Amorphization of Sodium Cobalt Oxide Active Materials for High

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Amorphization of sodium cobalt oxide active materials for high capacity all-solid-state sodium batteries Yuka Nagata, Kenji Nagao, Minako Deguchi, Atsushi Sakuda, Akitoshi Hayashi, Hirofumi Tsukasaki, Shigeo Mori, and Masahiro Tatsumisago Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b01872 • Publication Date (Web): 17 Sep 2018 Downloaded from http://pubs.acs.org on September 18, 2018

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Chemistry of Materials

aAmorphization of sodium cobalt oxide active materials for high ccapaci pacity allall-solidsolid-state sodium batteries Yuka Nagata,1 Kenji Nagao,1 Minako Deguchi,1 Atsushi Sakuda,1 Akitoshi Hayashi,1,* Hirofumi Tsukasaki,2 Shigeo Mori,2 Masahiro Tatsumisago1 1 Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1-1, Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan 2 Department of Materials Science, Graduate School of Engineering, Osaka Prefecture University, 1-1, Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan.

ABSTRACT: Amorphous Na0.7CoO2-NaxMOy (M=N, S, P, B, or C) positive electrode active materials were synthesized by a mechanochemical technique to achieve high capacities and improved cyclabilities owing to their open and random structures. As none of the X-ray diffraction peaks are attributable to the starting materials, it was clear that the reaction between Na0.7CoO2 and NaxMOy had been successful. The prepared Na0.76Co0.8N0.2O2.2 (80Na0.7CoO2·20NaNO3 (mol%)) was easily densified by pressing at room temperature, and then applied as a positive electrode in bulk-type all-solid-state sodium cells (Na15Sn4/Na3PS4 glass-ceramic/Na0.7CoO2-NaxMOy). The cell based on the Na0.76Co0.8N0.2O2.2 active material without any conductive additives in an ultra-thick positive electrode layer (~50 μm thickness) operated as a secondary battery at 25 °C. The average discharge voltage was 2.9 V, and the initial discharge capacity was 70 mAh per gram of the positive electrode. This cell exhibited a higher discharge voltage and a larger capacity than cells employing crystalline Na0.7CoO2 or milled Na0.7CoO2 as the positive electrode. The electrochemical properties of Na0.7CoO2 were therefore improved by amorphization with NaNO3. Furthermore, the cell with the composite electrode containing a conducting additive gave a discharge capacity of 170 mAh per gram of Na0.76Co0.8N0.2O2.2, which is the highest reported to date for all-solid-state sodium cells based on oxide positive electrodes. Therefore, the amorphization of layered transition metal oxides with sodium oxy-acids is an effective way to achieve novel active materials with high capacities.

1. Introduction Lithium-ion batteries have been widely used as energy storage devices in portable electronic equipment. However, due to the concerns regarding the cost and future availability of lithium, the development of sodium-ion batteries is desirable due to the comparable abundance of sodium. In addition, although cells based on organic liquid electrolytes have been widely studied, their safety is still a concern. For applications in large-scale batteries, including those of electric vehicles and dispersed individual residential power sources, all-solid-state batteries with nonflammable inorganic solid electrolytes have attracted attention due to their improved safety and reliability.1–3 Higher energy densities can also be achieved by the stacking of cells. So, all-solid-state sodium batteries based on such a structure may be promising next generation batteries. In addition, as the design of the electrode-electrolyte interface is key in the successful operation of all-solid-state batteries, it is particularly important to develop solid electrolytes with high formabilities, high conductivities, and excellent stabilities. Indeed, we have previously developed an Na3PS4 glass-ceramic electrolyte with highly sodium ion conductivity, which could be easily densified by pressing at room temperature.4,5 The pressing step also forms a close

contact between the solid electrolyte and the active material. The resultant all-solid-state sodium cell (NaSn/crystalline TiS2) containing our Na3PS4 glass-ceramic electrolyte worked as a secondary battery with a reversible capacity of ~100 mAh g−1. However, to achieve higher energy densities for practical applications, the development of a novel positive electrode active material with a high capacity and a high operation voltage is necessary. Layered transition metal oxides have been studied as popular active materials for sodium-ion batteries. For example, NaxCoO2 was first reported by Delmas et al. as a positive electrode active material,6 as NaxCoO2 can undergo phase transition. Indeed, P2-Na0.7CoO2 gave the most promising battery performance.6,7 Alternative layered NaTMO2 (TM=transition metal)-type positive electrode active materials such as NaFeO28,9 and NaMnO210,11 have also been reported. To date, the majority of reported active materials are crystalline in nature, while those of amorphous active materials are limited. However, such amorphous materials have many advantages,12–18 including additional stable sites for cations due to their open and random structures, and so this can lead to larger capacities and improved cyclabilities. As such, we expect that the energy densities of active materials could be improved through amorphization. In-

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deed, we previously reported an amorphous TiS3 active material for use in all-solid-state batteries.19,20 The allsolid-state lithium cell based on amorphous TiS3 exhibited a higher capacity than that based on crystalline TiS3.19 However, sulfide-based active materials generally have lower operating potentials than oxide-based ones, and so amorphous oxide compounds can be considered suitable targets for use as high energy density positive electrode active materials. In this context, we proposed the amorphization of lithium transition metal oxides with lithium oxy-acids in the LiCoO2-Li2SO4 system.21,22 The all-solidstate cell using only 80LiCoO2·20Li2SO4 (mol%) as the active material in the positive electrode layer was employed as a secondary battery, and exhibited a large capacity of 163 mAh g−1 at 100 °C. This active material also exhibited high ionic and electronic conductivities in addition to good formability. Indeed, the performance of the cell with amorphous 80LiCoO2·20Li2SO4 was superior to those of cells containing crystalline LiCoO2 or milled LiCoO2. This indicates that the electrochemical performance of LiCoO2 was improved upon amorphization with Li2SO4. We therefore expect that in the sodium system, similar amorphization of the Na0.7CoO2 active material with sodium oxy-acids will improve the electrode performance, and that the prepared materials will possess suitable ionic and electronic conductivities, in addition to sufficient formability for application in all-solid-state sodium cells. Thus, we herein report the synthesis of novel amorphous oxide active materials for application in all-solid-state sodium batteries. The Na0.7CoO2-NaxMOy (M = N, S, P, B, or C) active materials are prepared by a mechanochemical technique from a mixture of Na0.7CoO2 and a sodium oxy-acid, such as NaNO3, Na2SO4, Na3PO4, Na3BO3, or Na2CO3. Allsolid-state sodium batteries based on Na0.7CoO2-NaxMOy are then prepared, and their electrochemical performances are evaluated.

2. Materials and methods Synthesis of Na0.7CoO2-NaxMOy positive electrode acactive materials Na0.7CoO2 crystals were synthesized by a conventional solid-state reaction. A mixture of Co3O4 (99.8%; SigmaAldrich) and Na2CO3 (>99.5%; Wako Pure Chemical Co.) was heated in an alumina crucible at 500 °C for 2 h. The obtained powder was then sintered at 500 °C for 12 h to form the Na0.7CoO2 crystals. Crystals of NaNO3 (>98.0%; Wako Pure Chemical Co.), Na2SO4 (>99.0%; Wako Pure Chemical Co.), and Na2CO3 (>99.5%; Wako Pure Chemical Co.) were heated at 150 °C for 12 h in a vacuum to remove any surface-absorbed water. Similarly, Na3PO4·12H2O (>98.0%; Wako Pure Chemical Co.) was heated at 160 °C for 12 h in a dry Ar atmosphere to remove the water and yield crystals of Na3PO4. In addition, Na3BO3 crystals were synthesized by a conventional solid-state reaction.23 In this case, a mixture of NaOH (>97%; Wako Pure Chemical Co.) and H3BO3 (>99%; Wako Pure Chemical Co.) was heated in an alumina crucible at 400 °C for 1 h in a dry Ar atmosphere. The resulting powder was then calcined at 400 °C for 3 h prior to grinding in an agate mortar, and sintered at 450 °C for 2 h to form crystals of Na3BO3. Finally, the

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80Na0.7CoO2·20NaxMOy (mol%) active materials were synthesized by a mechanochemical technique using planetary ball-milling apparatus (Pulverisette 7; Fritsch GmbH). A mixture (0.6 g) of Na0.7CoO2 and a sodium oxy-acid (i.e., NaNO3, Na2SO4, Na3PO4, Na3BO3, or Na2CO3) was placed in a zirconia pot (45 mL) with 100 zirconia balls (5 mm in diameter) and milled at 370 rpm for 40 h to give the desired active materials. All processes were conducted in a dry Ar atmosphere.

Characterization X-ray diffraction (XRD) measurements (CuKα) were carried out for the Na0.7CoO2-NaxMOy positive electrode active materials (SmartLab; Rigaku), while the morphology of 80Na0.7CoO2·20NaNO3 (mol%) (Na0.76Co0.8N0.2O2.2) was observed by field emission scanning electron microscopy (FE-SEM, SU8220, Hitachi High-Technologies) and highresolution transmission electron microscopy (HR-TEM, JEM-2100F, JEOL). N1s X-ray photoelectron spectroscopy (XPS) analysis was conducted on Na0.76Co0.8N0.2O2.2 and NaNO3 using a Thermo Fisher Scientific K-Alpha apparatus equipped with a monochromatic AlKα source (1486.6 eV). The observed binding energies were calibrated with the adventitious C1s peak at 284.7 eV. All samples were mounted on a sample stage in a glove box filled with dry Ar gas, then transferred to the analysis chamber using an Ar-filled transfer vessel. The electronic and ionic conductivities of the Na0.76Co0.8N0.2O2.2 active material were then measured. To achieve this, the Na0.76Co0.8N0.2O2.2 powder was pressed into a pellet (diameter = 10 mm, thickness = 0.3 mm) under a pressure of 360 MPa. Stainless-steel disks were then attached to the pellet to act as current collectors, and the electronic conductivity was measured via the DC polarization method using a potentiostat/galvanostat (SI-1287; Solartron) with an applied voltage and measurement time of 1 mV and 120 s, respectively. To evaluate the sodium ionic conductivity of the Na0.76Co0.8N0.2O2.2 active material, an electron blocking cell with a five-layer pellet (i.e., Na15Sn4/Na3PS4 glass-ceramic/Na0.76Co0.8N0.2O2.2/Na3PS4 glass-ceramic/Na15Sn4) was fabricated by pressing at room temperature and 360 MPa. For this pellet, the electron blocking layer was composed of a Na3PS4 glass-ceramic. The five-layer pellet was then sandwiched by stainlesssteel disks, and DC polarization measurements were conducted at 25 °C using an applied voltage and a measurement time of 100 mV and 3600 s, respectively. Following polarization, the terminal current was detected, and the total resistance was calculated by Ohm’s law. Similarly, the resistance of a cell containing a Na3PS4 glass-ceramic component and an interfacial component between Na3PS4 and Na15Sn4 was also calculated using a symmetric cell (i.e., Na15Sn4/Na3PS4 glass-ceramic/Na15Sn4). Finally, the ionic conductivity of the active material was calculated from the difference between the resistances of the two cells.

Fabrication of the allall-solidsolid-state sodium batteries The Na3PS4 glass-ceramic electrolyte powder was prepared via mechanochemistry and consecutive heat treat-


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ment.4,24 The Na15Sn4 negative electrode powder was also prepared by the same technique, but using a mixture of Na metal, Sn metal, and KB (the conducting additive) in a stainless-steel pot (45 mL) with 10 stainless-steel balls (10 mm in diameter).24,25 The weight ratio of Na15Sn4:KB was 19:2. Subsequently, the all-solid-state cells were prepared as follows. The Na15Sn4-KB powder (~40 mg, negative electrode), Na3PS4 glass-ceramic powder (~80 mg, solid electrolyte), and the prepared Na0.7CoO2-NaxMOy (~10 mg, positive electrode) were set in a polycarbonate tube (10 mm diameter) and pressed together under a pressure of 360 MPa. The resulting three-layer pellets were then sandwiched between two stainless-steel disks as current collectors. Galvanostatic charge-discharge tests were subsequently carried out at 25 °C in a dry Ar atmosphere using a charge-discharge measuring device (VMP3, Bio-Logic Co.). All cell fabrication processes and electrochemical tests were performed in a dry Ar-filled glove box. Finally, a cross-section of the cell was prepared using an Ar ion milling system (E-3500, Hitachi High-Technologies) and observed by FE-SEM.

3. Results and discussion The mechanochemical synthesis of 80Na0.7CoO2·20NaMOy (M = N, S, P, B, or C) (mol%) was carried out using planetary ball milling, and the XRD patterns of the products after 40 h milling are shown in Figure 1. At all compositions examined, no diffraction peaks attributable to the sodium oxy-acids were observed, thereby confirming that the reaction was complete. In addition, although broad peaks corresponding to P2-Na0.7CoO2 (JCPDS#01-079-5270) with the P63/mmc space group were observed for some compositions, amorphization was found to mainly proceed via a mechanochemical route.

Figure 1. XRD patterns of the Na0.7CoO2-NaxMOy (M=N, S, P, B, or C) samples after milling for 40 h. To investigate the detailed structure of the 80Na0.7CoO2·20NaNO3 (Na0.76Co0.8N0.2O2.2) sample, TEM observations were carried out. The corresponding HR-TEM image, shown in Figure 2(a), confirmed that nano-sized crystalline particles were present. The electron diffraction (ED) pattern is also shown as an inset in Figure 2(a), where the presence of weak Debye rings further confirmed the presence of nano-crystals. In addition, Figure 2(b) gives the intensity profile obtained along the dotted yellow arrow shown in the ED pattern. Although the peak positions of the intensity profile were similar to those of Na0.7CoO2, a number of peaks could not be assigned, thereby indicating that the layered structure of Na0.7CoO2 was not maintained, and that amorphization had taken place. Furthermore, the absence of peaks attributable to NaNO3 confirmed the formation of an amorphous compound in the Na0.7CoO2NaNO3 system. It can therefore be assumed from the XRD and HR-TEM results that the obtained materials are nanocomposites of nano-sized crystals and an amorphous matrix.

Figure 2. (a) HR-TEM image and electron diffraction pattern, and (b) intensity profile for the electron diffraction pattern of the milled 80Na0.7CoO2·20NaNO3 (Na0.76Co0.8N0.2O2.2). To investigate the electronic state of N in the amorphous matrix, XPS measurements were carried out for the Na0.76Co0.8N0.2O2.2 and NaNO3 particles. Figure 3 shows the N1s XPS spectra for both species. In the spectrum of Na0.76Co0.8N0.2O2.2, an intense peak was observed at 407 eV in addition to a broad peak at 403 eV. These peaks could be attributed to the NO3 and NO2 units, respectively,26,27 which originate from the amorphous Na0.76Co0.8N0.2O2.2 particles containing NO3 as a main component, and NO2 units derived from the starting NaNO3 crystals. Figure 3. N1s XPS spectra for Na0.76Co0.8N0.2O2.2 and NaNO3. Figure 4(a) shows an SEM image of the milled Na0.76Co0.8N0.2O2.2 particles, where the size of a secondary particle was found to range between 1 and 5 μm. In addition, Figures 4(b) and 4(c) show a cross-sectional SEM image and its N, O, Na, and Co EDX mappings for the pellet pressed under 360 MPa for 5 min at room temperature. The SEM image shows that the particles were deformed, and close contacts were formed through this simple pressing procedure. Indeed, a densification process known as “room temperature pressure sintering” appeared to take place here, which is known to proceed in sulfide-based materials.28 It is noted that this high deformability was achieved by the addition of NaNO3 with a low melting temperature.29 Furthermore, EDX mapping signals confirmed that all mapped elements were uniformly overlapped, thereby confirming the successful preparation of the milled Na0.76Co0.8N0.2O2.2 particles. The SEM images of Na0.7CoO2-Na2SO4 and crystalline and milled Na0.7CoO2 particles, and cross-sectional SEM images of the corresponding pellets are also shown in Figure S1. The cross-sectional SEM image of crystalline Na0.7CoO2 pellet showed many cracks inside particles, indicating that crystalline Na0.7CoO2 has lower deformability. Figure 4. (a) SEM image of the Na0.76Co0.8N0.2O2.2 particle, (b) cross-sectional SEM image of the Na0.76Co0.8N0.2O2.2 pellet, and (c) corresponding EDX mappings for N, O, Na, and Co. To quantitatively evaluate the deformability, the relative density of Na0.7CoO2-NaxMOy was measured, and the results are shown in Table 1. Crystalline Na0.7CoO2 has a lower relative density than amorphous active materials. Especially, amorphous active materials with sodium oxy-acids showed high relative density of more than 80%. It is expected that these amorphous active materials will show good battery performances when they are used alone in the positive electrode layers. The electronic conductivities of the prepared Na0.76Co0.8N0.2O2.2 pellets were then investigated using DC polarization measurements. More specifically, a value of >10−2 S cm−1 was obtained at room temperature, and the pellet was found to maintain a high electronic conductivity following the addition of NaNO3. To measure the ionic conductivity of the pellet, the symmetric cell



(Na15Sn4/Na3PS4/Na0.76Co0.8N0.2O2.2/Na3PS4/Na15Sn4) was fabricated by pressing at room temperature. The calculated sodium ionic conductivity for the Na0.76Co0.8N0.2O2.2 pellet was 1.9×10−6 S cm−1 at room temperature, which is significantly higher than that of oxide sodium ionic conductors such as Na3BO3 and 50Na3BO3·50Na2SO4 (mol%) glasses, namely 1.5×10−8 and 5.9×10−8 S cm−1 at room temperature, respectively.23 Therefore, the Na0.76Co0.8N0.2O2.2 sample was a mixed conductor that exhibits both electronic and sodium ionic conductivities. The conductivities of other active materials were also measured in the same way, and the results are summarized in Table 1. In the amorphous active materials, a linear relationship is observed between the relative density and ionic conductivity. The sodium ionic conductivity is plotted against relative density in Figure S2. It is indicated that the relative density has a stronger effect on increasing the ionic conductivity than the detailed structure of the active material. Table 1. Density and electronic and sodium ionic conductivities of the Na0.7CoO2-NaxMOy (M=N, S, or P) active materials, together with those of milled Na0.7CoO2 and crystalline Na0.7CoO2

All-solid-state cells (Na15Sn4/Na3PS4/80Na0.7CoO2·20NaxMOy (M = N, S, P, B, or C) (mol%)) were then fabricated to investigate their electrochemical performances. As 80Na0.7CoO2·20NaxMOy exhibited both electronic and ionic conductivities, only this active material was used in the positive electrode layer, without adding solid electrolyte or conductive carbon species. All-solidstate cells based on crystalline Na0.7CoO2 positive electrodes both with and without ball milling were also prepared for comparison. These cells were charged and discharged at 25 °C between 2.0 and 4.0 V under a constant current density of 0.013 mA cm−2. Figure 5(a) shows the second charge-discharge curves of the cells prepared using Na0.76Co0.8N0.2O2.2, milled Na0.7CoO2, and crystalline Na0.7CoO2. The Na0.76Co0.8N0.2O2.2 cell gave an initial discharge capacity of 70 mAh g−1 and an average discharge voltage of 2.9 V, both of which are larger than the corresponding values for cells based on crystalline or milled Na0.7CoO2 as positive electrodes. Herein, crystalline and milled Na0.7CoO2 differ in their charge-discharge curves. The overpotential during cycles varies depending on the ionic conductivity and relative density of the positive electrode. At the first charge, milled Na0.7CoO2 has a larger overpotential because of its lower ionic conductivity. At the first discharge, crystalline Na0.7CoO2 has a larger overpotential. This may be due to the poor contact between the crystalline particles, and the loss of ionic pathway after Na+ ion extraction due to the low deformability. Furthermore,

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Figure 5(b) shows the cross-sectional SEM image of the cell prepared using the Na0.76Co0.8N0.2O2.2 electrode layer. A dense positive electrode layer (thickness ~50 μm) was obtained, and a close interface was formed with the solid electrolyte layer. We attribute the increase in the operating voltage to an inductive effect30 from the addition of NaNO3. Similarly, the charge and discharge capacities increased due to formation of a dense electrode and the corresponding abundant conduction paths. These results indicate that the electrochemical properties of Na0.7CoO2 were improved by the addition of NaNO3. To find the optimal composition, the molar ratio of NaNO3 was varied in the range of 0 to 30%, and the results are shown in Figure S3. When the molar ratio was 20%, the best charge-discharge performance was obtained. Thus, we used the same molar ratio of NaxMOy for the other sodium oxy-acids. Figure 5. (a) 2nd Charge-discharge curves for the cells containing only the active materials (i.e., Na0.76Co0.8N0.2O2.2, milled Na0.7CoO2, and crystalline Na0.7CoO2) in the voltage range 2.0–4.0 V. (b) Cross-sectional SEM image of the allsolid-state cell produced using the Na0.76Co0.8N0.2O2.2 active material. The charge-discharge properties were also evaluated for the other 80Na0.7CoO2·20NaxMOy active materials in secondary batteries, and the results are summarized in Table 2. Interestingly, all cells containing active materials based on the sodium oxy-acids exhibited larger capacities than those prepared using milled Na0.7CoO2 without sodium oxy-acids. In addition, the obtained capacities after the 10th cycle were larger for all cells using milled Na0.7CoO2NaxMOy and milled Na0.7CoO2, compared to those based on crystalline Na0.7CoO2. It is noteworthy that both amorphization and the addition of sodium oxy-acids contributed to improving the electrochemical performance of Na0.7CoO2. Table 2. 1st Charge and discharge capacities, and 10th discharge capacities of the all-solid-state cells containing Na0.7CoO2-NaxMOy (M=N, S, P, B, or C) active materials, milled Na0.7CoO2, or crystalline Na0.7CoO2 as the positive electrode layer

Herein, the sodium ionic conductivity is plotted against the 1st discharge capacity in Figure 6, showing a strong correlation between the two. Therefore, the sodium ionic conductivity and the relative density are very important for achieving higher capacity.


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Figure 6. Sodium ionic conductivity vs 1st discharge capacity of the Na0.7CoO2-NaxMOy (M=N, S, or P) active materials, milled Na0.7CoO2, and crystalline Na0.7CoO2. We found that Na0.76Co0.8N0.2O2.2 exhibited the best cyclability and the largest capacity among the various 80Na0.7CoO2·20NaxMOy active materials. Thus, chargedischarge measurements were carried out for the Na0.76Co0.8N0.2O2.2 cell using cut-off voltages of 2.0–4.0 V and 1.0–4.5 V. Figure 7(a) shows the charge-discharge curves with a cut-off voltage of 2.0–4.0 V, where the capacities decreased gradually during cycling, with approximately 70% of the initial discharge capacity remaining after the 10th cycle. The rate performance was also investigated, and the results are shown in Figure S4. The cell using active material alone as the positive electrode could work as a secondary battery under the current density of 0.064 mA cm−2. On the other hand, the cell with only crystalline Na0.7CoO2 showed little capacity at the 9th cycle under 0.013 mA cm−2. Similarly, Figure 7(b) shows the charge-discharge curves obtained with a cut-off voltage of 1.0–4.5 V. Interestingly, the initial coulombic efficiency increased from 80 to 93% upon changing the cut-off voltage. In this case, the discharge capacity of the second cycle was 80 mAh g−1, although this capacity gradually decreased during cycling. Furthermore, Figure 7(c) shows the charge-discharge curves of the cell containing a composite electrode prepared by mixing Na0.76Co0.8N0.2O2.2 with the Na3PS4 glass-ceramic electrolyte in a weight ratio of 7:3. In the corresponding charge-discharge test under a cut-off voltage of 1.0–4.5 V, the initial discharge capacity was twice as much as that without the glass-ceramic electrolyte. Moreover, the cell capacity increased upon cycling, achieving a value of 170 mAh g−1 for the 5th discharge. Indeed, this value represents the highest capacity reported to date for all-solid-state sodium cells based on oxide positive electrodes, and is comparable to the capacity of cells based on liquid electrolytes.6 To investigate whether the solid electrolyte in the positive electrode layer contributes to the redox reaction, a cell was prepared using a composite of Na3PS4 and acetylene black (AB, 4:1 weight ratio) as the positive electrode. According to the corresponding charge-discharge curves, shown in Figure 7(d), operation of the cell based on Na3PS4-AB composite as a secondary battery was challenging. Therefore, the cell capacity shown in Figure 7(c) must be derived from the Na0.76Co0.8N0.2O2.2 active material. The improved ionic conductivity of the positive electrode layer with Na3PS4 was assumed to contribute to the enhanced cell cyclability. As the added Na3PS4 electrolyte exhibits a high ionic conductivity and excellent deformability,5 Na+ ions can be supplied to the active material throughout the entire electrode between the electrolyte layer and the current collector. As a result, the uneven reaction current distribution in the electrode is suppressed, and satisfactory cell performance is achieved. Cross-sectional SEM images of the positive electrode before and after charge-discharge test are shown in Figure S5. After the 1st discharge, the interfaces between particles disappeared, and the structure was maintained after the 10th discharge. Therefore, the particle-to-particle contact

became better during the cycles; a similar phenomenon often occurs in all-solid-state batteries. 31 This is also a reason for the high cyclability observed here. Figure 7. Charge-discharge curves for the cell prepared using Na0.76Co0.8N0.2O2.2 in the voltage ranges of (a) 2.0– 4.0 V and (b) 1.0–4.5 V. (c) Charge-discharge curves for the cell prepared using the Na0.76Co0.8N0.2O2.2 composite electrode and a Na3PS4 electrolyte in the voltage range of 1.0– 4.5 V. (d) Charge-discharge curves for the cell using a composite electrode of Na3PS4 and acetylene black in the voltage range of 1.0–4.5 V.

4. Conclusions We report the successful synthesis of amorphous 80Na0.7CoO2·20NaxMOy (M=N, S, P, B, or C) (mol%) positive electrode active materials via a mechanochemical technique. All-solid-state cells based on the amorphous 80Na0.7CoO2·20NaxMOy active materials were then fabricated, and their electrochemical performances were examined. We found that all cells prepared using the 80Na0.7CoO2·20NaxMOy (M=N, S, P, B, or C) positive electrode active materials functioned as secondary batteries at 25 °C, exhibiting larger capacities than the cell based on milled Na0.7CoO2 without sodium oxy-acids. In particular, the cell containing 80Na0.7CoO2·20NaNO3 (Na0.76Co0.8N0.2O2.2) gave the largest initial discharge capacity (i.e., 70 mAh g−1) among the cells examined. This improved electrochemical performance is attributed to the increase in both the deformability and the ionic conductivity upon the addition of NaNO3 to Na0.7CoO2. Furthermore, the cell capacity increased when using a composite electrode of Na0.76Co0.8N0.2O2.2 and Na3PS4 electrolyte. Interestingly, the capacity increased during cycling, reaching 170 mAh g−1 in the 5th cycle. Therefore, the amorphization of layered transition metal oxides with sodium oxy-acids seems to be effective for producing novel active materials with high capacities and cyclabilities for use in all-solid-state batteries, likely due to the improved conductivities and formabilities of the amorphous electrodes.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publication website at DOI: XXXXX. Characterization of active materials (SEM images, electrochemical performance, conductivity-relative density relationship) (PDF)

AUTHOR INFORMATION Corresponding Author * Akitoshi Hayashi (Professor). Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan. Tel.: +81-72-2549334; Fax.: +81-72-2549910; E-mail address: [email protected].

ABBREVIATIONS



AB, acetylene black; ED, electron diffraction; FE-SEM, field emission scanning electron microscopy; HR-TEM, highresolution transmission electron microscopy; XPS, X-ray photoelectron spectroscopy; XRD, X-ray diffraction.

REFERENCES REFERENCES [1] Minami, T.; Hayashi, A.; Tatsumisago, M. Recent progress of glass and glass-ceramics as solid electrolytes for lithium secondary batteries. Solid State Ion., 2006, 177, 2715–2720. [2] Ohta, N.; Takada, K.; Zhang, L.; Ma, R.; Osada, M.; Sasaki, T. Enhancement of the high-rate capability of solid-state lithium batteries by nanoscale interfacial modification. Adv. Mater., 2006, 18, 2226–2229. [3] Tatsumsiago, M.; Hayashi, A. All-solid-state lithium secondary batteries using sulfide-based glass ceramic electrolytes. Funct. Mater. Lett., 2008, 1, 31–36. [4] Hayashi, A.; Noi, K.; Sakuda, A.; Tatsumisago, M. Superionic glass-ceramic electrolytes for room-temperature rechargeable sodium batteries. Nat. Commun., 2012, 3, 856–860. [5] Nose, M.; Kato, A.; Sakuda, A.; Hayashi, A.; Tatsumisago, M. Evaluation of mechanical properties of Na2S-P2S5 sulfide glass electrolytes. J. Mater. Chem. A, 2015, 3, 22061–22065. [6] Delmas, C.; Braconnier, J.-J.; Fouassie, C.; Hagenmuller, P. Electrochemical intercalation of sodium in NaxCoO2 bronzes. Solid State Ion., 1981, 3–4, 165–169. [7] Baster, D.; Maziarz, W.; Świerczek, K.; Stokłosa, A.; Molenda, J. Structural and electrochemical properties of Na0.72CoO2 as cathode material for sodium-ion batteries. J. Solid State Electrochem., 2015, 19, 3605–3612. [8] Okada, S.; Takahashi, Y.; Kiyabu, T.; Doi, T.; Yamaki, J.-I.; Nishida, T. LayeRED TRANSITION METAL OXIDES AS CATHODES FOR SODIUM SECONDARY Battery. 210th ECS Meeting Abstracts, 2006, MA2006-02, p 201 (available at http://ma.ecsdl.org/content/MA2006-02/4/201.abstract). [9] Zhao, J.; Zhao, L.; Dimov, N.; Okada, S.; Nishida, T. Electrochemical and thermal properties of α-NaFeO2 cathode for Na-ion batteries. J. Electrochem. Soc., 2013, 160, A3077–A3081. [10] Mendiboure, A.; Delmas, C.; Hagenmuller, P. Electrochemical intercalation and deintercalation of NaxMnO2 bronzes. J. Solid State Chem., 1985, 57, 323–331. [11] Caballero, A.; Hernan, L.; Morales, J.; Sanchez, L.; Santos Pena, J.; Aranda, M. A. G. Synthesis and characterization of hightemperature hexagonal P2-Na0.6MnO2 and its electrochemical behaviour as cathode in sodium cells. J. Mater. Chem., 2002, 12, 1142–1147. [12] Sabi, Y.; Sato, S.; Hayashi, S.; Furuya, T.; Kusanagi, S. A new class of amorphous cathode active material LixMyPOz (M = Ni, Cu, Co, Mn, Au, Ag, Pd). J. Power Sources, 2014, 258, 54–60. [13] Kercher, A. K.; Kolopus, J. A.; Carroll, K. J.; Unocic, R. R.; Kirklin, S.; Wolverton, C.; Stooksbury, S. L.; Boatner, L. A.; Dudney, N. J. Mixed polyanion glass cathodes: Iron phosphate vanadate glasses. J. Electrochem. Soc., 2014, 161, A2210–A2215. [14] Kercher, A. K.; Kolopus, J. A.; Carroll, K. J.; Unocic, R. R.; Kirklin, S.; Wolverton, C.; Stooksbury, S. L.; Boatner, L. A.; Dudney, N. J. Mixed polyanion glass cathodes: Glass-state conversion reactions. J. Electrochem. Soc., 2016, 163, A131-A137. [15] Kercher, A. K.; Kolopus, J. A.; Sacci, R. L.; Ruther, R. E.; Gallego, N. C.; Stooksbury, S. L.; Boatner, L. A.; Dudney, N. J. Mixed polyanion glass cathodes: Effect of polyanion content. J. Electrochem. Soc., 2017, 164, A804–A809. [16] Togashi, T.; Honma, T.; Shinozaki, K.; Komatsu, T. Electrochemical performance as cathode of lithium iron silicate, borate

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and phosphate glasses with different Fe2+ fractions. J. Non-Cryst. Solids, 2016, 436, 51-57. [17] Nakata, S.; Togashi, T.; Honma, T.; Komatsu, T. Cathode properties of sodium iron phosphate glass for sodium ion batteries. J. Non-Cryst. Solids, 2016, 450, 109–115. [18] Isono, M.; Okada, S.; Yamaki, J. Synthesis and electrochemical characterization of amorphous Li-Fe-P-B-O cathode materials for lithium batteries. J. Power Sources, 2010, 195, 593-598. [19] Matsuyama, T.; Hayashi, A.; Ozaki, T.; Mori, S.; Tatsumisago, M. Improved electrochemical performance of amorphous TiS3 electrodes compared to its crystal for all-solid-state rechargeable lithium batteries. J. Ceram. Soc. Jpn., 2016, 124, 242-246. [20] Tanibata, N.; Matsuyama, T.; Hayashi, A.; Tatsumisago, M. All-solid-state sodium batteries using amorphous TiS3 electrode with high capacity. J. Power Sources, 2015, 275, 284–287. [21] Nagao, K.; Hayashi, A.; Deguchi, M.; Tsukasaki, H.; Mori, S.; Tatsumisago, M. Amorphous LiCoO2-Li2SO4 active materials: Potential positive electrodes for bulk-type all-oxide solid–state lithium batteries with high energy density. J. Power Sources, 2017, 348, 1–8. [22] Nagao, K.; Nagata, Y.; Sakuda, A.; Hayashi, A.; Tatsumisago, M. Amorphous LiCoO2-based positive electrode active materials with good formability for all-solid-state rechargeable batteries. MRS Adv., 2018, 3, 1319-1327. [23] Suzuki, K.; Nakamura, Y.; Tanibata, N.; Hayashi, A.; Tatsumisago, M. Preparation and characterization of Na3BO3-Na2SO4 glass electrolytes with Na+ ion conductivity prepared by a mechanical milling technique. J. Asian Ceram. Soc., 2016, 4, 6–10. [24] Hayashi, A.; Noi, K.; Tanibata, N.; Nagao, M.; Tatsumisago, M. High sodium ion conductivity of glass-ceramic electrolytes with cubic Na3PS4. J. Power Sources, 2014, 258, 420-423. [25] Yamamoto, T.; Nohira, T.; Hagiwara, R.; Fukunaga, A.; Sakai, S.; Nitta, K.; Inazawa, S. Charge-discharge behavior of tin negative electrode for a sodium secondary battery using intermediate temperature ionic liquid sodium bis(fluorosulfonyl) amidepotassium bis(fluorosulfonyl)amide. J. Power Sources, 2012, 217, 479–484. [26] Aduru, S.; Contarini, S.; Rabalais, J. W. Electron-, X-ray-, and ion-stimulated decomposition of nitrate salts. J. Phys. Chem., 1986, 90, 1683-1688. [27] Datta, M.; Mathieu, H. J.; Landolt, D. Characterization of transpassive films on nickel by sputter profiling and angle resolved AES/XPS. Appl. Surf. Sci., 1984, 18, 299–314. [28] Sakuda, A.; Hayashi, A.; Tatsumisago, M. Sulfide solid electrolyte with favorable mechanical property for all-solid-state lithium battery. Sci. Rep., 2013, 3, 2261. [29] Tatsumisago, M.; Takano, R.; Nose, M.; Nagao, K.; Kato, A.; Sakuda, A.; Tadanaga, K.; Hayashi, A. Electrical and mechanical properties of glass and glass-ceramic electrolytes in the system Li3BO3–Li2SO4. J. Ceram. Soc. Jpn., 2017, 125, 433-437. [30] Padhi, A. K.; Manivannan, V.; Goodenough, J. B. Tuning the position of the redox couples in materials with NASICON structure by anionic substitution. J. Electrochem. Soc., 1998, 145, 1518– 1520. [31] Sakuda, A.; Takeuchi, T.; Shikano, M.; Sakaebe, H.; Kobayashi, H. High reversibility of “soft” electrode materials in all-solid-state batteries. Front. Energy Res., 2016, 4, 19: 1-7.


Page 7 of 14 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



7


M=N (Na0.76Co0.8N0.2O2.2) M=S M=P

Intensity (arb. unit)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56

Page 8 of 14

M=B

M=C Milled Na0.7CoO2

Crystalline Na0.7CoO2

10

20

30

40 50 60 2 / o (CuK)

Fig. 1 ACS Paragon Plus Environment

70

80

Page 9 of 14

(a)

20 nm (b)

Peak intensity

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56


Na0.7CoO2 0

26

52 Scattering vector [1/nm]

Fig. 2


78

104


NO3-

N1s Na0.76Co0.8N0.2O2.2

Intensity (arb.unit)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56

Page 10 of 14

NO2-

NaNO3

410

408

406

404

Binding energy / eV

Fig. 3


402

Page 11 of 14 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56


(a)

(c)

N

O

5 mm

(b)

5 mm

Na

Co

5 mm

5 mm

Fig. 4

5 mm


5 mm


6

(a) Cell voltage / V

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56

Milled Crystalline 2nd cycle Na CoO 5 0.7 2 Na0.7CoO2 Na0.76Co0.8N0.2O2.2

4

2 1 0

(b) Positive electrode Layer (Na0.76Co0.8N0.2O2.2)

3

Page 12 of 14

2.0-4.0 V 0.013 mA cm-2, 25oC

Na3PS4 layer

0 20 40 60 80 100 120 Capacity / mAh g-1 (positive electrode)


50 mm

10 mm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56


Sodium ionic conductivity / S cm -1

Page 13 of 14

10-5 Crystalline Na0.7CoO2

10-6

Na2SO4

NaNO3

Na3PO4 Milled Na0.7CoO2

-7

10

10-8

0

25 50 75 100 -1 1st discharge capacity / mAh g



6

0

0.4

6

Na0.76Co0.8N0.2O2.2

5 Cell voltage / V

0.1

(b)

10 5 2 1

4 3 2

10 5 2 1

1 0

0.4

10 5 1 2

4 3 2

0


0.1

Na+ / mol 0.2 0.3

Na0.76Co0.8N0.2O2.2

1

2.0-4.0 V 0.013 mA cm-2, 25oC

(c)

1.0-4.5 V 10 5 1 2 0.013 mA cm-2, 25oC


(d) 6

0

0.2

0.4

0.6

0.8

6

Na0.76Co0.8N0.2O2.2–Na3PS4 composite

5 Cell voltage / V

0

5 Cell voltage / V

(a)

Na+ / mol 0.2 0.3

1

2 15 5

4 3 2 1 1.0-4.5 V 0

0.013 mA cm-2, 25oC

0

Na3PS4–AB composite

5

Cell voltage / V

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56

Page 14 of 14

52

4 3 2 5←1 1.0-4.5 V 0.013 mA cm-2, 25oC

1

1 15 2 5

50 100 150 200 250 Capacity / mAh g-1 (active material)

1

0

0


20 40 60 80 Capacity / mAh g-1 (Na3PS4)

100

Amorphization of Sodium Cobalt Oxide Active Materials for High

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