Factors influencing the thermal stability of li-ion ... · Markus Börner | 08.03.2018 | Workshop...

Factors Influencing the Thermal Stability of Lithium Ion Batteries-

From Active Materials to State-of-Charge and Degradation

JRC Exploratory Research Workshop – Safer Li-Ion Batteries by Preventing Thermal Propagation? M. Börner, A. Friesen, F. Schappacher, M. Winter – 08./09.03.2018 – Petten

Factors Influencing the Thermal Stability of Lithium Ion Batteries - From Active Materials to State-of-Charge and Degradation

Markus Börner | 08.03.2018 | Workshop - Thermal Propagation

AgingState-of-Charge (SOC)

Positive Active

Materials

External HeatThermal Propagation

ThermalStability

Internal HeatExo. Decomposition

External Short-circuit

Internal Short-circuit

Dendrites

IntrusionNail Penetration

Crush

Correlation of Aging and the Thermal Stability


M. Börner et al., J Power Sources 342 (2017) 382-395A. Friesen et al., J Power Sources 342 (2017) 88-97X. Mönnighoff et al., J Power Sources 352 (2017) 56-63

Commercial 18650 cell

Cathode LiNi0.5Co0.2Mn0.3O2

Anode Graphite

Separator PE

Electrolyte solvents DMC, EC, PC

Electrolyte additives FEC, PS, SN

Nominal Capacity 2.2 Ah

Charge4.2 V

CCCV - 1C; < C/20

Discharge2.5 V

CC - 1C

20°C Strong capacity fading after 100 cycles (<400 cycles to an SOH* of 70%)

45°C Almost linear capacity fading indicates the formation of an effective SEI and homogeneous degradation effects(>1100 cycles to an SOH* of 70%)

* State of health: SOH

Reference Insufficient formation by cell manufacturer 20°C Deposition of “mossy” metallic lithium 45°C Formation of an effective SEI confirmed



M. Börner et al., J Power Sources 342 (2017) 382-395

10 µm

Reference (100% SOH) 20°C, 80% SOH 45°C, 80% SOH

FIB

10 µm 10 µm

20 µm 1 µm

20°C Insufficient SEI formation Co-intercalation Exfoliation Decomposition layer Potential lithium plating

• 7Li MAS nuclear magnetic resonance (NMR)

A. Friesen et al., J Power Sources 342 (2017) 88-97X. Mönnighoff et al., J Power Sources 352 (2017) 56-63

2 µm

• Heat-wait-search (HWS) experiments in an accelerating rate calorimeter (ARC)

• Heat - 5 K steps Wait - 30 min Search - Identification of exothermic reactions (≥ 0.02 K min-1)

Heating according to self-heating rate of the cell quasi-adiabatic conditions




Tonset for self-sustaining exothermic reactions*: dT/dt ≥ 0.02 K min-1

TTR thermal runaway*: dT/dt ≥ 10 K min-1

Tonset depends on aging behavior and state of charge (SOC)

PE separator melting at 130°C Gas evolution triggers burst disk

(155°C < T < 170°C) Decomposition of the cathode

active material and subsequent reactions thermal runaway

* D. Doughty et al., Electrochem Soc Interfaces 21 (2012) 37-44

• SOC* dependency



Reference Direct correlation of SOC and Tonset

(determined by thermally induced release of intercalated lithium)

20°C Lower Tonset attributed to the presence of metallic lithium (less pronounced correlation with the SOC)

M. Börner et al., J Power Sources 342 (2017) 382-395 * State of charge: SOC

• SOH* dependency (at 100% SOC)



20°C Reduced Tonset due to presence of metallic lithium

45°C Higher Tonset assigned to thermally stable SEI

M. Börner et al., J Power Sources 342 (2017) 382-395 * State of health: SOH

• SOH* dependency (at 100% SOC)



20°C Reduced Tonset due to presence of metallic lithium

45°C Higher Tonset assigned to thermally stable SEI


No significant effect of the SOH on TTR

(moderate cycling conditions)

* State of health: SOH


0 100 200 300 400 500 600 700 800 900 10001.2

1.4

1.6

1.8

2.0

2.2

2.4

55°C 45°C 20°C 0°C

Dis

char

ge C

apac

ity /

Ah

Cycle no. / 1

i = 2.2 A


Anode Graphite

Separator PE




0°C Drastically reduced thermal stability due to the presence of high surface area lithium (HSAL) Tonset below 30°C

Influence of Low Temperature Cycling on the Thermal Stability

A. Friesen et al., J Power Sources 334 (2016) 1-11

*at 20°C to 80% SOH

Degradation Effects on the Surface of Commercial LiNi0.5Co0.2Mn0.3O2 Cathodes



freshaged*

10 µm

2 µm

LiNi1/3Co1/3Mn1/3O2

2 µm

LiNi0.82Co0.15Al0.03O2

2 µm

LiNi0.5Co0.2Mn0.3O2

2 µm

Irregularly distributed particle cracking


Anode Graphite

Separator PE




• Local inhomogeneity of the composite electrode lead to deviations in the state-of-charge (SOC) and current density:

Highly delithiated structure on the particle surface local overcharge conditions

Repulsive Coulombic interaction of adjacent layers mechanical stress

Lithium vacancies phase transition to rock-salt or disordered spinel structure

Transition metal migration in defective structure dissolution into the electrolyte




T < 150°C: Residual electrolytecomponents and their decompositionproducts

275°C < T < 375°C: Phase change + oxygen release from cathode activematerial

350°C < T < 450°C: PVdF binder

T > 450°C: Phase change + oxygenrelease




Thermogravimetric analysis* (TGA)

* Ar-atmosphere; 100 mL min-1 flow rate; 5 K min-1 heating rate

• Increasing C-rate results in a larger surface area consisting of a highly unstable delithiated structure

Increasing oxygen loss from the unstable overcharged LiNi0.5Co0.2Mn0.3O2 structure

High reactivity of oxygen in presence of electrolyte can lead to fatal consequences (explosion, fire)

• Minor influence of the upper cut-off potential on the thermal stability compared to high C-rates




Thermogravimetric analysis* (TGA)

* Ar-atmosphere; 100 mL min-1 flow rate; 5 K min-1 heating rate

• Decreased thermal stability in the charged (delithiated) state.

• Increased mass loss due to charge/discharge cycling regardless of the SOC.

Thermal Stability of Different Cathode Active Materials –Electrochemical Performance, Degradation, Influence of SOC


NCM111 NCM622

• Reduced thermal stability due to higher nickel content

• Increased influence of aging on the thermal stability of the active material

1 , 3 – Structural change + oxygen evolution2 – PVdF binder decomposition

1 , 3 – Structural change + oxygen evolution2 – PVdF binder decomposition

Thermal decomposition route of NCM materials

Structural degradation facilitatesphase changes (TM-migration)accompanied by oxygen evolution.

Unstable charged (delithiated)structure accelerates phasechanges and oxygen evolution.

Thermal Stability of Different Cathode Active Materials –Electrochemical Performance, Degradation, Influence of SOC


• Onset of exothermic reactions is determined by the anode side

Deposition of metallic lithium on the anode surface should be prevented

Effective and thermally stable SEI is key for a safe LIB

• High reactivity of the cathode in presence of electrolyte dominates the kinetics during thermal runaway

Layered and spinel-type electrodes exhibit a decreased thermal stability in the charged state

The presence of nickel largely reduces the thermal stability of positive active materials (especially in the charged state; Ni4+) independent of the structure (layered/spinel)

An increasing nickel content in the NCM structure intrinsically reduces the thermal stability

Overall, aging effects have a larger influence on the thermal stability of layered transition metal oxides like NCM compared to spinel-type or olivine-type active materials

Conclusions


Factors influencing the thermal stability of li-ion ... · Markus Börner | 08.03.2018 | Workshop...

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