CN104858020B - A kind of vacuum ball grinding method for improving LiFePO 4 material compacted density - Google Patents

Abstract

The present invention is a vacuum ball milling method for increasing the compaction density of lithium iron phosphate material, which comprises: step a, put the pre-fired or sintered lithium iron phosphate into the ball mill through the feed port; step b, through pumping The nozzle is connected to a vacuum pump to evacuate the ball mill into an ultimate vacuum, and then filled with nitrogen, and then close the gas nozzle after repeated three times to ensure that the air pressure is at -0.1MPa; step c, inject circulating cooling water into the cooling water jacket, and start ball milling; step d. After the ball milling is completed, feed inert gas, and when the temperature in the ball mill drops below 80 degrees, open the discharge port to discharge the material. In this way, not only can the compaction density of the lithium iron phosphate be increased, but also the oxidation of the lithium iron phosphate material can be prevented.

Description

A vacuum ball milling method for increasing the compaction density of lithium iron phosphate material

technical field

The invention relates to the technical field of ball milling, in particular to a vacuum ball milling method for increasing the compacted density of lithium iron phosphate materials.

Background technique

The low compaction density of lithium iron phosphate material will reduce the energy density of the battery. In order to increase the compaction density, methods such as mechanical fusion, mechanical crushing or airflow crushing are used to control the particle size distribution, and ball milling is also used to improve the lithium iron phosphate. method of compaction density. However, if the ordinary ball milling method is used to process lithium iron phosphate, the kinetic energy will be converted into heat energy due to the continuous impact of the balls on the powder, and the temperature inside the ball mill will be too high during long-term operation, and the local temperature will be too high during the impact, thus The lithium iron phosphate material is oxidized to affect the performance of the material.

In view of the above defects, the inventor of the present invention finally proposed a vacuum ball milling method for increasing the compacted density of lithium iron phosphate material after long-term research and experiments.

Contents of the invention

The object of the present invention is to provide a vacuum ball milling method for increasing the compacted density of lithium iron phosphate material, so as to overcome the above-mentioned technical defects.

In order to achieve the above object, the technical solution adopted by the present invention is to provide a vacuum ball milling method for improving the compacted density of lithium iron phosphate material, which includes:

Step a, putting the pre-burned or sintered lithium iron phosphate into the ball mill through the feed port;

Step b, evacuate the ball mill into an ultimate vacuum by connecting the vacuum pump to the vacuum pump, then fill it with nitrogen, close the gas nozzle after repeating three times, and ensure that the air pressure is at -0.1MPa;

Step c, feed circulating cooling water into the cooling water jacket, and start ball milling;

In step d, after the ball milling is completed, an inert gas is introduced, and when the temperature inside the ball mill drops below 80° C., the discharge port is opened to discharge the material.

Preferably, the ball mill is a vertical ball mill or a horizontal ball mill.

Preferably, the step b includes: before evacuating the ball mill to an ultimate vacuum, connecting the air pipe connected to the vacuum pump to the suction nozzle on the vacuum tank, and opening the ball valve after turning on the vacuum pump.

Preferably, the step c includes: after starting the ball milling, setting the rotational speed of the ball mill, the milling time and the ball-to-material ratio.

Preferably, the ball milling time is 0.5-5h.

Preferably, the range of the grinding ball material ratio is 1-5:1.

Preferably, the material ratio of the grinding balls is 3:1.

Preferably, the inert gas is nitrogen or argon.

Preferably, said step c includes:

Step c1, setting the speed of the ball mill, the material ratio of the balls to be milled and the milling time;

Step c2, setting the temperature range of the ball mill for ball milling, passing circulating cooling water into the cooling water jacket, and starting ball milling;

Step c3, using multiple temperature sensors to simultaneously measure the temperature values at various positions of the ball mill, and calculate the average temperature;

Step c4, compare the average temperature with the set temperature range, and determine the subsequent steps according to the size of the average temperature and the set temperature range; if the average temperature is less than the minimum value of the set temperature range, then perform step c5; if the average temperature is greater than the set temperature range If the maximum value of the temperature range is set, then perform step c6; if the average temperature is within the set temperature range, then perform step c7;

Step c5, reducing the flow rate of cooling water in the cooling water jacket or stopping the flow of cooling water, and performing step c7;

Step c6, increasing the flow rate of cooling water in the cooling water jacket, and performing step c7;

Step c7, judging whether the ball milling time has been reached, if yes, end the ball milling; otherwise, return to step c3.

Preferably, in the step c3, the calculation formula of the average temperature is:

Among them, the calculation formula of M _j is:

In the above formula, Represents the average temperature sought, i and j both represent the serial number of the temperature sensor, k represents the serial number of the temperature sensor corresponding to the optimal central value, n represents the number of the temperature sensor, m represents the percentage value, T _i , T _j , T _k represent the temperature values measured by the i, j, and k temperature sensors respectively, M _j represents the coefficient value corresponding to the temperature value T _j , N _i , N _j represent the temperature values T _i , T _j and the rest The sum of the differences of all temperature values, P _i represents the judgment value corresponding to the temperature value T _i , α represents the correction factor, and β represents the correction value.

Compared with the prior art, the beneficial effect of the present invention is that it provides a vacuum ball milling method for increasing the compacted density of lithium iron phosphate material, which can not only improve the compacted density of lithium iron phosphate, but also prevent the lithium iron phosphate material from being oxidized; Repeating three times can completely convert the air in the tank and drain the residual oxygen in the tank; the high temperature resistant sealing gasket can maintain the vacuum in the vacuum tank and prevent the lithium iron phosphate material from being oxidized; The value is converted into the corresponding judgment value, and the central value sequence number is directly obtained by rounding the judgment value, which is simple and convenient to calculate, can quickly obtain the result, and improves the speed of temperature measurement. At the same time, the simple calculation process saves system time. resources; in addition, the difference between the temperature value and the central value is converted into a corresponding coefficient through the absolute value and rounding the original acid, thus directly excluding the temperature values outside the range of the central value (1-m%, 1+m%), And it makes the coefficient of the temperature value close to the central value larger, so that the obtained average temperature value is closer to the actual temperature value, which improves the measurement accuracy and reduces the error; the calculation formula is simple and convenient, and the result can be obtained quickly, which improves the temperature The speed of the measurement improves the speed of the entire temperature control reaction. At the same time, the simple calculation process saves computing resources; during the ball milling process, the temperature of the ball mill is kept relatively stable. Under certain circumstances, it is ensured that the compacted density of lithium iron phosphate will fluctuate within the required range, so as to obtain a lithium iron phosphate material with more stable compacted density.

Description of drawings

Fig. 1 is the flowchart of the vacuum ball milling method that the present invention improves the compacted density of lithium iron phosphate material;

Fig. 2 is a flow chart of the ball milling process of the vacuum ball milling method for increasing the compacted density of lithium iron phosphate material according to the present invention.

detailed description

The above and other technical features and advantages of the present invention will be described in more detail below in conjunction with the accompanying drawings.

As shown in Figure 1, it is a flow chart of the vacuum ball milling method for improving the compacted density of lithium iron phosphate material according to the present invention, wherein the vacuum ball milling method includes:

Step a, putting the pre-burned or sintered lithium iron phosphate into the ball mill through the feed port;

The lithium iron phosphate needs to be pre-sintered before grinding, and after the pre-sintering or sintering is completed, the lithium iron phosphate is put into a ball mill; the ball mill is a vertical ball mill or a horizontal ball mill. The grinding cylinder in the vertical ball mill or horizontal ball mill used in the present invention is a vacuum tank that can be vacuumed.

Step b, evacuate the ball mill into an ultimate vacuum by connecting the vacuum pump to the vacuum pump, then fill it with nitrogen, close the gas nozzle after repeating three times, and ensure that the air pressure is at -0.1MPa;

The purpose of this step is to keep the vacuum state in the vacuum tank of the ball mill at last. Keeping in the ultimate vacuum can minimize the oxygen content in the tank environment, 100% guarantee that the material will not be oxidized.

Repeating three times is the minimum number of times that the air in the tank can be completely converted and the residual oxygen in the tank can be exhausted. If the number of times is less than three times, the gas in the tank may not be completely converted, and too many replacement times waste gas and time.

There is a ball valve and an air nozzle on the vacuum tank. Before vacuuming, connect the air pipe connected to the vacuum pump to the air nozzle on the vacuum tank. After turning on the vacuum pump, open the ball valve; after the vacuuming process is completed, close the ball valve and unplug the air pipe. . In order to prevent air leakage, two ball valves can be connected in series or plugged with a plug at the air nozzle after the trachea is unplugged. A high-temperature-resistant sealing gasket is used, and high-temperature-resistant vacuum grease is applied between the gasket and the contact surface, which can prevent air leakage, maintain the vacuum in the vacuum tank, and prevent the lithium iron phosphate material from being oxidized.

Step c, feed circulating cooling water into the cooling water jacket, and start ball milling;

The shell of the ball mill is made of 5-12mm stainless steel with a cooling water jacket. After the ball mill starts ball milling, the rotational speed of the ball mill and the ball-to-material ratio can be set, and ball milling can be performed for a predetermined time after setting.

Grinding ball: The ratio of materials is 1-5:1, and the ball milling time is 0.5-5h.

As shown in Figure 2, it is a flow chart of the ball milling process of the vacuum ball milling method for improving the compacted density of lithium iron phosphate material in the present invention; wherein, the step c includes the following steps:

Step c1, setting the speed of the ball mill, the material ratio of the balls to be milled and the milling time;

When the speed, ball material ratio, temperature, and ball milling time are fixed, the compacted density of lithium iron phosphate will fluctuate within a certain range, and the fluctuation is caused by the mechanical reasons of the ball milling process. In order to obtain the compacted density Relatively stable lithium iron phosphate materials need to set and control the speed, ball material ratio, temperature, and ball milling time during the entire ball milling process; among them, the speed, ball material ratio, and ball milling time can be set in advance.

This step is a preparatory step before starting ball milling, and can be used as the preparatory part of step a, can be used as the preparatory part of step b, or can be used as the preparatory part of step c, and its order with steps a and b is variable.

When setting the rotational speed of the ball mill, the ratio of balls to materials and the milling time, the rotational speed of the ball mill can be determined according to the rotational speed range of the ball mill, and a more suitable rotational speed is selected within the attainable rotational speed range of the ball mill.

For the setting of the ball milling time, it can be set in advance, and other timers can also be used for timing during the ball milling process, and the ball milling can be stopped after the timing is completed.

The material ratio of grinding balls refers to the weight ratio between the grinding balls and lithium iron phosphate added in the ball mill, and the preferred range is 1-5:1, that is, one part of lithium iron phosphate can correspond to 1-5 parts of grinding balls ; The preferred ball-to-material ratio is 3:1.

Step c2, setting the temperature range of the ball mill for ball milling, feeding circulating cooling water into the cooling water jacket, and starting ball milling;

When the rotational speed, ball material ratio, and ball milling time have been set, the temperature range during ball milling is determined according to the compacted density of lithium iron phosphate that needs to be achieved after ball milling, and then the ball milling temperature is kept at the required level during ball milling. Within the temperature range, the compacted density of the lithium iron phosphate obtained after ball milling will meet the expectation.

After setting the temperature range of the ball mill during ball milling, circulate cooling water into the cooling water jacket to cool the ball mill during ball milling.

The flow rate of the cooling water in the cooling water jacket sink is controllable, increasing the flow rate can improve the cooling effect of the cooling water on the ball mill, on the contrary, reducing the flow rate or stopping the flow of the cooling water can reduce the cooling effect of the cooling water on the ball mill. The cooling effect of the ball mill.

Step c3, using multiple temperature sensors to simultaneously measure the temperature values at various positions of the ball mill, and calculate the average temperature;

During the ball milling process, the rotational speed, ball material ratio, and ball milling time can be set in advance, but the temperature changes rapidly during the ball milling process. It is necessary to measure the temperature in the vacuum tank in time and take relevant measures quickly to reduce the grinding process. The temperature fluctuation keeps the temperature stable throughout the grinding process.

Multiple temperature sensors are installed in various parts of the ball mill, so that the temperature of each part of the ball mill can be measured simultaneously during the ball milling process, and then the average temperature can be obtained.

Wherein, the calculation formula of described average temperature is:

Among them, the calculation formula of M _j is:

In the above formula, Indicates the average temperature to be obtained, i and j both indicate the serial number of the temperature sensor, k indicates the serial number of the sensor corresponding to the optimal central value, n indicates the number of the temperature sensor, m indicates the percentage value, T _i , T _j , T _k represents the temperature values measured by the i, j and k temperature sensors respectively, M _j represents the coefficient value corresponding to the temperature value T _j , N _i and N _j represent the temperature values T _i , T _j and all other temperature values respectively The sum of the differences, P _i represents the judgment value corresponding to the temperature value T _i , α represents the correction factor, and β represents the correction value.

The above idea is: according to the temperature measured by each temperature sensor, first find the sum of the absolute value of the difference between a certain temperature and the other temperatures, as the sum of the differences corresponding to the temperature; the sum of all differences and a certain difference The ratio of the sum of the values is rounded down and summed, and the sum is divided by the number of sensors (that is, the number of sums of differences) to obtain the judgment value corresponding to the sum of the differences; different temperatures correspond to different judgment values, The judgment value corresponding to the temperature corresponding to the smallest sum of difference values is rounded down to 1, and the rest are all 0. In this way, the judgment value is rounded down and then multiplied by the corresponding sensor number, and the result obtained after continuous addition is The serial number of the temperature with the smallest sum of corresponding differences, that is, the serial number of the central value of the required temperature value (the central value here refers to the value closest to other values); a certain temperature value and the temperature as the central value The absolute value of the value difference divided by the percentage of the central value is used as the coefficient value of the temperature value, where m is the percentage value, which can be adjusted according to the actual situation; the coefficient value is rounded down as the coefficient of the corresponding temperature value, The coefficient of the fluctuation range outside the range of the central value (1-m%, 1+m%) is 0, which excludes the temperature values with large fluctuations, thereby obtaining a more accurate actual average temperature value.

The beneficial effect is: the temperature value is converted into the corresponding judgment value by calculating the sum of the differences and the rounding operation, and the central value serial number is directly obtained by rounding the judgment value, so that the calculation is simple and convenient, and the result can be obtained quickly. The speed of temperature measurement is improved, and at the same time, the simple calculation process saves system resources; in addition, the difference between the temperature value and the central value is converted into a corresponding coefficient through the absolute value and rounded original acid, which directly excludes the central value (1 -m%, 1+m%) outside the temperature range, and make the coefficient of the temperature value close to the central value larger, so that the average temperature value obtained is closer to the actual temperature value, which improves the accuracy of the measurement and reduces the error. The calculation formula is simple and convenient, the result can be obtained quickly, the speed of temperature measurement is improved, and the speed of the entire temperature regulation reaction is improved, and at the same time, the simple calculation process saves computing resources.

Step c4, compare the average temperature with the set temperature range, and determine the subsequent steps according to the size of the average temperature and the set temperature range; if the average temperature is less than the minimum value of the set temperature range, then perform step c5; if the average temperature is greater than the set temperature range If the maximum value of the temperature range is set, then step c6 is performed; if the average temperature is equal to the set temperature range (i.e. within the set temperature range), then step c7 is performed;

As the ball mill progresses, the temperature of the ball mill increases gradually; the average temperature is lower than the minimum value of the set temperature range, which proves that the temperature of the ball mill has not yet reached the set temperature, and it is necessary to speed up the temperature rise of the ball mill to make it reach the set temperature as soon as possible. Set the temperature, so execute step c5; the average temperature is greater than the maximum value of the set temperature range, which proves that the temperature of the ball mill has exceeded the set temperature, and it is necessary to reduce the temperature of the ball mill to make it return to the set temperature as soon as possible, so perform step c6; average If the temperature is equal to the set temperature range (that is, within the set temperature range), it proves that the temperature of the ball mill has reached the set temperature and needs to be maintained as much as possible, so step c7 is executed.

Step c5, reducing the flow rate of cooling water in the cooling water jacket or stopping the flow of cooling water, and performing step c7;

The temperature of the ball mill has not yet reached the set temperature, and it is necessary to speed up the temperature rise rate of the ball mill to make it reach the set temperature as soon as possible. Within the range of the set speed), on the other hand, various measures to reduce temperature can be stopped or slowed down, such as ventilation, cooling water, etc.; among them, reducing the flow rate of cooling water in the cooling water jacket or stopping the flow of cooling water is the The most effective method can also take other measures such as no ventilation.

Step c6, increasing the flow rate of cooling water in the cooling water jacket, and performing step c7;

The temperature of the ball mill has exceeded the set temperature. It is necessary to reduce the temperature of the ball mill so that it can return to the set temperature as soon as possible. On the other hand, various measures to reduce temperature can be accelerated, such as ventilation, cooling water, etc. Among them, increasing the flow rate of cooling water in the cooling water jacket is the most effective method, and can also be adopted to speed up Ventilate or align the tuyere with the ball mill and other measures to cool down.

Step c7, judging whether the ball milling time has been reached, if yes, end the ball milling; otherwise, return to step c3.

The measurement and calculation of the average temperature should be carried out at any time, or once every period of time. The period of time here can be 1 minute, 5 minutes, or any other suitable time period, which can be set according to the actual situation. Certainly.

Before the ball milling time is reached, it is necessary to calculate the average temperature according to the measurement, adjust the cooling water flow rate (speed, ventilation, etc.) according to the average temperature, then measure and calculate the average temperature, and then adjust the cooling water flow rate (speed, ventilation, etc. Other measures) cycle like this until the end of ball milling.

In this way, during the ball milling process, the temperature of the ball mill can be kept relatively stable, and the compacted density of lithium iron phosphate can be guaranteed to fluctuate within the required range under the condition that the rotational speed, the ball material ratio and the ball milling time are fixed, so as to obtain Lithium iron phosphate material with more stable compacted density.

In step d, after the ball milling is completed, the inert gas is introduced, and when the temperature in the ball mill drops below 80 degrees, the discharge port is opened to discharge the material.

The inert gas can be nitrogen or argon, or other inert gases that are difficult to react with lithium iron phosphate at high temperature; the cost of nitrogen is lower in production, so nitrogen can be preferred.

Heat transfer is relatively slow in a vacuum state. In order to reduce the temperature to the reclaiming temperature as soon as possible, nitrogen gas is introduced after ball milling to enhance heat transfer, so that the temperature in the tank can drop below 80 degrees faster and improve efficiency. If it is not lowered below 80 degrees, the material will be oxidized when it is in contact with the air during retrieving.

If the air is passed in, the material will react with the oxygen in the air at high temperature, and the electrical properties of the material will be reduced. The purpose of using vacuum ball milling is to prevent the material from being oxidized. If other materials are not afraid of oxidation during the ball milling process, ordinary ball milling can be used.

If other materials adopt the method of ball milling, it can also improve the compaction density to a certain extent. However, if other materials are not afraid of being oxidized, ordinary ball milling can be used instead of vacuum. As far as the applicant is aware, there is currently no other material that uses vacuum ball milling to increase compaction density.

The vacuum ball milling method for increasing the compaction density of the lithium iron phosphate material in the present invention can not only increase the compaction density of the lithium iron phosphate material, but also prevent the lithium iron phosphate material from being oxidized.

In the case where the vacuum ball milling method is not applicable, the compacted density of the lithium iron phosphate material can reach 2.2 through other ball milling measures, but the vacuum ball milling method of the present invention can reach 2.4 under the same conditions, which greatly improves the density of the lithium iron phosphate material. The compacted density of the material.

The above descriptions are only preferred embodiments of the present invention, and are only illustrative rather than restrictive to the present invention. Those skilled in the art understand that many changes, modifications, and even equivalents can be made within the spirit and scope defined by the claims of the present invention, but all will fall within the protection scope of the present invention.

Claims (8)

1. A kind of 1. vacuum ball grinding method for improving LiFePO 4 material compacted density, it is characterised in that including：

   Step a, by after pre-burning or sintering complete LiFePO4 be put into by charging aperture in ball mill；

   Step b, vavuum pump is connected by air exhaust nozzle the ball mill is pumped into end vacuum, be then charged with nitrogen, repeatedly for three times After close valve, ensure air pressure in -0.1MPa；

   Step c, recirculated cooling water is passed through in cooling water jecket, starts ball milling；

   Step d, after ball milling, inert gas is passed through, when the ball milling built-in temperature is down to below 80 DEG C, opens discharging opening Discharging；

   The step c includes：

   Step c1, set the rotating speed, abrading-ball material ratio and Ball-milling Time of the ball mill；

   Step c2, the temperature range during ball mill ball milling is set, be passed through recirculated cooling water in the cooling water jecket, open Beginning ball milling；

   Step c3, measure the temperature value of each position of the ball mill simultaneously using multiple temperature sensors, and calculate average temperature Degree；

   Step c4, by mean temperature compared with the temperature range set, according to the big of mean temperature and design temperature scope Small determination subsequent step；Step c5 is performed if mean temperature is less than the minimum value of design temperature scope；If mean temperature is more than The maximum of design temperature scope, then perform step c6；If mean temperature in the range of design temperature, performs step c7；

   Step c5, reduce the flow velocity of cooling water in the cooling water jecket or stop flow of cooling water, and perform step c7；

   Step c6, increases the flow velocity of cooling water in the cooling water jecket, and performs step c7；

   Step c7, judge whether to reach Ball-milling Time, be, terminate ball milling；Otherwise return to step c3.

   In the step c3, the calculation formula of the mean temperature is：

   Wherein, M_jCalculation formula be：

   <mrow> <msub> <mi>N</mi> <mi>i</mi> </msub> <mo>=</mo> <munderover> <mo>&amp;Sigma;</mo> <mrow> <mi>j</mi> <mo>=</mo> <mn>1</mn> </mrow> <mi>n</mi> </munderover> <mo>|</mo> <msub> <mi>T</mi> <mi>i</mi> </msub> <mo>-</mo> <msub> <mi>T</mi> <mi>j</mi> </msub> <mo>|</mo> </mrow>

   <mrow> <msub> <mi>P</mi> <mi>i</mi> </msub> <mo>=</mo> <mfrac> <mn>1</mn> <mi>n</mi> </mfrac> <munderover> <mo>&amp;Sigma;</mo> <mrow> <mi>j</mi> <mo>=</mo> <mn>1</mn> </mrow> <mi>n</mi> </munderover> <mfrac> <msub> <mi>N</mi> <mi>j</mi> </msub> <msub> <mi>N</mi> <mi>i</mi> </msub> </mfrac> </mrow>

   <mrow> <msub> <mi>M</mi> <mi>j</mi> </msub> <mo>=</mo> <mfrac> <mrow> <msub> <mi>m%T</mi> <mi>k</mi> </msub> </mrow> <mrow> <mo>|</mo> <msub> <mi>T</mi> <mi>j</mi> </msub> <mo>-</mo> <msub> <mi>T</mi> <mi>k</mi> </msub> <mo>|</mo> </mrow> </mfrac> <mo>,</mo> <mi>j</mi> <mo>&amp;NotEqual;</mo> <mi>k</mi> </mrow>

   In above formula,Required mean temperature is represented, i, j represent the sequence number of the temperature sensor, and k represents optimal central value The corresponding temperature sensor sequence number, n represent the quantity of the temperature sensor, and m represents percent value, T_i、T_j、T_kRespectively Represent the temperature value that i-th, j, k temperature sensors measure, M_jRepresent temperature value T_jCorresponding coefficient value, N_i、N_jTemperature is represented respectively Angle value T_i、T_jWith the difference sum of remaining all temperature value, P_iRepresent temperature value T_iCorresponding judgment value, α represent modifying factor, β Represent correction value.
2. 2. a kind of vacuum ball grinding method as claimed in claim 1, it is characterised in that the ball mill is vertical ball mill or crouched Formula ball mill.
3. 3. a kind of vacuum ball grinding method as claimed in claim 2, it is characterised in that the step b includes：By the ball milling Machine is pumped into before end vacuum, the tracheae for connecting the vavuum pump is connected with the air exhaust nozzle on vacuum tank, described in opening Ball valve is opened after vavuum pump.
4. 4. the vacuum ball grinding method as described in any in a kind of 1-3 such as claim, it is characterised in that the step c includes：Start After ball milling, the rotating speed of the ball mill, Ball-milling Time and abrading-ball material ratio are configured.
5. 5. a kind of vacuum ball grinding method as claimed in claim 4, it is characterised in that the Ball-milling Time is 0.5-5h.
6. 6. a kind of vacuum ball grinding method as claimed in claim 4, it is characterised in that the scope of the abrading-ball material ratio is 1-5: 1。
7. 7. a kind of vacuum ball grinding method as claimed in claim 6, it is characterised in that the abrading-ball material ratio is 3:1.
8. 8. a kind of vacuum ball grinding method as claimed in claim 5, it is characterised in that the inert gas is nitrogen or argon Gas.