Supporting Information Atomic Layered Coating Enabled Ultrafast ...
Supporting Information Atomic Layered Coating Enabled Ultrafast Surface Kinetics of Silicon Electrodes in Lithium Ion Batteries Juchuan Li,†‡* Xingcheng Xiao,‡* Yang-Tse Cheng,† Mark W. Verbrugge‡
†
Department of Chemical & Materials Engineering, University of Kentucky, Lexington, KY 40506, USA ‡ Chemical Sciences and Materials Systems Laboratory, General Motors Research and Development Center, Warren, MI 48090, USA
* Emails: [email protected]; [email protected]
1. The Coulombic efficiency of Al2O3-ALD coated Si electrodes. The Coulombic efficiency, defined as the ratio of de-lithiation capacity to lithiation capacity, of Al2O3-ALD coated Si electrodes cycled galvanostaticly (C/10 rate) is compared in Figure S1. A clear trend can be seen that Al2O3 coatings help stabilizing the Coulombic efficiency of the electrodes during cycling, while the Coulombic efficiency of bare Si shows gradual decay. Coulombic efficiency can be strongly affected by surface chemistry and the electrolyte, as well as other factors such as unwanted side reactions in the electrolyte. 1, 2 Thus, a quantitative comparison of the effect of Al2O3 coating on the Coulombic efficiency requires careful design of the electrochemical cells and precise measurement of the current and charge, 1, 3 which is crucial in evaluating the long-term cycling performance of electrodes and is beyond the scope of this work.
1
Figure S1. Coulombic efficiency of Al2O3-ALD coated Si electrodes cycled under a rate of C/10.
2. Differential capacity-voltage profiles of Al2O3-ALD coated Si electrodes. Differential capacity-voltage (dQ/dV vs. V) profiles for the first cycle (C/10 rate) are compared in Figure S2. R1 and R2 denote two reduction peaks during lithiation, and O1 and O2 are the corresponding oxidation peaks during de-lithiation. The peak voltage is related to the sum of ohmic polarization, charge-transfer polarization, and diffusion polarization. 4 Take R1 as an example. The peak potential of R1 for bare Si is 0.258V; and it is pushed down to 0.226 V after 2 atomic layers of Al2O3 are coated on Si. Increasing Al2O3 coating thickness leads to further decay of the peak potential. Other reduction and oxidation peaks show similar trends. R0 represents the major peak for SEI formation. The formation of SEI film on bare Si starts at 0.7V during lithiation and has a peak at 0.54V. By adding Al2O3 coating, the potential range of SEI formation is shifting downwards. It is interesting that when Al2O3 coating is 20 layers thick, there is no obvious peak between 0.7 and 0.3 V, indicating that electrolyte decomposition (or reduction) has been significantly suppressed due to the Al2O3 coating, which substantially blocks electrons from reaching the electrolyte. Similar observations have been made elsewhere. 5
2
Figure S2. dQ/dV vs. V plot of ALD-Al2O3 coated Si thin films.
3. The modified PITT and the exchange current density. The modified PITT treats systems that are controlled by both diffusion and interfacial kinetics. Three parameters, the electrochemical Biot number , the diffusion coefficient , and the exchange current density can be obtained simultaneously from a single measurement. 6 When the interfacial kinetics are fast compared to diffusion processes, the system is limited only by diffusion and can be analyzed using the original PITT developed by Huggins et al. 6, 7 (corresponding to an infinitely large electrochemical Biot number). The Al2O3-ALD coating can change the overall charge-transfer process by modifying the surface chemistry and by suppressing in situ SEI formation. In this work, the Al2O3 coating (up to 4.4 nm) is much thinner than the Si electrode thickness (100 nm), and we assume that the main effect of the Al2O3-ALD coating is modifying the interfacial kinetics rather than the diffusion profile. This allows us to use the modified PITT for analyzing the experimental data. An example of the current response under PITT measurement and the fitted curve is given is Figure S3 (20 ALD).
3
Figure S3. Experimental data and fitting results for PITT measurement (20 ALD).
The definition of the electrochemical Biot number
is given as 6
|
|
(S1) | is the exchange current density at the electrode
where is the thickness of the electrode, surface,
| is the potential gradient with respect to the lithium concentration at the electrode
surface, is the diffusion coefficient, is the gas constant, and The exchange current density can be calculated by
is the absolute temperature.
| | (S2)
4
The value of
| is obtained from quasi-equilibrium cycling (C/300) profiles of Si vs. Li
6
and
considering the linear volume expansion of Li in LiSi alloys. 8 A value of 7.95 V (mol cm-3)-1 is used in this work.
4. The reaction rate constant. The reaction rate constant of lithiation/de-lithiation of Si electrodes can also be determined by the modified PITT measurement. We consider the electrochemical reaction of the type | (S3) which represents the lithiation and de-lithiation of alloying-type LIB electrodes. The exchangecurrent density, , is related to the anodic and cathodic rate constants, and , respectively:
(S4) Here is the concentration of lithium in solution, is the equilibrium Li concentration in Li-Si electrodes, is the Faraday constant, and is the symmetry factor. The standard rate constant transfer process: 9
with units of [cm s-1] is often used to assess the interfacial charge-
(S5) For the PITT experiments, we provide a small potential excitation step, giving rise to small changes in the lithium concentration within the Si from their initial concentration values and no significant change to the lithium ion concentration in the electrolyte phase (0.001 mol cm-3 of Li+ from the fully dissociated LiPF6 salt). For the exchange current density values provided in Table 1, the equilibrium Li concentration in electrodes prior to the PITT experiment corresponded -3 to 0.0646 mol cm . This value is calculated from the state of charge (SOC) in WEs for the PITT measurement and by considering linear volume expansion of Si electrodes during lithiation. 10, 11 Using these values and the above equation, and assuming the symmetry factor is 0.5, we calculated the standard rate constant values in Table 1. The rate constants obtained for lithiation/de-lithiation of Al2O3 coated Si electrodes are in the range of 4.8 × 10-8 to 3.1 × 10-7 cm s-1. To the best of our knowledge, this is first reported values 5
of rate constant involved in Si as LIB anodes. The lithiation/de-lithiation of Si is relatively sluggish compared to many other reactions in aqueous solutions, such as Na+/Na(Hg). 9
5. Surface morphology of Si films. The surface morphology of 100 nm Si thin film on Cu substrate is shown in Figure S4. The substrate is roughened by electrodepositing a Cu layer for improved adhesion.
Figure S4. SEM image of pristine 100 nm Si on Cu.
6
References (1)
(2)
(3) (4)
(5) (6)
(7)
(8)
(9) (10)
(11)
Sinha, N. N.; Burns, J. C.; Sanderson, R. J.; Dahn, J. Comparative Studies of Hardware Corrosion at High Potentials in Coin-Type Cells with Non Aqueous Electrolytes. J. Electrochem. Soc. 2011, 158, A1400-A1403. Chen, X. L.; Xu, W.; Xiao, J.; Engelhard, M. H.; Ding, F.; Mei, D. H.; Hu, D. H.; Zhang, J.; Zhang, J. G. Effects of Cell Positive Cans and Separators on the Performance of HighVoltage Li-Ion Batteries. J. Power Sources 2012, 213, 160-168. Smith, A. J.; Burns, J. C.; Xiong, D.; Dahn, J. R. Interpreting High Precision Coulometry Results on Li-ion Cells. J. Electrochem. Soc. 2011, 158, A1136-A1142. Liu, J.; Yang, Y. F.; Yu, P.; Li, Y.; Shao, H. X. Electrochemical Characterization of LaNi5-xAlx (x=0.1-0.5) in the Absence of Additives. J. Power Sources 2006, 161, 14351442. Ahn, D.; Xiao, X. C. Extended Lithium Titanate Cycling Potential Window with Near Zero Capacity Loss. Electrochem. Commun. 2011, 13, 796-799. Li, J. C.; Xiao, X. C.; Yang, F. Q.; Verbrugge, M. W.; Cheng, Y. T. Potentiostatic Intermittent Titration Technique for Electrodes Governed by Diffusion and Interfacial Reaction. J. Phys. Chem. C 2012, 116, 1472-1478. Wen, C. J.; Ho, C.; Boukamp, B. A.; Raistrick, I. D.; Weppner, W.; Higgins, R. A. Use of Electrochemical Methods to Determine Chemical-Diffusion Coefficients in Alloys: Application to 'LiAl'. Int. Mater. Rev. 1981, 5, 253-268. Timmons, A.; Dahn, J. R. Isotropic Volume Expansion of Particles of Amorphous Metallic Alloys in Composite Electrodes for Li-Ion Batteries. J. Electrochem. Soc. 2007, 154, A444-A448. Bard, A. J.; Faulkner, L. R., Electrochemical Methods & Applications. 2nd Edition. 2000, New York: Wiley. Beaulieu, L.; Hatchard, T.; Bonakdarpour, A.; Fleischauer, M.; Dahn, J. Reaction of Li with Alloy Thin Films Studied by in situ AFM. J. Electrochem. Soc. 2003, 150, A1457A1464. He, Y.; Yu, X.; Li, G.; Wang, R.; Li, H.; Wang, Y.; Gao, H.; Huang, X. Shape Evolution of Patterned Amorphous and Polycrystalline Silicon Microarray Thin Film Electrode Caused by Lithium Insertion and Extraction. J. Power Sources 2012, 216, 131-138.
7
†
Department of Chemical & Materials Engineering, University of Kentucky, Lexington, KY 40506, USA ‡ Chemical Sciences and Materials Systems Laboratory, General Motors Research and Development Center, Warren, MI 48090, USA
* Emails: [email protected]; [email protected]
1. The Coulombic efficiency of Al2O3-ALD coated Si electrodes. The Coulombic efficiency, defined as the ratio of de-lithiation capacity to lithiation capacity, of Al2O3-ALD coated Si electrodes cycled galvanostaticly (C/10 rate) is compared in Figure S1. A clear trend can be seen that Al2O3 coatings help stabilizing the Coulombic efficiency of the electrodes during cycling, while the Coulombic efficiency of bare Si shows gradual decay. Coulombic efficiency can be strongly affected by surface chemistry and the electrolyte, as well as other factors such as unwanted side reactions in the electrolyte. 1, 2 Thus, a quantitative comparison of the effect of Al2O3 coating on the Coulombic efficiency requires careful design of the electrochemical cells and precise measurement of the current and charge, 1, 3 which is crucial in evaluating the long-term cycling performance of electrodes and is beyond the scope of this work.
1
Figure S1. Coulombic efficiency of Al2O3-ALD coated Si electrodes cycled under a rate of C/10.
2. Differential capacity-voltage profiles of Al2O3-ALD coated Si electrodes. Differential capacity-voltage (dQ/dV vs. V) profiles for the first cycle (C/10 rate) are compared in Figure S2. R1 and R2 denote two reduction peaks during lithiation, and O1 and O2 are the corresponding oxidation peaks during de-lithiation. The peak voltage is related to the sum of ohmic polarization, charge-transfer polarization, and diffusion polarization. 4 Take R1 as an example. The peak potential of R1 for bare Si is 0.258V; and it is pushed down to 0.226 V after 2 atomic layers of Al2O3 are coated on Si. Increasing Al2O3 coating thickness leads to further decay of the peak potential. Other reduction and oxidation peaks show similar trends. R0 represents the major peak for SEI formation. The formation of SEI film on bare Si starts at 0.7V during lithiation and has a peak at 0.54V. By adding Al2O3 coating, the potential range of SEI formation is shifting downwards. It is interesting that when Al2O3 coating is 20 layers thick, there is no obvious peak between 0.7 and 0.3 V, indicating that electrolyte decomposition (or reduction) has been significantly suppressed due to the Al2O3 coating, which substantially blocks electrons from reaching the electrolyte. Similar observations have been made elsewhere. 5
2
Figure S2. dQ/dV vs. V plot of ALD-Al2O3 coated Si thin films.
3. The modified PITT and the exchange current density. The modified PITT treats systems that are controlled by both diffusion and interfacial kinetics. Three parameters, the electrochemical Biot number , the diffusion coefficient , and the exchange current density can be obtained simultaneously from a single measurement. 6 When the interfacial kinetics are fast compared to diffusion processes, the system is limited only by diffusion and can be analyzed using the original PITT developed by Huggins et al. 6, 7 (corresponding to an infinitely large electrochemical Biot number). The Al2O3-ALD coating can change the overall charge-transfer process by modifying the surface chemistry and by suppressing in situ SEI formation. In this work, the Al2O3 coating (up to 4.4 nm) is much thinner than the Si electrode thickness (100 nm), and we assume that the main effect of the Al2O3-ALD coating is modifying the interfacial kinetics rather than the diffusion profile. This allows us to use the modified PITT for analyzing the experimental data. An example of the current response under PITT measurement and the fitted curve is given is Figure S3 (20 ALD).
3
Figure S3. Experimental data and fitting results for PITT measurement (20 ALD).
The definition of the electrochemical Biot number
is given as 6
|
|
(S1) | is the exchange current density at the electrode
where is the thickness of the electrode, surface,
| is the potential gradient with respect to the lithium concentration at the electrode
surface, is the diffusion coefficient, is the gas constant, and The exchange current density can be calculated by
is the absolute temperature.
| | (S2)
4
The value of
| is obtained from quasi-equilibrium cycling (C/300) profiles of Si vs. Li
6
and
considering the linear volume expansion of Li in LiSi alloys. 8 A value of 7.95 V (mol cm-3)-1 is used in this work.
4. The reaction rate constant. The reaction rate constant of lithiation/de-lithiation of Si electrodes can also be determined by the modified PITT measurement. We consider the electrochemical reaction of the type | (S3) which represents the lithiation and de-lithiation of alloying-type LIB electrodes. The exchangecurrent density, , is related to the anodic and cathodic rate constants, and , respectively:
(S4) Here is the concentration of lithium in solution, is the equilibrium Li concentration in Li-Si electrodes, is the Faraday constant, and is the symmetry factor. The standard rate constant transfer process: 9
with units of [cm s-1] is often used to assess the interfacial charge-
(S5) For the PITT experiments, we provide a small potential excitation step, giving rise to small changes in the lithium concentration within the Si from their initial concentration values and no significant change to the lithium ion concentration in the electrolyte phase (0.001 mol cm-3 of Li+ from the fully dissociated LiPF6 salt). For the exchange current density values provided in Table 1, the equilibrium Li concentration in electrodes prior to the PITT experiment corresponded -3 to 0.0646 mol cm . This value is calculated from the state of charge (SOC) in WEs for the PITT measurement and by considering linear volume expansion of Si electrodes during lithiation. 10, 11 Using these values and the above equation, and assuming the symmetry factor is 0.5, we calculated the standard rate constant values in Table 1. The rate constants obtained for lithiation/de-lithiation of Al2O3 coated Si electrodes are in the range of 4.8 × 10-8 to 3.1 × 10-7 cm s-1. To the best of our knowledge, this is first reported values 5
of rate constant involved in Si as LIB anodes. The lithiation/de-lithiation of Si is relatively sluggish compared to many other reactions in aqueous solutions, such as Na+/Na(Hg). 9
5. Surface morphology of Si films. The surface morphology of 100 nm Si thin film on Cu substrate is shown in Figure S4. The substrate is roughened by electrodepositing a Cu layer for improved adhesion.
Figure S4. SEM image of pristine 100 nm Si on Cu.
6
References (1)
(2)
(3) (4)
(5) (6)
(7)
(8)
(9) (10)
(11)
Sinha, N. N.; Burns, J. C.; Sanderson, R. J.; Dahn, J. Comparative Studies of Hardware Corrosion at High Potentials in Coin-Type Cells with Non Aqueous Electrolytes. J. Electrochem. Soc. 2011, 158, A1400-A1403. Chen, X. L.; Xu, W.; Xiao, J.; Engelhard, M. H.; Ding, F.; Mei, D. H.; Hu, D. H.; Zhang, J.; Zhang, J. G. Effects of Cell Positive Cans and Separators on the Performance of HighVoltage Li-Ion Batteries. J. Power Sources 2012, 213, 160-168. Smith, A. J.; Burns, J. C.; Xiong, D.; Dahn, J. R. Interpreting High Precision Coulometry Results on Li-ion Cells. J. Electrochem. Soc. 2011, 158, A1136-A1142. Liu, J.; Yang, Y. F.; Yu, P.; Li, Y.; Shao, H. X. Electrochemical Characterization of LaNi5-xAlx (x=0.1-0.5) in the Absence of Additives. J. Power Sources 2006, 161, 14351442. Ahn, D.; Xiao, X. C. Extended Lithium Titanate Cycling Potential Window with Near Zero Capacity Loss. Electrochem. Commun. 2011, 13, 796-799. Li, J. C.; Xiao, X. C.; Yang, F. Q.; Verbrugge, M. W.; Cheng, Y. T. Potentiostatic Intermittent Titration Technique for Electrodes Governed by Diffusion and Interfacial Reaction. J. Phys. Chem. C 2012, 116, 1472-1478. Wen, C. J.; Ho, C.; Boukamp, B. A.; Raistrick, I. D.; Weppner, W.; Higgins, R. A. Use of Electrochemical Methods to Determine Chemical-Diffusion Coefficients in Alloys: Application to 'LiAl'. Int. Mater. Rev. 1981, 5, 253-268. Timmons, A.; Dahn, J. R. Isotropic Volume Expansion of Particles of Amorphous Metallic Alloys in Composite Electrodes for Li-Ion Batteries. J. Electrochem. Soc. 2007, 154, A444-A448. Bard, A. J.; Faulkner, L. R., Electrochemical Methods & Applications. 2nd Edition. 2000, New York: Wiley. Beaulieu, L.; Hatchard, T.; Bonakdarpour, A.; Fleischauer, M.; Dahn, J. Reaction of Li with Alloy Thin Films Studied by in situ AFM. J. Electrochem. Soc. 2003, 150, A1457A1464. He, Y.; Yu, X.; Li, G.; Wang, R.; Li, H.; Wang, Y.; Gao, H.; Huang, X. Shape Evolution of Patterned Amorphous and Polycrystalline Silicon Microarray Thin Film Electrode Caused by Lithium Insertion and Extraction. J. Power Sources 2012, 216, 131-138.
7
