Preparation method and application of lithium nitride

Abstract

The application discloses a preparation method and application of lithium nitride. The preparation method comprises the following steps: after surface treatment, metal lithium is placed in a nitrogen atmosphere to generate incomplete lithium nitride through a nitriding reaction; and the incomplete lithium nitride is crushed in an inert gas atmosphere and a plasma atmosphere to obtain lithium nitride. The lithium nitride obtained through the method is applied to lithium ion batteries as a lithium supplement additive. The lithium nitride obtained through the preparation method has small particle size, uniform distribution, high activity and high purity, and has high decomposition rate and low decomposition potential as the lithium supplement additive.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology, specifically relating to a method for preparing lithium nitride and its application. Background Technology

[0002] Lithium-ion batteries, with their numerous advantages such as high energy density, low self-discharge, good cycle performance, and no memory effect, are now widely used in various portable electronic devices, electric vehicles, and large-scale energy storage devices, gradually becoming the mainstream rechargeable battery. With the booming development of the consumer market, the demand for lithium-ion batteries with higher energy density and lower cost is becoming increasingly urgent.

[0003] During the first charge and discharge cycle of a lithium-ion battery, the organic solvent in the electrolyte undergoes a reduction reaction at the negative electrode, forming a solid electrolyte intercalation (SEI) film. This process partially consumes the reversibly intercalated and deintercalated active lithium ions in the battery, resulting in a loss of initial discharge capacity and a reduction in battery cycle life. This is particularly pronounced for negative electrode materials with high specific capacity, such as silicon and tin, where the consumption of positive electrode lithium source during the initial cycle is even more severe. To mitigate this issue, existing methods mainly fall into two categories: positive electrode lithium replenishment and negative electrode lithium replenishment, both aimed at pre-replenishing the active lithium ions in the battery system. Negative electrode lithium replenishment often involves the use of metallic lithium, which, in addition to its high production cost, also imposes stringent environmental requirements, limiting its application in production. Positive electrode lithium replenishment involves adding a small amount of positive electrode lithium replenishment additive during the positive electrode homogenization process. During the first charge cycle, this additive releases active lithium ions to compensate for the lithium consumed in forming the SEI film. A perfect cathode lithium replenishment material usually needs to meet three conditions: First, the irreversible delithiation potential of the lithium replenishment additive should be lower than the upper limit of the cathode material, and the lithium insertion potential should be lower than the lower limit of the cathode material; Second, the irreversible capacity should be greater than 350 mAh / g; Third, it should have good environmental stability and be compatible with the homogenization process and battery system.

[0004] Among numerous cathode lithium supplements, lithium nitride (LN) possesses an extremely high theoretical specific capacity (2309 mAh / g) and a low theoretical decomposition potential (~0.44 V vs. Li+ / Li), with decomposition products being lithium ions and nitrogen gas, attracting considerable attention and research. However, due to its high chemical reactivity and the fact that its raw material is metallic lithium, LN readily reacts with air and water during synthesis to produce lithium carbonate and lithium hydroxide, resulting in high residual alkali content and posing significant challenges in practical production. Achieving a lithium nitride product purity ≥98% requires extremely stringent conditions, necessitating a highly dry and inert environment. Furthermore, when the purity is below 95%, the delithiation potential of LN exceeds 4.7 V, and the decomposition rate is only around 30%, failing to meet the requirements for industrial applications.

[0005] Patent CN 114466817 A discloses a method for manufacturing lithium nitride. This method involves embedding inorganic particles into a lithium structure and exposing it to a nitrogen atmosphere, enabling a rapid nitridation reaction to generate lithium nitride material. The product is then pulverized and collected. However, this process requires the introduction of additional inorganic particles, making the lithium nitride product prone to introducing other impurities. Furthermore, heating is required during the nitridation process to accelerate the reaction, and this heating process necessitates further pulverization. The process is cumbersome and complex, requiring three steps, and the particle size of the lithium nitride product is unstable. CN115397770A discloses a lithium nitride manufacturing apparatus and a method for manufacturing lithium nitride. The lithium nitride manufacturing apparatus is characterized by having a nitridation reaction tank, a heating mechanism, an environmental control mechanism, and an environmental cooling mechanism, directly synthesizing lithium nitride from lithium components within a nitrogen atmosphere. However, this process has the following drawbacks: lithium has high activity and is very easy to react with reaction equipment or reaction carrier, requiring a harsh reaction environment; the lithium component nitride rate is low, resulting in low purity of the final product; the particle size of lithium nitride is still unstable after further product crushing; and multiple mechanisms are needed to control the environment and temperature during equipment operation, resulting in high energy consumption. Summary of the Invention

[0006] To address the problems of high cost, cumbersome processes, low product purity, unstable particle size, high decomposition potential, low decomposition rate, and low capacity in existing lithium nitride preparation technologies, this invention provides a method for preparing lithium nitride and its application as a positive electrode lithium supplementation additive in lithium-ion batteries. The lithium nitride product obtained by this method has low particle size, uniform particle size, and high purity. After electric field treatment, its activity is even higher. When used as a positive electrode lithium supplementation additive in lithium-ion batteries, it exhibits excellent lithium supplementation effect.

[0007] To achieve the above objectives, the present invention adopts the following technical solution.

[0008] The first aspect of the present invention provides a method for preparing lithium nitride, comprising the following steps:

[0009] (1) After surface treatment, lithium metal is placed in a nitrogen atmosphere to undergo a nitriding reaction to generate incomplete lithium nitride.

[0010] (2) The incomplete lithium nitride generated in step (1) is crushed in an inert gas atmosphere containing nitrogen and a plasma atmosphere to obtain lithium nitride.

[0011] This invention creatively introduces a low-temperature plasma atmosphere into the crushing process, that is, introduces cold field discharge plasma into the mechanical crushing process. The synergistic effect of high-energy non-equilibrium plasma and mechanical crushing can promote the refinement of powder structure, activation, chemical reaction and accelerate in-situ gas-solid phase reaction, which can greatly improve crushing efficiency, form lithium nitride with a specific α phase and β phase ratio, and significantly improve the material performance, such as improving material purity, reducing decomposition voltage and increasing decomposition rate.

[0012] In the above preparation method, the lithium metal can be any form of lithium metal used in the conventional preparation of lithium nitride, and the lithium metal can typically be one or more combinations of lithium powder, lithium strip, lithium sheet, lithium ingot, lithium rope, and lithium alloy.

[0013] In the above preparation method, the surface treatment of lithium metal refers to destroying the surface structure of lithium metal in any feasible way. As a preferred embodiment, the surface treatment of lithium metal involves physically destroying the surface oxide layer or organic film of lithium metal. For example, the surface structure of lithium metal can be destroyed by physical shearing or physical crushing impact. The purpose of surface treatment is to destroy the surface structure and accelerate the nitriding rate of the lithium sheet. As an example, the surface of lithium metal can be struck or scratched with tools, such as by hammering or scissors.

[0014] In the above preparation method, as a preferred embodiment, in step (1), the nitriding reaction time is 1h to 120h (e.g., 5h, 10h, 20h, 30h, 50h, 70h, 90h, 100h, 110h or 115h).

[0015] In the above preparation method, as a preferred embodiment, the temperature of the nitriding reaction is 0℃~100℃ (e.g., 0℃, 15℃, 20℃, 30℃, 40℃, 50℃, 70℃, 80℃, 90℃ or 95℃). To ensure that lithium metal does not react with other containers or contact materials, the temperature is preferably 15℃~35℃ (e.g., 12℃, 15℃, 18℃, 20℃, 23℃, 25℃ or 28℃). In this invention, if the nitriding reaction temperature is too low, the lithium metal nitriding rate is slow and the nitriding efficiency is low; if the temperature is too high, lithium metal is very likely to react with the nitriding container, introducing impurities.

[0016] In the above preparation method, as a preferred embodiment, the crushing environment temperature is 10-150℃ (e.g., 15℃, 20℃, 40℃, 50℃, 70℃, 90℃, 100℃, 120℃, 140℃ or 145℃), which is determined by the plasma atmosphere and crushing process conditions.

[0017] In this invention, if the ambient temperature is too low during crushing, it will affect the gas-solid phase reaction and lead to a decrease in the purity of lithium nitride; if the temperature is too high, the material will easily stick to the wall, affecting the ball milling efficiency and causing unstable particle size.

[0018] In the above preparation method, as a preferred embodiment, the discharge frequency of the plasma atmosphere is set to 6–13 kHz (e.g., 7 kHz, 8 kHz, 9 kHz, 10 kHz, 11 kHz, or 13 kHz), preferably 8.5–11 kHz (e.g., 8.8 kHz, 9 kHz, 9.5 kHz, 10 kHz, 10.5 kHz, or 10.8 kHz). The discharge frequency affects the strength of the electric field in the crushing environment; too high a frequency leads to an increase in the temperature of the crushing environment, while too low a frequency may result in uneven particle size distribution and low surface activity of the material.

[0019] In the above preparation method, as a preferred embodiment, the working current of the plasma atmosphere is 70-200mA (e.g., 80mA, 100mA, 120mA, 150mA, 170mA, 190mA), and the excitation voltage is 3-20kV (e.g., 5kV, 8kV, 10kV, 15kV or 18kV); preferably, the working current is 100-150mA (e.g., 110mA, 120mA, 130mA, 140mA, 145mA), and the excitation voltage is 10-15kV (e.g., 11kV, 12kV, 13kV, 14kV or 14.5kV). The stronger the current and voltage, the stronger the plasma electric field, the higher the proportion of the α phase of lithium nitride products, the greater the activity, and the easier it is to spontaneously combust; the lower the current and voltage, the weaker the plasma electric field, the higher the proportion of the β phase of lithium nitride products, the lower the activity of lithium nitride, and the higher the decomposition potential.

[0020] In the above preparation method, as a preferred embodiment, in step (2), the inert gas also includes other inert gases, namely, a mixture of nitrogen and other inert gases, such as a mixture of nitrogen and argon; the relative pressure of the inert gas atmosphere is set in the range of -0.1MPa to 5MPa (for example, 0MPa, 0.5MPa, 1MPa, 2MPa, 3MPa, 4MPa or 4.5MPa), and the purity of the inert gas is 99.99% or higher.

[0021] In the above preparation method, as a preferred embodiment, in step (2), the crushing includes one or more processes such as sand milling, ball milling, roller milling, or pulverizing.

[0022] In the above preparation method, as a preferred embodiment, the material of the crushing beads is selected from at least one of stainless steel balls, zirconia balls, or agate balls. Preferably, the size of the crushing beads can be selected from one or more different size ratios of 0.1 to 20 mm (e.g., 0.3 mm, 0.5 mm, 1 mm, 3 mm, 5 mm, 8 mm, 10 mm, 15 mm, or 18 mm).

[0023] In the above preparation method, as a preferred embodiment, the weight ratio of the crushed pellets is 1:1 to 100:1 (e.g., 5:1, 10:1, 20:1, 30:1, 50:1, 70:1, 80:1, 90:1 or 95:1), preferably 10:1 to 25:1 (e.g., 12:1, 15:1, 18:1, 20:1, 22:1 or 24:1).

[0024] In the above preparation method, as a preferred embodiment, the crushing speed is set to 300–1500 rpm (e.g., 400 rpm, 500 rpm, 700 rpm, 900 rpm, 1000 rpm, 1100 rpm, 1200 rpm, 1300 rpm, 1400 rpm, or 1450 rpm), preferably 1000–1300 rpm (e.g., 1050 rpm, 1100 rpm, 1150 rpm, 1200 rpm, or 1250 rpm). In this invention, excessively high speeds cause temperature increases and, in addition, increase the impact force on the material, but decrease the number of impacts, affecting ball milling efficiency; excessively low speeds result in uneven particle size distribution and low activity of the material.

[0025] In the above preparation method, as a preferred embodiment, the crushing time is set to 10-2000 min (e.g., 50 min, 100 min, 400 min, 800 min, 1300 min, 1500 min, 1700 min, 1900 min), preferably 120-300 min (e.g., 150 min, 180 min, 200 min, 220 min, 2500 min, 270 min, 290 min).

[0026] In the above preparation method, as a preferred embodiment, the lithium nitride obtained in step (2) has an α phase and a β phase, and the mass ratio between the α phase and the β phase is 99:1 to 60:40 (e.g., 95:5, 90:10, 85:15, 80:20, 75:25, 70:30, 65:35 or 62:38), preferably 95:5 to 80:20 (e.g., 92:8, 90:10, 88:12, 85:15, 83:17 or 81:19).

[0027] This invention obtains lithium nitride with the aforementioned α-phase to β-phase ratio by controlling parameters such as the excitation voltage, discharge frequency, and rotational speed during the fragmentation process in the plasma atmosphere. When the α-phase proportion exceeds 99%, the lithium nitride exhibits high activity and readily reacts with moisture and carbon dioxide in the air, leading to spontaneous combustion and gelation during application. When the β-phase proportion exceeds 60%, the lithium nitride cannot release Li₂ in the low voltage range. + This fails to provide adequate lithium replenishment. Preferably, the ratio of α-phase to β-phase is 95:5-80:20, allowing lithium nitride to achieve a stable state while also releasing a large amount of Li in low-voltage applications. + Active lithium to compensate for the loss in SEI film formation.

[0028] A second aspect of the present invention provides the application of the above-mentioned lithium nitride material as a lithium supplementation additive in lithium-ion batteries.

[0029] In the above applications, the amount of lithium nitride material is preferably 0.1% to 15% of the total mass of the positive electrode active material (e.g., 0.5%, 1%, 2%, 5%, 7%, 10%, 12%, 14%, 14.5%).

[0030] In the above applications, the positive electrode active material preferably includes lithium iron phosphate, lithium manganese oxide, lithium cobalt oxide, or ternary materials.

[0031] In this invention, the above-mentioned technical features can be freely combined to form new technical solutions without conflict.

[0032] Compared with the prior art, the present invention has the following beneficial effects.

[0033] This invention provides a preparation process for high-capacity cathode lithium nitride additive. Through nitriding and crushing processes, and the introduction of a low-temperature plasma atmosphere during mechanical crushing, lithium nitride material with uniform particle size, high purity (up to 98%), and high capacity can be directly obtained. The process is simple and has low energy consumption. After adding the cathode material, the decomposition rate of lithium nitride can reach more than 60%, preferably more than 80%, or even more than 90%.

[0034] (2) The lithium nitride particles obtained by the preparation method of the present invention have smaller particle size and uniform distribution and higher activity. Compared with the lithium nitride prepared by conventional methods (the purity of the prepared lithium nitride is low, resulting in a higher decomposition potential, above 4.3V), it has a higher decomposition rate as a lithium supplement additive, and the decomposition potential is reduced. Under optimal conditions, a more suitable decomposition potential (around 3.7V) can be obtained. Attached Figure Description

[0035] Figure 1 This is the XRD pattern of lithium nitride, the positive electrode lithium replenishment material prepared in Example 1.

[0036] Figure 2 This is a scanning electron microscope (SEM) image of lithium nitride, the positive electrode lithium replenishment material prepared in Example 1. Detailed Implementation

[0037] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely for the purpose of helping to understand the present invention and should not be considered as specific limitations of the present invention.

[0038] Example 1

[0039] Weigh 20g of lithium metal sheets inside a nitrogen glove box. Use a stainless steel block to strike the lithium metal sheets to break their surface structure. After standing in the nitrogen glove box for 24 hours, weigh the nitrided lithium metal sheets. Select 3mm stainless steel grinding balls and, at a ball-to-material mass ratio of 20:1, load the nitrided lithium sheets into a ball mill jar inside the nitrogen glove box and seal it. After installing the ball mill jar into the ball mill, introduce nitrogen through the machine's built-in gas pipeline system, controlling the nitrogen pressure inside the ball mill jar to 0.15MPa, to crush the nitrided lithium sheets. Set the ball mill parameters: each ball milling operation runs for 30 minutes, then stops for 30 minutes, repeating 4 times; motor speed 1020rpm; plasma atmosphere is added during ball milling (i.e., plasma treatment is performed simultaneously with ball milling); plasma discharge frequency is set to 9.2kHz, excitation voltage to 12kV, operating current to 110mA, and ambient temperature during crushing treatment to 72℃. After the ball mill has run for 4 hours, let it stand for 2 hours, then place the ball mill jar in an inert glove box and open it to collect the lithium nitride material and sieve it through a 400-mesh sieve. The material that passes through the sieve will be used as a lithium supplement additive for the positive electrode of lithium-ion batteries.

[0040] Figure 1 The XRD pattern of the positive electrode lithium nitride additive prepared in this embodiment is shown. The XRD characterization results show that the lithium nitride material obtained in Example 1 mainly consists of α and β phases, with no lithium hydroxide, lithium carbonate, or lithium oxide impurities present. The α phase content is 86%, the β phase content is 14%, and the ratio between the α and β phases is 86:14.

[0041] Figure 2 The SEM image of the lithium nitride cathode lithium supplementation additive prepared in this embodiment is shown. As can be seen from the SEM image, the lithium nitride particles prepared in Example 1 have a uniform particle size, generally between 2 and 4 μm.

[0042] Example 2

[0043] Weigh 20g of lithium sheets inside a nitrogen glove box. Use a blade to scratch the surface of the lithium sheets, disrupting the surface structure. After standing in the nitrogen glove box for 96 hours, weigh the nitrided lithium sheets. Select 10mm stainless steel grinding balls and, at a ball-to-material mass ratio of 30:1, load the nitrided lithium sheets into a ball mill jar inside the nitrogen glove box and seal it. After installing the ball mill jar into the ball mill, introduce nitrogen through the machine's built-in gas pipeline system, controlling the pressure inside the ball mill jar to -0.01MPa, and perform crushing treatment on the nitrided lithium sheets. Set the ball mill parameters: each ball milling operation runs for 30 minutes, then stops for 30 minutes, repeating 10 times; motor speed is 1140rpm; plasma treatment is added during the ball milling process (i.e., plasma treatment is performed simultaneously with ball milling); the discharge frequency is set to 11.5kHz, the excitation voltage to 13kV, the operating current to 130mA, and the ambient temperature during the crushing treatment to 87℃. After the ball mill has run for 10 times, let it stand for 2 hours, then place the ball mill jar in an inert glove box and open it to collect the lithium nitride material and sieve it through a 400-mesh sieve. Take the sieved material as lithium nitride as a lithium-ion battery cathode additive.

[0044] This lithium nitride material mainly consists of α and β phases, with no lithium hydroxide, lithium carbonate, or lithium oxide impurities present. The α phase content is 88%, and the β phase content is 12%, with a ratio of 88:12 between the α and β phases. The lithium nitride particles prepared in this embodiment have a uniform particle size, primarily between 2 and 4 μm.

[0045] Example 3

[0046] Weigh 150g of lithium rope inside a nitrogen glove box. Tap the lithium rope with a stainless steel block to break its surface structure. After standing in the nitrogen glove box for 12 hours, weigh the nitrided lithium rope. Select a mixed combination of 1, 3, 5, 10, and 20mm stainless steel grinding balls, and load the nitrided lithium rope into a ball mill jar inside the nitrogen glove box, sealing it. After installing the ball mill jar into the ball mill, introduce nitrogen through the machine's built-in gas pipeline system, controlling the pressure inside the ball mill jar to 0.8MPa, and crush the nitrided lithium sheets. Set the ball mill parameters: each crushing operation runs for 10 minutes, then stops for 5 minutes, with 24 cycles. The motor speed is 960rpm. Plasma treatment is added during the ball milling process, with a discharge frequency of 8.5kHz, an excitation voltage of 10kV, an operating current of 100mA, and a temperature of 89℃ during the crushing process. After the ball mill has run 24 times, let it stand for 2 hours, then place the ball mill jar in an inert glove box and open it to collect the lithium nitride material and sieve it through a 400-mesh sieve. Take the sieved material as lithium nitride as a lithium-ion battery cathode additive.

[0047] This lithium nitride material is mainly composed of α phase and β phase, with no lithium hydroxide, lithium carbonate and lithium oxide impurities present. The α phase content is 72% and the β phase content is 28%, with a ratio of 72:28 between the α and β phases.

[0048] Example 4

[0049] Weigh 20g of lithium sheets inside a nitrogen glove box. Use a blade to scratch the surface of the lithium sheets, disrupting the surface structure. After standing in the nitrogen glove box for 96 hours, weigh the nitrided lithium sheets. Select 5mm zirconia balls and, at a ball-to-material mass ratio of 30:1, load the nitrided lithium sheets into a ball mill jar inside the nitrogen glove box and seal it. After installing the ball mill jar into the ball mill, introduce nitrogen through the machine's built-in gas pipeline system, controlling the pressure inside the ball mill jar to 0.8MPa, and crush the nitrided lithium sheets. Set the ball mill parameters: each crushing operation runs for 30 minutes, then stops for 30 minutes, repeated 5 times, with a motor speed of 600rpm. After ball milling, the lithium nitride was transferred to a sand mill under nitrogen atmosphere for further sand milling. The sand mill was set to run for 8 hours, with 3mm grinding beads. Plasma treatment was added during the sand milling process (i.e., plasma treatment was performed simultaneously with sand milling). The discharge frequency was set to 10.0kHz, the excitation voltage to 11kV, and the operating current to 120mA. The ambient temperature during the sand milling process was 84℃. The lithium nitride material was collected and sieved through a 400-mesh sieve. The undersize material was used as lithium nitride additive for the positive electrode of lithium-ion batteries.

[0050] The lithium nitride material is mainly composed of α phase and β phase, with no lithium hydroxide, lithium carbonate and lithium oxide impurities present. The α phase content is 90% and the β phase content is 10%, with a ratio of 90:10 between the α phase and β phase.

[0051] Example 5

[0052] The lithium nitride preparation method in this embodiment is the same as in Example 1, except that the low-temperature plasma treatment conditions during ball milling are different. The low-temperature plasma treatment conditions in this embodiment are: discharge frequency of 7 kHz, excitation voltage of 4 kV, and operating current of 70 mA. The temperature during the crushing process is 65°C.

[0053] The lithium nitride material obtained through this embodiment is mainly composed of α phase and β phase, with no lithium hydroxide, lithium carbonate and lithium oxide impurities present. The ratio between the α phase and β phase is 67:33.

[0054] Example 6

[0055] The lithium nitride preparation method in this embodiment is the same as in Example 1, except that the low-temperature plasma treatment conditions during ball milling are different. The low-temperature plasma treatment conditions in this embodiment are: discharge frequency of 12kHz, excitation voltage of 20kV, and operating current of 200mA. The temperature during the crushing process is 74℃.

[0056] The lithium nitride material obtained through this embodiment is mainly composed of α phase and β phase, with no lithium hydroxide, lithium carbonate and lithium oxide impurities present. The ratio between the α phase and β phase is 82:18.

[0057] Example 7

[0058] This embodiment provides a method for preparing lithium nitride, which differs from Embodiment 1 in that the plasma discharge frequency is set to 13 kHz. The temperature during the crushing process is 77°C.

[0059] In the lithium nitride material prepared in this embodiment, the ratio between the α phase and the β phase is 74:26, and no lithium hydroxide, lithium carbonate, or lithium oxide impurity phases appear.

[0060] Example 8

[0061] This embodiment provides a method for preparing lithium nitride, which differs from Embodiment 1 in that the plasma discharge frequency is set to 6 kHz. The temperature during the crushing process is 67°C.

[0062] In the lithium nitride material prepared in this embodiment, the ratio between the α phase and the β phase is 66:34, and no lithium hydroxide, lithium carbonate, or lithium oxide impurity phases appear.

[0063] Example 9

[0064] This comparative example provides a method for preparing lithium nitride, which differs from Example 1 in that the plasma excitation voltage is set to 2kV and the operating current is 65mA.

[0065] In the lithium nitride material prepared in this comparative example, the ratio between the α phase and the β phase is 55:45. In addition to the α phase and the β phase, the lithium nitride material also contains a small amount of lithium hydroxide, lithium carbonate and lithium oxide impurities.

[0066] Example 10

[0067] This comparative example provides a method for preparing lithium nitride, which differs from Example 1 in that the plasma excitation voltage is set to 22kV and the operating current is 210mA.

[0068] In the lithium nitride material prepared in this comparative example, the ratio between the α phase and the β phase is 59:41. In addition to the α phase and the β phase, the lithium nitride material also contains a small amount of lithium hydroxide, lithium carbonate and lithium oxide impurities.

[0069] Comparative Example 1

[0070] This comparative example provides a method for preparing lithium nitride, which differs from Example 1 in that no plasma atmosphere is introduced during the ball milling process.

[0071] In the lithium nitride material prepared in this comparative example, the ratio between the α phase and the β phase is 56:44. In addition to the α phase and the β phase, the lithium nitride material also contains a small amount of lithium hydroxide, lithium carbonate and lithium oxide impurities.

[0072] Comparative Example 2

[0073] This comparative example provides a method for preparing lithium nitride, which differs from Example 3 in that no plasma atmosphere is introduced during the ball milling process.

[0074] In the lithium nitride material prepared in this comparative example, the ratio between the α phase and the β phase is 52:48. In addition to the α phase and the β phase, the lithium nitride material also contains a small amount of lithium hydroxide, lithium carbonate and lithium oxide impurities.

[0075] Application examples

[0076] The lithium nitride additives for positive electrode lithium supplementation prepared in Examples 1-10 and Comparative Examples 1-2 were subjected to particle size analysis using a laser particle size analyzer. At the same time, the prepared lithium nitride and positive electrode materials were used to make lithium iron phosphate half-cells containing lithium nitride, wherein the amount of lithium nitride material was 3% of the total mass of the positive electrode active material, and its related electrochemical performance was tested. The test results are shown in Table 1.

[0077] In Table 1, the test conditions for the specific capacity of the first charge cycle are as follows: the test is conducted at a charge / discharge cutoff voltage of 2.0-4.5V vs. Li+ / Li, and at 0.1C.

[0078] In Table 1, the formula for calculating the lithium nitride decomposition rate is:

[0079] Lithium nitride decomposition rate = (First-cycle charge specific capacity of lithium iron phosphate half-cell containing lithium nitride - First-cycle charge specific capacity of lithium iron phosphate half-cell alone) / (Lithium nitride addition amount * Theoretical capacity of lithium nitride)

[0080] The preparation method of the lithium iron phosphate half-cell containing lithium nitride is as follows: 1.94g of lithium iron phosphate is dispersed in an appropriate amount of DMF, stirred and mixed evenly, and then 0.06g of lithium nitride (a positive electrode lithium supplement additive), 0.0526g of PVDF (a binder added in the form of a PVDF-DMF gel), and 0.0526g of SP (a conductive agent) are added. The mixture is then homogenized until it has good fluidity; the negative electrode is a lithium sheet. After coating, drying, and assembly, it is assembled into a half-cell.

[0081] The preparation method of a pure lithium iron phosphate half-cell is as follows: 2g of lithium iron phosphate is dispersed in an appropriate amount of DMF, stirred and mixed thoroughly, and then 0.0526g of PVDF binder (added in the form of a PVDF-DMF gel) and 0.0526g of SP conductive agent are added. Homogenization continues until the slurry has good fluidity; lithium foil is used as the negative electrode. The mixture is then coated, dried, and assembled into a half-cell.

[0082] Table 1

[0083]

[0084] Based on the data recorded in Table 1, a comparison of Example 1 with Comparative Example 1, and Example 3 with Comparative Example 2, shows that, compared to the crushing process without introducing a plasma atmosphere, introducing a plasma atmosphere in the mechanical crushing process of this invention can reduce the particle size of lithium nitride powder, increase powder uniformity, and improve lithium nitride purity. When the lithium nitride of this invention is used as a lithium replenishment material in lithium batteries, it can increase the specific capacity of the battery's first charge, increase the decomposition rate of lithium nitride, and reduce the decomposition voltage of lithium nitride. This indicates that introducing a low-temperature plasma atmosphere in the crushing process can promote powder microstructure refinement, activation, chemical reactions, and accelerate in-situ gas-solid phase reactions, which can greatly improve crushing efficiency, form lithium nitride with a specific α-phase and β-phase ratio, and significantly improve material performance.

[0085] Comparing Example 1 with Examples 9-10, it can be seen that the lithium nitride prepared under the preferred plasma treatment conditions of the present invention has a higher proportion of α phase, a more reasonable ratio of α phase to β phase, high activity and stable state of lithium nitride, and the prepared lithium nitride material is basically free of lithium hydroxide, lithium carbonate and lithium oxide impurities, with a purity of over 99%, and the lithium nitride particles are smaller and more uniform and stable.

[0086] Furthermore, when the lithium nitride of this invention is used as a lithium replenishment material in lithium-ion batteries, no spontaneous combustion or gelation occurs during the homogenization process. Battery electrochemical data analysis shows that more than 60% of the lithium nitride of this invention undergoes a decomposition reaction during the first charging cycle, releasing active lithium ions and nitrogen gas. Only a small amount of lithium nitride needs to be added to compensate for the lithium consumed by the SEI film. Moreover, the nitrogen gas is extracted during the battery formation stage and will not affect the battery internally.

[0087] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Any simple modifications, alterations, and equivalent transformations made to the above embodiments based on the technical essence of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A method for preparing lithium nitride, characterized in that, Includes the following steps: (1) After surface treatment, lithium metal is placed in a nitrogen atmosphere to undergo a nitriding reaction to generate incomplete lithium nitride; (2) The incomplete lithium nitride generated in step (1) is crushed in an inert gas atmosphere containing nitrogen and a plasma atmosphere to obtain lithium nitride; The discharge frequency of the plasma atmosphere is 6 to 13 kHz; the operating current of the plasma atmosphere is 70 to 200 mA, and the excitation voltage is 3 to 20 kV.

2. The method for preparing lithium nitride according to claim 1, characterized in that, The surface treatment of the lithium metal involves physically destroying the surface oxide layer or organic film of the lithium metal. And / or, in step (1), the nitriding reaction time is 1 h to 120 h; the nitriding reaction temperature is 0℃ to 100℃.

3. The method for preparing lithium nitride according to claim 2, characterized in that, The nitriding reaction is carried out at a temperature of 15°C to 35°C.

4. The method for preparing lithium nitride according to claim 1, characterized in that, In step (2), the ambient temperature for crushing is 10~150℃.

5. The method for preparing lithium nitride according to claim 4, characterized in that, The discharge frequency of the plasma atmosphere is 8.5 ~ 11 kHz.

6. The method for preparing lithium nitride according to claim 4, characterized in that, The plasma atmosphere operates at a current of 100-150 mA and an excitation voltage of 10-15 kV.

7. The method for preparing lithium nitride according to claim 1, characterized in that, In step (2), the relative pressure of the inert gas atmosphere is -0.1 MPa ~ 5 MPa.

8. The method for preparing lithium nitride according to claim 1, characterized in that, In step (2), the crushing includes one or more processes such as sand milling, ball milling, roller milling, or pulverizing. And / or, the material of the crushing beads in the crushing process is at least one of stainless steel balls, zirconia balls, or agate balls.

9. The method for preparing lithium nitride according to claim 8, characterized in that, In step (2), the diameter of the broken beads is 0.1~20 mm; And / or, the weight ratio of the crushed pellets is 1:1 to 100:

1.

10. The method for preparing lithium nitride according to claim 9, characterized in that, The weight ratio of the crushed pellets is 10:1 to 25:

1.

11. The method for preparing lithium nitride according to claim 1, characterized in that, The crushing speed is 300~1500 rpm; And / or, the crushing time is 10 to 2000 min.

12. The method for preparing lithium nitride according to claim 11, characterized in that, The crushing speed is 1000 ~ 1300 rpm.

13. The method for preparing lithium nitride according to claim 11, characterized in that, The crushing time is 120 to 300 minutes.

14. The method for preparing lithium nitride according to any one of claims 1-13, characterized in that, The mass ratio of α phase to β phase in the lithium nitride obtained in step (2) is 99:1 to 60:

40.

15. The method for preparing lithium nitride according to claim 14, characterized in that, The mass ratio of α phase to β phase in the lithium nitride obtained in step (2) is 95:5~80:20.

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