The Surface Coating of Commercial LiFePO4 by Utilizing ZIF-8 for High Electrochemical Performance Lithium Ion Battery
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KeywordsLiFePO4 Zeolitic imidazolate frameworks-8 Surface coating Cathode Lithium ion battery
A surface modification layer, which has 10 nm with metal zinc and graphite-like carbon, was synthesized on commercial LiFePO4 (LFP) using ZIF-8.
As-prepared LFP/CZIF-8 possesses prominent electrochemical performances with a discharge specific capacity of 159.3 mAh g−1 at 0.1C and a discharge specific energy of 141.7 mWh g−1 after 200 cycles at 5.0C.
As the world’s population grows and the natural non-renewable resource consumes, developing the lithium ion batteries (LIBs) with high electrochemical performances become increasingly important [1, 2, 3]. Recently, LIBs with LiFePO4 (LFP) as cathodes have matured significantly and serviced the markets of modern electronics and electric vehicles [4, 5, 6]. However, the LFP with high specific energy and high cycle stability is still facing a great challenge, because the electron/ion transfer and structural stability of commercial electrode materials cannot meet the harsh ultrahigh-rate working environment [7, 8]. Therefore, it is the assiduous goal to develop advanced electrode materials with high performances.
High specific energy electrodes are highly related to voltage platforms [9, 10] and specific capacities of electrodes [11, 12]. For LFP cathode material, we have demonstrated that the increased voltage platform depends on the improvement of electrode conductivity . In the past study, it was demonstrated that the carbon coating was an effective strategy to improve the conductivity of electrode material . In general, the coating layer is amorphous carbon [15, 16], because the crystallization temperature of carbon is much higher than the crystal growth temperature of LFP. Meanwhile, the sp2 carbon material is conducive to the transmission of electrons , which may be beneficial to improve the conductive electrode material. Recently, some progress has been made in the surface coating of electrode material by using crystalline carbonaceous material at low temperature . Moreover, metal elements are also used to improve the electrode conductivity due to their free electrons . The improvement of the conductivity also has a significant effect on the specific capacity of the electrode material [1, 18]. These studies provide a theoretical basis for the improvement of specific energy of LFP cathode material.
The high cycle stability mainly depends on the structural stability of the electrode material at different charge and discharge current rates [19, 20]. For LFP cathode material, both volume shrinkage (charge/lithium ion deintercalation) and expansion (discharge/lithium ion intercalation) will lead to structural damage and thus cause capacity decline . It is demonstrated that the suitable coating layer with porous structure can effectively improve the structural stability of the active material by buffering volume change [22, 23]. This may be an appropriate modification method for commercial electrode materials.
The surface coating plays an important role in the improvements of the electrochemical performances. The choice of coating materials is the key to improve the electrode performance. Zeolitic imidazolate frameworks-8 (ZIF-8) is a class of porous materials, which attracted considerable interest due to its regular polyhedral morphology and ordered pore structure [24, 25]. The pyrolysis products of ZIF-8 under anaerobic conditions are porous carbon materials with high specific surface areas and high conductivities [26, 27]. Torad et al.  and Zhang et al.  reported that graphite-type carbon was prepared at 800 °C by using ZIF-8 as raw materials, and they also proved that crystalline carbon can be prepared at lower temperature. Moreover, the formation of metallic zinc annealed in an inert atmosphere is also conducive to the improvement of conductivity. Although ZIF-8 as coating material used to modify LIB anodes and showed excellent electrochemical performances [30, 31], ZIF-8 has not been used for surface coating of LFP cathode so far. Therefore, it is necessary to explore the effect of ZIF-8 in LFP cathode coating for improving the electrochemical performances of commercial LIB.
In this work, we modify the commercial LFP by the growth and carbonization of ZIF-8 on the surface of LFP particles. The prepared LiFePO4/carbonized ZIF-8 (LFP/CZIF-8) was demonstrated evidently to improve the electrochemical performances compared with pristine commercial LFP materials.
3.1 Coating Process
The raw materials used in this experiment include methanol (99.5%, Beijing Chemical Works), zinc nitrate (99%, Tianjin Fuchen Chemical Reagent Factory), 2-methylimidazole (99%, Sinopharm Group Chemical Reagent Co., Ltd.), and commercial LiFePO4 powder (Qinghai Taifeng First Lithium Technology Co., Ltd.).
The coating processes were performed as described below: LFP was dispersed into methanol to form a uniform slurry by ultrasonic. Then, zinc nitrate and 2-methylimidazole were dissolved into the above slurry by magnetic stirring for 1 h. The mixture was aged at room temperature for 24 h and the gray powders were precipitated. The ratio of LFP dispersion: zinc nitrate: 2-methylimidazole was 100 mL:1.029 g:1.314 g. And the ratio of LFP: methanol was 8(g):x(mL) (x = 0, 188, 282, 376). The powders were washed very carefully with methanol and annealed at 500/600/700/800/900 °C with a heating rate of 5 °C min−1 under nitrogen flow to obtain the LFP/CZIF-8 samples. As the blank sample, the ZIF-8 was synthesized using the same method without LFP.
The phase compositions of the synthesized products were analyzed by X-ray diffraction (XRD) employing a Cu-Kα X-ray diffractometer (D8 ADVANCE Bruker AXS). The morphologies of grains were characterized by transmission electron microscopy (TEM) with an energy dispersive spectrometer (EDS), TEM images were obtained using a Philips Tecnai 20U-TWIN microscope. Surface composition of the sample is investigated using X-ray photoelectron spectroscopy (XPS, ESCALAB 250 XPS using Al Kα (1486.6 eV) radiation). The N2 adsorption and desorption isotherms and Barrett–Joyner–Halenda pore-size-distribution were obtained at 77 K using an automatic surface area analyzer (Micromeritics, Gemini V2380, USA) under continuous adsorption conditions. Raman spectroscopic analysis was performed with a 532 nm FILTER In-Via Raman microscopic instrument.
3.3 Electrochemical Evaluation
The charge and discharge performances were determined with CR 2032 coin cells. The cathode materials were prepared by mixing the LFP/CZIF-8 with acetylene black and poly-(vinylidene fluoride) in a weight ratio of 8:1:1 in N-methyl pyrrolidone to ensure homogeneity. Then, the slurry was coated on an Al-foil with about 0.02 mm thickness, dried under the air atmosphere at 60 °C for 5 h and the vacuum atmosphere at 120 °C for 10 h, and cut into circular strips of 15 mm in diameter. The loading densities are ∼2.3 mg cm−2. The cells were assembled in a glove box filled with high purity argon, where lithium metal was used as an anode, polypropylene film as a separator, and 1 M LiPF6 as an electrolyte consisting of ethylene carbonate/dimethyl carbonate/ethylene methyl carbonate in a volume ratio of 1:1:1 (1 M LiPF6/EC/DEC/EMC) as lithium ion electrolyte. Under each condition, five identical samples were synthesized.
The charge and discharge performances of the LFP/CZIF-8 were tested on a channels battery analyzer (CT3008W) at different current densities between 2.5 and 4.2 V cut-off voltage using the coin cells. The electrochemical impedance spectroscopy (EIS) measurements were performed on a PARSTAT 4000 electrochemical workstation. EIS was also recorded with frequencies ranging from 100 kHz to 10 mHz and an AC signal of 5 mV in amplitude as the perturbation. All the tests were performed at room temperature.
4 Results and Discussion
The TEM and line scanning EDS analysis were employed to probe the composition and structure of the coating layer. The yellow line in Fig. 1b marks the line scanning EDS test range from 0 to 250 μm. The element distribution curve in Fig. 1c shows that there are C and Zn in the coating layer on the particle surface because of the existent curves of carbon and Zn in the range of 60–200 μm. The Zn content is less because the annealing temperature is close to its boiling point (908 °C), it causes the volatilization of metal Zn at 800 °C. A nano-scaled LFP/CZIF-8 particle with the particle size of about 300 nm is shown in Fig. 1d. There is a 10 nm coating layer on the surface of LFP/CZIF-8 particle. Figure 1e shows the clear regular lattice fringes with a d-spacing of 0.3008 nm which corresponds to the (121) plane of LFP. The regular lattice fringes with the d-spacing of 0.3348 nm correspond to the (006) plane of graphite, and the lattice fringes of Zn were also found in the white dashed box (Fig. 1f). The enlarged image displays that the d-spacing of 0.2091 nm corresponds to the (101) plane of elemental zinc. These results demonstrate the in situ growth of elemental zinc and graphite-like carbon on LFP surface. Moreover, the XPS results also confirm the existing of elemental zinc (Fig. S5). Two peaks at 1021.78 eV (2p3/2) and 1044.88 eV (2p1/2) and the peak splitting of 23.1 eV assure the presence of elemental zinc, which is beneficial to enhance the conductivity of LFP/CZIF-8.
Moreover, we investigate the effects of annealing temperature and the ratio of LFP/methanol on the ion conductivity by using EIS test (Fig. S6; Table S1). We find that the increase of the coating material is beneficial to the enhancements of the ion conductivity and lithium diffusion coefficient (Fig. S6a, c). And the LFP/CZIF-8 cathode synthesized at 800 °C delivers the highest conductivity and lithium diffusion coefficient (Fig. S6b, d). The Raman spectra (Fig. S7) show that the peak intensity ratio of the G-band and the D-band (IG/ID) is the highest when the annealing temperature is 800 °C. The results suggest that the 800 °C annealed sample has the largest degree of graphitization without damaging the coating layer. Therefore, 800 °C is the optimized annealing temperature with the highest conductivity.
The discharge specific energies of LFP and LFP/CZIF-8 cathodes’ active materials are shown in Fig. 6e. The discharge specific energies at different C rates are significantly improved, and the average value of LFP/CZIF-8 cathodes’ discharge specific energy is 355.3 mWh g−1 at 1.0C, while that of LFP is 212.7 mWh g−1. Moreover, the discharge specific energy retention rate of LFP/CZIF-8 cathode is approximate 99% after 200 cycles at 5.0C, while LFP’s discharge specific energy retention rate is only 40%. These results are attributed to the synergy improvements of voltage platform, specific capacity, and freedom degree for volume change after surface coating.
In summary, we studied the surface modification of commercial LiFePO4 (LFP) by utilizing ZIF-8 and synthesized the LFP/CZIF-8 cathodes by growth and carbonization of ZIF-8 on the surface of LFP. The coating layer with metal Zn and graphite-like carbon is about 10 nm. Using as the cathode material, LFP/CZIF-8 clearly improves the conductivity, the lithium ion diffusion coefficient, and the degree of freedom for volume change. Therefore, LFP/CZIF-8 delivers a discharge specific capacity of 159.3 mAh g−1 at 0.1C and a discharge specific energy of 141.7 mWh g−1 after 200 cycles at 5.0C (the retention rate is approximately 99%).
Although we demonstrated that ZIF-8 was a better surface modification material for commercial LIB cathodes, several key aspects still need to be explored in the future. (1) The theoretical description and prediction of the mechanism of the enhancement of electrochemical performances should be established in future studies. (2) It is necessary to further study the controllability of the ratio of LFP, graphite, and zinc. Potentially, our approach for modifying LFP cathode materials could also be applied in modifying other cathodes for commercial LIBs.
This work is supported by the Scientific and Technological Development Project of the Beijing Education Committee (No. KZ201710005009).
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