The Morphology of TiO₂ (B) Nanoparticles

The morphology of a nanomaterial (geometric shape and dimension) has a significant impact on its physical and chemical properties. It is, therefore, essential to determine the morphology of nanomaterials so as to link shape with performance in specific applications. In practice, structural features with different length scales are encoded in a specific angular range of the X-ray or neutron total scattering pattern of the material. By combining small- and wide-angle scattering (typically X-ray) experiments, the full angular range can be covered, allowing structure to be determined accurately at both the meso- and the nanoscale. In this Article, a comprehensive morphology analysis of lithium-ion battery anode material, TiO₂ (B) nanoparticles (described in Ren, Y.; Liu, Z.; Pourpoint, F.; Armstrong, A. R.; Grey, C. P.; Bruce, P. G. <i>Angew. Chem. Int. Ed.</i> <b>2012</b>, <i>51</i>, 2164), incorporating structure modeling with small-angle X-ray scattering (SAXS), pair distribution function (PDF), and X-ray powder diffraction (XRPD) techniques, is presented. The particles are oblate-shaped, contracted along the [010] direction, this particular morphology providing a plausible rationale for the excellent electrochemical behavior of these TiO₂(B) nanoparticles, while also provides a structural foundation to model the strain-driven distortion induced by lithiation. The work demonstrates the importance of analyzing various structure features at multiple length scales to determine the morphologies of nanomaterials

Polymer-Templated LiFePO₄/C Nanonetworks as High-Performance Cathode Materials for Lithium-Ion Batteries

Lithium iron phosphate (LFP) is currently one of the main cathode materials used in lithium-ion batteries due to its safety, relatively low cost, and exceptional cycle life. To overcome its poor ionic and electrical conductivities, LFP is often nanostructured, and its surface is coated with conductive carbon (LFP/C). Here, we demonstrate a sol–gel based synthesis procedure that utilizes a block copolymer (BCP) as a templating agent and a homopolymer as an additional carbon source. The high-molecular-weight BCP produces self-assembled aggregates with the precursor-sol on the 10 nm scale, stabilizing the LFP structure during crystallization at high temperatures. This results in a LFP nanonetwork consisting of interconnected ∼10 nm-sized particles covered by a uniform carbon coating that displays a high rate performance and an excellent cycle life. Our “one-pot” method is facile and scalable for use in established battery production methodologies

Mesoporous Titania Microspheres with Highly Tunable Pores as an Anode Material for Lithium Ion Batteries

Mesoporous titania microspheres (MTMs) have been employed in many applications, including (photo)­catalysis as well as energy conversion and storage. Their morphology offers a hierarchical structural design motif that lends itself to being incorporated into established large-scale fabrication processes. Despite the fact that device performance hinges on the precise morphological characteristics of these materials, control over the detailed mesopore structure and the tunability of the pore size remains a challenge. Especially the accessibility of a wide range of mesopore sizes by the same synthesis method is desirable, as this would allow for a comparative study of the relationship between structural features and performance. Here, we report a method that combines sol–gel chemistry with polymer micro- and macrophase separation to synthesize porous titania spheres with diameters in the micrometer range. The as-prepared MTMs exhibit well-defined, accessible porosities with mesopore sizes adjustable by the choice of the polymers. When applied as an anode material in lithium ion batteries (LIBs), the MTMs demonstrate excellent performance. The influence of the pore size and an in situ carbon coating on charge transport and storage is examined, providing important insights for the optimization of structured titania anodes in LIBs. Our synthesis strategy presents a facile one-pot approach that can be applied to different structure-directing agents and inorganic materials, thus further extending its scope of application

New Insights into the Crystal and Electronic Structures of Li_1+<i>x</i>V_1–<i>x</i>O₂ from Solid State NMR, Pair Distribution Function Analyses, and First Principles Calculations

Pair distribution function (PDF) analyses of synchrotron data obtained for the anode materials Li_1+<i>x</i>V_1–<i>x</i>O₂ (0 ≤ <i>x</i> ≤ 0.1) have been performed to characterize the short to medium range structural ordering. The data show clear evidence for the magnetically-induced distortion of the V sublattice to form trimers, the distortion persisting at even the highest excess Li content considered of <i>x</i> = 0.1. At least three distinct local environments were observed for the stoichiometric material LiVO₂ in ⁶Li nuclear magnetic resonance (NMR) spectroscopy, the environments becoming progressively more disordered as the Li content increases. A two-dimensional Li–Li correlation NMR experiment (POST-C7) was used to identify the resonances corresponding to Li within the same layers. NMR spectra were acquired as a function of the state of charge, a distinct environment for Li in Li₂VO₂ being observed. The results suggest that disorder within the Li layers (in addition to the presence of Li within the V layers as proposed by Armstrong et al. <i>Nat. Mater.</i> <b>2011</b>, <i>10</i>, 223–229) may aid the insertion of Li into the Li_1+<i>x</i>V_1–<i>x</i>O₂ phase. The previously little-studied Li₂VO₂ phase was also investigated by hybrid density functional theory (DFT) calculations, providing insights into magnetic interactions, spin–lattice coupling, and Li hyperfine parameters

Comprehensive Study of the CuF₂ Conversion Reaction Mechanism in a Lithium Ion Battery

Conversion materials for lithium ion batteries have recently attracted considerable attention due to their exceptional specific capacities. Some metal fluorides, such as CuF₂, are promising candidates for cathode materials owing to their high operating potential, which stems from the high electronegativity of fluorine. However, the high ionicity of the metal–fluorine bond leads to a large band gap that renders these materials poor electronic conductors. Nanosizing the active material and embedding it within a conductive matrix such as carbon can greatly improve its electrochemical performance. In contrast to other fluorides, such as FeF₂ and NiF₂, good capacity retention has not, however, been achieved for CuF₂. The reaction mechanisms that occur in the first and subsequent cycles and the reasons for the poor charge performance of CuF₂ are studied in this paper via a variety of characterization methods. In situ pair distribution function analysis clearly shows CuF₂ conversion in the first discharge. However, few structural changes are seen in the following charge and subsequent cycles. Cyclic voltammetry results, in combination with in situ X-ray absorption near edge structure and ex situ nuclear magnetic resonance spectroscopy, indicate that Cu dissolution is associated with the consumption of the LiF phase, which occurs during the first charge via the formation of a Cu¹⁺ intermediate. The dissolution process consequently prevents Cu and LiF from transforming back to CuF₂. Such side reactions result in negligible capacity in subsequent cycles and make this material challenging to use in a rechargeable battery