electrolytes Kristina Edström, Bertrand Philippe (Pau/Uppsala), Fredrik Lindgren, Remi Dedryvère, Håkan Rensmo and Danielle Ångström Lab.

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1 Silicon in Li-ion ion batteries and its reaction different electrolytes Kristina Edström, Bertrand Philippe (Pau/Uppsala), Fredrik Lindgren, Remi Dedryvère, Håkan Rensmo and Danielle Gonbeau Ångström Lab., Uppsala IPREM, Pau

2 Graphite LiC mah/g 818 mah/ml Negative electrodes for Li-ion batteries: why silicon? Silicon Li 15 Si mah/g 8335 mah/ml (Larcher et al, J. Mater. Chem.. 17 (2007) 3759 Low operating voltage ( V) Abundant and cheap Large volume expansion upon lithiation ( 270 %) Strategies to reduce this expansion: 15-20% - Nanosized particles higher battery - Optimisation of the potential window capacity - Use of binder (cellulose l Na-CMC, etc ) 2

3 Capa acity (mah/g of Si) The goals of this study: To understand the role of the lithium salt on SEI formation and on the stability of the silicon LiFSI > LiPF LiFSI 6 LiPF 6 6 ~ 50 cycles ~ 150 cycles Cycling between 1V V Limitation of the discharge capacity: 1200 mah/g * Nano-Si powder ( < 50 nm) 80% 200 CB (super P) 12% CMC-Na 8% Cycles * Oumellal et al, J. Mater. Chem. 21 (2011) 6201.

4 Other cut-off potential and other electrode Ch arge capacity [mah/g of S i] Electrode from Bernard Lestrier, see his talk tomorrow! 10% FEC Cycling 1200 mah/glifsi in 10% FEC, 1%VC, EC/DEC Henriklyte (cocktail electrolyte) LiPF6 EC:DEC LiPF 6 in EC/DEC Cycle number

5 1) The first cycle of a silicon electrode cycled with LiPF 6 in EC/DEC 2) Long term cycling of a silicon electrode cycled with LiPF 6 in EC/DEC 3) Comparison with a silicon electrode cycled with LiFSI (Li(N(SO 2 F) 2 ) 4) The first cycle cle of a silicon electrode cycled cled with the TDI salt in EC/DEC Han et al., J. Power Sources 2011, 196,

6 New Salts LiTDI Scheers et al.pccp 2011, 13, LiFSI salt and its benefits LiPF 6 LiFSI Han et al., J. Power Sources 2011, 196,

7 A non destructive depth-profiling using PES (P)ES/XPS platform From the extreme surface to the ʺbulkʺ Soft X-ray h = 100 to 1500 ev h = 230 ev from ~1nm to ~30nm Core peak Si2p Pristine electrode Al K ray h = ev h = 710 ev h = ev O 2 SiO Bu ulk Si-Si Hard X-ray h = 2000 to ev In-house XPS h = 2300 ev Sputtering is detrimental for the analysis!!! h = 6900 ev The SEI is too complex for angle resolved depth profiling Binding energy (ev)

8 The relation between photon energy and depth Prob bing dept th / nm SOXPES XPS HAXPES Kinetic energy / ev

9 PES analysis A safe transfer between the glovebox and the spectrometer is necessary In-house XPS (Kratis Axis Ultra) Glovebox Glovebox connected to the XPS At synchrotrons Transfer system Glovebox 9

10 1- First cycling of silicon with LiPF 6 First cycle (LiPF 6 ) 1,0 soaked electrode 0.9V V at 150 ma/g of Si SEI + Li Li 4 SiO 4 Si (crystalline) + /Li Potential vs Li + SiO 2 0,5 0.5V 0.5V 0.06V 0.1V 0,0 0.01V Capacity (mah/g of Si) + Li + Li -Li Li 2 O Discharge Discharge Full discharge Charge 0.5V 01V 0.1V 0.01V01V 09V 0.9V - SEI formation - SEI composition homogeneous and stable upon the 1st cycle - Growth of the SEI - Formation of Li 4 SiO 4 at the surface - Formation of Li 2 O at the surface - Li insertion: Si + x Li + +xe - Li Si the 1st cycle - Li insertion: Si + x Li + x e - Li extraction B. Philippe et al., Chem. Mater 2012, 24, Dissolution of a part of the SEI -Li 4 SiO 4 still present -Li 2 O disappears - Li extraction

11 2- Long term cycling with LiPF 6 Potential (V) 1,4 1,2 10 1,0 0,8 0,6 0, V at 700 ma/g of Si* 100 cycles Cap pacity (mah/g g) st 10 th 50 th discharge charge 100 th 1,00 0,95 0,90 0,85 Efficiency 0, , Capacity (mah/g of Si) Cycles 0,80 1- Four pre-cycles : Electrodes discharged to 500, 1000, 1500 and 2000 mah/g and charged to 0.9V ** 2- Cycling between 0.12V and 0.9V *700 ma/g of Si C/2 ** Li et al., Electrochem. Solid-State Lett. 2007, 10, A17.

12 SEI thickness when cycling with LiPF 6 Measured amount of Si in the first 5 nm of the surface: evolution as a function of cycle number 30 28% % 15 B. Philippe, R. Dedryvère, M. Gorgoi, H. Rensmo, D. Gonbeau, and K. Edström. Chem. Mater., 25 (2013) Stabilization of the SEI thickness 5 3.2% 0.7% 0.8% 0.6% 0.7% 0 Starting electrode 1st discharge 10th discharge 50th discharge 100th discharge 1014th discharge

13 The silicon content as a function cycle number Si 2p h = 1487 ev ,00 SiO 2 Li 4 SiO 4 1 st discharge Ah/g) Capacity (m discharge charge 0,95 0,90 0,85 Eff ficiency th discharge 50 th discharge Cycles 0,80 A new component appears ev = fluorinated environment of silicon Reaction of HF with SiO 2 SiO x F y SiO x F y H H SiO 2 Li 4 SiO th discharge O Si O +HF - H 2 O F Si F Binding energy (ev) O O O O

14 Surface composition after the 100 th cycle (LiPF 6 ) 100 th discharge (Si 2p) deeper analysis h = 230 ev h = 690 ev h = ev h = 2300 ev SiO x F y SiO Binding energy (ev) Binding energy (ev) Binding energy (ev) Binding energy (ev) The new fluorinated component is found at the extreme surface B. Philippe, R. Dedryvère, M. Gorgoi, H. Rensmo, D. Gonbeau, and K. Edström. Chem. Mater., 25 (2013) 394.

15 h = 1487 ev deeper analysis h = 2300 ev O1s (100 th discharge) h = 6900 ev 10 th discharge SEI oxygen Li 4 SiO 4 SEI oxygen ₓ 5 SEI ₓ 5 oxygen Li 2 O Li 2 O th discharge Li 4 SiO 4 Li 4 SiO th discharge B. Philippe, et al. Chem. Mater., 25 (2013) Binding energy (ev) Binding energy (ev) Binding energy (ev) Li 2 O disappears on cycling Li 2 O + HF LiF + H 2 O

16 A summary Long-term cycling (LiPF 6 ) SEI Li 4 SiO 4 SiO x F y SiO 2 Li 2 O 1 st discharge 10 th discharge 100 th discharge Formation of a fluorinated compound attack of SiO 2 by HF Li 2 O Li 2 O + HF LiF + H 2 O Stabile SEI thickness B. Philippe, et al. Chem. Mater., 25 (2013) 394.

17 Soaking time in LiPF 6 1M, EC:DEC (2:1) Chemistry or electrochemistry? SiO x F y SiO 2 Si (crystalline) 96 days Fluorination upon time (no SEI) 1 discharge until 500mAh/g of Si (4h) Li 4 SiO 4 Cyclin ngtime SEI Li 2 O Lix Si 182 days 100 discharges [ V] (11 days) Fluorination upon long cycling SiO x F y 41 days x Si The SEI protects the surface against fluorination by HF upon time (contact between electrode/electrolyte) It does not protect against fluorination by HF upon cycling

18 3 - Now the LiFSI comparison 1,0 ~ 360 mah/g of Si ~ 550 mah/g of Si LiFSI 1M, EC:DEC (2:1) LiPF 6 1M, EC:DEC (2:1) i + /Li) Pote ential (V vs L 0,9 0,8 0,7 06 0,6 0,5 0,4 0,3 0,2 LiFSI LiPF6 Coulombic efficiency 1 st cycle: LiPF 6 : ~ 85% LiFSI: ~ 89% 0,1 Higher polarization with LiPF 6 0, Capacity (mah/g of Si)

19 Electrochemical results Cycling between 1V V Limitation of the discharge capacity: 1200 mah/g * * Oumellal et al, J. Mater. Chem. 2011, 21, Si) (mah/g of LiFSI LiPF 6 ~ 50 cycles ~ 150 cycles LiFSI > LiPF 6 Capacity Cycles

20 SEI study: composition (C1s) LiPF 6 LiFSI h = 6900 ev h = 6900 ev CH 2 CO 3 CO 2 CO CH 2 Carbon black SEI not in the SEI CO 3 CO 2 CO Carbon black 1st discharge (0.9V-0.01V) 1 st discharge Carbonaceous species of the SEI No significant change of the SEI composition upon cycling LiPF6 10th discharge (0.9V V) 10 th discharge Growing of the SEI upon long term cycling with LiFSI 50th discharge (0.9V V) 100th discharge (0.9V V) 50 th discharge 100 th discharge Binding energy (ev) Binding energy (ev)

21 LiFSI 1st cycle: Li insertion into Si deeper analysis h = 230 ev h = 690 ev h = 2300 ev Starting electrode Si-Si 1 st discharge (0.01V) SiO 2 O y 1 st charge (0.9V) Binding energy gy( (ev) Binding energy (ev) Binding energy (ev) During the 1 st cycle, same species observed with LiPF 6 and LiFSI

22 Evolution of silicon upon long cycling (LiFSI) h = 2300 ev SiO 2 O y 1 st discharge No appearance of a fluorinated compound 10 th discharge SiO x F y compound 50 th discharge Absence of HF due to the higher stability towards hydrolysis of LiFSI 100 th discharge Binding energy (ev)

23 Conclusion SEI Li 4 SiO 4 SiO x F y LiPF 6 Li 2 O 1 st discharge 10 th discharge 100 th discharge SEI Li 4SiO 4 LiFSI 1 st discharge Li 2 O Li 2 O 10 th discharge 100 th discharge No fluorinated compound No Li 2 O consumption 2 p Absence of HF Growing of the SEI

24 Now compare with TDI results 1, M LiPF 6 (EC:DEC 2:1 by volume) ~0.6M LiTDI (EC:DMC 1:2 by weight) vs Li + /Li) Potential (V 0,9 0,8 0,7 0,6 05 0,5 0,4 0,3 0,2 LiFSI LiPF6 Cell poten ntial [V] LiTDI 01 0, , Capacity (mah/g of Si) Capacity [mah g -1 ]

25 Summary of the first cycle with the TDIsalt Si SiO 2 Si SiO 2 SiO 2 Si SEI SEI Si SiO 2 SEI Si SiO 2 SEI

26 Degradation of silicon anodes during cycling a binder or an electrolyte problem? YES Silicon electrodes is also an enginnering problem Bernard Lestriez IMN-CNRS and Marek Marcinek WUT

27 Thank you for your attention!

28 Electrode preparation and battery assembly Electrode formulation (mechano synthesis): Nano-Si powder ( < 50 nm) 80% CB (super P) 12% CMC-Na 8% Solvent: H 2 O/ethanol (70:30) Electrolyte: LiPF 6 1M, EC:DEC (2:1) "coffee bag" assembly Lithium electrode Nickel connectors separator (polyethylene, l Solupor ) Si electrode 28