Redox-Driven Spin Transition in a Layered Battery Cathode Material

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Redox-Driven Spin Transition in a Layered Battery Cathode Material Eriko Watanabe, Wenwen Zhao, Akira Sugahara, Benoit Mortemard de Boisse, Laura Lander, Daisuke Asakura, Yohei Okamoto, Takashi Mizokawa, Masashi Okubo, and Atsuo Yamada Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b04775 • Publication Date (Web): 04 Mar 2019 Downloaded from http://pubs.acs.org on March 5, 2019

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Chemistry of Materials

Redox-Driven Spin Transition in a Layered Battery Cathode Material Eriko Watanabe,1† Wenwen Zhao,1† Akira Sugahara,1 Benoit Mortemard de Boisse,1 Laura Lander,1 Daisuke Asakura,2 Yohei Okamoto,3 Takashi Mizokawa3 Masashi Okubo,1,4 and Atsuo Yamada*1,4 1Department

of Chemical System Engineering, School of Engineering, The University of Tokyo, Hongo 7-3-1, Bunkyoku, Tokyo 113-8656, Japan 2National Institute of Advanced Industrial Science and Technology, Umezono 1-1-1, Tsukuba, Ibaraki 305-8568, Japan 3Department of Applied Physics, School of Advanced Science and Engineering, Waseda University, Okubo 3-4-1, Shinjuku-ku, Tokyo 169-8555, Japan 4Elements Strategy Initiative for Catalysts & Batteries (ESICB), Kyoto University, Nishikyo-ku, Kyoto 615-8245, Japan *Correspondence to: [email protected] †These authors contributed equally.

ABSTRACT: A spin transition between high-spin (HS) and low-spin (LS) states in a solid can occur when the energies of two spin configurations intersect, which is usually caused by external perturbations such as temperature, pressure, and magnetic fields, with substantial influence to its physical and chemical properties. Here we discover the electrochemical “redox reaction” as a new driving-force to induce reversible HS-LS spin transition. Although reversible solid-state redox reaction has been thoroughly investigated as the fundamental process in battery electrode materials, coupling between redox reactions and spin transitions has not been explored. Using density functional theory calculations, we predicted the existence of redox-driven spin transition occurring exclusively for the Co3+/Co2+ redox couple in layered transition-metal oxides, leading to a colossal potential hysteresis (> 1 V) between the cathodic (LS Co3+ to LS Co2+) and anodic (HS Co2+ to HS Co3+) reactions. The predicted potential hysteresis associated with the spin transition of Co was experimentally verified for NaxTi0.5Co0.5O2 by monitoring the electrochemical potential, local coordination structure, electronic structure, and magnetic moment.

INTRODUCTION Electronic spin plays a crucial role in determining the physical and chemical properties of transition-metal compounds.1-6 In particular, d4−d7 electronic configurations of first-row transition-metals in an octahedral coordination have either high spin (HS) or low spin (LS) states, governed by the competition between the ligand-field splitting and electron paring energy. When two spin states are thermally accessible, the spin state changes (also known as spin crossover or spin transition) owing to external stimuli such as temperature,7-9 pressure,10-11 light,12-13 and chemical reactions.14-16 Because the alteration of spin states drastically modulates frontier orbitals that are fully responsible for chemical reactivity, coupling phenomena between spin transitions and chemical reactions are of great interest in chemistry. In an electrochemical system, the number of d electrons at a redox center is variable. Therefore, a stable spin state may occasionally be altered after oxidation/reduction, leading to a spin transition. Redox-driven spin transitions are important processes, for example in biochemistry,17 and have also been studied for possible applications for molecular devices.18-20 Theoretically, this phenomenon is described by a square scheme involving two ErCi reactions where Er and Ci represent a reversible electron transfer (Er) followed by an irreversible change (Ci) in the spin configuration state:21-22

LS + e - ⇄ LS ↑ ↓ HS + e - ⇄ HS where, notably, the cathodic and anodic potentials are determined by the LS/LS- and HS/HS- redox couples, respectively. Redox-driven spin transitions (RDSTs) have been reported for transition-metals in liquids and many rely on the redox couples of Co3+/Co2+ and Fe3+/Fe2+ in coordination complexes where a stable spin state changes upon redox reaction such as Co2+(HS) ⇄ Co3+(LS) + e− or Fe2+(HS) ⇄ Fe3+(LS) + e−.23-26 This can be explained by the high LS stability of the d6 configuration in Co3+ and Fe2+. In battery ceramics electrode materials containing transitionmetals M, electrochemical ion (de)intercalation modulates the valence of M, which in turn changes the magnitude of the ligand-field splitting and the number of d electrons.27 In analogy with liquid state transition-metal complexes, it is likely that M in battery electrode materials exhibits a hitherto unreported solid-state electrochemical spin transition, which might considerably influence the operating potential of the electrodes. However, despite considerable interest in the redox behavior of M in battery electrode materials, materials featuring a RDST has not been explored to date. One suitable material for exploring this new concept is the layered transition-metal oxide NaMO2 where MO6 octahedra form a triangular two-dimensional slab and Na ions reside in

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the interlayer space.28 NaMO2 has been investigated as a promising cathode material for sodium-ion batteries with a large theoretical capacity.29-34 Importantly, NaMO2 maintains a layered structure with various Mn+ owing to the large ionic size difference between Na+ and Mn+.35-39 These features make the NaMO2 system suitable for investigating the electrochemical properties of various redox couples M(n+1)+/Mn+ in an identical solid-state matrix. Taking advantage of the remarkable structural tolerance of NaMO2 towards the variation of the electronic structure, we explored the possibility of spin transitions in NaxMO2 during charging/discharging reactions. Using computational calculations assuming various d4−d7 configurations in NaxMO2, we predict Mn+ species that can cause the HS-LS transition in a battery cell and verify these predictions in multiple experiments. METHODS Calculation Details. All the calculations were performed using Vienna Ab-initio simulation package (VASP).40-41 The projector-augmented-wave (PAW) method and a plane-wave basis set with a kinetic energy cutoff of 550 eV were used.42-43 The k-point sampling on (5 × 5 × 2) grid was used. To correct self-interaction error, we applied GGA+U approach presented by Dudarev et al.45 Values of Ueff = 3.9, 4.0, 3.4, 6.0 eV were used for the d electrons of Mn, Fe, Co, and Ni atoms, respectively. These values were determined to reproduce the experimental oxidation enthalpy of several compounds and are suitable for the calculations of metal ions with various valence states.44-45 The effect of the van der Waals interaction was included through the optB88-vdW functional,46-47 which reproduces structural and thermodynamic properties of LixCoO2.48 The calculated redox potentials of several systems agreed well with experiments based on the same transition metals as redox centers (Table S1). The spin states were controlled by fixing the total number of spin multiplet with ferromagnetic ordering. The spin quantum number of transition metal ions having d4, d5, d6 or d7 configurations were set to be S = 2.0, 2.5, 2.0, and 1.5 and S = 0.0/1.0, 0.5, 0.0, and 0.5 for HS and LS configurations, respectively. We note that because the S values of both 0.0 and 1.0 are possible LS states in d4 configurations, we used the energetically more stable one. The redox-driven spin transition of Co3+/Co2+ predicted by our calculations was further confirmed at several U values and HSE06 calculations (Fig. S1).49-51 The crystal structures were visualized by VESTA.52 The initial structures of NaxMO2 (x = 1.0, 0.5) were built based on Rietveld refinements on O3-type NaTi0.5Co0.5O2. (Fig. S2 and Table S2). Because of the partial occupations of Ti and Co atoms at 3b site, we first determined the stable cation ordering of the sodiated phase. We used a supercell and the stable cation ordering was determined by calculating the total energies of probable orderings found by supercell program.53 The stable cation ordering was found to be zig-zag type [Fig. S3(a)]. For the desodiated phase, the same zigzag-type ordering was used. The positions of Na-ions were determined so as to minimize the Coulomb repulsion between Na+ and Ti4+ atoms [Fig. S3(b)]. The initial structures of other compounds were built simply by replacing Co and Ti atoms and then optimizing their lattice parameters and atomic positions. The detailed mechanism of RDST was investigated as follows. First, we performed the geometry optimizations listed in Table S3. Then, intermediate structures along the reaction

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coordinates were obtained by the linear interpolation. The free energy per formula unit (G’) of each structure was obtained via G(Na1.0Ti0.5Co0.5O2 :Co2+ compounds) = E(Na1.0Ti0.5Co0.5O2)/nM G(Na0.5Ti0.5Co0.5O2 :Co3+ compounds) = (E(Na0.5Ti0.5Co0.5O2) + 1/2𝜇Na + + 𝑒 ― )/nM where E is the total energy of the system and nM is the number of Co ions per formula unit (=0.5). Here we ignore the contributions of entropic term and zero-point energy because their changes are negligible in the solid-state reactions. The 𝜇Na + + 𝑒 ― is the chemical potential of Na+ and e-, 𝜇Na + + 𝑒 ― = E(Na) - eU (vs. Na/Na+) where U is the electrode potential. Then relative free energy (ΔG) was evaluated with respect to G of optimized Na1.0Ti0.5Co0.5O2 (Co2+ HS). Materials synthesis. NaTi0.5Co0.5O2 and NaTi0.5Ni0.5O2 were synthesized by a conventional solid-state reaction. Typically, for the synthesis of NaTi0.5Co0.5O2, stoichiometric amounts of Na2CO3, TiO2 and Co3O4 were thoroughly hand-milled in an Ar filled glove box. The mixture was pressed into pellet under a pressure of 10 MPa, and the resulting pellet was transferred to furnace and sintered at 900 for 12 h under Ar. After cooling to room temperature, the obtained reddish sample was transferred into glove box and ground thoroughly into fine powder. To avoid air exposure, all samples were stored in an Ar filled glove box. Materials characterization. Powder X-ray diffraction patterns were recorded between 10 and 80° (0.021° steps) on a D8 Advance (Bruker-AXS) powder diffractometer with Co Kα radiation. Rietveld refinements were performed using RietanFP software. The magnetic susceptibility was measured by a MPMS-5S SQUID magnetometer (Quantum Design) in the temperature range 2–300 K under an applied field of 0.1 T. The obtained magnetic susceptibility was corrected for the background and the core diamagnetism estimated from Pascal’s constants and the Pauli paramagnetic contribution from the acetylene black. Electrochemistry. Electrochemical measurements were performed in CR2032-type coin cells. The working electrode was a mixture of active material (80 wt%),10 wt% of acetylene black (AB), and 10 wt% of polytetrafluoroethylene (PTFE). 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 with a volume ratio of 1:1. The electrochemical properties were characterized by galvanostatic charge-discharge measurements. The galvanostatic charge-discharge measurements were performed between 3.8−0.5 V vs. Na/Na+ at a specific current of 5 mA/g. For an ex-situ measurement, a cell charged/discharged to a desired state-of-charge was disassembled in an Ar-filled glove box to collect an electrode. The electrode was then washed by dimethyl carbonate (DMC) and dried under vacuum before further investigation. X-ray absorption spectroscopy (XAS). XAS was performed at the Beamlines BL-7A (for the Ti and Co L-edges) and BL-12C (for the Co K-edge) of the Photon Factory, KEK, Japan. The samples were sealed in a water-resistant polymer film in an Ar filled glove box. For the soft X-ray measurement at BL-7A, sample holders were transferred from an Ar-filled glove box to a vacuum chamber without air exposure. Surface-sensitive total-electron yield (TEY) mode was used for the Ti and Co L2,3edge XAS. The energy resolution of the incident beam was


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Chemistry of Materials approximately 100 meV. All XAS experiments were performed at room temperature. The Ti and Co L-edge XAS spectra were simulated by configuration interaction calculation. In the configuration interaction calculations, the ground state was determined by a linear combination of dn and dn+1L configurations, where L denotes an O 2p hole. The energy difference between dn and dn+1L corresponds to the charge-transfer energy Δ. The final states are given by linear combinations of the cdn+1 and cdn+2L configurations (c denotes a Co 2p hole). The energy difference between cdn+1 and cdn+2L is Δ−Q+U where U and Q represent multiplet-averaged Co 3d−3d and Co 2p−3d Coulomb interactions, respectively. The off-diagonal Coulomb interaction terms between the Co 3d electrons are expressed by the Racah parameters B and C, which are fixed at 0.12 and 0.48 eV. The off-diagonal Coulomb interaction terms between the Co 2p core hole and the Co 3d electron are given by the Slater integrals F2(2p,3d) and G1(2p,3d), which were fixed to 80% of the atomic Hartree–Fock values. The off-diagonal hybridization terms between dn and dn+1L and between cdn+1 and cdn+2L were described by pdσ and pdπ with a fixed pdσ/pdπ ratio of −2.16. We used 10Dq = 1.5 eV, Δ = 4.0 eV, and pdσ/pdπ = −1.6 for the simulation of the Ti L-edge XAS spectra.

Na/Na+ for HS + e− ⇄ HS− and 2.1 V vs. Na/Na+ for LS + e− ⇄ LS−, which suggest that NaxTi0.5Co0.5O2 undergoes electrochemical reactions according to the square scheme shown below: Co3 + (LS) + e ↑ Co3 + (HS) + e -

calc. 2.1 V

calc. 3.1 V

Co2 + (LS) ↓ Co2 + (HS)

On the basis of this predicted scheme, Na+ deintercalation should occur at 3.1 V vs. Na/Na+ and Na+ intercalation should occur at 2.1 V vs. Na/Na+. This redox-driven spin transition character is exclusively observed for Co3+/Co2+ in NaxTi0.5Co0.5O2.

RESULTS AND DISCUSSION Theoretical prediction of redox-driven spin transition in NaxMO2. We conducted systematic density functional theory (DFT) calculations for O3-type NaMO2 and P3-type Na0.5MO2 to determine the spin state and predict the redox potential. Here, O3 and P3 denote the coordination environment of Na and the oxygen stacking sequence (Fig. 1a).54 The O3-type NaMO2 is known to transform into a P3-type structure after desodiation.55-57 All Mn+ (n = 2, 3, and 4) that potentially have two spin states (d4−d7) were considered with the chemical compositions of M = Ti4+0.5Mʹ0.5 (Mʹ = Mn, Fe and Co) and Al3+0.5Mʹ0.5 (Mʹ = Fe, Co and Ni) (Fig. 1a, Fig. S3, and Table S4). The former uses Mʹ3+/Mʹ2+ and the latter uses Mʹ4+/Mʹ3+ upon charging/discharging. Fig. 1b shows the stable spin state of each Mʹn+. Focusing on a valence change (e.g., Mn2+ vs. Mn3+), the increase in the number of d electrons tends to make the HS state more stable relative to the LS state. As for isoelectronic M (e.g., Mn2+ vs. Fe3+ vs. Co4+), the HS state is also prone to be stabilized with decreasing the atomic number. We attribute these trends of HS state stabilization to the decrease in the splitting energy that arises from the increase in the ionic size of M. In general, many redox reactions proceed while maintaining their spin configuration; for example, we predicted that NaxTi0.5Mn0.5O2 exhibits Mn3+(HS)/Mn2+(HS) and 4+ NaxAl0.5Ni0.5O2 exhibits Ni (LS)/Ni3+(LS). However, Co3+/Co2+ presents a unique case; the predicted stable spin state of Co3+ in Na0.5Ti0.5Co0.5O2 is LS whereas that of Co2+ in NaTi0.5Co0.5O2 is HS. From this result, we predict that the stable spin-state transformation of Co occurs upon charging/discharging of NaxTi0.5Co0.5O2, which potentially leads to a redox-driven spin transition. The calculated redox potentials of various Mʹ(n+1)+/Mʹn+ are shown in Fig. 1c. In the case of Co3+/Co2+ and Fe4+/Fe3+, we calculated two redox potentials for HS + e− ⇄ HS− and LS + e− ⇄ LS−. Note that the energies of the HS and LS states in Fe4+ (Fig. 1b) lie within a range of approximately 50 meV (as indicated by the pale grey color). Hence, we cannot clearly identify a stable spin state for the Fe4+/Fe3+ redox reaction. The calculated redox potentials of Co3+/Co2+ in NaxTi0.5Co0.5O2 are 3.1 V vs.

Figure 1. Theoretical prediction of redox-coupled spin transition in NaxMO2. (a) Schematic illustration of NaxMO2 upon Na+ (de)intercalation. (b) Stable spin states of Mʹn+ in NaxTi0.5Mʹ0.5O2 and NaxAl0.5Mʹ0.5O2. We considered all transition metals that potentially have two spin states (d4−d7) during charge/discharge. The color scale represents the energy difference between the HS and LS, defined as ΔE = {E(LS) ― E(HS)}/nM where 𝐸 represents the total energy and 𝑛𝑀 represents the number of Mʹ in a unit cell. (c) Calculated redox potentials of Mʹ(n+1)+/Mʹn+ in NaxTi0.5Mʹ0.5O2 and NaxAl0.5Mʹ0.5O2. Two redox potentials of LS ⇄ LS and HS ⇄ HS are plotted for Co3+/Co2+ and Fe4+/Fe3+ (see text). The redox potential of HS ⇄ LS was not considered because such a redox reaction is spin-forbidden.



Experimental verification. Having theoretically predicted the redox-driven spin transition for Co3+/Co2+, we verified our findings experimentally. Generally, NaxMO2 exhibits reversible electrochemical Na-ion (de)intercalation with a small polarization owing to high Na-ion diffusivity and high electron mobility. 58 For example, NaTi0.5Ni0.5O2 shows reversible charge/discharge curves over a narrow voltage region (dotted lines in Fig. 2a).59 Conversely, the charge/discharge curves for NaxTi0.5Co0.5O2 show a large voltage hysteresis of approximately 2.8 V for the Na+ (de)intercalation (Fig. 2a). This phenomenon is not caused by sluggish conversion/phase segregation because ex situ X-ray diffraction experiments confirmed a commonly-observed smooth reversible transformation between the O3 and P3 phases upon Na-ion (de)intercalation (Fig. 2b and Fig. S4). Indeed, a similar potential hysteresis for NaxTi0.5Co0.5O2 was also reported very recently by S .Maletti et al.60 Therefore, we attribute this large voltage hysteresis to specific energetics of the electron configuration.

interaction (CI) calculation of Ti4+ in octahedral TiO6. Clearly, Ti is not redox active during charging/discharging NaxTi0.5Co0.5O2. We further confirmed the redox inactivity of Ti by X-ray photoelectron spectroscopy measurements of the Ti 2p region (Fig. S6). Conversely, the Co L-edge spectra for NaxTi0.5Co0.5O2 showed notable and reversible changes upon charge/discharge (Fig. 3a). The spectra for NaTi0.5Co0.5O2 and Na0.5Ti0.5Co0.5O2 were well reproduced by CI calculations of Co2+ HS and Co3+ LS in octahedral CoO6, respectively. The change in electronic structure is also verified by the projected density of states (pDOS) calculations (Fig. S7). Clearly, Co ions take part in the redox reaction whereas Ti ions are inactive in all structures. Therefore, as predicted by our DFT calculations (Fig. 1b), the stable spin state of Co reversibly changed during the charging/discharging of NaxTi0.5Co0.5O2.

Figure 2. Electrochemical properties of NaxTi0.5Co0.5O2. (a) Galvanostatic charge/discharge potential profile at 5 mA/g in 1 M NaPF6/EC-DMC. The charge/discharge curves for NaxTi0.5Ni0.5O2, which exhibit Ni3+/Ni2+ are also plotted as dotted lines for comparison. (b) Ex situ X-ray diffraction patterns for NaxTi0.5Co0.5O2. To clarify the change in the electronic-structure of NaxTi0.5Co0.5O2 upon desodiation (charging) and subsequent sodiation (discharging) processes, we measured L-edge X-ray absorption spectroscopy of the transition metals M (Fig. S5 and Fig. 3a). No significant change of the Ti L-edge spectra (Fig. S5) was observed upon charge/discharge. The spectral shapes for all samples were well reproduced by the configuration

Figure 3 Electronic, structural, and magnetic changes of NaxTi0.5Co0.5O2 upon Na+ (de)intercalation. (a) X-ray absorption spectra for Co L-edge (solid lines: experimental results, dotted lines: simulated results). (b) Fourier transform magnitudes weighted by k2 of Co K-edge EXAFS (black dots:


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Chemistry of Materials experimental results, red solid line: simulated results). (c) Temperature dependence of the inverse of the magnetic susceptibility χ.

In general, spin transition from a HS to LS state shortens the metal-ligand distance due to fewer antibonding electrons in eg orbitals, and this change can help to identify alternate spin states.61 Thereby, we measured the Co K-edge extended X-ray absorption fine structure (EXAFS) for NaxTi0.5Co0.5O2 (Fig. 3b) to quantify the local coordination environment of Co. The EXAFS signals, which we analyzed with two nearest neighbor shells for Co−O and Co−Co (Fig. S8), revealed that the Co−O distance decreased from 2.14 Å for NaTi0.5Co0.5O2 to 1.93 Å for Na0.5Ti0.5Co0.5O2 (Fig. 3b). These values are in complete agreement with those from the DFT calculations (2.13 Å for NaTi0.5Co0.5O2 and 1.94 Å for Na0.5Ti0.5Co0.5O2). Furthermore, the decrease of 0.21 Å is consistent with a difference in the ionic radii of Co2+ HS (0.89 Å) and Co3+ LS (0.69 Å),62 confirming that a HS to LS transition of Co occurred during the desodiation process.

equilibrated r(Co-O) for Co2+ HS, Co3+ HS, Co2+ LS and Co3+ LS are 2.13, 2.04, 2.11, and 1.94 Å, respectively. The reactioncoordinate separation in r(Co-O) between Co3+ LS and Co2+ LS is 0.17 Å, which is almost twice as large as that for Co2+ HS and Co3+ HS (0.09 Å). This significant separation in reaction coordinate causes a large vertical free-energy difference between Co3+ LS and Co2+ LS at the equilibrium potential of 2.1 V vs. Na/Na+ in Fig. 4b, leading to a large cathodic overpotential during the discharging process. From the electronic point of view, the transition from Co3+ LS to Co2+ LS is an electron injection process into anti-bonding Co eg orbital, which is at the origin of the large bond-length change.

The magnetic susceptibility χ for NaTi0.5Co0.5O2 and Na0.5Ti0.5Co0.5O2 further confirmed the electronic structures (Fig. 3c). The temperature dependence of χ−1 for NaTi0.5Co0.5O2 was well fitted by the Curie–Weiss law with a Curie constant (C) of 1.47 emu K−1 mol−1 and a Weiss constant (Θ) of 20.3 K, where C is close to the value expected for Co2+ HS (S = 3/2, 1.13 emu K−1 mol−1 under the assumption of g = 2.0). However, the Curie–Weiss fitting for Na0.5Ti0.5Co0.5O2 gave a smaller C value of 0.58 emu K−1 mol−1 and a smaller Θ value of 6.4 K. The decrease of both C and Θ was attributed to the generation of diamagnetic Co3+ LS (S = 0, 0 emu K−1 mol−1) in Na0.5Ti0.5Co0.5O2. After Na+ re-intercalation, both C and Θ almost recovered to their initial values for Co2+ HS, which further supports our proposal of a reversible spin-state transition between Co2+ HS and Co3+ LS in NaxTi0.5Co0.5O2. Mechanism of RDST. We now discuss the detailed mechanism of RDST along the reaction coordinates (Table S3). Fig. 4a and 4b plot the calculated diabatic free energy of each electronic state for charging (Co2+ HS to Co3+ LS) and discharging (Co3+ LS to Co2+ HS) processes, respectively. On the charging process, the spin-multiplicity conservation rule and adiabatic theorem apply to require oxidation from Co2+ HS to Co3+ HS. As shown in the left-hand side of Fig. 4a, the vertical free-energy difference between Co2+ HS and Co3+ HS is 0.5 eV under 3.1 V vs Na/Na+ (equilibrium potential, Ueq, charge, between Co3+ HS and Co2+ HS), which gives rise to the anodic overpotential of 0.5 V. Once Co3+ HS is generated under the anodic overpotential (3.6 V vs. Na/Na+), it starts to be relaxed toward shorter r(Co-O) (Fig. 4a, the right-hand side). At r(Co-O) = 2.04 Å, Co3+ HS transforms into Co3+ LS. This process is a downhill reaction on diabatic potential surfaces. On the discharging process in Fig. 4b, the Co3+ LS is reduced to Co2+ LS under the overpotential of 1.1 V (1.0 V vs. Na/Na+). Once Co2+ LS is produced, the remaining diabatic process is the downhill reaction. These predicted overpotentials of 0.5 V and 1.1 V for the charging and discharging processes reasonably explain the difference between DFT and experimental reaction voltages, i.e. 3.5 V (exp.) vs. 3.1 V (DFT) (charging) and 0.7 V(exp.) and 2.1 V(DFT) (discharging), respectively. We attribute the origin of the specifically larger overpotential for the discharging process to the short r(Co-O) of Co3+ LS. The

Figure 4 Reaction coordinates for (a) charging process under calculated equilibrium potential between Co3+ HS and Co2+ HS (i.e. Ueq, charge (=3.1 V vs. Na/Na+)) and under anodic overpotential (3.6 V vs. Na/Na+), and (b) discharging process under calculated equilibrium potential between Co3+ LS and Co2+ LS (i.e. Ueq, discharge (= 2.1 vs. Na/Na+)) and under cathodic overpotential (1.0 V vs. Na/Na+). The circle symbols represent the DFT results and solid colored lines are interpolated quadratic functions, assuming the harmonic approximation. The proposed reaction paths for RDST are shown in black arrows. Initial and final states are highlighted in yellow circles. The left panels represent the diabatic free energies at the potential corresponding to equilibrium, while the right ones consider the overpotential. Thus, all the experimental observations support the occurrence of the RDST of NaxTi0.5Co0.5O2 (Fig. 5), as predicted by the DFT calculations. Upon charging, under the constraint that direct oxidation of Co2+ HS (S = 3/2) to Co3+ LS (S = 0) is spin-forbidden due to the spin-multiplicity conservation rule,



Co2+ HS (S = 3/2) is first oxidized to unstable Co3+ HS (S = 2) at approximately 3.5 V vs. Na/Na+, which immediately undergoes a spin transition to the ground state Co3+ LS (S = 0). Chronopotentiometric analysis based on a linear diffusion model gives the rate constant (k) of 9.0×10−4 s−1 for Co3+ HS → Co3+ LS during charging NaxTi0.5Co0.5O2 (Fig. S9), suggesting the slow kinetics of spin transitions in solids (ca. 105−107 s−1 in solution).63 Upon discharge, direct reduction of Co3+ LS (S = 0) to Co2+ HS (S = 3/2) is again spin-forbidden. Therefore, Co3+ LS (S = 0) is first reduced to unstable Co2+ LS (S = 1/2) at approximately 0.7 V vs. Na/Na+, which immediately relaxes to the ground state Co2+ HS (S = 3/2). The observed cathodic potential is lower than the calculated value (2.1 V vs. Na/Na+), which, we suggest, is caused by the overpotential during Co3+(LS) → Co2+(LS). However, in principle, Co3+ HS/Co2+ HS and Co3+ LS/Co2+ LS dominate the peculiar large voltage hysteresis during charge and discharge processes.

ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Stable spin states of NaxTi0.5Co0.5O2 as a function of U value; Powder X-ray diffraction pattern for O3-type NaTi0.5Co0.5O2; Optimized structures of NaxTi0.5Co0.5O2; Ex situ X-ray diffraction patterns for NaxTi0.5Co0.5O2 upon charging/discharging; Ti Ledge X-ray absorption spectra of NaxTi0.5Co0.5O2; Ti 2p X-ray photoelectron spectra for pristine, charged, and discharged NaxTi0.5Co0.5O2; Projected density of states of NaxTi0.5Co0.5O2; Co K-edge k2χ(k) EXAFS signals for pristine, charged, and discharged NaxTi0.5Co0.5O2 (open circles) at room temperature; Potential profile (black dots) for NaxTi0.5Co0.5O2 obtained by GITT; Comparison of redox potentials with experiments; Rietveld refinement results for O3-type NaTi0.5Co0.5O2; Initial and final structures used to obtain the reaction coordinate; List of chemical compositions with their stable spin states (ΔE) and redox potentials

AUTHOR INFORMATION Corresponding Author * Atsuo Yamada. E-mail: [email protected]

Author Contributions M.O. and A.Y. conceived and directed the project. E.W. performed all DFT calculations. W.Z. and B.M.d.B. synthesized NaTi0.5Co0.5O2 and NaTi0.5Ni0.5O2. W.Z., B.M.d.B., and L.L. evaluated the electrochemical properties, and conducted the Xray diffraction experiments. M.O., Y.O., T.M., and D.A. measured and analyzed the X-ray absorption spectra. A.S. measured the magnetic properties. E.W., M.O., W.Z., and A.Y. wrote the manuscript. All authors commented on the manuscript.

Notes Figure 5. Square scheme of NaxTi0.5Co0.5O2. Schematic diagram of electron chemical potential of NaxTi0.5Co0.5O2 phases. Na+ deintercalation induces oxidation (3.5 V vs. Na/Na+) of high spin Co2+ to high spin Co3+ which undergoes a spin transition to low spin Co3+. Upon Na+ intercalation, low spin Co3+ is reduced at 0.7 V vs. Na/Na+ to low spin Co2+, which undergoes a spin transition to high spin Co2+. CONCLUSIONS In summary, we discovered a solid-state redox-driven spinstate transition of Co3+/Co2+ in layered transition metal oxides upon reversible sodium intercalation reaction in an electrochemical cell. Our work points to the considerable influence of the spin-state variance of transition metals with d4−d7 configurations on their solid-state electrochemistry. Further exploration in various host structures might reveal the missing link between the spin transition and electrochemical modulations. In a practical sense, redox-driven spin transitions cause a large voltage hysteresis (> 1 V) upon electrochemical charging/discharging processes and a large loss in energy efficiency. Thus, Co3+/Co2+, and possibly Fe4+/Fe3+, in oxides should be used with care as the redox couples in battery electrodes.

The authors declare no competing financial interest.

ACKNOWLEDGMENT We acknowledge the financial supports from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan; the Grant-in-Aid for Specially Promoted Research No. 15H05701 and “Elemental Strategy Initiative for Catalysts and Batteries (ESICB).” E. W. was financially supported by Grant-inAid for Young Scientists (B). M.O. was financially supported by MEXT, Japan; Grant-in-Aid for challenging Exploratory Research, and the Iketani Science and Technology Foundation. B.M.dB. and L.L. are grateful to the Japan Society for the Promotion of Science (JSPS) for their respective fellowship. The synchrotron X-ray absorption experiments at Photon Factory were performed under the approval of the Photon Factory Program Advisory Committee (Proposal Nos. 2016G031, 2018G082, and 2016G108). A part of X-ray absorption spectra was measured by the joint research in Synchrotron Radiation Research Organization and the Institute for Solid State Physics, the University of Tokyo (Proposal Nos. 2015B7500). The computation in this work was performed at the Supercomputer Center, Institute for Solid State Physics, The University of Tokyo.

REFERENCES 1. Prinz, G. A., Magnetoelectronics. Science 1998, 282 (5394), 1660-1663.


Page 6 of 9

Page 7 of 9 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Chemistry of Materials 2. Kimura, T.; Goto, T.; Shintani, H.; Ishizaka, K.; Arima, T.; Tokura, Y., Magnetic control of ferroelectric polarization. Nature 2003, 426 (6962), 55-58. 3. Murakami, S.; Nagaosa, N.; Zhang, S. C., Spin-Hall insulator. Phys. Rev. Lett. 2004, 93 (15), 156804. 4. Snyder, B. E.; Vanelderen, P.; Bols, M. L.; Hallaert, S. D.; Bottger, L. H.; Ungur, L.; Pierloot, K.; Schoonheydt, R. A.; Sels, B. F.; Solomon, E. I., The active site of low-temperature methane hydroxylation in iron-containing zeolites. Nature 2016, 536 (7616), 317-321. 5. Breeze, J. D.; Salvadori, E.; Sathian, J.; Alford, N. M.; Kay, C. W. M., Continuous-wave room-temperature diamond maser. Nature 2018, 555 (7697), 493-496. 6. Halcrow, M. A., Spin-crossover materials properties and applications. Wiley: Chichester, 2013. 7. Baker, W. A.; Bobonich, H. M., Magnetic Properties of Some High-Spin Complexes of Iron(II). Inorg. Chem. 1964, 3 (8), 1184-1188. 8. Real, J. A.; Andres, E.; Munoz, M. C.; Julve, M.; Granier, T.; Bousseksou, A.; Varret, F., Spin crossover in a catenane supramolecular system. Science 1995, 268 (5208), 265-267. 9. Moritomo, Y.; Akimoto, T.; Takeo, M.; Machida, A.; Nishibori, E.; Takata, M.; Sakata, M.; Ohoyama, K.; Nakamura, A., Metal-insulator transition induced by a spin-state transition inTbBaCo2O5+δ (δ=0.5). Phys. Rev. B 2000, 61 (20), R13325R13328. 10. Takano, M.; Nasu, S.; Abe, T.; Yamamoto, K.; Endo, S.; Takeda, Y.; Goodenough, J. B., Pressure-induced high-spin to lowspin transition in CaFeO3. Phys. Rev. Lett. 1991, 67 (23), 32673270. 11. Kawakami, T.; Tsujimoto, Y.; Kageyama, H.; Chen, X. Q.; Fu, C. L.; Tassel, C.; Kitada, A.; Suto, S.; Hirama, K.; Sekiya, Y.; Makino, Y.; Okada, T.; Yagi, T.; Hayashi, N.; Yoshimura, K.; Nasu, S.; Podloucky, R.; Takano, M., Spin transition in a four-coordinate iron oxide. Nat Chem 2009, 1 (5), 371-376. 12. Decurtins, S.; Gutlich, P.; Kohler, C. P.; Spiering, H.; Hauser, A., Light-Induced Excited Spin State Trapping in a Transition-Metal Complex - the Hexa-1-Propyltetrazole-Iron (Ii) Tetrafluoroborate Spin-Crossover System. Chem. Phys. Lett. 1984, 105 (1), 1-4. 13. Ohkoshi, S.; Imoto, K.; Tsunobuchi, Y.; Takano, S.; Tokoro, H., Light-induced spin-crossover magnet. Nat Chem 2011, 3 (7), 564-569. 14. Halder, G. J.; Kepert, C. J.; Moubaraki, B.; Murray, K. S.; Cashion, J. D., Guest-dependent spin crossover in a nanoporous molecular framework material. Science 2002, 298 (5599), 17621765. 15. Matsumoto, T.; Newton, G. N.; Shiga, T.; Hayami, S.; Matsui, Y.; Okamoto, H.; Kumai, R.; Murakami, Y.; Oshio, H., Programmable spin-state switching in a mixed-valence spincrossover iron grid. Nat Commun 2014, 5, 3865. 16. Simonato, J. P.; Pecaut, J.; Le Pape, L.; Oddou, J. L.; Jeandey, C.; Shang, M.; Scheidt, W. R.; Wojaczynski, J.; Wolowiec, S.; Latos-Grazynski, L.; Marchon, J. C., An integrated approach to the mid-spin state (S = 3/2) in six-coordinate iron(III) chiroporphyrins. Inorg. Chem. 2000, 39 (18), 3978-3987. 17. Sligar, S. G., Coupling of spin, substrate, and redox equilibria in cytochrome P450. Biochemistry 1976, 15 (24), 53995406. 18. Schneider, B.; Demeshko, S.; Dechert, S.; Meyer, F., A double-switching multistable Fe4 grid complex with stepwise spin-crossover and redox transitions. Angew. Chem. Int. Ed. Engl. 2010, 49 (48), 9274-9277.

19. Cowan, M. G.; Olguin, J.; Narayanaswamy, S.; Tallon, J. L.; Brooker, S., Reversible switching of a cobalt complex by thermal, pressure, and electrochemical stimuli: abrupt, complete, hysteretic spin crossover. J. Am. Chem. Soc. 2012, 134 (6), 2892-2894. 20. Kuzu, I.; Krummenacher, I.; Hewitt, I. J.; Lan, Y.; Mereacre, V.; Powell, A. K.; Hofer, P.; Harmer, J.; Breher, F., Syntheses, structures and electronic properties of zwitterionic iron(II) and cobalt(II) complexes featuring ambidentate tris(pyrazolyl)methanide ligands. Chemistry 2009, 15 (17), 43504365. 21. Bard, A. J.; Faulkner, L. R., Electrochemical methods and applications. Wiley-Interscience: New York; London, 2000. 22. Jacq, J., Schema carre: Etablissement et Discussion de L'equation Generale de la Courbe Intensite-Potentiel en Regime Stationnaire et Diffusion Convective. J. Electroanal. Chem. Interfacial Electrochem. 1971, 29 (1), 149-180. 23. Turner, J. W.; Schultz, F. A., Coupled electron-transfer and spin-exchange reactions. Coord. Chem. Rev. 2001, 219, 8197. 24. Lord, R. L.; Schultz, F. A.; Baik, M. H., Spin crossovercoupled electron transfer of [M(tacn)2]3+/2+ complexes (tacn = 1,4,7-triazacyclononane; M = Cr, Mn, Fe, Co, Ni). J. Am. Chem. Soc. 2009, 131 (17), 6189-6197. 25. Harding, P.; Harding, D. J.; Daengngern, R.; Thurakitsaree, T.; Schutte, B. M.; Shaw, M. J.; Tantirungrotechai, Y., Redox coupled-spin crossover in cobalt β-diketonate complexes: Structural, electrochemical and computational studies. Polyhedron 2012, 42 (1), 291-301. 26. Turner, J. W.; Schultz, F. A., Electrochemical activation parameters of coupled electron-transfer and spin-exchange reactions. Experimental studies of [M(Tacn)2]3+/2+ and [Fe(Pzb)2]+/0 redox systems. J. Phys. Chem. B 2002, 106 (8), 20092017. 27. Reed, J.; Ceder, G., Role of electronic structure in the susceptibility of metastable transition-metal oxide structures to transformation. Chem. Rev. 2004, 104 (10), 4513-4533. 28. Delmas, C., Sodium and sodium‐ion batteries: 50 years of research. Advanced Energy Materials 2018, 1703137. 29. Yabuuchi, N.; Kubota, K.; Dahbi, M.; Komaba, S., Research development on sodium-ion batteries. Chem. Rev. 2014, 114 (23), 11636-11682. 30. Yabuuchi, N.; Kajiyama, M.; Iwatate, J.; Nishikawa, H.; Hitomi, S.; Okuyama, R.; Usui, R.; Yamada, Y.; Komaba, S., P2type Nax[Fe1/2Mn1/2]O2 made from earth-abundant elements for rechargeable Na batteries. Nat. Mater. 2012, 11 (6), 512. 31. Yoshida, J.; Guerin, E.; Arnault, M.; Constantin, C.; de Boisse, B. M.; Carlier, D.; Guignard, M.; Delmas, C., New P2Na0.70Mn0.60Ni0.30Co0.10O2 Layered Oxide as Electrode Material for Na-Ion Batteries. J. Electrochem. Soc. 2014, 161 (14), A1987A1991. 32. Sathiya, M.; Hemalatha, K.; Ramesha, K.; Tarascon, J.M.; Prakash, A., Synthesis, structure, and electrochemical properties of the layered sodium insertion cathode material: NaNi1/3Mn1/3Co1/3O2. Chem. Mater. 2012, 24 (10), 1846-1853. 33. Berthelot, R.; Carlier, D.; Delmas, C., Electrochemical investigation of the P2-NaxCoO2 phase diagram. Nat Mater 2011, 10 (1), 74-80. 34. Komaba, S.; Yabuuchi, N.; Nakayama, T.; Ogata, A.; Ishikawa, T.; Nakai, I., Study on the reversible electrode reaction of Na1–xNi0.5Mn0.5O2 for a rechargeable sodium-ion battery. Inorg. Chem. 2012, 51 (11), 6211-6220.



35. Delmas, C.; Braconnier, J.-J.; Fouassier, C.; Hagenmuller, P., Electrochemical intercalation of sodium in NaxCoO2 bronzes. Solid State Ionics 1981, 3, 165-169. 36. Didier, C.; Guignard, M.; Denage, C.; Szajwaj, O.; Ito, S.; Saadoune, I.; Darriet, J.; Delmas, C., Electrochemical NaDeintercalation from NaVO2. Electrochemical and Solid State Letters 2011, 14 (5), A75-A78. 37. Maazaz, A.; Delmas, C.; Hagenmuller, P., A study of the NaxTiO2 system by electrochemical deintercalation. Journal of inclusion phenomena 1983, 1 (1), 45-51. 38. Komaba, S.; Nakayama, T.; Ogata, A.; Shimizu, T.; Takei, C.; Takada, S.; Hokura, A.; Nakai, I., Electrochemically reversible sodium intercalation of layered NaNi0.5Mn0.5O2 and NaCrO2. Ecs Transactions 2009, 16 (42), 43-55. 39. Mendiboure, A.; Delmas, C.; Hagenmuller, P., Electrochemical Intercalation and Deintercalation of NaMnO2 Bronzes. J. Solid State Chem. 1985, 57 (3), 323-331. 40. Kresse, G.; Furthmuller, J., Efficiency of ab-initio total energy calculations for metals and semiconductors using a planewave basis set. Comp. Mater. Sci. 1996, 6 (1), 15-50. 41. Kresse, G.; Furthmuller, J., Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys. Rev. B 1996, 54 (16), 11169-11186. 42. Blochl, P. E., Projector Augmented-Wave Method. Phys. Rev. B 1994, 50 (24), 17953-17979. 43. Kresse, G.; Joubert, D., From ultrasoft pseudopotentials to the projector augmented-wave method. Phys. Rev. B 1999, 59 (3), 1758-1775. 44. Jain, A.; Hautier, G.; Ong, S. P.; Moore, C. J.; Fischer, C. C.; Persson, K. A.; Ceder, G., Formation enthalpies by mixing GGA and GGA+U calculations. Phys. Rev. B 2011, 84 (4), 045115. 45. Wang, L.; Maxisch, T.; Ceder, G., Oxidation energies of transition metal oxides within the GGA+U framework. Phys. Rev. B 2006, 73 (19), 195107. 46. Klimeš, J.; Bowler, D. R.; Michaelides, A., Chemical accuracy for the van der Waals density functional. J. Phys.: Condens. Matter 2010, 22 (2), 022201. 47. Thonhauser, T.; Cooper, V. R.; Li, S.; Puzder, A.; Hyldgaard, P.; Langreth, D. C., Van der Waals density functional: Self-consistent potential and the nature of the van der Waals bond. Phys. Rev. B 2007, 76 (12), 125112. 48. Aykol, M.; Kim, S.; Wolverton, C., Van der Waals Interactions in Layered Lithium Cobalt Oxides. J. Phys. Chem. C 2015, 119 (33), 19053-19058. 49. Heyd, J.; Scuseria, G. E.; Ernzerhof, M., Hybrid functionals based on a screened Coulomb potential. J. Chem. Phys. 2003, 118 (18), 8207-8215. 50. Heyd, J.; Scuseria, G. E.; Ernzerhof, M., Erratum: “Hybrid functionals based on a screened Coulomb potential” [J. Chem. Phys. 118, 8207 (2003)]. The Journal of Chemical Physics 2006, 124 (21). 51. Paier, J.; Marsman, M.; Hummer, K.; Kresse, G.; Gerber, I. C.; Angyan, J. G., Screened hybrid density functionals applied to solids. J. Chem. Phys. 2006, 124 (15), 154709.

52. Momma, K.; Izumi, F., VESTA 3 for three-dimensional visualization of crystal, volumetric and morphology data. J. Appl. Crystallogr. 2011, 44 (6), 1272-1276. 53. Okhotnikov, K.; Charpentier, T.; Cadars, S., Supercell program: a combinatorial structure-generation approach for the local-level modeling of atomic substitutions and partial occupancies in crystals. J Cheminform 2016, 8, 17. 54. Delmas, C.; Fouassier, C.; Hagenmuller, P., Structural Classification and Properties of the Layered Oxides. Physica B & C 1980, 99 (1-4), 81-85. 55. Mortemard de Boisse, B.; Cheng, J. H.; Carlier, D.; Guignard, M.; Pan, C. J.; Bordère, S.; Filimonov, D.; Drathen, C.; Suard, E.; Hwang, B. J.; Wattiaux, A.; Delmas, C., O3– NaxMn1/3Fe2/3O2 as a positive electrode material for Na-ion batteries: structural evolutions and redox mechanisms upon Na+ (de)intercalation. Journal of Materials Chemistry A 2015, 3 (20), 10976-10989. 56. Han, M. H.; Gonzalo, E.; Casas-Cabanas, M.; Rojo, T., Structural evolution and electrochemistry of monoclinic NaNiO2 upon the first cycling process. J. Power Sources 2014, 258, 266271. 57. Didier, C.; Guignard, M.; Suchomel, M. R.; Carlier, D.; Darriet, J.; Delmas, C., Thermally and Electrochemically Driven Topotactical Transformations in Sodium Layered Oxides NaxVO2. Chem. Mater. 2016, 28 (5), 1462-1471. 58. Ong, S. P.; Chevrier, V. L.; Hautier, G.; Jain, A.; Moore, C.; Kim, S.; Ma, X. H.; Ceder, G., Voltage, stability and diffusion barrier differences between sodium-ion and lithium-ion intercalation materials. Energy Environ. Sci. 2011, 4 (9), 36803688. 59. Nanba, Y.; Iwao, T.; de Boisse, B. M.; Zhao, W. W.; Hosono, E.; Asakura, D.; Niwa, H.; Kiuchi, H.; Miyawaki, J.; Harada, Y.; Okubo, M.; Yamada, A., Redox Potential Paradox in NaxMO2 for Sodium-Ion Battery Cathodes. Chem. Mater. 2016, 28 (4), 1058-1065. 60. Maletti, S.; Giebeler, L.; Oswald, S.; Tsirlin, A. A.; Senyshyn, A.; Michaelis, A.; Mikhailova, D., Irreversible Made Reversible: Increasing the Electrochemical Capacity by Understanding the Structural Transformations of NaxCo0.5Ti0.5O2. ACS Appl Mater Interfaces 2018. 61. Bersuker, I. B., Electronic structure and properties of transition metal compounds: introduction to the theory. John Wiley & Sons: 2010. 62. Shannon, R. D., Revised Effective Ionic-Radii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides. Acta Crystallographica Section A 1976, 32 (Sep1), 751-767. 63. Dose, E. V.; Hoselton, M. A.; Sutin, N.; Tweedle, M. F.; Wilson, L. J., Dynamics of intersystem crossing processes in solution for six-coordinate d5, d6, and d7 spin-equilibrium metal complexes of iron (III), iron (II), and cobalt (II). J. Am. Chem. Soc. 1978, 100 (4), 1141-1147.


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Chemistry of Materials

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Redox-Driven Spin Transition in a Layered Battery Cathode Material

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