135-triphenylbenzene-corannulene-as-electron-receptors-for-lithium

1,3,5-Triphenylbenzene and Corannulene as Electron Receptors for Lithium Solvated Electron Solutions

The authors report on conductivity studies carried out on lithium solvated electron solutions (LiSES) prepared using two types of polyaromatic ...

Nanyang Technological University

Samples of Li(x)Ni0.5Mn0.5O2 and Li(x)Ni(1/3)Mn(1/3)Co(1/3)O2 were prepared as active materials in electrochemical half-cells and were cycled electrochemically to obtain different values of Li concentration, x. Absorption edges of Ni, Mn, Co, and O in these materials of differing x were measured by electron energy loss spectrometry (EELS) in a transmission electron microscope to determine the changes in local electronic structure caused by delithiation. The work was supported by electronic structure calculations with the VASP pseudopotential package, the full-potential linear augmented plane wave code WIEN2K, and atomic multiplet calculations that took account of the electronic effects from local octahedral symmetry. A valence change from Ni2+ to Ni4+ with delithiation would have caused a 3 eV shift in energy of the intense white line at the Ni L3 edge, but the measured shift was less than 1.2 eV. The intensities of the "white lines" at the Ni L-edges did not change enough to account for a substantial change of Ni valence. No changes were detectable at the Mn and Co L-edges after delithiation either. Both EELS and the computational efforts showed that most of the charge compensation for Li+ takes place at hybridized O 2p states, not at Ni atoms.

Local electronic structure of layered Li(x)Ni0.5Mn0.5O2 and Li(x)Ni(1/3)Mn(1/3)Co(1/3)O2.

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.

Local electronic structure of olivine phases of LixFePO4.

We report on the electric conductivity measurements and FTIR studies carried out on lithium solvated electron solutions (Li-SES) using biphenyl as the electron receptor and tetrahydrofuran (THF) as the solvent. Conductivity measurements carried out on samples with varying compositions of lithium, biphenyl (β), and THF reveal the highest conductivity of 12.0 mS/cm is achieved with the solution composition Li(1.0)β(THF)(8.2) and a metallic behavior. Characteristic fingerprint peaks attributed to the solvated electron solution appear in the FTIR spectra. Also, from the FTIR spectra, we propose a reaction mechanism of lithium reaction with biphenyl in THF in that Li(1)β(THF)(n1) (1 Li atom pairing with 1 biphenyl molecule) is formed first followed by Li(2)β(THF)(n1) (2 Li atoms pairing with 1 biphenyl molecule). A better understanding of the physical properties of the Li-SES is a prerequisite in view of their optimal applications as liquid anodes in lithium batteries.

Synthesis and characterization of biphenyl-based lithium solvated electron solutions.

Olivine-type LiMPO4 (M = Fe, Mn, Co, Ni) has become of great interest as cathodes for next-generation high-power lithium-ion batteries. Nevertheless, this family of compounds suffers from poor electronic conductivities and sluggish lithium diffusion in the [010] direction. Here, we develop a liquid-phase exfoliation approach combined with a solvothermal lithiation process in high-pressure high-temperature (HPHT) supercritical fluids for the fabrication of ultrathin LiMPO4 nanosheets (thickness: 3.7-4.6 nm) with exposed (010) surface facets. Importantly, the HPHT solvothermal lithiation could produce monodisperse nanosheets while the traditional high-temperature calcination, which is necessary for cathode materials based on high-quality crystals, leads the formation of large grains and aggregation of the nanosheets. The as-synthesized nanosheets have features of high contact area with the electrolyte and fast lithium transport (time diffusion constant in at the microsecond level). The estimated diffusion time for Li(+) to diffuse over a [010]-thickness of <5 nm (L) was calculated to be less than 25, 2.5, and 250 μs for LiFePO4, LiMnPO4, and LiCoPO4 nanosheets, respectively, via the equation of t = L(2)/D. These values are about 5 orders of magnitude lower than the corresponding bulk materials. This results in high energy densities and excellent rate capabilities (e.g., 18 kW kg(-1) and 90 Wh kg(-1) at a 80 C rate for LiFePO4 nanosheets).

Olivine-type nanosheets for lithium ion battery cathodes.

Three dimensional (3D) ordered hierarchically porous electrodes with an entrapped active nanoparticles configuration afford an extremely effective conductive 3D network from the micrometer to the nano meter scale for fast electron and Li-ion transport, and also allow the development of a stable solid electrolyte interphase over the electrode materials, therefore exhibiting extraordinary rate capabilities.

Ultrahigh rate capabilities of lithium-ion batteries from 3D ordered hierarchically porous electrodes with entrapped active nanoparticles configuration.

The fundamental understanding of the relationship between the nanostructure of an electrode and its electrochemical performance is crucial for achieving high-performance lithium-ion batteries (LIBs). In this work, the relationship between the nanotubular aspect ratio and electrochemical performance of LIBs is elucidated for the first time. The stirring hydrothermal method was used to control the aspect ratio of viscous titanate nanotubes, which were used to fabricate additive-free TiO2 -based electrode materials. We found that the battery performance at high charging/discharging rates is dramatically boosted when the aspect ratio is increased, due to the optimization of electronic/ionic transport properties within the electrode materials. The proof-of-concept LIBs comprising nanotubes with an aspect ratio of 265 can retain more than 86 % of their initial capacity over 6000 cycles at a high rate of 30 C. Such devices with supercapacitor-like rate performance and battery-like capacity herald a new paradigm for energy storage systems.

Unravelling the correlation between the aspect ratio of nanotubular structures and their electrochemical performance to achieve high-rate and long-life lithium-ion batteries.

Article	Year
Local electronic structure of layered Li(x)Ni0.5Mn0.5O2 and Li(x)Ni(1/3)Mn(1/3)Co(1/3)O2. The journal of physical chemistry. B\| PubMed ID: 16375321	2005
Local electronic structure of olivine phases of LixFePO4. The journal of physical chemistry. A\| PubMed ID: 17444619	2007
Synthesis and characterization of biphenyl-based lithium solvated electron solutions. The journal of physical chemistry. B\| PubMed ID: 22747199	2012
Olivine-type nanosheets for lithium ion battery cathodes. ACS nano\| PubMed ID: 23713414	2013
Ultrahigh rate capabilities of lithium-ion batteries from 3D ordered hierarchically porous electrodes with entrapped active nanoparticles configuration. Advanced materials (Deerfield Beach, Fla.)\| PubMed ID: 24449491	2014
Unravelling the correlation between the aspect ratio of nanotubular structures and their electrochemical performance to achieve high-rate and long-life lithium-ion batteries. Angewandte Chemie (International ed. in English)\| PubMed ID: 25168684	2014

Rachid Yazami

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