Boric acid induced synergistically modified cobalt-free lithium-rich manganese-based cathode material and preparation method thereof

Abstract

The application discloses a borate-induced synergistically modified cobalt-free lithium-rich manganese-based positive electrode material and a preparation method thereof. 0.75 Ni 0.25 CO3-based co-precipitated carbonate as a precursor, mixing the precursor with a lithium source and boric acid by ball milling to obtain a mixed powder; the obtained mixed powder is calcined to obtain a boron phase-doped cobalt-free lithium-rich manganese-based positive electrode material; and (2) dispersing the boron phase-doped cobalt-free lithium-rich manganese-based positive electrode material obtained in the step (1) into a boric acid solution, heating and stirring, centrifuging, drying and annealing to obtain a cobalt-free lithium-rich manganese-based positive electrode material synergistically modified by boron phase doping and surface spinel coating. The application realizes the synergistic modification of boron phase doping and surface spinel coating based on the same modifier (boric acid), and has excellent electrochemical performance.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery cathode material technology, specifically relating to a cobalt-free lithium-rich manganese-based cathode material, particularly a cobalt-free lithium-rich manganese-based cathode material and its preparation method that achieves synergistic modification of bulk doping and surface spinel coating through boric acid induction. Background Technology

[0002] With the rapid development of electric vehicles and large-scale energy storage technologies, higher demands are being placed on the energy density and cost control of lithium-ion batteries. Lithium-rich manganese-based cathode materials (xLi₂MnO₃·(1-x)LiTMO₂, TM=Ni, Mn, etc.) are considered one of the most promising cathode materials for next-generation high-energy-density lithium-ion batteries due to their reversible specific capacity exceeding 250 mAh / g. However, their commercial application is constrained by problems such as severe voltage decay during cycling, poor rate performance, and intense interfacial side reactions.

[0003] To reduce costs and alleviate the pressure of cobalt resource scarcity, cobalt-free materials have become an important development direction for lithium-rich manganese-based materials. However, the structural stability of cobalt-free systems faces greater challenges. To address these issues, existing technologies mainly employ bulk doping (such as boron, aluminum, and magnesium doping) to stabilize lattice oxygen and suppress the irreversible transformation of layered structures into spinel or rock salt phases; or surface coating (such as spinel phase coating, metal oxide coating, etc.) to isolate the electrolyte and protect the surface of the active material. For example, some studies have achieved uniform boron doping on the surface and inside of materials by introducing boron-containing compounds during co-precipitation (CN110429268A A modified boron-doped lithium-rich manganese-based cathode material and its preparation method and application). Furthermore, a synergistic modification approach using ball milling combined with solid-state sintering to achieve boron doping, followed by the introduction of an organic coating layer (such as c-PAN) through wet coating, has also been reported (Synergistic modification via c-PAN coating and B 3+ doping enhances the cycling and rate performance of cobalt-free lithium-rich manganese-based materials, Journal of Energy Storage).

[0004] However, the above-mentioned technical solutions are either limited to a single modification method (doping or coating only), or although they adopt synergistic modification, they require the introduction of two or more different modifiers, resulting in complex processes and prominent interfacial compatibility issues between different components. Summary of the Invention

[0005] To address the problems existing in the prior art, the present invention aims to provide a boric acid-induced synergistic modification of a cobalt-free lithium-rich manganese-based cathode material and its preparation method. Based on the same modifier (boric acid), it achieves synergistic modification of bulk doping and surface spinel coating, resulting in excellent electrochemical performance.

[0006] To achieve the above-mentioned technical objectives, the present invention adopts the following technical solution: A method for preparing a boric acid-induced synergistic modification cobalt-free lithium-rich manganese-based cathode material includes the following steps: (1) With Mn 0.75 Ni 0.25 CO3-based coprecipitated carbonate is used as a precursor. The precursor is directly ball-milled and mixed with lithium source and boric acid to obtain a mixed powder. The obtained mixed powder is calcined to obtain a boron bulk doped cobalt-free lithium-rich manganese-based cathode material. (2) The boron bulk doped cobalt-free lithium-rich manganese-based cathode material obtained in step (1) is dispersed in boric acid solution, heated and stirred, and then centrifuged, dried and annealed to obtain the cobalt-free lithium-rich manganese-based cathode material with boron bulk doping and surface spinel coating synergistic modification.

[0007] In a preferred embodiment, in step (1), the lithium source is one or more of lithium carbonate and lithium hydroxide; the molar ratio of the precursor to the lithium source is 1.55~1.65:1 based on Li / (Mn+Ni).

[0008] In a preferred embodiment, in step (1), the amount of boric acid added is 1.0~5.0 wt% of the total mass of the precursor and the lithium source.

[0009] In a preferred embodiment, in step (1), the roasting process is as follows: first, maintain at 400~600 ℃ for 2~4 h, and then maintain at 800~900 ℃ for 10~20 h.

[0010] In a preferred embodiment, in step (2), the concentration of the boric acid solution is 0.2~0.8 mol / L.

[0011] In a preferred embodiment, in step (2), the heating temperature is 50~80 ℃ and the stirring time is 1~3h.

[0012] In a preferred embodiment, in step (2), the annealing temperature is 300~500℃ and the time is 3~8 h.

[0013] In this invention, boric acid is used as the sole modifier. A two-step process, "solid-phase co-firing followed by solution heat treatment," sequentially achieves bulk boron doping and surface spinel coating. In the first step of solid-phase co-firing, boron, being an "oxygen-loving element," forms the strongest BO covalent bonds, thus stabilizing the oxygen framework of the bulk phase and inhibiting oxygen loss. Furthermore, during high-temperature calcination, some boron tends to accumulate at grain boundaries or near-surface regions. This "pre-enrichment" effect ensures that the material surface itself contains "active boron sites" during the second step of boric acid treatment. These sites can induce heterogeneous nucleation of the spinel phase. In the second step of solution heat treatment, the boron concentration in the solution is extremely high, while the boron concentration in the bulk phase is relatively low (but already has a foundation). This internal-external boron concentration gradient drives the "floating" of bulk boron and the "infiltration" of surface boron to meet at the interface, forming a gradient interface layer with gradual changes in composition and structure. This gradient structure, transitioning from a boron-doped layered phase to a spinel phase, exhibits no significant lattice mismatch, minimal interfacial stress, and is resistant to peeling during cycling. Therefore, the invention chose boric acid as the sole modifier based on the inventors' profound understanding of its electronic structure, chemical bond characteristics, and interfacial reaction mechanism. The boron doping in the first step not only stabilizes the lattice oxygen but also forms a boron-enriched layer on the surface, providing active sites for the second reaction. During the second step of boric acid treatment, the boron in the solution and the boron enriched in the bulk phase form a concentration gradient-driven chemical relay, generating a seamlessly connected spinel gradient layer stabilized by BO-TM bonds in situ.

[0014] When other elements (such as Al and Mg) are used to replace boron in the first step of doping, these elements cannot form pre-enrichment on the surface, nor can they form chemical bonds with boron in the second step. This results in an uneven spinel layer, poor interfacial bonding, and limited performance improvement. This synergistic mechanism of "bulk pre-enrichment-in-situ surface induction" based on the same element cannot be reproduced by any other combination of elements, and it is also the inventive aspect of this invention.

[0015] The present invention also provides a cobalt-free lithium-rich manganese-based cathode material prepared by any of the above methods.

[0016] The beneficial effects of this invention are: (1) Synergistic modification mechanism of "one source, two effects": This invention uses only boric acid as a modifier and achieves bulk boron doping and surface spinel coating in sequence through a two-step process of "solid phase co-firing followed by solution heat treatment". This modification strategy based on homologous elements makes the bulk doping and surface coating highly chemically compatible.

[0017] (2) In-situ induced gradient interface structure: In this invention, the boron doped in the bulk phase in the first step forms a pre-enrichment in the near-surface region, providing active sites for spinel nucleation in the second step; during the second step, the boron in the solution and the boron enriched in the bulk phase form a concentration gradient-driven chemical relay, generating in-situ a seamless gradient interface from the "boron-doped layered phase" to the "boron-doped spinel phase" stabilized by strong BO-TM bonds. This interface has no lattice mismatch and is not easily peeled off during cycling.

[0018] (3) Excellent electrochemical performance: The cobalt-free lithium-rich manganese-based cathode material modified by the present invention has significantly better cycle stability and rate performance than materials that only undergo bulk doping, only undergo surface coating, or other element doping combined with boric acid coating, demonstrating the technical effect of "1+1>2". Attached Figure Description

[0019] Figure 1 The image shows the XRD pattern of the cobalt-free lithium-rich manganese-based cathode material prepared in Example 1 of this invention.

[0020] Figure 2 The images show SEM and TEM images of the cobalt-free lithium-rich manganese-based cathode material prepared in Example 1 of this invention.

[0021] Figure 3 The above are charge-discharge curves of the cobalt-free lithium-rich manganese-based cathode materials prepared in Example 1 and Comparative Examples 1-3 of this invention.

[0022] Figure 4 The diagram shows the cycle performance of the cobalt-free lithium-rich manganese-based cathode materials prepared in Example 1 and Comparative Examples 1-3 of this invention.

[0023] Figure 5 The rate performance diagrams are for the cobalt-free lithium-rich manganese-based cathode materials prepared in Example 1 and Comparative Examples 1-3 of this invention. Detailed Implementation

[0024] Example 1

[0025] Coprecipitate the precursor Mn 0.75 Ni 0.25CO3, Li2CO3 (Li / (Mn+Ni)=1.575:1), and H3BO3 (3 wt% of the total mass of precursor and lithium source) were accurately weighed and added to an agate ball mill jar containing an appropriate amount of grinding beads and anhydrous ethanol. The jar was sealed and placed in a planetary ball mill at 450 r / min for 5 h. After the program was completed, the jar was removed and dried. The resulting mixed powder was ground evenly and transferred to an alumina crucible. The crucible was then calcined in a muffle furnace at 500 °C for 3 h and 850 °C for 12 h. The powder was then dispersed in a 0.5 mol / L boric acid solution (solid-liquid ratio of 1:100) and stirred continuously at 60 °C for 1 h. After centrifugation and washing, the cathode material was dried in an 80 °C vacuum drying oven for 12 h. The resulting mixed powder was ground evenly and transferred to an alumina crucible. The crucible was then calcined in a muffle furnace at 400 °C for 5 h. Finally, a cobalt-free lithium-rich manganese-based cathode material, denoted as LMNO-B / HB, was obtained.

[0026] Comparative Example 1 Coprecipitate the precursor Mn 0.75 Ni 0.25 CO3, Li2CO3 (Li / (Mn+Ni)=1.575:1), and H3BO3 (3 wt% of the total mass of precursor and lithium source) were accurately weighed and added to an agate ball mill jar containing an appropriate amount of grinding beads and anhydrous ethanol. The jar was sealed and placed in a planetary ball mill at a speed of 450 r / min for 5 h. After the program was completed, the jar was removed, dried, and the resulting mixed powder was ground evenly and transferred to a corundum crucible. The crucible was then placed in a muffle furnace and calcined at 500 ℃ for 3 h and then at 850 ℃ for 12 h to obtain a cobalt-free lithium-rich manganese-based cathode material, denoted as LMNO-B.

[0027] Comparative Example 2 Coprecipitate the precursor Mn 0.75 Ni 0.25 CO3 and Li2CO3 (Li / (Mn+Ni)=1.575:1) were accurately weighed and added to an agate ball mill jar containing appropriate amounts of grinding beads and anhydrous ethanol. The jar was sealed and placed in a planetary ball mill at 450 r / min for 5 h. After the program was completed, the jar was removed and dried. The resulting mixed powder was ground evenly and transferred to an alumina crucible. The crucible was then calcined in a muffle furnace at 500 °C for 3 h and 850 °C for 12 h. The powder was then dispersed in a 0.5 mol / L boric acid solution (solid-liquid ratio of 1:100) and stirred continuously at 60 °C for 1 h. After centrifugation and washing, the cathode material was dried in an 80 °C vacuum drying oven for 12 h. The resulting mixed powder was ground evenly and transferred to an alumina crucible. The crucible was then calcined in a muffle furnace at 400 °C for 5 h. Finally, a cobalt-free lithium-rich manganese-based cathode material, denoted as LMNO / HB, was obtained.

[0028] Comparative Example 3 Coprecipitate the precursor Mn 0.75 Ni 0.25 CO3, Li2CO3 (Li / (Mn+Ni)=1.575:1), and Al2O3 (1% of the precursor molar amount) were accurately weighed and added to an agate ball mill jar containing an appropriate amount of grinding beads and anhydrous ethanol. The jar was sealed and placed in a planetary ball mill at 450 r / min for 5 h. After the program was completed, the jar was removed and dried. The resulting mixed powder was ground evenly and transferred to an alumina crucible. The crucible was then placed in a muffle furnace and calcined at 500 °C for 3 h and 850 °C for 12 h. The powder was then dispersed in a 0.5 mol / L boric acid solution (solid-liquid ratio of 1:100) and stirred continuously at 60 °C for 1 h. After centrifugation and washing, the cathode material was placed in an 80 °C vacuum drying oven and dried for 12 h. The resulting mixed powder was ground evenly and transferred to an alumina crucible. The crucible was then placed in a muffle furnace and calcined at 400 °C for 5 h. Finally, a cobalt-free lithium-rich manganese-based cathode material, denoted as LMNO-Al / HB, was obtained.

[0029] like Figure 1 The image shows the XRD pattern of the material prepared in Example 1. It can be seen that the LMNO-B / HB sample has a typical layered α-NaFeO2 structure with space group R-3m. At the same time, a relatively obvious Li2MnO3 superlattice diffraction peak appears between 20º-25º, which is a characteristic peak of layered lithium-rich manganese-based cathode material with space group C2 / m and I(003) / I(104) ratio greater than 1.2. The XRD test results show that the cobalt-free lithium-rich manganese-based cathode material with good structure and low cation mixing degree was successfully prepared by the technical method of the present invention.

[0030] like Figure 2 As shown, the EDS-mapping test results proved the presence of Mn, Ni, O, and B elements in LMNO-B / HB; the SEM image showed that LMNO-B / HB had a relatively regular morphology, a smooth surface, and no rough adhering substances or impurity phases; the HR-TEM image further observed the microstructure of LMNO-B / HB, with a lattice spacing of 0.481 nm in the bulk phase, corresponding to the (003) crystal plane of the R-3m space group of the layered structure; a crystal phase structure completely different from the clear layered structure of the bulk phase appeared on the sample surface. When subjected to FFT transformation, diffraction spots belonging to the spinel phase appeared. The lattice spacing of the surface reconstruction layer was 0.286 nm, corresponding to the (220) crystal plane of the Fd-3m space group of the cubic spinel phase, proving the formation of the spinel coating layer, which was about 5 nm thick. It was closely connected to the bulk phase and smoothly transitioned, with no obvious phase boundary observed, indicating that there was good structural compatibility between the two phases.

[0031] like Figure 3As shown, the charge-discharge curves of the materials are as follows. Within the voltage range of 2.0-4.8V, all materials exhibit typical charge-discharge curves of lithium-rich manganese-based cathode materials. The sloping region below 4.5V is attributed to Ni. 2+ / Ni 4+ The redox pair and the long plateau region near 4.5V are attributed to the electrochemical activation of the Li2MnO3 component. In addition, LMNO-B / HB, LMNO / HB, and LMNO-Al / HB treated with boric acid solution have a distinct spinel phase discharge plateau at ~2.6V, which further proves that boric acid solution induces the formation of surface spinel phase. Among them, LMNO-B / HB exhibits the highest initial discharge specific capacity of 283.32 mAh / g.

[0032] like Figure 4 The figure shows the cycling performance of the material. LMNO-B / HB has a discharge specific capacity of 231.68 mAh / g at a current density of 1C and a capacity retention of 86% after 200 cycles.

[0033] like Figure 5 As shown, the rate performance diagram of the material shows that LMNO-B / HB still has a discharge specific capacity of 195.15 mAh / g and 169.89 mAh / g at high current densities of 5C and 10C, respectively.

[0034] The above electrochemical tests demonstrate that the cobalt-free lithium-rich manganese-based cathode material prepared by the method of the present invention, which achieves synergistic modification of bulk doping and surface spinel coating based on boric acid induction, has a large discharge specific capacity, excellent rate performance, and improved cycle stability, exhibiting excellent electrochemical performance.

Claims

1. A method for preparing a boric acid-induced synergistic modification of a cobalt-free, lithium-rich manganese-based cathode material, characterized in that, Includes the following steps: (1) With Mn 0.75 Ni 0.25 CO3-based coprecipitated carbonate is used as a precursor. The precursor is directly ball-milled and mixed with lithium source and boric acid to obtain a mixed powder. The obtained mixed powder is calcined to obtain a boron bulk doped cobalt-free lithium-rich manganese-based cathode material. (2) The boron bulk doped cobalt-free lithium-rich manganese-based cathode material obtained in step (1) is dispersed in boric acid solution, heated and stirred, and then centrifuged, dried and annealed to obtain the cobalt-free lithium-rich manganese-based cathode material with boron bulk doping and surface spinel coating synergistic modification.

2. The preparation method according to claim 1, characterized in that, In step (1), the lithium source is one or more of lithium carbonate and lithium hydroxide; the molar ratio of the precursor to the lithium source is 1.55~1.65:1 based on Li / (Mn+Ni).

3. The preparation method according to claim 1, characterized in that, In step (1), the amount of boric acid added is 1.0~5.0 wt% of the total mass of the precursor and the lithium source.

4. The preparation method according to claim 1, characterized in that, In step (1), the roasting process is as follows: first, maintain at 400~600 ℃ for 2~4 h, and then maintain at 800~900 ℃ for 10~20 h.

5. The preparation method according to claim 1, characterized in that, In step (2), the concentration of the boric acid solution is 0.2~0.8 mol / L.

6. The preparation method according to claim 1, characterized in that, In step (2), the heating temperature is 50~80℃ and the stirring time is 1~3 h.

7. The preparation method according to claim 1, characterized in that, In step (2), the annealing temperature is 300~500℃ and the time is 3~8 h.

8. The cobalt-free lithium-rich manganese-based cathode material prepared by any one of claims 1-7.

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