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Advanced Na-NiCl2 Battery using Nickel-Coated Graphite with Core–Shell Microarchitecture Hee Jung Chang, Nathan L. Canfield, Keeyoung Jung, Vincent L. Sprenkle, and Guosheng Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b00271 • Publication Date (Web): 16 Mar 2017 Downloaded from http://pubs.acs.org on March 16, 2017

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ACS Applied Materials & Interfaces

Advanced Na-NiCl2 Battery using Nickel-Coated Graphite with Core–Shell Microarchitecture Hee-Jung Chang,† Nathan L. Canfield,† Keeyoung Jung,‡ Vincent L. Sprenkle† and Guosheng Li†,* †

Stationary Energy Storage Group, Energy and Environmental Directorate, Pacific Northwest National Laboratory, Richland, WA 99352, USA ‡

Materials Research Division, Research Institute of Industrial Science & Technology (RIST), Pohang, Republic of Korea Corresponding: [email protected]

Keywords: Stationary energy storage, sodium metal halide battery, ZEBRA battery, core-shell, Ni

Abstract: Stationary electric energy storage devices (rechargeable batteries) have gained increasing prominence due to great market needs, such as smoothing the fluctuation of renewable energy resources and supporting the reliability of the electric grid. With regarding to raw materials availability, sodium based batteries are better positioned than lithium batteries due to the abundant resource of sodium in the earth’s crust. However, the sodium-nickel chloride (NaNiCl2) battery, one of the most attractive stationary battery technologies, is hindered from further market penetration by its high material cost (Ni cost) and fast material degradation at its high operating temperature. Here, we demonstrate the design of a core–shell microarchitecture − nickel-coated graphite − with a graphite core to maintain electrochemically active surface area and structural integrity of the electron percolation pathway while using 40% less Ni than conventional Na-NiCl2 batteries. An initial energy 1 ACS Paragon Plus Environment


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density of 133 Wh/kg (at ~C/4) and energy efficiency of 94% are achieved at an intermediate temperature of 190°C.

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1. Introduction Demand for stationary electric energy storage systems has increased because they are crucial to stimulate the successful integration of renewable energy resources and increase reliability of the electric power grid.1-3 The vast majority of battery technologies have been studied and investigated from technical and cost perspectives to meet this new opportunity, which could revolutionize electricity generation and distribution.2-3 Recently, molten-sodium (Na) beta-alumina batteries (NBBs), such as sodium-sulfur (Na-S) and sodium-metal halide (Na-MH) batteries, have drawn interest for use as stationary energy storage devices due to their long cycle life.4-8 Despite the common features shared by Na-S and Na-MH batteries − molten sodium anode and β″alumina solid-state electrolyte (BASE) − Na-MH batteries have several advantages over Na-S batteries, including lower operating temperature, higher output voltage, superior battery safety, and ease of assembly in the discharged state without preloading metallic sodium in the anode.7, 9-10 Sodium-nickel chloride (Na-NiCl2) is the most widely investigated redox couple among various Na-MH battery redox chemistries reported in the literature.11-15 The overall redox reaction of a NaNiCl2 battery during charge and discharge processes is as follows:9 (Discharged state) 2NaCl + Ni ↔ 2Na + NiCl2 (Charged state),

E0 = 2.58 V at 300°C (1)

Despite the relatively simple conversion-type redox reaction shown above, many challenges still remain for developing a practical Na-NiCl2 battery to achieve further market penetration. Over the years, extensive effort has been devoted for developing low operating temperature Na-NiCl2 batteries to retard material degradation in the cathode.16-19 However, reducing the Ni content in the cathode, which accounts for more than 60% of total materials cost in Na-NiCl2 batteries, has not been entirely successful. It is well known that the Ni content in conventional Na-NiCl2 batteries is much higher than the stoichiometric amount from reaction (1). This is because excess Ni content is necessary to provide sufficient surface area for electrochemical reactions, maintain the electron conduction pathway, and suppress 3 ACS Paragon Plus Environment


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particle growth in the current conventional Na-NiCl2 batteries.15, 20-21 During the charging process, layers of NiCl2 form on the surfaces of bare Ni particles in the cathode (Figure 1a). The majority of the Ni particles also play an important role as electron percolating pathways, which are critical to collect and pass electrons in the cathodes (Figure 1a). For this reason, the Ni/NaCl ratio in conventional Na-NiCl2 batteries is about 1.8, which is more than three times larger than the stoichiometric ratio (0.5). In light of this, simply reducing the Ni content in Na-NiCl2 battery cathodes may not be an option, since it is reported that battery performance was substantially affected in Na-NiCl2 batteries operated with lower Ni/NaCl ratios at 280°C.20 Overall, reducing Ni content in Na-NiCl2 batteries has remained as a challenge. Here, for the first time, we demonstrate a core–shell microarchitecture Ni-coated graphite (NCG) for stable cycling in Na-NiCl2 batteries. The unique feature of core-shell microarchitecture NCG is using electrochemically stable and electrically conducting graphite to replace the excess Ni, thus preserving the structural integrity of the electron percolation pathway with minimum Ni content (Figure 1b). Na-NCG cells are tested at two different temperatures at 280oC and 190oC, respectively, to see how the operating temperature changes battery performances. 280oC is a typical operating temperature for tubular type Na-NiCl2 battery, and 190oC could offer cost effective solutions for the cell manufacture and maintenance. Recently, we have also reported that low temperature (190oC) operation can greatly improve battery long term stability. Finally, we reports that Na-NCG batteries using the core–shell microarchitecture NCG can achieve an initial energy density of 133 Wh/kg and an excellent energy efficiency of 94% with a stable battery performance at 190oC. 2. Experimental Section Material preparation: NCG and Ni (Novamet and Sulzer Metco), NaCl (Alfa Aesar, 99.99%), and cathode additives (Sigma Aldrich, FeS, aluminum powder, NaI, and NaF) were used as received. Two different types of NCGs were studied in this work. NCG-20 and NCG-100 4 ACS Paragon Plus Environment

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represent NCG powders with average particle sizes of 20 µm and 100 µm, respectively. Detailed compositions for different cathode formulas (NCG and Ni cathodes) tested in this work are summarized in Table S1. For example, sample #3 consisted of NCG-20, NaCl, and additives. The weight ratio between NCG-20 and NaCl was 0.83:1. Since the Ni content of NCG-20 is about 60 wt% (40 wt% graphite), the mole ratio of Ni/NaCl is about 0.5, which is same as the stoichiometric ratio of reaction (1). Sample #5 was made of pure Ni powders, and Ni cells were tested for comparing the battery performance with that of NCG cells. Each ingredient of cathode materials was weighed in a nitrogen-purged glove box and put into a polyethylene bottle with zirconia milling media (dia. 2 mm). Hermetically sealed PE bottles were brought out from the glove box and were rolled on a low-energy roller for 8 h. Then, the well mixed cathode powders were brought into the glove box for battery assembly. The secondary electrolyte (NaAlCl4) used in this work was custom synthesized according to the method that was also described in our previous report.20 Cell assembly: The button cell consisted of two battery cases (stainless steel) for cathode and anode sides, a molybdenum cathode-current collector, a stainless steel anode-current collector, an α-alumina fixture, and a composite yttria-stabilized zirconia (YSZ)/BASE (3 cm2 active area) disk. The BASE disks were fabricated by the vapor phase conversion process and had a thickness around 500 µm.20, 22 The detailed ionic conductivities of BASE have been reported elsewhere.19 Then, BASE disks were glass-sealed to an α-alumina fixture. The anode side of the BASE disk was treated with lead acetate solution and subjected to a thermal treatment in an inert environment to enhance Na wettability. The fixture was then moved into a glove box for cell assembly. 1 g of cathode powder was added to the cathode side of the fixture and 0.7 g of melt was vacuum infiltrated into the powders at an elevated temperature of 200°C. A small amount of metallic sodium ( 95%) shown in Figure 3a (black) has two likely 7 ACS Paragon Plus Environment


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causes: (1) depletion of active material (NaCl), and (2) passivation of the active surface areas of NCG-20 from formation of NiCl2 layers, or a combination of both. Either cause will increase the cell overall polarization at the EOC, and the charge voltage will rise quickly to the cutoff voltage (2.8 V). During the discharge process, the plateau appears at 2.57 V (may vary slightly), which is a good indication of the reduction reaction of NiCl2 to Ni. As shown in Figure 3a (black), the cell voltage drops quickly at the end of discharge (EOD), where the SOC is below 10%. The sharp voltage drop at EOD can be attributed to the depletion of NiCl2 (active material) at the EOD. Clearly, the cell with the cathode having NCG-20/NaCl = 1.8 can be charged to full capacity (100% SOC), and shows excellent voltage plateaus for both charge and discharge processes. In contrast, the cells having lower NCG-20/NaCl ratios show quite different results. As shown in Figure 3a (red), the cell with NCG-20/NaCl = 1.25 shows a limited charge capacity (up to 70% SOC). Compared to the cell with NCG-20/NaCl = 1.8, the voltage rise of the charge process begins at a much lower SOC, which indicates a greater polarization in the cells with NCG-20/NaCl = 1.25. Further decreasing the amount of NCG-20 in the cathode, such as to NCG-20/NaCl = 0.83 (blue in Figure 3a), the battery can barely be charged to 10% SOC and has much higher cell polarization. Apparently, the inferior battery performance of the cells with NCG-20/NaCl = 1.25 and 0.83 in the cathode can be attributed to the low NCG content. This is because the lower NCG content in the cathode is unable to provide sufficient surface area and electron pathways for batteries to operate with a normal performance. As shown in Figure 3b, a cell with NCG-100 cathode (NCG-100/NaCl = 1.8) was tested in a conditioning cycle. Surprisingly, the NCG-100 cell can only utilize 20% of its theoretical capacity despite having a Ni content similar to that of the NCG-20 cell (NCG20/NaCl = 1.8). Considering that all cell conditions are the same for both cells except the particle sizes of NCG powders used in the cathode, we believe that large particle size (~100 µm) of NCG-100 is responsible for the limited battery capacity shown in Figure 3b. More specifically, the lower surface area of NCG-100, which is due to the larger particle size, 8 ACS Paragon Plus Environment

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causes the limited cell capacity and the rapid increase in cell polarization (thicker NiCl2 passivation layers on NCG-100 particles) during the charge process. Hence, more detailed Na-NCG battery tests were conducted on the cells having NCG-20 cathodes (NCG-20/NaCl = 1.8). To evaluate the cycling performance of NCG-20 cells with NCG-20/NaCl = 1.8 in cathodes, we cycled the cells in a fixed capacity window of 60% SOC (90 mAh) between 20% SOC (30 mAh) and 80% (120 mAh), after the conditioning cycles were done. (The results of conditioning cycles for NCG-20 cells can be found in Figure 3a and Figure S1 for 190°C and 280°C, respectively). Figure 4 shows detailed results of the fixed capacity window tests of NCG-20 cells at 190°C and 280°C. The capacity plot indicates that no decay was observed up to the 150th cycle for the cells tested at 190°C (Figure 4a, black). However, identical NCG-20 cells tested at 280°C (Figure 4a, red) show a drastic decay within 30 cycles. To further understand the effect of temperature on cell performance, voltage profiles vs. SOC of cells tested at 190°C and 280°C are shown in Figure 4b and 4c, respectively. Consistent with the trend of the capacity plots, voltage profiles of NCG-20 cells tested at 190°C show stable charge and discharge plateaus. In contrast, the voltage profiles of NCG-20 cells tested at 280°C show a drastic increase in cell polarization, as shown in Figure 4c. The cell cycling window is also shifted to a more discharged state (low SOC) at 280°C due to the limited charging capability over cycles. The voltages at EOC and EOD, which are typically used for presenting the degradation rate of Na-NiCl2 batteries, are shown in Figure 4d. Similarly, the EOC and EOD voltages at 190°C show a stable trend over 150 cycles, but the EOC and EOD voltages at 280°C quickly degrade and reach the cutoff voltage within less than 30 cycles. 3.3 SEM measurements of NCG cathodes NCG-20 cathodes retrieved from the cells cycled at 190°C and 280°C in fixedcapacity tests (Figure 4) were examined using SEM/energy dispersive spectroscopy (EDS). 9 ACS Paragon Plus Environment


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SEM images obtained in backscatter mode are shown in Figure 5a, 5d, and 5g for untested cells and cells tested 190°C (30 cycles) and 280°C (30 cycles), respectively. White spots in Figure 5d indicate Ni particles (white arrows) and dark areas indicate graphite particles (pink ovals). The corresponding images of carbon mapping (C-mapping) shown in Figure 5b (uncycled), 5e (190°C), and 5h (280°C), and Ni-mapping shown in Figure 5c (uncycled), 5f (190°C), and 5i (280°C) further confirm the particle shape. Compared with uncycled NCG-20 cathodes, the cells tested at 190°C show a partial delamination of Ni layers from graphite particles. In contrast, the SEM images for the cathodes retrieved from the cells tested at 280°C revealed a complete delamination of Ni layers from graphite particles. Indeed, large Ni particles (~10 µm in Figure S2) were observed in NCG-20 cathodes retrieved from the cells tested at 280°C. SEM image and its corresponding carbon/Ni mappings for the cells (150 cycles at 190oC) are shown in Figure S3, respectively. Although it is difficult to give a quantitative comparison, Ni-mapping (Figure S3, 150 cycles) shows a slightly more Ni delamination compared to the cells at 30 cycles (Figure 5f). This observation is in a good agreement with voltage profiles shown in Figure 4d, where EOC and EOD profiles show a slight degradation (ascending of EOC voltage and descending of EOD voltage) over 150 cycles. 4. Discussion Energy efficiency plots in Figure 6a show excellent performance. The initial discharge energy density of a Na-NCG battery was more than 133 Wh/kg, which was calculated based on the total weight of cathode materials including the mass of graphite and melts. As a result of replacing costly Ni powders with NCG, which has 40% lower Ni content, the Na-NCG batteries demonstrated in this work can significantly reduce the materials cost for the current Na-NiCl2 battery technology. Na-NCG batteries operated at 190°C also have an excellent energy efficiency of about 94% (Figure 6a), which is another advantage for Na-NCG batteries. However, it should be noted that the energy efficiency could be greatly affected by battery 10 ACS Paragon Plus Environment

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operating conditions. The high energy efficiency shown in here was obtained in relatively mild operating conditions, using a charge rate of C/4.5 and a discharge rate of C/3. The battery performances for the cells consisting of NCG-20 (black) and Ni (red) cathodes are shown in Figure 6b. By monitoring the trends of EOC and EOD, NCG-20 cathode obviously shows a little faster rate (faster degradation) for EOC ascending and EOD descending than that of Ni cathode. However, it should be noted that NCG-20 cells have 40% less Ni content compared with Ni cells. It is quite clear that the morphologies of NCG-20 cathodes show a strong dependence on the cell operating temperature. For instance, NCG-20 cathodes tested at 280°C showed a complete delamination of Ni layers from the graphite surface (Figure 5g). Furthermore, large pure Ni particles, which were originated from delaminated Ni layers, were observed in the cathodes (Figure S2). The morphology evolution of cathode materials is the main cause of the fast degradation of the Na-NCG battery operated at 280°C, since the delamination of Ni layers will substantially reduce the surface area of Ni and disrupt the electron percolation path. In contrast, only a partial delamination of Ni layers was observed in the Na-NCG cells tested at 190°C. This partial delamination may also lead to a decrease in the surface area of Ni, which will increase the over potential for charge and discharge processes. The overpotential of voltage profiles will be more pronounced for the cells at high SOC, where more NiCl2 are formed. Evidently, an increase of overpotential at EOC and beginning of discharge (BOD) are shown in Figure 4b. The voltage drop observed at EOD could be attributed to disruption of the electron pathway due to the high population of NaCl particles in the cathode and partial delamination of Ni from graphite particles. Impedance measurements (Figure S4) show an increase in ohmic resistance (the high frequency interception) for the cells tested at 190oC and 280oC, which is a good indication for the disruption of electron pathway over the battery cycles.



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It is worth noting that delamination of Ni layers from graphite particle is due to easier electrodeposition process (discharge) of Ni on the Ni surface compared to the surface of graphite particles. This is most likely due to existence of the overpotential, which is necessary to overcome the heterogeneous electroplating barrier between Ni and graphite. The heterogeneous electroplating barrier relates to the interfacial energy, and increases with crystal structure mismatch between two phases (Ni and graphite in here).23 For instance, crystal structure of Ni is a face-centered cube with a lattice parameter of 0.352 nm, which is much larger than the lattice parameter (0.142 nm) of graphite arranged in a honeycomb. In this regard, the delamination of Ni from core graphite particles could be mitigated by reducing Ni plating overpotential through replacing graphite with other materials, which has stable electrochemical properties and a lower interfacial energy. Similarly, adding a secondary coating layer between Ni and graphite to reduce Ni plating overpotential could be another way to further improve battery performance of Na-NCG batteries. Active research for designing and modifying core-shell microarchitecture cathodes is underway.

5. Conclusion To summarize, we have developed a novel Na-NCG battery that can be operated at 190°C with a high energy density (133 Wh/kg) with an excellent capacity retention. Use of NCG reduced the Ni loading in the Na-NCG battery cathode by 40% compared to conventional Na-NiCl2 batteries. By demonstrating lower materials cost and stable performance for Na-NiCl2 battery, this work provides a new solution toward making NBB battery technology a more suitable and sustainable solution for stationary energy storage applications.

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] 12 ACS Paragon Plus Environment

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ORCID Guosheng Li: 0000-0001-5193-5488 Notes The authors declare no competing financial interest.

Acknowledgements This work was supported by the U.S. Department of Energy (DOE) Office of Electricity Delivery and Energy Reliability under Contract No. 57558 and the International Collaborative Energy Technology R&D Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted financial resources from POSCO and the Ministry of Trade, Industry & Energy of the Republic of Korea. (No. 20158510050010). PNNL is a multiprogram laboratory operated by Battelle Memorial Institute for the DOE under Contract DE-AC05-76RL01830.

Supporting information The supporting information is available free of charge on the ACS Publication website at DOI: Na-NCG cells tested 280oC and its SEM images, SEM images for Na-NCG cells tested up to 150 cycle at 190oC, Impedance spectra, and Table for cathode compositions.

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Figure 1. Schematic view of the charge process for nickel (Ni) particles and NCG particles. (a) Bare Ni particles undergo NiCl2 passivation on the particle surface upon charge. Electrons are conducted along the Ni chains, which consist of physically integrated Ni particles. (b) The core–shell morphology of NCG can provide sufficient surface area for NiCl2 formation during charging and can maintain the electron pathway without using excess Ni.


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Figure 2. Morphology of NCG. (a) and (c) backscatter and cross-section SEM images of NCG-20 with scale bar 10 µm. (b) and (d) backscatter and cross-section SEM images of NCG-100 with scale bar 100 µm. The average particle sizes (D50) are 20 µm and 100 µm for NCG-20 and NCG-100, respectively. (e) High magnification cross-section SEM image of NCG-20 with scale bar 1 µm. (f) High magnification cross-section SEM image of NCG-100 with scale bar 5 µm. The average thicknesses of Ni layers are about 0.3 µm and 2 µm for NCG-20 and NCG-100, respectively.



3.0

(a)

E (V)

Charge, 2.64 V

2.5

2.6 V (OCV)

Discharge, 2.57V

NCG-20/NaCl = 1.8 NCG-20/NaCl = 1.25 NCG-20/NaCl = 0.83

2.0

1.5 0

50

SOC (%)

100

3.0

(b)

E (V)

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2.5

NCG-100/NaCl=1.8

2.0

1.5 0

50

100

SOC (%) Figure 3. Typical voltage profiles vs. SOC for NCG cathodes with core–shell structure at 190°C (conditioning cycles, ~C/15). (a) Charge and discharge voltage profiles of NCG-20 cathodes with NCG-20/NaCl =1.8 (black), 1.25 (red), and 0.83 (blue). The open circuit voltage (OCV) is about 2.6 V (black dashed line). Charge and discharge plateaus were observed at 2.64 and 2.57 V, respectively. (b) Charge and discharge voltage profiles of NCG100 cathodes (NCG-100/NaCl = 1.8).


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3.0

150

(b)

(a)

190oC

o

190 C

100

E (V)

Capacity (mAh/g)

EOC BOD

2.5

1st 30th 150th

EOD

2.0

50

90 mAh/g o

280 C

1.5

0 0

50

100

0

150

Cycle number

50

100

SOC (%) 3.0

3.0

(d)

280oC

(c)

EOC (280oC)

EOC (190oC)

2.5

E (V)

2.5

E (V)

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


st

1 th 20 30th

2.0

EOD (190oC) 2.0

90 mAh/g

EOD (280oC)

1.5

1.5

0

50

100

0

SOC (%)

50

100

150

Cycle number

Figure 4. Cell tests in a fixed capacity window from 19% SOC (30 mAh) to 76% SOC (120 mAh) for NCG-20 cells (NCG-20/NaCl = 1.8) at 190°C and 280°C. (a) Cell capacity plots at 190°C (black) and 280°C (red), (b) Voltage profiles (1st, 30th, and 150th cycles) for cells tested at 190°C, (c) Voltage profiles (1st, 20th, and 30th cycles) for cells tested at 280°C, (d) Voltage plots of EOC and EOD for 190°C (black) and 280°C (red).



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

SEM

Page 20 of 22

C-mapping

Ni-mapping

(a)

(b)

(c)

(d)

(e)

(f)

(h)

(i)

Uncycled

Nickel

190oC

Graphite

(g) 280oC

Graphite

Graphite

Figure 5. SEM images and element maps for cathode materials (NCG-20) retrieved from an uncycled cell and cells operated with a fixed capacity (90 mAh). (a), (b), and (c) are for an uncycled cathode (scale bar 10 µm). (d), (e), and (f) are for cells tested at 190°C for 30 cycles (scale bar 10 µm). (g), (h) and (i) are for cells tested at 280°C for 30 cycles (scale bar 10 µm). (a), (d), and (g) are SEM (backscatter mode) images; (b), (e) and (h) are C (carbon) maps; (c), (f), and (i) are Ni maps.


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100

200

3.0 EOC, NCG-20

Energy Efficiency Charge Energy Discharge Energy

EOC, Ni

2.5

EOD, Ni

E (V)

50

100

Energy Efficiency (%)

Energy Density (Wh/kg)

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


EOD, NCG-20

2.0

(a)

(b)

0

0 0

50

100

1.5

150

0

Cycle Number

50

100

150

Cycle Number

Figure 6. (a) Energy efficiency and energy densities of NCG-20 cells. (b) Voltage plots of EOC and EOD for NCG-20 (black) and Ni cathode (red) at 190oC, respectively.



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

Abstract Graphic:


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Advanced Na-NiCl2 Battery Using Nickel-Coated ... - ACS Publications

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