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

Abstract

The invention relates to the technical field of lithium ion batteries, in particular to a formation method and a liquid injection method of a lithium ion battery and the lithium ion battery, and the formation method comprises the following steps: S1, taking unformed battery cells of the same batch, and carrying out test formation under low-current constant-current charging to obtain a dQ / dV-V curve graph; s2, low-current constant-current low-frequency charging is adopted to activate the to-be-formed battery cell; s3, segmenting based on peak valleys and peak tops of the dQ / dV-V curve graph obtained in the step S1, and then performing multi-current step formation on the to-be-formed battery cell; and S4, after the last peak is completed, carrying out constant-current charging on the to-be-formed battery cell with n times of the formation current of the last working step in the step S3, and stopping charging after a cut-off voltage is reached. According to the formation method provided by the invention, the components of the SEI film are uniform, and the electrochemical properties are more stable, so that the lithium ion battery has excellent storage performance and rate capability.

Description

Technical Field

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

[0002] Formation is an extremely critical step in the later process of lithium-ion battery manufacturing, and is often associated with injection, especially in the design of segmented injection scheme, that is, the first injection electrolyte contains film-forming additives and other ingredients, and the later injection electrolyte contains functional additives such as low temperature, high temperature, high pressure, flame retardant, and conductivity to ensure the quality of early formation and film formation and later cycle performance. However, the current formation step design is generally set by experience, without considering the peak voltage range and appropriate current density of the new additives in the electrolyte, resulting in the same peak but different steps in multi-step formation, that is, there is a staged reaction of the same substance at different current densities in the battery formation, resulting in different reaction conditions and different reaction products. The formed SEI film is unstable, resulting in poor storage performance and rate performance of the battery. Summary of the invention

[0003] In view of the above-mentioned deficiencies in the prior art, the object of the present invention is to provide a lithium ion battery formation method and a liquid injection method and a lithium ion battery. The present invention adopts dQ / dV prefabricated peaks for segmented formation during formation, thereby ensuring the same peak and current, making the SEI film formation reaction conditions the same, and the generated SEI film components are unified and the electrochemical properties are more stable, so that the lithium ion battery has excellent storage performance and rate performance.

[0004] In order to achieve the above object, the present invention adopts the following technical solutions:

[0005] In a first aspect, the present invention provides a formation method for a lithium ion battery, the formation method comprising the following steps:

[0006] S1. Take the unformed cells from the same batch and perform test formation by constant current charging at a low current to obtain a dQ / dV-V curve.

[0007] S2, empty-charge pulse activation: use small current constant current low-frequency charging to activate the battery to be formed;

[0008] S3, intermittent peak charging: based on the peaks and valleys of the dQ / dV-V curve obtained in step S1, segmentation is performed, and then the battery cell to be formed is subjected to multi-current process step formation to ensure the consistency of the same peak reaction conditions;

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

[0010] The present invention adopts dQ / dV prefabricated peaks for segmented formation during formation, which can ensure the same peak and current, make the SEI film formation reaction conditions the same, and the generated SEI film components are unified and the electrochemical properties are more stable, so that the lithium ion battery has excellent storage performance and rate performance.

[0011] Furthermore, the battery cell to be formed is a square battery cell;

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

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

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

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

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

[0017] Peak-valley ratio principle: In the peak-free section of the dQ / dV-V curve, the first peak-valley starting from the peak-free section and then climbing to the first peak is the peak-free section. The charging ratio I1 of the peak-free section is determined by the peak intensity of the first peak-valley of the first peak. I1 is calculated by formula (1). The cut-off voltage of the peak-free section is the voltage corresponding to the peak intensity of the first peak-valley of the first peak.

[0018] Peak-to-peak rate principle: In a peak segment, a segment is defined as a section starting from a peak valley, passing through the peak top of the peak and then to the peak valley of the peak. The charging rate I2 of this segment is determined by the peak intensity of the peak top. I2 is calculated by formula (2). The cutoff voltage of this segment is the voltage corresponding to the peak intensity of the second peak valley of this peak.

[0019] Multi-peak rate principle: if multiple peaks overlap or the voltage between the peaks of multiple peaks does not exceed the threshold, the charging rate of this interval is determined by the rate determined by the first peak of the multiple peaks according to formula (2), and the cut-off voltage of this interval is the voltage corresponding to the peak strength of the last peak valley in this interval;

[0020] Termination rate principle: The charging rate corresponding to the last peak is determined by the previous peak and valley according to formula (1), and the cut-off voltage is the same as the cut-off voltage in step S4;

[0021] Interval rate principle: when there is no peak at the beginning, the charging is carried out according to the rate determined by the first peak valley according to formula (1); when a multi-peak interval is charged at a rate, the rate determined by the peak top of the first peak according to formula (2) is used as the rate of the interval;

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

[0023] I2 = (peak-to-peak intensity × voltage value corresponding to peak-to-peak intensity ÷ designed capacity of the battery cell to be formed × charging coefficient β) 0.5 (2);

[0024] Among them, the peak-to-valley peak intensity is the value of the ordinate dQ / dV at the peak-to-valley position, which is tested by the test equipment; the peak-to-top peak intensity is the value of the ordinate dQ / dV at the peak-to-top position, which is tested by the test 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.

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

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

[0027] Furthermore, in step S3, the battery cell to be formed is placed on hold after each formation step, the temperature and pressure of the placement are the same as the temperature and pressure of the next formation step, and the placement time is 2 to 4 minutes;

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

[0029] And / or, in step S4, the battery cell to be formed is placed on hold after formation, the temperature and pressure of the placement being the same as the temperature and pressure of the formation in step S4, and the placement time is 2 to 4 minutes;

[0030] And / or, during the formation process of the battery cell to be formed, the liquid injection port of the battery cell to be formed adopts negative pressure suction to heat and pressurize the large surface of the battery cell to be formed.

[0031] Furthermore, a negative pressure pipe (5) in a liquid injection and replenishment device is used to perform negative pressure suction on the liquid injection port (12) of the battery cell to be formed, and the liquid injection and replenishment device also includes an air chamber (8), which is arranged at the connection between the negative pressure pipe (5) and the liquid injection port (12). During the formation process of the battery cell to be formed, the air chamber (8) contains an electrolyte (9) to achieve the replenishment of the electrolyte in the battery cell to be formed;

[0032] And / or, during the formation process of the battery cell to be formed, 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 to 5 times the diameter of the negative pressure pipe (5);

[0034] And / or, a heating plate (2) is used to heat the battery cell to be formed, and a silica gel pad (1) is arranged around the heating plate (2).

[0035] Furthermore, in step S2, the battery cell to be formed is heated and pressurized, the heating temperature is 60-90° C., and the pressurizing pressure is 150-200 kgf;

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

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

[0038] In a second aspect, the present invention provides a liquid injection method, the liquid injection method comprising the following steps:

[0039] S1, determine the original weight M1 of the battery cell to be filled with liquid;

[0040] S2, injecting the first electrolyte into the battery cell to be injected, and then letting it stand to obtain the battery cell to be formed;

[0041] S3, using the formation method described in the first aspect to form the battery cell to be formed;

[0042] S4, after the formation is completed, let it stand, then determine the weight M2 of the battery cell after the formation is completed, determine the amount of the first injection of electrolyte based on M1 and M2, and determine the amount of the post-injection electrolyte based on the amount of the first injection of electrolyte and the post-injection coefficient.

[0043] Further, the determining of the original weight M1 of the battery cell to be injected includes: packaging the prepared dry battery cell package in a designed battery cell shell, and after welding and helium inspection and meeting the standards, vacuum drying and weighing, which is recorded as M1;

[0044] And / or, in step S2, the standing temperature is 50 to 130° C., and the standing time is 4 to 24 hours;

[0045] And / or, in step S4, the standing temperature is 50-80° C., and the standing time is 4-24 hours;

[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 invention provides a lithium ion battery, which is obtained by forming the lithium ion battery according to the first aspect, or by injecting liquid according to the second aspect.

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

[0049] (1) The present invention uses dQ / dV pre-formed peaks for segmented formation during formation, which can ensure the same peak and current, so that the SEI film formation reaction conditions are the same, the generated SEI film components are unified and the electrochemical properties are more stable, thereby enabling the lithium ion battery to have excellent storage performance and rate performance.

[0050] (2) The amount of liquid injected for the first time is determined according to the amount of liquid absorbed after formation, and the amount of liquid injected for the second time is determined accordingly. In addition, sufficient electrolyte is ensured during the formation process. This can accurately control the electrolyte demand of the battery cell and the amount of additives used in each stage, ensure the uniformity of the added electrolyte composition, and improve the functional efficiency of the additive components in the electrolyte.

[0051] (3) Adjusting the formation conditions in phases in conjunction with the formation steps can ensure that the formation process is smoother and in line with mechanical and thermal characteristics, and is more conducive to adjusting the properties of the SEI film - loose or dense.

[0052] (4) Using gas chamber injection can not only reduce the impact of negative pressure and electrolyte immersion on the core package during the first injection, thereby achieving a buffering and regulating effect, but also keep the core package constantly infiltrated with electrolyte during the formation stage to inhibit the generation of gas paths, and will not affect the formation effect due to lack of liquid caused by excessive electrolyte absorption due to negative pressure.

[0053] (5) The use of a combination of silica gel and a heating plate can increase the friction on the battery cell while taking into account the inherent characteristics of the battery cell, which is thick in the middle and thin at the edges, so that the battery cell is evenly stressed during the formation process. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0055] Figure 1 It is a schematic diagram of the peak and valley positioning of the dQ / dV-V curve diagram of the present invention;

[0056] Figure 2 This is a schematic diagram of the multi-peak rate principle based on the dQ / dV-V curve diagram of the present invention;

[0057] Figure 3 This is a schematic diagram of the combination of the silicone pad and the heating plate of the present invention;

[0058] Figure 4 It is a schematic diagram of the combination of the liquid injection and replenishment device of the present invention and the battery cell to be formed.

[0059] Icons: 1-silicone pad; 2-heating plate; 3-vacuum valve; 4-sealing rod; 5-negative pressure pipe; 6-fluid filling port; 7-fluid filling valve; 8-gas chamber; 9-electrolyte; 10-positive electrode; 11-negative electrode; 12-liquid filling port; 13-explosion-proof valve. DETAILED DESCRIPTION

[0060] In order to make the purpose, technical scheme and advantages of the present invention clearer, the technical scheme in the embodiment of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. It should be understood by those skilled in the art that the embodiments are only to help understand the present invention and should not be regarded as specific limitations of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in the field without making creative work are within the scope of protection of the present invention. The process parameters of the following embodiments that do not specify specific conditions are usually based on conventional conditions.

[0061] The endpoints and any values ​​of the ranges disclosed in the present invention are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of each range, the endpoint values ​​of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be regarded as specifically disclosed in the present invention.

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

[0063] S1. Take the unformed cells from the same batch and perform test formation by constant current charging at a low current to obtain a dQ / dV-V curve.

[0064] S2, empty-charge pulse activation: use small current constant current low-frequency charging to activate the battery to be formed;

[0065] S3, intermittent peak charging: Figure 1 As shown, based on the peaks and valleys of the dQ / dV-V curve obtained in step S1, segmentation is performed, and then the battery cell to be formed is subjected to multi-current process step formation to ensure the consistency of the same peak reaction conditions;

[0066] S4, planned charging termination: after the last peak is completed, the battery cell to be formed is charged with a constant current of n times the formation current of the last step in step S3, and the charging is stopped after the cut-off voltage is reached, wherein n is not less than 1.

[0067] The present invention adopts dQ / dV prefabricated peaks for segmented formation during formation, which can ensure the same peak and current, make the SEI film formation reaction conditions the same, and the generated SEI film components are unified and the electrochemical properties are more stable, so that the lithium ion battery has excellent storage performance and rate performance.

[0068] The dQ / dV-V curve in step S1 is obtained by testing the formation equipment. This step is equivalent to pre-formation, which is used to determine the peak state of the batch of battery cells.

[0069] In the above-mentioned lithium-ion battery formation method, as an optional implementation, the battery cell to be formed is a square battery cell.

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

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

[0072] In the above-mentioned lithium ion battery formation method, as an optional implementation mode, 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 is charged to 40%-60% SOC (for example, it can be 40% SOC, 45% SOC, 50% SOC, 55% SOC or 60% SOC).

[0073] In the formation method of the above-mentioned lithium ion battery, as an optional embodiment, in step S2, a small current constant current low-frequency charging of 0.001 to 0.05C (for example, 0.001C, 0.01C, 0.02C, 0.03C, 0.04C or 0.05C) is adopted, and the frequency of low-frequency charging is 1 to 10Hz (for example, 1Hz, 3Hz, 5Hz, 7Hz or 10Hz), and the total charging time is 10s to 120s (for example, 10s, 20s, 40s, 60s, 80s, 100s or 120s). Pulse current activation adopts 0.001 to 0.05C small current low-frequency charging to open a stable conduction path and pave the way for subsequent formation.

[0074] In the above-mentioned lithium ion battery formation method, as an optional implementation, in step S3, the principle of segmenting and determining the charging rate and cut-off voltage of the formation step corresponding to each segment includes:

[0075] Peak-valley ratio principle: In the peak-free section of the dQ / dV-V curve, the first peak-valley starting from the peak-free section and then climbing to the first peak is the peak-free section. The charging ratio I1 of the peak-free section is determined by the peak intensity of the first peak-valley of the first peak. I1 is calculated by formula (1). The cut-off voltage of the peak-free section is the voltage corresponding to the peak intensity of the first peak-valley of the first peak.

[0076] Peak-to-peak rate principle: In a peak segment, a segment is defined as a section starting from a peak valley, passing through the peak top of the peak and then to the peak valley of the peak. The charging rate I2 of this segment is determined by the peak intensity of the peak top. I2 is calculated by formula (2). The cutoff voltage of this segment is the voltage corresponding to the peak intensity of the second peak valley of this peak.

[0077] Multi-peak ratio principle: Figure 2 As shown, if multiple peaks overlap or the voltage between the peaks of multiple peaks does not exceed the threshold, the charging rate of this interval is determined by the rate determined by the first peak of the multiple peaks according to formula (2), and the cut-off voltage of this interval is the voltage corresponding to the peak strength of the last peak valley in this interval;

[0078] Termination rate principle: The charging rate corresponding to the last peak is determined by the previous peak and valley according to formula (1), and the cut-off voltage is the same as the cut-off voltage in step S4;

[0079] Interval rate principle: when there is no peak at the beginning, the charging is carried out at the rate determined by the first peak valley according to formula (1) (that is, after the empty charge pulse is activated, all intermittent charging starts with the rate corresponding to the first peak valley). When the multi-peak interval is charged at a rate, the rate determined by the peak of the first peak according to formula (2) is used as the rate of the interval.

[0080] I1 = (peak-to-valley peak intensity × voltage value corresponding to peak-to-valley peak intensity ÷ designed capacity of the battery cell to be formed × charging coefficient α) 0.5 (1);

[0081] I2 = (peak-to-peak intensity × voltage value corresponding to peak-to-peak intensity ÷ designed capacity of the battery cell to be formed × charging coefficient β) 0.5 (2);

[0082] Among them, the peak-to-valley peak intensity is the value of the ordinate dQ / dV of the peak-to-valley position, which is obtained by testing the test equipment; the peak-to-top peak intensity is the value of the ordinate dQ / dV of the peak-to-top position, which is obtained by testing the test equipment; the charging coefficient α 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 β 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 electrode active material and its theoretical specific capacity, which is generally 10Ah-800Ah (for example, it can be 10Ah, 100Ah, 200Ah, 400Ah, 600Ah or 800Ah).

[0083] In the above-mentioned lithium-ion battery formation method, as an optional implementation, the threshold value is not greater than 1.0V, for example, the threshold value may be 1.0V or 0.8V.

[0084] In the above-mentioned lithium-ion battery formation method, as an optional implementation, the charging coefficients α of different steps are the same, and the charging coefficients β of different steps are the same. The same charging coefficient is used to anchor different stages to achieve the purpose of coordinated change.

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

[0086] In the above-mentioned lithium ion battery formation method, as an optional implementation, in step S3, the battery cell to be formed is placed after each formation step, and the temperature and pressure of the placement are the same as the temperature and pressure of the next formation step, and the placement time is 2 to 4 minutes. The present invention adjusts the pressure and temperature during the placement time to meet the requirements of the next formation conditions.

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

[0088] In the above-mentioned lithium-ion battery formation method, as an optional implementation, in step S4, the battery cell to be formed is placed after formation, the temperature and pressure of the placement are the same as the temperature and pressure of the formation in step S4, and the placement time is 2 to 4 minutes.

[0089] In the above-mentioned lithium-ion battery formation method, as an optional implementation, during the formation process of the battery cell to be formed, the liquid injection port of the battery cell to be formed adopts negative pressure suction to heat and pressurize the large surface of the battery cell to be formed.

[0090] In the above-mentioned lithium ion battery formation method, as an optional embodiment, Figure 4 As shown, the negative pressure pipe 5 in the liquid injection and replenishment device is used to perform negative pressure suction on the liquid injection port 12 of the battery cell to be formed, and the liquid injection and replenishment device also includes an air chamber 8, which is arranged at the connection between the negative pressure pipe 5 and the liquid injection port 12. During the formation process of the battery cell to be formed, the air chamber 8 contains electrolyte 9 to achieve the replenishment of the electrolyte in the battery cell to be formed. In an embodiment of the present invention, from the beginning to the end of the formation, the electrolyte can be replenished by setting the negative pressure pipe 5 and the air chamber 8 connected to the negative pressure pipe, so that the gas (bubble) generated at the reaction site can quickly escape from the liquid surface under the action of pressure, and the liquid surface is quickly filled with excess electrolyte after gas production, ensuring that the electrolyte is sufficient to prevent the generation of gas paths leading to lithium precipitation or uneven formation reaction.

[0091] Liquid injection is a very important step in the production process of battery cells. The traditional one-time liquid injection is to give the liquid injection coefficient according to the design capacity based on experience, which can ensure the uniformity of the liquid injection composition, but it cannot effectively improve the utilization rate and function of various additives in the electrolyte. In addition, the different densities of different electrolytes and the different liquid absorption of the core package will also cause the empirical coefficient to be incompatible. The popular staged liquid injection now is also carried out according to the design capacity and then the liquid injection coefficient is given, but there are operations such as negative pressure formation in the middle, which causes the electrolyte in the battery cell to be sucked away, resulting in insufficient electrolyte during the formation, resulting in poor quality of the generated SEI film. If it also involves refilling after formation, the simple refilling method (only adding the first injection or the second injection) will make the electrolyte composition contained in each battery cell different, thereby adversely affecting the consistency of the battery. If the first injection and the second injection formula are the same, the function and utilization of the components in the electrolyte will be greatly reduced.

[0092] The present invention uses gas chamber injection. From the beginning to the end of formation, the gas chamber connected to the negative pressure tube maintains a certain liquid level of electrolyte by replenishing the first injected electrolyte, which can effectively improve the utilization rate and function of various additives in the electrolyte. At the same time, it can also solve the technical problem of poor quality of the generated SEI film due to insufficient electrolyte during formation in staged injection, and can also improve the consistency of the battery.

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

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

[0095] In the above-mentioned lithium-ion battery formation method, as an optional implementation, the diameter of the air chamber 8 is 2 to 5 times the diameter of the negative pressure pipe 5, for example, it can be 2 times, 3 times, 4 times or 5 times.

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

[0097] In the above-mentioned lithium-ion battery formation method, as an optional implementation mode, in step S3, the battery cell to be formed is heated and pressurized, the heating temperature is 50-90°C (for example, it can be 50°C, 52°C, 54°C, 56°C, 60°C, 70°C or 90°C), and the pressurizing pressure is 100-200kgf (for example, it can be 100kgf, 150kgf, 170kgf, 190kgf or 200kgf). Preferably, in the process step when the voltage has not reached the platform voltage, the formation pressure is 150-200kgf. At this time, the charging SOC is relatively small, and the electrode expansion force is small. The high pressure can make the gas escape from the battery cell quickly during the reaction gas production stage, and increase the density of the SEI film, improve the film quality and form 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 process step when the voltage reaches the platform voltage, the formation pressure is 100-150kgf. The pressure of this process step is relatively small, which is conducive to alleviating the expansion of the electrode caused by charging, and at the same time forming a dense middle layer SEI film, making the SEI film on the negative electrode side dense, thereby improving storage performance.

[0098] In the formation method of the above-mentioned lithium ion battery, as an optional embodiment, in step S4, the battery cell to be formed is heated and pressurized, the heating temperature is lower than the heating temperature in step S3, the heating temperature is 35-50°C (for example, it can be 35°C, 40°C, 45°C or 50°C), and the pressurization pressure is 50-150kgf (for example, it can be 50kgf, 100kgf or 150kgf). When the planned charging is terminated, the heating temperature ranges from 35 to 50°C and the pressure is 50 to 150kgf. At this time, the SOC increases, the pole piece expands greatly, and when the pressure is low, a loose outer SEI film can be formed at this stage to provide more lithium ion attachment sites. At this time, the current is further increased, and the temperature can be slightly reduced to save energy.

[0099] In the above-mentioned lithium ion battery formation method, as an optional embodiment, Figure 3 As shown, a heating plate 2 is used to heat the battery cell to be formed, and a silicone pad 1 is arranged around the heating plate 2. The embodiment of the present invention can increase the friction force on the battery cell by using a combination of silicone and a heating plate.

[0100] In the above-mentioned lithium-ion battery formation method, as an optional implementation, the silicone pad 1 can match the situation where the thickness of the electrode thinning area of ​​the battery cell to be formed is smaller. The embodiment of the present invention uses a combination of silicone and a heating plate to increase the friction force on the battery cell and take into account the characteristics of the battery cell being thick in the middle and thin at the edges, so that the battery cell is evenly stressed during the formation process.

[0101] In the above-mentioned lithium-ion battery formation method, as an optional implementation, the thickness of the silica gel pad 1 is 2 to 4 mm, for example, it can be 2 mm, 2.5 mm, 3 mm, 3.5 mm or 4 mm.

[0102] In a second aspect, the present invention provides a liquid injection method, the liquid injection method comprising the following steps:

[0103] S1, determine the original weight M1 of the battery cell to be filled with liquid;

[0104] S2, injecting the first electrolyte into the battery cell to be injected, and then letting it stand to obtain the battery cell to be formed;

[0105] S3, using the formation method described in the first aspect to form the battery cell to be formed;

[0106] S4, after the formation is completed, let it stand, then determine the weight M2 of the battery cell after the formation is completed, determine the amount of the first injection of electrolyte based on M1 and M2, and determine the amount of the post-injection electrolyte based on the amount of the first injection of electrolyte and the post-injection coefficient.

[0107] The present invention determines the amount of the first electrolyte injection according to the amount of liquid absorbed after formation, and determines the amount of the subsequent electrolyte injection based on this, and ensures that the electrolyte is sufficient during the formation process. The demand for electrolyte by the battery cell and the amount of additives used in each stage can be accurately regulated, ensuring that the added electrolyte components are uniform, and improving the functional efficiency of the additive components in the electrolyte.

[0108] In the above-mentioned liquid injection method, as an optional implementation, the determination of the original weight M1 of the battery cell to be injected includes: packaging the prepared dry battery cell package in a designed battery cell shell, and after welding and helium inspection processes and meeting the standards, vacuum drying and weighing again, recorded as M1. The purpose of weighing in the embodiment of the present invention is, on the one hand, to discover abnormal phenomena in the assembly process, to avoid weight errors caused by packaging problems and subsequent effects, and on the other hand, to record the original weight M1 of the battery cell when there is no electrolyte.

[0109] In the above-mentioned liquid injection method, as an optional implementation, in step S2, the standing temperature is 50-130°C (for example, it can be 50°C, 70°C, 90°C, 110°C or 130°C), and the standing time is 4-24h (for example, it can be 4h, 8h, 12h, 16h, 20h or 24h).

[0110] In the above-mentioned liquid injection method, as an optional implementation, in step S4, the standing temperature is 50-80°C (for example, it can be 50°C, 60°C, 70°C or 80°C), and the standing time is 4-24h (for example, it can be 4h, 8h, 12h, 16h, 20h or 24h).

[0111] In the above-mentioned injection method, as an optional implementation, in step S4, the amount of the first injection 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 electrolyte m2=xm1.

[0112] In a third aspect, the present invention provides a lithium ion battery, which is obtained by forming the lithium ion battery according to the first aspect, or by injecting liquid according to the second aspect.

[0113] In the above lithium-ion battery, as an optional implementation, the lithium-ion battery includes a square lithium-ion battery.

[0114] The present invention will be further described in detail below with reference to specific embodiments and comparative examples.

[0115] In the following examples and comparative examples:

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

[0117] 2. Peak-valley ratio principle: In the peak-free section of the dQ / dV-V curve, the first peak-valley starting from the peak-free section and then climbing to the first peak is the peak-free section. The charging ratio I1 of the peak-free section is determined by the peak intensity of the first peak-valley of the first peak. I1 is calculated by formula (1). The cut-off voltage of the peak-free section is the voltage corresponding to the peak intensity of the first peak-valley of the first peak.

[0118] Peak-to-peak rate principle: In a peak segment, a segment is defined as a section starting from a peak valley, passing through the peak top of the peak and then to the peak valley of the peak. The charging rate I2 of this segment is determined by the peak intensity of the peak top. I2 is calculated by formula (2). The cutoff voltage of this segment is the voltage corresponding to the peak intensity of the second peak valley of this peak.

[0119] Multi-peak rate principle: if multiple peaks overlap or the voltage between the peaks of multiple peaks does not exceed 1V, the charging rate of this interval is determined by the rate determined by the first peak of the multiple peaks according to formula (2), and the cut-off voltage of this interval is the voltage corresponding to the peak strength of the last peak valley in this interval;

[0120] Termination rate principle: The charging rate corresponding to the last peak is determined by the previous peak and valley according to formula (1), and the cut-off voltage is the standard cut-off voltage of the battery cell to be formed;

[0121] Interval rate principle: when there is no peak at the beginning, the charging is carried out at the rate determined by the first peak valley according to formula (1) (that is, after the empty charge pulse is activated, all intermittent charging starts with the rate corresponding to the first peak valley). When the multi-peak interval is charged at a rate, the rate determined by the peak of the first peak according to formula (2) is used as the rate of the interval.

[0122] I1 = (peak-to-valley peak intensity × voltage value corresponding to peak-to-valley peak intensity ÷ designed capacity of the battery cell to be formed × charging coefficient α) 0.5 (1);

[0123] I2 = (peak-to-peak intensity × voltage value corresponding to peak-to-peak intensity ÷ designed capacity of the battery cell to be formed × charging coefficient β) 0.5 (2);

[0124] Among them, the peak-to-valley peak intensity is the value of the ordinate dQ / dV at the peak-to-valley position, which is tested by the test equipment; the peak-to-top peak intensity is the value of the ordinate dQ / dV at the peak-to-top position, which is tested by the test 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, which is generally 10Ah~800Ah.

[0125] 3. Storage and rate performance tests were carried out in accordance with GB / T 31486-2015 standard. In the storage performance test, capacity retention rate = charge retention capacity / room temperature discharge capacity; capacity recovery rate = charge recovery capacity / room temperature discharge capacity; in the rate performance test, 4C charge retention rate = 4C charge capacity / 1C charge capacity; 4C discharge retention rate = 4C discharge capacity / 1C discharge capacity; 2C charge retention rate = 2C charge capacity / 1C charge capacity; 2C discharge retention rate = 2C discharge capacity / 1C discharge capacity.

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

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

[0128] S1. Weighing preparation: Place the dry battery cell packs of the same batch with uniform weight specifications and meeting the requirements into the square battery cell shell. After welding, helium inspection and other processes and meeting the standards, vacuum dry and weigh and record as M1.

[0129] S2. First injection of electrolyte: Use a 500ml air chamber injection and refilling device, the diameter of which is 4 times the diameter of the negative pressure pipe. First, open the vacuum valve, keep the sealing rod lifted, close the refilling valve, and slowly evacuate the battery cell and the air chamber to -100kPa; then close the vacuum valve, open the refilling valve, keep the sealing rod tightly closed, and drive type A electrolyte to be injected into the air chamber, with an injection volume of 400ml; finally, close the refilling valve and confirm that the vacuum valve is closed, and slowly open the sealing rod to allow type A electrolyte to enter the battery cell driven by the pressure difference. After maintaining the inhalation and infiltration for 20 minutes, close the sealing rod and suck the excess electrolyte into the residual liquid bottle through the refilling valve. Finally, seal the injection port with a sealing nail and let it stand at 80°C for 18 hours.

[0130] S3, formation process: Take the unformed cells of the same batch (for the same group of solutions), charge them to 50% SOC with a small current constant current of 0.05C for test formation, and finally obtain the dQ / dV-V curve. The dQ / dV-V curve is divided into peaks and segments, and the peaks G1 (1.5V, 0.207Ah / V), G2 (1.9V, 0.87Ah / V), G3 (3.2V, 50Ah / V) are found, and the peaks D1 (1.76V, 2.87Ah / V), D2 (2.43V, 11.77Ah / V), D3 (3.24V, 410.77Ah / V) are found. According to the principles of valley rate, peak rate, multi-peak rate and termination rate, the formation process step interval is composed of three sections: (0.1~1.5V), (1.5~3.2V) and (3.2~3.3V). Given α=0.2 and β=0.1, the rates of these three process step intervals are 0.02C, 0.06C and 0.5C according to formulas (1) and (2), and the cut-off voltages are 1.5V, 3.2V and 3.3V respectively. During the formation, the air chamber is connected to the injection port. After the negative pressure state of the battery cell and the air chamber is obtained through the vacuum valve, some electrolyte is injected from the refill port so that a small amount of electrolyte is contained in the air 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 an open state, and the sealing rod is in a lifted state, except that the refill valve is closed during the refilling. A heating plate is used to heat the battery cell to be formed. A silicone pad with a thickness of 2.5 mm is arranged around the heating plate. The silicone pad can match the smaller thickness of the pole piece thinning area of ​​the battery cell to be formed. According to the present invention, the formation steps and conditions are shown in Table 1, and the battery cell 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 pipe is recovered, and the injection port is blocked with a rubber plug. The battery cell is placed in a high temperature chamber for 12 hours 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 extracted. At this time, the weight of the battery cell is M2, that is, 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 liquid volume m2=1 / 9m1. Finally, the injection port is blocked, and the formation and injection are all completed.

[0134] After capacity division, storage and rate performance tests were carried out. The cell was stored at 100% SOC at 25°C for 28 days with a capacity retention rate of 97.6% and a recovery rate of 98.5%. The 4C rate discharge retention rate was 92.1% and the 4C rate charge retention rate was 95.2%.

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

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

[0137] S1. Weighing preparation: Place the dry battery cell packs of the same batch with uniform weight specifications and meeting the requirements into the square battery cell shell. After welding, helium inspection and other processes and meeting the standards, vacuum dry and weigh and record as M1.

[0138] S2. First injection of electrolyte: Use negative pressure to maintain pressure and cycle multiple injections to complete the injection. The injection volume is controlled at m1, and then the injection port is sealed with a sealing nail and left to stand at 80°C for 18 hours.

[0139] S3, formation process: take the unformed cells of the same batch (for the same group of solutions) and form them according to the conventional method. The formation steps and conditions are as follows:

[0140] step Step Name Charging rate Duration Formation temperature Negative pressure formation Cut-off conditions 1 Shelf / 30s / / / 2 Charge 0.05C 60min 45℃ -45kPa 60min 3 Shelf / 2min 45℃ -45kPa 2min 4 Charge 0.15C 120min 45℃ -45kPa 120min 5 Shelf / 2min 45℃ -45kPa 2min 6 Finish

[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 hours at a temperature of 50°C. Then the battery cell is taken out of the high temperature chamber and post-injection is performed at a ratio of first injection: post-injection = 9:1, that is, the post-injection amount m2 = 1 / 9m1. Finally, the injection port is blocked, and the formation and injection are all completed.

[0142] After capacity division, storage and rate performance tests were carried out. The cell was stored at 100% SOC at 25°C for 28 days with a capacity retention rate of 92.3% and a recovery rate of 94.3%. The 4C rate discharge retention rate was 89.7% and the 4C rate charge retention rate was 91.6%.

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

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

[0145] S1. Weighing preparation: Place the dry battery cell packs of the same batch with uniform weight specifications and meeting the requirements into the square battery cell shell. After welding, helium inspection and other processes and meeting the standards, vacuum dry and weigh and record as M1.

[0146] S2. First injection of electrolyte: Use an injection and rehydration device with an 80ml air chamber, the diameter of which is twice the diameter of the negative pressure pipe. First, open the vacuum valve, keep the sealing rod lifted, close the injection valve, and slowly evacuate the battery cell and the air chamber to -80kPa; then close the vacuum valve, open the injection valve, keep the sealing rod tightly closed, and drive the B-type electrolyte to be injected into the air chamber, with an injection volume of 60ml; finally, close the true injection valve and confirm that the vacuum valve is closed, and slowly open the sealing rod to allow the B-type electrolyte to enter the battery cell driven by the pressure difference. After maintaining the inhalation and infiltration for 10 minutes, close the sealing rod and suck the excess electrolyte into the residual liquid bottle through the injection valve. Finally, seal the injection port with a sealing nail and let it stand at 50°C for 4 hours.

[0147] S3, formation process: take the unformed cells of the same batch (for the same group of solutions), charge them to 50% SOC with a small constant current of 0.05C for experimental formation, and finally obtain the dQ / dV-V curve. The dQ / dV-V curve is divided into peaks and segments to find the peak valleys G1 (2.5V, 1.89Ah / V) and G2 (3.18V, 7.28Ah / V), and the peaks D1 (2.6V, 13.23Ah / V) and D2 (3.23V, 56.37Ah / V). According to the peak-to-valley ratio, peak-to-peak ratio, multi-peak ratio and termination ratio principles, the working step interval is composed of two sections (0.1-2.5V) and (2.5-3.3V), and α=0.05 and β=0.02 are assigned. According to the calculation formulas (1) and (2), the ratios of the two working step intervals are 0.11C and 0.18C respectively, and the cut-off voltages are 2.5V and 3.3V respectively. During the formation, the air chamber is connected to the injection port. After the negative pressure state of the battery cell and the air chamber is obtained through the vacuum valve, some electrolyte is injected from the refill port so that a small amount of electrolyte is contained in the air 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 an open state, and the sealing rod is in a lifted state, except that the refill valve is closed during the refilling. A heating plate is used to heat the battery cell to be formed. A silicone pad with a thickness of 2.5 mm is arranged around the heating plate. The silicone pad can match the smaller thickness of the pole piece thinning area of ​​the battery cell to be formed. According to the present invention, the formation steps and conditions 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 of 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 hours 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 extracted. The weight of the battery cell obtained at this time is M2, that is, the actual liquid content m1=M2–M1. The post-injection is performed at a ratio of first injection: post-injection=8:1, and the post-injection liquid volume m2=1 / 8m1. Finally, the injection port is blocked, and the formation and injection are all completed.

[0151] After capacity division, storage and rate performance tests were carried out. The cell was stored at 100% SOC at 25°C for 28 days with a capacity retention rate of 98.1% and a recovery rate of 98.5%. The 4C rate discharge retention rate was 93.7% and the 4C rate charge retention rate was 96.1%.

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

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

[0154] S1. Weighing preparation: Place the dry battery cell packs of the same batch with uniform weight specifications and meeting the requirements into the square battery cell shell. After welding, helium inspection and other processes and meeting the standards, vacuum dry and weigh and record as M1.

[0155] S2. First injection of electrolyte: Use an injection and refilling device with an 800ml air chamber, the diameter of which is 5 times the diameter of the negative pressure pipe. First, open the vacuum valve, keep the sealing rod lifted, close the refilling valve, and slowly evacuate the battery cell and the air chamber to -200kPa; then close the vacuum valve, open the refilling valve, keep the sealing rod closed, and drive the C-type electrolyte to be injected into the air chamber, with an injection volume of 700ml; finally, close the refilling valve and confirm that the vacuum valve is closed, and slowly open the sealing rod to allow the C-type electrolyte to enter the battery cell driven by the pressure difference. Repeat the above operation 3 times to ensure that the electrolyte fills the battery cell. After maintaining the inhalation and infiltration for 10 minutes, close the sealing rod and suck the excess electrolyte into the residual liquid bottle through the refilling valve. Finally, seal the injection port with a sealing nail and let it stand at 130°C for 24 hours.

[0156] S3, formation process: Take the unformed cells of the same batch (for the same group of solutions), charge them to 50% SOC with a small constant current of 0.05C for test formation, and finally obtain the dQ / dV-V curve. The dQ / dV-V curve is divided into peaks and segments, and the peaks G1 (1.65V, 2.24Ah / V), G2 (2.48V, 80.38Ah / V), G3 (2.8V, 241.54Ah / V) are found, and the peaks D1 (1.85V, 50.24Ah / V), D2 (2.63V, 322.79Ah / V), D3 (3.25V, 1932.31Ah / V) are found. According to the principles of peak-to-valley ratio, peak-to-peak ratio, multi-peak ratio and termination ratio, the working step interval is composed of three sections: (0.1-1.65V), (1.65-2.8V) and (2.8-3.3V), with α=0.02 and β=0.01. According to the calculation formulas (1) and (2), the ratios of these three working step intervals are 0.01C, 0.04C and 0.15C respectively, and the cut-off voltages are 1.65V, 2.8V and 3.3V respectively. During the formation, the air chamber is connected to the injection port. After the negative pressure state of the battery cell and the air chamber is obtained through the vacuum valve, some electrolyte is injected from the refill port so that a small amount of electrolyte is contained in the air 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 an open state, and the sealing rod is in a lifted state, except that the refill valve is closed during the refilling. A heating plate is used to heat the battery cell to be formed, and a silicone pad with a thickness of 2.5 mm is arranged around the heating plate. The silicone pad can match the smaller thickness of the pole piece thinning area of ​​the battery cell to be formed. According to the present invention, the formation steps and conditions are shown in Table 3, and the battery cell to be formed is formed according to the formation steps and conditions shown in Table 3.

[0157] Table 3

[0158]

[0159] S3, post-injection of 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 hours 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 extracted. The weight of the battery cell obtained at this time is M2, that is, the actual liquid content m1=M2–M1. The post-injection is performed at a ratio of first injection: post-injection=7:1, and the post-injection liquid volume m2=1 / 7m1. Finally, the injection port is blocked, and the formation and injection are all completed.

[0160] After capacity division, storage and rate performance tests were carried out. The cell was stored at 100% SOC at 25°C for 28 days with a capacity retention rate of 98.3% and a recovery rate of 98.6%. The 2C rate discharge retention rate was 95.7% and the 2C rate charge retention rate was 96.5%.

[0161] Comparative Example 2: Preparation of 140Ah square battery cell with a volume of 1000ml

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

[0163] After capacity division, storage and rate performance tests were carried out. The cell was stored at 100% SOC at 25°C for 28 days with a capacity retention rate of 97.7% and a recovery rate of 98.0%. The 4C rate discharge retention rate was 90.8% and the 4C rate charge retention rate was 93.1%.

[0164] Comparative Example 3: Preparation of 140Ah square battery cell with a volume of 1000ml

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

[0166] After capacity division, storage and rate performance tests were carried out. The cell was stored at 100% SOC at 25°C for 28 days with a capacity retention rate of 95.6% and a recovery rate of 96.5%. The 4C rate discharge retention rate was 91.5%, and the 4C rate charge retention rate was 94.8%.

[0167] Comparative Example 4: Preparation of 140Ah square battery cell with a volume of 1000ml

[0168] The preparation method of the square battery cell is substantially the same as that of Example 1, except that the shelving pressure in step 7 is 100 kgf, and steps 8 and 9 are not included.

[0169] After capacity division, storage and rate performance tests were carried out. The cell was stored at 100% SOC at 25°C for 28 days with a capacity retention rate of 95.3% and a recovery rate of 96.2%. 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, the storage performance and rate performance are both reduced. The applicant speculates that the reason may be that after charging to the cut-off voltage under high voltage, the pressure is not released and the voltage will be high, that is, the formation charge capacity is reduced and the film will be relatively thin.

[0171] Comparative Example 5: Preparation of 140Ah square battery cell with a volume of 1000ml

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

[0173] After capacity division, storage and rate performance tests were carried out. The cell was stored at 100% SOC at 25°C for 28 days with a capacity retention rate of 97.9% and a recovery rate of 98.5%. The 4C rate discharge retention rate was 91.3%, and the 4C rate charge retention rate was 93.8%.

[0174] Compared with Example 1, when the formation current of the planned charge termination step is less than the formation current of the last step in step S3, although the storage performance is equivalent, the rate performance deteriorates. The applicant speculates that the reason may be that the charging current decreases in the later stage of formation, which slows down the film-forming reaction and makes the film dense. Such an SEI film is similar to a model that is first dense, then loose, and then dense. The top layer is too dense, which is not conducive to the rate performance and the overall performance is not good. In addition, the execution time of this step in this comparative example is short, and the deterioration is not obvious. If in actual application, the execution time of this step is longer, 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 invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A formation method for a lithium ion battery, characterized in that: The formation method comprises the following steps: S1. Take the unformed cells from the same batch and perform test formation by constant current charging at a low current to obtain a dQ / dV-V curve. S2, empty-charge pulse activation: use small current constant current low-frequency charging to activate the battery to be formed; S3, intermittent peak charging: based on the peaks and valleys of the dQ / dV-V curve obtained in step S1, segmentation is performed, and then the battery cell to be formed is subjected to multi-current process step formation to ensure the consistency of the same peak reaction conditions; S4, planned charging termination: after the last peak is completed, the battery cell to be formed is charged with a constant current of n times the formation current of the last step in step S3, and the charging is stopped after the cut-off voltage is reached, wherein n is not less than 1.

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 the battery cell includes at least one of lithium iron phosphate, a ternary positive electrode material and lithium manganese iron phosphate; And / or, the negative electrode active material to be formed into the 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 test formation current is 0.02C-0.05C, and constant current charging is performed to 40%-60% SOC; And / or, in step S2, a low-current constant current low-frequency charging of 0.001 to 0.05C is adopted, the frequency of the low-frequency charging is 1 to 10 Hz, and the total charging time is 10s to 120s; And / or, in step S3, the principle of segmenting and determining the charging rate and cut-off voltage of the formation step corresponding to each segment includes: Peak-valley ratio principle: In the peak-free section of the dQ / dV-V curve, the first peak-valley starting from the peak-free section and then climbing to the first peak is the peak-free section. The charging ratio I1 of the peak-free section is determined by the peak intensity of the first peak-valley of the first peak. I1 is calculated by formula (1). The cut-off voltage of the peak-free section is the voltage corresponding to the peak intensity of the first peak-valley of the first peak. Peak-to-peak rate principle: In a peak segment, a segment is defined as a section starting from a peak valley, passing through the peak top of the peak and then to the peak valley of the peak. The charging rate I2 of this segment is determined by the peak intensity of the peak top. I2 is calculated by formula (2). The cutoff voltage of this segment is the voltage corresponding to the peak intensity of the second peak valley of this peak. Multi-peak rate principle: if multiple peaks overlap or the voltage between the peaks of multiple peaks does not exceed the threshold, the charging rate of this interval is determined by the rate determined by the first peak of the multiple peaks according to formula (2), and the cut-off voltage of this interval is the voltage corresponding to the peak strength of the last peak valley in this interval; Termination rate principle: The charging rate corresponding to the last peak is determined by the previous peak and valley according to formula (1), and the cut-off voltage is the same as the cut-off voltage in step S4; Interval rate principle: when there is no peak at the beginning, the charging is carried out according to the rate determined by the first peak valley according to formula (1); when a multi-peak interval is charged at a rate, the rate determined by the peak top of the first peak according to formula (2) is used as the rate of the interval; I1 = (peak-to-valley peak intensity × voltage value corresponding to peak-to-valley peak intensity ÷ designed capacity of the battery cell to be formed × charging coefficient α) 0.5 (1); I2 = (peak-to-peak intensity × voltage value corresponding to peak-to-peak intensity ÷ designed capacity of the battery cell to be formed × charging coefficient β) 0.5 (2); Among them, the peak-to-valley peak intensity is the value of the ordinate dQ / dV at the peak-to-valley position, which is tested by the test equipment; the peak-to-top peak intensity is the value of the ordinate dQ / dV at the peak-to-top position, which is tested by the test 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.

4. The formation method of a lithium ion battery according to claim 3, characterized in that: The threshold value is not greater than 1.0V; And / or, the charging coefficients α of different working steps are the same, and the charging coefficients β of different working steps are the same.

5. The lithium ion battery formation method according to claim 1, characterized in that: In step S3, after each formation step, the battery cell to be formed is placed on hold, the temperature and pressure of the placement are the same as the temperature and pressure of the next formation step, and the placement time is 2 to 4 minutes; And / or, in step S4, the cut-off voltage is the standard cut-off voltage of the battery cell to be formed, which is determined by the positive electrode active material of the battery cell to be formed, for example, when the positive electrode active material is lithium iron phosphate, the cut-off voltage is 3.3V, and when the positive electrode active material is a ternary positive electrode material or lithium iron manganese phosphate, the cut-off voltage is 3.9V; And / or, in step S4, the battery cell to be formed is placed on hold after formation, the temperature and pressure of the placement being the same as the temperature and pressure of the formation in step S4, and the placement 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 adopts negative pressure suction to heat and pressurize the large surface of the battery cell to be formed.

6. The lithium ion battery formation method according to claim 5, characterized in that: A negative pressure pipe (5) in a liquid injection and replenishment device is used to perform negative pressure suction on a liquid injection port (12) of the battery cell to be formed. The liquid injection and replenishment device further comprises an air chamber (8). The air chamber (8) is arranged at the connection between the negative pressure pipe (5) and the liquid injection port (12). During the formation process of the battery cell to be formed, the air chamber (8) contains an electrolyte (9) to replenish the electrolyte in the battery 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 pipeline (5) is -100 to -500 kPa; 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 battery cell to be formed, and a silica gel pad (1) is arranged around the heating plate (2).

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

8. A liquid injection method, characterized in that: The liquid injection method comprises the following steps: S1, determine the original weight M1 of the battery cell to be filled with liquid; S2, injecting the first electrolyte into the battery cell to be injected, and then letting it stand to obtain the battery cell to be formed; S3, using the formation method described in any one of claims 1 to 7 to form the battery cell to be formed; S4, after the formation is completed, let it stand, then determine the weight M2 of the battery cell after the formation is completed, determine the amount of the first injection of electrolyte based on M1 and M2, and determine the amount of the post-injection electrolyte based on the amount of the first injection of electrolyte and the post-injection coefficient.

9. The liquid injection method according to claim 8, characterized in that: Determining the original weight M1 of the battery cell to be injected includes: packaging the prepared dry battery cell package in a designed battery cell shell, and after welding and helium inspection and meeting the standards, vacuum drying and weighing, which is recorded as M1; And / or, in step S2, the standing temperature is 50 to 130° C., and the standing time is 4 to 24 hours; And / or, in step S4, the standing temperature is 50-80° C., and the standing time is 4-24 hours; 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.

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

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