Tuning the Phase Stability of Sodium Metal Pyrophosphates for

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Tuning the Phase Stability of Sodium Metal Pyrophosphates for Synthesis of High Voltage Cathode Material Heejin Kim, Chan Sun Park, Jang Wook Choi, and Yousung Jung Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b03185 • Publication Date (Web): 30 Aug 2016 Downloaded from http://pubs.acs.org on September 3, 2016

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Tuning the Phase Stability of Sodium Metal Pyrophosphates for Synthesis of High Voltage Cathode Material Heejin Kim,a,‡ Chan Sun Park,b,c,‡ Jang Wook Choi,b,* Yousung Jungb,* a

Suncheon Center, Korea Basic Science Institute, Suncheon 57922, Republic of Korea

b

Graduate School of EEWS, Korea Advanced Institute of Science and Technology, Daejeon 34141, Republic of Korea

c

Materials Development Group, LCR Division, Samsung Electro-Mechanics, Suwon 16674, Republic of Korea

ABSTRACT: Properties of the electrode materials are strongly influenced by their crystal structures, yet there is still a lack of design principles to control the polymorphism, showing multiple structures for a given composition with varying battery performance. Here, the underlying mechanism that governs the phase stability of Na2CoP2O7, which has two polymorphs with different electrochemical properties, and a strategy to control it via transition metal substitution are investigated. It is found that the relative stability between the triclinic and orthorhombic polymorphs of Na2MP2O7 (M = transition metals) is determined by two factors, the ionic size and crystal field stabilization energy. Based on this understanding, a computational strategy is devised for selecting the optimal substituents to produce a desired polymorph, from which the introduction of Ca, Ni or Mn into Na2CoP2O7 is identified to stabilize the preferred triclinic phase that has a higher voltage than the orthorhombic counterpart. This prediction of selective synthesis of a particular polymorph for improved battery performance is successfully verified by experimental syntheses, characterization, and electrochemical measurements. We expect that the current strategy can be generalized for other materials synthesis in which the functionalities of materials are dependent sensitively on the crystal polymorphs.

1. Introduction

for Na4M3(PO4)2P2O7 (M = Fe or Co).9,11 Similarly, Na2MnP2O7 exhibits a higher cell voltage than Na2FeP2O7 as much as 0.8 V just by changing TM elements.10,12 Due to its simplicity in developing the analogous materials with a higher energy density, experimental and theoretical investigations have also been performed for TM alterations in Na2+2xM2-x(SO4)3 and NaMPO4F.13-16

Sodium ion battery (SIB) is one of the promising alternatives to the lithium ion battery (LIB), but it has a drawback in terms of energy density due to the large ionic size (volumetric) and mass (gravimetric).1-5 While the atomic weight of Na is larger than that of Li roughly by a factor of 3, its influence on the cell capacity is marginal since the major portion of the total weight of the cell is determined by the host material itself, so that the theoretical capacities of LIB and SIB differ by less than 10 % for a given host framework. For a stable cell operation, however, the active material of SIBs should have a larger empty space in the host framework than that of LIBs to accommodate larger Na ions and provide their migration channels. This additional penalty in the inert part of the framework largely decreases the capacity of SIBs. Since those size and mass problems are intrinsic, strategies to increase the cell voltage have been applied diversely to obtain a higher net energy density.

As an extension, electrochemical properties of the triclinic Na2CoP2O7, namely the rose form, were reported recently.17 This Co-based cathode shows an increased cell voltage as designed by 1.4 V compared with its Fe counterpart. The Na2CoP2O7 compound, however, has another polymorphic phase, namely the blue form, which is dominantly produced over the rose form in most synthesis conditions due to its thermodynamic stability.17-19 Unfortunately, this blue form exhibits inferior electrochemical properties than the rose form, such that the average operation voltage of the rose and blue phases are 4.3 V and 3.0 V, respectively, vs. Na/Na+.17,20

For phosphates used as SIB cathode, the Fe2+/Fe3+ and V /V4+ redox couples are the most extensively studied ones since they exhibit relatively high yet suitable operation voltages for the stable window ( Mn > Fe > Zn in its decreasing order.

and blue polymorphs (See synthesis details in the Experimental Section above). Figure 4a is the X-ray diffraction (XRD) data of the Na2Co0.5M0.5P2O7 compounds, i.e. x = 0.5, for M = Ca, Ti, V, Cr, Mn, Fe, Ni and Zn. For the M = Ti, V and Cr (black patterns), impurity phases that are neither rose nor blue are dominant as inferred by their favorable oxidation states. In accord with calculation results, the Zn substituted and pure Co compounds crystalize into the blue form (blue patterns), while the Ca, Mn and Ni substituted samples lead to the rose form (red patterns). In the Fe substituted case, which locates in between the rose and blue stabilizing elements in Figure 3, both rose and blue phases are identified (green pattern). Note the black arrow in Figure 4a that the XRD peaks associated with the rose form shift toward a higher angle with decreasing the ionic size of substituents (Mn > Fe > Ni), indicating that the substituted TM and Co atoms are well mixed as a single phase in the host framework. See Table S2 in the SI for the fitted lattice parameters of these compounds. For the M = Ca, Ni and Mn, more detailed composition ranges (0.1 ≤ x ≤ 0.3) were tested for the substitution. In the Ca substituted compounds (Figure 4b), new peaks corresponding to the rose phase are beginning to appear at 10% substitution (x = 0.1) in addition to peaks from the blue phase (blue asterisks). With increasing Ca contents to 20%, only the rose form is identified with a small impurity of sodium phosphates (Na5P3O10 and NaPO3), but the blue form completely disappears. These results suggest that the composition range of 0 < x < 0.2 is two phase region, where the two end members are Na2CoP2O7 (blue) and Na2Co0.8Ca0.2P2O7 (rose).

Figure 4. (a) The powder XRD data of Na2Co0.5M0.5P2O7 for M = Ca, Ti, V, Cr, Mn, Fe, Ni, Zn and Co. Color code of XRD pattern: red (rose phase), blue (blue phase), green (mixture of rose and blue phases), black (mixture of other phases). Blue asterisks indicate the blue phase. (b–d) XRD patterns of Na2Co1-xMxP2O7 (0.1 ≤ x ≤ 0.3) for (b) M = Ca, (c) Ni and (d) Mn. (e) The fraction of blue phase quantified from the XRD data in Figure 4a–4d.

3.4. Synthesis and Characterization of Na2Co1Based on these theoretical understandings and predictions, we synthesized various TM substituted compounds under the fixed synthesis condition to verify the influences of TM elements on the phase stability of rose xMxP2O7.

The Ni and Mn substituted cases exhibit similar behavior, but the “critical composition” that can maximize Co contents with the lowest amount of M under the rose form are different. For the M = Ni shown in Figure 4c, both blue (major) and rose (minor) phases are identified at the composition of x = 0.1. The blue form decreases with increasing the Ni content; and thus, only a marginal extent of blue form is identified at x = 0.2. On the contrary, for 10% Mn substitution (x = 0.1 in Figure 4d), the primary phase is identified as the blue form with a small amount of rose form. At the composition of x = 0.2, i.e. for the Na2Co0.8Mn0.2P2O7, both rose and blue phases are clearly observed with the similar intensity. In overall, the relative intensity of blue phase vs. rose phase is higher in Mn substituted cases than that in the Ni substituted cases at the same composition, indicating that the Ni substitution stabilizes the rose form more significantly than the Mn substitution. From the XRD data presented in Figure 4a–4d, we performed the semi-quantitative analysis using the reference intensity ratio (RIR) of rose and blue phases (Figure 4e), for which the RIR values of 0.36 and 1.28 were used, respectively, as recorded in the inorganic crystal structure database (ICSD). The trend in stabilizing the rose form of Na2Co1-xMxP2O7 is indeed Ca > Ni > Mn > Fe > Zn substitutions, as proposed by the calculation in Figure 3. The cal-

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culated composition of substituents for producing the rose phase was not exactly matched with experiments (e.g. Na2Co0.5Fe0.5P2O7 is a blue phase in calculation but it is a mixture of rose and blue phases in experiment), since the materials synthesis depends on various conditions, such as annealing temperature/time and oxidizing/reducing atmospheres that are not addressed in this work. Nevertheless, qualitative agreements between computation and experiment suggest that the calculation can be successfully used to select proper elements for controlling the phase stability, and thus for synthesizing the specific material with desirable properties.

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kinetics due to a low electronic conductivity is a primary reason for this phenomenon.38 More electrochemical data for the Na2Co0.8Ca0.2P2O7 (rose) and pure Co-based blue phases are presented in the Figure S4 and S5 of SI, in which the rose form is more active than the blue form (70 mAh g-1 vs. 40 mAh g-1). The measured redox potential of the rose phase is in line with other pyrophosphate materials as summarized in Figure 5b. The average cell voltages of Na2MP2O7 (Li2MP2O7) are 3.0 (3.5), 3.8 (4.4), and 4.3 (4.9) V for M = Fe, Mn, and Co, respectively, in reference to each metal anode.10,12,39-41 In this set of measurements, the redox potential gap between Fe and Mn is 0.8–0.9 V and that between Mn and Co is 0.5 V for both Li and Na based compounds. Also, the voltage difference between the Li and Na based pyrophosphates is 0.5–0.6 V regardless of the TM elements; the latter difference is among the high values compared to other compounds (0.17–0.57 V) in the previous work.42 This comprehensive relationship between redox couples of various TM elements in related LIB and SIB systems will be useful for further investigations on the factors that affect the redox potential of electrode materials in general. 4. Conclusion

Figure 5. (a) The CV cycles of Na2Co0.8Ca0.2P2O7 (solid line) and differential capacity curve of Na1.63Co1.13P2O7 (dotted line; from reference 17). (b) The average cell voltages for Li and Na based pyrophosphate cathodes in reference to each metal anode.

3.5. Electrochemical test of Na2Co0.8Ca0.2P2O7. To further investigate the synthesized material, we analyzed electrochemical properties of the Ca substituted compound, Na2Co0.8Ca0.2P2O7, using the cyclic voltammetry (CV) measurements (Figure 5a). Since the Ca2+/Ca3+ couple is typically inactive in the tested voltage range (1.5– 4.65 V), all observed electrochemical signals are expected to arise from the redox reaction of Co alone, enabling transparent characterizations. The oxidation peaks are noticeable in the positive sweep at 4.0 and 4.4 V, while the reduction peak appears at 4.3 V in the negative sweep. These characteristic peak positions are similar to the recently reported Na1.63Co1.13P2O7,17 confirming that the synthesized compound is electrochemically analogous to the pure Co-based rose phase. Nevertheless, due to the inactivity of Ca, its theoretical capacity is lower (78 mAh g-1) than that of the pure Co-based sample by ~18 mAh g-1. The redox peaks gradually diminish during the cycles probably due to the electrolyte decomposition at a high operation voltage. Also, the disappearance of discharge plateau at ~4 V is observed; we expect that the inferior

We investigated the governing factors for the formation of a particular crystal structure for a given chemical composition Na2MP2O7. The relative stability between two polymorphs can be understood and described by two descriptors, the ionic size and crystal field stabilization energy. Since these factors can be easily evaluated in most oxide materials using conventional electronic structure methods, similar stability analysis will be applicable to understand the polymorphism of other materials in general. In an effort to further tune the latter phase stability for desired battery properties, it is shown that the phase stability of Na2Co1-xMxP2O7 binary mixture with a varying degree of substitution by M is satisfactorily approximated by the formation enthalpy estimated with the linear combination of two pure substances to avoid costly calculations at all intermediate compositions. These understandings and calculation results then led to a prediction that the substitution with M = Ca, Ni, or Mn can promote the formation of the triclinic (rose) phase with a desired high voltage, which is otherwise less favored than the orthorhombic (blue) phase with a lower cell voltage. This prediction is finally verified by controlled syntheses, characterization, and electrochemical measurements which showed the expected 4.3 V for the rose polymorph synthesized indeed. We expect that our understandings and demonstrations offer a new design principle for developing new materials, of which functionalities are affected by the crystal structure.

ASSOCIATED CONTENT Supporting Information. Comparison of blue and tet phases, Octahedral site preference energy, Stability analysis for

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NaMPO4, Percentage of oxidation states, Fitted lattice parameters, Galvanostatic data. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * [email protected]; *[email protected]

Author Contributions ‡These authors contributed equally.

ACKNOWLEDGMENT We acknowledge the financial support from the National Research Foundation of Korea Grant funded by the Korean Government (NRF-2014R1A4A1003712, NRF2015R1A2A1A15055539), Korea Basic Science Institute (C36270), and the R&D Convergence Program of the National Research Council of Science & Technology (CAP-15-02-KBSI). J. W. C. acknowledges the financial support of the National Research Foundation of Korea grants (NRF2015R1A2A1A05001737).

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