Search

A chip-sized passively Q-switched laser is demonstrated, in which a solid-state gain medium is intra-cavity pumped and vertically integrated with a semiconductor laser. A laser chip volume of 1 mm3 and peak power of 50 kW are achieved simultaneously.

Capacity failure due to the imbalance of coulombic efficiencies between positive and negative electrodes in a lithium-ion (shuttlecock) cell was examined by fabricating two types of cells, i.e., Li[Li1/3Ti5/3]O4/LiNiO2 and carbon/LiNiO2. Rechargeable capacity of a lithium-ion cell consisting of Li[Li1/3Ti5/3]O4 and LiNiO2 faded more rapidly than that of a cell based on natural graphite and LiNiO2 in spite of superior coulombic efficiency of Li[Li1/3Ti5/3]O4 (very close to 100%) compared with that of natural graphite (about 98%). The important role of the electrolyte upon the capacity retention of lithium-ion cells is described and specific problems to extend cycle life are discussed.

Carbon materials as possible alternatives to metallic lithium were described with emphasis on the electrochemical characters of carbon materials, such as (natural and artificial) graphite, petroleum coke, pitch-based carbon fibers, high-area carbons, and other carbonaceous materials. Well-defined graphite showed the lowest operating voltage (0-0.3 V versus Li) and the highest volumetric capacity (about 0.6 Ah·cm−3 based on the observed density and rechargeable capacity) in addition to excellent rechargeability. The theoretical capacity of graphite was 372 mAh·g−1 or 850 mAh·cm−3 based on the graphite sample for the reaction to form the first-stage compound (LiC6). Among the materials examined, some of them showed the capacity more than 372 mAh·g−1, while the operating voltage was higher and the density was lower than that of graphite. To understand such an anomalous behavior of carbonaceous materials, we modeled carbon electrodes and explained why some of carbonaceous materials exceed a capacity limit of 372 mAh·g−1. According to our model, high-capacity materials more than 500 mAh·g−1 were possible but traded off volumetric capacity, operating voltage, and consequently energy density. A lithium-ion (shuttlecock) cell consisting of natural graphite and LiNiO2 was also reported and the specific problems in applying carbon materials to lithium-ion cells were discussed.

Electrochemical reduction of natural graphite was carried out in 1M LiClO[sub 4] ethylene carbonate (EC)/1,2-dimethoxyethane (DME) solution at 30 C. Natural graphite was reduced stepwise to LiC[sub 6]. The staging phenomenon was observed by X-ray diffraction (XRD). The first stage and the second stage compounds were identified as a commensurate structure in which lithium atoms form a close-packed two-dimensional array. A second-stage compound (LiC[sub 18]) with a different in-plane lithium ordering based on a LiC[sub 9] two-dimensional packing in lithium intercalated sheets also was observed; also third, fourth-stage compounds were identified. The electrochemical oxidation of the first-stage compound (LiC[sub 6]) was examined and shown to reversible over the entire range, i.e., C[sub 6] + xLi [r reversible] Li[sub x]C[sub 6]. The reaction mechanism for the reduction of graphite and the oxidation of the first-stage compound are discussed in relation to the staging phenomenon from the detailed open-circuit voltage and XRD data. The chemical potential of LiC[sub 6] was estimated to be [minus]3.6 kcal mol from the observed reversible potential. The feasibility of using a lithium-graphite intercalation compound in lithium ion (shuttlecock) cells is described, and the innovative secondary systems, C[sub 6]/LiCoO[sub 2] and C[sub 6]/LiNiO[sub 2] fabricated in discharged states,more » are demonstrated.« less

The preparation and characterization of LiNi1−xCoxO2 compounds (0 ⩽ x ⩽ 1) having a space group R3m, for 4 volt secondary lithium cells were investigated. By developing processing methods, homogeneous LiNi1−xCoxO2 samples were obtained and characterized by XRD, ir and magnetic susceptibility measurements. When increasing x in LiNi1−xCoxO2, the unit cell dimensions a and c in hexagonal setting, decreased almost linearly as a function of x. Magnetic susceptibility measurements indicated that LiNi1−xCoxO2 consists of low-spin states of Co3+ (t62g e0g) and Ni3+ (t62ge1g). All samples may be used as positive electrodes in nonaqueous lithium cells. Of these, LiCoO2 showed the highest working voltage and about 120 mAh g−1 of rechargeable capacity, and LiNi12Co12O2 showed the lowest working voltage and about 130 mAh g−1 of rechargeable capacity in the voltage range 2.5–4.2 V in 1 M LiClO4 propylene carbonate solution. LiNiO2 has more than 150 mAh g−1 of rechargeable capacity with working voltages above 3.5 V. Secondary lithium ion cells which consisted of LiNi1−xCoxO2 cathodes and natural graphite anodes, were also examined and the specific problems of establishing an innovative secondary lithium cell were discussed.

Electrochemical reduction of natural graphite was carried out in 1M LiClO[sub 4] ethylene carbonate (EC)/1,2-dimethoxyethane (DME) solution at 30 C. Natural graphite was reduced stepwise to LiC[sub 6]. The staging phenomenon was observed by X-ray diffraction (XRD). The first stage and the second stage compounds were identified as a commensurate structure in which lithium atoms form a close-packed two-dimensional array. A second-stage compound (LiC[sub 18]) with a different in-plane lithium ordering based on a LiC[sub 9] two-dimensional packing in lithium intercalated sheets also was observed; also third, fourth-stage compounds were identified. The electrochemical oxidation of the first-stage compound (LiC[sub 6]) was examined and shown to reversible over the entire range, i.e., C[sub 6] + xLi [r reversible] Li[sub x]C[sub 6]. The reaction mechanism for the reduction of graphite and the oxidation of the first-stage compound are discussed in relation to the staging phenomenon from the detailed open-circuit voltage and XRD data. The chemical potential of LiC[sub 6] was estimated to be [minus]3.6 kcal mol from the observed reversible potential. The feasibility of using a lithium-graphite intercalation compound in lithium ion (shuttlecock) cells is described, and the innovative secondary systems, C[sub 6]/LiCoO[sub 2] and C[sub 6]/LiNiO[sub 2] fabricated in discharged states,more » are demonstrated.« less

Li[Ni 1/2 Mn 3/2 ]O 4 was prepared by a two-step solid state reaction and characterized by X-ray diffraction (XRD) infrared (IR)-Raman, and electron diffraction (ED). Li[Ni 1/2 Mn 3/2 ]O 4 having characteristic eight absorption bands in 400-800 cm -1 in IR spectrum, extra lines in XRD, and extra spots in ED was analyzed in terms of a superlattice structure. Analytical results on the structural data indicated that Li[Ni 1/2 Mn 3/2 ]O 4 (cubic: a = 8.167 A) was a superlattice structure based on a spinel framework structure having a space group of P4 3 32 (or P4 1 32) in which nickel ions were located at the octahedral 4(b) sites, manganese ions were at the octahedral 12(d) sites, and lithium ions were at the 8(c) sites in a cubic-close packed oxygen array consisting of the 8(c) and 24(e) sites. Well-defined Li[Ni 1/2 Mn 3/2 ]O 4 was examined in nonaqueous lithium cells and showed that the cell exhibited extremely flat operating voltage of about 4.7 V with rechargeable capacity of 135 mAh/g based on the sample weight. The reaction mechanism of Li[Ni 1/2 Mn 3/2 ]O 4 was examined and shown that the reaction at ca. 4.7 V consisted of two cubic/cubic two-phase reactions, i.e., □[Ni 1/2 Mn 3/2 ]O 4 (a = 8.00 A) was reduced to Li[Ni 1/2 Mn 3/2 ]O 4 (a = 8.17 A) via □ 1/2 Li 1/2 [Ni 1/2 Mn 3/2 ]O 4 (a = 8.09 A). Results on the detailed reversible potential measurements indicated that the flat voltage at ca. 4.7 V consisted of two voltages of 4.718 and 4.739 V. The reaction of Li[Ni 1/2 Mn 3/2 ]O 4 to Li 2 [Ni 1/2 Mn 3/2 ]O 4 is also examined and showed that the reaction proceeded in a cubic (a = 8.17 A)/tetragonal (a = 5.74 A, c = 8.69 A) two-phase reaction with the reversible potential of 2.795 V. From these results, characteristic features of topotactic two-phase reactions of Li[Ni 1/2 Mn 3/2 ]O 4 (P4 3 32) were discussed by comparing with the results on LiMn 2 O 4 (Fd3m).