Work Package 1 Batteries FUTURE/VESI Seminar 14 th Jan 2015, London Dr Gregory Offer*, Dr Edmund Noon, Billy Wu, Sam Cooper, Vladimir Yufit, Farid Tariq, Marie-Therese Srbik, Monica Marinescu, Ricardo Martinez-Botas, Nigel Brandon * gregory.offer@imperial.ac.uk, Lecturer, Department Mechanical Engineering, Imperial College London
Aims To develop the knowledge & tools that industry needs: To make better systems around electrochemical energy storage devices (i.e. batteries & supercapacitors) By Creating degradation aware models Designing new experiments Pushing the boundaries of our knowledge
Current / Equally split current All cells are not always equal Parallel cells Rarely in the same electrical environment Even if in the same temperature environment If T is not managed, things can go badly wrong Even if T is managed cells can still be unequally stressed Series cells Same problem as uneven current distribution in large form factor cells R(I 1 ) R(I 1 +I 2 ) Battery 1 (B 1 ) Battery 2 (B 2 ) Battery 3 (B 3 ) 5 4 3 2 1 R(I 2 +I 3 ) R(I 3 ) V app = OCV 1 + η 1 + R IC (2I 1 + I 2 ) V app = OCV 2 + η 2 + R IC (I 1 + 2I 2 + I 3 ) V app = OCV 3 + η 3 + R IC (I 2 + 2I 3 ) Maximum/ Minimum / 4 cells / 6 cells / 8 cells / 10 cells / 12 cells Equal loading 0 0.0 0.2 0.4 0.6 0.8 1.0 Ratio of interconnect to battery resistance Applied potential difference Wu B, Yufit V, Marinescu M, et al., 2013, Coupled thermal electrochemical modelling of uneven heat generation in lithium-ion battery packs, Journal of Power Sources, 43, 2013, 544 554
Dynamic rebalancing Current must be conserved between cells Cell voltage / V 4.0 3.8 3.6 3.4 Isopotential curves 3.2-4 0 Current / A -2 0 0 20 40 0 20 SOC converges to mid point due to balancing Cell voltage / V 40 60 SOC 80 100 4.10 4.05 4.00 3.95 3.90 3.85 3.80 3.75 3.70 3.65 3.60 3.55 3.50 3.45 3.40 3.35 3.30 3.25 3.20 3.15 3.10 Wu B, Yufit V, Marinescu M, et al., 2013, Coupled thermal electrochemical modelling of uneven heat generation in lithium-ion battery packs, Journal of Power Sources, 43, 2013, 544 554
-Z imag [m ] -Z imag [m ] The effect of thermal gradients Under thermal gradients, a cell behaves like one with a higher average temperature Effect is due to nonlinear temperature dependence on charge transfer resistance -200-150 -100-50 0-13 C -5 C 5 C 15 C 25 C 35 C 45 C 55 C 10 Hz -9-6 -3 0 0 3 6 9 12 15 1 Hz Decreasing temperature 0.1 Hz 0 50 100 150 200 Z real [m ] 10 8 6 4 2 0-2 220 Hz 15 o C, isothermal 10 o C < T < 20 o C 5 o C < T < 25 o C 0 o C < T < 30 o C -5 o C < T < 35 o C 30 Hz increasing thermal gradient 0 2 4 6 8 10 12 14 Z real [m ] 5 Hz...... 0.3 Hz Troxler Y, Wu B, Marinescu M, Yufit V, Patel Y, Marquis AJ, Brandon NP, Offer GJ, The effect of thermal gradients on the performance of lithium-ion batteries, Journal of Power Sources, 247, 2014, 1018-1025
3C discharge of 20Ah EIG NCM battery There is a need to manage thermal boundary conditions properly during experiments
Voltage / V Temperature / o C 3C discharge of 20Ah EIG NCM battery 55 50 45 40 35 Channel 1 Channel 2 Channel 3 Channel 4 Channel 5 Channel 6 Channel 7 Channel 8 Channel 9 Channel 10 Channel 11 Channel 12 Channel 13 Channel 14 Channel 15 Cold Junction Cold Junction dt ~ 10 o C Strong effect thermal boundaries Opposite behaviour to normally reported Caused by bus bars Similar to cell tab cooling Region high current is the coolest Significant extra heating 3-4oC 30 4.2 25 4.0 20 0 500 1000 1500 2000 2500 Time / s 3.8 3.6 3.4 Constant current, or real life load cycle = similar thermal behaviour 3.2 3.0 0 500 1000 1500 2000 Time / s
Temperature / o C 3C discharge of 9 cell x 20Ah EIG NCM battery 55 50 45 40 35 30 Prototype for testing Thermally conducting base plate * Insulated other boundaries Air exposed cell tabs * No thermal mgmt. system * Ambient TC1 TC3 TC4 TC5 TC6 TC7 TC8 dt ~ 10 o C 25 20 15 0 500 1000 1500 2000 2500 3000 Time / s * All different in actual battery pack
Effect of cooling strategy In-between cells <1mm total thickness Slip between cells in a stack Design of cooling channels important.
Plate cooling Temperature difference is much smaller. Design of cooling channels could be optimised further. Will lead to temperature gradients in z direction.
Degradation in lithium-ion batteries Lithium-ion batteries degrade Various irreversible losses over the lifetime of a cell lead to capacity fade and power fade Monitoring of degradation in batteries is of critical importance 11 Schematics of various degradation mechanisms at the anode-electrolyte interface and the cathode region Aging mechanisms in lithium-ion batteries. Vetter et. al Journal of Power Sources doi:10.1016/j.jpowsour.2005.01.006.
Existing degradation diagnostics Electrochemical Impedance Spectroscopy (EIS) Slow-Rate Cyclic Voltammetry (SRCV) Incremental Capacity method (IC) EIS SRCV IC -Imaginary impedance / m 50 40 30 20 10 4.2 V 4.0 V 3.8 V 3.6 V 3.4 V 3.2 V 3.0 V 2.8 V Decreasing state-of-charge 0 0 20 40 60 80 Real impedance / m Experimental results of a 4.8 Ah Kokam lithium-polymer cell at different voltages/states of capacity fade. EIS (left), SRCV (middle), IC curve (right). Wu B, Yufit V, Merla Y, Martinez-Botas RF, Brandon NP, Offer GJ, Differential thermal voltammetry for tracking of degradation in lithium-ion batteries, Journal of Power Sources 2015, 273, Pages 495-501
Wu B, Yufit V, Merla Y, Martinez-Botas RF, Brandon NP, Offer GJ, Differential thermal voltammetry for tracking of degradation in lithium-ion batteries, Journal of Power Sources 2015, 273, Pages 495-501 Differential Thermal Voltammetry New in-situ battery diagnosis method for tracking degradation Uses the temperature profile under constant current discharge to infer the same data as SRCV/IC but in a much reduced time DTV = dt dv = dt dt dt dv Plotted against cell voltage, it may be possible to give an indication of the entropy changes in carefully controlled conditions Peaks represent phase transformation dv dt dt dv dt dt
Wu B, Yufit V, Merla Y, Martinez-Botas RF, Brandon NP, Offer GJ, Differential thermal voltammetry for tracking of degradation in lithium-ion batteries, Journal of Power Sources 2015, 273, Pages 495-501 Advantages / Disadvantages Made for application in real world systems Extract the same amount of information as a SRCV/IC Takes only a few minutes Simple computation : dt/dv Only requires voltage and temperature measurements (useful in parallel packs) Does not require iso-thermal condition
Wu B, Yufit V, Merla Y, Martinez-Botas RF, Brandon NP, Offer GJ, Differential thermal voltammetry for tracking of degradation in lithium-ion batteries, Journal of Power Sources 2015, 273, Pages 495-501 Accelerated Degradation Experiment 4.8Ah Lithium polymer batteries made by Dow Kokam Carbon anode and Nickel-Manganese-Cobalt cathode 3 cells stored/operated at various conditions EIS, SRCV, IC and DTV carried out at every 50 cycles at room temperature to track degradation Cell Temperature Loading 1 55 o C 4.2V Hold 2 55 o C 1C cycling 3 Room 1C cycling 4.8Ah lithium polymer Kokam cell
Wu B, Yufit V, Merla Y, Martinez-Botas RF, Brandon NP, Offer GJ, Differential thermal voltammetry for tracking of degradation in lithium-ion batteries, Journal of Power Sources 2015, 273, Pages 495-501 SRCV vs. DTV Similar peak potentials and changes through capacity loss between the two methods 3 hours for SRCV and 30 minutes for DTV Cell held at 4.2V: significant change in both peaks, anode and cathode Cycled cell: small change in the anode peak but negligible for the cathode Primary (anode) Secondary (cathode) Comparison of experimental SRCV and DTV results for a 4.8Ah lithium polymer Kokam cell at various stages of capacity fade
Current Research Problem Modelling Lithium Iron Phosphate (LFP) cathodes Phase change material with flat voltage profile & large hysteresis effect Competing models, shrinking core vs. domino cascade, both work but the latter is more physically correct Zeng, Y. I., Bazant, Martin Z. Delmas, C., Maccario, M., Croguennec, L., Le Cras, F., Weill, F
Thank You for your time! Contact: gregory.offer@imperial.ac.uk www.futurevehicles.ac.uk