Quantitative safety characterization of li-ion cells · 2018-04-06 · Reaction time correlates with the observed weak light-blue flame coming from inside the cell. Gas pressure @

QUANTITATIVE SAFETY CHARACTERIZATION OF LI-ION CELLS.DR. SEBASTIAN SCHARNER

JRC Exploratory Research Workshop

March 8th, 2018

OUTLINE.

JRC Exploratory Research Workshop, 8-9 March 2018 © BMW Group Page 2

Challenges caused by Li-ion cells.

Quantitative safety characterization of Li-ion cells.

Estimation of the reactions occurring at the „thermal event“.

POSSIBLE RISK OF LI-ION CELLS.


Required Location

Oxygen Cathode: LMO, NMC, NCA, etc.

Combustible

substance

Liquid electrolyte, separator,

graphite (anode)

Heat Inside or outside of the cell

Environment

Heat

Li-ion cell contain oxygen chemically bound in the cathode and combustible

substances in the other components, e.g. electrolyte solvents.

During normal operation Li-ion cells are safe, as long as heat sources are absent.

POSSIBLE CONSEQUENCE OF A THERMAL EVENT.


A thermal event can result in an uncontrolled release of heat from a cell ( thermal runaway).

Thermal runaway

Uncontrolled release

of heat

Cell starts to produce

heat of its own

Heat source

PROPAGATION OF THE THERMAL RUNAWAY.


Thermal propagation: Transfer of the thermal runaway event from one cell to the next cell.

Thermal Propagation

Uncontrolled release

of heat and heat input into

neighboring cells

Uncontrollable heat

release of

neighboring cells

Heat source Cell starts to produce

heat of its own

FAILURES LEADING TO THERMAL EVENTS (FAILURE ANALYSIS).


Quality defects can neither be predicted nor intercepted by any BMS.

Heat Reason Background of the reason Countermeasure

Cell-external Short circuit between cells Improved isolation of cells

External fire Different reasons Thermal isolation

Over discharge BMS: Voltage monitoringCu-Dendrites

Traffic accidentCrush Mechanical design

Quality defects Different reasonsInline Inspection(Limited possibilities)

Overcharge/

Li-DendritesFast charge BMS: Current monitoring

Cell-internal

External short

circuit

Failure of the external fuse Cell-internal fuse

Inline inspection(Limited possibilities)

Different reasonsQuality defects

CONSEQUENCE OF QUALITY DEFECTS.


Time

Cel

l tem

pera

ture

160 °C

… 200 °C

100 °C

700 °C

Decomposition of electrolyte (~ 250 J/g)

Reaction of O2 from the cathode, e.g. with

electrolyte solvents (~ 450 - 600 J/g)

„thermal runaway “

Uncontrollable heat release caused

by internal combustion

maximum thermal effectTemperature curves

dependent on short circuit

resistance, internal cell

resistance and thermal cell

environment

Decomposition of SEI (~150 J/g?)

Cell failure = „Initialization“ electric power (heat): PW, el. = [RSC + Ri] · I2

Decomposition of the Anode (~ 350 J/g)

Def

ect i

s

mor

e se

vere Sho

rt c

ircui

t

resi

stan

ce

small

large

Quality failures can lead to consequential errors of varying severity.

SAFETY EVALUATION OF LI-ION CELLS.


Classification according to EUCAR (Nail test, hotbox, hotplate, crush…)

Storages are regarded as safe, if the cells comply with a certain risk level in the test.

EUCAR HAZARD LEVEL, mostly ≤ 4

Hazard level (EUCAR) Classification criteria & effect

0 No effect No effect, no loss of functionality

pas

s

1Passive protection

activated

No defect; no leakage; no venting, fire, or flame; no rupture; no

explosion; no exothermic reaction or thermal runaway. Cell

reversibly damaged. Repair of protection device needed.

2 Defect, damageNo leakage; no venting, fire, or flame; no rupture; no explosion;

no exothermic reaction or thermal runaway. Cell irreversibly

damaged. Repair needed.

3Leakage

D mass < 50%

No venting, fire, or flame*; no rupture; no explosion. Weight

loss <50% of electrolyte weight (electrolyte = solvent + salt).

4Venting

D mass ≥ 50%

No fire or flame*; no rupture; no explosion. Weight loss ≥50%

of electrolyte weight (electrolyte = solvent + salt).

fail

5 Fire or flame No rupture; no explosion (i.e., no flying parts).

6 Rupture No explosion, but flying parts of the active mass.

7 Explosion Explosion (i.e., disintegration of the cell).

• Qualitative evaluation

• Boundary conditions not defined

• Acceptance criterion arbitrary

• Definition of fire unclear

• Only rough classification

• Not relevant to storages

(Influence of atmospheric oxygen)

Cell evaluations according to EUCAR additionally require a safety validation of the storage.

Does this make sense?

UN ECE SAFETY REQUIREMENTS.


Vehicle level

Similar safety behavior: Electric vehicle ↔ Conventional vehicle.

Storage level

UN ECE R100, Part 2: “Safety requirements with respect to the Rechargeable Energy Storage System”.

UN ECE GTRxxx IWG TF 5: Special regulations „Thermal Propagation“ still in the development stage.

Possible requirement:

Documented system description with risk analysis on the basis of internal tests to be carried out with criteria, e.g.

no fire, no explosion for a certain time, e.g. 5 min.

Cell level

The safer the cell, the less security measures are necessary at a higher level.

To design a safe storage, quantified information on cell safety is needed.

OUTLINE.





PARAMETERS FOR DETERMINING THE SAFETY OF CELLS.


O2-content of the measurement chamber and thermal boundary conditions need to be specified.

(1) Measurement of the total heat release

(2) Determination of the reaction time

(3) Gas pressure over time

(4) Determination of the total amount of released gases

(5) Determination of the gas production rate

(6) Mass balance of the cell

(7) Precise temperature profiles at several measuring points

(8) Nature and amount of every reaction product

Target: Quantified description of the safety behavior of cells being subjected to thermal runaway.

MEASUREMENT OF HEAT RELEASE UNDER INERT GAS ATMOSPHERE.


Metal block with implemented cell Autoclave for cell measurements

Cell

Thermal isolation

Metal block

Hea

t tr

ansf

er

Nail

Gas

ou

tlet

Headspace (argon) Gas pressure sensor

Autoclave

Determination of the heat transferred to the metal block, measurement of the pressure curve.

Measurement (under inert gas atmosphere)

• Heat: Q1 = (cp,Me · mMe + cp,Zelle · mZelle)· DT

• Pressure curve

• Temperature curves

• Mass loss

• Analysis of reacted materials (solids, liquids, gases)

Thermal

control

ESTIMATION OF THE TOTAL HEAT RELEASE Q.


Assumption: Measured heat Q1 ~ remaining mass m1 and heat loss Q2 ~ mass loss m2.

Cell

Heat transfer to the

metal block Q1

(measured)

Only the heat, which is transferred to the metal block can be measured with the experiment set-up!

Heat loss Q2

?

Assumption for Q2

Cell Cell(after reaction)

Example: m: 1.000g = m1: 750g + m2: 250g

Q: 100% = Q1 75% + Q2 25%

+

weight loss

heat loss

Metal block

MASS LOSS AND MEASURED HEAT Q1.


Measured heat Q1 is proportional to the weight loss assumption is useful.

Deviation caused by:

• Cell format

• Rupture disk design

• Amount of electrolyte/ Ah

• etc.Similar cell geometry

Different cell sizes

R² = 0,7884

Weight loss [wt.-%]

Nor

mal

ized

hea

t Q1

[kJ/

Ah]

EXTRAPOLATION OF THE CHEMICALLY STORED ENERGY.


Chemical energy per Ah: Q = Q1 + Q2 = 21.0 kJ

Electric energy per Ah: 3.65 V x 1 Ah = 13.1 kJ

Energy-ratio factor

Echemical / Eelectric =

21.0 kJ / 13.1 kJ ~ 1.6

Energy-ratio factor mostly found in the range of 1.6.

Deviation caused by:

• Cell format

• Rupture disk design

• Amount of electrolyte/ Ah

• etc.Similar cell geometry

Different cell sizes

Weight loss [wt.-%]

Nor

mal

ized

hea

t Q1

[kJ/

Ah]

DETERMINATION OF REACTION TIME.


time

pEQLStart of reaction

Idealized pressure curve

Gas cooling outweighs

gas production

Def. End of reaction: (pmax - pEQL)/ 2

pmax

Reaction time

End of reaction

The reaction time can be calculated from the gas pressure curve.

Reaction time correlates with the observed weak light-blue flame coming from inside the cell.

Gas pressure @

temperature equilibrium

Gas pressure (overpressure)

Cell voltage

Nail insertion

DETERMINATION OF THE TOTAL GAS AMOUNT.


Gas pressure (overpressure)

Cell voltage

time

pGGWStart of reaction

Gas pressure @

temperature equilibrium

The amount of gas can be calculated from the equation for ideal gases.

Amount of gas under standard conditions (101,3 kPa; 25 °C): ~ 1 - 2 Liter / Ah

pStart

p … pressure

V … volume (autoclave)

R … universal gas constant

T … temperature

Amount of gas: 𝑛 =∆𝑝∙𝑉

𝑅∙𝑇

∆𝑝

Nail insertion

ENERGY DENSITY AND CELL TEMPERATURE.


Linear relationship between energy density and temperature.

The maximum temperature of the cell surface increases linearly to the energy density.

R² = 0,8680

Energy density [Wh/ kg]

Tem

pera

ture

[°C

]

ENERGY DENSITY AND HEAT FLUX.


Exponential relationship between energy density and heat flux.


The heat flux averaged over the reaction time increases exponentially to the energy density.

R² = 0,8680

R² = 0,9049

Energy density [Wh/ kg]

Tem

pera

ture

[°C

]

Hea

t flu

x[W

/cm

2 ]

CELL TEMPERATURE AND HEAT FLUX.


Exponential relationship between temperature and heat flux.


The heat flux averaged over the reaction time increases exponentially to the energy density.

The heat flux increases exponentially to the temperature

Temperature increases the reaction rate.

R² = 0,8249

Temperature [°C]

Hea

t flu

x[W

/cm

2 ]

OUTLINE.





TEST & RESULT.


The weight loss is 30 %.

Cell

Housing: Aluminum

Cathode: similar to 111-NMC

Anode: Graphit

Energy content (electric): ~ 690 kJ

Energy density: ~ 200 Wh/kg

State of charge (Start of test condition)

100% SoC (4.200 V)

Experimental parameters

Nail: Steel, hardened

Diameter: 3,2 mm

Length: 75 mm

Point angle: 60°Feed: 8 cm/s

Intrusion depth: ca. 15 mm

The cell after the experiment

Measured Heat Q1: 741 kJ

Heat Q2 (estimated): 330 kJ

Total heat Q: 1.071 kJ

Reaction time: 14,8 s

Average power: 72,4 kW

Energy-ratio factor: ~ 1,55

Dp (autoclave): ~ 0,5 bar

Amount of gas: ~ 1,2 l/ Ah

POST MORTEM ANALYSIS OF THE J/R INSIDE THE CELL.


During the thermal runaway strongly reducing conditions can be observed inside the cell.

X-ray diffractogram of solids taken from the J/R

inside the cell (homogenized sample)

Copper does not melt or react.

Employed Analytics: ICP, X-Ray

Solids Graphit, Li2CO3, LiAlO2, MnO, Ni, Co, Al, Cu


Assignment to layered oxide difficult Assumption: Mn as LiMnO2; Ni, Co as divalent oxides.

Gases H2, CO, CO2, CH4, C2H4

Solids Graphite, layered oxide „(Li0.79Ni0.01)NiO2“ Li(Mn, Ni?,Co? )O2, (Ni, Co)O, Al

Solvents DMC, EMC

Employed Analytics: ICP, GC, X-Ray

X-ray diffractogram of substances

taken from outside the cell

(homogenized samples)

POST MORTEM ANALYSIS OF EJECTED SUBSTANCES.

CALCULATION OF THE MEASURED REACTION ENTHALPY.


Assumptions

• Isobaric reaction (Dpautoclave < 1 atm) DUr ~ DHr

• Calculation must reflect the analysis results

• Conservation of mass with respect to each element

• Maximization of released heat of reaction

The reaction enthalpy can be calculated by a detour via the elements (Hess’s law).

Elements

- Hf0(1) Hf

0(2)

Li-ion cell Reaction products

DHr0

REACTION ENTHALPY OF CELL CONTENTS.


The calculated energy for decomposing the starting materials into the elements is 3.145 kJ.

DHf0

@298,15 K, 101,3 kPa, normalized to the exchange of one electron

Decomposing the cell contents into its elements

Conducting salt: 1/6 LiPF6 (s) → 1/6 Li (s) + 1/6 P (s, r) + 1/2 F2 (g) + 383 kJ

Ceram. Coating: 1/6 Al2O3 (s) → 1/3 Al (s) + ¼ O2 (g) + 280 kJ

Cathode binder: ¼ -[CH2-CF2]n- (s) → n/4 [C (s, G) + H2 (g) + F2 (g)] + 205 kJ

Cathode material: ¼ Li0,45[Ni1/3Mn1/3Co1/3]O2 (s) → 0,11 Li (s) + 1/12 [(Ni (s) + Mn (s) + Co (s)] + ¼ O2 (g) + 130 kJ

Solvent EC: 1/10 C3H4O3 (s) → 3/10 C (s, G) + 1/5 H2 (g) + 3/20 O2 (g) + 60 kJ

Anode binder*: 1/52 -[C18H26O16]n- (s) → n/52 [ 18 C (s, G) + 13 H2 (g) + 8 O2 (g)] + 59 kJ

Solvent DMC: 1/12 C3H6O3 (l) → ¼ C (s, G) + ¼ H2 (g) + 1/8 O2 (g) + 45 kJ

Solvent EMC: 1/14 C4H8O3 (l) → 2/7 C (s, G) + 2/7 H2 (g) + 3/28 O2 (g) + 37 kJ

Anode material: 8,55 Li0,117C (s) → Li (s) + 8,55 C (s, G) + 17 kJ

Separator: ½ -[CH2]n- (s) → n/2 [C (s, G) + H2 (g)] + 14 kJ

Elements

Li-ion cell

- Hf0(1)

Required energy

supply

*CMC (DS = 1,5)

REACTION ENTHALPY OF THE REACTION PRODUCTS.


The calculated energy to synthesize reaction products is - 4.212 kJ.

DHf0

@298,15 K, 101,3 kPa, normalized to the exchange of one electron

Formation of reaction products (validated)

¼ Li (s) + ¼ Al (s) + ¼ O2 (g) → ¼ LiAlO2 (s) - 297 kJ

¼ Li (s) + ¼ Mn (s) + ¼ O2 (g) → ¼ LiMnO2 (s) - 210 kJ

1/3 Li (s) + 1/6 C (s, G) + ¼ O2 (g) → 1/6 Li2CO3 (s) - 203 kJ

½ Mn (s) + ¼ O2 (g) → ½ MnO (s) - 193 kJ

½ Ni (s) + ¼ O2 (g) → ½ NiO (s) - 120 kJ

½ Co (s) + ¼ O2 (g) → ½ CoO (s) - 119 kJ

¼ C (s, G) + ¼ O2 (g) → ¼ CO2 (g) - 98 kJ

½ C (s, G) + ¼ O2 (g) → ½ CO (g) - 55 kJ

¼ C (s, G) + ½ H2 (g) → ¼ CH4 (g) - 19 kJ

½ C (s, G) + ½ H2 (g) → ¼ C2H4 (g) + 13 kJ

… postulated reaction products (not validated)

½ Li (s) + 1/6 Al (s) + ½ F2 (g) → 1/6 Li3AlF6 (s) more stable than LiF and AlF3 - 564 kJ

Li (s) + ¼ O2 (g) → ½ Li2O (s) - 299 kJ

½ H2 (g) + ½ F2 (g) → HF (g) (traces, according to literature) - 271 kJ

½ H2 (g) + ¼ O2 (g) → ½ H2O (l) - 143 kJ

Li (s) + 1/3 P (r, s) → 1/3 Li3P (s) - 112 kJ

Elements

Reaction products

Hf0(2)

Energy release

Energy

release

RESULT.

JRC Exploratory Research Workshop, 8-9 March 2018 © BMW Group

The calculation covers approximately 99.6% of the observed value.

The calculated reaction enthalpy is:

DHr0

= - Hf0(1) + Hf

0(2) = 3.145 kJ – 4.212 kJ = - 1.067 kJ

Measured value: Q = - 1.071 kJ

Reasons for deviations, e.g.:

• The exact amount of additives within the starting materials is not exactly known

• The complete qualitative and quantitative identification of all reaction products is difficult

Li-ion cell Reaction products

DHr0

Page 28

HYDROFLUORIC ACID DURING THERMAL RUNAWAY?


HF is produced at most in traces, since it will find many reaction partners in the cell interior.

A. Hydrogen fluoride HF („Hydrofluoric acid“)

1) Hydrogen fluoride is formed by binder decomposition (PVDF) inside the cathode (> 400 °C).

2) The conducting salt LiPF6 decomposes at elevated temperature in LiF und PF5; PF5 forms HF with solvents.

½ Li2CO3 (s) + HF (g) → LiF (s) + ½ H2O (l) + ½ CO2 (g) - 78 kJ

1/6 Al2O3 (s) + HF (g) → 1/3 AlF3 (s) + ½ H2O (l) -96 kJ

½ Li2O (s) + HF (g) → LiF + ½ H2O (l) - 190 kJ

1/3 Al (s) + HF (g) → 1/3 AlF3 (s) + ½ H2 (g) - 232 kJ

Analytical evidence of abundant Li2CO3 inside the cell.

Aluminum is available in excess.

Analytical detection of H2 in the emission gas.

Reaction enthalpy DHr0

per formula turnover @ 298,15 K; 101,3 kPa

Energy release

REACTIONS OF ALUMINUM.


Aluminum reduces most fluorine- or oxygen-containing compounds to the element.

B. Aluminum

The thermal event with temperatures around ~ 700 °C provides the activation energy for kinetically hindered

reactions. Thermodynamically stable reaction products are preferred:

¾ MnO2 + Al → ½ Al2O3 + ¾ Mn - 448 kJ

3/2 NiO + Al → ½ Al2O3 + 3/2 Ni - 478 kJ

3/2 CoO + Al → ½ Al2O3 + 3/2 Co - 481 kJ

3/5 POF3 + Al → 3/5 AlF3 + 1/5 Al2O3 + 3/5 P -499 kJ

3/5 LiPF6 + Al → 1/5 Li3AlF6 + 4/5 AlF3 + 3/5 P - 508 kJ

1,29 Li[Ni1/3Mn1/3Co1/3]O2 + Al → 0,14 Li2O + LiAlO2 + 0,43 Ni + 0,43 MnO + 0,43 Co - 517 kJ

3/5 PF5 + Al → AlF3 + 3/5 P - 564 kJ

1,04 Li0,45[Ni1/3Mn1/3Co1/3]O2 + Al → 0,27 Al2O3 + 0,47 LiAlO2 + 0,35 Ni + 0,35 MnO + 0,35 Co - 595 kJ

3 HF + Al → AlF3 + 3/2 H2 - 696 kJ

Analytical proof of Ni(0) and Co(0): thermite-like conversion of the cathodic aluminum collector!

Reaction enthalpy DHr0

per formula turnover @ 298,15 K; 101,3 kPa

Energy release

1,04 Li0,45[Ni1/3Mn1/3Co1/3]O2 + Al → 0,27 Al2O3 + 0,47 LiAlO2 + 0,35 Ni + 0,35 MnO + 0,35 Co - 595 kJ

1,29 Li[Ni1/3Mn1/3Co1/3]O2 + Al → 0,14 Li2O + LiAlO2 + 0,43 Ni + 0,43 MnO + 0,43 Co - 517 kJ

THE INFLUENCE OF OXYGEN IN THE PRESENCE OF AIR.


Air access increases the heat release by a factor between 3 and 4.

C. Combustion of combustible substances emitted from the cell

The following substances were detected outside the cell:

Solids: Graphite 15% by weight of the amount used

Solvents: DMC, EMC 56% by weight of the amount used

Gases: H2, CO, C2H4, CH4

Ele

ctric

ally

sto

red

ener

gy

1,0

The

rmal

eve

nt

w/o

air

acce

ss

The

rmal

eve

nt

with

air

acce

ss

1,6 5,4

Hea

t qua

ntity

[kJ]

SUMMARY.


The quantitative safety characterization of Li-ion cells provides valuable information with direct

reference to the cell and storage system safety:

1. Thermal information for a more robust design of storages against a thermal propagation

event.

2. Chemical information about the reactions taking place during thermal runaway of Li-ion

cells. From this, cell materials can be optimized with regard to minimizing the heat release.

THANK YOU FOR YOUR ATTENTION!

JRC Exploratory Research Workshop, 8-9 March 2018 © BMW Group Seite 33

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Quantitative safety characterization of li-ion cells · 2018-04-06 · Reaction time correlates with the observed weak light-blue flame coming from inside the cell. Gas pressure @