A cathode material for sodium-ion batteries, a preparation method thereof, and applications

Abstract

The present invention provides a cathode material for a sodium-ion battery, a preparation method and an application thereof. A coating layer containing A3PO4 and T2P2O7 is coated on the surface of the main material. T2P2O7 in the coating layer can act synergistically with A3PO4 and the main material, making the coating layer and the main material combine more firmly, improving the compatibility between the coating layer and the main material, and enabling the coating layer to remain stable during battery cycling and high-temperature storage, and not being prone to cracking and shedding. This coating layer does not participate in the electrochemical reaction within the voltage range of 0-4V, can inhibit the high-temperature swelling of the battery, and improve the high-temperature performance and cycling performance of the battery. The preparation method involved in the present invention is simple, the coating materials used are inexpensive, the comprehensive cost is relatively low, and large-scale industrial production can be carried out.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and particularly to a cathode material for sodium-ion batteries, a preparation method thereof, and an application thereof. Background Art

[0002] Sodium-ion batteries have advantages such as rich resources, low cost, and good low-temperature performance. With the substantial increase in the price of lithium resources in recent years, sodium-ion batteries have received attention from the capital market, and the industrialization process has been accelerating. Currently, there are three technical routes for the preparation of cathode materials for sodium-ion batteries. Among them, sodium layered oxide materials are the most promising technical route.

[0003] However, due to the high residual alkali content on the surface of sodium layered oxide materials, the pH value is generally greater than 13, and side reactions are likely to occur when contacting with the electrolyte or water, which will lead to the following technical problems:

[0004] (1) Poor high-temperature performance and easy gas swelling: At high temperatures, side reactions occur between the sodium layered oxide material and the electrolyte, generating gas, resulting in serious gas swelling of the battery, battery deformation, increased internal resistance, and even safety problems;

[0005] (2) Poor cycle performance: During the cycling process, side reactions occur between the sodium layered oxide material and the electrolyte, degrading the cathode material and generating gas, resulting in battery thickening, increased internal resistance, and attenuation of the cycling capacity;

[0006] (3) Poor processing performance: It is extremely easy to absorb water during the preparation of the slurry, forming a gel.

[0007] In response to the above problems, researchers have proposed the following solutions:

[0008] The patent with the publication number CN115692653A discloses a cathode material for sodium-ion batteries and a preparation method thereof. By coating the surface of the sodium layered oxide material with an oxide M y O z (where M includes at least one of Ge, Rb, Cd, Zn, Zr, Mg, Al, Nb, Ti, Sn, Ca, Ru, Mo, Sb, Sr, and Ag, 0 < y ≤ 3, 0 < z ≤ 5), the coating layer can reduce the volume change stress of the overall material during cycling, and improve the cycling stability and capacity retention rate.

[0009] The patent with the publication number CN115472796A discloses a cathode material with a coating layer, a preparation method thereof, and a sodium-ion battery. By coating the surface of the sodium layered oxide material with Na 3-2p P 4-p and / or Na 3-q PS 4-q H q, which is used to solve the problem of high alkali content on the surface of sodium-ion cathode materials in the prior art, improve the gas generation performance and high-temperature performance of sodium-ion cathode batteries, and enhance the specific capacity and rate performance of sodium-ion batteries.

[0010] However, the above patents all have some defects and deficiencies. For example, the patent oxide M of CN115692653A y O z coating layer will be corroded by HF at high temperature, resulting in damage to the coating layer and unable to solve the problem of poor high-temperature performance of the battery; the patent Na of CN115472796A 3-2p PS 4-p and / or Na 3-q PS 4-q H q The residual sulfide in it will dissolve in the electrolyte and migrate to the negative electrode, occupying the active sites of the negative electrode and blocking the sodium ion insertion channels, unable to solve the problem of poor cycle performance of the battery; and the compatibility between the coating layer and the sodium layered oxide material is not good. After long-term cycling, the coating layer will crack and peel off, resulting in a sudden drop in the cycle capacity. Summary of the Invention

[0011] In view of this, the present invention proposes a sodium-ion battery cathode material, its preparation method and application. By coating a coating layer containing A3PO4 and T2P2O7 on the surface of the main material, the gas swelling of the battery can be inhibited, and the cycle performance and high-temperature performance of the battery can be improved simultaneously.

[0012] The main idea of the present invention is as follows: The main reasons for the poor cycle performance, high-temperature performance and easy gas swelling of sodium-ion batteries are that the sodium layered oxide cathode material will have side reactions with the electrolyte. On the one hand, the side reactions will cause the destruction of the material structure and the attenuation of the cycle capacity; on the other hand, the side reactions will cause the decomposition of the electrolyte to generate gas, resulting in gas swelling, deformation of the battery and an increase in internal resistance. Therefore, to solve the problems of poor cycle performance, high-temperature performance and easy gas swelling of sodium-ion batteries, the side reactions must be blocked first.

[0013] Adopting the coating technology to separate the sodium layered oxide cathode material from the electrolyte through the coating layer so that the two cannot be in direct contact is a feasible method to block the side reactions. The coating layer should have the following properties:

[0014] (1) Good chemical stability, does not react with the cathode material, electrolyte, current collector, etc. during the operation of the battery and is insoluble in the electrolyte;

[0015] (2) Good electrochemical stability, does not participate in electrochemical reactions within the voltage range of 0-4V;

[0016] (3) Good compatibility with the sodium layered oxide material and not easy to fall off;

[0017] (4) It has a relatively high sodium ion conductivity.

[0018] Existing conventional coating materials such as alumina, lithium iron phosphate, carbon materials, etc. cannot fully meet the above conditions. Therefore, it is necessary to develop new coating materials. After screening and experiments on various materials, the present invention finally determines to use A3PO4 and T2P2O7 as the coating materials.

[0019] The technical solution of the present invention is realized as follows:

[0020] On the one hand, the present invention provides a cathode material for a sodium ion battery, which is composed of a main material and a coating layer;

[0021] The chemical general formula of the main material is NaMO2;

[0022] Among them, M is one or more elements of Ni, Fe, Ti, Co, Cu, Mn, V, Cr;

[0023] The composition of the coating layer contains A3PO4 and T2P2O7, wherein T2P2O7 is generated by the high-temperature decomposition of THPO4;

[0024] Among them, A is one or more alkali metal elements, and T is one or more alkaline earth metal elements.

[0025] Preferably, A is one or more elements of Li, Na, K, Rb, Cs.

[0026] Further preferably, A contains A1 and A2, wherein A1 is Li, and A2 is one or more elements of Li, Na, K, Rb, Cs. In this way, the sodium ion conductivity of the coating layer can be further improved.

[0027] Preferably, T is one or two elements of Mg or Ca.

[0028] Preferably, the interface between the coating layer and the main material contains T-O-M bonds.

[0029] In the above solution, the coating layer does not participate in the electrochemical reaction within the voltage range of 0 - 4V.

[0030] Preferably, the weight ratio of the main material to the coating layer is 1:(0.01 - 0.1). If the content of the coating layer is too low, it will result in an incomplete coating layer and cannot block the contact between the main material and the electrolyte; if the content of the coating layer is too high, it will lead to a decrease in the specific capacity of the material.

[0031] Preferably, the weight ratio of the main material to the coating layer is 1:(0.02 - 0.08).

[0032] Further preferably, the molar ratio of A3PO4 to T2P2O7 is 1:(0.01 - 0.3). The introduction of T2P2O7 can improve the compatibility between the coating layer and the main material. However, if the content of T2P2O7 is too high, it will cause a decrease in the specific capacity of the material; if the content of T2P2O7 is too low, the binding stability between the coating layer and the main material will decrease.

[0033] Further preferably, the molar ratio of A3PO4 to T2P2O7 is 1:(0.03 - 0.2).

[0034] On the other hand, the present invention also provides a method for preparing the cathode material for a sodium-ion battery described in the first aspect of the present invention, comprising the following steps:

[0035] Step 1: Mix sodium carbonate and the M source in proportion to form a mixture, where M is one or more elements selected from Ni, Fe, Ti, Co, Cu, Mn, V, and Cr. Ball-mill the mixture to mix it evenly to obtain a precursor.

[0036] Step 2: Sinter the precursor at a high temperature, then crush and screen it to obtain Product 1.

[0037] Step 3: Mix and ball-mill Product 1 with A3PO4 and THPO4, disperse it in ethanol to make a slurry, and then spray-dry it to obtain Product 2.

[0038] Step 4: Sinter Product 2 at a high temperature, then crush and screen it to obtain the cathode material for the sodium-ion battery.

[0039] Preferably, in Step 1, the molar ratio of sodium carbonate to the M source is (1.01 - 1.1):1.

[0040] Preferably, in Step 1, the M source is an oxide of M, a carbonate of M, or an oxalate of M.

[0041] Preferably, in Step 2, the sintering temperature is 800°C - 1200°C, and the sintering time is 6h - 24h.

[0042] Preferably, in Step 3, first mix and ball-mill A3PO4 and THPO4, and then mix and ball-mill with Product 1.

[0043] Preferably, dry air is introduced during the sintering process in Step 4 to remove the water vapor generated by the decomposition of THPO4.

[0044] Preferably, in step 4, the sintering temperature is 550°C - 750°C, and the sintering time is 2h - 6h. During the sintering process, THPO4 will decompose to generate T2P2O7 and H2O. The THPO4 at the interface between the coating layer and the main material will react with the main material while decomposing, generating T-O-M bonds. The coating layer is connected to the main material through the T-O-M bonds and pyrophosphate groups, enhancing the binding stability between the coating layer and the main material.

[0045] Since the formation of T-O-M bonds will reduce the valence state of M, resulting in a decrease in the specific capacity of the cathode material, the addition amount of THPO4 should not be too much.

[0046] In a third aspect, the present invention also provides an application of a sodium-ion battery cathode material, using the cathode material obtained by the method for preparing a sodium-ion battery cathode material described in the second aspect in a sodium-ion battery.

[0047] The sodium-ion battery cathode material, preparation method and application of the present invention have the following beneficial effects compared with the prior art:

[0048] (1) By constructing a coating layer containing A3PO4 and T2P2O7 on the surface of the main material, the present invention can prevent side reactions between the main material and the electrolyte, thereby inhibiting battery swelling and degradation of the main material, and improving the high-temperature performance and cycling performance of the battery;

[0049] (2) T2P2O7 in the coating layer of the present invention can act synergistically with A3PO4 and the main material. Through pyrophosphate groups and the generated T-O-M bonds, the coating layer is more firmly combined with the main material, improving the compatibility between the coating layer and the main material. By regulating the ratio of the two, a coating layer with both high stability and high sodium-ion conductivity can be constructed, enabling the coating layer to remain stable during battery cycling and high-temperature storage, and not easily cracking or peeling off, thereby improving the high-temperature performance and cycling performance of the battery, and especially improving the high-temperature performance after long-term cycling of the battery;

[0050] (3) The preparation method involved in the present invention is simple, the coating materials used are inexpensive, and the comprehensive cost is low, and large-scale industrial production can be carried out. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0052] Figure 1 It is the XRD spectrum of the sodium-ion battery cathode material prepared in Example 1.

[0053] Figure 2 Cycling curves of the sodium-ion batteries prepared in Example 1 and Comparative Example 1.

[0054] Figure 3 High-temperature discharge curves of the sodium-ion batteries prepared in Example 1 and Comparative Example 1. Detailed implementation manners

[0055] Next, in combination with the implementation manners of the present invention, the technical solutions in the implementation manners of the present invention will be clearly and completely described. Obviously, the described implementation manners are only a part of the implementation manners of the present invention, rather than all of the implementation manners. Based on the implementation manners in the present invention, all other implementation manners obtained by those of ordinary skill in the art without creative efforts fall within the scope of protection of the present invention.

[0056] Example 1

[0057] This example provides a cathode material for a sodium-ion battery, the main material is NaNi 0.4 Fe 0.2 Mn 0.4 O2, the coating layer components include Li3PO4, Na3PO4, Mg2P2O7, and the molar ratio is 0.5:0.5:0.3. The weight ratio of the main material to the coating layer is 1:0.01. The preparation method is as follows:

[0058] Step 1: Mix sodium carbonate with nickel oxide, iron oxide, and manganese oxide in proportion to form a mixture, and ball-mill the mixture to mix evenly to obtain a precursor.

[0059] Step 2: Sinter the precursor at 1200 °C for 6 h, then crush and screen it to obtain Product 1.

[0060] Step 3: Mix Product 1 with Li3PO4, Na3PO4, and MgHPO4 in proportion and ball-mill, disperse it in ethanol to make a slurry, and then spray-dry it to obtain Product 2.

[0061] Step 4: Sinter Product 2 at 750 °C for 2 h, introduce dry air during the sintering process, then crush and screen it to obtain the cathode material for the sodium-ion battery.

[0062] Battery preparation method:

[0063] Mix the cathode material for the sodium-ion battery prepared in this example with a binder PVDF and a conductive agent SP in a weight ratio of 94:4:2, and mix it with a solvent NMP to prepare a slurry. Then coat it on an aluminum foil, and after drying, rolling, cutting and other processes, prepare a cathode sheet for the sodium-ion battery.

[0064] Using hard carbon as the anode material, it is mixed with binder LA133 and conductive agent SP in a weight ratio of 93:4.5:2.5, and mixed with deionized water as the solvent to prepare a slurry. Then it is coated on aluminum foil, and after drying, rolling, cutting and other processes, the anode sheet of the sodium-ion battery is prepared.

[0065] The above-mentioned anode sheet, cathode sheet and separator of the sodium-ion battery are wound together to form an electrode group. Then the electrode tabs are welded and encapsulated with an aluminum-plastic film. After baking, liquid injection, activation, formation and other processes, a soft-pack sodium-ion battery is prepared.

[0066] Example 2

[0067] This example provides a cathode material for a sodium-ion battery. The main material is NaNi 0.4 Fe 0.2 Mn 0.4 O2, and the coating layer composition includes Li3PO4 and Mg2P2O7 with a molar ratio of 1:0.01. The weight ratio of the main material to the coating layer is 1:0.1. The preparation method is as follows:

[0068] Step 1: Mix sodium carbonate with nickel oxide, iron oxide and manganese oxide in proportion to form a mixture, and ball-mill the mixture to mix evenly to obtain a precursor;

[0069] Step 2: Sinter the precursor at 800 °C for 24 h, then crush and screen it to obtain Product 1;

[0070] Step 3: Mix Product 1 with Li3PO4 and MgHPO4 in proportion, ball-mill them, disperse them in ethanol to make a slurry, and then spray-dry to obtain Product 2;

[0071] Step 4: Sinter Product 2 at 550 °C for 6 h, introduce dry air during the sintering process, then crush and screen it to obtain the cathode material for the sodium-ion battery.

[0072] Prepare a sodium-ion battery according to the method of Example 1.

[0073] Example 3

[0074] This example provides a cathode material for a sodium-ion battery. The main material is NaNi 0.4 Fe 0.2 Mn 0.4 O2, and the coating layer composition includes Li3PO4, K3PO4 and Ca2P2O7 with a molar ratio of 0.5:0.5:0.07. The weight ratio of the main material to the coating layer is 1:0.06. The preparation method is as follows:

[0075] Step 1: Mix sodium carbonate with nickel oxide, iron oxide and manganese oxide in proportion to form a mixture, and ball-mill the mixture to mix evenly to obtain a precursor;

[0076] Step 2: Sinter the precursor at 900 °C for 18 h, then crush and screen it to obtain Product 1;

[0077] Step 3: Mix Product 1 with Li3PO4, K3PO4, and CaHPO4 in proportion, ball-mill them, disperse them in ethanol to make a slurry, and then spray-dry to obtain Product 2;

[0078] Step 4: Sinter Product 2 at 600 °C for 5 h, introduce dry air during the sintering process, then crush and screen it to obtain the positive electrode material for the sodium-ion battery.

[0079] Prepare a sodium-ion battery according to the method of Example 1.

[0080] Example 4

[0081] This example provides a positive electrode material for a sodium-ion battery. The main material is NaNi 0.4 Fe 0.2 Mn 0.4 O2, and the coating layer composition includes Li3PO4, Na3PO4, K3PO4, and Mg2P2O7, with a molar ratio of 0.5:0.25:0.25:0.14. The weight ratio of the main material to the coating layer is 1:0.04. The preparation method is as follows:

[0082] Step 1: Mix sodium carbonate with nickel oxide, iron oxide, and manganese oxide in proportion to form a mixture, ball-mill the mixture to mix it evenly to obtain a precursor;

[0083] Step 2: Sinter the precursor at 1100 °C for 10 h, then crush and screen it to obtain Product 1;

[0084] Step 3: Mix Product 1 with Li3PO4, Na3PO4, K3PO4, and MgHPO4 in proportion, ball-mill them, disperse them in ethanol to make a slurry, and then spray-dry to obtain Product 2;

[0085] Step 4: Sinter Product 2 at 700 °C for 3 h, introduce dry air during the sintering process, then crush and screen it to obtain the positive electrode material for the sodium-ion battery.

[0086] Prepare a sodium-ion battery according to the method of Example 1.

[0087] Example 5

[0088] This example provides a positive electrode material for a sodium-ion battery. The main material is NaNi 0.4 Fe 0.2 Mn 0.4O2, the coating composition contains Li3PO4, Na3PO4, K3PO4, Rb3PO4, Mg2P2O7, with a molar ratio of 0.4:0.2:0.2:0.2:0.2, and the weight ratio of the main material to the coating is 1:0.02. The preparation method is as follows:

[0089] Step 1: Mix sodium carbonate with nickel oxide, iron oxide, and manganese oxide in proportion to form a mixture. Ball-mill the mixture to mix evenly to obtain a precursor;

[0090] Step 2: Sinter the precursor at 1200 °C for 6 h, then crush and screen it to obtain Product 1;

[0091] Step 3: Mix Product 1 with Li3PO4, Na3PO4, K3PO4, Rb3PO4, and MgHPO4 in proportion and ball-mill them. Disperse them in ethanol to make a slurry, and then spray-dry to obtain Product 2;

[0092] Step 4: Sinter Product 2 at 750 °C for 2 h. Pass dry air during the sintering process, then crush and screen it to obtain the positive electrode material for the sodium-ion battery.

[0093] Prepare a sodium-ion battery according to the method of Example 1.

[0094] Example 6

[0095] This example provides a positive electrode material for a sodium-ion battery. The main material is NaNi 0.4 Fe 0.2 Mn 0.4 O2, the coating composition contains Li3PO4, K3PO4, Mg2P2O7, Ca2P2O7, with a molar ratio of 0.5:0.5:0.03:0.03, and the weight ratio of the main material to the coating is 1:0.06. The preparation method is as follows:

[0096] Step 1: Mix sodium carbonate with nickel oxide, iron oxide, and manganese oxide in proportion to form a mixture. Ball-mill the mixture to mix evenly to obtain a precursor;

[0097] Step 2: Sinter the precursor at 900 °C for 18 h, then crush and screen it to obtain Product 1;

[0098] Step 3: Mix Product 1 with Li3PO4, K3PO4, MgHPO4, and CaHPO4 in proportion and ball-mill them. Disperse them in ethanol to make a slurry, and then spray-dry to obtain Product 2;

[0099] Step 4: Sinter Product 2 at 700 °C for 3 h. Pass dry air during the sintering process, then crush and screen it to obtain the positive electrode material for the sodium-ion battery.

[0100] Prepare a sodium-ion battery according to the method of Example 1.

[0101] Comparative Example 1

[0102] This comparative example provides a cathode material for a sodium-ion battery, which is different from that of Example 1 in that uncoated NaNi 0.4 Fe 0.2 Mn 0.4 O2 is used as the cathode material, and other steps are the same as those in Example 1 to prepare a sodium-ion battery.

[0103] Comparative Example 2

[0104] This comparative example provides a cathode material for a sodium-ion battery, which is different from that of Example 1 in that LiFePO4 is used as the coating layer, and the weight ratio of the main material to the coating layer is 1:0.05. The preparation method is as follows:

[0105] Step 1: Mix sodium carbonate with nickel oxide, iron oxide, and manganese oxide in proportion to form a mixture, and ball-mill the mixture to mix evenly to obtain a precursor.

[0106] Step 2: Sinter the precursor at 1000 °C for 15 h, then crush and screen it to obtain Product 1.

[0107] Step 3: Mix Product 1 with LiFePO4 in proportion and ball-mill them, disperse them in ethanol to make a slurry, and then spray-dry to obtain Product 2.

[0108] Step 4: Sinter Product 2 at 650 °C for 4 h, introduce dry air during the sintering process, then crush and screen it to obtain the LiFePO4-coated cathode material for the sodium-ion battery.

[0109] Prepare a sodium-ion battery according to the method of Example 1.

[0110] Comparative Example 3

[0111] This comparative example provides a cathode material for a sodium-ion battery, which is different from that of Example 1 in that Na 2.92 PS 3.96 is used as the coating layer, and the weight ratio of the main material to the coating layer is 1:0.05. The preparation method is as follows:

[0112] Step 1: Mix Na2S and P2S5 in a molar ratio of 73:25 and grind them, then transfer the pre-ground material into a ball mill and ball-mill it at a rotation speed of 400 r / min for 10 h to obtain a mixed material.

[0113] Step 2: Perform low-temperature annealing treatment on the mixed material at 200 °C to obtain Na 2.92 PS 3.96 ;

[0114] Step 3: Mix sodium carbonate with nickel oxide, iron oxide, and manganese oxide in proportion to form a mixture. Ball-mill the mixture to mix it evenly to obtain a precursor. Sinter the precursor at 1000 °C for 15 h, then crush and screen it to obtain Product 1;

[0115] Step 4: Dissolve Na 2.92 PS 3.96 and Product 1 in ethanol and stir to obtain a mixed slurry;

[0116] Step 5: Spray-dry the mixed slurry to obtain mixed powder; and place the mixed powder in a muffle furnace and heat-treat it at 450 °C for 10 h;

[0117] Step 6: Grind the heat-treated powder to obtain Na 2.92 PS 3.96 -coated sodium-ion battery cathode material.

[0118] Prepare a sodium-ion battery according to the method of Example 1.

[0119] Comparative Example 4

[0120] This comparative example provides a sodium-ion battery cathode material, which is different from that of Example 1 in that the coating component is Na3PO4 and does not contain T2P2O7, and the weight ratio of the main material to the coating layer is 1:0.05. The preparation method is as follows:

[0121] Step 1: Mix sodium carbonate with nickel oxide, iron oxide, and manganese oxide in proportion to form a mixture. Ball-mill the mixture to mix it evenly to obtain a precursor;

[0122] Step 2: Sinter the precursor at 1000 °C for 15 h, then crush and screen it to obtain Product 1;

[0123] Step 3: Mix Product 1 with Na3PO4 in proportion, ball-mill them, disperse them in ethanol to make a slurry, and then spray-dry it to obtain Product 2;

[0124] Step 4: Sinter Product 2 at 650 °C for 4 h. Pass dry air during the sintering process, then crush and screen it to obtain the sodium-ion battery cathode material.

[0125] Prepare a sodium-ion battery according to the method of Example 1.

[0126] Comparative Example 5

[0127] This comparative example provides a sodium-ion battery cathode material, which is different from that of Example 1 in that the coating component is Mg2P2O7 and does not contain A3PO4, and the weight ratio of the main material to the coating layer is 1:0.05. The preparation method is as follows:

[0128] Step 1: Mix sodium carbonate with nickel oxide, iron oxide, and manganese oxide in proportion to form a mixture, and ball-mill the mixture to mix evenly to obtain a precursor;

[0129] Step 2: Sinter the precursor at 1000 °C for 15 h, then crush and screen it to obtain Product 1;

[0130] Step 3: Mix Product 1 with MgHPO4 in proportion and ball-mill it, disperse it in ethanol to make a slurry, and then spray-dry it to obtain Product 2;

[0131] Step 4: Sinter Product 2 at 650 °C for 4 h, introduce dry air during the sintering process, then crush and screen it to obtain the positive electrode material for the sodium-ion battery.

[0132] Prepare a sodium-ion battery according to the method of Example 1.

[0133] Table 1 Statistical Table of Components in Examples and Comparative Examples

[0134] Main material component Coating layer component Example 1 <![CDATA[NaNi 0.4 Fe 0.2 Mn 0.4 O2]]> <![CDATA[Li3PO4, Na3PO4, Mg2P2O7]]> Example 2 <![CDATA[NaNi 0.4 Fe 0.2 Mn 0.4 O2]]> <![CDATA[Li3PO4, Mg2P2O7]]> Example 3 <![CDATA[NaNi 0.4 Fe 0.2 Mn 0.4 O2]]> <![CDATA[Li3PO4, K3PO4, Ca2P2O7]]> Example 4 <![CDATA[NaNi 0.4 Fe 0.2 Mn 0.4 O2]]> <![CDATA[Li3PO4, Na3PO4, K3PO4, Mg2P2O7]]> Example 5 <![CDATA[NaNi 0.4 Fe 0.2 Mn 0.4 O2]]> <![CDATA[Li3PO4, Na3PO4, K3PO4, Rb3PO4, Mg2P2O7]]> Example 6 <![CDATA[NaNi 0.4 Fe 0.2 Mn 0.4 O2]]> <![CDATA[Li3PO4, K3PO4, Mg2P2O7, Ca2P2O7]]> Comparative example 1 <![CDATA[NaNi 0.4 Fe 0.2 Mn 0.4 O2]]> None Comparative example 2 <![CDATA[NaNi 0.4 Fe 0.2 Mn 0.4 O2]]> <![CDATA[LiFePO4]]> Comparative example 3 <![CDATA[NaNi 0.4 Fe 0.2 Mn 0.4 O2]]> <![CDATA[Sodium 2.92 PS 3.96 > Comparative example 4 <![CDATA[NaNi 0.4 Fe 0.2 Mn 0.4 O2]]> <![CDATA[Na3PO4]]> Comparative example 5 <![CDATA[NaNi 0.4 Fe 0.2 Mn 0.4 O2]]> <![CDATA[Mg2P2O7]]>

[0135] Cycling performance test

[0136] Charge the sodium-ion soft-pack battery prepared in each example at a constant current of 400 mA to 4 V, let it stand for 10 min, then discharge it at a current of 400 mA to 1.5 V, let it stand for 10 min, and perform a cycling test for 300 cycles. The test data are shown in Table 2 and Figure 2 .

[0137] Table 2 Statistical Table of Cycling Test Data

[0138]

[0139] High-temperature performance test

[0140] Fully charge the battery that has been cycled 300 times above, measure the battery thickness (recorded as H1), place it in a high-temperature oven at 55 °C for 48 h, observe the gas generation situation of the battery, measure the battery thickness (recorded as H2), and then discharge it at a current of 400 mA to 1.5 V. The test data are shown in Table 3 and Figure 3 .

[0141] Battery thickness expansion rate = (H2 - H1) / H1 * 100%

[0142] Table 3 Statistical Table of High-temperature Test Data

[0143]

[0144] From the above data, it can be seen that the capacity retention rates of Examples 1-6 are all above 93.9% after 300 cycles of testing. By extrapolation, the cycle life (cut-off capacity retention rate of 80%) of the examples can reach more than 1000 times, showing good cycle performance. Moreover, after long-term cycling, the high-temperature capacity retention rate of Examples 1-6 at 55 °C can reach about 97%, and the thickness expansion rate is not more than 5%, indicating excellent high-temperature performance. In contrast, the capacity retention rates of Comparative Examples 1-5 are only about 80% after 300 cycles of testing, showing poor cycle performance. Moreover, after long-term cycling, the battery swells severely at high temperature, the high-temperature capacity retention rate is low, and the high-temperature performance is very poor.

[0145] Among them, Comparative Example 1 uses an uncoated NaNi 0.4 Fe 0.2 Mn 0.4 O2 cathode material, which has poor stability and is prone to side reactions with the electrolyte, resulting in the degradation of the active material, gas generation, and an increase in internal resistance, leading to very poor cycle performance. The capacity retention rate is only 74.1% after 300 cycles of testing. Moreover, at high temperature, serious side reactions occur between the electrolyte and the cathode material, resulting in severe gas swelling of the battery and a significant reduction in the high-temperature discharge capacity.

[0146] Comparative Example 2 uses a LiFePO4-coated NaNi 0.4 Fe 0.2 Mn 0.4 O2 cathode material. On the one hand, since LiFePO4 participates in the electrochemical reaction in the 0-4V range, strain will occur during the cycle, deteriorating the interface between the coating layer and the main material. And due to the poor compatibility between LiFePO4 and NaNi 0.4 Fe 0.2 Mn 0.4 O2 materials, after long-term stress accumulation, the coating layer cracks and detaches from the main material, resulting in an increase in the internal resistance of the battery and a rapid decay of the cycle capacity. On the other hand, due to the poor lithium storage reversibility of the hard carbon negative electrode of the sodium-ion battery, Li + in LiFePO4 will undergo side reactions on the surface of the hard carbon during the charging process, blocking the transmission channel of Na + and occupying the active sites for storing Na + , resulting in a rapid decay of the cycle capacity of the battery. The capacity retention rate is only 77.5% after 300 cycles of testing. Moreover, since the coating layer has cracked after long-term cycling, the electrolyte directly contacts the main material, and side reactions occur at high temperature, resulting in severe gas swelling of the battery and a significant reduction in the high-temperature discharge capacity.

[0147] Comparative Example 3 uses a Na 2.92 PS 3.96 -coated NaNi 0.4 Fe 0.2 Mn0.4 The O2 cathode material. Due to the sulfides remaining during the preparation of Na 2.92 PS 3.96 process being soluble in the electrolyte and gradually migrating to the anode, occupying the active sites of the anode and blocking the Na + transport channels, resulting in a relatively rapid decay of the battery's cycle capacity; and Na 2.92 PS 3.96 The coating layer has poor compatibility with the sodium layered oxide material. After long-term cycling, the coating layer will crack and detach, leading to a relatively rapid decay of the battery's cycle capacity. After 300 cycle tests, the capacity retention rate is only 76.1%. Moreover, due to the cracking of the coating layer after long-term cycling, the electrolyte directly contacts the main material, causing side reactions at high temperatures, resulting in serious bloating of the battery and a significant reduction in the high-temperature discharge capacity.

[0148] Comparative Example 4 used a NaNi 0.4 Fe 0.2 Mn 0.4 O2 cathode material. Due to the poor compatibility between the Na3PO4 coating layer and the sodium layered oxide material, after long-term cycling, the coating layer will crack and detach, resulting in a relatively rapid decay of the battery's cycle capacity. After 300 cycle tests, the capacity retention rate is only 79.7%. Moreover, due to the cracking of the coating layer after long-term cycling, the electrolyte directly contacts the main material, causing side reactions at high temperatures, resulting in serious bloating of the battery and a significant reduction in the high-temperature discharge capacity.

[0149] Comparative Example 5 used a NaNi 0.4 Fe 0.2 Mn 0.4 O2 cathode material. Although Mg2P2O7 has good compatibility with the sodium layered oxide material, due to the low conductivity of Mg2P2O7 and the relatively large interfacial impedance of the material, the secondary particles of the material break during cycling, having an adverse impact on the cycling performance of the battery; in addition, due to the reaction between Mg2P2O7 and the sodium layered oxide material, the specific capacity of the cathode material decreases. The capacity of the battery prepared in this comparative example is about 70 mAh lower than that of other groups; due to the combined action of the above factors, the cycling performance of the battery in Comparative Example 5 is better than that of other comparative examples, but there is still a large gap compared with the examples. Due to the good compatibility between Mg2P2O7 and the sodium layered oxide material, although after long-term cycling, the coating layer still has a good protective effect, compared with Comparative Example 1, the gas generation at high temperatures is significantly reduced. However, due to the breakage of the secondary particles of the material during cycling, at high temperatures, the electrode liquid reacts with the material at the cracks, resulting in battery bloating and a reduction in the high-temperature discharge capacity.

[0150] Table 4 Data Analysis Table

[0151]

[0152] It can be found from Table 4 that the cyclic capacity retention rate of Example 1 is increased by 21% compared with that of Comparative Example 1, while the improvement effect of Comparative Example 4 is 5.6%, and the improvement effect of Comparative Example 5 is 11.1%. The sum of the individual effects of the two is 16.7%. It shows that the technical effect (21%) of improving the cyclic performance of the present invention is better than the sum of the individual effects of the two components (16.7%), indicating that there is indeed a synergistic effect in the technical solution of the present invention.

[0153] The high-temperature capacity retention rate of Example 1 is increased by 49.1% compared with that of Comparative Example 1, while the improvement effect of Comparative Example 4 is 17.1%, and the improvement effect of Comparative Example 5 is 22.7%. The sum of the individual effects of the two is 39.8%. It shows that the technical effect (49.1%) of improving the high-temperature performance of the present invention is better than the sum of the individual effects of the two components (39.8%), indicating that there is indeed a synergistic effect in the technical solution of the present invention.

[0154] In summary, after 300 cycles of cyclic testing, the capacity retention rates of Examples 1-6 are all above 93.9%, showing good cyclic performance. Moreover, after long-term cycling, the high-temperature capacity retention rate of the examples can reach about 97% at 55 °C, and the thickness expansion rate is not more than 5%, showing good high-temperature performance. It shows that the present invention can inhibit battery swelling and effectively improve the cyclic performance and high-temperature performance of the battery.

[0155] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for preparing a cathode material for a sodium-ion battery, characterized in that: The positive electrode material of the sodium-ion battery is composed of a main material and a coating layer; The chemical general formula of the main material is NaMO2; Among them, M is one or more elements of Ni, Fe, Ti, Co, Cu, Mn, V, Cr; The composition of the coating layer includes A3PO4 and T2P2O7; Among them, A is one or more alkali metal elements, and T is one or more alkaline earth metal elements; The preparation method includes the following steps: Step 1: Mix sodium carbonate and the M source in proportion to form a mixture, ball-mill the mixture, and mix evenly to obtain a precursor; Step 2: Sinter the precursor at a high temperature, then crush and screen it to obtain Product 1; Step 3: Mix and ball-mill Product 1 with A3PO4 and THPO4, disperse it in ethanol to make a slurry, and then spray-dry it to obtain Product 2; Step 4: Sinter Product 2 at a high temperature, then crush and screen it to obtain the positive electrode material of the sodium-ion battery.

2. The preparation method of the cathode material for sodium ion battery according to claim 1, characterized in that: Among the components of the coating layer, A is one or more elements of Li, Na, K, Rb, Cs.

3. The preparation method of the positive electrode material for a sodium ion battery according to claim 1, characterized in that: Among the components of the coating layer, T is one or two elements of Mg or Ca.

4. The preparation method of the cathode material for a sodium-ion battery according to claim 1, wherein: The weight ratio of the main material to the coating layer is 1:(0.01 - 0.1).

5. The preparation method of the cathode material for a sodium-ion battery according to claim 1, wherein: The molar ratio of A3PO4 to T2P2O7 in the coating layer is 1:(0.01 - 0.3).

6. The preparation method of the positive electrode material of the sodium ion battery according to claim 5, wherein: The M source is an oxide, carbonate or oxalate of M.

7. The preparation method of the positive electrode material of the sodium ion battery according to claim 5, characterized in that: In Step 2, the sintering temperature is 800°C - 1200°C, and the sintering time is 6 - 24 h.

8. The preparation method of the cathode material for a sodium-ion battery according to claim 5, characterized in that: In Step 4, the sintering temperature is 550°C - 750°C, and the sintering time is 2 - 6 h.

9. Application of the positive electrode material obtained by the preparation method of the positive electrode material of the sodium-ion battery according to any one of claims 5 - 8 in a sodium-ion battery.

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