Supporting Information for. A Water-in-Salt Electrolyte for Potassium-Ion Batteries

Transcription

1 Supporting Information for A Water-in-Salt Electrolyte for Potassium-Ion Batteries Daniel P. Leonard #, Zhixuan Wei #, Gang Chen, Fei Du *, Xiulei Ji * Department of Chemistry, Oregon State University, Corvallis, Oregon, , United States Key Laboratory of Physics and Technology for Advanced Batteries (Ministry of Education), College of Physics, Jilin University, Changchun, , People s Republic of China * david.ji@oregonstate.edu, dufei@jlu.edu.cn Experimental Methods: Material synthesis: KTi 2 (PO 4 ) 3 was prepared by an electrospray method. Analytically pure K 2 CO 3, (CH 3 CH 3 CHO) 4 Ti, NH 4 H 2 PO 4 and citric acid with stoichiometric ratio of 1: 4: 6: 2 were dissolved into a mixture of deionized water, ethyl alcohol and acetic 1

2 acid and stirred to form a homogeneous solution. Then a uniform adhesive solution of 1g Polyvinyl Pyrrolidone (PVP, Mw = , Aladdin) in 10 ml deionized water were mixed with the above solution through vigorous stirring for 24 h. After that, the precursor solution was poured into a 10mL plastic syringe with a blunt-tip needle and a grounded aluminum foil was placed 18 cm below for collection. A high voltage of 22 kv was applied between the needle and the foil by a direct-current power supply with a 0.5 ml/h flow rate of the solution. The as-collected electrosprayed precursor powder was calcined at 800 C for 4 h with a heating rate of 5 C min -1 under N 2 atmosphere to obtain KTP. Material characterization: X-ray diffraction (XRD) patterns of the KTP nanocomposite were recorded using a DX-2700B diffractometer with Cu Kα radiation. The morphology was investigated using a field emission scanning electron microscope (FESEM, JEOL JSM-6700F). TEM was obtained on a JEM 2200FS with an accelerating voltage of 200 kv. Electrochemical measurements: KTP was mixed with C-45 and polyvinylidene fluoride (PVdF) with a mass ratio of 7: 2: 1 in N-Methyl-2-pyrrolidone (NMP), then cast on aluminum foil to make working electrodes. The counter electrodes were self-standing films of activated carbon, which were composed of 70 wt.% activated-carbon, 20 wt.% C-45 and 10 wt.% polyvinylidene flouride (PVDF) as binder. The electrochemical performance was tested in a three-electrode Swagelok cell (a T-cell), which comprised KTP as the working electrode, activated carbon as the counter electrode, an Ag/AgCl reference electrode (sat. KCl, V vs. SHE), and 2

3 Whatman filter paper as separators. The electrolytes were made by dissolving 1 m, 10 m and 30 m potassium acetate (Fisher Chemical) in de-ionized H 2 O, with ph values of 7.8, 9.4, and 10.8, respectively. The 30 m electrolyte was warmed to 80 C to facilitate the complete dissolution of KAc in H 2 O. All electrochemical testing was conducted at 25 C The cyclic voltammetry (CV) tests, the galvanostic charge/discharge test and rate performance test were performed on a BioLogic EC-lab VMP-3 multi-channel workstation. The long cycling test was carried out on a Land BT2000 battery test system (Wuhan, China). All the electrochemical tests were carried out at room temperature (25 C). For the non-aqueous tests, KTP was mixed with the super P and carboxyl methylated cellulose binder dissolved in water, in a weight ratio of 7:2:1, which was then cast on a copper foil current collector. Potassium metal served as the counter and reference electrode. KPF 6 (0.8 M) in a mixture of ethylene carbonate and diethyl carbonate (1: 1 by volume) functions as the electrolyte, and the glass fiber membrane (Whatman GF/F) serves as the separator. The electrode was assembled into a 2032-type coin cell in a glovebox. CV was performed at a scan rate of 0.1 mv/s. The electrochemical test was also performed at room temperature (25 C). Conductivity measurements: Electrolyte conductivity experiments were carried out using electrochemical impedance spectroscopy (EIS) on the Biologic EC-Lab VMP3 multi-channel workstation. The test cell consisted of two parallel copper foil electrodes on either 3

4 side of a 1 cm x 1 cm x 0.5 cm electrolyte reservoir. The room temperature (25 C) electrolytes were transferred to the test cell tested at a frequency range of 200 khz to 100 Hz. The uncompensated resistance from the Nyquist plot was assumed to be dominated by electrolyte resistance and that value was used to calculate conductivity. Physical characterization of KTP: Figure S1. XRD pattern of KTP with the crystal structure (inset). 4

5 Figure S2. SEM (a, b) and TEM (c, d) images of KTP in different scale bars, showing microscale secondary spheres, consisting of numerous primary nanocrystals coated with carbon layer. 5