Supplementary Information

Towards Succinonitrile-based Lithium Metal Batteries with Long Cycle Life: The Influence of

Fluoroethylene Carbonate Loading and the Separator

Mohammed B. Effata, Ziheng Lua,d, Alessio Belottia, Jing Yua, Yu-Qi Lyua, and Francesco

Ciuccia,b,c*

aThe Hong Kong University of Science and Technology, Department of Mechanical and

Aerospace Engineering, Hong Kong, China

bThe Hong Kong University of Science and Technology, Department of Chemical and Biological

Engineering, Hong Kong, China

cGuangzhou HKUST Fok Ying Tung Research Institute, China

dShenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen

518055, China

*E-mail: francesco.ciucci@ust.hk (Francesco Ciucci)

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1 Thermal stability and ignition behavior of the prepared electrolyte

(a)

(b) (c)

Figure S1 TG and ignition of commercial carbonate-based and SN-based electrolytes. (a) TG curves of SN-based and commercial carbonate electrolytes. Ignition of (b) carbonate electrolyte and (c) SN-10 electrolyte, when exposed to a direct flame. The high flammability of the carbonate electrolyte is reflected by its rapid ignition. Instead, SN-10 could not be ignited even after direct contact with the flame for over 45 s. The videos are available at the publisher website.

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2 The electrolyte resistance decrease during cycling

(a)

(b)

Figure S2 The electrolyte resistance decrease during cycling. (a) EIS of the LM-LFP battery with SN-10 before and after 30 cycles. The inset shows the discharge capacity versus cycle number.

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(b) Overpotential versus time obtained by the RT galvanostatic cycling of the symmetric LM cells with SN-5 and and SN-10 electrolytes at 0.3 mA cm-2 for 200 hours.

3 Microstructure of the separators

(a) (b)

Figure S3 Microstructure of (a) GF and (b) C/3501separators.

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4 Black spots observed on the surface of glass fiber separator

(a) (b)

(c)

Fig. S4 Optical images showing (a) black spots on the surface of GF separators observed after disassembling the batteries, suggesting the growth of lithium dendrites 1. Unfortunately, we were not able to recover the full separator of the SN-5 based battery as it was strongly sticking to the stainless steel spacer of the coin cell, (b) the C/3501 separator covering the black LFP cathode, and (c) the cycled LM of LM-LFP battery with SN-5 electrolyte. We note the existence of an SEI layer covering the grey surface of the LM. The two layers are highlighted by the blue dashed circle and magnified by SEM. For the corresponding images with SN-10, the reader is referred to Fig. 7 in the main text. The metallic tip appearing in the picture is for the tweezer used to hold the sample.

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5 Voltage spikes observed during cycling of GF based batteries

(a) (b)

Figure S5 voltage spikes observed during cycling of LM-LFP battery using GF separator at (a) the 190th cycle with SN-5 electrolyte and (b) the 130th cycle with SN-10 electrolyte.

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References

(1) Wu, B.; Lochala, J.; Taverne, T.; Xiao, J. The Interplay Between Solid Electrolyte Interface (SEI) and Dendritic Lithium Growth. Nano Energy 2017, 40, 34-41.

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