CN106784625A - A kind of anode material for lithium-ion batteries and preparation method thereof - Google Patents

Abstract

The invention belongs to the technical field of lithium-ion power battery electrode materials, and relates to a lithium-ion battery positive electrode material and a preparation method thereof. The lithium-ion battery positive electrode material is silicon-site-doped lithium ferrous silicate, and its chemical formula is Li ₂ FeSi _1‑x M _x O _4+δ , where M is M=B, S, P, Mn, Be, Al, Mo, Cr, W, Nb, lithium: iron: silicon: M=2:1: (1‑ x): x, the value range of x is 0~0.2. One of the present invention achieves the purpose of improving material charge-discharge specific capacity, high-rate charge-discharge performance, and improving particle aggregation morphology and surface morphology by doping dissimilar element ions at the silicon site of lithium ferrous silicate.

Description

A kind of positive electrode material of lithium ion battery and preparation method thereof

technical field

The invention belongs to the technical field of lithium ion power battery electrode materials, and relates to a lithium ion battery positive electrode material and a preparation method thereof.

Background technique

In order to cope with the increasing shortage of oil resources and the increasing environmental pollution, people are turning to develop alternative energy sources to traditional petrochemical energy. Among them, it is particularly urgent to realize the clean energy supply of mainstream vehicles such as automobiles. At present, power batteries with high energy density, large capacity, low cost, safety and environmental protection have become the bottleneck of large-scale commercialization of electric vehicles. As a polyanionic structural material, unlike the widely used metal oxide types such as LiCoO ₂ , LiMn ₂ O ₄ , LiNi _1/3 Co _1/3 Mn _1/3 O ₂ and other positive electrode materials, lithium iron phosphate (LiFePO ₄ ) has the characteristics of stable structure, long cycle life, good safety performance, low cost, and environmental friendliness, and has become the preferred battery type for electric vehicle power sources.

In 2005, Nyten et al. [Nyten A, et al., Electrochem. Commun., 2005, 7(2):156-160.] reported for the first time a kind of polyanionic structural material synthesized by solid phase method Cross-structure lithium ferrous silicate. As a cathode material for lithium-ion batteries, Li ₂ FeSiO ₄ has similar structure and performance characteristics to LiFePO ₄ . In addition to lower cost, the structure contains 2 Li ⁺ for extraction/intercalation, and the theoretical specific capacity is as high as 331.2mA.hg ^{- 1} , which is much higher than the 169.9 mA.hg ^-1 of LiFePO ₄ , showing great potential for lithium-ion power batteries. However, the material still has the positional blending of Li and Fe during the first charge-discharge cycle, which leads to a decrease in voltage, and the difficulty of oxidation of Fe ³⁺ leads to only one Li ⁺ deintercalation/intercalation, relatively low actual specific capacity, electronic and Li ⁺ conductivity Low, unable to charge and discharge at a high rate and other issues.

In order to improve and enhance the low-voltage, low-capacity, and high-rate charge-discharge performance of Li ₂ FeSiO ₄ , three methods are currently used in the world, namely, the synthesis of nanoparticles, carbon coating, and Mn, Ni, Mg and other heterogeneous element ions. Substitution at the Fe site to stabilize the structure and improve the conductivity of the material. The 3D carbon network formed by burning Li ₂ FeSiO ₄ materials and then ball milling carbon coating, using various organic polymer precursors to form carbon coatings on site, and sol-gel, spray pyrolysis and other synthetic methods The thin layer greatly enhances the electron transport between the crystallites of the material, and at the same time inhibits the growth of the crystallites during synthesis, and the synthesized nanoparticles also shorten the diffusion path of Li ⁺ , which greatly enhances the electron and electron transport in the electrochemical process. Lithium-ion transport ability has achieved remarkable results. Substituting or doping with different kinds of ions is an effective means to improve the structural stability of materials and improve the transport performance of lithium ions and electrons. For example, X. Wu et al. [X. Wu, et al., Electrochimica Acta, 2012, 80: 50-55.] of Wuhan University used polyoxyethylene-polypropylene-polyoxyethylene triblock copolymer P123 precursor on-site The carbon-coated material has a particle size of 30nm, a specific capacity at room temperature of 224 mA.hg ^-1 , 1.39 Li ⁺ released, and a capacity retention rate of 98% for the first 15 discharges. At the same time, the work of S. Zhang et al [S. Zhang, et al., Journal of Electronalytical Chemistry., 2010, 644: 150-154.] of Harbin Institute of Technology showed that by doping 3% Mg at the Fe site, the The Li ⁺ diffusion coefficient and electronic conductivity are more than 2.5 times, respectively 1.69×10 ^-13 cm ² s- ¹ and 8.96×10 ^-3 S cm ^-1 , thus improving the specific capacity and cycle stability of the material.

There are not many international studies on the silicon-site doping of Li ₂ FeSiO ₄ . At the earliest, ME Arroyo-de Dompablo et al [ME Arroyo-de Dompable, et al., Electrochem. Commun., 2006, 8: 1292-1298.] proposed the possibility of Si substitution through theoretical calculations. Subsequently, A. Liivat [A. Liivat, et al., Computational Materials Science, 2010, 50: 191-197.] calculated through density functional theory and discussed the substitution of P and V for the Si position in Li ₂ FeSiO ₄ the resulting impact. The results show that due to the potential electrochemical activity of (VO ₄ ) ^3- , the substitution of part of Si by V has the potential to improve the electron transfer between Fe tetrahedrons, thereby improving the charge-discharge specific capacity of the material; and the Li in LiFeVO ₄ Intercalation and deintercalation only cause expansion and contraction of 6% and 10%, which will not cause the collapse of the crystal structure of the material.

Contents of the invention

In order to solve the problem that the second Li ⁺ extraction/intercalation potential of lithium ferrous silicate is higher than 4.5V vs. Li ⁺ /Li, the actual charge-discharge specific capacity is low, the electronic and ion conductivity itself is low, and the high-rate charge-discharge performance is poor. And the problem of particle agglomeration and unevenness. The first object of the present invention is to provide a lithium ion battery cathode material.

The specific technical solution of the present invention is a lithium-ion battery positive electrode material, the lithium-ion battery positive electrode material is silicon-site doped lithium ferrous silicate, and its chemical formula is Li ₂ FeSi _1-x M _x O _4+δ , Where M is M=B, S, P, Mn, Be, Al, Mo, Cr, W, Nb, lithium: iron: silicon: M=2:1: (1-x): x, the value range of x is 0~0.2.

Another object of the present invention is to provide a method for preparing a positive electrode material of a lithium ion battery, comprising the following steps, step 1, with the stoichiometric ratio of lithium: iron: silicon: M being 2:1: (1-x): x, The value range of x is 0~0.2. Weigh lithium salt, iron salt, silicon compound and M element compound, and dissolve them in distilled water in order to obtain a mixed solution;

Step 2, adding a saturated solution of a carbon-containing compound to the mixed solution obtained in step 1, and stirring magnetically for 12 hours at room temperature;

Step 3: Stir the mixed solution obtained in step 2 in an 80 ^o C oil bath, reflux and condense for 24 hours, vacuum rotary evaporate the excess water until viscous, and then vacuum dry at 100 o ^C for 12 hours to obtain a dry powder;

In step 4, the obtained dry powder is placed in an inert atmosphere or a weakly reducing atmosphere, and pre-fired and sintered to prepare silicon-site-doped lithium ferrous silicate.

Wherein, the carbon-containing compound is citric acid, sucrose, glucose, polyvinyl alcohol or polymer resin.

Wherein, the pre-calcination includes: heating the dry powder from room temperature to 300 ^o C at a heating rate of 5 ^o C/min, keeping it under the protection of an inert atmosphere or a weak reducing atmosphere for 3 hours, and cooling to room temperature to obtain a pre-calcined precursor; The sintering includes: heating the pre-fired precursor dry powder from room temperature to 600-800 ^o C at a heating rate of 5 ^o C/min, keeping it under the protection of an inert atmosphere or a weak reducing atmosphere for 10 hours, and cooling to room temperature to obtain silicon sites. Doped with lithium ferrous silicate.

Wherein, the inert atmosphere is nitrogen.

Wherein, the weakly reducing atmosphere is a mixed gas of nitrogen and hydrogen.

Wherein, the lithium salt is lithium acetate dihydrate.

Wherein, the iron salt is iron nitrate nonahydrate.

Wherein, the silicon compound is nano silicon dioxide.

Wherein, the M element compound is aluminum nitrate nonahydrate and chromium nitrate nonahydrate.

Principle of the present invention is:

Using the lithium ferrous silicate modification method and preparation means provided by the present invention can improve the chemical environment of Li and Fe in lithium ferrous silicate, reduce the oxidation-reduction potential of the second platform Fe ³⁺ /Fe ⁴⁺ , and realize the first The extraction/intercalation of 2 Li ⁺ at the potential below 4.5V vs. Li ⁺ /Li, improving the specific capacity performance of the material, improving the electronic and ionic conductivity of lithium ferrous silicate, improving the high rate charge and discharge performance of the material, and To achieve the purpose of achieving a charge-discharge specific capacity of more than 165.5mA.hg ^-1 theoretical specific capacity of a single lithium ion, and at the same time improve the high-rate charge-discharge performance.

The characteristic of the doping element M in the present invention is that the ion radii are respectively located on both sides of Si ⁴⁺ , the radii are not much different, both are 4-coordination, and the electronegativity has a large difference. Attached Figure 1 summarizes the ionic radii and electronegativity properties of some ions with the 4-coordination configuration of oxygen ions, and provides theoretical guidance for the specific selection of silicon-site doping ions.

The preparation of the dissimilar element ion silicon-site-doped lithium ferrous silicate positive electrode material according to the present invention, according to the degree of difficulty of dissolving, dissolves lithium salt, iron salt, silicon compound and dissimilar element compound sequentially in order from easy to difficult, In this way, the mixing uniformity of the raw materials can be improved, the phase purity of the target product can be improved, and the synthesis temperature can be reduced.

In the preparation of the dissimilar element ion silicon-site-doped lithium ferrous silicate cathode material in the present invention, the so-called inert atmosphere or weakly reducing atmosphere is a nitrogen atmosphere or a mixed atmosphere of nitrogen and hydrogen to ensure that the chemical state of Fe ions is +2.

The preparation of the dissimilar element ion silicon site-doped lithium ferrous silicate positive electrode material in the present invention is characterized in that the dry powder is heated from room temperature to 300 ^oC at a heating rate of 5 ^o C/min, and is heated in an inert atmosphere or a weakly reducing atmosphere. Keep under protection for 3 hours, cool to room temperature to obtain a pre-burned precursor; to obtain a pre-burned precursor, heat the dry powder from room temperature to 600-800 ^oC at a heating rate of 5 ^oC /min, and keep it under the protection of an inert atmosphere or a weak reducing atmosphere After 10 hours, it was cooled to room temperature to obtain silicon-site-doped lithium ferrous silicate (Li ₂ FeSi _1-x M _x O _4+δ /C).

Compared with the prior art, the present invention has the following advantages and beneficial effects:

The invention provides a method of doping dissimilar element ions on the silicon site of lithium ferrous silicate to achieve the purpose of improving material charge-discharge specific capacity, high-rate charge-discharge performance, and improving particle aggregation morphology and surface morphology. Combining the traditional high-temperature solid-state method and the sol-gel method to obtain silicon-site-doped lithium ferrous silicate, the aggregation shape, morphology and particle size of the material can be effectively controlled; at the same time, the substitution of Si ⁴⁺ ions by ions in different valence states will also A considerable number of holes and interstitial electrons are generated in the crystal lattice. The changes in these properties have a positive effect on the structural stability of the material, the migration of lithium ions and electrons, effectively reduce the polarization of the electrode, and improve the charge-discharge specific capacity of the material. Performance: Silicon doped with metal cations or ions of different electronegativity can effectively improve the electronic conductivity and lithium ion transport capacity of lithium ferrous silicate, and improve the high rate charge and discharge performance of the material.

The invention provides a method for preparing silicon-site-doped lithium ferrous silicate, which has the characteristics of low raw material cost, simple steps, easy-to-obtain product, low synthesis temperature and the like.

Description of drawings

Accompanying drawing 1 Ionic radius r _M and electronegativity x _{M of oxygen ion 4 coordination ion M} ;

Accompanying drawing 2 Example 1 The scanning electron microscope image of Li ₂ FeSi _0.9 Al _0.1 O _4+δ /C doped with 10% aluminum on the silicon site;

Accompanying drawing 3 Example 2 The scanning electron microscope image of Li ₂ FeSi _0.9 Cr _0.1 O _4+δ /C doped with 10% chromium on the silicon site;

Accompanying drawing 4 comparative example 1 undoped Li ₂ FeSiO ₄ /C, the scanning electron microscope picture;

Figure 5 Example 1 Li ₂ FeSi _0.9 Al _0.1 O _4+δ /C doped with 10% aluminum on the silicon site, Example 2 Li ₂ FeSi _0.9 Cr _0.1 O _4+δ /C doped with 10% chromium on the silicon site , and the discharge specific capacity-cycle number curves of undoped Li ₂ FeSiO ₄ /C in Comparative Example 2 at different discharge rates;

Figure 6 Example 3 Li ₂ FeSi _0.97 Cr _0.03 O _4+δ /C doped with 3% chromium on the silicon site, Example 4 Li ₂ FeSi _0.94 Cr _0.06 O _4+δ /C doped with 6% chromium on the silicon site , Example 2 Li ₂ FeSi _0.9 Cr _0.1 O _4+δ /C doped with 10% chromium on the silicon site, Li ₂ FeSi _0.8 Cr _0.2 O _4+δ /C doped with 20% chromium on the silicon site in Example 5, and The XRD pattern of comparative example 1 undoped Li ₂ FeSiO ₄ /C, and comparative example 2 undoped Li ₂ FeSiO ₄ ;

Figure 7 Example 6 Li ₂ FeSi _0.94 Al _0.06 O _4+δ /C doped with 6% aluminum on the silicon site, comparative example 3 undoped Li ₂ FeSiO ₄ /C, the first time, the 13th time, and the 14th time , 15 and 16 charge and discharge voltage-specific capacity curves;

Figure 8 Example 7 Li ₂ FeSi _0.97 Cr _0.03 O _4+δ /C doped with 3% cadmium at the silicon site, Comparative Example 4 Li ₂ Fe _0.97 Cr _0.03 SiO _4+δ /C doped with 3% cadmium at the iron site , the charge-discharge voltage-specific capacity curves for the 1st, 18th, 19th, 20th and 21st times.

detailed description

The present invention is further described below in conjunction with example and accompanying drawing.

Example 1

Set the doping amount of aluminum at the silicon position to be 10% by mole, and weigh lithium acetate dihydrate (CH ₃ COOLi 2H ₂ O) 6.2433 according to the stoichiometric ratio lithium:iron:silicon:aluminum ion=0.0612:0.0306:0.0275:0.0031 g. Ferric nitrate nonahydrate (Fe(NO ₃ ) ₃ 9H ₂ O) 12.3621g, nano silicon dioxide (SiO ₂ ) 1.6547g, aluminum nitrate nonahydrate (Al(NO ₃ ) ₃ 9H ₂ O) 1.1479g , added to 50mL deionized water in a certain order, stirred magnetically, one reactant was dissolved and then another reactant was added; 12.86g of citric acid was weighed as a carbon source to prepare a saturated solution of citric acid, and the mixed solution was added drop by drop , stirred magnetically at room temperature for 12 hours, transferred the solution to a flask, heated it in an oil bath at 80 ^oC , refluxed and condensed the mixed solution for 24 hours, and then rotary evaporated the excess water in the mixed solution to a viscous state, and the obtained substance was left standing Gel-like material was formed overnight, and dried at 100 ^oC for 12 hours. The obtained precursor was pre-calcined by heating the dry powder from room temperature to 300 ^oC at a heating rate of 5 ^oC /min in a nitrogen atmosphere, and cooled to room temperature to obtain Pre-burn the precursor, grind and pulverize the dry powder from room temperature to 700 ^oC at a heating rate of 5 ^oC /min, keep it under nitrogen atmosphere protection for 10 hours, and cool to room temperature to obtain silicon-site-doped lithium iron aluminosilicate / carbon composite (Li ₂ FeSi _0.9 Al _0.1 O _4+δ /C).

After the sample was taken out and ground, the particle aggregation and surface morphology of the sample were observed under a scanning electron microscope, and the results are shown in Figure 2.

Weigh 0.3g of the above sample (the content of the sample in the pole piece is 70wt.%), add 0.0857g of acetylene black (the content of the conductive agent in the pole piece is 20wt.%), use a spatula to stir and disperse for more than 20min, and mix it evenly. Put the above mixture into a 50mL beaker containing 0.3106g of PTFE emulsion with a concentration of 13.8 wt.% (the binder content in the pole piece is 10wt.%), add 4mL of isopropanol, stir and disperse with a spatula, and Alcohol volatilizes to plasticine. Then use a manual roller machine to repeatedly roll, fold, and roll until the surface of the mixture sheet is dense and uniform. Adjust the slit of the roller machine to 20 μm to roll the mixture into a sheet with regular edges, smooth surface, and uniform thickness. Dry in a 50 ^o C oven for 2 hours. Then use a puncher to punch into a disc with a diameter of 12mm, use a tablet press to press it on an aluminum mesh disc with a known quality of 12mm in diameter at a pressure of 500kgf.cm ^-2 , and place it in a vacuum tank for 12 ^hours of vacuum drying , weigh the mass of the pole piece in the glove box after cooling.

Put the above pole piece in a glove box filled with a high-purity argon atmosphere, use lithium metal as the negative electrode, and a solution of 1M LiPF ₆ in a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) as the electrolyte. Assemble the R2032 button half battery, and then conduct a charge and discharge test. Charge and discharge at 0.1C, 0.2C, 0.3C, 0.5C, 0.3C, 0.2C, 0.1C current, the specific capacity-cycle number curve of the material is shown in Figure 5, and the measured battery performance of the material is as follows:

The first discharge specific capacity is 122.59mA.hg ^-1 , after 0.1C, 0.2C, 0.3C, 0.5C, 0.3C, 0.2C, 0.1C rate cycles, the 70th discharge specific capacity is 131.25 mA.hg ^-1 , compared to the first discharge The discharge specific capacity increases by 7.06%, and at 0.5C, the relative discharge specific capacity of 0.1C decreases by 15.51%.

Example 2

Set the chromium doping amount at the silicon position to be 10% by mole, and weigh lithium acetate dihydrate (CH ₃ COOLi 2H ₂ O) 6.2433 according to the stoichiometric ratio lithium:iron:silicon:chromium ion=0.0612:0.0306:0.0275:0.0031 g. Ferric nitrate nonahydrate (Fe(NO ₃ ) ₃ 9H ₂ O) 12.3621g, nano silicon dioxide (SiO ₂ ) 1.6547g, chromium nitrate nonahydrate (Al(NO ₃ ) ₃ 9H ₂ O) 1.2244g According to the operation of Example 1, the silicon site doped with 10% chromium iron ferrous silicate/carbon composite (Li ₂ FeSi _0.9 Cr _0.1 O _4+δ /C) was synthesized.

After the sample was taken out and ground, the particle aggregation and surface morphology of the sample were observed under a scanning electron microscope, and the results are shown in Figure 3; the phase structure and phase purity of the sample were investigated by measuring the powder X-ray diffraction pattern, and the results are shown in Figure 6.

Weigh 0.3g of the above sample (the content of the sample in the pole piece is 70wt.%), add 0.0857g of acetylene black (the content of the conductive agent in the pole piece is 20wt.%), and prepare the pole piece according to the operation steps of Example 1, and assemble the R2032 button half battery, and then conduct a charge and discharge test. Charge and discharge at currents of 0.1C, 0.2C, 0.3C, 0.5C, 0.3C, 0.2C, and 0.1C. The specific capacity-cycle curve of the material is shown in Figure 5. The six charge and discharge voltage-specific capacity curves are shown in Figure 5, and the performance of the measured material battery is as follows:

The specific capacity of the first discharge is 146.44mA.hg ^-1 , after 0.1C, 0.2C, 0.3C, 0.5C, 0.3C, 0.2C, 0.1C rate cycles, the specific capacity of the 70th discharge is 138.54 mA.hg ^-1 , which is relatively the first time The discharge specific capacity decreased by 5.40%, and at 0.5C, the relative discharge specific capacity of 0.1C decreased by 13.68%.

Comparative example 1

According to the stoichiometric ratio lithium:iron:silicon=0.0618:0.0309:0.0309, weigh lithium acetate dihydrate (CH ₃ COOLi 2H ₂ O) 6.3046g, iron nitrate nonahydrate (Fe(NO ₃ ) ₃ 9H ₂ O) 12.4837 g. Nano silicon dioxide (SiO ₂ ) 1.8566g, according to the operation of Example 1, synthesize undoped lithium ferrous silicate/carbon composite (Li ₂ FeSiO _4+δ /C).

After the sample was taken out and ground, the particle aggregation and surface morphology of the sample were observed under a scanning electron microscope, and the results are shown in Figure 4; the phase structure and phase purity of the sample were investigated by measuring the powder X-ray diffraction pattern, and the results are shown in Figure 6.

Weigh 0.3g of the above sample (the content of the sample in the pole piece is 70wt.%), add 0.0857g of acetylene black (the content of the conductive agent in the pole piece is 20wt.%), prepare the pole piece and assemble the R2032 button half-cell according to the operation steps of Example 1 , and then carry out the charge and discharge test. Charge and discharge at 0.1C, 0.2C, 0.3C, 0.5C, 0.3C, 0.2C, 0.1C current, the specific capacity-cycle number curve of the material is shown in Figure 5, and the measured battery performance of the material is as follows:

The first discharge specific capacity is 111.10mA.hg ^-1 , after 0.1C, 0.2C, 0.3C, 0.5C, 0.3C, 0.2C, 0.1C rate cycles, the 70th discharge specific capacity is 99.21mA.hg ^-1 , which is relatively the first time The discharge specific capacity decreased by 10.70%, and at 0.5C, the relative discharge specific capacity of 0.1C decreased by 37.77%.

As shown in Figures 2, 3 and 4, from Example 1 (Li ₂ FeSi _0.9 Al _0.1 O _4+δ /C), Example 2 (Li ₂ FeSi _0.9 Cr _0.1 O _4+δ /C) and Comparative Example 1 (Li ₂ FeSiO ₄ /C) scanning electron microscope photo comparison shows that the Si position in Li ₂ FeSiO ₄ is doped with different element ions Al and Cr, the material particles are uniform, the particle size is reduced, and the photo shows that the maximum particle size is from From 6 µm to 3.5 µm, the aggregation state of the secondary particles of the material has been greatly improved.

As shown in Figure 5, from Example 1 (Li ₂ FeSi _0.9 Al _0.1 O _4+δ /C), Example 2 (Li ₂ FeSi _0.9 Cr _0.1 O _4+δ /C) and Comparative Example 1 (Li ₂ FeSiO ₄ /C) The comparison of battery charge and discharge performance shows that the doping of dissimilar element ions Al and Cr at the Si position in Li ₂ FeSiO ₄ can improve the charge and discharge specific capacity and rate performance of the material to varying degrees.

Example 3

Set the chromium doping amount at the silicon position to be 3% by mole, and weigh lithium acetate dihydrate (CH ₃ COOLi 2H ₂ O) 6.2861 according to the stoichiometric ratio lithium:iron:silicon:chromium ion=0.0616:0.0308:0.0299:0.0009 g. Ferric nitrate nonahydrate (Fe(NO ₃ ) ₃ 9H ₂ O) 12.4470g, nano silicon dioxide (SiO ₂ ) 1.7956g, chromium nitrate nonahydrate (Cr(NO ₃ ) ₃ 9H ₂ O) 1.1657g According to the operation of Example 1, the silicon site doped with 3% chromium iron ferrous silicate/carbon composite (Li ₂ FeSi _0.97 Cr _0.03 O _4+δ /C) was synthesized. After the sample was taken out and ground, the powder X-ray diffraction pattern was measured to investigate the phase structure and phase purity of the sample, and the results are shown in Figure 6.

Example 4

Set the chromium doping amount at the silicon position to be 6% by mole, and weigh lithium acetate dihydrate (CH ₃ COOLi 2H ₂ O) 6.2677 according to the stoichiometric ratio lithium:iron:silicon:chromium ion=0.0614:0.0307:0.0289:0.0018 g. Ferric nitrate nonahydrate (Fe(NO ₃ ) ₃ 9H ₂ O) 12.4105g, nano silicon dioxide (SiO ₂ ) 1.7350g, chromium nitrate nonahydrate (Cr(NO ₃ ) ₃ 9H ₂ O) 0.7375g According to the operation of Example 1, the silicon site doped with 6% chromium iron ferrous silicate/carbon composite (Li ₂ FeSi _0.94 Cr _0.06 O _4+δ /C) was synthesized. After the sample was taken out and ground, the powder X-ray diffraction pattern was measured to investigate the phase structure and phase purity of the sample, and the results are shown in Figure 6.

Example 5

Set the chromium doping amount at the silicon position to be 20% by mole, and weigh lithium acetate dihydrate (CH ₃ COOLi 2H ₂ O) 6.1831 according to the stoichiometric ratio of lithium:iron:silicon:chromium ion=0.0606:0.0303:0.0242:0.0061 g. Ferric nitrate nonahydrate (Fe(NO ₃ ) ₃ 9H ₂ O) 12.2429g, nano silicon dioxide (SiO ₂ ) 1.4567g, chromium nitrate nonahydrate (Cr(NO ₃ ) ₃ 9H ₂ O) 2.4253g According to the operation of Example 1, the silicon site doped with 20% chromium iron ferrous silicate/carbon composite (Li ₂ FeSi _0.8 Cr _0.2 O _4+δ /C) was synthesized. After the sample was taken out and ground, the powder X-ray diffraction pattern was measured to investigate the phase structure and phase purity of the sample, and the results are shown in Figure 6.

Comparative example 2

According to the stoichiometric ratio lithium:iron:silicon=0.0618:0.0309:0.0309, weigh lithium acetate dihydrate (CH ₃ COOLi 2H ₂ O) 6.3046g, iron nitrate nonahydrate (Fe(NO ₃ ) ₃ 9H ₂ O) 12.4837 g. Nano silicon dioxide (SiO ₂ ) 1.8566g was added into 50mL deionized water in a certain order, stirred magnetically, one reactant was dissolved and another reactant was added; magnetically stirred at room temperature for 12 hours, and the solution was transferred to The flask was heated in an 80 ^°C oil bath, and the mixed solution was refluxed and condensed for 24 hours, and then the moisture in the mixed solution was evaporated by rotary evaporation to a viscous shape. The obtained material was placed in a petri dish and dried at 100 ^°C for 12 hours to obtain In a nitrogen atmosphere, heat the dry powder from room temperature to 300 ^oC at a heating rate of 5 ^oC /min for pre-sintering, cool to room temperature to obtain a pre-sintered precursor, grind and pulverize it, and heat it at a heating rate of 5 ^oC /min The dry powder was heated from room temperature to 700 ^oC , kept under a nitrogen atmosphere for 10 hours, and cooled to room temperature to obtain undoped lithium ferrous silicate (Li ₂ FeSiO ₄ ).

After the sample was taken out and ground, the powder X-ray diffraction pattern was measured to investigate the phase structure and phase purity of the sample, and the results are shown in Figure 6.

As shown in Figure 6, from Example 3 (Li ₂ FeSi _0.97 Cr _0.03 O ₄ /C), Example 4 (Li ₂ FeSi _0.94 Cr _0.06 O _4+δ /C), Example 2 (Li ₂ FeSi _0.9 Cr _0.1 O _4+δ /C), Example 5 (Li ₂ FeSi _0.8 Cr _0.2 O _4+δ /C), Comparative Example 1 (Li ₂ FeSiO ₄ /C), Comparative Example 2 (Li ₂ FeSiO ₄ ) XRD It can be seen from the comparison of the spectra that the synthesized pyrolytic carbon-coated Li ₂ FeSiO ₄ /C composite has significantly broader spectral lines and lower intensity than the uncoated Li ₂ FeSiO ₄ standard sample, indicating that the Li ₂ FeSiO ₄ /C composite The C composite is a secondary composite particle, which is formed by the agglomeration of Li ₂ FeSiO ₄ primary nanoparticle crystallites with fine particle size; on the other hand, the ratio of Li ₂ FeSiO 4 When the Si site in FeSiO ₄ is doped with different element ions Cr, as shown in Figure 3, the characteristic peaks of the LiCrO ₂ impurity phase appear at 16.58 degrees and 16.66 degrees 2θ of the doped materials except for 10% and 20% silicon sites, respectively. In addition, the XRD pattern curves of the materials are consistent, indicating that the Si-site Cr doping of Li ₂ FeSiO ₄ between the doping amount of x=0-10% will not affect the phase structure of the parent material, and when the doping amount More than 10%, the impurity phase LiCrO ₂ appears, and with the increase of doping amount x, the impurity peak is enhanced.

Example 6

Set the aluminum doping amount at the silicon level to be 6% by mole, and weigh lithium acetate dihydrate (CH ₃ COOLi 2H ₂ O) 6.3260 according to the stoichiometric ratio of lithium:iron:silicon:aluminum ion=0.0620:0.0310:0.0291:0.0019 g. Ferric nitrate nonahydrate (Fe(NO ₃ ) ₃ 9H ₂ O) 12.5260g, nano silicon dioxide (SiO ₂ ) 1.7511g, aluminum nitrate nonahydrate (Al(NO ₃ ) ₃ 9H ₂ O) 0.6979g According to the operation of Example 1, the silicon site doped with 6% lithium iron aluminosilicate/carbon composite (Li ₂ FeSi _0.94 Al _0.6 O _4+δ /C) was synthesized.

Weigh 0.3g of the above sample (the content of the sample in the pole piece is 70wt.%), add 0.0857g of acetylene black (the content of the conductive agent in the pole piece is 20wt.%), and prepare the pole piece according to the operation steps of Example 1, and assemble the R2032 button half battery, and then conduct a charge and discharge test. Charge and discharge at a current of 0.1C, the charge and discharge voltage-specific capacity curves of the 13th to 16th charge and discharge for the first time are shown in Figure 7, and the performance of the measured material battery is as follows:

The specific capacity of the first discharge was 139.37 mA.hg ^-1 , and the specific capacities of the 13th to 16th discharges were 140.47, 143.37, 144.82, and 142.44 mA.hg ^-1 , respectively.

Comparative example 3

Weigh 0.3g of the sample synthesized in Comparative Example 1 (the sample content in the pole piece is 70wt.%), add 0.0857g of acetylene black (the content of the conductive agent in the pole piece is 20wt.%), and prepare the pole piece according to the operation steps of Example 1. Assemble the R2032 button half battery, and then conduct a charge and discharge test. Charge and discharge at a current of 0.1C, the charge and discharge voltage-specific capacity curves of the 13th to 16th charge and discharge for the first time are shown in Figure 7, and the performance of the measured material battery is as follows:

The specific capacity of the first discharge was 110.30 mA.hg ^-1 , and the specific capacities of the 13th to 16th discharges were 103.10, 104.92, 105.02, and 107.77 mA.hg ^-1 , respectively.

As shown in Figure 7, it can be seen from the comparison of the battery charge and discharge performance of Example 6 (Li ₂ FeSi _0.94 Al _0.06 O _4+δ /C) and Comparative Example 3 (Li ₂ FeSiO ₄ /C), in Li ₂ FeSiO ₄ The Si position is doped with different element ions Al, and the 15th discharge specific capacity of the material is increased by 37.90%; further, it can be seen from the shape of the charging curve of Li ₂ FeSi _0.94 Al _0.06 O _4+δ /C in Figure 7 that at 4.5V A new plateau emerged, indicating that the doping of dissimilar element ions Al on the Si site effectively reduced the redox potential of the second plateau Fe ³⁺ /Fe ⁴⁺ , and to some extent achieved the second Li ⁺ at 4.5 V vs . Extraction/insertion of the potential below Li ⁺ /Li, improving the specific capacity performance of the material.

Example 7

Weigh 0.3g of the sample synthesized in Example 3 (Li ₂ FeSi _0.97 Cr _0.03 O _4+δ /C) (the content of the sample in the pole piece is 70wt.%), add 0.0857g of acetylene black (the content of the conductive agent in the pole piece is 20wt. %), according to the operation steps of Example 1 to prepare pole pieces, assemble R2032 button half cells, and then conduct charge and discharge tests. Charge and discharge with a current of 0.1C, the charge-discharge voltage-specific capacity curves of the 1st and 18th to 21st times of charge and discharge for the first time are shown in Figure 8, and the performance of the measured material battery is as follows:

The specific capacity of the first discharge was 102.40 mA.hg ^-1 , and the specific capacities of the 18th to 21st discharges were 108.87, 109.10, 108.17, and 111.57 mA.hg ^-1 , respectively.

Comparative example 4

Set the doping amount of chromium in the iron position to be 3% by mole, and weigh lithium acetate dihydrate (CH ₃ COOLi 2H ₂ O) 6.2998 according to the stoichiometric ratio lithium:iron:chromium:silicon ion=0.0618:0.0300:0.0009:0.0309 g. Iron nitrate nonahydrate (Fe(NO ₃ ) ₃ 9H ₂ O) 12.0999g, chromium nitrate nonahydrate (Cr(NO ₃ ) ₃ 9H ₂ O) 0.3707g, nano silicon dioxide (SiO ₂ ) 1.8552g According to the operation of Example 1, iron-doped 3% chromosilicate lithium iron/carbon composite (Li ₂ Fe _0.97 Cr _0.03 SiO _4+δ /C) was synthesized.

Weigh 0.3g of the above sample (the content of the sample in the pole piece is 70wt.%), add 0.0857g of acetylene black (the content of the conductive agent in the pole piece is 20wt.%), prepare the pole piece and assemble the R2032 button half-cell according to the operation steps of Example 1 , and then carry out the charge and discharge test. Charge and discharge with a current of 0.1C, the charge-discharge voltage-specific capacity curves of the 1st and 18th to 21st times of charge and discharge for the first time are shown in Figure 8, and the performance of the measured material battery is as follows:

The specific capacity of the first discharge was 92.94 mA.hg ^-1 , and the specific capacities of the 18th to 21st discharges were 92.48, 93.64, 90.63, and 89.20 mA.hg ^-1 , respectively.

As shown in Figure 8, it can be seen from the comparison of the battery charge and discharge performance of Example 7 (Li ₂ FeSi _0.97 Cr _0.03 O _4+δ /C) and Comparative Example 4 (Li ₂ Fe _0.97 Cr _0.03 SiO _4+δ /C). The Si site in Li ₂ FeSiO ₄ is doped with 3% dissimilar element ions Cr. Compared with the Fe site doped with 3% dissimilar element ions Cr, the specific capacity of the 20th discharge of the material is increased by 19.35%; further, Li ₂ FeSi _0.97 Cr _0.03 O _4+δ /C material has a plateau at 4.5V, but the doping of dissimilar element ions Cr on the Fe site does not produce a plateau, which shows that the Si site doping has an effect on the structure of the parent material and the Fe site doping. different effects.

The above is only a preferred embodiment of the present invention, and is not intended to limit the present invention. Any modifications, equivalent replacements and improvements made within the spirit and principles of the present invention should be included in the present invention. within the scope of protection.

Claims (10)

1. A cathode material for a lithium-ion battery, characterized in that: the anode material for a lithium-ion battery is silicon-site-doped lithium ferrous silicate, and its chemical formula is Li ₂ FeSi _1-x M _x O _4+δ , wherein M M=B, S, P, Mn, Be, Al, Mo, Cr, W or Nb, lithium: iron: silicon: M=2:1: (1-x): x, x ranges from 0 to 0.2. 2. a preparation method of lithium-ion battery cathode material as claimed in claim 1, is characterized in that: comprise the steps, step 1, with stoichiometric ratio lithium: iron: silicon: M is 2:1: (1- x): x, the value range of x is 0~0.2, weigh lithium salt, iron salt, silicon compound and M element compound, and dissolve them in distilled water to obtain a mixed solution; Step 2, adding a saturated solution of a carbon-containing compound to the mixed solution obtained in step 1, and stirring magnetically for 12 hours at room temperature; Step 3: Stir the mixed solution obtained in step 2 in an oil bath at 80 ^oC , reflux and condense for 24 hours, evaporate the excess water in a vacuum until it becomes viscous, and then dry it in vacuum at 100 ^oC for 12 hours to obtain a dry powder; In step 4, the obtained dry powder is placed in an inert atmosphere or a weakly reducing atmosphere, and pre-fired and sintered to prepare silicon-site-doped lithium ferrous silicate. 3. The preparation method of a lithium ion battery positive electrode material according to claim 2, characterized in that: the carbon-containing compound is citric acid, sucrose, glucose, polyvinyl alcohol or polymer resin. 4. The preparation method of a kind of positive electrode material of lithium ion battery according to claim 2, it is characterized in that: said pre-calcination comprises, with the heating rate of 5 ^oC /min, dry powder is heated from room temperature to 300 ^oC , in inert atmosphere or a weak reducing atmosphere for 3 hours, cooled to room temperature to obtain a pre-sintered precursor; the sintering includes heating the dry powder of the pre-sintered precursor from room temperature to 600-800 ^oC at a heating rate of 5 ^oC /min, and then Keep it under the protection of an inert atmosphere or a weak reducing atmosphere for 10 hours, and cool to room temperature to obtain silicon-site doped lithium ferrous silicate. 5 . The preparation method of a cathode material for a lithium ion battery according to claim 2 or 4 , wherein the inert atmosphere is nitrogen. 6 . 6. The preparation method of a cathode material for a lithium ion battery according to claim 2 or 4, wherein the weakly reducing atmosphere is a mixed gas of nitrogen and hydrogen. 7. The preparation method of a kind of lithium-ion battery cathode material according to claim 2, characterized in that: the lithium salt is lithium acetate dihydrate. 8. The preparation method of a lithium ion battery cathode material according to claim 2, characterized in that: the iron salt is iron nitrate nonahydrate. 9. The preparation method of a lithium-ion battery positive electrode material according to claim 2, characterized in that: the silicon compound is nano silicon dioxide. 10. The preparation method of a lithium ion battery positive electrode material according to claim 2, characterized in that: the M element compound is aluminum nitrate nonahydrate and chromium nitrate nonahydrate.