Polyvinylidene fluoride (pvdf)

PVDF was purchased from Solvay (Seoul, Republic of Korea)(Solef 1015, Mw = 570,000 g/mol). Gamma-butyrolactone (GBL, diluent), N-methyl-2-pyrrolidone (NMP, solvent), dimethylacetamide (DMAc, solvent), and ethylene glycol (EG, non-solvent) were purchased from the SAMCHUN Chemical (Pyeongtaek, Republic of Korea). NA11 (nucleating agent; NA) was purchased from Asahi Denka (Tokyo, Japan) (ADK STAB NA11, Sodium 2,2’-methylene bis-(4,6-di-tert-butylphenyl) phosphate). PVP (pore-forming additive) was purchased from BASF (Florham Park, NJ, USA) (LUVITEC K-30). The coagulation bath was composed of tap water as the non-solvent. All chemicals were used as received.

Electrode preparation: In an Ar-filled glove box, positive electrodes were prepared using 80 wt.% of the active material, 10 wt.% Carbon SuperP (Timcal, Willebroek, Belgium), 10 wt.% of PVdF (Solvay, Brussels, Belgium). An adequate amount of NMP (Sigma Aldrich) was added, to form a uniform slurry, then it was coated on an aluminum foil (20 μm) using a 100 µm doctor blade and left to dry in the glovebox for three days. The electrodes were cut (14 mm diameter), pressed (10 t), weighed (loaded > 2 mg) in the glovebox, and dried in a Buchi^® oven at 60 °C for 12 h under vacuum.
Electrochemical testing: Coin cells were prepared using 14-mm diameter positive electrodes, 16-mm diameter Li foil as a negative electrode, and 150 μL electrolyte containing 1 M LiPF₆ in EC:PC:DMC, where EC: ethylene carbonate; PC: propylene carbonate; DMC: dimethyl carbonate, in a volumetric ratio of 1:1:3, respectively. A propylene separator (Celgard^® 2400, 16.5 mm diameter) and polyolefin separator (Viledon^®, 16.5 mm diameter) were used in the coin cells. Three-coin cells were cycled at 22 °C, at a rate of C/10 between 3 V and 1.5 V, using ARBIN cycling instrumentation.

LiNi_0.6Mn_0.2Co_0.2O₂ cathode (90% NMC622, Toda Kogyo Corp.; 5% C45 conductive carbon, Timcal; 5% PVdF, Solvay 5130) was fabricated by the CAMP Facility at Argonne National Laboratory. Active material loading is 8.8 mg/cm²e. 1-Methyl-1-propyl-3-fluoropyrrolidinium bis(fluorosulfonyl)imide (PMpyr_fFSI) was synthesized following the literature procedure^{35 (link)} by the one-step reaction of 1-propyl-3-fluoropyrrolidine with MeFSI. The resultant ionic liquid was dried in a lyophilizer for at least two days, then stored in 4 Å molecular sieves and filtered before use. The water content was <20 ppm, measured by Karl–Fischer titrator C30. (PMpyr_f)_0.8Li_0.2FSI and (PMpyr_f)_0.5Li_0.5FSI electrolytes were prepared by dissolving LiFSI (Nippon ShokuBai Co., Ltd.) in PMpyr_fFSI with 4/1 and 1/1 molar ratio in an argon-filled glovebox.

The electrode slurries were prepared with a composition ratio of LiNi_0.8Co_0.1Mn_0.1O₂(NCM811, LG energy solution)/carbon black additive (Super C65)/binder = 92/3/5 (w/w/w). For the control PVDF electrode, polyvinylidene fluoride (PVDF, Solvay) was solely used as binder and dissolved in N-methyl-2-pyrrolidone (NMP, Aldrich). For the control n-IPN and c-IPN electrodes, PVDF/trimethylolpropane triacrylate (TMPTA) (= 3/2 (w/w)) and PVDF/TMPTA/VAI-TFSI ( = 3/0.7/1.3 (w/w/w)) were dissolved in NMP, respectively, in which benzyol peroxide (BPO, 1 wt%) was used as a thermal initiator. The electrodes were fabricated by casting the electrode slurry on an Al current collector. The casted electrode slurry was dried at 100 °C for 1 h and followed by roll-pressing at 90 °C. The electrode density of the electrodes examined herein was set at 2.8 g cc⁻¹ for a fair comparison. The electrodes were vacuum-dried at 120 °C for 12 h before the cell assembly.

PVDF with an average molecular weight of M_W ~ 370,000 g·mol⁻¹ was purchased from Solvay, Republic of Korea. Copper sulfate pentahydrate (CuSO₄·H₂O) and sodium hydroxide (NaOH; 98%) were supplied by Sigma-Aldrich (St. Louis, MO, USA). Acetone and dimethylformamide (DMF) were bought from Merck (Darmstadt, Germany). No further purification was performed before using the materials.

The NMC811 active cathode material was prepared via a solid-state
reaction of the Ni_0.8Mn_0.1Co_0.1(OH)₂ precursor (Umicore Finland Oy) and 5% excess LiOH (Sigma-Aldrich,
98%). The materials were thoroughly mixed and then heated at 800 °C
in a tube furnace (Nabertherm) for 12 h under an O₂ atmosphere.
To fabricate positive electrodes, an electrode slurry was mixed using
a dispergator (Dispermat) at 500 rpm and then coated on 20 μm
thick aluminum foil (MTI). The dry content of the positive electrode
slurry is composed of 95 wt % NMC811 active material, 3 wt % conducting
carbon black (Timcal Super C65), and 2 wt % polyvinylidene fluoride
(PVDF, Solvay, Sole 5130) binder dissolved in N-methyl-2-pyrrolidone
(NMP, Alfa Aesar) as the solvent. The printed electrode was dried
in a fume hood overnight and subsequently in an oven at 80 °C
for 4 h. Afterward, circular electrodes with a diameter of 14 mm for
the coin cell configuration and 18 mm for the three-electrode cell
configuration were cut. The prepared electrode had a loading of 10–10.5
mg cm^–2, which corresponds to ∼2 mA h cm^–2 based on 200 mA h g^–1 for NMC811.

NMC cathodes were prepared following the protocol described below. First, a slurry was prepared using as-received NMC powder (LiNi_0.5Mn_0.3Co_0.2O₂, Toda America), carbon black (CB, Denka), polyvinylidene fluoride (PVDF, Solvay, 5130), and n-methyl-2-pyrrolidone (NMP, Sigma Aldrich). For the optical imaging experiment, the slurry was cast onto a battery-grade aluminum sheet using a slot-die coating method, with an aerial loading of 12.5 mg per cm² and with weight ratios of 90 wt.% NMC, 5 wt.% PVDF, and 5 wt.% CB. To expose a flat and smooth surface (Fig. S3) suitable for high magnification optical imaging, a strip of 0.8 cm² area was cut out of the cathode and subjected to ion polishing using a JEOL IB-19500CP ion polisher with a rotating stage. In the CMCD experiment, the thickness of the electrode was controlled at 10 μm.