S.A. Hackney

Advances in manganese-oxide ‘composite’ electrodes for lithium-ion batteries

Recent advances to develop manganese-rich electrodes derived from ‘composite’ structures in which a Li2MnO3 (layered) component is structurally integrated with either a layered LiMO2 component or a spinel LiM2O4 component, in which M is predominantly Mn and Ni, are reviewed. The electrodes, which can be represented in two-component notation as xLi2MnO3·(1 − x)LiMO2 and xLi2MnO3·(1 − x)LiM2O4, are activated by lithia (Li2O) and/or lithium removal from the Li2MnO3, LiMO2 and LiM2O4 components. The electrodes provide an initial capacity >250 mAh g−1 when discharged between 5 and 2.0 V vs. Li0 and a rechargeable capacity up to 250 mAh g−1 over the same potential window. Electrochemical charge and discharge reactions are followed on compositional phase diagrams. The data bode well for the development and exploitation of high capacity electrodes for the next generation of lithium-ion batteries.

Structural fatigue in spinel electrodes in high voltage (4 V) Li/Li{sub x}Mn{sub 2}O{sub 4} cells.

Evidence of structural fatigue has been detected at the surface of discharged Li{sub x}[Mn{sub 2}]O{sub 4} spinel electrodes in (4 V) Li/Li{sub x}[Mn{sub 2}]O{sub 4} cells. Under nonequilibrium conditions, domains of tetragonal Li{sub 2}[Mn{sub 2}]O{sub 4} coexist with cubic Li[Mn{sub 2}]O{sub 4}, even at 500mV above the thermodynamic voltage expected for the onset of the tetragonal phase. The presence of Li{sub 2}[Mn{sub 2}]O{sub 4} on the particle surface may contribute to some of the capacity fade observed during cycling of Li/Li{sub x}[Mn{sub 2}]O{sub 4} cells.

Preferred Orientation of Polycrystalline LiCoO2 Films

Polycrystalline films of deposited by radio frequency magnetron sputtering exhibited a strong preferred orientation or texturing after annealing at 700°C. For films thicker than about 1 μm, more than 90% of the grains were oriented with their (101) and (104) planes parallel to the substrate and less than 10% with their (003) planes parallel to the substrate. As the film thickness decreased below 1 μm, the percentage of (003)‐oriented grains increased until at a thickness of about 0.05 μm, 100% of the grains were (003) oriented. These extremes in texturing were caused by the tendency to minimize volume strain energy for the thicker films or the surface energy for the very thin films. Films were deposited using different process gas mixtures and pressures, deposition rates, substrate temperatures, and substrate bias. Of these variables, only changes in substrate temperature could cause large changes in texturing of thick films from predominately (101)–(104) to (003). Although lithium ion diffusion should be much faster through cathodes with a high percentage of (101)‐ and (104)‐oriented grains than through cathodes with predominately (003)‐oriented grains, it was not possible to verify this expectation because the resistance of most cells was dominated by the electrolyte and electrolyte‐cathode interface. Nonetheless, cells with cathodes thicker than about 2 μm could deliver more than 50% of their maximum energies at discharge rates of or higher.

Observation and measurement of grain rotation and plastic strain in nanostructured metal thin films

Abstract The deformation behavior of nanostructured gold thin films, with grain diameters of 10 nm and film thicknesses of 10–20 nm, has been studied by means of in situ high resolution transmission electron microscopy. Grain rotation was observed by measuring the changes in the angular relationships between the lattice fringes of different grains during deformation at low strain rates. The strain tensor was calculated by measuring the relative displacements of three material points, and using an analysis similar to that for strain gage rosettes. Relative grain rotations of up to 15 degrees, along with effective plastic strains on the order of 30%, were measured. No evidence of dislocation activity was detected during or after straining. Identical experiments on coarser-grained silver thin films, with grain diameters around 110 nm, yielded clear evidence of dislocation activity. These results indicate that grain rotation and grain boundary sliding can make significant contributions to the deformation of nanostructured thin films at low homologous temperatures.

ZrO2- and Li2ZrO3-stabilized spinel and layered electrodes for lithium batteries

Strategies for countering the solubility of LiMn{sub 2}O{sub 4} (spinel) electrodes at 500 {sup o}C and for suppressing the reactivity of layered LiMO{sub 2} (M = Co, Ni, Mn, Li) electrodes at high potentials are discussed. Surface treatment of LiMn{sub 2}O{sub 4} with colloidal zirconia (ZrO{sub 2}) dramatically improves the cycling stability of the spinel electrode at 50 {sup o}C in Li/LiMn{sub 2}O{sub 4} cells. ZrO{sub 2}-coated LiMn{sub 0.5}Ni{sub 0.5}O{sub 2} electrodes provide a superior capacity and cycling stability to uncoated electrodes when charged to a high potential (4.6 V vs Li{sup 0}). The use of Li{sub 2}ZrO{sub 3}, which is structurally more compatible with spinel and layered electrodes than ZrO{sub 2} and which can act as a Li{sup +}-ion conductor, has been evaluated in composite 0.03Li{sub 2}ZrO{sub 3} - 0.97LiMn{sub 0.5}Ni{sub 0.5}O{sub 2} electrodes; glassy Li{sub x}ZrO{sub 2 + x/2} (0

A simple, mixtures-based model for the grain size dependence of strength in nanophase metals

Abstract A model is presented for the strength of nanophase metals. The model assumes that polycrystalline metals consist of two phases: the “bulk” intragranular regions, and the “grain boundaries”. The boundary phase is treated as a glassy, but not highly rate-dependent material with a constant strength equal to that of the amorphous metal. The crystalline phase is assumed to follow a Hall-Petch equation for the grain-size dependence of strength. Treating the material as a composite, with a rule of mixtures approach, predicts a change in the Hall-Petch slope at small grain sizes, as has been observed. Grain size softening is predicted, but not until sizes below 5 nm. The model is compared to data in the literature.

The Electrochemical Stability of Spinel Electrodes Coated with ZrO2 , Al2 O 3 , and SiO2 from Colloidal Suspensions

Stoichiometric LiMn 2 O 4 and substituted Li 1.05 M 0.05 Mn 1.9 O 4 (M = Al,Ni) spinel electrodes, coated with ZrO 2 , Al 2 O 3 , and SiO 2 from colloidal suspensions, have been evaluated in lithium cells. ZrO 2 -coated Li 1.05 Ni 0.05 Mn 1.9 O 4 electrodes provide the best cycling stability at 50°C. The excellent cycling stability is attributed to a porous network of amphoteric ZrO 2 particles, less than 4 nm in dimension, that protect the spinel surface from acid attack by scavenging HF and H 2 O from the electrolyte, while still allowing access of the electrolyte to the active electrode.

Nanocrystalline Li{sub x}Mn{sub 2{minus}y}O{sub 4} cathodes for solid-state thin-film rechargeable lithium batteries

Thin-film cathodes of lithium manganese oxide, 0.3-3 μm thick, were deposited by rf magnetron sputtering of a LiMn 2 O 4 ceramic target onto unheated substrates. The resulting films were dense, ∼4.2 g/cm 3 , with a ∼50 A nanocrystalline spinel structure. The film composition was typically Li x Mn 2-y O 4 with y ∼ 0.3 and 1.2 < x < 2.2. When cycled in a thin-film rechargeable lithium battery, specific cathode capacities of 145 ± 23 and ∼270 mAh/g were realized for discharge from 4.5 V to either 2.5 or 1.5 V, respectively. The discharge and charge current densities were limited by the resistivity of lithium transport into and through the cathode. After thousands of cycles at 25°C, there was a small increase in cell resistance. After several hundred cycles at 100°C, the discharge curves developed a stable knee at ∼4 V characteristic of crystalline LiMn 2 O 4 cathodes. The polarization of the discharge/charge cycles were interpreted in terms of free energy of mixing curves.

IN SITU STUDIES OF DEFORMATION AND FRACTURE IN NANOPHASE MATERIALS

Abstract Nanocrystalline gold (8–25 nm grain size) and gold/silicon nanocomposites were prepared by sputtering and then strained to fracture in a transmission electron microscope. In situ and post mortem analyses revealed that the nanophase gold films were ductile, and significant plasticity was associated with fracture. Observations of pore formation, as well as a strain-rate effect on deformation behavior and direct lattice imaging of deformation, all indicated that the deformation occurred by diffusion-based mechanisms. Fracture was intergranular, but not brittle. Gold/silicon nanocomposites containing large volume fractions of brittle, amorphous Si and nanocrystalline gold precipitates exhibited behavior indicating significant toughness.

The pearlite-austenite growth interface in an Fe-0.8 C-12 Mn alloy

Abstract The interphase boundary structure and interface processes at the pearlite-retained austenite growth interface in Fe-0.8 wt% C-12 wt% Mn alloy have been investigated by transmission electron microscopy. Facetting, misfit correcting dislocations, and ledge defects are all observed at the previously assumed disordered boundary. Hot stage electron microscopy revealed that the ledge defects are mobile, indicating the migration of the growth interface occurs by the lateral movement of steps. It is found that the growth ledges are continuous across the growth interfaces of the pearlitic ferrite and cementite. This provides a mechanism by which the interface processes of the two pearlite phases may be coupled.