Ribbon-Ordered Superlattice Enables Reversible Anion Redox and Stable High-Voltage Na-Ion Battery Cathodes.

High-voltage layered oxide cathodes attract great attention for sodium-ion batteries (SIBs) due to the potential high energy density, but high voltage usually leads to rapid capacity decay. Herein, a stable high-voltage NaLi0.1Ni0.35Mn0.3Ti0.25O2 cathode with a ribbon-ordered superlattice is reported, and the intrinsic coupling mechanism between structure evolution and the anion redox reaction (ARR) is revealed. Li introduction constructs a special Li-O-Na configuration activating reversible nonbonded O 2p (|O2p)-type ARR and regulates the structure evolution way, enabling the reversible Li ions out-of-layer migration instead of the irreversible transition metal ions out-of-layer migration. The reversible structure evolution enhances the reversibility of the bonded O 2p (O2p)-type ARR and inhibits the generation of oxygen dimers, thus suppressing the irreversible molecular oxygen (O2)-type ARR. After the structure regulation, the structure evolution becomes reversible, |O2p-type ARR is activated, O2p-type ARR becomes stable, and O2-type ARR is inhibited, which largely suppresses the capacity degradation and voltage decay. The discharge capacity is increased from 154 to 168 mA h g-1, the capacity retention after 200 cycles significantly increases from 35 to 84%, and the voltage retention increases from 78 to 93%. This study presents some guidance for the design of high-voltage, O3-type oxide cathodes for high-performance SIBs.

K-Doping Suppresses Oxygen Redox in P2-Na0.67Ni0.11Cu0.22Mn0.67O2 Cathode Materials for Sodium-Ion Batteries.

In P2-type layered oxide cathodes, Na site-regulation strategies are proposed to modulate the Na+ distribution and structural stability. However, their impact on the oxygen redox reactions remains poorly understood. Herein, the incorporation of K+ in the Na layer of Na0.67Ni0.11Cu0.22Mn0.67O2 is successfully applied. The effects of partial substitution of Na+ with K+ on electrochemical properties, structural stability, and oxygen redox reactions have been extensively studied. Improved Na+ diffusion kinetics of the cathode is observed from galvanostatic intermittent titration technique (GITT) and rate performance. The valence states and local structural environment of the transition metals (TMs) are elucidated via operando synchrotron X-ray absorption spectroscopy (XAS). It is revealed that the TMO2 slabs tend to be strengthened by K-doping, which efficiently facilitates reversible local structural change. Operando X-ray diffraction (XRD) further confirms more reversible phase changes during the charge/discharge for the cathode after K-doping. Density functional theory (DFT) calculations suggest that oxygen redox reaction in Na0.62K0.03Ni0.11Cu0.22Mn0.67O2 cathode has been remarkably suppressed as the nonbonding O 2p states shift down in the energy. This is further corroborated experimentally by resonant inelastic X-ray scattering (RIXS) spectroscopy, ultimately proving the role of K+ incorporated in the Na layer.

A Unique Wide-Spacing Fence-Type Superstructure for Robust High-Voltage O3-Type Sodium Layered Cathode.

Enhancing the energy density of layered oxide cathode materials is of great significance for realizing high-performance sodium-ion batteries and promoting their commercial application. Lattice oxygen redox at high voltage usually enables a high capacity and energy density. But the structural degradation, severe voltage decay, and the resultant poor cycling performance caused by irreversible oxygen release seriously restrict the practical application. Herein we introduce a novel fence-type superstructure (2a×3a type supercell) into O3-type layered cathode material Na0.9Li0.1Ni0.3Mn0.3Ti0.3O2 and achieve a stable cycling performance at a high voltage of 4.4 V. The fence-type superstructure effectively inhibits the formation of the vacancy clusters resulting from out-of-plane Li migration and in-plane transition metal migration at high voltage due to the wide d-spacing, thereby significantly reducing the irreversible release of lattice oxygen and greatly stabilizing the crystal structure. The cathode exhibits a high energy density of 545 Wh kg-1, a high rate capability (112.8 mAh g-1 at 5 C) and a high cycling stability (85.8 %@200 cycles with a high initial capacity of 148.6 mAh g-1 at 1 C) accompanied by negligible voltage attenuation (98.5 %@200 cycles). This strategy provides a distinct spacing effect of superstructure to design stable high-voltage layered cathode materials for Na-ion batteries.

Achieving the High Capacity and High Stability of Li-Rich Oxide Cathode in Garnet-Based Solid-State Battery.

Solid-state batteries (SSBs) based on Li-rich Mn-based oxide (LRMO) cathodes attract much attention because of their high energy density as well as high safety. But their development was seriously hindered by the interfacial instability and inferior electrochemical performance. Herein, we design a three-dimensional foam-structured GaN-Li composite anode and successfully construct a high-performance SSB based on Co-free Li1.2 Ni0.2 Mn0.6 O2 cathode and Li6.5 La3 Zr1.5 Ta0.5 O12 (LLZTO) solid electrolyte. The interfacial resistance is considerably reduced to only 1.53â€…Ω cm2 and the assembled Li symmetric cell is stably cycled more than 10,000 h at 0.1-0.2 mA cm-2 . The full battery shows a high initial capacity of 245 mAh g-1 at 0.1 C and does not show any capacity degradation after 200 cycles at 0.2 C (≈100 %). The voltage decay is well suppressed and it is significantly decreased from 2.96 mV/cycle to only 0.66 mV/cycle. The SSB also shows a very high rate capability (≈170 mAh g-1 at 1 C) comparable to a liquid electrolyte-based battery. Moreover, the oxygen anion redox (OAR) reversibility of LRMO in SSB is much higher than that in liquid electrolyte-based cells. This study offers a distinct strategy for constructing high-performance LRMO-based SSBs and sheds light on the development and application of high-energy density SSBs.

Reducing Co/O Band Overlap through Spin State Modulation for Stabilized High Capability of 4.6 V LiCoO2.

High-voltage LiCoO2 (LCO) attracts great interest because of its large specific capacity, but it suffers from oxygen release, structural degradation, and quick capacity drop. These daunting issues root from the inferior thermodynamics and kinetics of the triggered oxygen anion redox (OAR) at high voltages. Herein, a tuned redox mechanism with almost only Co redox is demonstrated by atomically engineered high-spin LCO. The high-spin Co network reduces the Co/O band overlap, eliminates the adverse phase transition of O3 → H1-3, delays the exceeding of the O 2p band over the Fermi level, and suppresses excessive O → Co charge transfer at high voltages. This function intrinsically promotes Co redox and restrains O redox, fundamentally addressing the issues of O2 release and coupled detrimental Co reduction. Moreover, the chemomechanical heterogeneity caused by different kinetics of Co/O redox centers and the inferior rate performance limited by slow O redox kinetics is simultaneously improved owing to the suppression of slow OAR and the excitation of fast Co redox. The modulated LCO delivers ultrahigh rate capacities of 216 mAh g-1 (1C) and 195 mAh g-1(5C), as well as high capacity retentions of 90.4% (@100 cycles) and 86.9% (@500 cycles). This work sheds new light on the design for a wide range of O redox cathodes.

Facilitating Reversible Cation Migration and Suppressing O2 Escape for High Performance Li-Rich Oxide Cathodes.

High-capacity Li-rich Mn-based oxide cathodes show a great potential in next generation Li-ion batteries but suffer from some critical issues, such as, lattice oxygen escape, irreversible transition metal (TM) cation migration, and voltage decay. Herein, a comprehensive structural modulation in the bulk and surface of Li-rich cathodes is proposed through simultaneously introducing oxygen vacancies and P doping to mitigate these issues, and the improvement mechanism is revealed. First, oxygen vacancies and P doping elongates OO distance, which lowers the energy barrier and enhances the reversible cation migration. Second, reversible cation migration elevates the discharge voltage, inhibits voltage decay and lattice oxygen escape by increasing the Li vacancy-TM antisite at charge, and decreasing the trapped cations at discharge. Third, oxygen vacancies vary the lattice arrangement on the surface from a layered lattice to a spinel phase, which deactivates oxygen redox and restrains oxygen gas (O2 ) escape. Fourth, P doping enhances the covalency between cations and anions and elevates lattice stability in bulk. The modulated Li-rich cathode exhibits a high-rate capability, a good cycling stability, a restrained voltage decay, and an elevated working voltage. This study presents insights into regulating oxygen redox by facilitating reversible cation migration and suppressing O2 escape.

Enhanced Thermoelectric Performance in Ternary Skutterudite Co(Ge0.5Te0.5)3 via Band Engineering.

We report the phase evolution and thermoelectric properties of a series of Co(Ge0.5Te0.5)3-xSbx (x = 0-0.20) compositions synthesized by mechanical alloying. Pristine ternary Co(Ge0.5Te0.5)3 skutterudite crystallizes in the rhombohedral symmetry (R3̅), and Sb doping induces a structural transition to the cubic phase (ideal skutterudite, Im3̅). The Sb substitution increases the carrier concentration while maintaining a high thermopower even at higher doping levels owing to an increased effective mass. The exceptional electronic properties exhibited by Co(Ge0.5Te0.5)3 upon doping are attributed to the carrier transport from both the primary and secondary conduction bands, as shown by theoretical calculations. The enhanced electrical conductivity and high thermopower increase the power factor by more than 20 times. Because the dominant phonon propagation modes in binary skutterudites are associated with the vibrations of pnictogen rings, twisting the latter through the isoelectronic replacement of Sb4 rings with Ge2Te2 ones, as done in this study, can effectively reduce the thermal conductivity. This leads to an increase in the dimensionless figure-of-merit (zT) by a factor of 30, reaching 0.65 at 723 K for Co(Ge0.5Te0.5)2.9Sb0.1.

Tuning Bulk O2 and Nonbonding Oxygen State for Reversible Anionic Redox Chemistry in P2-Layered Cathodes.

Improving the reversibility of oxygen redox is quite significant for layered oxides cathodes in sodium-ion batteries. Herein, we for the first time simultaneously tune bulk O2 and nonbonding oxygen state for reversible oxygen redox chemistry in P2-Na0.67 Mn0.5 Fe0.5 O2 through a synergy of Li2 TiO3 coating and Li/Ti co-doping. O2- is oxidized to molecular O2 and peroxide (O2 )n- (n<2) during charging. Molecular O2 derived from transition metal (TM) migration is related to the superstructure ordering induced by Li doping. The synergy mechanism of Li2 TiO3 coating and Li/Ti co-doping on the two O-redox modes is revealed. Firstly, Li2 TiO3 coating restrains the surface O2 and inhibits O2 loss. Secondly, nonbonding Li-O-Na enhances the reversibility of O2- →(O2 )n- . Thirdly, Ti doping strengthens the TM-O bond which fixes lattice oxygen. The cationic redox reversibility is also enhanced by Li/Ti co-doping. The proposed insights into the oxygen redox reversibility are insightful for other oxide cathodes.

Solar to hydrocarbon production using metal-free water-soluble bulk heterojunction of conducting polymer nanoparticle and graphene oxide.

This work demonstrates the first example of interfacial manipulation in a hybrid photocatalyst based on poly(3-hexylthiophene-2,5-diyl) (P3HT) nanoparticle and graphene oxide (GO) bulk heterojunctions to efficiently reduce CO2 into selective industrial hydrocarbons under gas-phase reaction and visible-light illumination. High selectivity of chemical products (methanol and acetaldehyde) was observed. Moreover, the hybrid photocatalyst's solar-to-fuel conversion efficiency was 13.5 times higher than that of pure GO. The increased production yield stems from the co-catalytic and sensitizing role of P3HT in the hybrid system due to its ability to extend light absorption to the visible range and improve interfacial charge transfer to GO. The hybrid P3HT-GO formed a type II heterojunction, and its static and dynamic exciton behaviors were examined using fluorescence spectroscopy and exciton lifetime mapping. A reduced fluorescence decay time was observed by interfacial manipulation for improved dispersion, indicating a more efficient charge transfer from the excited P3HT to GO. Thus, the conducting polymer nanoparticles, 2D nanocarbon, have demonstrated superior performance as a metal-free, non-toxic, low-cost, and scalable heterogeneous photocatalyst for CO2 reduction to solar fuel, a solid-gas system.

Tailoring Co3d and O2p Band Centers to Inhibit Oxygen Escape for Stable 4.6†V LiCoO2 Cathodes.

High-voltage LiCoO2 delivers a high capacity but sharp fading is a critical issue, and the capacity decay mechanism is also poorly understood. Herein, we clarify that the escape of surface oxygen and Li-insulator Co3 O4 formation are the main causes for the capacity fading of 4.6 V LiCoO2 . We propose the inhibition of the oxygen escape for achieving stable 4.6 V LiCoO2 by tailoring the Co3d and O2p band center and enlarging their band gap with MgF2 doping. This enhances the ionicity of the Co-O bond and the redox activity of Co and improves cation migration reversibility. The inhibition of oxygen escape suppresses the formation of Li-insulator Co3 O4 and maintains the surface structure integrity. Mg acts as a pillar, providing a stable and enlarged channel for fast Li+ intercalation/extraction. The modulated LiCoO2 shows almost zero strain and achieves a record capacity retention at 4.6 V: 92 % after 100 cycles at 1C and 86.4 % after 1000 cycles at 5C.

Characterization of the soft X-ray spectrometer PEAXIS at BESSY†II.

The performance of the recently commissioned spectrometer PEAXIS for resonant inelastic soft X-ray scattering (RIXS) and X-ray photoelectron spectroscopy and its hosting beamline U41-PEAXIS at the BESSY II synchrotron are characterized. The beamline provides linearly polarized light from 180 eV to 1600 eV allowing for RIXS measurements in the range 200-1200 eV. The monochromator optics can be operated in different configurations to provide either high flux with up to 1012 photons s-1 within the focal spot at the sample or high energy resolution with a full width at half maximum of <40 meV at an incident photon energy of ∼400 eV. The measured total energy resolution of the RIXS spectrometer is in very good agreement with theoretically predicted values obtained by ray-tracing simulations. PEAXIS features a 5 m-long RIXS spectrometer arm that can be continuously rotated about the sample position by 106° within the horizontal photon scattering plane, thus enabling the study of momentum-transfer-dependent excitations. Selected scientific examples are presented to demonstrate the instrument capabilities, including measurements of excitations in single-crystalline NiO and in liquid acetone employing a fluid cell sample manipulator. Planned upgrades of the beamline and the RIXS spectrometer to further increase the energy resolution to ∼100 meV at 1000 eV incident photon energy are discussed.

The spin-forbidden transition in iron(IV)-oxo catalysts relevant to two-state reactivity.

Quintet oxoiron(IV) intermediates are often invoked in nonheme iron enzymes capable of performing selective oxidation, while most well-characterized synthetic model oxoiron(IV) complexes have a triplet ground state. These differing spin states lead to the proposal of a two-state reactivity model, where the complexes cross from the triplet to an excited quintet state. However, the energy of this quintet state has never been measured experimentally. Here, magnetic circular dichroism is used to assign the singlet and triplet excited states in a series of triplet oxoiron(IV) complexes. These transition energies are used to determine the energies of the quintet state via constrained fitting of 2p3d resonant inelastic x-ray scattering. This allowed for a direct correlation between the quintet energies and substrate C─H oxidation rates.

Boosting Thermoelectric Performance in Nanocrystalline Ternary Skutterudite Thin Films through Metallic CoTe2 Integration.

Metal-semiconductor nanocomposites have emerged as a viable strategy for concurrently tailoring both thermal and electronic transport properties of established thermoelectric materials, ultimately achieving synergistic performance. In this investigation, a series of nanocomposite thin films were synthesized, embedding metallic cobalt telluride (CoTe2) nanophase within the nanocrystalline ternary skutterudite (Co(Ge1.22Sb0.22)Te1.58 or CGST) matrix. Our approach harnessed composition fluctuation-induced phase separation and in situ growth during thermal annealing to seamlessly integrate the metallic phase. The distinctive band structures of both materials have developed an ohmic-type contact characteristic at the interface, which raised carrier density considerably yet negligibly affected the mobility counterpart, leading to a substantial improvement in electrical conductivity. The intricate balance in transport properties is further influenced by the metallic CoTe2 phase's role in diminishing lattice thermal conductivity. The presence of the metallic phase instigates enhanced phonon scattering at the interface boundaries. Consequently, a 2-fold enhancement in the thermoelectric figure of merit (zT ∼ 1.30) is attained with CGST-7 wt. % CoTe2 nanocomposite film at 655 K compared to that of pristine CGST.

Competing Mechanisms Determine Oxygen Redox in Doped Ni-Mn Based Layered Oxides for Na-Ion Batteries.

Cation doping is an effective strategy for improving the cyclability of layered oxide cathode materials through suppression of phase transitions in the high voltage region. In this study, Mg and Sc are chosen as dopants in P2-Na0.67Ni0.33Mn0.67O2, and both have found to positively impact the cycling stability, but influence the high voltage regime in different ways. Through a combination of synchrotron-based methods and theoretical calculations it is shown that it is more than just suppression of the P2 to O2 phase transition that is critical for promoting the favorable properties, and that the interplay between Ni and O activity is also a critical aspect that dictates the performance. With Mg doping, the Ni activity can be enhanced while simultaneously suppressing the O activity. This is surprising because it is in contrast to what has been reported in other Mn-based layered oxides where Mg is known to trigger oxygen redox. This contradiction is addressed by proposing a competing mechanism between Ni and Mg that impacts differences in O activity in Na0.67MgxNi0.33- xMn0.67O2 (x < 0 < 0.33). These findings provide a new direction in understanding the effects of cation doping on the electrochemical behavior of layered oxides.

Oxygen Anion Redox Chemistry Correlated with Spin State in Ni-Rich Layered Cathodes.

Despite the low cost and high capacity of Ni-rich layered oxides (NRLOs), their widespread implementation in electric vehicles is hindered by capacity decay and O release. These issues originate from chemo-mechanical heterogeneity, which is mainly related to oxygen anion redox (OAR). However, what to tune regarding OAR in NRLOs and how to tune it remains unknown. In this study, a close correlation between the OAR chemistry and Li/Ni antisite defects is revealed. Experiments and calculations show the opposite effects of aggregative and dispersive Li/Ni antisite defects on the NiO6 configuration and Ni spin state in NRLOs. The resulting broad or narrow spans for the energy bands caused by spin states lead to different OAR chemistries. By tuning the Li/Ni antisite defects to be dispersive rather than aggregative, the threshold voltage for triggering OAR is obviously elevated, and the generation of bulk-O2 -like species and O2 release at phase transition nodes is fundamentally restrained. The OAR is regulated from irreversible to reversible, fundamentally addressing structural degradation and heterogeneity. This study reveals the interaction of the Li/Ni antisite defect/OAR chemistry/chemo-mechanical heterogeneity and presents some insights into the design of high-performance NRLO cathodes.

Synergistic activation of anionic redox via cosubstitution to construct high-capacity layered oxide cathode materials for sodium-ion batteries.

As a potential substitute for lithium-ion battery, sodium-ion batteries (SIBs) have attracted a tremendous amount of attention due to their advantages in terms of cost, safety and sustainability. Nevertheless, further improvement of the energy density of cathode materials in SIBs remains challenging and requires the activation of anion redox reaction (ARR) activity to provide additional capacity. Herein, we report a high-performance Mn-based sodium oxide cathode material, Na0.67Mg0.1Zn0.1Mn0.8O2 (NMZMO), with synergistic activation of ARR by cosubstitution. This material can deliver an ultra-high capacity of âˆ¼233 mAh/g at 0.1 C, which is significantly higher than their single-cation-substituted counterparts and among the best in as-reported MgMn or ZnMn-based cathodes. Various spectroscopic techniques were comprehensively employed and it was demonstrated that the higher capacity of NMZMO originated from the enhanced ARR activity. Neutron pair distribution function and resonant inelastic X-ray scattering experiments revealed that out-of-plane migration of Mg/Zn occurred upon charging and oxygen anions in the form of molecular O2 were trapped in vacancy clusters in the fully-charged-state. In NMZMO, Mg and Zn mutually interacted with each other to migrate toward tetrahedral sites, which provided a prerequisite for further ARR activity enhancement to form more trapped molecular O2. These findings provide unique insight into the ARR mechanism and can guide the development of high-performance cathode materials through ARR enhancement strategies.

Improving the oxygen redox reversibility of Li-rich battery cathode materials via Coulombic repulsive interactions strategy.

The oxygen redox reaction in lithium-rich layered oxide battery cathode materials generates extra capacity at high cell voltages (i.e., >4.5 V). However, the irreversible oxygen release causes transition metal (TM) dissolution, migration and cell voltage decay. To circumvent these issues, we introduce a strategy for tuning the Coulombic interactions in a model Li-rich positive electrode active material, i.e., Li1.2Mn0.6Ni0.2O2. In particular, we tune the Coulombic repulsive interactions to obtain an adaptable crystal structure that enables the reversible distortion of TMO6 octahedron and mitigates TM dissolution and migration. Moreover, this strategy hinders the irreversible release of oxygen and other parasitic reactions (e.g., electrolyte decomposition) commonly occurring at high voltages. When tested in non-aqueous coin cell configuration, the modified Li-rich cathode material, combined with a Li metal anode, enables a stable cell discharge capacity of about 240 mAh g-1 for 120 cycles at 50 mA g-1 and a slower voltage decay compared to the unmodified Li1.2Mn0.6Ni0.2O2.

Role of Dopants on the Local Electronic Structure of Polymeric Carbon Nitride Photocatalysts.

Polymeric carbon nitride (PCN) is a promising class of materials for solar-to-chemical energy conversion. The increase of the photocatalytic activity of PCN is often achieved by the incorporation of heteroatoms, whose impact on the electronic structure of PCN remains poorly explored. This work reveals that the local electronic structure of PCN is strongly altered by doping with sulfur and iron using X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS). From XAS at the carbon and nitrogen K-edges, sulfur atoms are found to mostly affect carbon atoms, in contrast to iron doping mostly altering nitrogen sites. In RIXS at the nitrogen K-edge, a vibrational progression, affected by iron doping, is evidenced, which is attributed to a vibronic coupling between excited electrons in nitrogen atoms and C-N stretching modes in PCN heterocycling rings. This work opens new perspectives for the characterization of vibronic coupling in polymeric photocatalysts.

Addressing voltage decay in Li-rich cathodes by broadening the gap between metallic and anionic bands.

Oxygen release and irreversible cation migration are the main causes of voltage fade in Li-rich transition metal oxide cathode. But their correlation is not very clear and voltage decay is still a bottleneck. Herein, we modulate the oxygen anionic redox chemistry by constructing Li2ZrO3 slabs into Li2MnO3 domain in Li1.21Ni0.28Mn0.51O2, which induces the lattice strain, tunes the chemical environment for redox-active oxygen and enlarges the gap between metallic and anionic bands. This modulation expands the region in which lattice oxygen contributes capacity by oxidation to oxygen holes and relieves the charge transfer from anionic band to antibonding metal-oxygen band under a deep delithiation. This restrains cation reduction, metal-oxygen bond fracture, and the formation of localized O2 molecule, which fundamentally inhibits lattice oxygen escape and cation migration. The modulated cathode demonstrates a low voltage decay rate (0.45 millivolt per cycle) and a long cyclic stability.

Nanoscale redox mapping at the MoS2-liquid interface.

Layered MoS2 is considered as one of the most promising two-dimensional photocatalytic materials for hydrogen evolution and water splitting; however, the electronic structure at the MoS2-liquid interface is so far insufficiently resolved. Measuring and understanding the band offset at the surfaces of MoS2 are crucial for understanding catalytic reactions and to achieve further improvements in performance. Herein, the heterogeneous charge transfer behavior of MoS2 flakes of various layer numbers and sizes is addressed with high spatial resolution in organic solutions using the ferrocene/ferrocenium (Fc/Fc+) redox pair as a probe in near-field scanning electrochemical microscopy, i.e. in close nm probe-sample proximity. Redox mapping reveals an area and layer dependent reactivity for MoS2 with a detailed insight into the local processes as band offset and confinement of the faradaic current obtained. In combination with additional characterization methods, we deduce a band alignment occurring at the liquid-solid interface.