(LiFePO4, LPF) is widely used as a low-cost cathode material for lithium-ion batteries, but its low ionic conductivity and electronic conductivity limit its rate performance. A lot of efforts have been made to overcome this limitation, such as coating LFP with carbon, mixing LFP with a conductive agent, and reducing the particle size of LFP. These methods improve conductivity and shorten the ion diffusion length. However, such carbon coatings are generally amorphous, and their electrical conductivity is significantly lower than that of graphite. At the same time, the electrode is usually made of a mixture containing LFP particles, a binder, and a conductive agent, and they are in close contact to form a conductive network. Excessive reduction in the size of the LFP particles will reduce the tap density and may produce more interface contacts, which will increase the interface resistance and destroy the overall conductivity.
Recently, the Li Shen, Runwei Mo, and Yunfeng Lu teams of the University of California, Los Angeles published their latest research results on Nano Letters, reporting on the synthesis of lithium iron phosphate/graphite composites, in which lithium iron phosphate nanoparticles are grown in a graphite matrix. The graphite matrix has the characteristics of porosity, high conductivity and high mechanical strength, and the prepared electrode has excellent cycle performance and rate performance. This research provides a new strategy for low-cost, long-life and high-power batteries.
Research highlights:
(1) Embed LFP particles between graphite layers to obtain a high-performance lithium battery cathode material;
(2) LFP/graphite composite material has high conductivity, low interface resistance and porous structure;
(3) The volume energy density of electrodes made of LFP/graphite composite materials at 10C and 60C are as high as 560W h L−1 and 427 W h L−1, respectively, which is significantly better than commercial lithium iron phosphate.
Figure 1a shows the synthesis process of LFP/graphite composite material. First, the molten salt method is used to insert FeCl3 into natural graphite, and then it is solvothermal reacted in ethylene glycol containing lithium and phosphate to form LFP particles in the graphite layer. Further annealing at high temperature obtains nano-LFP particles embedded in micron graphite particles.
As shown in Figure 1b, the formation of nano-LFP minimizes the ion diffusion length in the particles, embeds the LFP nanoparticles in a continuous graphite sheet, minimizes the interface resistance, and creates a porous structure during the formation of the LFP, allowing the effective electrolyte transmission.
Figure 1 Schematic diagram of the synthesis and structure of LFP/graphite composite material.
(a) Synthesis of LFP/graphite composite materials;
(b) Schematic diagram of LFP/graphite cathode and commercial LFP cathode.
Figure 2a shows the XRD patterns of graphite, FeCl3/graphite and LFP/graphite composites. Graphite presents characteristic peaks at 26.5° and 54.3°, corresponding to (002) and (004) crystal planes. After inserting iron trichloride, a series of diffraction peaks were observed at 16.2°, 18.6°, 32.1°, 33.6°, 37.8°, 47.2° and 51.4°, and the peak of the graphite (002) plane shifted from 26.5° to 25.8 °, and almost disappeared in the LFP/graphite composite material, indicating that the formation of LFP particles destroyed the layered structure of graphite. Since the formation of LFP particles starts between graphite layers, the growth of the particles expands the layered structure, allowing the formation of LFP/graphite composites with a tight LFP/graphite interface and minimal contact resistance.
Figure 2b shows the thermogravimetric analysis of graphite and LFP/graphite composite in air. The results show that the composite contains 93.2 wt% LFP and 6.8 wt% graphite.
The structure evolution of graphite in the process of ferric chloride intercalation was also characterized by Raman spectroscopy (Figure 2c). Embedding FeCl3 between graphite layers resulted in a G band ranging from 1581 to 1623 cm-1, which was attributed to the doping effect caused by the charge transfer from graphite to FeCl3. The 2D band of graphite also shows a change from a multimodal structure to a unimodal structure, which proves the loss of the electronic coupling effect between adjacent graphite sheets based on the presence of ferric chloride. The LFP/graphite composite shows a Raman spectrum similar to that of ferric chloride/graphite.
The N2 adsorption and desorption test was used to study the pore structure of LFP/graphite. Figure 2d, e shows the N2 adsorption/desorption isotherm and pore size distribution of graphite and LFP/graphite particles. Compared with non-porous graphite, the average pore diameter of LFP/graphite particles is 28 nm (Figure 2e), and the surface area calculated by the BET method is 109 m2 g-1, which is significantly higher than that of graphite (graphite is 8 m2 g-1) (Figure 2d).
Figure 2 Intercalation of ferric chloride and pore structure of ferric chloride/graphite composite. (a) XRD patterns of graphite, ferric chloride/graphite composite materials and LFP/graphite composite materials;
(b) Thermogravimetric analysis of graphite and LFP/graphite composite materials;
(c) Raman spectra of graphite, ferric chloride/graphite composite material and LFP/graphite composite material;
(d) N2 adsorption and desorption isotherm and corresponding (e) pore size distribution.
Using SEM electron microscope and TEM electron microscope, the structure and morphology of the LFP/graphite composite were characterized. The energy dispersive X spectroscopy (EDS) of the LFP/graphite composite material confirmed the uniform distribution of LFP particles in the graphite matrix (Figure 3a, b), which was further confirmed by SEM observation (Figure 3c, d). Figure 3e-g shows the TEM images of the LFP/graphite composite material, confirming that the interplanar spacing of the embedded LFP particles is 0.429 nm, which can be attributed to the (101) crystal plane of LFP. In order to further confirm that the LFP particles were indeed embedded in the graphite matrix, the embedded LFP particles were etched away by acid washing, and the resulting samples were studied by SEM and TEM (Figure 3h, i). The etched LFP/graphite showed a highly porous networked graphite structure, confirming that the LFP particles were indeed embedded in the graphite matrix.
Figure 3 The structure and morphology of LFP/graphite composite material.
(A, B) SEM and energy spectrum of LFP/graphite composite material,
(C, D) SEM image and (e-g) TEM image; SEM (H) and TEM images (I) of the porous graphite particles after etching away the LFP particles from the LFP/graphite composite material.
Figure 4a shows the rate performance of commercial LFP and LFP/graphite cathodes. LFP/graphite electrodes show much better reversible capacity than commercial LFP. It is worth noting that even at 60C, the LFP/graphite electrode can still release a reversible capacity of 107.5 mAh g-1, while the commercial LFP electrode only shows a reversible capacity of 15 mAh g-1.
The improvement of the cycle stability and rate performance of the LFP/graphite electrode can be attributed to its unique structure, which improves the conductivity, reduces the polarization of the electrochemical process, and shortens the diffusion distance of lithium ions. For further research, the electrochemical process of commercial LFP and LFP/graphite cathodes was studied using cyclic voltammetry and electrochemical impedance spectroscopy. As shown in Figure 4b, the CV curve shows that at a scan rate of 0.1 mV s-1, the potential difference between the anode and cathode peaks of the LFP/graphite electrode is 158 mV, which is much lower than the commercial LFP electrode (262 mV). The redox peak of the LFP/graphite electrode is sharper and more symmetrical, indicating that the redox kinetics has been improved. The Nyquist curve (Figure 4c) shows that the charge transfer resistance of the LFP/graphite positive electrode is lower and the electronic conductivity is higher.
As shown in Figure 4d, after 2000 cycles, the LFP/graphite electrode can still provide large reversible capacities of 116 and 106 mAh g-1 at 30C and 60C, and the capacity retention rates are about 95.7% and 91.5%, respectively. Figure 4e compares the reported capacity and cycle times of various LFP-based cathodes at different rates, including carbon-coated LFP, conductive polymer-coated LFP, graphene-modified LFP and LFP/carbon nanotubes The composite material, LFP/graphite electrode has the best performance.
Figure 4 Electrochemical performance of LFP/graphite electrode.
(a) Rate performance. The capacity is based on the total mass of the composite material;
(b) Cyclic voltammetry curve;
(c) Nyquist diagram;
(d) Cycle stability;
(e) Comparison of the rate performance of the LFP/graphite electrode with the previously reported LFP cathode.
Figure 5a shows the capacity of LFP/graphite electrodes with different mass loads. It is noteworthy that at 60C, LFP/ graphite electrodes with 3 and 6 mg cm-2 can still achieve 96 and 80 mAh g-1 reversible capacities, respectively, indicating the feasibility of manufacturing high-power energy positive electrodes with high-quality loads.
Figure 5c compares the reported reversible capacities of LFP cathodes under different mass loads, including carbon-coated LFP, conductive polymer-coated LFP, graphene-modified LFP, and LFP/carbon nanotube composites. The high-quality load LFP/graphite cathode provides the best rate performance and reversible capacity. Figure 5d evaluates the volumetric energy density of the LFP/graphite composite, which is estimated based on its working voltage, tap density, and reversible capacity under different magnifications. The LFP/graphite composite material can still reach a volumetric energy density of 427 W h L−1 at 60C, which is very important for electric vehicles and other applications.
Figure 5
(a) The specific capacity of LFP/graphite electrodes with different mass loads;
(b) The active material utilization rate of LFP/graphite electrodes with different mass loads at different magnifications;
(c) the electrochemical performance comparison with the previously reported LFP-based electrodes;
(e) the volume energy density comparison.
A new type of cathode material was designed by embedding LFP nanoparticles in the graphite matrix. This unique structure allows electrons and ions to be efficiently transported through the strong, highly conductive graphite matrix. Compared with commercial LFP, this LFP/graphite composite material can provide large reversible capacity, ultra-high rate capacity and excellent cycle performance. This work provides a high-performance LFP material for lithium-ion batteries with long life and high power energy density, which is very meaningful for electric vehicles and other applications.
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