Development of Na-beta alumina batteries at Pacific Northwest National Laboratory：From tubular to planar

LU Xiaochuan, LI Guosheng, MEINHARDT Kerry D, SPRENKLEVincent L



Development of Na-beta alumina batteries at Pacific Northwest National Laboratory：From tubular to planar

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(Pacific Northwest National Laboratory, Richland, WA 99352, USA)

Na-beta alumina batteries are one of the most promising technologies for renewable energy storage and grid applications. Na-beta alumina batteries can be constructed in either tubular or planar designs, depending on the shape of the beta-alumina solid electrolyte. The tubular designs have been widely studied and developed since the 1960s primarily because of their ease of sealing. However, planar designs are considered superior to tubular designs in terms of power output, cell packing, ease of assembly, thermal management, and other characteristics. Recently, Pacific Northwest National Laboratory has begun to develop high-performance planar Na-beta alumina batteries. In this paper, we provide an overview on the basic battery electrochemistry, solid electrolyte synthesis and fabrication, and our recent progress in developing planar batteries. Future trends for further technology improvement will also be presented.

Na-beta alumina batteries; tubular or planar designs; beta-alumina solid electrolyte; battery electrochemistry

1 Introduction

Renewable energy generation systems and energy storage devices for transportation and stationary applications have received much attention in the past few decades due to environmental concerns about the use of fossil fuels [1-4]. Among various energy storage devices, Na-beta alumina batteries (NBBs), based on a molten Na anode and β"-Al2O3solid electrolyte, are one of the most promising technologies for renewable energy storage and grid applications [3, 5-7]. Due to the high energy density, high efficiency, and good calendar and cycle life, the NBB technologies have been widely investigated and significant progress has been achieved in the technologies. However, the batteries are still facing challenges in performance, safety, and cost for market penetration. This paper provides a brief overview on the basic electrochemistry and solid electrolyte, followed by recent progress at Pacific Northwest National Laboratory (PNNL) in battery design, performance, and scale-up.

2 NBB electrochemistry

Two major types of NBB electrochemistry have been widely studied, distinguished by their cathode materials. One is the sodium-sulfur (Na-S) battery, which uses molten sulfur as the cathode. The structure of a tubular Na-S cell is schematically shown in Fig.1. The half- and overall-cell reactions are as follows:

anode:

cathode:

overall-cell reaction:

The Na-S chemistry offers a high theoretical energy density (about 760 W·h·kg–1), high energy efficiency, excellent rate capability, and acceptable cycle life [6-7]. The materials of a Na-S cell primarily include alumina, sulfur, and sodium. All of these materials are relatively non-toxic, inexpensive, and readily available. This combination of features makes the Na-S chemistry extremely attractive compared to other technologies such as lithium-ion, Ni-metal hydride, or lead-acid batteries for grid-scale energy storage.

The drawbacks of the Na-S battery include: ①Intrinsic corrosive behavior of polysulfide melts, which limits material selections for both cathode current collector and battery casing; ②Safety issue and cell failure mode, which potentially results in a fire or even an explosion [6-7].

The other type is the so-called ZEBRA batteries, in which solid transition metal halides (e.g., NiCl2, FeCl2, or ZnCl2)along with a molten salt (i.e., NaAlCl4) are typically used as cathode materials[6,8-11]. The molten salt of NaAlCl4ensures facile sodium ion transport between the β"-Al2O3electrolyte and solid active materials in the cathode. The electrochemical reactions of a Na-NiCl2cell are as follows:

anode:

cathode:

overall-cell reaction:

The ZEBRA batteriesexhibit a number of advantages over the Na-S technology, including higher cell voltage, facile assembly in a discharged state, less corrosive cathode materials, lower operating temperature, and safer cell failure mode[6]. One notable disadvantage of the current ZEBRA technologies is lower energy density. The energy density of a Na-NiCl2battery is as low as ~260 W·h·kg–1when excess Ni current collector and molten NaAlCl4in the cathode are taken into account[12].

3 Beta-alumina solid electrolyte (BASE)

The beta-alumina group of oxides is characterized by structures of alternating closely packed slabs and loosely packed layers, as shown in Fig.2. The closely packed oxide slabs are layers of oxygen ions with aluminum ions sitting in both octahedral and tetrahedral interstices. The loosely packed layers contain mobile cations of Na+and are called conduction planes or slabs. In these planes or slabs, the Na+ions are free to move under an electric field. The closely packed slabs are referred to as a spinel block, which is bonded to two adjacent spinel blocksthe conduction planes or slabs. The Na+ions diffuse exclusively within the conduction layers perpendicular to theaxis (see Fig.2).

There are two distinct crystal structures in the beta-alumina group: β-Al2O3(hexagonal; P63/mmc;0=0.559 nm,0=2.261 nm) [13-14]and β"-Al2O3(rhombohedral; R3m;0=0.560 nm,0=3.395 nm)[15-16]. The two structures differ in chemical stoichiometry, stacking sequence of oxygen ions (refer to Fig.2), electrical properties, and mechanical performance. With its higher Na+concentration, β"-Al2O3exhibits a superior Na+conductivity over β-Al2O3and is the preferred phase for battery electrolyte applications.

β"-Al2O3powders can be synthesized by conventional solid-state reaction [17-20]. The solid-state reaction processes are typically carried out with the starting materials of α-Al2O3, Na2CO3or NaNO3, and a small amount of MgO or Mg(NO3)2and/or Li2CO3or LiNO3. The procedures involve multiple ball-milling and calcination steps, followed by final sintering treatment above 1600℃. The disadvantages of the process include sodium loss during the high-temperature sintering, formation of secondary phases such as NaAlO2, and low yield rate of β"-Al2O3[6]. In addition to the solid-state reaction route, solution-based methods such as the sol-gel process offer several advantages, such as producing powders with a higher degree of homogeneity and purity, and yielding powders with a higher surface area that facilitates the subsequent sintering [21-24]. Similar to the solid-state approach, however, the β-Al2O3phase may not be completely eliminated from the final productthe chemical methods.

Sintering of the β"-Al2O3green body to achieve a high density and adequate electrical/mechanical properties is also challenging [6]. The sintering temperature typically needs to be above 1600 ℃ to achieve a full density. However, a major issue during such high-temperature sintering is the formation of a duplex microstructure with large grains (50～500 μm) randomly distributed in a fine-grained matrix (grain size≤10 μm) [17, 25-26], which significantly reduces the mechanical strength of β"-Al2O3. Grain growth is extremely sensitive to the maximum temperature and the hold time [6-7]. There are several techniques can be employed to suppress the exaggerated grain growth during sintering, such as a short hold time at the high temperatures, a two-peak firing schedule, hot pressing at lower temperatures, zone sintering with a reduced time in the high-temperature zone, and lower temperature sintering with sintering aids [27].

4 Research at Pacific Northwest National Laboratory (PNNL) on planar NBBs

In both Na-S and ZEBRA batteries, the BASE could be either planar or tubular, depending on the cell designs. A cell with a tubular electrolyte was first reported by KUMMER and WEBER[28] and the design has been retained by most researchers today. Over the last seven years, PNNL has attempted to develop planar NBBs to replace the traditional tubular ones[29]. Fig.3 compares a planar button cell developed at PNNL with a tubular cell. In the planar design, a planar BASE disc is glass-sealed to an outer α-Al2O3ring with an activecell area of ～3 cm2. The cell assembly then is moved into an argon-filled glove box. After the active materials are loaded into the anode and cathode compartment chambers, the chambers are compression-sealed to two stainless-steel end plates using metal (e.g., aluminum, silver, or gold) O-rings. Compare to the tubular designs, the advantages of the planar design include adjustable power-to-energy ratio, simplified interconnects between single cells, improved cell packing efficiency, and better thermal management [29].

Fig. 4(a) shows details of a planar Na-NiCl2button cell (～3 cm2) assembly. After assembly, the cell was heated in air to 190 ℃ for testing. During testing, the cell was charged under a constant current of 20 mA to a cut-off voltage of 2.8 V. In the subsequent discharge, a constant power of 75 mW was employed, which is equivalent to an average current of 40 mA and a rate of～ C/3. The discharge was stopped at 2.0 V. Fig.4(b) shows the cycling performance of the cell. The cell capacity increased continuously during the initial 200 cycles, which could be related to the improved sodium wettability at the BASE/anode interface with cycling. This type of behavior is typically observed when the battery is operated at temperatures below 200℃. After the initialization, the cell performance stabilized with negligible degradation in cell capacity for almost 800 cycles, as shown in Fig.4(b). This excellent cycling performance clearly demonstrates durability of the cell materials and components, effectiveness of the metal seals, and feasibility of the cell design.

In addition to the 3 cm2button cell, larger cells with planar BASEs have also been fabricated and tested at PNNL. Fig. 5(a) shows a planar BASE with an active area of 64 cm2. The larger BASEs were fabricateda method similar to that for the small BASEs, i.e. a gas-phase conversion process[11-12,29-32]. In the first step, discs of α-Al2O3/yttria stabilized zirconia (YSZ) (volume ratio of 70 to 30) were fired at 1600 ℃ for 2 h in air to achieve full density (>99%). Thesintered α-Al2O3/YSZ discs then were placed in a loose β"-Al2O3powder and heat-treated at 1450 ℃ for 10 h in air to complete the conversion from α- to β"-Al2O3. The ionic conductivity of the larger BASEs with active areas of64 and 100 cm2was measured and the results are compared with that of the 3 cm2BASE in Fig.5(b). The conductivity of the 64 and 100 cm2BASEs is 0.059 and 0.077 S·cm–1at 300 ℃, respectively, which is significantly higher than that of the 3 cm2BASE. The higher conductivity could be related to more-homogeneous composition and higher sodium concentration in the larger BASEs achieved during the conversion process.

Fig.6 shows the mechanical strength of the converted BASEs with different α-Al2O3/YSZ ratios and sintering/conversion conditions. The general trend is that converted BASEs with higher YSZ contents show higher mechanical strength, and all of them are significantly stronger than those of the β"-Al2O3and β"-Al2O3/YSZ from a commercial supplier (Ionotec). Also, according to Fig.6, the strength of the samples made with a standard process (STP) is higher than 500 MPa, which is almost twice that of the Ionotec samples. In the STP, α-Al2O3/YSZ parts were sintered at 1600 ℃ for 2h to achieve full density, and then converted in β"-Al2O3powders at 1450℃ for 10h with a rate of 3 ℃·min–1(refer to Table 1 for the detailed sintering and conversion conditions). We have also tried fabricating BASE using a slow process (SLP). In this modified process, the heating and cooling rate during conversion was decreased from 3 to 0.5/min in the range from 1200 to 1450 ℃, and the hold time at 1450 ℃ was decreased from 10 to 1h. The major benefit is that significant improvement in flatness of the final electrolyte was obtained with such a slow heating/cooling process, which is particularly useful when the part size is scaled up from an active area of 3 to 64 cm2. Our conductivity measurements showed no significant difference in the conductivity of the BASE samples prepared via the STP and the SLP.However, as shown in Fig.6, the slower conversion profile does result in a 10 to 30% reduction of the BASE flexural strength. A lower sintering temperature of 1510 ℃ compared to 1600 ℃ in the STP along with the SLP (LS-SLP) has also been investigated and the mechanical strength is even lower, as show in Fig.6. Clearly, more work is needed to further optimize the sintering/conversion process to achieve high mechanical/electrical performance and desired flatness for the BASE samples.

Table 1 Sintering and conversion conditions for the BASE samples

Once a large-size BASE sample was prepared, it was glass-sealed to an α-Al2O3ring, as shown in Fig.7(a). The assembly of the large cell was similar to that of small cell shown in Fig.4(a). Testing of the large cell was carried out at 280 ℃. As shown in Fig.7(c1), the cell was charged and discharged at a 1 C rate with a cycling energy density of 100 W·h·kg–1. Negligible fade in cell capacity and energy was observed over 700 cycles. And the energy efficiency was 91%. The cell was further cycled at a higher energy density of 150 W·h·kg–1at a rate of C/4 and it was stable without performance degradation for 150 cycles.

The path for developing planar NBBs at PNNL is shown schematically in Fig.8. Over the past few years, we have performed cell material development and optimization, cell design, and cell performance testing using 3 cm2button cells. Material scale-up with large-scale performance and life testing have also been carried out with larger 64 cm2button cells. Currently, we are teaming with EaglePicherTechnologies, LLC,and other entities to design, fabricate, and test multicell stacks with active areas of 64 and 100 cm2. The next step is fabrication of 200 cm2cells and module stacks, and the stack design, performance and life testing will be reported in the near future.

5 Concluding remarks

NBBs have attracted great interests in the past several decades for energy storage applications because of their high energy density, high efficiency, low material cost, and good cycle life. Tubular NBBs have been widely studied and developed since the invention of Na-S in the late 1960s. Currently, several companies including NGK Insulator, Inc., and General Electric (GE) are involved in efforts to commercialize tubular NBBs. The planar NBBs have also been investigated in the early stage. Compared to the tubular designs, the planar designs have overwhelming advantages, including power output, cell packing, ease of assembly, and thermal management. The two major areas where planar NBBs are considered inferior to the tubular ones are sealing between the α-Al2O3ring (or holder) and the metal parts, and mechanical strength of the BASE. Owing to the inherent weakness of the thin α-Al2O3ring, sealing technologies such as compression or thermocompression seals cannot be applied to large planar cells. Both the thin, wide BASE and α-Al2O3ring are also unable to withstand pressure differentials in the electrode chambers during battery heating and cooling, as well as charging and discharging. These problems have halted the commercialization efforts for planar NBBs.

Recently, PNNL has worked on developing high-performance planar NBBs because of their advantages over tubular cells. With operating temperatures decreased from above 300 ℃ for Na-S batteries to below 300 ℃or even 200 ℃ for ZEBRA batteries, low-cost polymer seals can be used to replace the thermocompression seals, which thus eliminates the need to apply a mechanical load to the α-Al2O3ring during sealing. Reduced temperature also alleviates the pressure difference between the anode and cathode chambers. As shown in Fig.6, the mechanical strength of the converted BASE is almost twice that of material from Ionotec, which also makes it possible for the BASE to withstand pressure differentials. Other designs such as duplex BASE with a thin, dense layer of BASE supported by a thicker, porous BASE would maintain the good mechanical strength while significantly decreasing the cell internal resistance, which can realize the operation of NBBs at lower temperatures. Apparently, novel designs, effective seals, and new materials/components eventually will be needed to commercialize planar NBBs.

Acknowledgements: The work was supported by the US Department of Energy’s (DOE’s) Advanced Research Projects Agency-Energy (ARPA-E), and Office of Electricity Delivery & Energy Reliability (OE). We appreciate the useful discussions with Dr. I. Gyuk of the DOE-OE Grid Storage Program. PNNL is a multiprogram laboratory operated by Battelle Memorial Institute for the DOE under Contract DE-AC05-76RL01830.

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10.3969/j.issn.2095-4239.2016.03.007

TM912 Document code：A Article ID：2095-4239（2016）03-309-08

date: 2016-04-14.

LU Xiaochuan (1978—), senior research scientist, fields of research：sodium batteries, Li-ion batteries, redox flow batteries, solid-state electrolytes, etc., E-mail：Xiaochuan.Lu@pnnl.gov.