NiCl2

Article 3

2

The self-repairing function of NiS layer on Ni particles in the Na/NiCl cells with the addition of sulfur in the catholyte 2

Xin Ao, Zhaoyin Wen, Xiangwei Wu, Tian Wu, and Meifen Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 02 Jun 2017 Downloaded from http://pubs.acs.org on June 6, 2017

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ACS Applied Materials & Interfaces

The Self-repairing Function of Ni3S2 Layer on Ni Particles in the Na/NiCl2 Cells with the Addition of Sulfur in the Catholyte Xin Aoa,b, Zhaoyin Wena,b,*, Xiangwei Wua,*, Tian Wua,b, Meifen Wua a

CAS Key Laboratory of Materials for Energy Conversion, Shanghai Institute of

Ceramics, Chinese Academy of Sciences, 1295 DingXi Road, Shanghai 200050, P.R. China b

University of Chinese Academy of Sciences, 19 A Yuquan Rd, Shijingshan District,

Beijing, 100049, P.R. China Abstract The role of Ni3S2 layer on Ni particles in the electrochemical performance of Na/NiCl2 cells with the addition of sulfur in the cathode is studied. It was found that Ni3S2 layer could be in-situ generated on nickel particles and exhibit a self-repairing function during cycling when sulfur exist in the cathode due to the reaction between sulfur and nickel particle. The self-repairing function of Ni3S2 layer could enhance the blocking effect and improve the battery cycle performance. The capacity of the cell with the optimum amount of sulfur (with self-repairing function) after 50 cycles is about 12% greater than that of the cell with optimum level of Ni3S2 (without self-repairing function). The effect of self-repairing function of Ni3S2 layer is determined by the amount of sulfur in the cathode. Keywords: Sulfur additive, Self-repairing function, Sodium-nickel chloride battery, Zebra battery, Ni particle growth, cell cycling behavior, mechanism analysis Corresponding

Author

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+86-21-52411704,

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+86-21-52413903 *E-mail address: [email protected], [email protected]

1. Introduction The sodium/nickel chloride (Na/NiCl2 or Zebra) battery with high specific energy density (790 Wh kg-1), high cyclability, and better safety feature as compared to sodium/sulfur (Na/S) battery1-3 is one of the alternative candidates for transportation 1

ACS Paragon Plus Environment


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applications4-6 and large-scale energy storage systems for renewable power integration and grid applications7-8. Na/NiCl2 battery, like Na/S battery, consists of a molten sodium negative electrode9 and β″-Al2O3 solid electrolyte10-12. However, different from Na/S battery, nickel chloride is used as the cathode ingredient and a molten secondary electrolyte of sodium tetrachloroaluminate (NaAlCl4 with a melting point of 156.7 °C 13) is added into the cathode to accelerate the movement of Na+ between the solid electrolyte and cathode materials14-17. The operating temperature of the battery is typically around 200 ~ 350 °C18-20 corresponding to better ionic conductivity of the solid electrolyte (β″-Al2O3) and the secondary electrolyte (NaAlCl4) as well as the wettability of liquid sodium on the solid electrolyte. The overall electrochemical reaction of a Na/NiCl2 battery is described as follows: (charged state) NiCl2 + 2Na ↔ Ni + 2NaCl (discharged state), E = 2.58V at 300 °C. The Na/NiCl2 battery is typically assembled in discharged state with the cathode consisting of the mixture of Ni, NaCl, NaAlCl4 and a few additives14-17 in order to avoid using metallic sodium and anhydrous nickel chloride due to their reactivity and hazardous properties. During the charging process, metallic sodium is generated in the negative electrode after transportation of Na+ through the β″-Al2O3 solid electrolyte and nickel is oxidized to generate nickel chloride on the surface of nickel particles in the positive electrode. The charging step is regarded as the rate-determining step caused by the formation of poorly conductive NiCl2 and the slow dissolution of NaCl particles in NaAlCl4.21 The surface area of nickel particles, mainly determined by nickel particle size, is one of the critical factors for battery performance owing to the fact that the electrochemical reaction can only occur on the surface of nickel particles20-21. In practice, a few additives were added into the cathode to stabilize the nickel particles size and improve battery cycle performance. Sudworth pointed out that FeS in the positive electrode can poison the surface of nickel particles and prevent nickel particle growth during cycling16. Another study by Li et al.22 found that the addition of FeS in the cathode can remove the passivation layer on Ni particles, which suppressed nickel particle growth and improved the battery cycling characteristics. Furthermore, they viewed that FeS was probably decomposed to elemental sulfur and polysulfide to 2


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enhance the battery performance rather than directly reacting with Ni. Prior to this, elemental sulfur added to the cathode found to prevent the agglomeration of nickel particles and improved the cycle performance effectively23-24. Unfortunately, the mechanisms of the improvement of cycling performance of the cell have not been clearly identified. In our previous work, the Ni3S2 layer on nickel particles was found to suppress the nickel particle growth and improve the cycling performance of the cell owing to the blocking effect of Ni3S2 surface layer25. However, the Ni3S2 layer will break into pieces and get separated away from nickel particles during cycling, which lead to the attenuation of the blocking effect and degradation of the capacity at the later stage of cycling. In this work, based on the results of electrochemical test, XRD, TEM, XPS and SEM measurements, we found the Ni3S2 layer could be in-situ generated on the surface of nickel particles and exhibit a self-repairing function during cycling in the sulfur containing cathode due to the reaction between sulfur and nickel particles. The self-repairing function of Ni3S2 layer could enhance its blocking effect and improve the battery cycle performance. 2. Experimental 2.1 Material preparation Materials used in this work consisted of nickel powder (Ni-255, Novamet), NaCl (Aladdin, 99.8%), NaAlCl4 (self-made), aluminum powder (Sinopharm Chemical Reagent, 99.99%), elemental sulfur (Spectrum pure) and sodium (Sinopharm Chemical Reagent, 99.5%). NaCl was pulverized into fine crystals of 2~5 µm. NaAlCl4 was prepared by mixing NaCl and high purity anhydrous AlCl3 (purified from AlCl3, Sinopharm Chemical Reagent, 99%) in the molar ratio of 1.05:1 and homogenized at 300 °C 25. Excessive NaCl was employed to maintain basic electrolyte in the Na/NiCl2 cells

13, 26-27

. As-prepared NaAlCl4 was mixed uniformly with 1, 3, 5 and 7 wt%

elemental sulfur, respectively in argon glove box. The nickel particles encapsulated with optimum level25 of Ni3S2 layer were prepared by mixing of nickel powder with 15 mol% elemental sulfur. The cathode materials were prepared by mixing nickel particles (or nickel particles 3



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encapsulated with optimum level of Ni3S2 layer ), NaCl fine crystals and Al powders evenly in the mass ratio of 6:4:0.115 and then heated at 300 °C for 2 hours under a hydrogen-reducing atmosphere (5 wt% hydrogen + 95 wt% argon) to dehydrate

26-27

.

The cathode electrodes were prepared by mixing of 0.32g of cathode materials (corresponding to a theoretical capacity of 58.6mAh) and 0.48g of NaAlCl4 with different amounts of elemental sulfur homogeneously. It should be noted that the cathode materials with surface modified nickel particles are mixed with non-sulfur NaAlCl4. Subsequently, the final materials were pressed into a pellet with a diameter of 18mm in argon glove box for assembling battery. 2.2 Research cell configuration Electrochemical properties of Na/NiCl2 battery were carried out in homemade research cells (as seen in Fig. S1). The research cells consisted of positive electrodes with different amounts of sulfur, sodium negative electrode, β″-Al2O3 ceramic electrolyte (active area of 2.5 cm2), metal current conductor and Pyrex glass shell. The β″-Al2O3 discs were fabricated by a double zeta process as described previously 28-29, the thickness of which was around 0.8 mm. Before loading any positive materials, the as-prepared β″-Al2O3 disc was glass-sealed to an α-Al2O3 ring to separate the positive and negative electrode cases. The anode side of the ceramic electrolyte was treated by spraying gold in order to enhance the wettability of the liquid sodium. Five Na/NiCl2 cells with different contents of sulfur in NaAlCl4 were prepared. According to the mass percentage of sulfur in NaAlCl4, these five different Na/NiCl2 cells were referred to as 0S (without sulfur in NaAlCl4), 1S (1 wt% sulfur in NaAlCl4), 3S, 5S, 7S for simplicity. 2.3 Electrochemical tests The Na/NiCl2 research cells were assembled in argon glove box and then tested at 300 °C. Cell tests were conducted using a LAND CT2001A battery test system. The cycling was performed at a constant current (10mA, 4mA cm-2) between 2.45V and 2.65V (V.S. Na/Na+). AC impedance measurements were conducted by using a Frequency Response Analyzer (FRA) technique on an Autolab Electrochemical Workstation over the frequency range from 0.1 Hz to 1 MHz with the amplitude of 10 4


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mV. 2.4 Characterization The positive materials of the cell 1S, 3S, 5S, 7S were sealed in a closed glass vase filled with high purity argon and then heated at 300 °C for 2 hours, which simulated the battery aging process in order to study the changes occurred in the sulfur-doped cathode during the process. After cooling down, some of these materials were rinsed with distilled water to remove the salts (NaCl and NaAlCl4). The remaining powders were identified by X-ray diffraction (XRD, Rigaku RAD-C with monochromatic Cu Kα radiation, λ= 1.5418 Å), scanning electron microscope (SEM, Hitachi S-3400), energy dispersive spectroscopy (EDS, X-max 20-011), transmission electron microscopy (TEM, Hitachi H800) and X-ray photo-electron spectroscopy (XPS, AXIS UltraDLD). The cells were destructively disassembled in a glove box after cycling test. Part of the positive electrode was ground into powder and then identified by XRD measurements. Other cathode materials were washed with distilled water to remove the salts and the remaining particles were examined by SEM (Hitachi S-3400 or 4800). 3. Results and discussion 3.1 Thermochemical behavior of the cathode materials in sulfur containing Na/NiCl2 cells Thermochemical behavior of the sulfur containing cathode of Na/NiCl2 cell was investigated to identify the chemical reactions happened in the cathode before cell cycling or during the cell aging stage. The tests were carried out with all the sulfur containing cathode materials in Argon atmosphere at the working temperature of the cell. Fig. 1 shows the XRD patterns of the reacted positive materials and the remaining particles after washing with distilled water. As seen, the obvious and sharp peaks of Ni3S2 are observed no matter whether the materials washed with distilled water or not. Additionally, the intensity of Ni3S2 peaks is generally increased with the addition of sulfur in the cathode. Based on above analysis, it is concluded that Ni3S2 would be formed in the sulfur doped cathode even before the electrochemical tests and the amount of Ni3S2 increases with the addition of sulfur. 5



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Fig. 1. XRD patterns of (a) the positive materials of the cells with different sulfur heated in Argon atmosphere at 300 °C for 2 hours without any electrochemical tests and (b) the remaining particles prepared by the above materials after being washed with distilled water. Fig. 2a illustrates the SEM images of the remaining particles prepared by the positive materials of the cell 5S being heated in Argon atmosphere at 300 °C for 2 hours and then rinsed with distilled water. The surface atomic distribution of the remaining particles were mapped out by performing EDS mapping analysis (Fig. 2b and c). As seen, S and Ni elements distribute homogeneously. To further certify the surface compositions, XPS measurement was conducted to examine the valance state of the element S and Ni. Fig. 2d displays the XPS spectra of Ni 2p in the binding energy range of 840~890 eV, in which two major fitted peaks at the binding energy of 856.0 and 873.7 eV with two accompanied satellite peaks at 861.5 and 879.7 eV were observed, in good accordance with the Ni 2p3/2 and Ni 2p1/2 of the Ni3S2 30-31. The two weak fitted peaks at 852.2 and 869.8 eV can be assigned to metallic Ni0 32-33, which is ascribed to the unreacted surface of the nickel particles. The minor bump at 853.4 eV probably originates from a small amount of NiO on the surface 22, 34. Fig. 2e elucidates the XPS spectrum of the S 2p. The fitted peaks at 161.6 and 162.8 eV in the spectrum are indexed to the S 2p3/2 and S 2p1/2 of Ni3S2, respectively 35-36. The fitted peak at 163.8 eV is related with S0 37, which is due to the residual sulfur from the cathode additive. The above XPS results are consistent with the results of XRD and EDS confirming the formation of Ni3S2 on the surface of nickel particles. Obviously, the Ni3S2 has 6


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originated from the reaction between sulfur and nickel owing to the strong corrosiveness of sulfur at 300 °C.

Fig. 2. (a) SEM image of the remaining particles prepared by the positive materials of the cell 5S being heated in Argon atmosphere at 300 °C for 2 hours and then rinsed with distilled water. (b) & (c) EDS mapping images for elements S (red dots) & Ni (green dots) respectively with their corresponding SEM image on the left. (d) and (e) XPS spectra of Ni 2p and S 2p respectively.

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Fig. 3. TEM images of the remaining particles prepared by the positive materials of the cell (a) 1S, (b) 3S, (c) 5S, (d) 7S being heated in Argon atmosphere at 300 °C for 2 hours without any electrochemical tests and then washed with distilled water. The TEM results of the remaining particles is shown in Fig. 3a-d. As we can see, there is a layer (light-colored) generated on the surface of four different remaining particles (dark-colored). According to the XRD results of the remaining particles, the light-colored surface layer could be Ni3S2 layer. In the figure, the Ni3S2 layer thickness is not homogeneous, but the average thickness of the layer is generally increased with the addition of sulfur. It should be noted that, not all the surface of nickel particles is encapsulated with Ni3S2 layer in the sample 1S. Another TEM image of the sample 1S is shown in Fig. S2. In the figure, part of the nickel particle surface isn’t encapsulated with Ni3S2 layer, which might be caused by the lower sulfur content and inadequate reaction between sulfur and nickel. It can be inferred that the area unencapsulated with Ni3S2 will decrease with the increase of sulfur content in the cathode.

Based on above XRD, EDS, XPS and TEM results, it is confirmed that the Ni3S2 layer could be formed in-situ on the surface of nickel particles in sulfur doped cathode 8


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at 300 °C, which is caused by the reaction between nickel and highly corrosive sulfur. As a result, we can infer that fresh Ni3S2 layer would be generated on the fresh surface of the nickel and the damaged Ni3S2 layer could be repaired during cycling. 3.2 Electrochemical performance of the Na/NiCl2 cell with sulfur additive The cycle performance of the Na/NiCl2 cells with and without sulfur doped cathode and the cell with optimum level of Ni3S2 layer on Ni particles is displayed in Fig. 4a. The discharge capacity of 0S (without sulfur) degrades rapidly as the cycle proceeded and remains at only 10.8 mAh (18.6% of theoretical capacity) after 50 cycles. As observed in the figure, the capacity retention of the cells with sulfur doped cathode is much better than the cell without sulfur. The cell 5S presents the best cycling performance. Even though the discharge capacity of the cell 5S degrades gradually, it remains at 39.8 mAh after 50 cycles (68.6% of theoretical capacity). It is remarkable that the cycle performance of the cell 5S is better than the cell with optimum level of Ni3S2 layer on nickel particles (pentagram point) which is probably caused by the self-repairing function of Ni3S2 layer on nickel particles. The rate performances of the cell 0S, 5S and the cell with optimum level of Ni3S2 layer on Ni particles are evaluated in the Fig. S3. The current density was increased from 10mA/cm2 to 40mA/cm2 stepwise, and finally returned to 10mA/cm2. As seen, the discharge capacity of the cell 5S is higher than that of other two cells at all tested current densities, which is demonstrated that the cell 5S can afford better rate performance than other cells. The 20th charge and discharge curve of the cell 0S, 1S, 3S, 5S and 7S are plotted in Fig. 4b. In addition, the voltage plots (V vs SOC) of the 20th charge and discharge process for the above cells are shown in Fig.S4. As seen, the charge and discharge curves present only one plateau at ~2.58V which is attributed to the redox reaction of Ni and Ni2+. Based on the analysis of potential difference between the charging and discharging plateau, it is shown that the polarization voltage of the cells is generally increased with the increasing of sulfur addition amount in cathode, which is probably due to the poor electrical conductivity of sulfur and the Ni3S2 product. The electrochemical impedance spectroscopy (EIS) plots of the cells with and without sulfur in the 10th discharge state are shown in Fig. 4c. As seen, no obvious 9



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semicircle is observed in the Nyquist plots of 0S and 1S. It indicates that the chemical system is so kinetically facile that mass transfer always plays a role, and the semicircular region is not well defined (charge transfer resistance, Rct, is small) 38. The semicircles in the Nyquist plots of 3S, 5S and 7S are clear, suggesting that a new interface layer formed in the cathode. In addition, the size of the semicircles increases with the increasing of sulfur content in the cathode, implying the thickness and surface area of interface layer increased with the addition of sulfur. Unconspicuous semicircle in the Nyquist plot of 1S is probably due to the relatively thin Ni3S2 layer and plentiful nickel surface without Ni3S2. Above EIS plots could be analyzed in terms of an equivalent circuit shown in Fig. 4f. In this circuit, Re, Rct and RN are the resistance of electrolyte, charge transfer resistance and the new interface layer (Ni3S2 layer), respectively. As seen in Fig. 4d, the Nyquist plots of the cell 5S in different cycles show that, despite the radius of the semicircle decreases with cycling, the semicircle of Ni3S2 layer still exists, which illustrated that some Ni3S2 layer is still on the surface of nickel particles. As shown in Fig. 4e, the Nyquist plots of the cell with optimum level of Ni3S2 in different cycles show that, the semicircle of Ni3S2 layer decreases with cycling and eventually disappears after 40 cycles, which indicated the damage of the Ni3S2 layer during cycling. A comparison of Fig. 4d and Fig. 4e reveals that Ni3S2 surface layer on nickel particles of the cell with the addition of sulfur in the cathode could maintain for a longer time, which may probably be caused by the self-repairing function of Ni3S2 layer.

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Fig. 4. The electrochemical performance of five different Na/NiCl2 cells. (a) The cycling performance, (b) 20th charge and discharge curves of 0S, 1S, 3S, 5S and 7S. Electrochemical impedance spectroscopy plots of (c) the cells with and without sulfur in 10th cycle, (d) the cell 5S in different cycles at discharge state and (e) the cell with optimum level of Ni3S2 layer on Ni particles 25, (f) equivalent circuit. 3.3 Phase and morphology evolution of the sulfur containing active cathode materials during cycling.

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Fig. 5. X-ray diffraction patterns of the cathode materials in the cell of (a) 7S, (b) 5S, (c) 3S, (d) 1S, (e) 0S and (f) the cell with optimum level of Ni3S2 after 50 cycles in discharged state. XRD patterns of the five cathode materials with different sulfur content and the cell with optimum level of Ni3S2 after cycling in discharged state are shown in Fig. 5. As seen, the obvious diffraction peaks of Ni3S2 (corresponding to JCPDS No.71-1682) are clearly observed in the XRD patterns of the sulfur doped cathodes. Additionally, the intensity of Ni3S2 diffraction peaks increases with the increasing of sulfur content in the cathode (Fig. 5a-d). In contrast, no obvious Ni3S2 peaks were observed in the XRD pattern of the cell without sulfur (Fig. 5e). It is confirmed that Ni3S2 is formed in the sulfur added cathodes during cycling. In our previous work 25, it was proved that Ni3S2 won’t be involved in any electrochemistry reaction in the voltage range between 2.4 and 2.7V. In this work, the voltage range is between 2.45 and 2.65V, as a result, Ni3S2 will remain stable during test. In order to observe the morphology of nickel particles after cycling, the positive electrode was washed with deionized water to remove the water-soluble salt, and the remaining substances were examined by SEM measurement. The morphology of nickel particles of the cell 0S (without sulfur in NaAlCl4) before and after cycle test is shown 12


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in Fig. 6a&b. The pristine nickel particles have a size of 2-4 µm (Fig. 6a). After 50 cycles, the size of nickel particles of 0S increases to the range of 15~20 µm. As seen in Fig. 6c, the size of nickel in the cell 5S after cycling is in the range of 3-6 µm. A comparison of Fig. 6b and Fig. 6c revealed that the grain size of nickel in the cell 5S is much smaller than that in the cell 0S after 50 cycles. It is indicated that the addition of sulfur to the cathode could prevent the nickel particle growth efficiently. In addition, both rectangular areas in Fig. 6c were conducted to EDS elemental analysis. The result of area 1 (red) shows that the atomic percentage of nickel is 98% and the result of area 2 (blue) shows that the atomic percentage of nickel and sulfur is 62.3% and 37.7% respectively, which is in close agreement with the stoichiometric ratio of Ni3S2. According to the XRD results (Fig. 5b), the Ni3S2 exists in the sulfur doped cathode. Combining with EDS and XRD results, it may be concluded that the material in rectangular area 2 is Ni3S2. It is confirmed that the Ni3S2 layer still exist on the surface of the nickel particles after cycling test (Fig. 6d), which is in accordance with the analysis of the Nyquist plot of the cell 5S (Fig. 4d). Fig. 6d shows the morphology of the cathode materials rinsed with distilled water in the cell without self-repairing function which contains the optimum level of Ni3S2 layer after cycling test 25. As seen in Fig. 6d, no Ni3S2 layer is observed on the surface of the Ni particles, by contrast, Ni3S2 exist in fragments and disperse among the nickel crystals, which is ascribed to the disappearance of the semicircle of Ni3S2 layer in the EIS plot (Fig. 4e). The Ni3S2 fragments were originated from the damaged Ni3S2 layer during cycling. Comparing the morphology of Ni3S2 shown in Fig.6c and 6d, we may suggest that the self-repairing function of Ni3S2 in the sulfur containing cell could prevent the damage of the Ni3S2 layer and enhance the blocking effect. As a result, the size of nickel particles of the cell 5S (Fig. 6c) is smaller than that of the cell (Fig.6d) with optimum level of Ni3S2 layer (without self-repairing function).

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Fig. 6. SEM image of (a) pristine nickel particles and (b) the nickel in the cell of 0%S (without sulfur in NaAlCl4) after cycling test. Morphology of cathode materials rinsed with distilled water in the cell (c) with self-repairing function (the cell 5S) and (d) without self-repairing function which contains the optimum level of Ni3S2 layer after cycling test. 3.4 The self-repairing function of Ni3S2 layer in sulfur containing Na/NiCl2 cell The mechanism of nickel particle growth in the cell without sulfur and Ni3S2 modification layer was illustrated in our previous study 25. During charging process, the formation of low-density NiCl2 layer on nickel particles (Ni@NiCl2) would lead to the volume expansion of particles and the aggregation of the adjacent Ni@NiCl2 particles. During discharging process, the integrated Ni@NiCl2 particles would be eventually converted to integrated nickel crystals and the size of nickel crystals increases. Therefore, the active surface area of nickel particles decreases dramatically and the capacity degrades rapidly. When nickel particles were encapsulated with Ni3S2 layer, the Ni@NiCl2 particles were hard to integrate together due to the blocking effect of the layer and thus the growth of nickel particle were suppressed. Unfortunately, the Ni3S2 layer would gradually break into pieces and get separated away from nickel particles 14


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during cycling due to the volume expansion and shrinkage of particles. Therefore, the blocking effect of the layer get attenuated, and the battery performance degrades. When sulfur were added into the cathode, the Ni3S2 layer could be in-situ generated on the surface of nickel particles and the layer could be constantly formed to repair the damaged layer during cycling. As a result, the cell with Ni3S2 self-repairing function (5S) exhibit even better cycle performance than the cell with optimum level of Ni3S2 layer.

Fig. 7. Schematic illustration of the process of Ni3S2 layer self-repairing and the mechanism of sulfur additive in preventing the growth of nickel particles in the cathode. The schematic representation of the mechanism of sulfur additive to the cathode in preventing nickel particle growth and the process of Ni3S2 self-repairing are shown in Fig. 7. As noted above, Ni3S2 layer is in-situ generated on the surface of nickel particles before testing (Fig. 7b). During charging, total volume of Ni@NiCl2 particles expand due to the conversion of high-density nickel to low-density NiCl2, which could lead to the damage of some of the surface Ni3S2 layers (Fig. 7c). However, for the discharging process, fresh Ni3S2 layer would be formed on the newly generated nickel surface by 15



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the reaction between sulfur and nickel when residual sulfur exists in the cathode, which is noted as self-repairing function (Fig.7e-f). Fig. 7e illustrates the cathode materials in the fully charged state (100% SoC), Ni3S2 layer is partly damaged due to particle expansion and NaCl is depleted during charging. Fig. 7f&g shows the cathode materials after discharging to x% SoC. Since nickel is generated from the outside to the inside of Ni@NiCl2 particles in the process of discharging 21, there is a newly generated nickel layer encapsulating Ni@NiCl2 particles (Fig. 7f). When sulfur exists in the cathode, sulfur will react with nickel, a fresh Ni3S2 layer would be formed on the newly generated nickel surface (Fig. 7g). Fig. 7h illustrates the cathode materials in the fully discharged state (0% SoC), almost NiCl2 is reduced to nickel and damaged Ni3S2 is repaired. Actually, Ni3S2 layer continues to be formed on the nickel surface during whole discharging process, so it exhibits a self-repairing function. Due to the consumption of sulfur, the amount of sulfur decreases and the Ni3S2 repairing function attenuates with cycling. As a result, the thickness and amount of the Ni3S2 layer on nickel particles decreases with cycling, which is in accordance with the result of the EIS plot in Fig. 4d where the semicircle of Ni3S2 interface decreases with cycling. The effect of Ni3S2 self-repairing function is related to the amount of sulfur addition in the cathode. The more sulfur added in the cathode, the longer time the self-repairing function could sustain. On the other hand, the thickness of the layer increases with the addition of sulfur. Therefore, the cycle performance of the cell enhanced with the addition of sulfur within a certain range (as seen in Fig. 4a). However, excessive sulfur would result in the degradation of cycle performance owing to the poor electrical conductivity of sulfur and its derivative Ni3S2. As seen in Fig. 4b, the polarization voltage increased with amount of sulfur in the cathode, which was harmful to the cell performance. These two opposing effects of sulfur on battery performance imply that there is an optimum value for sulfur addition. According to the above mentioned experimental results, the optimum sulfur addition amount is around the amount that contained in the cell 5S. The particle growth of Ni and NaCl in cathodes is considered to be the two most important factors that lead to the degradation of Na/NiCl2 battery 21. The Ni3S2 layer on 16


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the surface of nickel particles could prevent the growth of nickel particles but couldn’t inhibit the particle growth of NaCl in the cathode during cycling. Fig.S5a illustrates the SEM image of NaCl particles in the cell of 5S after cycling test. The composition analysis were studied by the EDS elemental mapping (Fig.S5b and c), in which the homogeneous existence of Na and Cl elements is illustrated. It is confirmed that the material in the Fig.S5a is NaCl. As we can see, the size of NaCl after 50 cycles is in the range of 20~30 µm. The SEM image of pristine NaCl particles is shown in Fig. S6. A comparison of Fig.S5 and Fig.S6 reveals an obvious growth of NaCl particles during cycling, which is in agreement with the reported results by Li. and Lu. et al. 20-21. Larger particle size of active ingredients in the cathode will lead to less active surface area for electrochemical reactions. Therefore, particle growth of NaCl could lead to the degradation of the sulfur contained cell in which the growth of nickel particle was suppressed in the cathode.

4. Conclusions In this work, it was found that Ni3S2 layer could be in-situ generated on nickel particles and exhibit a self-repairing function during cycling when sulfur exists in the cathode due to the reaction between sulfur and nickel particle. The self-repairing function of Ni3S2 layer could enhance the blocking effect and improve the battery cycle performance. The effect of self-repairing function of Ni3S2 layer is determined by the amount of sulfur in the cathode. There is an optimum value for sulfur addition amount. Therefore, below the optimum amount, the more the sulfur added in the cathode, the function could sustain longer time and lead to better cycle performance of the cell, while, excessive sulfur would result in battery performance degradation owing to the poor electrical conductivity of sulfur and the Ni3S2 product. The findings in this work could provide a logical basis for designing high durable Na/NiCl2 batteries.

Supporting information Supporting Information Available: Schematic illustration of self-made research Na/NiCl2 cell, supplementary TEM image of the sample 1S, rate performances of the cell 0S, 5S and the cell with optimum level of Ni3S2 layer on Ni particles, voltage plots 17



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of the 20th charge and discharge process for the cell of 0S, 1S, 3S, 5S, 7S, SEM/EDS observations for the cathode with the elimination of NaAlCl4 in the cell of 5S after cycling test, and the SEM image of pristine NaCl particles

Acknowledgments This work was financially supported by the National Natural Science Foundation of China (NFSC) project No. 51402333 and No. 51672300. The authors thank Prof. B. V. R. Chowdari (School of Materials Science and Engineering, Nanyang Technological University, Singapore) for the helpful discussion.

AUTHOR INFORMATION Corresponding Author Tel: +86-21-52411704, Fax: +86-21-52413903 *E-mail: [email protected], *E-mail: [email protected]

Notes The authors declare no competing financial interest.

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Table of contents Graphic

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Graphics for manuscript

Fig. 1. XRD patterns of (a) the positive materials of the cells with different sulfur heated in Argon atmosphere at 300 °C for 2 hours without any electrochemical tests and (b) the remaining particles prepared by the above materials after being washed with distilled water.

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Fig. 2. (a) SEM image of the remaining particles prepared by the positive materials of the cell 5S being heated in Argon atmosphere at 300 °C for 2 hours and then rinsed with distilled water. (b) & (c) EDS mapping images for elements S (red dots) & Ni (green dots) respectively with their corresponding SEM image on the left. (d) and (e) XPS spectra of Ni 2p and S 2p respectively.

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Fig. 3. TEM images of the remaining particles prepared by the positive materials of the cell (a) 1S, (b) 3S, (c) 5S, (d) 7S being heated in Argon atmosphere at 300 °C for 2 hours without any electrochemical tests and then washed with distilled water.

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Fig. 4. The electrochemical performance of five different Na/NiCl2 cells. (a) The cycling performance, (b) 20th charge and discharge curves of 0S, 1S, 3S, 5S and 7S. Electrochemical impedance spectroscopy plots of (c) the cells with and without sulfur in 10th cycle, (d) the cell 5S in different cycles at discharge state and (e) the cell with optimum level of Ni3S2 layer on Ni particles, (f) equivalent circuit.

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Fig. 5. X-ray diffraction patterns of the cathode materials in the cell of (a) 7S, (b) 5S, (c) 3S, (d) 1S, (e) 0S and (f) the cell with optimum level of Ni3S2 after 50 cycles in discharged state.

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Fig. 6. SEM image of (a) pristine nickel particles and (b) the nickel in the cell of 0%S (without sulfur in NaAlCl4) after cycling test. Morphology of cathode materials rinsed with distilled water in the cell (c) with self-repairing function (the cell 5S) and (d) without self-repairing function which contains the optimum level of Ni3S2 layer after cycling test.

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Fig. 7. Schematic illustration of the process of Ni3S2 layer self-repairing and the mechanism of sulfur additive in preventing the growth of nickel particles in the cathode.

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NiCl2

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