MnO6 Octahedron Research Articles

Mitigating Long Range Jahn‐Teller Ordering to Stabilize Mn Redox Reaction in Biphasic Layered Sodium Oxide

AbstractTo develop the next‐generation commercial oxide cathodes for sodium‐ion batteries, it is crucial to reduce the expensive Ni element content, and further regulate redox reaction of cheap transition metal elements such as Mn to elevate specific capacity. Nevertheless, the activation of Mn redox reaction (MRR) remains a challenge, and notably, MRR induces pronounced Jahn‐Teller effect, resulting in severe structural distortion and fast performance decay. Herein, activated by Na vacancies and weakened hybridization of O (2p)‐TM (3d‐t2g) orbital, a biphasic low‐Ni Mn‐based oxide P2/O3‐Na0.8Ni0.23Fe0.34Mn0.43O2 (P2/O3) exhibits reversible MRR, which performs the transition between Mn4+ and Mn3+ during charging and discharging. Due to the interlaced arrangement of P2‐type and O3‐type crystal domains in P2/O3, the long range Jahn‐Teller ordering is restricted to mitigate the cooperative distortion of MnO6 octahedron induced by MRR and the Jahn‐Teller effect is suppressed, ensuring sustained stable involvement of MRR in charge compensation. In addition, owing to the introduction of P2‐type phase, there is a significant reduction of the migration barrier for sodium ions and no obvious capacity decline after air exposure, leading to a marked enhancement in dynamic performance and air stability of P2/O3, respectively. Consequently, P2/O3 exhibits excellent electrochemical and processing performance.

Mitigating Jahn–Teller effect of MnO2 via charge regulation of Mn-local environment for advanced calcium storage

Manganese dioxide (MnO2) stands out as a prospective cathode material for calcium ion batteries (CIBs) owing to its elevated theoretical capacity and high operating voltage. Nevertheless, MnO2 with Jahn-Teller (J-T) twisted MnO6 octahedra experiences severe Mn dissolution during cycling, leading to the destabilization of the transition metal layer and resulting in poor cycling performance. In this work, Mo-substitution is purposely proposed to suppress the J-T effect in MnO2 for CIBs. The local charge of Mn is regulated by Mo doping, which enhances the electrical conductivity of MnO2 and suppresses the J-T distortion of the MnO6 octahedron. Meanwhile, density-functional theory (DFT) calculations manifest that Mo doping enhances the structural integrity of MnO2, making it difficult for Mn to escape from the MnO6 structure and inhibiting the dissolution of Mn. Accordingly, Mo-MnO2 demonstrates impressive rate capability (112 mAh g−1 at 1 A g−1) and excellent cycling stability, maintaining approximately 93.9 % capacity after 900 cycles at 1 A g−1. As a result, the introduction of heteroatoms through doping offers a novel design approach for the advancement of cathode materials for advanced CIBs.

Pivotal Role of Modifiable A-Site Doping in Enhancing Valence Stability and Excited State Dynamics of MnO6.

The controlled regulation of A-site in rare earth manganate perovskites can orderly arrange the electronic states, leading to the emergence of unique transport properties. However, it is challenging to balance crystal structure stability and property variations during the multi-ion doping. In this study, a series of multivalent manganate perovskites are synthesized by hydrothermal method through the A-site multielement doping, which enables the manganese atoms with varying valence states to orderly arrange at the B site. Powder X-ray diffraction (PXRD) and X-ray absorption spectra (XAS) confirm that the splitting of the K─O hybrid orbitals in the crystal effectively prevents any distortion of the MnO6 octahedron, thereby facilitating the ordered arrangement of Mn (III) -Mn (IV) -Mn (V) at the B-site and promoting superstructure formation. The transient absorption spectra (TAS) reveals that the sequential arrangement of Mn (III) - Mn (IV) - Mn(V) better forms the charge transfer channels, and thereby makes the photodynamic properties of the sample composition-dependent. These photodynamic properties will facilitate the study of exciton-electron coupling behavior in LCKMO crystals during electrical transport.

Stable high energy density in orthogonal layered cathode achieved by trace-substitution strategy

P′2-type Na0.67MnO2 is considered as one of the most promising cathode materials due to its high theoretical capacities and the low cost of sodium-ion batteries (SIBs). However, the multiple phase transitions and distortion of MnO6 octahedron during Na+ extraction/insertion cause poor structural stability and electrochemical properties. Here, a trace-substitution strategy of electronegative Zn2+ and Ti4+ was applied to balance the high capacity and structural stability. The obtained Na0.67Zn0.04Ti0.06Mn0.9O2 (NZTM4) maintains a high capacity of up to 204.3 and 109 mAh g−1 at 0.1 and 10 C rate, respectively, simultaneously achieving excellent capacity retention of 90.6% after 300 cycles. The Mn-O-Zn-O-Ti local structure formed after Zn incorporation inhibits the distortion of MnO6 octahedron and provides lower activation barrier for Na+ diffusion. With the addition of sodium supplements, this enables a high energy density of 241 Wh kg−1 and satisfactory cycle performance in full cells. These findings provide a promising strategy for designing high-capacity layered cathodes of SIBs.

Facile and efficient fabrication of Cu1.04Mn0.96O2 nanosheet anodes with superior electrochemical lithium storage capability

In this work, Cu1.04Mn0.96O2 nanosheets were synthesized via a simple hydrothermal method, and their electrochemical lithium storage properties and reaction mechanisms were investigated. The nanosheet structure effectively promotes electron transfer and shortens the transport path. Additionally, the partial substitution of Cu for Mn decreases the Jahn-Teller distortion of the MnO6 octahedron. Employing as an anode for Li-ion batteries, the specific capacity reached 610.91 mAh g−1 after 100 cycles at a current density of 100 mA g−1.

Defect engineering of copper-doped mesoporous manganese dioxide nanoflowers for enhancing electrocatalytic detection of hydrogen peroxide

Birnessite-type δ-MnO2 nanoflowers doped with copper ions (Cu-MnO2) can enhance the catalytic performance of MnO2-based catalysts for efficient electrochemical sensing of H2O2. There are no significant changes in the surface morphology and lattice structure of bulk MnO2 after Cu doping via engineered cation exchange. The catalytic Cu species are effectively incorporated into the tunnel entrance of δ-MnO2, the Mn site in the MnO6 octahedron, and surface defects. The Cu doping enhances the electrical and ionic conductivities, and generates numerous Mn3+ defects, oxygen vacancies, and Cu+/Cu2+ redox sites. Thus, the Cu doping facilitates the transport and adsorption of more H2O2 molecules to the catalytic sites, significantly increasing the catalytic rate. In contrast, pristine MnO2 exhibits negligible catalytic performance for H2O2 reduction because of its slow catalytic kinetics and low electrical conductivity. The Cu-MnO2 catalyst exhibits higher sensitivity (105.7 μA·mM−1·cm−2), a lower detection limit (0.6 μM), and a broader linear range (0.005 ∼ 4.2 mM) compared to pristine δ-MnO2.

Bond Structure Engineering Induced the Mn Redox Stabilization Toward High‐Energy Mn‐Based Phosphate Cathode

AbstractManganese (Mn) ‐based phosphate is poised for commercial applications driven by its cost‐effectiveness, robust NASICON framework, and multi‐dimensional Na+ pathways. However, it encounters insufficient redox reactions and rapid structural collapse with severe lattice distortion as the culprit. Herein, one meticulously engineered substitutional solid solution cathode (integrating Na4MnCr(PO4)3 and Na3MnTi(PO4)3, denoted NMCTP) is proposed to regulate the local crystal structure of Mn─O bond to stabilize and promote the Mn redox reaction for optimizing the electrochemical performance. It is uncovered that the bulk framework with structural stability is constructed by strongly symmetric Mn─O bond lengths of MnO6 octahedrons and strengthened Mn─O covalency. In addition, the sufficient utilization of Mn redox is tightly correlated with redistributed Na2 occupancy and enhanced diffusion kinetics with accelerated electron transportation. By virtue of the above merits, The NMCTP performs ultra‐high capacity (150.3 mAh g−1 at 0.1 C) and appealing cycling stability (84.7% retention over 1000 cycles). Sodium storage mechanisms and potential factors at high potentials are unveiled in NMCTP materials. This work sheds light on fire‐new solid solution strengthening in view of the Mn─O bond structure for high‐performance Mn‐based phosphate cathodes.

Triggering cationic/anionic hybrid redox stabilizes high-temperature Li-rich cathodes materials via three-in-one strategy

Efficiently advancing lithium-ion batteries toward high energy density entails overcoming challenges in Li-rich layered oxides, including severe voltage and capacity fading, irreversible oxygen escape and compromised thermal robustness due to the uncontrollable oxygen anionic redox (OAR). Herein, a three-in-one modification strategy, aim at strengthening the reversibility of OAR and structural integrity of MnO6 octahedron, is rationally introduced on Li1.2Mn0.53Ni0.20Co0.07O2 cathode through a universal H3BO3-treatment. Through the introduction of oxygen vacancies and gradient B-doping, the energy level of unhybridized O 2p states is decreased to narrow the band energy gap with transition metal (TM) band, which triggers cationic/anionic hybrid redox at high voltage, thereby stabilizing the peroxo-like (O2)n- species and hindering irreversible oxygen escape, while the lithium borate nano-coating serves to mitigate side reactions, particularly during charging at elevated temperatures. As a result, the modulated cathode exhibits notable capacity retention of 96.3 % after 500 cycles at 1 C with limited voltage fading rate (1.73 mV per cycle) and even stable cyclic performance at high temperature (60 °C). This approach provides an effective and straightforward method to tackle the voltage decay and capacity fading of high-energy Li-rich layered cathode materials.

Experimental and theoretical investigation on the high activity La0.8-xSr0.2KxMn1-yByO3-δ perovskite catalyst with combined A and B site modifications

In this work, a series of LaMnO3 (LM) perovskite catalysts modified at A and B sites were prepared to accelerate soot combustion. The La0.68Sr0.2K0.12Mn0.9Cu0.1O3-δ (LSK0.12MCu0.1) modified by Sr, K and Cu has good catalytic activity and cycle stability, the presence of NO can obviously promote the oxidation of soot. A variety of characterization results show that doping the A and B sites of perovskite will not destroy the original structure of perovskite, and the doped ions (Sr2+, K+, and Cu2+) can enter the perovskite catalyst well, which may be conducive to the formation of more oxygen defects and active oxygen species. According to the DFT calculation, adding Sr and K to the A sites can alter how atoms interact in a perovskite catalyst, increase the amount of unsaturated coordination bonds, and encourage the development of high-valence active ions (Mn4+) and oxygen defects. The doping of transition metal to the B sites results in the formation of the Cu-O-Mn link, which forms the Mn(Cu)O6 octahedron and stimulates the development of oxygen vacancies and the release of active oxygen. This study will provide an important reference for developing non-precious metal-based perovskite materials with high activity and low cost for catalytic combustion.

Band Structure Engineering Promotes Anionic Redox Reversibility of Cobalt-Free Li-Rich Layered Oxides Cathodes.

Li-rich layered oxides cathodes (LLOs) have prevailed as the promising high-energy-density cathode materials due to their distinctive anionic redox chemistry. However, uncontrollable anionic redox process usually leads to structural deterioration and electrochemical degradation. Herein, a Mo/Cl co-doping strategy is proposed to regulate the relative position of energy band for modulating the anionic redox chemistry and strengthening the structural stability of Co-free Li1.16Mn0.56Ni0.28O2 cathodes. The incorporation of Mo with high d state orbit and Cl with low electronegativity can narrow the band energy gap between bonding and antibonding bands via increasing the filled lower-Hubbard band (LHB) and decreasing the non-bonding O 2p energy bands, promoting the anionic redox reversibility. In addition, strong covalent Mo─O and Mn─Cl bonding further increases the covalency of Mn─O band to further stabilize the O2 n- species and enhance the reversible distortion of MnO6 octahedron. The strengthening electronic conductivity, together with the epitaxial structure Li2MoO4 facilitates the fast Li+ kinetics. As a result, the dual doping material exhibits enhanced anionic redox reversibility and suppressed oxygen release with increased cyclic stability and excellent rate performance. This strategy provides some guidance to design high-energy-density LLOs with desirable anionic redox reversibility and stable crystal structure via band structure engineering.

Theoretical simulation of pre-sintering effect and improvement of electrical transport properties of La0.7Ca0.18Sr0.12MnO3 film prepared by spin coating method

Perovskite manganites Lax(Ca1-ySry)1-xMnO3 have drawn the plentiful attention owing to their distinctive physical properties. Their unique properties make them an ideal candidate for high-sensitivity and uncooled bolometric applications. Optimized La0.7Ca0.18Sr0.12MnO3 (LCSMO) films were grown on (00l) LaAlO3 substrate at different pre-sintering temperature (Tps) via sol–gel spin coating method. According to the test results of microstructure, internal strain, Mn ion valences and surface morphology, electrical transport properties are restricted by MnO6 octahedron distortion and film crystallinity affected by the change in Tps. The microstructure growth was improved and the surface morphology tended to grow in a cross-like structure with high crystallinity by optimizing Tps. Double exchange mechanism and internal strain were enhanced with the increase in Tps. Thereby, due to the enlarging MnOMn bond angle and enhancing MnO covalency, the temperature (Tk) corresponding to temperature coefficient of resistivity (TCR) was increased gradually as Tps increased. The TCR value of LCSMO film reached a maximum value (10.82 %/K) and Tk reached room temperature (302.03 K) at Tps = 1123 K. The theoretical foundation of the pre-sintering effect has been supplemented, helping to understand the influence of pre-sintering on film growth and electrical transport properties.

Jahn–Teller Distortions Induced by in situ Li Migration in λ‐MnO2 for Boosting Electrocatalytic Nitrogen Fixation

AbstractLithium‐mediated electrochemical nitrogen reduction reaction (Li‐NRR) completely eschews the competitive hydrogen evolution reaction (HER) occurred in aqueous system, whereas the continuous deposition of lithium readily blocks the active sites and further reduces the reaction kinetics. Herein, we propose an innovative in situ Li migration strategy to realize that Li substitutes Mn sites in λ‐MnO2 instead of evolving into the dead Li. Comprehensive characterizations corroborate that the intercalation of Li+ at high voltage breaks the structural integrity of MnO6 octahedron and further triggers unique Jahn–Teller distortions, which promotes the spin state regulation of Mn sites to generate the ameliorative eg orbital configuration and accelerates N≡N bond cleavage via eg‐σ and eg‐π* interaction. To this end, the resulted cationic disordered LiMnO4 delivers the recorded highest NH3 yield rate of 220 μg h−1 cm−2 and a Faradaic efficiency (FE) 83.80 % in organic electrolyte.

Jahn-Teller Distortions Induced by in situ Li Migration in λ-MnO2 for Boosting Electrocatalytic Nitrogen Fixation.

Lithium-mediated electrochemical nitrogen reduction reaction (Li-NRR) completely eschews the competitive hydrogen evolution reaction (HER) occurred in aqueous system, whereas the continuous deposition of lithium readily blocks the active sites and further reduces the reaction kinetics. Herein, we propose an innovative in situ Li migration strategy to realize that Li substitutes Mn sites in λ-MnO2 instead of evolving into the dead Li. Comprehensive characterizations corroborate that the intercalation of Li+ at high voltage breaks the structural integrity of MnO6 octahedron and further triggers unique Jahn-Teller distortions, which promotes the spin state regulation of Mn sites to generate the ameliorative eg orbital configuration and accelerates N≡N bond cleavage via eg -σ and eg -π* interaction. To this end, the resulted cationic disordered LiMnO4 delivers the recorded highest NH3 yield rate of 220 μg h-1 cm-2 and a Faradaic efficiency (FE) 83.80 % in organic electrolyte.

Effect of Zn doping on the structure and electrical conductivity of Mn1.5Co1.5O4 spinel

Electrical conductivity is one of the most important concerns in spinel-structured antioxidant films coated on a solid oxide fuel cell separator, and the mechanism of electrical conduction involves the small polaron hopping through corner-shared octahedral sites. Arranging favorable ions in the octahedral sites rather than in the tetrahedral sites is better for achieving higher electrical conduction. In this study, Zn, which prefers the tetrahedral site, was substituted for the Mn1.5Co1.5O4 spinel to arrange site distribution. The crystal structure, electrical conductivity, and thermal expansion behavior of Zn-substituted ZMCOs [ZnxMn1.5−0.5xCo1.5−0.5xO4 (0.0 ≤ x ≤ 0.6)] were investigated. At Zn = 0.2, the highest electrical conductivity of 42.1 S/cm was observed, while at Zn > 0.2, the electrical conductivity decreased due to decreased Co2+/CoIII and Mn3+/Mn4+ pair concentration because Zn ions started to occupy the octahedral site. The higher the Zn concentration of ZMCO, the lower the distortion of the MnO6 octahedron via the Jahn–Teller effect, resulting in a higher symmetry of the crystal structure. Therefore, with the increase in the Zn concentration, a mixture of tetragonal-cubic phase changed to cubic phase. In addition, the phase transition temperature of the tetragonal-cubic phase decreased.

Improving Rate Performance by Inhibiting Jahn–Teller Effect in Mn‐Based Phosphate Cathode for Na‐Ion Batteries

AbstractManganese‐based phosphate cathodes are promising candidates for developing advanced sodium‐ion batteries, primarily driven by their reliable elemental abundance, low toxicity, and desirable cycling performance. However, the cooperative Jahn–Teller effect of Mn3+ will inevitably lead to structural disorder and irreversible phase transition, thus greatly harming the reversible capacity, rate, and cycling performance. Herein, a stable NASICON‐type Na3MnHf(PO4)3 cathode is demonstrated with a volume variation of 1.9% upon the process of Na+ extraction/insertion based on the robust Hf─O bond and symmetrical MnO6 octahedron. Moreover, making full use of the stepwise redox reactions of Mn2+/Mn3+/Mn4+, this cathode reveals excellent cycling stability with a capacity retention of 85.4% after 2500 cycles at 10 C. Matching with commercial hard carbon anodes, the assembled full cell keeps a capacity retention of 92.1% with the Coulombic efficiency close to 100% after 600 cycles at 1 C. The research promises opportunities for the structural amelioration of manganese‐based phosphate cathodes toward the application in high‐performance sodium‐ion batteries.

Optimization of oxygen evolution electrocatalytic activity of metal oxide nanosheet via surface modification

AbstractA surface modification route to high‐performance oxygen evolution electrocatalysts was developed by immobilization of highly‐oxidized SeO42− species on the surface of exfoliated MnO2 nanosheets. The enhanced stability of tetragonally‐distorted MnO6 octahedron could be achieved by the weakening of axial Mn–O bond due to competition with highly covalent Se6+–O bonds. The selenate anchoring was found to be quite effective in improving the electrocatalyst functionality of MnO2 nanosheets for oxygen evolution reaction, which could be ascribed to the improvement of charge transfer kinetics and the enhancement of Mn3+ stability. This study emphasized that the fine‐control of bonding nature via surface modification with selenate anchoring can offer useful means to explore efficient electrocatalysts.

Low-temperature synthesis of doped MnO2–carbon dots nanocomposite: an analysis of nanostructure and electrical properties

The synthesis of δ-MnO2, δ-MnO2 carbon dots nanocomposite, and Fe/Cu-doped δ-MnO2 carbon dots nanocomposite has been successfully carried out through a stirring process at room temperature and 80 °C. The synthesized powder shows a low crystallization determined through XRD and TEM analysis. Furthermore, the carbon dots are well attached to MnO2 performing a core–shell composite material, while the doping ions Fe and Cu were incorporated into the matrix substitute Mn in the MnO6 octahedron, although potassium ions were also detected. The manganese possess an oxidation state of + 3 and + 4, which promotes the oxygen vacancy creation {V}_{mathrm{O}}^{cdotcdot} denoting the conductivity decrease.

Co-optimization of Ag and K doping to regulate room-temperature coefficient of resistivity of La0.7Ag0.3-xKxMnO3 films

In this study, La0.7Ag0.3-xKxMnO3 (LAKMO, x = 0.15, 0.16, 0.17, 0.18, and 0.19) films with excellent electrical properties and low surface roughness were prepared on SrTiO3 (001) substrates using the spin-coating method. The crystal structures and ion valence states of the LAKMO films were modified by adjusting the doping amounts of Ag and K. The Jahn–Teller effect and electron–lattice coupling were enhanced with the MnO6 octahedron undergoes distortion. Moreover, the double-exchange interaction was improved with an increase in the number of Mn4+/Mn3+ ion pairs. Under the influence of these factors, the electrical transport properties especially the temperature coefficient of resistivity (TCR) of the LAKMO films can be refined modulated or optimized via A-site Ag and K co-doping. When x = 0.17, the maximum TCR of LAKMO reached 11.23 % K−1 at 293.23 K. Thus, La0.7Ag0.13K0.17MnO3 film with a large TCR exhibited excellent thermal properties at room temperature and was considered material for application to uncooled infrared bolometers.

Multiferroic properties induced by defect dipoles in thin Ca3Mn2O7 films at room temperature

Improper ferroelectrics are a class of materials with the potential to design multiferroic properties. In this paper, we report on the effect of defect dipoles on the ferromagnetic and ferroelectric properties of Ca3Mn2O7 improper ferroelectric thin films. Two kinds of defects dipoles, Mn3+-oxygen vacancy (Mn3+-VO∙∙) and Li+-Al3+ doping, were studied. The experimental results show that both Mn3+-VO∙∙ and Li+-Al3+ defect dipoles can introduce ferromagnetism in the films, and Li+-Al3+ defect dipole can also improve ferroelectricity with self-polarization. The first-principle calculations based on local density approximation demonstrate the local lattice distortion, including title MnO6 octahedron and off-center displacement of Mn ion, plays crucial role in multiferroic properties of the film. This work gives an effective method to design the ferroelectricity and ferromagnetic properties in improper ferroelectric oxides with layered structure through constructing defect dipoles.

Unveiling the role of Ni doping in the electrochemical performance improvement of the LiMn2O4 cathodes

Ni doping is a commonly used strategy to improve the electrochemical performance of LiMn2O4. Uncovering the structural changes of Ni-doped LiMn2O4 during electrochemical cycling, particularly at the atomic level, is important to understand the mechanism of the relationship between Ni doping and performance improvement. Herein, we investigate the chemical and structural evolutions of the Ni-doped LiMn2O4 electrodes before and after 1000 cycles at 10 C with atomic resolution and correlate them with changes in the electrochemical properties. We found that Ni2+ ions doped into the Mn 16d sites to form a more steady LiNixMn2-xO4 phase construction, which can effectively stabilize the spinel structure by hindering Jahn − Teller distortion. Comprehensive atomic imaging demonstrated that Ni doping can suppress the formation of the unexpected Mn3O4, and rock-salt MnO phases on the surface and subsurface of LiMn2O4 during cycling, which is conducive to stabilizing the MnO6 octahedron and inhibiting the migration of Mn. Resultantly, splendid long cycling stability of 88.92 % after 1000 cycles at 10 C (1 C = 148 mAh g−1) for the optimized LiNi0.05Mn1.95O4 sample is presented. Our study provides an ingenious avenue for regulating the surface optimization and reconstruction of electrode materials.