Modified lithium iron phosphate, preparation method thereof, lithium ion battery and method for judging energy efficiency of positive electrode material of lithium ion battery

Abstract

The invention relates to the technical field of lithium ion batteries, in particular to modified lithium iron phosphate, a preparation method of the modified lithium iron phosphate, a lithium ion battery and a method for judging the energy efficiency of a positive electrode material of the lithium ion battery. The surface of the doped lithium iron phosphate inner core is coated with the carbon coating layer; the doped lithium iron phosphate core is fluorine and sodium co-doped lithium iron phosphate. According to the modified lithium iron phosphate provided by the invention, fluorine and sodium are co-doped, so that a lithium ion battery prepared from the modified lithium iron phosphate has relatively high energy efficiency.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium - ion batteries, and in particular, to a modified lithium iron phosphate, a preparation method thereof, a lithium - ion battery, and a method for judging the energy efficiency of a positive electrode material of a lithium - ion battery. Background Art

[0002] The electrochemical energy storage industry has risen rapidly. As the most important component of electrochemical energy storage means, the lithium - ion batteries for energy storage are mainly lithium iron phosphate lithium - ion batteries. Energy efficiency is a very important parameter of energy storage batteries, directly reflecting their excellent performance. Compared with the old version of the energy storage national standard, the new version of the energy storage national standard puts forward higher requirements and standards for the energy efficiency of batteries. Therefore, further improving the energy efficiency of lithium - ion batteries is an urgent problem to be solved at present.

[0003] Lithium iron phosphate materials have the advantages of good cycle stability and high safety, and are widely used in electric vehicles, electronic products, energy storage and other fields, becoming one of the main positive electrode materials of lithium - ion batteries. Due to its relatively low working voltage platform, large polarization during charge - discharge process and large internal resistance, the energy efficiency of lithium iron phosphate materials is relatively low. Therefore, how to improve the energy efficiency of lithium - ion batteries prepared from lithium iron phosphate materials is a technical problem jointly faced by the lithium iron phosphate material industry and the lithium - ion battery cell manufacturing industry at present. Summary of the Invention

[0004] Aiming at the above - mentioned deficiencies in the prior art, the purpose of the present invention is to provide a modified lithium iron phosphate, a preparation method thereof, a lithium - ion battery, and a method for judging the energy efficiency of a positive electrode material of a lithium - ion battery. The modified lithium iron phosphate provided by the present invention has a high energy efficiency for the lithium - ion battery prepared therefrom through co - doping of fluorine and sodium.

[0005] In order to achieve the above purpose, the present invention adopts the following technical solutions:

[0006] In the first aspect, the present invention provides a modified lithium iron phosphate, which comprises:

[0007] A doped lithium iron phosphate core; and

[0008] A carbon coating layer coated on the surface of the doped lithium iron phosphate core;

[0009] The doped lithium iron phosphate core is lithium iron phosphate co - doped with fluorine and sodium.

[0010] The modified lithium iron phosphate provided by the present invention has a high energy efficiency for the lithium - ion battery prepared therefrom through co - doping of fluorine and sodium.

[0011] Further, the raw materials for preparing the doped lithium iron phosphate core include a phosphorus source, an iron source, a lithium source, a fluorine source, and a sodium source. The molar ratio of fluorine element in the fluorine source to iron element in the iron source is 1:(100 - 600), and the molar ratio of sodium element in the sodium source to iron element in the iron source is 1:(150 - 700);

[0012] And / or, the iron source includes iron phosphate and iron oxide.

[0013] Further, the molar ratio of the iron phosphate to the iron oxide is (8 - 9):(2.0 - 1.0);

[0014] And / or, the lithium source includes at least one of lithium carbonate and lithium hydroxide;

[0015] And / or, the phosphorus source includes at least one of iron phosphate and ammonium dihydrogen phosphate;

[0016] And / or, the fluorine source includes at least one of ammonium fluoride, 2 - fluoroethanol, and lithium hexafluorophosphate;

[0017] And / or, the sodium source includes at least one of sodium carbonate and sodium gluconate.

[0018] In a second aspect, the present invention provides a method for preparing the modified lithium iron phosphate as described in the first aspect. The preparation method includes the following steps:

[0019] Mix the lithium source, iron source, phosphorus source, fluorine source, sodium source, and carbon source in a solvent, and then dry and calcine to obtain the modified lithium iron phosphate.

[0020] Further, the carbon source includes at least one of glucose, sucrose, starch, polystyrene, polyethylene glycol, polyurethane, graphene, and asphalt;

[0021] And / or, the dosage of the carbon source is 3wt% - 60.0wt% of the mass of iron element in the iron source;

[0022] And / or, the solvent includes water.

[0023] Further, drying is carried out under the protection of a protective gas, and the protective gas includes at least one of nitrogen, argon, and helium. The drying temperature is 60 - 100°C, and the drying time is 10 - 24h;

[0024] And / or, the calcination atmosphere includes one of nitrogen, argon, and helium. The calcination conditions are: heating to 700 - 800°C at a heating rate of 1 - 4°C / min and holding for 10 - 15 hours;

[0025] And / or, the iron source, phosphorus source, and lithium source are mixed at a molar ratio of Fe:P:Li = (0.90 - 1.00):(0.95 - 1.05):(1.00 - 1.10);

[0026] And / or, after the calcination, the preparation method further includes pulverizing and sieving.

[0027] In a third aspect, the present invention provides a lithium-ion battery, including a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte. The active material of the positive electrode sheet includes the modified lithium iron phosphate described in the first aspect or the modified lithium iron phosphate prepared by the preparation method of the modified lithium iron phosphate described in the second aspect.

[0028] In a fourth aspect, the present invention provides a method for judging the energy efficiency of a positive electrode material for a lithium-ion battery. The judging method includes the following steps:

[0029] S1. Preparation of the positive electrode sheet: A standard positive electrode sheet and a test positive electrode sheet are respectively prepared based on a standard positive electrode material and a test positive electrode material, where the difference between the standard positive electrode sheet and the test positive electrode sheet is only the positive electrode material;

[0030] S2. Assembly of the coin cell half-cell: The standard positive electrode sheet and the test positive electrode sheet in step S1 are respectively assembled with a lithium sheet and a separator to form a coin cell half-cell, which are respectively denoted as the standard coin cell half-cell and the test coin cell half-cell;

[0031] S3. Coin cell test: The standard coin cell half-cell and the test coin cell half-cell are respectively tested as follows:

[0032] a. Activation;

[0033] b. 0.5C charging: Let it stand, and then charge at a constant current of 0.5C to a known upper charging voltage, and charge at a constant voltage with the known upper charging voltage until the cut-off rate is the known charging cut-off current;

[0034] S4. Calculate the ratio of the charging gram capacity of the standard positive electrode material and the test positive electrode material when charging at a constant current of 0.5C to the known upper charging voltage to the total of the charging gram capacity in the 0.5C constant current section and the charging gram capacity in the constant voltage section, which are respectively denoted as standard P and test P;

[0035] S5. Analysis and judgment:

[0036] a. If test P ≥ standard P, it is considered that the energy efficiency of the full cell made of the test positive electrode material is better than or equal to that of the standard positive electrode material;

[0037] b. If test P < standard P, it is considered that the energy efficiency of the full cell made of the test positive electrode material is worse than that of the standard positive electrode material.

[0038] Further, the preparation of the standard positive electrode sheet and the positive electrode sheet to be tested respectively based on the standard positive electrode material and the positive electrode material to be tested includes:

[0039] Mix the standard positive electrode material, conductive agent and binder in proportion and stir evenly, then apply it on both sides of the aluminum foil by double-sided coating, and obtain the standard positive electrode sheet after drying and rolling; mix the positive electrode material to be tested, conductive agent and binder in proportion and stir evenly, then apply it on both sides of the aluminum foil by double-sided coating, and obtain the positive electrode sheet to be tested after drying and rolling;

[0040] And / or, in step S3, the test temperature is 25 °C;

[0041] And / or, the activation includes:

[0042] 0.1C charging: Stand still in the cabinet, then charge at a constant current of 0.1C to the known upper charging limit voltage, and charge at a constant voltage with the known upper charging limit voltage, and the cut-off rate is the known charging cut-off current;

[0043] 0.1C discharging: Stand still, then discharge at a constant current of 0.1C to the known lower discharging voltage;

[0044] And / or, in step S3, after step b, it further includes: c. 0.5C discharging: Stand still, then discharge at a constant current of 0.5C to the known lower discharging voltage;

[0045] And / or, in step b, the standing time is 5 - 15 min, and in step c, the standing time is 5 - 15 min.

[0046] Further, when the positive electrode material of the lithium-ion battery is lithium iron phosphate, the known upper charging limit voltage is 3.75V, the known charging cut-off current is 0.05C, and the known lower discharging voltage is 2.0V;

[0047] And / or, the positive electrode material includes the modified lithium iron phosphate described in the first aspect or the modified lithium iron phosphate prepared by the preparation method of the modified lithium iron phosphate described in the second aspect.

[0048] Compared with the prior art, the beneficial effects of the present invention at least include one of the following:

[0049] (1) The modified lithium iron phosphate provided by the present invention is co-doped with fluorine and sodium, so that the lithium-ion battery prepared therefrom has a higher energy efficiency.

[0050] (2) The method for judging the energy efficiency of the positive electrode material of the lithium-ion battery provided by the present invention can predict the change trend of the energy efficiency data of the battery cell through specific characterization data, reduce the characterization time of the energy efficiency of the modified positive electrode material, and thus shorten the R & D cycle for improving the energy efficiency of the positive electrode material. Description of the Drawings

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

[0052] Figure 1 It is a linear fitting graph of the charging specific capacity ratio in the constant current section and the 0.5P energy efficiency of the lithium iron phosphate materials provided in Examples 1-6 and Comparative Examples 1-6 of the present invention. Specific Embodiments

[0053] To make the objectives, technical solutions, and advantages of the present invention clearer, the following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the embodiments of the present invention. Those skilled in the art should understand that the described embodiments are only for helping to understand the present invention and should not be regarded as specific limitations on the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the scope of protection of the present invention. The process parameters without specific conditions noted in the following embodiments are usually in accordance with conventional conditions.

[0054] In the ranges disclosed in the present invention, the endpoints and any values are not limited to the exact ranges or values. These ranges or values should be understood to include values close to these ranges or values. For numerical ranges, between the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values, they can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed in the present invention.

[0055] In a first aspect, the present invention provides a modified lithium iron phosphate, which includes:

[0056] A doped lithium iron phosphate core; and

[0057] A carbon coating layer coated on the surface of the doped lithium iron phosphate core;

[0058] The doped lithium iron phosphate core is lithium iron phosphate co-doped with fluorine and sodium.

[0059] The modified lithium iron phosphate provided by the present invention has a high energy efficiency for the lithium ion battery prepared therefrom through co-doping with fluorine and sodium.

[0060] In the above-mentioned modified lithium iron phosphate, as an alternative embodiment, the raw materials for preparing the doped lithium iron phosphate core include a phosphorus source, an iron source, a lithium source, a fluorine source, and a sodium source. The molar ratio of fluorine element in the fluorine source to iron element in the iron source is 1:(100 - 600) (for example, it can be 1:100, 1:200, 1:300, 1:450, or 1:600), and the molar ratio of sodium element in the sodium source to iron element in the iron source is 1:(150 - 700) (for example, it can be 1:150, 1:300, 1:450, or 1:700). If the fluorine source or sodium source is too little, the improvement effect of energy efficiency is not obvious; if the fluorine source or sodium source is too much, the main element of lithium iron phosphate: the lithium content decreases, affecting the specific capacity. In addition, too much doping may generate impurity phases.

[0061] In the above-mentioned modified lithium iron phosphate, as an alternative embodiment, the iron source includes iron phosphate and iron oxide. By limiting the iron source to include iron phosphate and iron oxide, the energy efficiency of the lithium-ion battery can be improved in the present invention.

[0062] In the above-mentioned modified lithium iron phosphate, as an alternative embodiment, the molar ratio of the iron phosphate to the iron oxide is (8 - 9):(2.0 - 1.0). If the proportion of iron oxide is too small, the improvement effect will not be obvious; if the content of iron oxide is too much, the degree of particle adhesion will increase, which has an adverse effect on the energy efficiency of lithium iron phosphate.

[0063] In the above-mentioned modified lithium iron phosphate, as an alternative embodiment, the lithium source includes at least one of lithium carbonate and lithium hydroxide.

[0064] In the above-mentioned modified lithium iron phosphate, as an alternative embodiment, the phosphorus source includes at least one of iron phosphate and ammonium dihydrogen phosphate.

[0065] In the above-mentioned modified lithium iron phosphate, as an alternative embodiment, the fluorine source includes at least one of ammonium fluoride, 2-fluoroethanol, and lithium hexafluorophosphate.

[0066] In the above-mentioned modified lithium iron phosphate, as an alternative embodiment, the sodium source includes at least one of sodium carbonate and sodium gluconate.

[0067] In the second aspect, the present invention provides a preparation method of the modified lithium iron phosphate as described in the first aspect. The preparation method includes the following steps:

[0068] Mix the lithium source, iron source, phosphorus source, fluorine source, sodium source, and carbon source in a solvent, and then obtain the modified lithium iron phosphate through drying and calcination.

[0069] In the above-mentioned preparation method of the modified lithium iron phosphate, as an alternative embodiment, the lithium source includes at least one of lithium carbonate and lithium hydroxide.

[0070] In the above preparation method of modified lithium iron phosphate, as an alternative embodiment, the iron source includes iron phosphate and iron oxide. By limiting the iron source to include iron phosphate and iron oxide, the energy efficiency of the lithium-ion battery can be improved in the present invention.

[0071] In the above preparation method of modified lithium iron phosphate, as an alternative embodiment, the molar ratio of the iron phosphate to the iron oxide is (8 - 9):(2.0 - 1.0).

[0072] In the above preparation method of modified lithium iron phosphate, as an alternative embodiment, the phosphorus source includes at least one of iron phosphate and ammonium dihydrogen phosphate.

[0073] In the above preparation method of modified lithium iron phosphate, as an alternative embodiment, the fluorine source includes at least one of ammonium fluoride, 2-fluoroethanol, and lithium hexafluorophosphate.

[0074] In the above preparation method of modified lithium iron phosphate, as an alternative embodiment, the sodium source includes at least one of sodium carbonate and sodium gluconate.

[0075] In the above preparation method of modified lithium iron phosphate, as an alternative embodiment, the carbon source includes at least one of glucose, sucrose, starch, polystyrene, polyethylene glycol, polyurethane, graphene, and asphalt.

[0076] In the above preparation method of modified lithium iron phosphate, as an alternative embodiment, the molar ratio of fluorine element in the fluorine source to iron element in the iron source is 1:(100 - 600) (for example, it can be 1:100, 1:200, 1:300, 1:450, or 1:600), the molar ratio of sodium element in the sodium source to iron element in the iron source is 1:(150 - 700) (for example, it can be 1:150, 1:300, 1:450, or 1:700), and the dosage of the carbon source is 3wt% - 60.0wt% of the mass of iron element in the iron source (for example, it can be 3wt%, 10wt%, 20wt%, 30wt%, 40wt%, 50wt%, or 60wt%).

[0077] In the above preparation method of modified lithium iron phosphate, as an alternative embodiment, the solvent includes water.

[0078] In the above method for preparing the modified lithium iron phosphate, as an alternative embodiment, drying is carried out under the protection of an inert gas, and the inert gas includes at least one of nitrogen, argon, and helium. The drying temperature is 60-100 °C (for example, it can be 60 °C, 65 °C, 70 °C, 75 °C, 80 °C, 85 °C, 90 °C, or 100 °C), and the drying time is 10-24 h (for example, it can be 10 h, 12 h, 14 h, 16 h, 18 h, 20 h, 22 h, or 24 h).

[0079] In the above method for preparing the modified lithium iron phosphate, as an alternative embodiment, the calcination atmosphere includes one of nitrogen, argon, and helium. The calcination conditions are as follows: heating up to 700-800 °C (for example, it can be 700 °C, 720 °C, 740 °C, 760 °C, 780 °C, or 800 °C) at a heating rate of 1-4 °C / min (for example, it can be 1 °C / min, 2 °C / min, 3 °C / min, or 4 °C / min) and holding for 10-15 hours (for example, it can be 10 hours, 11 hours, 12 hours, or 15 hours).

[0080] In the above method for preparing the modified lithium iron phosphate, as an alternative embodiment, the iron source, phosphorus source, and lithium source are mixed in a molar ratio of Fe:P:Li = (0.90-1.00):(0.95-1.05):(1.00-1.10).

[0081] In the above method for preparing the modified lithium iron phosphate, as an alternative embodiment, after the calcination, the preparation method further includes pulverizing and sieving.

[0082] In a third aspect, the present invention provides a lithium-ion battery, including a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte. The active material of the positive electrode sheet includes the modified lithium iron phosphate described in the first aspect or the modified lithium iron phosphate prepared by the method for preparing the modified lithium iron phosphate described in the second aspect.

[0083] In a fourth aspect, the present invention provides a method for judging the energy efficiency of a lithium-ion battery positive electrode material. The judging method includes the following steps:

[0084] S1. Prepare positive electrode sheets: respectively prepare a standard positive electrode sheet and a test positive electrode sheet based on a standard positive electrode material and a test positive electrode material, where the difference between the standard positive electrode sheet and the test positive electrode sheet is only the positive electrode material;

[0085] S2. Assemble coin half-cells: respectively assemble the standard positive electrode sheet and the test positive electrode sheet in step S1 with lithium sheets and separators to form coin half-cells, which are respectively denoted as a standard coin half-cell and a test coin half-cell;

[0086] S3. Coin cell test: Test the standard coin cell half - cell and the coin cell half - cell to be tested respectively, as follows:

[0087] a. Activation;

[0088] b. 0.5C charging: Let it stand still, then charge at a constant current of 0.5C until the well - known upper charging voltage, and charge at a constant voltage with the well - known upper charging voltage, and the cut - off rate is the well - known cut - off charging current;

[0089] S4. Calculate the ratio of the charging gram capacity of the standard cathode material and the cathode material to be tested when charging at a constant current of 0.5C to the well - known upper charging voltage to the sum of the charging gram capacity in the 0.5C constant - current charging section and the charging gram capacity in the constant - voltage charging section (the total gram capacity of 0.5C charging in step S3), and record them as standard P and tested P respectively;

[0090] S5. Analysis and judgment:

[0091] a. If tested P ≥ standard P, it is considered that the energy efficiency of the full cell made of the cathode material to be tested is better than or equal to that of the standard cathode material;

[0092] b. If tested P < standard P, it is considered that the energy efficiency of the full cell made of the cathode material to be tested is worse than that of the standard cathode material.

[0093] Compared with the traditional judgment method, the cycle for making a full cell is long (about 12 days) and the cost is high (including material cost, process cost, etc.). The method for judging the energy efficiency of the lithium - ion battery cathode material provided by the present invention makes the cathode material into a coin cell half - cell, with a short test cycle (about 3 days). Through specific characterization data, the change trend of the energy efficiency data of the battery cell can be predicted, reducing the characterization time of the energy efficiency of the modified cathode material, thus shortening the R & D cycle for improving the energy efficiency of the cathode material and greatly reducing the test cost.

[0094] For the improvement of the energy efficiency of lithium iron phosphate materials in lithium - ion batteries, it often needs to be prepared into a battery product for characterization. The normal preparation cycle of a finished lithium - ion battery is long. The normal process from slurry mixing, coating, rolling, slicing, stacking, baking, liquid injection to formation and grading takes about 10 days. Plus the test time, it takes about 12 days from lithium iron phosphate powder to the output of the energy efficiency result. The present invention uses a new coin - cell process and characterization method, which can feedback the large - degree improvement of energy efficiency on the coin - cell electrical performance, shortening the feedback time from about 12 days to about 3 days.

[0095] In the above - mentioned method for judging the energy efficiency of the lithium - ion battery cathode material, as an optional implementation manner, the preparation of the standard cathode sheet and the cathode sheet to be tested based on the standard cathode material and the cathode material to be tested respectively includes:

[0096] Mix the standard cathode material, conductive agent, and binder in proportion and stir evenly. Then coat both sides on the aluminum foil, and obtain the standard cathode sheet after drying and rolling. Mix the cathode material to be tested, conductive agent, and binder in proportion and stir evenly. Then coat both sides on the aluminum foil, and obtain the cathode sheet to be tested after drying and rolling.

[0097] In the method for judging the energy efficiency of the lithium-ion battery cathode material, as an alternative embodiment, the conductive agent includes at least one of carbon nanotubes, graphene, and conductive carbon black, and the binder includes polyvinylidene fluoride.

[0098] In the method for judging the energy efficiency of the lithium-ion battery cathode material, as an alternative embodiment, in step S3, the test temperature is 25 °C.

[0099] In the method for judging the energy efficiency of the lithium-ion battery cathode material, as an alternative embodiment, the activation includes:

[0100] 0.1C charge: Place it on the shelf and let it stand. Then charge at a constant current of 0.1C until the well-known charging upper limit voltage, and charge at a constant voltage with the well-known charging upper limit voltage, and the cut-off rate is the well-known charging cut-off current.

[0101] 0.1C discharge: Let it stand, and then discharge at a constant current of 0.1C until the well-known discharge lower limit voltage.

[0102] After the coin cell is activated at a rate of 0.1C, the accuracy of the 0.5C test can be guaranteed, thereby improving the accuracy of predicting the energy efficiency result of the battery cell.

[0103] In the method for judging the energy efficiency of the lithium-ion battery cathode material, as an alternative embodiment, the cathode material includes the modified lithium iron phosphate described in the first aspect or the modified lithium iron phosphate prepared by the preparation method of the modified lithium iron phosphate described in the second aspect.

[0104] In the method for judging the energy efficiency of the lithium-ion battery cathode material, as an alternative embodiment, in step S3, after step b, it further includes: c. 0.5C discharge: Let it stand, and then discharge at a constant current of 0.5C until the well-known discharge lower limit voltage. After the battery cell completes the test, discharging the battery cell to a low power level can improve the storage safety of the battery cell.

[0105] In the method for judging the energy efficiency of the lithium-ion battery cathode material, as an alternative embodiment, in step b, the standing time is 5 - 15 min, and in step c, the standing time is 5 - 15 min.

[0106] In the method for judging the energy efficiency of the above lithium-ion battery cathode material, as an alternative implementation, the known charging upper limit voltage, the known charging cut-off current, and the known discharging lower limit voltage are related to the lithium-ion battery cathode material. When the lithium-ion battery cathode material is lithium iron phosphate, the known charging upper limit voltage is 3.75 V, the known charging cut-off current is 0.05 C, and the known discharging lower limit voltage is 2.0 V.

[0107] The present invention will be further described in detail below with specific examples and comparative examples.

[0108] In the following examples and comparative examples:

[0109] The raw materials of the lithium iron phosphate materials in Examples 1-6 and Comparative Examples 1-6 are shown in Table 1.

[0110] Table 1

[0111]

[0112]

[0113] Example 1

[0114] This example provides a modified lithium iron phosphate, which includes: a doped lithium iron phosphate core; and a carbon coating layer coated on the surface of the doped lithium iron phosphate core; the doped lithium iron phosphate core is lithium iron phosphate co-doped with fluorine and sodium.

[0115] The preparation method of the modified lithium iron phosphate provided in this example includes the following steps: Lithium source, iron source, phosphorus source, fluorine source, sodium source, and carbon source are put into a sand mill at one time, an appropriate amount of water is added as a grinding medium, and after grinding for 5 hours, the slurry is taken out. The slurry is dried in a drying oven at 80 °C for 12 hours under nitrogen protection. The dried powder is sieved through a 200-mesh sieve to remove large particle impurities. The powder is transferred to a tube furnace, and under nitrogen protection, the heating rate is 2 °C / min, heated to 700 °C, held for 14 hours, and cooled with a normal-temperature nitrogen gas flow. After the powder is cooled to room temperature, physical pulverization is carried out. The pulverized powder is sieved through a 200-mesh sieve to remove large particle impurities to obtain the modified lithium iron phosphate, where the lithium source is lithium carbonate, the iron source and the phosphorus source are iron phosphate, the fluorine source is lithium hexafluorophosphate, the sodium source is sodium carbonate, and the carbon source is glucose; the molar ratio of iron element in iron phosphate to lithium element in lithium carbonate is 1.00:1.10, the molar ratio of F element in lithium hexafluorophosphate to Fe element in iron phosphate is 1:600, the molar ratio of Na element in sodium carbonate to Fe element in iron phosphate is 1:700, and the addition amount of glucose is 10.0 wt% of the addition amount of iron phosphate.

[0116] Example 2

[0117] This embodiment provides a modified lithium iron phosphate, which includes: a doped lithium iron phosphate core; and a carbon coating layer coated on the surface of the doped lithium iron phosphate core; the doped lithium iron phosphate core is lithium iron phosphate co-doped with fluorine and sodium.

[0118] The preparation method of the modified lithium iron phosphate provided in this embodiment is basically the same as that in Example 1, except that the molar ratio of the F element in lithium hexafluorophosphate to the Fe element in iron phosphate is 1:450, and the molar ratio of the Na element in sodium carbonate to the Fe element in iron phosphate is 1:450.

[0119] Example 3

[0120] This embodiment provides a modified lithium iron phosphate, which includes: a doped lithium iron phosphate core; and a carbon coating layer coated on the surface of the doped lithium iron phosphate core; the doped lithium iron phosphate core is lithium iron phosphate co-doped with fluorine and sodium.

[0121] The preparation method of the modified lithium iron phosphate provided in this embodiment is basically the same as that in Example 1, except that the molar ratio of the F element in lithium hexafluorophosphate to the Fe element in iron phosphate is 1:200, and the molar ratio of the Na element in sodium carbonate to the Fe element in iron phosphate is 1:150.

[0122] Example 4

[0123] This embodiment provides a modified lithium iron phosphate, which includes: a lithium iron phosphate core; and a carbon coating layer coated on the surface of the lithium iron phosphate core.

[0124] The preparation method of modified lithium iron phosphate provided in this embodiment comprises the following steps: putting a lithium source, an iron source, a phosphorus source and a carbon source into a sand mill at one time, adding an appropriate amount of water as a grinding medium, and taking out the slurry after grinding for 5 hours. The slurry is dried at 80°C for 12 hours in a drying oven under nitrogen protection. The dried powder is sieved with a 200-mesh sieve to remove large particles of impurities. The powder is transferred to a tubular furnace, and under nitrogen protection, the heating rate is 2°C / min, the temperature is raised to 700°C, kept warm for 14 hours, and cooled with a nitrogen flow at room temperature. After the powder cools to room temperature, it is physically crushed. The crushed powder is sieved with a 200-mesh sieve to remove large particle impurities to obtain the modified lithium iron phosphate, wherein the lithium source is lithium carbonate, the iron source is iron phosphate and iron oxide, the phosphorus source is iron phosphate and diammonium phosphate, and the carbon source is glucose; the molar ratio of iron phosphate to iron oxide is 9:1, the molar ratio of iron element in the iron source, phosphorus element in the phosphorus source and lithium element in the lithium source is 1.00:1:1.10, and the amount of glucose added is 27.0wt% of the mass of iron element in the iron source.

[0125] Example 5

[0126] This embodiment provides a modified lithium iron phosphate, which includes: a lithium iron phosphate core; and a carbon coating layer coated on the surface of the lithium iron phosphate core.

[0127] The preparation method of the modified lithium iron phosphate provided in this embodiment is basically the same as that in Example 4, except that the molar ratio of iron phosphate to iron oxide is 8:2.

[0128] Example 6

[0129] This embodiment provides a modified lithium iron phosphate, which includes: a doped lithium iron phosphate core; and a carbon coating layer coated on the surface of the doped lithium iron phosphate core; the doped lithium iron phosphate core is lithium iron phosphate co-doped with fluorine and sodium.

[0130] The preparation method of the modified lithium iron phosphate provided in this embodiment includes the following steps: Put the lithium source, iron source, phosphorus source, fluorine source, sodium source and carbon source into a sand mill at one time, add an appropriate amount of water as the grinding medium, grind for 5 hours, and then take out the slurry. The slurry is dried in a drying oven under nitrogen protection at 80 °C for 12 hours. The dried powder is sieved through a 200-mesh sieve to remove large particle impurities. The powder is transferred to a tubular furnace, and under nitrogen protection, the heating rate is 2 °C / min, heated to 700 °C, kept warm for 14 hours, and cooled with a normal temperature nitrogen gas flow. After the powder is cooled to room temperature, physical pulverization is carried out. The pulverized powder is sieved through a 200-mesh sieve to remove large particle impurities to obtain the modified lithium iron phosphate, where the lithium source is lithium carbonate, the iron source is iron phosphate and iron oxide, the phosphorus source is iron phosphate and ammonium dihydrogen phosphate, the fluorine source is lithium hexafluorophosphate, the sodium source is sodium carbonate, and the carbon source is glucose; the molar ratio of iron phosphate to iron oxide is 8:2, the molar ratio of iron in the iron source, phosphorus in the phosphorus source to lithium in the lithium source is 1.00:1:1.10, the molar ratio of F element in lithium hexafluorophosphate to Fe element in the iron source is 1:450, the molar ratio of Na element in sodium carbonate to Fe element in the iron source is 1:450, and the addition amount of glucose is 27.0 wt% of the mass of iron element in the iron source.

[0131] Comparative Example 1

[0132] This comparative example provides a lithium iron phosphate, which includes: a lithium iron phosphate core; and a carbon coating layer coated on the surface of the lithium iron phosphate core.

[0133] The preparation method of lithium iron phosphate provided in this comparative example includes the following steps: Lithium source, iron source, phosphorus source and carbon source are put into a sand mill at one time, an appropriate amount of water is added as a grinding medium, and after grinding for 5 hours, the slurry is taken out. The slurry is dried in an oven under nitrogen protection at 80 °C for 12 hours. The dried powder is sieved through a 200-mesh sieve to remove large particle impurities. The powder is transferred to a tubular furnace, and under nitrogen protection, the heating rate is 2 °C / min, heated to 700 °C, held for 14 hours, and cooled with a normal-temperature nitrogen gas flow. After the powder is cooled to normal temperature, physical pulverization is carried out. The pulverized powder is sieved through a 200-mesh sieve to remove large particle impurities to obtain lithium iron phosphate, wherein the lithium source is lithium carbonate, the iron source and the phosphorus source are iron phosphate, and the carbon source is glucose; the molar ratio of iron element in iron phosphate to lithium element in lithium carbonate is 1.00:1.10, and the addition amount of glucose is 10.0 wt% of the addition amount of iron phosphate.

[0134] Comparative Example 2

[0135] The modified lithium iron phosphate provided in this comparative example is basically the same as that in Example 2, except that the doped lithium iron phosphate core is sodium-doped lithium iron phosphate.

[0136] The preparation method of the modified lithium iron phosphate provided in this comparative example is basically the same as that in Example 2, except that lithium hexafluorophosphate is not added.

[0137] Comparative Example 3

[0138] The modified lithium iron phosphate provided in this comparative example is basically the same as that in Example 2, except that the doped lithium iron phosphate core is fluorine-doped lithium iron phosphate.

[0139] The preparation method of the modified lithium iron phosphate provided in this comparative example is basically the same as that in Example 2, except that sodium carbonate is not added.

[0140] Comparative Example 4

[0141] The modified lithium iron phosphate provided in this comparative example is basically the same as that in Example 2, except that the doped lithium iron phosphate core is aluminum-doped lithium iron phosphate.

[0142] The preparation method of the modified lithium iron phosphate provided in this comparative example is basically the same as that in Example 2, except that sodium carbonate and lithium hexafluorophosphate are not added, and alumina is added, and the molar ratio of aluminum element in alumina to Fe element in iron phosphate is 1:450.

[0143] Comparative Example 5

[0144] The modified lithium iron phosphate provided in this comparative example is basically the same as that in Example 2, except that the doped lithium iron phosphate core is lithium iron phosphate co-doped with fluorine and aluminum.

[0145] The preparation method of the modified lithium iron phosphate provided in this comparative example is basically the same as that in Example 2, except that sodium carbonate is not added, alumina is added, and the molar ratio of aluminum element in alumina to Fe element in iron phosphate is 1:450.

[0146] Comparative Example 6

[0147] This comparative example provides a modified lithium iron phosphate, which includes: a doped lithium iron phosphate core; and a carbon coating layer coated on the surface of the doped lithium iron phosphate core; the doped lithium iron phosphate core is sodium-doped lithium iron phosphate, and the carbon coating layer contains F element.

[0148] The preparation method of the modified lithium iron phosphate provided in this comparative example includes the following steps: Put the lithium source, iron source, phosphorus source, sodium source and carbon source into a sand mill at one time, add an appropriate amount of water as the grinding medium, and after grinding for 5 hours, take out the slurry. The slurry is dried in a drying oven at 80 °C for 12 hours under nitrogen protection. The dried powder is sieved through a 200-mesh sieve to remove large particle impurities. The powder is transferred to a tubular furnace, and under nitrogen protection, the heating rate is 2 °C / min, heated to 700 °C, held for 14 hours, and cooled with a normal temperature nitrogen gas flow. After the powder is cooled to room temperature, physical pulverization is carried out. The pulverized powder is sieved through a 200-mesh sieve to remove large particle impurities to obtain a sintered material. The sintered material is fully mixed with a carbon source accounting for 4 wt% of the iron phosphate addition, placed near the air outlet, and a fluorine source is placed in another quartz boat so that the fluorine source is close to the air inlet. Subsequently, it is heated to 700 °C at a rate of 2 °C / min in a nitrogen atmosphere and held for 10 h, the nitrogen gas flow rate is controlled at 0.3 L / min, and after cooling, mechanical pulverization is carried out to obtain the modified lithium iron phosphate, where the lithium source is lithium carbonate, the iron source and the phosphorus source are iron phosphate, the sodium source is sodium carbonate, the fluorine source is lithium hexafluorophosphate, and the carbon source is glucose; the molar ratio of iron element in the iron source, phosphorus element in the phosphorus source to lithium element in the lithium source is 1.00:1:1.10, the molar ratio of F element in lithium hexafluorophosphate to Fe element in iron phosphate is 1:450, the molar ratio of Na element in sodium carbonate to Fe element in iron phosphate is 1:450, and the addition amount of glucose is 10.0 wt% of the iron phosphate addition amount.

[0149] Performance test

[0150] For Comparative Examples 1-6 and Examples 1-6 of the present invention, lithium-ion soft-pack batteries (3.2 Ah) were used for energy efficiency testing. The cathode material was selected from lithium iron phosphate in Comparative Examples 1-6 and Examples 1-6, and the anode material was artificial graphite. The brief process is as follows: Lithium iron phosphate, conductive carbon black, and binder PVDF were mixed evenly in a mass ratio of 90:5:5, N-methylpyrrolidone was added, and further mixed evenly. Then, coating was carried out, and after drying, rolling and slitting were performed to obtain the positive electrode sheet; artificial graphite, conductive carbon black, CMC, and SBR were mixed evenly in a mass ratio of 96:1:1.5:1.5, deionized water was added, and mixed evenly. Then, coating was carried out, and after drying, rolling and slitting were performed to obtain the negative electrode sheet; then, the positive and negative electrode sheets were stacked, and the electrolyte was injected respectively to make lithium-ion soft-pack batteries; after processes such as formation, aging, and grading, the production was completed.

[0151] The energy efficiency test method was as follows: Constant current charging was carried out at 0.5C to 3.65V, constant voltage charging was carried out at 3.65V, and the cut-off rate was 0.05C; after standing for 10 min, constant current discharging was carried out at 0.5C to 2.5V, and then the discharging energy and charging energy were calculated. Energy efficiency = discharging energy / charging energy * 100%. The test results are shown in Table 2.

[0152] Table 2

[0153]

[0154]

[0155] It can be seen from Table 2 at least the following points:

[0156] (1) By comparing Example 1 and 2 with Comparative Example 1, it can be known that the modified lithium iron phosphate provided by the present invention has a higher energy efficiency for the lithium-ion battery prepared therefrom through co-doping with fluorine and sodium.

[0157] (2) By comparing Example 2 and 6 with Comparative Example 1, it can be known that the present invention can further improve the energy efficiency of the lithium-ion battery by restricting the iron source to include iron phosphate and iron oxide.

[0158] (3) By comparing Example 2 with Comparative Examples 2 and 3, it can be known that compared with single doping, co-doping with fluorine and sodium can play a synergistic effect in improving the energy efficiency of lithium iron phosphate batteries, which is significantly better than single doping; by comparing Comparative Examples 3-5, it can be known that single doping with fluorine or aluminum can improve the energy efficiency of lithium iron phosphate batteries, but when co-doping with fluorine and aluminum, no synergistic effect can be achieved in terms of the energy efficiency of lithium iron phosphate batteries, and it is even worse than single doping.

[0159] Example 7

[0160] This embodiment provides a method for judging the energy efficiency of lithium iron phosphate, and the judging method includes the following steps:

[0161] S1. Prepare the positive electrode sheet: According to the mass ratio of lithium iron phosphate: conductive carbon black: polyvinylidene fluoride = 90:5:5, respectively mix and stir evenly the lithium iron phosphate materials provided in Examples 1-6 and Comparative Examples 1-6 with conductive carbon black and polyvinylidene fluoride, and apply them on both sides of the aluminum foil. After drying and rolling, 12 positive electrode sheets are obtained. Among them, the thickness of the aluminum foil is 15 μm, the thickness of the electrode sheet is 50 μm, the diameter of the electrode sheet is 14 mm, and the compaction density is 2.3 g / cm 3 .

[0162] S2. Assemble the coin-type half-cell: Assemble the 12 positive electrode sheets in step S1 with lithium sheets (negative electrode, diameter 16 mm, thickness 0.6 mm) and diaphragms (diameter 19 mm) respectively to form coin-type half-cells, and the electrolyte dosage is 60 μl to obtain 12 coin-type half-cells;

[0163] S3. Coin-type test: At 25 °C, test the 12 coin-type half-cells obtained in step S2 respectively, specifically as follows:

[0164] a. Activation: Stand still in the cabinet for 3 hours, charge at a constant current of 0.1C to 3.75V, charge at a constant voltage of 3.75V, and the cut-off rate is 0.05C; stand still for 10 min, and discharge at a constant current of 0.1C to 2.0V; the discharge capacity per gram at 0.1C is shown in Table 3;

[0165] b. 0.5C charge: Stand still for 10 min, and then charge at a constant current of 0.5C to 3.75V, charge at a constant voltage of 3.75V, and the cut-off rate is 0.05C;

[0166] c. 0.5C discharge: Stand still for 10 min, and then discharge at a constant current of 0.5C to 2.0V;

[0167] S4. Calculate the proportion of the charge capacity per gram of charging at a constant current of 0.5C to 3.75V in the total charge capacity of the constant current section and the constant voltage section (the total charge capacity per gram of charging at 0.5C in step S3) for the 12 coin-type half-cells obtained in step S2 respectively, and the results are shown in Table 4;

[0168] S5. Analysis and judgment:

[0169] Through the test method of the present invention, predict the improvement of energy efficiency. As can be seen from Table 4, the proportion of the charge capacity per gram in the constant current section of the lithium iron phosphate material provided in Example 6 is the highest, and the proportion of the charge capacity per gram in the constant current section of the lithium iron phosphate material provided in Comparative Example 1 is the lowest. Therefore, the energy efficiency of the full cell made of the lithium iron phosphate material provided in Example 6 is the best, and the energy efficiency of the full cell made of the lithium iron phosphate material provided in Comparative Example 1 is the worst.

[0170] Table 3

[0171]

[0172]

[0173] Table 4

[0174]

[0175] Verification method:

[0176] By comparing the proportion of the charging capacity per gram in the constant current section of Comparative Examples 1-6 and Examples 1-6 in Table 4 with the 0.5P energy efficiency of Comparative Examples 1-6 and Examples 1-6 in Table 2, it can be seen that the predicted result of the change trend of the energy efficiency of lithium iron phosphate obtained by using the method for judging the energy efficiency of lithium iron phosphate provided in Example 7 is basically consistent with the change trend of the energy efficiency after making a full cell, indicating that the method for judging the energy efficiency of the positive electrode material of the lithium ion battery provided by the present invention is feasible.

[0177] Taking the data of the proportion of the charging capacity per gram in the constant current section of the coin cell half cells made of the lithium iron phosphate materials provided in Comparative Examples 1-6 and Examples 1-6 in Table 4 as the abscissa, and taking the data of the 0.5P energy efficiency of the full cells made of the lithium iron phosphate materials provided in Comparative Examples 1-6 and Examples 1-6 in Table 2 as the ordinate, a linear fitting is performed, and the result is as Figure 1 shown. The square of R is greater than 0.90, and there is a strong correlation between the proportion of the charging capacity per gram in the 0.5C constant current section and the energy efficiency of the battery cell, that is, the change trend of the energy efficiency of the battery cell can be predicted in advance by the proportion of the charging capacity per gram in the 0.5C constant current section.

[0178] Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A modified lithium iron phosphate, characterized in that, The modified lithium iron phosphate includes: A doped lithium iron phosphate core; and A carbon coating layer coated on the surface of the doped lithium iron phosphate core; The doped lithium iron phosphate core is lithium iron phosphate co-doped with fluorine and sodium.

2. The modified lithium iron phosphate according to claim 1, wherein The raw materials for preparing the doped lithium iron phosphate core include a phosphorus source, an iron source, a lithium source, a fluorine source, and a sodium source. The molar ratio of fluorine element in the fluorine source to iron element in the iron source is 1:(100 - 600), and the molar ratio of sodium element in the sodium source to iron element in the iron source is 1:(150 - 700); And / or, the iron source includes iron phosphate and iron oxide.

3. The modified lithium iron phosphate according to claim 2, wherein The molar ratio of the iron phosphate to the iron oxide is (8 - 9):(2.0 - 1.0); And / or, the lithium source includes at least one of lithium carbonate and lithium hydroxide; And / or, the phosphorus source includes at least one of iron phosphate and ammonium dihydrogen phosphate; And / or, the fluorine source includes at least one of ammonium fluoride, 2-fluoroethanol, and lithium hexafluorophosphate; And / or, the sodium source includes at least one of sodium carbonate and sodium gluconate.

4. A method for preparing modified lithium iron phosphate as described in any one of claims 1-3, characterized in that, The preparation method includes the following steps: Mix the lithium source, iron source, phosphorus source, fluorine source, sodium source, and carbon source in a solvent, and then dry and calcine to obtain the modified lithium iron phosphate.

5. The preparation method of the modified lithium iron phosphate according to claim 4, wherein The carbon source includes at least one of glucose, sucrose, starch, polystyrene, polyethylene glycol, polyurethane, graphene, and asphalt; And / or, the dosage of the carbon source is 3wt% - 60.0wt% of the mass of iron element in the iron source; And / or, the solvent includes water.

6. The preparation method of the modified lithium iron phosphate according to claim 4, characterized in that Dry under the protection of a protective gas, the protective gas includes at least one of nitrogen, argon, and helium. The drying temperature is 60 - 100°C, and the drying time is 10 - 24h; And / or, the calcination atmosphere includes one of nitrogen, argon, and helium. The calcination conditions are: heating to 700 - 800°C at a heating rate of 1 - 4°C / min and holding for 10 - 15 hours; And / or, the iron source, phosphorus source, and lithium source are mixed in a molar ratio of Fe:P:Li = (0.90 - 1.00):(0.95 - 1.05):(1.00 - 1.10); And / or, after the calcination, the preparation method further includes pulverizing and sieving.

7. A lithium-ion battery, comprising a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, characterized in that, The active material of the positive electrode sheet includes the modified lithium iron phosphate according to any one of claims 1 - 3 or the modified lithium iron phosphate prepared by the preparation method of the modified lithium iron phosphate according to any one of claims 4 - 6.

8. A method for judging the energy efficiency of a cathode material of a lithium-ion battery, characterized in that, The judgment method includes the following steps: S1. Prepare positive electrode sheets: Prepare a standard positive electrode sheet and a test positive electrode sheet based on a standard positive electrode material and a test positive electrode material respectively, where the difference between the standard positive electrode sheet and the test positive electrode sheet is only the positive electrode material; S2. Assemble coin half-cells: Assemble the standard positive electrode sheet and the test positive electrode sheet in step S1 with lithium sheets and diaphragms respectively to form coin half-cells, which are respectively denoted as a standard coin half-cell and a test coin half-cell; S3. Coin cell test: Test the standard coin half-cell and the test coin half-cell respectively, specifically as follows: a. Activation; b. 0.5C charging: Stand still, then charge at a constant current of 0.5C until the well-known charging upper limit voltage, and charge at a constant voltage with the well-known charging upper limit voltage, with the cut-off rate being the well-known charging cut-off current; S4. Calculate the proportions of the charging gram capacity of the standard cathode material and the test cathode material when charged at a constant current of 0.5C until the well-known charging upper limit voltage, respectively accounting for the total of the charging gram capacity in the 0.5C constant current section and the charging gram capacity in the constant voltage section, and record them as standard P and test P respectively; S5. Analysis and judgment: a. If test P ≥ standard P, it is considered that the energy efficiency of the full cell made of the test cathode material is better than or equal to that of the standard cathode material; b. If test P < standard P, it is considered that the energy efficiency of the full cell made of the test cathode material is worse than that of the standard cathode material.

9. The method for judging the energy efficiency of the positive electrode material of a lithium-ion battery according to claim 8, characterized in that, The preparation of the standard cathode sheet and the test cathode sheet respectively based on the standard cathode material and the test cathode material includes: Mix the standard cathode material, conductive agent and binder in proportion and stir evenly, then apply it on both sides of the aluminum foil by coating, and obtain the standard cathode sheet after drying and rolling; mix the test cathode material, conductive agent and binder in proportion and stir evenly, then apply it on both sides of the aluminum foil by coating, and obtain the test cathode sheet after drying and rolling; And / or, in step S3, the test temperature is 25°C; And / or, the activation includes: 0.1C charging: Place on the shelf and stand still, then charge at a constant current of 0.1C until the well-known charging upper limit voltage, and charge at a constant voltage with the well-known charging upper limit voltage, with the cut-off rate being the well-known charging cut-off current; 0.1C discharging: Stand still, then discharge at a constant current of 0.1C until the well-known discharging lower limit voltage; And / or, in step S3, after step b, it further includes: c. 0.5C discharging: Stand still, then discharge at a constant current of 0.5C until the well-known discharging lower limit voltage; And / or, in step b, the standing time is 5 - 15 min, and in step c, the standing time is 5 - 15 min.

10. The method for judging the energy efficiency of the positive electrode material of a lithium-ion battery according to claim 8 or 9, characterized in that, When the positive electrode material of the lithium-ion battery is lithium iron phosphate, the well-known charging upper limit voltage is 3.75V, the well-known charging cut-off current is 0.05C, and the well-known discharging lower limit voltage is 2.0V; And / or, the positive electrode material includes the modified lithium iron phosphate described in any one of claims 1 - 3 or the modified lithium iron phosphate prepared by the preparation method of the modified lithium iron phosphate described in any one of claims 4 - 6.

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