Supporting Information A Novel Potassium-Ion Hybrid Capacitor Based on an Anode of K 2 Ti 6 O 13 Micro-Scaffolds Shengyang Dong,, Zhifei Li, Zhenyu Xing, Xianyong Wu, Xiulei Ji*, and Xiaogang Zhang*, Jiangsu Key Laboratory of Electrochemical Energy Storage Technologies, College of Material Science and Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing, 210016, P. R. China Department of Chemistry, Oregon State University, Corvallis, Oregon 97331-4003, United States *Corresponding author. Tel: +86-025-5211291; Fax: +86-025-52112626; E-mail address: azhangxg@nuaa.edu.cn (X. G. Zhang); david.ji@oregonstate.edu (X. L. Ji) S-1
Materials Characterization The crystal structures of the obtained samples were characterized by X-ray diffraction (XRD) (Rigaku Ultima IV diffractometer) with Cu Kα radiation (λ = 1.5406 Å). For ex situ XRD measurements, half cells were disassembled in an Ar-filled glovebox. Electrodes were first washed with propylene carbonate (PC) several times and then dried at room temperature under vacuum. All samples were airtight by Kapton films. The microscopic morphologies were characterized by field-emission scanning electron microscopy (FESEM, FEI NOVA 230) and transmission electron microscopy (TEM, FEI Titan 80-200). Nitrogen adsorption/desorption data were collected on Micromeritics TriStar II 3020 analyzer. Thermogravimetric (TG) measurement was carried out on a thermal analyzer (NETZSCH, STA 409 PC) under air with a temperature ramp of 5 min 1. The X-ray photoelectron spectroscopy (XPS) data were collected on a PerkinElmer spectrometer (PHI 550) with the Al-Ka (1486.6 ev) as X-ray source. Electrochemical Measurement All the half-cell measurements were test in standard CR2032-type coin cells and used potassium foil as reference and counter electrodes and glass microfiber filter paper (Whatman GF/D) as separator. All the cells were assembled in a glove box (MBRAUN) filled with high-purity Ar gas. The electrolyte was 0.8 mol L 1 KPF 6 dissolved in propylene carbonate (PC) and 5% fluoroethylene carbonate (FEC) (vol/vol). The KTO electrode was obtained by mixing active materials, conductive carbon black (TIMCAL Super C-45), and binder sodium carboxymethyl cellulose (CMC) in DI water at a mass ratio of 8:1:1. Then, the above homogeneous slurry was casted on Cu foil. Afterwards, the electrode was dried at 110 C for 12 h under vacuum oven. The NGC and C-45 electrode were prepared by the same method except that the weight ratio of NGC (or C-45)/CMC is 9: 1 and the current collector is Al (or Cu) foil. The galvanostatic S-2
charge/discharge (GCD) measurements were conducted on an Arbin BT2000 system. For halfcells, the capacities and current densities were calculated based on the mass of electrode materials (KTO or NGC). The current densities and capacities of the KIC full cells were normalized by the total active mass (KTO and NGC) of both electrodes. The KIC full cell balance was obtained by setting the electrode mass ratio of cathode/anode to about 1.5. Cyclic voltammetry (CV) tests were conducted on a CHI 760E electrochemical workstation. Electrochemical impedance spectra (EIS) were performed on the ZIVE SP2 electrochemical workstation (WonATech) with the frequency range 0.01 Hz to 100 khz, while the disturbance amplitude was 5 mv. S-3
Figure S1. The XRD pattern (a) and TG plot (b) of the KTO precursor. Figure S2. (a, b) The SEM images of the KTO precursor. S-4
Figure S3. The SEM images of the KTO precursor with different hydrothermal time (a) 12 h and (b) 36 h. We also investigated the effects of hydrothermal time on the structures of the as-prepared KTO precursor. If the hydrothermal time is reduced to 9 h, the interwoven structure has partially grown (Fig. S3a). However, if the hydrothermal time is extended to 36 h, the micro-scaffolds structure has been broken to some extent (Fig. S3b). S-5
Figure S4. (a) N 2 adsorption-desorption isotherms of KTO, (b) pore size distribution data through the Barrett-Joyner-Halenda (BJH) method. Figure S5. The first cycle charge/discharge profile of KTO electrode. S-6
Figure S6. Galvanostatic discharge/charge profiles of C-45 at 50 ma g -1. Figure S7. A representative SEM image of KTO after GCD test of 1000 cycles. S-7
Figure S8. Z as a function of ω -1/2 plot in low frequency range (the slope of fitting curves is the Warburg factor: σ w ) The estimated methods of the potassium ion of as-prepared KTO electrode using in this work: The lithium ion diffusion coefficient (D K, cm 2 s -1 ), can be calculated from the Warburg region in eqation: 1-2 D = R 2 T 2 /2An 4 F 4 C 2 σ w 2 (1) Z = R e + R ct + σ w ω -1/2 (2) Where R is the gas constant, T is the absolute temperature, A is assumed as electrode cross section of the electrode, n is the number of electrons, F is the Faraday constant, C is the concentration of potassium ions, σ w is the Warburg factor and ω is the angular frequency. The Z- ω -1/2 plots in the low frequency region of the electrodes after 20 cycles are presented in Figure S7. S-8
Figure S9. The first GCD profiles at a current density of 50 ma g 1 with various state of charge (SOC) marked by Arabic numerals (1, discharge to 1.0 V; 2, discharge to 0.4 V; 3, discharge to 0.1 V; 4, charge to 0.4 V; 5, charge to 2.5 V; 6, discharge to 0.1 V; 7, charge to 2.5 V). (b) Ex situ XRD patterns of the KTO electrode at the SOC marked in (a). (c) Ex situ XRD patterns showing 2θ regions between 11 and 12 to demonstrates peak shift during the charge/discharge process. S-9
Figure S10. ex situ XPS spectra of the Ti 2p peaks at the different SOC (pristine, discharge to 0.01 V and charge to 2.5 V vs. K + /K). S-10
Figure S11. XRD pattern of NGC. Figure S12. (a) CV curve of KIC at 2 mv s 1. (b) Electrochemical impedance spectra of KIC before cycling and after 500 cycles. S-11
References (1) Zhao, X.; Wang, H-E.; Chen, X.; Cao, J.; Zhao, Y.; Neale, Z.; Cai, W.; Sui, J.; Cao, G. Tubular MoO 2 Organized by 2D Assemblies for Fast and Durable Alkali-Ion Storage. Energy Storage Mater. 2018, 11, 161-169. (2) Lee, S.; Kim, J.; Lee J.; Cho, B. H. State-of-charge and Capacity Estimation of Lithium-Ion Battery Using a New Open-circuit Voltage Versus State-of-charge. J. Power Sources, 2008, 185, 1367-1373. S-12