Developing efficient sodium-ion-storage mechanisms to increase the energy capacity in organic electrodes is a critical issue even after this period of prolonged effort. Uric acid (UA), a simple organic compound with three carbonyl groups is demonstrated to be electrochemically active in the insertion/extraction of Na ions. Theoretical calculations and experimental characterizations reveal that the sodium-ion storage by UA is a result of the stepwise mechanisms of p−π conjugation and the carbon anion. Aside from C═O, the functional group C═C(NH—)2 also provides an efficient Na-storage activated site in which the lone-pair electrons is stabilized through the planar-to-tetrahedral structural transition and low-energy orbital hybridization of N atoms. For further improvement of the electrochemical performance, a uric acid and carbon nanotube (UA@CNT) composite is prepared via a vacuum solution impregnation method. When employed as an anode material for sodium-ion batteries, the UA@CNT composite exhibits high specific capacity, excellent rate capability, and long cycling life even at high current densities. A reversible capacity of over 163 mA h g–1 is maintained even after 150 cycles at a current density of 200 mA g–1. The present study paves a way to develop reversible high-capacity organic electrode materials for sodium-ion batteries by a carbon-anion stabilization mechanism.Keywords: CNT composite; electrochemical performance; sodium ion; theoretical calculation; uric acid;

The electrochemical performance of rechargeable lithium–oxygen (Li–O2) batteries relies on the catalysts used in the cathode to a great extent. Herein, a series of CuxCo3−xO4 nanorods with different Cu2+ concentrations was prepared by a convenient hydrothermal method. The corresponding structures and electrochemistries were further characterized in order to reveal the composition effect of catalyst on catalytic activity. Experimental characterization and theoretical calculations indicate that more Cu2+ occupying Co2+ positions and dispersing on the catalyst surface has an enhanced catalytic activity in terms of higher capacity, lower overpotential and better reversibility. This implies that suitable charge transfer from Li2O2 to the catalyst plays an important role in improving the electrochemical performance of Li–O2 batteries. The present study is helpful for designing a highly active catalyst by tuning the composition of the catalyst surface.

N2 reduction to ammonia by solar light represents a green and sustainable ammonia synthesis approach which helps to suppress the global warming and energy crisis. However, conventional semiconductors usually suffer from low activity or poor stability, largely suppressing the application of this technology. Here, we report that bismuth monoxide (BiO) quantum dots with an average size of 2–5 nm exhibited efficient photocatalytic activity for ammonia synthesis under simulated solar light. A highly efficient ammonia synthesis rate of 1226 μmol g−1 h−1 is achieved without the assistance of any sacrificial agent or co-catalyst, which is about 1000 times higher than that using the traditional Fe-TiO2 photocatalyst. Kinetic analysis reveals that the synergy of three low valence surface Bi(II) species markedly enhances N2 activation by electron donation, which finally resulted in the highly efficient N2 photoreduction performance. This work will shed light on designing efficient and robust N2 reduction photocatalysts.

Porous activated carbons (PACs) are promising candidates to capture CO2 through physical adsorption because of their chemical stability, easy-synthesis, cost-effectiveness and good recyclability. However, their low CO2 adsorption capacity, especially low CO2/N2 selectivity, has limited their practical applications. In this work, an optimized PAC with a large specific surface area, a small micropore size, and a large micropore volume has been synthesized by one-step carbonization/activation of casein using K2CO3 as a mild activation agent. It showed a remarkably enhanced CO2 adsorption capacity as high as 5.78 mmol g−1 and an excellent CO2/N2 selectivity of 144 (25 °C, 1 bar). Based on DFT calculations and experimental results, the coexistence of adjacent pyridinic N and –OH/–NH2 species was proposed for the first time to make an important contribution to the ultra-high CO2 adsorption performance, especially CO2/N2 selectivity. This work provides effective guidance to design PAC adsorbents with high CO2 adsorption performance. The content of pyridine N combined with –OH/–NH2 was further elevated by additional nitrogen introduction, resulting in a further enhanced CO2 adsorption capacity up to 5.96 mmol g−1 (25 °C, 1 bar). All these results suggest that, in addition to the well-defined pore structure, pyridinic N with neighboring OH or NH2 species played an important role in enhancing the CO2 adsorption performance of PACs, thus providing effective guidance for the rational design of CO2 adsorbents.

Tuning the composition of discharge products is an important strategy to reduce charge potential, suppress side reactions, and improve the reversibility of metal–oxygen batteries. In the present study, first-principles calculations and experimental confirmation were performed to unravel the influence of O2 pressure, particle size, and electrolyte on the composition of charge products in Na–O2 batteries. The electrolytes with medium and high donor numbers (>12.5) are favorable for the formation of sole NaO2, while those with low donor numbers (<12.5) may permit the formation of Na2O2 by disproportionation reactions. Our comparative experiments under different electrolytes confirmed the calculation prediction. Our calculations indicated that O2 pressure and particle size hardly affect discharge products. On the electrode, only one-electron-transfer electrochemical reaction to form NaO2 takes place, whereas two-electron-transfer electrochemical and chemical reactions to form Na2O2 and Na3O4 are prevented in thermodynamics. The present study explains why metastable NaO2 was identified as a sole discharge product in many experiments, while thermodynamically more stable Na2O2 was not observed. Therefore, to achieve low overpotential, a high-donor-number electrolyte should be applied in the discharge processes of Na–O2 batteries.

Lithium–sulfur (Li–S) batteries have been considered as next-generation rechargeable energy storage systems due to their high theoretical energy densities and low cost; however, the capacity decay resulting from the shuttle of lithium polysulfides (LiPSs) hinders their practical application. Herein, we describe a strategy to synthesize highly pyridinic-N-doped three-dimensional (3D) carbons for the chemisorption of LiPSs, which consist of zeolitic imidazolate framework-8-derived carbon (ZIF-8(C)) coated on the surface of N-doped carbon nanotubes supported by carbon nanosheets (NCNTs–CS–ZIF-8(C)). Using the obtained carbons as sulfur hosts, the S/NCNTs–CS–ZIF-8(C) cathodes show a high sulfur utilization of 86% at 0.1 C, a low capacity decay rate of 0.052% per cycle over 700 cycles at 1 C and impressive cycling life that is 564 mA h g−1 after 700 cycles at 1 C. First principles calculations based on the Vienna Ab-Initio Simulation Package (VASP) reveal that increasing the amount of the pyridinic-N component can enhance the adsorption of LiPSs, which yields effective suppression of the LiPS shuttle.

•Distribution and component of discharge products are manipulated by ZnO/VACNTs.•Li-deficient Li2−xO2 is stabilized by the nanosize effect.•The ZnO/VACNT interfaces play the key role in the Li2−xO2 formation.•Reduced overpotential and prolonged cycles are achieved with the ZnO/VACNTs.Control of discharge products with respect to composition, size and morphology is of importance to reduce charge potential, suppress side reactions and improve reversibility of Li-O2 batteries, but faces significant challenge due to complicated electrochemical reactions. Here, we report a cathode architecture composed of ZnO nanoparticles anchored on vertically aligned carbon nanotubes (ZnO/VACNTs) that significantly suppresses LiO2 disproportionation, forming the composites of LiO2, Li3O4 and Li2O2 in nanometer size as the final discharge products. The exposed surface of VACNTs provides the electrochemical reaction sites for reducing O2 to O2−. Transmission electron microscopy measurements in combination with first-principles calculations reveal that the Li2−xO2 compounds nucleated at the interfaces between the ZnO and the VACNTs are stabilized by the nanosize effect (in the scale of 6 nm), which is related to the semiconductive behavior of ZnO. This discharge chemistry leads to the reduced charge overpotential and extended cycle life. The results here demonstrate that the electrochemical reaction tuned by the cathode architecture is a powerful tool to improve Li-O2 cell performance.

Unraveling the descriptor of catalytic activity, which is related to physical properties of catalysts, is a major objective of catalysis research. In the present study, the first-principles calculations based on interfacial model were performed to study the oxygen evolution reaction mechanism of Li2O2 supported on active surfaces of transition-metal compounds (TMC: oxides, carbides, and nitrides). Our studies indicate that the O2 evolution and Li+ desorption energies show linear and volcano relationships with surface acidity of catalysts, respectively. Therefore, the charging voltage and desorption energies of Li+ and O2 over TMC could correlate with their corresponding surface acidity. It is found that certain materials with an appropriate surface acidity can achieve the high catalytic activity in reducing charging voltage and activation barrier of rate-determinant step. According to this correlation, CoO should have as active catalysis as Co3O4 in reducing charging overpotential, which is further confirmed by our comparative experimental studies. Co3O4, Mo2C, TiC, and TiN are predicted to have a relatively high catalytic activity, which is consistent with the previous experiments. The present study enables the rational design of catalysts with greater activity for charging reactions of Li-O2 battery.

Unraveling the catalytic mechanism of transition-metal oxides (TMOs) for the charging reaction in a Li–O2 battery and characterizing their surface structures and electronic structure properties of active sites are of great importance for the development of an effective catalyst to improve low round-trip efficiency and power density. In the current study, an interfacial model is first constructed to study the decomposition reaction mechanism of Li2O2 supported on Co3O4 surfaces. The computational results indicate that the O-rich Co3O4 (111)C with a relatively low surface energy in high O2 concentration has a high catalytic activity in reducing overpotential and O2 desorption barrier due to the electron transfer from the Li2O2 layer to the underlying surface. Meanwhile, the basic sites of Co3O4 (110)B surface induce Li2O2 decomposition into Li2O and a dangling Co–O bond, which further leads to a high charging voltage in the subsequent cycles. The calculations for transition-metal (TM)-doped Co3O4 (111) indicate that P-type doping of Co3O4 (111) exhibits significant catalysis in decreasing both charging overpotential and O2 desorption barrier. The ionization potential of doped TM is determined as an important parameter to regulate the catalytic activity of metal oxides.Keywords: catalytic mechanism; Co3O4; doping transition metal; first-principles calculations; lithium-O2 battery

Numerous lithium ion battery cathode materials containing trace amounts of water accommodated in Li+ transportation tunnels have been experimentally synthesized. However, the impact of water on structural stability and electrochemical performance of cathode materials is still unclear. Here, the first-principles calculations combining thermodynamic analysis of LixFeF3·0.33H2O were performed to unravel the interaction mechanism among frameworks of FeF3, H2O, and Li+. The FeF3 framework structure distortion is mitigated by hydrogen bonding between isolated H2O and F− ions, bringing opposite effects on the stability of hydrogen bonding and instability of structural distortion. The hydrogen bonding strength of F−⋯H2O can be further mediated by the Li+-inserted amount, which indirectly results in a wide discharge voltage window of 2.2 to 3.6 V. The Li+ transportation barrier in cooperative mode is also tuned by the flexible hydrogen bonding strength due to different occupied positions. Li0.66FeF3·0.33H2O is determined as the most stable species and more Li+ insertion directly leads to the conversion reaction FeF63− → FeF4− + 2F−. Therefore, stabilizing Fe–F bonds and reducing octahedral chain distortion are important to improve the electrochemical performance of FeF3 cathode materials with water.

The efficient removal of low-concentration nitric oxide at room temperature from a semi-closed space is becoming a crucial but challenging issue in the context of increasingly serious air pollution. A novel nanoflower-like weak crystallization manganese oxide (WMO) has been synthesized via a facile and scalable strategy for low-concentration nitric oxide oxidation at room temperature. The prepared WMO shows the nanoflower-like morphology with abundant water molecules and Mn vacancies inside. Such WMO could easily adsorb NO and quickly convert it into NO2via catalytic oxidization. Herein, the weak crystallization structure and the presence of Mn vacancies are identified to be mainly responsible for the adsorption and catalytic oxidation of NO. More importantly, it shows much longer lifetime in a moist stream of simulated feed gas than that under dry conditions, which can be attributed to the relative stability of the catalyst with hydrated surfaces. Comparative DFT-calculations are performed to reveal the catalytic effect of Mn-vacancies and hydrated surfaces in reducing the reaction barriers of rate-determining steps.

A lithium–air battery as an energy storage technology can be used in electric vehicles due to its large energy density. However, its poor rate capability, low power density and large overpotential problems limit its practical usage. In this paper, the first-principles thermodynamic calculations were performed to study the catalytic activity of X-doped graphene (X = B, N, Al, Si, and P) materials as potential cathodes to enhance charge reactions in a lithium–air battery. Among these materials, P-doped graphene exhibits the highest catalytic activity in reducing the charge voltage by 0.25 V, while B-doped graphene has the highest catalytic activity in decreasing the oxygen evolution barrier by 0.12 eV. By combining these two catalytic effects, B,P-codoped graphene was demonstrated to have an enhanced catalytic activity in reducing the O2 evolution barrier by 0.70 eV and the charge voltage by 0.13 V. B-doped graphene interacts with Li2O2 by Li-sited adsorption in which the electron-withdrawing center can enhance charge transfer from Li2O2 to the substrate, facilitating reduction of O2 evolution barrier. In contrast, X-doped graphene (X = N, Al, Si, and P) prefers O-sited adsorption toward Li2O2, forming a X–O22−⋯Li+ interface structure between X–O22− and the rich Li+ layer. The active structure of X–O22− can weaken the surrounding Li–O2 bonds and significantly reduce Li+ desorption energy at the interface. Our investigation is helpful in developing a novel catalyst to enhance oxygen evolution reaction (OER) in Li–air batteries.

Basin-hopping global searching and quantum chemistry calculations were performed to predict global minimum structures of SinAl5−n+1,0,−1 (n = 0–5) clusters in order to explore the intrinsic mechanism of 3D-to-planar structural transition and structural characteristics of planar tetracoordinate species. A structural similarity of isoelectronic SinAl5−n+1,0,−1 (n = 0–5) clusters exists, and may generally be extended to similar clusters containing B, C, N, O, and P. The structural diagram and molecular orbital analysis reveal that a 3D-to-planar structural transition should be related to composition and total valence electron number of clusters. Our calculations indicate that the global minima of SinAl5−n+1,0,−1 (n = 0–5) with planar tetracoordinate Si (ptSi) should meet the 18-electron rule and take the charge of −1, 0, +1. Further, it is found that 18-electron M–Al4 (M = B, C, N, O, and F) planar clusters prefer a central atom M with a low electronegativity and peripheral Al substituted with a lower-electronegativity atom. Based on the structural characteristics of planar clusters, we firstly predict a novel planar tetracoordinate C (PtC) structure C2Al3− which is more stable in energy than the experimentally observed CAl42−. C2Al3− may become a building block to assemble some larger supermolecule containing multiple planar hypercoordinate C (phC).

The facet-dependent performance has aroused great interest in the fields of catalyst, lithium ion battery and electrochemical sensor. In this study, the well-defined Co3O4 cubes with exposed (001) plane and octahedrons with exposed (111) plane have been successfully synthesized and the facet-dependent electrocatalytic performance of Co3O4 for rechargeable Li–O2 battery has been comprehensively investigated by the combination of experiments and theoretical calculations. The Li–O2 battery cathode catalyzed by Co3O4 octahedron with exposed (111) plane shows much higher specific capacity, cycling performance, and rate capability than Co3O4 cube with exposed (001) plane, which may be largely attributed to the richer Co2+ and more active sites on (111) plane of Co3O4 octahedrons. The DFT-based first-principles calculations further indicate that Co3O4 (111) has a lower activation barrier of O2 desorption in oxygen evolution reaction (OER) than Co3O4 (001), which is very important to refresh active sites of catalyst and generate a better cyclic performance. Also, our calculations indicate that Co3O4 (111) surface has a stronger absorption ability for Li2O2 than Co3O4 (001), which may be an explanation for a larger initial capacity in Co3O4 (111) plane by experimental observation.

Unraveling the metal–insulator transition (MIT) mechanism of VO2 becomes tremendously important for understanding strongly correlated character and developing switching applications of VO2. First-principles calculations were employed in this work to map the reduced-dimension potential energy surface of the MIT of VO2. In the beginning stage of MIT, a significant orbital switching between σ-type dz2 and π-type dx2–y2/dyz accompanied by a large V–V dimerization and a slight twisting angle change opens a band gap of ∼0.2 eV, which can be attributed to the electron-correlation-driven Mott transition. After that, the twisting angle of one chain quickly increases, which is accompanied by the appearance of a larger change in band gap from 0.2 to 0.8 eV, even though orbital occupancy is maintained. This finding can be ascribed to the structure-driven Peierls transition. The present study reveals that a staged electron-correlation-driven Mott transition and structure-driven Peierls transition are involved in MIT of VO2.

Tailoring effectively the hysteresis loop width of VO2 semiconductor-metal transition (SMT) is crucial to develop thermal sensor devices. The Ti-doping is known as the most effective method to reduce ΔTc and has been considered as the prototypical model in understanding the mechanism by which the ΔTc of VO2 MST could be manipulated. Here we present a joint experimental and first-principles computational study on nondoped and Ti-doped VO2 to clarify the mechanism of Ti-doping on narrowing the hysteresis loop width of VO2. On the basis of the analyses of differential scanning calorimetry (DSC), we found that phase transition temperatures in the cooling circle increase faster than those in the heating circle with increasing Ti concentrations, exhibiting a hysteresis width reduction at 2 °C per Ti at. %. First-principles calculations reveal that dopant Ti atoms break the octahedral symmetry of local structure in VO2 (R) phase. This distortion is propagated in anisotropy and exhibits an obvious nonlocal effect. In contrast, the Ti-doping-induced structural change in VO2 (M) phase is only constrained in Ti-involved chain along the a-axis. The calculated energy profiles for TixV1–xO2 phase transition shows that structural stability and activation energies increased with the increase of Ti concentration. The activation barriers in Ti-doped VO2 (R) phases are increased more remarkably than that in Ti-doped VO2 (M) phases, which is consistent with experimental observation of concentration-dependent reduction of thermal hysteresis width.

The lithium–air battery as an energy storage technology can be used in electric vehicles due to its large energy density, while its poor rate capability limits its practical usage under large current density. According to first-principles thermodynamics calculation, we predict B-doped graphene can be a potential catalyst to improve the charge rate of lithium–air battery. The lowest-energy reaction pathway for oxygen evolution reaction (OER) is predicted as Li+ → Li+ → O2. The rate-determining step (RDS) is predicted as the O2 evolution step. B-doped graphene can reduce the RDS barrier by 0.40 eV, indicating that charge rate may be significantly improved. B-doping can increase charge transferring of Li2O2 to the substrate by 0.36 e–, which helps to activate Li–O bonds and oxidize O22– to O2. We suggest a good OER catalytic substrate that can reduce the O2 evolution barrier should show p-type surface behavior.

The first-principles and thermodynamic calculations were performed to study structural stability, thermodynamic properties, and Li+ migration mechanism of the superionic conductor Li10GeP2S12 (LGPS). Our calculations show that the zigzag and parallel arrangements of GeS44– and P(1)S43– units form three types of stable structures. Among them, zigzag-type structures with 2–4 Li+ occupied in the Li4 position were found to be the most stable. Our thermodynamic calculations show that LGPS may be stable at >276 K when configuration and vibration entropies are considered based on disordered arrangement of GeS44– and P(1)S43– units, and partially occupied Li+. Based on the calculation for minimum energy paths, we found that Li+ migration along the c axis may be more favorable than that in the ab plane, indicating a very weak anisotropy for Li+ migration of LGPS. These structural and mechanistic studies are helpful to design a novel superionic conductor with high performance.

The high-activity electrocatalysts for the hydrogen evolution reaction (HER) are highly desired to replace precious Pt, but difficult to achieve. Herein, we report the loading of ultrafine tungsten carbide (WC) nanoparticles (NPs) on cobalt-embedded, bamboo-like, nitrogen-doped carbon nanotubes (WC/Co@NCNTs) with high-level N doping via a one-step strategy, leading to a desirable multicomponent nanocomposite with superior activity and stability when used as the HER electrocatalyst. The optimized WC/Co@NCNTs showed a very low onset overpotential (Uonset) of ~18 mV, a small Tafel slope of 52 mV dec−1, a small η10 of only 98 mV to reach a current of 10 mA cm−2, and a large exchange current density (j0) of 0.103 mA cm−2, which also retained its high activity for at least 12.5 h operation in acidic electrolyte. The DFT calculations revealed an important role of the N dopants in the HER as well as a favorable ΔGH* for the adsorption and desorption of hydrogen derived from the synergistic effects between WC NPs and Co@NCNTs.A multicomponent nanocomposite of ultrafine WC nanoparticles anchored on Co-encased, N-doped carbon nanotubes prepared by one-step pyrolysis of low-cost precursors can serve as hydrogen-generating electrocatalyst with excellent activity and superior stability in acidic electrolyte.Download high-res image (191KB)Download full-size image

N2 reduction to ammonia by solar light represents a green and sustainable ammonia synthesis approach which helps to suppress the global warming and energy crisis. However, conventional semiconductors usually suffer from low activity or poor stability, largely suppressing the application of this technology. Here, we report that bismuth monoxide (BiO) quantum dots with an average size of 2–5 nm exhibited efficient photocatalytic activity for ammonia synthesis under simulated solar light. A highly efficient ammonia synthesis rate of 1226 μmol g−1 h−1 is achieved without the assistance of any sacrificial agent or co-catalyst, which is about 1000 times higher than that using the traditional Fe-TiO2 photocatalyst. Kinetic analysis reveals that the synergy of three low valence surface Bi(II) species markedly enhances N2 activation by electron donation, which finally resulted in the highly efficient N2 photoreduction performance. This work will shed light on designing efficient and robust N2 reduction photocatalysts.

The efficient removal of low-concentration nitric oxide at room temperature from a semi-closed space is becoming a crucial but challenging issue in the context of increasingly serious air pollution. A novel nanoflower-like weak crystallization manganese oxide (WMO) has been synthesized via a facile and scalable strategy for low-concentration nitric oxide oxidation at room temperature. The prepared WMO shows the nanoflower-like morphology with abundant water molecules and Mn vacancies inside. Such WMO could easily adsorb NO and quickly convert it into NO2via catalytic oxidization. Herein, the weak crystallization structure and the presence of Mn vacancies are identified to be mainly responsible for the adsorption and catalytic oxidation of NO. More importantly, it shows much longer lifetime in a moist stream of simulated feed gas than that under dry conditions, which can be attributed to the relative stability of the catalyst with hydrated surfaces. Comparative DFT-calculations are performed to reveal the catalytic effect of Mn-vacancies and hydrated surfaces in reducing the reaction barriers of rate-determining steps.

Lithium–sulfur (Li–S) batteries have been considered as next-generation rechargeable energy storage systems due to their high theoretical energy densities and low cost; however, the capacity decay resulting from the shuttle of lithium polysulfides (LiPSs) hinders their practical application. Herein, we describe a strategy to synthesize highly pyridinic-N-doped three-dimensional (3D) carbons for the chemisorption of LiPSs, which consist of zeolitic imidazolate framework-8-derived carbon (ZIF-8(C)) coated on the surface of N-doped carbon nanotubes supported by carbon nanosheets (NCNTs–CS–ZIF-8(C)). Using the obtained carbons as sulfur hosts, the S/NCNTs–CS–ZIF-8(C) cathodes show a high sulfur utilization of 86% at 0.1 C, a low capacity decay rate of 0.052% per cycle over 700 cycles at 1 C and impressive cycling life that is 564 mA h g−1 after 700 cycles at 1 C. First principles calculations based on the Vienna Ab-Initio Simulation Package (VASP) reveal that increasing the amount of the pyridinic-N component can enhance the adsorption of LiPSs, which yields effective suppression of the LiPS shuttle.

Numerous lithium ion battery cathode materials containing trace amounts of water accommodated in Li+ transportation tunnels have been experimentally synthesized. However, the impact of water on structural stability and electrochemical performance of cathode materials is still unclear. Here, the first-principles calculations combining thermodynamic analysis of LixFeF3·0.33H2O were performed to unravel the interaction mechanism among frameworks of FeF3, H2O, and Li+. The FeF3 framework structure distortion is mitigated by hydrogen bonding between isolated H2O and F− ions, bringing opposite effects on the stability of hydrogen bonding and instability of structural distortion. The hydrogen bonding strength of F−⋯H2O can be further mediated by the Li+-inserted amount, which indirectly results in a wide discharge voltage window of 2.2 to 3.6 V. The Li+ transportation barrier in cooperative mode is also tuned by the flexible hydrogen bonding strength due to different occupied positions. Li0.66FeF3·0.33H2O is determined as the most stable species and more Li+ insertion directly leads to the conversion reaction FeF63− → FeF4− + 2F−. Therefore, stabilizing Fe–F bonds and reducing octahedral chain distortion are important to improve the electrochemical performance of FeF3 cathode materials with water.

A lithium–air battery as an energy storage technology can be used in electric vehicles due to its large energy density. However, its poor rate capability, low power density and large overpotential problems limit its practical usage. In this paper, the first-principles thermodynamic calculations were performed to study the catalytic activity of X-doped graphene (X = B, N, Al, Si, and P) materials as potential cathodes to enhance charge reactions in a lithium–air battery. Among these materials, P-doped graphene exhibits the highest catalytic activity in reducing the charge voltage by 0.25 V, while B-doped graphene has the highest catalytic activity in decreasing the oxygen evolution barrier by 0.12 eV. By combining these two catalytic effects, B,P-codoped graphene was demonstrated to have an enhanced catalytic activity in reducing the O2 evolution barrier by 0.70 eV and the charge voltage by 0.13 V. B-doped graphene interacts with Li2O2 by Li-sited adsorption in which the electron-withdrawing center can enhance charge transfer from Li2O2 to the substrate, facilitating reduction of O2 evolution barrier. In contrast, X-doped graphene (X = N, Al, Si, and P) prefers O-sited adsorption toward Li2O2, forming a X–O22−⋯Li+ interface structure between X–O22− and the rich Li+ layer. The active structure of X–O22− can weaken the surrounding Li–O2 bonds and significantly reduce Li+ desorption energy at the interface. Our investigation is helpful in developing a novel catalyst to enhance oxygen evolution reaction (OER) in Li–air batteries.

Tuning the composition of discharge products is an important strategy to reduce charge potential, suppress side reactions, and improve the reversibility of metal–oxygen batteries. In the present study, first-principles calculations and experimental confirmation were performed to unravel the influence of O2 pressure, particle size, and electrolyte on the composition of charge products in Na–O2 batteries. The electrolytes with medium and high donor numbers (>12.5) are favorable for the formation of sole NaO2, while those with low donor numbers (<12.5) may permit the formation of Na2O2 by disproportionation reactions. Our comparative experiments under different electrolytes confirmed the calculation prediction. Our calculations indicated that O2 pressure and particle size hardly affect discharge products. On the electrode, only one-electron-transfer electrochemical reaction to form NaO2 takes place, whereas two-electron-transfer electrochemical and chemical reactions to form Na2O2 and Na3O4 are prevented in thermodynamics. The present study explains why metastable NaO2 was identified as a sole discharge product in many experiments, while thermodynamically more stable Na2O2 was not observed. Therefore, to achieve low overpotential, a high-donor-number electrolyte should be applied in the discharge processes of Na–O2 batteries.