A magnesium-doped calcium stannate composite material for negative electrode active material and preparation method thereof

Abstract

The present invention provides a negative electrode active material comprising a magnesium-doped calcium stannate composite material, wherein the mass fraction of magnesium in the magnesium-doped calcium stannate composite material is 0.5-2%. By introducing magnesium into the calcium stannate, the present invention overcomes the intrinsic defects of calcium stannate (CaSnO3) to a certain extent, thereby improving the reversible capacity and cycle performance of the calcium stannate.

Description

Technical Field

[0001] The present invention belongs to the field of negative electrode materials for lithium batteries, and specifically comprises a magnesium-doped calcium stannate composite material for negative electrode active materials and a preparation method thereof. Background Art

[0002] With the widespread development of consumer electronics and new energy vehicles, the market has placed higher demands on the energy density of lithium-ion batteries. High-energy-density lithium-ion batteries require anode materials with high specific capacity. Graphite, currently the most widely used anode material, has an actual specific capacity below 372 mAh / g, making it difficult to meet the energy density requirements.

[0003] Compared with traditional graphite negative electrode materials, metal oxides have excellent lithium extraction / insertion capabilities and generally have higher theoretical specific capacities. Among the many candidate materials, transition metal oxides, such as FeO, CoO, NiO, Cu2O, etc., can be used as negative electrode materials for lithium-ion batteries. Electrochemical performance tests have shown that these materials exhibit higher specific capacities (>600mA·h / g), which is about twice the theoretical capacity of carbon materials. x O y (M=Fe, Co, Ni, Ti, etc.) has attracted wide attention as negative electrode materials for lithium-ion batteries. However, metal oxides have poor electronic conductivity. + The process of extraction / insertion will produce irreversible volume expansion, causing lattice collapse, leading to problems such as crystal crushing and falling off, which greatly affects the cycle stability of lithium batteries.

[0004] Perovskite oxides (ABO3) have the advantages of high theoretical capacity, high ionic conductivity, and abundant resource reserves, meeting the requirements for the development of future lithium-ion battery negative electrode materials. In the perovskite crystal structure, different A and B ions will have different degrees of distortion of the BO6 octahedron, which creates a large number of holes at the top corners of the cube, which is conducive to ion migration. Among them, calcium stannate (CaSnO3) as a perovskite oxide is a new type of negative electrode material with a theoretical specific capacity of approximately 994mAh / g, far exceeding the currently commonly used negative electrode materials. In addition, the preparation process is mature and simple, the price is low, and it is environmentally friendly, making it a great potential to become a new generation of negative electrode materials.

[0005] In order to reduce the defects of tin-based electrode materials caused by volume change, such as electrode cracking and pulverization, which in turn leads to poor electrical contact of the material and capacity decay during cycling, the following measures are usually taken: (1) preparing tin oxides with special morphologies (such as thin films, nanoparticles or amorphous states) to minimize the volume expansion rate; (2) trying to form the alloy in a network structure composed of different crystal system oxides or intermetallic compounds (which can be active or inactive for metallic lithium) or carbon derivatives to prevent the separation of metallic tin and buffer the stress caused by volume expansion during the alloying process; (3) selecting a suitable battery operating voltage window to reduce the occurrence of side reactions. Summary of the Invention

[0006] In order to solve the problems and shortcomings of the existing technology, the present invention provides a negative electrode active material, which is a magnesium-doped calcium stannate composite material, which overcomes the intrinsic defects of calcium stannate (CaSnO3) to a certain extent and improves the reversible capacity and cycle performance of calcium stannate.

[0007] According to a first aspect of the present invention, a negative electrode active material is provided, which is a magnesium-doped calcium stannate composite material. In the magnesium-doped calcium stannate composite material, the mass fraction of the magnesium element in the magnesium-doped calcium stannate composite material is 0.5-2%, for example, it can be 0.5%, 0.8%, 1%, 1.2%, 1.5%, 1.8%, or 2%, but is not limited to the listed values. Other values not listed within the numerical range are also applicable.

[0008] Calcium stannate (CaSnO3) is a perovskite oxide. Although we know that its theoretical specific capacity is high, its reversible capacity is greatly reduced due to the irreversible generation of metallic tin and lithium oxide during the first discharge (CaSnO3+4Li + +4e-→CaO+2Li2O+Sn); on the other hand, lithium ions and tin further react Generated Li xSn causes a serious volume change in the material, leading to the collapse of the lattice structure, and ultimately pulverizing the electrode material, affecting the adhesion as a negative electrode material, and causing a large amount of capacity decay. Based on this, domestic and foreign researchers have done little research on tin-based perovskite negative electrode materials. Therefore, calcium stannate is further modified so that calcium stannate can be used as a negative electrode active material to overcome the defects of cracking and pulverizing the electrode caused by volume change, which in turn leads to poor electrical contact of the material and capacity decay during the cycle. Doping modification is currently one of the most widely used means of modifying various materials. In order to improve the electrochemical properties of existing positive and negative electrode active materials, there are many reports showing that appropriate doping can improve material performance. However, as far as calcium stannate is used as a negative electrode material, there are few reports on its doping modification to improve the electrochemical performance of the material. In this scheme, by introducing magnesium into calcium stannate, the crystal structure of calcium stannate is improved, the DC resistance of the material is reduced, and the doping of magnesium can effectively buffer the stress caused by volume expansion of calcium stannate during the electrochemical reaction process, thereby reducing the irreversible capacity loss of the material and greatly improving the cycle stability of the material. Therefore, when the magnesium-doped calcium stannate composite material in this scheme is used as the negative electrode material, the first effect and cycle performance of the battery are improved. In addition, the amount of magnesium doping affects the first effect and crystallinity of the material battery. Magnesium doping consumes more oxygen to generate magnesium salts. Magnesium doping will produce magnesium thermal reaction and release more heat, resulting in an increase in the crystallinity of the calcium stannate material. The oxygen element is consumed, which can reduce the consumption of active lithium in the high-temperature storage process, which is beneficial to the storage performance of the battery. When the doping amount of magnesium meets the above values, the intrinsic defects in calcium stannate, such as vacancy defects, interstitial atomic defects, and dislocation defects, are all better improved. The obtained magnesium-doped calcium stannate composite material has more excellent structural stability, which can well improve the volume expansion effect of the calcium stannate material in the battery cycle, so that it has better reversible capacity and cycle performance.

[0009] Preferably, the magnesium content of the magnesium-doped calcium stannate composite material is 1% by mass. At this magnesium content, the magnesium atoms can largely compensate for the intrinsic defects of the calcium stannate material. The resulting composite material exhibits optimal structural stability, significantly reduces volume expansion during battery charging and discharging, and exhibits optimal overall performance as a negative electrode active material.

[0010] According to a second aspect of the present invention, a method for preparing a negative electrode active material is provided, which is characterized in that it comprises the following steps: S1. dissolving a calcium source and a tin source in water to obtain a first mixed solution, adding an alkaline solution to the first mixed solution to adjust the pH of the reaction system to 10.5-11.5, reacting for 5-15 minutes, and then adding a dispersant thereto to obtain a second mixed solution; S2. adding a magnesium source to the second mixed solution to obtain a third mixed solution, reacting the third mixed solution at 180-220°C for 18-30 hours, and centrifuging, washing, and drying the obtained reaction product in sequence to obtain a magnesium-doped calcium stannate precursor; and S3. calcining the magnesium-doped calcium stannate precursor at 700-900°C for 4-8 hours to obtain a magnesium-doped calcium stannate composite material. In the above preparation method, a calcium source and a tin source are first used as reaction raw materials for a preliminary reaction to form a suspension (a first mixed solution), and then a magnesium source is added to further carry out a coprecipitation reaction, so that the magnesium element can be fully dispersed and evenly distributed in the calcium stannate material, eliminating the lattice defects of the calcium stannate itself as much as possible, and reducing the volume expansion effect of the calcium stannate material during the battery charge and discharge process; in the calcination solid phase reaction carried out after the coprecipitation reaction, the ions in the material undergo solid phase migration, and then the magnesium element can be more stably doped into the material, and can simultaneously make up for some defects of the calcium stannate crystals in the formation process as the calcium stannate crystals are formed, further improving the structural stability of the calcium stannate material, reducing the lattice collapse problem and volume effect problem caused by lattice defects, reducing the irreversible capacity of the calcium stannate material as a negative electrode material, and improving the electrochemical performance of the battery. At the same time, after the above-mentioned calcium source, tin source, and magnesium source coprecipitation reaction, only one calcination is required, eliminating the process of secondary calcination required for traditional element doping, thereby reducing production costs.

[0011] In the above S1, the pH can be, for example, 10.5, 10.8, 11, 11.2, or 11.5, and the reaction time can be, for example, 5 minutes, 6 minutes, 7 minutes, 8 minutes, 9 minutes, or 10 minutes; in the above S2, the reaction temperature can be, for example, 180°C, 190°C, 200°C, 210°C, or 220°C, and the reaction time can be, for example, 18 hours, 20 hours, 22 hours, 24 hours, 26 hours, 28 hours, or 30 hours; in the above S3, the calcination temperature can be, for example, 700°C, 750°C, 800°C, 850°C, or 900°C, and the calcination time can be, for example, 4 hours, 5 hours, 6 hours, 7 hours, or 8 hours; the reaction conditions of the above steps are not limited to the listed values, and other values not listed within the numerical range are equally applicable.

[0012] Preferably, in S1, the calcium source includes at least one of calcium chloride, calcium acetate, and calcium nitrate.

[0013] Preferably, the calcium source is calcium chloride.

[0014] Preferably, in S1, the tin source includes at least one of tin tetrachloride and tin tetrafluoride.

[0015] Preferably, the tin source is tin tetrachloride.

[0016] Preferably, in S1, the magnesium source comprises magnesium oxide.

[0017] Preferably, the particle size of magnesium oxide is 50 to 200 nm, for example, 50 nm, 70 nm, 90 nm, 100 nm, 120 nm, 140 nm, 160 nm, 180 nm, or 200 nm, but is not limited to the listed values, and other values not listed within the numerical range are also applicable.

[0018] Preferably, the particle size of magnesium oxide is 100 nm.

[0019] Preferably, in S3, the magnesium-doped calcium stannate precursor is heated to 700-900°C at the following heating program: first heated to 350-450°C at a first heating rate, held constant for 1-3 hours, and then heated to 700-900°C at a second heating rate; the first heating rate is 3-6°C / min; the second heating rate is 7-10°C / min. A slower heating rate is first used to increase the temperature so that the magnesium element has sufficient time to fill the defects formed by the calcium stannate crystals, and the calcium stannate crystals can overcome more stress defects at a slower heating rate, thereby avoiding the rapid volatilization of volatile components causing cracking of the composite material and forming weak points inside or outside the composite material that are not conducive to the stability of the composite material structure. Then, a higher heating rate is used to ensure the densification of the composite material structure and further enhance the structural stability of the material, thereby reducing the volume expansion and lattice collapse problems of the calcium stannate material during the battery charge and discharge process and improving the battery charge and discharge performance.

[0020] In the above S3, the first heating rate can be, for example, 3°C / min, 4°C / min, 5°C / min, or 6°C / min, and the temperature can be raised to 350-450°C, for example, 350°C, 370°C, 390°C, 410°C, 430°C, or 450°C, and the constant temperature time can be 1 hour, 2 hours, or 3 hours; the second heating rate can be, for example, 7°C / min, 8°C / min, 9°C / min, or 10°C / min, and the temperature can be raised to 700-900°C, for example, 700°C, 750°C, 800°C, 850°C, or 900°C; the reaction conditions of the above steps are not limited to the listed values, and other values not listed within the numerical range are equally applicable.

[0021] Preferably, the first heating rate is 4°C / min; the second heating rate is 8°C / min. At these heating rates, the resulting magnesium-doped calcium stannate composite material not only exhibits excellent compactness, but also significantly reduces defects in its crystal structure, as well as surface or internal defects. This results in a smaller irreversible capacity during actual battery charge and discharge, making it a superior negative electrode material.

[0022] According to a third aspect of the present invention, a negative electrode sheet is provided, comprising a negative electrode active material. Using the magnesium-doped calcium stannate composite material as the negative electrode material for a battery significantly improves the battery's initial efficiency and cycle performance, maintaining a high specific capacity after multiple cycles.

[0023] According to a fourth aspect of the present invention, a lithium-ion battery is provided, comprising a positive electrode sheet, an electrolyte, a separator, and the negative electrode sheet. DETAILED DESCRIPTION

[0024] In order to enable those skilled in the art to better understand the solutions of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments.

[0025] Example 1

[0026] 1. Preparation of negative electrode active materials

[0027] The negative electrode active material of this embodiment was prepared as follows:

[0028] S1. 8.8 g of CaCl2 (0.1 mol) and 17.5 g of SnCl4·H2O were dissolved in 500 mL of deionized water to obtain a first mixed solution. NaOH solution was added to the first mixed solution under continuous stirring until the pH of the reaction system was 11 and a large amount of white precipitate was formed. After continuous stirring for 10 min, 0.8 g of CTAB (cetyltrimethylammonium bromide) was added and stirred to obtain a second mixed solution.

[0029] S2. The second mixed solution in S1 is transferred to a reactor, and magnesium oxide is added (the amount is calculated based on the mass fraction of magnesium element in the final magnesium-doped calcium stannate composite material being 1%), the particle size of the magnesium oxide being 100 nm, to obtain a third mixed solution, and the third mixed solution is stirred at 200° C. for 24 h. The obtained reaction product is centrifuged and collected, washed three times with deionized water and an organic solvent, respectively, and then dried in an oven (60° C.) to obtain a magnesium-doped calcium stannate precursor (Mg / CaSn(OH)6); the organic solvent is selected from at least one of methanol, ethanol, and acetone;

[0030] S3. Place the magnesium-doped calcium stannate precursor prepared in S2 in a muffle furnace, first heat it to 400°C at a heating rate of 4°C / min, keep it warm for 2 hours, then heat it to 800°C at a heating rate of 8°C / min, calcine it at 800°C for 6 hours, and cool it to room temperature to obtain a magnesium-doped calcium stannate composite material, which is used as the negative electrode active material, wherein the mass fraction of magnesium element in the magnesium-doped calcium stannate composite material is 1%.

[0031] 2. Battery Preparation

[0032] The negative electrode sheet was prepared using the negative electrode active material and negative electrode current collector obtained above. The negative electrode sheet, positive electrode sheet (NCM811 as the positive electrode active material) and separator (Celgard separator 2400 microporous multilayer polymer) were assembled, and the electrolyte (1 mol / L LiPF6 ethylene carbonate EC + dimethyl carbonate EDC (volume ratio 1:1)) was injected to prepare a battery.

[0033] Example 2

[0034] 1. Preparation of negative electrode active materials

[0035] In the preparation of the negative electrode active material of this embodiment, the difference from Example 1 is that the amount of magnesium oxide added in S2 is different, so that the mass fraction of magnesium element in the magnesium-doped calcium stannate composite material prepared in S3 is 2%; the rest is the same as Example 1.

[0036] 2. Battery Preparation

[0037] The preparation of the battery of this embodiment is the same as that of Example 1.

[0038] Example 3

[0039] 1. Preparation of negative electrode active materials

[0040] In the preparation of the negative electrode active material of this embodiment, the difference from Example 1 is that the amount of magnesium oxide added in S2 is different, so that the mass fraction of magnesium element in the magnesium-doped calcium stannate composite material prepared in S3 is 0.5%; the rest is the same as Example 1.

[0041] 2. Battery Preparation

[0042] The preparation of the battery of this embodiment is the same as that of Example 1.

[0043] Example 4

[0044] 1. Preparation of negative electrode active materials

[0045] In the preparation of the negative electrode active material of this embodiment, the difference from that of Example 1 is that the calcium source used in S1 is calcium acetate (the input amount is converted according to the mass of the calcium element); the rest is the same as that of Example 1.

[0046] 2. Battery Preparation

[0047] The preparation of the battery of this embodiment is the same as that of Example 1.

[0048] Example 5

[0049] 1. Preparation of negative electrode active materials

[0050] In the preparation of the negative electrode active material of this embodiment, the difference from that of Example 1 is that the calcium source used in S1 is calcium nitrate (the input amount is converted according to the mass of calcium element); the rest is the same as that of Example 1.

[0051] 2. Battery Preparation

[0052] The preparation of the battery of this embodiment is the same as that of Example 1.

[0053] Example 6

[0054] 1. Preparation of negative electrode active materials

[0055] In the preparation of the negative electrode active material of this embodiment, the difference from that of Example 1 is that the tin source used in S1 is tin tetrafluoride (the input amount is converted according to the mass of the tin element); the rest is the same as that of Example 1.

[0056] 2. Battery Preparation

[0057] The preparation of the battery of this embodiment is the same as that of Example 1.

[0058] Example 7

[0059] 1. Preparation of negative electrode active materials

[0060] In the preparation of the negative electrode active material of this embodiment, the difference from that of Example 1 is that the magnesium source used in S1 is magnesium chloride (the input amount is converted according to the mass of the magnesium element); the rest is the same as that of Example 1.

[0061] 2. Battery Preparation

[0062] The preparation of the battery of this embodiment is the same as that of Example 1.

[0063] Example 8

[0064] 1. Preparation of negative electrode active materials

[0065] In the preparation of the negative electrode active material of this embodiment, the difference from Example 1 is that in S3, the calcium stannate precursor is heated to 800°C according to the following heating program: first, the temperature is increased to 400°C at 8°C / min, kept constant at this temperature for 2 hours, and then the temperature is increased to 800°C at 8°C / min; the rest is the same as Example 1.

[0066] 2. Battery Preparation

[0067] The preparation of the battery of this embodiment is the same as that of Example 1.

[0068] Example 9

[0069] 1. Preparation of negative electrode active materials

[0070] In the preparation of the negative electrode active material of this embodiment, the difference from Example 1 is that in S3, the calcium stannate precursor is heated to 800°C according to the following heating program: first, the temperature is increased to 400°C at 4°C / min, kept constant at this temperature for 2 hours, and then the temperature is increased to 800°C at 4°C / min; the rest is the same as Example 1.

[0071] 2. Battery Preparation

[0072] The preparation of the battery of this embodiment is the same as that of Example 1.

[0073] Example 10

[0074] 1. Preparation of negative electrode active materials

[0075] In the preparation of the negative electrode active material of this embodiment, the difference from Example 1 is that in S3, the final sintering temperature of the calcium stannate precursor is too low. Specifically, the calcium stannate precursor is heated to 600°C according to the following heating program: first, the temperature is increased to 400°C at 4°C / min, and the temperature is kept constant for 2 hours, and then the temperature is increased to 600°C at 8°C / min; then, the temperature is calcined at 600°C for 6 hours. The rest is the same as in Example 1.

[0076] 2. Battery Preparation

[0077] The preparation of the battery of this embodiment is the same as that of Example 1.

[0078] Example 11

[0079] 1. Preparation of negative electrode active materials

[0080] In the preparation of the negative electrode active material of this embodiment, the difference from Example 1 is that in S3, the final sintering temperature of the calcium stannate precursor is too low. Specifically, the calcium stannate precursor is heated to 1000°C according to the following heating program: first, the temperature is increased to 400°C at 4°C / min, and the temperature is kept constant for 2 hours, and then the temperature is increased to 1000°C at 8°C / min; then, the temperature is calcined at 1000°C for 6 hours. The rest is the same as Example 1.

[0081] 2. Battery Preparation

[0082] The preparation of the battery of this embodiment is the same as that of Example 1.

[0083] Example 12

[0084] 1. Preparation of negative electrode active materials

[0085] In the preparation of the negative electrode active material of this embodiment, the difference from that of Example 1 is that the particle size of the magnesium oxide added in S2 is 25 nm, and the magnesium-doped calcium stannate composite material prepared in S3; the rest is the same as that of Example 1.

[0086] 2. Battery Preparation

[0087] The preparation of the battery of this embodiment is the same as that of Example 1.

[0088] Example 13

[0089] 1. Preparation of negative electrode active materials

[0090] In the preparation of the negative electrode active material of this embodiment, the difference from Example 1 is that the particle size of the magnesium oxide added in S2 is 300 nm, and the magnesium-doped calcium stannate composite material prepared in S3; the rest is the same as Example 1.

[0091] 2. Battery Preparation

[0092] The preparation of the battery of this embodiment is the same as that of Example 1.

[0093] Comparative Example 1

[0094] 1. Preparation of negative electrode materials

[0095] In the preparation of the negative electrode active material of this embodiment, the difference from that of Example 1 is that magnesium oxide is not added to S2; the rest is the same as that of Example 1.

[0096] 2. Battery Preparation

[0097] The preparation of the battery of this comparative example is consistent with that of Example 1.

[0098] Comparative Example 2

[0099] 1. Preparation of negative electrode materials

[0100] In the preparation of the negative electrode active material of this embodiment, the difference from Example 1 is that the amount of magnesium oxide added in S2 is different, so that the mass fraction of magnesium element in the magnesium-doped calcium stannate composite material prepared in S3 is 0.2%; the rest is the same as Example 1.

[0101] 2. Battery Preparation

[0102] The preparation of the battery of this comparative example is consistent with that of Example 1.

[0103] Comparative Example 3

[0104] 1. Preparation of negative electrode active materials

[0105] In the preparation of the negative electrode active material of this embodiment, the difference from that of Example 1 is that the mass fraction of the magnesium element in the magnesium-doped calcium stannate composite material prepared in S3 is 2.5%; the rest is the same as that of Example 1.

[0106] 2. Battery Preparation

[0107] The preparation of the battery of this comparative example is consistent with that of Example 1.

[0108] Test Case

[0109] 1. Experimental Construction Method

[0110] The batteries prepared in Examples 1 to 13 and Comparative Examples 1 to 3 were tested for direct current resistance (DCIR), first efficiency, initial reversible capacity, reversible capacity after 300 cycles, and cycle performance;

[0111] The DC resistance test method is as follows: the battery is charged to 50% SOC and discharged at a current rate of 2C for 10 seconds. The battery voltage U2 and current I before discharge are recorded, as well as the battery voltage U1 after the battery voltage stabilizes. The DC internal resistance R is calculated according to the formula R = (U2-U1) / I. The DC internal resistance before and after the battery cycle is recorded as R0 and R1, respectively. The DC resistance change rate = (R1-R0) / R0. The DC resistance rate is the rate of change of the DC resistance at the time of discharge relative to the initial DC resistance.

[0112] The first efficiency test method is as follows: discharge the battery at 0.1C to 2.8V, let it stand for 15 minutes, and obtain the first lithium insertion capacity; then charge it at 0.1C to 4.25V to obtain the first lithium removal capacity; first efficiency = first lithium removal capacity / first lithium insertion capacity × 100%;

[0113] The test method for the reversible capacity after 100 cycles is as follows: the capacity of the battery is tested after 100 cycles at a rate of 0.1C;

[0114] The test method for the reversible capacity after 300 cycles is: test the capacity of the battery after 300 cycles at a 0.1C rate.

[0115] 2. Experimental Results

[0116] The electrochemical performance characterization results of the batteries prepared in Examples 1 to 13 and Comparative Examples 1 to 3 are shown in Table 1 below.

[0117] Table 1 Characterization results of electrochemical performance of batteries prepared in Examples 1 to 13 and Comparative Examples 1 to 3

[0118]

[0119] As can be seen from Table 1, it can be seen from Examples 1 to 13 and Comparative Examples 1 to 3 that the battery prepared by the magnesium-doped calcium stannate composite material obtained by the preparation method of the present invention not only has a low DC resistance, but also has a high reversible capacity; It can be seen from Examples 1 to 3 that, under the condition of a suitable magnesium doping amount, the battery prepared by the magnesium-doped calcium stannate composite material has a low DC resistance, a high first efficiency and a high reversible capacity, and the overall performance of the battery is good; It can be seen from Examples 1 and 4 to 5 that when the tin source is not the preferred tin source provided by the present invention, the battery prepared by the negative electrode material has poor DC internal resistance and first efficiency performance; It can be seen from Examples 1 and 6 to 7 that, When the magnesium source is not the preferred magnesium source provided by the present invention, the reversible capacity of the battery prepared by the obtained negative electrode material is relatively low; it can be seen from Example 1 and Examples 8 to 11 that a reasonable heating program and calcination temperature can help to exert the performance of the negative electrode material and improve the electrochemical performance of the battery; it can be seen from Examples 1 and Examples 12 to 13 that when the particle size of the magnesium oxide used is not within an appropriate range, the performance of the battery prepared by the formed negative electrode material is correspondingly reduced. It may be that the combination effect of magnesium oxide with too small or too large particle size and calcium stannate is poor, the structural stability of the formed negative electrode material is poor, and the DC resistance is large. It can be seen from Example 1 and Comparative Example 1 that the doping of magnesium oxide can effectively improve the crystal structure of calcium stannate, and can effectively buffer the stress caused by volume expansion of calcium stannate during the electrochemical reaction process, thereby reducing the irreversible capacity loss of the material and improving the battery performance; it can be seen from Example 1 and Comparative Example 2 that insufficient magnesium doping will reduce the oxygen consumption during the preparation of the composite material, and insufficient oxygen consumption will increase the active lithium consumption during storage, which is not conducive to the battery storage performance and reversible capacity; it can be seen from Example 1 and Comparative Example 3 that too high magnesium content will lead to an intensified magnesium thermal reaction, thereby increasing the grain size of the composite material, increasing the polarization of the material, and causing the DC resistance of the battery to increase rapidly.

[0120] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents, but these modifications or replacements are all within the scope of protection of the present invention.

Claims

1. A negative electrode active material, characterized in that: The negative electrode active material is a magnesium-doped calcium stannate composite material. In the magnesium-doped calcium stannate composite material, the mass fraction of magnesium element in the magnesium-doped calcium stannate composite material is 0.5-2%.

2. The negative electrode active material according to claim 1, wherein: The mass fraction of magnesium element in the magnesium-doped calcium stannate composite material is 1%.

3. A method for preparing the negative electrode active material according to claim 1, characterized in that: The steps include: S1. The calcium source and the tin source are dissolved in water to obtain a first mixed solution, an alkali solution is added to the first mixed solution to give a pH of the reaction system of 10.5 to 11.5, and after reacting for 5 to 15 minutes, a dispersant is added thereto to obtain a second mixed solution; S2. A magnesium source is added to the second mixed solution to obtain a third mixed solution, and the third mixed solution is reacted at 180 to 220 ° C for 18 to 30 hours, and the obtained reaction product is centrifuged, washed, and dried to obtain a magnesium-doped calcium stannate precursor; S3. calcining the magnesium-doped calcium stannate precursor at 700-900° C. for 4-8 hours to obtain a magnesium-doped calcium stannate composite material.

4. The method according to claim 3, wherein: In S1, the calcium source includes at least one of calcium chloride, calcium acetate, and calcium nitrate.

5. The method according to claim 3, wherein: In S1, the tin source includes at least one of tin tetrachloride and tin tetrafluoride.

6. The method according to claim 3, wherein: In S1, the magnesium source includes magnesium oxide.

7. The method according to claim 6, wherein: The particle size of the magnesium oxide is 50 to 200 nm.

8. The method according to claim 3, wherein: In S3, the magnesium-doped calcium stannate precursor is controlled to be heated to 700-900° C. according to the following heating program: first heating to 350-450° C. at a first heating rate, keeping the temperature constant for 1-3 hours, and then heating to 700-900° C. at a second heating rate; the first heating rate is 3-6° C. / min; the second heating rate is 7-10° C. / min.

9. A negative electrode sheet, characterized in that: The negative electrode active material comprises the negative electrode active material according to any one of claims 1 to 2.

10. A lithium-ion battery comprising a positive electrode sheet, an electrolyte, a separator, and the negative electrode sheet according to claim 9.

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