Impact of Electrolyte on the Cycling of Si-Based Materials

Transcription

1 Impact of Electrolyte on the Cycling of Si-Based Materials 228 th ECS Meeting Phoenix As presented: October 14, 2015

2 Impact of Electrolyte on the Cycling of Si-Based Materials Vincent L. Chevrier, Connor Aiken, Larry J. Krause 3M Corporate Research Materials Laboratory Xiaohua Ma, Zhonghua Lu, Ang Xiao, Matt Triemert, Li Liu, Dinh Ba Le, Kevin W. Eberman 3M Electronic Materials Solutions Division Remi Petibon, Jeff R. Dahn Dalhousie University 228 th ECS Meeting Phoenix 2015/10/14

3 3M Si Alloy Design Nano-Si in inactive matrix Active/inactive domains < 15nm High first cycle efficiency Remains amorphous on cycling Minimizes surface area Dilutes volume expansion Cost effective synthesis active inactive Multifaceted approach to optimize energy, cycle-life, and cost JES 161, A783 (2014) 3

4 3M Si Alloy Performance Long cycle life demonstrated in pouch cell (>600 Wh/L) by customer >80% after 500 cycles 4

5 Cross Section SEM s

6 Electrolyte Reactivity 3M Si Alloy and Pure Si Cross section study in alloy/binder electrode Ion mill cross section and SEM before cycling Ion mill cross section and SEM after 100 cycles Cycling Cycle 1 Lith: C/10, 5 mv, C/40 trickle Delith: C/10 to 1.5V Cycle 2+ Lith: C/4, 5 mv, C/20 trickle Delith: C/4 to 0.9V Electrolyte EC/EMC 3/7 1M LiPF %FEC 3M Alloy Fresh 1kX 6

7 Cross Section 3M Alloy Fresh 1kX 3M Alloy Fresh 10kX Before cycling 3M alloy coating similar to fresh Si coating ~12 um Si Fresh 1kX Si Fresh 1kX ~12 um 7

8 Cross Section 3M Alloy 100 cycles 1kX 3M Alloy 100 cycles 10kX After 100 cycles Cycled 3M alloy has small SEI Cycled Si : there are no more particles! Only reacted material Major expansion with Si ~ 7x Si 100 cycles 1kX ~16 um 3EC/7EMC +10%FEC Si 100 cycles 10kX ~85 um 3EC/7EMC +10%FEC 8

9 Effect of Electrolyte Same cycling protocol (100 cycles, 0.005V-0.9V) Electrolytes tested: 1M LiPF6 3EC/7EMC 1M LiPF6 3EC/7EMC + 2%FEC 1M LiPF6 3EC/7EMC + 2%VC 1M LiPF6 3EC/7EMC + 2%FEC + 2%VC 1M LiPF6 3EC/7EMC + 5%FEC + 5%VC 1M LiPF6 3EC/7EMC + 10%VC After cycling electrodes were cross sectioned by ion-mill and imaged by SEM 9

10 Effect of Electrolyte After 100 cycles 10

11 Pouch Cells for Research s

12 Research Pouch Cells with 3M Si Alloy Anode coated at 3M 15wt% 3M Si alloy ~ 30% of the capacity comes from the alloy ~ 3 mah/cm 2 All other components and assembly by LiFun LCO/SP/PVdF 95/2.5/2.5 N/P ~ 1.15 Nominally 200 mah (3V 4.35V) Dry cells: filled, formed, degassed at 3M Two groups of cells: CMC/SBR anode binder; LiPAA anode binder 12

13 Importance of Electrolyte and Binder Base electrolyte EC/EMC 3/7, 1M LiPF 6 Additives dramatically impact capacity retention. What is happening? Li plating? Electrolyte dry out? Impedance rise? Binder failure? Etc? RT, 2.75V-4.35V, C/2(C/20) Unclamped 13

14 GCMS Studies s

15 Electrolyte Extraction Method Extraction of electrolyte for GCMS studies For further details see paper by R. Petibon et al. in JES 161, A1167 (2014). 15

16 GCMS Before and After Sudden Failure Electrolyte EC/EMC 3/7 + 10%FEC Cycling stopped before and after sudden failure GCMS studies performed on both LiPAA RT, 2.75V-4.35V, C/2(C/20) Unclamped 16

17 GCMS Before and After Sudden Failure Starting electrolyte EC/EMC 3/7 + 10%FEC Before failure (Cell 1) EC, EMC, FEC present After failure (Cell 2) EC, EMC, DMC, DEC present Half of the EMC is gone! FEC is gone! 17

18 Transesterification of EMC After FEC is gone: EMC DEC EMC DMC Which one is bad for the cycling? JES 161, A1167 (2014). 18

19 Co-solvent effect Electrolyte EC/[cosolvent] 3/7 + 5%FEC + 2% 3MZ Clearly DMC is worse DMC EMC DEC Transesterification of EMC accelerates failure through the production of DMC CMC/SBR binder RT, 2.75V-4.35V, C/2(C/20) Unclamped 19

20 FEC Consumption Starting electrolyte EC/EMC 3/7 + 10%FEC +2% 3MY FEC content decreases with cycle life Cell cycles CMC/SBR binder RT, 2.75V-4.35V, C/2(C/20) unclamped Cell cycles Cell 1 Cell 2 Cell 3 Cycles %FEC 5.3% 0.9% 0.0% EMC transesterification 0.2% 0.2% 48% EC/(EC+EMC+DEC+DMC) 26% 25% 24% Cell cycles 20

21 FEC Consumption Mechanism on Formation GC-TCD gas quantification in formation cycle Absence of ethylene may indicate FEC suppresses the reduction of EC H 2 consistent with ~20ppm level of water in electrolyte CO 2 consistent with 1 electron reduction of FEC as proposed by Leung et al. in JES 161, A213 (2014) O O O Li + + e Li + O O O F CO 2 O + F 21

22 FEC Consumption Time or Cycle? Accelerated failure test at 40 C, clamped, 3.0V-4.35V, C/2(C/20) Electrolyte EC/EMC 3/7 + 10%FEC Pouches cycled to various cycle numbers at C/2 and C/4 FEC consumed quantified by GCMS 22

23 FEC Consumption Time or Cycle? FEC consumed quantified by GCMS Graphite will also have a time dependent consumption of FEC CMC/SBR binder 40 C, 3V-4.35V, C/2(C/20) clamped Dominant factor is cycle number 23

24 Recap 3M Si alloy electrolyte reactivity is much better than pure Si Electrolyte reactivity is highly dependent on solvents and additives FEC is gradually consumed during cycling Consumption is dependent primarily on cycle number Additives can reduce FEC consumption (e.g. 3M additive 3MZ) When FEC is completely consumed the cell undergoes sudden failure Sudden failure is correlated with the transesterification of EMC But what is causing fade and sudden failure? 24

25 dv/dq Analysis Origin of Fade From fitting obtain Capacity loss from slippage (electrolyte reactivity) Capacity loss from mass loss (binder failure) Fitting done for C/20 and 40 C. Fit is decent. No significant material loss Fade is explained by relative slippage dv/dq fitting: JES 159, A1405 (2012) 25

26 10 C EIS Pouch Cell 40 C cycling Conditions EC/EMC 3/7 + 10%FEC CMC/SBR binder 40 C, 3V-4.35V, C/2(C/20) clamped EIS at 10 C with cells at 3.8 V (~50 DoD) Clear impedance increase with cycle number Symmetric cell data will show which electrode is responsible for this impedance rise EIS at 10 C CMC/SBR binder 40 C, 3V-4.35V, C/2(C/20) clamped 26

27 10 C EIS Symmetric Cells 40 C cycling Pouch cells discharged to 3.5V Corresponds to when Si is about half full Symmetric cells reconstructed with 10% FEC electrolyte EIS measured at 10 C Clearly most of the impedance increase is due to the positive electrode For more details of EIS with symmetric cells see JPS 251, 187 (2014) EIS at 10 C clamped 27

28 Recap 3M Si alloy electrolyte reactivity is much better than pure Si Electrolyte reactivity is highly dependent on solvents and additives FEC is gradually consumed during cycling Consumption is dependent primarily on cycle number Additives can reduce FEC consumption (e.g. 3M additive 3MZ) Cathode impedance rises When FEC is completely consumed the cell undergoes sudden failure Cathode impedance grows out of control Transesterification of EMC But what is causing cathode impedance, high voltage or anode interaction? 28

29 Limited Capacity Cycling s

30 Limited Capacity Cycling Full range cycling: 2.75V, 4.35V, C/2(C/20) Limited capacity cycling with 80% DoD and 50% DoD. (9 shallow, 1 full) Most of the alloy capacity is below ~3.6V (depends on cell build) 4.35V 9 shallow cycles 1 full cycle 2.75V Fixed capacity discharge 30

31 Limited Capacity Cycling Limited capacity cycling dramatically extends cycle life Real world impact The cells have cycled for approximately the same amount of time The cathode in 50% DoD has spent more time at high voltage What does GCMS reveal? 31

32 Limited Capacity Cycling What does GCMS reveal? Alloy cycling is required for FEC consumption 50% DoD 80% DoD Full range Cycles %FEC 1.9% 0.7% 0.0% EMC transesterification 0.2% 0.2% 48% 32

33 Time or Cycle? Time Dependence Full Cell Upper Voltage Dependence Cycle Dependence Graphite Electrolyte Reactivity LCO Electrolyte Reactivity Si Electrolyte Reactivity Strong Moderate Weak Weak Strong Weak Weak Weak Strong 33

34 Time or Cycle? Favors limited capacity cycling Time Dependence Full Cell Upper Voltage Dependence Cycle Dependence Graphite Electrolyte Reactivity LCO Electrolyte Reactivity Si Electrolyte Reactivity Strong Moderate Weak Weak Strong Weak Weak Weak Strong Favors fast, full DoD cycling 34

35 How does this compare to Graphite? Reputable 2.6Ah commercial LCO/Gr 3.0V 4.2V Charge: 0.7C(C/20) Discharge: C Fade is time based. 50% DoD cycling dies earlier Pouch cells with 3M Si alloy 2.75V 4.35V 50% DoD cycling last longer than graphite 18650! 35

36 Recap 3M Si alloy electrolyte reactivity is much better than pure Si FEC is gradually consumed during cycling Consumption is dependent primarily on cycle number Additives can reduce FEC consumption (e.g. 3M additive 3MZ) Cathode impedance rises Si must be cycled When FEC is completely consumed the cell undergoes sudden failure Cathode impedance grows out of control Transesterification of EMC Si electrolyte reactivity causes impedance rise on the cathode. 36

37 Looking Forward: Additive 3MZ Biggest barrier to using high levels of FEC in a cell is gassing at high T EC/EMC 3/7 + 10%FEC shows major gassing at 45 C No gassing after 170 cycles with 2% 3MZ C/10 CMC/SBR binder 45 C, 2.75V-4.35V, C/10 unclamped C/5 C/2 C/20 C 2% 3MZ 37

38 Looking Forward: Additive 3MZ Base electrolyte EC/EMC 3/7, 1M LiPF 6 Reduced FEC consumption with 3MZ Sudden failure occurs 100 cycles later LiPAA RT, 2.75V-4.35V, C/2(C/20) unclamped 38

39 Looking Forward: Impedance Control Preliminary results with additives to control impedance rise on the cathode Compare EC/EMC 3/7 + 10% FEC EC/EMC 3/7 + 10% X After 200 cycles impedance is lower with X than after 10 cycles with FEC 39

40 Conclusion 3M Si alloy electrolyte reactivity is much better than pure Si FEC is gradually consumed during cycling only if Si is cycled Consumption is dependent primarily on cycle number Shallow cycling reduces the life of a graphite-only cell and increases the life of a Si cell Electrolyte is key to controlling capacity retention in full cells with Sibased materials New 3M additive reduces FEC gassing and consumption (3MZ) Exploring additives to reduce cathode impedance growth caused by Si cycling 40

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