Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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Rhombohedral Potassium−Zinc Hexacyanoferrate as a Cathode Material for Nonaqueous Potassium-Ion Batteries Jongwook W. Heo,† Munseok S. Chae,† Jooeun Hyoung, and Seung-Tae Hong* Department of Energy Science and Engineering, Daegu Gyeongbuk Institute of Science and Technology, Daegu 42988, Republic of Korea
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ABSTRACT: Rhombohedral potassium−zinc hexacyanoferrate K1.88Zn2.88[Fe(CN)6]2(H2O)5 (KZnHCF) synthesized using a precipitation method is demonstrated as a high-voltage cathode material for potassium-ion batteries (PIBs), exhibiting an initial discharge capacity of 55.6 mAh g−1 with a discharge voltage of 3.9 V versus K/K+ and a capacity retention of ∼95% after 100 cycles in a nonaqueous electrolyte. All K ions are extracted from the structure upon the initial charge process. However, only 1.61 out of 1.88 K ions per formula unit are inserted back into the structure upon discharge, and it becomes the reversible ion of the second cycle onward. Despite the large ionic size of K, the material exhibits a lattice-volume change (∼3%) during a cycle, which is exceptionally small among the cathode materials for PIBs. The distinct feature of the material seems to come from the unique porous framework structure built by ZnN4 and FeC6 polyhedra linked via the CN bond and a Zn/Fe atomic ratio of 3/2, resulting in high structural stability and cycle performance.
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than that for Na+ (0.23 e/Å3), enabling higher mobility of K+ ions in a nonaqueous solution.15 During the past few years, new cathode materials for PIBs have been actively studied, such as TiS2,17 K0.3MnO2,15 KTi2(PO4)3,18 K0.7Fe0.5Mn0.5O2,19 KxCoO2,20,21 KVP2O7,22 KVPO4F, KVOPO4,23 K0.5V2O5,24 K3V2(PO4)3,25 MoS2,26 K0.23V2O5,27 Na0.52CrO2,28 poly(anthraquinonyl sulfide),13 3,4,9,10-perylenetetracarboxylic acid dianhydride,29 and cubic Prussian blue (PB) analogues, KxM[Fe(CN)6]1−y(H2O)z (M = Mn,30,31 Fe,14,32−34 Ni16). An ideal cubic PB analogue can be represented as A2MM′(CN)6 (A = zeolitic water, metal ions; M = Mn, Fe, Co, Ni, Cu, V, Mo; M′ = Co, Cr, Fe, Ru), as shown in Figure 1a. The ordering of M and M′ sites forms a face-centered rocksalt arrangement, and the two atoms are connected through CN, forming an MNCM′ sequence. Consequently, the M and M′ are located in the octahedral site composed of N and C, respectively.35 The cations of A are accommodated in the large cavity sites formed by 12 CN groups. Such cavities generate three-dimensional (3D) diffusion paths so that the A ions or water may be transported easily. Because of their openframework structure, high durability, and high redox potentials, cubic PB analogues have been studied as host materials for various cations.16,36−44 When Zn and Fe ions occupy the M and M′ sites, respectively, i.e., in the case of zinc hexacyanoferrate, the cubic
INTRODUCTION
Lithium-ion batteries (LIBs) are some of the best-performing rechargeable batteries in terms of energy and power density because of its commercialization by Sony in 1991.1 However, LIBs are expected to experience difficulties in supplying lithium because of the uneven distribution of its resources,2 leading to extensive studies of sodium-ion batteries (SIBs) as a potential replacement for LIBs because of lower cost and toxicity and abundance of sodium.3,4 Besides, because SIBs have not been commercialized yet, potassium-ion batteries (PIBs) have also attracted increasing interest.5 K ions are known to intercalate into a graphitic carbon anode to form KC8,6 unlike Na ions,7,8 which can be a significant advantage compared to SIBs because nongraphitic carbon or alloys still have many problems that need to be resolved, such as substantial volume changes during cycles, low initial Coulombic efficiency, and large polarization voltage.9,10 Potassium intercalation into graphite shows good cycle performance, low polarization, low first-cycle irreversible capacity, and low operating voltage.6,11,12 Also, potassium is earth-abundant, and the redox potential for K/K+ (−2.94 V) is lower than that for Na/Na+ (−2.71 V), closer to Li/Li+ (−3.04 V). The lower redox potential of potassium can lead to a higher cell voltage and energy density.4,6,11−16 Aluminum can be used instead of copper as an anode current collector because potassium does not form an alloy with aluminum, unlike lithium, lowering battery cost. Furthermore, the intercalant ion charge density for K+ (0.09 e/Å3) is lower © XXXX American Chemical Society
Received: October 31, 2018
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DOI: 10.1021/acs.inorgchem.8b03081 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Sigma-Aldrich; 60 mL, 50 mM) was gradually added to a K4Fe(CN)6· 3H2O (≥99.5%, Sigma-Aldrich; 40 mL, 50 mM) aqueous solution with stirring at 400 rpm under 70 °C, producing insoluble colloidal products. The initial colors of the solution and products were transparent yellow and opaque white, respectively. After stirring for 30 min, the centrifugation (4500 rpm for 10 min) and washing process were repeated five times using deionized water in order to separate the unreacted reagents from the KZnHCF powder. The powder was collected and dried at 25 °C in a vacuum oven overnight. Materials Characterization. The morphological and elemental analyses were investigated using ultrahigh-resolution field-emission scanning electron microscopy (SEM; Hitachi SU-8020), fieldemission transmission electron microscopy (FE-TEM; Hitachi HF3300) with an energy-dispersive X-ray spectrometry (EDX), and inductively coupled plasma optical emission spectroscopy (ICP-OES; Varian 700-ES). In order to analyze the water content of the host materials, thermogravimetric analysis (TGA; Rigaku TG 8120) was performed. X-ray photoelectron spectroscopy (XPS; Thermo Scientific ESCALAB 250Xi) analysis was also carried out to probe the oxidation state changes of Fe in KZnHCF upon cycling. Electrochemical Characterization. Electrochemical experiments were performed with 2032 coin-type cells. The working electrode consisted of the as-synthesized KZnHCF, conductive carbon (Super P, Timcal Graphite & Carbon), and a binder [poly(vinylidene fluoride), W#1300, Kureha Co.] in a weight ratio of 80:10:10, which were blended and dispersed in N-methyl-2-pyrrolidone, cast onto ∼20 μm aluminum foil, and dried 12 h under ambient temperature in a vacuum oven to remove the solvent. The electrodes were roll-pressed and stored in an argon-atmosphere glovebox. The loading of a host material on each electrode (electrode area of 1.53 cm2) was 1.6 mg. A glass microfiber separator (GF/A, Whatman) was used, and 0.5 M potassium hexafluorophosphate (99+%, Alfa Aesar) in diethyl carbonate (99+%, Alfa Aesar) and ethylene carbonate (99%, Alfa Aesar) (1:1, v/v) containing 2 wt % of fluoroethylene carbonate (FEC; Panaxetec) was used as the electrolyte.31,52 The FEC electrolyte additive was used because it is known as an efficient electrolyte additive to reduce the electrolytic side reaction on the electrode surface.53,54 The water content of the electrolyte determined by Karl Fischer titration (Metrohm, 831 KF coulometer) was about 3 ppm. Coin-shaped metal plates made from K metal (99.5%, SigmaAldrich) was used as an anode. Galvanostatic charge−discharge measurements and cyclic voltammetry were carried out with EC-Lab software on a Biologic VMP3 multichannel potentiostat (Biologic Science Instruments SAS). Structural Analysis. Powder X-ray diffraction (XRD) data were collected at ambient temperature using an X-ray diffractometer (Rigaku MiniFlex 600) with a Cu Kα (λ = 1.5418 Å) X-ray source. Crystal structures for K1.88Zn2.88[Fe(CN)6]2·5H2O (KZnHCF), K 0.79Zn2.88 [Fe(CN)6]2·5H 2O (K0.79 ZnHCF), and K0 Zn2.88[Fe(CN)6]2·5H2O (ZnHCF) were refined via the refinement program GSAS,55 where the initial structural model for KZnHCF was referred to from a previous report.46 The Fourier electron density maps were calculated from powder XRD data via CRYSTALS56 and MCE,57 as described in our previous reports.58,59 Crystal structures were drawn using software ATOMS.60 For in situ powder XRD measurements, a homemade in situ cell was used (Figure S1). KZnHCF was loaded on an Al-coated Be metal foil (35 μm). Be metal is easily oxidized in the operating voltage range of the electrochemical cell. To protect Be, the thin Al (200 nm) was deposited on the Be using thermal evaporation in a vacuum thermal evaporator (
