Supplementary Information

High Capacity Utilization of Li Metal Anodes by Application of

Celgard® Separator-Reinforced Ternary Polymer Electrolyte

Mengyi Zhang 1, Alicia Lei Gui 1, Wei Sun 1,Z, Jens Becking 1, Olga Riedel 1, Xin He 2, Debbie

Berghus 1, Vassilios Siozios 1, Dong Zhou 1, Tobias Placke 1, Martin Winter 1,2, and Peter Bieker 1,Z

1 MEET Battery Research Center, Institute of Physical Chemistry, University of Münster,

Corrensstr. 46, 48149 Münster, Germany

2 Helmholtz-Institute Münster (HI MS), IEK-12, Forschungszentrum Jülich GmbH, Corrensstr.

46, 48149 Münster, Germany

zWei Sun: wei.sun@uni-muenster.de

zPeter Bieker: peter.bieker@uni-muenster.de

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SI Appendix

Appendix S1

A method to estimate the Filling Fraction (FF) of polymer electrolyte in the voids of the separator . The

porosity defined as the ratio of void volume to apparent geometric volume.1 For Celgard®2500 PP

monolayer separator, the porosity ϕ is calculated as:

ϕ (% )=(1−W separator / ρPP

V separator)× 100 % , [A1]

where W separator is the weight of dry separator, ρPP the density of semi-crystalline polypropylene and

V separator apparent geometric volume of a 25 µm thick separator. ρPP is proposed to be estimated according

to the reported degree of crystallinity (X c , %) of this type of separator (X c=35 2),3 that is:

ρPP=100

X c / ρc+(100−Xc )/ ρa , [A2]

where ρc and ρa terms refer to the density of the crystalline phase (0.936-0.946 g/cm3) and amorphous

phase (0.850-0.855 g/cm3).4 Therefore, the value of ρPP is accordingly estimated to be ≈0.88 g/cm3. Then,

the Filling Fraction (FF) of TSPE in separator can be defined as:

FF=W TSPE/ ρTSPE

ϕV separator× 100 % , [A3]

where W TSPE is the weight of TSPE inside the voids of separator, ρTSPE measured density of TSPE with

value of 1.40 g/cm3. By measuring the average thickness of CgTSPE samples (≈47 µm) and average

weight of CgTSPE and Celgard®2500 separator with diameter of 1.6 cm (11.94 mg and 1.98 mg

respectively), the value of ϕCelgard ®2500is 55% and FFCgTSPE is estimated to be ≈97%.

The Uptake ability (η) was measured by recording the average weight of CgTSPE and Celgard®2500

separator, then η was calculated as:

η=(W CgTSPE−W separator)/W separator ×100 % , [A4]

where W CgTSPE and W separator are the average weight of sample CgTSPE and Celgard®2500 separator.

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Appendix S2

The relationship of the elastic modulus (E) between non-porous polymers, e.g. polypropylene and

microporous polymer separators can be explained by the ROM estimation (Voigt model) of fiber-

reinforced polymers proposed by Corey T. Love.5 Approximately, the relationship between the elastic

modulus of TSPE and Celgard in the CgTSPE sample is considered based on the Voigt model without

accounting the difference of pore dimensions along MD or TD, as illustrated in Figure SA1:

ECelgard , MD /TD=(1−ϕCelgard ) ∙ EPolymer ,MD /TD, [A5]

E Interlayer , MD /TD=ECelgard , MD /TD+ϕ∙ ETSPE , [A6]

ECgTSPE , MD /TD=V interlayer ∙E interlayer , MD /TD+V TPSE 1+ 2 ∙ ETSPE , MD /TD , [A7]

where V is volume fraction of interlayer or TSPE, which is determined by measuring the thickness of the

membranes (average thickness of measured CgTSPE is 40 µm). Nevertheless, the accuracy of this model,

based on ideal assumptions is limited to some extent, therefore, the calculated results should be only

interpreted qualitatively.

Figure SA1. Illustration of 3D structure of CgTSPE membrane assumed for Voigt model (MD: machine

direction, TD: transverse direction).

As shown in Table 2, the Celgard domains inside CgTSPE are additionally compared with untreated

Celgard®2500 separator. It is found that the predicted moduli of Celgard inside the CgTSPE are similar to

that of Celgard®2500 along TD dimension. In contrast, those are much larger along MD dimension. It is

likely due to the possible scenario that the partially cross-linked PP nanofibers-amorphous PEO micro-

network6 along MD inside the micro-voids (containing aligned PP nanofibers1) contributes to the

additional elastic modulus.

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Appendix S3

In the model of the membrane’s 3D structure (Figure SA1), Li+ mitigate through three layers of

membranes in series, meaning that the ionic conductance κ of CgTSPE and TSPE should be,

κCgTSPE−1=κTSPE 1

−1+κTSPE2−1+κ interlayer

−1 , [A8]

κCgTSPE=σCgTSPEA

LCgTSPE , [A9]

κTSPE=σTSPEA

LTSPE , [A10]

where σ TSPE or σ CgTSPE is measured as the ionic conductivity of the TSPE or CgTSPE membrane with a

thickness LTSPE of 60 µm and LCgTSPE of 47 µm, respectively, and A area of the punched membrane. It is

also reasonable to assume that σ TSPE=σ TSPE 1=σTSPE 2, and, therefore, equation A8 can be expressed as

after rearrangement,

σ TSPE

σCgTSPE=

LTSPE1+LTSPE 2

LCgTSPE+

NM Linterlayer

LCgTSPE , [A11]

by introducing the MacMullin number N M=σ TSPE/σ interlayer in this polymer electrolyte system, where

LCgTSPE=LTSPE1+LTSPE2+Linterlayer. In Figure SA2, the temperature dependence of the MacMullin number

is illustrated: N M can be regarded as a constant number ≈4.5 in the temperature window of 20 °C and

80 °C, which is similar to the reported value (4.5 ± 0.3) for Celgard® 2500 measured in liquid

electrolytes,7 as determined by impedance measurements.

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Figure SA2. Temperature dependence of the MacMullin number (NM) of Celgard® 2500 for TSPE in the

range of 0 °C and 80 °C.

Appendix S4

The Butler-Volmer equation can describe the current-potential relationship in the case of small current

density j for the reaction Li+¿+ e−¿⇌ Li¿ ¿, and the transfer coefficient is supposed to be equal to 0.5 if only a

simple electron transfer process is involved. In the common low DC polarization studies for liquid

electrolyte systems, the Ohmic drop can be ignored, but due to the relatively low ionic conductivity of

polymer electrolytes, the contribution of the Ohmic drop should be considered. When the electrode

overpotential ηe is smaller than RT/nF (≈29 mV at 60 °C), the Butler-Volmer equation can be expressed in

a linear form,8

j=− jexchange

F ηe

RT , [A12]

where F is Faraday constant, R is the universal gas constant and T is the temperature.

The measured cell voltage (with the total overvoltage, ηt) in symmetric Lipolymer electrolyteLi coin

cell by applying a current density j is influenced by contributions from the overpotential ηa at the anode,

from the overpotential ηc at the cathode and also from the voltage drop ( jA Rb, here A is the area of

electrode and Rb is the bulk resistance). Assuming that the reaction proceeds as simple electron transfer

process at the interface between Li metal and the polymer electrolyte without any other complex

mechanism (e.g. decomposition of polymer electrolyte) involved,

ηt=(ηc−ηa )+ jA Rb , [A13]

ηa=−ηc , [A14]

Thus, equation A12 becomes

ηt

j= −2 RT

F jexchange+ A Rb , [A15]

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SI Figures and Tables

Figure S1. Thermomechanical behavior of membranes under ultra-low static tensile load (0.005 N) with a

constant heating rate (5 K/min). Load is applied in the MD in the samples of Celgard® 2500 and CgTSPE.

The inset shows the shrinkage onset temperatures.

Celgard®2500 is a typical anisotropic monolayer separator with MD and TD.1 Especially in MD

orientation, it shows characteristic one-dimensional shrinkage responding to elevated temperature. As

shown in Figure S1, as the temperature increases until the membranes experience rapture, one-

dimensional changes undergo quasi strain-free phase under ultra-low static tensile load. As the

temperature goes higher, it reaches shrinkage onset temperature (T shrink), which is defined as the

temperature where −2% strain occurs in this study (see the inset view). The improvement of T shrink in

CgTSPE can be explained by the generated counter stresses from TSPE (in the voids and on the surface)

that are against relieving internal stresses in the membrane. However, at temperatures above 150 °C, this

advantage is not dominant anymore due to dramatically decreased stresses from TSPE. Starting from 160

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°C, which is near the melting temperature of Celgard®2500, the CgTSPE membrane experiences

deformation, elongation (positive strain), and finally rupture.

Figure S2. Thermal shrinkage in MD direction at different temperatures via DMA under a low static force

of 0.01 N, which could simulate placing the sample without tension; (a) 60 °C for 60 min; (b) 120 °C for

30 min.

Figure S3. Representative stress-strain curves measured via DMA with constant strain ramp of 6%/min,

(a-c) obtained at 20 °C, (d-f) obtained at 60 °C; dimensions of (a) (d) TD and (b)(e) MD for sample

Celgard®2500 and CgTSPE; (c) and (f) are from sample TSPE which is mechanically isotropic.

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(d) (e) (f)

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Table S1. Comparative results of the exchange current densities (jexchange) and limiting current densities

(jlim) in the fresh or aged samples of Li|TSPE|Li and Li|CgTSPE|Li at 60 °C. Aged samples were obtained

after storage for seven days at 60 °C.

Current Density

[mA/cm2]TSPE-fresh TSPE-aged CgTSPE-fresh CgTSPE-aged

jexchange 0.10 0.08 0.06 0.03

jlim N.A.a) N.A.a) 0.36 0.34

a) Due to dendrite induced short-circuit in TSPE sample, the data is not available to obtain corresponding

limiting current densities.

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Figure S4. (a) High frequency region of Nyquist plots as a function of time in the first 6 days for the

samples CgTSPE and TSPE; (b) Voltage profiles of Li||Li at different rate various from 0.05 mA/cm 2 to

0.25 mA/cm2 with constant areal capacity of 0.05 mAh/cm2; (c) Nyquist plot of EIS measurement after

rate test.

As shown in Figure S4a, the bulk resistance Rb of the symmetric cell, derived from the intercept at the

high frequency region of the Nyquist plot, decreases during thermal ageing. This decrease could be

explained by the reason of improved effective contact area between the polymer electrolyte and the Li

metal surface with a certain roughness happening under high pressure.

When comparing the voltage of Li|TSPE|Li and Li|CgTSPE|Li in Figure S4b with different current

density ranging from 0.05 mA/cm2 to 0.25 mA/cm2, Li|CgTSPE|Li shows higher overvoltage. When

applied current density approaching to the limiting current density (≈0.36 mA/cm2), the concentration

polarization is becoming more prominent and overvoltage is on the increase. From the EIS in Figure S4c,

although lower interfacial/interphasial impedance in Li|CgTSPE|Li cell is observed, the bulk resistance Rb

of Li|CgTSPE|Li shows a higher value than that in Li|TSPE|Li. Therefore, in Li||Li cells with CgTSPE and

TSPE, we can conclude that difference in bulk resistance and concentration polarization could be the main

factors causing the higher polarization in Li|CgTSPE|Li.

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Figure S5. Short-circuiting time vs. current density curve, in which the samples in this work are compared

with PEO-based solid polymer electrolytes from published reports9-12, areal capacity utilization of

5 mAh/cm2 is regarded as the baseline for practical application of Li metal batteries.

Figure S6. Post mortem analysis of the TSPE surface facing to Cu foil in Li|TSPE|Cu coin cell after

galvanostatic cycling; the enlarged view of the SEM image on the right side shows that Li metal dendrites

grow into the TSPE.

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Figure S7. XPS characterization of pristine roll-pressed Li metal and roll-pressed Li metal after 50 cycles

in LiCgTSPELi cell; XPS spectra of C 1s, O 1s and F 1s region from samples (a) pristine roll-pressed Li

metal and (b) cycled roll-pressed Li metal.

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LiF

CF3

Li2CO3

LiOH

Li2CO3

COR

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CH/CC

COORLi2CO3

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COOR

Li2O

Li2OLi2CO3

COH

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Table S2. Comparative results of components ratio in atomic percentages (at. %) on the surface of the Li

metal anode from XPS measurements.

ComponentFresh Li metal

Li metal

after cycling and washingCgTSPE after cycling

C 1s O 1s F 1s C 1s O 1s F 1s S 2p C 1s O 1s F 1s S 2p N 1s

C-H, C-C 46.4 - - 44.9 - - - 43.5 - - - -

COH 25.4 0a - - - - - - - - -

LiOH - 5.2 - - - - - - - - - -

Li2O - 0.2 - - 0.7 - - - - - - -

COOR 1.1 0a - 2.7 0a - - 2.1 0a - - -

COR - - - 9.6 3.2a - - 17.0 10.7a - - -

Li2CO3 0.6 13.4a - 4.1 18.2a - - - - - - -

LiF - - - - - 0.3 - - - - - -

TFSI - - - 0b 0b 1.5 0.4 3.0 5.3 11.9 3.0 1.8

a) Due to overlapping of binding energy of organic CO bond and metal carbonates in C 1s, the value is

considered unreliable.13

b) Due to detection limit of XPS (0.1 – 1 at. %), peak is not detectable.

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Figure S8. Proposed model of Li metal deposition and dissolution against Cu foil during cycling.

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Figure S9. Representative SEM images of Li metal surface (a) after 500 h deposition (b) after 50 h

dissolution (c) after 50 h dissolution and 50 h deposition with a current density of 0.1 mA/cm 2 at 60 °C in

the CgTSPE sample.

In Figure S9a, the compressed tip of globular Li deposit with 50 mAh/cm2 shows that the PP polymer is

stiff enough to compress the dendrite penetration in the vertical direction, thus in other words the

mechanism, leading to the short-circuit (Figure 4d) of the CgTSPE membrane, could be the brittle

fracture5 rather than the piercing effect caused by globular deposits. Parallel to the electrodeposition, the

Li metal surface after dissolution with 5 mAh/cm2 capacity utilization is also investigated (Figure S9b). It

is found that the dissolution process on the fresh Li metal is much more homogeneous compared with the

deposition process. However, it differs in microscale, especially more dissolution at grain boundaries and

formed terrace-like morphology is observed.14 This could be the mechanism that the surface around the

grain boundaries and terrace are more energetically favorable according to the self-diffusion barriers

theory, proposed by Markus Jaeckle and Axel Gross.15, 16

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Figure S10. Operando electrochemical dilatometry analysis of LiTSPELi configuration at 60 °C with a

current density of 0.1 mA/cm2 and an areal capacity utilization of 3 mAh/cm2, height change

superimposed on voltage profile during first deposition/dissolution cycle, in which the onset of short-

circuit is indicated.

As shown in Figure S10, the height will decrease at the beginning with a rate of −0.6 µm/h,

corresponding to 6 µm per 1 mAh/cm2, which strongly indicates Scenario 3 (see manuscript), that is, a

wild growth of Li metal dendrites into polymer electrolyte until short-circuit occurs, when the cell voltage

drops suddenly. After short-circuiting, the height keeps constant corresponding to no further deposition

and dissolution reactions.

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References

1. P. Arora and Z. Zhang, Chem. Rev., 104 (10), 4419-4462 (2004).

2. C. T. Love, J. Power Sources, 196 (5), 2905-2912 (2011).

3. R. P. Sheldon, J. Polym. Sci., Part B: Polym. Lett., 1 (12), 655-657 (1963).

4. D. V. Howe, in Polymer Data Handbook, J. E. Mark, ed., p. 780–786, Oxford University Press,

New York, USA, (1999).

5. C. T. Love, J. Electrochem. Energy Convers. Storage, 13 (3), 031004-031004-031005 (2016).

6. P. Zamotaev, E. Shibirin, and Z. Nogellova, Polym. Degrad. Stab., 47 (1), 93-107 (1995).

7. J. Landesfeind, J. Hattendorff, A. Ehrl, W. A. Wall, and H. A. Gasteiger, J. Electrochem. Soc.,

163 (7), A1373-A1387 (2016).

8. N. Munichandraiah, L. G. Scanlon, R. A. Marsh, B. Kumar, and A. K. Sircar, J. Electroanal.

Chem., 379 (1), 495-499 (1994).

9. M. Rosso, T. Gobron, C. Brissot, J. N. Chazalviel, and S. Lascaud, J. Power Sources, 97-98 804-

806 (2001).

10. S. Liu, H. Wang, N. Imanishi, T. Zhang, A. Hirano, Y. Takeda, O. Yamamoto, and J. Yang, J.

Power Sources, 196 (18), 7681-7686 (2011).

11. S. Liu, N. Imanishi, T. Zhang, A. Hirano, Y. Takeda, O. Yamamoto, and J. Yang, J. Power

Sources, 195 (19), 6847-6853 (2010).

12. S. Liu, N. Imanishi, T. Zhang, A. Hirano, Y. Takeda, O. Yamamoto, and J. Yang, J. Electrochem.

Soc., 157 (10), A1092-A1098 (2010).

13. O. Wijaya, P. Hartmann, R. Younesi, I. I. E. Markovits, A. Rinaldi, J. Janek, and R. Yazami, J.

Mater. Chem. A, 3 (37), 19061-19067 (2015).

14. X. Xin, K. Ito, A. Dutta, and Y. Kubo, Angew. Chem. Int. Ed., 57 (40), 13206-13210 (2018).

15. M. Jäckle and A. Groß, The Journal of Chemical Physics, 141 (17), 174710 (2014).

16. M. Jäckle, K. Helmbrecht, M. Smits, D. Stottmeister, and A. Groß, Energy Environ. Sci., 11 (12),

3400-3407 (2018).

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