonflammable electrolyte for lithium ion batteries. The charge and discharge performances of graphite negative and lithiated metal oxide (LiCr_0.1Mn_1.9O₄) positive electrodes were examined in the EFE-based electrolyte. High reversible capacity was obtained for the graphite negative electrode but less rechargeability of the metal oxide positive electrode was observed in the EFE-based electrolyte. Addition of a small amount of biphenyl in the electrolyte improved the rechargeability of the oxide. The AC impedance responses proved that the organic additive modifies the properties of the interface between the EFE-based electrolyte and the lithiated metal oxide cathode.

A multilayered Sn-Zn/Zn/Cu film electrode was prepared by an electroplating method and the electrode performance was investigated. The heat-treated electrode showed an excellent charge-discharge performance; a discharge capacity of around 300 mAh g⁻¹ was maintained even after 100 cycles. The electrode was mainly composed of a Cu₆Sn₅ phase due to interdiffusion of Sn and Cu during heat-treatment process. At the same time, two Zn enriched layers were formed. This optimized electrode structure would relax volume expansion and phase transition during cycling, resulting in an enhanced electrode performance.

Charge-discharge performance of chemically modified LiMn₂O₄ with rare earth oxide was estimated for the cathodes of secondary lithium batteries. The capacity fade during charge and discharge cycling was investigated by use of chemically modified LiMn₂O₄. Among rare earth oxides CeO₂ showed the most superior effect on the cathode performance.

The effects of nonionic surfactants on electrochemical behaviors of graphite electrodes in propylene carbonate solutions containing Li ion were studied with a cyclic voltammetry. The reductive decomposition of propylene carbonate was markedly suppressed by the addition of nonionic surfactants such as polyethylene glycol monolaurate or polyethylene glycol monomethyl ether, and the anodic peak due to the deintercalation of Li ion from graphite electrode was clearly observed in the presence of surfactants. The addition of such kind of surfactants is highly useful for the development of Li ion secondary battery utilizing propylene carbonate.

A kind of artificial graphite particles, so called as mesocarbon microbeads (MCMB) particles was treated in Ar-H₂ plasma atmosphere to create the functional surface, and to improve electrochemical properties of graphite particles. The electrochemical properties of plasma-treated MCMB particles and the influence of post-processing atmosphere on electrochemical properties were investigated as anode of lithium-ion rechargeable battery. The modification of surface morphology and structure by H₂ plasma treatment, and the protection of adsorbing species on the surface by controlling post-processing atmosphere of dry Ar lead to the improvement of the first charge/discharge efficiency and the reversible capacity.

The iron-based vanadium oxides with different amount of vanadium and sulfur have been prepared by the hydrolysis and the solid-state methods for use as positive active material of lithium secondary cells. The reduction of the amount of vanadium in the oxide from 24 mass% to 13 mass% changed its structure from that of the hydrated iron ortho-vanadate to the other one like a mixture of the vanadate and β-FeOOH, which improved the cycle performance of the positive electrode. The incorporation of sulfur of 8 mass% in the oxide further improved this performance, and also shifted its discharge potential to the noble direction of 0.2 V.

A liquid organic electrolyte system for lithium-ion batteries with graphite anode containing vinyl group compounds (vinyl butyrate, vinyl hexanoate, vinyl benzoate, vinyl crotonate and vinyl pivalate) as electrolyte additives has been studied. The decomposition of a propylene carbonate (PC) based electrolyte on the graphite electrode could be remarkably suppressed by the addition of vinyl butyrate (VB) additive, leading to the improvement of electrochemical performances of the cell. The correlation between the lowest unoccupied molecular orbitals (LUMO) energy and the reduction potential of the additives is roughly linear.

In order to get homogeneous partially substituted lithium manganese oxide positive electrode material, we applied carbonate co-precipitation method to the synthesis of multi-doped Li_1+xM_yMn_2−x−yO₄(M = Co, Ni) spinel compounds. X-ray diffraction experiment and B.E.T measurement revealed that the Li_1+xM_yMn_2−x−yO₄(M = Co, Ni) spinel compounds having small surface area (0.16-0.22 m² g⁻¹) were successfully synthesized using carbonate co-precipitation method. The spinel compounds showed good capacity retention upon cycling test at an elevated temperature, and it was remarkably increased with increasing lithium doping amount. Furthermore, we found that rate capability of spinel compounds also depend on lithium doping amount.

We developed a high power battery pack module composed of 18650-type high-power lithium-ion batteries. The newly designed unit cell drives a maximum constant 6C-rate discharge current. The cell retains the same level of safety as the conventional type. We also developed a cycle simulation method to design a pack module for high power discharge. It predicts the cyclic behavior of the battery pack module derived from the properties of individual cells. It also makes it possible to determine the permissible range of unit cell capacity and impedance to compose high power battery pack modules.

The electrical conductivity of LiClO₄-EC solution coexisting with the monodisperse spherical SiO₂ powder was measured. The composition dependence and the activation energy of the conductivity were discussed. The conductivity depends on the solid/liquid composition, especially impregnating condition in the macro-pore among the spherical particles according to the percolation theory. The inner nano-pore contributes as the conducting path besides the supporting performance for the electrolyte solution.

The charge-discharge behaviour of tin-iron (13% by weight) alloy films plated on rolled copper foils was investigated. X-ray diffraction patterns of the tin-iron alloy films contained peaks corresponding to FeSn₂ and tin. This negative electrode possessed a discharge capacity of 356 mAh/g after 50 cycles using metallic lithium as the counter electrode and was superior to tin-plated copper and carbon negative electrodes. These results suggest that the dispersion of iron in tin deposits separated from the tin-iron alloy during the first charge reaction improves the electrical conductivity between regions of tin-lithium intermetallic compounds formed during discharge and leads to the suppression of repeated contraction and expansion of the plated film.

“Pure” β-NiOOH, a positive active material for 3 volt class lithium secondary cells, has been successfully prepared by ozonization process as an alkali-free synthetic method though electrochemical characteristics of nickel oxyhydroxide has been influenced by alkali species due to NiOOH_1−xLi_x formation by ion-exchange side reaction in an organic electrolyte. The discharge curve of the NiOOH_1−xLi_x electrode involved two potential plateaux corresponding to Ni(II)/Ni(III) and Ni(III)/Ni(IV) redox pairs. The positive electrode with this pure active material was found to show a monotonous potential variation in the discharge process.

An in-situ gas analysis was carried out using a quadrupole mass spectrometer during the overcharge test in a typical lithium ion cell, and the flow rate of the evolved gas was estimated. The mass spectrum derived from the electrolyte solvent was observed at a cell voltage over 5.0 V, and the spectrum at the mass numbers 44 and 28 was observed much higher than that of the electrolyte solvent. The spectrum of mass numbers 44 and 28 appeared to be from the reaction products of the electrolyte solution.

Effects of doping Cr and coating carbon nanotube on Si powder were studied as negative electrode of Li ion rechargeable battery. Although the large Li intercalation capacity higher than 1500 mAh/g is exhibited on pure Si, it decreased drastically with the cycle number. Increasing the electrical conductivity by doping chromium is effective for increasing the initial capacity and the cycle stability of Si for Li intercalation, and the reversible capacity of 250 mAh/g was sustained after 5 cycles of charge and discharge. Coating Cr doped Si with the carbon nanotube obtained by decomposition of CH₄ is effective for increasing the cycle stability, though the initial Li intercalation capacity decreased to ca. 1900 from 2500 mAh/g. Reversible Li intercalation capacity of 750 mAh/g was sustained after 5 cycles of charge and discharge on Si coated with carbon nanotubes.

Synthesis and the electrochemical Li de-intercalation properties of LiVOPO₄ as the alternative cathode material for Li ion rechargeable battery were investigated. Two types of LiVOPO₄ structures, orthorhombic and triclinic phases can be prepared by changing calcination temperature. At current density of 0.04 mAcm⁻² (C/50), orthorhombic phase shows the discharge capacity of 90 mAhg⁻¹ with flat potential of 3.7 V vs. Li/Li⁺, however, almost no Li de-intercalation capacity was observed on triclinic phase. Orthorhombic phase is also exhibited a good cycle stability for Li de-intercalation and intercalation.

Carbonaceous thin film electrodes were prepared by plasma-assisted chemical vapor deposition (plasma CVD). Surface of the thin film electrodes were modified by O₂ plasma. The effect of surface modification was studied by Raman spectroscopy and XPS measurements. O₂ plasma modification decreased the reduction current at about 0.6 V vs. Li/Li⁺ in the cyclic voltammograms at the 1st cycle, which was attributed to the formation of solid electrolyte interface (SEI), which was clarified by using the electrolyte containing vinylene carbonate (VC) and ethylene sulfite (ES) as additives. Effect of surface plasma modification for the thin film electrodes on SEI formation was discussed.

Several organic solvent electrolytes used in lithium batteries were added to the electrolyte based on 1-ethyl-3-methyl imidazolium tetrafluoroborate (EMIBF₄) in order to improve the deposition/dissolution behavior of lithium metal. The selected solvents for this purpose were ethylene carbonate (EC), dimethyl carbonate (DMC), and vinylene carbonate (VC). All of the selected electrolytes were effective in improving the reversible lithium deposition/dissolution on a steinless-steel electrode when added in 10 w/o. Among the selected solvents, VC provided the best behavior due to its good film-forming ability on the lithium surface in EMIBF₄.

The performance enhancing effect of a graphitized carbon fiber treated by Ag film deposition has been examined by impedance spectroscopy analysis. The surface modification was performed by depositing Ag films in vacuum at various thicknesses, followed by heat-treatment. For the case of a 200 Å-thick film the spectra indicated that both the film resistance and the charge transfer resistance increased. The heat-treated sample showed larger film resistance but smaller charge transfer resistance, implying that the increase in the reaction rate is caused by the activation of the reaction site.

In order to determine the mass transfer rate of Li through a metal foil electrode, a tungsten blue electrode doped with lithium has been developed for sensing the Li⁺ ion in an organic electrolyte. The electrode potential changed linearly with the logarithmic change in Li⁺ concentration in an organic electrolyte containing LiClO₄. By sandwiching a test foil electrode of Ni or Cu a bipolar cell was constructed for the determination of Li mass transfer rate in the metal foil. The rate determined by this method showed good agreement with that obtained by potential step current measurement.

Several kinds of metal films have a rate enhancing effect for Li insertion/extraction reaction when deposited on graphitized carbon. Ag, In, and Zn improved the reaction rates, whereas Au, Sn and Cu caused cycleability degradation. For Au and Sn this poor cycleability could be attributed for Au and Sn to the crystal collapse caused by a volume expansion due to the formation of Li alloys, whereas Cu yielded no alloys. Deposition of a Ag film over the surface of the Cu film recovered cycleability to a favorable level. Further deposition of a Cu film resulted in poor cycleability, which was again recovered by further covering with a Ag film. These results can be explained based on the insufficient property of the SEI formed on the Cu surface.

In an attempt to improve the high power capability of the Li insertion/extraction reaction of a vacuum deposited Si film on a Ni foil, we tried to increase the electron conductivity of the deposited film by sandwiching a metal conducting film of Cu and Ag with the Si film. The metal-sandwiched film showed the rate to be improved over 1.5 times as compared a single layer deposited film. The second trial for improving the rate was to use a carrier doped Si having a high conductivity. Deposition of 5 Ω cm Si film on a Ni foil resulted in an attainment of not only the high power capability over with 30 C rate but also the cycle life over 3000 cycles. The issue to be solved was to attain the capability with a very thick deposited film.

Li_3−2xM_xInBr₆ (M = Ca, Sr, Ba; x = 0-1.0) was synthesized and the cation substitution effect was investigated by means of X-ray diffraction and AC conductivity measurements. In Li_3−2xSr_xInBr₆(x≤0.7), the conductivity of high temperature phase decreased with an increase of x and LiSrInBr₆ show almost the same conductivity to Li₃InBr₆. Intensity of XRD peaks of LiSrInBr₆ was different from those of Li₃InBr₆ and the substitution of Sr was expected to affect the structure and ionic conductivity. In Li_3−2xCa_xInBr₆, the conductivity decreased monotonously with an increase of x.

In situ atomic force microscope observation coupled with cyclic voltammetry of the basal plane of highly oriented pyrolytic graphite was carried out at room and elevated temperatures to investigate the surface film formation on graphite negative electrode in lithium-ion cells. The observation revealed that solvent decomposition and precipitate layer formation were enhanced at elevated temperatures. The thickness of the resulting precipitate layer on the surface increased with an increase in temperature. It was concluded that enhanced solvent decomposition and the growth of the surface film are the major reasons for the increase in irreversible capacity at elevated temperatures.

The structure of ordered-olivine LiCoPO₄ was identified by neutron diffraction measurements. A 4.8 V discharge profile was demonstrated using coin-type cells. XAFS and magnetic measurements indicated that the high-voltage plateau of Li_1−xCoPO₄ corresponds to the Co²⁺/Co³⁺ redox reaction.

It is known that LiPF₆ electrolytes react with water. In this study, LiCl, LiF, LiBr and LiI were added to 1 M LiPF₆/EC + DEC electrolyte with about 5000 ppm water to inhibit the reaction of LiPF₆. When 0.1 M LiCl was added, the LiPF₆ electrolyte did not react with the water for 50 hours. Differential scanning calorimetry (DSC) measurement indicated that the amount of exothermic heat at 265°C, which is related to a reaction with LiPF₆, did not change at all for 48 hours when LiCl was added. Based on this result, we confirmed that LiPF₆ salt did not react with water when LiCl was added.

We investigated the crystal structure change during the charge-discharge process of LiMn_1.5Ni_0.5O₄ as a 5 V class cathode active material, which was prepared by the sol-gel method. The lithium content of Li_1−xMn_1.5Ni_0.5O₄ (x = 0.5, 0.7, 1.0) was controlled by electrochemical lithium extraction. The crystal structure was determined by Rietveld analysis using powder neutron diffraction. As a result, all samples consisted of three phases (space group: P4₃32) of different lattice constant and Ni valences. The each phase showed a different distortion of the Mn,Ni (12d)-O octahedral site and the Madelung energy.

We investigated the electronic states of Li_yMn_2−xM_xO₄ (M = Mn, Mg, Ni, Co) as cathode active materials for the 4 V class lithium secondary battery. We calculated the electronic states of Li_yMn_1.75M_0.25O₄ (M = Mn; y = 0, Mg, Ni; y = 0.5, Co; y = 0.25) using first-principles calculations by the DV-Xα method. The net charge of each atom, and the bond overlap populations of Mn-O and M-O were calculated. The strong covalency between Mn and O existed in the Li defect model and Li_yMn_1.75M_0.25O₄ (M = Mg, Ni, Co). The bonding change in Mn-O is small and maintained the spinel structure during the charge-discharge process by substitution of Mn with M (= Mg, Ni, Co).

Vanadium pentoxide (V₂O₅) xerogels containing some foreign metal cations, Na⁺, Ca²⁺ and La³⁺, have been prepared by ion-exchange of hydroxonium ions, H₃O⁺, and investigated as a cathode material for rechargeable lithium battery. The discharge capacities of the cation-exchanged V₂O₅ xerogels are identical to that of V₂O₅ xerogel containing H₃O⁺ at low current density. However, the capacities of the cation-exchanged V₂O₅ xerogels became lower than that of V₂O₅ xerogel containing H₃O⁺ with an increase in current density, suggesting the columbic interaction between the foreign metal cations and lithium ions hinders the diffusion of lithium ions between the layers.

Sydnone is known as a typical mesoionic compound, which can be represented by the resonance hybrid of different polarized ionic structures. We have newly synthesized 3-methyl-4-acetylsydnone (3-MASD) and describe the thermal characteristics. We have also investigated the physicochemical properties of 3-MASD – based binary solutions and the addition effect of the 3-MASD on cycling efficiency of a lithium anode in ethylene carbonate (EC) – chain ester (mole ratio 1 : 1) systems. The addition of the 3-MASD in a mole ratio of 0.05 is found to be the most effective for an increase of the cycling efficiency.

LiPF₆/Methyl difluoroacetate (MFA) shows a very good thermal stability with Li metal. However, LiPF₆/Ethyl difluoroacetate (EFA) did not show any improvement of the thermal stability. Using FTIR and XPS, the main component of SEI in the MFA electrolyte was found to be CHF₂COOLi. However, CHF₂COOLi does not exist in the SEI of the EFA electrolyte, because CHF₂COOLi dissolves in EFA electrolyte. CHF₂COOLi was found to dissolve in EC, DMC, and DEC. Therefore, the addition of a small amount of MFA in 1 M LiPF₆/EC + DMC, DMC, and DEC electrolytes was not effective to improve thermal stability.

Thin carbon fibers (TCF) were prepared by the spinning/stabilization/carbonization process of the polymer blend, which consists of phenolic resin (carbon precursor) and polyethylene (pyrolysing polymer without carbon residue). TCF had the diameter of 100∼400 nm and microporous structure. The conventional carbon fibers (Ref-CF) with 10∼20 µm diameter were also obtained from phenolic resin in the same process. The TCF electrode showed different properties for electrochemical Li⁺ insertion/extraction compared with the Ref-CF electrode. The cycleability and the rate properties for TCF were better than those for Ref-CF. TCF and Ref-CF had the similar carbon structure for pore structure and crystallinity. Therefore, the characteristic properties for TCF can be attributed to the size dimension effect.

A novel Rietveld analytical method for neutron diffraction data was developed with internal Si standard to adjust the scale factor and led to the accurate defective structure of lithium manganese spinel cathode after high temperature storage. An abrupt decrease of the site occupancies for all elements, g(Li), g(Mn) and g(O), was confirmed at the temperature range above 50°C, together with the corresponding shrinkage in the unit-cell volume. This supports the degradation model that the host atoms in bulk migrate to the particle surfaces to cause charge disproportionation.