The document discusses aging in Ni-MH batteries. It presents an advanced electrochemical model to simulate aging mechanisms like corrosion and electrochemical grinding. Experimental tests on new and 3-year-old battery packs show the aged pack has higher power performance, likely due to grinding reducing particle sizes. Modeling fits experimental data on the aged pack by reducing the particle size parameter, validating the model's ability to represent aging effects.

1. Controlled CO2 | Diversified fuels | Fuel-efficient vehicles | Clean refining | Extended reserves
Ni-
Ni-MH battery ageing:
from comprehensive study to
state-of-
electrochemical modeling for state-of-charge
state-of-
(SOC) and state-of-health (SOH) estimation
Eric PRADA
Julien BERNARD
Valérie SAUVANT-MOYNOT
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E.Prada, J. Bernard, V. Sauvant-Moynot – Les Rencontres Scientifiques de l'IFP – 30th November – 2nd December 2009
INTRODUCTION
Hybrid vehicle lifetime should last 15 years...
What about Batteries?
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2 E.Prada, J. Bernard, V. Sauvant-Moynot – Les Rencontres Scientifiques de l'IFP – 30th November – 2nd December 2009
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2. INTRODUCTION
Ageing is a critical and open issue
Battery long term operation and failure are governed by ageing
mechanisms occurring at the molecular scale
They are numerous, depending on battery chemistry and use
→ ageing modeling is a rather difficult task
Battery inner state estimation (SoC, SoH) is sensitive to ageing:
Accurate SoC and SoH are required to optimize battery performances
on-
→ on-board application in the Battery Management System:
Kalman filtering to estimate the inner state of a system. It uses a
mathematical model in addition to measurements of the accessible
quantities (Battery voltage, Current, Temperature)
Expected benefits of an advanced electrochemical
model are to better understand battery ageing & improve
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control strategies
3 E.Prada, J. Bernard, V. Sauvant-Moynot – Les Rencontres Scientifiques de l'IFP – 30th November – 2nd December 2009
OUTLINE
Ni-
Ni-MH batteries
Principle
Ageing mechanisms: state-of-the art
mechanisms: state-of-
lumped-
Advanced lumped-parameter electrochemical model
Presentation and specific ageing developments
set-
Experimental set-up
Results
Comparison of fresh / old commercial Ni-MH packs
Ni-
Validation of model versus experimental data
Perspectives
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4 E.Prada, J. Bernard, V. Sauvant-Moynot – Les Rencontres Scientifiques de l'IFP – 30th November – 2nd December 2009
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3. OUTLINE
Ni-
Ni-MH batteries
Principle
Ageing mechanisms: state-of-the art
mechanisms: state-of-
lumped-
Advanced lumped-parameter electrochemical model
Presentation and specific ageing developments
Experimental set-up
set-
Results
Comparison of fresh / old commercial Ni-MH packs
Ni-
Validation of model versus experimental data
Perspectives
© IFP
5 E.Prada, J. Bernard, V. Sauvant-Moynot – Les Rencontres Scientifiques de l'IFP – 30th November – 2nd December 2009
Ni-MH System
Principle
POSITIVE electrode
NiOOH+ H2O + e− ⇔ Ni(OH )2 + HO−
− 1
2 HO → O 2 + H 2O + 2e −
2
NEGATIVE electrode
MH + HO − ⇔ M + H 2O + e −
1
O 2 + H 2 O + 2 e − → 2 HO −
2
ALKALINE (KOH) electrolyte
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6 E.Prada, J. Bernard, V. Sauvant-Moynot – Les Rencontres Scientifiques de l'IFP – 30th November – 2nd December 2009
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4. Ni-MH ageing mechanisms: state-of-the art
MH electrodes are mostly responsible for ageing
charge/discharge
Electrochemical grinding due to intensive charge/discharge cycling
Absorption / desorption reactions lead to expansion / reduction of the
electrode and induce a mechanical particle breakage
(AB
Corrosion of MH (ABx) due to agressive concentrated alkaline media
Growth of resistive corrosion layer A(OH)3 on the electrode particles
Self-
Self-Discharge
Consumption of active electrode material and water from electrolyte
Overcharge / Overdischarge mechanisms
Gas production and O2 partial recombination at each electrode
Loss of Capacity (material consumption)
consumption) Energy loss
resistance)
Impedance Rise (higher resistance) Power loss
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7 E.Prada, J. Bernard, V. Sauvant-Moynot – Les Rencontres Scientifiques de l'IFP – 30th November – 2nd December 2009
OUTLINE
Ni-
Ni-MH batteries
Principle
Ageing mechanisms: state-of-the art
mechanisms: state-of-
lumped-
Advanced lumped-parameter electrochemical model
Presentation and specific ageing developments
set-
Experimental set-up
Results
Comparison of fresh / old commercial Ni-MH packs
Ni-
Validation of model versus experimental data
Perspectives
© IFP
8 E.Prada, J. Bernard, V. Sauvant-Moynot – Les Rencontres Scientifiques de l'IFP – 30th November – 2nd December 2009
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5. Electrochemical lumped-parameter model
0D model without ageing
Sauvant et al. EVS 24, May 13-16 2009, Stavanger
Mass conservation Model
output data
Butler-
Butler-Volmer Cell Voltage
kinetics
Overpotential Cell Temperature
expressions
State Of Charge
Charge conservation
Energy Balance (Thermal)
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9 E.Prada, J. Bernard, V. Sauvant-Moynot – Les Rencontres Scientifiques de l'IFP – 30th November – 2nd December 2009
Electrochemical lumped-parameter model
Proposition of an ageing model
Electrochemical grinding due to
charge/discharge cycling:
charge/discharge cycling:
Reduction of the spherical particle
radius & electrochemical active
surface evolution
material:
Corrosion of MH active material:
Growth of a corrosion layer and
integration of additional over voltage
Notten et al. J. Alloys & Compounds 231 (1995) 604-610
Development of a combined-coupled ageing model
where MH particle radius becomes time dependant and a
corrosion layer growing mechanism on them
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10 E.Prada, J. Bernard, V. Sauvant-Moynot – Les Rencontres Scientifiques de l'IFP – 30th November – 2nd December 2009
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6. Electrochemical lumped-parameter model
set-
Experimental set-up
Work was performed on two
commercial Prius 2 battery packs
(Ni-MH 168 cells, 202 V nominal, 6.5Ah)
Ni- cells,
a new pack
year-
a 3 year-old pack (70 000 km)
Tests at various isotherms
Charge / Discharge cycling
(Energetic performances)
Hybrid Power Pulse Characterization
(HPPC, Freedom Car 2003)
500A/500V (± 120 kW)
(Power performances)
50A/50V
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IFP Lyon test Bench
11 E.Prada, J. Bernard, V. Sauvant-Moynot – Les Rencontres Scientifiques de l'IFP – 30th November – 2nd December 2009
Electrochemical lumped-parameter model
Calibration on cells (1.2V 6.5 Ah)
3-electrode configuration Reference
electrode RE Resin for Sealing
WE
CE
+ - Added
electrolyte
J. Bernard et al. Conference on Advances in Hybrid Powertrains 25-26/11/2008
Charge / Discharge cycling
Electrochemical experiments (EIS)
post-
Material post-mortem analysis Multi-potentiostat 20V, 400mA
(SEM, EDS) (boost 5A to 20A) + EIS
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12 E.Prada, J. Bernard, V. Sauvant-Moynot – Les Rencontres Scientifiques de l'IFP – 30th November – 2nd December 2009
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7. OUTLINE
Ni-
Ni-MH batteries
Principle
Ageing mechanisms: state-of-the art
mechanisms: state-of-
lumped-
Advanced lumped-parameter electrochemical model
Presentation and specific ageing developments
Experimental set-up
set-
Results
Comparison of fresh / old commercial Ni-MH packs
Ni-
Validation of model versus experimental data
Perspectives
© IFP
13 E.Prada, J. Bernard, V. Sauvant-Moynot – Les Rencontres Scientifiques de l'IFP – 30th November – 2nd December 2009
Comparison between New & In-life packs
HPPC tests on the 2 Prius battery packs
Discharge/Regen Pulse Power Capabilities Discharge/Regen Pulse Power Capabilities
as a function of DOD as a function of Energy Removed
in-
Gain of power and useable energy for the in-life
pack and recharge resistance 5 to 15% lower
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14 E.Prada, J. Bernard, V. Sauvant-Moynot – Les Rencontres Scientifiques de l'IFP – 30th November – 2nd December 2009
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8. Comparison between New & In-Life Packs (2/2)
Material Analysis
Scanning Electron Microscopy (SEM)
Fresh Negative Electrode "Aged" Negative Electrode
Increase of small particles number that may be fully used for the energy
storage ?
Chemical composition EDS:
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No modification observed at this stage
15 E.Prada, J. Bernard, V. Sauvant-Moynot – Les Rencontres Scientifiques de l'IFP – 30th November – 2nd December 2009
Results on the new Battery Pack
Parameter identification through EIS & material analysis
Fresh (-) electrode :
Blue Dots
Fresh (+) electrode :
Red Triangles
Full Cell impedance:
Green Squares
Validation of the electrochemical model on HPPC test
Focus on Discharge &
Regenerative Power Peaks
@ 50% SOC @ 25°C
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16 E.Prada, J. Bernard, V. Sauvant-Moynot – Les Rencontres Scientifiques de l'IFP – 30th November – 2nd December 2009
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9. Results on the 70 000 km battery Pack (1/2)
EIS measurements on the 70 000 km battery cells
Full impedance of aged cell
decreased compared to the new one
Positive electrode impedance was
not modified
-Im(Z)
Negative electrode impedance
decreased compared to the new one
Double Layer Capacitance Cdl is
multiplied by 2 for the negative
electrode Re(Z)
Fresh (-) electrode :
→ Active surface is multiplied by 2 Blue Diamonds
meaning that particles size is divided by 2 Aged (-) electrode :
Red Triangles
Electrochemical grinding seems to have occurred but
corrosion process does not seem significantly
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developed at this stage
17 E.Prada, J. Bernard, V. Sauvant-Moynot – Les Rencontres Scientifiques de l'IFP – 30th November – 2nd December 2009
Results on the 70 000 km battery Pack (2/2)
"Aged" Parameters were tested with the Electrochemical model and
Aged"
compared with experimental HPPC Test Data
Negative electrode grain size is divided by 2
Focus on Discharge &
Regenerative Power Peaks
@ 50% SOC @ 25°C
The model fits in a good way the experimental voltage
data on the In-life battery pack
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18 E.Prada, J. Bernard, V. Sauvant-Moynot – Les Rencontres Scientifiques de l'IFP – 30th November – 2nd December 2009
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10. Conclusions & Perspectives
Focus on ageing mechanisms of Ni-MH systems as case studies
Ni-
HPPC and EIS measurements showed an improvement of Power
performances after 70 000 km service for a Prius 2 battery pack
Only positive aspects of electrochemical grinding seem to have
occured at this stage of ageing ... but corrosion reactions may take
profit later from an increase of the negative electrode area
Physics-
Physics-based 0D electrochemical modeling offers opportunity to
discuss ageing process
Model parameters of the aged battery point out the increase of
negative particle surface in agreement with SEM measurements
Ongoing model developments intend to include ageing kinetics as
a basis for SoC and SoH estimation in Kalman filter based BMS
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19 E.Prada, J. Bernard, V. Sauvant-Moynot – Les Rencontres Scientifiques de l'IFP – 30th November – 2nd December 2009
THANK YOU FOR YOUR ATTENTION
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20 E.Prada, J. Bernard, V. Sauvant-Moynot – Les Rencontres Scientifiques de l'IFP – 30th November – 2nd December 2009
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