Thermal stability, surface characteristics, and electrode performance for LiNi1−x−yCoxAlyO2-based materials in a 1.5 M LiBF4 ethylene carbonate/γ-butyrolactone (1: 2) electrolyte have been investigated in order to develop laminated thin Li-ion batteries using LiNi0.8−yCo0.2AlyO2 cathodes. Using a LiNi0.74Co0.2Al0.06O2 cathode with low basicity, the laminated thin Li-ion batteries provided high energy density, long cycle life, and very low swelling. It was demonstrated that the prototype laminated thin Li-ion battery with a thickness of 3.8 mm achieved the energy densities of 200 Wh/kg and 407 Wh/l. The value of pH (definition is in experimental section) for LiNi0.74Co0.2Al0.06O2 should be less than 11 for the practical application because of a long cycle life of 500 cycles and very low swelling at an even high temperature of 85°C. Native alkaline impurities in LiNi0.8−yCo0.2AlyO2 led to CO2 gas evolution and cycle degradation. The results of impedance and x-ray photoelectron spectroscopy measurements for LiNi0.74Co0.2Al0.06O2 and LiCoO2 indicated that the surface film formation on LiNi0.74Co0.2Al0.06O2 cathode is more inactive and thermally stable than that of LiCoO2 cathode. The films formation on LiNi0.8−yCo0.2AlyO2 cathode suppressed further oxidation of the electrolyte and the gas evolution at high-temperature condition.
The degradations of thin-film LiMn2O4 electrodes during prolonged exposure to 1 M LiPF6/EC-DMC at room temperature and 55°C, were assessed by in situ Raman spectroscopy. Our results demonstrate that different deterioration mechanisms of the spinel were found depending on the temperature. Exposure of a thin-film spinel LiMn2O4 electrode to LiPF6/EC-DMC electrolyte at room temperature and 55°C led to spontaneous formation of a thin insulating surface layer. This process is accompanied by the transformation of the LiMn2O4 electrode surface into λ-MnO2 and Mn2O3 at room temperature and 55°C, respectively.
A mixture of α-fluoro and β fluoro-γ-butyrolactone (30 : 70 mol%; F-γ-BLmix) was prepared by the direct fluorination of γ-butyrolactone (γ-BL) and its physical properties and its performances as an electrolyte solvent were examined for lithium rechargeable cell application. Its relative permittivity and viscosity was more than two times of those of γ-BL. Its lithium cycling efficiency on a Ni electrode was compared with γ-BL and propylene carbonate (PC) using LiClO4, LiBF4, and LiPF6. Among every combination, LiPF6/F-γ-BLmix electrolyte afforded the best cycling efficiency due to the smoothest surface formation, however, the cycle performance of Li/LiCoO2 cell using LiPF6/F-γ-BLmix was not as good as the one using LiPF6/PC presumably due to its higher film resistance.
The prismatic lithium-ion batteries of LiCo1/3Ni1/3Mn1/3O2 with graphite were fabricated and examined in terms of the rate capability, low-temperature discharge characteristics, cycleability, and safety inspection by an accelerating rate calorimeter (ARC). Electricity of 94.0% or 88.2% was delivered at 1 C or 2 C-rate, respectively, against the nominal capacity of 640 mAh determined at 0.2 C-rate at 23°C. When the cells were operated at 0°, −10° or −20°C, the cells respectively delivered 89.5%, 84.9% or 66.5% of electricity based on the capacity determined at 1 C-rate at 23°C. The ARC results on the charged LiCo1/3Ni1/3Mn1/3O2 prepared at 4.4 and 4.7 V in the lithium-ion batteries with graphite indicated that self-heating began at about 160°C and then its heating rate reach maximum at 200°C for charged sample at 4.4 V or 195°C for that at 4.7 V. From these results, we have discussed whether or not novel lithium-ion batteries of LiCo1/3Ni1/3Mn1/3O2 with graphite can be used toward an expanded need for advanced lithium-ion batteries.
We propose a polymer blending method for preparing the PEO (polyethylene oxide)-LiBF4 complex electrolyte for lithium secondary battery applying to the IPN (interpenetrated polymer network) gel electrolyte. The polymer blend mixture of PEO-PS (polystyrene)-LiBF4 was prepared as a film by the hot-pressing method. The resulting IPN film was plasticized with the electrolyte solution of 0.5 M LiBF4/EC (ethylene carbonate)-PC (propylene carbonate) (1 : 1 vol.), in which the formation of PEO-LiBF4 complex was confirmed by the Raman spectroscopy. The basic properties as an electrolyte of Ii metal batteries, i.e., ionic conductivity, chemical stability at the polymer gel electrolyte/lithium metal interface, and charge-discharge performance of the Li/(PEO-LiBF4/PS) gel electrolyte/LiCoO2 cell were studied and discussed.
Li[LiyMn2−y]O4 (0≦y<1/3) having a spinel-framework structure was prepared and examined in nonaqueous lithium cells in wide voltage range of 1.0 to 5.2 V. Li[LiyMn2−y]O4 can be used as a positive electrode with rechargeable capacities of 50-120 mAh/g depending on y in Li[LiyMn2−y]O4 for 4-volt class of lithium-ion batteries with a graphite negative-electrode. A redox signal at 5.0 V was observed for all samples except stoichiometric LiMn2O4. The X-ray examinations on the charged samples at 5.2 V vs. Li gave a structural parameter of 8.04-8.05 Å regardless of y in Li[LiyMn2−y]O4. In addition to the redox potentials of 3.0, 4.0 and 5.0 V, we found a redox potential of about 1.8 V at which one-phase reaction proceeded in a tetragonal phase.
Single phase LiCoPO4 powders with olivine structure were synthesized from solution containing lithium acetate, cobalt acetate and phosphoric acid. Single phase of LiCoPO4 was obtained with a heat treatment at 700°C for 2 h. When the LiCoPO4 powders were used as cathode materials of electrochemical cell with organic electrolytes, a plateau around 5.0 V against lithium metal was observed for the charge and discharge curves. The charge-discharge efficiency was about 85% after 2nd cycle. When the all-solid-state lithium battery was constructed, discharge at around 4.1 V against indium metal was observed with small discharge capacity.
All-solid-state In/LiCoO2 cells using the 80Li2S·20P2S5 (mol%) glass-ceramic with high ambient temperature conductivity of about 10−3 S cm−1 as a solid electrolyte were fabricated, and their charge-discharge behaviors were investigated. The cells worked as lithium secondary batteries at room temperature, although they showed large irreversible capacities at the 1st cycle and capacity fading after the 2nd cycle. Open circuit voltage and cyclic voltammogram (CV) revealed that the cell could not be charged completely at the 1st cycle, because an irreversible anodic reaction except for the Co3+/Co4+ redox reaction occurred only at the 1st cycle. X-ray diffraction patterns and Raman spectra suggested that LiCoO2 and the glass-ceramic did not largely change in structure before and after the CV measurements. Cycling performances were improved by charging under capacity-controlled conditions during the first several cycles. The cells kept high charge-discharge capacities over 100 mAh g−1 with charge-discharge efficiencies of 100% up to the 200th cycle by charging up to x = 0.40 in Li1−xCoO2 until the 5th cycle.
The physical and electrolytic properties of the fluoroethyl methyl carbonate (FEMC) obtained chemically were examined. The dielectric constant, density and viscosity of FEMC were much higher than those of ethyl methyl carbonate (EMC). This means that the molecular interaction in FEMC becomes large with introducing a fluorine atom with high electron withdrawing into EMC molecule. The oxidative decomposition voltage (oxidation durability) of FEMC is slightly higher than that of EMC. At relatively low electrolyte concentrations, the specific conductivities in FEMC solutions are higher than those in EMC solutions because of high dielectric constant for FEMC. The conductivity in EC-FEMC binary solution decreases gradually with increasing FEMC concentration. The lithium electrode cycling efficiencies in EC-FEMC equimolar binary solutions containing 1.0 mol dm−3 LiPF6 and LiBF4 are higher than those in EC-EMC solutions. In particular, the LiBF4 solution shows the high efficiency of more than 70% at a high range of cycle number. The surface of the films formed on the electrode after cycling in EC-FEMC solutions is homogeneous, and consists of an uniform and small grain size.
The use of a series of multifunctional compounds as chelate ligands enables us to control charge delocalization around a central atom in the chelate anions and to investigate various properties systematically. We have investigated the substituent effect on the thermal stability and electrolytic properties of lithium bis[5-bromosalicylato (2-)]borate (5-BLBSB) and lithium bis[5-chlorosalicylato (2-)]borate (5-CLBSB), which we have newly synthesized, as well as lithium bis[salicylate (2-)] borate (LBSB), lithium bis[3-methylsalicylato(2-)]borate (3-MLBSB), lithium bis[3,5-dichlorosalicylato(2-)]borate (DCLBSB), and lithium bis[3,5,6-trichlorosalicylato(2-)]borate (TCLBSB). The thermal decomposition temperature decreases in approximately the following descending order: 5-BLBSB ≈3-MLBSB (330°C) >DCLBSB (310°C) >5-CLBSB (300°C) >LBSB (290°C) >TCLBSB (260°C). The stability to oxidative decomposition decreases in the sequence TCLBSB ≈ DCLBSB > 5-CLBSB > 5-BLBSB >LBSB > 3-MLBSB, and the reductive stability increases in the opposite order except for 5-CLBSB. Furthermore, we have applied the lithium chelatoborates to lithium batteries and have investigated cycling efficiency of a lithium anode, discharge characteristics of Li/V2O5 prototype cells, and performance of Li/LiCoO2 and C/LiCoO2 coin cells with respect to retention of discharge capacity and specific energy, using EC–DEC (mole ratio 1:1) binary solutions containing LiPF6. The addition of a small amount of the lithium chelatoborate improves the cell performance.
Layered lithium nickel manganese oxides described as Li[MxNi(1−x)/2Mn(1−x)/2]O2 (M = Ni and Co, 0≦x≦1/3) were induced from superlattice layered Li[Ni1/2Mn1/2]O2. XAS results suggested that Li[NixNi(1−x)/2Mn(1−x)/2]O2 compounds contained Ni2,3+ and Mn4+, and that Li[CoxNi(1−x)/2Mn(1−x)/2]O2 contained Co3+, Ni2+ and Mn4+. Li[Co1/3Ni1/3Mn1/3]O2 showed almost the same reversible capacity as LiCoO2. From results on DSC of the electrode charged by 4.3 V versus Li metal, Li[Co1/3Ni1/3Mn1/3]O2 exhibited higher thermal stability than LiCoO2. ARC measurements of the full-charged Li-ion batteries were carried out. The battery with Li[Co1/3Ni1/3Mn1/3]O2 exhibited lower self-heat rate for thermal runaway than that with LiCoO2, suggesting that the battery with Li[Co1/3Ni1/3Mn1/3]O2 was superior to that with LiCoO2 in terms of thermal stability at full-charged state.
Hydrogen fluoride is a detrimental impurity in nonaqueous electrolyte solutions for electrochemical energy storage devices. Linear sweep voltammetry using a Pt rotating disk electrode was applied to the quantitative analysis of HF. The HF content in an Et3MeNBF4− based electrolyte was successfully measured by this technique. However, PO2F2− cogenerated by the hydrolysis of PF6− prevented the detection of HF in LiPF6− based electrolyte.
Research on architectures of positive electrodes for rapid charging/discharging performances of lithium ion secondary batteries have been carried out by analyzing the contact resistance in the electrode composite. For the high charging/discharging rate, it was found that the decrease in the contact resistance between aluminum current collector and the active material composite was a key issue. The increase in the amount of point defect in the passivation film on aluminum and the increase in the contacting points at the terminal end of the carbon conductive additive network to the passivation film are effective for this purpose. The simple equation that determines the maximum amount of active material in the electrode composite was derived. By applying this equation, we concluded that LiMn2O4 active material had the maximum high charging/discharging rate of 720 C at least.
It is well-known that an aromatic compound such as biphenyl is added into electrolyte solutions to prevent lithium-ion cells from overcharging, where it generates hydrogen gas under overcharging conditions. We have examined the oxidative behaviors of one-benzene-ring aromatic compounds including benzene, toluene, ethylbenzene, cumene, tert-butylbenzene, and cyclohexylbenzene under the overcharging conditions. We have found that aromatic compounds without hydrogen atom at the benzylic position such as tert-butylbenzene generated mainly carbon dioxide gas, whereas those with hydrogen atom at the benzylic position showed polymerization accompanied by hydrogen gas evolution. It was considered that tert-butylbenzene works as a redox mediator, which mediates the oxidative decomposition of carbonate solvents evolving the carbon dioxide gas.
Electrochemical characteristics of the spinel Li1.05M0.2Mn1.75O4 (M = A1, Co, and Cr) and manganese dissolution from the spinel cathode materials into an electrolyte solution were investigated in order to understand and suppress the dissolution. As we described previously, the dissolution is one of the origins of a significant capacity fading with cycling for rechargeable lithium ion batteries, that is, severe degradation of a carbon anode was caused by deposition of the soluble Mn(II) species released from the spinel. For the purpose of suppressing the manganese dissolution, the lithium-rich spinel, Li1.05Mn1.95O4, was doped with a small amount of Al, Co, and Cr on the Mn 16d site, and the relationship between their cycle performances and Mn dissolution was investigated using 1 mol dm−3 LiClO4/EC + DMC (volume ratio 1:1) electrolyte solution at 25 and 50°C. By doping with Co and Cr, manganese dissolution from the spinel was successfully suppressed rather than Li1.05Mn1.95O4. Furthermore, the substitution with Co and Cr was remarkably effective to the enhancement in cycling behavior due to the structural stabilization and the suppression of manganese dissolution.