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Electrochemically activated Na–ZnCl2 battery using a carbon matrix in the cathode compartment
Journal of Power Sources 440 (2019) 227110
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Electrochemically activated Na–ZnCl2 battery using a carbon matrix in the cathode compartment Younki Lee a, 1, Han-Jun Kim b, c, 1, Dong-Jin Byun c, Kwon-Koo Cho a, Jou-Hyeon Ahn a, Chang-Sam Kim b, * a b c
Department of Materials Engineering and Convergence Technology, Gyeongsang National University, Jinju, Gyeongnam, 52828, Republic of Korea Center for Energy Storage Research, Korea Institute of Science and Technology, Seoul, 02792, Republic of Korea Department of Material Science & Engineering, Korea University, Seoul, 02841, Republic of Korea
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� A new cathode architecture with carbon felt improves Na/ZnCl2 battery performance. � ZnCl2–NaCl eutectic chemistry allows the use of a preformed carbon felt at 260 � C. � A new cathode provides significantly reduced cathodic resistances. � A carbon matrix also efficiently sup presses microstructural degradation.
A R T I C L E I N F O
A B S T R A C T
Keywords: Na/ZnCl2 batteries Sodium metal chloride cells Charge transfer resistance Carbon felt Cathode architecture
Sodium-metal chloride batteries have been highlighted as one of the massive energy storage systems for its intrinsically excellent safety and the use of abundant sodium. Nickel and sodium chloride have been the most studied for cathode materials because this chemistry allows high open-circuit voltage and high energy density among the candidates. However, there is a need to reduce the material cost due to costly nickel powders which are used in large quantities for the electrical connection in the cathode. This study proposes an electrochemically activated Na/ZnCl2 battery using less-expensive carbon felt to maintain efficient electron percolation in the cathode and evaluates the charge-discharge behavior and cell impedance. The Na/ZnCl2 cell, which has a ca pacity of 220 mAh with a new cathode configuration, significantly reduces the charge transfer resistance compared to conventional cells by approximately 42% and 62% at the 10th and 51st cycles, respectively. When the designed capacity increases to 440 mAh with the constant active area of an electrolyte, the reduction of resistance becomes apparent. This enhancement occurs because the carbon felt sufficiently conducts electrons in the cathode compartment and results in a uniform electrode reaction, which is also revealed in our analysis of the microstructure of the electrode.
* Corresponding author. E-mail address: [email protected] (C.-S. Kim). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.jpowsour.2019.227110 Received 14 May 2019; Received in revised form 22 August 2019; Accepted 3 September 2019 Available online 24 September 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
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1. Introduction
Researchers also investigated the optimum ratio of Zn to NaCl in the cathode material to improve the cell performance of Na/ZnCl2 batteries [25]. To maximize the benefits of the Na/ZnCl2 cell, we focused more on one of its inherent properties, the phase-transition into eutectic liquid above approx. 245 � C. When a preformed electron-conducting frame work such as less-expensive carbon felt is used to maintain electric percolation, the eutectic salt liquid can penetrate and disperse relatively easily, while the conventional metal and metal chloride particles do not. This method can lead to activate electronic transport toward active materials and effectively suppress unwanted reactions such as grain growth (Fig. 1). However, to the best of our knowledge, the cathode architectures in previous studies merely depended on the percolated particle-salt mixture [16,17,19–26]. Furthermore, because of the resti tution force of the carbon felt, the electrical contact between the cell components can be improved with a simplified battery configuration (Fig. 2). Here, a new cathode architecture for Na/ZnCl2 batteries with a preformed carbon network was first investigated in order to enhance the cathodic electrochemical performance. A carbon felt was applied as an efficient electronic-conductive route, and it also suggested a simplified cell configuration due to its spring-like nature. The newly designed cell is demonstrated and its electrochemical performance is evaluated to the charge-discharge behavior and electrochemical impedance spectra. The cell performances were compared to those of the conventional one.
Sodium beta-alumina batteries (SBBs) have been highlighted for the integrated energy storage systems on a large scale because the abun dance of sodium dramatically supports the reduction of the cost of materials, and the use of a molten electrode allows a significant increase in unit cell capacity compared to conventional lithium-based batteries [1–10]. Since a non-flammable solid electrolyte, beta-alumina, is used, the risk of catching fire due to massive Joule heat generated by charging and discharging at high current is low, and thus the cells are tolerant of their high-temperature operation (190–350 � C) [1,4–7,11–13]. Sodium-sulfur (Na/S) batteries are one of the well-commercialized SBBs that use molten sulfur as an anode material; sulfur is much less expen sive than other electrode candidates, and its molten state offers an extended reaction area by convection and difference in wetting prop erties of reaction product [1,7,8]. However, leakage of highly corrosive and active electrode materials at high temperature has a fatal disad vantage: it can lead to a fire accident [6,8]. On the other hand, sodium metal chloride (Na/MCl2) batteries, the other type of representative SBBs, have significant advantages for safety due to their non-flammable cathode chemistry. In fabrication, typically, the active materials of the cathode are comprised of three types of starting materials: (i) sodium chloride, e.g., NaCl, which works as a sodium source and can react with metal particles; (ii) catholyte, e.g., sodium tetrachloroaluminate (NaAlCl4), which facilitates sodium ion conduction as a liquid form at operating temperatures [14,15]; and (iii) metal particles, which provide a fast electron conduction pathway while being a reactant, are commonly used [2–6,16–18]. Under a charging process, sodium ions are beginning to dissolve from NaCl and conduct toward the anode through the ionic liquid. At the same time, the metal source is chlorinated and forms metal chloride from the surface of the particles. As charging and discharging are repeated, growth of solid particles occurs [6]. Sodium/nickel chloride (Na/NiCl2) is the most preferred chemistry pair among the candidates, Fe, Zn, Cu, Ag and so on, due to its higher open-circuit voltage (OCV) and high specific energy [4–6,16,17,19,20]. However, it is essential to use a large amount of nickel ($18.6 per kg) to maintain the electron pathway during charging and discharging [5,6,16,17,21,22]; thus, this high content of nickel serves as an obstacle to battery cost reduction. In the previous re searches, to overcome the high materials cost of the cathode, nickel-coated graphite was utilized as an inexpensive framework to support electronic conduction. The nickel content and the loading level of the active materials had also been optimized to avoid overuse and to obtain high capacity under high discharging power [17,19,21]. It is also a feasible approach to use alternative chemistry using lowcost metals [16,20,22–26]. Zinc is one of the suitable candidate ele ments for the active materials of sodium metal halide batteries because Zn satisfies the cell reactions entirely and the cost of Zn is only approximately 10% of Ni [20,22,23,25,26]. However, consider that the Na/ZnCl2 chemistry has several drawbacks: The composition exhibits a lower free energy change upon discharge than that of a nickel-based composition and presents a significantly lower OCV compared to the other candidates discussed earlier [16,20,22]. According to the previous report, the specific energy for Na/ZnCl2 at 90% efficiency was estimated as 516 Wh kg 1 when the specific energy for Na/NiCl2 was calculated to 697.9 Wh kg 1 [27]. Interestingly, notwithstanding these shortcomings, this Zn-chemistry has another essential feature that is different from other compositions. There is a regime where the eutectic salt liquid of NaCl–ZnCl2 can be generated during charging and discharging, repre sentatively, above 245 � C [20,22–25,28]. The phase transition to eutectic liquid has a chance to suppress the grain growth of Zn and NaCl so that can promote stable cell cycle performance. The molten salt can lead to ionic conduction in the cathode. However, due to the restricted range of composition in which the salt exists as a liquid phase, it is recommended to use the additional catholyte, e.g., NaAlCl4, to have sufficient ion transport at the operating temperature [22,25,26].
2. Experimental Zinc (Kojundo chem., 99.9%) and sodium chloride (Kojundo chem., 99.9%) were used in a powder form as the cathode materials of Na/ ZnCl2 batteries. Zinc was sieved with 25 μm-opening in an Ar-filled glove-box. For NaCl, the powder was pulverized using a mortar and a pestle in a glove-box and was subsequently screened with a 70 μm sieve. Sodium tetrachloroaluminate (NaAlCl4, Sigma-Aldrich, 99.99%) and metallic sodium (Sigma-Aldrich, 99.9%) were prepared as the catholyte and anode materials, respectively. The screened Zn and NaCl, as well as NaAlCl4, were weighed in a glove-box as 0.91 g, 0.48 g, and 2.8 g, respectively, for a 220 mAh-cell. The weight of active materials was doubled to fabricate 440 mAh-cells. Zn and NaCl were mixed in the molar ratio of 1.7:1, and the NaAlCl4 catholyte powder was incorporated in a weight ratio to the cathode material of 1:2. The weighed powders were transferred to the outside of the glove-box with a sealed vial and then evenly mixed through a SPEX mill for 30 min. The theoretical ca pacities of the cells calculated in terms of the NaCl capacity were 220 mAh and 440 mAh, respectively. The carbon felt with a diameter of 25 mm and a thickness of 10 mm was heated in a furnace to 600 � C at a rate of 5 � C min 1, and further heat-treated in an Ar atmosphere for 1 h, followed by natural cooling to 100 � C. The heat-treated carbon felt was immediately transferred to the glove box at 100 � C to minimize exposure to air. Commercial Na β’’-alumina (1609-L1, Finetech Ltd., Republic of Korea) disks were used as a solid electrolyte in this study. The solid electrolyte has a thickness of 1.2 mm and resistivity of 10.6 Ω cm at 259 � C that measured by Pt-blocking method (Fig. S1). To assemble with the other parts, the electrolyte was glass-bonded onto an α-alumina (99.9%) header (Fig. 2). The dimensions of the α-alumina header are 45.15 mm in outer diameter, 24.85 mm in inner diameter of cathode compartment, and 10.0 mm in the depth 24.85 mm in inner diameter, and 10.0 mm in the depth of cathode compartment respectively. The active area of the electrolyte was up to 4.85 cm2. All the Na/ZnCl2 cells, the conventional and the newly designed cells, were assembled in a glove-box for a comparison. Fig. 2a is a scheme of the conventional cell configuration without carbon felt. For the conventional cell, the well-mixed cathode powders were filled into the upper opening of the part of Na β’’-alumina electrolyte-α-alumina header, and a piece of a nickel mesh was placed on the flattened powders as a current collector. Two sets of cells with different capacities, 220 and 2
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Journal of Power Sources 440 (2019) 227110
Fig. 1. Schematic view of (a) the conventional and (b) the new cathode architecture of Na/ZnCl2 batteries.
Fig. 2. The Na/ZnCl2 cells with (a) a conventional configuration and (b) a new cathode architecture using carbon felt.
440 mAh, were fabricated (referred to as C220 and C440, respectively). Then, the assembly was heated up to 200 � C on a hot-plate to liquefy the NaAlCl4 catholyte and frequently tapped to achieve a well-distribution. After solidification of the catholyte at room temperature, a nickel pin connected to the upper case (Ni-coated STS316L) was used as a current collector and put in contact with a nickel mesh. A graphite gasket was used to seal between the α-alumina header and the upper case. To improve the sodium wetting, the anode side of Na β’’-alumina was entirely marked with a graphite pencil. An aluminum foil was used as a sealing gasket between the α-alumina and the lower case (Ni-coated STS316L). A part of sodium was pre-filled into the shim to facilitate ion migration from the electrolyte. The sodium-filled shim was inserted into the anode, and then covered with an anode case. Finally, the anode and cathode were sealed entirely by tightening the bolts and nuts connected to the covers with the appropriate pressure with a wrench (Fig. S2). An insulating plate on the upper case was additionally used to achieve electric disconnection between the caps. For comparison, the cell configuration was newly designed to adopt carbon felt in the cathode (Fig. 2b and Fig. S2). The cell with a new cathode architecture was considerably simplified but assembled in a way similar to the conventional one. Two sets of new cells with different capacities, 220 and 440 mAh, were also fabricated (referred to as F220 and F440, respectively). Because of the different cell designs, several manufacturing processes were changed. When the NaAlCl4 catholyte was fully liquefied at 200 � C, then a piece of carbon felt was inserted into the cathode materials and then consolidated at room temperature. No additional pin-type current collector was used on the cathode side. On the anode side, similar to the conventional one, the solid electrolyte was also coated by a graphite pencil, and metallic sodium was pre-filled. A
metal pin on the anode cap was used as a current collector for the so dium electrode. In this configuration, the sodium anode was facing upward in order to achieve gravity assistance during wetting. The graphite gaskets were used in both the anode and cathode side and finished in the same manner as in the conventional method [29]. Note that the eutectic temperature of the ZnCl2–NaCl is near 245 � C [28]. The higher the temperature, the wider the liquid region. However, the lowered operating temperature can retard cathode degradation [6, 26,30]. Therefore, the operation temperature of 260 � C was chosen in consideration of the operating margin for the liquid region. All the assembled cells were heated to 260 � C in a furnace at the rate of approximately 0.4 � C min 1, where they were initially charged under a current of 3 mA (0.62 mA cm 2) up to 80% of the state of charge (SoC) and discharged back to 20% of the SoC. The cells were further cycled between 20 and 80% SoC (cycling capacity of 132 mAh for the 220mAh-cells) to compare the voltage changes at the end-of-charge (EoC) and the end-of-discharge (EoD). After the initial charging-discharging, the cells were cycled twice under currents of 5 mA (1.03 mA cm 2) and 10 mA (2.06 mA cm 2), respectively. Consequently, the cells were charged under a constant current of 30 mA (6.19 mA cm 2) and discharged under the various currents of 30, 40, 60, 80, 100, 120, 140, and 160 mA (6.19–33 mA cm 2). Every five cycles, the conditioning cycle was performed with a current of 10 mA for charging and discharging to evaluate the retention of cell properties. However, the C440 cell was cycled with a reduced charge/discharge current due to instability in the voltage behavior. Cell cycling was car ried out using a battery testing system (WBCS 3000, WonATech Co.) and electrochemical impedance spectra were obtained with a potentiostat (VMP3, BioLogic). The microstructure of the cathode was analyzed 3
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through a scanning electron microscope (SEM, Inspect F50, FEI) with a backscattered electron detector (BSED). Subsequently, the distribution of Zn and Na was evaluated with energy-dispersive X-ray spectroscopy (EDS).
(Step 3), and (iv) liquid salt and ZnCl2 (Step 4), respectively [22,26,28]. Based on this phase transformation, Lu proposed that the voltage pro files can be distinguished by four different steps during the charging/ discharging processes under the full SoC range, and the theoretical NaCl utilization rates at the end of each step under 280 � C were suggested as 50%, 76.5%, 87.6%, and 100%, respectively [22]. At the lowered operation temperature, 260 � C, the region of liquid salt shrank (65.7–69.7 mol% of ZnCl2) [28], so the values of SoC at the end of steps 2 and 3 changed to 78.3% and 81.3%, respectively. All the voltage profiles in Fig. 3a nicely present three multiple steps as expected in the SoC range of 20%–80%; however, the broad range of phase transition of Step 1 into Step 2 at the initial charging and the SoC shift of transition points were observed. These results might have originated from in homogeneity of the cathode constituent at the reaction front during initial charge-discharge. The cathode materials might be localized due to insufficient liquefaction and irregular salt liquid penetration into the carbon matrix. Therefore, the further phase transition might unevenly proceed. The difference of voltage profiles of 3 mA compared to those of 30 mA and 80 mA infers that the uneven active materials were distrib uted during charge-discharge cycle. The conventional cell with 220mAh of capacity, C220, was identi cally tested at 260 � C and the voltage profiles are also compared to those of F220 cells in Fig. 3b. Under the higher current, the conventional cell exhibited higher charging voltages and lower discharging voltages, and the deviation was more obvious in the charging process. This higher overpotential might be caused by electronic disconnection. During the
3. Results and discussion As mentioned above, the advanced Na/ZnCl2 cells with a carbon network (F220 and F440) were demonstrated and compared to the conventional ones (C220 and C440) (Fig. 2). At the cathode side, the carbon felt was applied to improve the electron conduction toward cathodic active materials but it also acted as a current collector that simultaneously contacted the cathode case and the electrolyte; there fore, no additional probe into the cathode was required. A metallic shim at the anode was omitted when the anode was faced on the upper cap because the gravity performed the equivalent role, which improved the sodium wettability and holding of the molten sodium at the surface of the solid electrolyte. To understand the current effect on cell performance, voltage pro files over state-of-charge (SoC) for the new architecture of a cell with a capacity of 220 mAh, F220, tested in the SoC range of 20%–80% at 260 � C and the results are shown in Fig. 3a. The cell was initially charged and further discharged with a 3 mA (a black line). As previously re ported, the NaCl–ZnCl2 cathode chemistry above 245 � C presents four different phases depending on the mole fraction of NaCl: (i) Na2ZnCl4 and NaCl (Step 1), (ii) Na2ZnCl4 and liquid salt (Step 2), (iii) liquid salt
Fig. 3. The voltage profiles of the Na/ZnCl2 batteries (220mAh) at 260 � C: (a) the charge-discharge curves of the cell with a carbon matrix for discharging currents of 3, 10, and 80 mA (b) the voltage profiles of the cells with or without a carbon matrix (dotted lines) (c) end-of-charge and the end-of-discharge voltages as a function of cycle number and discharging currents (d) average discharge voltages and powers under various discharging current. 4
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Journal of Power Sources 440 (2019) 227110
charging processes, metallic zinc was gradually consumed by reacting with sodium-contained salts. Although a large amount of zinc was used in the cathode, the electronic percolation at the end might locally collapse. Accordingly, this lowered overpotential of the new architec ture supported that a carbon matrix conducts electrons efficiently. The cell voltages at the end-of-charge (EoC) and the end-of-discharge (EoD) under the various currents, which typically represent the degra dation rate [6,21,22,26], are plotted in Fig. 3c. After the phase con version into the liquid salt through the conditioning cycles under 10 mA, the evaluation was carried out at 30 mA of fixed charging current, while the discharging current was varied from 30 mA to 160 mA. For the every five cycles, the cells were charged and discharged with the conditioning current of 10 mA and there conditioning cycles were marked as ’*’. The EoC voltages of the conditioned F220 cell were approximately 2.2 V, while the EoD voltage was 1.38 V under 160 mA. For the conditioning cycles with 10 mA of charging and discharging, the cells showed the stable cell retention even for the cycles resulted irregular EoD or EoC changes. When the discharge current increased to 120 mA or higher, there was an abrupt change in the EoC voltages of the C220 cell; this degradation inferred that the localized zinc precipitation under fast discharging was unable to provide sufficient current distribution on the active materials. In the good correspondence with Fig. 3b, the EoD voltages of the F220 cell were higher than those of the C220 cell at all current intervals, and the voltage difference increased under the higher current discharging. At the 51st cycle, the EoD voltage of the F220 cell was 1.372 V, which is about 15% higher than that of the C220 cell, 1.193 V. This trend is more clearly shown in Fig. 3d and Fig. S3; the graph presents the average discharge voltages and corresponding powers as a function of the discharge current. The slope of the voltage curves indicates the cell resistance. For the F220 cell, the average volt ages showed a linear trend up to 100 mA for discharging; however, the voltage curve was gradually deviated from the linear trend toward the high discharge current. The voltage degradation was more severely observed in the C220 cell; the maximum power of the F220 cell was 240.7 mW, enhanced approximately 11% compared to the conventional one. It is evident that the use of a carbon matrix in the cathode allowed high-power discharging, especially under a high current. Note that the active area of the solid electrolyte was 4.85 cm2. The power density of the F220 cell is 49.6 mWcm-2 at the current density of 28.9 mAcm 2. It is reported that the prominent planar Na/NiCl2 batteries powered approximately 200 mWcm 2 at the width of 60% of the theoretical ca pacity [19]. Since the operating voltage of Na/ZnCl2 cells is relatively lower compared to that of Na/NiCl2 cells, it is hard to compare the performance directly, however, there still be need to enhance the performance. The electrochemical impedance spectra (EIS) of the secondary bat teries can be typically analyzed as Randles’ equivalent circuit based on one-dimensional diffusion into a semi-infinite solid in an ideal solution [17,26,31,32]. The Ohmic resistance (ROhm) is found by reading the real axis value at the high-frequency intercept. The overall Ohmic resistance can be originated from the solid electrolyte, the NaAlCl4 catholyte, the molten sodium, and the metallic case and wires [26,33]. The semicircles at mid-frequencies are originated from both the charge transfer re sistances (RCT) and the double layer capacitances (Cdl) at the electrode-electrolyte interfaces, and the straight line with a slope of 45� at low frequencies was due to the Warburg impedance (W). The redox reactions that occurring at the interface of the electrode can directly cause on the amount of RCT and Cdl so the analysis of the RCT is necessary to evaluate the activation of the electrochemical reaction [32]. Fig. 4a and Fig. 4b show the electrochemical impedance spectra of both the C220 and F220 cells at the 10th cycle and 51st cycle, respectively. In all cases, the Ohmic difference did not mainly depend on the charge/di scharge and the cathode architecture, but the charge transfer resistances were obviously changed. As shown in Fig. 4a, at least two types of charge transfer reactions were involved: (i) a tens of kHz (Reaction I) region and (ii) a several hundred Hz-region (Reaction II). Reaction I exhibited an
Fig. 4. The electrochemical impedance spectra of the Na/ZnCl2 cells, C220 and F220, with or without a carbon matrix at 260 � C: (a) 10th cycle and (b) 51st cycle.
almost constant resistance regardless of charging and discharging, while Reaction II showed a resistance increase of about 50% or more when discharging compared with charging. In the case of the F220 cell, Re action I was predominantly observed, and the semicircle corresponding to Reaction II was hardly distinguished. As a result, due to the use of a carbon matrix, the sum of the charge transfer resistance was signifi cantly reduced at approximately 42% and 62% at the 10th and 51st cycle, respectively (Fig. S4). This result proves that Reaction II, which was much affected by charge and discharge, was significantly activated by using a carbon matrix. It also shows good agreement with the reac tion mechanism suggested in discussions for Figs. 1 and 3. If the carbon matrix activates the charge transfer reaction in the cathode, the effect will be more apparent when the cell capacity in creases due to expanded reaction front. Fig. 5 show the charge and discharge curves when the cell capacity is doubled, 220 to 440 mAh; the cells were evaluated in the same SoC range as the previous results. As shown in Fig. 5, the voltage profile of the F440 cell was compared to that of the F220 cell as a function of current. Interestingly, both results are considerably similar at all of the currents; the voltage difference be tween the two cells was not greater than that of the carbon matrix effect on 220mAh-cells. This result means that the resistance change according to the capacity is not substantial for the cells with the new cathode ar chitecture. In other words, the electronic path belonging to the carbon matrix was still sufficient to the doubled cell capacity. Furthermore, the phase transition broadening discussed for the F220 cell was not observed in the equivalent process of the F440 cell; this broadening might have been due to the initial mixing status of the cathode materials, not the inherent characteristics of the new cathode architecture. On the other hand, for the C440 cell, the overpotential increase of 0.36 V at a 5
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Journal of Power Sources 440 (2019) 227110
cycle progressed, zinc unevenly precipitated and exhibited poor con nectivity and thus caused high overpotential in the cell. In principle, the phase transition into the eutectic liquid salts can suppress the localiza tion of the cathode material, but it is supposed to be insufficient under the present conditions; this interpretation is in good agreement with the results of impedance (Fig. 7). When the carbon matrix was applied to the 440 mAh-cells, the sum of the charge transfer resistances considerably decreased, and the resis tance difference between after charging and discharging also remark ably decreased compared to that of the 220 mAh-cells (Fig. 7). Similar to Fig. 4, the carbon matrix did not significantly change the higherfrequency semicircle, but it sharply shrank the lower-frequency one. The Ohmic resistance was relatively constant regardless of the cell configuration and the SoC state, which means that the contribution of the carbon matrix to the ohmic resistance was small and mainly affected the electrode response. The cathode constituents at the discharged state were examined with SEM-BSED to investigate the correspondence between the charge transfer resistance and the microstructure of the cathode; since both cells, C220 and F220, were disassembled at a SoC of 20%, excess Zn, NaCl, NaAlCl4, and Na2ZnCl4 could exist. Sodium appears as a green area, while a grey area represents zinc. Fig. 8 and Fig. S5 are the images of the cathode without and with a carbon matrix, respectively. The main difference at a glance is in the size of particles. For the conventional cathode, the Na2ZnCl4 particles, the aggregated ones, grew to a size of about 100 μm or more and were not uniformly distributed. On the other hand, the new cathode architecture provided the well-distributed fine microstructure in Fig. 8b. During the charging and discharging, the cathode material passes through the region where only the liquid salt exists on the phase diagram of NaCl–ZnCl2 [22,26,28] so that the par ticles are uniformly distributed. However, if the electronic pathway is insufficient and the liquid salt-only zone (66–70 mol% ZnCl2) is rela tively narrow at 260 � C, the formation of the NaCl–ZnCl2 molten salt and the precipitation of the metallic Zn may occur locally so the sufficient dispersion may be hardly achieved kinetically as the charge and discharge occur more rapidly. The direction of the cathode compartment (upward for the conven tional cells and downward for the newly designed cells, respectively) may have influenced the performance difference. When the cathode compartment is located at the beneath of the solid electrolyte, the liq uefied salts, including ZnCl2–NaCl and NaAlCl4, can be withdrawn from the interface. However, since the cells with a carbon matrix shows lower cell degradation, the effect seems to be insignificant.
Fig. 5. The voltage profiles of the Na/ZnCl2 batteries with (red lines) or without (black lines) a carbon matrix under a discharging current of 80 mA at 260 � C. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
discharging current of 80 mA was exceptionally remarkable compared to the C220 cell (Fig. 5). Fig. 6 shows the EoC and EoD voltages of 440 mAh-cells according to the charging and discharging current and cycle number. The C440 cell exhibited very unstable EoC voltages under 30 mA of charge current compared to the half-capacity cell. The charge current decreased to 20 mA from the 20th cycle to achieve the stability of EoC voltages, but a voltage fluctuation occurred when the discharge current was more than 100 mA. On the other hand, the F440 cell exhibited a stable EoC of about 2.3 V even at a charging current of 30 mA and a discharging current of 30–160 mA. Under a discharge current of 100 mA for the C440 and F440 cells, the EoD voltage (1.606 V) of the F440 cell was enhanced by about 0.508 V compared to that of the C440 cell under the equivalent current due to the use of the carbon matrix in the cathode. These results are even more apparent and fit well with the discussion in Figs. 3c and 4. The sufficient electronic path in the cathode might alleviate the insuf ficient distribution of cathodic salt and the weak connection of metallic zinc. Without carbon felt, zinc ions in the cathodic salts would be rapidly reduced only near the zinc particles during high-rate discharge; as the
4. Conclusions In this research, we demonstrate electrochemically activated Na/
Fig. 6. The end-of-charge (black squares) and end-of-discharge (red circles) voltages as a function of current for the 440 mA-cells with (closed symbols) or without (open symbols) a carbon matrix. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 7. The electrochemical impedance spectra of the Na/ZnCl2 cells, C440 and F440, with or without a carbon matrix at 260 � C as functions of SoC status and cycle number. 6
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Fig. 8. Backscattered electron detector images with elemental mapping (Zn and Na) for the 220mAh-cathode (a) without a carbon matrix and (b) with a car bon matrix.
ZnCl2 cells using a carbon matrix in the cathode compartment. The newly designed batteries provided enhanced rate performance and cycling stability at 260 � C compared to those of the conventional cell. The ZnCl2–NaCl chemistry allowed the use of a carbon matrix as a wellstructured electronically conductive network due to its unique phase transition into a penetrative eutectic liquid salt above 245 � C. With carbon felt, the voltages at the end-of-discharge of the cell was enhanced 15 and 50% compared to those of the conventional cells for the 220 mAh (1.37 V)- and 400mAh (1.61 V)- cell, respectively. The total charge transfer resistance of the 220 mAh-cell was also significantly decreased as 62% at the 51st cycle under 160 mA of discharging current compared to the conventional one. This tendency was even more apparent in high capacity (440 mAh) cells. This prominent performance was likely due to the effective electron transfer through the carbon felt, which provided sufficient distribution of Zn precipitates and sodium-containing salts kinetically. By redesigning the cathode with inexpensive materials and an efficiently conductive carbon matrix, the advanced Na/ZnCl2 battery
offers a beneficial approach to developing a cost-compatible massive energy storage system. Acknowledgment This research was supported by the Energy Efficiency & Resources Core Technology R & D Program (No. 20142010102460) funded by the Ministry of Trade, Industry & Energy (MOTIE) and the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (No. NRF2017R1A4A1015711). This work was also supported by the fund of research promotion program (2016-04-005), Gyeongsang National University, 2016. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. 7
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org/10.1016/j.jpowsour.2019.227110.
[17] H.J. Chang, X. Lu, J.F. Bonnett, N.L. Canfield, S. Son, Y.C. Park, K. Jung, V. L. Sprenkle, G. Li, “Ni-Less” cathodes for high energy density, intermediate temperature Na–NiCl2Batteries, Adv. Mater. Interfaces. 5 (2018) 1–8, https://doi. org/10.1002/admi.201701592. [18] G. Li, X. Lu, J.Y. Kim, M.H. Engelhard, J.P. Lemmon, V.L. Sprenkle, The role of FeS in initial activation and performance degradation of Na-NiCl2batteries, J. Power Sources 272 (2014) 398–403, https://doi.org/10.1016/j.jpowsour.2014.08.106. [19] X. Lu, H.J. Chang, J.F. Bonnett, N.L. Canfield, K. Jung, V.L. Sprenkle, G. Li, Effect of cathode thickness on the performance of planar Na-NiCl2battery, J. Power Sources 365 (2017) 456–462, https://doi.org/10.1016/j.jpowsour.2017.07.029. [20] G. Tao, N. Weber, A High Temperature (400 to 650oC) Secondary Storage Battery Based on Liquid Sodium and Potassium Anodes, 2007, pp. 235–238, https://doi. org/10.2172/908547. [21] H.J. Chang, N.L. Canfield, K. Jung, V.L. Sprenkle, G. Li, Advanced Na-NiCl2 battery using nickel-coated graphite with core-shell microarchitecture, ACS Appl. Mater. Interfaces 9 (2017) 11609–11614, https://doi.org/10.1021/acsami.7b00271. [22] X. Lu, G. Li, J.Y. Kim, J.P. Lemmon, V.L. Sprenkle, Z. Yang, A novel low-cost sodium-zinc chloride battery, Energy Environ. Sci. 6 (2013) 1837–1843, https:// doi.org/10.1039/c3ee24244g. [23] C.W. Ahn, M. Kim, B.D. Hahn, I. Hong, W. Kim, G. Moon, H. Lee, K. Jung, Y. C. Park, J.H. Choi, Microstructure design of metal composite for active material in sodium nickel-iron chloride battery, J. Power Sources 329 (2016) 50–56, https:// doi.org/10.1016/j.jpowsour.2016.08.068. [24] P. Parthasarathy, N. Weber, A.V. Virkar, High temperature sodium - zinc chloride batteries with sodium beta" - alumina solid electrolyte (BASE), ECS Trans 6 (2007) 67–76, https://doi.org/10.1149/1.2811944. [25] H.-T. Kim, S.-I. Kim, H.-L. Choi, W.-I. Park, C.-S. Kim, Effect of Zn/NaCl ratios on the charge/discharge performance in Na-ZnCl2 battery, J. Korean Cryst. Growth Cryst. Technol. 25 (2015) 74–79, https://doi.org/10.6111/ JKCGCT.2015.25.2.074. [26] X. Lu, H.J. Chang, J.F. Bonnett, N.L. Canfield, K. Jung, V.L. Sprenkle, G. Li, An intermediate-temperature high-performance Na–ZnCl 2 battery, ACS Omega 3 (2018) 15702–15708, https://doi.org/10.1021/acsomega.8b02112. [27] G. Tao, N. Weber, A high temperature (400 to 650 � c) secondary storage battery based on liquid sodium and potassium anodes. http://www.osti.gov/servlets/pur l/908547/, 2007. [28] NaCl-ZnCl2 Phase diagram (n.d.), http://www.crct.polymtl.ca/fact/phase_diagram .php?file¼NaCl-ZnCl2.jpg&dir¼FTsalt. [29] J.P. Lemmon, Planar ZEBRA battery for renewable integration and grid applications, in: IBA Meet. Cape T. South Africa, 2011. https://energyenvironment. pnnl.gov/presentations/pdf/Planar.Zebra.pdf. [30] G. Li, X. Lu, J.Y. Kim, K.D. Meinhardt, H.J. Chang, N.L. Canfield, V.L. Sprenkle, Advanced intermediate temperature sodium-nickel chloride batteries with ultrahigh energy density, Nat. Commun. 7 (2016) 1–6, https://doi.org/10.1038/ ncomms10683. [31] R.A. Huggins, Advanced Batteries : Materials Science Aspects, Springer, 2009. [32] J.-K. Park, Principles and Applications of Lithium Secondary Batteries, Wiley-VCH, 2012. https://www.wiley.com/en-us/PrinciplesþandþApplicationsþofþLithiumþ SecondaryþBatteries-p-9783527331512. (Accessed 14 February 2019). [33] K. Jung, H.J. Chang, J.F. Bonnett, N.L. Canfield, V.L. Sprenkle, G. Li, An advanced Na-NiCl2battery using bi-layer (dense/micro-porous) β00 -alumina solid-state electrolytes, J. Power Sources 396 (2018) 297–303, https://doi.org/10.1016/j. jpowsour.2018.06.039.
References [1] J.L. Sudworth, The sodium/sulphur battery, J. Power Sources 11 (1984) 143–154. [2] Z. Yang, J. Zhang, M.C.W. Kintner-meyer, X. Lu, D. Choi, J.P. Lemmon, Electrochemical energy storage for green grid.pdf, Chem. Rev. (2011) 3577–3613, https://doi.org/10.1021/cr100290v. [3] B. Dunn, H. Kamath, J. Tarascon, Electrical energy storage for the Grid : a battery of choices, Science (80-.) 334 (2011) 928–936. [4] J.L. Sudworth, The sodium/nickel chloride (ZEBRA) battery, J. Power Sources 100 (2001) 149–163. [5] C.H. Dustmann, Advances in ZEBRA batteries, J. Power Sources 127 (2004) 85–92, https://doi.org/10.1016/j.jpowsour.2003.09.039. [6] G. Li, X. Lu, J.Y. Kim, K.D. Meinhardt, H.J. Chang, N.L. Canfield, V.L. Sprenkle, Advanced intermediate temperature sodium-nickel chloride batteries with ultrahigh energy density, Nat. Commun. 7 (2016) 1–6, https://doi.org/10.1038/ ncomms10683. [7] T. Oshima, M. Kajita, A. Okuno, Development of sodium-sulfur batteries, Int. J. Appl. Ceram. Technol. 1 (2004) 269–276, https://doi.org/10.1111/j.17447402.2004.tb00179.x. [8] Z. Wen, Y. Hu, X. Wu, J. Han, Z. Gu, Main challenges for high performance NAS battery: materials and interfaces, Adv. Funct. Mater. 23 (2013) 1005–1018, https://doi.org/10.1002/adfm.201200473. [9] X. Lu, J.P. Lemmon, J.Y. Kim, V.L. Sprenkle, Z. Yang, High energy density Na e S/ NiCl 2 hybrid battery, J. Power Sources 224 (2013) 312–316, https://doi.org/ 10.1016/j.jpowsour.2012.09.108. [10] P.A. Nelson, Advanced high-temperature batteries, J. Power Sources 29 (1990) 565–577, https://doi.org/10.1016/0378-7753(90)85026-9. [11] M. Hosseinifar, A. Petric, High temperature versus low temperature Zebra (Na/ NiCl 2) cell performance, J. Power Sources 206 (2012) 402–408, https://doi.org/ 10.1016/j.jpowsour.2012.01.125. [12] X. Ao, Z. Wen, Y. Hu, T. Wu, X. Wu, Q. He, Enhanced cycle performance of a Na/ NiCl2 battery based on Ni particles encapsulated with Ni3S2 layer, J. Power Sources 340 (2017) 411–418, https://doi.org/10.1016/j.jpowsour.2016.11.091. [13] H.J. Chang, X. Lu, J.F. Bonnett, N.L. Canfield, S. Son, Y.C. Park, K. Jung, V. L. Sprenkle, G. Li, Development of intermediate temperature sodium nickel chloride rechargeable batteries using conventional polymer sealing technologies, J. Power Sources 348 (2017) 150–157, https://doi.org/10.1016/j. jpowsour.2017.02.059. [14] J.S. Weaving, S. Walter Orchard, Experimental studies of transition metal chloride electrodes in undivided cells using molten NaAlCl4 electrolyte, J. Power Sources 36 (1991) 537–546, https://doi.org/10.1016/0378-7753(91)80079-D. [15] G. Li, X. Lu, C.A. Coyle, J.Y. Kim, J.P. Lemmon, V.L. Sprenkle, Z. Yang, Novel ternary molten salt electrolytes for intermediate-temperature sodium/nickel chloride batteries, J. Power Sources 220 (2012) 193–198, https://doi.org/ 10.1016/j.jpowsour.2012.07.089. [16] G. Li, X. Lu, J.Y. Kim, V.V. Viswanathan, K.D. Meinhardt, M.H. Engelhard, V. L. Sprenkle, An advanced Na–FeCl2 ZEBRA battery for stationary energy storage application, Adv. Energy Mater. 5 (2015) 1500357, https://doi.org/10.1002/ aenm.201500357.
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Electrochemically activated Na–ZnCl2 battery using a carbon matrix in the cathode compartment Younki Lee a, 1, Han-Jun Kim b, c, 1, Dong-Jin Byun c, Kwon-Koo Cho a, Jou-Hyeon Ahn a, Chang-Sam Kim b, * a b c
Department of Materials Engineering and Convergence Technology, Gyeongsang National University, Jinju, Gyeongnam, 52828, Republic of Korea Center for Energy Storage Research, Korea Institute of Science and Technology, Seoul, 02792, Republic of Korea Department of Material Science & Engineering, Korea University, Seoul, 02841, Republic of Korea
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� A new cathode architecture with carbon felt improves Na/ZnCl2 battery performance. � ZnCl2–NaCl eutectic chemistry allows the use of a preformed carbon felt at 260 � C. � A new cathode provides significantly reduced cathodic resistances. � A carbon matrix also efficiently sup presses microstructural degradation.
A R T I C L E I N F O
A B S T R A C T
Keywords: Na/ZnCl2 batteries Sodium metal chloride cells Charge transfer resistance Carbon felt Cathode architecture
Sodium-metal chloride batteries have been highlighted as one of the massive energy storage systems for its intrinsically excellent safety and the use of abundant sodium. Nickel and sodium chloride have been the most studied for cathode materials because this chemistry allows high open-circuit voltage and high energy density among the candidates. However, there is a need to reduce the material cost due to costly nickel powders which are used in large quantities for the electrical connection in the cathode. This study proposes an electrochemically activated Na/ZnCl2 battery using less-expensive carbon felt to maintain efficient electron percolation in the cathode and evaluates the charge-discharge behavior and cell impedance. The Na/ZnCl2 cell, which has a ca pacity of 220 mAh with a new cathode configuration, significantly reduces the charge transfer resistance compared to conventional cells by approximately 42% and 62% at the 10th and 51st cycles, respectively. When the designed capacity increases to 440 mAh with the constant active area of an electrolyte, the reduction of resistance becomes apparent. This enhancement occurs because the carbon felt sufficiently conducts electrons in the cathode compartment and results in a uniform electrode reaction, which is also revealed in our analysis of the microstructure of the electrode.
* Corresponding author. E-mail address: [email protected] (C.-S. Kim). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.jpowsour.2019.227110 Received 14 May 2019; Received in revised form 22 August 2019; Accepted 3 September 2019 Available online 24 September 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
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1. Introduction
Researchers also investigated the optimum ratio of Zn to NaCl in the cathode material to improve the cell performance of Na/ZnCl2 batteries [25]. To maximize the benefits of the Na/ZnCl2 cell, we focused more on one of its inherent properties, the phase-transition into eutectic liquid above approx. 245 � C. When a preformed electron-conducting frame work such as less-expensive carbon felt is used to maintain electric percolation, the eutectic salt liquid can penetrate and disperse relatively easily, while the conventional metal and metal chloride particles do not. This method can lead to activate electronic transport toward active materials and effectively suppress unwanted reactions such as grain growth (Fig. 1). However, to the best of our knowledge, the cathode architectures in previous studies merely depended on the percolated particle-salt mixture [16,17,19–26]. Furthermore, because of the resti tution force of the carbon felt, the electrical contact between the cell components can be improved with a simplified battery configuration (Fig. 2). Here, a new cathode architecture for Na/ZnCl2 batteries with a preformed carbon network was first investigated in order to enhance the cathodic electrochemical performance. A carbon felt was applied as an efficient electronic-conductive route, and it also suggested a simplified cell configuration due to its spring-like nature. The newly designed cell is demonstrated and its electrochemical performance is evaluated to the charge-discharge behavior and electrochemical impedance spectra. The cell performances were compared to those of the conventional one.
Sodium beta-alumina batteries (SBBs) have been highlighted for the integrated energy storage systems on a large scale because the abun dance of sodium dramatically supports the reduction of the cost of materials, and the use of a molten electrode allows a significant increase in unit cell capacity compared to conventional lithium-based batteries [1–10]. Since a non-flammable solid electrolyte, beta-alumina, is used, the risk of catching fire due to massive Joule heat generated by charging and discharging at high current is low, and thus the cells are tolerant of their high-temperature operation (190–350 � C) [1,4–7,11–13]. Sodium-sulfur (Na/S) batteries are one of the well-commercialized SBBs that use molten sulfur as an anode material; sulfur is much less expen sive than other electrode candidates, and its molten state offers an extended reaction area by convection and difference in wetting prop erties of reaction product [1,7,8]. However, leakage of highly corrosive and active electrode materials at high temperature has a fatal disad vantage: it can lead to a fire accident [6,8]. On the other hand, sodium metal chloride (Na/MCl2) batteries, the other type of representative SBBs, have significant advantages for safety due to their non-flammable cathode chemistry. In fabrication, typically, the active materials of the cathode are comprised of three types of starting materials: (i) sodium chloride, e.g., NaCl, which works as a sodium source and can react with metal particles; (ii) catholyte, e.g., sodium tetrachloroaluminate (NaAlCl4), which facilitates sodium ion conduction as a liquid form at operating temperatures [14,15]; and (iii) metal particles, which provide a fast electron conduction pathway while being a reactant, are commonly used [2–6,16–18]. Under a charging process, sodium ions are beginning to dissolve from NaCl and conduct toward the anode through the ionic liquid. At the same time, the metal source is chlorinated and forms metal chloride from the surface of the particles. As charging and discharging are repeated, growth of solid particles occurs [6]. Sodium/nickel chloride (Na/NiCl2) is the most preferred chemistry pair among the candidates, Fe, Zn, Cu, Ag and so on, due to its higher open-circuit voltage (OCV) and high specific energy [4–6,16,17,19,20]. However, it is essential to use a large amount of nickel ($18.6 per kg) to maintain the electron pathway during charging and discharging [5,6,16,17,21,22]; thus, this high content of nickel serves as an obstacle to battery cost reduction. In the previous re searches, to overcome the high materials cost of the cathode, nickel-coated graphite was utilized as an inexpensive framework to support electronic conduction. The nickel content and the loading level of the active materials had also been optimized to avoid overuse and to obtain high capacity under high discharging power [17,19,21]. It is also a feasible approach to use alternative chemistry using lowcost metals [16,20,22–26]. Zinc is one of the suitable candidate ele ments for the active materials of sodium metal halide batteries because Zn satisfies the cell reactions entirely and the cost of Zn is only approximately 10% of Ni [20,22,23,25,26]. However, consider that the Na/ZnCl2 chemistry has several drawbacks: The composition exhibits a lower free energy change upon discharge than that of a nickel-based composition and presents a significantly lower OCV compared to the other candidates discussed earlier [16,20,22]. According to the previous report, the specific energy for Na/ZnCl2 at 90% efficiency was estimated as 516 Wh kg 1 when the specific energy for Na/NiCl2 was calculated to 697.9 Wh kg 1 [27]. Interestingly, notwithstanding these shortcomings, this Zn-chemistry has another essential feature that is different from other compositions. There is a regime where the eutectic salt liquid of NaCl–ZnCl2 can be generated during charging and discharging, repre sentatively, above 245 � C [20,22–25,28]. The phase transition to eutectic liquid has a chance to suppress the grain growth of Zn and NaCl so that can promote stable cell cycle performance. The molten salt can lead to ionic conduction in the cathode. However, due to the restricted range of composition in which the salt exists as a liquid phase, it is recommended to use the additional catholyte, e.g., NaAlCl4, to have sufficient ion transport at the operating temperature [22,25,26].
2. Experimental Zinc (Kojundo chem., 99.9%) and sodium chloride (Kojundo chem., 99.9%) were used in a powder form as the cathode materials of Na/ ZnCl2 batteries. Zinc was sieved with 25 μm-opening in an Ar-filled glove-box. For NaCl, the powder was pulverized using a mortar and a pestle in a glove-box and was subsequently screened with a 70 μm sieve. Sodium tetrachloroaluminate (NaAlCl4, Sigma-Aldrich, 99.99%) and metallic sodium (Sigma-Aldrich, 99.9%) were prepared as the catholyte and anode materials, respectively. The screened Zn and NaCl, as well as NaAlCl4, were weighed in a glove-box as 0.91 g, 0.48 g, and 2.8 g, respectively, for a 220 mAh-cell. The weight of active materials was doubled to fabricate 440 mAh-cells. Zn and NaCl were mixed in the molar ratio of 1.7:1, and the NaAlCl4 catholyte powder was incorporated in a weight ratio to the cathode material of 1:2. The weighed powders were transferred to the outside of the glove-box with a sealed vial and then evenly mixed through a SPEX mill for 30 min. The theoretical ca pacities of the cells calculated in terms of the NaCl capacity were 220 mAh and 440 mAh, respectively. The carbon felt with a diameter of 25 mm and a thickness of 10 mm was heated in a furnace to 600 � C at a rate of 5 � C min 1, and further heat-treated in an Ar atmosphere for 1 h, followed by natural cooling to 100 � C. The heat-treated carbon felt was immediately transferred to the glove box at 100 � C to minimize exposure to air. Commercial Na β’’-alumina (1609-L1, Finetech Ltd., Republic of Korea) disks were used as a solid electrolyte in this study. The solid electrolyte has a thickness of 1.2 mm and resistivity of 10.6 Ω cm at 259 � C that measured by Pt-blocking method (Fig. S1). To assemble with the other parts, the electrolyte was glass-bonded onto an α-alumina (99.9%) header (Fig. 2). The dimensions of the α-alumina header are 45.15 mm in outer diameter, 24.85 mm in inner diameter of cathode compartment, and 10.0 mm in the depth 24.85 mm in inner diameter, and 10.0 mm in the depth of cathode compartment respectively. The active area of the electrolyte was up to 4.85 cm2. All the Na/ZnCl2 cells, the conventional and the newly designed cells, were assembled in a glove-box for a comparison. Fig. 2a is a scheme of the conventional cell configuration without carbon felt. For the conventional cell, the well-mixed cathode powders were filled into the upper opening of the part of Na β’’-alumina electrolyte-α-alumina header, and a piece of a nickel mesh was placed on the flattened powders as a current collector. Two sets of cells with different capacities, 220 and 2
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Fig. 1. Schematic view of (a) the conventional and (b) the new cathode architecture of Na/ZnCl2 batteries.
Fig. 2. The Na/ZnCl2 cells with (a) a conventional configuration and (b) a new cathode architecture using carbon felt.
440 mAh, were fabricated (referred to as C220 and C440, respectively). Then, the assembly was heated up to 200 � C on a hot-plate to liquefy the NaAlCl4 catholyte and frequently tapped to achieve a well-distribution. After solidification of the catholyte at room temperature, a nickel pin connected to the upper case (Ni-coated STS316L) was used as a current collector and put in contact with a nickel mesh. A graphite gasket was used to seal between the α-alumina header and the upper case. To improve the sodium wetting, the anode side of Na β’’-alumina was entirely marked with a graphite pencil. An aluminum foil was used as a sealing gasket between the α-alumina and the lower case (Ni-coated STS316L). A part of sodium was pre-filled into the shim to facilitate ion migration from the electrolyte. The sodium-filled shim was inserted into the anode, and then covered with an anode case. Finally, the anode and cathode were sealed entirely by tightening the bolts and nuts connected to the covers with the appropriate pressure with a wrench (Fig. S2). An insulating plate on the upper case was additionally used to achieve electric disconnection between the caps. For comparison, the cell configuration was newly designed to adopt carbon felt in the cathode (Fig. 2b and Fig. S2). The cell with a new cathode architecture was considerably simplified but assembled in a way similar to the conventional one. Two sets of new cells with different capacities, 220 and 440 mAh, were also fabricated (referred to as F220 and F440, respectively). Because of the different cell designs, several manufacturing processes were changed. When the NaAlCl4 catholyte was fully liquefied at 200 � C, then a piece of carbon felt was inserted into the cathode materials and then consolidated at room temperature. No additional pin-type current collector was used on the cathode side. On the anode side, similar to the conventional one, the solid electrolyte was also coated by a graphite pencil, and metallic sodium was pre-filled. A
metal pin on the anode cap was used as a current collector for the so dium electrode. In this configuration, the sodium anode was facing upward in order to achieve gravity assistance during wetting. The graphite gaskets were used in both the anode and cathode side and finished in the same manner as in the conventional method [29]. Note that the eutectic temperature of the ZnCl2–NaCl is near 245 � C [28]. The higher the temperature, the wider the liquid region. However, the lowered operating temperature can retard cathode degradation [6, 26,30]. Therefore, the operation temperature of 260 � C was chosen in consideration of the operating margin for the liquid region. All the assembled cells were heated to 260 � C in a furnace at the rate of approximately 0.4 � C min 1, where they were initially charged under a current of 3 mA (0.62 mA cm 2) up to 80% of the state of charge (SoC) and discharged back to 20% of the SoC. The cells were further cycled between 20 and 80% SoC (cycling capacity of 132 mAh for the 220mAh-cells) to compare the voltage changes at the end-of-charge (EoC) and the end-of-discharge (EoD). After the initial charging-discharging, the cells were cycled twice under currents of 5 mA (1.03 mA cm 2) and 10 mA (2.06 mA cm 2), respectively. Consequently, the cells were charged under a constant current of 30 mA (6.19 mA cm 2) and discharged under the various currents of 30, 40, 60, 80, 100, 120, 140, and 160 mA (6.19–33 mA cm 2). Every five cycles, the conditioning cycle was performed with a current of 10 mA for charging and discharging to evaluate the retention of cell properties. However, the C440 cell was cycled with a reduced charge/discharge current due to instability in the voltage behavior. Cell cycling was car ried out using a battery testing system (WBCS 3000, WonATech Co.) and electrochemical impedance spectra were obtained with a potentiostat (VMP3, BioLogic). The microstructure of the cathode was analyzed 3
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through a scanning electron microscope (SEM, Inspect F50, FEI) with a backscattered electron detector (BSED). Subsequently, the distribution of Zn and Na was evaluated with energy-dispersive X-ray spectroscopy (EDS).
(Step 3), and (iv) liquid salt and ZnCl2 (Step 4), respectively [22,26,28]. Based on this phase transformation, Lu proposed that the voltage pro files can be distinguished by four different steps during the charging/ discharging processes under the full SoC range, and the theoretical NaCl utilization rates at the end of each step under 280 � C were suggested as 50%, 76.5%, 87.6%, and 100%, respectively [22]. At the lowered operation temperature, 260 � C, the region of liquid salt shrank (65.7–69.7 mol% of ZnCl2) [28], so the values of SoC at the end of steps 2 and 3 changed to 78.3% and 81.3%, respectively. All the voltage profiles in Fig. 3a nicely present three multiple steps as expected in the SoC range of 20%–80%; however, the broad range of phase transition of Step 1 into Step 2 at the initial charging and the SoC shift of transition points were observed. These results might have originated from in homogeneity of the cathode constituent at the reaction front during initial charge-discharge. The cathode materials might be localized due to insufficient liquefaction and irregular salt liquid penetration into the carbon matrix. Therefore, the further phase transition might unevenly proceed. The difference of voltage profiles of 3 mA compared to those of 30 mA and 80 mA infers that the uneven active materials were distrib uted during charge-discharge cycle. The conventional cell with 220mAh of capacity, C220, was identi cally tested at 260 � C and the voltage profiles are also compared to those of F220 cells in Fig. 3b. Under the higher current, the conventional cell exhibited higher charging voltages and lower discharging voltages, and the deviation was more obvious in the charging process. This higher overpotential might be caused by electronic disconnection. During the
3. Results and discussion As mentioned above, the advanced Na/ZnCl2 cells with a carbon network (F220 and F440) were demonstrated and compared to the conventional ones (C220 and C440) (Fig. 2). At the cathode side, the carbon felt was applied to improve the electron conduction toward cathodic active materials but it also acted as a current collector that simultaneously contacted the cathode case and the electrolyte; there fore, no additional probe into the cathode was required. A metallic shim at the anode was omitted when the anode was faced on the upper cap because the gravity performed the equivalent role, which improved the sodium wettability and holding of the molten sodium at the surface of the solid electrolyte. To understand the current effect on cell performance, voltage pro files over state-of-charge (SoC) for the new architecture of a cell with a capacity of 220 mAh, F220, tested in the SoC range of 20%–80% at 260 � C and the results are shown in Fig. 3a. The cell was initially charged and further discharged with a 3 mA (a black line). As previously re ported, the NaCl–ZnCl2 cathode chemistry above 245 � C presents four different phases depending on the mole fraction of NaCl: (i) Na2ZnCl4 and NaCl (Step 1), (ii) Na2ZnCl4 and liquid salt (Step 2), (iii) liquid salt
Fig. 3. The voltage profiles of the Na/ZnCl2 batteries (220mAh) at 260 � C: (a) the charge-discharge curves of the cell with a carbon matrix for discharging currents of 3, 10, and 80 mA (b) the voltage profiles of the cells with or without a carbon matrix (dotted lines) (c) end-of-charge and the end-of-discharge voltages as a function of cycle number and discharging currents (d) average discharge voltages and powers under various discharging current. 4
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charging processes, metallic zinc was gradually consumed by reacting with sodium-contained salts. Although a large amount of zinc was used in the cathode, the electronic percolation at the end might locally collapse. Accordingly, this lowered overpotential of the new architec ture supported that a carbon matrix conducts electrons efficiently. The cell voltages at the end-of-charge (EoC) and the end-of-discharge (EoD) under the various currents, which typically represent the degra dation rate [6,21,22,26], are plotted in Fig. 3c. After the phase con version into the liquid salt through the conditioning cycles under 10 mA, the evaluation was carried out at 30 mA of fixed charging current, while the discharging current was varied from 30 mA to 160 mA. For the every five cycles, the cells were charged and discharged with the conditioning current of 10 mA and there conditioning cycles were marked as ’*’. The EoC voltages of the conditioned F220 cell were approximately 2.2 V, while the EoD voltage was 1.38 V under 160 mA. For the conditioning cycles with 10 mA of charging and discharging, the cells showed the stable cell retention even for the cycles resulted irregular EoD or EoC changes. When the discharge current increased to 120 mA or higher, there was an abrupt change in the EoC voltages of the C220 cell; this degradation inferred that the localized zinc precipitation under fast discharging was unable to provide sufficient current distribution on the active materials. In the good correspondence with Fig. 3b, the EoD voltages of the F220 cell were higher than those of the C220 cell at all current intervals, and the voltage difference increased under the higher current discharging. At the 51st cycle, the EoD voltage of the F220 cell was 1.372 V, which is about 15% higher than that of the C220 cell, 1.193 V. This trend is more clearly shown in Fig. 3d and Fig. S3; the graph presents the average discharge voltages and corresponding powers as a function of the discharge current. The slope of the voltage curves indicates the cell resistance. For the F220 cell, the average volt ages showed a linear trend up to 100 mA for discharging; however, the voltage curve was gradually deviated from the linear trend toward the high discharge current. The voltage degradation was more severely observed in the C220 cell; the maximum power of the F220 cell was 240.7 mW, enhanced approximately 11% compared to the conventional one. It is evident that the use of a carbon matrix in the cathode allowed high-power discharging, especially under a high current. Note that the active area of the solid electrolyte was 4.85 cm2. The power density of the F220 cell is 49.6 mWcm-2 at the current density of 28.9 mAcm 2. It is reported that the prominent planar Na/NiCl2 batteries powered approximately 200 mWcm 2 at the width of 60% of the theoretical ca pacity [19]. Since the operating voltage of Na/ZnCl2 cells is relatively lower compared to that of Na/NiCl2 cells, it is hard to compare the performance directly, however, there still be need to enhance the performance. The electrochemical impedance spectra (EIS) of the secondary bat teries can be typically analyzed as Randles’ equivalent circuit based on one-dimensional diffusion into a semi-infinite solid in an ideal solution [17,26,31,32]. The Ohmic resistance (ROhm) is found by reading the real axis value at the high-frequency intercept. The overall Ohmic resistance can be originated from the solid electrolyte, the NaAlCl4 catholyte, the molten sodium, and the metallic case and wires [26,33]. The semicircles at mid-frequencies are originated from both the charge transfer re sistances (RCT) and the double layer capacitances (Cdl) at the electrode-electrolyte interfaces, and the straight line with a slope of 45� at low frequencies was due to the Warburg impedance (W). The redox reactions that occurring at the interface of the electrode can directly cause on the amount of RCT and Cdl so the analysis of the RCT is necessary to evaluate the activation of the electrochemical reaction [32]. Fig. 4a and Fig. 4b show the electrochemical impedance spectra of both the C220 and F220 cells at the 10th cycle and 51st cycle, respectively. In all cases, the Ohmic difference did not mainly depend on the charge/di scharge and the cathode architecture, but the charge transfer resistances were obviously changed. As shown in Fig. 4a, at least two types of charge transfer reactions were involved: (i) a tens of kHz (Reaction I) region and (ii) a several hundred Hz-region (Reaction II). Reaction I exhibited an
Fig. 4. The electrochemical impedance spectra of the Na/ZnCl2 cells, C220 and F220, with or without a carbon matrix at 260 � C: (a) 10th cycle and (b) 51st cycle.
almost constant resistance regardless of charging and discharging, while Reaction II showed a resistance increase of about 50% or more when discharging compared with charging. In the case of the F220 cell, Re action I was predominantly observed, and the semicircle corresponding to Reaction II was hardly distinguished. As a result, due to the use of a carbon matrix, the sum of the charge transfer resistance was signifi cantly reduced at approximately 42% and 62% at the 10th and 51st cycle, respectively (Fig. S4). This result proves that Reaction II, which was much affected by charge and discharge, was significantly activated by using a carbon matrix. It also shows good agreement with the reac tion mechanism suggested in discussions for Figs. 1 and 3. If the carbon matrix activates the charge transfer reaction in the cathode, the effect will be more apparent when the cell capacity in creases due to expanded reaction front. Fig. 5 show the charge and discharge curves when the cell capacity is doubled, 220 to 440 mAh; the cells were evaluated in the same SoC range as the previous results. As shown in Fig. 5, the voltage profile of the F440 cell was compared to that of the F220 cell as a function of current. Interestingly, both results are considerably similar at all of the currents; the voltage difference be tween the two cells was not greater than that of the carbon matrix effect on 220mAh-cells. This result means that the resistance change according to the capacity is not substantial for the cells with the new cathode ar chitecture. In other words, the electronic path belonging to the carbon matrix was still sufficient to the doubled cell capacity. Furthermore, the phase transition broadening discussed for the F220 cell was not observed in the equivalent process of the F440 cell; this broadening might have been due to the initial mixing status of the cathode materials, not the inherent characteristics of the new cathode architecture. On the other hand, for the C440 cell, the overpotential increase of 0.36 V at a 5
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cycle progressed, zinc unevenly precipitated and exhibited poor con nectivity and thus caused high overpotential in the cell. In principle, the phase transition into the eutectic liquid salts can suppress the localiza tion of the cathode material, but it is supposed to be insufficient under the present conditions; this interpretation is in good agreement with the results of impedance (Fig. 7). When the carbon matrix was applied to the 440 mAh-cells, the sum of the charge transfer resistances considerably decreased, and the resis tance difference between after charging and discharging also remark ably decreased compared to that of the 220 mAh-cells (Fig. 7). Similar to Fig. 4, the carbon matrix did not significantly change the higherfrequency semicircle, but it sharply shrank the lower-frequency one. The Ohmic resistance was relatively constant regardless of the cell configuration and the SoC state, which means that the contribution of the carbon matrix to the ohmic resistance was small and mainly affected the electrode response. The cathode constituents at the discharged state were examined with SEM-BSED to investigate the correspondence between the charge transfer resistance and the microstructure of the cathode; since both cells, C220 and F220, were disassembled at a SoC of 20%, excess Zn, NaCl, NaAlCl4, and Na2ZnCl4 could exist. Sodium appears as a green area, while a grey area represents zinc. Fig. 8 and Fig. S5 are the images of the cathode without and with a carbon matrix, respectively. The main difference at a glance is in the size of particles. For the conventional cathode, the Na2ZnCl4 particles, the aggregated ones, grew to a size of about 100 μm or more and were not uniformly distributed. On the other hand, the new cathode architecture provided the well-distributed fine microstructure in Fig. 8b. During the charging and discharging, the cathode material passes through the region where only the liquid salt exists on the phase diagram of NaCl–ZnCl2 [22,26,28] so that the par ticles are uniformly distributed. However, if the electronic pathway is insufficient and the liquid salt-only zone (66–70 mol% ZnCl2) is rela tively narrow at 260 � C, the formation of the NaCl–ZnCl2 molten salt and the precipitation of the metallic Zn may occur locally so the sufficient dispersion may be hardly achieved kinetically as the charge and discharge occur more rapidly. The direction of the cathode compartment (upward for the conven tional cells and downward for the newly designed cells, respectively) may have influenced the performance difference. When the cathode compartment is located at the beneath of the solid electrolyte, the liq uefied salts, including ZnCl2–NaCl and NaAlCl4, can be withdrawn from the interface. However, since the cells with a carbon matrix shows lower cell degradation, the effect seems to be insignificant.
Fig. 5. The voltage profiles of the Na/ZnCl2 batteries with (red lines) or without (black lines) a carbon matrix under a discharging current of 80 mA at 260 � C. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
discharging current of 80 mA was exceptionally remarkable compared to the C220 cell (Fig. 5). Fig. 6 shows the EoC and EoD voltages of 440 mAh-cells according to the charging and discharging current and cycle number. The C440 cell exhibited very unstable EoC voltages under 30 mA of charge current compared to the half-capacity cell. The charge current decreased to 20 mA from the 20th cycle to achieve the stability of EoC voltages, but a voltage fluctuation occurred when the discharge current was more than 100 mA. On the other hand, the F440 cell exhibited a stable EoC of about 2.3 V even at a charging current of 30 mA and a discharging current of 30–160 mA. Under a discharge current of 100 mA for the C440 and F440 cells, the EoD voltage (1.606 V) of the F440 cell was enhanced by about 0.508 V compared to that of the C440 cell under the equivalent current due to the use of the carbon matrix in the cathode. These results are even more apparent and fit well with the discussion in Figs. 3c and 4. The sufficient electronic path in the cathode might alleviate the insuf ficient distribution of cathodic salt and the weak connection of metallic zinc. Without carbon felt, zinc ions in the cathodic salts would be rapidly reduced only near the zinc particles during high-rate discharge; as the
4. Conclusions In this research, we demonstrate electrochemically activated Na/
Fig. 6. The end-of-charge (black squares) and end-of-discharge (red circles) voltages as a function of current for the 440 mA-cells with (closed symbols) or without (open symbols) a carbon matrix. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 7. The electrochemical impedance spectra of the Na/ZnCl2 cells, C440 and F440, with or without a carbon matrix at 260 � C as functions of SoC status and cycle number. 6
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Fig. 8. Backscattered electron detector images with elemental mapping (Zn and Na) for the 220mAh-cathode (a) without a carbon matrix and (b) with a car bon matrix.
ZnCl2 cells using a carbon matrix in the cathode compartment. The newly designed batteries provided enhanced rate performance and cycling stability at 260 � C compared to those of the conventional cell. The ZnCl2–NaCl chemistry allowed the use of a carbon matrix as a wellstructured electronically conductive network due to its unique phase transition into a penetrative eutectic liquid salt above 245 � C. With carbon felt, the voltages at the end-of-discharge of the cell was enhanced 15 and 50% compared to those of the conventional cells for the 220 mAh (1.37 V)- and 400mAh (1.61 V)- cell, respectively. The total charge transfer resistance of the 220 mAh-cell was also significantly decreased as 62% at the 51st cycle under 160 mA of discharging current compared to the conventional one. This tendency was even more apparent in high capacity (440 mAh) cells. This prominent performance was likely due to the effective electron transfer through the carbon felt, which provided sufficient distribution of Zn precipitates and sodium-containing salts kinetically. By redesigning the cathode with inexpensive materials and an efficiently conductive carbon matrix, the advanced Na/ZnCl2 battery
offers a beneficial approach to developing a cost-compatible massive energy storage system. Acknowledgment This research was supported by the Energy Efficiency & Resources Core Technology R & D Program (No. 20142010102460) funded by the Ministry of Trade, Industry & Energy (MOTIE) and the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (No. NRF2017R1A4A1015711). This work was also supported by the fund of research promotion program (2016-04-005), Gyeongsang National University, 2016. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. 7
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org/10.1016/j.jpowsour.2019.227110.
[17] H.J. Chang, X. Lu, J.F. Bonnett, N.L. Canfield, S. Son, Y.C. Park, K. Jung, V. L. Sprenkle, G. Li, “Ni-Less” cathodes for high energy density, intermediate temperature Na–NiCl2Batteries, Adv. Mater. Interfaces. 5 (2018) 1–8, https://doi. org/10.1002/admi.201701592. [18] G. Li, X. Lu, J.Y. Kim, M.H. Engelhard, J.P. Lemmon, V.L. Sprenkle, The role of FeS in initial activation and performance degradation of Na-NiCl2batteries, J. Power Sources 272 (2014) 398–403, https://doi.org/10.1016/j.jpowsour.2014.08.106. [19] X. Lu, H.J. Chang, J.F. Bonnett, N.L. Canfield, K. Jung, V.L. Sprenkle, G. Li, Effect of cathode thickness on the performance of planar Na-NiCl2battery, J. Power Sources 365 (2017) 456–462, https://doi.org/10.1016/j.jpowsour.2017.07.029. [20] G. Tao, N. Weber, A High Temperature (400 to 650oC) Secondary Storage Battery Based on Liquid Sodium and Potassium Anodes, 2007, pp. 235–238, https://doi. org/10.2172/908547. [21] H.J. Chang, N.L. Canfield, K. Jung, V.L. Sprenkle, G. Li, Advanced Na-NiCl2 battery using nickel-coated graphite with core-shell microarchitecture, ACS Appl. Mater. Interfaces 9 (2017) 11609–11614, https://doi.org/10.1021/acsami.7b00271. [22] X. Lu, G. Li, J.Y. Kim, J.P. Lemmon, V.L. Sprenkle, Z. Yang, A novel low-cost sodium-zinc chloride battery, Energy Environ. Sci. 6 (2013) 1837–1843, https:// doi.org/10.1039/c3ee24244g. [23] C.W. Ahn, M. Kim, B.D. Hahn, I. Hong, W. Kim, G. Moon, H. Lee, K. Jung, Y. C. Park, J.H. Choi, Microstructure design of metal composite for active material in sodium nickel-iron chloride battery, J. Power Sources 329 (2016) 50–56, https:// doi.org/10.1016/j.jpowsour.2016.08.068. [24] P. Parthasarathy, N. Weber, A.V. Virkar, High temperature sodium - zinc chloride batteries with sodium beta" - alumina solid electrolyte (BASE), ECS Trans 6 (2007) 67–76, https://doi.org/10.1149/1.2811944. [25] H.-T. Kim, S.-I. Kim, H.-L. Choi, W.-I. Park, C.-S. Kim, Effect of Zn/NaCl ratios on the charge/discharge performance in Na-ZnCl2 battery, J. Korean Cryst. Growth Cryst. Technol. 25 (2015) 74–79, https://doi.org/10.6111/ JKCGCT.2015.25.2.074. [26] X. Lu, H.J. Chang, J.F. Bonnett, N.L. Canfield, K. Jung, V.L. Sprenkle, G. Li, An intermediate-temperature high-performance Na–ZnCl 2 battery, ACS Omega 3 (2018) 15702–15708, https://doi.org/10.1021/acsomega.8b02112. [27] G. Tao, N. Weber, A high temperature (400 to 650 � c) secondary storage battery based on liquid sodium and potassium anodes. http://www.osti.gov/servlets/pur l/908547/, 2007. [28] NaCl-ZnCl2 Phase diagram (n.d.), http://www.crct.polymtl.ca/fact/phase_diagram .php?file¼NaCl-ZnCl2.jpg&dir¼FTsalt. [29] J.P. Lemmon, Planar ZEBRA battery for renewable integration and grid applications, in: IBA Meet. Cape T. South Africa, 2011. https://energyenvironment. pnnl.gov/presentations/pdf/Planar.Zebra.pdf. [30] G. Li, X. Lu, J.Y. Kim, K.D. Meinhardt, H.J. Chang, N.L. Canfield, V.L. Sprenkle, Advanced intermediate temperature sodium-nickel chloride batteries with ultrahigh energy density, Nat. Commun. 7 (2016) 1–6, https://doi.org/10.1038/ ncomms10683. [31] R.A. Huggins, Advanced Batteries : Materials Science Aspects, Springer, 2009. [32] J.-K. Park, Principles and Applications of Lithium Secondary Batteries, Wiley-VCH, 2012. https://www.wiley.com/en-us/PrinciplesþandþApplicationsþofþLithiumþ SecondaryþBatteries-p-9783527331512. (Accessed 14 February 2019). [33] K. Jung, H.J. Chang, J.F. Bonnett, N.L. Canfield, V.L. Sprenkle, G. Li, An advanced Na-NiCl2battery using bi-layer (dense/micro-porous) β00 -alumina solid-state electrolytes, J. Power Sources 396 (2018) 297–303, https://doi.org/10.1016/j. jpowsour.2018.06.039.
References [1] J.L. Sudworth, The sodium/sulphur battery, J. Power Sources 11 (1984) 143–154. [2] Z. Yang, J. Zhang, M.C.W. Kintner-meyer, X. Lu, D. Choi, J.P. Lemmon, Electrochemical energy storage for green grid.pdf, Chem. Rev. (2011) 3577–3613, https://doi.org/10.1021/cr100290v. [3] B. Dunn, H. Kamath, J. Tarascon, Electrical energy storage for the Grid : a battery of choices, Science (80-.) 334 (2011) 928–936. [4] J.L. Sudworth, The sodium/nickel chloride (ZEBRA) battery, J. Power Sources 100 (2001) 149–163. [5] C.H. Dustmann, Advances in ZEBRA batteries, J. Power Sources 127 (2004) 85–92, https://doi.org/10.1016/j.jpowsour.2003.09.039. [6] G. Li, X. Lu, J.Y. Kim, K.D. Meinhardt, H.J. Chang, N.L. Canfield, V.L. Sprenkle, Advanced intermediate temperature sodium-nickel chloride batteries with ultrahigh energy density, Nat. Commun. 7 (2016) 1–6, https://doi.org/10.1038/ ncomms10683. [7] T. Oshima, M. Kajita, A. Okuno, Development of sodium-sulfur batteries, Int. J. Appl. Ceram. Technol. 1 (2004) 269–276, https://doi.org/10.1111/j.17447402.2004.tb00179.x. [8] Z. Wen, Y. Hu, X. Wu, J. Han, Z. Gu, Main challenges for high performance NAS battery: materials and interfaces, Adv. Funct. Mater. 23 (2013) 1005–1018, https://doi.org/10.1002/adfm.201200473. [9] X. Lu, J.P. Lemmon, J.Y. Kim, V.L. Sprenkle, Z. Yang, High energy density Na e S/ NiCl 2 hybrid battery, J. Power Sources 224 (2013) 312–316, https://doi.org/ 10.1016/j.jpowsour.2012.09.108. [10] P.A. Nelson, Advanced high-temperature batteries, J. Power Sources 29 (1990) 565–577, https://doi.org/10.1016/0378-7753(90)85026-9. [11] M. Hosseinifar, A. Petric, High temperature versus low temperature Zebra (Na/ NiCl 2) cell performance, J. Power Sources 206 (2012) 402–408, https://doi.org/ 10.1016/j.jpowsour.2012.01.125. [12] X. Ao, Z. Wen, Y. Hu, T. Wu, X. Wu, Q. He, Enhanced cycle performance of a Na/ NiCl2 battery based on Ni particles encapsulated with Ni3S2 layer, J. Power Sources 340 (2017) 411–418, https://doi.org/10.1016/j.jpowsour.2016.11.091. [13] H.J. Chang, X. Lu, J.F. Bonnett, N.L. Canfield, S. Son, Y.C. Park, K. Jung, V. L. Sprenkle, G. Li, Development of intermediate temperature sodium nickel chloride rechargeable batteries using conventional polymer sealing technologies, J. Power Sources 348 (2017) 150–157, https://doi.org/10.1016/j. jpowsour.2017.02.059. [14] J.S. Weaving, S. Walter Orchard, Experimental studies of transition metal chloride electrodes in undivided cells using molten NaAlCl4 electrolyte, J. Power Sources 36 (1991) 537–546, https://doi.org/10.1016/0378-7753(91)80079-D. [15] G. Li, X. Lu, C.A. Coyle, J.Y. Kim, J.P. Lemmon, V.L. Sprenkle, Z. Yang, Novel ternary molten salt electrolytes for intermediate-temperature sodium/nickel chloride batteries, J. Power Sources 220 (2012) 193–198, https://doi.org/ 10.1016/j.jpowsour.2012.07.089. [16] G. Li, X. Lu, J.Y. Kim, V.V. Viswanathan, K.D. Meinhardt, M.H. Engelhard, V. L. Sprenkle, An advanced Na–FeCl2 ZEBRA battery for stationary energy storage application, Adv. Energy Mater. 5 (2015) 1500357, https://doi.org/10.1002/ aenm.201500357.
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