A modified sodium bismuth titanate-coated ternary cathode material and its preparation method

Abstract

The present invention discloses a modified sodium bismuth titanate coated ternary positive electrode material and a preparation method thereof. The modified sodium bismuth titanate coated ternary positive electrode material comprises: a ternary positive electrode material matrix and a modified sodium bismuth titanate coating layer, and the chemical formula of the modified sodium bismuth titanate is Na 0.5 Bi 0.5‑δ Ti 1‑z M z O 3‑ε ; Wherein, M is one or more of Mg, Zn, Cu, and La; 0

Description

Technical Field

[0001] The present invention relates to the field of lithium batteries, and in particular to a ternary positive electrode material coated with modified sodium bismuth titanate and a preparation method thereof. Background Art

[0002] In recent years, lithium-ion batteries have been widely used in consumer electronics, energy storage, and power batteries as a new type of green energy storage device. As the demand for driving range of pure electric passenger vehicles continues to increase, nickel-cobalt-manganese ternary cathode materials have shown broad application prospects due to their high theoretical specific capacity. In order to further improve the energy density of batteries, the nickel content in ternary materials has been continuously increased. High nickel ternary materials, such as LiNi 0.6 Co 0.2 Mn 0.2 O2(NCM622) and LiNi 0.8 Co 0.1 Mn 0.1 O2 (NCM811) and other products have been put into mass production. However, the high specific capacity characteristics of high-nickel ternary materials come at the expense of thermal stability and safety characteristics. Their structural stability and thermal stability decrease with the increase of nickel content. Under special operating conditions such as high temperature, overcharge, high rate, and mechanical external force, they are prone to overheating, leading to thermal runaway of lithium batteries, posing a safety hazard that cannot be ignored. Existing research shows that thermal runaway of lithium-ion batteries is mainly caused by the thermal reaction between the positive electrode material and the electrolyte, which in turn causes an internal short circuit. When the battery heats up due to an internal short circuit, if its discharge current is not limited in time, the heat accumulation will cause the internal temperature of the battery to rise, causing the positive electrode material to decompose due to an irreversible phase change, releasing O2. O2 and the electrolyte react violently instantly, producing a large amount of CO2 gas, which breaks through the battery shell and causes an explosion.

[0003] Surface coating is one of the effective strategies to improve the structural stability and thermal stability of ternary positive electrode materials. This method can form a protective layer on the surface of the electrode material, which has many beneficial effects on the electrode material, such as preventing the positive electrode material from direct contact with the electrolyte to inhibit side reactions, inhibiting the phase transformation of the positive electrode material, stabilizing the surface structure, and preventing gas precipitation. Chinese patent CN113161547 discloses a preparation method for a ternary positive electrode material coated with a PTC (positive temperature coefficient) material, which selects sodium bismuth titanate as the main component to form the PTC material layer and coats it on the surface of the ternary positive electrode material, so that the resistance of the PTC material layer increases stepwise with the increase of temperature, reducing the current and inhibiting the occurrence of side reactions, thereby effectively reducing the conductivity at high temperature, significantly reducing the internal short-circuit current, and thus significantly reducing the risk of thermal runaway inside the positive electrode material, and ultimately significantly improving the cycle and storage performance of lithium-ion batteries at high temperatures. However, the room temperature conductivity of pure sodium bismuth titanate is also poor, which is not conducive to the transmission of lithium ions and electrons during normal operation of the battery, and cannot effectively inhibit the phase change of the positive electrode material and the release of O2, ultimately resulting in limited improvement in the electrochemical performance of the battery during normal operation and safety at high temperatures. Summary of the Invention

[0004] The purpose of the present invention is to overcome the above-mentioned technical deficiencies, propose a modified sodium bismuth titanate-coated ternary positive electrode material and a preparation method thereof, and solve the technical problem in the prior art that the pure sodium bismuth titanate-coated ternary positive electrode material has limited improvement on the electrochemical performance of the battery during normal operation and the safety at high temperature.

[0005] In the first aspect, the present invention provides a ternary positive electrode material coated with modified sodium bismuth titanate, comprising: a ternary positive electrode material matrix and a modified sodium bismuth titanate coating layer, wherein the chemical formula of the modified sodium bismuth titanate is Na 0.5 Bi 0.5-δ Ti 1-z M z O 3-ε ; Wherein, M is one or more of Mg, Zn, Cu, and La; 0<δ≤0.1, 0 <z≤0.1,0<ε≤0.25。

[0006] In a second aspect, the present invention provides a method for preparing a modified sodium bismuth titanate-coated ternary cathode material, comprising the following steps:

[0007] S1. Provide ternary cathode materials and modified sodium bismuth titanate;

[0008] S2. Evenly mix the modified sodium bismuth titanate and the ternary positive electrode material, and then perform heat treatment to obtain a ternary positive electrode material coated with the modified sodium bismuth titanate.

[0009] Compared with the prior art, the present invention has the following beneficial effects:

[0010] The present invention uses sodium bismuth titanate modified by Bi-site deficiency and low-valent metal doping at the Ti-site to coat the ternary cathode material, comprehensively improving the structural stability, interfacial stability, thermal stability and cycling performance of the ternary material, and significantly enhancing the electrochemical performance of the battery during normal operation and the safety at high temperatures. Brief Description of the Drawings

[0011] Figure 1 It is a preparation schematic diagram of an embodiment of the preparation method of the ternary cathode material coated with modified sodium bismuth titanate provided by the present invention;

[0012] Figure 2 It is a curve graph showing the change of the resistance of the coating material used in Example 1 and Comparative Example 2 of the present invention with temperature. Detailed Embodiments

[0013] In order to make the purpose, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0014] In a first aspect, the present invention provides a ternary cathode material coated with modified sodium bismuth titanate, including: a ternary cathode material matrix and a modified sodium bismuth titanate coating layer, and the chemical formula of the modified sodium bismuth titanate is Na 0.5 Bi 0.5-δ Ti 1-z M z O 3-ε ; wherein, M is one or more of low-valent metals substituting for the Ti-site in sodium bismuth titanate such as Mg, Zn, Cu, La, etc.; 0 < δ ≤ 0.1, 0 < z ≤ 0.1, 0 < ε ≤ 0.25. If there is no Bi-site deficiency or Ti-site doping, the oxygen vacancy concentration of the modified sodium bismuth titanate near room temperature will be relatively low, which cannot provide good conductivity and is not conducive to the performance of the rate performance of the battery during normal temperature operation; if the Bi-site deficiency amount or Ti-site doping is excessive, the structure of the modified sodium bismuth titanate will be unstable, and the excessive doping amount of the doping element M will also lead to a decrease in the proportion of Ti, affecting its ability to form a strong Ti-O bond with the matrix, making the coating material prone to fall off during the electrochemical cycling process and damaging the capacity retention rate.

[0015] In this embodiment, the ternary cathode material is a nickel-cobalt-manganese ternary cathode material.

[0016] In some preferred embodiments of the present invention, the chemical general formula of the ternary cathode material is LiNi 1-x-y Co x Mn y O2, where 0.02 ≤ x + y ≤ 0.67.

[0017] In this embodiment, the modified sodium bismuth titanate material accounts for 0.05% to 3.0% of the total mass of the modified sodium bismuth titanate-coated ternary cathode material, preferably 0.1% to 1%. During this process, the coating amount of the modified sodium bismuth titanate must be strictly controlled. If the coating amount is too small, the ternary cathode material may not be fully coated, and the battery performance may not be sufficiently improved; if the coating amount is too large, the battery capacity and lithium ion / electron transport (rate performance) will be affected.

[0018] In a second aspect, the present invention provides a method for preparing a modified sodium bismuth titanate-coated ternary cathode material, comprising the following steps:

[0019] S1. Provide ternary cathode materials and modified sodium bismuth titanate;

[0020] S2. Evenly mix the modified sodium bismuth titanate and the ternary positive electrode material, and then perform heat treatment to obtain a ternary positive electrode material coated with the modified sodium bismuth titanate.

[0021] In this embodiment, the above-mentioned ternary positive electrode material is obtained by the following steps: a lithium source and a ternary positive electrode material precursor are uniformly mixed, and then a first sintering treatment is performed to obtain the ternary positive electrode material.

[0022] In some preferred embodiments of the present invention, the ternary cathode material precursor is a nickel-cobalt-manganese ternary cathode material precursor, and the chemical formula of the nickel-cobalt-manganese ternary cathode material precursor is Ni 1-x-y Co x Mn y (OH)2, where 0.02≤x+y≤0.67.

[0023] In some preferred embodiments of the present invention, the particle size D50 of the ternary cathode material precursor is 5 to 15 μm.

[0024] In some preferred embodiments of the present invention, the lithium source is one or more of lithium carbonate, lithium hydroxide, lithium oxide, lithium acetate, and lithium nitrate; when the nickel content is high (Ni≥0.8), lithium hydroxide is more preferred.

[0025] In some preferred embodiments of the present invention, the ratio of the ternary cathode material precursor to the lithium source is 1.01 to 1.10, more preferably 1.03 to 1.06, according to the molar ratio of lithium to metal element Li / Me.

[0026] In some preferred embodiments of the present invention, during the first sintering process, the reaction atmosphere is dried air or oxygen; when the nickel content is high (Ni≥0.8), oxygen is more preferred.

[0027] In some preferred embodiments of the present invention, during the first sintering process, the first target temperature is 600-900° C., the heating rate is 1-10° C. / min, and after reaching the first target temperature, the temperature is kept constant for 6-30 hours.

[0028] In this embodiment, the modified sodium bismuth titanate is obtained by the following steps:

[0029] The sodium source, bismuth source, titanium source and dopant are mixed evenly, followed by a second sintering process, and the sintered product is ground into nano-scale powder to obtain modified sodium bismuth titanate.

[0030] In some preferred embodiments of the present invention, the sodium source is one or more of sodium carbonate, sodium oxide, and sodium hydroxide.

[0031] In some preferred embodiments of the present invention, the bismuth source is one or more of bismuth oxide and bismuth hydroxide.

[0032] In some preferred embodiments of the present invention, the titanium source is titanium dioxide.

[0033] In some preferred embodiments of the present invention, the dopant is one or more of the low-valent metal oxides such as magnesium oxide (MgO), zinc oxide (ZnO), copper oxide (CuO), and lanthanum oxide (La2O3) that replace the Ti position in sodium bismuth titanate.

[0034] In some preferred embodiments of the present invention, the sodium source, bismuth source, titanium source and dopant are prepared according to the chemical formula Na 0.5 Bi 0.5-δ Ti 1-z M z O 3-ε The molar ratio of the metal elements is used for mixing; where 0<δ≤0.1, 0 <z≤0.1,0<ε≤0.25。

[0035] In some preferred embodiments of the present invention, the sodium source, bismuth source, titanium source and dopant are baked at 200-400° C. for 4-6 hours before sintering the mixture to remove moisture.

[0036] In some preferred embodiments of the present invention, during the second sintering process, the atmosphere is dried air or oxygen.

[0037] In some preferred embodiments of the present invention, during the second sintering process, the second target temperature is 900-1100° C., the heating rate is 1-10° C. / min, and the temperature is kept constant for 3-12 hours after reaching the second target temperature.

[0038] In some preferred embodiments of the present invention, the sintered product is ground into nano-scale powder by a nano-sand mill.

[0039] In some preferred embodiments of the present invention, the particle size of the nano-scale modified sodium bismuth titanate powder is less than 200 nm.

[0040] In this embodiment, during the heat treatment, the atmosphere is dried air or oxygen; when the nickel content is high (Ni≥0.8), oxygen is more preferred.

[0041] In this embodiment, during the heat treatment, the third target temperature is 400-900° C., the heating rate is 1-10° C. / min, and after reaching the third target temperature, the temperature is maintained constant for 3-12 hours.

[0042] In some preferred embodiments of the present invention, the modified sodium bismuth titanate-coated ternary cathode material needs to be crushed and then passed through a 300-mesh sieve.

[0043] Example 1

[0044] Step 1: Ni with particle size D50 of 10 μm 0.82 Co 0.05 Mn 0.13 The (OH)2 precursor and lithium hydroxide powder were weighed in a molar ratio of Li / Me=1.05:1, mixed evenly in a high-speed mixer, and then the mixture was heated to 800°C in an oxygen atmosphere at a heating rate of 5°C / min and calcined for 12 hours to obtain LiNi 0.82 Co 0.05 Mn 0.13 O2 ternary cathode material;

[0045] Step 2: Bake industrial grade sodium carbonate, bismuth oxide, titanium dioxide and zinc oxide at 300 ° C for 5 hours to remove moisture before sintering; then 0.5 Bi 0.48 Ti 0.97 Zn 0.03 O 2.94 The molar ratio of the metal elements was weighed, mixed evenly with a high-speed mixer, and then heated to 960 ° C in a dry air atmosphere at a rate of 5 ° C / min and calcined for 6 hours to obtain a composite modified Na 0.5 Bi 0.48 Ti 0.97 Zn 0.03 O 2.94 The material is then ground into nano-scale powder with a particle size of about 100nm using a nano sand mill;

[0046] Step 3: LiNi obtained in step 1 0.82 Co 0.05 Mn 0.13 O2 positive electrode material and nanoscale Na obtained in step 2 0.5 Bi 0.48 Ti 0.97Zn 0.03 O 2.94 The powder is mixed evenly with a high-speed mixer, and the nano-scale Na 0.5 Bi 0.48 Ti 0.97 Zn 0.03 O 2.94 The powder accounts for 1.0% of the total mass of the mixed material. The mixed material is heated to 400℃ in a muffle furnace at a heating rate of 5℃ / min and kept at this temperature for 6h for heat treatment. The heat-treated sample is crushed and sieved after cooling to obtain Na 0.5 Bi 0.48 Ti 0.97 Zn 0.03 O 2.94 Coated LiNi 0.82 Co 0.05 Mn 0.13 O2 positive electrode material.

[0047] Example 2

[0048] Step 1: Ni with particle size D50 of 5μm 0.6 Co 0.2 Mn 0.2 The (OH)2 precursor and lithium hydroxide powder were weighed in a molar ratio of Li / Me = 1.01:1, mixed evenly with a high-speed mixer, and then the mixture was heated to 900 ° C in an oxygen atmosphere at a heating rate of 2 ° C / min and calcined for 6 h to obtain LiNi 0.6 Co 0.2 Mn 0.2 O2 ternary cathode material;

[0049] Step 2: Bake industrial grade sodium carbonate, bismuth oxide, titanium dioxide and copper oxide at 200 ° C for 6 hours to remove moisture before sintering; then 0.5 Bi 0.4 Ti 0.9 Cu 0.1 O 2.75 The molar ratio of the metal elements was weighed, mixed evenly with a high-speed mixer, and then heated to 1100 ° C in a dry air atmosphere at a rate of 10 ° C / min and calcined for 3 hours to obtain a composite modified Na 0.5 Bi 0.4 Ti 0.9 Cu 0.1 O 2.75 The material is then ground into nano-scale powder with a particle size of about 200nm using a nano sand mill;

[0050] Step 3: LiNi obtained in step 1 0.6 Co 0.2 Mn 0.2O2 positive electrode material and nanoscale Na obtained in step 2 0.5 Bi 0.4 Ti 0.9 Cu 0.1 O 2.75 The powder is mixed evenly with a high-speed mixer, and the nano-scale Na 0.5 Bi 0.4 Ti 0.9 Cu 0.1 O 2.75 The powder accounts for 0.05% of the total mass of the mixed material. The mixed material is heated to 750℃ in a muffle furnace at a heating rate of 2℃ / min and kept at this temperature for 3h for heat treatment. The heat-treated sample is crushed and sieved after cooling to obtain Na 0.5 Bi 0.4 Ti 0.9 Cu 0.1 O 2.75 Coated LiNi 0.6 Co 0.2 Mn 0.2 O2 positive electrode material.

[0051] Example 3

[0052] Step 1: Ni with particle size D50 of 15μm 0.95 Co 0.02 Mn 0.03 The (OH)2 precursor and lithium hydroxide powder were weighed in a molar ratio of Li / Me=1.1:1, mixed evenly with a high-speed mixer, and then the mixture was heated to 600°C in an oxygen atmosphere at a heating rate of 10°C / min and calcined for 30h to obtain LiNi 0.95 Co 0.02 Mn 0.03 O2 ternary cathode material;

[0053] Step 2: Bake industrial grade sodium carbonate, bismuth oxide, titanium dioxide and magnesium oxide at 400 ° C for 4 hours to remove moisture before sintering; then 0.5 Bi 0.45 Ti 0.99 Mg 0.01 O 2.915 The molar ratio of the metal elements was weighed, mixed evenly with a high-speed mixer, and then heated to 900 ° C at a rate of 2 ° C / min in a dry air atmosphere and calcined for 12 hours to obtain a composite modified Na 0.5 Bi 0.45 Ti 0.99 Mg 0.01 O 2.915 The material is then ground into nano-scale powder with a particle size of about 150nm using a nano sand mill;

[0054] Step 3: LiNi obtained in step 1 0.95 Co 0.02 Mn 0.03 O2 positive electrode material and nanoscale Na obtained in step 2 0.5 Bi 0.45 Ti 0.99 Mg 0.01 O 2.915 The powder is mixed evenly with a high-speed mixer, and the nano-scale Na 0.5 Bi 0.45 Ti 0.99 Mg 0.01 O 2.915 The powder accounts for 3.0% of the total mass of the mixed material. The mixed material is heated to 900℃ in a muffle furnace at a heating rate of 10℃ / min and kept at this temperature for 12h for heat treatment. The heat-treated sample is crushed and sieved after cooling to obtain Na 0.5 Bi 0.45 Ti 0.99 Mg 0.01 O 2.915 Coated LiNi 0.95 Co 0.02 Mn 0.03 O2 positive electrode material.

[0055] Comparative Example 1

[0056] No surface coating was performed, and the rest was the same as in Example 1.

[0057] Comparative Example 2

[0058] According to the chemical formula Na 0.5 Bi 0.5 Weighing and sintering TiO3 to prepare pure Na 0.5 Bi 0.5 TiO3, and then pure Na 0.5 Bi 0.5 The surface coating of TiO3 and other conditions are the same as those in Example 1.

[0059] Comparative Example 3

[0060] According to the chemical formula Na 0.5 Bi 0.48 TiO 2.97 Weigh the materials and sinter to prepare Na 0.5 Bi 0.48 TiO 2.97 , and then the cathode material is subjected to Na 0.5 Bi 0.48 TiO 2.97 The other conditions are the same as those in Example 1.

[0061] Comparative Example 4

[0062] According to the chemical formula Na 0.5 Bi 0.48 Ti 0.85 Zn 0.15 O 2.82 Weigh the materials and sinter to prepare Na 0.5 Bi 0.48 Ti 0.85 Zn 0.15 O 2.82 , and then the cathode material is subjected to Na 0.5 Bi 0.48 Ti 0.85 Zn 0.15 O 2.82 The other conditions are the same as those in Example 1.

[0063] Comparative Example 5

[0064] According to the chemical formula Na 0.5 Bi 0.5 Ti 0.97 Zn 0.03 O 2.97 Weigh the materials and sinter to prepare Na 0.5 Bi 0.5 Ti 0.97 Zn 0.03 O 2.97 , and then the cathode material is subjected to Na 0.5 Bi 0.5 Ti 0.97 Zn 0.03 O 2.97 The other conditions are the same as those in Example 1.

[0065] Comparative Example 6

[0066] According to the chemical formula Na 0.5 Bi 0.35 Ti 0.97 Zn 0.03 O 2.745 Weigh the materials and sinter to prepare Na 0.5 Bi 0.35 Ti 0.97 Zn 0.03 O 2.745 , and then the cathode material is subjected to Na 0.5 Bi 0.35 Ti 0.97 Zn 0.03 O 2.745 The other conditions are the same as those in Example 1.

[0067] Comparative Example 7

[0068] According to nanoscale Na 0.5 Bi 0.48Ti 0.97 Zn 0.03 O 2.94 The powder accounts for 5.0% of the total mass of the mixed material for surface coating, and other conditions are the same as those in Example 1.

[0069] experimental group

[0070] The composite cathode materials obtained in the above examples and comparative examples were assembled into button cells and 2Ah soft pack cells, and the relevant electrochemical performance parameters of the batteries were tested. The results are shown in Table 1. The specific battery assembly process is as follows:

[0071] (1) Button battery

[0072] The prepared composite positive electrode material was mixed with conductive carbon black and binder PVDF in a mass ratio of 92:4:4 using NMP as solvent, and then coated on Al foil. The mixture was dried at 120°C for 12 h, rolled and punched into 12 mm discs. In an argon-protected glove box, CR2032 button batteries were assembled using metal lithium sheets as negative electrodes. The electrochemical performance was tested at 25°C and a voltage window of 2.8-4.3.

[0073] (2) 2Ah soft pack battery

[0074] The prepared composite cathode material was added into NMP solvent according to the mass ratio of cathode material: Super P: KS-6: PVDF = 96:2:0.5:1.5 and evenly mixed to prepare a slurry with a solid content of 68%. The slurry was then made into a compacted density of 3.5 g / cm 3 The positive electrode active material is copper foil coated with graphite, the separator is Celgard 2400, and the electrolyte is 1M LiPF6 with a ratio of EC:EMC:DEC = 4:4:2. It is assembled with the positive electrode active material to form a 2Ah soft pack battery. The above batteries were subjected to high temperature cycle performance tests (2.8V-4.3V, 1C / 4C rate, 45℃), storage gas generation test (1C rate full charge 4.3V, 85℃ high temperature storage for 24h) and hot box test (first fully charge the battery at 1C rate, place it in a hot box, then heat it from room temperature to 150℃±2℃ at a rate of 3℃ / min and keep it warm for 30min, and observe the battery explosion and fire).

[0075] Table 1 Performance data of button batteries and 2Ah soft pack batteries of Examples 1 to 3 and Comparative Examples 1 to 7

[0076]

[0077]

[0078] As can be seen from Table 1, the modified sodium bismuth titanate-coated ternary positive electrode materials prepared in Examples 1 to 3 of the present invention have good structural stability, interface stability, thermal stability and cycle performance, significantly improving the electrochemical performance of the battery during normal operation and the safety at high temperatures.

[0079] Compared with Example 1, the high and low temperature cycle performance and thermal stability of Comparative Example 1 are very poor. The reason is that Comparative Example 1 is not coated with modified sodium bismuth titanate, and cannot isolate the direct contact between the electrolyte and the positive electrode active material. There are no stable oxygen vacancies to compensate for the oxygen vacancies generated by the positive electrode material during the cycle, and it cannot effectively inhibit the phase change process of the positive electrode material, the release of oxygen, and the side reaction with the electrolyte. As a result, its structural stability, thermal stability and cycle performance are very poor, and it is easy to have thermal runaway at high temperature.

[0080] Compared with Example 1, the rate performance (capacity ratio of 0.2C / 1C), cycle performance and thermal stability of Comparative Example 2 at room temperature are all poor. The reason is that Comparative Example 2 is coated with pure sodium bismuth titanate, which has a large resistance near room temperature (see Figure 2 ), poor conductivity, there is a large surface impedance after coating on the surface of the positive electrode material, which affects the rate performance; the oxygen vacancy concentration of pure sodium bismuth titanate near room temperature is small, which cannot effectively compensate for the oxygen vacancies generated by the positive electrode material during the cycle, thereby failing to significantly improve the structural stability of the material and affecting the cycle performance; in addition, the PTC effect of pure sodium bismuth titanate is significantly worse than that of modified sodium bismuth titanate (see Figure 2 The lift-to-resistance ratio of modified sodium bismuth titanate is significantly higher than that of pure sodium bismuth titanate), and the temperature at which the resistance step increases is relatively high, which makes it impossible to respond to the abnormal temperature increase of the system earlier and adjust the surface resistance and reduce the current in time.

[0081] Compared with Example 1, the electrochemical improvement effect of Comparative Example 3 is slightly worse than that of Comparative Example 1. This is because the coating of Comparative Example 3 only performs Bi site deletion on sodium bismuth titanate without performing Ti site substitution modification. Its room temperature resistivity, oxygen vacancy concentration and PTC effect are lower than those of Example 1.

[0082] Compared with Example 1, the improvement effect of Comparative Example 4 on the cycle performance and thermal stability is slightly worse. This is because excessive Zn doping at the Ti position will lead to structural instability of the modified sodium bismuth titanate and reduce the proportion of Ti, affecting its ability to form a strong Ti-O bond with the matrix, making the coating material easy to fall off during the electrochemical cycle, which is not conducive to the improvement of cycle performance, interface stability and thermal stability.

[0083] Compared with Example 1, the electrochemical improvement effect of Comparative Example 5 is slightly worse than that of Comparative Example 1. This is because the coating of Comparative Example 5 only replaces the Ti site of sodium bismuth titanate and the Bi site ions are not missing. Its room temperature resistivity, oxygen vacancy concentration and PTC effect are lower than those of Example 1.

[0084] Compared with Example 1, the improvement effect of Comparative Example 6 on the cycle performance and thermal stability is slightly worse. This is because excessive ion deficiency of Bi will lead to structural instability of the modified sodium bismuth titanate, which is not conducive to the improvement of cycle performance, interface stability and thermal stability.

[0085] Compared with Example 1, Comparative Example 7 has a slightly better improvement effect on the cycle performance and thermal stability, but the discharge capacity and rate performance deteriorate. This is mainly because the coated modified sodium bismuth titanate itself does not participate in the charge and discharge process and does not provide capacity, and the conductivity of lithium ions is weaker than that of the lithium nickel cobalt manganese oxide core. Therefore, when the coating amount is too large, the surface coating layer will be too thick, which will affect the battery charge and discharge process and the transport of lithium ions / electrons, which is not conducive to the capacity and rate performance of the battery.

[0086] In summary, the beneficial effects of the present invention include:

[0087] (1) Compared with pure sodium bismuth titanate, sodium bismuth titanate modified by Bi deficiency and Ti doping with low-valent metals has a higher oxygen vacancy concentration and better conductivity near room temperature. Its coating on the surface of the ternary positive electrode material not only avoids direct contact between the positive electrode material and the electrolyte, inhibits side reactions, but also reduces the surface impedance of the electrode material near room temperature, thereby improving the cycle stability and rate performance of the battery during normal charge and discharge;

[0088] (2) Compared with pure sodium bismuth titanate, sodium bismuth titanate modified by Bi deficiency and Ti-site doping of low-valent metals has more stable oxygen vacancies. When coated on the surface of the ternary cathode material, it can clamp the highly active lattice oxygen generated by the irreversible reaction of the ternary material, inhibit the cathode material from transforming from layered to spinel-like phase and finally to rock salt phase as the temperature rises under high lithium removal state, and inhibit the release of oxygen, thereby comprehensively improving the structural stability and cycle performance of the ternary cathode material;

[0089] (3) Compared with pure sodium bismuth titanate, the resistance value of sodium bismuth titanate modified by Bi deficiency and Ti doping of low-valent metals increases stepwise with increasing temperature, and the temperature point moves toward low temperature. It also has a large lift-to-resistance ratio and a strong PTC effect, which enables it to respond earlier to the temperature rise caused by internal short circuit of the battery, adjust the surface resistance in time to reduce the current, and better avoid thermal runaway of the battery.

[0090] The specific embodiments of the present invention described above do not limit the scope of protection of the present invention. Any other corresponding changes and modifications made based on the technical concept of the present invention should be included in the scope of protection of the claims of the present invention.

Claims

1. A modified sodium bismuth titanate-coated ternary cathode material, characterized in that: include: The ternary cathode material matrix and the modified sodium bismuth titanate coating layer, and the chemical formula of the modified sodium bismuth titanate is Na 0.5 Bi 0.5-δ Ti 1-z M z O 3-ε ; Wherein, M is one or more of Mg, Zn, Cu, and La, 0<δ≤0.1, 0 <z≤0.1,0<ε≤0.25。 2. The modified sodium bismuth titanate-coated ternary cathode material according to claim 1, characterized in that: The ternary positive electrode material is a nickel-cobalt-manganese ternary positive electrode material.

3. The modified sodium bismuth titanate-coated ternary cathode material according to claim 2, characterized in that: The chemical formula of the ternary cathode material is LiNi 1-x-y Co x Mn y O2; where 0.02≤x+y≤0.

67.

4. The modified sodium bismuth titanate-coated ternary cathode material according to claim 1, characterized in that: The modified sodium bismuth titanate accounts for 0.05% to 3.0% of the total mass of the ternary positive electrode material coated with the modified sodium bismuth titanate.

5. A method for preparing a ternary positive electrode material coated with modified sodium bismuth titanate according to any one of claims 1 to 4, characterized in that: The following steps are involved: Provide ternary cathode materials and modified sodium bismuth titanate; The modified sodium bismuth titanate and the ternary positive electrode material are mixed evenly, and then heat treated to obtain the modified sodium bismuth titanate-coated ternary positive electrode material.

6. The method for preparing the modified sodium bismuth titanate-coated ternary cathode material according to claim 5, characterized in that: The ternary cathode material is obtained by the following steps: uniformly mixing a lithium source and a ternary cathode material precursor, and then performing a first sintering process to obtain the ternary cathode material; wherein, The particle size D50 of the ternary cathode material precursor is 5 to 15 μm; The lithium source is one or more of lithium carbonate, lithium hydroxide, lithium oxide, lithium acetate, and lithium nitrate; The ratio of the ternary cathode material precursor to the lithium source is such that the molar ratio of lithium to metal element Li / Me is 1.01 to 1.10; During the first sintering treatment, the reaction atmosphere is dried air or oxygen, the first target temperature is 600-900°C, the heating rate is 1-10°C / min, and after reaching the first target temperature, the sintering is carried out at a constant temperature for 6-30 hours.

7. The method for preparing the modified sodium bismuth titanate-coated ternary cathode material according to claim 5, characterized in that: The modified sodium bismuth titanate is obtained by the following steps: The sodium source, bismuth source, titanium source and dopant are mixed evenly, followed by a second sintering process, and the sintered product is ground into nano-scale powder to obtain modified sodium bismuth titanate.

8. The method for preparing the modified sodium bismuth titanate-coated ternary cathode material according to claim 7, characterized in that: The sodium source is one or more of sodium carbonate, sodium oxide, and sodium hydroxide; the bismuth source is one or more of bismuth oxide and bismuth hydroxide; the titanium source is titanium dioxide; and the dopant is one or more of magnesium oxide, zinc oxide, copper oxide, and lanthanum oxide.

9. The method for preparing the modified sodium bismuth titanate-coated ternary cathode material according to claim 7, characterized in that: The sodium source, bismuth source, titanium source and dopant need to be baked at 200-400° C. for 4-6 hours before sintering the mixture to remove moisture.

10. The method for preparing the modified sodium bismuth titanate-coated ternary cathode material according to claim 7, characterized in that: During the second sintering process, the atmosphere is dried air or oxygen, the second target temperature is 900-1100° C., the heating rate is 1-10° C. / min, and after reaching the second target temperature, the sintering is carried out at a constant temperature for 3-12 hours.

11. The method for preparing the modified sodium bismuth titanate-coated ternary cathode material according to claim 7, characterized in that: The particle size of the nano-scale modified sodium bismuth titanate powder is less than 200 nm.

12. The method for preparing the modified sodium bismuth titanate-coated ternary cathode material according to claim 5, characterized in that: During the heat treatment, the atmosphere is dried air or oxygen, the third target temperature is 400-900° C., the heating rate is 1-10° C. / min, and after reaching the third target temperature, the temperature is kept constant for 3-12 hours.

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