The role of anionic processes in Li1−xNi0.44Mn1.56O4 studied by resonant inelastic X-ray scattering

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Anion Redox Chemistry in the Cobalt Free 3d Transition Metal Oxide Intercalation Electrode Li[Li0.2_{0.2}Ni0.2_{0.2}Mn0.6_{0.6}]O2_2

Conventional intercalation cathodes for lithium batteries store charge in redox reactions associated with the transition metal cations, e.g., Mn3+/4+ in LiMn2O4, and this limits the energy storage of Li-ion batteries. Compounds such as Li[Li0.2Ni0.2Mn0.6]O2 exhibit a capacity to store charge in excess of the transition metal redox reactions. The additional capacity occurs at and above 4.5 V versus Li+/Li. The capacity at 4.5 V is dominated by oxidation of the O2– anions accounting for ?0.43 e–/formula unit, with an additional 0.06 e–/formula unit being associated with O loss from the lattice. In contrast, the capacity above 4.5 V is mainly O loss, ?0.08 e–/formula. The O redox reaction involves the formation of localized hole states on O during charge, which are located on O coordinated by (Mn4+/Li+). The results have been obtained by combining operando electrochemical mass spec on 18O labeled Li[Li0.2Ni0.2Mn0.6]O2 with XANES, soft X-ray spectroscopy, resonant inelastic X-ray spectroscopy, and Raman spectroscopy. Finally the general features of O redox are described with discussion about the role of comparatively ionic (less covalent) 3d metal–oxygen interaction on anion redox in lithium rich cathode materials

What Triggers Oxygen Loss in Oxygen Redox Cathode Materials?

It is possible to increase the charge capacity of transition-metal (TM) oxide cathodes in alkali-ion batteries by invoking redox reactions on the oxygen. However, oxygen loss often occurs. To explore what affects oxygen loss in oxygen redox materials, we have compared two analogous Na-ion cathodes, P2-Na0.67Mg0.28Mn0.72O2 and P2-Na0.78Li0.25Mn0.75O2. On charging to 4.5 V, >0.4e– are removed from the oxide ions of these materials, but neither compound exhibits oxygen loss. Li is retained in P2-Na0.78Li0.25Mn0.75O2 but displaced from the TM to the alkali metal layers, showing that vacancies in the TM layers, which also occur in other oxygen redox compounds that exhibit oxygen loss such as Li[Li0.2Ni0.2Mn0.6]O2, are not a trigger for oxygen loss. On charging at 5 V, P2-Na0.78Li0.25Mn0.75O2 exhibits oxygen loss, whereas P2-Na0.67Mg0.28Mn0.72O2 does not. Under these conditions, both Na+ and Li+ are removed from P2-Na0.78Li0.25Mn0.75O2, resulting in underbonded oxygen (fewer than 3 cations coordinating oxygen) and surface-localized O loss. In contrast, for P2-Na0.67Mg0.28Mn0.72O2, oxygen remains coordinated by at least 2 Mn4+ and 1 Mg2+ ions, stabilizing the oxygen and avoiding oxygen loss

Oxygen redox chemistry without excess alkali-metal ions in Na2/3_{2/3}[Mg0.28_{0.28}Mn0.72_{0.72}]O2_2

The search for improved energy-storage materials has revealed Li- and Na-rich intercalation compounds as promising high-capacity cathodes. They exhibit capacities in excess of what would be expected from alkali-ion removal/reinsertion and charge compensation by transition-metal (TM) ions. The additional capacity is provided through charge compensation by oxygen redox chemistry and some oxygen loss. It has been reported previously that oxygen redox occurs in O 2$p$ orbitals that interact with alkali ions in the TM and alkali-ion layers (that is, oxygen redox occurs in compounds containing Li$^+$–O(2$p$)–Li$^+$ interactions). Na$_{2/3}$[Mg$_{0.28}$Mn$_{0.72}$]O$_2$ exhibits an excess capacity and here we show that this is caused by oxygen redox, even though Mg$^{2+}$ resides in the TM layers rather than alkali-metal (AM) ions, which demonstrates that excess AM ions are not required to activate oxygen redox. We also show that, unlike the alkali-rich compounds, Na$_{2/3}$[Mg$_{0.28}$Mn$_{0.72}$]O$_2$ does not lose oxygen. The extraction of alkali ions from the alkali and TM layers in the alkali-rich compounds results in severely underbonded oxygen, which promotes oxygen loss, whereas Mg$^{2+}$ remains in Na$_{2/3}$[Mg$_{0.28}$Mn$_{0.72}$]O$_2$, which stabilizes oxygen

Anion redox processes in novel battery cathode materials investigated by soft X-ray spectroscopy

This thesis presents experimental investigations of the electronic structure of emerging and novel cathode materials used in lithium- and sodium-ion batteries. The investigated materials include a range of oxide materials containing the elements nickel and manganese. Central goals are to find fundamental explanations for favorable, respectively, unfavorable electrochemical cycling behavior and to arrive at a better understanding of the roles that the different elemental constituents of the compounds play. The experiments are based on the application of X-ray Absorption Spectroscopy (XAS) and Resonant Inelastic X-ray Scattering (RIXS) in the soft X-ray region and have been performed at synchrotron radiation facilities such as The Advanced Light Source (USA), The Swiss Light Source (Switzerland) and SPring-8 (Japan).  XAS and RIXS of spinel LiNi0.44Mn1.56O4 at the O K-edge as well as the Ni and Mn L-edges were measured for two different crystal structures, namely, transition-metal-ordered and -disordered, respectively. The results show that both Ni and O contribute strongly as redox centers for the charge compensation during electrochemical cycling. The Ni L-RIXS spectra show evidence of a more stable Ni--O bond in the disordered material.  In the layered manganese oxide materials Li[Li0.2Ni0.2Mn0.6]O2, Na0.67[Mg0.28Mn0.72]O2, and Na0.78[Li0.25Mn0.75]O2, as well as the disordered Li1.9Mn0.95O2.05F0.95 one observes that reversible O redox leads to two distinct features in O K-RIXS. Both features resonate in a narrow incident energy range suggesting that localized O hole states are formed, one close to the elastic peak and the other as a strong emission peak at an energy loss of about 8 eV. These features appear reversibly on the voltage plateau of the charge-discharge curve and can be used to identify a certain type of O redox reactions. The work also includes investigations that compare two different compositions of the structurally related material Li2MnO3 grown epitaxially as thin films. Evidence is found for anionic activity during the initial cycle that is of a different kind than the above as no evidence for localized O holes is found. Instead, excess Li in the transition metal layer is shown to lead to a more rapid loss of covalency in the Mn--O bonds. In short, this work presents some of the first explorations into the role of different types of anionic redox centers in cathodes, by means of XAS and RIXS thereby also demonstrating the utility and power of synchrotron based techniques for gaining atomic-level understanding of battery electrode materials

Новые процессы и материалы в металлургии

The high corrosion resistance of copper is a key feature in the design of copper-lined canisters that will be utilized to protect people and the environment from dangers of spent nuclear fuel far into the future. Our present study sheds light on the effects that sulfide ions in otherwise relatively benign anoxic groundwater may have on the copper of the container material. Using soft X-ray spectroscopy, we have studied the chemistry of the transformation of single-phase copper oxide cover layers (cuprite, tenorite, paratacamite) as well as single-phase oxide powders (paratacamite and malachite) when exposed to aqueous sulfide solutions. While X-ray diffraction shows that the main bulk of the oxides are nearly unaffected, Cu L-edge absorption spectroscopy shows that a cover layer of about 100 nm thickness on the metal substrate is transformed from Cu(II)- to Cu(I)-species. By contrast, paratacamite and malachite powders exposed to the same kind of aqueous sulfide solutions show much less transformation to Cu(I)-species. We conclude that the main mechanism for reduction of Cu(II) on copper is the comproportionation reaction between divalent copper ions from the covering oxide and the underlying metallic copper atoms to form monovalent copper ions. By contrast, the absence of metallic copper inhibits this mechanism in the powders

Sulfidation of Single-Phase Oxide on Copper and as Powder Studied Using Soft X-Ray Spectroscopy

The high corrosion resistance of copper is a key feature in the design of copper-lined canisters that will be utilized to protect people and the environment from dangers of spent nuclear fuel far into the future. Our present study sheds light on the effects that sulfide ions in otherwise relatively benign anoxic groundwater may have on the copper of the container material. Using soft X-ray spectroscopy, we have studied the chemistry of the transformation of single-phase copper oxide cover layers (cuprite, tenorite, paratacamite) as well as single-phase oxide powders (paratacamite and malachite) when exposed to aqueous sulfide solutions. While X-ray diffraction shows that the main bulk of the oxides are nearly unaffected, Cu L-edge absorption spectroscopy shows that a cover layer of about 100 nm thickness on the metal substrate is transformed from Cu(II)- to Cu(I)-species. By contrast, paratacamite and malachite powders exposed to the same kind of aqueous sulfide solutions show much less transformation to Cu(I)-species. We conclude that the main mechanism for reduction of Cu(II) on copper is the comproportionation reaction between divalent copper ions from the covering oxide and the underlying metallic copper atoms to form monovalent copper ions. By contrast, the absence of metallic copper inhibits this mechanism in the powders

Understanding charge compensation mechanisms in Na0.56Mg0.04Ni0.19Mn0.70O2

Sodium-ion batteries have become a potential alternative to Li-ion batteries due to the abundance of sodium resources. Sodium-ion cathode materials have been widely studied with particular focus on layered oxide lithium analogues. Generally, the capacity is limited by the redox processes of transition metals. Recently, however, the redox participation of oxygen gained a lot of research interest. Here the Mg-doped cathode material P2-Na0.56Mg0.04Ni0.19Mn0.70O2 is studied, which is shown to exhibit a good capacity (ca. 120 mAh/g) and high average operating voltage (ca. 3.5 V vs. Na+/Na). Due to the Mg-doping, the material exhibits a reversible phase transition above 4.3 V, which is attractive in terms of lifetime stability. In this study, we combine X-ray photoelectron spectroscopy, X-ray absorption spectroscopy and resonant inelastic X-ray scattering spectroscopy techniques to shed light on both, cationic and anionic contributions towards charge compensation

How Mn/Ni Ordering Controls Electrochemical Performance in High-Voltage Spinel LiNi0.44Mn1.56O4 with Fixed Oxygen Content

The crystal structure of LiNi0.5O4 (LNMO) can adopt either low-symmetry ordered (Fd (3) over barm) or high-symmetry disordered (P4(3)32) space group depending on the synthesis conditions. A majority of published studies agree on superior electrochemical performance of disordered LNMO, but the underlying reasons for improvement remain unclear due to the fact that different thermal history of the samples affects other material properties such as oxygen content and particle morphology. In this study, ordered and disordered samples were prepared with a new strategy that renders samples with identical properties apart from their cation ordering degree. This was achieved by heat treatment of powders under pure oxygen atmosphere at high temperature with a final annealing step at 710 degrees C for both samples, followed by slow or fast cooling. Electrochemical testing showed that cation disordering improves the stability of material in charged (delithiated) state and mitigates the impedance rise in LNMO parallel to LTO (Li4Ti5O12) and LNMO parallel to Li cells. Through X-ray photoelectron spectroscopy (XPS), thicker surface films were observed on the ordered material, indicating more electrolyte side reactions. The ordered samples also showed significant changes in the Ni 2p XPS spectra, while the generation of ligand (oxygen) holes was observed in the Ni-O environment for both samples using X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS). Moreover, high-resolution transmission electron microscopy (HRTEM) images indicated that the ordered samples show a decrease in ordering near the particle surface after cycling and a tendency toward rock-salt-like phase transformations. These results show that the structural arrangement of Mn/Ni (alone) has an effect on the surface and "nearsurface" properties of LNMO, particularly in delithiated state, which is likely connected to the bulk electronic properties of this electrode material