Supplementary Information

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1 Supplementary Information An all-integrated bifunctional separator for Li dendrite detection via solution synthesis of thermostable polyimide nanoporous membrane Dingchang Lin, Denys Zhuo, Yayuan Liu, Yi Cui*,, Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, USA. Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA *Correspondence to:

2 SUPPLEMENTARY METHODS Synthesis of PI separators Solution preparation It is noted that the dissolution of LiBr in the solvent is an exothermic reaction. The release heat might heat up the solvent to above boiling point. As a consequence, it is necessary to keep the reaction vessel in a water bath to keep ambient temperature during the LiBr dissolution. After mechanically stirred for ~5 hrs, the solution became totally clear and was ready to be coated. Membrane coating Doctor blading was performed to coat the as-obtained solution on the glass. The gap depth varied from 3 mil to 10 mil was used to control the thickness of the film. A thin layer of 30% LiBr solution was applied on the top surface of the film before the film is dried. After coating, the film was kept on the glass for ~20 min to guarantee that THF and MeOH were fully evaporated, followed by the rinsing of the film with DI water and ethanol for 5 times for LiBr removal and pores creation. The asobtained PAA nanoporous membrane was dried in vacuum at ambient temperature for ~4 hrs to fully remove the residue water. Thermal imidization The dried nano-porous PAA membrane was imidized in a box furnace at air atmosphere. The temperature ramping program was set as: (1) Ramp up from room temperature (RT) to 100 C at 3 C min -1 ; (2) Keep at 100 C for 30 min; (3) Ramp up to 200 C at 3 C min -1 ; (4) Keep at 200 C for 30 min; (5) Ramp up to 300 C at 3 C min -1 ; (6) Keep at 300 C for 30 min; (7) Cool down to RT in furnace. Post-treatment The as-obtained PI nano-porous membrane was washed with isopropyl alcohol (IPA) for 3 times to remove the trace amount of residue LiBr. The membrane was then dried and punched into appropriate size for test. Characterizations Liquid absorption test To determine the porosity of the separator, liquid absorption test was performed here. Mineral oil was used as the liquid for the absorption. The weight of the separator was first measured before mineral oil absorption. Afterwards, separator was

3 immersed into mineral oil and kept for 10 minutes to make sure the complete absorption. Then, separator was taken out and wiped with Kimwipes to completely remove the surface mineral oil residue, followed by weighting the separator after absorption. The weight of separator and mineral oil can be calculated, respectively. Since the densities of separator s material and mineral oil are known. We can calculate the volume fraction of mineral oil (originally it is pore) within the separator. Brunauer-Emmett-Teller (BET) surface area and pore distribution measurement N 2 sorption studies were performed in a Micromeritics ASAP 2020 adsorption apparatus at 77 K and at pressure up to 1 bar after the samples were first degassed at 80 C overnight. The Brunauer-Emmett-Teller (BET) surface area was calculated using the adsorption data in a relative pressure ranging from 0.1 to 0.3. The mesopore size distribution was determined by the Barrett Joyner Halenda (BJH) method using the adsorption branch of the isotherm. The total pore volume (V t ) was determined using the adsorption branch of the N 2 isotherm curve at a relative pressure of Electrochemical characterizations Rate capability test in LiFePO 4 (LFP)/Li cells To test the rate capability in LFP/Li cells, LFP electrode was firstly fabricated with standard slurry process. LFP, carbon black and polyvinylidene fluoride (PVDF) were premixed in the weight ratio of 8:1:1 and N-Methyl-2-pyrrolidone (NMP) was used as the solvent. The slurry was then bladed on the Al foils to render uniform coating, which was further dried in vacuum oven at 60 C. The areal mass loading of the cathode is ~ 1.0 mg. To carry out the electrochemical test, 2032 type coin cells were assembled. Li foil (99.9%, Alfa Aesar) was used as the anode. 1 M LiPF 6 in 50/50 (v/v) ethylene carbonate (EC)/ diethyl carbonate (DEC) was used as the electrolyte here. For the control cell, all the parameters are the same except the separator used, where Celgard 2325 PP/PE/PP separator was used for comparison.

4 Full-cell test For the full-cell test, NMC 532 was used as the cathode material while graphite was used as the anode material. Standard slurry coating process was applied to coat the both electrodes. For the cathode, NMC 532, super P, KS-6 and PVDF were mixed in the weight ratio of 93:2:2:3 for slurry preparation and NMP was used as the solvent. For the anode, MCMB, super P, CMC and SBR were mixed in the weight ratio of 92.5:1.25:1.25:5 for slurry preparation and H 2 O was used as the solvent. The areal mass loading of the cathode is ~18.0 mg cm -2, and the areal mass loading of the anode is ~10.5 mg cm type coin cell is used, and 14 mm-diameter electrodes were punched for the test. 1 M LiPF 6 in 50/50 (v/v) EC/DEC with 2% of vinylene carbonate (VC) was used as the electrolyte here.the cycling program was set as: (1) Constant-current (CC) charging at 0.2 C to 4.2 V; (2) Hold at 4.2 V for 1 hrs; (3) CC discharging at 0.2 C to 2.75 V. AC impedance spectroscopy measurement In the measurement, the tested separator was sandwiched between two stainless steel electrodes.. 1 M LiPF 6 in 50/50 (v/v) EC/DEC was used as the electrolyte. The frequency was scanned from 1 MHz to 100 mhz. Biologic VMP3 system was used to carry out the measurement. Cyclic voltammetry (CV) measurement In the CV measurement, Li foil was used as the counter electrode while a stainless steel electrodes was used as the working electrode. Tested separator was sandwiched by the electrodes and in direct contact with the both electrode. 1 M LiPF 6 in 50/50 (v/v) EC/DEC was used as the electrolyte. The scanning was performed in the range of -0.3 to 6 V versus Li + /Li. The scanning rate was set at 1 mv s -1. Fabrication of PI/Cu/PI trilayer bifunctional separators Metallic Cu layer coating With an PI separator fabricated with the previously mentioned method, ~50 nm of Cu layer was sputtered onto the separator s surface with a sputtering system (AJA, Inc). Top layer coating With the PAA-SiO 2 -LiBr precursor solution, it was coated onto a glass inside a glove box with argon atmosphere and low humidity (sub ppm H 2 O level).

5 Another layer of 30% LiBr was coated on the top. Before it was dried, the Cu coated PI separator was adhered to the surface with Cu layer facing the glass. It is noted that it is important to carry out the process at a dry atmosphere, which will prevent the absorption of H 2 O by LiBr onto the interface and damage the interface adhesion. After the solvent was fully evaporated, the membrane was transferred back to ambient atmosphere and rinsed with DI water to remove LiBr. Afterwards, the membrane was dried in vacuum before thermal imidization. Thermal imidization The dried nano-porous PAA membrane was imidized in a tube furnace at argon atmosphere to prevent the oxidation of Cu in air. The temperature ramping program was set as: (1) Ramp up from room temperature (RT) to 100 C at 3 C min -1 ; (2) Keep at 100 C for 30 min; (3) Ramp up to 200 C at 3 C min -1 ; (4) Keep at 200 C for 30 min; (5) Ramp up to 300 C at 3 C min -1 ; (6) Keep at 300 C for 30 min; (7) Cool down to RT in furnace. Dendrite detection test To carry out dendrite detection test, pouch cells were assembled and tested. Symmetric cell configuration was used with Li foils as the electrodes at both sides. One piece of PI/Cu/PI trilayer bifunctional separator was sandwiched between the two electrodes and connected to a third electrode for the Cu potential measurement. 1 M LiPF 6 in 50/50 (v/v) EC/DEC was used as the electrolyte. During the test, Li was stripped from the positive electrode and deposited onto the negative one. High current density of 4 ma cm -2 was used to accelerate the Li dendrite formation and penetration. Two individual test systems were used for Li deposition and Cu potential monitor.

6 SUPPLEMENTARY FIGURES Figure S1. Mechanism for the formation of porous membrane a. The formation of porous membrane by using low-boiling-point solvent for PAA-SiO 2 -LiBr precursor synthesis. b, The formation of pulverized PAA powder by using high-boiling-point solvent for for PAA-SiO 2 -LiBr precursor synthesis.

7 Figure S2. PIs with different dianhydrides PI separators with different dianhydrides of 4,4'- Oxydiphthalic anhydride (a,b, OPDA), 3,3',4,4'-Benzophenone tetracarboxylic dianhydride (c,d, BTDA) and 2,2'-Bis-(3,4-Dicarboxyphenyl) hexafluoropropane dianhydride (e,f, 6FDA) were synthesized with the same process. b, d, f shows the FTIR specta of OPDA-ODA, BTDA-ODA and 6FDA-ODA PIs, respectively. All of the samples show strong characteristic signals of PI.

8 Figure S3. Different thickness of PI separators SEM images showing the cross sections of the as-fabricated PI separator with various thickness of ~10 µm (a), ~15 µm (b), ~20 µm (c); and ~25 µm (d), which covers the major range of the commercial separators. Figure S4. Interfaces of the PI separators Tilted SEM images showing the edges of a typical PI separator s surfaces facing air (a) and facing glass (b). The yellow dash lines mark the boundary of the surface and the cross section.

9 Figure S5. Thermogravimetric analysis Thermogravimetric analysis (TGA) of the PI separator in both air (blue) and N 2 (red) atmosphere. The weight loss of PP separator in air (black) was measured as comparison. It is noted that above ~520 C, PI separator in air still retain ~22 % of the initial weight. This part is the residual fumed SiO 2 added into the PI separator. The weight is consistent with the addition amount of SiO 2. Figure S6. Brunauer Emmett Teller (BET) surface area measurement. Comparison of BET surface area (a) and pore size distribution (b) of Celgard 2400 separator with PI separator (2g LiBr/1g PAA).

10 Figure S7. Tensile test of the PI separator a, The stress-strain curve of PI separator under tensile strength. The strain rate was kept constant at 10% min -1. b, Digital camera image showing the DMA setup and breaking phenomenon. Figure S8. Flexibility of PI separator. Digital camera images showing the bending (a) and twisting (b) modes of PI separator, which indicates good flexibility of the PI separator.

11 Figure S9. Electrolyte wettability in EC/DEC Digital camera photos of the Celgard 2325 separator (a) and the nanoporous PI separator (b) with a droplet of EC/DEC electrolyte on the top. The time for electrolyte diffusion is the same for both cases. It is clearly shown that the electrolyte droplet cannot be efficiently absorbed by the commercial Celgard 2325 separator. In contrast, the nanoporous PI separator shows good electrolyte absorption across the whole separator. Figure S10. Rate capability of the Celgard 2325 cell The voltage profiles of the LFP/Li cells with Celgard 2325 separator at different rates varied from C/4 to 10 C.

12 Figure S11. Ionic conductivity measurement Nyquist plots comparing the ionic conductivity of different separators. Separators with the same thickness (25 um) were used for the measurement. The intersections with the x axis indicate the ionic resistance of the separators where the PI separators have consistently lower resistance. Figure S12. Electrochemical stability window of the PI separator Cyclic voltammetry characterization showing the electrochemical stability of the PI separator. The potential was scanned from -0.3 V to 6 V versus Li + /Li. The peaks near 0 V are corresponding to the Li plating/stripping signals, while the small peaks at ~4.2 V can be attributed to the anodic decomposition of the liquid electrolyte. No other peak can be identified, which indicates that the PI separator is stable in the full range.

13 Figure S13. PI separator after battery cycling. Digital camera photos comparing the pristine PI separator and the separator after 80 cycles in NMC/MCMB full cell. The cycled separator was rinsed with ethanol to remove the residual Li salt. No obvious morphology change of PI separator can be observed after battery cycling. Figure S14. All-integrated PI/Cu/PI trilayer bifunctional separator Digital camera photos showing the both surfaces of the all-integrated PI/Cu/PI bifunctional separator. The Cu line in the left image is not covered by the top PI layer because it is left for the connection of the dendrite detection electrode.