Lithium manganese iron phosphate composite electrode material as well as preparation method and application thereof

Abstract

The invention discloses an electrode material, which is characterized in that the electrode material is a carbon-coated lithium manganese iron phosphate composite material, the composition of the coated material is represented as LiMnxFeyNi (1-x-y) PO4, x is equal to 0.48-0.50, y is equal to 0.48-0.50, and 1-x-y is equal to 0.01-0.03. The invention also discloses a preparation method and application of the electrode material. According to the prepared electrode material, due to doping of Ni < 2 + >, on one hand, anti-position defects in the obtained material can be effectively reduced, a more efficient lithium ion transmission channel is promoted, on the other hand, preferential growth of crystal grains along an a-c plane is facilitated, the particle size is reduced, and the stability of an olivine structure in the composite material is improved; therefore, the diffusion kinetics and the cycling stability of Li ions are improved, and the cyclicity and the rate capability of the material are remarkably improved.

Description

Technical Field

[0001] The present invention belongs to the field of electrochemistry technology, and particularly relates to a lithium iron manganese phosphate composite electrode material, a preparation method thereof, and an application thereof. Background Art

[0002] Lithium-ion batteries (LIBs) are the most popular energy storage devices and are essential in various fields, including portable electronic products, medical devices, hybrid electric vehicles, and electric vehicles. Driven by the rapid development of the electric vehicle industry, the global lithium-ion battery market is expected to grow by 15% - 30% from 2020 to 2030. Improving the power density and energy density of lithium-ion batteries to meet the requirements of high-performance lithium-ion batteries is the main task of current research. LiFePO 4 has an olivine structure and has been used as a cathode material for lithium-ion batteries for many years, with a relatively high theoretical specific capacity and cycle stability. LiFePO 4 has a redox potential of about 3.45V, and its theoretical energy density is relatively low, unable to meet the long-range requirements of new energy electric vehicles. Compared with LiFePO 4 , LiMnPO 4 with the same olivine structure has a redox potential of about 4.1V, and its theoretical specific capacity is nearly 20% higher than that of LiFePO 4 . Nevertheless, the conductivity and lithium-ion diffusion coefficient of LiMnPO 4 are not as good as those of LiFePO 4 , and the Jahn-Teller distortion and structural instability of Mn 3+ during the cycling process limit its further application. To alleviate the problems of insufficient electronic conductivity and lithium-ion diffusion of LiMnPO 4 materials, researchers mainly adopt methods such as reducing the particle size of the material, adding a carbon coating on its surface, and doping various metal cations. Among various modification strategies, iron-doped LiMnPO 4 electrode materials are an efficient and direct method, which can improve the charge transfer ability, enhance the lithium-ion diffusion efficiency, and improve the electrochemical performance of the LiMnPO 4 electrode. In addition, since the ionic radii of Mn 2+ and Fe 2+ are quite similar, they can be doped in any proportion to produce LiMn 1-x Fe x PO 4 solid solution. This material not only retains the advantages of LiFePO 4 , but also exhibits a higher energy density, making it a promising cathode material for the next generation of lithium-ion batteries. Fe 2+ / Fe 3+with a low redox potential (3.45 V vs. Li / Li + ) indicates that excessive substitution of Fe 2+ for Mn 2+ will significantly reduce the energy density of the material. Yamada et al. (Journal of The Electrochemical Society, 2001, 148(10): A1153 - A1158) established a two - dimensional phase diagram of Li x (Fe 1-y Mn y )PO 4 , where the lithium content is on the x - axis and the manganese content is on the y - axis. They observed that when the manganese content exceeds 0.8, due to the lattice distortion caused by Mn 3+ , the lattice of the brittle phase becomes unstable. This instability leads to the kinetic deceleration of the LiMn 1-x Fe x PO 4 material. Therefore, they concluded that LiMn 1-x Fe x PO 4 is not suitable as a cathode material for lithium - ion batteries. Xiao et al. (JOURNAL OF THE ELECTROCHEMICAL SOCIETY, 2013, 160(6): A918 - A926) synthesized LiMn 1-x Fe x PO 4 materials with different Mn doping levels (x = 0, 0.25, 0.5, 0.75, 1) using a polymer - assisted high - temperature solid - state technique. Their research shows that when the Mn content is 0.75, the energy density of the sample increases, and at high current densities, the capacity remains moderate. Therefore, they believe that LiMn 0.75 Fe 0.25 PO 4 is only suitable for applications that require high working voltage and energy density conditions, while LiMn 0.5 Fe 0.5 PO 4 exhibits fast charge / discharge capabilities and is the best choice for large - scale applications. Although the iron - doped LiMnPO 4 electrode material improves the electrochemical performance of LiMnPO 4 , the synthesized LiMn 1-x Fe x PO 4The electronic conductivity and lithium-ion diffusion efficiency of materials are still insufficient, severely limiting their practical applications (A UniformConductive Carbon Coating of Nitrogen-Doped Carbon Improves theElectrochemical Performance of LiMn 0.7 Fe 0.3 PO 4 Cathode Material for Lithium-ionBatteries [J]. CHEMELECTROCHEM, 2024, 11(21)). Doping LiMn 2+ with metal cations such as Mg 2+ and Co 1-x Fe x PO 4 materials can significantly improve the electronic conductivity and lithium-ion diffusion efficiency, thereby improving the electrochemical properties of the materials. In cation-doped or substituted LiMn 1-x Fe x PO 4 materials, Mg 2+ has been widely studied, revealing a significant synergistic effect of co-doping or substitution of iron and magnesium, which has a positive impact on the lithium storage performance of LiMnPO 4 . Fang et al. first reported the synthesis of LiFe 0.05 Mn 0.9 Mg 0.05 PO 4 / C materials by the traditional solid-state method and pointed out that iron and magnesium uniformly replaced manganese in the obtained materials. The research results show that the discharge capacities of LiMnPO 4 / C, LiFe 0.1 Mn 0.9 PO4 / C and LiFe 0.05 Mn 0.9 Mg 0.05 PO 4 / C at 0.2C are 67 mAh g -1 , 74 mAh g -1 and 121 mAh g -1 respectively, and the specific capacities and multiple performances of the doped materials are significantly improved. Jang et al. successfully synthesized LiFe 0.48 Mn 0.48 Mg 0.04 PO 4 , revealing a small amount of Mg 2+The incorporation reduces the lattice mismatch and converts the two-phase reaction into a pseudo single-phase reaction during charging, thereby improving the capacity of the material. Kisu et al. synthesized single-walled carbon nanotube-coated LiMn 0.792 Fe 0.198 Mg 0.010 PO 4 materials. Compared with the undoped sample, the charge transfer resistance of the sample doped with Mg 2+ was reduced by 50%, and the Li + diffusion coefficient was increased by 200%. Wang et al. synthesized nickel-doped LiMn 0.8 Fe 0.2 PO 4 nanosheets. After 200 charge-discharge cycles at a rate of 0.5C, the capacity retention rate was 94.1%. At temperatures of 10°C and 20°C, the capacity retention rates were 75.7% and 65.2% respectively, indicating excellent electrochemical performance. Although there are many studies on metal cation-doped LiMn 1-x Fe x PO 4 materials, the rate performance and cycle stability of the existing electrode materials still need to be further improved. Summary of the Invention

[0003] The first technical problem to be solved by the present invention is to provide an electrode material with excellent rate performance and cycle stability.

[0004] The second technical problem to be solved by the present invention is to provide a preparation method of the above electrode material.

[0005] To solve the above first technical problem, the electrode material provided by the present invention is a carbon-coated lithium iron manganese phosphate composite material, and the composition of the material to be coated is expressed as LiMn x Fe y Ni 1-x-y PO 4 , where x = 0.48 - 0.50, y = 0.48 - 0.50, and 1 - x - y = 0.01 - 0.03.

[0006] To solve the above second technical problem, the method designed by the present invention includes the following steps: 1) Dissolve H 3 PO 4 and ascorbic acid in a mixed solvent of water and ethylene glycol. Under vigorous stirring, gradually add the pre-prepared LiOH solution dropwise to the mixture for 15 - 45 minutes.

[0007] 2) Sequentially add the pre-prepared divalent manganese salt, divalent iron salt, and divalent nickel salt solutions to the above mixture. React for 3 - 6 hours under stirring conditions.

[0008] 3) Transfer the mixture to a reaction kettle and carry out a hydrothermal reaction at 180 - 250 °C for 6 - 15 hours.

[0009] 4) After taking out the reaction kettle, cool it to room temperature. After the formed precipitate is separated, washed and dried, LiMn x Fe y Ni 1-x-y PO 4 material is obtained, where x = 0.48 - 0.50, y = 0.48 - 0.50, and 1 - x - y = 0.01 - 0.03.

[0010] 5) Mix the above - obtained LiMn x Fe y Ni 1-x-y PO 4 and glucose evenly and calcine to obtain carbon - coated LiMn x Fe y Ni 1-x-y PO 4 material, that is, LiMn x Fe y Ni 1-x-y PO 4 @C.

[0011] Preferably, in the step 2), the divalent manganese salt, divalent iron salt and divalent nickel salt are chlorides, nitrates or sulfates of divalent manganese, divalent iron and divalent nickel.

[0012] Preferably, in the step 4), x = 0.5, y = 0.485 - 0.49.

[0013] The present invention also provides the application of the above electrode material as a cathode electrode material in a battery.

[0014] The inventors of the present invention found that in the LiMn x Fe y Ni 1-x-y PO 4 electrode material, when the manganese and iron contents are approximately equal and the nickel doping is less, the doping of Ni 2+ can effectively reduce the anti - site defects in the obtained material and promote a more efficient lithium - ion transport channel, and the cycle performance and rate performance of the material are significantly improved. Based on this discovery, the prepared LiMn x Fe y Ni 1-x- y PO 4 material, due to Ni 2+The doping is also beneficial to the preferential growth of grains along the a–c plane, reduces the particle size, and improves the stability of the olivine structure in the composite material, thereby improving the Li-ion diffusion kinetics and cycle stability, and is also conducive to improving the cycling performance and rate performance of the material, thus greatly improving the charge-discharge performance of this type of electrode material when used as a battery material. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 are the XRD patterns of LMFP, LMFP-N1, LMFP-N2, and LM-N3.

[0016] Figure 2 are the FTIR spectra of LMFP, LMFP-N1, LMFP-N2, and LMFP3.

[0017] Figure 3 are the SEM images of LMFP, LMFP-N1, LMFP-N2, and LMFP3.

[0018] Figure 4 are the electrochemical performance diagrams of LMFP, LMFP-N1, LMFP-N2, and LMFP3.

[0019] Figure 5 are the cycling performances of LMFP, LMFP-N1, LMFP-N2, and LMFP3. DETAILED DESCRIPTION OF THE INVENTION

[0020] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. Embodiment

[0021] LiOH⋅H 2 O, H 3 PO 4 , (MnSO 4 + FeSO 4 +NiSO 4 ) have a stoichiometric ratio of 2.7:1:1, where the stoichiometric ratio of MnSO 4 , FeSO 4 , NiSO 4 is 50:m:n, and m + n = 50. First, H 3 PO 4 and ascorbic acid are dissolved in a solvent mixture of water and ethylene glycol. Under vigorous stirring, a LiOH solution pre-prepared from LiOH⋅H 2 O is added dropwise to the mixture over 30 minutes. Then, the pre-prepared MnSO 4 , FeSO 4 and NiSO 4The solution was added to the above mixture. After being stirred for 4 hours, the mixture was transferred to a stainless steel reactor lined with polytetrafluoroethylene and subjected to a hydrothermal reaction at 200°C for 10 hours. The reactor was taken out and naturally cooled to room temperature. The formed precipitate was centrifuged and washed with ethanol and deionized water several times. The product was vacuum dried at 80°C for 4 hours to obtain LiMn 0.5 Fe 0.5-n Ni n PO 4 Material.

[0022] Changing LiMn 0.5 Fe 0.5-n Ni n PO 4 MediumFeSO 4 and NiSO 4 The stoichiometric ratio of n = 0, 0.01, 0.02 and 0.03 were synthesized and labeled as LMFP, LMFP-N1, LMFP-N2 and LMFP-N3, respectively.

[0023] The LiMn 0.5 Fe 0.5-n Ni n PO 4 and glucose in a mass ratio of 4:1 and ball-milled, and then heated in an Ar / H 2 The LiMn 0.5 Fe 0.5-n Ni n PO 4 @C, namely carbon-coated LMFP, LMFP-N1, LMFP-N2 and LMFP-N3 materials.

[0024] X-ray diffractometer (XRD) (DX-600, Shimadzu) was used to analyze LiMn 0.5 Fe 0.5-n Ni n PO 4 @C The crystallinity and phase purity of the powder were measured at 30 kV, 10 mA, Cu Kα radiation (λ = 0.15406 nm), 10–80° scanning, and 5° min -1 . Figure 1 The XRD patterns of LMFP, LMFP-N1, LMFP-N2 and LMFP-N3 are presented, and all four exhibit similar diffraction patterns and can be completely indexed as orthorhombic structures belonging to the Pmnb space group. Although the introduction of exogenous cations may lead to the formation of impurities, the lower doping levels are completely dissolved in the olivine lattice, and therefore, Figure 1No impurity phase was observed in (a). Figure 1 (b) shows that as the Ni 2+ doping level increases, the diffraction peaks of the material shift slightly towards higher angles, which is attributed to the decrease in the ionic radius of Ni 2+ , resulting in lattice contraction and causing the diffraction peaks to shift towards higher angles, which also indicates that Ni 2+ is effectively incorporated into the LMFP material.

[0025] Fourier transform infrared (FT-IR) spectra were obtained using a Nico IS 10 spectrometer. The FTIR spectra of LMFP, LMFP-N1, LMFP-N2, and LMFP-N3 shown in Figure 2 indicate that the symmetric stretching vibration peaks of the P-O bond appear at 993, 985, 984, and 985 cm -1 respectively. The maximum shift of the P-O bond peak corresponds to the highest concentration of antisite defects. The change in the P-O bond in LMFP-N1, LMFP-N2, and LMFP-N3 is significantly less than that in LMFP, which indicates that the incorporation of Ni²⁺ effectively reduces the antisite defects in the material and promotes a more efficient lithium-ion transport path. Minor small peaks also appear between 400 and 800 cm -1 , which are caused by the stretching vibrations of the Mn-O bond and Fe-O.

[0026] The morphology of the materials was observed using a Zeiss GeminiSEM360 scanning electron microscope. Figure 3 shows the SEM images of LMFP samples with different nickel contents. As shown in Figure 3(a), the LMFP sample exhibits a rod-like structure with a length of approximately 200 nm and a width of approximately 50 nm. Manganese ions have an impact on the selective growth of LMFP crystals because the electronegativity of Mn 2+ (8.88) is greater than that of Fe 2+ (8.43). Therefore, the adsorption of Mn 2+ cations on the nuclei is enhanced, which may affect the growth rates of different crystal planes. However, with a moderate increase in the Ni 2+ doping percentage (Figure 3b-d), the particle size rapidly decreases to dozens of nanometers, and the sample gradually changes from a rod-like structure to a spherical structure. Ni 2+ doping significantly reduces the size of the LMFP bulk, which may be due to the smaller ionic radius of Ni 2+ (69 pm), which is smaller than that of Mn 2+ (80 pm) and Fe 2+ (76 pm), and Ni 2+has the highest electronegativity (9.60) among the three cations. These properties inhibit the preferential growth in one direction, thus indicating that Ni 2+ doping effectively reduces the particle size and shortens the lithium-ion diffusion path in the LMFP material. In addition, SEM / S analysis of LMFP, LMFP-N1, LMFP-N2, and LMFP-N3 samples shows that the Ni 2+ contents are 0%, 1.2%, 1.4%, and 1.8% respectively, indicating that Ni 2 + can be effectively incorporated into the LMFP material, and the doping level is basically consistent with the target molar ratio.

[0027] By assembling LMFP, LMFP-N1, LMFP-N2, and LMFP-N3 cathode materials with a lithium anode, 2032-type coin cells were prepared, and a series of electrochemical performance evaluations were carried out on the effect of Ni 2+ doping on the electrochemical performance of the LMFP material. Figure 4 (a) shows the initial charge-discharge curves of LMFP, LMFP-N1, LMFP-N2, and LMFP-N3 cathode materials at 0.1 C. The battery was charged to 4.5 V at a current density of 0.1 C, held at 4.5 V until the current dropped to 0.02 C, and then discharged to 2.5 V at a current density of 0.1 C. All samples showed similar curve characteristics, with two obvious voltage plateaus at 3.5 V and 4.0 V, which correspond to the reduction processes of Fe 2+ / Fe 3+ and Mn 2+ / Mn 3+ . LMFP-N1 exhibited the largest discharge capacity of 162.7 mAh g-1, while the capacities of LMFP, LMFP-N2, and LMFP-N3 were 152 mAh g -1 , 155.4 mAh g -1 and 149.1 mAh -1 respectively. As the Ni 2+ content increased, the discharge capacity first increased and then decreased. This phenomenon is mainly attributed to the fact that Ni 2+ does not participate in the reduction reaction within the material, so it does not increase the capacity. A lower concentration of Ni 2+ has no effect on the proportion of active substances (Mn 2+ , Fe 2+ ), which is beneficial to optimizing the lattice, improving the electronic conductivity and lithium-ion mobility. However, excessive Ni 2+ doping will cause severe lattice distortion, reduce the proportion of active substances, and reduce the capacity of the material. Figure 4(b) shows the galvanostatic charge-discharge curves of LMFP, LMFP-N1, LMFP-N2, and LMFP-N3 at 1C. All samples exhibit typical discharge voltage plateaus around 3.5 V and 4.1 V. At 1C, the discharge capacities of LMFP, LMFP-N1, LMFP-N2, and LMFP-N3 were measured to be 142.8 mAh g -1 , 151 mAh g -1 , 146.8 mAh g -1 , and 142.2 mAh g -1 , respectively, among which LMFP-N1 exhibits the highest discharge capacity at 1C. The discharge voltage plateau at 4.1 V is shorter than that at 0.1C, which is due to the increase in the electrochemical polarization of the material with the increase in the charge-discharge current density. However, compared with LMFP, the Ni 2+ -doped samples exhibit longer voltage plateaus and lower polarization, indicating that Ni 2+ doping improves the rate performance of the electrode material. To further understand the effect of Ni 2+ doping on the electrochemical performance of the LMFP material, cyclic voltammetry tests were performed on the synthesized LMFP, LMFP-N1, LMFP-N2, and LMFP-N3. Figure 4 (c) shows the cyclic voltammograms of the Li / Li electrodes at a scanning rate of 0.1 mV / s in the voltage range of 2.0 - 4.5 V. All samples exhibit two pairs of redox peaks at approximately 3.5 V and 4.1 V, corresponding to the redox pairs of Fe 3+ / Fe 2+ and Mn 3+ / Mn 2+ , respectively, which is consistent with the charging electrode voltage plateau shown in Figure 4 (a). The potential difference between each pair of redox peaks reflects the polarization of the material. For Fe 3+ / Fe 2+ , the voltage differences between the cathodic and anodic peaks of LMFP, LMFP-N1, LMFP-N2, and LMFP-N3 are 113, 69, 75, and 79 mV, respectively. For Mn 3+ / Mn 2+ , the voltage differences between the cathodic and anodic peaks of LMFP, LMFP-N1, LMFP-N2, and LMFP-N3 are 173, 145, 148, and 150 mV, respectively. Among all samples, the LMFP-N1 electrode exhibits the smallest voltage gaps between the Fe 3+ / Fe 2+ and Mn 3+ / Mn 2+ anodic peaks, indicating appropriate Ni 2+Doping can significantly reduce polarization and improve the electrochemical performance of cathode materials. Figure 4 (d) shows the rate performance of LMFP, LMFP-N1, LMFP-N2, and LMFP-N3 at current densities of 0.1 C, 0.2 C, 0.5 C, 1 C, 2 C, 5 C, and 10 C, and then restored to 0.1 C. Compared with the other three samples, LMFP-N1 exhibits the highest capacity at all varying rates. At discharge rates of 0.1, 0.2, 0.5, 1, 2, 5, and 1 C, the capacities are measured as 162.7, 158.5, 154.2, 151, 144.7, 138.1, and 124.9 mAh g -1 . When the rate is restored to 0.1 C, the specific discharge capacity returns to its initial value. These findings indicate that appropriate Ni 2+ doping can greatly improve the rate performance and structural stability of the material. To evaluate the cycling stability, all samples were charged to 4.5 V at a current density of 1 C, held at 4.5 until the current decreased to 0.02 C, and then discharged to 2.5 V at a current density of 1 C, as Figure 5 (a) shows. Compared with the three samples, LMFP-N1 exhibits excellent reversible capacity and cycling stability, achieving a discharge capacity of 145.6 mAh g -1 after 200 cycles, and the capacity retention rate of its initial value reaches 96.87%. In contrast, the discharge capacities of LMFP, LMFP-N2, and LMFP-N3 after 200 cycles are 140.1 mAh g -1 , 136.5 mAh g -1 , and 34.3 mAh g -1 , respectively, and the corresponding capacity retention rates are 95.05%, 92.23%, and 90.7%. Figure 5 (b) shows the cycling performance of LMFP, LMFP-N1, LMFP-N2, and LMFP-N3 at a current density of 10 C. Although at a high current density of 10 C, LMFP-N1 exhibits an initial discharge capacity of 128.4 mAh g -1 , and the capacity after 20 cycles is 113.5 mAh g -1 , and the capacity retention rate is 88.4%. In contrast, the discharge capacities of LMFP, LMFP2, and LMFP-N3 after 200 cycles are 97.3 mAh g -1 , 105.7 mAh g -1 , and 101.5 mAh g -1 , respectively, and the capacity retention rates are 86.95%, 87.86%, and 87.92%.

Claims

1. An electrode material, characterized in that: The electrode material is a carbon-coated lithium manganese iron phosphate composite material, and the coated material composition is expressed as LiMn x Fe y Ni 1-x-y PO4, where x=0.48~0.50, y=0.48~0.50, 1-xy=0.01~0.

03.

2. The method for preparing the electrode material according to claim 1, characterized in that: The method comprises the following steps: 1) Dissolve H3PO4 and ascorbic acid in a mixed solvent of water and ethylene glycol. Add the pre-prepared LiOH solution dropwise into the mixture under vigorous stirring for 15 to 45 minutes. 2) adding the pre-prepared divalent manganese salt, divalent iron salt and divalent nickel salt solutions to the above mixture in sequence, and reacting for 3 to 6 hours under stirring conditions; 3) Transfer the mixture to a reactor and perform a hydrothermal reaction at 180-250°C for 6-15 hours; 4) After the reactor is taken out and cooled to room temperature, the precipitate formed is separated, washed and dried to obtain LiMn x Fe y Ni 1-x- y PO4 material, where x=0.48~0.50, y=0.48~0.50, 1-xy=0.01~0.03; 5) The LiMn x Fe y Ni 1-x-y After mixing PO4 and glucose, ball milling and calcination, carbon-coated LiMn x Fe y Ni 1-x-y PO4 material, namely LiMn x Fe y Ni 1-x-y PO4@C.

3. The preparation method according to claim 2, characterized in that: In the step 2), the divalent manganese salt, the divalent iron salt and the divalent nickel salt are chlorides, nitrates or sulfates of divalent manganese, divalent iron and divalent nickel.

4. The preparation method according to claim 2 or 3, characterized in that: In the step 4), x=0.5, y=0.485~0.

49.

5. Use of the electrode material according to claim 1 as a cathode electrode material in a battery.

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