⇦Chinese Physics Letters, 2021, Vol. 38, No. 6, Article code 068201 FeSO$_{4}$ as a Novel Li-Ion Battery Cathode Jiachao Yang (杨家超)1, Jian Zou (邹剑)1, Chun Luo (罗春)1, Qiwen Ran (冉淇文)1, Xin Wang (王鑫)1, Pengyu Chen (陈鹏宇)1, Chuan Hu (胡川)1, Xiaobin Niu (牛晓滨)1, Haining Ji (姬海宁)1*, and Liping Wang (王丽平)1,2* Affiliations 1School of Materials and Energy, University of Electronic Science and Technology of China, Chengdu 610054, China 2Tianmu Lake Institute of Advanced Energy Storage Technologies, Changzhou 213300, China Received 17 February 2021; accepted 30 March 2021; published online 25 May 2021 Supported by the Fundamental Research Funds for the Central Universities, China (Grant No. ZYGX2019Z008), and the National Natural Science Foundation of China (Grant No. 52072061).
*Corresponding authors. Email: hainingji@uestc.edu.cn; lipingwang@uestc.edu.cn Citation Text: Yang J C, Zou J, Luo C, Ran Q W, and Wang X et al. 2021 Chin. Phys. Lett. 38 068201    Abstract FeSO$_{4}$ has the characteristics of low cost and theoretical high energy density (799 W$\cdot$h$\cdot$kg$^{-1}$ with a two-electron reaction), which can meet the demand for next-generation lithium-ion batteries. Herein, FeSO$_{4}$ as a novel high-performance conversion-reaction type cathode is investigated. We use dopamine as a carbon coating source to increase its electronic conductivity. FeSO$_{4}$@C demonstrates a high reversible specific capacity (512 mA$\cdot$h$\cdot$g$^{-1}$) and a superior cycling performance (482 mA$\cdot$h$\cdot$g$^{-1}$ after 250 cycles). In addition, we further study its reaction mechanism. The FeSO$_{4}$ is converted to Fe and Li$_{2}$SO$_{4}$ during lithium ion insertion and the Fe|Li$_{2}$SO$_{4}$ grain boundaries further store additional lithium ions. Our findings are valuable in exploring other new conversion-type lithium ion battery cathodes. DOI:10.1088/0256-307X/38/6/068201 © 2021 Chinese Physics Society Article Text With the increasing concerns about energy and environmental security, renewable and green energies such as lithium-ion batteries (LIBs) are becoming more attractive.[1–3] After decades of development, LIBs have been widely used in portable devices (smartphones, Pads et al.) and electric cars. However, higher energy density LIBs, and especially the cathodes, urgently need to meet the demand for a longer endurance time.[4] Meanwhile, the cost is also a key issue for the commercialization of LIBs. Up to now, researchers have explored a series of cathode materials with both high energy density and low cost, such as Li-rich/Ni-rich layered oxides,[5] spinel oxides and conversion-type fluorides.[6–8] Apart from these cathodes, some polyanionic sulfate compounds also meet the requirements of low cost and high energy densities.[9] Sulfates have the advantages of non-toxicity and high operation potential, which have drawn a lot of concerns.[10–12] A typical sulfate can be expressed as M$_{a}$(SO$_{4}$)$_{b}$ (M = Fe, Cu, V, etc.),[13] where the [MO$_{6}$] octahedron and [SO$_{4}$] tetrahedron are packaged by oxygen atom point sharing. The strong bonding between S$^{6+}$ and O$^{2-}$ leads to a relatively weak interaction between M$^{2+}$ (or M$^{3+}$) and O$^{2-}$, resulting in a lower energy of M$^{2+}$ or M$^{3+}$.[13,14] In other words, the working potential can be very high when compared with other conversion type cathodes (FeS$_{2}$, MnO$_{2}$, etc.).[10,11,15] A common sulfate, Fe$_{2}$(SO$_{4}$)$_{3}$, was firstly studied by Manthiram et al. in 1989.[16] It was found that Fe$_{2}$(SO$_{4}$)$_{3}$ has two types of crystal structures (rhombohedral structure and orthorhombic framework) with a specific capacity of 100 mA$\cdot$h$\cdot$g$^{-1}$, while the work potential is 3.6 V (vs Li/Li$^{+}$). Wu et al. further studied the lithium ion insertion/extraction mechanism of the mikasaite-type Fe$_{2}$(SO$_{4}$)$_{3}$.[17] At initial discharge, two lithium ions were inserted in the Fe$_{2}$(SO$_{4}$)$_{3}$ to obtain Li$_{2}$Fe$_{2}$(SO$_{4}$)$_{3}$. However, only the second lithium ion was reversible in the subsequent cycles. Another sulfate, CuSO$_{4}$, also attracted attention because of its high operating voltage.[18] Schwieger et al. reported that the working potential for CuSO$_{4}\cdot$5H$_{2}$O is 3.2 V (vs Li/Li$^{+}$),[14] with a specific capacity of 156 mA$\cdot$h$\cdot$g$^{-1}$. They believed that the elimination of the crystal water may degrade its electrochemical performance. Apart from these sulfates, fluoridated sulfates were expected to have a much higher operation voltage because of the strong electronegativity of fluorine.[19,20] LiFeSO$_{4}$F was demonstrated to have an operation voltage of 3.6 V (vs Li/Li$^{+}$), with a specific capacity of 140 mA$\cdot$h$\cdot$g$^{-1}$.[21] Similarly, the introduction of the OH$^{-}$ group showed a comparable effect. LiFeSO$_{4}$OH exhibited an average potential of 3.6 V (vs Li/Li$^{+}$) with 0.7 mol reversible lithium lions during insertion/extraction processes.[22] Note that many sulphates have a higher working voltage but lower capacity which leads to unsatisfactory energy density.[23] In addition, Nakayama mentioned that polyanionic salts provide high operating voltages,[24] but they simultaneously lead to low electronic conductivities. The theoretical capacity of FeSO$_{4}$ for a two-electron conversion reaction is 352 mA$\cdot$h$\cdot$g$^{-1}$, and the working potential is 2.27 V according to the Nernst equation. Therefore, FeSO$_{4}$ is expected to provide an energy density of 799 W$\cdot$h$\cdot$kg$^{-1}$, which is higher than the afore-mentioned sulfates. However, there are lacks of related reports about FeSO$_{4}$ as a lithium ion battery cathode. Herein, FeSO$_{4}$ is explored as a cathode in high energy density lithium metal batteries. With an expanded voltage window (0.01–3.0 V), it delivers an initial discharge specific capacity of 858 mA$\cdot$h$\cdot$g$^{-1}$, much higher than its theoretical value of 352 mA$\cdot$h$\cdot$g$^{-1}$. Meanwhile, FeSO$_{4}$ has a reversible capacity of 512 mA$\cdot$h$\cdot$g$^{-1}$ and maintains a specific capacity of 482 mA$\cdot$h$\cdot$g$^{-1}$ after 250 cycles. FeSO$_{4}$ undergoes a reversible reaction to Fe and Li$_{2}$SO$_{4}$. Lithium ions are stored in grain boundaries of Fe and Li$_{2}$SO$_{4}$ below 0.56 V. Material Synthesis. FeSO$_{4}$$\cdot$$x$H$_{2}$O ($x=6.3$, Aladdin Industrial Corporation) was sintered in a nitrogen atmosphere at 300℃ for 3 h to obtain anhydrous FeSO$_{4}$. FeSO$_{4}$@C was prepared by polydopamine (PDA) coating followed by an annealing process. In a typical procedure, 400 mg FeSO$_{4}$ was added to 30 ml ethanol and stirred at room temperature for 1 h to prepare solution A. Meanwhile, 60 mg 3-hydroxytryptamine hydrochloride (dopamine HCl, Shanghai Titan Scientific Co. Ltd) was dissolved in 5 ml ethanol to prepare solution B. In addition, 102 mg ferric chloride (FeCl$_{3}$, Shanghai Titan Scientific Co. Ltd) was dissolved in 5 ml ethanol to prepare solution C. Then, solution B was added dropwise to A and stirred for 1 h. Afterward, solution C was added dropwise in the above mixture and stirred for 12 h. The FeSO$_{4}$@PDA composites were obtained by centrifuging the mixture with ethanol several times and dried in a vacuum oven at 80℃ for 8 h. Then FeSO$_{4}$@PDA was sintered at 300℃ for 6 h in nitrogen to obtain FeSO$_{4}$@C composites. Material Characterizations. The thermogravimetry analysis (TGA) was conducted on a thermal analyzer (Netzsch STA 449C) under a nitrogen atmosphere with a heating rate of 5 ℃/min. The crystal structure was analyzed by x-ray diffraction (XRD, Bruker D8 Advance). Microstructures were examined using the transmission electron microscope (TEM, FEI Talos F200x). Morphologies were carried out by a scanning electron microscope (SEM, Hitachi S-4800). To value their conductivity, the samples were pressed into a pellet with a diameter of 13 mm under a pressure of 30 kN. The thickness of the FeSO$_{4}$ and FeSO$_{4}$@C composites tablets is 1.002 mm and 0.725 mm, respectively. The conductivity was calculated by Ohm's law. Electrochemical Characterizations. The active material, super P, and polyvinylidene fluoride (PVDF) in N-methyl pyrrolidone (NMP) were mixed in the weight ratio of $ 6\!:\!3\!:\!1$ to make a homogenous slurry. The electrodes were prepared by coating the mixture on the copper foil and then vacuum drying (80℃, 12 h). The mass loading is about 2 mg/cm$^{2}$. The coin cells were assembled in an argon-filled glovebox. Metallic lithium was used as an anode, celgard 2400 was used as the separator, and l M LiPF$_{6}$ dissolved in ethylene carbonate (EC)/dimethyl carbonate (DMC) with a $1\!:\!1$ volume ratio was used as the electrolyte. The specific capacity for FeSO$_{4}$@C composites is based on a sum weight of FeSO$_{4}$ and carbon. The ex situ XRD patterns were measured with electrodes after cycling. The electrodes were discharged to the corresponding voltage of each state (A–G) at a current density of 0.5 C. Galvanostatic intermittent titration technique (GITT) was performed on LANHE CT2001A (LAND Electronic Co. Ltd) battery test system. In detail, cells were cycled at a current density of 0.5 C (1 C = 352 mA$\cdot$g$^{-1}$) with a voltage window of 0.01–3.0 V, and the cells were charge/discharged for a duration time of 5 min and a relaxation period of 2 h. The cyclic voltammetry (CV) was obtained using the CHI660E (CH Instruments) electrochemical workstation. The scanning rate was 0.1 mV$\cdot$s$^{-1}$. The electrochemical impedance spectroscopy (EIS) measurements were carried out with the same instrument. The amplitude was set as 5 mV with frequency range from $1 \times 10^{5}$ to $1 \times 10^{-2}$ Hz. Results and Discussion. FeSO$_{4}$$\cdot$$x$H$_{2}$O is heated from room temperature to 750℃ in nitrogen at a heating rate of 5℃/min. The TGA curve is shown in Fig. 1(a). With the increase of temperature, the decomposition process of FeSO$_{4}$$\cdot$$x$H$_{2}$O as it turns into Fe$_{2}$O$_{3}$ can be expressed as follows: FeSO$_{4}\cdot$7H$_{2}$O (room temperature) $\to$ FeSO$_{4}$$\cdot$$x$H$_{2}$O ($ < 77$ ℃) $\to$ FeSO$_{4}$$\cdot$5H$_{2}$O (86–140 ℃) $\to$ FeSO$_{4}$$\cdot$H$_{2}$O (213–260 ℃)  $\to$ FeSO$_{4}$ (500–650℃) $\to$ Fe$_{2}$O$_{3}$. Based on the weight loss, the value of $x$ is determined to be 6.3. The XRD pattern in Fig. 1(b) confirms that anhydrous FeSO$_{4}$@C (JCPDS No. 17-0873) can be synthesized by annealing FeSO$_{4}$@PDA at 300℃ for 6 h. The corresponding crystal structure is also shown in the inset. The basic structural unit of FeSO$_{4}$ is composed of iron oxide octahedron and sulfur oxygen tetrahedron. The iron element and sulfur element share the oxygen atom. This is a kind of layered structure with 3D lithium-ion channels, which can facilitate the transport of lithium ions. The carbon content in FeSO$_{4}$@C is calculated to be 8.9 wt% in Fig. S1 in the Supporting Information, and conductivities are calculated by Ohm's law. In Fig. 1(c), the resistances of FeSO$_{4}$ and FeSO$_{4}$@C composites are 10$^{9}\,\Omega$ and $1.34 \times 10^{5 }\,\Omega$, respectively. Thus, the FeSO$_{4}$ exhibits a conductivity of less than 10$^{-9}$ S/m, four orders of magnitude lower than that of FeSO$_{4}$@C composites ($3.78 \times 10^{-5}$ S/m). This result shows that carbon coating can improve the conductivity of FeSO$_{4}$.[25] The morphologies of FeSO$_{4}$@C are shown in Figs. 1(d)–1(e). Figure 1(d) reveals that FeSO$_{4}$@C composites are irregular particles with particle sizes less than 100 µm. In Fig. 1(e), many pores are observed, which can be assigned to the crystal water vapor vent during the annealing. In Fig. 1(f), an amorphous surface on FeSO$_{4}$ can be attributed to the carbon layer. Its thickness is estimated to be 3.1 nm. In addition, the crystallized part can be assigned to FeSO$_{4}$. The interlayer distance of 0.27 nm corresponds to its (111) plane.
cpl-38-6-068201-fig1.png
Fig. 1. (a) TGA curve of FeSO$_{4}$$\cdot$$x$H$_{2}$O ($x=6.3$) in N$_{2}$ at a heating rate of 5 ℃/min. (b) XRD pattern and crystal structure of FeSO$_{4}$@C composites. (c) The conductivity of FeSO$_{4}$@C composites and pristine FeSO$_{4}$. (d) and (e) SEM images of FeSO$_{4}$@C composites at different magnifications. (f) TEM image of FeSO$_{4}$@C.
cpl-38-6-068201-fig2.png
Fig. 2. (a) $C$–$V$ curves of the FeSO$_{4}$@C at 0.1 mV/s. (b) Charge/discharge profiles of FeSO$_{4}$@C (0.01–3.0 V). (c) Cycling performance of the FeSO$_{4}$@C and pristine FeSO$_{4}$. (d) EIS of the FeSO$_{4}$@C and FeSO$_{4}$. (e) Charge/discharge curves of FeSO$_{4}$@C (1.0–3.0 V from the 2nd cycle). (f) Cycling performance of FeSO$_{4}$@C (1.0–3.0 V from the 2nd cycle).
Figure 2(a) shows the $C$–$V$ curves of FeSO$_{4}$@C. For the initial scan, the reduction peak around 1.0 V indicates that FeSO$_{4}$ begins to decompose, and is completely decomposed at the peak of 0.56 V. Meanwhile, it is also accompanied with the formation of solid electrolyte interphase (SEI). The broad oxidation peak is located at 2.57 V. The highly overlapped 2nd and 3rd scans exhibit excellent reversibility. The oxidation peak and reduction peak are located at 2.76 V and 1.87 V, respectively, indicating that FeSO$_{4}$ is converted to Fe and Li$_{2}$SO$_{4}$ during the reduction while FeSO$_{4}$ is recovered during the oxidation. A detailed discussion will be conducted later. The misalignment between redox peaks indicates a voltage hysteresis, which is related to asymmetrical reaction pathways and sluggish kinetics.[7] Figure 2(b) displays the charge/discharge curves of FeSO$_{4}$@C at 0.5 C (0.01–3.0 V). The initial discharge capacity is 858 mA$\cdot$h$\cdot$g$^{-1}$. In the subsequent cycles, the lithium ion insertion reaction mechanism is slightly different from the initial cycle. The 2nd discharge capacity is 512 mA$\cdot$h$\cdot$g$^{-1}$, and the efficiency is 91% after 100 cycles (467 mA$\cdot$h$\cdot$g$^{-1}$). The irreversible capacity loss (346 mA$\cdot$h$\cdot$g$^{-1}$) in the initial discharge is attributed to the formation of the SEI layer.[26] Figure 2(c) compares the cycle performance of FeSO$_{4}$ and FeSO$_{4}$@C at 0.5 C. After 250 cycles, the capacity of pristine FeSO$_{4}$ fades (120.7 mA$\cdot$h$\cdot$g$^{-1}$) rapidly while FeSO$_{4}$@C exhibits excellent stability (482 mA$\cdot$h$\cdot$g$^{-1}$). This is due to the carbon coating, which increases the electronic conductance and suppresses the potential volume change during the conversion reaction of FeSO$_{4}$. AC impedance is employed to reveal the fading mechanism by analyzing their resistance variations. Figure 2(d) shows the testing and fitting results of EIS. The intercept between the semicircle and the $x$-axis is the inherent impedance $R_{\rm f}$ of the battery. In the high-frequency region, the curve is composed of two partially stacked semicircles. The first semicircle is the SEI impedance ($R_{\rm s}$//CPE$_{\rm s}$), while the second semicircle represents the charge transfer resistance ($R_{\rm ct}$//CPE$_{\rm ct}$). In the low-frequency region, the straight line represents Warburg impedance $W$, which is related to the lithium-ion diffusion. The charge transfer resistance of FeSO$_{4}$ decreases in the initial 10 cycles. This behavior can be explained by the activation of FeSO$_{4}$ originating from electrolyte infiltration.[3] Note that FeSO$_{4}$@C shows smaller resistances in both 1st and 10th cycles. This result demonstrates that the carbon layer can efficiently reduce the charge transfer resistance and leads to better rate performance. The influence of voltage window between 1.0–3.0 V is also studied. The initial voltage window is 0.01–3.0 V, which is to trigger the decomposition of FeSO$_{4}$ and the initial capacity is 874 mA$\cdot$h$\cdot$g$^{-1}$. Figure 2(e) is the charge/discharge curves at a current rate of 0.5 C (1.0–3.0 V from 2nd cycle), and the cycle performance is shown in Fig. 2(f). The discharge capacity of FeSO$_{4}$@C composites is about 210 mA$\cdot$h$\cdot$g$^{-1}$ at the 2nd cycle, while the discharge capacity after 100 cycles is only 63 mA$\cdot$h$\cdot$g$^{-1}$ at 0.5 C. Obviously, the capacity and the cycling stability between 1.0–3.0 V are worse than 0.01–3.0 V. The reason is not yet clear. It seems that grain boundary storage in the low voltage range contributes significantly to the cycle stability. Thus, for a higher capacity and energy density, using 0.01–3.0 V as the voltage range would be more proper.
cpl-38-6-068201-fig3.png
Fig. 3. (a) GITT curves of FeSO$_{4}$@C at a constant current at 0.5 C for 5 min and a relaxation of 2 h. (b) Polarization voltages as a function of specific capacity. (c) Specific state (A–G) for ex situ XRD tests. (d) The ex situ XRD patterns at specific states (A–G).
GITT measurements are used to determine the kinetic characteristics of FeSO$_{4}$@C. The curves of the first 2 cycles are shown in Fig. 3(a). In the GITT model, the discharge capacity increases to 999 mA$\cdot$h$\cdot$g$^{-1}$ at the initial cycle and 514 mA$\cdot$h$\cdot$g$^{-1}$ at the 2nd cycle. Figure 3(b) shows the overpotential variation during the cycling. At the early stage of initial discharge, the overpotential reaches up to 1.6 V. This value gradually decreases with insertion of lithium ions. At the end of the discharge, the overpotential is only 0.2 V. This overpotential drop can be attributed to the generation of Fe during the conversion process that is helpful for improving the conductivity. Thus, with Fe gradually disappearing in the charging process, the overpotential increases. Note that the overpotential of the second discharge is smaller than the initial discharge. This behavior is due to the nanosize effect after the discharge process.[27] To study the charge/discharge mechanism of FeSO$_{4}$, ex situ XRD patterns are collected at different stages of charge and discharge. Figures 3(c) and 3(d) show the specific states (A–G) and their corresponding XRD patterns, respectively. During the discharge process, the FeSO$_{4}$ gradually disappears (A $\to$ B). This process can be simply expressed as follows: $$ {\rm FeSO}_{4}+ 2{\rm Li} \to {\rm Fe} + {\rm Li}_{2}{\rm SO}_{4}.~~ \tag {1} $$ Note that the theoretical capacity of this reaction is only 352 mA$\cdot$h$\cdot$g$^{-1}$, while the experimental value at state B is 530 mA$\cdot$h$\cdot$g$^{-1}$. Considering the SEI layer would generate within the voltage range ($ < 1.0$ V), this extra capacity can be attributed to the formation of SEI.[28] Moreover, the theoretical potential of reaction (1) calculated by the Nernst equation is 2.27 V,[18] corresponding to the theoretical energy density of 799 W$\cdot$h$\cdot$kg$^{-1}$. However, it is observed that the initial discharge plateau is below 2.0 V in Fig. 3(a), much lower than the theoretical value. Such a deviation shows that the overpotential is needed to initiate and to pursue the decomposition reaction.[29] This phenomenon is quite common in conversion type materials.[29,30] Indeed, there is no obvious Fe and Li$_{2}$SO$_{4}$ reflections at state B, which confirms that these two products are amorphous. With a further lithium-ion insertion (B $\to$ C), there would be a long voltage slope. This slope has been widely observed in other conversion type materials. The different fermi levels of Fe and Li$_{2}$SO$_{4}$ particles make their grain boundaries highly reactive and thus enable extra lithium ion insertions.[31] The storage mechanism is known as the “job-sharing”,[32] where the electrons are accommodated by Fe and the lithium ions are stored in Li$_{2}$SO$_{4}$. Thus, the reaction between 0.01 V and 0.56 V can be expressed as $$ {\rm Fe+Li}_{2} {\rm SO}_{4}+x {\rm Li} \to {\rm Fe}|x {\rm Li}|{\rm Li}_{2}{\rm SO}_{4}.~~ \tag {2} $$ During the charging process, there is no obvious changes in the XRD patterns for C $\to$ D, corresponding to the lithium ion extraction from the Fe|Li$_{2}$SO$_{4}$ grain boundaries. The gradual strength of FeSO$_{4}$ reflections from D $\to$ E $\to$ F indicates that reaction (1) is reversible. Thus, the overall reversible capacity is contributed by the conversion reaction between FeSO$_{4}$ and Fe+Li$_{2}$SO$_{4}$, and the Fe|Li$_{2}$SO$_{4}$ grain boundaries storage. In summary, the electrochemical properties of FeSO$_{4}$ as a Li-ion battery cathode and its reaction mechanisms have been revealed. We use a simple carbon coating method to obtain FeSO$_{4}$@C for improving its conductivity. The FeSO$_{4}$@C exhibits a high initial discharge specific capacity of 858 mA$\cdot$h$\cdot$g$^{-1}$, with a reversible specific capacity of 512 mA$\cdot$h$\cdot$g$^{-1}$ and delivers a specific capacity of 482 mA$\cdot$h$\cdot$g$^{-1}$ after 250 cycles (0.01–3.0 V). With ex situ XRD tests, it can be concluded that the reversible capacity is contributed by the conversion reaction between FeSO$_{4}$ and Fe+Li$_{2}$SO$_{4}$, and due to lithium ion storage between Fe and Li$_{2}$SO$_{4}$ grain boundaries at low voltage. The research provides a comprehensive understanding of FeSO$_{4}$ and mechanism studying for other conversion type materials. 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