Understanding Impedance of Li-Ion Batteries with Battery Design Studio Robert Spotnitz Battery Consultant Thursday July 6, 2017 16:40-17:10
Understanding Impedance Li-Ion Batteries with Battery Design Studio Table of contents Battery Design Studio 3 Voltage Losses in Li-Ion Cells 4 Electrochemical Impedance Spectroscopy 5 Li-Ion Cell Characterization and Parameter Identification 6 EIS Experiment 14 Interpretation of EIS Results 15 Conclusion 30 Page 2
Battery Design Studio (BDS) Battery Design Studio is a unique tool that allows engineers to design battery cells, simulate performance, and analyze data from both simulation and test work. Page 3 BDS couples geometric models of cells to simulation models to provide physically realistic results.
Voltage Losses in a Lithium-Ion Cell Anode Active Binder Anode Collector SEI Conductive Additive Cathode Active 2 Cathode Collector Cathode Active 1 How to resolve/quantify different contributions? 1) Electronic ohmic drops Contact resistances at current collector/coating interfaces Current collectors Across porous electrodes. 2) Ionic ohmic drops in electrolyte Porous electrodes Separator 3) Diffusion overpotential Solid-phase in active materials Electrolyte in porous electrodes and separator 4) Kinetic overpotential at active/electrolyte interfaces (including SEI) Page 4
Electrochemical Impedance Spectroscopy (EIS) E. Barsukov (http://m.eet.com/media/1053386/ti_algorithms_fig1.gif) E. Barsoukov, J. Ross MacDonald Impedance Spectroscopy 2 nd Ed. Wiley (2005). Usual frequency range is 10 khz to 1 mhz. Page 5 EIS resolves ohmic, kinetic, and diffusion processes.
How to simulate EIS of Li-ion Cells? Equivalent circuit models + Easy to use : fast, automatic regression of model parameters - Difficult to include physical parameters such as electrode thickness, porosity Linearized physics models + Includes physical parameters, fairly fast simulations - Linearization eliminates non-linear behavior so may not fit EIS experiment Nonlinear physics models + Approximates actual cell - Computationally expensive EIS theory requires linear system to analyze impedance. Nonlinear simulation is comparable to experiment even without linearity. Page 6
Numerical Simulation of EIS Experiment Select frequency ( ), current (I) Discretize sine wave I = I o sin ωt t For each constant current step integrate voltage to obtain average V Numerically solve variant of Newman s DUALFOIL model with sinusoidal current applied. Page 7 Linearly regress voltage data to obtain magnitude (V o ) and phase angle ( ) V = V avg + V o sin ωt + φ Compute impedance magnitude, real and imaginary impedance Z Z mag real V I Z o o mag cos, Z Z sin imag mag
High-Power Cell UR18650W Publications using this cell 1) T. Waldmann, M. Wilka, M. Kasper, M. Fleischhammer, M. Wohlfahrt-Mehrens Temperature dependent ageing mechanisms in Lithium-ion batteries - A Post-Mortem study J. Power Src. 262 (2014) 129-135. 2) J. Wang, J. Purewal, P. Liu, J. Hicks- Garner, S. Soukazian, E. Sherman, A. Sorenson, L. Vu, H. Tataria, M. W. Verbrugge Degradation of lithium ion batteries employing graphite negatives and nickel-cobalt-manganese oxide + spinel manganese oxide positives: Part 1, aging mechanisms and life estimation J. Power Src. 269 (2014) 937-948. Page 8
Cell Characterization Independent testing gave discharge curves close to those on datasheet. Page 9
BDS Physical Cell Description Positive Negative Page 10
Amount of NCM/LMO Equilibrium voltage curves - NCM used Toda 3100 low rate discharge - LMO used curve from http://www.limn2o4.com/ - Graphite for negative NCM LMO http://www.todaamerica.com/nm3100.htm Used BDS formation routine to compute cell capacity versus amount of LMO; 80 wt. % NCM/ 20 wt. % LMO gave 1.52 Ah capacity Page 11
BDS Build Report cell wt w/o label 42.96 g Computed cell weight agrees very well with measured value. Page 12
Simulation of Discharge Cell Voltage (V) 1.5 A Discharge 10 A Discharge Temperature ( C) Reasonable fits with limited data set. Page 13
EIS Experiment: Nyquist Plot Ambient temperature Gamry Instruments 0.001 to 1000 Hz Galvanostatic mode 0.1 A ac Page 14
EIS Experiment Comparison with simulation (1D) Nyquist and Bode Plots To fit data adjusted kinetic resistance and increase tortuosity of electrodes. Page 15
Presentation of 2D Profiles Time Time = = 0 2.3 1 min min Page 16
Presentation of 3D Profiles Page 17
Presentation of 3D Profiles by Region Page 18
Presentation of 3D Solid-Phase Profiles by Region Graphite LMO NMC Page 19
Examination of Semicircle: Capacitance 38.6 100 cps Page 20 9000 cps 1000 cps Capacitive current grows into porous electrodes while decreasing in magnitude as frequency decreases. 400 cps
Examination of Semicircle: Kinetics 100 cps 9000 cps 1000 cps 400 cps Page 21 Kinetic current grows into porous electrodes while increasing in magnitude as frequency decreases.
Salt Concentration Profiles from 1 to 0.01 cps Liquid-phase gradient develops 0.001 cps 1 cps 0.1 cps 0.01 cps Page 22 Salt concentration profile develops in electrodes from 1 to 0.01 cps
Negative solid-phase average concentration from 1 to 0.001 cps Solid-phase Li concentration profile develops 0.001 cps 1 cps 0.1 cps 0.01 cps Page 23 Li concentration profile develops in negative from 1 to 0.01 cps
Graphite solid-phase concentration at separator from 1 to 0.01 cps 0.01 cps Solid-phase Li profile develops in C particle 1 cps 0.1 cps 0.02 cps Page 24 Li (Graphite) concentration profile in particle develops from 0.1 to 0.01 cps
LMO solid-phase average concentration from 1 to 0.01 cps Solid-phase gradient across electrode hardly changed 0.001 cps Page 25 1 cps 0.1 cps Li (LMO) concentration profile hardly changed from 1 to 0.01 cps 0.01 cps
LMO solid-phase average concentration from 0.01 to 0.001 cps Solid-phase gradient develops across positive 0.001 cps 0.01 cps 0.004 cps 0.002 cps Page 26 Li (LMO) concentration profile develops from 0.01 to 0.001 cps
NMC solid-phase average concentration from 0.01 to 0.001 cps Solid-phase gradient develops across positive 0.001 cps 0.01 cps 0.004 cps Page 27 Li (NMC) concentration profile develops from 0.01 to 0.001 cps 0.002 cps
NMC solid-phase concentration at separator from 1 to 0.01 cps Solid-phase Li profile develops in NMC particle 0.01 cps 1 cps 0.1 cps 0.02 cps Page 28 Li (NMC) concentration profile in particle develops from 0.1 to 0.01 cps
LMO solid-phase concentration at separator from 0.01 to 0.001 cps Solid-phase Li profile develops in LMO particle 0.001 cps 0.1 cps 0.01 cps 0.005 cps Page 29 Li (LMO) concentration profile in particle develops from 0.01 to 0.001 cps
Model-Based Analysis of Nyquist Plot NMC Particle Profile Graphite Particle Profile LMO Particle Profile Positive Electrode Li Solid-Phase Profiles +Impedance of positive and negative electrodes overlap +Resolve kinetics from masstransfer processes +Liquid-phase and solidphase mass-transfer overlap Negative Electrode Li Solid-Phase Profile Kinetics Capacitance Salt profile Page 30
Electrode ohmic drops, separator Double-layer charging Solid-phase gradient Filling solid-phase BDS now simulates Electrochemical Impedance Spectroscopy (EIS) +EIS +Used in R&D and mfg. +Non-destructive +Resolve and quantify Electrode kinetics Liquidphase gradient processes on scale of 10-3 to 10 4 s +BDS uses nonlinear model +Validate model parameters +Changes in curves can be used to diagnose problems, identify effects of aging Page 31
Thank You! Robert Spotnitz Consultant Battery Design LLC 2277 Delucchi Drive Pleasanton, CA 94588 Mobile: +1 925-895-4080 E-mail: rspotnitz@batdesign.com Page 32