Lithium iron phosphate positive electrode material and preparation method thereof

Abstract

The application relates to the technical field of lithium ion batteries, in particular to a lithium iron phosphate positive electrode material and a preparation method thereof. The preparation method of the lithium iron phosphate positive electrode material provided by the application comprises the following steps: S1, a hydrothermal reaction is carried out on an iron source, an additive and an alkaline precipitator to obtain a precursor material A; S2, after the precursor material A is annealed, a precursor material B is obtained; and S3, after a mixture of the precursor material B, a lithium source, a phosphorus source and a carbon source is sequentially subjected to grinding, drying and sintering, the lithium iron phosphate positive electrode material is obtained. The preparation method can be used to prepare a lithium iron phosphate precursor material with a micro-mesoporous structure and good sphericity, and the precursor material is used to prepare a lithium iron phosphate positive electrode material with good sphericity and high compactness, so that the electrochemical performances such as the capacity and the energy density of the lithium iron phosphate positive electrode material are effectively improved.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and in particular to a lithium iron phosphate cathode material and its preparation method. Background Technology

[0002] Lithium-ion batteries are widely used due to their advantages such as high operating voltage, long cycle life, wide operating temperature range, and environmental friendliness. Currently, commercially available lithium-ion battery cathode materials include lithium cobalt oxide, lithium nickel manganese cobalt oxide ternary materials, and lithium iron phosphate. With the rapid development of the new energy vehicle market and energy storage materials, lithium iron phosphate has quickly captured a large market share due to its good safety performance and long cycle life. However, because lithium iron phosphate itself has a low density, coating its surface with carbon materials to improve conductivity results in a lower compaction density of the lithium iron phosphate cathode material, thus reducing the battery's volumetric energy density and limiting its application in power and energy storage batteries.

[0003] Currently, improving the compaction density of lithium iron phosphate (LFP) cathode materials mainly focuses on the material morphology, size, and particle size distribution. Better sphericity and larger primary particle size result in higher compaction density. Increasing the primary particle size of LFP cathode materials is primarily achieved by raising the holding temperature or extending the holding time during the sintering stage. However, larger primary particles make it difficult for the electrolyte to penetrate the inner layer, leading to a longer lithium-ion migration path and severely impacting the material's electrochemical performance. When materials are packed with equal-sized particles, numerous pores exist between them, reducing the compaction density. Therefore, suitable small particles with appropriate particle size distribution are needed to fill these gaps. During LFP preparation, the precursor material acts as a template, gradually transforming into LFP material. Therefore, the morphology and size of the precursor material directly affect the morphology and size of the LFP.

[0004] In view of this, the present invention is hereby proposed. Summary of the Invention

[0005] The primary objective of this invention is to provide a method for preparing lithium iron phosphate cathode material, which solves, in whole or in part, the problems of poor sphericity of primary particles, low compaction density, and poor electrolyte wettability in existing lithium iron phosphate cathode materials.

[0006] The second objective of this invention is to provide a lithium iron phosphate cathode material with good sphericity, high compaction density, good electrolyte wettability, and high capacity.

[0007] In order to achieve the above-mentioned objectives of the present invention, the following technical solution is adopted:

[0008] This invention provides a method for preparing lithium iron phosphate cathode material, comprising the following steps:

[0009] S1, iron source, additives and alkaline precipitant undergo hydrothermal reaction to obtain precursor material A;

[0010] S2. After the precursor material A is annealed, precursor material B is obtained.

[0011] S3. The mixture of the precursor material B, lithium source, phosphorus source and carbon source is ground, dried and sintered in sequence to obtain the lithium iron phosphate cathode material.

[0012] Furthermore, step S1 includes at least one of the following features (1) to (3);

[0013] (1) The additives include at least one of polyethylene glycol, polyvinylpyrrolidone and methylcellulose;

[0014] (2) The molecular weight of the additive is 4000 to 30000;

[0015] (3) The molar ratio of the iron source to the additive is 500:(0.2 to 1.1).

[0016] Furthermore, step S1 includes at least one of the following features (1) to (3);

[0017] (1) The alkaline precipitant includes ammonia and / or urea;

[0018] (2) The iron source includes at least one of ferric chloride, ferric nitrate and ferric sulfate;

[0019] (3) The molar ratio of the iron source and the alkaline precipitant is 1:(3-7).

[0020] Furthermore, in step S1, the temperature of the hydrothermal reaction is 140–180°C, and the time of the hydrothermal reaction is 3–8 hours.

[0021] Furthermore, in step S2, the annealing temperature is 400–600°C, and the annealing time is 4–6 hours.

[0022] Furthermore, in step S3, the grinding includes wet grinding.

[0023] Preferably, the grinding includes grinding the mixture to a particle size of 0.3 to 0.8 μm.

[0024] Furthermore, in step S3, the drying includes spray drying.

[0025] Preferably, the spray drying temperature is 160–280°C.

[0026] Further, in step S3, the sintering includes: holding at 680–820°C for 8–11 hours in a sintering atmosphere.

[0027] Preferably, the sintering atmosphere includes at least one of nitrogen, argon, and carbon dioxide.

[0028] Furthermore, step S3 includes at least one of the following features (1) to (5);

[0029] (1) The molar ratio of the lithium source, the iron source and the phosphorus source is (1~1.1):(0.95~0.99):1;

[0030] (2) The mass ratio of the carbon source to the iron source is (0.1~0.2):1;

[0031] (3) The lithium source includes at least one of lithium carbonate, lithium hydroxide, lithium phosphate and lithium dihydrogen phosphate;

[0032] (4) The phosphorus source includes at least one of lithium dihydrogen phosphate, lithium phosphate and ammonium dihydrogen phosphate;

[0033] (5) The carbon source includes at least one of glucose, sucrose and starch.

[0034] The present invention also provides a lithium iron phosphate cathode material, which is prepared by the method described above for preparing lithium iron phosphate cathode materials.

[0035] Preferably, the lithium iron phosphate cathode material includes lithium iron phosphate and a carbon layer coated on the surface of the lithium iron phosphate, wherein the lithium iron phosphate has a microporous structure.

[0036] Preferably, the compaction density of the lithium iron phosphate cathode material is ≥2.55 g / cm³. 3 .

[0037] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0038] 1. The method for preparing lithium iron phosphate cathode material of the present invention involves obtaining a precursor material with good sphericity and a microporous structure through a one-step hydrothermal method; then, using the precursor material with a lithium source, a phosphorus source, and a carbon source, a lithium iron phosphate cathode material with good sphericity, an internal microporous structure, and a carbon layer on the surface is prepared; wherein, the microporous structure enables the electrolyte to fully wet, reduces the lithium ion insertion / extraction distance, and improves the lithium ion transport rate; the carbon layer can effectively improve the electron transport rate.

[0039] 2. This invention controls the particle size and particle size distribution of the precursor material by controlling the molecular weight of the additive, thereby controlling the particle size and particle size distribution of the lithium iron phosphate cathode material, thus improving the compaction density of the lithium iron phosphate cathode material.

[0040] 3. The lithium iron phosphate cathode material prepared by the preparation method of the present invention has the characteristics of good sphericity, high compaction density, good electrolyte wettability, high capacity, and excellent energy density. Attached Figure Description

[0041] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0042] Figure 1 This is a SEM image of the lithium iron phosphate cathode material of Example 1 of the present invention with a size of 1 μm.

[0043] Figure 2 This is a SEM image of the lithium iron phosphate cathode material of Example 1 of the present invention with a size of 100 nm.

[0044] Figure 3 This is a particle size distribution diagram of the primary particles of the lithium iron phosphate cathode material in Example 1 of the present invention.

[0045] Figure 4 The figures show the charge-discharge test results of the lithium iron phosphate cathode material at different rates in Example 1 of this invention. Detailed Implementation

[0046] The technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings and specific embodiments. However, those skilled in the art will understand that the embodiments described below are some embodiments of the present invention, but not all embodiments, and are only used to illustrate the present invention, and should not be regarded as limiting the scope of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall be followed. Where the manufacturers of reagents or instruments are not specified, they are all conventional products that can be purchased commercially.

[0047] The following is a detailed description of a lithium iron phosphate cathode material and its preparation method according to an embodiment of the present invention.

[0048] In some embodiments of the present invention, a method for preparing a lithium iron phosphate cathode material is provided, comprising the following steps:

[0049] S1, iron source, additives and alkaline precipitant undergo hydrothermal reaction to obtain precursor material A;

[0050] S2. After annealing the precursor material A, precursor material B is obtained.

[0051] S3. The mixture of the above-mentioned precursor material B, lithium source, phosphorus source and carbon source is ground, dried and sintered in sequence to obtain lithium iron phosphate cathode material.

[0052] In the preparation method of the lithium iron phosphate cathode material of the present invention, a precursor material with good sphericity and a microporous structure is obtained by a one-step hydrothermal method; then, the precursor material is used with a lithium source, a phosphorus source, and a carbon source to obtain a lithium iron phosphate cathode material with good sphericity, an internal microporous structure, and a carbon layer on the surface. The internal microporous structure of the lithium iron phosphate cathode material enables sufficient wetting of the electrolyte, reduces the lithium ion insertion / extraction distance, and improves the lithium ion transport rate; the carbon layer on the surface of the lithium iron phosphate cathode material can effectively improve the electron transport rate.

[0053] In the preparation method of lithium iron phosphate cathode material of the present invention, additives are used to control the particle size and particle size distribution of the precursor material, thereby controlling the particle size and particle size distribution of the lithium iron phosphate cathode material, thereby achieving the purpose of improving the compaction density of the lithium iron phosphate cathode material.

[0054] The preparation method of lithium iron phosphate cathode material of the present invention effectively solves the problems of poor sphericity of primary particles, low compaction density and poor electrolyte wettability of lithium iron phosphate cathode material, thereby improving the electrochemical performance of lithium iron phosphate cathode material such as capacity and energy density.

[0055] In some embodiments of the present invention, in step S1, the additive includes at least one of polyethylene glycol, polyvinylpyrrolidone, and methylcellulose.

[0056] In some embodiments of the present invention, in step S1, the molecular weight of the additive is 4,000 to 30,000; preferably, the molecular weight of polyethylene glycol is 4,000 to 10,000, the molecular weight of polyvinylpyrrolidone is 10,000 to 30,000, and the molecular weight of methylcellulose is 10,000 to 20,000.

[0057] This invention controls the growth degree of precursor materials by controlling the molecular weight of additives, thereby controlling their particle size and particle size distribution, and thus controlling the particle size and particle size distribution of lithium iron phosphate cathode materials.

[0058] When the molecular weight of the additive is small, the primary particle size of the prepared material is small, resulting in low space utilization of the finished lithium iron phosphate material and low compaction density. When the molecular weight of the additive is large, the primary particles may grow abnormally, with a large number of micron-sized particles. The presence of large particles can effectively improve the compaction density of the material, but micron-sized particles will cause the lithium ion insertion and extraction path to be prolonged, thereby deteriorating the electrochemical performance of the material.

[0059] In some embodiments of the present invention, in step S1, the molar ratio of the iron source to the additive is 500:(0.2 to 1.1); typically, but not limitingly, for example, the molar ratio of the iron source to the additive is a range of 500:0.2, 500:0.5, 500:1, or any combination thereof.

[0060] In some embodiments of the present invention, in step S1, the alkaline precipitant includes ammonia and / or urea.

[0061] In some embodiments of the present invention, in step S1, the iron source includes at least one of ferric chloride, ferric nitrate, and ferric sulfate.

[0062] In some embodiments of the present invention, in step S1, the molar ratio of the iron source to the alkaline precipitant is 1:(3 to 7); typically, but not limitingly, for example, the molar ratio of the iron source to the alkaline precipitant can be a range of 1:3, 1:5, 1:7 or any combination thereof.

[0063] In some embodiments of the present invention, in step S1, the alkaline precipitant is added in the form of an alkaline precipitant solution; preferably, the concentration of the ammonia solution is 25%, and the concentration of the urea solution is 2-5 mol / L.

[0064] In some embodiments of the present invention, in step S1, the ratio of the iron source to the alkaline precipitant solution is 1 mol: (1 to 20) L; typically, but not limitingly, for example, the ratio of the iron source to the alkaline precipitant is 1 mol: 2 L, 1 mol: 5 L, 1 mol: 20 L, or any combination thereof.

[0065] In some embodiments of the present invention, in step S1, the temperature of the hydrothermal reaction is 140–180°C, and the time of the hydrothermal reaction is 3–8 h; typically, but not limitingly, for example, the temperature of the hydrothermal reaction can be a range of 140°C, 150°C, 160°C, 170°C, 180°C, or any combination thereof; the time of the hydrothermal reaction can be a range of 3 h, 4 h, 5 h, 6 h, 7 h, 8 h, or any combination thereof.

[0066] In some embodiments of the present invention, in step S2, the annealing temperature is 400–600°C, and the annealing time is 4–6 hours; typically, but not limitingly, for example, the annealing temperature can be a range of 400°C, 450°C, 500°C, 550°C, 600°C, or any combination thereof; the annealing time can be a range of 4 hours, 4.5 hours, 5 hours, 5.5 hours, 6 hours, or any combination thereof.

[0067] In the preparation method of lithium iron phosphate cathode material of the present invention, an iron source is subjected to hydrothermal reaction with additives and appropriate pH to form a precursor A with suitable particle size, and after annealing treatment, a precursor B with good sphericity is obtained.

[0068] In some embodiments of the present invention, step S3 includes wet grinding.

[0069] In some embodiments of the present invention, in step S3, grinding includes grinding until the particle size of the mixture is 0.3 to 0.8 μm.

[0070] In some embodiments of the present invention, in step S3, drying includes spray drying; preferably, the temperature of spray drying is 160-280°C.

[0071] In some embodiments of the present invention, step S3 includes sintering at 680–820°C for 8–11 hours in a sintering atmosphere; typically, but not limitingly, for example, the sintering temperature can be a range of 680°C, 700°C, 720°C, 740°C, 760°C, 780°C, 800°C, 820°C or any combination thereof; the sintering time can be 8 hours, 9 hours, 10 hours, 11 hours or any combination thereof.

[0072] In some embodiments of the present invention, in step S3, the sintering atmosphere includes at least one of nitrogen, argon and carbon dioxide.

[0073] In some embodiments of the present invention, in step S3, the molar ratio of lithium source, iron source and phosphorus source is (1-1.1):(0.95-0.99):1.

[0074] In some embodiments of the present invention, in step S3, the mass ratio of carbon source to iron source is (0.1 to 0.2):1; typically, but not limitingly, for example, the mass ratio of carbon source to iron source can be 0.1:1, 0.12:1, 0.14:1, 0.16:1, 0.18:1, 0.2:1 or any combination thereof.

[0075] In some embodiments of the present invention, in step S3, the lithium source includes at least one of lithium carbonate, lithium hydroxide, lithium phosphate, and lithium dihydrogen phosphate.

[0076] In some embodiments of the present invention, in step S3, the phosphorus source includes at least one of lithium dihydrogen phosphate, lithium phosphate, and ammonium dihydrogen phosphate.

[0077] In some embodiments of the present invention, in step S3, the carbon source includes at least one of glucose, sucrose, and starch.

[0078] This invention achieves the preparation of lithium iron phosphate precursor materials with microporous structure and good sphericity by controlling the process steps, and realizes the preparation of lithium iron phosphate cathode materials with controllable morphology, high compaction density and good sphericity, effectively improving the electrochemical performance of lithium iron phosphate cathode materials such as capacity and energy density.

[0079] In some embodiments of the present invention, a lithium iron phosphate cathode material is also provided, which is prepared by the above-described method for preparing lithium iron phosphate cathode materials.

[0080] In some embodiments of the present invention, the lithium iron phosphate cathode material includes lithium iron phosphate and a carbon layer coated on the surface of the lithium iron phosphate, and the lithium iron phosphate has a microporous structure.

[0081] In some embodiments of the present invention, the size of the micropores is 0.3 to 10 nm.

[0082] In some embodiments of the present invention, the compaction density of the lithium iron phosphate cathode material is ≥2.55 g / cm³. 3 The preferred value is 2.55–2.6 g / cm³. 3 .

[0083] The average particle size of the primary particles in the lithium iron phosphate cathode material of the present invention is 360-380 nm, with small particles mainly concentrated around 290-310 nm. When the particle size of the primary particles is greater than 400 nm, their number gradually decreases, and there are a small number of particles with a size of about 1 μm, which can effectively improve the compaction density of the material.

[0084] Example 1

[0085] The method for preparing lithium iron phosphate cathode material provided in this embodiment includes the following steps:

[0086] S1. Add 10g of ferric chloride hexahydrate, 0.1480g of polyethylene glycol (molecular weight 4000), and 74mL of urea solution with a concentration of 3mol / L to a reaction vessel and react at 160℃ for 5h to obtain precursor material A.

[0087] S2. After washing and drying, precursor material A is annealed at 500℃ for 5 hours to obtain precursor material B.

[0088] S3. Water was added to a mixture of precursor material B, 2.9289 g lithium carbonate, 4.3423 g ammonium dihydrogen phosphate and 0.7258 g glucose, and the mixture was wet-milled until the particle size of the mixture was 0.3-0.8 μm. Then, it was spray-dried at 210 °C. After spray drying, it was sintered at 750 °C for 10 h under a nitrogen atmosphere to obtain lithium iron phosphate cathode material.

[0089] Example 2

[0090] The preparation method of the lithium iron phosphate cathode material provided in this embodiment is the same as that in Example 1, except that polyethylene glycol (molecular weight 4000) is replaced with methylcellulose (molecular weight 15000).

[0091] Example 3

[0092] The preparation method of the lithium iron phosphate cathode material provided in this embodiment is the same as that in Example 5, except that the amount of methylcellulose added is 0.0148g.

[0093] Experimental Example 1

[0094] The lithium iron phosphate cathode material prepared in Example 1 was subjected to scanning electron microscopy (SEM) testing, and the results are as follows: Figure 1 and Figure 2 As shown. Among them, Figure 1 The size is 1μm. Figure 2 The size is 100nm.

[0095] from Figure 1 and Figure 2 It can be seen that the lithium iron phosphate cathode material of the present invention has good sphericity.

[0096] The primary particles of the lithium iron phosphate cathode material prepared in Example 1 were measured, and the results are as follows: Figure 3 As shown.

[0097] The average particle size of primary particles in lithium iron phosphate cathode material is about 370 nm, with small particles mainly concentrated around 300 nm. When the particle size of primary particles is greater than 400 nm, their number gradually decreases, and there are a small number of particles with a size of about 1 μm, which can effectively improve the compaction density of the material.

[0098] The performance of the lithium iron phosphate cathode materials prepared in Examples 1-3 was tested, and the results are recorded in Table 1. Among them, the charge-discharge rates of the lithium iron phosphate cathode material in Example 1 are as follows: Figure 4 As shown.

[0099] Electrochemical performance testing: Positive electrode active material, conductive carbon black, carbon nanotubes, and polyvinylidene fluoride (PVDF) were mixed evenly in a mass ratio of 85:5:5:5. An appropriate amount of N-methylpyrrolidone was added to obtain a slurry. The slurry was coated onto aluminum foil, dried, rolled, and cut to obtain the positive electrode sheet. A button cell was assembled using lithium metal as the negative electrode. The assembled battery was tested at room temperature (25°C) using a Blue Electric testing system with a voltage range of 2.1–3.75V. After two cycles at 0.1C, electrochemical performance tests were conducted at rates of 0.2C / 0.2C, 0.2C / 0.5C, 0.2C / 1C, and 0.2C / 2C.

[0100] Compaction density test: The compaction density was measured using a compaction density meter under a pressure of 3 tons.

[0101] Table 1

[0102] D50(μm) D90(μm) <![CDATA[Compaction density (g / cm 3 )]]> 1C discharge specific capacity (mAh / g) Example 1 1.395 3.283 2.58 152 Example 2 1.28 3.748 2.57 151.2 Example 3 1.87 5.72 2.61 132.5

[0103] As can be seen from Table 1, a comparison between Examples 1 and 2 shows that when appropriate additives are used and the compaction density of the synthesized material is within a suitable range, the prepared lithium iron phosphate cathode material exhibits excellent electrochemical performance. A comparison between Examples 2 and 3 shows that when the additive level is too low, the compaction density exceeds the control range, resulting in poor electrochemical performance. This may be because when the additive level is low, the primary particle size of the precursor is larger, causing the lithium-ion transport path to be prolonged, thereby deteriorating the electrochemical performance.

[0104] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a lithium iron phosphate cathode material, characterized in that, Includes the following steps: S1, iron source, additives and alkaline precipitant undergo hydrothermal reaction to obtain precursor material A; S2. After the precursor material A is annealed, precursor material B is obtained. S3. The mixture of the precursor material B, lithium source, phosphorus source and carbon source is ground, dried and sintered in sequence to obtain the lithium iron phosphate cathode material. The additives include at least one of polyethylene glycol, polyvinylpyrrolidone, and methylcellulose, and the alkaline precipitant includes ammonia and / or urea. The molar ratio of the iron source to the additive is 500:(0.2~1.1), and the molar ratio of the iron source to the alkaline precipitant is 1:(3~7).

2. The method for preparing the lithium iron phosphate cathode material according to claim 1, characterized in that, In step S1, the molecular weight of the additive is 4000~30000.

3. The method for preparing the lithium iron phosphate cathode material according to claim 1, characterized in that, In step S1, the iron source includes at least one of ferric chloride, ferric nitrate, and ferric sulfate.

4. The method for preparing the lithium iron phosphate cathode material according to claim 1, characterized in that, In step S1, the temperature of the hydrothermal reaction is 140~180℃, and the time of the hydrothermal reaction is 3~8h.

5. The method for preparing lithium iron phosphate cathode material according to claim 1, characterized in that, In step S2, the annealing temperature is 400~600℃ and the annealing time is 4~6h.

6. The method for preparing the lithium iron phosphate cathode material according to claim 1, characterized in that, In step S3, the grinding includes wet grinding; The grinding includes grinding the mixture until the particle size is 0.3~0.8μm.

7. The method for preparing lithium iron phosphate cathode material according to claim 1, characterized in that, In step S3, the drying includes spray drying; The spray drying temperature is 160~280℃.

8. The method for preparing lithium iron phosphate cathode material according to claim 1, characterized in that, In step S3, the sintering includes: holding at 680~820℃ for 8~11 hours in a sintering atmosphere.

9. The method for preparing the lithium iron phosphate cathode material according to claim 8, characterized in that, The sintering atmosphere includes at least one of nitrogen, argon, and carbon dioxide.

10. The method for preparing the lithium iron phosphate cathode material according to claim 1, characterized in that, Step S3 includes at least one of the following features (1) to (5); (1) The molar ratio of the lithium source, the iron source and the phosphorus source is (1~1.1):(0.95~0.99):1; (2) The mass ratio of the carbon source to the iron source is (0.1~0.2):1; (3) The lithium source includes at least one of lithium carbonate, lithium hydroxide, lithium phosphate and lithium dihydrogen phosphate; (4) The phosphorus source includes at least one of lithium dihydrogen phosphate, lithium phosphate, and ammonium dihydrogen phosphate; (5) The carbon source includes at least one of glucose, sucrose and starch.

11. A lithium iron phosphate cathode material, characterized in that, The lithium iron phosphate cathode material was prepared using the preparation method according to any one of claims 1 to 10.

12. The lithium iron phosphate cathode material according to claim 11, characterized in that, The lithium iron phosphate cathode material includes lithium iron phosphate and a carbon layer coated on the surface of the lithium iron phosphate, and the lithium iron phosphate has a microporous structure.

13. The lithium iron phosphate cathode material according to claim 11, characterized in that, The compaction density of the lithium iron phosphate cathode material is ≥2.55 g / cm³. 3 .

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