Redox Potential Paradox in Nax ... - ACS Publications

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Redox Potential Paradox in NaxMO2 for Sodium-Ion Battery Cathodes Yusuke Nanba,† Tatsumi Iwao,‡ Benoit Mortemard de Boisse,‡ Wenwen Zhao,‡ Eiji Hosono,† Daisuke Asakura,† Hideharu Niwa,§,∥ Hisao Kiuchi,⊥ Jun Miyawaki,§,∥ Yoshihisa Harada,§,∥ Masashi Okubo,‡,# and Atsuo Yamada*,‡,# †

Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8568, Japan ‡ Department of Chemical System Engineering, School of Engineering, The University of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan § Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277-8581, Japan ∥ Synchrotron Radiation Research Organization, The University of Tokyo, Tatsuno, Hyogo 679-5165, Japan ⊥ Department of Applied Chemistry, School of Engineering, The University of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan # Elements Strategy Initiative for Catalysts & Batteries (ESICB), Kyoto University, Nishikyo-ku, Kyoto 615-8245, Japan S Supporting Information *

ABSTRACT: Raising the operating potential of the cathode materials in sodium-ion batteries is a crucial challenge if they are to outperform state-of-the-art lithium-ion batteries. Although the layered transition metal oxides, NaMO2 (M: transition metal), are the most promising cathode materials owing to their high theoretical capacity with much more stable nature than Li1−xMO2 system, factors influencing the redox potential have not yet been fully understood. Here, we identify redox potential paradox, E(Ni3+/Ni2+) > E(Ni4+/Ni3+), in an identical structural framework, namely, NaTi4+0.5Ni2+0.5O2 and NaFe3+0.5Ni3+0.5O2, which is induced by transition of the oxides from Mott−Hubbard to negative charge-transfer regimes. The origin of the unusually low E(Ni4+/Ni3+) is the surprisingly large contribution (over 80%) of oxygen orbital to the redox reaction, of which the primary effect on the electrochemical property is demonstrated for the first time, providing a firm platform to design better cathodes for advanced sodium-ion batteries.

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INTRODUCTION Sodium-ion batteries have recently attracted much attention as low-cost and sustainable alternatives to lithium-ion batteries, especially in large scale application. A notable advantage is that highly polarizable sodium ions as mobile charges can enable rapid charge/discharge reactions. However, at present, the gravimetric and volumetric energy densities of sodium-ion batteries are limited because a simple extension of lithium intercalation chemistry to sodium may lead to both lower capacity and lower operating potential due to heavier sodium and higher Na/Na+ potential than lithium.1−3 Therefore, development of a new strategy for designing the cathode materials specific to sodium intercalation is essential to realize enhanced energy densities.4−11 One of the most promising strategies to enhance the energy density is to raise the operating potential of the cathode materials. This strategy has already been adopted to polyanionic compounds based on a concept of an inductive effect: by incorporating ionic SO42− into a framework, Barpanda et al. have succeeded in achieving a 3.8 V (vs Na+/ Na) cathode material Na2Fe2(SO4)3 even with Fe3+/Fe2+ redox couple.6 In contrast, although the layered metal oxides NaxMO2 © XXXX American Chemical Society

(M: transition metal) are the most promising cathode materials due to their high theoretical capacity,12−17 a rational strategy to control the operating potential has not yet been developed. With this context, understanding the factors to determine the redox potential of the metal oxides is an essential issue. In this article, we focus on O3-type NaMO2, one of the most well-studied NaxMO2.18−26 O3 denotes a layered structure with sodium ions in octahedral sites where the close-packed oxideion layers stack in the manner of ABCABC (Scheme 1).27 Because NaMO2 possesses a layered structure that is highly stabilized by a large difference in ionic radii of Na and M, various transition metals can be incorporated for rational control of the electronic state. These compositional and electronic versatilities enable a comprehensive study on the operating potentials of the nominal redox couples, for example, Ni3+/Ni2+ and Ni4+/Ni3+ using NaxM0.5Ni0.5O2 with M3+ or M4+, which complements the orbital-level understandings of the NaxMO2 cathodes to tailor the electrode performance. Received: November 4, 2015 Revised: January 20, 2016

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DOI: 10.1021/acs.chemmater.5b04289 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials Scheme 1. Crystal Structure of O3-NaMO2a

Ernzerh generalized gradient approximation. In the present study, we consider the simplest model crystal where the atomic positions were relaxed. The antiferromagnetic ordering between the M1 and M2 atoms was assumed, because strong antiferromagnetic interaction occurs due to the large orbital overlap of Ni eg−O 2p (σ) and Fe/Ti t2g−O 2p (π) according to the Goodenough−Kanamori rule.31 The plane-wave cutoff was RMTKmax = 7.0, where RMT is the smallest atomic sphere radius in the unit cell and Kmax is the magnitude of the largest k vector. The self-consistency was carried out on 8 × 8 × 7 k-point grid in the Brillouin zone. On-site Coulomb potentials U were applied for the Ti, Fe, and Ni atoms.32−34 The optimized U values for NaMO2 have been proposed previously (0.0 eV for Ti, 4.0 eV for Fe, and 6.0 eV for Ni).35 However, for the solid solution systems (NaNiO2− NaFeO2 and NaNiO2−NaTiO2) in this work, the Ni 3d orbital expands due to the enhanced ligand-to-metal charge transfer (LMCT) in Ni−O, which is caused by the neighboring ionic Ti−O/Fe−O bonds. The expanded Ni 3d orbital should give a smaller Coulomb integral. Meanwhile, covalent Ni−O reduces the LMCT in the neighboring Ti−O bond, which shrinks the Ti 3d orbital to give a larger Coulomb integral. Therefore, U values are slightly modified to 1.5 eV for Ti, 4.0 eV for Fe, and 5.0 eV for Ni.

a

M = transition metal, gray. Oxide ions (red) stack in the manner of ABCABC while Na ions (yellow) occupy octahedral sites.

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RESULTS AND DISCUSSION NaFe0.5Ni0.5O2 that exhibits the Ni4+/Ni3+ redox reaction was synthesized by a conventional solid state reaction. NaTi0.5Ni0.5O2 was also synthesized as a comparative system that exhibits the Ni3+/Ni2+ redox reaction. Powder X-ray diffraction patterns for both compounds were fitted by Rietveld refinement based on the O3-type NaMO2 structure (R3̅m), indicating the formation of a single solid−solution phase (Figure 1 and Table S1, Supporting Information). The X-ray

Here, applying a combined assessment method of soft X-ray absorption spectroscopy (XAS) and electronic structure calculation to NaFe0.5Ni0.5O2 and NaTi0.5Ni0.5O2, we demonstrate that the redox potential paradox, E(Ni3+/Ni2+) > E(Ni4+/ Ni3+), arises in these NaxMO2 cathodes by the large contribution of the oxygen 2p orbital.

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EXPERIMENTAL SECTION

The precursor mixture of NaTi0.5Ni0.5O2 was prepared by hand-milling Na2CO3, NiO, and TiO2. Na2CO3 (8 wt %) was added to account for the high volatility of Na at high temperature. The mixture was pressed into pellets which were annealed at 900 °C for 12 h under pure O2. The precursor mixture of NaFe0.5Ni0.5O2 was prepared by hand-milling Na2O2, NiO, and Fe2O3. Na2O2 (3 wt %) was added to account for the Na volatility. Pellets were annealed at 700 °C for 12 h under pure O2. After the heat treatment, both materials were directly entered into an Ar-filled glovebox to prevent any moisture exposition. Powder X-ray diffraction patterns were recorded between 10° and 130° (0.021° steps, 9s/step) on a D8 Advance (Bruker-AXS) powder diffractometer with Co Kα radiation. Rietveld refinements were carried out using Jana software.28 Even though the refinements were carried out in the 10− 130° range, only the 15−80° one is shown for clarity purposes. 57Fe Mössbauer spectra with 57Co in Rh as a Mössbauer source were recorded at room temperature in transmission mode. The velocity was calibrated by using α-Fe. Mö ssbauer spectra were fitted with Mosswinn3.0 software. Electrochemical measurements were conducted by using CR2032type coin cells. Each sample (75 wt %) was mixed with 20 wt % of acetylene black and 5 wt % of polytetrafluoroethylene into a freestanding film as a working electrode. Na metal was used as counter electrode. Electrodes were separated by a glass fiber sheet soaked with NaPF6 (1 M) in ethylene carbonate (EC)−diethyl carbonate (DEC) electrolyte. The XAS experiments were carried out at BL07LSU of SPring-8. Sample holders were transferred from an Ar-filled glovebox to a vacuum chamber without air exposure. Bulk-sensitive partialfluorescence yield (PFY) mode was used for the O K-edge and Ni L2,3-edge XAS. Energy resolution of the incident beam was approximately 100 meV. The soft X-ray energy for O K-edge was calibrated using the well-established pre-edge peak energy of liquid H2O (535 eV), and the error is within ±0.1 eV.29 All XAS experiments were performed at room temperature. The density-of-state (DOS) calculations for NaFe0.5Ni0.5O2 and NaTi0.5Ni0.5O2 were performed using the full-potential linearized augmented-plane-wave as implemented in the Wien2k package.30 The exchange and correlation terms were treated by the Perdew-Burke-

Figure 1. Powder X-ray diffraction patterns and Rietveld refinement results of O3-NaM0.5Ni0.5O2 (M = Ti, Fe).

absorption spectrum for the Ti L-edge (Figure S1, Supporting Information) exhibits the four characteristic peaks of Ti4+; thus, the nominal valence state can be written as NaTi0.54+Ni0.52+O2.36 The 57Fe Mössbauer spectrum for NaFe0.5Ni0.5O2 (Figure 2a) shows a doublet with an isomer shift of 0.344 mm/ s and a quadrupole splitting of 0.511 mm/s, typical values for high−spin Fe3+, suggesting a nominal valence state of NaFe0.53+Ni0.53+O2.37 When incorporated in electrochemical Na cells, both NaTi0.5Ni0.5O2 and NaFe0.5Ni0.5O2 exhibit electrochemical desodiation/sodiation (charge/discharge), and show little B


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Chemistry of Materials

Figure 2. (a) Mössbauer spectra for the pristine, desodiated (x = 0.5), and sodiated (x = 1.0) NaxFe0.5Ni0.5O2. (b) Isomer shift and quadrupole splitting of various iron oxides. The red empty circles correspond to the values for NaxFe0.5Ni0.5O2. Figure 3. (a) Potential profiles as a function of x in NaxTi0.5Ni0.5O2 and NaxFe0.5Ni0.5O2. The dx/dV plots are also shown for comparison. (b) Average values of the redox potential for (de)sodiation of NaMO2 ↔ NaxMO2 (x ∼ 0.5).18−26 The potential range where (de)sodiation occurs is also shown as bars. The broken line is an eye guide.

irreversibility. Figure 3a shows the charge/discharge curves at a constant specific current of 20 mA/g, where the desodiation limit was set to Na0.5MO2. Of particular interest is that the average redox potential of NaxTi0.5Ni0.5O2 (3.11 V vs Na+/Na), nominally of Ni3+/Ni2+, is higher than that of NaxFe0.5Ni0.5O2 (2.99 V vs Na+/Na), nominally of Ni4+/Ni3+. It appears that the redox potential paradox, i.e., E(Ni3+/Ni2+) > E(Ni4+/Ni3+), occurs. Because both compounds undergo the same phase transformation from O3 to P3 upon desodiation, as evidenced by the ex situ X-ray diffraction patterns (Figure S2, Supporting Information), this redox potential paradox is not caused by the difference in the structural change. To determine the general trend in the redox potential of O3NaMO2, we plotted the average values of the redox potential for (de)sodiation of O3-NaMO2 ↔ NaxMO2 (x ≈ 0.5) in Figure 3b.18−26 In general, it can be expected that a later transition metal oxide exhibits a higher redox potential (i.e., Ti < V < Cr < Mn < Fe < Co < Ni). As shown in Figure 3b, this is true only for the early transition metals (Ti, V, and Cr). Another basic assumption is that a higher-valence transition metal oxide exhibits a higher redox potential (i.e., M3+/M2+ < M4+/M3+). However, this is not the case for the late transition metals: the nominal redox couple of Ni3+/Ni2+ (NaxTi0.5Ni0.5O2 and NaxMn0.5Ni0.5O2) exhibits a redox potential comparable to that of Ni4+/Ni3+ (NaxFe0.5Ni0.5O2 and NaxNiO2). To explain this contradiction, we hypothesize a large contribution from oxygen orbitals upon cycling NaxFe0.5Ni0.5O2 and NaxNiO2.

To test this hypothesis, we conducted ex situ X-ray absorption spectroscopy (XAS) for NaTi0.5Ni0.5O2 and NaFe0.5Ni0.5O2. We chose to examine the transition metal Ledge because this allows the 3d orbitals to be probed through the Laporte-allowed 2p → 3d transition. Furthermore, as demonstrated previously, the transition metal K-edge are highly sensitive to the changes in ligand-to-metal charge transfer (LMCT) induced by the neighboring atom oxidation/ reduction. For example, in NaxFe0.3Ni0.7O2, even when the oxidation state of Fe monitored by the Mössbauer spectroscopy does not change, the Fe K-edge shifts largely due to the oxidation of the neighboring Ni.22 Therefore, the L-edge spectroscopy was employed to monitor the 3d electronic structure more precisely in this study. The Ni L3-edge XAS spectrum for NaTi0.5Ni0.5O2 (Figure 4a) exhibits a main peak at 852.8 eV with a satellite peak at 854.8 eV, which is typical of high-spin Ni2+.38 On desodiation (sodiation), the intensity of the satellite peak increases (decreases) reversibly, suggesting a reversible change in the electronic state of the [NiO6]n− cluster in NaxTi0.5Ni0.5O2. The electronic state of Ti does not change during desodiation/sodiation because there is only a negligible change in the Ti L-edge XAS spectral shape (Figure S1, Supporting Information). C


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Chemistry of Materials /=

∑ εP(jγ )nP ,jγσ + ∑ Vpd(jγ )(dγσ† Pjγσ + hc) j,γ ,σ

+

j,γ ,σ

∑ [εd(γ ) − Q ∑ (1 − np,mσ ′)]nd ,γσ + ∑ εp,mσ np,mσ m,σ′

γ ,σ

+ U ∑ nd , γ ↑nd , γ ↓ + γ

m,σ

∑

(U ′ − Jδσ , σ ′)nd , γσ nd , γ ′ σ ′

γ>γ′,σ ,σ′

+ /multi

The following defines the terms in the equation: the first term is the O 2p states, the second term is Ni 3d-O 2p hybridization, the third term is the Ni 3d states, the fourth term is the Ni 2p states, the fifth term is the coulomb interaction, the sixth term is the exchange interactions between Ni 3d electrons, and the last term is the multipole part. The XAS spectrum was obtained using Fermi’s golden rule, expressed as I (E ) =

∑ |⟨fi |T |g ⟩|2 δ(E − E f

i

+ Eg )

i

where |g⟩ and |f⟩ are the ground (initial) and final states with energies of Eg and Ef. T denotes the electric dipole operator. Details of the CIFM calculation are described in the Supporting Information. Electronic structure parameters used for the CIFM calculation are listed in Tables S2 and S3. As shown in Figure 4a, the Ni L3-edge XAS spectrum for NaTi0.5Ni0.5O2 is successfully reproduced by the CIFM calculation for [NiO6]10−, where the main electronic configuration is Ni2+O62− (89%, Figure 4b). After desodiation, the spectrum contains a large contribution from [NiO6]9−, suggesting that the cluster underwent oxidation. However, the electronic state of [NiO6]9− (Figure 4b) contains a considerable fraction of the charge-transferred state Ni2+O52−O− (37%) in addition to the ground state Ni 3+ O 6 2− (60%). Thus, NaxTi0.5Ni0.5O2 is within the intermediate regime between the Mott−Hubbard type and the charge-transfer type oxides. Owing to the substantial contribution of compensational oxygen oxidation, Ni is not oxidized to the extent expected on the basis of the total input of the charging current for pristine NaTi0.5Ni0.5O2. The oxidation state of Ni in NaxFe0.5Ni0.5O2 upon cycling deviates more significantly from the nominal value than that in NaTi0.5Ni0.5O2. From the CIFM calculation of the Ni L3-edge XAS for NaFe0.5Ni0.5O2, it is evident that there is a large contribution from [NiO6]8− to the spectrum on desodiation. Note that a small misfit around 853 eV for the desodiated one arises from the surface Ni2+, as detected by the surface-sensitive total-electron-yield mode (Figure S4). More importantly, the electronic state of the [NiO6]8− cluster (Figure 4b) contains the oxygen-oxidized state, Ni3+O52−O− (57%), as the ground state, whereas the fraction of the Ni-oxidized state, Ni4+O62−, is only 17%. Thus, NaxFe0.5Ni0.5O2 is regarded as the negative chargetransfer system similar to NaCuO2.47 Therefore, during desodiation, Ni in NaFe0.5Ni0.5O2 is barely oxidized to Ni4+; rather, oxygen is mainly oxidized. Having inferred that oxygen plays a dominant role in the electrochemical behavior of NaxFe0.5Ni0.5O2, we measured the XAS spectra for the oxygen K-edge (Figure 5a). Because the oxygen K-edge corresponds to the excitation from the oxygen 1s orbital to the unoccupied oxygen 2p orbital, which mixes with the transition metal 3d orbital, the spectra can be used to monitor the hole on the oxygen 2p and transition metal 3d orbitals.48 For reference, the density of states (DOS) was

Figure 4. (a) Ni L3-edge X-ray absorption spectra obtained by the PFY mode for the pristine, desodiated (x = 0.5), and sodiated (x = 1.0) NaxTi0.5Ni0.5O2 and NaxFe0.5Ni0.5O2. Results of the CIFM calculation (red dotted lines) are also shown. The shaded area represents the CIFM-calculated fraction of each cluster. (b) Weight fraction of each electronic configuration in the [NiO6]10−, [NiO6]9−, and [NiO6]8− clusters for simulation of the X-ray absorption spectra of NaxTi0.5Ni0.5O2 and NaxFe0.5Ni0.5O2.

The Ni L3-edge XAS spectrum for NaFe0.5Ni0.5O2 exhibits two peaks at 853.8 and 855.4 eV, similar to the spectrum of low-spin Ni3+.39 The intensity of the peak at 855.4 eV increases on desodiation and decreases on sodiation. Thus, NaxFe0.5Ni0.5O2 also undergoes a reversible change in the electronic state of the [NiO6]n− cluster, whereas the 57Fe Mössbauer and Fe L-edge XAS spectra show little change (Figure 2 and Figure S3, Supporting Information), which indicates that the valence state of Fe does not change during desodiation/sodiation. Indeed, the isomer shift of the Mössbauer spectra for NaxFe0.5Ni0.5O2 ranges from 0.28 to 0.35 mm/s, which is within the range of the previously reported Fe3+ oxides (Figure 2b).22,40−43 The XAS spectra revealed that both compounds exhibit changes in the electronic structure of the [NiO6]n− cluster on charging/discharging. For a more quantitative analysis, we simulated the XAS spectra based on the calculated electronic states of [NiO6]n− clusters (n = 8, 9, and 10). In this calculation (configuration-interaction full-multiplet calculation; CIFM calculation),44−46 we employed the following Hamiltonian for the electronic state: D


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Fermi level in NaFe0.5Ni0.5O2, the contribution from the transition metal 3d orbital to the frontier orbital is considerably smaller than that in NaTi0.5Ni0.5O2. Therefore, the hole generated by desodiation localizes mainly on the oxygen 2p orbital, (over 80% based on the CIFM calculation), verifying the large oxygen orbital contribution to the redox reaction in NaFe0.5Ni0.5O2. In Scheme 2, the electronic structure of NaxM0.5Ni0.5O2 (M = Ti, Fe) is summarized. For NaxTi0.5Ni0.5O2, the oxygen 2p and Scheme 2. Schematic Electronic Structures of NaxM0.5Ni0.5O2 (M = Ti, Fe)a

a Oxygen 2p and Ni eg orbitals hybridize well to delocalize holes over both Ni and oxygen in NaxTi0.5Ni0.5O2. For NaxFe0.5Ni0.5O2, the frontier orbital with a dominant oxygen 2p character does not reflect the Ni4+/Ni3+ character significantly.

Ni eg orbitals hybridize well to delocalize the hole over both Ni and oxygen. In contrast, for NaxFe0.5Ni0.5O2, the frontier orbital with a dominant oxygen 2p character does not reflect the Ni4+/ Ni3+ character; rather, it reflects the oxygen redox reaction with a relatively lower potential, leading to the apparent redox potential paradox observed for NaTi0.5Ni0.5O2 and NaFe0.5Ni0.5O2. Although it has been well-known since the 1980s that the orbitals of later transition metals and heavier chalcogenides tend to hybridize each other to a greater extent and show redox participation by anions,47,49−52 the present result clearly demonstrates the hybridization in identical oxygen sublattice can lead to the redox potential paradox of E(Ni3+/Ni2+) > E(Ni4+/Ni3+). Therefore, the caution should be directed to the intuitive material design based on the simple order of higher voltage generation by higher valence state. Such redox flexibility and versatility seem to be provided by the stable nature of layered structure with high contrast in size of Na and M, encouraging further materials exploration toward better sodium batteries. Finally, we compare the electrode performance of NaTi0.5Ni0.5O2 and NaFe0.5Ni0.5O2. After 10 charge/discharge cycles (30 mA/g, 2.0−3.8 V vs Na+/Na), NaFe0.5Ni0.5O2 retains 91% of the initial discharge capacity (Figure 6a), which is slightly higher than the capacity retention of NaTi0.5Ni0.5O2 (82%). Furthermore, at a high charge/discharge rate of 300 mA/g, NaFe0.5Ni0.5O2 delivers 48% of the capacity at 30 mAh/

Figure 5. (a) Oxygen K-edge X-ray absorption spectra for the pristine (green lines), desodiated (red lines, x = 0.5), and sodiated (blue lines, x = 1.0) NaxTi0.5Ni0.5O2 and NaxFe0.5Ni0.5O2. Desodiation generates the hole, resulting in the increase in the intensity of the peaks A and D, respectively. (b) The density of states (black lines) and the partial density of states (red, green, blue, and emerald lines for O, Fe, Ni, and Ti, respectively) for NaTi0.5Ni0.5O2 and NaFe0.5Ni0.5O2.

calculated for each pristine compound using ab initio calculations (Figure 5b). Desodiation of NaTi 0.5 Ni0.5 O2 generates a new, intense peak (peak A) at 528 eV, which arises through hole generation at the Fermi level. On the basis of the calculated DOS, the partial DOS (pDOS) of the oxygen 2p orbital is almost the same as that of the Ni eg orbital, and the oxygen 2p orbital at the Fermi level exhibits significant orbital hybridization with the Ni eg orbital. Therefore, the hole generated by desodiation delocalizes over both Ni (ca. 60% based on the CIFM calculation) and oxygen (ca. 40% based on the CIFM calculation). In contrast, desodiation of NaFe0.5Ni0.5O2 increases the intensity of peak D in Figure 5a, which corresponds to the unoccupied oxygen 2p orbital hybridized with Ni eg and Fe eg; however, the change in intensity is not as significant as that for NaxTi0.5Ni0.5O2. Because the pDOS of the oxygen 2p orbital is much larger than that of the Fe eg and Ni eg orbitals at the E



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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b04289. Theoretical details, powder X-ray diffraction patterns, Ti L-edge X-ray absorption spectra, ex situ X-ray diffraction patterns, Fe L-edge X-ray absorption spectra, crystallographic data, and parameters of electronic structure (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan; Grant-in-Aid for Specially Promoted Research No. 15H05701 and the “Elemental Strategy Initiative for Catalysis and Batteries” (ESICB). Part of this work was conducted on the basis of the Japan−U.S. cooperation project for research and standardization of Clean Energy Technologies. M.O. was financially supported by the Hattori Hokokai Foundation. The XAS measurements were performed by the joint research in SRRO and ISSP, the University of Tokyo (Proposal Nos. 2013B7460 and 2014A7464).

Figure 6. (a) Cycle stability (30 mA/g, 2.0−3.8 V vs Na+/Na) and (b) high-charge/discharge-rate capability of O3-NaM0.5Ni0.5O2 (M = Ti, Fe).

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g, whereas NaTi0.5Ni0.5O2 retains 32% (Figure 6b). These results suggest that the large oxygen orbital contribution to the redox reaction is not disadvantageous to the performance of layered 3d metal oxide cathode materials. In comparison with the oxygen redox reaction reported in overlithiated Li1+xM1−xO2 such as xLi2MO3·(1−x)LiM′O2 or Li2Ru1−yMyO3,53−61 the oxygen redox reaction in NaxMO2 is more reversible in terms of the Coulombic efficiency and potential hysteresis (Figure 3a). One possible explanation for this reversibility is the stabilization of O(2−δ)− species in the layered NaxMO2 structure.

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CONCLUSIONS We have demonstrated that oxygen is the dominant redox species in the stable electrochemical reaction of NaxFe0.5Ni0.5O2, and the apparent redox potential paradox, E(Ni3+/Ni2+) > E(Ni4+/Ni3+), observed for NaFe0.5Ni0.5O2 and NaTi0.5Ni0.5O2, is explained by the difference in the oxygen orbital contribution, ca. 80% and ca. 40%, respectively, to the redox reaction. More generally, since the hybridization between the oxygen 2p and transition metal 3d orbitals plays a critical role in determining the redox potential of NaMO2, the operating potential is not necessarily determined by the nominal valence change of the transition metals. This work is an important step toward a systematic methodology to design the metal oxides as superior cathodes for sodium-ion batteries. F


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