Direct Observation of Reductive Coupling Mechanism between Oxygen and Iron/Nickel in Cobalt-Free Li-Rich Cathode Material: An in Operando X-Ray Absorption Spectroscopy Study

Li-rich cathodes possess high capacity and are promising candidates in next-generation high-energy density Li-ion batteries. This high capacity is partly attributed to its poorly understood oxygen-redox activity. The present Li-rich cathodes contain expensive and environmentally-incompatible cobalt as a main transition metal. In this work, cobalt-free, iron-containing Li-rich cathode material (nominal composition Li$_{1.2}$Mn$_{0.56}$Ni$_{0.16}$Fe$_{0.08}$O$_{2}$) is synthesized, which exhibits excellent discharge capacity (≈250 mAh g$^{-1}$ and cycling stability. In operando, X-ray absorption spectroscopy at Mn, Fe, and Ni K edges reveals its electrochemical mechanism. X-ray absorption near edge structure (XANES) features of Fe and Ni K edges show unusual behavior: when an electrode is charged to 4.5 V, Fe and Ni K edges’ XANES features shift to higher energies, evidence for Fe$^{3+}$→Fe$^{4+}$ and Ni$^{2+}$→Ni$^{4+}$ oxidation. However, when charged above 4.5 V, XANES features of Fe and Ni K edges shift back to lower energies, indicating Fe$^{4+}$→Fe$^{3+}$ and Ni$^{4+}$→Ni$^{3+}$ reduction. This behavior can be linked to a reductive coupling mechanism between oxygen and Fe/Ni. Though this mechanism is observed in Fe-containing Li-rich materials, the only electrochemically active metal in such cases is Fe. Li$_{1.2}$Mn$_{0.56}$Ni$_{0.16}$Fe$_{0.08}$O$_{2}$ has multiple electrochemically active metal ions; Fe and Ni, which are investigated simultaneously and the obtained results will assist tailoring of cost-effective Li-rich materials

Sodium vanadium titanium phosphate electrode for symmetric sodium-ion batteries with high power and long lifespan

Sodium-ion batteries operating at ambient temperature hold great promise for use in grid energy storage owing to their significant cost advantages. However, challenges remain in the development of suitable electrode materials to enable long lifespan and high rate capability. Here we report a sodium super-ionic conductor structured electrode, sodium vanadium titanium phosphate, which delivers a high specific capacity of 147 mA h g−1 at a rate of 0.1 C and excellent capacity retentions at high rates. A symmetric sodium-ion full cell demonstrates a superior rate capability with a specific capacity of about 49 mA h g−1 at 20 C rate and ultralong lifetime over 10,000 cycles. Furthermore, in situ synchrotron diffraction and X-ray absorption spectroscopy measurement are carried out to unravel the underlying sodium storage mechanism and charge compensation behaviour. Our results suggest the potential application of symmetric batteries for electrochemical energy storage given the superior rate capability and long cycle life

Spatially resolved studies in direct methanol fuel cells

The thesis mainly focuses on the spatially resolved characterization of a direct methanol fuel cell. Initially spatially resolved analyses were carried out on an end of life (5000 hrs operated) stack membrane electrode assembly (MEA) using various techniques, like X-ray diffraction (XRD), transmission electron microscope (TEM), energy dispersive X-ray (EDX) mapping and X-ray absorption spectroscopy (XAS). The fate of the Ru in the direct methanol fuel cell (DMFC) with ageing is carefully analyzed in these studies. It was found that the large oxidized ruthenium fraction in the anode catalyst plays a significant role in particle growth and ruthenium dissolution. Ru was also found in the form of precipitates in the Nafion membrane preferentially at the methanol outlet regions. Ex-situ studies were preceded by in-situ spatially resolved XAS studies. For these, in-situ cells for spatially resolved DMFC studies are developed and optimized. The relative OH and CO coverages on both the anode and cathode were followed using the XANES technique at different regions of a DMFC during operation at several current levels in dependence on the oxygen flow. For the first time, a very strong “cross-talk” between the anode and cathode is seen with the anode dictating at high O2 flow rate the OH coverage on the cathode. The fuel starvation studies on the single DMFC cell revealed a non-uniform degradation pattern with a high degradation at the methanol inlet and low degradation at methanol outlet. Finally, shape-selected Pt nanoparticles were synthesized using different surfactants like tetradecyltrimethylammonium bromide (TTAB) and polyvinylpyrrolidone (PVP) and tested fuel cell performance. These shape-selected Pt nanoparticles were characterized by TEM and their electrocatalytical activity tested by cyclic voltammetry. High potential cycling of the shape-selected particles revealed a preferential degradation of Pt (100) facets over Pt (110). The TEM analysis of the cycled samples showed predominantly shape-selected particles with very few spherical particles. Finally, supported shape-selected particles showed excellent fuel performance even with low Pt loading. Tuning of the shape of Pt nanoparticles is expected to increase the Pt utilization, i.e. Pt loading can be reduced in the MEA. Further higher durability is expected for the shape-selected particles than the commercial catalyst. Thus by tuning the shape of the Pt nanoparticles, cost reduction and increased durability can be achieved

Unveiling the Electrochemical Mechanism of High-Capacity Negative Electrode Model-System BiFeO 3 in Sodium-Ion Batteries: An In Operando XAS Investigation

Careful development and optimization of negative electrode (anode) materials for Na-ion batteries (SIBs) are essential, for their widespread applications requiring a long-term cycling stability. BiFeO$_3$ (BFO) with a LiNbO$_3$-type structure (space group R3c) is an ideal negative electrode model system as it delivers a high specific capacity (770 mAh g$^{–1}$), which is proposed through a conversion and alloying mechanism. In this work, BFO is synthesized via a sol–gel method and investigated as a conversion-type anode model-system for sodium-ion half-cells. As there is a difference in the first and second cycle profiles in the cyclic voltammogram, the operating mechanism of charge–discharge is elucidated using in operando X-ray absorption spectroscopy. In the first discharge, Bi is found to contribute toward the electrochemical activity through a conversion mechanism (Bi$^{3+}$ → Bi$^0$), followed by the formation of Na–Bi intermetallic compounds. Evidence for involvement of Fe in the charge storage mechanism through conversion of the oxide (Fe$^{3+}$) form to metallic Fe and back during discharging/charging is also obtained, which is absent in previous literature reports. Reversible dealloying and subsequent oxidation of Bi and oxidation of Fe are observed in the following charge cycle. In the second discharge cycle, a reduction of Bi and Fe oxides is observed. Changes in the oxidation states of Bi and Fe, and the local coordination changes during electrochemical cycling are discussed in detail. Furthermore, the optimization of cycling stability of BFO is carried out by varying binders and electrolyte compositions. Based on that, electrodes prepared with the Na-carboxymethyl cellulose (CMC) binder are chosen for optimization of the electrolyte composition. BFO–CMC electrodes exhibit the best electrochemical performance in electrolytes containing fluoroethylene carbonate (FEC) as the additive. BFO–CMC electrodes deliver initial capacity values of 635 and 453 mAh g$^{–1}$ in the Na-insertion (discharge) and deinsertion (charge) processes, respectively, in the electrolyte composition of 1 M NaPF$_6$ in EC/DEC (1:1, v/v) with a 2% FEC additive. The capacity values stabilize around 10th cycle and capacity retention of 73% is observed after 60 cycles with respect to the 10th cycle charge capacity

Improving the rate capability of high voltage lithium-ion battery cathode material LiNi0.5Mn1.5O4 by ruthenium doping

The citric acid-assisted solegel method was used to produce the high-voltage cathodes LiNi0.5Mn1.5O4and LiNi0.4Ru0.05Mn1.5O4 at 800 C and 1000 C final calcination temperatures. High resolution powderdiffraction using synchrotron radiation, inductively coupled plasma e optical emission spectroscopy andscanning electron microscopy analyses were carried out to characterize the structure, chemicalcomposition and morphology. X-ray absorption spectroscopy studies were conducted to confirm Rudoping inside the spinel as well as to compare the oxidation states of transition metals. The formation ofan impurity LixNi1xO in LiNi0.5Mn1.5O4 powders annealed at high temperatures (T 800 C) can besuppressed by partial substitution of Ni2þ by Ru4þ ion. The LiNi0.4Ru0.05Mn1.5O4 powder synthesized at1000 C shows the highest performance regarding the rate capability and cycling stability. It has an initialcapacity of ~139 mAh g1 and capacity retention of 84% after 300 cycles at C/2 chargingedischarging ratebetween 3.5 and 5.0 V. The achievable discharge capacity at 20 C for a charging rate of C/2 is~136 mAh g1 (~98% of the capacity delivered at C/2). Since the electrode preparation plays a crucial roleon parameters like the rate capability, the influence of the mass loading of active materials in the cathodemixture is discussed.© 2014 Elsevier B.V. All rights reserved

Fatigue of LiNi0.8_{0.8} Co0.15_{0.15}Al0.05_{0.05}O2_{2} in commercial Li ion batteries

The degradation of LiNi0.8Co0.15Al0.05O2 (LNCAO), a cathode material in lithium-ion-batteries, was studied using in situ powder diffraction and in situ Ni K edge X-ray absorption spectroscopy (XAS). The fatigued material was taken from a 7 Ah battery which was cycled for 34 weeks under defined durability conditions. Meanwhile, a cell was stored, as reference, under controlled conditions without electrochemical treatment. The fatigued LNCAO used in this study showed a capacity loss of 26% ± 9% compared to the non-cycled material. During charge and discharge the local and the overall structure of LNCAO was investigated by X-ray near edge structure (XANES) analysis, the extended X-ray absorption fine structure (EXAFS) analysis and by using Rietveld refinement of in situ powder diffraction patterns. Both powder diffraction and XAS revealed additional, rhombohedral phases which do not change with electrochemical cycling. Moreover, a phase with the lattice parameters of fully lithiated LNCAO was still present in the fatigued material at high potentials, while it was absent in the non-fatigued reference material. The coexistence of these phases is described by domains within the LNCAO particles, in correlation with the observed fatigue

Improving the rate capability of high voltage Lithium-ion battery cathode material LiNi0.5Mn1.5O4LiNi_{0.5}Mn_{1.5}O_{4} by ruthenium doping

The citric acid-assisted sol–gel method was used to produce the high-voltage cathodes LiNi0.5Mn1.5O4 and LiNi0.4Ru0.05Mn1.5O4 at 800 °C and 1000 °C final calcination temperatures. High resolution powder diffraction using synchrotron radiation, inductively coupled plasma – optical emission spectroscopy and scanning electron microscopy analyses were carried out to characterize the structure, chemical composition and morphology. X-ray absorption spectroscopy studies were conducted to confirm Ru doping inside the spinel as well as to compare the oxidation states of transition metals. The formation of an impurity LixNi1−xO in LiNi0.5Mn1.5O4 powders annealed at high temperatures (T ≥ 800 °C) can be suppressed by partial substitution of Ni2+ by Ru4+ ion. The LiNi0.4Ru0.05Mn1.5O4 powder synthesized at 1000 °C shows the highest performance regarding the rate capability and cycling stability. It has an initial capacity of ∼139 mAh g−1 and capacity retention of 84% after 300 cycles at C/2 charging–discharging rate between 3.5 and 5.0 V. The achievable discharge capacity at 20 C for a charging rate of C/2 is ∼136 mAh g−1 (∼98% of the capacity delivered at C/2). Since the electrode preparation plays a crucial role on parameters like the rate capability, the influence of the mass loading of active materials in the cathode mixture is discussed

Sodium vanadium titanium phosphate electrode for symmetric sodium-ion batteries with high power and long lifespan

Sodium-ion batteries operating at ambient temperature hold great promise for use in grid energy storage owing to their significant cost advantages. However, challenges remain in the development of suitable electrode materials to enable long lifespan and high rate capability. Here we report a sodium super-ionic conductor structured electrode, sodium vanadium titanium phosphate, which delivers a high specific capacity of 147 mA h $g^1$ at a rate of 0.1 C and excellent capacity retentions at high rates. A symmetric sodium-ion full cell demon-strates a superior rate capability with a specific capacity of about 49 mA h $g^1$ at 20 C rate and ultralong lifetime over 10,000 cycles. Furthermore, in situ synchrotron diffraction and X-ray absorption spectroscopy measurement are carried out to unravel the underlying sodium storage mechanism and charge compensation behaviour. Our results suggest the potential application of symmetric batteries for electrochemical energy storage given the superior rate capability and long cycle life