Cycling-Induced Degradation of Batteries

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1 Cycling-Induced Degradation of Batteries Michael Vallance & Andrey Meshkov, GE Global Research Ralph White, Meng Guo, Sean Rayman, & Long Cai, R.E. White and Associates Comsol Conference 2015 Boston Imagination at work. We acknowledge the financial support from the New York State Energy Research and Development Authority (NYSERDA) via the New York Battery and Energy Storage Technology Consortium (NY-BEST). NYSERDA has not reviewed the information contained herein, and the opinions expressed in this report do not necessarily reflect those of NYSERDA or the State of New York.

2 U.S. Energy Storage Deployments by Segment, E February 20, 2015 GTM Research The speed of battery integration depends on the cost of energy storage (MWh/$)... And the cost of energy storage depends on the lifetime (MWh throughput between failures)... And failures occur due to battery cell degradation 2

3 current collector (+ pole) nickel + iron + sodium chloride + sodium aluminum chloride cell case (- pole) sodium Sodium Metal Chloride Rechargeable Cell C operation β alumina electrolyte NaCl-buffered sodium aluminum chlorate cathode electrolyte Compound cathode: nickel & iron & NaCl Net electrochemical dischargereactions: NiCl Na Ni + 2 NaCl, U0 = 2.58 V charge ceramic electrolyte discharge 2 U charge 0 = FeCl + 2 Na Fe + 2 NaCl, 2.33 V 3

4 Test Geometry Cycling Protocol W to 2.00 V W to 600 s W to V M V M to A W to 600 s Repeat Design of Experiment V M (V) X X T ( C) 300 X X 340 X X 2 3 full factorial with partial replication 4 CT slice of as-built cell

5 Electrochemical Reactions (Bulter-Volmer) ( s) ( s) + 2 Ni + 2 Cl = NiCl + 2 e ( s) ( s) ( s) Fe + 6 NaCl + 2 Cl = Na FeCl + 2 e 1 4 Fe ( s) + Na6FeCl 8 ( s) + 2 Cl = FeCl 2 ( s) + 2 NaCl ( s) + 2 e Na + 2 e = 2 Na 6 8 ( ) Cathode Anode Non-electrochemical Reactions (First order or G) Na + + Cl = NaCl ( s ) + ( s) + + = ( s) FeCl 6 Na 6 Cl Na FeCl ( ) = ( ) 2 AlCl Al Cl ( ) = 2 7 AlCl AlCl Al Cl 4 = AlCl Al Cl Cl ( s) + ( ) ( s) = ( ) ( s ) + 6 NaCl ( s ) Na FeCl Ni Fe Cl Ni Fe Cl 6 8 A B-1 2 A+B-1 A B 2 A+B Precipitation & dissolution Cathode electrolyte rearrangement Metal chloride reorganization 5

6 Solid electrolyte anode Cathode Solid electrolyte Current collector Cathode Cathode Solid electrolyte Dependent variable Ionic charge Electronic charge AlCl concentration 4 Cl concentration Al Cl concentration 2 7 AlCl concentration 3 Al Cl concentration 2 6 Fe concentration NaCl concentration Na FeCl concentration 6 8 FeCl concentration 2 NiCl concentration 2 Flux constitutive law Nernst-Planck Ohm's law Nernst-Planck Nernst-Planck Nernst-Planck Nernst-Planck Nernst-Planck Not applicable Not applicable Not applicable Not applicable Not applicable 1-D, axisymmetric, time-dependent simulation 1 full cycle using experimental cycling protocol Reaction moduli used as fitting parameters 6

7 Simulations of initial degradation ( 400 cycles) Cycle # Fe FeCl 2 Charge Ni NiCl 2 Discharge Spikes correspond to a special characterization cycle. Cyclinginduced degradation Degradation not strong function of V M Initial rate of degradation increases with T Same mechanism for C Iron capacity strongly impacted T = 300 C, V M = 2.87 V 7

8 V M = 2.67 V T = 300 C Measured vs Simulated Charge & Discharge Typical of all T and V M 8

9 Key Learning from Simulation: Degradation manifests as uniform loss of Fe and NaCl from the cathode, in the molar ratio ~1:3. Known solid species: FeCl 2 (1:2), Na 6 FeCl 8 (1:8) Possible mobile species: Na 2 Fe 3 Cl 8 (3:8), Na 4 Fe 3 Cl 10 (3:10) How do Fe & NaCl move, and where do they go? 9

10 Cathode from disassembled cell. V M = 2.87 V and T = 300 C. Location 1 contains slag deposit formed at the top of electrolyte pool. Location 2, top third of granules packed bed, is devoid of granules structure. 10

11 Stacked X-ray diffraction patterns, by location. Discharged state. V M = 2.87 V and T = 300 C. Na 6 FeCl 8 & FeCl 2 : Locations 1 & 2. Should be absent in discharged cell. (Ni,Fe) fcc : Locations (2), 3 & 4. Should be uniformly distributed throughout. NaCl: Locations (1), (3) & 4. Should be uniformly distributed throughout. ( ) indicates low level 11

12 Summary Sodium / nickel chloride / iron(ii)chloride high T rechargeable cell Degradation mechanism in present geometry associated with iron species mobility Porous electrode, finite element model at different states of degradation Model: Iron & sodium chloride disappear Post-mortem XRD: Unreactive iron chloride species concentrate at top of cathode

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