Nanoconfined Iron Oxychloride Material as a High-Performance Cathode for Rechargeable Chloride Ion Batteries

As a group of attractive photoelectromagnetic and catalytic functional materials, metal oxychlorides have been attracting attention for electrochemical energy storage in rechargeable chloride ion battery (CIB) systems recently. Their application, however, is limited by the complicated synthesis and/or poor cycling stability. Herein, a facile strategy using vacuum impregnation and subsequent thermal decomposition at mild conditions has been developed to synthesize the FeOCl/ CMK-3 nanocomposite material. Benefiting from the nanoconfined structure, a high-performance FeOCl/CMK-3 cathode, which has a high discharge capacity of 202 mAh g−1, superior cycling stability, and significantly improved charge transfer and chloride ion diffusion, is achieved. The electrolyte component is found to show a high affinity with the chlorine layer in the FeOCl phase, inducing evident expansion of the FeOCl layers along the b-axis direction and thus boosting a new potential liquid exfoliation approach for preparing 2D FeOCl material. Importantly, reversible electrochemical reactions of the FeOCl cathode material based on the redox reactions of iron species and chloride ion transfer are revealed. Metal oxychlorides MOxCly (M = transition or rare earth metal) are a group of chemical compounds in which both oxygen and chlorine atoms are bonded to a metal element. They have strong in-plane M−O chemical bonds and weak out-of-plane van der Waals interaction between adjacent Cl−Cl layers. This structure could facilitate the separation and transport of charge carriers, contributing to the development of metal oxychlorides as high-efficiency photocatalysts for water splitting, environment remediation, and hydrogen production under UV or visible light. The electronic, magnetic, and photoluminescent behaviors of metal oxychlorides are also of great interest. Furthermore, applications of metal oxychlorides in glucose and gas sensors and Fenton-like catalysts for degradation of organic contaminants were also developed. Recently, owing to high stability, desired high theoretical energy density, and chlorine resources, metal oxychlorides such as BiOCl, FeOCl, and VOCl have been employed as cathode materials for chloride ion batteries (CIBs), which is a new rechargeable battery based on chloride ion transfer, using Li or Mg as the anode material. Besides the high theoretical energy density, FeOCl material possesses abundant elemental components. Currently, the synthesis of FeOCl is commonly performed by a complicated chemical vapor transport method, which requires a high temperature of 643 K (370 °C) for several days, leads to the formation of a large particle size of FeOCl, and brings about Received: August 3, 2017 Accepted: September 12, 2017 Published: September 12, 2017 Leter http://pubs.acs.org/journal/aelccp © XXXX American Chemical Society 2341 DOI: 10.1021/acsenergylett.7b00699 ACS Energy Lett. 2017, 2, 2341−2348 high pressure during synthesis because of the vaporization of FeCl3. 14,16,17 Therefore, a facile synthesis method of FeOCl is highly demanded. Second, FeOCl has a high electrical resistivity (ER) of 10 Ω·cm at ambient conditions, which is 2 orders of magnitude higher than that of pure Si. Conventional carbon incorporation at high temperature for improving the electrical conductivity of FeOCl would not be realized because the decomposition of FeOCl occurs at around 643 K (370 °C). Alternatively, mechanical milling of FeOCl with different carbon materials at room temperature was carried out, and the charge transfer resistance of the FeOCl cathode could be drastically decreased, resulting in an increase of the maximum discharge capacity. Nevertheless, decomposition of FeOCl by mechanical milling was observed, as shown in Figure S1, and good cycling stability was not obtained. The third challenge for the application of FeOCl lies in buffering the large volume change during cycling. Theoretically, removal of the chlorines in FeOCl would cause a volume contraction of 58.6%. Actually, the issues concerning the high ER and large volume variation also exist in utilization of the attractive sulfur (ER: 10 Ω·cm) or silicon electrode material in rechargeable batteries. A nanoconfinement approach using porous carbon as the matrix enabled realization of high electronic conduction and structural integrity of these electrode materials during cycling. Herein, we report a nanoconfined FeOCl cathode material, which was successfully prepared by a thermal decomposition method and the incorporation of CMK-3 under mild conditions below 473 K. The as-prepared FeOCl/CMK-3 nanocomposite cathode shows a maximum discharge capacity of 202 mAh g−1 and a high reversible capacity of 162 mAh g−1 after 30 cycles, owing to the drastically enhanced charge transfer and chloride ion diffusion. Intercalation of organic molecules and also the electrolyte into the layers of FeOCl before cycling has been observed. The electrochemical reaction mechanism based on chloride ion transfer of the FeOCl cathode material in CIBs is further elucidated. A schematic fabrication process of the FeOCl/CMK-3 material is shown in Figure 1a. First, the FeCl3·6H2O and CMK-3 powders were mixed together in a mortar. The mixture was loaded into an evacuated and sealed quartz tube and then treated at 353 K for 24 h. Thereafter, the obtained FeCl3· 6H2O/CMK-3 powders were heat treated at 453 K for 10 h in a flask under reduced pressure. The pure FeOCl material was prepared directly by this thermal decomposition method without the addition of CMK-3. Figure 1b−h shows the morphology and structure results of the as-prepared pure FeOCl and FeOCl/CMK-3 nanocomposites. The SEM images in Figure 1b,c represent an agglomerated morphology of the asprepared FeOCl material. Some nanoflakes with a thickness of about 40 nm were also observed and aggregated in the bulk FeOCl particles. In contrast, the as-prepared FeOCl/CMK-3 nanocomposites exhibit distinct morphologies, including wormlike CMK-3 particles coexisting with some highly dispersed nanosheets with a thickness of about 20 nm (Figure 1d,e). This suggests that the addition of CMK-3 can prevent the agglomeration of FeOCl during its crystallization process by the thermal decomposition of FeCl3·6H2O. The corresponding XRD patterns in Figure 1f show that all of the reflections of both FeOCl and FeOCl/CMK-3 samples can be indexed and assigned to the orthorhombic layered FeOCl phase (PDF card no. 72-619) with three characteristic peaks corresponding to (010), (110), and (021) lattice planes. The decrease in the intensity ratio between (010) and (110) lattice planes indicates Figure 1. (a) Schematic fabrication process of the FeOCl/CMK-3 material. SEM images and XRD patterns of (b,c,f) the as-prepared FeOCl and (d,e,f) FeOCl/CMK-3 nanocomposite. (g) TEM image of the CMK-3 powder. (h) TEM image and the corresponding SAED and elemental mapping images of the FeOCl@CMK-3 nanocomposite. ACS Energy Letters Letter DOI: 10.1021/acsenergylett.7b00699 ACS Energy Lett. 2017, 2, 2341−2348 2342 the formation of thinner FeOCl layers in the FeOCl/CMK-3 sample. This is consistent with the SEM results in Figure 1d,e. TEM analysis was investigated to further illustrate the structure and morphology of CMK-3 and the FeOCl/CMK-3 nanocomposite. CMK-3 shows its clear mesoporous channels with a pore size of about 4 nm in Figure 1g. After FeOCl loading, the apparent morphology of CMK-3 was retained, while the internal mesoporous channels were blocked, as shown in Figure 1h. The elemental mapping analysis of this sample reveals the uniform distribution of Fe, O, and Cl elements, and the corresponding EDS pattern in Figure S2 demonstrates the fine composition of the FeOCl in CMK-3, indicating the successful nanoconfinement of FeOCl in CMK-3 (FeOCl@CMK-3 nanohybrid) by vacuum impregnation and subsequent thermal decomposition. The corresponding selected area electron diffraction (SAED) image shows only the reflections of the semigraphitized CMK-3. This is consistent with the result of few-layer FeOCl nanosheets, which also showed no evident diffraction. Figure 2 shows the electrochemical properties of the FeOCl/ Li electrode systems. The typical discharge and charge profiles of the electrode system using the as-prepared FeOCl/CMK-3 cathode are presented in Figure 2a. The FeOCl/CMK-3 cathode shows a discharge capacity of 174 mAh g−1 and a dominant two-step discharge stage at the first cycle. A maximum discharge capacity of 202 mAh g−1 (81% of the theoretical discharge capacity) was received after electrochemical activation in the first cycle, which could be attributed to refinement of the particle/grain size by the conversion reaction. The voltage hysteresis between discharge and charge was narrowed by this activation. The CV patterns of this electrode system in Figure 2b exhibit two dominant pairs of cathodic and anodic stages under the potential window of 1.6− 3.5 V. The first reduction stage appears from 2.8 to 2.1 V and consists of a weak peak at 2.6 V and a distinct peak at 2.38 V. The corresponding oxidation stage is centralized at around 2.88 V. Another redox couple at the lower potential includes a reduction peak at 1.9 V and the corresponding oxidation peak at 2.19 V. The increase in the peak current and the decrease in the potential gap after the first cycle are in accordance with the aforementioned activation process. Similar discharge and charge profiles (Figure 2c) were observed when the asprepared FeOCl was used as the cathode. The pure FeOCl cathode has a maximum discharge capacity of 165 mAh g−1, which is much lower than that of the as-prepared FeOCl/ CMK-3 cathode. Upon cycling, an obvious capacity decay occurred and only 100 mAh g−1 (60.6% capacity retention rate) of the as-prepared FeOCl cathode was maintained after 30 cycles (Figure 2d). Still, this capacity is much higher than 60 mAh g−1 (37.5% capacity retention rate) of the FeOCl material fabricated by the chemical vapor transport method at high temperature, followed by mechanical milling with carbon black. Therefore, the thermal decomposition of FeCl3·6H2O at mild conditions facilitates the formation of high-performance FeOCl material. In comparison with pure FeOCl, the asprepared FeOC/CMK-3 cathode retained a high discharge capacity of 162 mAh g−1 (80.2% capacity retention rate) after 30 cycles. This indicates that the incorporation of CMK-3 in the FeOCl cathode contributes to significant improvements in the electrochemical activity and the structural stability of FeOCl, stimulating the addition of more CMK-3 in the FeOCl cathode. As shown in Figure S3, the diffraction peaks of the FeOCl were broaden and weakened by the increase in the CMK-3 content. This may be related to the decrease of FeOCl content and/or refinement in the particle/grain size of FeOCl. However, Fe2O3 was formed and chlorine content was decreased (Figure S3, EDS) when more CMK-3 was added, which may be caused by the delay in the release of water vapor during the decomposition of FeCl3·6H2O. The water vapor then reacted with the fresh FeOCl and led to the formation of Fe2O3, resulting in reduction of the active material content. As a consequence, the discharge capacity of the FeOCl cathode was evidently decreased when more CMK-3 was introduced, as shown in Figure S4. Figure 2. Discharge and charge curves (10 mA g−1) and CV patterns (60 μV s−1) of the as-prepared (a,b,d) FeOCl/CMK-3 and (c,d) FeOCl cathodes. (e) Nyquist plots and (f) the relationship between Z′ and ω−1/2 in the low-frequency region of the as-prepared FeOCl/CMK-3 and FeOCl cathodes. ACS Energy Letters Letter DOI: 10.1021/acsenergylett.7b00699 ACS Energy Lett. 2017, 2, 2341−2348 2343 To understand the reasons for the impressive charge and discharge performance of the FeOCl/CMK-3 nanocomposite, electrochemical impedance spectroscopy (EIS) measurement was carried out (Figure 2e,f). The electrochemical processes of both the as-prepared FeOCl and FeOCl/CMK-3 electrode systems show a controlling step of a mixed rate-determining process containing charge transfer and chloride ion diffusion steps. The EIS curves consist of three parts, a semicircle at high frequency followed by a further semicircle and a straight line at low frequency (Figure 2e). The former semicircle extended to the Z′ axis is reflected by a solution resistance (Rs), which includes ionic resistance from the separator and electrolyte. The former semicircle is considered to be the contact resistance (Rc), the later semicircle may be related to the charge transfer process (Rct), and the straight line should be associated with the Warburg impedance by chloride ion diffusion. The asprepared FeOCl cathode shows a notable Rc of 161.6 Ohm and an Rct of 573.9 Ohm. These values of the as-prepared FeOCl/CMK-3 nanocomposite were significantly decreased to 69.1 and 345.2 Ohm, implying that higher electrical conductivity and a faster charge transfer reaction were obtained for the as-prepared FeOCl/CMK-3 nanocomposite. From the straight line in the low-frequency range, the diffusion coefficient of the chloride ion is estimated according to the following equation σ = D R T A n F C ( )/(2 ) 2 2 2 4 4 2 2 (1) where D is the ionic diffusion coefficient, R is the gas constant, T is the absolute temperature, A is the surface area of the electrode, n is the number of electrons per molecule, F is the Faraday constant, C is the ionic concentration, and σ is the Warburg factor, which has a relationship with Z′