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

Transcription

1 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./ Petten

2 Factors Influencing the Thermal Stability of Lithium Ion Batteries - From Active Materials to State-of-Charge and Degradation Internal Shortcircuit Dendrites Intrusion Nail Penetration External Short-circuit Crush External Heat Thermal Propagation Thermal Stability Internal Heat Exo. Decomposition Aging State-of- Charge (SOC) Positive Active Materials Markus Börner Workshop - Thermal Propagation Page 2

3 Correlation of Aging and the Thermal Stability 20 C Strong capacity fading after 100 cycles (<400 cycles to an SOH* of 70%) Commercial cell 45 C Almost linear capacity fading indicates the formation of an effective SEI and homogeneous degradation effects (>1100 cycles to an SOH* of 70%) Cathode LiNi 0.5 Co 0.2 Mn 0.3 O 2 Anode Graphite Separator PE Electrolyte solvents DMC, EC, PC Electrolyte additives FEC, PS, SN Nominal Capacity Charge Discharge 2.2 Ah 4.2 V CCCV - 1C; < C/ V CC - 1C Markus Börner Workshop - Thermal Propagation A. Friesen et al., J Power Sources 342 (2017) Page 3 M. Börner et al., J Power Sources 342 (2017) X. Mönnighoff et al., J Power Sources 352 (2017) * State of health: SOH

4 Correlation of Aging and the Thermal Stability 7 Li MAS nuclear magnetic resonance (NMR) Reference (100% SOH) 20 C, 80% SOH 45 C, 80% SOH 10 µm 10 µm 10 µm FIB 20 µm 2 µm 1 µm Reference 20 C 45 C Insufficient formation by cell manufacturer Deposition of mossy metallic lithium Formation of an effective SEI confirmed 20 C Insufficient SEI formation Co-intercalation Exfoliation Decomposition layer Potential lithium plating Markus Börner Workshop - Thermal Propagation A. Friesen et al., J Power Sources 342 (2017) Page 4 M. Börner et al., J Power Sources 342 (2017) X. Mönnighoff et al., J Power Sources 352 (2017) 56-63

5 Correlation of Aging and the Thermal Stability 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 T TR thermal runaway*: dt/dt 10 K min -1 T onset for self-sustaining exothermic reactions*: dt/dt 0.02 K min -1 T onset 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 Markus Börner Workshop - Thermal Propagation Page 5 M. Börner et al., J Power Sources 342 (2017) * D. Doughty et al., Electrochem Soc Interfaces 21 (2012) 37-44

6 Correlation of Aging and the Thermal Stability SOC * dependency Reference Direct correlation of SOC and T onset (determined by thermally induced release of intercalated lithium) 20 C Lower T onset attributed to the presence of metallic lithium (less pronounced correlation with the SOC) Markus Börner Workshop - Thermal Propagation Page 6 M. Börner et al., J Power Sources 342 (2017) * State of charge: SOC

7 Correlation of Aging and the Thermal Stability SOH * dependency (at 100% SOC) 20 C 45 C Reduced T onset due to presence of metallic lithium Higher T onset assigned to thermally stable SEI Markus Börner Workshop - Thermal Propagation Page 7 M. Börner et al., J Power Sources 342 (2017) * State of health: SOH

8 Correlation of Aging and the Thermal Stability SOH * dependency (at 100% SOC) 20 C 45 C Reduced T onset due to presence of metallic lithium Higher T onset assigned to thermally stable SEI No significant effect of the SOH on T TR (moderate cycling conditions) Markus Börner Workshop - Thermal Propagation Page 8 M. Börner et al., J Power Sources 342 (2017) * State of health: SOH

9 Influence of Low Temperature Cycling on the Thermal Stability Commercial cell Cathode LiNi 0.5 Co 0.2 Mn 0.3 O 2 Anode Graphite Discharge Capacity / Ah C 45 C 20 C 0 C Cycle no. / 1 i = 2.2 A Separator PE Electrolyte solvents DMC, EC, PC 0 C Drastically reduced thermal stability due to the presence of Electrolyte additives FEC, PS, SN high surface area lithium (HSAL) T onset below 30 C Markus Börner Workshop - Thermal Propagation Page 9 A. Friesen et al., J Power Sources 334 (2016) 1-11

10 Degradation Effects on the Surface of Commercial LiNi 0.5 Co 0.2 Mn 0.3 O 2 Cathodes aged * fresh LiNi 1/3 Co 1/3 Mn 1/3 O 2 10 µm 2 µm LiNi 0.5 Co 0.2 Mn 0.3 O 2 Commercial cell 2 µm Cathode LiNi 0.5 Co 0.2 Mn 0.3 O 2 Anode Graphite Separator PE Electrolyte solvents DMC, EC, PC Electrolyte additives FEC, PS, SN 2 µm Irregularly distributed particle cracking LiNi 0.82 Co 0.15 Al 0.03 O 2 2 µm Markus Börner Workshop - Thermal Propagation Page 10 M. Börner et al., J Power Sources 335 (2016) *at 20 C to 80% SOH

11 Degradation Effects on the Surface of Commercial LiNi 0.5 Co 0.2 Mn 0.3 O 2 Cathodes 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 Markus Börner Workshop - Thermal Propagation Page 11 M. Börner et al., J Power Sources 335 (2016) 45-55

12 Degradation Effects on the Surface of Commercial LiNi 0.5 Co 0.2 Mn 0.3 O 2 Cathodes Thermogravimetric analysis* (TGA) T < 150 C: Residual electrolyte components and their decomposition products 275 C < T < 375 C: Phase change + oxygen release from cathode active material 350 C < T < 450 C: PVdF binder T > 450 C: Phase change + oxygen release Markus Börner Workshop - Thermal Propagation Page 12 M. Börner et al., J Power Sources 335 (2016) * Ar-atmosphere; 100 ml min -1 flow rate; 5 K min -1 heating rate

13 Degradation Effects on the Surface of Commercial LiNi 0.5 Co 0.2 Mn 0.3 O 2 Cathodes Thermogravimetric analysis* (TGA) Increasing C-rate results in a larger surface area consisting of a highly unstable delithiated structure Increasing oxygen loss from the unstable overcharged LiNi 0.5 Co 0.2 Mn 0.3 O 2 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 Markus Börner Workshop - Thermal Propagation Page 13 M. Börner et al., J Power Sources 335 (2016) * Ar-atmosphere; 100 ml min -1 flow rate; 5 K min -1 heating rate

14 Thermal Stability of Different Cathode Active Materials Electrochemical Performance, Degradation, Influence of SOC NCM111 NCM622 Decreased thermal stability in the charged (delithiated) state. Increased mass loss due to charge/discharge cycling regardless of the SOC. Reduced thermal stability due to higher nickel content Increased influence of aging on the thermal stability of the active material Markus Börner Workshop - Thermal Propagation Page 14 1, 3 Structural change + oxygen evolution 1, 3 Structural change + oxygen evolution 2 PVdF binder decomposition 2 PVdF binder decomposition

15 Thermal Stability of Different Cathode Active Materials Electrochemical Performance, Degradation, Influence of SOC Thermal decomposition route of NCM materials Structural degradation facilitates phase changes (TM-migration) accompanied by oxygen evolution. Unstable charged (delithiated) structure accelerates phase changes and oxygen evolution. Markus Börner Workshop - Thermal Propagation Page 15

16 Conclusions 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; Ni 4+ ) 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 Markus Börner Workshop - Thermal Propagation Page 16