Formation method of secondary battery and secondary battery

Abstract

The invention relates to a formation method of a secondary battery and the secondary battery. The invention relates to the technical field of batteries, and a formation method of a secondary battery adopts a negative pressure formation process and comprises the following steps: charging the secondary battery to a first voltage; charging the secondary battery at a first rate to increase the voltage of the secondary battery from the first voltage to a second voltage; charging at a second rate to maintain the secondary battery at a second voltage; charging at a third rate to maintain the secondary battery at the second voltage; wherein the first multiplying power is larger than the second multiplying power, and the second multiplying power is larger than the third multiplying power; the reaction voltage threshold value of the lithium supplementing agent is greater than the first voltage and less than the second voltage. And by utilizing the characteristics that the first multiplying power is greater than the second multiplying power and the second multiplying power is greater than the third multiplying power, the duration time of the lithium supplementing agent under the reaction voltage is longer, the lithium supplementing agent reacts more fully, and meanwhile, lithium precipitation on the surface of the negative electrode is reduced.

Description

Technical Field

[0001] The present application relates to the field of battery technology, and in particular to a formation method of a secondary battery and a secondary battery. Background Art

[0002] During the first charge formation of lithium-ion batteries, a violent reaction will occur, consuming a large amount of active substances to form a solid electrolyte interface (SEI) film, resulting in irreversible loss of lithium ions. The consumed active substances lead to irreversible capacity loss, resulting in a waste of lithium battery production costs. At the same time, during the charging cycle of lithium-ion batteries, side reactions will occur to varying degrees to consume active lithium, resulting in capacity decay and reduced cycle life of lithium-ion batteries.

[0003] In order to reduce production costs and increase the cycle life of lithium-ion batteries, positive electrode pre-lithiation technology has attracted attention from the industry. Positive electrode pre-lithiation refers to adding a certain proportion of lithium replenisher when the positive electrode plate is homogenized. The lithium replenisher is activated during formation to release a large amount of lithium ions to make up for the active lithium consumed in the film-forming reaction, and continue to act as a lithium source in subsequent cycles, thereby reducing the battery's cycle capacity attenuation and extending the cycle life.

[0004] However, lithium supplements usually cause black spots and lithium deposition on the negative electrode, affecting battery performance. Summary of the invention

[0005] The present application provides a secondary battery formation method and a secondary battery to solve the problems of black spots and negative electrode lithium deposition usually caused by lithium supplements in the related art.

[0006] In a first aspect, the present application provides a secondary battery formation method, using a negative pressure formation process, comprising the following steps:

[0007] charging the secondary battery to a first voltage;

[0008] Charging the secondary battery at a first rate so that the voltage of the secondary battery increases from a first voltage to a second voltage;

[0009] charging the secondary battery at a second rate so that the secondary battery is maintained at a second voltage;

[0010] charging the secondary battery at a third rate so that the secondary battery is maintained at a second voltage;

[0011] Wherein, the first magnification is greater than the second magnification, and the second magnification is greater than the third magnification;

[0012] The first voltage is less than the reaction voltage threshold of the lithium supplement agent and less than the second voltage.

[0013] The present application adopts a constant voltage charging process with a gradient drop rate in the voltage interval above the reaction voltage threshold of the lithium supplement, that is, the reaction stage of the lithium supplement, and utilizes the characteristics that the first rate is greater than the second rate and the second rate is greater than the third rate, so that the duration of the lithium supplement under the reaction voltage is longer, the lithium supplement reacts more fully, and the lithium precipitation on the negative electrode surface is reduced. Through the negative pressure formation process, it can help to discharge the gas generated by the lithium supplement during the reaction and reduce black spots. The reaction of the lithium supplement is related to the voltage value. Usually, the reaction voltage threshold of the lithium supplement is 3.8V. The reaction starts at a voltage above 3.8V. The larger the voltage value, the more thorough the reaction of the lithium supplement, and the voltage value is related to polarization. The larger the polarization, the higher the voltage generated, and the polarization is related to the rate. The higher the rate, the greater the polarization. Therefore, the largest first rate among the three rates can make the lithium supplement react quickly, and the second voltage of the secondary battery is maintained by the second rate and the third rate that are smaller than the first rate, which can make the secondary battery maintain the second voltage for a longer time, and the negative electrode is more fully embedded with lithium, thereby reducing the lithium precipitation of the negative electrode. Usually the second voltage is in an overcharge state, and its function is to allow the lithium supplement agent to fully react. The lithium ions released by the lithium supplement agent need a certain amount of lithium insertion time for the negative electrode. Maintaining the second voltage of the secondary battery through constant-voltage charging at the second rate and the third rate can provide time for the negative electrode to insert lithium, making the lithium insertion more complete and reducing lithium plating at the negative electrode.

[0014] It should be noted that the negative pressure forming process means that at least one of the various steps of the forming process uses negative pressure, rather than requiring that every step uses negative pressure.

[0015] In some embodiments, the first magnification is 0.5C to 1C; and / or,

[0016] The second magnification is 0.4C to 0.8C; and / or,

[0017] The third ratio is 0.3C to 0.5C.

[0018] At the same cut-off voltage, the greater the charging rate, the less capacity is charged, and the smaller the charging rate, the more capacity is charged. The reaction of the lithium supplement is determined by the battery voltage. The higher the voltage, the more complete the reaction of the lithium supplement. However, the larger the charging capacity, the more serious the lithium plating at the negative electrode. Therefore, when considering the lithium plating of the lithium supplement, a higher voltage and less capacity are required to achieve the full reaction of the lithium supplement and reduce lithium plating. The large rate of 0.5C to 1C allows the second voltage to be reached quickly, and the lithium supplement reacts quickly. At the same time, compared with the low rate of 0.1C, the capacity charged into the battery can be reduced, reducing lithium plating. By reducing the rate, the voltage of the secondary battery can be maintained at the second voltage for a longer time, which can provide time for lithium insertion at the negative electrode and further reduce lithium plating.

[0019] Since at the same cut-off voltage, the greater the rate, the less capacity charged into the battery, and in order to maintain the high voltage time, the charging rate is reduced in a step-by-step manner, the battery can continue to be charged, the voltage is increased to maintain the second voltage, i.e., the constant voltage time, to provide time for the negative electrode to insert lithium, reduce lithium precipitation, and at the same time make the lithium supplement agent react more fully, release more lithium ions, and increase the battery capacity. In addition, charging at a high rate can also speed up the entire charging time, using polarization to increase the voltage so that the lithium supplement agent reacts quickly, while reducing lithium precipitation, while improving the performance of the secondary battery, improving efficiency and increasing production capacity.

[0020] In some embodiments, the first voltage is 3.65-3.75V. The main purpose of the first voltage is to form a SEI film on the surface of the positive electrode, provide a lithium insertion depth for the negative electrode, and reduce lithium precipitation at the beginning of the formation stage. The lithium supplement agent basically does not react at this stage. Preferably, the first voltage is 3.7V, and the SEI film is more uniform; and / or,

[0021] The second voltage is 4V to 4.1V. The main purpose of the second voltage is to make the lithium supplement agent release lithium ions to supplement the secondary battery. At this stage, the negative electrode has basically completed most of the lithium insertion, and the remaining lithium ions released by the lithium supplement agent are embedded into the negative electrode during the charging process, thereby increasing the battery capacity. Preferably, the second voltage is 4.05V.

[0022] In some embodiments, before charging the secondary battery to the first voltage, the step further includes:

[0023] charging the secondary battery to a fourth voltage at a fourth rate;

[0024] charging the secondary battery to a fifth voltage at a fifth rate;

[0025] charging the secondary battery to a sixth voltage at a sixth rate;

[0026] Wherein, the fourth magnification is smaller than the fifth magnification, and the fifth magnification is smaller than the sixth magnification;

[0027] The fourth voltage is lower than the fifth voltage, and the fifth voltage is lower than the sixth voltage.

[0028] By increasing the voltage and the rate in steps, the negative electrode can be slowly intercalated with lithium and the negative electrode lithium deposition can be reduced. The voltage is increased from the fourth voltage to the sixth voltage in a stepwise manner, and the rate is increased from the fourth rate to the sixth rate. Different rates can be matched in voltage ranges with different formation gas production rates to slow down the formation gas production and help form a uniform and dense SEI film.

[0029] In some embodiments, the fourth magnification is 0.05C to 0.2C; and / or,

[0030] The fifth magnification is 0.1C to 0.5C; and / or,

[0031] The sixth magnification is 0.1C to 1C; and / or,

[0032] The fourth voltage is 3V to 3.2V, preferably, the fourth voltage is 3.1V; and / or,

[0033] The fifth voltage is 3.3V to 3.5V, preferably, the fifth voltage is 3.4V; and / or,

[0034] The sixth voltage is 3.65V to 3.75V. Preferably, the sixth voltage is 3.7V.

[0035] The purpose of charging the secondary battery to the fourth voltage at the fourth rate is to activate the negative electrode. In this formation voltage range, the formation reaction is the most intense, and it is also the initial stage of SEI film formation. A lower rate of charging is required to reduce the gas production rate and form a uniform SEI film. Therefore, the fourth rate is 0.05C~0.2C, and the fourth voltage is 3V~3.2V, which is the transition voltage of the formation gas production rate. Before this voltage, the formation gas production rate is high, and after this voltage, the formation gas production rate gradually decreases. The purpose of charging the secondary battery to the fifth voltage at the fifth rate is to continue the formation reaction to form the SEI film. At this time, the SEI film has been initially formed and stabilized after the fourth voltage, and the gas production rate is gradually reduced. At this time, the charging rate can be gradually increased. Therefore, the fifth rate is 0.1C~0.5C, and the fifth voltage is 3.3V~3.5V, which is the end voltage of formation gas production. Before this voltage, there is still gas production during formation, and after this voltage, the gas production is basically finished. The purpose of charging the secondary battery to the sixth voltage at the sixth rate is to continuously charge and increase the cell voltage to the lithium replenisher reaction stage, in preparation for the lithium replenisher reaction. Therefore, the sixth rate is 0.1C to 1C, and the sixth voltage is 3.65V to 3.75V, which is the full charge voltage of the lithium iron phosphate battery / graphite, and is slightly lower than the lower limit voltage of the lithium replenisher reaction.

[0036] In some embodiments, before charging the secondary battery to the first voltage, the step further includes:

[0037] discharging the secondary battery to a seventh voltage at a seventh rate;

[0038] charging the secondary battery to a first voltage at an eighth rate;

[0039] The discharge and charge between the seventh voltage and the first voltage are repeated.

[0040] Repeated charging and discharging between the seventh voltage and the first voltage can increase the lithium insertion depth of the negative electrode and reduce the lithium precipitation problem caused by the inability to insert lithium ions released by the subsequent lithium supplement reaction.

[0041] In some embodiments, the seventh magnification is 0.1C to 1C; and / or,

[0042] The eighth magnification is 0.5C to 1C; and / or,

[0043] The seventh voltage is 2.5V to 2.7V, preferably, the seventh voltage is 2.6V; and / or,

[0044] The number of repetitions is greater than or equal to three times.

[0045] The secondary battery is charged to a sixth voltage at a sixth rate, discharged to a seventh voltage at a seventh rate, and charged to a first voltage at an eighth rate, that is, charged to 3.65V to 3.75V at 0.1C to 1C, discharged to 2.5V to 2.7V at 0.1C to 1C, and charged to 3.65V to 3.75V at 0.5C to 1C. This arrangement allows slow charging, increased lithium insertion depth, fast discharge, reduced time consumption, improved formation efficiency, and reduced lithium insertion.

[0046] In some embodiments, the gas pressure in the gas production stage adopts high and low pressure alternating cycles, the low pressure is -100KPa to -30KPa, and the high pressure is -30KPa to 30KPa; wherein, the gas production stage includes:

[0047] charging the secondary battery to a fourth voltage at a fourth rate;

[0048] charging the secondary battery to a fifth voltage at a fifth rate;

[0049] Charging the secondary battery at a first rate so that the voltage of the secondary battery increases from a first voltage to a second voltage;

[0050] charging the secondary battery at a second rate so that the secondary battery is maintained at a second voltage;

[0051] The secondary battery is charged at the third rate so as to be maintained at the second voltage.

[0052] The battery formation process includes a formation gas production stage and a lithium supplement reaction gas production stage. The formation gas production stage is the aforementioned charging of the secondary battery to the fourth voltage stage at the fourth rate and the charging of the secondary battery to the fifth voltage stage at the fifth rate; the lithium supplement reaction gas production stage is the aforementioned charging of the secondary battery from the first voltage to the second voltage stage at the first rate, charging at the second rate to keep the secondary battery at the second voltage stage, and charging at the third rate to keep the secondary battery at the second voltage stage. It can be understood that in the battery formation process, each charging step includes a shelving step after each charging step, so, to be precise, the formation gas production stage includes charging the secondary battery to the fourth voltage stage at the fourth rate, a shelving stage, charging the secondary battery to the fifth voltage stage at the fifth rate, and a shelving stage; the lithium supplement reaction gas production stage includes charging the secondary battery from the first voltage to the second voltage stage at the first rate, a shelving stage, charging the secondary battery at the second rate to keep the secondary battery at the second voltage stage, a shelving stage, charging the secondary battery at the third rate to keep the secondary battery at the second voltage stage, and a shelving stage. In the above-mentioned gas production stage and lithium supplement reaction gas production stage, the gas pressure adopts high and low pressure alternating cycle, wherein the low pressure is -100KPa~-30KPa and the high pressure is -30KPa~30KPa. The high and low pressure cycle can promote the timely discharge of the gas generated in the gas production stage and the lithium supplement reaction gas production stage, and reduce the impact of gas residue on the battery.

[0053] In some embodiments, a clamp is used to restrain the secondary battery during charging, wherein the restraining force is 100Kgf to 1000Kgf. The battery can be clamped by a clamp tooling, and in the formation step, a restraining force of 100Kgf to 1000Kgf is applied to the battery, which can reduce the occurrence of black spots and lithium precipitation caused by the excessive gap between the pole pieces due to the gas generated during the formation process. At the same time, appropriate restraining force helps to exhaust gas. In addition, the presence of the clamp can also shape the battery to make the lithium insertion more uniform and avoid excessive battery thickness.

[0054] In a second aspect, the present application proposes a secondary battery obtained by the secondary battery formation method described in the first invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0055] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0056] Figure 1 Flow chart of a secondary battery formation method according to an embodiment of the present application.

[0057] Figure 2 Flow chart of a secondary battery formation method according to an embodiment of the present application.

[0058] Figure 3 Flow chart of a secondary battery formation method according to an embodiment of the present application.

[0059] Figure 4 Flow chart of a secondary battery formation method according to an embodiment of the present application.

[0060] Figure 5 This is the appearance of a secondary battery processed by the secondary battery formation method of Example 2 of the present application. DETAILED DESCRIPTION

[0061] In order to make the purpose, technical solution and advantages of the present application clearer, the technical solution of the present application will be clearly and completely described below in combination with the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present application.

[0062] During the first charge formation of lithium-ion batteries, a violent reaction will occur, consuming a large amount of active substances to form a solid electrolyte interface (SEI) film, resulting in irreversible loss of lithium ions. The consumed active substances lead to irreversible capacity loss, resulting in a waste of lithium battery production costs. At the same time, during the charging cycle of lithium-ion batteries, side reactions will occur to varying degrees to consume active lithium, resulting in capacity decay and reduced cycle life of lithium-ion batteries.

[0063] In order to reduce production costs and increase the cycle life of lithium-ion batteries, positive electrode pre-lithiation technology has attracted attention from the industry. Positive electrode pre-lithiation refers to adding a certain proportion of lithium replenisher when the positive electrode plate is homogenized. The lithium replenisher is activated during formation to release a large amount of lithium ions to make up for the active lithium consumed in the film-forming reaction, and continue to act as a lithium source in subsequent cycles, thereby reducing the battery's cycle capacity attenuation and extending the cycle life.

[0064] However, when activated, the lithium supplement will produce a large amount of gas and quickly release a large amount of lithium ions, which can easily cause black spots caused by insufficient exhaust and lithium plating at the negative electrode.

[0065] The usual method to reduce lithium plating is to use a small rate, such as 0.01C ~ 0.1C, to charge multiple times to reduce the polarization of the battery and thus reduce lithium plating. However, in terms of technology, small rate charging takes a long time, seriously affects production capacity, and cannot meet market demand.

[0066] In view of this, the present application provides a secondary battery formation method and a secondary battery to solve the problems of black spots and negative electrode lithium deposition usually caused by lithium supplements in the related art.

[0067] First, as Figure 1 As shown, the present application provides a secondary battery formation method, which adopts a negative pressure formation process and includes the following steps:

[0068] S100, charging the secondary battery to a first voltage;

[0069] S200, charging the secondary battery at a first rate so that the voltage of the secondary battery rises from a first voltage to a second voltage;

[0070] S300, charging the secondary battery at a second rate so that the secondary battery is maintained at a second voltage;

[0071] S400, charging the secondary battery at a third rate so that the secondary battery is maintained at a second voltage;

[0072] Wherein, the first magnification is greater than the second magnification, and the second magnification is greater than the third magnification;

[0073] The first voltage is less than the reaction voltage threshold of the lithium supplement agent and less than the second voltage.

[0074] The present application adopts a constant voltage charging process with a gradient drop rate in the voltage interval above the reaction voltage threshold of the lithium supplement, that is, the reaction stage of the lithium supplement, and utilizes the characteristics that the first rate is greater than the second rate, and the second rate is greater than the third rate, so that the lithium supplement lasts longer at the reaction voltage, the lithium supplement reacts more fully, and the lithium precipitation on the negative electrode surface is reduced. Through the negative pressure formation process, it can help to discharge the gas generated by the lithium supplement during the reaction and reduce black spots. The reaction of the lithium supplement is related to the voltage value. Usually, the reaction voltage threshold of the lithium supplement is 3.8V. The reaction starts at a voltage above 3.8V. The larger the voltage value, the more thorough the reaction of the lithium supplement, and the voltage value is related to polarization. The larger the polarization, the higher the voltage value generated, and the polarization is related to the rate. The higher the rate, the greater the polarization. Therefore, the largest first rate among the three rates can make the lithium supplement react quickly, and the second voltage of the secondary battery can be maintained by using the second rate and the third rate that are smaller than the first rate, which can make the secondary battery maintain the second voltage for a longer time, and the negative electrode is more fully embedded with lithium, thereby reducing the lithium precipitation of the negative electrode. Usually the second voltage is in an overcharge state, and its function is to allow the lithium supplement agent to fully react. The lithium ions released by the lithium supplement agent need a certain amount of lithium insertion time for the negative electrode. Maintaining the second voltage of the secondary battery through constant-voltage charging at the second rate and the third rate can provide time for the negative electrode to insert lithium, making the lithium insertion more complete and reducing lithium plating at the negative electrode.

[0075] It should be noted that the negative pressure forming process means that at least one of the various steps of the forming process uses negative pressure, rather than requiring that every step uses negative pressure.

[0076] In combination with the first aspect, in some embodiments provided in the present application, the first ratio is 0.5C to 1C.

[0077] In combination with the first aspect, in some embodiments provided in the present application, the second ratio is 0.4C to 0.8C.

[0078] In combination with the first aspect, in some embodiments provided in the present application, the third ratio is 0.3C to 0.5C.

[0079] At the same cut-off voltage, the greater the charging rate, the less capacity is charged, and the smaller the charging rate, the more capacity is charged. The reaction of the lithium supplement is determined by the battery voltage. The higher the voltage, the more complete the reaction of the lithium supplement. However, the larger the charging capacity, the more serious the lithium plating at the negative electrode. Therefore, when considering the lithium plating of the lithium supplement, a higher voltage and less capacity are required to achieve the full reaction of the lithium supplement and reduce lithium plating. The large rate of 0.5C to 1C allows the second voltage to be reached quickly, and the lithium supplement reacts quickly. At the same time, compared with the low rate of 0.1C, the capacity charged into the battery can be reduced, reducing lithium plating. By reducing the rate, the voltage of the secondary battery can be maintained at the second voltage for a longer time, which can provide time for lithium insertion at the negative electrode and further reduce lithium plating.

[0080] Since at the same cut-off voltage, the greater the rate, the less capacity is charged into the battery, and in order to maintain the high voltage time, the charging rate is reduced in a step-by-step manner, the battery can continue to be charged, and the time to maintain the second voltage, i.e., the constant voltage, is increased, providing time for the negative electrode to insert lithium, reducing lithium precipitation, and making the lithium supplement agent react more fully, releasing more lithium ions, and increasing the battery capacity. In addition, charging at a high rate can also speed up the entire charging time, using polarization to increase the voltage so that the lithium supplement agent reacts quickly, while reducing lithium precipitation, while improving the performance of the secondary battery, improving efficiency, and increasing production capacity.

[0081] In combination with the first aspect, in some embodiments provided in the present application, the first voltage is 3.65-3.75V. The main purpose of the first voltage is to form an SEI film on the surface of the negative electrode, provide a lithium insertion depth for the negative electrode, and reduce lithium precipitation at the beginning of the formation stage. The lithium supplement agent basically does not react at this stage. Preferably, the first voltage is 3.7V, and the SEI film is more uniform.

[0082] In combination with the first aspect, in some embodiments provided in the present application, the second voltage is 4V to 4.1V. The main purpose of the second voltage is to allow the lithium supplement agent to release lithium ions to supplement the secondary battery. At this stage, the negative electrode has basically completed most of the lithium insertion, and the remaining lithium ions released by the lithium supplement agent are embedded in the negative electrode during the charging process, thereby increasing the battery capacity. Preferably, the second voltage is 4.05V.

[0083] In combination with the first aspect, in some embodiments provided in the present application, such as Figure 2 As shown, before charging the secondary battery to the first voltage, the step further includes:

[0084] S001, charging the secondary battery to a fourth voltage at a fourth rate;

[0085] S002, charging the secondary battery to a fifth voltage at a fifth rate;

[0086] S003, charging the secondary battery to a sixth voltage at a sixth rate;

[0087] Wherein, the fourth magnification is smaller than the fifth magnification, and the fifth magnification is smaller than the sixth magnification;

[0088] The fourth voltage is lower than the fifth voltage, and the fifth voltage is lower than the sixth voltage.

[0089] By increasing the voltage and the rate in a step-by-step manner, the negative electrode can be slowly intercalated with lithium and lithium deposition at the negative electrode can be reduced. The voltage is increased step-by-step from the fourth voltage to the sixth voltage, and the rate is increased step-by-step from the fourth rate to the sixth rate. Different rates can be matched in voltage ranges with different formation gas production rates to slow down formation gas production and help form a uniform and dense SEI film.

[0090] In combination with the first aspect, in some embodiments provided in the present application, the fourth ratio is 0.05C to 0.2C.

[0091] In combination with the first aspect, in some embodiments provided in the present application, the fifth ratio is 0.1C to 0.5C.

[0092] In combination with the first aspect, in some embodiments provided in the present application, the sixth ratio is 0.1C to 1C.

[0093] In combination with the first aspect, in some embodiments provided in the present application, the fourth voltage is 3V to 3.2V, and preferably, the fourth voltage is 3.1V.

[0094] In combination with the first aspect, in some embodiments provided in the present application, the fifth voltage is 3.3V to 3.5V, and preferably, the fifth voltage is 3.4V.

[0095] In combination with the first aspect, in some embodiments provided in the present application, the sixth voltage is 3.65V to 3.75V, and preferably, the sixth voltage is 3.7V.

[0096] The purpose of charging the secondary battery to the fourth voltage at the fourth rate is to activate the negative electrode. In this formation voltage range, the formation reaction is the most intense, and it is also the initial stage of SEI film formation. A lower rate of charging is required to reduce the gas production rate and form a uniform SEI film. Therefore, the fourth rate is 0.05C~0.2C, and the fourth voltage is 3V~3.2V, which is the transition voltage of the formation gas production rate. Before this voltage, the formation gas production rate is high, and after this voltage, the formation gas production rate gradually decreases. The purpose of charging the secondary battery to the fifth voltage at the fifth rate is to continue the formation reaction to form the SEI film. At this time, the SEI film has been initially formed and stabilized after the fourth voltage, and the gas production rate is gradually reduced. At this time, the charging rate can be gradually increased. Therefore, the fifth rate is 0.1C~0.5C, and the fifth voltage is 3.3V~3.5V, which is the end voltage of formation gas production. Before this voltage, there is still gas production during formation, and after this voltage, the gas production is basically finished. The purpose of charging the secondary battery to the sixth voltage at the sixth rate is to continuously charge and increase the voltage to a stage close to the lithium replenisher reaction in preparation for the lithium replenisher reaction. Therefore, the sixth rate is 0.1C to 1C, and the sixth voltage is 3.65V to 3.75V, which is the full charge voltage of the lithium iron phosphate battery / graphite, and is slightly lower than the lower limit voltage of the lithium replenisher reaction.

[0097] In combination with the first aspect, in some embodiments provided in the present application, such as Figure 3 As shown, before charging the secondary battery to the first voltage, the step further includes:

[0098] S004, discharging the secondary battery to a seventh voltage at a seventh rate;

[0099] S005, charging the secondary battery to a first voltage at an eighth rate;

[0100] S006 , repeating the discharging and charging between the seventh voltage and the first voltage.

[0101] Repeated charging and discharging between the seventh voltage and the first voltage can increase the lithium insertion depth of the negative electrode and reduce the lithium precipitation problem caused by the inability to insert lithium ions released by the subsequent lithium supplement reaction.

[0102] In combination with the first aspect, in some embodiments provided in the present application, the seventh ratio is 0.1C to 1C.

[0103] In combination with the first aspect, in some embodiments provided in the present application, the eighth ratio is 0.5C to 1C.

[0104] In combination with the first aspect, in some embodiments provided in the present application, the seventh voltage is 2.5V to 2.7V, and preferably, the seventh voltage is 2.6V.

[0105] In combination with the first aspect, in some embodiments provided in the present application, the number of repetitions is greater than or equal to three times.

[0106] The secondary battery is charged to a sixth voltage at a sixth rate, discharged to a seventh voltage at a seventh rate, and charged to a first voltage at an eighth rate, that is, charged to 3.65V to 3.75V at 0.1C to 1C, discharged to 2.5V to 2.7V at 0.1C to 1C, and charged to 3.65V to 3.75V at 0.5C to 1C. This arrangement allows slow charging, increased lithium insertion depth, fast discharge, reduced time consumption, improved formation efficiency, and reduced lithium insertion.

[0107] In combination with the first aspect, in some embodiments provided in the present application, such as Figure 4 As shown, after charging the secondary battery at the third rate to maintain the secondary battery at the second voltage, the method further includes:

[0108] S500, discharging the secondary battery to an eighth voltage at a ninth rate;

[0109] S600 , charging the secondary battery at a ninth rate to a ninth voltage.

[0110] In combination with the first aspect, in some embodiments provided in the present application, the ninth ratio is 0.5C to 1C.

[0111] In combination with the first aspect, in some embodiments provided in the present application, the eighth voltage is 2.5V to 2.7V, and preferably, the eighth voltage is 2.6V.

[0112] In combination with the first aspect, in some embodiments provided in the present application, the ninth voltage is 3.3V to 3.5V, and preferably, the ninth voltage is 3.4V.

[0113] The secondary battery is charged at a third rate to maintain the second voltage, and then discharged at a ninth rate to an eighth voltage, and then charged at a ninth rate to a ninth voltage, that is, charged to 4V to 4.1V at 0.3C to 0.5C, and then discharged to 2.5V to 2.7V at 0.5C to 1C, and then charged to 3.3V to 3.5V at 0.5C to 1C. This setting can eliminate the internal polarization of the battery and repair the SEI film structure damaged by charging to the second voltage.

[0114] In combination with the first aspect, in some embodiments provided in the present application, the gas pressure in the gas production stage adopts a high-low pressure alternating cycle, the low pressure is -100KPa to -30KPa, and the high pressure is -30KPa to 30KPa, wherein the gas production stage includes:

[0115] charging the secondary battery to a fourth voltage at a fourth rate;

[0116] charging the secondary battery to a fifth voltage at a fifth rate;

[0117] Charging the secondary battery at a first rate so that the voltage of the secondary battery increases from a first voltage to a second voltage;

[0118] charging the secondary battery at a second rate so that the secondary battery is maintained at a second voltage;

[0119] The secondary battery is charged at the third rate so as to be maintained at the second voltage.

[0120] The battery formation process includes a formation gas production stage and a lithium supplement reaction gas production stage. The formation gas production stage is the aforementioned charging of the secondary battery to the fourth voltage stage at the fourth rate and the charging of the secondary battery to the fifth voltage stage at the fifth rate; the lithium supplement reaction gas production stage is the aforementioned charging of the secondary battery from the first voltage to the second voltage stage at the first rate, charging at the second rate to keep the secondary battery at the second voltage stage, and charging at the third rate to keep the secondary battery at the second voltage stage. It can be understood that in the battery formation process, each charging step includes a shelving step after each charging step, so, to be precise, the formation gas production stage includes charging the secondary battery to the fourth voltage stage at the fourth rate, a shelving stage, charging the secondary battery to the fifth voltage stage at the fifth rate, and a shelving stage; the lithium supplement reaction gas production stage includes charging the secondary battery from the first voltage to the second voltage stage at the first rate, a shelving stage, charging the secondary battery at the second rate to keep the secondary battery at the second voltage stage, a shelving stage, charging the secondary battery at the third rate to keep the secondary battery at the second voltage stage, and a shelving stage. In the above-mentioned gas production stage and lithium supplement reaction gas production stage, the gas pressure adopts high and low pressure alternating cycle, wherein the low pressure is -100KPa~-30KPa and the high pressure is -30KPa~30KPa. The high and low pressure cycle can promote the timely discharge of the gas generated in the gas production stage and the lithium supplement reaction gas production stage, and reduce the impact of gas residue on the battery.

[0121] In combination with the first aspect, in some embodiments provided in the present application, a clamp is used to restrain the secondary battery during charging, wherein the restraining force is 100Kgf to 1000Kgf. The battery can be clamped by a clamp tooling, and in the formation step, a restraining force of 100Kgf to 1000Kgf is applied to the battery, which can reduce the occurrence of black spots and lithium precipitation caused by the excessive gap between the pole pieces caused by the gas generated during the formation process. At the same time, appropriate restraining force helps to exhaust gas. In addition, the presence of the clamp can also shape the battery to make the lithium insertion more uniform and avoid excessive battery thickness.

[0122] In the second aspect, the present application proposes a secondary battery obtained by the secondary battery formation method described in the first invention. The secondary battery has all the technical solutions of the secondary battery formation method, and thus also has all the beneficial effects of the secondary battery formation method, which will not be described in detail in this application.

[0123] The technical solution provided by the present application is described in detail below in conjunction with the embodiments. The process parameters of embodiments 1 to 3 are shown in Table 1. In the entire formation stage, embodiments 1 to 3 all apply a restraining force of 500Kgf on the secondary battery. Steps 2-9 are the formation gas production stage, and steps 18-29 are the lithium supplement agent reaction gas production stage; the formation gas production stage and the lithium supplement agent reaction gas production stage, that is, steps 2-9 and steps 18-29 use high and low pressure alternating cycles to promote exhaust.

[0124] Table 1 Formation process parameters of the secondary batteries of Examples 1 to 3

[0125]

[0126]

[0127] Comparative Example 1

[0128] Comparative Example 1 is substantially the same as Example 1, except that the charging rates in step 18, step 22 and step 26 are equal, namely, 0.4C.

[0129] Comparative Example 2

[0130] Comparative Example 2 is substantially the same as Example 1, except that the charging rates in step 18, step 22 and step 26 are increased successively, namely 0.3C, 0.4C and 0.5C.

[0131] Comparative Example 3

[0132] Comparative Example 3 is substantially the same as Example 1, except that the voltage in step 18, step 22 and step 26 is 3.65V.

[0133] Comparative Example 4

[0134] Comparative Example 4 is basically the same as Example 1, except that the voltages in step 18, step 22 and step 26 are 3.7V, 4.0V and 4.1V respectively; step 18, step 22 and step 26 use small rate charging, and decrease in sequence, which are 0.1C, 0.05C and 0.02C respectively.

[0135] Performance Test:

[0136] The secondary batteries obtained by the formation methods of the secondary batteries of the above-mentioned Examples 1-3 and Comparative Examples 1-4 were subjected to performance tests, and the specific test methods were as follows:

[0137] Discharge capacity test: The secondary batteries obtained by the formation methods of the secondary batteries of the above-mentioned embodiments 1-3 and comparative examples 1-4 were placed at a constant temperature of 25°C for 6 hours, charged to 3.65V at a rate of 1C, and discharged to 2.5V at a rate of 1C after being placed at a constant temperature of 25°C for 10 minutes to obtain a 1C discharge capacity.

[0138] Cycle performance test: step 1. Place the secondary battery obtained by the formation method of the secondary battery of the above embodiments 1-3 and comparative examples 1-4 at a constant temperature of 25°C for 6 hours, step 2. Charge to 3.65V at a rate of 1C, step 3. Place for 10 minutes, step 4. Discharge to 2.5V at a rate of 1C, step 5. Place for 10 minutes, step 6. Cycle step 2 to step 5.

[0139] Lithium precipitation and black spot inspection: After the secondary batteries obtained by the formation methods of the secondary batteries of the above-mentioned Examples 1-3 and Comparative Examples 1-4 were divided into different capacities, they were charged and discharged 10 times at a rate of 1C, and then fully charged and disassembled to inspect the interface.

[0140] The results are shown in Table 2.

[0141] Table 2 Performance test results of Examples 1 to 3 and Comparative Examples 1 to 4

[0142]

[0143] After the secondary battery of Example 2 is treated by the formation process, its surface is as follows Figure 5 As shown in Table 2, it can be seen that Examples 1 to 3 do not precipitate lithium, and their discharge capacity and cycle performance are better than Comparative Examples 1 to 4.

[0144] In comparative example 1, since the charging rates in steps 18, 22 and 26 are the same, and in comparative example 2, since the charging rates in steps 18, 22 and 26 are increased successively, due to the influence of polarization, specifically, the polarization under high rate conditions is greater than the polarization under low rate conditions, resulting in that the capacity cannot be basically charged in the latter two charging steps, step charging is not achieved in the lithium replenisher reaction stage, the lithium replenisher activation reaction is not sufficient, and the battery performance is not greatly improved. The reaction continues to occur in the subsequent process, resulting in lithium precipitation at the interface and the generation of black spots.

[0145] In Comparative Example 3, since the voltage in steps 18, 22 and 26 is 3.65V, which does not reach the voltage threshold of the lithium supplement reaction, the lithium supplement does not react at all, which is equivalent to a battery without lithium supplementation. Therefore, no lithium precipitation will occur on the interface and no black spots will be generated, but the battery performance is not improved.

[0146] In Comparative Example 4, since the voltages in step 18, step 22, and step 26 are 3.7V, 4.0V, and 4.1V, respectively, and step 18, step 22, and step 26 are charged at a small rate, and are successively decreased to 0.1C, 0.05C, and 0.02C, respectively, the lithium supplement agent reaction stage is not reached during charging in step 18, and the lithium supplement agent reaction is only performed in step 22 and step 26. At the same time, since the charging cut-off voltage of step 22 is only 4.0V, the lithium supplement agent reaction is not complete, so the lithium supplement agent reaction in Comparative Example 4 is not complete, which is similar to Comparative Example 2, and the battery electrical performance is not fully improved. At the same time, since the lithium supplement agent that has not reacted completely will continue to partially react in the subsequent process, causing interface lithium precipitation. At the same time, since the charging rates are 0.1C, 0.05C, and 0.02C, respectively, the formation time is long, which affects the efficiency of the entire formation and affects the production capacity.

[0147] In summary, it can be seen that a constant voltage charging process with a gradient rate reduction is adopted in the voltage interval above the reaction voltage threshold of the lithium supplement, that is, in the reaction stage of the lithium supplement. By utilizing the characteristics that the first rate is greater than the second rate, and the second rate is greater than the third rate, the lithium supplement lasts longer at the reaction voltage, the lithium supplement reacts more fully, and the lithium precipitation on the negative electrode surface is reduced. Through the negative pressure formation process, the gas generated by the lithium supplement during the reaction can be discharged to reduce black spots. The reaction of the lithium supplement is related to the voltage value. Usually, the lithium supplement starts to react at a voltage above 3.8V. The larger the voltage value, the more thorough the reaction of the lithium supplement, and the voltage value is related to polarization. The greater the polarization, the higher the voltage generated, and the polarization is related to the rate. The higher the rate, the greater the polarization. Therefore, the use of the largest first rate among the three rates can make the lithium supplement react quickly, and the use of the second rate and the third rate that are smaller than the first rate to maintain the second voltage of the secondary battery can make the secondary battery maintain the second voltage for a longer time, and the negative electrode is more fully embedded with lithium, thereby reducing the lithium precipitation of the negative electrode. Usually the second voltage is in an overcharge state, and its function is to allow the lithium supplement agent to fully react. The lithium ions released by the lithium supplement agent need a certain amount of lithium insertion time for the negative electrode. Maintaining the second voltage of the secondary battery through constant-voltage charging at the second rate and the third rate can provide time for the negative electrode to insert lithium, making the lithium insertion more complete and reducing lithium plating at the negative electrode.

[0148] In the description of this specification, the description with reference to the terms "one embodiment / method", "some embodiments / methods", "example", "specific example" or "some examples" etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment / method or example are included in at least one embodiment / method or example of the present application. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment / method or example. Moreover, the specific features, structures, materials or characteristics described may be combined in a suitable manner in any one or more embodiments / methods or examples. In addition, those skilled in the art may combine and combine the different embodiments / methods or examples described in this specification and the features of the different embodiments / methods or examples, unless they are contradictory.

[0149] It should be noted that, in the present application, relational terms such as "first" and "second" are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply that there is any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that the process, method, article or equipment including a series of elements includes not only those elements, but also includes other elements not explicitly listed, or also includes elements inherent to such process, method, article or equipment. In the absence of more restrictions, the elements defined by the sentence "comprise one..." do not exclude the existence of other identical elements in the process, method, article or equipment including the elements. In the present application, the meaning of "multiple" is at least two, such as two, three, etc., unless otherwise clearly and specifically stipulated.

[0150] The above description is only a specific implementation of the present application, so that those skilled in the art can understand or implement the present application. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to the embodiments shown herein, but will conform to the widest range consistent with the principles and novel features applied for herein.

Claims

1. A secondary battery formation method, characterized in that: The negative pressure forming process includes the following steps: charging the secondary battery to a first voltage; Charging the secondary battery at a first rate so that the voltage of the secondary battery increases from a first voltage to a second voltage; charging the secondary battery at a second rate so that the secondary battery is maintained at a second voltage; charging the secondary battery at a third rate so that the secondary battery is maintained at a second voltage; Wherein, the first magnification is greater than the second magnification, and the second magnification is greater than the third magnification; The first voltage is less than the reaction voltage threshold of the lithium supplement agent and less than the second voltage.

2. The secondary battery formation method according to claim 1, wherein: The first magnification is 0.5C to 1C; and / or, The second magnification is 0.4C to 0.8C; and / or, The third ratio is 0.3C to 0.5C.

3. The secondary battery formation method according to claim 1, wherein: The first voltage is 3.65-3.75V; and / or, The second voltage is 4V to 4.1V.

4. The secondary battery formation method according to claim 1, characterized in that: Before charging the secondary battery to the first voltage, the method further includes: charging the secondary battery to a fourth voltage at a fourth rate; charging the secondary battery to a fifth voltage at a fifth rate; charging the secondary battery to a sixth voltage at a sixth rate; Wherein, the fourth magnification is smaller than the fifth magnification, and the fifth magnification is smaller than the sixth magnification; The fourth voltage is lower than the fifth voltage, and the fifth voltage is lower than the sixth voltage.

5. The secondary battery formation method according to claim 4, characterized in that: The fourth magnification is 0.05C to 0.2C; and / or, The fifth magnification is 0.1C to 0.5C; and / or, The sixth magnification is 0.1C to 1C; and / or, The fourth voltage is 3V to 3.2V; and / or, The fifth voltage is 3.3V to 3.5V; and / or, The sixth voltage is 3.65V to 3.75V.

6. The secondary battery formation method according to claim 4, characterized in that: Before charging the secondary battery to the first voltage, the method further includes: discharging the secondary battery to a seventh voltage at a seventh rate; charging the secondary battery to a first voltage at an eighth rate; The discharge and charge between the seventh voltage and the first voltage are repeated.

7. The secondary battery formation method according to claim 6, wherein: The seventh magnification is 0.1C to 1C; and / or, The eighth magnification is 0.5C to 1C; and / or, The seventh voltage is 2.5V to 2.7V; and / or, The number of repetitions is greater than or equal to three times.

8. The secondary battery formation method according to claim 4, characterized in that: The gas pressure in the gas production stage adopts high and low pressure alternating cycle, the low pressure is -100KPa~-30KPa, and the high pressure is -30KPa~30KPa; wherein, the gas production stage includes: charging the secondary battery to a fourth voltage at a fourth rate; charging the secondary battery to a fifth voltage at a fifth rate; Charging the secondary battery at a first rate so that the voltage of the secondary battery increases from a first voltage to a second voltage; charging the secondary battery at a second rate so that the secondary battery is maintained at a second voltage; The secondary battery is charged at the third rate so as to be maintained at the second voltage.

9. The secondary battery formation method according to claim 1, characterized in that: During charging, a clamp is used to restrain the secondary battery, wherein the restraining force is 100Kgf to 1000Kgf.

10. A secondary battery, characterized in that: Obtained by the secondary battery formation method according to any one of claims 1 to 9.

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