aging mechanisms in li-ion battery

Aging Mechanisms in Li-ion BatteriesBatteries

Shrikant C. Nagpure (ME)Prof. Bharat Bhushan (ME)

([email protected])

Prof. Sudarsanam Suresh Babu (IWSE)Prof. Giorgio Rizzoni (ME/ECE)

Prof. Yann Guezennec (ME/ECE)

Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics–Center for Automotive Research

1Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics

Center for Automotive Research

Outline

• Li-ion Batteries – Introduction and Objective

• Multi-scale Characterization Plan

• ResultsCell CyclingThermography and SEM

Heat Diffusion – Proposed MechanismAFM l t i l i t lAFM electrical resistance on nanoscale

Surface Resistance – Proposed Mechanism

• SummarySummary

Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics Center for Automotive Research 2

Li-ion Batteries - Introduction

Comparison with Different Battery Chemistries


i i C llLi-ion Cell • Anode and cathode with a separator and electrolyte in between, are rolled and packed in a can to form a cylindrical cell.a can to form a cylindrical cell.

• Positive Electrode (Cathode): Lithiated Metal Phosphate pnanoparticles coating with carbon bonded on an Al substrate with Polyvinylidene Difluoride (PVDf) binderbinder

• Negative Electrode (Anode): Graphitic Carbon bonded on copper substrate

• Organic Electrolyte: Lithium Hexafluorophosphate Alkylene Carbonates (LiPF6 salt in ethylene carbonate

Length = 5 feet ; Width = 2.5”Thickness = 0.2 mm

(LiPF6 salt in ethylene carbonate (EC) – diethyl carbonate (DEC))Nominal Operating Voltage: 3.3 V

Nominal Capacity: 2.3 AhNanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics

Center for Automotive Research 4

Li ion Cell OperationLi-ion Cell Operation

• Charging: Li in cathode ionizes and intercalates within anode layers

LiFePO + 6C Li FePO + Li CLiFePO4 + 6C Li1-xFePO4 + LixC6

(Cathode) (Anode) (Delithiated Cathode) (Lithiated Anode)

• Discharging: Li de intercalates from anode and migrates back to• Discharging: Li de-intercalates from anode and migrates back to cathode to form original compound

Harris S., “Lithium-ion batteries”, OSU, Seminar, 2009


T i l A i M h i f C th dTypical Aging Mechanisms for Cathode

• Structural disorderingDi d i f t l t tDisordering of crystal structure during synthesis

• Phase transitionPhase changes during repeated intercalation/de-intercalation of Li in the host lattice

• Metal dissolutionDissolved metal forms deposits on the cathode and also on anode

• Electrolyte decompositionSurface films due to electrolyte oxidation

Vetter, J., Novak, P., Wagner, M. R., Veit, C., Moller, K. –C., Besenhard, J. O., Winter, M., Wohlfahrt‐Mehrens, M., Volger, C., Hammouche, A., “Ageing mechanisms in lithium‐ion batteries”, J. Power Sources, 147 (2005) 269‐281


Problem StatementProblem Statement• A good understanding of relevant mechanisms does not exist. • Aging is known to be a result of several simultaneous g g

physiochemical processes.• Multi-scale characterization is needed to understand the aging

effects at various length scales, as well as a function of aging g , g gprocess.

Objectives• Understand the aging mechanisms in the Li-ion batteries

Understand the kinetics of material degradation in the cathode• Identify precursors in material degradation for improved

material selection. • Based on understanding of aging mechanisms, predict the

battery life as a function of design, geometry, materials and


service conditions

Multi-scale Characterization Plan

• Task 1: Generation of Cathode Surfaces from Batteries with Different Aging Characteristics - Batteries aged to 0, 25, 50, 75 100 % of their life

• Task 2: Thermography Mapping to study material degradation on millimeter scale. Also identify the potential regions (µm2) of material y p g (µ )degradation within a large cathode area (m2).

• Task 3: Micro- and Nano-scale Surface Characterization -InvestigateTask 3: Micro and Nano scale Surface Characterization Investigate the identified areas further for surface roughness, grain coarsening, chemical compositional changes or formation of nanocrystalline deposits, structural changes and surface electrical propertiesp , g p p


Proposed Experimental PlanProposed Experimental Plan


C ll C liCell Cycling • Nominal capacity (Ah) decreases as a function of aging cycles.aging cycles.

• The direct measure of cell aging is the increase in cell impedanceimpedance.

This increase can be attributed to the increase in surface resistance of the anode and cathode. The surface resistance affects the battery operation because

Cell is aged under very harsh electrical duty cycles at high temperature (55 °C) and high C-rate (16C ) (1C = 2 3Ah)

battery operation because battery reactions occur at the surfaces of anodes and cathodesand high C rate (16C ) (1C = 2.3Ah)

Nagpure S. C., Marano V., Rizzoni G., Babu. S. S., Guezennec and Bhushan B., “Aging and Material Characterization Studies of Lithium-ion Batteries at The Ohio State University”, 1st ANL conference on Advanced Lithium Battery for Automotive Applications


cathodes.

Angus

高亮度

ThermographyThermographyIt is believed that thermal properties are a measure of electrical impedance, thus iselectrical impedance, thus is of importance in aging studies on a mm scale.

It helps to understand whether aging process is uniform over the entire

• High intensity short duration thermal pulse incident on front face of the

cathode surface.

g y psample

• IR camera captures the thermal data from the rear face

ASTM Note: E 1461 - 92Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics

Center for Automotive Research 11

• The cathode is divided in 5 sections, 2 X 2 inch each

• Darker spots are colder.


Nagpure S. C., Dinwiddie R., Babu S. S., Rizzoni G., Bhushan B., and Frech T., “Thermal Diffusivity Study of Aged Li-ion Batteries Using Flash Method,” J. Power Sources, (in press)

Thermal diffusivity calculated from:where L = thickness, t1/2 = time for

213879.0 L

• Thermal diffusivity of aged samples is higher compared to unaged samples

, 1/2rear face to reach half-maximum

2/1t=α

samples



SEMSEM

SEM micrographs were taken to investigate gmechanisms for the increase in thermal diffusivity.

• Washed in 1-methyl-2-pyrolidinone to dissolve PVdF binder

• SEM micrographs reveal the coarsening of the nanoparticles forSEM micrographs reveal the coarsening of the nanoparticles for aged sample , believed to be due to sintering.

• Coarsening leads to a decrease in the effective surface area of the particles and porositythe particles and porosity.

• It can also lead to the de-bonding of these particles from the aluminum substrate



Heat Diffusion - Proposed Mechanismp

• Due to coarsening of the nanoparticles, porosity decreases - the effective surface area per unit volume decreases.

• A decrease in porosity of the medium leads to an increase in the th l diff i itthermal diffusivity.



AFM Surface Resistance MeasurementsAFM Surface Resistance Measurements

• As indicated earlier, an increase in impedance of a cell after aging can be attributed to the increase in thebe attributed to the increase in the surface resistance .

• SSRM is used to measure the surface resistance of a circular contactSpreading Surface Resistance resistance of a circular contact formed between the LiFePO4nanoparticles and the conductive AFM tip on a nanoscale

Spreading Surface Resistance Measurement (SSRM) Setup

AFM tip on a nanoscale. Nagpure. S. C., Bhushan B., Babu S. S., Rizzoni G., “Scanning Spreading Resistance Characterization of Aged Li-ion Batteries Using Atomic Force Microscopy,” Scripta Mater. 60, 933–936 (2009)


Surface height and change inSurface height and change in the surface resistance of LiFePO4 cathode as a result of aging.g g

• Surface height: The LiFePO4nanoparticles tend to coarsennanoparticles tend to coarsen in the aged samples.

• Surface resistance: The lower lt t t i th fvoltage output in the case of

aged samples indicates higher surface resistance as compared to unaged sampleBias: +1 V Note the compared to unaged sample.Bias: +1 V

Scale: V α 1/RNagpure. S. C., Bhushan B., Babu S. S., Rizzoni G., “Scanning Spreading Resistance Characterization of Aged Li-ion Batteries Using Atomic Force Microscopy,” Scripta Mater. 60, 933–936, (2009)


difference in scale

Surface Resistance - Proposed MechanismSurface Resistance Proposed Mechanism• The resistance of the LiFePO4

nanoparticles increases as the cells are cycled.

because of the coarsening of the particles, as is seen in the height image.The coarsening also causes loss ofThe coarsening also causes loss of the carbon coating which leads to the further increase in the resistance of these particles.In addition to this, the total surface resistance of the aged sample increases due to the additional resistance from the nanocrystallineTotal Resistance resistance from the nanocrystalline deposits (NCD) formed on the cathode surface. NCD is formed due to the chemical reactions taking

Total ResistanceUnaged (R1)u

Aged (R1)a + (R2)aplace at the surface of the cathode


Nagpure. S. C., Bhushan B., Babu S. S., Rizzoni G., “Scanning Spreading Resistance Characterization of Aged Li-ion Batteries Using Atomic Force Microscopy,” Scripta Mater. 60, 933–936, (2009)

Summary• The direct measure of cell aging is the increase in cell impedance.

This increase can be attributed to the increase in surface resistance of the anode and cathode. The surface resistance affects the battery operation because battery reactions occur at the surfaces of

d d th danodes and cathodes.

• Based on thermography, damage is non-uniform over the cathode surface

• Grain coarsening of cathode surface occurs during aging of cellsCoarsening is observed at nano-scale using SEM micrographs

Porosity decreases due to coarsening leading to change in the thermal diffusivity as measured by Flash method, a measure of surface resistance.

C i l l d t i i th f i tCoarsening also leads to increase in the surface resistance as observed in SSRM studies.

• Future plans:

Chemical analysis: EELS, EDX, Raman; Structural analysis: TEM; Surface analysis: KPM


References

• Nagpure S. C. and Bhushan B. , In Applied Scanning Probe Microscope Methods – Biomimetics and Industrial Applications Vol. 13, (B. Bhushan and H. Fuchs, eds.) pp. 203-233, (Springer-Verlag, Heidelberg, G 2009)Germany, 2009).

• Nagpure S. C., Bhushan B., Babu S. S., and Rizzoni G., “Scanning Spreading Resistance Characterization of Aged Li-ion Batteries Using Atomic Force Microscopy,” Scripta Mater. 60, 933–936, (2009)

• Nagpure S. C., Dinwiddie R., Babu S. S., Rizzoni G., Bhushan B., and Frech T., “Thermal Diffusivity Study of Aged Li-ion Batteries Using Flash , y y g gMethod,” J. Power Sources (in press)

http://www.mecheng.osu.edu/nlbb/http://car osu edu/http://car.osu.edu/


Acknowledgements

• Institute of Materials Research (IMR)

• Center for Automotive Research – Battery Consortium

• Oak Ridge National Laboratory (ORNL)

Edi W ldi I tit t (EWI)• Edison Welding Institute (EWI)

aging mechanisms in li-ion battery

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