factors influencing the thermal stability of li-ion
TRANSCRIPT
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 Page 2
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
Markus Börner | 08.03.2018 | Workshop - Thermal Propagation Page 3
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
Correlation of Aging and the Thermal Stability
Markus Börner | 08.03.2018 | Workshop - Thermal Propagation Page 4
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
Correlation of Aging and the Thermal Stability
Markus Börner | 08.03.2018 | Workshop - Thermal Propagation Page 5
M. Börner et al., J Power Sources 342 (2017) 382-395
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
Correlation of Aging and the Thermal Stability
Markus Börner | 08.03.2018 | Workshop - Thermal Propagation Page 6
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)
Correlation of Aging and the Thermal Stability
Markus Börner | 08.03.2018 | Workshop - Thermal Propagation Page 7
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)
Correlation of Aging and the Thermal Stability
Markus Börner | 08.03.2018 | Workshop - Thermal Propagation Page 8
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
No significant effect of the SOH on TTR
(moderate cycling conditions)
* State of health: SOH
Markus Börner | 08.03.2018 | Workshop - Thermal Propagation Page 9
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
Cathode LiNi0.5Co0.2Mn0.3O2
Anode Graphite
Separator PE
Electrolyte solvents DMC, EC, PC
Electrolyte additives FEC, PS, SN
Commercial 18650 cell
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
Markus Börner | 08.03.2018 | Workshop - Thermal Propagation Page 10
M. Börner et al., J Power Sources 335 (2016) 45-55
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
Cathode LiNi0.5Co0.2Mn0.3O2
Anode Graphite
Separator PE
Electrolyte solvents DMC, EC, PC
Electrolyte additives FEC, PS, SN
Commercial 18650 cell
• 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
Degradation Effects on the Surface of Commercial LiNi0.5Co0.2Mn0.3O2 Cathodes
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M. Börner et al., J Power Sources 335 (2016) 45-55
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
Degradation Effects on the Surface of Commercial LiNi0.5Co0.2Mn0.3O2 Cathodes
Markus Börner | 08.03.2018 | Workshop - Thermal Propagation Page 12
M. Börner et al., J Power Sources 335 (2016) 45-55
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
Degradation Effects on the Surface of Commercial LiNi0.5Co0.2Mn0.3O2 Cathodes
Markus Börner | 08.03.2018 | Workshop - Thermal Propagation Page 13
M. Börner et al., J Power Sources 335 (2016) 45-55
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
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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
Markus Börner | 08.03.2018 | Workshop - Thermal Propagation Page 15
• 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
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