Congcheng Wang,# Guoyin Zhu,# Pan Liu, and Qing Chen*

METHODS

Battery Assembly and Testing. Ni−Zn batteries were assembled into coin-type cells (CR2032) with a Zn anode, either the compressed nanoporous Zn (∼0.36 cm2 in size) or a pasted electrode comprising Zn powder (90 wt %, Aldrich), polyvinylidene fluoride (Arkema Kynar), and a NiOOH cathode (650 mAh, GP Batteries). Tin foil was used as the anode current collector. The anode was separated from the cathode by a nonwoven cellulose membrane and a microporous separator (Celgard 3501). A solution of 6.0 MKOH (VWR) saturated with ZnO (J&K Scientific) was used as the electrolyte. The battery electrolyte (Xiamen Tmaxcn Inc.)volume was controlled to be around 0.3mL. The cells were assembled under a pressure of 6.8 MPa with anMSK-110 hydraulic coin cell crimper(Xiamen Tmaxcn Inc.). Cycling tests were performed with a New are battery testing station (CT-3008W), with a charge and discharge current density of 5 and 25 mA/cm2, respectively. Voltage cutoffs were set at 1.35 and 1.9 V, respectively. EIS was carried out with a Biologic VMP3 potentiost at, with a frequency range from 10 mHz to7 MHz. The polarization tests were performed to a freshly assembled cell at a scan rate of 1 mV/s.