Electronically conductive phospho-olivines as lithium storage electrodes

Synthesis and electrochemical studies on surface-modified LiCoVO4 with La2O3 and malonic acid for cathode material of Li-ion cells

An inverse spinel LiCoVO4 cathode material was synthesized by a citric acid-urea polymeric method, calcined at 773 K for 5 h. The synthesized LiCoVO4 sample was surface-modified with La2O3 and/or malonic acid. The composite materials were comprehensively characterized with the aid of various spectroscopic and analytical techniques. X-ray diffraction (XRD) patterns revealed that single-phase crystallinity occurred when they were heated at 773 K for 5 h in air. For the La2O3-coated samples, there was no evident signal corresponding to secondary-phase peaks. Fourier transform infrared (FTIR) spectra showed the complete elimination of organic residues, nitrates, and the formation of pure LiCoVO4 at 773 K. The two strong peaks due to sp2 and sp3 carbon bonding in the Raman spectra clearly confirm the presence of carbon coating at the surface of LiCoVO4 particles. Transmission electron microscopy (TEM) images showed that the 0.5 wt % La2O3-coated LiCoVO4 material calcined with 60 wt % malonic acid was compact with an average thickness of about 15 nm. This composite cathode material also demonstrated the best cell performance with an initial capacity of 71 mAhg-1 and better thermal stability with lower heat evolution of 35 Jg-1, and a higher onset temperature of thermal decomposition at 475 K, vs. 176 Jg-1 and 452 K for the bare LiCoVO4 sample.

Comparison between Different LiFePO4 Synthesis Routes

Olivine-type lithium iron phosphate (LiFePO4) powders were synthesized applying three different methods: solid state reaction at high temperature, ultrasonic spray pyrolysis, and sonochemical treatment. The samples were characterized by X-ray powder diffraction (XRPD). Particle morphologies of the obtained powders were determined by scanning electron microscopy (SEM). It was found that structural and microstructural parameters of this material were strongly dependent on the synthesis conditions. We present here the results obtained upon optimization of each procedure for designing this cathode material.

Electronic structure of phospho-olivines Li(x)FePO4 (x = 0, 1) from soft-x-ray-absorption and -emission spectroscopies

The electronic structure of the phospho-olivine Li(x)FePO4 was studied using soft-x-ray-absorption (XAS) and emission spectroscopies. Characteristic changes in the valence and conduction bands are observed upon delithation of LiFePO4 into FePO4. In LiFePO4, the Fe-3d states are localized with little overlap with the O-2p states. Delithiation of LiFePO4 gives stronger hybridization between Fe-3d states and O-2p states leading to delocalization of the O-2p states. The Fe L-edge absorption spectra yield "fingerprints" of the different valence states of Fe in LiFePO4 and FePO4. Resonant soft-x-ray-emission spectroscopy at the Fe L edge shows strong contributions from resonant inelastic soft x-ray scattering (RIXS), which is described using an ionic picture of the Fe-3d states. Together the Fe L-edge XAS and RIXS study reveals a bonding character of the Fe 3d-O2p orbitals in FePO4 in contrast to a nonbonding character in LiFePO4.

Local Electronic Structure of Olivine Phases of LixFePO4

Changes in the local electronic structure at atoms around Li sites in the olivine phase of LiFePO4 were studied during delithiation. Electron energy loss spectrometry was used for measuring shifts and intensities of the near-edge structure at the K-edge of O and at the L-edges of P and Fe. Electronic structure calculations were performed on these materials with a plane-wave pseudopotential code and with an atomic multiplet code with crystal fields. It is found that both Fe and O atoms accommodate some of the charge around the Li+ ion, evidently in a hybridized Fe-O state. The O 2p levels appear to be fully occupied at the composition LiFePO4. With delithiation, however, these states are partially emptied, suggestive of a more covalent bonding to the oxygen atom in FePO4 as compared to LiFePO4. The same behavior is found for the white lines at the Fe L2,3-edges, which also undergo a shift in energy upon delithiation. A charge transfer of up to 0.48 electrons is found at the Fe atoms, as determined from white line intensity variations after delithiation, while the remaining charge is compensated by O atoms. No changes are evident at the P L2,3-edges.

Molecular Wiring of Insulators: Charging and Discharging Electrode Materials for High-Energy Lithium-Ion Batteries by Molecular Charge Transport Layers

Self-assembled monolayers (SAMs) of redox-active molecules on mesoscopic substrates exhibit two-dimensional conductivity if their surface coverage exceeds the percolation threshold. Here, we show for the first time that such molecular charge transport layers can be employed to electrochemically address insulating battery materials. The widely used olivine-structured LiFePO4 was derivatized with a monolayer of 4-[bis(4-methoxyphenyl)amino]benzylphosphonic acid (BMABP) in this study. Fast cross-surface hole percolation was coupled to interfacial charge injection, affording charging and discharging of the cathode material. These findings offer the prospect to greatly reduce the amount of conductive carbon additives necessary to electrochemically address present metal phosphate cathode materials, opening up the possibility for a much improved energy storage density. When compared at equal loading, the rate capability is also enhanced with respect to conventional carbon-based conductive additives.

Electrochemical kinetics of porous, carbon-decorated LiFePO4cathodes: separation of wiring effects from solid state diffusion

We try to identify the rate-determining step of electrochemical kinetics of a LiFePO(4)-carbon composite electrode by varying the mass of electrode and, additionally, by varying the charge-discharge current in a wide range. It is shown that the reversible capacity is almost independent of electrode mass at currents lower than ca. 1 C (170 mAh g(-1)). At higher currents, however, the reversible capacity starts to drop significantly. The electrode resistance determined from the corresponding polarization voltage shows inverse proportionality with mass at currents smaller than 1 C. At higher currents the electrode resistance is almost independent of electrode mass. We conclude that at lower currents (below 1 C) the main transport step is related to the active particles themselves (either to incorporation reaction or solid state diffusion of Li). At higher currents the contribution of electronic and ionic transport towards the active particles becomes substantial and should be taken into account when designing high-rate insertion electrodes.