Ion-intercalation mechanism and structural relaxation in layered iron phosphate Na3Fe3(PO4)4 cathodes

Layered Na3Fe3(PO4)4 can function as a positive electrode for both Li- and Na-ion batteries and may hold advantages from both classical layered and phosphate-based electrode materials. Using a combination of ex-situ and operando synchrotron radiation powder X-ray diffraction, void space analysis, and Mössbauer spectroscopy, we herein investigate the structural evolution of the Na3Fe3(PO4)4 framework during Li- and Na-ion intercalation. We show that during discharge, Li- and Na-intercalation into Na3Fe3(PO4)4 occurs via a solid solution reaction wherein Na-ions appear to be preferentially intercalated into the intralayer sites. The intercalation causes an expansion of the unit cell volume, however at open circuit conditions after ion-intercalation (i.e., after battery discharge), Na3+xFe3(PO4)4 and LixNa3Fe3(PO4)4 undergo a structural relaxation, wherein the unit volume contracts below that of the pristine material. Rietveld refinement suggests that the ions intercalated into the intra-layer sites diffuse to the sites in the inter-layer space during the relaxation. This behavior brings new perspectives to understanding structural relaxation and deviations between structural evolution observed under dynamic and static conditions

Dynamic charge-discharge phase transitions in Li3V2(P4)3\mathrm{Li_{3}V_{2}(P_{4})_{3}} cathodes

Monoclinic $\alpha-\mathrm{Li_{3}V_{2}(P_{4})_{3}}$ is a promising cathode material for future Li-ion batteries due to its high theoreticalcapacity, good capacity retention and relatively high ionic conductivity. The material undergoes a series ofcomplex phase transitions which depend on the number of Li-ions extracted during charge. The phase behaviorhas been extensively studied under (quasi-) equilibrium conditions, however insight into the phase evolutionduring dynamic conditions is lacking. Through operando synchrotron X-ray diffraction we report the complexseries of structural phase transitions under dynamic battery charge-discharge conditions in $\alpha-\mathrm{Li_{3}V_{2}(P_{4})_{3}}$ cathodes with extraction of both two and three Li-ions. For extraction of two Li-ions, the phase evolution followsthe series of expected two-phase transitions, while for extraction of three Li-ions the dynamic phase behaviordiffers significantly from that observed by equilibrium studies, e.g. we reveal unexpected solid solution behaviorduring removal of the last Li-ion and unforeseen structural hysteresis between charge and discharge. Our resultsare further reinforced by electrochemical analysis. This paper joins a series of recent reports, where extendedsolid solution behavior in battery electrode materials is observed under operando conditions, and reinforces theimportance of these types of measurements to provide a more realistic picture of working battery materials

Structural evolution dependency on depth-of-discharge in VO2_2(B) Li-ion battery electrodes

Bronze vanadium oxide, VO$_2$(B), has gained significant interest as electrode for Li-ion mainly due to the ease of preparation and the experimentally obtainable capacities of >325 mAh g$^{−1}$ with intercalation of >1Li. In this work, we investigate for the first time the effects of intercalating >0.5Li on the structural phase evolution. Using operando X-ray diffraction, we find that deep discharge (i.e. inserting >0.7Li), has a dramatic effect on the subsequent charge process by introducing significant solid-solution behavior in the two-phase transition between the Li-rich and Li-poor phases. Rietveld refinement shows that the discharge-charge asymmetry is caused by severe structural deformations in the Li-rich state. Furthermore, we find that deep discharge causes capacity fade. This appears to be linked to the structural deformation causing an irreversible decrease in the Li-ion diffusion coefficients, determined herein by galvanostatic intermittent titrations

Local and Global Structures in the Phase Evolution of P2-Nax_xFey_yMn1–y_{1–y}O2_2 Electrodes for Na-Ion Batteries

The limited availability of raw materials for the production of Li-ion batteries creates a strong incentive to develop Na-ion batteries due to the higher abundance of sodium raw materials. Layered transition-metal oxides are among the most promising electrode materials for Na-ion batteries due to their high capacities. Unfortunately, they still suffer from poor capacity retention. Furthermore, for the Na-ion battery technology to be truly sustainable, the Na-ion electrodes must be free of scarce elements like Ni, Co, and Li (often used as stabilizing dopants). This study investigates the sustainable materials P2-Na$_x$Fe$_y$Mn$_{1–y}$O$_2$ (y = 0.33 and 0.5) to correlate the structural evolution during Na-ion extraction and insertion (i.e., battery charge and discharge) to the Fe:Mn ratio. Using operando powder diffraction, we map the complete structural evolution during deep charge and discharge. Through a combination of Rietveld refinement and pair distribution function analysis, structural models for the distorted and disordered phases at deep discharge and charge are deduced at a global (average) and local scale. By combining the overview of the full structural evolution with the details from the structural analysis, insight into the effects of the Fe:Mn ratio and the origin of phase transitions are elucidated

Polymorphic Purity and Structural Charge–Discharge Evolution of β-LiVOPO4_4 Cathodes

Lithium vanadyl phosphate, LiVOPO$_4$, holds interest as a Li-ion battery electrode due to the multiple active redox states of vanadium ($V^{5+} ⇋ V^{4+} ⇋ V^{3+}$) and the high redox potential induced by vanadyl phosphate. The orthorhombic β-LiVOPO$_4$ polymorph is of special interest owing to its low Li-ion diffusion barrier and high stability. In this work, we synthesize β-LiVOPO$_4$ and investigate the polymorphic purity under different synthesis conditions and show that high-energy ball milling without postsintering can alter the sample morphology and thereby dramatically increase the electrochemical performance of β-LiVOPO$_4$. Through operando powder X-ray diffraction (PXRD), the structural evolution of β-LiVOPO$_4$ during charge and discharge is unraveled and unexpected solid solution behavior in the Li-poor β-VOPO$_4$ is revealed

Crystalline Disorder, Surface Chemistry, and Their Effects on the Oxygen Evolution Reaction (OER) Activity of Mass-Produced Nanostructured Iridium Oxides

In the present study, three mass-produced commercial IrOx samples from different suppliers were studied to establish correlations between various properties and their OER activities. The structures of the electrocatalysts at different scales were explored through laboratory instrumentation, powder X-ray diffraction, and synchrotron-based X-ray total scattering experiments combined with pair distribution function analysis. X-ray photoelectron spectroscopy and energy-dispersive X-ray spectroscopy using a transmission electron microscope were used to determine respectively the surface and the bulk elemental compositions of the samples. The coherent domain size (CDS) values of IrO$_x$ phases within the catalyst particles were estimated to be ∼10, ∼ 19, and ∼ 54 Å for the three IrO$_x$ samples. Surprisingly, the sample with a CDS of ∼19 Å turned out as the best OER electrocatalyst among the three in terms of mass-specific activity, I$_{OER(m)}$, followed by the 10 and 54 Å species. The amount of surface native compound oxygen was found to be a key parameter for the interface electrochemical accessibility. The intrinsic OER activity, evaluated using area-specific activity, I$_{OER(a)}$, suggests that the oxide with lattice disorder presenting a mixture of tetragonal and orthorhombic phases (70:20 w/w) is of superior intrinsic OER activity; however, the oxide with the presence of a monoclinic-like phase is of inferior intrinsic OER activity, which may also be due to the surface presence of Ir$^{3+}$ along with Ir$^{4+}$. The classic belief that the pure tetragonal phase is the best crystalline structure as the OER catalyst is challenged. Iridium oxides with disordered crystallinities may offer a class of highly active oxygen evolution electrocatalysts. The knowledge thus obtained should have a significant impact on the understanding, selection, and processing of IrO$_x$-based OER electrocatalysts

DANOISE; a 3D Printable Battery Cell for Laboratory Operando X-Ray Diffraction and Absorption Spectroscopy

Studying structural and electronic processes in the materials inside a battery as they occur during battery operation is key for developing novel improved materials for future generations of batteries. Such studies often entail X-ray diffraction and absorption spectroscopy, which have so far primarily been conducted at synchrotron facilities despite the widespread use of laboratory-based X-ray diffraction and absorption spectroscopy for day-to-day characterization. With the ?Developed in Aarhus: New Operando In-house Scattering Electrochemical? (DANOISE) cell, these experiments are brought into the laboratories making in-house operando X-ray diffraction and absorption spectroscopy on-demand additions to the existing facilities. The chic and facile design of the DANOISE cell provides high quality scattering and absorption data besides providing reliable electrochemical performance. In this work, we describe the design of the DANOISE cell and demonstrate the capabilities of the cell using different commercial technologies for Li-ion batteries and from a new Na-ion battery electrode material.Peer reviewe