Physical Chemistry Chemical Physics ! "# $ %& ! " #$ $ % #! & ( ) & & ' "& * + ,*+ & ,0 2 % " & ) 5 + ,"& + & ,0 0 & $ + $ 6 . / !+ %/ % )" 334" . / !+ & $ 3 1 %/ % )" ,. / !+ & $ % & 1!+ 1 % 3 1 & 1! Please do not adjust margins Physical Chemistry Chemical Physics Page 1 of 8 PCCP ARTICLE Mixed Iron-Manganese Based Pyrophosphate Cathode, Na2Fe0.5Mn0.5P2O7, for Rechargeable Sodium Ion Batteries Received 00th January 20xx, Accepted 00th January 20xx DOI: 10.1039/x0xx00000x www.rsc.org/ Rana A. Shakoor,* ,a,ǀǀ b,ǀǀ Chan Sun Park, c, Arsalan A. Raja b , Jaeho Shin, Ramazan Kahraman* ,c The development of secondary batteries based on abundant and cheap elements is vital. Among various alternatives to conventional lithium-ion batteries, sodium-ion batteries (SIBs) are promising due to the abundant resources and low cost of sodium. While there are many challenges associated with SIB system, cathode is an important factor in determining the electrochemical performance of this battery system. Accordingly, ongoing research in the field of SIBs is inclined towards the development of safe, cost effective cathode materials having improved performance. In particular, pyrophosphate cathodes have recently demonstrated decent electrochemical performance and thermal stability. Herein, we report the synthesis, electrochemical properties, and thermal behavior of a novel Na2Fe0.5Mn0.5P2O7 cathode for SIBs. The material was synthesized through a solid state process. The structural analysis reveals that the mixed substitution of manganese and iron has resulted in a triclinic crystal structure (P-1 space group). Galvanostatic charge/discharge measurements indicates that Na2Fe0.5Mn0.5P2O7 is electrochemically active with a reversible capacity of ~80 mAh/g at C/20 rate with an average redox potential of 3.2 V. (vs. Na/Na+). It is noticed that 84% of initial capacity is preserved over 90 cycles showing promising cyclability. It is also noticed that the rate capability of Na2Fe0.5Mn0.5P2O7 is better than Na2MnP2O7. Ex situ and and CV analyses indicate that Na2Fe0.5Mn0.5P2O7 undergoes a single phase reaction rather than a biphasic reaction due to different Na coordination environment and different Na sites occupancy when compared to other pyrophosphate mterials (Na2FeP2O7 and Na2MnP2O7).Thermogravimetric analysis (25-550 ̊C) confirms good thermal stability of Na2Fe0.5Mn0.5P2O7 with only 2% weight loss. Owing to promising electrochemical properties and decent thermal stability, Na2Fe0.5Mn0.5P2O7, can be an attractive cathode for SIBs. Introduction Lithium-ion batteries (LIBs) have succeeded to find a wide range of applications, from portable electronics to electric vehicles[1]. The recent trend of extending battery systems, especially towards electric vehicles and energy storage systems (ESS), has given rise to a serious concern of the cost and abundance of Li resources. Hence, recently sodium-ion batteries (SIBs) have emerged as an attractive replacements for LIBs [2, 3] based on cheap and easily accessible resources of sodium [3, 4]. In addition, much of the knowledge on the electrolyte and electrode phenomena accumulated during research of LIBs can be utilized to that of SIBs [5]. However, lower energy density and inferior kinetics, originating from the large size of Na ion, are serious challenges that must be overcome. Accordingly, a variety of crystal structures for cathode materials have been explored to address these drawbacks [6-8]. Among them, pyrophosphate materials are quite attractive due to their open and stable crystal structure enabling easier movement of sodium ions during the battery operation. There are few reported pyrophosphate cathode materials for sodium-ion batteries [8-12]. Among them, Na2FeP2O7 has shown promising reversible capacity (80 mAh/g) and rate capability [8]. Na2MnP2O7 has also exhibited a + reversible capacity of ~90 mAh/g at 3.8 V (vs. Na/Na ) with 96% capacity retention after 30 cycles at scan rate of C/20 [11]. However, Na2MnP2O7 is suffering from limited rate capability due to the intrinsic lower electronic conductivity of Mn. In addition, it is also reported that Mn-based polyanionic materials demonstrate inferior thermal stability than that of Fe-based polyanionic materials [13, 14]. Improving properties of materials by the addition of transition metals is quite attractive and many reports can be found in the literature [15, 16]. Thus, we designed a mixed transition metal (TM) pyrophosphate material to improve the rate performance and thermal stability of Mn-based pyrophosphate materials. The Na2Fe0.5Mn0.5P2O7 was synthesized via solid state reaction. This work describes the structural, thermal and electrochemical performance of novel Na2Fe0.5Mn0.5P2O7 cathode materials which has not been reported earlier. J. Name., 2013, 00, 1-3 | 1 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins Physical Chemistry Chemical Physics ARTICLE Page 2 of 8 Journal Name Results and discussion Fig. 1 a) A triclinic crystal structure (space group: P-1) b) The XRD profile and whole pattern indexing of Na2Fe0.5Mn0.5P2O7 (Rp = 2 2.88, Rwp = 4.01 χ = 1.77) Inset: SEM image showing the size of 3~5 μm as-synthesized particles c) The XRD profile and whole 2 pattern indexing of Na2MnP2O7 (Rp = 9.38, Rwp = 13.07 χ = 2.63) d) The XRD profile and whole pattern indexing of Na2FeP2O7 (Rp 2 = 4.06, Rwp = 6.00 χ = 1.72) Crystal Structure Sodium pyrophosphate materials adopt different crystal structures depending on the types of transition metals used and synthetic conditions: M=Ni, (triclinic), Zn (tetragonal), Co (both orthorhombic and triclinic) [17-19]. Among them, triclinic structures have open frameworks which facilitate Naion diffusion during battery operation. The Na2MP2O7 (M=Fe, Mn) family is indexed to triclinic framework which is [17] and isostructural with triclinic Na2CoP2O7 Na3.64Ni2.18(P2O7)2 [20]. The triclinic crystal structure can be described by a framework comprising corner-shared M2O11 [MO6-MO6] dimers, sharing one oxygen atom and each MO6 unit being separated by PO4 or P2O7 groups (Figure 1a). In the environment of MO6 sites, one MO6 octahedron (blue) is connected to P2O7 by corner-sharing and the other distorted MO6 (green) octahedron is linked by both corner-sharing and edge-sharing. The Na ions occupy six crystallographic sites (Na1-Na6). The X-ray diffraction (XRD) data for as-synthesized Na2Fe1-xMnxP2O7 (x=0,0.5,1) was indexed to a triclinic structure and the rietveld refined XRD spectra is presented in Figure 1 (b, c and d). The lattice parameters of Na2FeP2O7 and Na2MnP2O7 are also tabulated in Table 1 for a clear comparison. As shown in Figure 2, the values of the lattice parameters of Na2Fe0.5Mn0.5P2O7 lie in between the lattice constants of Na2FeP2O7 and Na2MnP2O7, indicating solid solution formation based on Vegard’s law. In the family of pyrophosphates, It has been reported that Na2FeP2O7 and Na2MnP2O7 have difference in occupancy of Na cations [21, 22]. All Na sites are fully occupied in Na2MnP2O7 while Na2FeP2O7 contains either full or partially occupied Na sites. Therefore, we anticipate that the Na occupancy of partially occupied Na sites may have changed by the mixing of Fe and Mn to form a solid solution of Na2Fe0.5Mn0.5P2O7. As shown in Fig. 1a, there are partially occupied Na4-Na6 sites and fully occupied Na1Na3 sties in the crystal structure of Na2Fe0.5Mn0.5P2O7. To clarify the change in the occupancy of Na sites, we performed galvanostatic measurements for first and second cycles (Figure 3). 2 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins Physical Chemistry Chemical Physics Page 3 of 8 Journal Name ARTICLE Table 1: The lattice parameters of Na2Fe1-xMnxP2O7 (x=0, 0.5, 1) 3 Materials a (Å) b (Å) c (Å) α (°) β (°) γ (°) V (Å ) Na2FeP2O7 6.4310 9.4264 10.992 64.687 79.707 72.989 574.9 Na2Fe0.5Mn0.5P2O7 6.4945 9.4666 11.031 64.941 79.826 72.802 584.6 Na2MnP2O7 6.5484 9.5347 11.068 64.444 79.830 73.155 596.6 In the first and second cycles of Na2FeP2O7, the charge capacities of the first and second cycles shows a difference of approximately 15 mAh/g, which indicates that 0.16 Na ion are unoccupied. On the other hand, the first and second charge capacity difference of Na2Fe0.5Mn0.5P2O7 is only 5 mAh/g (0.07 Na ion per formula unit), confirming increased Na occupancy in Na2Fe0.5Mn0.5P2O7 as compared to the case of Na2FeP2O7. Thus, from the charge-discharge profile, the changed occupancy of Na ions can be clearly observed. Electrochemical Measurements The galvanostatic initial charge/discharge analysis of Na2Fe0.5Mn0.5P2O7 at a scan rate of C/20 is presented in Figure 4a. It can be noticed that Na2Fe0.5Mn0.5P2O7 demonstrates a charge capacity of ~90 mAh/g and a discharge capacity of ~80mAh/g, respectively, in a potential range of 2 ~ 4.5 V (vs. Na/Na+). The 2.5 V and 3.1V plateaus represent the activation of the Fe+2/+3 redox couple while the plateau at 3.8 V confirms the activity of the Mn2+/Mn3+ redox couple. Fig. 3 First and second galvanostatic profile of Na2FeP2O7 and Na2Fe0.5Mn0.5P2O7 at C/20 2+/3+ Fig. 2 The linear variation of lattice parameters in the Na2Fe1xMnxP2O7 (x=0, 0.5, 1) Interestingly, unlike Li2Fe0.5Mn0.5P2O7 [23, 24] the Mn redox couple is active in Na2Fe0.5Mn0.5P2O7.The monoclinic Li2MnP2O7 has edge-shared Mn sites with multiple bonds which experience bond breakage during charging and discharging due to Jahn-teller distortion. On the other hand, triclinic Na2MnP2O7 has corner-shared Mn sites that enables it to accommodate Jahn-teller distortion [11]. In terms of cyclic performance, 84% of the initial capacity (80 mAh/g) is preserved as reversible capacity in 90 cycles, indicating the stability of the framework upon Na insertion/de-insertion. In addition, the coulombic efficiency reaches a satisfactory value of 95% even after 90 cycles (Figure 4b). The rate capability of Na2Fe0.5Mn0.5P2O7 is presented in Figure 4c. It can be seen that when the C-rate is increased ten-fold from 0.05C to 0.5C, the discharge capacity of Na2Fe0.5Mn0.5 P2O7 is dropped from 80mAh/g to 56mAh/g. Whereas Na2MnP2O7 experiences relatively a bigger drop in discharge capacity under the same conditions. The discharge capacity drops from 60 mAh/g to 47mAh/g (Figure 4d). This comparison reveals that the rate capability of Na2Fe0.5Mn0.5P2O7 is better than Na2MnP2O7. The rate capability of Na2Fe0.5Mn0.5P2O7 is dropped with mixing of Mn when compared with Na2FeP2O7 [8] which is in turn associated to the inferior kinetics of Mn. At the same time, it is pertinent to note that the discharge capacity of Na2Fe0.5Mn0.5P2O7 is higher than Na2MnP2O7 at all J. Name., 2013, 00, 1-3 | 3 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins Physical Chemistry Chemical Physics Page 4 of 8 PCCP ARTICLE Fig. 4 (a) Voltage profile of Na2Fe0.5Mn0.5P2O7 (b) Cycle performance of Na2Fe0.5Mn0.5P2O7 Na2Fe0.5Mn0.5P2O7 (d) Rate capability test of Na2MnP2O7 C-scan rates (from 0.05C to 0.5C). In particular, the Na2MnP2O7 shows 45 mAh/g at 0.5C which is lower than Na2Fe0.5Mn0.5P2O7 (60 mAh/g at 0.5C). It can be further noticed, although the rate capacity retention of Na2Fe0.5Mn0.5P2O7 is dropped as compared to Na2FeP2O7, the amount of Na ion’s transfer of Na2Fe0.5Mn0.5P2O7 is higher than Na2MnP2O7 at high C-rate. (0.5 vs. 0.7 Na ion per formula weight). Mn-based cathodes have inherently lower conductivity than Fe-based cathodes, causing relatively limited rate capability [25]. Thus, incorporation of more conductive metals such as Fe has improved the rate capability of Na2MnP2O7. In layered materials for SIB cathodes such as P2-NaxCO2, P2Na2/3Co2/3Mn1/3O2 phase transitions are known to occur depending on the amount of Na ions, which in turn affect the coordination of Na sites [26-28]. To investigate the structural rearrangement by Na occupancy change in Na2Fe0.5Mn0.5P2O7, we performed cyclic voltammetry (CV) measurements at a scan rate of 0.05 mV/s (Figure 5a). In the CV data, broad redox 2+/3+ peaks can be observed at 2.5 V, 3.1 V (Fe ) and 3.8 V 2+/3+ ), indicating a single-phase reaction. To further clarify (Mn the phase reaction, cyclic voltammetry (CV) measurements at a slower scan rate of 0.01mV/s were conducted (Figure 5a). (c) Rate capability test of .Fig. 5 Cyclic voltammetry (CV) data for Na2Fe0.5Mn0.5P2O7, Na2MnP2O7 and Na2FeP2O7 at scan rate of 0.05 mV/s. J. Name., 2013, 00, 1-3 | 4 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins Physical Chemistry Chemical Physics Page 5 of 8 PCCP ARTICLE It has been reported that region at 2.5V defines a single-phase reaction while the region at 3V region represents a two-phase transition [8, 12]. In particular, the three peaks around 3V transition depend on the scan rate conditions. At a slower scan rate, peak separation becomes sharper indicating a two-phase reaction. The CV data of Na2Fe0.5Mn0.5P2O7 shows broad redox peaks in the entire voltage range even at a slower rate suggesting existence of single phase reaction. To confirm the phase behavior, CV measurements of Na2FeP2O7 and Na2MnP2O7 are also compared (Figure 5b, 5c). This comparison indicates that intercalation/de-intercalation of sodium into/from Na2Fe0.5Mn0.5P2O7 takes place through a single phase reaction rather than a biphasic reaction. In Na2MnP2O7 and Na2FeP2O7, a two-phase transition is known to occur at ~3.8V and ~3V, respectively [8, 11]. This two-phase reaction region has changed into a single-phase reaction region in Na2Fe0.5Mn0.5P2O7 due to the modified Na 2+/3+ redox coordination. However, unlike the upshift of Fe potential in Li2Fe1-yMnyP2O7 [23, 24], the redox potential tune ability is not observed in Na2Fe0.5Mn0.5P2O7. The existence of single phase reaction during the intercalation/de-intercalation of sodium into/from the host structure was further confirmed by ex-situ XRD analysis (Figure 6). For more clarity the same ex-situ analysis is also presented in small “2θ” range (Figure 6b). The ex-situ XRD data shows small shifts in “2θ” values for all the crystal planes and confirms the absence of any new phase during extraction/insertion of sodium from the host structure. This can also be clearly visualized in ex-situ XRD data shown in Figure 6b. During the charging process, the extraction of sodium from the host structure (desodiation), peaks are slightly shifted to higher 2θ values depicting decrease in the lattice parameters due to contraction of the lattice planes. However, during discharging process (sodiation), the peaks are shifted towards small 2θ values undergoing lattice expansion without the formation of any new phase(s). These findings confirm the reversible nature of extraction/insertion of sodium into/from the host structure. The ex-situ analysis thus confirms that the extraction/insertion of sodium in Na2Fe0.5Mn0.5P2O7 takes place through a single phase reaction rather than a biphasic reaction. Hence, it can be concluded that a difference in the Na occupancy has a significant role on the structural transformation during charging/discharging and influences the mode of intercalation/de-intercalation of sodium into/from the host structure. Thermal Stability Thermogravimetric analyses (TGA) were conducted for Na2Fe0.5Mn0.5P2O7 and Na2MnP2O7 (Figure 7) to study the thermal stability of the synthesized materials. Upon heating up to 550 oC, TGA data indicates that both as-prepared Na2Fe0.5Mn0.5P2O7 and Na2MnP2O7 show negligible weight loss (~2%). These thermal stability results are comparable to that of as-prepared Na2FeP2O7 [8]. In partially desodiated state (Fig. 7a and 7c), exo/endothermic peaks are barely observable from Fig. 6 (a) Ex-situ XRD pattern during charging/discharging for Na2Fe0.5Mn0.5P2O7 (b) Ex-situ XRD pattern during charging/discharging for NaFe0.5Mn0.5P2O7 J. Name., 2013, 00, 1-3 | 5 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins Physical Chemistry Chemical Physics Page 6 of 8 PCCP ARTICLE Figure 7. TGA/DTA of: a) Na2Fe0.5Mn0.5P2O7 b) desodiated NaFe0.5Mn0.5P2O7 c) Na2MnP2O7 d) desodiated NaMnP2O7. differential thermal analysis (DTA) results indicating absence of any phase transformation in both the materials. However, it is evident that the weight loss of partially desodiated Na2MnP2O7 is higher than partially desodiated NaFe0.5Mn0.5P2O7 (Figure 7b and 7d). In fact, the thermal stability of the charged polyanionic materials is affected by transition metals. In general, the high redox potential materials have poor thermal stability [13, 29]. By considering the Li-based counterparts of charged (LiFeP2O7 and LiMnP2O7), weight loss (delithiated) electrode of Fe-based polyanionic materials tends to be less than Mn-based polyanionic materials in the same temperature range (e.g. LiFeP2O7 (~5%) > Li1.4MnP2O7 (~10%) [13, 30]. Thus, it is important to note that the thermal stability of the desodiated Na2MnP2O7 is improved by mixing with Fe. Conclusions The solid-solution phase Na2Fe1-xMnxP2O7(x=0, 0.5, 1) materials have been synthesized through a solid-state reaction. The synthesized materials adopt a triclinic crystal structure with P1 space group. The novel mixed pyrophosphate material (Na2Fe0.5Mn0.5P2O7) demonstrates a single phase reaction rather than a bi phasic reaction during intercalation/deintercalation of sodium into/from the host structure. The origin of this single phase behavior can be attributed to different Na coordination environment and different Na sites occupancy in the crystal structure. Both Fe+2/+3 and Mn+2/+3 redox couples are electrochemically active in Na2Fe0.5Mn0.5P2O7 in contrast to its counterpart Li2Fe0.5Mn0.5 P2O7. The activation of Mn+2/+3 redox couple in Na2Fe0.5Mn0.5 P2O7 can be presumably regarded as the effect of cornershared Mn sites in the crystal structure that enables it to accommodate Jahn-teller distortion. Na2Fe0.5Mn0.5P2O7 demonstrates decent discharge capacity (80mAh/g at C/20) in the voltage range of 2.0 to 4.5 V with average redox potential approximately 3.2V (vs. Na/Na+), good cyclibity (84% capacity retention over 90 cycles) and promising rate capability (70% J. Name., 2013, 00, 1-3 | 6 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins Physical Chemistry Chemical Physics Page 7 of 8 Journal Name ARTICLE capacity retention from 0.05 C to 0.5C). Finally, it can also be concluded that NaFe0.5Mn0.5P2O7 exhibits good thermal stability both in sodiated (~2% weight loss) and partially desodiated states (~10% weight loss) in the temperature range of 25-550 ̊C. In addition, thermal stability of partially desodiated NaFe0.5Mn0.5P2O7 is found superior to partially desodiated NaMnP2O7 (~15% weight loss). Therefore, mixed iron–manganese sodium based pyrophosphate cathodes can be attractive for SIBs. Experimental the counter electrode, electrolyte, and separator, respectively. The partially desodiated samples (NaFe0.5Mn0.5P2O7) for thermal analysis were prepared by charging at 4.5 V and held for 2h. After charging, the cells were disassembled inside a glove box and washed with PC solvent. The cleaned samples were then dried at room temperature. Most of electrochemical testing was performed based on the galvanostatic technique using a battery cycler (WonA Tech, WBCS 3000, Korea) in the potential range of 2.0~4.5 V (vs. + Na/Na ) at room temperature. Acknowledgements Materials and methods This paper was made possible by NPRP grant # NPRP-5-569-2232 from the Qatar National Research Fund (member of Qatar Foundation). The statements made herein are solely the responsibility of the authors. The stoichiometric amounts of precursors, Na2CO3 (Aldrich), (NH4)2HPO4 (Aldrich), MnC2O4.2H2O (Aldrich) and FeC2O4.2H2O (Aldrich), were thoroughly mixed to synthesize Na2Fe1xMnxP2O7 (x=0, 0.5, 1) through a solid-state reaction. The mixed powder was pelletized and calcined under argon atmosphere at 350 °C for 3 hours. After calcination, pellets were cooled to room temperature (in Argon) and were ground into fine powder. This powder was re-pelletized and sintered at 600 °C for 6 hours under argon atmosphere to attain the desired phase. The pellets were cooled to room temperature under argon environment. The powder of the synthesized phase was obtained after grinding. In order to improve the electrical conductivity, the powder was coated with carbon by a high energy ball-milling (500 rpm, 24 h, silicon carbide balls) to yield an 8:2 ratio (wt %) of active material (carbon coating + super P) to carbon. After ball milling, the carbon coated powder was pelletized and annealed at 600 °C for 10 hours under argon atmosphere to restore the crystallinity of the material. 3 Material Characterization 9 Powder X-Ray Diffraction (PXRD) measurements of the synthesized material were conducted using a D/MAX-2500 XRD machine (Rigaku, Japan) to analyze phase purity and crystal structure. Thermo-gravimetric analysis and Differential thermal analysis (TGA/DTA) were carried out at a heating rate -1 of 5 °C min in Ar atmosphere. Field emission scanning electron microscopy (FE-SEM) was used to study the size and morphology of the pristine material and carbon coated material by using XL30 FEG (Philips, The Netherlands). References 1 2 4 5 6 7 8 10 11 12 13 14 Electrochemical Characterization 15 The carbon-coated (C-coated) active material (75 wt%), carbon black (15 wt%), and polyvinylidene fluoride (PVDF) (10 wt%) were mixed in N-methyl-2-pyrrolidone (NMP) to prepare slurry. 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