Vacancy Ordering Research Articles

Electrocatalytic Properties of Oxygen-Deficient Perovskites Ca3Fe3-xMnxO8 (x = 1-2) for the Hydrogen Evolution Reaction.

We have demonstrated a systematic trend in the electrocatalytic activity for the hydrogen evolution reaction (HER) and its correlations with transition-metal type, structural order, and electrical conductivity. The materials studied in this work, Ca3FeMn2O8 (CaFe1/3Mn2/3O3-1/3), Ca3Fe1.5Mn1.5O8, and Ca3Fe2MnO8, belong to the family of oxygen-deficient perovskites and show a gradual increase in the ordering of oxygen vacancies. Ca3FeMn2O8 (CaFe1/3Mn2/3O3-1/3) contains randomly distributed oxygen vacancies, which begin to order in Ca3Fe1.5Mn1.5O8, and are fully ordered in Ca3Fe2MnO8. The gradual increase in the structural order is associated with a systematic enhancement of the electrocatalytic activity for HER in acidic conditions, Ca3FeMn2O8 < Ca3Fe1.5Mn1.5O8 < Ca3Fe2MnO8. While the improvement of the HER activity is also associated with an increase in the Fe content, we have shown that the type of structural order plays a more important role. We demonstrated this effect by control experiments on an analogous material where all Mn was substituted by Fe, leading to a different type of structural order and showing an inferior HER activity compared to the above three materials. Furthermore, electrical conductivity studies in a wide range of temperatures, 25-800 °C, indicate that the trend in the electrical conductivity is the same as that of the HER activity. These findings reveal several important structure-property relationships and highlight the importance of synergistic effects in enhancing the electrocatalytic properties.

Magnetic impedance in nonstichiometric manganese sulfide

The role of defects on the dynamic characteristics of manganese sulfide is studied by impedance spectroscopy in the frequency range 102–106 Hz and temperatures 80–500 K. Nonstoichiometry plays an important role in the formation of new transport and magnetic properties, as it leads to electrically inhomogeneous states. The phase composition and crystal structure of nonstoichiometric manganese sulfide were studied on a DRON-3 X-ray unit using CuKα – radiation at room temperature. According to X-ray diffraction analysis, the synthesized compound is single-phase and has a NaCl-type cubic lattice. From the frequency dependences of the impedance components measured in the absence of a field and in a magnetic field, the relaxation time of the current carriers in the Debye model is found. A sharp decrease in the relaxation time and its correlation with conductivity were found. The contribution to the impedance of the active and reactive parts of the impedance at frequencies below and above the relaxation time is established. The capacitance from the impedance hodograph in the equivalent circuit model is determined. In defective manganese sulfide, the temperature-dependent impedance has an activation character. The activation energy is determined in the range 250–500 K, which is attributed to the excitation energy of lattice polarons. The effect of a magnetic field on the dynamic characteristics of current carriers was studied as a result of a change in the impedance components in a magnetic field at fixed temperatures. The impedance increases in a magnetic field and reaches a maximum in the temperature range of charge ordering of vacancies. An increase in the impedance in a magnetic field is explained by a decrease in the diagonal component of the permittivity in a magnetic field in an electrically inhomogeneous medium. The experimental data are explained in the Debye model.

Vacancy ordering in zirconium carbide with different carbon contents

Zirconium carbide (ZrCx) ceramics with different carbon contents were prepared by reactive hot-pressing. The rock-salt structure of ZrCx was the only phase detected by x-ray diffraction of the hot pressed ceramics. The relative densities of ZrCx decreased as carbon content increased, in general. The actual carbon contents were measured by completely oxidizing the ZrCx ceramics to ZrO2. For most compositions, the actual carbon contents were higher than nominal batched compositions, presumably due to carbon uptake from the graphite furnace and hot press dies. Selected area electron diffraction and neutron powder diffraction revealed the presence of carbon vacancy ordering in ZrCx for 0.6 < x < 0.75. Rietveld refinement of the neutron diffraction patterns determined that the crystal structure of the ordered phase was hexagonal, and the carbon site occupancies were higher than nominal batched carbon stoichiometry.

Oxygen Vacancy Ordering and Molten Salt Corrosion Behavior of ZnO-Doped CeYSZ for Solid Oxide Membranes.

Although 4Ce4YSZ has high corrosion resistance, it faces challenges concerning its sinterability and ionic conductivity. Therefore, we studied destabilization behavior caused by corrosion and oxygen vacancy ordering according to ZnO doping. Powders of (4Ce4YSZ)1-x(ZnO)x (x = 0.5, 1, 2, 4 mol%) were synthesized using the sol-gel method. With the addition of ZnO, the cubic phase increased, and secondary phases were not observed. The (111) peak showed a higher angle shift in ZnO-doped 4Ce4YSZ compared to 4Ce4YSZ, and TEM-SAED revealed a reduction in the spacing of the (011)t plane, suggesting lattice contraction due to the substitution of the smaller Zn2+ (60 Å) for Zr4+ (84 Å) in the lattice. The local atomic structure analysis was conducted using EXAFS to investigate the oxygen vacancy ordering behavior. Zr K-edge Fourier transform data revealed a decrease in the Zr-O1 peak intensity with an increasing amount of ZnO doping, indicating an increase in oxygen vacancies. The Zr-O1 peak position shifted to the right, leading to an increase in the Zr-O1 interatomic distance. In the Y K-edge Fourier transform data, the Y-O1 peak intensity did not decrease, and there was little variation in the Y-O1 interatomic distance. These results suggest that the oxygen vacancies formed due to ZnO doping are located in the neighboring oxygen shell of Zn, rather than in the neighboring oxygen shells of Y and Zr. Impedance measurements were conducted to measure the conductivity, and as the amount of ZnO doping increased, the total conductivity increased, while the activation energy decreased. The increase in oxygen vacancies by ZnO doping contributed to the enhancement of conductivity, and it is considered that these created oxygen vacancies did not interact with Zn2+ and did not form defect associations. Fluoride-based molten salts were introduced to the specimens to assess the corrosion behavior in a molten salt environment. Yttrium depletion layers (YDLs) were formed on the surfaces of all specimens due to the leaching of yttrium. However, Ce remained relatively stable at the interface according to EDS line scans, suggesting a reduction in the phase transformation (cubic, tetragonal to monoclinic) typically associated with yttrium leaching in YSZ.

Manipulating Na occupation and constructing protective film of P2-Na0.67Ni0.33Mn0.67O2 as long-term cycle stability cathode for sodium-ion batteries

P2-Na0.67Ni0.33Mn0.67O2 (NNMO) is promising cathode material for sodium-ion batteries (SIBs) due to its high specific capacity and fast Na+ diffusion rate. Nonetheless, the irreversible P2-O2 phase transformation, Na+/vacancy ordering, and transition metal (TM) dissolution seriously damage its cycling stability and restrict its commercialization process. Herein, Na occupation manipulation and interface stabilization are proposed to strengthen the phase structure of NNMO by synergistic Zn/Ti co-doping and introducing lithium difluorophosp (LiPO2F2) film-forming electrolyte additive. The Zn/Ti co-doping regulates the occupancy ratio of Nae/Naf at Na sites and disorganizes the Na+/vacancy ordering, resulting in a faster Na+ diffusion kinetics and reversible P2-Z phase transition for P2-Na0.67Ni0.28Zn0.05Mn0.62Ti0.05O2 (NNZMTO). Meanwhile, the LiPO2F2 additive can form homogeneous and ultrathin cathode-electrolyte interphase (CEI) on NNZMTO surface, which can stabilize the NNZMTO-electrolyte interface to prevent TM dissolution, surface structure transformation, and micro-crack generation. Combination studies of in situ and ex situ characterizations and theoretical calculations were used to elucidate the storage mechanism of NNZMTO with LiPO2F2 additive. As a result, the NNZMTO displays outstanding capacity retention of 94.44% after 500 cycles at 1C with 0.3 wt% LiPO2F2, excellent rate performance of 92.5 mA h g−1 at 8C with 0.1 wt% LiPO2F2, and remarkable full cell capability. This work highlights the important role of manipulating Na occupation and constructing protective film in the design of layered materials, which provides a promising direction for developing high-performance cathodes for SIBs.

High Na-content P2 and P2-O3 layered oxide cathodes for high performance Na-ion batteries

P2-NaxTMO2, (x ≤0.67, TM-transition metal) oxides are promising cathode materials for Na-ion batteries, however they suffer from Na+-vacancy ordering and huge structural transformations (P2→O2→OP4). In contrast, high Na content in P2-type cathodes enables high structural stability and promotes active elements to their lower oxidation state and thereby achieving high capacity and cycling stability. Herein, light-weight elements boron and lithium doped in P2-type Na0.67Ni0.33Mn0.67O2 using cationic potential approach, aiming to improve the Na content. Li-B doped Na0.8Ni0.3Mn0.6Li0.05B0.05O2 delivers a capacity of 85 mAh/g and excellent capacity retention (88 % after 500 cycles). The effects of Li-B doping in P2-type Na0.67Ni0.33Mn0.67O2 are systematically investigated.

Ordered deficient perovskite La2/3TiO3 films grown via molecular beam epitaxy

As the parent compound of a promising solid electrolyte material Li3xLa2/3−xTiO3, the perovskite La2/3TiO3 has potential for advancing research on Li-intercalated ionic conductors. Epitaxial La2/3TiO3 films have been grown by molecular beam epitaxy using a growth process consisting of deposition and annealing cycles, with in situ monitoring by electron diffraction. X-ray absorption spectroscopy confirms the tetravalent state of Ti in La2/3TiO3, and the as-grown films are insulating. X-ray diffraction reveals the presence of half-order peaks, indicating a doubling of the pseudocubic perovskite unit cell due to the ordering of La vacancies in alternating A-site layers. These results demonstrate that single-phase, vacancy-ordered epitaxial films of La2/3TiO3 can be stabilized with excellent crystalline and electronic properties over wafer-sized areas, making possible Li-ion intercalation studies in films with well-defined domain boundary properties. Such boundaries are known to profoundly influence Li-ion conduction within the material. Understanding the effects of domain boundaries on Li-ion conduction could lead to improvements in solid-state battery technology and pave the way for the development of more efficient and safer energy storage devices.

Boosting the Ultrastable High-Na-Content P2-type Layered Cathode Materials with Zero-Strain Cation Storage via a Lithium Dual-Site Substitution Approach.

P2-type layered transition-metal (TM) oxides, NaxTMO2, are highly promising as cathode materials for sodium-ion batteries (SIBs) due to their excellent rate capability and affordability. However, P2-type NaxTMO2 is afflicted by issues such as Na+/vacancy ordering and multiple phase transitions during Na-extraction/insertion, leading to staircase-like voltage profiles. In this study, we employ a combination of high Na content and Li dual-site substitution strategies to enhance the structural stability of a P2-type layered oxide (Na0.80Li0.024[Li0.065Ni0.22Mn0.66]O2). The experimental results reveal that these approaches facilitate the oxidation of Mn ions to a higher valence state, thereby affecting the local environment of both TM and Na ions. The resulting modification in the local structure significantly improves the Na-ion storage capabilities as required for cathode materials in SIBs. Furthermore, it induces a solid-solution reaction and enables nearly zero-strain operation (ΔV = 0.7%) in the Na0.80Li0.024[Li0.065Ni0.22Mn0.66]O2 cathode during cycling. The assembled full cells demonstrate an exceptional rate performance, with a retention rate of 87% at 10 C compared to that of 0.1 C, as well as an ultrastable cycling capability, maintaining a capacity retention of 73% at 2 C after 1000 cycles. These findings offer valuable insights into the electronic and structural chemistry of ultrastable cathode materials with "zero-strain" Na-ion storage.

Suppressing the P2‐O2 phase transition and Na+/vacancy ordering in Na0.67Ni0.33Mn0.67O2 by a delicate multicomponent modulation strategy

AbstractP2‐type Na0.67Ni0.33Mn0.67O2 is a promising cathode for sodium‐ion batteries with features of high specific capacity and air resistance, whereas its cycling stability and rate performance are dissatisfactory suffering from the disastrous P2‐O2 phase transition and Na+/vacancy ordering during sodium‐ion de/intercalation, which makes it an obstruction for future practical applications. Herein, a delicate multicomponent modulation strategy is proposed to tackle these two issues simultaneously, in which Li+ and Ti4+ are introduced to replace the Ni2+ and Mn4+, respectively, whereas the Na+ content is also designed according to the principle of charge balance. Consequently, the designed cathode (Na0.72Ni0.28Li0.05Mn0.57Ti0.10O2) can deliver an enchanting cycling stability of 80% at 1 C after 200 cycles along with a considerable rate performance of 82.7 mAh g−1 at 5 C. In situ X‐ray diffraction measurement demonstrates the destructive P2‐O2 phase transition is suppressed and converted into a P2‐Z phase transition with superior reversibility as well as smooth charge/discharge curves with better Na+/vacancy disordering. In addition, the full cell matched with hard carbon anode delivers an excellent energy density of 263.4 Wh kg−1 at 37.3 W kg−1, exhibiting great practicality. Our work presents a mean to rationally design the component of layered oxide cathode and achieve fabulous performance for sodium ion batteries.

Hydrogen diffusion and storage in substoichiometric TiC

In this paper we investigate the nature of hydrogen diffusion in substoichiometric TiC using simulations. Specifically, we examine how well connected the carbon structural vacancies are in TiCx using percolation theory and how many structural carbon vacancies are connected to the surface of TiCx. This provides direct insight into the number of sites available for hydrogen storage and the interconnected nature of the carbon vacancy network. We also compute the binding energies and migration energies of hydrogen in TiCx using density functional theory. This is conducted over a range of carbon concentrations from nearly stoichiometric TiC to Ti2C and includes the differences in vacancy ordering. We find that the migration energy of hydrogen in TiCx generally decreases with carbon loss and is associated with the tetrahedral interstices near them but that the binding energy of hydrogen to TiCx is independent of carbon concentration. Furthermore, we show that this migration path can occur through the tetrahedral interstices, which is influenced by local carbon concentration. These results also demonstrate the vacancy ordered R3¯m Ti2C structure does not possess any more superior ability to store or transport hydrogen than the Fd3¯m Ti2C structure.

Tailoring Ion Ordering in Perovskite Oxide for High-Temperature Oxygen Evolution Reaction.

Perovskites exhibit excellent high-temperature oxygen evolution reaction (OER) activities as the anodes of solid oxide electrolysis cells (SOECs). However, the relationship between ion ordering and OER performances is rarely investigated. Herein, a series of PrBaCo2-x Fex O5+δ perovskites with tailored ion orderings are constructed. Physicochemical characterizations and density functional theory calculations confirm that the oxygen bulk migration and surface transport capacities as well as the OER activities are promoted by the A-site cation ordering, but weakened by the oxygen vacancy ordering. Hence, SOEC with the A-site-ordered and oxygen-vacancy-disordered PrBaCo2 O5+δ anode exhibits the highest performance of 3.40 A cm-2 at 800 °C and 2.0 V. This work sheds light on the critical role of ion orderings in the high-temperature OER performance and paves a new way for screening novel anode materials of SOECs.

Tailoring Ion Ordering in Perovskite Oxide for High‐Temperature Oxygen Evolution Reaction

AbstractPerovskites exhibit excellent high‐temperature oxygen evolution reaction (OER) activities as the anodes of solid oxide electrolysis cells (SOECs). However, the relationship between ion ordering and OER performances is rarely investigated. Herein, a series of PrBaCo2−xFexO5+δ perovskites with tailored ion orderings are constructed. Physicochemical characterizations and density functional theory calculations confirm that the oxygen bulk migration and surface transport capacities as well as the OER activities are promoted by the A‐site cation ordering, but weakened by the oxygen vacancy ordering. Hence, SOEC with the A‐site‐ordered and oxygen‐vacancy‐disordered PrBaCo2O5+δ anode exhibits the highest performance of 3.40 A cm−2 at 800 °C and 2.0 V. This work sheds light on the critical role of ion orderings in the high‐temperature OER performance and paves a new way for screening novel anode materials of SOECs.

Rational Design of a Complete Solid-Solution Reaction P2-Type Na-Ion Layered Oxide Cathode with High Performance

P2-type Na-ion layered oxides have received wide consideration as cathodes for Na-ion batteries (SIBs) attributing to their easy syntheses, high theoretical capacity, and convenient Na+ diffusion paths. However, the different Na+/vacancy orderings and large-volume phase transitions at high voltages cause poor cycling performance and rate capability, limiting the development and application of P2-type layered oxides. Herein, we propose a feasible strategy for rationally adjusting transition metals and combining different functionalities of multiple elements. Results showed that multiple redox couples provided sufficient charge compensation and moderately inactive Ti could help suppress occurrence of Na+/vacancy ordering and hinder the phase transitions from P2 to O2 with large volume variations. Consequently, the designed Na0.7Ni0.25Cu0.05Fe0.1Mn0.35Ti0.25O2 cathode presented a complete solid-solution reaction with small volume change (∼1%) upon charging and discharging. The highly reversible solid-solution behavior and inhibited P2–O2 phase transformation enabled this cathode to deliver a capacity of 121 mAh/g in a voltage between 2.0 and 4.2 V, outstanding cycling stability with a capacity retention of 89% after 1000 cycles, and rate performance (62% conservation rate at 10 C). This study provides a highly available approach to design high-performance layered oxide cathodes for advanced SIBs.

Optimization of Chemical Bonding through Defect Formation and Ordering─The Case of Mg7Pt4Ge4.

The new phase Mg7Pt4Ge4 (≡Mg8□1Pt4Ge4; □ = vacancy) was prepared by reacting a mixture of the corresponding elements at high temperatures. According to single crystal X-ray diffraction data, it adopts a defect variant of the lighter analogue Mg2PtSi (≡Mg8Pt4Si4), reported in the Li2CuAs structure. An ordering of the Mg vacancies results in a stoichiometric phase, Mg7Pt4Ge4. However, the high content of Mg vacancies results in a violation of the 18-valence electron rule, which appears to hold for Mg2PtSi. First principle density functional theory calculations on a hypothetical, vacancy-free "Mg2PtGe" reveal potential electronic instabilities at EF in the band structure and significant occupancy of states with an antibonding character resulting from unfavorable Pt-Ge interactions. These antibonding interactions can be eliminated through introduction of Mg defects, which reduce the valence electron count, leaving the antibonding states empty. Mg itself does not participate in these interactions. Instead, the Mg contribution to the overall bonding comes from electron back-donation from the (Pt, Ge) anionic network to Mg cations. These findings may help to understand how the interplay of structural and electronic factors leads to the "hydrogen pump effect" observed in the closely related Mg3Pt, for which the electronic band structure shows a significant amount of unoccupied bonding states, indicating an electron deficient system.

Topotactically induced oxygen vacancy order in nickelate single crystals

The strong structure-property coupling in rare-earth nickelates has spurred the realization of new quantum phases in rapid succession. Recently, topotactic transformations have provided a new platform for the controlled creation of oxygen vacancies and, therewith, for the exploitation of such coupling in nickelates. Here, we report the emergence of oxygen vacancy ordering in Pr$_{0.92}$Ca$_{0.08}$NiO$_{2.75}$ single crystals obtained via a topotactic reduction of the perovskite phase Pr$_{0.92}$Ca$_{0.08}$NiO$_{3}$, using CaH$_2$ as the reducing agent. We unveil a brownmillerite-like ordering pattern of the vacancies by high-resolution scanning transmission electron microscopy, with Ni ions in alternating square-pyramidal and octahedral coordination along the pseudocubic [100] direction. Furthermore, we find that the crystal structure acquires a high level of internal strain, where wavelike modulations of polyhedral tilts and rotations accommodate the large distortions around the vacancy sites. Our results suggest that atomic-resolution electron microscopy is a powerful method to locally resolve unconventional crystal structures that result from the topotactic transformation of complex oxide materials.

Impact of Ti and Zn Dual-Substitution in P2 Type Na2/3 Ni1/3 Mn2/3 O2 on Ni-Mn and Na-Vacancy Ordering and Electrochemical Properties.

High-entropy layered oxide materials containing various metals that exhibit smooth voltage curves and excellent electrochemical performances have attracted attention in the development of positive electrode materials for sodium-ion batteries. However, a smooth voltage curve can be obtained by suppression of the Na+ -vacancy ordering, and therefore, transition metal slabs do not need to be more multi-element than necessary. Here, the Na+ -vacancy ordering is found to be disturbed by dual substitution of TiIV for MnIV and ZnII for NiII in P2-Na2/3 [Ni1/3 Mn2/3 ]O2 . Dual-substituted Na2/3 [Ni1/4 Mn1/2 Ti1/6 Zn1/12 ]O2 demonstrates almost non-step voltage curves with a reversible capacity of 114mAhg-1 and less structural changes with a high crystalline structure maintained during charging and discharging. Synchrotron X-ray, neutron, and electron diffraction measurements reveal that dual-substitution with TiIV and ZnII uniquely promotes in-plane NiII -MnIV ordering, which is quite different from the disordered mixing in conventional multiple metal substitution.

Dual Site-Selective Substitution Strategy Achieving Anionic Redox Activity and Solid-Solution Reaction in P2-Type Cathode Materials

P2-type Na0.67Ni0.33Mn0.67O2 has been considered as the potential cathode for sodium-ion batteries. However, its practical application is plagued by Na+/vacancy ordering, harmful phase transition, and lattice oxygen loss. Herein, we develop a dual site-selective substitution strategy to fabricate a P2-type Na0.63Ca0.05(Ni0.26Li0.07Mn0.67)O2 cathode. The substitution of Li+ for Ni2+ introduces lone pair oxygen via forming a Li–O–Li configuration and make O 2p close to its Fermi level due to the weakened TM–O (TM: transition metal) bond, which triggers the anionic redox for charge compensation, while the introduction of Ca2+ in a Na layer enhances the electrostatic cohesion of neighboring TM layers by forming a strengthened O–Ca–O configuration, which suppresses the glide of adjacent TM layers and reduces the excessive lattice oxygen loss. Therefore, with a dual site-selective substitution strategy, the P2-type Na0.63Ca0.05(Ni0.26Li0.07Mn0.67)O2 cathode can suppress the Na+/vacancy ordering, P2–O2 phase transition, and lattice oxygen loss even at a potential of 4.35 V, achieving a reversible anionic redox and solid-solution reaction. The P2-type Na0.63Ca0.05(Ni0.26Li0.07Mn0.67)O2 cathode exhibits high discharge capacity (142.7 mA h g–1 at 20 mA g–1), excellent rate capability (57.1 mA h g–1 at 2 A g–1), and cyclic stability (a capacity retention of 83.2% after 700 cycles).

Structure, Electrochemical, and Transport Properties of Li- and F-Modified P2-Na2/3Ni1/3Mn2/3O2 Cathode Materials for Na-Ion Batteries

The development of cobalt-free P2-Na2/3Ni1/3Mn2/3O2 cathodes is hampered by poor electrochemical performance, resulting from structural instability during high-voltage cycling. Herein, Li+ and F− ions are introduced simultaneously via a simple sol–gel method. The F not only enters the lattice but forms chemically stable NaF on the surface. The modified electrode delivered significantly better electrochemical performance than the pristine one, including much-enhanced capacity retention (64% vs. 36%, 100 cycles) at 0.5 C and a four-time higher capacity output at 10 C. The ex situ XRD and in situ Raman analysis revealed cyclability enhancement mechanisms in terms of inhibiting the P2–O2 phase transition and Na+/vacancy ordering. The conductivity measurements (based on AC impedance and DC polarization) and GITT analysis proved, on both bulk material and electrode levels, that Na+ conduction and, thus, rate performance is notably promoted by doping. The individual contribution of Li and F to the overall performance improvement was also discussed. Furthermore, a solid-state sodium-metal battery was successfully demonstrated with the modified cathode. The above results verify that Li+/F− incorporation can enable enhancements in both cyclability and rate capability of the P2-Na2/3Ni1/3Mn2/3O2 cathodes and are expected to provide a new perspective for the rational design of high-performance layered oxide cathode materials for progressive sodium-ion batteries.

Crystal structure and oxygen nonstoichiometry of ErBa2Fe3O8

Ten samples across the range of oxygen nonstoichometry of the ErBa2FeIII3O8 triple perovskite were obtained by high-temperature equilibration and quenching from flowing gas of controlled oxygen partial pressure. The crystal structure accommodating additional oxygen upon oxidation, and oxygen vacancies in the central coordination octahedron upon reduction, was studied by synchrotron X-ray powder diffraction. Close to the homogeneity limit upon reduction, the tetragonal P4/mmm structure distorts orthorhombic upon ordering of vacancies. The structural compromises across the nonstoichiometry range are discussed in terms of bond-valence sums at the atoms.

Structural and electronic structure evolution of K x Fe2− y Se2 from semiconducting phase to superconducting phase

The structural parameters, magnetic property and electronic structure of K0.875+a Fe1.5+b Se2 have been studied by first principles calculations. An increase of K or Fe content leads to the evolution of the magnetic property and electronic structure of K0.875Fe1.5Se2. The introduction of extra K brings a slight rigid shift of the minority states, while the introduction of extra Fe breaks the vacancy order of Fe atoms, forming 1D chains, and causes the Fermi surfaces to display an obvious nesting feature. The K0.875Fe1.5Se2 is proven to be a semiconductor with a 61 meV gap, and the evolution from semiconducting phase to superconducting phase is mainly due to the suppression of a rhombus iron vacancy order and antiferromagnetic order with an increase in Fe content.