Polyanionic Solid-Solution Cathodes for Rechargeable Batteries

Article pubs.acs.org/cm

Polyanionic Solid-Solution Cathodes for Rechargeable Batteries Jiechen Lu,† Shin-ichi Nishimura,†,‡ and Atsuo Yamada†,‡,* †

Department of Chemical System Engineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-Ku, Tokyo 113-8656, Japan ‡ Unit of Elements Strategy Initiative for Catalysts & Batteries (ESICB), Kyoto University, Kyoto 615-8510, Japan S Supporting Information *

ABSTRACT: As an efficient rechargeable power source, lithium-ion batteries have dominated the consumer electronics and electric vehicle markets. Their analogues, sodium-ion batteries, are emerging as promising low-cost candidates for grid electricity storage. In the pursuit of better cathode materials for Li/Na-ion batteries, major design strategies have involved cation engineering, as represented by studies on mixed transition metals and/or Li/Na-rich layered oxides. Instead of focusing on the cation, here, we report an effective anion engineering strategy that allowed us to identify a surprisingly wide range of polyanionic complete solid solutions of alluaudite-type NaxFey(PO4)3−z(SO4)z (0 ≤ z ≤ 3). These materials exhibited tunable voltage and superior capacities, depending on the mixing ratio of the two polyanions. The existence of a continuous, wide compositional domain in a mixed-polyanion system will significantly enrich materials design strategies and contribute to the development of new cathode materials for Li/Na-ion batteries.

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INTRODUCTION To realize a sustainable and green society, two key technical issues, electricity generation from renewable energies (e.g., wind, hydro, and solar) and electricity storage, should be resolved. Lithium-ion batteries, which have been used with great success in small-scale consumer electronics, are considered realistic candidates for large-scale electricity storage in electric vehicles and grid electricity storage systems. However, due to the scarcity and uneven geographical distribution of lithium reserves, there is an increasing concern about whether Li-ion batteries will meet explosive market demands in the near future.1−3 Sodium, as the sixth most abundant element in the earth’s crust, has a lower cost and higher availability. Although the switch from Li to Na in batteries will result in slightly lower energy density due to the Na+ ion being heavier and larger than the Li+ ion, and a lower voltage generation by approximately 0.35 V,4 Na-ion batteries might be inexpensive and sustainable supplements for Li-ion batteries, especially in volume/weight-less-dependent applications such as grid electricity storage systems. Over the last two decades, intensive efforts have been devoted to developing higher energy density cathodes for Li/ Na-ion batteries. Cathode materials are mainly categorized into © 2017 American Chemical Society

two groups: layered oxides and polyanionic compounds. For both categories, cation engineering, such as of mixed transition metals5−7 and/or Li/Na-rich phases,8,9 acts as a general strategy to improve energy density. Compared with layered oxides, polyanionic compounds can provide longer cycling life and greater safety due to their stable polyanionic frameworks, characteristics that have helped olivine LiFePO4 secure great success in the market.10 However, the lower capacity arising from heavier and larger polyanionic groups is a disadvantage to their use. Therefore, anion engineering is considered essential for designing polyanionic compounds with improved cathode performance. Complex anions, in combination with F− or OH− ions (e.g., LiFeSO4F,11 LiFePO4OH,12 Na3V2(PO4)2F3,13 and Na1.5VOPO4F0.514), have been widely studied, but their synthesis usually needs peculiar synthesis methods11,12 instead of simple and conventional solid-state reactions, and the presence of additional atoms further sacrifices energy density. In contrast, XO4-based (where X = Si, P, S) polyanion substitution (mixed Received: January 18, 2017 Revised: March 30, 2017 Published: March 30, 2017 3597

DOI: 10.1021/acs.chemmater.7b00226 Chem. Mater. 2017, 29, 3597−3602

Article

Chemistry of Materials

of the Mössbauer spectra were conducted by using the MossWinn software (version 3.0). Electrochemical Characterization. For electrochemical charge− discharge measurements, 2032-type coin cells were assembled and cycled between 2.0 and 4.5 V at a C/50 current rate. The cathode was made of 85 wt % active material, 10 wt % carbon black (Ketjen Black ECP, Lion Corp.), and 5 wt % polytetrafluoroethylene binder. This electrode pellet was pressed on an Al mesh, which served as the current collector. A piece of Na metal and NaPF6 dissolved in propylene carbonate containing 2 vol % fluorinated ethylene carbonate (Kishida Chemical) served as the anode and electrolyte, respectively. A glass fiber filter (GB-1000R, Advantec) was used as a separator. In the galvanostatic intermittent titration protocol, the measurement was performed at the very slow current rate of C/50, with the intermittent charge−discharge mode (relaxation time of 2 h after each 1 h charge− discharge).

polyanions) appears to be synthetically easier and more effective but has been long neglected, with very few examples, such as the sodium superionic conductor NASICON-type LiFe2(SO4)2PO4,15 A4Fe3P2O7(PO4)216 (A = Li or Na) and olivine Li1−xFe1+x(PO4)1−x(SiO4)x (0 ≤ x < 1).17 The former two have restricted mixing ratios of polyanions, and deviating from the ratios causes impurities to form, whereas compositions that are developed using the strict ratios show exclusive thermodynamic stability. In contrast, the latter one shows a complete polyanionic solid-solution range. Nevertheless, if x increases beyond 0.2, the material becomes electrochemically inactive due to the substantial number of Li/Fe antisite defects, which block the one-dimensional lithium diffusion path, thus making this strategy unappealing.17 An alluaudite framework was considered to be suitable for the strategic design of a mixed polyanion system, due to the wide variety of cation/anion species and valence states in both synthetic and mineral compounds (e.g., NaMn(II)Fe(III)2(PO 4 ) 3 , 18 Na 0.84 Ca 0.32 Mn(II)Fe(II,III) 1.74 Al 0.26 (PO 4 ) 3 , 19 K 0. 1 3 Na 3 .8 7 Mg(MoO 4 ) 3 , 2 0 NaCaMn(II)(Mn(II),Mg) 2 (AsO4)3,21 and Na1.4Mn(II)0.3Fe(II,III)3(VO4)322). In addition, the recent discovery of sulfate-based alluaudite Na2.56Fe1.72(SO4)323,24 that has excellent properties as a sodium battery cathode has further broadened the scope of alluaudite compounds with respect to anion variety. Inspired by the compositional variety of accommodating various chemical species and valence states in an alluaudite framework, we decided to adopt a polyanion substitution strategy to explore a new series of polyanionic solid-solution alluaudite-type cathode materials, NaxFey(PO4)3−z(SO4)z (0 ≤ z ≤ 3), that might exhibit enhanced electrochemical properties.

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RESULTS AND DISCUSSION The general formula of alluaudite compounds can be denoted by A(1)A(2)B(1)B(2)2(XO4)3.26 The three-dimensional alluaudite framework consists of chains of B(2)2O10 dimers (two edge-sharing B(2)O6 octahedrons) that are linked by distorted

EXPERIMENTAL SECTION

Synthesis of Polyanionic Solid-Solution Compounds. Phospho-alluaudite Na2Fe3(PO4)3 was synthesized by sintering a mixture of maricite NaFePO4 and berlinite FePO4 (2:1) at 600 °C under a flow of Ar gas. Maricite NaFePO4 was prepared by a conventional solid-state reaction, as follows. Na2CO3 (Wako, 99+%), FeC2O4·2H2O (Kojundo, 99+%), and (NH4)2HPO4 (Wako, 99+%) were ball-milled and heated at 500 °C for 12 h under a flow of Ar gas. Berlinite FePO4 was also prepared by the solid-state reaction of Fe(NO 3)3·9H2O and (NH4)2HPO4 at 500 °C for 12 h under a flow of Ar gas. Sulfo-alluaudite Na2.56Fe1.72(SO4)3 was synthesized by the solidstate reaction of nonstoichiometric amounts of Na2SO4 (Wako, 99%) and FeSO4. The anhydrous FeSO4 precursor was prepared in-house by annealing FeSO4·7H2O (Wako, 99%) under vacuum at 200 °C for 12 h. After ball-milling the precursor mixture for 4 h, the Na2.56Fe1.72(SO4)3 compound was obtained by annealing at 350 °C for 24 h under a flow of Ar gas. Polyanionic solid-solution compounds NaxFey(PO4)3−z(SO4)z (z = 0, 1, 1.5, 2, 3) were prepared by dry ball-milling a mixture of Na2Fe3(PO4)3 and Na2.56Fe1.72(SO4)3 in stainless steel jars filled with an Ar atmosphere, followed by sintering at 350 °C in an Ar atmosphere for 20 h. Materials Characterization. Powder X-ray diffraction (XRD) patterns were measured on a Bruker AXS D8 ADVANCE powder diffractometer with a Co Kα radiation source operating at 35 kV and 40 mA. Synchrotron powder XRD data were obtained at the BL-8B beamline of Photon Factory, High Energy Accelerator Research Organization (KEK), Tsukuba, Japan. Structural parameters were obtained by Rietveld refinement using the software Z-Rietveld (version 1.0.2). All structures were illustrated using the VESTA program.25 Mössbauer spectra were measured on a Topologic System Inc. spectrometer equipped with a 57Co γ-source embedded in a Rh matrix and calibrated with an α-Fe foil standard. Fittings and analyses

Figure 1. XRD patterns of the alluaudite-type materials Na2Fe3(PO4)3 and Na2.56Fe1.72(SO4)3. These materials share an identical crystallographic framework with space group C2/c. Yellow spheres, green tetrahedrons, red tetrahedrons, and gray octahedrons represent Na, PO4, SO4, and FeO6, respectively. The major difference between (a) Na2Fe3(PO4)3 and (b) Na2.56Fe1.72(SO4)3 is the atomic occupancy of each crystallographic site, as summarized in Table 1. 3598


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b, angle β, and volume V gradually increase with SO4 substitution, following Vegard’s law, whereas axis c remains almost invariable. To ensure that the compounds were not apparent solid solutions with nanodomain composites, but that they indeed form a complete solid-solution system, scanning transmission electron microscopy (STEM) energy-dispersive spectroscopy mapping of Na2.37Fe2.15PO4(SO4)2 (z = 2) was performed (Figure 2c). We identified that all elements were homogeneously distributed, thus excluding the possibility that nanodomain composites were formed. Mössbauer spectroscopy was used to check the Fe valence states in the solid-solution compounds. The two end member compounds, the phosphate Na2Fe3(PO4)3 and the sulfate Na2.56Fe1.72(SO4)3, contain 30.4% and 0% Fe(III), respectively. If these two end members form solid-solution phases, theoretical Fe(III) content can be calculated based on the simple mixing rule. As shown in Figure S1, the measured Fe(III) percentages of solid-solution phases matched the expected theoretical values well, indicating that no oxidation or decomposition occurred during the preparation process. To confirm the phase stability of the solid-solution compounds, the compound Na2.37Fe2.15PO4(SO4)2 (z = 2) was subjected to Xray diffraction measurement after 6 months of storage in an Ar atmosphere; the diffraction profile did not show any signals for phase separation to the two end members (Figure S2).

Table 1. Differences in Atomic Occupancy between Phosphate Na2Fe3(PO4)3 and Sulfate Na2.56Fe1.72(SO4)3 A(1)A(2)B(1)B(2)2(XO4)3

NaFe3(PO4)3

Na2.56Fe1.72(SO4)3

A(1) A(2) B(1) B(2) X

Na Na Fe Fe P

Na (partially occupied) Na (partially occupied) Na Fe (partially occupied) S

B(1)O6 octahedrons and XO4 tetrahedrons. Large tunnels formed by A(1) and A(2) sites along the c axis, combined with pathways from the B(1) to A(1) sites, suggest a facile threedimensional diffusion network of Na+ ions.23 As summarized in Figure 1 and Table 1, phosphate-based alluaudite Na2Fe3(PO4)3 and sulfate-based alluaudite Na2.56Fe1.72(SO4)3 share an identical framework with the space group C2/c, with the major difference found in the atomic occupancy of the crystallographic sites B(1), with Fe for phosphate and Na for sulfate. New polyanionic solid-solution compounds NaxFey(PO4)3−z(SO4)z (z = 0, 1, 1.5, 2, 3) were prepared by ball-milling the two end member compounds Na2Fe3(PO4)3 and Na2.56Fe1.72(SO4)3, followed by sintering at 350 °C in an Ar atmosphere. As shown in Figure 2a, regardless of the mixing ratio, all of the X-ray diffraction patterns can be indexed to the space group C2/c, and the continuous peak shifts are distinctive, which indicates that the compounds are solid-solution phases. The variance of lattice parameters (Figure 2b) shows that axes a and

Figure 2. Structural characterization of NaxFey(PO4)3−z(SO4)z. (a) XRD patterns of NaxFey(PO4)3−z(SO4)z (z = 0, 1, 1.5, 2, 3). Dashed black lines indicate the peaks for z = 0, which shift with increasing SO4 substitution. (b) Lattice constant variance during SO4 substitution. (c) STEM-EDS elemental mapping of Na2.37Fe2.15PO4(SO4)2 (z = 2). The data confirm that the synthesized compounds are solid-solution phases with a homogeneous distribution of all elements rather than nanodomain composites. 3599


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Figure 4. Electrochemical performance of NaxFey(PO4)3−z(SO4)z (z = 0, 1, 1.5, 2, 3). (a) Discharging profiles cycled between 2.0 and 4.5 V at current rate C/50. (b) Variance of average discharging voltages. A significant increase can be observed with SO4 substitution. (c) Variance of theoretical and reversible capacities during SO 4 substitution. Colored areas and the values in brackets represent the reversible capacities, where the maximum point can be achieved when z equals 1.5, consistent with the trend of theoretical capacities.

After substituting SO4 into the structure, the voltage profiles become smooth, slopy curves, which suggests single-phase reaction mechanisms. It is worth noting that the average voltage increased markedly from 3.19 (z = 0) to 3.36 (z = 1.5) and, finally, to 3.72 V (z = 3) by SO4 substitution (Figure 4b). The origin for the elevated voltage is the inductive effect of the highly electronegative SO4 polyanion, which has been generally accepted to explain the overall trend of voltage generation in polyanionic compounds.15 The other distinctive feature in our polyanionic solidsolution system is that superior capacity can be acquired after mixing two polyanions in the structure. We calculated that the theoretical capacities of the two end compounds Na2Fe3(PO4)3 and Na2.56Fe1.72(SO4)3 are 107.4 and 104.0 mAh/g, respectively. The capacities of polyanionic solid-solution compounds of formula NaxFey(PO4)3−z(SO4)z are theoretically higher than those of the two end members (Figure 4c). If z = 1.5, the theoretical capacity is increased to 129.7 mAh/g, with an improvement of more than 25% compared to the two end members because the theoretical capacity is calculated based on the poorer species of Na (ion-limited) or Fe (electron-limited), as listed in Table S6. The measured reversible capacities at a current rate of C/50 (Figure 4c) confirmed that z = 1.5 (Na/Fe = 1) delivered a superior reversible capacity (113.2 mAh/g) than the two end members: Na2Fe3(PO4)3 with z = 0 (82.1 mAh/g) or Na2.56Fe1.72(SO4)3 with z = 3 (102.9 mAh/g). The end member Na2Fe3(PO4)3 with z = 0 exhibits poor measured capacity, much lower than its theoretical capacity of 107.4 mAh/g. This may be due to the larger particle size (0.5−2 um) as compared with that (below 200 nm) of the sulfate end member with z = 3, where phosphates tend to crystallize more facilely than sulfates under the identical synthetic conditions. As stated in the Introduction, the focus of our research is to adopt the polyanion substitution strategy to explore new

Figure 3. Atomic occupancy variance depending on PO4/SO4 mixing ratio. (a) Crystallographic sites A(1), A(2), B(1), and B(2) in an alluaudite framework denoted as A(1)A(2)B(1)B(2)2(XO4)3. Gray octahedrons represent B(1)O 6 and B(2)O 6 . For clarity, XO4 tetrahedrons are omitted. (b) Variance of atomic occupancy at crystallographic sites. Na and Fe occupancies at B(1) sites change systematically.

Synchrotron X-ray diffraction patterns of NaxFey(PO4)3−z(SO4)z (z = 0, 1, 1.5, 2, 3) were measured to determine the atomic occupancy of each crystallographic site (Figures S3− S7). The occupancy variance, which depends on the PO4/SO4 mixing ratio, was most notable at the B(1) site (Figure 3). In the pure phosphate Na2Fe3(PO4)3, the B(1) site was fully occupied by Fe. In contrast, in the pure sulfate Na2.56Fe1.72(SO4)3, the B(1) site was fully occupied by Na. Therefore, it is expected that, as the SO4 concentration increases, the occupancy ratio of Fe/Na at the B(1) site will gradually decrease. The refined atomic occupancy variance at the B(1) site matches this tendency well. Structural pictures to visualize the difference in atomic occupancy are shown in Figure S8. The electrode properties of NaxFey(PO4)3−z(SO4)z (z = 0, 1, 1.5, 2, 3) were measured at a C/50 current rate in the voltage range 2.0−4.5 V. Only the discharging profiles are shown in Figure 4a for an overall comparison (charge−discharge profiles measured using a galvanostatic intermittent titration protocol are summarized in Figure S9). The phosphate Na2Fe3(PO4)3 delivered a reversible capacity of approximately 80 mAh/g, and the voltage profile exhibits several distinct voltage plateaus. 3600


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center is thanked for the STEM-EDS measurements. The synchrotron X-ray diffraction experiments were performed at KEK-PF under program No. 2015G684.

cathode materials that might exhibit enhanced electrochemical properties. Our exploration of the PO4−SO4 binary alluaudite system shows that SO4 polyanion substitution into a PO4-based compound increases the average voltage monotonically and modulates the capacity with an up-convex variation trend (maximum at z = 1.5). To evaluate the electrochemical performance optimization, theoretical energy densities (experimental average voltage × theoretical capacity) of two end members and the intermediate compound z = 1.5 were compared with experimental energy density (experimental average voltage × obtained capacity) in Table S7. In respect of theoretical energy density, the intermediate compound z = 1.5 is remarkably higher than two end members z = 0 and z = 3 (436.8 versus 342.6 and 386.9 mWh/g). Although the enhancement in the present experimental energy density at z = 1.5 is not remarkable as compared to the theoretical ones, the overall trend clearly shows that polyanionic substitution is an extremely effective strategy to optimize the electrochemical performance of cathode materials.

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(1) Kubota, K.; Komaba, S. ReviewPractical Issues and Future Perspective for Na-Ion Batteries. J. Electrochem. Soc. 2015, 162, A2538−A2550. (2) Choi, J. W.; Aurbach, D. Promise and Reality of Post-Lithium-Ion Batteries with High Energy Densities. Nat. Rev. Mater. 2016, 1, 16013. (3) Kim, H.; Kim, H.; Ding, Z.; Lee, M. H.; Lim, K.; Yoon, G.; Kang, K. Recent Progress in Electrode Materials for Sodium-Ion Batteries. Adv. Energy Mater. 2016, 6, 1600943. (4) Ong, S. P.; Chevrier, V. L.; Hautier, G.; Jain, A.; Moore, C.; Kim, S.; Ma, X.; Ceder, G. Voltage, Stability and Diffusion Barrier Differences between Sodium-Ion and Lithium-Ion Intercalation Materials. Energy Environ. Sci. 2011, 4, 3680−3688. (5) Ohzuku, T.; Makimura, Y. Layered Lithium Insertion Material of LiCo1/3Ni1/3Mn1/3O2 for Lithium-Ion Batteries. Chem. Lett. 2001, 30, 642−643. (6) Yamada, A.; Takei, Y.; Koizumi, H.; Sonoyama, N.; Kanno, R.; Itoh, K.; Yonemura, M.; Kamiyama, T. Electrochemical, Magnetic, and Structural Investigation of the Lix(MnyFe1‑y)PO4 Olivine Phases. Chem. Mater. 2006, 18, 804−813. (7) Yabuuchi, N.; Kajiyama, M.; Iwatate, J.; Nishikawa, H.; Hitomi, S.; Okuyama, R.; Usui, R.; Yamada, Y.; Komaba, S. P2-type Nax[Fe1/2Mn1/2]O2 Made from Earth-Abundant Elements for Rechargeable Na Batteries. Nat. Mater. 2012, 11, 512−517. (8) Lu, Z.; Beaulieu, L. Y.; Donaberger, R. A.; Thomas, C. L.; Dahn, J. R. Synthesis, Structure, and Electrochemical Behavior of Li[NixLi1/3−2x/3Mn2/3−x/3]O2. J. Electrochem. Soc. 2002, 149, A778− A791. (9) Mortemard de Boisse, B.; Liu, G.; Ma, J.; Nishimura, S.; Chung, S.-C.; Kiuchi, H.; Harada, Y.; Kikkawa, J.; Kobayashi, Y.; Okubo, M.; Yamada, A. Intermediate Honeycomb Ordering to Trigger Oxygen Redox Chemistry in Layered Battery Electrode. Nat. Commun. 2016, 7, 11397−11405. (10) Padhi, A. K.; Nanjundaswamy, K. S.; Goodenough, J. B. Phospho-Olivines as Positive-Electrode Materials for Rechargeable Lithium Batteries. J. Electrochem. Soc. 1997, 144, 1188−1194. (11) Recham, N.; Chotard, J. N.; Dupont, L.; Delacourt, C.; Walker, W.; Armand, M.; Tarascon, J. M. A 3.6 V Lithium-Based Fluorosulphate Insertion Positive Electrode for Lithium-Ion Batteries. Nat. Mater. 2010, 9, 68−74. (12) Marx, N.; Croguennec, L.; Carlier, D.; Wattiaux, A.; Cras, F. L.; Suard, E.; Delmas, C. The Structure of Tavorite LiFePO4(OH) from Diffraction and GGA + U Studies and its Preliminary Electrochemical Characterization. Dalton Trans. 2010, 39, 5108−5116. (13) Barker, J.; Gover, R. K. B.; Burns, P.; Bryan, A. J. Hybrid-ion: A Lithium-Ion Cell Based on a Sodium Insertion Material. Electrochem. Solid-State Lett. 2006, 9, A190−A192. (14) Sauvage, F.; Quarez, E.; Tarascon, J. M.; Baudrin, E. Crystal Structure and Electrochemical Properties vs. Na+ of the Sodium Fluorophosphate Na1.5VOPO4F0.5. Solid State Sci. 2006, 8, 1215−1221. (15) Padhi, A. K.; Manivannan, V.; Goodenough, J. B. Tuning the Position of the Redox Couples in Materials with NASICON Structure by Anionic Substitution. J. Electrochem. Soc. 1998, 145, 1518−1520. (16) Kim, H.; Park, I.; Seo, D. H.; Lee, S.; Kim, S. W.; Kwon, W. J.; Park, Y. U.; Kim, C. S.; Jeon, S.; Kang, K. New Iron-Based MixedPolyanion Cathodes for Lithium and Sodium Rechargeable Batteries: Combined First Principles Calculations and Experimental Study. J. Am. Chem. Soc. 2012, 134, 10369−10372. (17) Recham, N.; Casas-Cabanas, M.; Cabana, J.; Grey, C. P.; Jumas, J. C.; Dupont, L.; Armand, M.; Tarascon, J. M. Formation of A Complete Solid Solution between the Triphylite and Fayalite Olivine Structures. Chem. Mater. 2008, 20, 6798−6809. (18) Trad, K.; Carlier, D.; Croguennec, L.; Wattiaux, A.; Ben Amara, M.; Delmas, C. NaMnFe2(PO4)3 Alluaudite Phase: Synthesis,

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CONCLUSIONS Polyanion substitution strategies for developing functional battery electrodes have long been under-appreciated due to a very limited system, with the existence of some exact stable compositional ratios. Here, we identified a PO4−SO4 binary system with the general chemical formula NaxFey(PO4)3−z(SO4)z (0 ≤ z ≤ 3), which provides the first material platform for a polyanionic complete solid-solution system with distinct functions as an intercalation electrode. A continuous voltage shift of over 0.5 V (from 3.19 to 3.72 V) upon compositional change from phosphate (z = 0) to sulfate (z = 3) and extensive capacity modulation, which was maximal at approximately z = 1.5, clearly show that the strategy is effective for optimizing the electrochemical properties of these materials. Materials design with an additional degree of freedom can greatly extend the pool of candidates from which new polyanion cathode materials can be selected.

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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.7b00226. Mössbauer spectra, phase stability, details of the Rietveld refinement results of synchrotron XRD, atomic occupancy variance, and GITT measurements (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jiechen Lu: 0000-0001-7091-1668 Shin-ichi Nishimura: 0000-0001-7464-8692 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT), under the “Element Strategy Initiative for Catalysts and Batteries” (ESICB) project is gratefully acknowledged. The JEOL test 3601


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Polyanionic Solid-Solution Cathodes for Rechargeable Batteries

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