Lattice O2 Research Articles

Quantifying Mixed Redox and Parasitic Processes in Li-Rich Disordered Rocksalt Li-Ion Battery Cathodes

Li-ion battery demand is expected to increase dramatically as the transportation and power generation sectors become electrified. To address the expense and scarcity of cobalt and nickel used in current layered cathodes, alternative transition metals must be explored. Disordered rocksalt (DRX) cathodes can be fully comprised of affordable, earth-abundant metals such as manganese and titanium and have shown high practical capacity but currently suffer from poor capacity retention and voltage fade [1]. Many researchers have attributed this to O redox, and the subsequent formation of deleterious reactive oxygen species, which can occur in DRX cathodes at states-of-charge as low as 40% [2,3]. Recently, researchers have increased the Mn content in DRX cathodes, with the intent of increasing Mn redox and suppressing O redox [4]. Furthermore, these Mn-rich DRX cathodes undergo a beneficial phase transformation upon electrochemical cycling, increasing capacity and almost eliminating voltage fade [4]. Yet, the redox contributions of Mn and O in these Mn-rich cathodes, especially after this phase transformation, are not understood.Here we decouple capacity contributions from Mn-redox, O-redox, and parasitic processes (e.g. electrolyte decomposition) for Mn-rich DRX using a combination of operando gas measurements and inductively coupled plasma – optimal emission spectrometry (ICP-OES). Mn-rich DRX cathodes were cycled to various states of charge in coin cells and the resulting Mn and O oxidation state were determined via titration mass spectrometry (TiMS) using oxalic acid and triflic acid, respectively. To account for lattice O2 loss, differential electrochemical mass spectrometry (DEMS) was used on select cells. To account for Mn dissolution during cycling, ICP-OES was used to quantify the amount of Mn remaining in the cathode as a function of cycle number. The results of this study provide insight into the various redox processes and interfacial degradation that occurs in DRX cathodes, thereby informing future design to improve high-voltage stability of these promising materials.[1]: Raphaële J. Clément et al 2020 Energy Environ. Sci. 2 345[2]: Tzu-Yang Huang et al 2023 Adv. Energy Mater. 13 2300241[3]: Roland Jung et al 2017 J. Electrochem. Soc. 164 A1361[4]: Zijian Cai et al 2023 Nat. Energy 1

Amphiphilic Polymer Electrolyte Blocking Lattice Oxygen Evolution from High‐Voltage Nickel‐rich Cathodes for Ultra‐Thermal Stabile Batteries

AbstractNi‐rich cathodes have been intensively adopted in Li‐ion batteries to pursuit high energy density, which still suffering irreversible degradation at high voltage. Some unstable lattice O2− species in Ni‐rich cathodes would be oxidized to singlet oxygen 1O2 and released at high volt, which lead to irreversible phase transfer from the layered rhombohedral (R) phase to a spinel‐like (S) phase. To overcome the issue, the amphiphilic copolymers (UMA‐Fx) electrolyte were prepared by linking hydrophobic C−F side chains with hydrophilic subunits, which could self‐assemble on Ni‐rich cathode surface and convert to stable cathode–electrolyte interphase layer. Thereafter, the oxygen releasing of polymer coated cathode was obviously depressed and substituted by the Co oxidation (Co3+→Co4+) at high volt (>4.2 V), which could suppressed irreversible phase transfer and improve cycling stability. Moreover, the amphiphilic polymer electrolyte was also stable with Li anode and had high ion conductivity. Therefore, the NCM811//UMA‐F6//Li pouch cell exhibited outstanding energy density (362.97 Wh/kg) and durability (cycled 200 times at 4.7 V), which could be stalely cycled even at 120°C without short circuits or explosions.

Nitrogen-based redox couple regulated anionic redox to long-term cycling stability of Li and Mn-rich layered oxide cathode for Li-ion batteries

Lithium and manganese-rich layered oxides (LMROs) have attracted extensive attention and are promising cathode materials for next-generation lithium ion batteries due to their high capacities and high energy densities. However, LMRO cathode suffers from severe capacity and voltage fading originating from irreversible surface oxygen evolution. Herein, we propose a facile redox couple strategy by introducing nitroxyl radicals species to regulate the surface anionic redox reaction of LMRO cathode. Differential electrochemical mass spectroscopy, X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy analyses demonstrate that during charge process, the peroxide ion O22− on the surface generated from the oxidation of lattice O2− could be reduced back to stable O2− by redox couple in time, thus avoiding oxygen evolution and structure degradation, as well as enhancing bulk oxygen redox activity. The enhanced LMRO electrode delivers a high capacity of 220.3 mAh g−1 at 1 C. An excellent cycling stability with a capacity retention of 94.4 % is achieved after 500 cycles, as well as a suppressed voltage decay with only 1.12 mV per cycle.

Alkali metal and alkali earth metal-modified La-Fe-based perovskite catalyzed coke combustion

Coke catalytic gasification is an efficient method of burning coke. The core of this technology is the preparation of efficient and stable catalysts. A series of different type La-Fe-based perovskite catalysts, including ABO3 (La1-xSrxFeO3, LaK0.04Sr0.25FeO3), R-P (LaK1.7Sr0.2Fe1.7O6) and A2B2O6 (LaKSr0.17Fe1.5O6, LaKSr0.2Fe1.8O6) were prepared by sol-gel method and calcination of metal-organic framework (MOF) precursor method. The activity of perovskite catalytic coke was tested by thermogravimetric analysis and tubular reactor. Among them, LaKSr0.17Fe1.5O6−MOF has the best catalytic activity, with T10, T50, and T90 values of 610.65 °C, 666.86 °C, and 705.84 °C, respectively. LaKSr0.17Fe1.5O6−MOF not only improves coke conversion but also reduces reaction temperature. DFT and experimental results show that doped K and Sr metals promote the formation of many active sites (oxygen vacancies) on the surface of LaKSr0.17Fe1.5O6−MOF, which reduces the adsorption energy of O2 molecules, realizes the oxygen exchange kinetics. The oxygen release regeneration process is achieved between perovskite lattice oxygen and O2 molecules. These processes synergistically promote coke conversion. In addition, it also has strong stable regeneration performance. These results provide a promising strategy for designing and preparing efficient and low-cost catalysts for the practical application of coke removal.

Solid-state synthesis improves stability and cycling performance of layered sodium oxide cathodes: A solid-state NMR study

Improving the electrochemical performance and stability of layered sodium oxide cathode materials is of essential importance for developing sodium-ion batteries (SIBs). Recent studies showed that additional capacity can be obtained by triggering the O2−/(O2)n− redox reactions at high voltages (>4.0 V) for a novel Li-doped P2-Na2/3Ni1/3Mn2/3O2. Here, we present that surprisingly, such an involvement of lattice oxygen redox reaction can be suppressed by using solid-state synthesis, which leads to improved cycling stability upon simplification of the preparation procedure. Specifically, P2-Na0.79Li0.11Ni0.21Mn0.67O2 was prepared via a solid-state synthetic approach and showed no sign of lattice O2−/(O2)n− redox reaction at high voltages (>4.0 V) compared to the sol-gel synthesized analogy. The former yields higher crystallinity and much less defects or impurities, as demonstrated by 23Na and 7Li solid-state NMR spectroscopy. Such a material remains more stable at > 4.0 V, evidenced by the inhibition of LiTM to LiAM migration (transition metal layer to alkaline metal layer), which is usually a sign of phase change or meta-phase changes upon charging.

Enhanced photocatalytic NH3 synthesis for Bi2MoO6 regulated by doping of fluorine and iodine

This study presents a novel fluorine and iodine co-doped Bi2MoO6 (Bi2MoO6–F–I) photocatalyst prepared using a solvothermal method. The photocatalytic performance of Bi2MoO6–F–I in N2 photocatalytic reduction to NH3 is considerably enhanced compared to that of pristine Bi2MoO6. The experimental findings and theoretical calculations reveal that F− substitute lattice O2− in Bi2MoO6, creating a -Mo-F-Mo- configuration, while I− modifies the surface, forming -Bi-I species in Bi2MoO6–F–I, which serve as high-adsorption centres and active surface sites, facilitating N2 and H2O molecule adsorption and activation on the surface. The synergistic effect of F− doping and -Bi-I surface species optimizes the band structure of Bi2MoO6–F–I, aligning more effectively with the redox potential of N2 photoreduction, which consequently promotes photogenerated carrier separation and redox capability. The remarkable improvement in the photocatalytic performance of Bi2MoO6–F–I in N2 photoreduction to NH3 lays foundation for designing and fabricating high-performance Bi2MoO6-based photocatalysts.

F− doped Bi2MoO6 nanosheets for photoreduction of CO2 with H2O

The issue of excessive CO2 emissions has emerged as a significant global challenge. Photocatalytic technology holds great promise for controlling CO2 emissions. In this paper, two-dimensional Bi2MoO6 nanosheets were prepared by the hydrothermal method and modified by CTAB and F- doping to impart specific morphological characteristics and an energy band structure. The results showed that F-2D BMO with a doping amount of 1.5 wt% exhibited the highest photocatalytic efficiency. The CO evolution rate can be increased from 1.8 μmol/g/h to 5.6 μmol/g/h, with the CO generation selectivity increasing from 52.94 % to 87.50 %. This enhancement can be attributed to the replacement of original lattice O2– by the doped F-, which altered the lattice parameters and energy band structure of the catalyst. Consequently, the band gap was reduced and the conduction band potential became more negative, rendering it more suitable for CO production. Additionally, the improved separation rate of photogenerated carriers, reduced complexation rate, and enhanced generation and transfer of active species all contribute to the enhanced photocatalytic CO2 reduction performance of the catalysts. Therefore, this study highlights the potential benefits and significance of using F- doped Bi2MoO6 nanosheets for addressing concerns regarding CO2 emission concerns and promoting sustainable energy solutions.

Blending Layered Cathode with Olivine: An Economic Strategy for Enhancing the Structural and Thermal Stability of 4.65 V LiCoO2

AbstractThe intrinsic poor structural and thermal stability of high‐voltage layered cathodes are aggravated as the charging depth increases, which severely threatens the cycle life and safety of the battery. Herein, without modifying the high‐voltage layered cathode itself, a simple and economic blending strategy is introduced, and an olivine‐LiCoO2 blended cathode featuring superior comprehensive performance (energy, power, cycle‐life, and safety) at 4.65 V is fabricated. The strong bonding affinity at the olivine/LiCoO2 contact interface suppresses lattice O2 release of LiCoO2, thus significantly improving the structural and thermal stability at high delithiation states. Meanwhile, by contacting with high‐electron‐conductivity LiCoO2, the redox activity of olivine is further activated. Therefore, a stable operation of both olivine‐LiCoO2 and olivine‐LiNi0.8Co0.1Mn0.1O2 blended cathode is achieved under harsh cycling conditions (high voltage or elevated temperature) and long‐life pouch‐cell (91.6% capacity retention after 1000 cycles) is harvested, demonstrating the feasibility and universality of this economic blending strategy on heterogenous cathode candidates.

Partially Reduced Ni-NiO-TiO2 Photocatalysts for Hydrogen Production from Methanol–Water Solution

The study compares the photocatalytic behavior of TiO2, NiO-TiO2, and Ni-NiO-TiO2 photocatalysts in photocatalytic hydrogen production from methanol–water solution. TiO2 and NiO-TiO2 photocatalysts with theoretical NiO loading of 0.5, 1.0, and 3.0 wt. % of NiO were prepared by the sol–gel method. The Ni-NiO-TiO2 photocatalysts were prepared by partial reduction of NiO-TiO2 in hydrogen at 450 °C. The Ni-NiO-TiO2 photocatalysts showed significantly higher hydrogen production than the NiO-TiO2 photocatalysts. The structural, textural, redox, and optical properties of all of the prepared photocatalysts were studied by using XRD, SEM, N2- adsorption, XPS, H2-TPR, and DRS. Attention is focused on the contribution of Ni loading, the surface composition (Ni2+, the lattice O2− species, and OH groups), the distribution of Ni species (dispersed NiO species, crystalline NiO phase, and the metallic Ni0 species), oxygen vacancies, TiO2 modification, the TiO2 crystallite size, and the specific surface area.

Dual Activation of Molecular Oxygen and Surface Lattice Oxygen in Single Atom Cu1 /TiO2 Catalyst for CO Oxidation.

The in-depth mechanism on the simultaneous activation of O2 and surface lattice O2- on one active metallic site has not been elucidated yet. Herein, we report a strategy for the construction of abundant oxygen activation sites by rational design of Cu1 /TiO2 single atom catalysts (SACs). The charge transfer between isolated Cu and TiO2 support generates abundant CuI and 2-coordinated Olat sites in Cu1 -O-Ti hybridization structure, which facilitates the chemisorption and activation of O2 molecules. Simultaneously, the Cu1 -O-Ti induced TiO2 lattice distortion activate the adjacent surface lattice O2- , achieving the dual activation of O2 and surface lattice O2- . The Cu1 -O-Ti active site switches the CO oxidation mechanism from Eley-Rideal (80 °C) to Mars-van Krevelen route (200 °C) with the increase of reaction temperature. The dual activation of O2 and surface lattice O2- can by modulating the electron properties of SACs can boost the heterogeneous catalytic oxidation activity.

Dual Activation of Molecular Oxygen and Surface Lattice Oxygen in Single Atom Cu1/TiO2 Catalyst for CO Oxidation

AbstractThe in‐depth mechanism on the simultaneous activation of O2 and surface lattice O2− on one active metallic site has not been elucidated yet. Herein, we report a strategy for the construction of abundant oxygen activation sites by rational design of Cu1/TiO2 single atom catalysts (SACs). The charge transfer between isolated Cu and TiO2 support generates abundant CuI and 2‐coordinated Olat sites in Cu1−O−Ti hybridization structure, which facilitates the chemisorption and activation of O2 molecules. Simultaneously, the Cu1−O−Ti induced TiO2 lattice distortion activate the adjacent surface lattice O2−, achieving the dual activation of O2 and surface lattice O2−. The Cu1−O−Ti active site switches the CO oxidation mechanism from Eley‐Rideal (80 °C) to Mars–van Krevelen route (200 °C) with the increase of reaction temperature. The dual activation of O2 and surface lattice O2− can by modulating the electron properties of SACs can boost the heterogeneous catalytic oxidation activity.

Improved photocatalytic activity of Bi2MoO6 by modifying the halogen ions (Cl−, Br−, or I−) for photoreduction of N2 into NH3

A series of halogen ions (Cl−, Br−, or I−) modified Bi2MoO6 (Bi2MoO6–Cl, Bi2MoO6–Br, or Bi2MoO6–I) are synthesized via a one-step solvothermal method. It is determined that the halogen ions (Cl−, Br−, or I−) are modified on the surface of Bi2MoO6 as the -Bi-Cl, -Bi-Br, or -Bi-I species (the connected Bi3+ is coordinated with three lattice O2- ions in Bi2MoO6). The -Bi-Cl, -Bi-Br, or -Bi-I species on the Bi2MoO6 surface can act as highly active sites to polarize and activate the N2 and H2O molecules adsorbed on the surface of catalysts. Moreover, due to the surface-modified -Bi-Cl, -Bi-Br, or -Bi-I species, the visible light absorption is improved, the separation of photogenerated charge carriers is promoted, as well the band structure is better matched with the redox potential of photoreduction of N2. As a result, the photocatalytic activity of Bi2MoO6–Cl, Bi2MoO6–Br, or Bi2MoO6–I is improved for photoreduction of N2 into NH3 compared with Bi2MoO6. Overall, this work provides new insights into the doping states of halogen ions, the surface-active sites, the band structure, the photocatalytic activity, and the mechanism for photoreduction of N2 into NH3 for Bi2MoO6 with modification of halogen ions (Cl−, Br−, or I−).

Promotional effect for SCR of NO with CO over MnO -doped Fe3O4 nanoparticles derived from metal-organic frameworks

Abstract MnOx-Fe3O4 nanomaterials were fabricated by using the innovative scheme of pyrolyzing manganese-doped iron-based metal organic framework in inert atmosphere and exhibited extraordinary performance of NO reduction by CO (CO-SCR). Multi-technology characterizations were conducted to ascertain the properties of fabricated materials (e.g., TGA, XRD, SEM, FT-IR, XPS, BET, H2-TPR and O2-TPD). Moreover, the interaction between reactants and catalysts was ascertained by in situ FT-IR. Experimental results demonstrated that Mn was an ideal promoter for iron oxides, resulting in decrease of crystallite size, improve reducibility property, enhance the mobility and the amount of lattice O2– species, as well as strength the adsorption ability of active NO and CO to form multiple species (e.g., nitrate and carbonate). The unprecedented enhancement of CO-SCR activity over Mn-Fe nanomaterials follows the Eley-Rideal (E-R) and Langmuir-Hinshelwood (L-H) reaction pathway.

Tailoring Co3O4 active species to promote propane combustion over Co3O4/ZSM-5 catalyst

Co3O4 species in Co3O4/ZSM-5 catalyst can be tailored to smaller particle size and higher content of surface lattice O2− and Co3+ species by pre-reduction and pre-oxidation (labeled as Co3O4/ZSM-5-A catalyst), resulting in the enhanced catalytic activity for propane combustion. The 25Co3O4/ZSM-5-A catalyst gives a high reaction rate of 0.379 μmol gcat−1 s−1, which is over two folds higher than that of the 25Co3O4/ZSM-5 catalyst (reaction rate of only 0.177 μmol gcat−1 s−1). The higher content and reducibility of surface lattice O2− in the Co3O4/ZSM-5-A catalyst facilitated its activity for propane combustion through the Mars-Van Krevelen mechanism as evidenced by in-situ diffuse reflectance infrared spectra. The 25Co3O4/ZSM-5-A catalyst (Ea = 37.1 ± 2.9 kJ mol−1) gives much lower apparent activation energy than 25Co3O4/ZSM-5 catalyst (Ea = 85.5 ± 6.7 kJ mol−1), suggesting the easier occurrence of propane combustion over the 25Co3O4/ZSM-5-A catalyst. The atmospheres (2% O2 + 0.2% C3H8 + N2; 9% O2 + 0.2% C3H8 + N2; Air) for pre-oxidation process also significantly affect the catalytic activity of the finally obtained catalysts (labeled as 25Co3O4/ZSM-5-A, 25Co3O4/ZSM-5-B, 25Co3O4/ZSM-5-C), which is attributed to their differences in Co3O4 species (nanoparticle size, and the content of surface lattice O2− and Co3+). Besides, the acid sites of ZSM-5 also play important role on the catalytic combustion propane.

Role of Bound Magnetic Polaron Model in Sm Doped ZnO: Evidence from Magnetic and Electronic Structures

In the present study, the role of the Bound Magnetic Polaron (BMP) model is understood in Sm-doped ZnO nanoparticles synthesized via a facile co-precipitation method based on the experimental results from magnetic properties and electronic structures. Experimentally, these compounds exhibit a typical M vs H curve at room temperature depicting ferromagnetic (FM) ordering which decreases exponentially upon doping. Investigation using X-ray diffraction, Raman, and X-ray Photoelectron Spectroscopy (XPS) rule out the role of any impurity phases or micro-clusters of Sm, in the observed FM ordering. Further, O 1s XPS spectra reveal enhancement in the lattice O2− anions in Zn-O bond and reduction in oxygen vacancies (VO) resulting in the modification of oxygen defect structure. In the BMP model, the formation of Sm3+-VO-Sm3+, upon doping is assumed to be an important factor that mediates and stabilizes the FM ordering. By increasing the Sm concentration, the VO decreases, thus reducing the total number of BMPs to 1017cm−3, well below the percolation threshold of 1020 cm−3 resulting in reducing the FM component. This study shows that the BMP model explains the FM ordering in the ZnO host and is sensitive to the concentration of Sm ions and the induced oxygen vacancies play a vital role in stabilizing the FM behavior.

Design of strontium stannate perovskites with different fine structures for the oxidative coupling of methane (OCM): Interpreting the functions of surface oxygen anions, basic sites and the structure–reactivity relationship

To seek more feasible catalysts for OCM reaction, SrSnO3, Sr2SnO4 and Sr3Sn2O7 perovskite compounds have been purposely designed and synthesized. It is discovered that the fine crystal structure change influences the reaction performance of the catalysts significantly, which follows the order of Sr2SnO4 > Sr3Sn2O7 > SrSnO3.On these perovskite catalysts, both the electrophilic chemisorbed O2–/O22– and surface lattice O2– species are active and selective for C2 product formation. In addition, both of their moderate and strong surface basic sites contribute vitally to the reaction. It is found that the moderate sites are associated with the M+−O- ion pairs and oxygen vacancy, and the strong basic sites are related to the coordinatively unsaturated surface lattice O2–. Due to the phase structure difference of the three perovskites, their Sn-O bond length changes in the order of Sr2SnO4 > Sr3Sn2O7 > SrSnO3. Furthermore, both the electrical conductivity and DFT calculation results have testified that it is easier for Sr2SnO4 to generate surface oxygen vacancies than Sr3Sn2O7 and SrSnO3, which explains that this sample has the largest quantities of active surface oxygen and basic sites, as well as the best OCM performance.

Manipulating Surface Termination of Perovskite Manganate for Oxygen Activation

AbstractFor ABO3 perovskite oxides, one of the key issues limiting their utilization in heterogeneous catalysis is the dominant presence of catalytically inactive A‐site cations at the surface. The engineering of B‐site terminated perovskites is considered as an effective method to address this issue, especially when dealing with Mn/Co‐based perovskite catalysts. However, to date, such a strategy has not been fully successful and remains a major challenge in the field. Herein, a Mn‐terminated La0.45Sr0.45MnO3 (B‐LSM) is successfully synthesized via a one‐pot hydrothermal method, in which low‐valence Mn ions partially occupy the A site to form the active Mn‐excess phase. Experimental results and theoretical calculations reveal that the presence of the surface Mn termination in B‐LSM optimizes the hybrid orbitals of Mn 3d‐O 2p and promotes the activation of surface lattice oxygen, where the pristine inert lattice O2− is evolved into active and stable lattice O2−x. Such structural optimization significantly reduces the activation energy barriers on going from O2− species to important intermediate O− species during O2 activation. Moreover, this results in good stability and Pt‐like activity for the B‐LSM during CO oxidation. This work offers a new chemical route for the design of advanced perovskite‐type oxides possessing novel functions.

Promoting the surface active sites of defect BaSnO3 perovskite with BaBr2 for the oxidative coupling of methane

In this work, to develop better OCM catalysts, especially with improved ethylene selectivity, the promotional effects of BaBr2 on defect BaSnO3 has been investigated by changing the BaBr2/BaSnO3 molar ratio. XRD and Raman have substantiated that BaBr2 and BaSnO3 co-exists in all the BaBr2-modified catalysts. Compared with pure defect BaSnO3, all the modified catalysts exhibit significantly improved OCM performance, particularly with extremely high ethylene selectivity. Electrical conductivity measurement have proven that defect BaSnO3 is a p-type semi-conductor favouring OCM reaction, whose surface possesses abundant vacancies. O2-TPD, CO2-TPD and electrical conductivity dependence on O2 partial pressure results have indicated that BaBr2 addition can enhance the surface oxygen vacancy concentration of BaSnO3, which facilitates the conversion of surface lattice O2− into OCM reactive oxygen sites, such as O22− and O2- anions. At the same time, a larger amount of moderate strength basic sites have been formed. EPR, in situ Raman and XPS results have demonstrated that the quantities of these active oxygen sites can be evidently improved by BaBr2 addition. More importantly, a type of new O2δ- (0 <δ < 1) sites beneficial to OCM reaction have also been formed. Notably, XPS results have revealed that the amount of O22− sites, which can directly convert CH4 into ethylene through carbene intermediates, has been remarkably increased due to the interaction between BaBr2 and BaSnO3. As a consequence, the OCM reaction performance, especially ethylene selectivity and yield on the BaBr2-modified BaSnO3, has been significantly improved.

Photoinduced Adsorption and Oxidation of SO2 on Anatase TiO2(101).

Adsorption of molecules is a fundamental step in all heterogeneous catalytic reactions. Nevertheless, the basic mechanism by which photon-mediated adsorption processes occur on solid surfaces is poorly understood, mainly because they involve excited catalyst states that complicate the analysis. Here we demonstrate a method by which density functional theory (DFT) can be used to quantify photoinduced adsorption processes on transition metal oxides and reveal the fundamental nature of these reactions. Specifically, the photoadsorption of SO2 on TiO2(101) has been investigated by using a combination of DFT and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The combined experimental and theoretical approach gives a detailed description of the photocatalytic desulfurization process on TiO2, in which sulfate forms as a stable surface product that is known to poison the catalytic surface. This work identifies surface-SO42– as the sulfate species responsible for the surface poisoning and shows how this product can be obtained from a stepwise oxidation of SO2 on TiO2(101). Initially, the molecule binds to a lattice O2– ion through a photomediated adsorption process and forms surface sulfite, which is subsequently oxidized into surface-SO42– by transfer of a neutral oxygen from an adsorbed O2 molecule. The work further explains how the infrared spectra associated with this oxidation product change during interactions with water and surface hydroxyl groups, which can be used as fingerprints for the surface reactions. The approach outlined here can be generalized to other photo- and electrocatalytic transition metal oxide systems.

Charge Compensation Mechanism of Li2MnO3 Cathode in All-Solid-State Thin Film Battery Investigated By Using Operando HAXPES

Lithium battery has been used as a power source for not only small devices but also large equipment like an electric vehicle because of its potentially high energy density and high rate performance. All-solid-state battery is a promising battery form to achieve high energy density. Since solid-electrolyte has a wider potential window than a liquid one, high voltage cathode material can be implemented. Among novel cathode materials, we focus on a Li-rich manganese oxide Li2MnO3, which has a layered rock-salt structure with a honeycomb-ordered LiMn2 layer. Although having a high theoretical capacity of ~459 mAh g-1, the cathode delivers a smaller reversible capacity than the theoretical value. Recently, we have found that an all-solid-state battery with a Li2MnO3 thin film can deliver a reversibly capacity of 270 mAh g-1, and the rate and cyclic performance are extremely high [1]. However, the redox species of the Li2MnO3 during the electrochemical charge/discharge reactions are still unknown for this battery system. In this study, the charge compensation mechanism of the Li2MnO3 cathode in an all-solid-state thin film battery was investigated by using an operando hard X-ray photoelectron spectroscopy (HAXPES) method. Although X-ray photoelectron spectroscopy is a powerful method to investigate the electronic structure of the cathode, there is an issue that the probing depth is about a few nm when using Al or Mg Ka source. On the other hand, HAXPES has a high probing depth of several 10 nm, which makes it possible to detect a signal from the buried region of the battery. Solid-electrolyte supported thin film batteries were developed for HAXPES measurement as illustrated in Fig (a). Using solid-electrolyte as substrate, cathode, and anode layers can be separated on the front and the back of the substrate, respectively, which enables us to detect the HAXPES signal of the cathode layer. The battery composed of a polycrystalline Li2MnO3 cathode, an amorphous Li3PO4 buffer layer, a lithium anode, an Al current collector, and a LICGC glass-ceramics solid-electrolyte substrate (manufactured by Ohara Inc.) was prepared by using pulsed laser deposition, magnetron sputtering, and thermal or electron-beam vapor deposition methods. Charge-discharge experiments were performed galvanostatically at a 0.2 C rate between 2.0 and 5.0 V (vs. Li/Li+) at room temperature. After the cell voltage reached at 5.0 V, the voltage was kept for 30 minutes.An operando HAXPES measurement was conducted at BL28XU in SPring-8. The incident photon energy of 7940 eV was used. The operando O1s, Mn3s, and Li1s spectra were measured during the 1st and the 5th charge-discharge processes. The charge-discharge curves showed a plateau at 4.6 V in the 1st charge and slope-like curves from the 1st discharge. Since these characteristics are also identified in the case of the battery using the Li2MnO3 epitaxial thin film [1], the prepared battery is suitable to conduct a HAXPES measurement for the purpose. The figures (b, c) show O1s operando spectra during the 1st charge-discharge process. During the 1st charge, a new peak at 531.1 eV is generated from 3.7 V in addition to a lattice oxygen O2- at 529.8 eV. According to previous reports, the new peak can be assigned as higher oxidized lattice oxygen such as O- [2]. After 3.7 V, the area of the new peak increases while that of O2- decreases as the voltage increases. During the 1st discharge, the new peak gradually decreases and then disappears at 3.3 V. Therefore, the reversible oxygen redox could contribute the charge compensation for the battery reactions. The average valence state of Mn was analyzed from Mn3s operando spectra. Manganese is reduced from 4+ to 3.5+ during the 1st charge process then Mn is further reduced to 3.1+ from 3.3 V during 1st discharging process. Upon the 5th charge-discharge process, the Mn redox occurred reversibly below 3.5 V. These results indicate that the oxygen as well as Mn redox reactions occur reversibly in Li2MnO3 cathode. Furthermore, the oxygen redox occurs on the higher voltage region between ~3.3 and 5.0 V and the Mn redox occurs on the lower region between 2.0 and ~3.3 V. Therefore, it is evident that the large reversible capacity of Li2MnO3 thin film results from the oxygen redox activity. Realizing the stable oxygen redox reaction on batteries with a larger scale will be a key for practical application.[1] K. Hikima et al., Chem. Lett. 48, 192 (2019).[2] M. Sathiya et al., Nature Materials 12, 827 (2013). Figure 1