Microstructure and Conductivity of ZEBRA Battery Cathode Investigation on Microstructure and Conductivity of ZEBRA Battery Cathode Tannaz Javadi Dr. Anthony Petric Dr. Gianluigi Botton 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

Solid ceramic electrolyte 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 Na NaCl + Ni NiCl2 NaAlCl4 Liquid electrolyte +ve Current Collector Charged area Discharged Reaction front -ve Cell case Solid ceramic electrolyte ions route 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)

Cycling reactions: The net reaction: Na = Na+ + e- Negative electrode NiCl2 + 2Na+ + 2 e- = Ni + 2NaCl Positive electrode The net reaction: 2NaCl + Ni NiCl2 + 2Na E = 2.58 V @ 300 ˚C Charge 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: Vacuum Distillation: Heated up to 450˚C, under vacuum for 4 h. 1- The cathode-β” alumina interface 2- The cross section of the cathode from β”- alumina to current collector 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 β“-Al2O3 Current collector 1

SEM & FIB Images: Charged cell Discharged cell FIB cross section 1 µ D 10 µ Discharged cell 2 µ E D A C B FIB cross section A: NaCl, B: Ni, C: NaAlCl4 D: NiCl2 E: Na6FeCl8

Thermodynamic modeling Room temperature microstructure deviates from real phases present during operation at high temperature FactSage database are appropriate for modeling ZEBRA chemistry Examination of cell reaction during cycling Phase changes during cooling

Thermodynamic modeling charging Discharging Overcharge (L +NiCl2) Increase in solubility of NiCl2 in molten salt 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 A B C

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 (K (Ω-1cm-1)) Conductivity Cell Calibration Measuring cell constant using different concentration KCl at different temperatures Concentration (Molarity) Conductivity (K (Ω-1cm-1)) 18 ˚C 25 ˚C 1 0.09783 0.11134 0.1 0.011166 0.012856 0.01 0.0012205 0.0014087 R = Resistance (Ω) ρ = Resistivity (Ω.cm) l = length A = area ≈ 400 cm-1

Results Conductivity of pure Electronic conductivity of pure NaAlCl4 with temperature I (Amps) E (Volts) Electronic conductivity of pure NaAlCl4 with temperature Conductivity (Ω-1cm-1) T (˚C)

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) log 10 (activity) Alpha NbCl5 + <a> Bi Bi (mol)

Results Electrical Conductivity with Temp. Conductivity (Ω-1cm-1) T (˚C)

Results Electrical Conductivity NaAlCl4+NbCl5+Bi (300 ˚C) Electrical conductivity (Ω-1cm-1) Bi (mole %) Effect of different concentrations of Bi added to 30% NbCl5 and NaAlCl4 mixtures at 300 ˚C. Results Electrical Conductivity NaAlCl4+NbCl5+Bi (300 ˚C) Electrical conductivity (Ω-1cm-1) Log 10 (activity) (Mole)Bi Phases present at different concentrations of Bi in the mixture at 300 ˚C and their effect on conductivity Results Possible phases at different Mole fraction Bi

The I-E curve for different mixtures of NbCl5 + Bi +NaAlCl4. Results Electronic conductivity (300 ˚C) Conductivity (Ω-1cm-1) Pure NaAlCl4 0.038 30%NbCl5+0.2 mole Bi 0.53 30%NbCl5+0.5 mole Bi 0.572 30%NbCl5+0.75 mole Bi 0.57 30%NbCl5+0.9 mole Bi 0.50 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.

Dr. Anthony Petric Dr. Gianluigi Botton Dr. Gary Purdy Dr. Gu Xu Acknowledgement Dr. Anthony Petric Dr. Gianluigi Botton Dr. Gary Purdy Dr. Gu Xu CCEM staff Jim Garrett

Thank you