To cite this article: Ming Wang et al 2019 J. Electrochem. Soc. 166 A2151

Preparation of electrodes.—A manually operated powder coatingspray gun (Encore LT Manual Powder Coating System, Nordson) was used for the ESD process. One piece of 16μm thick aluminum foil was used as the electrode substrate, one piece of electrically grounded Cuplate (250μm thick) was used as the platform. The operation voltage was 25 kV. Both atomizing and flow pressures were 7 psi. The spraying time was 45 seconds. The spray direction was 45o with a 20 cm distance between the spray gun tip and the aluminum substrate. The thermal activation was done by transferring the dry-powder-coated electrodes to an oven (DKN812, Yamato) and baking in air. Different baking durations and temperatures were used as the thermal activation process and baking at 200°C for 1 hour was selected for the optimal results.

The baked electrodes were calendered at room temperature with a10μm gap spacing by a compact electrode calender machine (Xiamen Tmaxcn Inc.).The calendered electrodes were punched into 12 mm diameter discs by a coin cell disc punching machine(Xiamen Tmaxcn Inc.)and transferred to a glove box. The porosity of the dry-coated electrodes was about 30% after calendering.The average loading of the electrode discs was 2.4 mAh/cm2. The average thickness after calendering was 59μm.

Characterization.—Morphology characterization.—The surface morphology was investigated using a Scanning Electron Microscope (SEM) with Energy Dispersive Spectroscopy (EDS)(FEG 250, Quanta).Mechanical measurements.—The binding strength of the drypowder-coated electrodes was measured by peel test and scratch test.The binding strength was measured by a 180o peel test (MH2-110,Imada, Inc.) based on the ASTM-D903 standard. The speed was 60 mm per min. The Al foil was cut into 1 cm width strips ahead of the drypowder-coating process. The coated strips were heated at 200°C in an oven and calendered individually. The calendered strip was attached to the sample stage by a double-sided adhesive tape. The cohesive strength between NMC particles was measured by a scratch test (NanoTest Vantage, Micro Materials Limited) using a conical diamond stylus. The tip radius was 10μm, and the cone angle was 60o. A 12 mm diameter electrode disc was glued to a flat sample stage as the scratch sample. A 2000μm scratch was made by sliding the sample against the tip. The applied load was from 0.1 mN to 20 mN.

Coin cell fabrication and electrochemical measurements.—The coin cells (CR2025) were assembled using an coin cell crimper (Xiamen Tmaxcn Inc.)inside an argon-filled glove box(MBraun). A lithium metal foil (99.9%, Sigmae-Aldrich) was used as the counter electrode. One piece of polypropylene membrane (Celgard 2400) was used as the separator. The electrolyte was 1 M LiPF6 inethylene carbonate/ethyl methyl carbonate (EC/EMC 3:7 by volume,Gotion) with 2% vinylene carbonate (VC, Gotion). The electrochemical characterization of the assembled cells was performed using apotentiost at (BCS-805, Bio-logic). The current density for 1C is defined as 160 mA g−1. The cycling tests were performed at specified rates, ranging from 0.1 to 5C, between 3.0 and 4.3 V. The coin cell washeld at 4.3 V until the current dropped below 0.05C after each charge cycle. The electrochemical impedance spectroscopy (EIS) measurements were conducted using a potentiost at (VMP-3, Bio-logic) from10 mHz to 1 MHz, 10 mV AC signal. The cycled coin cells were discharged to 3.0 V and hold at 3 V until the current dropped below0.0025C. All cells were rest for 12 hours before the EIS measurement.