Defects and nanostrain gradients control phase transition mechanisms in single crystal high-voltage lithium spinel.

Lithiation dynamics and phase transition mechanisms in most battery cathode materials remain poorly understood, because of the challenge in differentiating inter- and intra-particle heterogeneity. In this work, the structural evolution inside Li1-xMn1.5Ni0.5O4 single crystals during electrochemical delithiation is directly resolved with operando X-ray nanodiffraction microscopy. Metastable domains of solid-solution intermediates do not appear associated with the reaction front between the lithiated and delithiated phases, as predicted by current phase transition theory. Instead, unusually persistent strain gradients inside the single crystals suggest that the shape and size of solid solution domains are instead templated by lattice defects, which guide the entire delithiation process. Morphology, strain distributions, and tilt boundaries reveal that the (Ni2+/Ni3+) and (Ni3+/Ni4+) phase transitions proceed through different mechanisms, offering solutions for reducing structural degradation in high voltage spinel active materials towards commercially useful durability. Dynamic lattice domain reorientation during cycling are found to be the cause for formation of permanent tilt boundaries with their angular deviation increasing during continuous cycling.

Driving Force of the Initial Step in Electrochemical Pt(111) Oxidation.

The first step of electrochemical surface oxidation is extraction of a metal atom from its lattice site to a location in a growing oxide. Here we show by fast simultaneous electrochemical and in situ high-energy surface X-ray diffraction measurements that the initial extraction of Pt atoms from Pt(111) is a fast, potential-driven process, whereas charge transfer for the related formation of adsorbed oxygen-containing species occurs on a much slower time scale and is evidently uncoupled from the extraction process. It is concluded that potential plays a key independent role in electrochemical surface oxidation.

Ca Cations Impact the Local Environment inside HZSM-5 Pores during the Methanol-to-Hydrocarbons Reaction.

The methanol-to-hydrocarbons (MTH) process is an industrially relevant method to produce valuable light olefins such as propylene. One of the ways to enhance propylene selectivity is to modify zeolite catalysts with alkaline earth cations. The underlying mechanistic aspects of this type of promotion are not well understood. Here, we study the interaction of Ca2+ with reaction intermediates and products formed during the MTH reaction. Using transient kinetic and spectroscopic tools, we find strong indications that the selectivity differences between Ca/ZSM-5 and HZSM-5 are related to the different local environment inside the pores due to the presence of Ca2+. In particular, Ca/ZSM-5 strongly retains water, hydrocarbons, and oxygenates, which occupy as much as 10% of the micropores during the ongoing MTH reaction. This change in the effective pore geometry affects the formation of hydrocarbon pool components and in this way directs the MTH reaction toward the olefin cycle.

Selenium Nanowire Formation by Reacting Selenate with Magnetite.

The mobility of 79Se, a fission product of 235U and long-lived radioisotope, is an important parameter in the safety assessment of radioactive nuclear waste disposal systems. Nonradioactive selenium is also an important contaminant of drainage waters from black shale mountains and coal mines. Highly mobile and soluble in its high oxidation states, selenate (Se(VI)O42-) and selenite (Se(IV)O32-) oxyanions can interact with magnetite, a mineral present in anoxic natural environments and in steel corrosion products, thereby being reduced and consequently immobilized by forming low-solubility solids. Here, we investigated the sorption and reduction capacity of synthetic nanomagnetite toward Se(VI) at neutral and acidic pH, under reducing, oxygen-free conditions. The additional presence of Fe(II)aq, released during magnetite dissolution at pH 5, has an effect on the reduction kinetics. X-ray absorption spectroscopy analyses revealed that, at pH 5, trigonal gray Se(0) formed and that sorbed Se(IV) complexes remained on the nanoparticle surface during longer reaction times. The Se(0) nanowires grew during the reaction, which points to a complex transport mechanism of reduced species or to active reduction sites at the tip of the Se(0) nanowires. The concomitant uptake of aqueous Fe(II) and Se(VI) ions is interpreted as a consequence of small pH oscillations that result from the Se(VI) reduction, leading to a re-adsorption of aqueous Fe(II) onto the magnetite, renewing its reducing capacity. This effect is not observed at pH 7, where we observed only the formation of Se(0) with slow kinetics due to the formation of an oxidized maghemite layer. This indicates that the presence of aqueous Fe(II) may be an important factor to be considered when examining the environmental reactivity of magnetite.

Electrochemical Strain Dynamics in Noble Metal Nanocatalysts.

The theoretical design of effective metal electrocatalysts for energy conversion and storage devices relies greatly on supposed unilateral effects of catalysts structure on electrocatalyzed reactions. Here, by using high-energy X-ray diffraction from the new Extremely Brilliant Source of the European Synchrotron Radiation Facility (ESRF-EBS) on device-relevant Pd and Pt nanocatalysts during cyclic voltammetry experiments in liquid electrolytes, we reveal the near ubiquitous feedback from various electrochemical processes on nanocatalyst strain. Beyond challenging and extending the current understanding of practical nanocatalysts behavior in electrochemical environment, the reported electrochemical strain provides experimental access to nanocatalysts absorption and adsorption trends (i.e., reactivity and stability descriptors) operando. The ease and power in monitoring such key catalyst properties at new and future beamlines is foreseen to provide a discovery platform toward the study of nanocatalysts encompassing a large variety of applications, from model environments to the device level.

Reactivity and Potential Profile across the Electrochemical LiCoO2-Li3PS4 Interface Probed by Operando X-ray Photoelectron Spectroscopy.

All-solid-state lithium batteries are a promising alternative for next-generation safe energy storage devices, provided that parasitic side reactions and the resulting hindrances in ionic transport at the electrolyte-electrode interface can be overcome. Motivated by the need for a fundamental understanding of such an interface, we present here real-time measurements of the (electro-)chemical reactivity and local surface potential at the electrified interface (Li2S)3-P2S5 (LPS) and LiCoO2 (LCO) using operando X-ray photoelectron spectroscopy (XPS) supplemented by X-ray photoemission electron microscopy (XPEEM). We identify three main degradation mechanisms: (i) reactivity at open circuit potential leading to the formation of reduced Co in the +2 oxidation state at the LCO surface, detected in the Co L-edge, which is further increased upon cycling, (ii) onset of electrochemical oxidation of the LPS at 2.3 V vs InLix detected in the S 2p and P 2p core levels, and (iii) Co-ion diffusion into the LPS forming CoSx species at 3.3 V observed in both S 2p and Co 2p core levels. Concurrently, a local surface overpotential of 0.9 V caused by a negative localized charge layer is detected at the LPS-LCO interface. Furthermore, in agreement with previous theoretical results, the presence of a sharp potential drop at the interface between active materials and solid electrolyte is demonstrated in all-solid-state batteries.

Unveiling the Complex Redox Reactions of SnO2 in Li-Ion Batteries Using Operando X-ray Photoelectron Spectroscopy and In Situ X-ray Absorption Spectroscopy.

We experimentally determine the redox reactions during (de-)lithiation of the SnO2 working electrode cycled in (Li2S)3-P2S5 solid electrolyte by combining operando X-ray photoelectron spectroscopy and in situ X-ray absorption spectroscopy. Specifically, we have accurately determined the composition changes in the SnO2 working electrode upon cycling and identified the onset voltage formation of the various phases. Starting from the open-circuit potential, we find that, on lithiation, the Sn M-edge absorption spectra reveal unequivocally the formation of SnOx (x ≤ 1) and Li2SnO3 already at a potential of 1.6 V vs Li+/Li, while Sn 3d/Sn 4d, O 1s, and Li 1s core-level spectra show the formation of Sn0 and Li2O along the first potential plateau at 0.8 V vs Li+/Li and of Li8SnO6 at lower potentials. Below 0.6 V vs Li+/Li, an alloying reaction takes place until the end of the lithiation process at 0.05 V vs Li+/Li, as shown by the formation of LixSn. During delithiation, both the conversion and alloying reactions are found to be partially reversible, starting by the re-formation of Sn0 at 0.3 V vs Li+/Li and followed by the re-formation of Li8SnO6 and SnOx above 0.5 V vs Li+/Li. The conversion and alloying reactions are found to overlap during both lithiation and delithiation. Finally, we validate the theoretical prediction for the SnO2 conversion and alloy (de-)lithiation reactions and clarify the open questions about their reaction mechanism.

Multi-length-scale x-ray spectroscopies for determination of surface reactivity at high voltages of LiNi0.8Co0.15Al0.05O2 vs Li4Ti5O12.

The surface evolution of LiNi0.8Co0.15Al0.05O2 (NCA) and Li4Ti5O12 (LTO) electrodes cycled in a carbonate-based electrolyte was systematically investigated using the high lateral resolution and surface sensitivity of x-ray photoemission electron microscopy combined with x-ray absorption spectroscopy and x-ray photoelectron spectroscopy. On the cathode, we attest that the surface of the pristine particles is composed of adventitious Li2CO3 together with reduced Ni and Co in a +2 oxidation state, which is directly responsible for the overpotential observed during the first de-lithiation. This layer decomposes at 3.8 V vs Li+/Li, leaving behind a fresh surface with Ni and Co in a +3 oxidation state. The charge compensation upon Li+ extraction takes place above 4.0 V and is assigned to the oxidation of both Ni and oxygen, while Co remains in a +3 oxidation state during the whole redox process. We also identified the formation of an inactive surface layer already at 4.3 V, rich in reduced Ni and depleted in oxygen. However, at 4.9 V, NiO-like species are detected accompanied with reduced Co. Despite the highly oxidative potential, the surface of the cathode after long cycling is free of oxidized solvent byproducts but contains traces of LiPF6 byproducts (LiF and POxFy). On the LTO counter electrode, transition metals are detected only after long cycling vs NCA to 4.9 V as well as PVdF and LiPF6 byproducts originating from the cathode. Finally, harvested cycled electrodes prove that the influence of the crosstalk on the electrochemical performance of LTO is limited.

Post Mortem and Operando XPEEM: a Surface-Sensitive Tool for Studying Single Particles in Li-Ion Battery Composite Electrodes.

X-ray photoemission electron microscopy (XPEEM), with its excellent spatial resolution, is a well-suited technique for elucidating the complex electrode-electrolyte interface reactions in Li-ion batteries. It provides element-specific contrast images that allows the study of the surface morphology and the identification of the various components of the composite electrode. It also enables the acquisition of local X-ray absorption spectra (XAS) on single particles of the electrode, such as the C and O K-edges to track the stability of carbonate-based electrolytes, F K-edge to study the electrolyte salt and binder stability, and the transition metal L-edges to gain insights into the oxidation/reduction processes of positive and negative active materials. Here we discuss the optimal measurement conditions for XPEEM studies of Li-ion battery systems, including (i) electrode preparation through mechanical pressing to reduce surface roughness for improved spatial resolution; (ii) corrections of the XAS spectra at the C K-edge to remove the carbon signal contribution originating from the X-ray optics; and (iii) procedures for minimizing the effect of beam damage. Examples from our recent work are provided to demonstrate the strength of XPEEM to solve challenging interface reaction mechanisms via post mortem measurements. Finally, we present a first XPEEM cell dedicated to operando/in situ experiments in all-solid-state batteries. Representative measurements were carried out on a graphite electrode cycled with LiI-incorporated sulfide-based electrolyte. This measurement demonstrates the strong competitive reactions between the lithiated graphite surface and the Li2O formation caused by the reaction of the intercalated lithium with the residual oxygen in the vacuum chamber. Moreover, we show the versatility of the operando XPEEM cell to investigate other active materials, for example, Li4Ti5O12.

Controlling the growth of Bi(110) and Bi(111) films on an insulating substrate.

We demonstrate the controlled growth of Bi(110) and Bi(111) films on an α-Al2O3(0001) substrate by surface x-ray diffraction and x-ray reflectivity using synchrotron radiation. At temperatures as low as 40 K, unanticipated pseudo-cubic Bi(110) films are grown with thicknesses ranging from a few to tens of nanometers. The roughness at the film-vacuum as well as the film-substrate interface, can be reduced by mild heating, where a crystallographic orientation transition of Bi(110) towards Bi(111) is observed at 400 K. From 450 K onwards high quality ultrasmooth Bi(111) films form. Growth around the transition temperature results in the growth of competing Bi(110) and Bi(111) domains.