Lithium-ion transfer through interfaces between electrodes of lithium-ion batteries and electrolyte is investigated. The interfacial lithium-ion transfer is a slow process in reactions of lithium-ion batteries. De-solvation reactions require high activation energy. The activation energy of interfacial lithium-ion transfer depends on the electrolyte solvents. Intercalation of solvated lithium-ion into graphite gives a lower activation energy than that of de-solvated lithium-ion. Surface modification by oxide on LiCoO2 decreases the activation energy. Ion-pairing at high salt concentration enhances the activation energy.
The electrochemical properties of air electrodes for lithium-air batteries using various kinds of carbon materials were investigated in an organic electrolyte of 1 mol l−1 LiPF6/PC. High-surface-area carbons such as Ketjen Black EC600JD exhibited large discharge capacities of more than 700 mAh g−1, which were roughly proportional to the surface areas. The high-performance carbons had very similar pore distributions. Moreover, pore distribution measurements revealed that carbons with larger numbers of mesopores delivered larger discharge capacities.
Thick film electrodes of Cu-coated Si particles were fabricated by an electroless deposition and a successive gas-deposition, and were evaluated their electrochemical properties as an anode of Li-ion battery. The discharge capacity and its retention at the 1000th cycle were 570 mAh g−1 and 61%. The excellent cycle life performance is attributed to an enhanced electrical conductivity due to the Cu-coating and a reversible conversion reaction between Li and Cu2O which was formed on the Cu surface. The high capacity retention is caused by a high fracture toughness of the coated Cu which can effectively relax a stress induced by volume expansion of Si as a buffer matrix.
The electrochemical and thermal properties of non-flammable polymer gel electrolyte (NPGE) have been studied to examine its potential application as an electrolyte in lithium ion batteries. Diphenyl phosphite (DPP) was used as a fire retardant component of NPGE. The electrochemical studies showed that NPGE has high ionic conductivity of 0.9 mS cm−1 at 30°C and a wide electrochemical window of ca. 4 V vs. Li/Li+, which are suitable for a practical battery system. Good thermal properties are confirmed by thermal safety calorimetry (TSC) measurements.
In this study, we investigated the electrochemical behavior of various types of activated carbons with different properties, such as the specific surface area and pore size distribution in an electrolyte containing TEA-BF4 and in that containing a spiro-type quaternary ammonium salt (SBP-BF4). EDLC cells using 1 M SBP-BF4/DMC+PC showed higher capacitances for the various activated carbons and good rate capabilities even for an activated carbon with a small pore size distribution compared to EDLC cells using 1 M TEA-BF4/PC. These results may be attributed to the higher mobility of SBP with a smaller cation size and the lower viscosity of 1 M SBP-BF4/DMC+PC due to the addition of DMC.
α-Fe2O3 fine powder that the particle size is ca. 0.3 µm were estimated as negative electrode materials for lithium cells using lithium-ion conductive organic electrolytes. The sheet-type electrodes were fabricated by using the two differential binder solutions of conventional polyvinylidene fluoride (PVDF) and polyamic acid solutions dissolved in N-methyl pyrrolidinone (NMP). The electrodes exhibited high capacities of over 1000 mAh g−1 corresponding to 6 Li per Fe2O3 at potentials ranging from the open circuit potential to 0.005 V (vs. Li/Li+) in the first charging (lithium insertion) process. First discharge capacities during lithium extraction process exhibited high capacities of over 700 mAh g−1. However, cycle life of α-Fe2O3 electrodes made using the conventional binder (PVDF) solution was poor. On the other hand, Fe2O3 electrodes made using polyamic acid solution showed better cycling performance than electrodes made from the PVDF binder solution during charge-discharge cycles and high charge-discharge efficiencies (coulombic efficiencies) over 85%. We suggest that new binders will be needed to improve cycle performance and first charge-discharge efficiencies of high-capacity iron oxide negative electrodes.
We performed a long-term test to determine the float charging durability of prismatic cells with Mn containing LiFePO4-based cathode material at various temperatures. The cell maintained more than 70% of its initial discharge capacity after float charging at 4.0 V and 25°C for 24 months. In contrast, the capacity decreased rapidly in a few months for a cell float charged at 55°C. To investigate the capacity fading mechanisms, we analyzed the cathode and the anode sheets removed from the prismatic cell used for the float charging test at 55°C with electrochemical, XRD and EPMA techniques.
We applied fluorine-containing solvents (FCSs) to an electrolyte for lithium (Li)-ion batteries with a cobalt oxide (LiCoO2) cathode to enhance the upper voltage limit and investigated its electrochemical properties in a proposed FCS-based electrolyte. FCSs were expected to have high oxidation resistance by introducing electron withdrawing fluorine atoms. A test cell containing a FCS-based electrolyte exhibited high and stable discharge capacity in 30 cycles even with a high upper cutoff voltage of 4.5 V when compared with a cell containing a conventional electrolyte. This improvement in the cycle performance was confirmed by an electrochemical impedance measurement. We found that the FCS-based electrolyte provides a favorable solid electrolyte interface (SEI) at the LiCoO2 cathode, leading to reversible charge-discharge behavior of the cathode.
The relationship between Li(I) transport property and the limiting current density was investigated in N-methyl-N-propylpyrrolidinium bis(trifluoromethylsulfonyl)amide (P13TFSA) at 30°C. The limiting current density in a LiTFSA/P13TFSA binary ionic liquid electrolyte was evaluated by chronoamperometry for a symmetric cell of Li|ionic liquid|Li, where the ionic liquid was incorporated in a porous polyethylene separator that was sandwiched between two Li electrodes. The diffusion process of Li(I) in the ionic liquid dominates the limiting current density in the cell. In addition to the Li(I) transport property of the electrolyte, the porosity and tortuosity of the separator has a significant effect on the limiting current density.
Lithium-air batteries using Ketjen Black-EC600JD for their air electrodes exhibited a rather large discharge capacity of about 3000 mAh g−1 at a current density of 0.05 mA cm−2. However, the cycle performance of the batteries was very poor due to the high overpotential for charging. The cycle properties were greatly improved by adding the Fe–Mn-based perovskite-type oxide, La0.6Sr0.4Fe0.6Mn0.4O3, to the air electrodes as an electrocatalyst.
The development of “LAMINELIGHT®” laminated stainless steel foil for the packaging of lithium-ion batteries is now complete. As a result of simulation, the degree of the deformation of battery packaging made from laminated stainless steel foil caused by rising inner pressure is less than that made from laminated aluminum foil. The simulation also shows that stainless steel foil packaging has a high level of stability and can withstand stress as the laminated resin film is smaller than that of laminated aluminum foil. In addition, the result of laminated stainless steel foil’s critical piercing load is about four times higher than that of laminated aluminum foil. These results show that laminated stainless steel foil can make packaging tougher and more durable than laminated aluminum foil.
A water-stable lithium electrode (WSLE) consisting of a lithium sheet as active layer, a nanocomposite polymer membrane as buffer layer, and lithium-ion conducting glass ceramics as water-stable protective layer, was fabricated as the anode for aqueous lithium-air secondary batteries. The WSLE with the polymer electrolyte containing BaTiO3 nano-filler (average particle size 40 nm) showed a total electrode resistance of 118 Ω cm2 in the WSLE/1 M LiCl/Pt black, air cell at 60°C, and exhibited a good cyclic performance for the dissolution and deposition of the lithium ions.
Li2FeSiO4 was prepared by the hydrothermal method, and the influences of synthesis conditions (temperature, time, and lithium-ion concentration) were systematically examined. Low-temperature phase of Li2FeSiO4 (space group; Pmn21, a=6.23, b=5.36, and c=4.98 Å) was hydrothermally prepared from LiOH, FeCl2, and amorphous SiO2 at 150°C for 3 day. As-prepared Li2FeSiO4 (ca. 1 µm size) shows poor electrochemical reactivity with 20 mAh g−1 of discharge capacity at room temperature in aprotic Li cell. Electrochemical reactivity of the as-prepared Li2FeSiO4 has been considerably improved by the ballmilling process with acetylene black because particle size of Li2FeSiO4 is reduced to 50–200 nm size, and the individual particles are uniformly covered by acetylene black. The ballmilled Li2FeSiO4 can deliver 130 mAh g−1 of the rechargeable capacity at room temperature or 150 mAh g−1 at 60°C.
We investigated the crystal and electronic structures of Li1+xNi0.5Mn0.5O2 by Rietveld analysis and first-principle calculations. The crystal structure was well refined by Rietveld analysis using R-3m as the space group, and it was found with MEM using synchrotron X-ray diffraction that the sample showing better cycle performance had higher electron density at a saddle point of 3b–6c. The first-principle calculation clarified that the covalent bond of Ni–O and Mn–O in near a cation mixing position was stronger than that of normal position.
The compatibility of the mixed electrolyte of two ionic liquids based on 1,3-substituted imidazolium cations with lithium manganese oxide LiMn2O4, lithium iron phouphate LiFePO4, graphite, and a hard carbon has been confirmed. For LiMn2O4 and LiFePO4, the mixed imidazolium ionic liquid electrolyte provides slightly higher plateau in discharge curves, while the initial capacity is slightly lower, when compared with piperidinium ionic liquid. In case of LiMn2O4, the capacity in the mixed ionic liquid is recovered by subsequent cycling. The thermal stabilities of charged positive electrodes with the mixed ionic liquid, as well as with the piperidinium one, from accelerating rate calorimetry are far over those in conventional carbonate electrolyte. A hard carbon electrode is compatible with the mixed ionic liquid electrolyte.
Instead of W. R. McKinnon1) theory based on interaction energy in lattice-gas model, which has been used widely to analyze the charge-discharge curve for electrode materials of lithium ion battery, Laviron’s7) and Matsuda’s8) theoretical treatments about the cyclic voltammetry (CV) for the electro-active molecular confined on an electrode are applied for clarifying the experimentally accessible physical quantities, i.e. the parameters of electrostatic interaction between redox sites and of electrode kinetics for the Li4Ti5O12 (LTO) solid-state layers. From the width at half height (ΔEp 1/2) of the reversible CV at slowest potential scan-rate, the corresponding attractive force in their active-materials was evaluated as W/RT=1.64 (4.07 kJ mol−1), being an identical factor for the interaction energy evaluated by the lattice-gas model. From the shift of peak potential and the changes of ΔEp 1/2 and peak current for the irreversible CVs against the moderate potential scan-rates, the values of the rate constant (k°’ s−1) and the transfer coefficients (α) regarding a redox-electrode reaction process were evaluated as 6.7×10−3 and 0.47–0.48, respectively. The interaction parameter ΔW‡/RT being related to the kinetics of the electrode process was evaluated as −4.28∼−4.68.
The local structure of Li[Ni0.17Li0.2Co0.07Mn0.56]O2 was analyzed by high resolution scanning transmission electron microscopy (HR-STEM) and X-ray absorption spectroscopy (XAS). The HR-STEM images clearly showed a new type of stacking fault, as well as visual patterns of Li-ordering in the transition-metal (TM) layers. The XAS data showed that the valence states of the component TMs are Ni(II), Co(III) and Mn(IV), suggesting that some of the Li(I) in the TM layers of Li2MnO3 structure is replaced by Ni(II). Based on the results obtained, a detailed structural model for Li-rich solid-solution cathode materials was proposed.
We proposed accelerated life estimation test methods for high-power lithium-ion batteries used in electrical vehicle. The effects of temperature and state of charge on the degradation of full-scale prototype cells (>5 Ah) were investigated from the viewpoints of capacity performance and output performance by carrying out storage tests over more than 100 days. The most appropriate charging method during storage was also investigated.
Solid polymer electrolytes, the electrolytes without a flammable solvent, can solve a safety problem of rechargeable lithium (Li) batteries. We prepared poly(oxetane) with nitrile groups, P(CYAMEO), by ring-opening polymerization of the oxetane derivative and used it for a matrix of solid polymer electrolytes. Charge-discharge performance of the Li|P(CYAMEO)-based electrolyte|LiCoO2 cells was tested under constant current charge-discharge cycling condition. The test cells with the P(CYAMEO)-based electrolyte that contained LiN(CF3SO2)2, showed better discharge performance than those with the electrolytes that contained other Li salts. The polarization behaviors of the Li electrode in the electrolyte suggest that performance of the test cells partially depend on the structure of the solid electrolyte interphase on the Li electrode surface in the P(CYAMEO)-based electrolyte.
In this study, the behavior of carbon radicals has been observed in a model cell of an electric double layer capacitor (EDLC) by using an in situ electron spin resonance (ESR) method and ac cyclic voltammetry (CV). The maximum ESR signal intensity of the carbon radical was observed during the anodic sweep of the applied voltage using the simultaneous measurement of the ESR, the impedance and the CV. It was found that the degradation progress of the EDLC was mainly due to the change in the concentration of the carbon radical, and that its phenomenon was not observed by CV.
Silicon powder was encased within carbon gel microspheres with different porosities by simply adding silicon powder to the water phase during the inverse emulsion polymerization of resorcinol with formaldehyde, followed by drying and carbonization. From the results of charge-discharge experiments, it was confirmed that the reversible capacity of silicon and its cycle ability could be improved through such encasement and that the degree of improvement depended on the porosity of the carbon gel matrix.
A blend of several boric esters was adopted as a novel electrolyte solvent exhibiting the high oxidation potential and favorable lithium ion conductivity. Spontaneous transesterification between boric esters takes place resulting in a mixed boric ester. The electrochemical properties that the mixed ester acquired seem to be descended from the properties of parent boric esters: thus B(OCH3)3 (1) or B(OCH2CH3)3 (2), and B(OCH2CH2CN)3 (3) were low viscous, and a good solvent for lithium salts respectively.
In this study, we investigated the results based on the “C.R.M. resistance” of positive electrodes (Co-, Ni- and Mn-based materials), which was directly related to the input-output power characteristics, using the “four-electrode cell”.1,2) By comparing the resistance of these positive electrodes, we found that the Ni-based electrode had the lowest C.R.M. resistance in the range of 4.0 V–3.5 V and the Mn-based electrode had a stationary C.R.M. resistance over a wide range of both the charge and discharge states.
Rechargeable Li-air battery is a candidate for post Li-ion battery with high energy density. In this paper, the rechargeability of Li-air battery over 100 cycles was confirmed and its capacity retention over 60% was achieved. Nevertheless, a large voltage gap between the discharge-charge profiles was observed. Here, a discharged product formed on a cathode was investigated by TEM observation and FT-IR spectroscopy. It was found that the main product formed in discharge was not an ideal compound, Li2O2, but was carbonate species issued from the decomposition of carbonate-based electrolyte solvent.
The use of nonflammable electrolytes containing a solvent that includes alkyl fluoride groups to improve the safety of lithium-ion cells has been investigated. The positive electrodes of the cells comprise a mixture of a multi-component transition metal oxide and a polyanionic compound. The nonflammability of the electrolyte is shown by adding 30 vol% of a phosphate solution to the base electrolyte. It is observed that the cycle-life performances of the cells with the nonflammable electrolyte at 45°C are higher than that of the cell containing the base electrolyte.
The deformation behavior of copper foil collectors electrodeposited with Ni3Sn4 during the charge-discharge cycle was observed in situ using an optical cantilever method. It was found that in the high yield strength copper foil, cyclical deformation due to charge-discharge were observed, while in low yield strength copper foil, the deformation which occurred during the first charge period remained. The former is elastic deformation and the latter is plastic deformation. As plastic deformation promotes determination through peeling and shedding of the active material during sequential cycles, the high yield strength copper should be adopted. For example, 3 µm Ni3Sn4 film requires a 31 µm pure copper foil collector with a yield strength of 350 MPa or a 18 µm Corson copper alloy foil collector with the yield strength of 700 MPa.
The electrode surface of a lithium battery was characterized using in situ reflectivity techniques and epitaxial thin films. Epitaxial LiFePO4 thin films were fabricated by pulsed laser deposition. Changes in interfacial structures on the surface are determined by X-ray and neutron reflectivity (NR) measurements using an X-ray-transmission electrochemical cell. The LiFePO4 surface is stable during the first charge/discharge process. NR analysis indicates a reversible change in the concentration gradient of lithium ions at the LiFePO4/electrolyte interface on lithium deintercalation.
We measured impedance spectra for lithium-ion batteries with a large capacity over 10 Ah and a small capacity under 1 Ah. We assigned the impedance spectra to the electrochemical parameters for the positive and negative electrode interfaces without disassembling. We proposed an advanced impedance analysis technique with constant phase element normalized by battery capacity for lithium-ion batteries of 10 Ah-class.
Effect of discharging before initial activation (pretreatment) of Co(OH)2-coated Ni(OH)2 positive electrodes for use in Ni-metalhydride (Ni-MH) batteries on their electrochemical properties was investigated. The high-rate dischargeability (HRD) of the positive electrode was improved by the pretreatment. The effect of the pretreatment was more conspicuous as the amount of the Co(OH)2-coated Ni(OH)2 as an active material increased. The charge transfer resistance (Rct) of the positive electrode evaluated by AC impedance measurements significantly increased at the depths of charge (DOCs) less than 20%, while the increase in Rct was effectively suppressed by the pretreatment. Moreover, the cyclic voltammogram of the pretreated positive electrode showed that metallic Co was produced during the pretreatment and oxidized to CoOOH. The formation of the conductive oxidation products from not only the pristine Co(OH)2 but also metallic Co can lead to the formation of more dense conductive network which may be responsible for the improvement of HRD and Rct.
The influence of the component of the gel polymer electrolyte containing imidazolium ionic liquid on its cathodic stability and mobility of lithium ion has been investigated. The selection of the mixed ionic liquid based on 1-ethyl-3-methyl (EMI) imidazolium and 1-cyanomethyl-3-methyl imidazolium (CmMI) cations exhibits higher cathodic stability for the gel electrolyte than the one containing EMI ionic liquid alone. The addition of alumina filler increases the mobility of lithium ion in the gel, and promotes the reversible lithium deposition-dissolution.
Multi-layered solid electrolyte sheets (OHARA sheet (Ohara Inc., Kanagawa, Japan)/lithium phosphorus oxynitride (LiPON)) with a copper current collector film on the LiPON side were prepared, and the effects of Ti modification at the OHARA sheet/LiPON interface on the electrochemical lithium deposition-dissolution reaction were investigated. Lithium plating-stripping current in cyclic voltammograms increased with an appropriate amount of Ti modification, indicating that Li+ transfer resistance at the OHARA sheet/LiPON interface decreased due to the modification. In addition, the Ti modification stabilized the electrochemical lithium deposition-dissolution reaction. This suggested that Ti modification increase the number of Li+ transfer sites at the interface, resulting in a good distribution of nucleation reaction sites on the copper film as well as the stabilization of the current collector film.
Li+ ion insertion/extraction properties of TiO2(B) powder, which was prepared from a K2Ti4O9 precursor by ion-exchange and dehydration, was investigated as a high-potential negative electrode. The irreversible capacity greatly decreased from 124 mAh g−1 to ca. 40 mAh g−1 when the lower potential limit was raised from 1.0 V to E≥1.2 V, which indicated that the irreversible capacity mainly originated from surface reactions, such as solvent decomposition, at E≤1.2 V in the first cycle. In galvanostatic intermittent titration tests, large polarization was observed at the beginning and the end of charging. This clearly indicated that a slow process is involved in the insertion reaction at the beginning and the end of charging, which causes solvent decomposition at potentials lower than 1.4 V. The reversible capacity was greatly improved to 246 and 276 mAh g−1 at an elevated temperature of 60°C and at a lower rate of C/60, respectively. The TiO2(B) sample contained ca. 14 wt% of the anatase phase as an impurity, and a reduction of the anatase phase was found to be effective for an increase in reversible capacity at moderate charge/discharge rates.
An investigation of the thermal deterioration characteristics of a lithium-ion secondary cell is inevitable for in utilization for electric vehicles. An accelerating rate calorimeter study revealed that the thermal runaway of the cell occurs over 130°C. In the present study, the thermal deterioration characteristics of a lithium-ion secondary cell during a 100°C storage have been investigated by varying the state of charge (SOC). The measured impedance spectra have been analyzed based on an equivalent circuit, which consists of resistances of the anode and cathode (R1 and R2). Based on the analyses, the increasing rates of R1 and R2 can be defined. The rate of R1 is lower than that of R2 for an SOC of 87%; conversely, that of R1 is greater than that of R2 for SOCs of 66 and 44%. Moreover, the increasing rates of both R1 and R2 are found to become significant when the magnitude of the SOC increases. The increasing rate of R2 more strongly depends on the SOC than that of R1. These results mean that the cathode material is thermally unstable when the extent of lithium deterioration is high.
Pure Si platelets (Si-LP, thickness: 100 nm) and Ni layer-laminated Si platelets with different thicknesses (Si/Ni/Si-LP50 (50/50/50 nm) and Si/Ni/Si-LP30 (30/30/30 nm)) were prepared by a vapor deposition method, and their charge/discharge properties were investigated as alternative negative electrode materials to graphite for lithiumion batteries. The shape of thin platelet effectively relieved the stress during the alloying and de-alloying processes, and improved the charge/discharge cycleability as compared with a commercially available Si powder. In particular, the laminated platelet samples showed very good cycleability, probably because of a shorter diffusion length of Li+ ions in the Si layers and an improved mechanical strength by the presence of the Ni layer.
Sydnones are known as typical mesoionic compounds, which can be represented by the resonance hybrid of different polarized ionic structures. We have newly synthesized 3-ethyl-4-propylsydnone (EPSD) and investigated its physical, chemical, and electrochemical properties. The electron-pair donability for EPSD was higher than that for propylene carbonate (PC), though the electron-pair acceptability for EPSD was lower. The addition of EPSD to an ethylene carbonate (EC)-diethyl carbonate (DEC) equimolar binary mixture at the molar ratio of 1:1:0.05 was effective for an increase in cycling efficiency of a lithium anode.
Ethyl acetate (EA) shows low viscosity for its relative permittivity. Monofluorinated solvents exert the polar effect on the physical and electrochemical properties. Relative permittivity of 2-fluoroethyl acetate (2FEA) was lower than that of ethyl fluoroacetate (EFA), though viscosity of 2FEA was higher. Ionic conductivity of 2FEA was lower than that of EFA, but higher than that of EA at high temperatures. The anodic stability of 2FEA and EFA was higher than that of EA. The use of 2FEA as a co-solvent improved cycling efficiency of a lithium anode.
Difluorination of 3-methyl-2-oxazolidinone (3Me2Ox or NMO (N-methyloxazolidinone)) decreased the relative permittivity and the ionic conductivity and increased the dynamic viscosity and the anodic stability. The difluorinated NMOs (DFNMOmix) consisted of a mixture of 4,4′-difluoro-3-methyl-2-oxazolidinone (4,4′-DFNMO) and 4,5-difluoro-3-methyl-2-oxazolidinone (4,5-DFNMO), and its molar ratio was 3:7. The addition of DFNMOmix to an ethylene carbonate (EC)-diethyl carbonate (DEC) equimolar binary mixture suppressed discharge capacity fading in a Li/LiCoO2 coin cell.
X-ray absorption near edge (XANES) measurements at K-edges of Ni and Co were carried out on the Li deintercalated Li1−xNi0.82Co0.15M0.03O2 (M=Nb, Ti) cathode materials for Li secondary batteries. Samples were prepared by two different ways of Li deintercalation. One was cathode materials removed from the coin cell charged in the different degree of Li deintercalation and the other was chemically processed with hydrochloric acid in the aqueous solution. Comparisons of XANES spectra between LNCO and Ti or Nb doped LNCO were made, and it was found that the formation of Ni4+ was greatly reduced in the charged cathode materials with the dope of Ti or Nb. The similar results were obtained within samples prepared by the real charge and the chemical charge. Valence of Co was invariable during the charge, and was supposed to be trivalent.
The extended X-ray absorption fine structure (EXAFS) was measured to study the effects of the local structures on the Li-ion transfer in the La2/3−xLi3xTiO3 perovskite model compound with different lanthanum ion concentrations. The results indicate that the local structure is more distorted and the interatomic distances of Ti–La and Ti–O–Ti become shorter with the decrease of lanthanum ion concentration. The change in Li-ion conductivity with different lanthanum ion concentration is well explained by the relation between the estimation of activation energy with which Li ions hop through the bottleneck and the results of local distortion by EXAFS analysis.
Sn and Sn-based compounds have attracted great interest as a candidate for anode materials of Li-ion batteries. In the present work, in a Sn–Li battery system we have investigated effects of the volume change associated with the formation of Li–Sn compounds on the electrode potential from the viewpoint of the Gibbs free energy and associated elastic-strain energy. Our experimental results show that the experimental electrode potential is much lower than the value expected thermodynamically, when a Sn plate-shape electrode is lithiated. These experimental results can be consistently explained by considering the contribution of the elastic-strain energy to the chemical free energy of formation.
Spherical C/Li4Ti5O12 fine powders were successfully prepared by the ultrasonic spray pyrolysis using aqueous solution of Li and Ti with organic compound. C/Li4Ti5O12 particles were crystallized to spinel structure with Fd3m space group at more than 600°C. The carbon content of C/Li4Ti5O12 powders was 5 wt% from DTG analysis. Chemical analysis revealed that the molar ratio of Li/Ti of C/Li4Ti5O12 powders was agreement with the starting solution component. The electrochemical properties of C/Li4Ti5O12 anode were also estimated by rechargeable capacity, cycle life, thermal stability and high rechargeable rate. The use of lactic acid as a carbon source was most effective for improvement of them. The rechargeable capacity of C/Li4Ti5O12 anode increased with increasing carbon contents and it exhibited 165 mAh g−1 at 1 C. The rechargeable capacity of C/Li4Ti5O12 decreased with increasing charge rate and that of C/Li4Ti5O12 was 150 mAh g−1 at 10 C, but the stable cycle life was also maintained at 10 C. The cycle stability of C/Li4Ti5O12 anode was also kept at 50°C.
The physical and electrolytic properties, and charge-discharge characteristics for secondary lithium batteries of partially fluorinated methyl propyl carbonate (MPC) derivatives were examined. Dielectric constants of the fluorinated MPC derivatives except 2,2,3,3,3-pentafluoropropyl methyl carbonate (PFPMC) tend to increase with increasing the number of fluorine atom. Viscosity of PFPMC with the highest molar volume is lower than those of other fluorinated MPC derivatives. Donor numbers (DN) of the fluorinated MPC derivatives were estimated by 29Si NMR chemical shifts of triphenylsilanol. DN of the fluorinated MPC derivatives except PFPMC tends to decrease with increasing the number of fluorine atom. 3-Fluoropropyl methyl carbonate (FPMC) shows the highest specific conductivity at the temperature above about 25°C. Oxidative decomposition potentials of the fluorinated MPC derivatives tend to increase with increasing the number of fluorine atom. PC-FPMC electrolyte shows higher lithium electrode cycling efficiency. Every PC-based fluorinated MPC derivative electrolyte shows higher discharge capacities for Li|LiCoO2 cells.
The electrochemical properties of lithium hexafluorometalates as positive electrode materials were studied. Trirutile-type Li2TiF6 was synthesized through a complex-formation reaction under mild temperature conditions. In addition, a spray-drying technique was used to obtain electrodes with a large surface area. The electrochemical performance of the resultant Li2TiF6 powders was investigated vs. Li metal as negative electrode. Li2TiF6 exhibited an initial discharge capacity of 100 mAh g−1 at 2.7 V.
We investigated the average, local and electronic structures of LiMn1/3Ni1/3Co1/3O2 prepared by various methods. Their electrode characteristics were also studied by charge-discharge test. As a result, it was demonstrated that the sample synthesized by solution method exhibited the most excellent cycle performance. In order to clarify how the preparation processes affected on the crystal structures in detail, local structure, i.e. cation mixing and transition-metal ordering, in the samples were analyzed by Pair Distribution Function (PDF) method using a neutron source and X-ray absorption fine structure measurements. From this analysis, it was found that the local structure in LiMn1/3Ni1/3Co1/3O2 depended on the preparation process. Such a difference in the local structures may result in different cathode properties among the samples synthesized.
Large capacity Li-ion cells with 100 Ah for satellite application had been developed in 1999, and calendar and cycle life characteristics of the cells had 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). These tests were started in 1999, and the data have been accumulated until 2008 for around ten years. From these results, we have confirmed that GYT Space Li-ion cells have sufficient capability to achieve the mission life requirements for several kinds of artificial satellites. Furthermore, we have discovered that our simple life estimation model needs to be modified to consider SEI growth blocking mechanism. It means that the SEI growth is blocked by the adjacent SEI layers, therefore calendar capacity loss is affected by not only its test term, temperature, and state of charge but also its calendar capacity loss values. Our modified estimation formula is that a rate of calendar capacity loss is decreased in proportion to the 2.4th power of the calendar capacity retention. By using the modified formula, the estimation results show very good fitting with the long term cell test data for ten years.
The Japan Aerospace Exploration Agency (JAXA) developed the 200 Ah-class lithium-ion secondary cell. Using the cell, the survivability on the Moon using chemical rechargeable battery was tested. The charge at 50°C and discharge at −20°C were repeated three times simulating the energy management on the Moon over nights. One of the cells was stored with higher SOC than the other at 50°C. The lower discharge voltage was observed after the storage with high SOC after the third cycle, but the difference was 58 mV after 150 Ah was discharged at −20°C. The degradation of the cell was not significant after the cycles, and the applicability of the lithium-ion secondary cells over nights on the Moon could be demonstrated.
Coin-type of air secondary batteries using hydrogen storage alloys as the negative electrode’s material were designed and fabricated, and the charge-discharge characteristics were evaluated under constant current operation. The positive electrode consisted of a nickel-based gas diffusion electrode using Ir2Bi2O7−z as oxygen evolution and reduction catalyst. A stable charge and discharge voltages were observed, and the discharge voltage and power density were improved by using a membrane separator, in which an alkaline solution was impregnated, between the positive and negative electrodes. A high power density up to 81 W dm−3 and a good charge-discharge cycling behaviors were also obtained.