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Phase Transformations in TiS2 during K Intercalation Bingbing Tian, Wei Tang, Kai Leng, Zhongxin Chen, Sherman Jun Rong Tan, Chengxin Peng, Guo-Hong Ning, Wei Fu, Chengliang Su, Guangyuan Wesley Zheng, and Kian Ping Loh ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b00529 • Publication Date (Web): 21 Jul 2017 Downloaded from http://pubs.acs.org on July 23, 2017
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ACS Energy Letters
Phase Transformations in TiS2 during K Intercalation
Bingbing Tian†,‡,#, Wei Tang§,#, Kai Leng‡,#, Zhongxin Chen‡, Sherman Jun Rong Tan‡, Chengxin Peng‡, Guo-Hong Ning‡, Wei Fu‡, Chenliang Su†,‡, Guangyuan Wesley Zheng§, Kian Ping Loh*,†,‡
†
International Collaborative Laboratory of 2D Materials for Optoelectronics Science and Technology,
Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China. ‡
Department of Chemistry, Centre for Advanced 2D Materials (CA2DM) and Graphene Research
Centre, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore. §
Institute of Materials Research and Engineering, A*STAR, 2 Fusionopolis Way, Innovis, Singapore
138634, Singapore. * Corresponding Authors: Kian Ping Loh, Email:
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Abstract: The electrochemical performances of TiS2 in potassium ion batteries (KIBs) are poor due to the large size of K ions, which induces irreversible structural changes and poor kinetics. To obtain detailed insights into the kinetics of phase changes, we have investigated the electrochemical properties, phase transformations and stability of potassium intercalated TiS2 (KxTiS2, 0≤x≤0.88). In-situ XRD reveals staged transitions corresponding to distinct crystalline phases during K ions intercalation, which are distinct from that of Li and Na ions. Electrochemical (cyclic voltammetry and galvanostatic charge/discharge) studies show that the phase transitions among various intercalated stages slow down the kinetics of the discharge/charge in bulk TiS2 host. By chemically pre-potassiating the bulk TiS2 (K0.25TiS2) to reduce the domain size of the crystal, these phase transitions are bypassed and more facile ion insertion kinetic can be obtained, which leads to improved coulombic efficiency, rate capability and cycling stability.
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Since the pioneering work by Whittingham on the concept of intercalation electrode, TiS2 has been widely studied as a promising cathode for rechargeable lithium ion batteries (LIBs) due to its high energy density, high electrical conductivity and superior rate capability.1 However, the use of this material for LIBs gradually fell into decline due to safety problems caused by lithium dendrites during repeated charge/discharge cycles. On the other hand, the scarcity and rising cost of lithium resources makes it cogent to exploit alternative energy storage devices that are based on earth-abundant elements, i.e. sodium or potassium. Compared to the other alkali metal ion batteries, potassium ion batteries (KIBs) offer some unique advantages. Firstly, similar to sodium, potassium can be used with minimum free electrolyte between the electrodes as it can be evenly deposited on the electrode surface;2 Secondly, due to the exceptionally negative potential of the K/K+ redox couple (-2.93 V), K-based batteries can deliver a higher cell voltage than that of sodium ion batteries when using cathodes with the same redox cores. Indeed, the standard potential of K/K+ is even advantageous compared to Li/Li+ in commonly used non-aqueous electrolyte;3-10 Thirdly, potassium is an abundant element, unlike lithium.11-15 Moreover, potassium as well as sodium do not alloy with aluminum at low potentials, thus allowing the latter to replace copper current collector and reduces the cost of the battery.16 Ji’s group17 reported electrochemically intercalated K into graphite as anode in a non-aqueous electrolyte. More recently, Mai’s group18 reported a Fe/Mn-based layered oxide (K0.7Fe0.5Mn0.5O2) interconnected nanowires for KIBs cathodes, which delivers an initial discharge capacity of 178 mAh g-1 and good cycling performance. Goodenough et. al.19 reported a low-cost high-energy potassium cathode of cyanoperovskite KxMnFe(CN)6 (0≤x≤2), which also delivers an attractive specific capacity of 142 mAh g-1. However, the electrochemical performances of these materials in KIBs are far from satisfactory due to the structural changes and poor kinetics caused by the insertion of large size K ions. Moreover, insights into reaction mechanisms are hampered by the lack of systematic structural studies. The advantages of potassium ion batteries motivate researchers to relook at some of the older electrode materials such as TiS2 cathode, which were deemed promising at one time. Despite numerous studies on the electrochemical performance of TiS2 cathode in Li-ion20-23 and Na-ion batteries,24-27 there 3 ACS Paragon Plus Environment
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are no studies on the charge/discharge mechanisms and phase transformations of TiS2 cathodes during the insertion/extraction of K ions. Whittingham was the first to demonstrate fast, reversible Li insertion into TiS2 over the solid-solution range 0≤x≤1 of LixTiS2.1 Hibma showed 3 phases in NaxTiS2 (0≤x≤1) during Na insertion into TiS2, with the existence of a well-defined three-dimensional super-lattice.28 Due to the larger ionic radius of K+, its phase transformations are expected to be different from Li+ and Na+ ions. The large ionic size of K+ is also expected to impose lattice strains and results in low coulombic efficiency, inferior rate performances and poor cyclic properties for K-ion batteries. To date, there is a lack of systematic studies of the different electrochemical behaviors of TiS2 during intercalation of Li+, Na+ and K+ ions. Herein, to improve the electrochemical performances of bulk TiS2 in KIBs, we introduce a chemical pre-potassiation strategy which induces the nano-restructuring of TiS2 crystal domains. To shed light on the intercalation mechanisms of TiS2 for KIBs, the ionic size-dependent electrochemical properties, stability, phase transformations and voltage profiles during the intercalation of alkali metal ions (Li, Na and K) in TiS2 are investigated using in-situ X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques. Multiple phase transformations were observed by in-situ XRD studies during potassiation of TiS2. We discover that by chemically pre-potassiating bulk TiS2, the samples can be rendered nanocrystalline. Nano-restructuring of the crystal domain can improve the performance of potassium battery, as misfit strain can be relieved by structural dislocations. Furthermore, the pre-potassiated TiS2 (K0.25TiS2) bypassed the buffer phase in KIBs, which is extremely important to improve the electrochemical performances. The particle morphology and crystalline structure of TiS2 powders were characterized by XRD and SEM, as shown in Figure S1. The XRD patterns indicate that the pristine TiS2 powders have the hexagonal structure of pure TiS2, which match JCPDS No. 88-1967 (Figure S1a). The SEM image of crystalline TiS2 powders (Figure S1b) shows typical crystal morphology with an irregular particle size (from 4-10 µm). The electrochemical performances (CVs and galvanostatic discharge/charge profiles) of ACS Paragon Plus Environment
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TiS2 electrodes for LIBs, NIBs and KIBs are evaluated and shown in Figure S2. In the CVs of a K/TiS2 cell, five cathodic peaks (located at 2.37, 2.19, 1.84, 1.26 and 1.19 V) and five anodic peaks (located at 2.73, 2.51, 2.25, 1.90 and 1.36 V) are observed, indicating that continuous multiphase changes occur during the potassiation/depotassiation processes. Plateaus at similar potentials, consistent with CVs, are also observed in the discharge and charge profiles. The initial potassiation discharge (210.7 mA h g-1) corresponds to 0.88 mole of K intercalated into 1 mole TiS2, forming K0.88TiS2. The following depotassiation charge capacity is only 145.8 mA h g-1, with a low initial coulombic efficiency of 69.2%. The second discharge/charge capacities are 150.4 and 144.5 mA h g-1, respectively, with a coulombic efficiency higher than 96%. Comparing the initial and second discharge profiles, the capacity loss mainly exists in the plateau of 2.5-2.3 V, which is consistent with the intensity decrease of the first cathode peak (C1) in CV curves. This partially irreversible potassiation/depotassiation process is responsible for the low coulombic efficiency in the first cycle. As determined from the loss capacity (64.9 mA h g-1) of the first discharge to charge, about 0.25 molar equivalent K is deactivated in TiS2 (formed K0.25TiS2) after the first cycle. We associate this recurring deactivation behavior in the first discharge/charge cycle to the energy loss expended during the formation of an intermediate phase, which is elaborated in later sections. This provides a potential avenue for improving coulombic efficiency and cycling performance of TiS2 for KIBs. For comparison, there are fewer redox peaks in the CV curves of LIBs and NIBs (two pairs of broad peaks centered at 2.3 and 1.95 V for Li/TiS2 cell and approximately two groups of symmetrical redox peaks centered at 2.1 and 1.6 V for Na/TiS2 cell), which indicate a different phase transformation from K/TiS2 cell during the electrochemical process. Li+, Na+ and K+ alkali ions are distinguished by their ionic size, which leads to markedly different electrochemical properties.
The
phase
transformations,
which
occur
during
the
different
stages
of
lithiation/sodiation/potassiation, are important to the stability, capacity and voltage profiles of the TiS2 electrode as cathode in rechargeable batteries. Previously, we have shown that the pre-intercalation of alkali metal ions (Li and Na) in layered transition metal dichalcogenides (MoS2 and WS2) transforms the bulk crystal into a quasi-2D 5 ACS Paragon Plus Environment
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nanocrystalline crystal, in which this phase conversion is responsible for excellent rechargeable battery performances.29 Herein, the chemical pre-potassiation of the bulk TiS2 was carried out using potassium naphthalenide as the intercalating agent to nano-restructure the crystal domains of TiS2 (Figure 1a). To study the effect of pre-potassiation, we performed XRD, XPS and TEM on the chemically prepotassiated TiS2 sample. The (001) peaks of the pristine TiS2 is shifted completely toward lower angles (Figure 1b), consistent with the formation of K0.25TiS2 as observed in the in-situ XRD study. Moreover, a mixed nano-polycrystalline, potassiated TiS2 (K0.25TiS2) phase, with an expended d-spacing (~0.762 nm), is observed as revealed by SAED and HRTEM images (Figure 1c). The expended d-spacing facilitates the diffusion of large cations, i.e. K+, reversible diffusion. It is also interesting to note that the potassium content in TiS2 does not vary with the pre-potassiation time and the concentration of potassium naphthalenide solution (as shown in Figure S9), revealing that the intercalation process is driven by the thermodynamics to form K0.25TiS2. Nanocrystalline K0.25TiS2 is obtained after the chemical pre-potassiation process, as revealed by in-situ XRD where the pre-potassiated TiS2 (here considered as K0.25TiS2) bypassed the buffer phase to the reversible phase. Figure 2 shows the electrochemical behaviors (initial CVs and galvanostatic discharge/charge profiles, cycling and rate performances) of the TiS2 and K0.25TiS2 electrodes. The K0.25TiS2 cell presents an OCP of ~2.4 V vs. K/K+, which is lower than that of bulk TiS2 cell (~2.8 V), thus the pre-potassiation TiS2 cell bypassed the first cathodic peak (C1) in the initial CV curve. The K0.25TiS2 cell delivers an initial discharge capacity of 145.0 mA h g-1 and subsequent charge capacity of 151.1 mA h g-1 at 0.1 C, with an initial coulombic efficiency of 104.2% due to pre-intercalation of K ions. Thus, the low initial coulombic efficiency of the TiS2 electrode for KIBs can be significantly increased by the prepotassiation process. Moreover, we found that the cycling and rate performances of TiS2 electrode for KIBs (Figure 2c, d) are also largely improved due to the nano-restructuring of bulk TiS2 by the chemical pre-intercalation.
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To shed light on the intercalation mechanisms of K in TiS2, in-situ XRD measurements were performed to monitor the phase changes accompanying the discharge and recharge of the K/TiS2 cell. In the K/TiS2 cell, as can be seen in Figure 3, the periodical shifting of (001) peak suggest the occurrence of reversible phase changes during K+ ion intercalation/extraction. In the early stages of K+ ion intercalation (x
