Formation method and liquid injection method of lithium ion battery and lithium ion battery

Abstract

The present application relates to the technical field of lithium ion batteries, and particularly relates to a formation method of a lithium ion battery, a liquid injection method of the lithium ion battery and the lithium ion battery, the formation method comprising the following steps: S1, taking unformed battery cells of the same batch, performing test formation under constant current charging at a small current to obtain a dQ / dV-V curve diagram; S2, adopting low-frequency constant current charging at a small current to activate the battery cells to be formed; S3, based on the peak, valley and peak top of the dQ / dV-V curve diagram obtained in step S1, performing segmentation, and then performing multi-current step formation on the battery cells to be formed; S4, after completing the last peak, performing constant current charging on the battery cells to be formed at n times of the formation current of the last step in step S3, and stopping charging after reaching the cut-off voltage. The formation method provided in the present application makes the SEI film composition uniform and the electrochemical properties more stable, so that the lithium ion battery has excellent storage performance and rate performance.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion batteries, in particular to a formation method, a liquid injection method and a lithium ion battery. BACKGROUND

[0002] Formation is a key step in the later stage process of lithium ion battery manufacturing, and is often associated with liquid injection, especially in the design of segmented liquid injection scheme, that is, the first injection of electrolyte contains film-forming additives and other components, and the later injection of electrolyte contains additives with functions such as low temperature, high temperature, high pressure, flame retardant, and electrical conductivity. The formation film quality in the early stage and the cycle performance in the later stage are ensured. However, the current formation step design is generally set by experience, without considering the voltage range of the new type of additive in the electrolyte and the appropriate current density, resulting in the same peak in different steps during multi-step formation, that is, there are phase reactions of the same substance under different current densities in the battery formation, which leads to different reaction conditions and different reaction products, resulting in unstable SEI film, thereby making the storage performance and rate performance of the battery poor. SUMMARY

[0003] In view of the above-mentioned deficiencies in the prior art, the purpose of the present application is to provide a formation method, a liquid injection method and a lithium ion battery. The present application adopts dQ / dV pre-prepared peak for segmented formation during formation, which can ensure the same peak and current, make the SEI film formation reaction conditions the same, and generate SEI film with uniform composition and more stable electrochemical properties, thereby making the lithium ion battery have excellent storage performance and rate performance.

[0004] In order to achieve the above-mentioned purpose, the present application adopts the following technical scheme:

[0005] In a first aspect, the present application provides a formation method of a lithium ion battery, which comprises the following steps:

[0006] S1, taking the same batch of unformed cells, performing test formation under small current constant current charging to obtain a dQ / dV-V curve;

[0007] S2, empty pulse activation: using small current constant current low frequency charging to activate the cells to be formed;

[0008] S3, intermittent peak charging: based on the peak valley and peak top of the dQ / dV-V curve obtained in step S1, the cells to be formed are segmented and subjected to multi-current step formation to ensure the consistency of the same peak reaction conditions;

[0009] S4, planned charging termination: after the last peak is completed, the to-be-formed battery cell is charged at n times of the formation current of the last step in step S3, and charging is stopped after reaching the cut-off voltage, wherein n is not less than 1.

[0010] The application can ensure the same peak and current by adopting the dQ / dV pre-peak segmented formation during formation, so that the SEI film forming reaction conditions are the same, the generated SEI film composition is uniform, and the electrochemical properties are more stable, thereby making the lithium ion battery have excellent storage performance and rate performance.

[0011] Further, the to-be-formed battery cell is a square cell.

[0012] And / or, the positive active material of the to-be-formed battery cell includes at least one of lithium iron phosphate, ternary positive material and lithium iron manganese phosphate.

[0013] And / or, the negative active material of the to-be-formed battery cell includes at least one of graphite, silicon-based material and hard carbon material.

[0014] Further, in step S1, the current of the test formation is 0.02C-0.05C, and constant current charging is performed to 40%-60% SOC.

[0015] And / or, in step S2, a small current constant current low frequency charging of 0.001-0.05C is adopted, the frequency of low frequency charging is 1-10Hz, and the total charging time is 10s-120s.

[0016] And / or, in step S3, the principle of determining the charging rate and cut-off voltage of each segment of the segmented formation includes:

[0017] Peak-valley rate principle: in the peak-free segment of the dQ / dV-V curve, the peak-free segment starts from the peak-free point and passes through the first peak valley of the first peak, the charging rate I1 of the peak-free segment is determined by the peak strength of the first peak valley of the first peak, the I1 is calculated by formula (1), and the cut-off voltage of the peak-free segment is the voltage corresponding to the peak strength of the first peak valley of the first peak.

[0018] Peak top rate principle: in the peak segment, a segment starts from the peak valley, passes through the peak top of the peak, and then reaches the peak valley of the peak, the charging rate I2 of the segment is determined by the peak strength of the peak top, the I2 is calculated by formula (2), and the cut-off voltage of the segment is the voltage corresponding to the peak strength of the second peak valley of the peak.

[0019] Principle of peak interval: if the interval between the peak top of the first peak and the peak top of the last peak is not more than the threshold value, the charging rate of the interval is determined by the charging rate of the first peak according to formula (2), and the cut-off voltage of the interval is the voltage corresponding to the peak strength of the last peak;

[0020] Principle of termination rate: the charging rate of the last peak is determined according to formula (1) based on the peak valley in front of the last peak, and the cut-off voltage is the same as that in step S4;

[0021] Principle of interval rate: when there is no peak at the beginning, the charging rate is determined according to formula (1) based on the first peak valley, and when a multi-peak interval is charged at a rate, the rate of the interval is determined according to formula (2) based on the peak top of the first peak;

[0022] I1 = (peak valley strength × voltage value corresponding to the peak valley strength ÷ design capacity of the to-be-formation battery cell × charging coefficient α) 0.5 (1);

[0023] I2 = (peak top strength × voltage value corresponding to the peak top strength ÷ design capacity of the to-be-formation battery cell × charging coefficient β) 0.5 (2);

[0024] Wherein, the peak valley strength is the value of the ordinate dQ / dV at the peak valley position, which is obtained by testing with a testing device; the peak top strength is the value of the ordinate dQ / dV at the peak top position, which is obtained by testing with a testing device; the charging coefficient α is 0.02-0.2, the charging coefficient β is 0.01-0.1, and the design capacity is the capacity calculated from the mass of the positive active material and the theoretical specific capacity.

[0025] Further, the threshold value is not more than 1.0V;

[0026] And / or, the charging coefficients α of different steps are the same, and the charging coefficients β of different steps are the same.

[0027] Further, in step S3, the to-be-formation battery cell is placed after each formation, the temperature and pressure of the placement are the same as the temperature and pressure of the next formation, and the time of the placement is 2-4min;

[0028] And / or, in step S4, the cut-off voltage is the standard cut-off voltage of the to-be-formation battery cell, which is determined by the positive active material of the to-be-formation battery cell, for example, when the positive active material is lithium iron phosphate, the cut-off voltage is 3.3V, and when the positive active material is ternary positive material or lithium iron manganese phosphate, the cut-off voltage is 3.9V;

[0029] And / or, in step S4, the to-be-formation battery cell is rested after formation, the temperature and pressure of the resting are the same as the temperature and pressure of the formation in step S4, and the time of the resting is 2-4 min.

[0030] And / or, in the formation of the to-be-formation battery cell, the injection port of the to-be-formation battery cell is subjected to negative pressure suction, and the large surface of the to-be-formation battery cell is heated and pressurized.

[0031] Further, the injection port (12) of the to-be-formation battery cell is subjected to negative pressure suction by using the negative pressure pipeline (5) in the liquid injection and supplementing device, the liquid injection and supplementing device further comprises an air chamber (8), the air chamber (8) is arranged at the connection between the negative pressure pipeline (5) and the injection port (12), and the air chamber (8) contains electrolyte (9) during the formation of the to-be-formation battery cell, so that the electrolyte in the to-be-formation battery cell is supplemented.

[0032] And / or, in the formation of the to-be-formation battery cell, the negative pressure range of the negative pressure pipeline (5) is-100 to-500 kPa.

[0033] And / or, the diameter of the air chamber (8) is 2-5 times the diameter of the negative pressure pipeline (5).

[0034] And / or, the to-be-formation battery cell is heated by using a heating plate (2), and the heating plate (2) is surrounded by a silica gel pad (1).

[0035] Further, in step S2, the to-be-formation battery cell is heated and pressurized, the heating temperature is 60-90℃, and the pressurizing pressure is 150-200 kgf.

[0036] And / or, in step S3, the to-be-formation battery cell is heated and pressurized, the heating temperature is 50-90℃, and the pressurizing pressure is 100-200 kgf.

[0037] And / or, in step S4, the to-be-formation battery cell is heated and pressurized, the heating temperature is less than the heating temperature in step S3, the heating temperature is 35-50℃, and the pressurizing pressure is 50-150 kgf.

[0038] In a second aspect, the application provides a liquid injection method, which comprises the following steps:

[0039] S1, determining the original weight M1 of a to-be-injected battery cell;

[0040] S2, injecting a first injection electrolyte into the to-be-injected battery cell, and then standing to obtain a to-be-formation battery cell;

[0041] S3, performing formation on the to-be-formed battery cell by using the formation method in the first aspect;

[0042] S4, after the formation, standing, then determining the weight M2 of the battery cell after the formation, determining the amount of the first injection electrolyte based on the M1 and the M2, determining the amount of the post-injection electrolyte based on the amount of the first injection electrolyte and the post-injection coefficient.

[0043] Further, the determining the original weight M1 of the to-be-injected battery cell comprises: encapsulating the prepared dry battery cell core package in the designed battery cell shell, after passing the welding and helium detection process and meeting the standards, performing vacuum drying and weighing, and recording as M1.

[0044] And / or, in step S2, the standing temperature is 50-130℃, and the standing time is 4-24h.

[0045] And / or, in step S4, the standing temperature is 50-80℃, and the standing time is 4-24h.

[0046] And / or, in step S4, the amount of the first injection electrolyte m1=M2-M1, the post-injection coefficient x is 0.05-0.2, and the amount of the post-injection electrolyte m2=xm1.

[0047] In a third aspect, the present application provides a lithium ion battery, which is obtained by the formation method of the lithium ion battery in the first aspect or obtained by the injection method in the second aspect.

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

[0049] (1) The present application can ensure the same peak and current by using dQ / dV preformed peak for segmented formation during formation, so that the SEI film forming reaction conditions are the same, the generated SEI film composition is uniform, and the electrochemical properties are more stable, thereby making the lithium ion battery have excellent storage performance and rate performance.

[0050] (2) The amount of the first injection electrolyte is determined according to the amount of the absorbed electrolyte after formation, and the amount of the post-injection electrolyte is determined based on the same, and the electrolyte is sufficient during the formation process, which can accurately control the demand amount of the battery cell for the electrolyte and the amount of the additive at each stage, ensure the uniformity of the added electrolyte composition, and improve the function of the additive components in the electrolyte.

[0051] (3) The formation conditions are adjusted in stages with the formation steps, which can ensure that the formation process is smoother and meets the mechanical and thermal characteristics, and is more conducive to adjusting the properties of the SEI film, i.e., loose or dense.

[0052] (4) Using the air chamber to inject liquid, not only can reduce the negative pressure and electrolyte immersion impact on the core package in the first injection, to achieve the effect of buffering adjustment, but also can keep the core package always immersed in electrolyte to inhibit the air path during the formation stage, and will not lack of liquid due to negative pressure absorption too much electrolyte and affect the formation effect.

[0053] (5) Using the combination of silica gel and heating plate, which not only increases the friction force on the battery cell, but also takes into account the characteristics of the battery cell, which is thick in the middle and thin on the edge, so that the battery cell is evenly stressed during the formation process. BRIEF DESCRIPTION OF DRAWINGS

[0054] In order to more clearly illustrate the specific embodiments of the present application or the technical solutions in the prior art, the following will briefly introduce the drawings needed to be used in the specific embodiments or prior art description. Obviously, the drawings described below are some embodiments of the present application, and those skilled in the art can also obtain other drawings according to these drawings without creative labor.

[0055] Figure 1 The peak valley and peak top positioning diagram of the dQ / dV-V curve diagram described in the present application;

[0056] Figure 2 The multi-peak rate principle diagram based on the dQ / dV-V curve diagram described in the present application;

[0057] Figure 3 The combination diagram of the silica gel pad and the heating plate described in the present application;

[0058] Figure 4 The combination diagram of the liquid injection and liquid supplement device and the battery to be formed described in the present application.

[0059] Figure: 1-silica gel pad; 2-heating plate; 3-vacuum valve; 4-sealing rod; 5-negative pressure pipeline; 6-liquid supplement port; 7-liquid supplement valve; 8-air chamber; 9-electrolyte; 10-positive electrode; 11-negative electrode; 12-liquid injection port; 13-explosion-proof valve. DETAILED DESCRIPTION

[0060] In order to make the purpose, technical scheme and advantages of the present application more clear, the following will combine the embodiments of the present application to clearly and completely describe the technical scheme in the embodiments of the present application. Those skilled in the art should understand that the described embodiments are only to help understand the present application, and should not be regarded as a specific limitation on the present application. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application. The process parameters not specified in the following examples are usually according to the conventional conditions.

[0061] The endpoints of the ranges and any values disclosed in the present application are not limited to the precise values recited. The endpoints of the ranges and values are approximations that are understood to encompass values approximating these ranges and values. For example, a range from 1 to 10 should be interpreted to include not only the precise ranges from 1 to 10, but also other ranges that are approximately equivalent, such as from 0.9 to 10.1, or from 0.95 to 10.05, or from 1.1 to 9.9, etc. Similarly, a value disclosed from 1 to 10 should be interpreted to include not only the precise values of 1 to 10, but also other values that are approximately equivalent, such as from 0.9 to 10.1, or from 0.95 to 10.05, or from 1.1 to 9.9, etc.

[0062] In a first aspect, the present application provides a formation method of a lithium ion battery, the formation method comprising the following steps:

[0063] S1, taking unformed cells of the same batch, performing test formation under small current constant current charging to obtain a dQ / dV-V curve;

[0064] S2, empty pulse activation: using small current constant current low frequency charging to activate the cells to be formed;

[0065] S3, intermittent peak charging: as shown in the formula (1), based on the peak and valley and peak top of the dQ / dV-V curve obtained in step S1, segmenting, and then performing multi-current step formation on the cells to be formed to ensure the consistency of the same peak reaction condition; Figure 1

[0066] S4, planned charging termination: after completing the last peak, performing constant current charging on the cells to be formed at n times of the formation current of the last step in step S3, and stopping charging after reaching the cut-off voltage, wherein the n is not less than 1.

[0067] The present application can ensure the same peak and current by using the dQ / dV preformed peak for segmented formation during formation, so that the SEI film formation reaction conditions are the same, the generated SEI film composition is uniform, and the electrochemical properties are more stable, thereby making the lithium ion battery have excellent storage performance and rate performance.

[0068] The dQ / dV-V curve in step S1 is obtained by testing with a formation device, and this step is equivalent to pre-formation, so as to determine the peak type state of the cells of the batch.

[0069] In the formation method of the lithium ion battery, as an optional implementation manner, the cells to be formed are square cells.

[0070] In the formation method of the lithium ion battery, as an optional implementation manner, the positive active material of the cells to be formed includes at least one of lithium iron phosphate, ternary positive electrode material and lithium iron manganese phosphate.

[0071] In the formation method of the lithium ion battery, as an optional implementation manner, the negative active material of the cells to be formed includes at least one of graphite, silicon-based material and hard carbon material.​

[0072] As an optional embodiment of the formation method of the lithium ion battery, in step S1, the current of the test formation is 0.02C-0.05C (for example, it can be 0.02C, 0.03C, 0.04C or 0.05C), and the constant current charging is to 40%-60% SOC (for example, it can be 40% SOC, 45% SOC, 50% SOC, 55% SOC or 60% SOC).

[0073] As an optional embodiment of the formation method of the lithium ion battery, in step S2, a small current of 0.001-0.05C (for example, it can be 0.001C, 0.01C, 0.02C, 0.03C, 0.04C or 0.05C) is used for low-frequency constant current charging, the frequency of the low-frequency charging is 1-10Hz (for example, it can be 1Hz, 3Hz, 5Hz, 7Hz or 10Hz), and the total charging time is 10s-120s (for example, it can be 10s, 20s, 40s, 60s, 80s, 100s or 120s). The pulse current activation uses a small current of 0.001-0.05C for low-frequency charging to open the stable conduction path and lay the foundation for the subsequent formation.

[0074] As an optional embodiment of the formation method of the lithium ion battery, in step S3, the principle of segmenting and determining the charging rate and the cut-off voltage of each segment includes:

[0075] Peak-valley rate principle: in the peak-free segment of the dQ / dV-V curve, the peak-free segment starts from the peak-free point and goes through the first peak valley of the first peak, the charging rate I1 of the peak-free segment is determined by the peak strength of the first peak valley of the first peak, the I1 is calculated by formula (1), and the cut-off voltage of the peak-free segment is the voltage corresponding to the peak strength of the first peak valley of the first peak;

[0076] Peak-top rate principle: in the peak segment, a segment is taken from the peak valley to the peak top of the peak and then to the peak valley of the peak, the charging rate I2 of the segment is determined by the peak strength of the peak top, the I2 is calculated by formula (2), and the cut-off voltage of the segment is the voltage corresponding to the peak strength of the second peak valley of the peak;

[0077] Multi-peak rate principle: as shown in Figure 2 if the peak tops of the multi-peaks are separated by a voltage not more than a threshold value, the charging rate of the interval is determined by the rate of the first peak top of the multi-peaks according to formula (2), and the cut-off voltage of the interval is the voltage corresponding to the peak strength of the last peak valley of the interval;

[0078] Termination rate principle: the charge rate corresponding to the last peak is determined according to formula (1) based on the peak valley in front of it, and the cut-off voltage is the same as that in step S4;

[0079] Interval rate principle: when there is no peak at the beginning, the rate is determined according to formula (1) based on the first peak valley (i.e. after the empty pulse is activated, all intermittent charging processes start with the rate corresponding to the first peak valley), and when a multi-peak interval is charged at a rate, the rate determined according to formula (2) based on the peak of the first peak is used as the rate of the interval;

[0080] I1= (peak valley peak strength x peak valley peak strength corresponding voltage value ÷ design capacity of the to-be-formed battery x charging coefficient a) 0.5 (1);

[0081] I2= (peak peak strength x peak peak strength corresponding voltage value ÷ design capacity of the to-be-formed battery x charging coefficient b) 0.5 (2);

[0082] Wherein, the peak valley peak strength is the value of the ordinate dQ / dV at the peak valley position, which is obtained by testing with a testing device; the peak peak strength is the value of the ordinate dQ / dV at the peak position, which is obtained by testing with a testing device; the charging coefficient a is 0.02-0.2 (for example, it can be 0.02, 0.04, 0.06, 0.08, 0.1, 0.15 or 0.2), the charging coefficient b is 0.01-0.1 (for example, it can be 0.01, 0.03, 0.05, 0.07 or 0.1), and the design capacity is the capacity calculated from the mass of the positive active material and its theoretical specific capacity, which is generally 10 Ah-800 Ah (for example, it can be 10 Ah, 100 Ah, 200 Ah, 400 Ah, 600 Ah or 800 Ah).

[0083] In the above formation method of the lithium ion battery, as an optional embodiment, the threshold value is not greater than 1.0 V, for example, the threshold value can be 1.0 V, 0.8 V.

[0084] In the above formation method of the lithium ion battery, as an optional embodiment, the charging coefficient a of different steps is the same, and the charging coefficient b of different steps is the same. The same charging coefficient is used for different stages to achieve the purpose of coordinated change.

[0085] In the above formation method of the lithium ion battery, as an optional embodiment, in step S4, the n is 1-1.5.

[0086] As an optional embodiment of the formation method of the lithium ion battery, in step S3, the to-be-formed battery cell is rested after each formation, the temperature and pressure of the resting are the same as the temperature and pressure of the next formation, and the resting time is 2-4 min. In the present application, the pressure and temperature are adjusted during the resting time to meet the requirements of the next formation conditions.

[0087] As an optional embodiment of the formation method of the lithium ion battery, in step S4, the cut-off voltage is the standard cut-off voltage of the to-be-formed battery cell, which is determined by the positive active material of the to-be-formed battery cell. For example, when the positive active material is lithium iron phosphate, the cut-off voltage is 3.3 V, and when the positive active material is ternary positive material or lithium iron manganese phosphate, the cut-off voltage is 3.9 V.

[0088] As an optional embodiment of the formation method of the lithium ion battery, in step S4, the to-be-formed battery cell is rested after formation, the temperature and pressure of the resting are the same as the temperature and pressure of the formation in step S4, and the resting time is 2-4 min.

[0089] As an optional embodiment of the formation method of the lithium ion battery, in the process of forming the to-be-formed battery cell, negative pressure suction is used for the liquid injection port of the to-be-formed battery cell, and the large surface of the to-be-formed battery cell is heated and pressurized.

[0090] As an optional embodiment of the formation method of the lithium ion battery, as shown in Figure 4 As an optional embodiment of the formation method of the lithium ion battery, as shown in

[0091] The liquid injection is a very important step in the process of making the battery cell. The traditional one-time liquid injection is according to the design capacity and the experience given injection coefficient, which can ensure the uniformity of the injection composition, but cannot effectively improve the utilization rate and function of various additives in the electrolyte, and the different densities of different electrolytes and the different liquid absorption of the core package will also cause the inapplicability of the experience coefficient. The popular phased liquid injection is also according to the design capacity and the given injection coefficient, but there are negative pressure formation operations in the middle, which leads to the electrolyte in the battery cell being absorbed and the electrolyte being insufficient during the formation, resulting in the poor quality of the SEI film generated. If the liquid supplement is carried out after the formation, the simple liquid supplement method (only the first injection or the post-injection) will cause the difference of the electrolyte composition contained in each battery cell, thereby adversely affecting the consistency of the battery. If the first injection and the post-injection have the same formula, the function and utilization rate of the components in the electrolyte will be greatly reduced.

[0092] The present application can effectively improve the utilization rate and function of various additives in the electrolyte by using the gas chamber liquid injection, and can also solve the technical problem of the poor quality of the SEI film generated due to the insufficient electrolyte during the formation in the phased liquid injection, and can improve the consistency of the battery.

[0093] According to the capacity and volume of the battery cell, different sizes of the gas chamber 8 are selected. In the formation method of the above lithium ion battery, as an optional implementation manner, the volume of the gas chamber 8 is 50-800 mL, for example, which can be 50 mL, 100 mL, 200 mL, 300 mL, 400 mL, 600 mL or 800 mL.

[0094] In the formation method of the above lithium ion battery, as an optional implementation manner, the negative pressure range of the negative pressure pipeline 5 is-100 to-500 kPa during the formation of the battery cell to be formed, for example, which can be 100 kPa, 200 kPa, 300 kPa, 400 kPa or 500 kPa.

[0095] In the formation method of the above lithium ion battery, as an optional implementation manner, the diameter of the gas chamber 8 is 2-5 times the diameter of the negative pressure pipeline 5, for example, which can be 2 times, 3 times, 4 times or 5 times.

[0096] In the formation method of the above lithium ion battery, as an optional implementation manner, in step S2, the battery cell to be formed is heated and pressurized, the heating temperature is 60-90℃ (for example, which can be 60℃, 70℃, 80℃ or 90℃), and the pressurization pressure is 150-200 kgf (for example, which can be 150 kgf, 170 kgf or 200 kgf).

[0097] As an optional embodiment of the formation method of the lithium ion battery, in step S3, the to-be-formed battery cell is heated and pressurized, the heating temperature is 50-90℃ (for example, it can be 50℃, 52℃, 54℃, 56℃, 60℃, 70℃ or 90℃), and the pressurizing pressure is 100-200kgf (for example, it can be 100kgf, 150kgf, 170kgf, 190kgf or 200kgf). Preferably, in the step when the voltage does not reach the platform voltage, the formation pressure is 150-200kgf, at this time, the charging SOC is relatively small, the expansion force of the pole piece is small, the high pressure can make the gas quickly escape from the battery cell in the reaction gas production stage, and increase the compactness of the SEI film formation, improve the film formation quality and produce a close connection with the negative electrode, at this time, the current is increased, and the temperature can be slightly reduced to save energy; in the step when the voltage reaches the platform voltage, the formation pressure is 100-150kgf, and the pressure in this step is small, which is beneficial to relieve the pole piece expansion caused by charging, and form a dense middle layer SEI film, so as to make the SEI film on the negative electrode side dense and improve the storage performance.

[0098] As an optional embodiment of the formation method of the lithium ion battery, in step S4, the to-be-formed battery cell is heated and pressurized, the heating temperature is less than the heating temperature in step S3, the heating temperature is 35-50℃ (for example, it can be 35℃, 40℃, 45℃ or 50℃), and the pressurizing pressure is 50-150kgf (for example, it can be 50kgf, 100kgf or 150kgf). When the planned charging termination stage, the heating temperature range is 35-50℃, and the pressure is 50-150kgf, at this time, the SOC is increased, the pole piece expansion is large, and the loose outer layer SEI film can be formed in this stage when the pressure is low, providing more lithium ion attachment sites, at this time, the current is further increased, and the temperature can be slightly reduced to save energy.

[0099] As an optional embodiment of the formation method of the lithium ion battery, as shown in Figure 3 The heating plate 2 is used to heat the to-be-formed battery cell, and the four sides of the heating plate 2 are provided with silica gel pads 1. The silica gel and heating plate combination form can increase the friction force on the battery cell.

[0100] As an optional embodiment of the formation method of the lithium ion battery, the silica gel pad 1 can match the condition that the thickness of the pole piece thinning area of the to-be-formed battery cell is smaller. The silica gel and heating plate combination form can increase the friction force on the battery cell and also take into account the characteristics of the battery cell, that is, the middle thickness and the edge thinness, so that the battery cell is uniformly stressed during the formation process.

[0101] As an optional implementation form of the formation method of the lithium ion battery, the thickness of the silica gel pad 1 is 2-4 mm, for example, 2 mm, 2.5 mm, 3 mm, 3.5 mm or 4 mm.

[0102] In a second aspect, the application provides a liquid injection method, which comprises the following steps:

[0103] S1, determining an original weight M1 of a to-be-injected battery;

[0104] S2, injecting a first injection electrolyte into the to-be-injected battery, and then standing to obtain a to-be-formed battery;

[0105] S3, forming the to-be-formed battery by using the formation method of the first aspect;

[0106] S4, after the formation is completed, standing, and then determining a weight M2 of the battery after the formation is completed, determining an amount of the first injection electrolyte based on the M1 and the M2, and determining an amount of a post-injection electrolyte based on the amount of the first injection electrolyte and a post-injection coefficient.

[0107] The application determines the amount of the first injection electrolyte according to the liquid absorption amount after the formation, and determines the amount of the post-injection electrolyte, so that the electrolyte is sufficient during the formation, the demand amount of the battery for the electrolyte and the amount of the additive at each stage can be accurately controlled, the added electrolyte components are uniform, and the function exertion efficiency of the additive components in the electrolyte is improved.

[0108] As an optional implementation form of the liquid injection method, the determination of the original weight M1 of the to-be-injected battery comprises: packaging a prepared dry battery core package in a designed battery shell, performing welding and helium detection processes and reaching the standard, and then performing vacuum drying and weighing again, which is recorded as M1. In the embodiment of the application, the purpose of weighing is to find abnormal phenomena in the assembly process, to avoid weight errors and subsequent effects caused by packaging problems, and to record the original weight M1 of the battery without electrolyte.

[0109] As an optional implementation form of the liquid injection method, in step S2, the standing temperature is 50-130℃ (for example, 50℃, 70℃, 90℃, 110℃ or 130℃), and the standing time is 4-24h (for example, 4h, 8h, 12h, 16h, 20h or 24h).

[0110] As an optional implementation form of the liquid injection method, in step S4, the standing temperature is 50-80℃ (for example, 50℃, 60℃, 70℃ or 80℃), and the standing time is 4-24h (for example, 4h, 8h, 12h, 16h, 20h or 24h).

[0111] In the above injection method, as an optional embodiment, in step S4, the amount of the first injection of electrolyte m1 = M2 - M1, the post-injection coefficient x is 0.05-0.2 (for example, it can be 0.05, 0.1, 0.15, 0.2), and the amount of the post-injection of electrolyte m2 = xm1.

[0112] In a third aspect, the present application provides a lithium ion battery, which is obtained by the formation method of the lithium ion battery according to the first aspect or obtained by the injection method according to the second aspect.

[0113] In the above lithium ion battery, as an optional embodiment, the lithium ion battery comprises a square lithium ion battery.

[0114] The present application will be further described in detail below in combination with specific examples and comparative examples.

[0115] In the following examples and comparative examples:

[0116] 1. The positive active material of the cell is lithium iron phosphate, and the negative active material is artificial graphite.

[0117] 2. Peak-valley rate principle: in the peak-free section of the dQ / dV-V curve, the first peak-valley from the beginning of the peak-free section to the first peak is the peak-free section, the charge rate I1 of the peak-free section is determined by the peak intensity of the first peak-valley, the I1 is calculated by formula (1), and the cut-off voltage of the peak-free section is the voltage corresponding to the peak intensity of the first peak-valley;

[0118] Peak-top rate principle: in the peak section, a section is taken from the peak-valley to the peak-top of the peak and then to the peak-valley of the peak, the charge rate I2 of the section is determined by the peak intensity of the peak-top, the I2 is calculated by formula (2), and the cut-off voltage of the section is the voltage corresponding to the peak intensity of the second peak-valley of the peak;

[0119] Multi-peak rate principle: if the multi-peaks overlap or the peak-tops of the multi-peaks are separated by a voltage of no more than 1V, the charge rate of the interval is determined by the rate determined according to formula (2) for the first peak-top of the multi-peaks, and the cut-off voltage of the interval is the voltage corresponding to the peak intensity of the last peak-valley of the interval;

[0120] Termination rate principle: the charge rate corresponding to the last peak is determined according to formula (1) from the peak-valley in front of it, and the cut-off voltage is the standard cut-off voltage of the cell to be formed;

[0121] Interval rate principle: when there is no peak at the beginning, the rate is determined according to formula (1) based on the first peak-valley, that is, after the empty pulse is activated, all intermittent charging is started with the rate corresponding to the first peak-valley; when there are multiple peaks in the interval, the rate determined according to formula (2) based on the peak of the first peak is used as the rate of the interval;

[0122] I1=(peak-valley peak strength x peak-valley peak strength corresponding voltage value ÷ design capacity of the to-be-formation battery x charging coefficient a) 0.5 (1);

[0123] I2=(peak peak strength x peak peak strength corresponding voltage value ÷ design capacity of the to-be-formation battery x charging coefficient b) 0.5 (2);

[0124] Wherein, the peak-valley peak strength is the value of the ordinate dQ / dV at the peak-valley position, which is obtained by testing equipment; the peak peak strength is the value of the ordinate dQ / dV at the peak position, which is obtained by testing equipment; the charging coefficient a is 0.02-0.2, the charging coefficient b is 0.01-0.1, and the design capacity is the capacity calculated from the mass of the positive active material and the theoretical specific capacity, which is generally 10Ah-800Ah.

[0125] 3. According to GB / T 31486-2015 standard, the storage and rate performance test is carried out, in the storage performance test, the capacity retention rate = the charged capacity / room temperature discharge capacity; the capacity recovery rate = the charged recovery capacity / room temperature discharge capacity; in the rate performance test, the 4C charge retention rate = the 4C charge capacity / 1C charge capacity; the 4C discharge retention rate = the 4C discharge capacity / 1C discharge capacity; the 2C charge retention rate = the 2C charge capacity / 1C charge capacity; the 2C discharge retention rate = the 2C discharge capacity / 1C discharge capacity.

[0126] Example 1: Preparation of a square battery with a volume of 1000ml and a capacity of 140Ah

[0127] The preparation method of the square battery comprises the following steps:

[0128] S1, weighing preparation: put the dry battery core package with uniform weight specification and meeting the requirements in the square battery shell, after welding, helium detection and other processes and meeting the standards, vacuum drying and weighing record as M1.

[0129] S2, first injection of electrolyte: select 500ml gas chamber of injection liquid supplement device, the diameter of the gas chamber is 4 times of the diameter of the negative pressure pipeline. First open the vacuum valve, keep the sealing rod raised, close the supplement valve, slowly vacuumize the battery and the gas chamber to-100kPa; then close the vacuum valve, open the supplement valve, keep the sealing rod closed, drive the injection of type A electrolyte into the gas chamber, the injection volume is 400ml; finally, close the supplement valve and confirm the vacuum valve is closed, slowly open the sealing rod to make the type A electrolyte enter the inside of the battery under the driving of pressure difference. After keeping inhaling and infiltrating for 20min, close the sealing rod, and the excess electrolyte is sucked into the liquid supplement bottle through the supplement valve. Finally, seal the injection port with sealing glue nails, and stand still at 80℃ for 18h.

[0130] S3, formation process: take the unformed battery of the same batch (for the same group scheme) to test formation by charging at a small current of 0.05C to 50% SOC, and finally obtain a dQ / dV-V curve. Peak and segment the dQ / dV-V curve to find peak valley G1 (1.5V, 0.207Ah / V), G2 (1.9V, 0.87Ah / V), G3 (3.2V, 50Ah / V), peak top D1 (1.76V, 2.87Ah / V), D2 (2.43V, 11.77Ah / V), D3 (3.24V, 410.77Ah / V), according to the peak valley rate, peak top rate, multi-peak rate and termination rate principle, the formation step interval is composed of three segments (0.1-1.5V), (1.5-3.2V), (3.2-3.3V), and α=0.2, β=0.1, then the rate of the three step intervals is 0.02C, 0.06C and 0.5C in turn according to the calculation formulas (1) and (2), and the cutoff voltage is 1.5V, 3.2V and 3.3V respectively. The gas chamber and the injection port are kept connected during the formation, after obtaining the negative pressure state of the battery and the gas chamber through the vacuum valve, some electrolyte is injected from the supplement port to make the gas chamber contain a small amount of electrolyte to ensure that the electrolyte can completely infiltrate the pole piece after the formation is completed. The vacuum valve is always in the open state, the sealing rod is in the raised state, and the supplement valve is in the closed state except during the supplement, a heating plate is used to heat the battery to be formed, a silica gel pad with a thickness of 2.5mm is arranged around the heating plate, and the silica gel pad can match the condition that the pole piece thinning area of the battery to be formed is smaller. The formation steps and conditions obtained according to the application are shown in Table 1, and the battery to be formed is formed according to the formation steps and conditions shown in Table 1.

[0131] Table 1

[0132]

[0133] S3, post-injection of electrolyte: after the formation is completed, the electrolyte in the gas chamber and its negative pressure pipeline is recovered, and the injection port is blocked with a rubber plug. The battery cell is placed in a high-temperature chamber for 12 h at a temperature of 50°C. Then the battery cell is taken out of the high-temperature chamber, and all the free electrolyte in the battery cell is pumped out. At this time, the weight of the battery cell is M2, and the actual liquid content m1 = M2 - M1 is obtained. The post-injection is performed at a ratio of first injection: post-injection = 9:1, and the post-injection amount m2 = 1 / 9 m1. Finally, the injection port is sealed, and the formation and injection are completed.

[0134] Then, after the equalization, the storage and rate performance test is performed. The capacity retention rate of the battery cell is 97.6% and the recovery rate is 98.5% after being stored at 100% SOC for 28 days at 25°C. The 4C rate discharge retention rate is 92.1%, and the 4C rate charge retention rate is 95.2%.

[0135] Comparative Example 1: Preparation of a square battery cell with a volume of 1000 ml and a capacity of 140 Ah

[0136] The preparation method of the square battery cell includes the following steps:

[0137] S1, weighing preparation: the dry battery cell core package of the same batch, uniform weight specification and meeting the requirements is placed in a square battery cell shell. After welding, helium detection and other processes and meeting the standards, vacuum drying is performed and the weight is recorded as M1.

[0138] S2, first injection of electrolyte: the injection is completed by using a negative pressure pressure maintaining cycle multiple injection method, and the injection amount is controlled at m1. Then the injection port is sealed with a sealing glue nail, and the battery cell is placed at a temperature of 80°C for 18 h.

[0139] S3, formation process: the unformed battery cell of the same batch (for the same group scheme) is taken, and the formation is performed according to the conventional method. The formation steps and conditions are as follows:

[0140] Step Step name Charge rate Duration Formation temperature Negative pressure formation Cut-off condition 1 Rest / 30s / / / 2 Charge 0.05C 60 min 45℃ -45 kPa 60 min 3 Rest / 2 min 45℃ -45 kPa 2 min 4 Charge 0.15C 120 min 45℃ -45 kPa 120 min 5 Rest / 2 min 45℃ -45 kPa 2 min 6 End

[0141] S3, post-injection of electrolyte: after the formation is completed, the injection port is blocked with a rubber plug, and the battery cell is placed in a high-temperature chamber for 12 h at a temperature of 50°C. Then the battery cell is taken out of the high-temperature chamber, and the post-injection is performed at a ratio of first injection: post-injection = 9:1, that is, the post-injection amount m2 = 1 / 9 m1. Finally, the injection port is sealed, and the formation and injection are completed.

[0142] Then, after the equalization, the storage and rate performance test is performed. The capacity retention rate of the battery cell is 92.3% and the recovery rate is 94.3% after being stored at 100% SOC for 28 days at 25°C. The 4C rate discharge retention rate is 89.7%, and the 4C rate charge retention rate is 91.6%.

[0143] Example 2: Preparation of a square battery cell with a volume of 150 ml and a capacity of 20 Ah

[0144] The preparation method of the square-shaped battery cell comprises the following steps:

[0145] S1, weighing preparation: the dry battery cell core package of the same batch weight specification is uniformly placed into the square-shaped battery cell shell, and after welding, helium detection and other processes and reaching the standard, vacuum drying is performed and the weight is recorded as M1.

[0146] S2, first injection of electrolyte: an electrolyte injection and supplementing device with a 80ml gas chamber is selected, the diameter of the gas chamber is 2 times the diameter of the negative pressure pipeline. First, the vacuum valve is opened, the sealing rod is lifted, the injection valve is closed, the battery cell and the gas chamber are slowly vacuumed to-80kPa; then the vacuum valve is closed, the injection valve is opened, the sealing rod is closed tightly, the B-type electrolyte is driven to be injected into the gas chamber, the injection volume is 60ml; finally, the injection valve is closed and the vacuum valve is confirmed to be closed, the sealing rod is slowly opened, the B-type electrolyte is driven into the battery cell under the pressure difference. After 10 minutes of suction and infiltration, the sealing rod is closed, and the excess electrolyte is sucked into the residual liquid bottle through the injection valve. Finally, the injection port is sealed with sealing glue nails, and is placed at a temperature of 50℃ for 4 hours.

[0147] S3, formation process: the unformed battery cell of the same batch (for the same group scheme) is tested for formation by charging at a small current of 0.05C to 50% SOC, and finally a dQ / dV-V curve diagram is obtained. The dQ / dV-V curve diagram is peak and segmented, and peak valleys G1 (2.5V, 1.89Ah / V) and G2 (3.18V, 7.28Ah / V) are found, peak tops D1 (2.6V, 13.23Ah / V) and D2 (3.23V, 56.37Ah / V) are found, according to the peak valley rate, the peak top rate, the multi-peak rate and the termination rate principle, the step interval is composed of two segments (0.1-2.5V) and (2.5-3.3V), α=0.05 and β=0.02 are given, the rates of the two step intervals are 0.11C and 0.18C respectively according to the calculation formulas (1) and (2), and the cut-off voltages are 2.5V and 3.3V respectively. During the formation, the gas chamber is connected with the injection port, after the negative pressure state of the battery cell and the gas chamber is obtained through the vacuum valve, a certain amount of electrolyte is injected from the supplementing port, so that a small amount of electrolyte is contained in the gas chamber to ensure that the electrolyte can completely infiltrate the pole piece after the formation is completed. From the beginning to the end of the formation, the vacuum valve is always in the open state, the sealing rod is in the lifted state, except that the supplementing valve is in the closed state during the supplementing, a heating plate is used to heat the battery cell to be formed, a silica gel pad with a thickness of 2.5mm is arranged around the heating plate, and the silica gel pad can match the pole piece thinning area with a smaller thickness. The formation steps and conditions obtained according to the application are shown in Table 2, and the battery cell to be formed is formed according to the formation steps and conditions shown in Table 2.

[0148] Table 2

[0149]

[0150] S3, post-injection electrolyte: after the formation is completed, the electrolyte in the gas chamber and its pipeline is recovered, and the injection port is blocked with a rubber plug. The battery cell is placed in a high-temperature chamber for 12 h at a temperature of 50°C. Then, the battery cell is taken out of the high-temperature chamber, and all the free electrolyte in the battery cell is pumped out. The weight of the battery cell obtained at this time is M2. The actual liquid content m1 = M2 - M1 is obtained. The post-injection is performed at a ratio of first injection: post-injection = 8: 1, and the post-injection liquid amount m2 = 1 / 8 m1. Finally, the injection port is sealed, and thus the formation and injection are completed.

[0151] Then, after the capacity test, the storage and rate performance test are performed. The capacity retention rate of the battery cell is 98.1% at 25°C for 28 days at 100% SOC, the recovery rate is 98.5%, the 4C rate discharge retention rate is 93.7%, and the 4C rate charge retention rate is 96.1%.

[0152] Example 3: Preparation of a square battery cell with a volume of 4800 ml and a capacity of 628 Ah

[0153] The preparation method of the square battery cell includes the following steps:

[0154] S1, weighing preparation: the dry battery cell package of the same batch, uniform weight specification and meeting the requirements, is placed in a square battery cell shell. After welding, helium detection and other processes and meeting the requirements, vacuum drying is performed, and the weight is recorded as M1.

[0155] S2, first injection of electrolyte: an injection liquid supplementing device with a gas chamber of 800 ml is selected, and the diameter of the gas chamber is 5 times the diameter of the negative pressure pipeline. First, the vacuum valve is opened, the sealing rod is lifted, the supplementing valve is closed, and the battery cell and the gas chamber are slowly vacuumed to-200 kPa. Then, the vacuum valve is closed, the supplementing valve is opened, the sealing rod is closed tightly, and the C-shaped electrolyte is driven to be injected into the gas chamber. The injection volume is 700 ml. Finally, the supplementing valve is closed, and the vacuum valve is confirmed to be closed. The sealing rod is slowly opened, so that the C-shaped electrolyte enters the inside of the battery cell under the driving of the pressure difference. The above operation is repeated 3 times to ensure that the electrolyte is fully injected into the battery cell. After 10 min of absorption and infiltration, the sealing rod is closed, and the excess electrolyte is pumped into the residual liquid bottle through the supplementing valve. Finally, the injection port is sealed with sealing glue nails, and the battery cell is placed at a temperature of 130°C for 24 h.

[0156] S3, formation process: take the same batch of unformed battery (for the same group of scheme), with a small current of 0.05C constant current charging to 50% SOC for test formation, finally get dQ / dV-V curve. Peak and segment the dQ / dV-V curve, find the peak valley G1 (1.65V, 2.24Ah / V), G2 (2.48V, 80.38Ah / V), G3 (2.8V, 241.54Ah / V), find the peak top D1 (1.85V, 50.24Ah / V), D2 (2.63V, 322.79Ah / V), D3 (3.25V, 1932.31Ah / V), according to the peak valley rate, the peak top rate and the multi-peak rate and the termination rate principle, the step interval is composed of three segments (0.1-1.65V), (1.65-2.8V), (2.8-3.3V), α=0.02, β=0.01, the rate of the three step intervals is 0.01C, 0.04C, 0.15C in turn according to the calculation formula (1) and (2), and the cutoff voltage is 1.65V, 2.8V and 3.3V respectively. During the formation, the gas chamber and the liquid injection port are connected, after obtaining the negative pressure state of the battery and the gas chamber through the vacuum valve, a certain amount of electrolyte is injected from the liquid supplement port, so that a small amount of electrolyte is contained in the gas chamber to ensure that the electrolyte can completely soak the pole piece after the formation is completed. From the beginning to the end of the formation, the vacuum valve is always in the open state, the sealing rod is in the lifted state, except that the liquid supplement valve is in the closed state during the liquid supplement, the heating plate is used to heat the battery to be formed, the heating plate is provided with a silica gel pad with a thickness of 2.5mm around the heating plate, and the silica gel pad can match the pole piece thinning area with smaller thickness of the battery to be formed. The formation steps and conditions obtained according to the application are shown in table 3, and the battery to be formed is formed according to the formation steps and conditions shown in table 3.

[0157] Table 3

[0158]

[0159] S3, post-injection electrolyte: after the formation is completed, the electrolyte in the gas chamber and its pipeline is recovered, and the liquid injection port is blocked with a rubber plug. The battery is placed in a high temperature chamber for 12h at a temperature of 50℃. Then the battery is taken out of the high temperature chamber, and all the free electrolyte in the battery is pumped out. The weight of the battery obtained at this time is M2, and the actual liquid content m1=M2-M1 is obtained. The post-injection is carried out in a ratio of first injection:post-injection=7:1, and the post-injection amount m2=1 / 7m1. Finally, the liquid injection port is sealed, and the formation and liquid injection are completed.

[0160] After that, after the capacity is divided, the storage and rate performance test is carried out. The capacity retention rate of the battery is 98.3% and the recovery rate is 98.6% after being stored at 25℃ for 28 days at 100% SOC, the 2C rate discharge retention rate is 95.7%, and the 2C rate charge retention rate is 96.5%.

[0161] Preparation of a square battery cell with a volume of 1000 ml of 140 Ah

[0162] The preparation method of the square battery cell was basically the same as that of Example 1, except that the cut-off voltage was 1.76 V in the second formation step.

[0163] After the capacity distribution, storage and rate performance tests were performed, the capacity retention rate of the battery cell was 97.7% and the recovery rate was 98.0% when stored at 100% SOC at 25°C for 28 days, the 4C rate discharge retention rate was 90.8%, and the 4C rate charge retention rate was 93.1%.

[0164] Preparation of a square battery cell with a volume of 1000 ml of 140 Ah

[0165] The preparation method of the square battery cell was basically the same as that of Example 1, except that the charge rate was 0.04C in the second formation step.

[0166] After the capacity distribution, storage and rate performance tests were performed, the capacity retention rate of the battery cell was 95.6% and the recovery rate was 96.5% when stored at 100% SOC at 25°C for 28 days, the 4C rate discharge retention rate was 91.5%, and the 4C rate charge retention rate was 94.8%.

[0167] Preparation of a square battery cell with a volume of 1000 ml of 140 Ah

[0168] The preparation method of the square battery cell was basically the same as that of Example 1, except that the pressure was 100 kgf in the 7th step, and the 8th and 9th steps were not included.

[0169] After the capacity distribution, storage and rate performance tests were performed, the capacity retention rate of the battery cell was 95.3% and the recovery rate was 96.2% when stored at 100% SOC at 25°C for 28 days, the 4C rate discharge retention rate was 91.0%, and the 4C rate charge retention rate was 94.1%.

[0170] Compared with Example 1, when S4, the planned charge termination step, is not included, both the storage performance and the rate performance decrease, and the applicant speculates that the reason may be that after charging to the cut-off voltage at high pressure, the voltage is too high, that is, the formation charge capacity decreases, and the film is relatively thin.

[0171] Preparation of a square battery cell with a volume of 1000 ml of 140 Ah

[0172] The preparation method of the square battery cell was basically the same as that of Example 1, except that the charge rate was 0.1C in the 8th step.

[0173] After the capacity distribution, storage and rate performance test, the capacity retention rate of the battery cell is 97.9% and the recovery rate is 98.5% when the battery cell is stored at 25℃ with 100% SOC for 28 days, the 4C rate discharge retention rate is 91.3%, and the 4C rate charge retention rate is 93.8%.

[0174] Compared with Example 1, when the formation current of the planned charging termination step is less than the formation current of the last step in step S3, the storage performance is equivalent, but the rate performance deteriorates. The applicant speculates that the reason may be that the reduction of the charging current in the later stage of formation slows down the film formation reaction, and the film formation becomes dense. Such SEI film is similar to the model of first dense, then loose, and then dense again. The uppermost layer is too dense to be conducive to the rate performance, and the comprehensive performance is not good. In addition, the execution time of the step in the present comparative example is shorter, and the deterioration is not obvious. If the execution time of the step is longer in actual application, the deterioration will be more obvious.

[0175] Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present application, and not to limit them; although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that: it can still modify the technical solutions recorded in the foregoing embodiments, or make equivalent replacement to part or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A method for forming a lithium-ion battery, characterized in that, The formation method includes the following steps: S1. Take unformed cells from the same batch and perform experimental formation by constant current charging under low current to obtain the dQ / dV-V curve. S2. Empty-charge pulse activation: Low-frequency constant current charging with a small current is used to activate the battery cell to be formed. S3. Intermittent peak charging: Based on the peaks and valleys and peaks of the dQ / dV-V curve obtained in step S1, the cells to be formed are segmented, and then multi-current step formation is performed on the cells to be formed to ensure the consistency of the reaction conditions at the same peak. S4. Planned charging termination: After the last peak is completed, the cell to be formed is charged with constant current at n times the formation current of the last step in step S3. Charging is stopped after the cutoff voltage is reached, where n is not less than 1. In step S3, the principles for segmenting the process and determining the charging rate and cutoff voltage for each segment's corresponding formation step include: Peak-valley ratio principle: In the peakless segment of the dQ / dV-V curve, the first peak-valley after climbing from the peakless segment to the first peak is the peakless segment. The charging ratio I1 of the peakless segment is determined by the peak intensity of the first peak-valley of the first peak. I1 is calculated by formula (1). The cutoff voltage of the peakless segment is the voltage corresponding to the peak intensity of the first peak-valley of the first peak. Peak-top ratio principle: In a peak segment, the segment is defined as the point from the peak to the valley of the peak. The charging ratio I2 of this segment is determined by the peak strength of the peak. I2 is calculated by formula (2). The cutoff voltage of this segment is the voltage corresponding to the peak strength of the second valley of the peak. Multi-peak rate principle: If multiple peaks overlap or the voltage between the peaks of multiple peaks does not exceed the threshold, then the charging rate of this interval is determined by the rate of the first peak in the multiple peaks according to formula (2), and the cutoff voltage of this interval is the voltage corresponding to the peak strength of the last valley of this interval. Termination rate principle: The charging rate corresponding to the last peak is determined by the preceding peaks and valleys according to formula (1), and the cutoff voltage is the same as the cutoff voltage in step S4. Interval multiplier principle: When there is no peak at the beginning, charge according to the multiplier determined by the first peak and valley according to formula (1). When a multi-peak interval is charged according to a multiplier, the multiplier determined by the first peak according to formula (2) is used as the multiplier of the interval. I1 = (peak-valley peak intensity × voltage value corresponding to peak-valley peak intensity ÷ design capacity of the cell to be formed × charging coefficient α) 0.5 (1); I2 = (Peak intensity × Voltage value corresponding to peak intensity ÷ Design capacity of the cell to be formed × Charging coefficient β) 0.5 (2); Among them, the peak intensity is the value of dQ / dV at the ordinate of the peak and valley positions, which is obtained by testing equipment; the peak intensity is the value of dQ / dV at the ordinate of the peak position, which is obtained by testing equipment; the charging coefficient α is 0.02~0.2, the charging coefficient β is 0.01~0.1, and the design capacity is the capacity calculated from the mass of the positive electrode active material and its theoretical specific capacity.

2. The formation method of a lithium-ion battery according to claim 1, characterized in that, The battery cell to be formed is a square battery cell; And / or, the positive electrode active material to be formed into a battery cell includes at least one of lithium iron phosphate, ternary positive electrode material and lithium manganese iron phosphate; And / or, the negative electrode active material to be formed into a battery cell includes at least one of graphite, silicon-based materials, and hard carbon materials.

3. The formation method of a lithium-ion battery according to claim 1, characterized in that, In step S1, the current generated in the test is 0.02C-0.05C, and the constant current is used to charge to 40%-60% SOC.

4. The formation method of a lithium-ion battery according to claim 1, characterized in that, The threshold value is no greater than 1.0V; And / or, the charging coefficient α is the same for different steps, and the charging coefficient β is the same for different steps.

5. The formation method of a lithium-ion battery according to claim 1, characterized in that, In step S3, after each formation step, the cell to be formed is placed for rest. The temperature and pressure of the resting are the same as those of the next formation step, and the resting time is 2 to 4 minutes. And / or, in step S4, the cutoff voltage is the standard cutoff voltage of the cell to be formed, which is determined by the positive electrode active material of the cell to be formed. When the positive electrode active material is lithium iron phosphate, the cutoff voltage is 3.3V; when the positive electrode active material is ternary positive electrode material or lithium manganese iron phosphate, the cutoff voltage is 3.9V. And / or, in step S4, after formation, the cell to be formed is placed for rest, the temperature and pressure of the rest are the same as the temperature and pressure of formation in step S4, and the rest time is 2 to 4 minutes; And / or, during the formation process of the battery cell to be formed, the liquid injection port of the battery cell to be formed is subjected to negative pressure suction to heat and pressurize the large surface of the battery cell to be formed.

6. The formation method of a lithium-ion battery according to claim 5, characterized in that, The negative pressure pipe (5) in the liquid injection and replenishment device is used to draw air into the liquid injection port (12) of the cell to be formed. The liquid injection and replenishment device also includes an air chamber (8), which is located at the connection between the negative pressure pipe (5) and the liquid injection port (12). During the formation process of the cell to be formed, the air chamber (8) is filled with electrolyte (9) to replenish the electrolyte in the cell to be formed. And / or, during the formation process of the battery cell to be formed, the negative pressure range of the negative pressure pipe (5) is -100~-500kPa; And / or, the diameter of the air chamber (8) is 2 to 5 times the diameter of the negative pressure pipe (5); And / or, a heating plate (2) is used to heat the cell to be formed, and a silicone pad (1) is provided around the heating plate (2).

7. The formation method of a lithium-ion battery according to claim 1, characterized in that, In step S2, the cell to be formed is heated and pressurized. The heating temperature is 60~90℃ and the pressurization pressure is 150~200kgf. And / or, in step S3, the cell to be formed is heated and pressurized, wherein the heating temperature is 50~90℃ and the pressurization pressure is 100~200kgf; And / or, in step S4, the cell to be formed is heated and pressurized, wherein the heating temperature is lower than the heating temperature in step S3, the heating temperature is 35~50℃, and the pressurization pressure is 50~150kgf.

8. A method for injecting liquid, characterized in that, The injection method includes the following steps: S1. Determine the original weight M1 of the cell to be injected with electrolyte; S2. Inject the first electrolyte into the cell to be injected, and then let it stand to obtain the cell to be formed. S3. Form the battery cell to be formed using the formation method according to any one of claims 1-7; S4. After the formation is completed, the cell is left to stand. Then, the weight M2 of the cell after the formation is completed is determined. Based on M1 and M2, the amount of the first electrolyte injection is determined. Based on the amount of the first electrolyte injection and the subsequent injection coefficient, the amount of the subsequent electrolyte injection is determined.

9. The injection method according to claim 8, characterized in that, The determination of the original weight M1 of the battery cell to be injected includes: encapsulating the prepared dried battery cell core package in the designed battery cell shell, and after welding, helium testing and meeting the standards, vacuum drying and weighing, which is recorded as M1; And / or, in step S2, the settling temperature is 50~130℃, and the settling time is 4~24h; And / or, in step S4, the settling temperature is 50~80℃, and the settling time is 4~24h; And / or, in step S4, the amount of the first injected electrolyte m1 = M2 – M1, the subsequent injection coefficient x is 0.05~0.2, and the amount of the subsequent injected electrolyte m2 = xm1.

10. A lithium-ion battery, characterized in that, The lithium-ion battery is obtained by forming the lithium-ion battery according to any one of claims 1-7, or by injecting liquid according to the liquid injection method according to claim 8 or 9.

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