Investigation on Microstructure and Conductivity of ZEBRA Battery Cathode TannazJavadi Dr. Anthony Petric Dr. GianluigiBotton MTLS 702
Contents: 1- Microstructure of the cathode 2- Thermodynamic modeling of ZEBRA cycling 3- Conductivity measurement of the liquid electrolyte with temperature. 4- Effect of adding additives on liquid electrolyte conductivity
Introduction: 1978 ZEBRA battery ZEolite Battery for Research in Africa Anode (-): Na metal Ni- Cu composite Current Collector Solid electrolyte: β“-Alumina (≥ 0.2 Ω -1cm-1 at 260 ˚C) Liquid electrolyte: NaAlCl4 (0.6 Ω -1cm-1 at 250 ˚C) Electrolyte Cathode (+): Transition metal chloride + Excess metal FeCl2 NiCl2 2.35 V @ 250 ˚C 2.58 V @ 300 ˚C (200- 300 ˚C) (200- 400 ˚C) J.L . Sudworth, J. Pow. Sour., 100 (2001) +ve Current Collector Na ions route -ve Cell case Charged area Discharged area Reaction front Solid ceramic electrolyte NaAlCl4 Liquid electrolyte NiCl2 NaCl + Ni Na
Cycling reactions: Na = Na+ + e- Negative electrode NiCl2 + 2Na+ + 2 e- = Ni + 2NaCl Positive electrode The net reaction: Charge NiCl2 + 2Na E = 2.58 V @ 300 ˚C 2NaCl + Ni Discharge Micron size Anhydrous NiCl2 and Na Loading in discharged state
Experimental materials: Cell 1: charge=48.3 Ah at 325 ˚C, discharge= 40.8 Ah at 295 ˚C. Cell 3: charge=Similar to Cell1, discharge= 38 Ah at 295 ˚C. Cell 763: First 12 cycles similar to Cell 3, discharge= 26.2 Ah at 295 ˚C. Sample preparation: Current collector β“-Al2O3 Vacuum Distillation: Heated up to 450˚C, under vacuum for 4 h. 1 15 14 2 13 12 11 10 7 6 5 4 3 9 8 1- The cathode-β” alumina interface 2- The cross section of the cathode from β”- alumina to current collector 2 1
Charged cell SEM & FIB Images: D Discharged cell 1 µ 10 µ FIB cross section D A: NaCl, B: Ni, C: NaAlCl4 D: NiCl2 E: Na6FeCl8 C E A 2 µ B
Thermodynamic modeling Room temperature microstructure deviates from real phases present during operation at high temperature Examination of cell reaction during cycling Phase changes during cooling FactSage database are appropriate for modeling ZEBRA chemistry
Thermodynamic modeling Discharging charging Increase in solubility of NiCl2 in molten salt Overcharge (L +NiCl2) Ni grain growth • Tannaz Javadi, Anthony Petric, J. Electrochem. Soc., V.158, Issue 6, p. A700-A704, (2011).
Incentive to improve electrolyte conductivity SEM micrographs show that there are Ni particles that are isolated. In these cases charge transfer may have problems C A B B
Conductivity measurement of NaAlCl4 Potentiostat and Frequency Analyzer Nyquist plot The U-shaped capillary The dip-type capillary design The non-capillary type High cell constant Conductance cell design • Molten salts have relatively low resistivity • Reactive nature of Sodium Chloroaluminate • to moisture • Volatile The U-shaped capillary High cell constant
Conductivity Cell Tungsten wire
Conductivity Cell Calibration Measuring cell constant using different concentration KCl at different temperatures R = Resistance (Ω) ρ = Resistivity (Ω.cm) l = length A = area ≈ 400 cm-1
Results Conductivity of pure NaAlCl4 with temperature Electronic conductivity of pure NaAlCl4 with temperature Conductivity (Ω-1cm-1) I (Amps) T (˚C) E (Volts)
Results The conductivity of different percentage of NbCl5 in NaAlCl4 Conductivity (Ω-1cm-1) T (˚C)
Thermodynamic modeling; Possible phases at different Mole fraction Bi NbCl3(S) NbCl4(S) NbCl4(S) Nb3Cl8(S) Bi (mol) NbCl5 + <a> Bi log 10 (activity) Alpha
Results Electrical Conductivity with Temp. Conductivity (Ω-1cm-1) T (˚C)
Electrical conductivity (Ω-1cm-1) Log 10 (activity) (Mole)Bi Results Electrical Conductivity NaAlCl4+NbCl5+Bi (300 ˚C) Results Possible phases at different Mole fraction Bi Electrical conductivity (Ω-1cm-1) • Bi(mole %) Phases present at different concentrations of Bi in the mixture at 300 ˚C and their effect on conductivity Effect of different concentrations of Bi added to 30% NbCl5 and NaAlCl4 mixtures at 300 ˚C.
Results Electronic conductivity (300 ˚C) I (Amps) E (volts) The I-E curve for different mixtures of NbCl5 + Bi +NaAlCl4. The scan rate is 1 mV/s and the range of voltage is 0-0.2V vs. Reference.
Summery • 1- Thermodynamic modelling predicts the presence of different phases at operating temperature and confirmed the SEM results. • 2- SEM micrographs from ZEBRA cell cathode reveal the existence of isolated Ni particles that may not contribute to the cycling reaction as they are all surrounded by merely ionic conductors. • 3- A special conductivity cell with high cell constant was designed to measure the conductivity of hygroscopic and volatile NaAlCl4. • 4- The effect of different additives on conductivity of the liquid electrolyte was examined by using EIS. • 5- Among different additives, 30 % NbCl5 + 0.2 mole Bi shows the best conductivity. • 6- The conductivity of the liquid electrolyte approximately doubles between 190 and 490 ˚C. • 7- The electronic conductivity of the mixtures were measured by using DC technique. Results show the presence of electronic conductivity in electrolyte by adding dopants.
Acknowledgement • Dr. Anthony Petric • Dr. GianluigiBotton • Dr. Gary Purdy • Dr. GuXu • CCEM staff • Jim Garrett