Xiaosong Liu

Phase Transformation and Lithiation Effect on Electronic Structure of LixFePO4: An In-Depth Study by Soft X-ray and Simulations

Through soft X-ray absorption spectroscopy, hard X-ray Raman scattering, and theoretical simulations, we provide the most in-depth and systematic study of the phase transformation and (de)lithiation effect on electronic structure in Li(x)FePO(4) nanoparticles and single crystals. Soft X-ray reveals directly the valence states of Fe 3d electrons in the vicinity of Fermi level, which is sensitive to the local lattice distortion, but more importantly offers detailed information on the evolution of electronic states at different electrochemical stages. The soft X-ray spectra of Li(x)FePO(4) nanoparticles evolve vividly with the (de)lithiation level. The spectra fingerprint the (de)lithiation process with rich information on Li distribution, valency, spin states, and crystal field. The high-resolution spectra reveal a subtle but critical deviation from two-phase transformation in our electrochemically prepared samples. In addition, we performed both first-principles calculations and multiplet simulations of the spectra and quantitatively determined the 3d valence states that are completely redistributed through (de)lithiation. This electronic reconfiguration was further verified by the polarization-dependent spectra collected on LiFePO(4) single crystals, especially along the lithium diffusion direction. The evolution of the 3d states is overall consistent with the local lattice distortion and provides a fundamental picture of the (de)lithiation effects on electronic structure in the Li(x)FePO(4) system.

Distinct charge dynamics in battery electrodes revealed by in situ and operando soft X-ray spectroscopy

Developing high-performance batteries relies on material breakthroughs. During the past few years, various in situ characterization tools have been developed and have become indispensible in studying and the eventual optimization of battery materials. However, soft X-ray spectroscopy, one of the most sensitive probes of electronic states, has been mainly limited to ex situ experiments for battery research. Here we achieve in situ and operando soft X-ray absorption spectroscopy of lithium-ion battery cathodes. Taking advantage of the elemental, chemical and surface sensitivities of soft X-rays, we discover distinct lithium-ion and electron dynamics in Li(Co1/3Ni1/3Mn1/3)O2 and LiFePO4 cathodes in polymer electrolytes. The contrast between the two systems and the relaxation effect in LiFePO4 is attributed to a phase transformation mechanism, and the mesoscale morphology and charge conductivity of the electrodes. These discoveries demonstrate feasibility and power of in situ soft X-ray spectroscopy for studying integrated and dynamic effects in batteries.

Plasma etch removal of poly(methyl methacrylate) in block copolymer lithography

Polystyrene-block-poly(methyl methacrylate), (PS-b-PMMA) diblock copolymer is a promising lithography alternative for nanometer scale features. The two components segregate into nanoscale domains when the polymer solution is spun on to form a thin film and annealed above the glass transition temperatures of both components. Preferential removal of PMMA domains through plasma etching to leave behind a PS mask for subsequent etching of underlying layers is the focus of this work. The quality of the PS mask is characterized by the thickness and lateral dimension of the PS structures after removal of the PMMA, as well as the smoothness of their surfaces. The effects of different plasma chemistries including O2, Ar/O2, Ar, CF4, and CHF3/O2 on etch selectivity and surface/sidewall roughness for PS and PMMA have been characterized. Ar/O2 produced the overall best results for the range of conditions studied.

Rechargeable aqueous zinc-manganese dioxide batteries with high energy and power densities

Although alkaline zinc-manganese dioxide batteries have dominated the primary battery applications, it is challenging to make them rechargeable. Here we report a high-performance rechargeable zinc-manganese dioxide system with an aqueous mild-acidic zinc triflate electrolyte. We demonstrate that the tunnel structured manganese dioxide polymorphs undergo a phase transition to layered zinc-buserite on first discharging, thus allowing subsequent intercalation of zinc cations in the latter structure. Based on this electrode mechanism, we formulate an aqueous zinc/manganese triflate electrolyte that enables the formation of a protective porous manganese oxide layer. The cathode exhibits a high reversible capacity of 225 mAh g−1 and long-term cyclability with 94% capacity retention over 2000 cycles. Remarkably, the pouch zinc-manganese dioxide battery delivers a total energy density of 75.2 Wh kg−1. As a result of the superior battery performance, the high safety of aqueous electrolyte, the facile cell assembly and the cost benefit of the source materials, this zinc-manganese dioxide system is believed to be promising for large-scale energy storage applications.The development of rechargeable aqueous zinc batteries are challenging but promising for energy storage applications. With a mild-acidic triflate electrolyte, here the authors show a high-performance Zn-MnO2 battery in which the MnO2 cathode undergoes Zn2+ (de)intercalation.

Recent Progress on Synchrotron‐Based In‐Situ Soft X‐ray Spectroscopy for Energy Materials

Soft X-ray spectroscopy (SXS) techniques such as photoelectron spectroscopy, soft X-ray absorption spectroscopy and X-ray emission spectroscopy are efficient and direct tools to probe electronic structures of materials. Traditionally, these surface sensitive soft X-ray techniques that detect electrons or photons require high vacuum to operate. Many recent in situ instrument developments of these techniques have overcome this vacuum barrier. One can now study many materials and model devices under near ambient, semi-realistic, and operando conditions. Further developments of integrating the realistic sample environments with efficient and high resolution detection methods, particularly at the high brightness synchrotron light sources, are making SXS an important tool for the energy research community. In this progress report, we briefly describe the basic concept of several SXS techniques and discuss recent development of SXS instruments. We then present several recent studies, mostly in situ SXS experiments, on energy materials and devices. Using these studies, we would like to highlight that the integration of SXS and in situ environments can provide in-depth insight of materials functionality and help researchers in new energy material developments. The remaining challenges and critical research directions are discussed at the end.

Universal mechanism for breaking amide bonds by ionizing radiation

The photodissociation of the amide bond by UV light and soft x-rays is investigated by x-ray absorption spectroscopy at the C, N, and O 1s edges. Irradiation leaves a clear and universal signature for a wide variety of amides, ranging from oligopeptides to large proteins and synthetic polyamides, such as nylon. As the π∗ peak of the amide bond shrinks, two new π∗ peaks appear at the N 1s edge with a characteristic splitting of 1.1 eV. An additional characteristic is the overall intensity reduction of both the π∗ and σ∗ features at the O 1s edge, which indicates loss of oxygen. The spectroscopic results are consistent with the release of the O atom from the amide bond, followed by the migration of the H atom from the N to one of its two C neighbors. Migration to the carbonyl C leads to an imine, and migration to the C(α) of the amino acid residue leads to a nitrile. Imine and nitrile produce the two characteristic π∗ transitions at the N 1s edge. A variety of other models is considered and tested against the N 1s spectra of reference compounds.

Unoccupied states in Cu and Zn octaethyl-porphyrin and phthalocyanine

Copper and zinc phthalocyanines and porphyrins are used in organic light emitting diodes and dye-sensitized solar cells. Using near edge x-ray absorption fine structure (NEXAFS) spectroscopy at the Cu 2p and Zn 2p edges, the unoccupied valence states at the Cu and Zn atoms are probed and decomposed into 3d and 4s contributions with the help of density functional calculations. A comparison with the N 1s edge provides the 2p states of the N atoms surrounding the metal, and a comparison with inverse photoemission provides a combined density of states.

The local atomic structure and chemical bonding in sodium tin phases

To understand the electrochemically-derived Na–Sn we have reinvestigated the formation of Na–Sn alloys to identify all the phases which form when x ≥ 1 (NaxSn) and characterized the local bonding around the Sn atoms with X-ray diffraction, 119Sn Mossbauer spectroscopy, and X-ray absorption spectroscopies. The results from the well-defined crystallographic materials were compared to the spectroscopic measurements of the local Sn structures in the electrochemically prepared materials. The reinvestigation of the Na–Sn compounds yields a number of new results: (i) Na7Sn3 is a new thermodynamically-stable phase with a rhombohedral structure and Rm space group; (ii) orthorhombic Na9Sn4 (Cmcm) has relatively slow formation kinetics suggesting why it does not form at room temperature during the electrochemical reaction; (iii) orthorhombic ‘Na14.78Sn4’ (Pnma), better described as Na16−xSn4, is Na-richer than cubic Na15Sn4 (I3d). Characterization of electrochemically prepared Na–Sn alloys indicate that, with the exception of Na7Sn3 and Na15Sn4, different crystal structures than similar Na–Sn compositions prepared via classic solid state reactions are formed. These phases are composed of disordered structures characteristic of kinetic-driven solid-state amorphization reactions. In these structures, Sn coordinates in asymmetric environments, which differ significantly from the environments present in Na–Sn model compounds.

Influence of molecular ordering on electrical and friction properties of omega-(trans-4-stilbene)alkylthiol self-assembled monolayers on Au (111)

The electrical and friction properties of ω-(trans-4-stilbene)alkylthiol self-assembled monolayers (SAMs) on Au(111) were investigated using atomic force microscopy (AFM) and near edge X-ray absorption fine structure spectroscopy (NEXAFS). The sample surface was uniformly covered with a molecular film consisting of very small grains. Well-ordered and flat monolayer islands were formed after the sample was heated in nitrogen at 120 °C for 1 h. While lattice resolved AFM images revealed a crystalline phase in the islands, the area between islands showed no order. The islands exhibit substantial reduction (50%) in friction, supporting the existence of good ordering. NEXAFS measurements revealed an average upright molecular orientation in the film, both before and after heating, with a narrower tilt-angle distribution for the heated fim. Conductance-AFM measurements revealed a 2 orders of magnitude higher conductivity on the ordered islands than on the disordered phase. We propose that the conductance enhancement is a result of a better π-π stacking between the trans-stilbene molecular units as a result of improved ordering in islands.

A Rechargeable Li-Air Fuel Cell Battery Based on Garnet Solid Electrolytes

Non-aqueous Li-air batteries have been intensively studied in the past few years for their theoretically super-high energy density. However, they cannot operate properly in real air because they contain highly unstable and volatile electrolytes. Here, we report the fabrication of solid-state Li-air batteries using garnet (i.e., Li6.4La3Zr1.4Ta0.6O12, LLZTO) ceramic disks with high density and ionic conductivity as the electrolytes and composite cathodes consisting of garnet powder, Li salts (LiTFSI) and active carbon. These batteries run in real air based on the formation and decomposition at least partially of Li2CO3. Batteries with LiTFSI mixed with polyimide (PI:LiTFSI) as a binder show rechargeability at 200 °C with a specific capacity of 2184 mAh g−1carbon at 20 μA cm−2. Replacement of PI:LiTFSI with LiTFSI dissolved in polypropylene carbonate (PPC:LiTFSI) reduces interfacial resistance, and the resulting batteries show a greatly increased discharge capacity of approximately 20300 mAh g−1carbon and cycle 50 times while maintaining a cutoff capacity of 1000 mAh g−1carbon at 20 μA cm−2 and 80 °C. These results demonstrate that the use of LLZTO ceramic electrolytes enables operation of the Li-air battery in real air at medium temperatures, leading to a novel type of Li-air fuel cell battery for energy storage.