Article pubs.acs.org/cm
Synthesis, Structure, and Na-Ion Migration in Na4NiP2O7F2: A Prospective High Voltage Positive Electrode Material for the Na-Ion Battery Dipan Kundu, Rajesh Tripathi, Guerman Popov, W. R. M. Makahnouk, and Linda F. Nazar* Department of Chemistry, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, Canada N2L3G1 S Supporting Information *
ABSTRACT: In the recent hunt for novel Na-ion cathode hosts, a variety of sodium analogues of classic Li-ion structures have been thoroughly explored. However, Na-ion analogues generally possess modified structures and dissimilar Na-ion energetics compared to their Li-ion analogues due to the large size of Na+ (102 pm) vs Li+ (76 pm), often resulting in sluggish Na+ kinetics. Materials development targeted toward new and different specific host structures possessing optimum properties for Na-ion migration is crucial. Here, we report the first sodium metal fluoropyrophosphate Na-ion host with a three-dimensional frameworkNa4NiP2O7F2which is predicted to have a high voltage (∼5 V) based on its Ni2+/3+/4+ redox couple and composition. Structure solution from single crystal diffraction data combined with atomistic simulation computation suggests the presence of low activation energy Na-ion migration pathways (10−9 S cm−1).47 Therefore, the conductivity obtained from the ac impedance spectroscopy measurement, which exceeds that of electrical conductivity, is expected to be predominantly ionic in nature. This ionic conductivity is quite substantial and can potentially be enhanced by introducing Na ion vacancies or by substitution at the transition metal site. Na+/Li+-Ion Exchange of Na4NiP2O7F2. Ion exchange is an effective way of preparing novel metastable phases that are otherwise inaccessible via traditional synthetic routes. Moreover, successful ion exchange clearly demonstrates the presence of mobile and replaceable ions in the structure. In this work, Na+/Li+ ion exchange was carried out in 1 M LiBr/acetonitrile at 70 °C for 24−36 h. This leads to the expulsion of about 3 Na+ for Li+ per formula unit as revealed by energy dispersive Xray spectroscopy in the SEM (Supporting Information Figures S5 and S6). The exchange of more than two Na+ ions directly indicates the ability to exchange both Na1 and Na2 sodium in E
DOI: 10.1021/cm504058k Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials
Figure 5. (a) Cyclic voltammogram of 0.5 M NaTFSI in P13-TFSI with Toray paper as the working electrode at a scan rate of 1 mV/S against metallic sodium as the counter electrode. (b) Galvanostatic charge−discharge profile of Na4NiP2O7F2 cathode against Na anode at a current density of 5 mA/g with 0.5 M NaTFSI in P13-TFSI as the electrolyte.
phosphate polyanionic compound. Prior to our work, no Ni− pyrophosphate systems have been reported nor computationally studied. In a fluoro-pyrophosphate material, the strong inductive effect exerted by the P−O moieties and fluorine atoms would affect the Ni2+/Ni4+ redox energy,35 thus predictably increasing the average redox voltage to >5 V vs Na. Following the initial charge region between 4.7 and 5.2 V, the profile exhibits a steady slope which most likely represents predominantly electrolyte decomposition catalyzed by the cathode. Furthermore, an insignificant amount of Na+ reinserts back in the subsequent discharge. Clearly, even for an oxidatively stable ionic liquid electrolyte, oxidation at the cathode surface is quite probable at high voltages50,51 and likely aggravated by interfacial parasitic reactions at the conductive carbon interface.52 The electrochemical performance of this material is most likely also limited by kinetic factors, such as poor electrical conductivity and large particle dimensions which can be countered by synthesizing nanocrystals and/or better conductive composite formation. However, realizing the full potential of this compound would require electrochemistry in conjunction with electrochemically and thermally stable solid ionic conductors in an all-solid-state battery, thus eliminating any possibilities of cathode catalyzed liquid electrolyte decompositions.
Na4NiP2O7F2, pointing to their mobile nature in agreement with the computational studies. The morphology of the ion exchanged sample remained similar to that of the pristine Na phase, proving the physically nondestructive nature of the process. Structural investigation of the lithiated phase was performed using XRD and time-of-flight neutron diffraction (ND). Details are presented in the Supporting Information. In both cases, the reflections were indexed to a noncentrosymmetric space group Pna21. Although structure determination of the lithiated phase was not possible, the change in the space group together with the EDX data clearly indicates ion exchange under mild conditions, confirming the presence of mobile Na+ ions with a low activation barrier to migration. This is a prerequisite for both Na-ion battery cathode and solid electrolyte applications. Electrochemical Performance of Na4NiP2O7F2. This material is expected to undergo Ni2+/Ni4+ redox at high potential based on its polyanionic framework as explained above. Its electrochemical performance was therefore evaluated against a Na metal anode using 0.5 M NaTFSI in P13-TFSI ((N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl) imide) as the electrolyte. The ionic liquid (IL) based electrolyte was chosen because ILs are theoretically predicted48 to have much higher anodic stability (>6 V against Li+/Li) by comparison to organic carbonate based electrolytes or even sulfones. A Teflon treated carbon paper was used as the cathode current collector to prevent electrochemical corrosion by TFSI based salts or ionic liquids (see details in the Experimental Section).49 Information on overall electrolyte stability in the absence of a cathode was deduced from cyclic voltammetry measurement in a potential window of 3 and 5.5 V, as shown in Figure 5a. A high anodic stability is observed without any major decomposition current both in the oxidation and in the reduction sweep. The same feature is reproducible over consecutive CV scans, confirming long-term electrochemical stability of the chosen electrolyte. Galvanostatic charge−discharge protocols were used to measure electrochemical sodium (de)intercalation from Na4NiP2O7F2 as shown in Figure 5b. On the basis of an expected 2 e− redox for Ni2+/Ni4+ a theoretical capacity of 150 mA h g−1 is expected. In the first electrochemical charge, however, only about 0.35 Na+ is extracted at an average voltage of ∼5−5.2 V. Such a high average voltage is anticipated from the theoretically calculated voltage of LiNiPO4, namely, 5.2 V vs lithium34 (hence ∼4.9 V vs sodium, accounting for a 0.3 V difference in the redox potential between Na+/Na and Li+/Li). This is the only predicted value for Ni2+/Ni3+ couple in a
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CONCLUSIONS The surge of NIB research has been mainly dominated by exploration of structural chemistries that are analogous to the well-established Li-ion systems. Although this approach is undoubtedly advantageous to speed up materials development, targeted discovery of novel structural chemistries is crucial to tap into the prospects of highly demanding applications such as grid storage or electrified transportation. In this regard, the novel open framework Na4NiP2O7F2 with its low energy Naion conduction pathways, good sodium ionic conductivities, and impressive thermal stability is particularly attractive as a prospective rechargeable NIB cathode. Although investigations with ionic liquid based electrolytes are preliminary, we note that electrochemical activity is observed at the highest reported operating voltage ever, inviting further exploration of this compound in an all solid state NIB. This work is expected to provide the impetus for the discovery of a family of new sodium metal fluoropyrophosphate compounds as novel high voltage cathode host materials for NIBs. F
DOI: 10.1021/cm504058k Chem. Mater. XXXX, XXX, XXX−XXX
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Chemistry of Materials
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ASSOCIATED CONTENT
S Supporting Information *
Detailed single crystal data collection, powder diffraction refinement results, thermogravimetric analysis, ac impedance spectroscopy, SEM-EDX, XRD, and neutron diffraction data of the Li-exchanged sample. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS We gratefully acknowledge NSERC for generous funding through its Discovery Grant and Canada Research Chair programs. Dr. Jalil Assoud in the UWaterloo Crystallographic Laboratories is warmly acknowledged for assistance with collection of the single crystal data and structure solution. The neutron diffraction data was collected at the Oak Ridge National Laboratory’s Spallation Neutron Source; research sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.
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