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Artificially coated NaFePO4 for aqueous rechargeable sodium-ion batteries
Journal of Alloys and Compounds 784 (2019) 720e726
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Artificially coated NaFePO4 for aqueous rechargeable sodium-ion batteries Seonghun Jeong a, 1, Byung Hoon Kim b, 1, Yeong Don Park a, Chang Yeon Lee a, Junyoung Mun a, **, Artur Tron a, * a
Department of Energy and Chemical Engineering, Innovation Center for Chemical Engineering, Incheon National University, 12-1, Songdo-dong, Yeonsu-gu, Incheon, 22012, Republic of Korea Department of Physics and Research Institute of Basic Science, Incheon National University, 12-1, Songdo-dong, Yeonsu-gu, Incheon, 22012, Republic of Korea
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 September 2018 Received in revised form 3 January 2019 Accepted 4 January 2019 Available online 6 January 2019
Aqueous rechargeable sodium-ion batteries are attractive as alternative materials to replace conventional lithium-ion batteries for the development of next-generation devices due to the abundance of sodium resources. Among various positive electrode materials for sodium-ion batteries, olivine NaFePO4 is attractive due to its high theoretical capacity and large electrochemical stability window. In this work, AlF3-coated olivine NaFePO4 is obtained by a facile electrochemical ion-exchange process in an aqueous electrolyte that is more environmentally friendly than conventional carbonate electrolytes. The robust AlF3-coated NaFePO4 displays better cycling stability and surface stability in the electrochemically fragile aqueous electrolyte. Physicochemical and electrochemical measurements confirm that the optimal improvement in the electrochemical performance of NaFePO4 is obtained using 0.5 wt% AlF3 in an aqueous electrolyte solution. Furthermore, the AlF3 coating on the surface of olivine NaFePO4 reduces surface failure by stabilizing the NaFePO4 surface in an aqueous electrolyte solution and facilitates lithium ion diffusion, leading to a NaFePO4 material with lower surface resistance and better surface stability. © 2019 Elsevier B.V. All rights reserved.
Keywords: NaFePO4 Cathode AlF3 coating Sodium Aqueous rechargeable sodium-ion battery
1. Introduction Concerns about global warming and fluctuating oil prices have led to the development of highly energy efficient systems, such as energy storage systems (ESSs) and electric vehicles (EVs), to reduce CO2 generation and fossil fuel combustion. Fundamentally, these systems require rechargeable batteries to obtain high energy efficiency by using charge and discharge behavior to avoid the loss of excess energy. Unlike conventional applications of rechargeable batteries for cell phones and laptop computers, which include lithium-ion batteries (LIBs), ESSs and EVs require extremely large batteries, at the kWh-MWh scale. The large size of these batteries has significantly increased the importance of LIBs, and their use has expanded since their commercialization by SONY in 1991 [1e3].
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Mun), [email protected] (A. Tron). 1 These authors equally contributed. https://doi.org/10.1016/j.jallcom.2019.01.046 0925-8388/© 2019 Elsevier B.V. All rights reserved.
However, LIBs may not be the best solution in large-scale applications due to their high cost and safety issues. Thus, large-scale facilities for LIBs are facing various critical problems for broad applications, and these issues must be solved to enable new applications [4,5]. Among the alternative rechargeable battery systems proposed for large applications, aqueous rechargeable batteries have very important advantages, such as being low cost, environmentally friendly and safe. Aqueous solutions are excellent electrolytes for electrochemical systems because of their high conductivity, nonflammability, environmental friendliness and abundant supply. Therefore, aqueous rechargeable battery systems have been considered promising energy resources for large-scale applications. However, their poor electrochemical stabilities limit the operating potential, energy density and electrochemical performance of these rechargeable batteries. Recently, many researchers have developed aqueous rechargeable battery systems with lithium ions which mitigate the limitations of the aqueous electrolyte [6e10]. Considering the future application of rechargeable batteries for EVs
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and ESSs, sodium ions may be more suitable than lithium ions for use in these batteries because lithium is usually only mined from limited deposits of Li2CO3 and LiOH. Aqueous sodium-ion batteries are attractive because of the low cost of sodium sources and the abundance of sodium compared to that of lithium, as shown in Fig. S1 (Supporting Information) [11e14]. In the development of next-generation sodium-ion battery systems ranging from portable devices to large-scale applications, the inferior high energy density of sodium compared to that of lithium becomes less critical due to the low cost and environmental friendliness of sodium materials [15e19]. Recently, many studies have focused on the development of electrode materials for sodium-ion batteries (NIBs) [20e23]. Similar to LIBs based on the various intercalation/deintercalation behavior of Li ions with a diameter of 0.69 Å, active materials for NIBs also have diverse electrochemical characteristics. In aqueous rechargeable battery systems, appropriate electrode materials should be selected to have an operating potential within the electrochemical stability window of water considering the pH; i.e., the window where H2 and O2 evolution reactions do not occur, as shown in Fig. S2 (Supporting Information) [7,24]. Considering that iron is a very affordable element, which is consistent with the lowcost advantages of aqueous rechargeable batteries, the following materials satisfy the above requirements: Na2FeP2O7, Na2FePO4F, Na4Fe3(PO4)2(P2O7), and olivine NaFePO4 [25e32]. Due to the many advantages of iron-based phosphate materials, such as easy synthesis, long cycle life and high rate cyclability, we selected olivinetype NaFePO4 as a candidate material for use in aqueous NIBs. Although olivine NaFePO4 has the same crystalline structure as well-known LiFePO4, it has a different electrochemical intercalation/deintercalation behavior of Na ions compared to that in the same process in LiFePO4. The electrochemical process of Li ions in the LiFePO4 structure proceeds via a distinct two-phase reaction in contrast to that in olivine NaFePO4. Sodiation and desodiation of Na ions are accompanied by the formation of a Na0.7FePO4 intermediate phase that displays a lower cycling stability than LiFePO4, which is related to the thermodynamically unstable phase of olivine NaFePO4 and the reaction kinetics of the electrochemical process [26,33,34]. Therefore, the synthesis of olivine NaFePO4 by the conventional solid-state reaction is accompanied by the formation of an unfavorable phase that limits the diffusion of Na ions [35,36]. To overcome this problem, many researchers have used an electrochemical ion-exchange process in an electrochemical system to replace the Li ions in LiFePO4 with Na ions [37,38]. This electrochemical ion-exchange process is usually carried out as a two-step transformation from LiFePO4 (LiFePO4 transforms to FePO4 in a lithium-containing aqueous electrolyte) to NaFePO4 (FePO4 transforms to NaFePO4 in a sodium-containing aqueous electrolyte). Only a few researchers have investigated aqueous NIBs [39e41]. Additionally, surface failure, which is a critical problem in aqueous electrolytes, has not been studied in detail. In this work, to obtain high-performance aqueous rechargeable NIBs with NaFePO4, AlF3 was coated on the active material to reduce the surface electrolyte decomposition. We report that pristine or AlF3-coated LiFePO4based materials were transformed into pristine NaFePO4 materials via an electrochemical ion-exchange process in an aqueous electrolyte. This ion-exchange process is attractive due to its facile processing, low cost, and use of environmentally friendly materials instead of nonaqueous electrolyte. The NaFePO4 materials obtained by this electrochemical ion-exchange process display improved cycling stability due to the AlF3 coating enhancing the surface stability in an aqueous electrolyte; the coating material is an attractive surface agent for anode and cathode materials of ARLBs that prevents the formation of unwanted side reaction components
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in an aqueous medium [42e44]. 2. Experimental LiFePO4 was obtained from the MTI Corporation and used without additional treatment. AlF3-coated LiFePO4 was prepared using a previously reported chemical deposition method [42]. To prepare AlF3-LiFePO4, starting materials such as NH4F (Aldrich) and Al(NO3)3 9H2O (Aldrich) were added to LiFePO4 and then mixed for 20 min with water with the same mass as LiFePO4; the Al to F molar ratio was controlled at 1:3. The total content of AlF3 on LiFePO4 was 0.5 and 1 wt%, and the obtained samples were denoted AlF3-0.5-LFP and AlF3-1-LFP, respectively. Then, the obtained slurry containing the coating precursors and LiFePO4 was dried at 120 C for 1 h and heated at 400 C for 2 h under an argon flow. The working electrode was prepared by mixing the active materials, acetylene black and poly(tetrafluoroethylene) (PTFE) with a weight ratio of 80:10:10 on a stainless-steel mesh of 0.25 cm2 with a loading amount of 2 mg cm2. To prepare the counter electrode, activated carbon was prepared using the same method used for the working electrode, namely, coating the surface of a stainless-steel mesh of 1 cm2 using a loading amount of 25 mg cm2. A silver chloride electrode (Ag/AgCl, þ0.197 V vs. SHE) was used as the reference electrode. Then, 1 M Li2SO4 and 1 M Na2SO4 aqueous electrolyte solutions were used for the electrochemical ionexchange process and electrochemical measurements. The electrochemical ion-exchange process to transform LiFePO4 into NaFePO4 was carried out in three-electrode cells with working, counter and reference electrodes. Then, the bare and AlF3-coated LiFePO4 electrodes were galvanostatically delithiated at a rate of 1 C (1 C ¼ 170 mAh g1) to 0.8 V (vs. Ag/AgCl) in a 1 M Li2SO4 aqueous electrolyte solution; then, the same electrodes (FePO4) were washed and subsequently galvanostatically sodiated at 0.1 C to 0.5 V (vs. Ag/AgCl) in a 1 M Na2SO4 aqueous electrolyte solution to obtain olivine NaFePO4. The obtained pristine and coated NaFePO4 electrodes were washed by distilled water and then dried at 80 C. For all of the electrodes in the aqueous ion-exchange process, the applied current and resulting capacity were calculated from the weights of the pristine and coated LiFePO4 electrodes. Electrodes of NaFePO4 with or without the AlF3 coating were used for electrochemical measurements in the three-electrode cells described above. Repeated charge-discharge cycles were carried out using a WonaTech potentiostat/galvanostat instrument in the potential range from 0.5e0.85 V (vs. Ag/AgCl) in a 1 M Na2SO4 aqueous electrolyte solution. The electrochemical behavior of LiFePO4 and NaFePO4 was investigated by cyclic voltammetry (CV) using a WonaTech potentiostat/galvanostat in the potential range from 0.2e0.8 V (for LiFePO4) and 0.7e1.1 V (for NaFePO4) at a scan rate of 0.1 mV s1 (vs. Ag/AgCl). Electrochemical impedance spectroscopy (EIS) measurements were carried out using a multichannel electrochemical workstation (Zive Lab, MP1) in the frequency range from 100 kHz to 10 mHz at a voltage amplitude of 10 mV. The EIS data were obtained after charge/discharge cycling. To investigate the obtained pristine and AlF3-coated FePO4 and NaFePO4 after delithiation and sodiation, respectively, X-ray diffraction (XRD) and field scanning electron microscopy (FE-SEM) with energy dispersive X-ray spectroscopy (EDS) analyses were performed. The crystalline structures of the bare and AlF3-coated FePO4 and NaFePO4 were determined by XRD using a Rigaku SmartLab diffractometer with Cu Ka radiation (40 kV, 250 mA) in the 2q range of 10e80 at a scan rate of 0.03 s1. The surface morphologies of these samples were observed using FE-SEM (JSM 7800F) with EDS and transmission electron microscopy (TEM, JEOL ARM-200F).
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The ionic conductivity of the aqueous electrolyte solutions (1 M Li2SO4 and 1 M Na2SO4) was determined in a cell (Sanxin, 2401-M Glass conductivity electrode) with two symmetric platinum electrodes and a cell constant of 1 cm1 using a multichannel electrochemical workstation from Zive Lab (MP1) at a temperature of 25 C in a calorstat that maintained the set temperature for 1 h.
3. Results and discussion An electrochemical ion-exchange process was used to transform olivine LiFePO4 into NaFePO4, as shown in Fig. 1a. During the initial delithiation process in the 1 M Li2SO4 aqueous electrolyte solution, the initial delithiation capacity was recorded as 134.9 mAh g1 with a typical voltage plateau at 0.25 V vs. Ag/AgCl. Then, the sodiation process of the electrode in the 1 M Na2SO4 aqueous electrolyte solution was performed, and a sodiation capacity of 122.6 mAh g1 with a plateau at 0.15 V was determined. The crystalline structures of FePO4 and NaFePO4 obtained after the delithiation and sodiation processes were elucidated by XRD analyses, and the results are presented in Fig. 1b. After the initial delithiation process, the structure of FePO4 was detected (JCPDF card no. 70-1793). Similarly, after sodiation, the XRD pattern is in excellent agreement with that of NaFePO4 [45,46]. These findings indicate that the crystalline structure of olivine LiFePO4 was successfully transformed into NaFePO4 through an FePO4 intermediate. The morphologies of the FePO4 and NaFePO4 materials obtained after the aqueous ion-exchange process were characterized by SEM and TEM (Fig. S3, Supporting Information and Fig. 2, respectively).
The SEM images of the pristine NaFePO4 material displayed the expected surface morphology and particle size while the images of the AlF3-coated NaFePO4 surface showed changes. The TEM analysis revealed that the surface of the AlF3-coated NaFePO4 cathode is distinguishable from that of pristine NaFePO4 with a regular surface morphology. The AlF3-coating layer on NaFePO4 is approximately 10 nm, as observed by TEM analysis. EDS analysis also confirmed the atomic ratios of FePO4 (Na: Fe: P: O of 0: 1: 1: 4) and NaFePO4 (Na: Fe: P: O of 1: 1: 1: 4), as shown in Fig. S4a and b (Supporting Information). In addition, these ratios were valid for the AlF3coated NaFePO4 obtained via electrochemical aqueous ionexchange processes (Fig. S4c and d, Supporting Information). Moreover, the uniform elemental distributions of Na, P, Fe, O, Al, F, and C on the surfaces of FePO4 and the pristine and AlF3-coated NaFePO4 are shown in Fig. 3. It should be noted that the presence of C on the surface of the obtained samples is common for all electrode materials because the inert synthetic conditions result in residual carbon from the precursors. Additionally, the presence of a possible carbon coating on the surface of the pristine and coated LiFePO4, NaFePO4 and FePO4 was observed in the EDS analysis. The presence of carbon on the surface of the olivine materials was confirmed by thermogravimetric analysis (TGA) [42]. The mass of the samples during TGA decreased to a small amount due to the possible formation of unwanted compounds above 600 C in air [47]. Therefore, the carbon amount on the surface of the samples did not have a crucial effect on the electrochemical performance. Thus, on the basis of the XRD, SEM, TEM and EDS analyses, pristine and coated NaFePO4 were obtained via an aqueous ion-exchange
Fig. 1. (a) Illustration of the aqueous electrochemical ion-exchange process from olivine LiFePO4 to FePO4 in 1 M Li2SO4 aqueous medium at 1 C rate to 0.8 V and FePO4 to NaFePO4 in 1 M Na2SO4 aqueous medium at 0.1 C rate to 0.5 V. (b) XRD patterns of the FePO4 and NaFePO4 samples obtained at the galvanostatic cycling stages as shown in Fig. 1a.
Fig. 2. Surface analysis by TEM of (a) pristine NaFePO4 and (b) AlF3-1-NFP samples.
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Fig. 3. EDS mapping of the elements (a) O, (b) Na, (c) Fe, (d) P, (e) C, (f) Al, and (g) F, after delithiation: FePO4, after sodiation: pristine NaFePO4, AlF3-0.5-NFP and AlF3-1-NFP during the aqueous electrochemical ion-exchange process.
process due to the strong PO bonds in the olivine structure and the similar ionic radii of Liþ and Naþ ions enabling retention of the olivine crystal structure. This method was also successful for NaFePO4 coated with 0.5 and 1 wt% AlF3. As shown in Fig. 4a, the cyclic voltammogram of olivine LiFePO4 has a single pair of reduction and oxidation peaks at 0.03 (1st peak) and 0.32 V (2nd peak), respectively, which are related to the intercalation and deintercalation of Li ions into/from the structure in 1 M Li2SO4; the FePO4 þ Liþ þ e / LiFePO4 reaction causes the 1st peak and the LiFePO4 - Liþ - e / FePO4 reaction results in the 2nd peak. For NaFePO4, a single peak was observed at 0.25 V, which was attributed to the sodiation process (1st peak: FePO4 þ Naþ þ e / NaFePO4), and two peaks were observed at 0.04 V and 0.22 V, which were attributed to the desodiation processes (NaFePO4 - Naþ - e / Na2/3FePO4 for the 2nd peak and Na2/ þ 3FePO4 - Na - e / FePO4 for the 3rd peak). The CV data indicated that the electrochemical intercalation and deintercalation process of olivine LiFePO4 with Li ions in an aqueous solution is faster than that of Na ions into/from the structure of NaFePO4. This difference is related to the phase transition during the electrochemical process, resulting in the larger charge transfer resistance and slower reaction kinetics of the olivine structure for Na ions than Li ions. It is important to mention that NaFePO4 coated with 0.5 and 1 wt% AlF3 shows a similar electrochemical process as the pristine material, as shown in Fig. 4b. However, the areas of the voltammograms for the
coated materials were smaller than those for pristine NaFePO4, indicating a positive effect on the rapid reaction kinetics of the intercalation and deintercalation of Na ions into/from the NaFePO4 structure. Thus, the AlF3 coating on NaFePO4 has a beneficial role in improving the cycling performance of the material in an aqueous electrolyte. In addition, the CV data obtained in the aqueous electrolyte are in agreement with the results obtained in an organic electrolyte [28,48]. Moreover, an examination of the voltage curves presented in Fig. 5 shows that the electrochemical process of intercalation and deintercalation of Na ions into/from the NaFePO4 structure is strongly correlated with the obtained CV data (Fig. 4). Fig. 5a shows the initial charging curve of the bare sample, which exhibited two plateaus at 0.25 and 0.75 V vs. Ag/AgCl with a total capacity of 101.7 mAh g1. The initial plateau at 0.25 V corresponds to the desodiation reactions from NaFePO4, but the two desodiation steps shown in Fig. 4a are not clearly observed due to high polarization. However, the other plateau at 0.8 V in the initial charging curve does not correspond to a desodiation reaction. Similarly, the discharge curve exhibits only one plateau, which is a reversible peak of the charging plateau at 0.25 V because it has a specific capacity of 76.5 mAh g1, and this capacity is almost the same as the capacity of the initial charging curve. The initial Coulombic efficiency was also very poor and is only 75.5% for the bare NaFePO4 in the aqueous electrolyte. Subsequently, the 2nd charging capacity
Fig. 4. (a) Cyclic voltammetry of the pristine LiFePO4 and NaFePO4 at a scan rate of 1 mV s1 in the potential range of 0.2e0.8 V for LiFePO4 and -0.7e1.1 V for NaFePO4. (b) Cyclic voltammetry of the pristine and AlF3-coated NaFePO4 (0.5 and 1 wt% AlF3) at a scan rate of 1 mV s1 in the potential range of 0.7e1.1 V. Note that the reference electrode is Ag/AgCl (þ0.197 V vs. SHE).
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Fig. 5. Charge/discharge voltage profiles of the (a) pristine NaFePO4, (b) AlF3-0.5-NFP, (c) AlF3-1-NFP materials at the rate of 1 C (154 mAh g1) in the potential range of 0.5e0.85 V in 1 M Na2SO4 aqueous electrolyte solution.
Fig. 6. (a) Cyclic performance and (b) Nyquist plots of pristine and coated NaFePO4 materials at selected cycle (20th cycle) in Fig. 6a in 1 M Na2SO4 aqueous electrolyte solution at the rate of 1 C (154 mAh g1), (c) rate capabilities of the NaFePO4 material at the rates of 0.1, 0.2, 0.5, 1, and 2 C in the potential range of 0.5e0.85 V.
suddenly decreased to 69.8 mAh g1, corresponding to the discharge capacity. It is believed that the unexpected plateau originated from a side reaction of the electrochemically fragile aqueous electrolyte, which caused surface deterioration, to generate compounds such as FeO or FeOH [39]. Surface passivation creates a protective layer to minimize electrolyte side reactions, and therefore, the 2nd charging capacity suddenly decreased due to high polarization. However, the initial charging curves of the coated sample shown in Fig. 5b and c do not exhibit the second plateau at 0.85 V. In addition to the 2nd plateau, desodiation at approximately 0.25 V had higher capacities (95.4 and 87.1 mAh g1) than the bare sample. With the 0.5 wt% AlF3 coating, the polarization between charge and discharge was reduced. Therefore, the 1st specific discharge capacity was recorded as 83.9 mAh g1, which is higher than that of the bare sample (76.2 mAh g1). Despite the absence of the 2nd plateau in the 1st desodiation curve, the initial Coulombic efficiencies of the coated samples were not very good (87.9% and 87.5%). As the coating amount increased to 1 wt%, the polarization increased again. With the similar polarization of AlF3-1-NFP, the thick coating of 1 wt% AlF3 resulted in high polarization and showed an initial discharge capacity similar to that of the bare sample. Fig. 6a displays the cyclability of the prepared pristine and coated NaFePO4 (0.5 and 1 wt% AlF3) samples in the 1 M Na2SO4 aqueous electrolyte solution. The initial discharge capacity of pristine NaFePO4 is 101.7 mAh g1, while the 0.5 and 1 wt% AlF3-coated samples have an initial discharge capacity of 95.6 and 86.3 mAh g1, respectively. These capacities are lower than the capacity of the pristine material due to the surface resistance of the AlF3 coating on the surface of NaFePO4 hindering sodium transportation processes through the surface. For the sample with a coating layer of 1 wt%, the initial capacity was lower than that of the pristine and AlF3-0.5-NFP samples. Moreover, the presence of a large amount of AlF3
coating material on the surface of NaFePO4 can result in resistance and high polarization, which hinder the sodium transport processes through the interface of the electrode and aqueous electrolyte during cycling because the AlF3 coating has nonionic conducting nature. The capacity of the coated samples was 55.8 and 51.9 mAh g1 for 0.5 and 1 wt% AlF3, respectively, at the 50th cycle, while the pristine sample had a lower capacity of 40.3 mAh g1. Therefore, a suitable amount of AlF3 coating on the surface of NaFePO4 can play a very important role in improving the electrochemical performance, leading to a stabilization process during repeated charge and discharge cycling that can effectively reduce polarization and enhance the kinetic processes through the interface. It was clearly observed that the bare sample had the poorest cyclability of the samples. The AlF3-0.5-NFP sample exhibited the best cyclability. EIS measurements were performed after the 20th cycle and were fitted to the equivalent circuit presented in the inset of Fig. 6b. The diameter of the semicircle in the Nyquist plot represents the surface resistance, as described in the equivalent circuit [42,43]. The surface resistance of the AlF3-0.5-NFP sample was fitted as 45 Ohm, which is much lower than the surface resistance of pristine NaFePO4 and AlF3-1-NFP, i.e., 500 and 321 Ohm, respectively. Moreover, the AlF3-0.5-NFP sample has a smaller cycling resistance than the pristine NaFePO4 and AlF3-1-NFP samples. As indicated in Ref. [42], olivine materials with an AlF3 coating can better reduce the surface resistance and facilitate lithium diffusion through an electrode and aqueous electrolyte interface than organic electrolytes with a conventional SEI layer. However, the surface of the active electrode materials in the aqueous electrolyte can have a passivation layer [42,44]. Therefore, the EIS data were consistent with the electrochemical performance, as shown in Fig. 6a. This result is due to the AlF3 coating on the surface of olivine NaFePO4 leading to surface stabilization and improving the reaction kinetics
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Table 1 The comparison of the olivine NaFePO4 in the aqueous electrolyte solutions for aqueous rechargeable sodium ion battery (ARLBs). Capacity in the 1st cycle (mAh g1) Capacity in the nth cycle (mAh g1) Coulombic efficiency (%) Electrolyte solution Reference
Materials
Anode materials
NaFePO4/C NaFePO4/C NaFePO4 NaFePO4/AlF3
Activated carbon 87 (1 C) Pt 118.3 (10 mV s1) Activated carbon 101.7 (1 C) Activated carbon 95.6 (1 C)
59 (1 C, 30 cycles)
92 (1st cycle)
40.3 (1 C, 50 cycles) 55.8 (1 C, 50 cycles)
75.5 (1st cycle) 87.9 (1st cycle)
through a coating material interface. The amount of the AlF3 coating should be optimized because these two mechanisms compete, as shown by the opposite trends with increasing coating amounts. Previously, we reported that the optimal AlF3 coating is 1 wt% for LiFePO4 in the same aqueous electrolyte [42]. In this work, the lower 0.5 wt% AlF3 content was the best for NaFePO4. The larger Na ions (compared to Li ions) have a higher ionic transport resistance through the passivation layer. Therefore, the successful application of AlF3-coating materials to improve the cycling stability of olivine Na-based electrode materials in an aqueous electrolyte requires the consideration of the amount of coating material due to its highly nonionic conducting behavior. The obtained pristine and AlF3-coated NaFePO4 samples were subjected to rate capability tests, as presented in Fig. 6c. The AlF30.5-NFP sample maintained a higher specific discharge capacity at various current densities than the pristine and AlF3-1-NFP samples, and this difference is related to the large electrode polarization (Fig. 5aec). It should be noted that during the electrochemical process, the particles of NaFePO4 need fast Naþ ion transfer through the electrolyte/electrode interface. The additional layer on the surface of the electrode (for the pristine sample, the layer formed from unwanted side reactions [42]; for the coated sample, the AlF3coating layer) can add an energy barrier to the transfer of Naþ ions. Therefore, the higher ionic conductivity of the 1 M Na2SO4 aqueous electrolyte solution (68.2 mS cm1) compared to that of 1 M Li2SO4 (43.6 mS cm1) is a crucial factor to ensure enhanced Naþ ion migration through the interfaces and improved cycling performance (Fig. S5, Supporting Information). Thus, NaFePO4 coated with 0.5 wt% AlF3 displays an enhanced electrochemical performance and lower surface resistance and electrode polarization than the pristine and 1 wt% AlF3-coated NaFePO4 due to better Naþ ion transfer from the bulk electrolyte to the electrode/aqueous electrolyte interface, indicating good cycling stability in the aqueous electrolyte solution of the AlF3-0.5-NFP cathode. Table 1 shows a comparison of the cycling stability of the olivine NaFePO4 cathodes in this study with previous reference data for aqueous rechargeable NIBs [29,49]. Previous data also showed the same electrochemical behavior shown in Fig. 5, which appears to be typical for olivine NaFePO4-based electrode materials [28e33]. The electrochemical performance shows that the discharge capacity is 95.6 mAh g1 at a 1 C rate and the capacity is 55.8 mAh g1 at 50 cycles. CanasCabanas reported that a NaFePO4/C sample showed a low initial discharging capacity of 87 mAh g1 at a 1 C rate and cycle stability of 59 mAh g1 after 30 cycles [29]. However, Vujkovic reported that an FePO4 material after sodiation at 10 mV s1 maintained a discharge capacity of 118 mAh g1 [49]. Table 1 shows that the AlF3coated NaFePO4 maintains a better electrochemical performance in the aqueous electrolyte. 4. Conclusion In summary, NaFePO4 cathode materials with surfaces modified by AlF3 were obtained by an electrochemical ion-exchange process in an aqueous electrolyte, which is a simple, low cost, and more environmentally friendly material than nonaqueous electrolytes. This olivine NaFePO4 material displays a discharge capacity of
Na2SO4 (1 M) NaNO3 (sat) Na2SO4 (1 M) Na2SO4 (1 M)
[29] [49] In this paper In this paper
55.8 mAh g1 at 1 C with better surface stability in an aqueous electrolyte solution with the 0.5 wt% AlF3 coating. Physicochemical and electrochemical measurements confirmed the improvement in the electrochemical performance of NaFePO4 with the AlF3 coating, which reduces surface failure by stabilizing the surface of pristine NaFePO4 in an aqueous electrolyte, leading to the reduced surface resistance and improved surface stability of iron-based phosphate materials. Therefore, we suggest that AlF3 coatings are attractive for the surface stabilization of anode and cathode materials in aqueous rechargeable lithium- and sodium-ion batteries. Acknowledgments This work was also supported through Incheon National University (INU), Incheon, Republic of Korea (2015-1831). This research was also supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2018R1D1A1B07048144). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.01.046. References [1] M.R. Palacin, Recent advances in rechargeable battery materials: a chemist's perspective, Chem. Soc. Rev. 38 (2009) 2565e2575. [2] A. Yoshino, The birth of the lithium-ion battery, Angew. Chem. Int. Ed. 51 (2012) 5798e5800. [3] B. Dunn, H. Kamath, J.M. Tarascon, Electrical energy storage for the grid: a battery of choices, Science 334 (2011) 928e935. [4] J.B. Goodenough, Y. Kim, Challenges for rechargeable Li batteries, Chem. Mater. 22 (2010) 587e603. [5] C.P. Grey, J.M. Tarascon, Sustainability and in situ monitoring in battery development, Nat. Mater. 16 (2016) 45e56. [6] W. Li, J.R. Dahn, D.S. Wainwright, Rechargeable lithium batteries with aqueous electrolytes, Science 264 (1994) 1115e1118. [7] H. Kim, J. Hong, K.-Y. Park, H. Kim, S.-W. Kim, K. Kang, Aqueous rechargeable Li and Na ion batteries, Chem. Rev. 114 (2014) 11788e11827. [8] C. Vaalma, D. Buchholz, M. Weil, S. Passerini, A cost and resource analysis of sodium-ion batteries, Nat. Rev. Mater. 3 (2018) 18013. [9] D. Bin, F. Wang, A.G. Tamirat, L. Suo, Y. Wang, C. Wang, Y. Xia, Progress in aqueous rechargeable sodium-ion batteries, Adv. Energy Mater. (2018), 1703008. [10] K. Kubota, M. Dahbi, T. Hosaka, S. Kumakura, S. Komaba, Towards K-Ion and Na-Ion batteries as “beyond Li-Ion”, Chem. Rec. 18 (2018) 1e22. [11] H. Zhang, S. Jeong, B. Qin, D.V. Carvalho, D. Buchholz, S. Passerini, Towards high-performance aqueous sodium-ion batteries: stabilizing the solid/liquid interface for NASICON-type Na2VTi(PO4)3 using concentrated electrolytes, ChemSusChem 11 (2018) 1382e1389. [12] H. Long, W. Zeng, H. Wang, M. Qian, Y. Liang, Z. Wang, Self-assembled biomolecular 1D nanostructures for aqueous sodium-ion battery, Adv. Sci. 5 (2018), 1700634. [13] W. Li, F. Zhang, X. Xiang, X. Zhang, Nickel-substituted copper hexacyanoferrate as a superior cathode for aqueous sodium-ion batteries, ChemElectroChem 5 (2018) 350e354. [14] P. Lei, Y. Wang, F. Zhang, X. Wan, X. Xiang, Carbon-coated Na2.2V1.2Ti0.8(PO4)3 cathode with excellent cycling performance for aqueous sodium-ion batteries, ChemElectroChem 5 (2018) 2482e2487. [15] H. Chen, H. Mi, L. Sun, P. Zhang, Y. Li, One-step synthesis of 3D-sandwiched Na3V2(PO4)2O2F@rGO composites as cathode material for high-rate sodiumion batteries, ChemElectroChem 5 (2018) 2593e2599. [16] J. Wang, C. Mi, P. Nie, S. Dong, S. Tang, X. Zhang, Sodium-rich iron hexacyanoferrate with nickel doping as a high performance cathode for aqueous
726
S. Jeong et al. / Journal of Alloys and Compounds 784 (2019) 720e726
sodium ion batteries, J. Electroanal. Chem. 818 (2018) 10e18. [17] F. Zhang, W. Li, X. Xiang, M. Sun, Highly stable Na-storage performance of Na0.5Mn0.5Ti0.5O2 microrods as cathode for aqueous sodium-ion batteries, J. Electroanal. Chem. 802 (2017) 22e26. [18] W. Li, F. Zhang, X. Xiang, X. Zhang, High-efficiency Na-storage performance of a nickel-based ferricyanide cathode in high-concentration electrolytes for aqueous sodium-ion batteries, ChemElectroChem 4 (2017) 2870e2876. [19] Y. Wang, Z. Feng, D. Laul, W. Zhu, M. Provencher, M.L. Trudeau, A. Guerfi, K. Zaghi, Ultra-low cost and highly stable hydrated FePO4 anodes for aqueous sodium-ion battery, J. Power Sources 374 (2018) 211e216. [20] J.-Y. Hwang, S.-T. Myung, Y.-K. Sun, Sodium-ion batteries: present and future, Chem. Soc. Rev. 46 (2017) 3529e3614. [21] H. Kim, H. Kim, Z. Ding, M.H. Lee, K. Lim, G. Yoon, K. Kang, Recent progress in electrode materials for sodium-ion batteries, Adv. Energy Mater. (2016), 1600943. [22] S.Y. Hong, Y. Kim, Y. Park, A. Choi, N.-S. Choi, K.T. Lee, Charge carriers in rechargeable batteries: Na ions vs. Li ions, Energy Environ. Sci. 6 (2013) 2067e2081. [23] M.I. Jamesh, A.S. Prakash, Advancement of technology towards developing Na-ion batteries, J. Power Sources 378 (2018) 268e300. [24] Y. Wang, L. Mu, J. Liu, Z. Yang, X. Yu, L. Gu, Y.-S. Hu, H. Li, X.-Q. Yang, L. Chen, X. Huang, A novel high capacity positive electrode material with tunnel-type structure for aqueous sodium-ion batteries, Adv. Energy Mater. 5 (2015), 1501005. [25] H. Kim, R.A. Shakoor, C. Park, S.Y. Lim, J.-S. Kim, Y.N. Jo, W. Cho, K. Miyasaka, R. Kahraman, Y. Jung, J.W. Choi, Na2FeP2O7 as a promising iron-based pyrophosphate cathode for sodium rechargeable batteries: a combined experimental and theoretical study, Adv. Funct. Mater. 23 (2013) 1147e1155. [26] V. Palomares, P. Serras, I. Villaluenga, K.B. Hueso, J. Carretero-Gonzalez, T. Rojo, Na-ion batteries, recent advances and present challenges to become low cost energy storage systems, Energy Environ. Sci. 5 (2012) 5884e5901. [27] H. Kim, I. Park, D.-H. Seo, S. Lee, S.-W. Kim, W.J. Kwon, Y.-U. Park, C.S. Kim, S. Jeon, K. Kang, New iron-based mixed-polyanion cathodes for lithium and sodium rechargeable batteries: combined first principles calculations and experimental study, J. Am. Chem. Soc. 134 (2012) 10369e10372. [28] Y. Zhu, Y. Xu, Y. Liu, C. Luo, C. Wang, Comparison of electrochemical performances of olivine NaFePO4 in sodium-ion batteries and olivine LiFePO4 in lithium-ion batteries, Nanoscale 5 (2013) 780e787. ndez-Ropero, D. Saurel, B. Acebedo, T. Rojo, M. Casas-Cabanas, [29] A.J. Ferna Electrochemical characterization of NaFePO4 as positive electrode in aqueous sodium-ion batteries, J. Power Sources 291 (2015) 40e45. [30] D. Bin, F. Wang, A.G. Tamirat, L. Suo, Y. Wang, C. Wang, Y. Xia, Progress in aqueous rechargeable sodium-ion batteries, Adv. Energy Mater. 8 (2018), 1703008. [31] M. Hilder, P.C. Howlett, D. Saurel, H. Anne, M. Casas-Cabanas, M. Armand, T. Rojo, D.R. MacFarlane, M. Forsyth, Stable cycling of NaFePO4 cathodes in high salt concentration ionic liquid electrolytes, J. Power Sources 406 (2018) 70e80. [32] D. Saurel, M. Galceran, M. Reynaud, H. Anne, M. Casas-Cabanas, Rate dependence of the reaction mechanism in olivine NaFePO4 Na-ion cathode material, Int. J. Energy Res. 42 (2018) 3258e3265.
[33] K.T. Lee, T.N. Ramesh, F. Nan, G. Botton, L.F. Nazar, Topochemical synthesis of sodium metal phosphate olivines for sodium-ion batteries, Chem. Mater. 23 (2011) 3593e3600. [34] P. Moreau, D. Guyomard, J. Gaubicher, F. Boucher, Structure and stability of sodium intercalated phases in olivine FePO4, Chem. Mater. 22 (2010) 4126e4128. [35] M. Galceran, D. Saurel, B. Acebedo, V.V. Roddatis, E. Martin, T. Rojo, M. CasasCabanas, The mechanism of NaFePO4 (de)sodiation determined by in situ Xray diffraction, Phys. Chem. Chem. Phys. 16 (2014) 8837e8842. [36] J. Lu, S.C. Chung, S.-I. Nishimura, A. Yamada, Phase diagram of olivine NaxFePO4 (0 < x< 1), Chem. Mater. 25 (2013) 4557e4565. [37] W. Tang, X. Song, Y. Du, C. Peng, M. Lin, S. Xi, B. Tian, J. Zheng, Y. Wu, F. Pan, K.P. Loh, High-performance NaFePO4 formed by aqueous ion-exchange and its mechanism for advanced sodium ion batteries, J. Mater. Chem. A 4 (2016) 4882e4892. [38] Y. Fang, Q. Liu, L. Xiao, X. Ai, H. Yang, Y. Cao, High-performance olivine NaFePO4 microsphere cathode synthesized by aqueous electrochemical displacement method for sodium ion batteries, ACS Appl. Mater. Interfaces 7 (2015) 17977e17984. [39] M. Zhao, W. Zhang, F. Qu, F. Wang, X. Song, Good discharge capacities of NaV6O15 material for an aqueous rechargeable lithium battery, Electrochim. Acta 138 (2014) 187e192. [40] S.S. Zhang, The redox mechanism of FeS2 in non-aqueous electrolytes for lithium and sodium batteries, J. Mater. Chem. A 3 (2015) 7689e7694. [41] F. Zhang, W. Li, X. Xiang, M. Sun, Nanocrystal-assembled porous Na3MgTi(PO4)3 aggregates as highly stable anode for aqueous sodium-ion batteries, Chem. Eur. J. 23 (2017) 12944e12948. [42] A. Tron, Y.N. Jo, S.H. Oh, Y.D. Park, J. Mun, Surface modification of the LiFePO4 cathode for the aqueous rechargeable lithium ion battery, ACS Appl. Mater. Interfaces 9 (2017) 12391e12399. [43] A. Tron, Y.D. Park, J. Mun, AlF3-coated LiMn2O4 as cathode material for aqueous rechargeable lithium battery with improved cycling stability, J. Power Sources 325 (2016) 360e364. [44] A. Tron, H. Kang, J. Kim, J. Mun, Electrochemical performance of AlF3-coated LiV3O8 for aqueous rechargeable lithium ion batteries, J. Electrochem. Sci. Technol. 9 (1) (2018) 60e68. [45] J. Hwang, K. Matsumoto, T. Nohira, R. Hagiwara, Electrochemical sodiationdesodiation of maricite NaFePO4 in ionic liquid electrolyte, Electrochemistry 85 (10) (2017) 675e679. [46] N. Wongittharom, T.C. Lee, C.H. Wang, Y.C. Wang, J.K. Chang, Electrochemical performance of Na/NaFePO4 sodium-ion batteries with ionic liquid electrolytes, J. Mater. Chem. A 2 (2014) 5655e5661. [47] I. Belharouak, C. Johnson, K. Amine, Synthesis and electrochemical analysis of vapor-deposited carbon-coated LiFePO4, Electrochem. Commun. 7 (2005) 983e988. lez, [48] M. Casas-Cabanas, V.V. Roddatis, D. Saurel, P. Kubiak, J. Carretero-Gonza V. Palomares, P. Serras, T. Rojo, Crystal chemistry of Na insertion/deinsertion in FePO4NaFePO4, J. Mater. Chem. 22 (2012) 17421e17423. [49] M. Vujkovic, S. Mentus, Fast sodiation/desodiation reactions of electrochemically delithiated olivine LiFePO4 in aerated aqueous NaNO3 solution, J. Power Sources 247 (2014) 184e188.
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Artificially coated NaFePO4 for aqueous rechargeable sodium-ion batteries Seonghun Jeong a, 1, Byung Hoon Kim b, 1, Yeong Don Park a, Chang Yeon Lee a, Junyoung Mun a, **, Artur Tron a, * a
Department of Energy and Chemical Engineering, Innovation Center for Chemical Engineering, Incheon National University, 12-1, Songdo-dong, Yeonsu-gu, Incheon, 22012, Republic of Korea Department of Physics and Research Institute of Basic Science, Incheon National University, 12-1, Songdo-dong, Yeonsu-gu, Incheon, 22012, Republic of Korea
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 September 2018 Received in revised form 3 January 2019 Accepted 4 January 2019 Available online 6 January 2019
Aqueous rechargeable sodium-ion batteries are attractive as alternative materials to replace conventional lithium-ion batteries for the development of next-generation devices due to the abundance of sodium resources. Among various positive electrode materials for sodium-ion batteries, olivine NaFePO4 is attractive due to its high theoretical capacity and large electrochemical stability window. In this work, AlF3-coated olivine NaFePO4 is obtained by a facile electrochemical ion-exchange process in an aqueous electrolyte that is more environmentally friendly than conventional carbonate electrolytes. The robust AlF3-coated NaFePO4 displays better cycling stability and surface stability in the electrochemically fragile aqueous electrolyte. Physicochemical and electrochemical measurements confirm that the optimal improvement in the electrochemical performance of NaFePO4 is obtained using 0.5 wt% AlF3 in an aqueous electrolyte solution. Furthermore, the AlF3 coating on the surface of olivine NaFePO4 reduces surface failure by stabilizing the NaFePO4 surface in an aqueous electrolyte solution and facilitates lithium ion diffusion, leading to a NaFePO4 material with lower surface resistance and better surface stability. © 2019 Elsevier B.V. All rights reserved.
Keywords: NaFePO4 Cathode AlF3 coating Sodium Aqueous rechargeable sodium-ion battery
1. Introduction Concerns about global warming and fluctuating oil prices have led to the development of highly energy efficient systems, such as energy storage systems (ESSs) and electric vehicles (EVs), to reduce CO2 generation and fossil fuel combustion. Fundamentally, these systems require rechargeable batteries to obtain high energy efficiency by using charge and discharge behavior to avoid the loss of excess energy. Unlike conventional applications of rechargeable batteries for cell phones and laptop computers, which include lithium-ion batteries (LIBs), ESSs and EVs require extremely large batteries, at the kWh-MWh scale. The large size of these batteries has significantly increased the importance of LIBs, and their use has expanded since their commercialization by SONY in 1991 [1e3].
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Mun), [email protected] (A. Tron). 1 These authors equally contributed. https://doi.org/10.1016/j.jallcom.2019.01.046 0925-8388/© 2019 Elsevier B.V. All rights reserved.
However, LIBs may not be the best solution in large-scale applications due to their high cost and safety issues. Thus, large-scale facilities for LIBs are facing various critical problems for broad applications, and these issues must be solved to enable new applications [4,5]. Among the alternative rechargeable battery systems proposed for large applications, aqueous rechargeable batteries have very important advantages, such as being low cost, environmentally friendly and safe. Aqueous solutions are excellent electrolytes for electrochemical systems because of their high conductivity, nonflammability, environmental friendliness and abundant supply. Therefore, aqueous rechargeable battery systems have been considered promising energy resources for large-scale applications. However, their poor electrochemical stabilities limit the operating potential, energy density and electrochemical performance of these rechargeable batteries. Recently, many researchers have developed aqueous rechargeable battery systems with lithium ions which mitigate the limitations of the aqueous electrolyte [6e10]. Considering the future application of rechargeable batteries for EVs
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and ESSs, sodium ions may be more suitable than lithium ions for use in these batteries because lithium is usually only mined from limited deposits of Li2CO3 and LiOH. Aqueous sodium-ion batteries are attractive because of the low cost of sodium sources and the abundance of sodium compared to that of lithium, as shown in Fig. S1 (Supporting Information) [11e14]. In the development of next-generation sodium-ion battery systems ranging from portable devices to large-scale applications, the inferior high energy density of sodium compared to that of lithium becomes less critical due to the low cost and environmental friendliness of sodium materials [15e19]. Recently, many studies have focused on the development of electrode materials for sodium-ion batteries (NIBs) [20e23]. Similar to LIBs based on the various intercalation/deintercalation behavior of Li ions with a diameter of 0.69 Å, active materials for NIBs also have diverse electrochemical characteristics. In aqueous rechargeable battery systems, appropriate electrode materials should be selected to have an operating potential within the electrochemical stability window of water considering the pH; i.e., the window where H2 and O2 evolution reactions do not occur, as shown in Fig. S2 (Supporting Information) [7,24]. Considering that iron is a very affordable element, which is consistent with the lowcost advantages of aqueous rechargeable batteries, the following materials satisfy the above requirements: Na2FeP2O7, Na2FePO4F, Na4Fe3(PO4)2(P2O7), and olivine NaFePO4 [25e32]. Due to the many advantages of iron-based phosphate materials, such as easy synthesis, long cycle life and high rate cyclability, we selected olivinetype NaFePO4 as a candidate material for use in aqueous NIBs. Although olivine NaFePO4 has the same crystalline structure as well-known LiFePO4, it has a different electrochemical intercalation/deintercalation behavior of Na ions compared to that in the same process in LiFePO4. The electrochemical process of Li ions in the LiFePO4 structure proceeds via a distinct two-phase reaction in contrast to that in olivine NaFePO4. Sodiation and desodiation of Na ions are accompanied by the formation of a Na0.7FePO4 intermediate phase that displays a lower cycling stability than LiFePO4, which is related to the thermodynamically unstable phase of olivine NaFePO4 and the reaction kinetics of the electrochemical process [26,33,34]. Therefore, the synthesis of olivine NaFePO4 by the conventional solid-state reaction is accompanied by the formation of an unfavorable phase that limits the diffusion of Na ions [35,36]. To overcome this problem, many researchers have used an electrochemical ion-exchange process in an electrochemical system to replace the Li ions in LiFePO4 with Na ions [37,38]. This electrochemical ion-exchange process is usually carried out as a two-step transformation from LiFePO4 (LiFePO4 transforms to FePO4 in a lithium-containing aqueous electrolyte) to NaFePO4 (FePO4 transforms to NaFePO4 in a sodium-containing aqueous electrolyte). Only a few researchers have investigated aqueous NIBs [39e41]. Additionally, surface failure, which is a critical problem in aqueous electrolytes, has not been studied in detail. In this work, to obtain high-performance aqueous rechargeable NIBs with NaFePO4, AlF3 was coated on the active material to reduce the surface electrolyte decomposition. We report that pristine or AlF3-coated LiFePO4based materials were transformed into pristine NaFePO4 materials via an electrochemical ion-exchange process in an aqueous electrolyte. This ion-exchange process is attractive due to its facile processing, low cost, and use of environmentally friendly materials instead of nonaqueous electrolyte. The NaFePO4 materials obtained by this electrochemical ion-exchange process display improved cycling stability due to the AlF3 coating enhancing the surface stability in an aqueous electrolyte; the coating material is an attractive surface agent for anode and cathode materials of ARLBs that prevents the formation of unwanted side reaction components
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in an aqueous medium [42e44]. 2. Experimental LiFePO4 was obtained from the MTI Corporation and used without additional treatment. AlF3-coated LiFePO4 was prepared using a previously reported chemical deposition method [42]. To prepare AlF3-LiFePO4, starting materials such as NH4F (Aldrich) and Al(NO3)3 9H2O (Aldrich) were added to LiFePO4 and then mixed for 20 min with water with the same mass as LiFePO4; the Al to F molar ratio was controlled at 1:3. The total content of AlF3 on LiFePO4 was 0.5 and 1 wt%, and the obtained samples were denoted AlF3-0.5-LFP and AlF3-1-LFP, respectively. Then, the obtained slurry containing the coating precursors and LiFePO4 was dried at 120 C for 1 h and heated at 400 C for 2 h under an argon flow. The working electrode was prepared by mixing the active materials, acetylene black and poly(tetrafluoroethylene) (PTFE) with a weight ratio of 80:10:10 on a stainless-steel mesh of 0.25 cm2 with a loading amount of 2 mg cm2. To prepare the counter electrode, activated carbon was prepared using the same method used for the working electrode, namely, coating the surface of a stainless-steel mesh of 1 cm2 using a loading amount of 25 mg cm2. A silver chloride electrode (Ag/AgCl, þ0.197 V vs. SHE) was used as the reference electrode. Then, 1 M Li2SO4 and 1 M Na2SO4 aqueous electrolyte solutions were used for the electrochemical ionexchange process and electrochemical measurements. The electrochemical ion-exchange process to transform LiFePO4 into NaFePO4 was carried out in three-electrode cells with working, counter and reference electrodes. Then, the bare and AlF3-coated LiFePO4 electrodes were galvanostatically delithiated at a rate of 1 C (1 C ¼ 170 mAh g1) to 0.8 V (vs. Ag/AgCl) in a 1 M Li2SO4 aqueous electrolyte solution; then, the same electrodes (FePO4) were washed and subsequently galvanostatically sodiated at 0.1 C to 0.5 V (vs. Ag/AgCl) in a 1 M Na2SO4 aqueous electrolyte solution to obtain olivine NaFePO4. The obtained pristine and coated NaFePO4 electrodes were washed by distilled water and then dried at 80 C. For all of the electrodes in the aqueous ion-exchange process, the applied current and resulting capacity were calculated from the weights of the pristine and coated LiFePO4 electrodes. Electrodes of NaFePO4 with or without the AlF3 coating were used for electrochemical measurements in the three-electrode cells described above. Repeated charge-discharge cycles were carried out using a WonaTech potentiostat/galvanostat instrument in the potential range from 0.5e0.85 V (vs. Ag/AgCl) in a 1 M Na2SO4 aqueous electrolyte solution. The electrochemical behavior of LiFePO4 and NaFePO4 was investigated by cyclic voltammetry (CV) using a WonaTech potentiostat/galvanostat in the potential range from 0.2e0.8 V (for LiFePO4) and 0.7e1.1 V (for NaFePO4) at a scan rate of 0.1 mV s1 (vs. Ag/AgCl). Electrochemical impedance spectroscopy (EIS) measurements were carried out using a multichannel electrochemical workstation (Zive Lab, MP1) in the frequency range from 100 kHz to 10 mHz at a voltage amplitude of 10 mV. The EIS data were obtained after charge/discharge cycling. To investigate the obtained pristine and AlF3-coated FePO4 and NaFePO4 after delithiation and sodiation, respectively, X-ray diffraction (XRD) and field scanning electron microscopy (FE-SEM) with energy dispersive X-ray spectroscopy (EDS) analyses were performed. The crystalline structures of the bare and AlF3-coated FePO4 and NaFePO4 were determined by XRD using a Rigaku SmartLab diffractometer with Cu Ka radiation (40 kV, 250 mA) in the 2q range of 10e80 at a scan rate of 0.03 s1. The surface morphologies of these samples were observed using FE-SEM (JSM 7800F) with EDS and transmission electron microscopy (TEM, JEOL ARM-200F).
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The ionic conductivity of the aqueous electrolyte solutions (1 M Li2SO4 and 1 M Na2SO4) was determined in a cell (Sanxin, 2401-M Glass conductivity electrode) with two symmetric platinum electrodes and a cell constant of 1 cm1 using a multichannel electrochemical workstation from Zive Lab (MP1) at a temperature of 25 C in a calorstat that maintained the set temperature for 1 h.
3. Results and discussion An electrochemical ion-exchange process was used to transform olivine LiFePO4 into NaFePO4, as shown in Fig. 1a. During the initial delithiation process in the 1 M Li2SO4 aqueous electrolyte solution, the initial delithiation capacity was recorded as 134.9 mAh g1 with a typical voltage plateau at 0.25 V vs. Ag/AgCl. Then, the sodiation process of the electrode in the 1 M Na2SO4 aqueous electrolyte solution was performed, and a sodiation capacity of 122.6 mAh g1 with a plateau at 0.15 V was determined. The crystalline structures of FePO4 and NaFePO4 obtained after the delithiation and sodiation processes were elucidated by XRD analyses, and the results are presented in Fig. 1b. After the initial delithiation process, the structure of FePO4 was detected (JCPDF card no. 70-1793). Similarly, after sodiation, the XRD pattern is in excellent agreement with that of NaFePO4 [45,46]. These findings indicate that the crystalline structure of olivine LiFePO4 was successfully transformed into NaFePO4 through an FePO4 intermediate. The morphologies of the FePO4 and NaFePO4 materials obtained after the aqueous ion-exchange process were characterized by SEM and TEM (Fig. S3, Supporting Information and Fig. 2, respectively).
The SEM images of the pristine NaFePO4 material displayed the expected surface morphology and particle size while the images of the AlF3-coated NaFePO4 surface showed changes. The TEM analysis revealed that the surface of the AlF3-coated NaFePO4 cathode is distinguishable from that of pristine NaFePO4 with a regular surface morphology. The AlF3-coating layer on NaFePO4 is approximately 10 nm, as observed by TEM analysis. EDS analysis also confirmed the atomic ratios of FePO4 (Na: Fe: P: O of 0: 1: 1: 4) and NaFePO4 (Na: Fe: P: O of 1: 1: 1: 4), as shown in Fig. S4a and b (Supporting Information). In addition, these ratios were valid for the AlF3coated NaFePO4 obtained via electrochemical aqueous ionexchange processes (Fig. S4c and d, Supporting Information). Moreover, the uniform elemental distributions of Na, P, Fe, O, Al, F, and C on the surfaces of FePO4 and the pristine and AlF3-coated NaFePO4 are shown in Fig. 3. It should be noted that the presence of C on the surface of the obtained samples is common for all electrode materials because the inert synthetic conditions result in residual carbon from the precursors. Additionally, the presence of a possible carbon coating on the surface of the pristine and coated LiFePO4, NaFePO4 and FePO4 was observed in the EDS analysis. The presence of carbon on the surface of the olivine materials was confirmed by thermogravimetric analysis (TGA) [42]. The mass of the samples during TGA decreased to a small amount due to the possible formation of unwanted compounds above 600 C in air [47]. Therefore, the carbon amount on the surface of the samples did not have a crucial effect on the electrochemical performance. Thus, on the basis of the XRD, SEM, TEM and EDS analyses, pristine and coated NaFePO4 were obtained via an aqueous ion-exchange
Fig. 1. (a) Illustration of the aqueous electrochemical ion-exchange process from olivine LiFePO4 to FePO4 in 1 M Li2SO4 aqueous medium at 1 C rate to 0.8 V and FePO4 to NaFePO4 in 1 M Na2SO4 aqueous medium at 0.1 C rate to 0.5 V. (b) XRD patterns of the FePO4 and NaFePO4 samples obtained at the galvanostatic cycling stages as shown in Fig. 1a.
Fig. 2. Surface analysis by TEM of (a) pristine NaFePO4 and (b) AlF3-1-NFP samples.
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Fig. 3. EDS mapping of the elements (a) O, (b) Na, (c) Fe, (d) P, (e) C, (f) Al, and (g) F, after delithiation: FePO4, after sodiation: pristine NaFePO4, AlF3-0.5-NFP and AlF3-1-NFP during the aqueous electrochemical ion-exchange process.
process due to the strong PO bonds in the olivine structure and the similar ionic radii of Liþ and Naþ ions enabling retention of the olivine crystal structure. This method was also successful for NaFePO4 coated with 0.5 and 1 wt% AlF3. As shown in Fig. 4a, the cyclic voltammogram of olivine LiFePO4 has a single pair of reduction and oxidation peaks at 0.03 (1st peak) and 0.32 V (2nd peak), respectively, which are related to the intercalation and deintercalation of Li ions into/from the structure in 1 M Li2SO4; the FePO4 þ Liþ þ e / LiFePO4 reaction causes the 1st peak and the LiFePO4 - Liþ - e / FePO4 reaction results in the 2nd peak. For NaFePO4, a single peak was observed at 0.25 V, which was attributed to the sodiation process (1st peak: FePO4 þ Naþ þ e / NaFePO4), and two peaks were observed at 0.04 V and 0.22 V, which were attributed to the desodiation processes (NaFePO4 - Naþ - e / Na2/3FePO4 for the 2nd peak and Na2/ þ 3FePO4 - Na - e / FePO4 for the 3rd peak). The CV data indicated that the electrochemical intercalation and deintercalation process of olivine LiFePO4 with Li ions in an aqueous solution is faster than that of Na ions into/from the structure of NaFePO4. This difference is related to the phase transition during the electrochemical process, resulting in the larger charge transfer resistance and slower reaction kinetics of the olivine structure for Na ions than Li ions. It is important to mention that NaFePO4 coated with 0.5 and 1 wt% AlF3 shows a similar electrochemical process as the pristine material, as shown in Fig. 4b. However, the areas of the voltammograms for the
coated materials were smaller than those for pristine NaFePO4, indicating a positive effect on the rapid reaction kinetics of the intercalation and deintercalation of Na ions into/from the NaFePO4 structure. Thus, the AlF3 coating on NaFePO4 has a beneficial role in improving the cycling performance of the material in an aqueous electrolyte. In addition, the CV data obtained in the aqueous electrolyte are in agreement with the results obtained in an organic electrolyte [28,48]. Moreover, an examination of the voltage curves presented in Fig. 5 shows that the electrochemical process of intercalation and deintercalation of Na ions into/from the NaFePO4 structure is strongly correlated with the obtained CV data (Fig. 4). Fig. 5a shows the initial charging curve of the bare sample, which exhibited two plateaus at 0.25 and 0.75 V vs. Ag/AgCl with a total capacity of 101.7 mAh g1. The initial plateau at 0.25 V corresponds to the desodiation reactions from NaFePO4, but the two desodiation steps shown in Fig. 4a are not clearly observed due to high polarization. However, the other plateau at 0.8 V in the initial charging curve does not correspond to a desodiation reaction. Similarly, the discharge curve exhibits only one plateau, which is a reversible peak of the charging plateau at 0.25 V because it has a specific capacity of 76.5 mAh g1, and this capacity is almost the same as the capacity of the initial charging curve. The initial Coulombic efficiency was also very poor and is only 75.5% for the bare NaFePO4 in the aqueous electrolyte. Subsequently, the 2nd charging capacity
Fig. 4. (a) Cyclic voltammetry of the pristine LiFePO4 and NaFePO4 at a scan rate of 1 mV s1 in the potential range of 0.2e0.8 V for LiFePO4 and -0.7e1.1 V for NaFePO4. (b) Cyclic voltammetry of the pristine and AlF3-coated NaFePO4 (0.5 and 1 wt% AlF3) at a scan rate of 1 mV s1 in the potential range of 0.7e1.1 V. Note that the reference electrode is Ag/AgCl (þ0.197 V vs. SHE).
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Fig. 5. Charge/discharge voltage profiles of the (a) pristine NaFePO4, (b) AlF3-0.5-NFP, (c) AlF3-1-NFP materials at the rate of 1 C (154 mAh g1) in the potential range of 0.5e0.85 V in 1 M Na2SO4 aqueous electrolyte solution.
Fig. 6. (a) Cyclic performance and (b) Nyquist plots of pristine and coated NaFePO4 materials at selected cycle (20th cycle) in Fig. 6a in 1 M Na2SO4 aqueous electrolyte solution at the rate of 1 C (154 mAh g1), (c) rate capabilities of the NaFePO4 material at the rates of 0.1, 0.2, 0.5, 1, and 2 C in the potential range of 0.5e0.85 V.
suddenly decreased to 69.8 mAh g1, corresponding to the discharge capacity. It is believed that the unexpected plateau originated from a side reaction of the electrochemically fragile aqueous electrolyte, which caused surface deterioration, to generate compounds such as FeO or FeOH [39]. Surface passivation creates a protective layer to minimize electrolyte side reactions, and therefore, the 2nd charging capacity suddenly decreased due to high polarization. However, the initial charging curves of the coated sample shown in Fig. 5b and c do not exhibit the second plateau at 0.85 V. In addition to the 2nd plateau, desodiation at approximately 0.25 V had higher capacities (95.4 and 87.1 mAh g1) than the bare sample. With the 0.5 wt% AlF3 coating, the polarization between charge and discharge was reduced. Therefore, the 1st specific discharge capacity was recorded as 83.9 mAh g1, which is higher than that of the bare sample (76.2 mAh g1). Despite the absence of the 2nd plateau in the 1st desodiation curve, the initial Coulombic efficiencies of the coated samples were not very good (87.9% and 87.5%). As the coating amount increased to 1 wt%, the polarization increased again. With the similar polarization of AlF3-1-NFP, the thick coating of 1 wt% AlF3 resulted in high polarization and showed an initial discharge capacity similar to that of the bare sample. Fig. 6a displays the cyclability of the prepared pristine and coated NaFePO4 (0.5 and 1 wt% AlF3) samples in the 1 M Na2SO4 aqueous electrolyte solution. The initial discharge capacity of pristine NaFePO4 is 101.7 mAh g1, while the 0.5 and 1 wt% AlF3-coated samples have an initial discharge capacity of 95.6 and 86.3 mAh g1, respectively. These capacities are lower than the capacity of the pristine material due to the surface resistance of the AlF3 coating on the surface of NaFePO4 hindering sodium transportation processes through the surface. For the sample with a coating layer of 1 wt%, the initial capacity was lower than that of the pristine and AlF3-0.5-NFP samples. Moreover, the presence of a large amount of AlF3
coating material on the surface of NaFePO4 can result in resistance and high polarization, which hinder the sodium transport processes through the interface of the electrode and aqueous electrolyte during cycling because the AlF3 coating has nonionic conducting nature. The capacity of the coated samples was 55.8 and 51.9 mAh g1 for 0.5 and 1 wt% AlF3, respectively, at the 50th cycle, while the pristine sample had a lower capacity of 40.3 mAh g1. Therefore, a suitable amount of AlF3 coating on the surface of NaFePO4 can play a very important role in improving the electrochemical performance, leading to a stabilization process during repeated charge and discharge cycling that can effectively reduce polarization and enhance the kinetic processes through the interface. It was clearly observed that the bare sample had the poorest cyclability of the samples. The AlF3-0.5-NFP sample exhibited the best cyclability. EIS measurements were performed after the 20th cycle and were fitted to the equivalent circuit presented in the inset of Fig. 6b. The diameter of the semicircle in the Nyquist plot represents the surface resistance, as described in the equivalent circuit [42,43]. The surface resistance of the AlF3-0.5-NFP sample was fitted as 45 Ohm, which is much lower than the surface resistance of pristine NaFePO4 and AlF3-1-NFP, i.e., 500 and 321 Ohm, respectively. Moreover, the AlF3-0.5-NFP sample has a smaller cycling resistance than the pristine NaFePO4 and AlF3-1-NFP samples. As indicated in Ref. [42], olivine materials with an AlF3 coating can better reduce the surface resistance and facilitate lithium diffusion through an electrode and aqueous electrolyte interface than organic electrolytes with a conventional SEI layer. However, the surface of the active electrode materials in the aqueous electrolyte can have a passivation layer [42,44]. Therefore, the EIS data were consistent with the electrochemical performance, as shown in Fig. 6a. This result is due to the AlF3 coating on the surface of olivine NaFePO4 leading to surface stabilization and improving the reaction kinetics
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Table 1 The comparison of the olivine NaFePO4 in the aqueous electrolyte solutions for aqueous rechargeable sodium ion battery (ARLBs). Capacity in the 1st cycle (mAh g1) Capacity in the nth cycle (mAh g1) Coulombic efficiency (%) Electrolyte solution Reference
Materials
Anode materials
NaFePO4/C NaFePO4/C NaFePO4 NaFePO4/AlF3
Activated carbon 87 (1 C) Pt 118.3 (10 mV s1) Activated carbon 101.7 (1 C) Activated carbon 95.6 (1 C)
59 (1 C, 30 cycles)
92 (1st cycle)
40.3 (1 C, 50 cycles) 55.8 (1 C, 50 cycles)
75.5 (1st cycle) 87.9 (1st cycle)
through a coating material interface. The amount of the AlF3 coating should be optimized because these two mechanisms compete, as shown by the opposite trends with increasing coating amounts. Previously, we reported that the optimal AlF3 coating is 1 wt% for LiFePO4 in the same aqueous electrolyte [42]. In this work, the lower 0.5 wt% AlF3 content was the best for NaFePO4. The larger Na ions (compared to Li ions) have a higher ionic transport resistance through the passivation layer. Therefore, the successful application of AlF3-coating materials to improve the cycling stability of olivine Na-based electrode materials in an aqueous electrolyte requires the consideration of the amount of coating material due to its highly nonionic conducting behavior. The obtained pristine and AlF3-coated NaFePO4 samples were subjected to rate capability tests, as presented in Fig. 6c. The AlF30.5-NFP sample maintained a higher specific discharge capacity at various current densities than the pristine and AlF3-1-NFP samples, and this difference is related to the large electrode polarization (Fig. 5aec). It should be noted that during the electrochemical process, the particles of NaFePO4 need fast Naþ ion transfer through the electrolyte/electrode interface. The additional layer on the surface of the electrode (for the pristine sample, the layer formed from unwanted side reactions [42]; for the coated sample, the AlF3coating layer) can add an energy barrier to the transfer of Naþ ions. Therefore, the higher ionic conductivity of the 1 M Na2SO4 aqueous electrolyte solution (68.2 mS cm1) compared to that of 1 M Li2SO4 (43.6 mS cm1) is a crucial factor to ensure enhanced Naþ ion migration through the interfaces and improved cycling performance (Fig. S5, Supporting Information). Thus, NaFePO4 coated with 0.5 wt% AlF3 displays an enhanced electrochemical performance and lower surface resistance and electrode polarization than the pristine and 1 wt% AlF3-coated NaFePO4 due to better Naþ ion transfer from the bulk electrolyte to the electrode/aqueous electrolyte interface, indicating good cycling stability in the aqueous electrolyte solution of the AlF3-0.5-NFP cathode. Table 1 shows a comparison of the cycling stability of the olivine NaFePO4 cathodes in this study with previous reference data for aqueous rechargeable NIBs [29,49]. Previous data also showed the same electrochemical behavior shown in Fig. 5, which appears to be typical for olivine NaFePO4-based electrode materials [28e33]. The electrochemical performance shows that the discharge capacity is 95.6 mAh g1 at a 1 C rate and the capacity is 55.8 mAh g1 at 50 cycles. CanasCabanas reported that a NaFePO4/C sample showed a low initial discharging capacity of 87 mAh g1 at a 1 C rate and cycle stability of 59 mAh g1 after 30 cycles [29]. However, Vujkovic reported that an FePO4 material after sodiation at 10 mV s1 maintained a discharge capacity of 118 mAh g1 [49]. Table 1 shows that the AlF3coated NaFePO4 maintains a better electrochemical performance in the aqueous electrolyte. 4. Conclusion In summary, NaFePO4 cathode materials with surfaces modified by AlF3 were obtained by an electrochemical ion-exchange process in an aqueous electrolyte, which is a simple, low cost, and more environmentally friendly material than nonaqueous electrolytes. This olivine NaFePO4 material displays a discharge capacity of
Na2SO4 (1 M) NaNO3 (sat) Na2SO4 (1 M) Na2SO4 (1 M)
[29] [49] In this paper In this paper
55.8 mAh g1 at 1 C with better surface stability in an aqueous electrolyte solution with the 0.5 wt% AlF3 coating. Physicochemical and electrochemical measurements confirmed the improvement in the electrochemical performance of NaFePO4 with the AlF3 coating, which reduces surface failure by stabilizing the surface of pristine NaFePO4 in an aqueous electrolyte, leading to the reduced surface resistance and improved surface stability of iron-based phosphate materials. Therefore, we suggest that AlF3 coatings are attractive for the surface stabilization of anode and cathode materials in aqueous rechargeable lithium- and sodium-ion batteries. Acknowledgments This work was also supported through Incheon National University (INU), Incheon, Republic of Korea (2015-1831). This research was also supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2018R1D1A1B07048144). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.01.046. References [1] M.R. Palacin, Recent advances in rechargeable battery materials: a chemist's perspective, Chem. Soc. Rev. 38 (2009) 2565e2575. [2] A. Yoshino, The birth of the lithium-ion battery, Angew. Chem. Int. Ed. 51 (2012) 5798e5800. [3] B. Dunn, H. Kamath, J.M. Tarascon, Electrical energy storage for the grid: a battery of choices, Science 334 (2011) 928e935. [4] J.B. Goodenough, Y. Kim, Challenges for rechargeable Li batteries, Chem. Mater. 22 (2010) 587e603. [5] C.P. Grey, J.M. Tarascon, Sustainability and in situ monitoring in battery development, Nat. Mater. 16 (2016) 45e56. [6] W. Li, J.R. Dahn, D.S. Wainwright, Rechargeable lithium batteries with aqueous electrolytes, Science 264 (1994) 1115e1118. [7] H. Kim, J. Hong, K.-Y. Park, H. Kim, S.-W. Kim, K. Kang, Aqueous rechargeable Li and Na ion batteries, Chem. Rev. 114 (2014) 11788e11827. [8] C. Vaalma, D. Buchholz, M. Weil, S. Passerini, A cost and resource analysis of sodium-ion batteries, Nat. Rev. Mater. 3 (2018) 18013. [9] D. Bin, F. Wang, A.G. Tamirat, L. Suo, Y. Wang, C. Wang, Y. Xia, Progress in aqueous rechargeable sodium-ion batteries, Adv. Energy Mater. (2018), 1703008. [10] K. Kubota, M. Dahbi, T. Hosaka, S. Kumakura, S. Komaba, Towards K-Ion and Na-Ion batteries as “beyond Li-Ion”, Chem. Rec. 18 (2018) 1e22. [11] H. Zhang, S. Jeong, B. Qin, D.V. Carvalho, D. Buchholz, S. Passerini, Towards high-performance aqueous sodium-ion batteries: stabilizing the solid/liquid interface for NASICON-type Na2VTi(PO4)3 using concentrated electrolytes, ChemSusChem 11 (2018) 1382e1389. [12] H. Long, W. Zeng, H. Wang, M. Qian, Y. Liang, Z. Wang, Self-assembled biomolecular 1D nanostructures for aqueous sodium-ion battery, Adv. Sci. 5 (2018), 1700634. [13] W. Li, F. Zhang, X. Xiang, X. Zhang, Nickel-substituted copper hexacyanoferrate as a superior cathode for aqueous sodium-ion batteries, ChemElectroChem 5 (2018) 350e354. [14] P. Lei, Y. Wang, F. Zhang, X. Wan, X. Xiang, Carbon-coated Na2.2V1.2Ti0.8(PO4)3 cathode with excellent cycling performance for aqueous sodium-ion batteries, ChemElectroChem 5 (2018) 2482e2487. [15] H. Chen, H. Mi, L. Sun, P. Zhang, Y. Li, One-step synthesis of 3D-sandwiched Na3V2(PO4)2O2F@rGO composites as cathode material for high-rate sodiumion batteries, ChemElectroChem 5 (2018) 2593e2599. [16] J. Wang, C. Mi, P. Nie, S. Dong, S. Tang, X. Zhang, Sodium-rich iron hexacyanoferrate with nickel doping as a high performance cathode for aqueous
726
S. Jeong et al. / Journal of Alloys and Compounds 784 (2019) 720e726
sodium ion batteries, J. Electroanal. Chem. 818 (2018) 10e18. [17] F. Zhang, W. Li, X. Xiang, M. Sun, Highly stable Na-storage performance of Na0.5Mn0.5Ti0.5O2 microrods as cathode for aqueous sodium-ion batteries, J. Electroanal. Chem. 802 (2017) 22e26. [18] W. Li, F. Zhang, X. Xiang, X. Zhang, High-efficiency Na-storage performance of a nickel-based ferricyanide cathode in high-concentration electrolytes for aqueous sodium-ion batteries, ChemElectroChem 4 (2017) 2870e2876. [19] Y. Wang, Z. Feng, D. Laul, W. Zhu, M. Provencher, M.L. Trudeau, A. Guerfi, K. Zaghi, Ultra-low cost and highly stable hydrated FePO4 anodes for aqueous sodium-ion battery, J. Power Sources 374 (2018) 211e216. [20] J.-Y. Hwang, S.-T. Myung, Y.-K. Sun, Sodium-ion batteries: present and future, Chem. Soc. Rev. 46 (2017) 3529e3614. [21] H. Kim, H. Kim, Z. Ding, M.H. Lee, K. Lim, G. Yoon, K. Kang, Recent progress in electrode materials for sodium-ion batteries, Adv. Energy Mater. (2016), 1600943. [22] S.Y. Hong, Y. Kim, Y. Park, A. Choi, N.-S. Choi, K.T. Lee, Charge carriers in rechargeable batteries: Na ions vs. Li ions, Energy Environ. Sci. 6 (2013) 2067e2081. [23] M.I. Jamesh, A.S. Prakash, Advancement of technology towards developing Na-ion batteries, J. Power Sources 378 (2018) 268e300. [24] Y. Wang, L. Mu, J. Liu, Z. Yang, X. Yu, L. Gu, Y.-S. Hu, H. Li, X.-Q. Yang, L. Chen, X. Huang, A novel high capacity positive electrode material with tunnel-type structure for aqueous sodium-ion batteries, Adv. Energy Mater. 5 (2015), 1501005. [25] H. Kim, R.A. Shakoor, C. Park, S.Y. Lim, J.-S. Kim, Y.N. Jo, W. Cho, K. Miyasaka, R. Kahraman, Y. Jung, J.W. Choi, Na2FeP2O7 as a promising iron-based pyrophosphate cathode for sodium rechargeable batteries: a combined experimental and theoretical study, Adv. Funct. Mater. 23 (2013) 1147e1155. [26] V. Palomares, P. Serras, I. Villaluenga, K.B. Hueso, J. Carretero-Gonzalez, T. Rojo, Na-ion batteries, recent advances and present challenges to become low cost energy storage systems, Energy Environ. Sci. 5 (2012) 5884e5901. [27] H. Kim, I. Park, D.-H. Seo, S. Lee, S.-W. Kim, W.J. Kwon, Y.-U. Park, C.S. Kim, S. Jeon, K. Kang, New iron-based mixed-polyanion cathodes for lithium and sodium rechargeable batteries: combined first principles calculations and experimental study, J. Am. Chem. Soc. 134 (2012) 10369e10372. [28] Y. Zhu, Y. Xu, Y. Liu, C. Luo, C. Wang, Comparison of electrochemical performances of olivine NaFePO4 in sodium-ion batteries and olivine LiFePO4 in lithium-ion batteries, Nanoscale 5 (2013) 780e787. ndez-Ropero, D. Saurel, B. Acebedo, T. Rojo, M. Casas-Cabanas, [29] A.J. Ferna Electrochemical characterization of NaFePO4 as positive electrode in aqueous sodium-ion batteries, J. Power Sources 291 (2015) 40e45. [30] D. Bin, F. Wang, A.G. Tamirat, L. Suo, Y. Wang, C. Wang, Y. Xia, Progress in aqueous rechargeable sodium-ion batteries, Adv. Energy Mater. 8 (2018), 1703008. [31] M. Hilder, P.C. Howlett, D. Saurel, H. Anne, M. Casas-Cabanas, M. Armand, T. Rojo, D.R. MacFarlane, M. Forsyth, Stable cycling of NaFePO4 cathodes in high salt concentration ionic liquid electrolytes, J. Power Sources 406 (2018) 70e80. [32] D. Saurel, M. Galceran, M. Reynaud, H. Anne, M. Casas-Cabanas, Rate dependence of the reaction mechanism in olivine NaFePO4 Na-ion cathode material, Int. J. Energy Res. 42 (2018) 3258e3265.
[33] K.T. Lee, T.N. Ramesh, F. Nan, G. Botton, L.F. Nazar, Topochemical synthesis of sodium metal phosphate olivines for sodium-ion batteries, Chem. Mater. 23 (2011) 3593e3600. [34] P. Moreau, D. Guyomard, J. Gaubicher, F. Boucher, Structure and stability of sodium intercalated phases in olivine FePO4, Chem. Mater. 22 (2010) 4126e4128. [35] M. Galceran, D. Saurel, B. Acebedo, V.V. Roddatis, E. Martin, T. Rojo, M. CasasCabanas, The mechanism of NaFePO4 (de)sodiation determined by in situ Xray diffraction, Phys. Chem. Chem. Phys. 16 (2014) 8837e8842. [36] J. Lu, S.C. Chung, S.-I. Nishimura, A. Yamada, Phase diagram of olivine NaxFePO4 (0 < x< 1), Chem. Mater. 25 (2013) 4557e4565. [37] W. Tang, X. Song, Y. Du, C. Peng, M. Lin, S. Xi, B. Tian, J. Zheng, Y. Wu, F. Pan, K.P. Loh, High-performance NaFePO4 formed by aqueous ion-exchange and its mechanism for advanced sodium ion batteries, J. Mater. Chem. A 4 (2016) 4882e4892. [38] Y. Fang, Q. Liu, L. Xiao, X. Ai, H. Yang, Y. Cao, High-performance olivine NaFePO4 microsphere cathode synthesized by aqueous electrochemical displacement method for sodium ion batteries, ACS Appl. Mater. Interfaces 7 (2015) 17977e17984. [39] M. Zhao, W. Zhang, F. Qu, F. Wang, X. Song, Good discharge capacities of NaV6O15 material for an aqueous rechargeable lithium battery, Electrochim. Acta 138 (2014) 187e192. [40] S.S. Zhang, The redox mechanism of FeS2 in non-aqueous electrolytes for lithium and sodium batteries, J. Mater. Chem. A 3 (2015) 7689e7694. [41] F. Zhang, W. Li, X. Xiang, M. Sun, Nanocrystal-assembled porous Na3MgTi(PO4)3 aggregates as highly stable anode for aqueous sodium-ion batteries, Chem. Eur. J. 23 (2017) 12944e12948. [42] A. Tron, Y.N. Jo, S.H. Oh, Y.D. Park, J. Mun, Surface modification of the LiFePO4 cathode for the aqueous rechargeable lithium ion battery, ACS Appl. Mater. Interfaces 9 (2017) 12391e12399. [43] A. Tron, Y.D. Park, J. Mun, AlF3-coated LiMn2O4 as cathode material for aqueous rechargeable lithium battery with improved cycling stability, J. Power Sources 325 (2016) 360e364. [44] A. Tron, H. Kang, J. Kim, J. Mun, Electrochemical performance of AlF3-coated LiV3O8 for aqueous rechargeable lithium ion batteries, J. Electrochem. Sci. Technol. 9 (1) (2018) 60e68. [45] J. Hwang, K. Matsumoto, T. Nohira, R. Hagiwara, Electrochemical sodiationdesodiation of maricite NaFePO4 in ionic liquid electrolyte, Electrochemistry 85 (10) (2017) 675e679. [46] N. Wongittharom, T.C. Lee, C.H. Wang, Y.C. Wang, J.K. Chang, Electrochemical performance of Na/NaFePO4 sodium-ion batteries with ionic liquid electrolytes, J. Mater. Chem. A 2 (2014) 5655e5661. [47] I. Belharouak, C. Johnson, K. Amine, Synthesis and electrochemical analysis of vapor-deposited carbon-coated LiFePO4, Electrochem. Commun. 7 (2005) 983e988. lez, [48] M. Casas-Cabanas, V.V. Roddatis, D. Saurel, P. Kubiak, J. Carretero-Gonza V. Palomares, P. Serras, T. Rojo, Crystal chemistry of Na insertion/deinsertion in FePO4NaFePO4, J. Mater. Chem. 22 (2012) 17421e17423. [49] M. Vujkovic, S. Mentus, Fast sodiation/desodiation reactions of electrochemically delithiated olivine LiFePO4 in aerated aqueous NaNO3 solution, J. Power Sources 247 (2014) 184e188.