Visualization of the Phase Propagation within Carbon-Free Li4Ti5O12 Battery Electrodes

The electrochemical reactions occurring in batteries involve the transport of ions and electrons among the electrodes, the electrolyte, and the current collector. In Li-ion battery electrodes, this dual functionality is attained with porous composite electrode structures that contain electronically conductive additives. Recently, the ability to extensively cycle composite electrodes of Li4Ti5O12without any conductive additives generated questions about how these structures operate, the answers to which could be used to design architectures with other materials that reduce the amount of additives that do not directly store energy. Here, the changes occurring in carbon-free Li4Ti5O12 electrodes during lithiation were studied by a combination of ex situ and operando optical microscopy and microbeam X-ray absorption spectroscopy (μ-XAS). The measurements provide visualizations of the percolation of lithiated domains through the thick (∼40-μm) structure after a depth of discharge of only 1%, followed by a second wave of propagation starting with regions in closest contact with the current collector and progressing toward regions in contact with the bulk electrode. These results emphasize the interplay between the electronic and ionic conductivities of the phases involved in a battery reaction and the formation of the phases in localized areas in the electrode architecture. They provide new insights that could be used to refine the design of these architectures to minimize transport limitations while maximizing energy density

Dependence of Li2FeSiO4 Electrochemistry on Structure

Small differences in the FeO4 arrangements (orientation, size, and distortion) do influence the equilibrium potential measured during the first oxidation of Fe2+ to Fe3+ in all polymorphs of Li2FeSiO4.</p

Polymorphism in Li-2(Fe,Mn)SiO4: A combined diffraction and NMR study

Li2MSiO4 compounds have been attracting significant attention as lithium intercalation compounds for the next generation of rechargeable lithium batteries. Their structures consist of slightly distorted close packed oxygen slabs between which cations occupy half the tetrahedral sites, leading to a range of polymorphs. In this paper we describe the rich polymorphism observed across the Li2FezMn(1-z)SiO4 solid solution, characterized by detailed powder neutron and X-ray diffraction studies, combined with solid state NMR. This polymorphism reflects that seen in the end-members, with a gradual transition from Fe-like behaviour for iron-rich compositions, to Mn-like behaviour with increasing manganese content.</p

Polymorphism in Li₂<em>M</em>SiO₄ ( <em>M</em> = Fe, Mn):a variable temperature diffraction study

Lithium transition metal silicates, Li2MSiO4 (M = Fe, Mn, Co), have attracted much interest as positive electrodes for Li-ion batteries due to their high theoretical capacities and their rich crystal chemistry. Low () and high () temperature forms of these tetrahedral structures differ in the ordering/distribution of cations within tetrahedral sites of an hcp based packing of oxygen. We have carried out VT powder neutron and X-ray diffraction on Li2MSiO4 (M = Fe, Mn) to characterize the rich polymorphism observed across the Li2MSiO4 (M = Fe, Mn) system as a function of temperature. For Li2FeSiO4 below 500 degrees C a (II) phase was observed with slight Li/Fe disorder. Above this temperature there was a sluggish transition to the (s) phase, which was observed up to ca. 820 degrees C, with the (II) polymorph seen at highest temperatures. Excellent agreement with phases obtained by quenching was observed, except that the phases exhibit a statistical distribution of Li/Fe, in contrast to the well-ordered structures of quenched phases. The Li2MnSiO4 polymorphism is less complex, the (II) phase is stable from room temperature to 750 degrees C, at which temperature it transforms to the structure with disordered Li/Mn.</p

Crystal Structure of a New Polymorph of Li2FeSiO4

We report on the crystal structure of a new polymorph of Li2FeSiO4 (prepared by annealing under argon at 900 degrees C and quenching to 25 degrees C) characterized by electron microscopy and powder X-ray and neutron diffraction. The crystal structure of Li2FeSiO4 quenched from 900 degrees C is isostructural with Li2CdSiO4, described in the space group Pmnb with lattice parameters a=6.2836(1) angstrom, b = 10.6572(1) angstrom, and c = 5.0386(1) angstrom. A close comparison is made with the structure of Li2FeSiO4 quenched from 700 degrees C, published recently by Nishimura et al. (J. Am. Chem. Soc. 2008, 130, 13212). The two polymorphs differ mainly on the respective orientations and alternate sequences of corner-sharing FeO4 and SiO4 tetrahedra.</p

Visualization of the Phase Propagation within Carbon-Free Li₄Ti₅O₁₂ Battery Electrodes

The electrochemical reactions occurring in batteries involve the transport of ions and electrons among the electrodes, the electrolyte, and the current collector. In Li-ion battery electrodes, this dual functionality is attained with porous composite electrode structures that contain electronically conductive additives. Recently, the ability to extensively cycle composite electrodes of Li₄Ti₅O₁₂ without any conductive additives generated questions about how these structures operate, the answers to which could be used to design architectures with other materials that reduce the amount of additives that do not directly store energy. Here, the changes occurring in carbon-free Li₄Ti₅O₁₂ electrodes during lithiation were studied by a combination of ex situ and operando optical microscopy and microbeam X-ray absorption spectroscopy (μ-XAS). The measurements provide visualizations of the percolation of lithiated domains through the thick (∼40-μm) structure after a depth of discharge of only 1%, followed by a second wave of propagation starting with regions in closest contact with the current collector and progressing toward regions in contact with the bulk electrode. These results emphasize the interplay between the electronic and ionic conductivities of the phases involved in a battery reaction and the formation of the phases in localized areas in the electrode architecture. They provide new insights that could be used to refine the design of these architectures to minimize transport limitations while maximizing energy density