ZEBRA Battery Flat Plate Cell Design

Transcription

1 ZEBRA Battery Flat Plate Cell Design Cord-H. Dustmann, Michael Bayer Battery Consult AG, Switzerland Introduction The ZEBRA battery chemistry was discovered by Johan Coetzer in CSIR 1986 [1]. The principle is illustrated in fig.1 with the summary formula of the cell reaction, the electronic and ionic current flow during charge and discharge, the OCV and the theoretical specific energy. To date the commercialized cell design has a tubular shape with a thermal compression bond seal (TCB). This seal technology is constrained to a diameter of about 40 mm [2]. The reason is the unavoidable pealing stress due to the different thermal expansion coefficients of alumina and nickel as demonstrated in fig.2. 2 NaCl + Ni NiCl Na Ni: Wh/kg Wh/kg = -790 Wh/kg, OCV=2,58V Fe: Wh/kg Wh/kg = -719 Wh/kg, OCV=2,35V - + e- e- Inverter Charge Discharge b -Al 2 O 3 Charge: NiCl 2 2e- Ni Ni backbone 2Cl - 2Na + NaCl NaCl Na + Na NaAlCl 4 + Additives 250 C Figure 1 Principle of the ZEBRA Cell Figure 2 TCB in assembly state (left) and at ambient temperature (right) 1

2 Flat plate geometries need larger diameters for economic reasons. Flat Plate Cell Design Fig 3 shows the design of the flat plate cell with the negative electrode formed by sodium, the separator and solid electrolyte by a β"-alumina disk and the positive electrode by the pressed mix of metal (Ni and/or Fe), salt (NaCl), additives and melt (NaAlCl4). 110 mm 20 mm Melt reservoir Pos. Electrode α- ring β"- disk Neg. Electrode Bipolar plate Figure 3 Flat Plate Cell For tubular cells, the active mass is dry granulated and vacuum impregnated with melt at 200 C. The dry granulation is necessary to form the electronically conductive nickel backbone shown in fig.1. The active mass of the flat plate cell is carefully premixed with powdery melt and pressed as a tablet (fig.4). The pressing generates the conductive backbone which is confirmed by a comparison of the active mass specific performance with ZEBRA cells after 100 cycles (fig. 5). 2

3 Figure 4 Active mass and melt pressed as a tablet Figure 5 Active mass specific performance of FPC vs ZEBRA after 100 cycles Figure 6 Negative electrode aluminum During assembly the negative compartment is empty, and first sodium is only generated during the first charge. The grooves (fig. 6) are in electronic contact with the β"-disk and have space for the sodium which is kept in contact with the β"-disk by capillary forces. The third important component of the cell is the subassembly of the β"-disk, the α-ring and the metallic sealing rings. All three components are prefabricated in a common glassing process (fig.7). The cells are hermetically closed by laser welding in vacuum to avoid inside gas pressure during operation at 250 C to 300 C and pressure variation during cycling. 3

4 According to present planning 18 cells form a stack with ca. 1kWh energy content. 7 stacks are assembled in an evacuated box, which assures very good thermal isolation and reliable containment. By this combination of safe chemistry, double containment and electronic control by the BMS this battery is simply very safe [3]. Battery and Inverter Integrated System Figure 7 Subassembly parts and after glassing Solar power and battery both work with DC but on different voltage levels whereas grid and consumption require 220 or 110 VAC. For this reason, a battery matched inverter is being developed with 2 MPPT inputs for 2 PV strings each with 3kWp, a 1Ø 3kW bi- Our produkt: Battery with BMS and integrated inverter AC DC DC BMS Energy meter 7 kwh OCV=325V Figure 8 Solar home battery with integrated solar and battery inverter directional grid inverter with off-grid capability and the bidirectional battery inverter all integrated with the BEMS (Battery Energy Management System) in the metal box at the front of the battery. [1] A New High Energy Density Battery System, J. Coetzer, Journal of Power Sources 18 (1986) [2] The Sodium Sulfur Battery, J.L. Sudworth, A.R. Tilley, Chapman & Hall 1985, p

5 [3] Encyclopedia of Electrochemical Power Sources, J. Garche et al, Newnes 2013 p