Cu-catalyzed Si-NWS grown on “carbon paper” as anodes for Li-ion cells

The very high theoretical capacity of the silicon (4200mAh/g more than 10 times larger than graphite), environmental-friendly, abundant and low-cost, makes it a potential candidate to replace graphite in high energy density Li-ion batteries. As a drawback, silicon suffers from huge volume changes (300%) on alloying and dealloying with Li, leading a structural deformation that induces disruption. The use of nanostructured silicon materials has been shown to be an effective way to avoid this mechanical degradation of the active material. In this paper the synthesis of silicon nanowires, grown on a highly porous 3D-like carbon paper substrate by CVD using Cu as the catalyst, is presented. The use of carbon paper allows to achieve remarkable loadings of active material (2-5 mg/cm2) and, consequently, high capacity densities. The silicon electrode was investigated both morphologically and electrochemically. To improve the electrochemical performance various strategies have been carried out. It was observed that a very slow first cycle (C/40), which helps the formation of a stable solid electrolyte interphase on the silicon surface, improves the performance of the cells; nevertheless, their cycle life has been found not fully satisfactory. Morphological analysis of the Si-NWs electrodes before and after cycling showed the presence of a dense silicon layer below the nanowires which could reduce the electrical contact between the active material and the substrate

Phase Separation in Lix_xFePO4_4 Induced by Correlation Effects

We report on a significant failure of LDA and GGA to reproduce the phase stability and thermodynamics of mixed-valence Li$_x$FePO$_4$ compounds. Experimentally, Li$_x$FePO$_4$ compositions ($0 \leq x \leq 1$) are known to be unstable and phase separate into Li FePO$_4$ and FePO$_4$. However, first-principles calculations with LDA/GGA yield energetically favorable intermediate compounds an d hence no phase separation. This qualitative failure of LDA/GGA seems to have its origin in the LDA/GGA self-interaction which de localizes charge over the mixed-valence Fe ions, and is corrected by explicitly considering correlation effects in this material. This is demonstrated with LDA+U calculations which correctly predict phase separation in Li$_x$FePO$_4$ for $U-J \gtrsim 3.5$eV. T he origin of the destabilization of intermediate compounds is identified as electron localization and charge ordering at different iron sites. Introduction of correlation also yields more accurate electrochemical reaction energies between FePO$_4$/Li$_x$FePO$_ 4$ and Li/Li$^+$ electrodes.Comment: 12 pages, 5 figures, Phys. Rev. B 201101R, 200

Improved Electrochemical Performance of a LiFePO4-Based Composite Cathode

LiFePO4 was synthesized in the presence of high-surface area carbon-black. The carbon was added to the precursors before the formation of the crystalline phase. SEM micrographs confirmed that the addition of the fine carbon powder reduces the LiFePO4 grain size. The carbon is uniformly dispersed between the grains, ensuring a good electronic contact. Electrochemical tests showed that the material obtained by adding 10 wt.% of carbon gives enhanced performance in terms of improved practical capacity and charge/discharge rate. The specific capacity was seen to increase on increasing temperatures. The full capacity (170 mA h g− 1) was delivered when discharging the cell at 80 °C and C/10 rate. The cyclability of the material was tested at room temperature and C/2 rate. The cell was cycled for over 230 cycles with an average specific capacity of about 95 mA h g− 1. © 2001 Elsevier Science Ltd. All rights reserved

Anisotropy and non-universality in kinetic roughening

We introduce a new model for kinetic roughening which exhibits a non-universal behavior for the roughness exponent, in agreement with many experimental findings. The model, inspired by the chemical etching processes, takes explicitly into account the effect of anisotropy, say the dependence of the growth rule on the local environment conditions. The interplay between anisotropy and non-universality is investigated as well as the relationship with the known universality classes

Fitting of the Voltage-Li+ insertion curve of LiFePO4

Fitting of the voltage vs. insertion curves of the LiFePO4 electrode was based on theoretical expressions describing the Li+ diffusive process in a solid medium. The noninteracting gas model for the chemical potential of ions distributed in a solid matrix was taken into account, and the diffusion coefficient and the energy activation for the diffusion process were accordingly calculated. The polari- zation curves at various discharge stages were theoretically obtained, and a good agreement was found with the experimental data at all discharge rates. A mathematical relation describing the trend of the diffusion resistance vs. insertion degree was also developed

Anisotropy and non-universality in kinetic roughening

We introduce a new model for kinetic roughening which exhibits a non-universal behavior for the roughness exponent, in agreement with many experimental findings. The model, inspired by the chemical etching processes, takes explicitly into account the effect of anisotropy, say the dependence of the growth rule on the local environment conditions. The interplay between anisotropy and non-universality is investigated as well as the relationship with the known universality classes

Determination of Chemical Diffusion Coefficient of Lithium in LiFePO4

The lithium insertion in the ordered olivine-type structure of LiFePO4 was analyzed as an insertion process with a Frumkin- type sorption isotherm. A minimum in the chemical diffusion coefficient of lithium (DLi) was predicted by the model for strong attractive interactions between the intercalation species and the host matrix. The DLi in the material was measured as a function of the lithium content by using the galvanostatic intermittent titration technique (GITT). The diffusion coefficient was found 1.8 10 14 and 2.2 10 16 cm2 s 1 for LiFePO4 and FePO4, respectively, with a minimum in correspondence of the peak of the differential capacity. The DLi has also been measured by AC impedance method for various lithium contents. The calculated values are in very good agreement with the previous calculated ones. D 2002 Elsevier Science B.V. All rights reserved

Effect of milling and doping on decomposition of NH3BH3 complex

In this work we considered three different samples: borane ammonia (BA) complex as received, ball milled BA and, finally, BA subjected to ball milling and doped with 1 mol% of hydrogen hexachloroplatinate hydrate. Their crystalline structure was described by XRD and morphology was investigated using SEM and EDS techniques. In order to describe samples behaviour in hydrogen release we plotted TGA curves and thermal decomposition tests were realized. We verified that ball milling increases the BA cristallinity and changes its morphology enhancing the amount of evolved gas. Both the milled and the doped samples show an increase of the pre-exponential factor in the Arrhenius equation. The activation energy remains almost unchanged for the doped sample and it increases for the milled one. As result it was found that the decomposition of 1 mol% doped sample could be provided by waste heat coming from a PEM fuel cell