Simple Experiments Giving Deep Insights into Capacity Fade and Capacity Loss Mechanisms of Li Battery Materials

Transcription

1 Chemistry Symposium, AABC Europe, 30 January 2 February, 2017, Mainz, GER Simple Experiments Giving Deep Insights into Capacity Fade and Capacity Loss Mechanisms of Li Battery Materials Florian Holtstiege 1, Johannes Kasnatscheew 1, Ralf Wagner 1, Tobias Placke 1 and Martin Winter 1,2 1 MEET Battery Research Center, Institute of Physical Chemistry, Univ. of Muenster, GER, martin.winter@uni-muenster.de & 2 Helmholtz Institute Muenster; Ionics in Energy Storage, IEK-12 of Forschungszentrum Juelich m.winter@fz-juelich.de

2 Acknowledgements (General) Federal Ministry of Economics and Technology (BMWi) Federal Ministry for the Environment, Nature Conservation & Nuclear Safety (BMU) Federal Ministry of Education and Research (BMBF) North-Rhine-Westphalia (NRW) University of Muenster (WWU) Helmholtz Association (HGF) and Forschungszentrum Jülich Page 2

3 Acknowledgements (Specific) Funding: BMW Group German Ministry of Education and Research (BMBF) within the project Benchbatt Material support by BASF Page 3

4 Battery Materials Analysis Many Nice to Have s - Electrochemical Characterization is Essential and Routine Numerous (often newly designed) materials under investigation Chemical and structural characterization: o Before, during and after electrochemical operation o Plenty of equipment and techniques (New) materials are formulated into electrodes or electrolytes and then characterized in cells o Half and full cell studies Electrochemical Characterization: o Cycling Discharge capacity, Capacity loss, Capacity fade o Rate capability, Impedance, etc. o Aging, reliability and safety Page 4

5 Common Belief in the Community Irreversible capacities (Q irr ) depend on the material s BET surface area Low Coulombic efficiency of high voltage cathodes is due to electrolyte oxidation The higher the cathode potential, the more capacity fade Irreversible capacities (Q irr ) at the anode are due to the loss of active lithium This loss of active lithium is higher for Si/C composites than for graphite These beliefs have the status of common wisdom in the community Here we show, that these beliefs are (at least partially, sometimes completely) wrong Page 5

6 Capacity A Closer Look: Capacity Fade and Capacity Loss Li battery materials show capacity fade and capacity loss during operation Possible origins: Loss of active lithium Kinetic effects Decomposition layers Resistance Active host material loss etc. Challenge: Mechanisms of Capacity Fade and Capacity Loss cannot be determined within the standard constant current cycling procedure Charge capacity Discharge capacity Cycle number Capacity Loss/ Irreversible Capacity Page 6

7 1 st cycle capacity loss / mah g -1 Common Knowledge: 1 st Cycle Capacity Losses Depend on the BET Surface Area of the Graphite Anode* NMC/Graphite Full Cell Total Constant 36.5 mah g -1 for low BET surface area graphites Anode SEI Electrolyte LiC n Carbon surface BET surface area / m 2 g -1 Li + Li + (solv) y This seems to be not true for full cells! NMC; various graphites 1M LiPF 6 in EC/EMC (1/1) Cut-off: V vs. Li/Li + Constant Potential Step: 3.0 V vs. Li/Li + for 24 h Rate: 0.2C, 0.2D MW.; Novák, P.; Monnier, A. J. Electrochem. Soc., 1998, 145, Page 8

8 Potential vs. Li/Li + / V Charging NMC to Higher Potentials Higher Discharge Capacity & Potential, thus More Energy But: Higher Capacity Loss In addition: At higher upper cut-off potential, the potential difference between the charge and discharge curves increases: Lower voltage efficiency* Lower energy efficiency* Increase of potential Increase of voltage Increase of Discharge Cap Increase of capacity loss Specific capacity / mah g -1 *Meister, P.; Jia, H.; Li, J.; Kloepsch, R.; MW.; Placke, T.; Chem. Mater., 2016, 28, Page 8

9 Common Knowledge: Li + /Ni 2+ Mixing Because of Similar Ionic Radii Li et al. Chem. Mater. 2007, 19, Lee et al. J. ofthe Electrochem. Soc., 2007, 154 (10), A971. Ni 2+ in the Li + layer leads impeded Li + diffusion, thus to irreversibility* Kawaguchi et al., Phys. Chem. Chem. Phys. 2015, 17, Page 10

10 Potential vs. Li/Li + / V Is it Really Irreversible? Facilitating Lithiation (I): Constant Potential Holding Step during Discharge at 3.0 V vs. Li/Li WE: NMC; CE, Re: Li 1M LiPF 6 in EC/EMC (1/1) Cut-off: V vs. Li/Li + Constant Potential Step: 3.0 V vs. Li/Li + for 24 h Rate: 30 ma g CP step at 3.0V vs. Li/Li + gains extra 22.1 mah g -1 in the 1 st cycle Specific Capacity / mah g Charge capacity: mah g Discharge capacity (without CP step): mah g Discharge capacity (with CP step): mah g Coulombic Eff.: 83.4 % 93.5 % CP step: practical and effective tool for facilitating lithiation 22.1 mah g -1 of overall capacity loss of 36.3 mah g -1 in the first cycle could be regenerated by a simple kinetic measure. Page 11

11 Potential vs. Li/Li + / V Facilitating Lithiation (II): Increasing the Temperature C (0.01 C, 0.2 D) 60 C (0.2 C, 0.2 D) WE: NMC; CE, Re: Li 1M LiPF 6 in EC/EMC (1/1) Cut-off: V vs. Li/Li + Rate: X C, 0.2 D Temperature increase gains 28.6 mah g -1 in the 1 st cycle Specific Capacity / mah g -1 Nearly the same charge capacity and polarizations, but charge at 60 C 20 times faster 28.6 mah g -1 higher discharge capacity Temperature increase (20 C 60 C) facilitates kinetics, as well. Page 12

12 Potential vs. Li/Li + / V Facilitating Lithiation (III): Increasing the Temperature + CP Step, 4.6 V Upper Cut-Off Potential WE: NMC; CE, Re: Li 1M LiPF 6 in EC/EMC (1/1) Cut-off: V vs. Li/Li + Rate: 0.2 C, 0.2 D C 40 C 60 C Specific Capacity / mah g -1 Temperature increase in 1 st cycle 1. Lowers Polarization 2. Increases charge capacity 3. Increases discharge capacity 4. Lowers capacity loss 5. Increases Coulombic efficiency No CP, 20 C; CE = 83.5% With CP, 60 C; CE = 96.9% Electrochemical performance strongly depends on temperature. Page 13

13 Potential vs. Li/Li + / V Facilitating Lithiation (IV): Temperature Increase to 40 C with 4.3V Upper Cut-Off C WE: NMC; CE, Re: Li 1M LiPF 6 in EC/EMC (1/1) Cut-off: V vs. Li/Li + (Constant Potential Step: 3.0V vs. Li/Li + for 24 h) Rate: 0.2 C, 0.2 D Charge capacity: mah g Discharge capacity (without CP step): mah g Discharge capacity (with CP step): mah g Coulombic Eff.: 90.2 % 99.7 % Specific Capacity / mah g -1 Nearly complete elimination of the capacity loss at 4.3V vs. Li/Li +. Li + extraction ratio is very close to the charge capacity. Compared to the upper cut-off potential of 4.6V vs. Li/Li + : less electrolyte oxidation and thus higher CE Page 13

14 CP Step at Higher Temperature is a Proof for a Larger Li + Extraction Degree Increase by 20% Specific Energy / Wh kg % Li V vs. Li/Li + 79% Li + 80% Li Temperature / C No CP CP WE: NMC; CE, Re: Li 1M LiPF 6 in EC/EMC (1/1) Cut-off: 4.6V vs. Li/Li + Constant Potential Step: 3.0V vs. Li/Li + for 24 h Rate: 0.2C, 0.2D Important Consequence: The Li + -extraction degree defines the stability limits of the cathode during charge and at the same time also the max. reversible extraction capacity. So far, the Li + extraction degree has been determined by referring to the discharge capacity (w/o CP step and at RT). Here we show, that the correct way to is to refer to the charge capacity.

15 Potential vs. Li/Li + / V Potential vs. Li/Li + / V Potential vs. Li/Li + / V Capacity Losses of LiFePO 4 (LFP), LiCoO 2 (LCO), and LiNi 0.5 Mn 1.5 O 2 (LNMO): Always Kinetic Measures Do Help LFP: +5.9 mahg -1 by CP step Coul. Eff.: 95.0 % 98.7 % Specific Capacity / mah g LCO: +4.8 mahg -1 by CP step Coul. Eff.: 95.6 % 98.6 % Specific Capacity / mah g LNMO: +1.8 mahg -1 by CP step Coul. Eff.: 96.1 % 97.4 % WE: LFP; LCO; LNMO CE, Re: Li 1M LiPF 6 in EC/EMC (1/1) Cut-off: V vs. Li/Li + (LFP) V vs. Li/Li + (LCO) V vs. Li/Li + (LNMO) Constant Potential Step: 3.0V vs. Li/Li + for 24 h Specific Capacity / mah g -1 Page 16

16 Sp. capacity retention / % Capacity Fade by Oxidation of the Electrolyte at High Potentials? No Direct Connection V 1M LiPF 6 in EC/EMC (1/1) Sp. current: 30 ma g -1 (cycles: 1-3) 150 ma g -1 (cycles: 4-53) WE: NMC; CE, Re: Li Cut-off: V vs. Li/Li + WE: LNMO; CE, Re: Li Cut-off: V vs. Li/Li LNMO NMC NCM Cycle no. Capacity Fading: Depends rather on type of active material than on operation and upper cut-off potentials Page 16

17 1 st cycle capacity loss / mah g -1 Coming back to 1 st Cycle Capacity Losses vs. BET Surface Area NMC/Graphite Full Cell Total BET surface area / m 2 g -1 Page 18

18 1 st cycle capacity loss / mah g -1 1 st Cycle Capacity Losses and BET Surface Area: Li/Graphite Half Cell vs. NMC/Graphite Full Cell Li/graphite Graphite half cell NMC/graphite Total full cell mah g -1 capacity loss from NMC BET surface area / m 2 g -1 Page 19

19 1 st cycle capacity loss / mah g -1 1 st Cycle Capacity Losses and BET Surface Area: Li/Graphite Half Cell vs. NMC/Graphite Full Cell NMC determines < cell capacity loss Graphite/Li half cell NMC/graphite Total full cell 40 Note: The capacity loss of graphite is irreversible 30 > Graphite determines cell capacity loss BET surface area / m 2 g -1 The capacity loss of NMC (and other cathode materials) can be mostly recovered Page 20

20 Both, the Graphite Anode and the NMC Cathode Show Capacity Losses E / V vs. Li/Li + vs. Li/Li + Potential vs Li/Li + / V Cap. loss C irr charge graphite 1st cycle Discharge capacity discharge C rev (186) (372) Graphite Anode in Half Cell 1 st cycle 2nd cycle Cap. Loss = Irreversible Capacity: Q irr x in Li x C 6 (C / Ah kg -1 ) Q irr = Q Charge Q Discharge Kinetic Cap. loss Specific Capacity / mah g -1 NMC Cathode in Half Cell 1 st cycle Page 21

21 Sp. discharge capacity / mah g -1 Many More Surprising and Belief-Changing Findings Literature Reports Kasnatscheew, J.; Evertz, M.; Streipert, B.; Wagner, R.; Klöpsch, R.; Vortmann, B.; Hahn, H.; Nowak, S.; Amereller, M.; Gentschev, A-C.; Lamp, P.; Winter, M.; The Truth about 1st Cycle Coulombic Efficiency of LiNi 1/3 Co 1/3 Mn 1/3 O 2 (NCM) Cathodes, Phys.Chem.Chem.Phys. 2016, 18, Kasnatscheew, J.; Rodehorst, U.; Streipert, B.; Wiemers- Meyer, S.; Jakelski, R.; Winter, M.; Learning from Overpotentials in Lithium Ion Batteries: A Case study on the LiNi 1/3 Mn 1/3 O 2 (NCM) Cathode, J. Electrochem. Soc., 2016, 163, A2943-A2950 Kasnatscheew, J.; Evertz, M.; Streipert, B.; Wagner, R.; Nowak, S.; Cekic-Laskovic, I.; Winter, M.; Changing Established Belief on Capacity Fade Mechanisms: Thorough Investigation of LiNi1/3Co1/3Mn1/3O2 (NCM111) under High Voltage Conditions, J. Phys. Chem C, 2017, DOI: /acs.jpcc.6b11746 Kasnatscheew, J.; Evertz, M.; Streipert, B.; Wagner, R.; Nowak, S.; Cekic-Laskovic, I.; Winter, M.; Improving cycle life of layered lithium transition metal oxide (LiMO 2 ) cathodes for Li ion batteries by smart selection of the electrochemical charging conditions, submitted Charge cut off potential vs. Li/Li + / V NMC111 NMC532 NMC622 NMC811 NCA Sp. discharge cap. retention / %

22 Accumulated Irreversible Capacity (Q AIC ) and Active Li losses (Q ALL ) Focus on LIB Anodes Irreversible Capacity (Q Irr ) / Accumulated irreversible capacities (Q AIC ): Q irr = Q Charge Q Discharge Q AIC = C=x C=1 c = cycle number Q Irr (c) There are two types of parasitic reactions in LIBs, that can consume charge, thus contribute to Q irr : Parasitic reactions, consuming lithium Active Li Loss (Q ALL ) SEI formation and repair, involving Li + Irreversible lithium plating Active material loss Etc. Q irr = Q ALL + Q irr ALL Parasitic reactions, NOT consuming lithium No Q ALL SEI formation and repair, NOT involving Li + (Transition metal) metal dissolution/deposition Gas formation Etc. So far, no simple method to detect Q ALL! Page 22

23 Active Lithium Loss (Q ALL ): Determination of Capacities Reflecting the Initial and the Remaining Active Lithium Content (IRLC-Method) 1 st step: 3-electrode set-up: Anode (variable) is the WE, Cathode (LFP) is the CE, Li is the RE 2 nd step: 2-electrode set-up: Anode disconnected; the Li electrode serves as counter electrode for the cathode Remaining capacity of cathode is discharged Q ALL = Q IC Q RC Anode (WE) Reference (RE) CE (LFP) n ALL = Q ALL F n ALL : F : Q ALL : Q IC : Q RC : Active lithium loss (ALL) in mol Faraday constant Capacity reflecting ALL Initial capacity Remaining capacity Page 23

24 IRLC-Method Determination of Q ALL Purpose: Determination of the contribution of Q ALL to Q irr (in the respective cycle) and of Q ALL to Q AIC (over cycling) Lithium content of a fresh positive electrode Lithium content of a cycled positive electrode Q ALL = Q IC Q RC Initial active lithium content Unused lithium Remaining active lithium content n ALL n ALL : F : Q ALL : Q IC : Q RC : n ALL = Q ALL F Active lithium loss (ALL) in mol Faraday constant Capacity reflecting ALL Initial capacity Remaining capacity Page 24

25 Model Anode Materials: Combination with a Capacity Oversized LFP Cathode * * Graphite (C) Standard anode Moderate Li immobilization = Li loss Dimensionally stable SEI Stable cycling Silicon/Graphite (Si/C) Advanced anode Large Li immobilization = Li loss Dimensionally unstable SEI Ongoing lithium consumption during cycling Unstable cycling Electrolyte: 1 M LiPF 6 ethylene carbonate (EC) : dimethyl carbonate (DMC) 1:1, (Electrolyte LP30, BASF). *same magnification Page 25

26 Ratio of Q ALL to Q AIC / % IRLC Determination of the Ratio of Q ALL to Q AIC Q ALL determined by the IRLC-method, in ratio to the Accumulated Irreversible Capacity (Q AIC ) over cycling 1 st Cycle: Q AIC is almost completely = Q ALL o e.g. Graphite (1 st cycle): 97% of Q AIC corresponds to Q ALL (= Li loss detected after 1 cycle) For higher cycle numbers: Difference between Q AIC and Q ALL increases o e.g. Graphite (103 rd cycle): Only 78% of Q AIC correspond to Q ALL (= Li losses detected after 103 cycles) 60 The parasitic reactions, NOT consuming lithium, become more important during cycling Graphite Cycle No. Page 26

27 Balancing of Electrodes in Full Cells: Capacity Balancing! NOT: Mass Balancing is Required Example: Si/C: 1000 mah g -1 C: 372 mah g -1 LFP:150 mah g -1 Normal balancing: 1.1 : 1 Capacities are decisive Apply to Q ALL C: 3.3 mah cm -2 Si/C: 3.3 mah cm -2 LFP: 3.0 mah cm -2 C: 8.9 mg cm -2 Si/C: 3.3 mg cm -2 LFP: 20 mg cm -2 Page 27

28 Capacity-Related Specific Lithium Li Loss Loss / / mmol Ah -1 Specific Lithium Loss / mmol g -1 n ALL : Graphite vs. Silicon/Graphite n ALL related to mass / (mmol g -1 ) n ALL related to capacity / (mmol Ah -1 ) 1 st Cycle 103 rd Cycle 1 st Cycle 103 rd Cycle C Si/C C Si/C n ALL can be related to mass or capacity o Capacity balancing in the cell is decisive! Cycle Contrary to what is frequently assumed: The n ALL of pure C is higher than that of the Si/C composite, if it is related to electrode capacity, at least for the first two cycles. During long-term cycling, the n ALL of the Si/C composite increases continuously. mmol Ah C Si/C Cycle Page 28

29 Capacity / mah Capacity / mah Distinguishing Between the Origins of Capacity Fade Determination of Q ALL : Determination of the remaining active Li content Further benefit: Distinguishing between origins of capacity fade Contrary to C, the Si/C composite shows a strong capacity fade With the IRLC-method, it can be shown that even after 103 cycles there would be still enough Li in the cathode to lithiate the anode In conclusion, the decrease in Li content in the cathode is not the decisive reason for capacity fade in this case Discharge capacity (C) Remaining capacity in the LFP electrode (C) 2.4 Discharge capacity (Si/C) Remaining capacity in the LFP electrode (Si/C) ! Cycle number Cycle number Page 29

30 Conclusions In full cells, anode and cathode may contribute to capacity fade and capacity loss Electrolyte oxidation makes a minor contribution to low Coulombic efficiencies of cathodes at high voltage The type of active material, rather than the cut-off and operation potentials determine HV performance The practical cathode capacity should be derived from the charge capacity, and not from the discharge capacity The contribution of Active Lithium Losses (Q ALL ) to Irrev. Capacities (Q irr and Q AIC ) can be determined IRLC-Method Simple, but in the community uncommon electrochemical techniques reveal that common wisdom may need a thorough re-thinking Page 30

31 March 28 30, 2017 Welcome in Aachen Dr. Michael M. Thackeray, Argonne National Laboratory Dr. Kai Vuorilehto, EAS Germany GmbH Prof. Jeff Dahn, Dalhousie University Dr. Ann Laheäär, Skeleton Technologies Prof. Jiang Jiuchun, Beijing Jiaotong University Prof. Bernd Friedrich, RWTH Aachen University

32 Simplicity is the ultimate sophistication." Leonardo da Vinci ( ); Italian artist, anatomist, engineer and natural philosopher