A negative pulse capacity division method for lithium iron phosphate cylindrical batteries

Abstract

The present invention discloses a negative pulse capacity division method for lithium iron phosphate cylindrical batteries, which includes dividing the charging process into multiple charging cycles during the formation process or the capacity division process, wherein the charging current value changes in a step-like manner in two adjacent charging cycles; each charging cycle includes a charging stage, a discharging stage, and a shelving stage; within the same charging cycle, the charging stage and the discharging stage are both constant current charging or constant current discharging, and the current value of the discharging stage is greater than the current value of the charging stage, and the discharge duration of the discharging stage is less than the charging duration of the charging stage. In the intervals of step charging, a negative pulse is introduced to alleviate concentration polarization and electrochemical polarization, and the negative pulse is then shelved to eliminate ohmic polarization. Through this method, the current of step charging can be increased, and the charging efficiency of the formation or capacity division stage can be improved.

Description

Technical Field

[0001] The invention belongs to the field of battery manufacturing, and in particular relates to a negative pulse capacity division method for a lithium iron phosphate cylindrical battery. Background Art

[0002] After assembly, lithium-ion batteries must undergo charging and formation to activate the active materials in the positive and negative electrodes, form a SEI film on the negative electrode surface, and simultaneously complete the initial lithium insertion into the negative electrode and the delithiation of the positive electrode. Formation is a highly complex process and a crucial step influencing battery performance. Following formation, capacity separation is performed, where the battery undergoes a 100% DOD charge and discharge, testing the battery capacity to determine if it meets the required capacity requirements and providing cells of the same capacity level for subsequent battery pack assembly. Therefore, formation and capacity separation are crucial steps in battery activation and separation, and their quality and efficiency significantly impact lithium-ion batteries.

[0003] The formation time of power batteries varies depending on the material system and process flow. In the battery manufacturing process, the formation and capacity separation processes are relatively long, occupying a large amount of equipment resources and process time.

[0004] The positive electrode is primarily composed of ternary materials and lithium iron phosphate. Due to the low diffusion coefficient of lithium in lithium iron phosphate, the charge and discharge currents are lower than those of ternary materials. Furthermore, the half-cell charge curve shows that the lithium iron phosphate half-cell has a longer charging plateau, while the charging curve of the ternary material has a relatively smaller plateau. This results in the lithium iron phosphate battery's negative electrode potential dropping more rapidly at the end of the full-cell split charge to increase the full-cell voltage at the same current density, making overpotential more likely to cause lithium deposition. Therefore, the NP of the lithium iron phosphate battery needs to be larger to ensure that lithium deposition is not prevented at the negative electrode potential during split-cell charging.

[0005] Currently, the formation time for lithium iron phosphate batteries is generally 4-9 hours, and the capacity separation time is generally 6-12 hours. Under the same conditions, the capacity separation time of ternary material batteries is 10%-30% shorter. Therefore, while ensuring accuracy, improving the efficiency of lithium iron phosphate battery formation and capacity separation and shortening the process time have significant economic benefits. Summary of the Invention

[0006] Purpose of the invention: In order to overcome the deficiencies in the prior art, the present invention provides a negative pulse formation and capacity division method for lithium iron phosphate cylindrical batteries, which can greatly improve the efficiency of formation and capacity division of lithium iron phosphate batteries and shorten the process time.

[0007] Technical solution: To achieve the above purpose, the technical solution of the present invention is as follows:

[0008] A negative pulse capacity separation method for lithium iron phosphate cylindrical batteries includes a formation process and a capacity separation process. During the formation process or the capacity separation process:

[0009] Place the battery cell in a corresponding formation temperature or capacity temperature environment, and divide the charging process into multiple charging cycles. In two adjacent charging cycles, the charging current value changes in a step-by-step manner.

[0010] Each charging cycle includes a charging phase, a discharging phase, and a rest phase. The charging voltage limit of the charging phase in several of the charging cycles is the same, and the discharging voltage limit of the discharging phase is the same. After the charging phase reaches the charging voltage limit, the charging phase enters the discharging phase, and after reaching the discharging voltage limit, the charging phase enters the rest phase.

[0011] In the same charging cycle, the charging stage and the discharging stage are both constant current charging or constant current discharging, and the current value of the discharging stage is greater than the current value of the charging stage, and the discharge time of the discharging stage is less than the charging time of the charging stage.

[0012] Furthermore, during the formation process or the capacity separation process, the charging current values ​​in several charging stages first increase from small to large and then decrease from large to small.

[0013] Furthermore, the discharge current range of the discharge stage in the formation process is 1C-3C; the discharge current range of the discharge stage in the capacity separation process is 2C-5C.

[0014] Furthermore, the holding time of the holding stage in the formation process or the volume separation process ranges from 5s to 20s.

[0015] Furthermore, the formation temperature range in the formation process is 40°C-50°C; the separation temperature range in the separation process is 20°C-30°C.

[0016] Furthermore, the formation process includes the following steps:

[0017] Step 1: Before charging, let it sit for 3 minutes;

[0018] Step 2: Charge at a constant current of 0.05C, with a time limit of 120 minutes and a voltage limit of 3.65V;

[0019] Step 3: Set aside for 10 minutes;

[0020] Step 4: Charge at 0.2C constant current for 22 minutes, with a voltage limit of 3.65V;

[0021] Step 5: Discharge at 1.5C constant current for 10s, voltage limit 2.0V;

[0022] Step 6: Wait for 10 seconds;

[0023] Step 7: Charge at 0.5C constant current for 12 minutes, with a voltage limit of 3.65V;

[0024] Step 8: Discharge at 2C constant current for 10s, voltage limit 2.0V;

[0025] Step 9: Wait for 10 seconds;

[0026] Step 10: Charge at 1.0C constant current for 9 minutes, with a voltage limit of 3.65V.

[0027] Step 11: Discharge at 2.5C constant current for 10s, voltage limit 2.0V;

[0028] Step 12: Wait for 10 seconds;

[0029] Step 13: Charge at 0.5C constant current for 12 minutes, with a voltage limit of 3.65V;

[0030] Step 14: Discharge at 2C constant current for 10s, voltage limit 2.0V;

[0031] Step 15: Set aside for 10 minutes;

[0032] Step 16: Finishing.

[0033] Furthermore, the volume separation process includes the following steps:

[0034] Step 1: Before charging, let it sit for 3 minutes;

[0035] Step 2: Charge at 1.5C constant current for 6 minutes, with a voltage limit of 3.65V;

[0036] Step 3: Discharge at 3C constant current for 10s, voltage limit 2.0V;

[0037] Step 4: Wait for 10 seconds;

[0038] Step 5: Charge at 1C constant current for 9 minutes, with a voltage limit of 3.65V;

[0039] Step 6: Discharge at 3C constant current for 10s, voltage limit 2.0V;

[0040] Step 7: Wait for 10 seconds;

[0041] Step 8: Charge at 0.5C constant current for 6 minutes, with a voltage limit of 3.65V;

[0042] Step 9: Discharge at 3C constant current for 10s, voltage limit 2.0V;

[0043] Step 10: Wait for 10 seconds;

[0044] Step 11: Charge at 0.33C constant current and constant voltage, cut-off current 0.05C, time limit 90min, and voltage limit 3.65V;

[0045] Step 12: Set aside for 10 minutes;

[0046] Step 13: Discharge at a constant current of 0.33C, with a time limit of 200 minutes and a voltage limit of 2.0V;

[0047] Step 14: Set aside for 10 minutes;

[0048] Step 15: Charge at a constant current of 0.33C, a cut-off current of 0.01C, a time limit of 30 minutes, and a voltage limit of 3.3V;

[0049] Step 16: Set aside for 3 minutes;

[0050] Step 17: The separation is completed.

[0051] Beneficial effects: The present invention introduces a negative pulse method in the gap of step charging to alleviate concentration polarization and electrochemical polarization, and then leaves the negative pulse to eliminate ohmic polarization; through this method, the current of step charging can be increased and the charging efficiency of the formation or capacity division stage can be improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Attachment Figure 1 It is a schematic diagram of the process of the present invention. DETAILED DESCRIPTION

[0053] The present invention will be further described below with reference to the accompanying drawings.

[0054] As attached Figure 1 As shown, a negative pulse capacity separation method for lithium iron phosphate cylindrical batteries includes a formation process and a capacity separation process. During the formation process or the capacity separation process:

[0055] The battery cell is placed in a corresponding formation temperature or capacity temperature environment, and the charging process is divided into multiple charging cycles. In two adjacent charging cycles, the charging current value changes in a step-by-step manner; that is, a step-by-step charging method is adopted to perform multi-cycle charge and discharge on the battery cell to form a negative pulse charging method. The current values ​​of two adjacent charging cycles are different, thereby reducing the phenomena of ohmic polarization, concentration polarization and electrochemical polarization.

[0056] Each charging cycle includes a charging phase, a discharging phase, and a rest phase. The charging voltage limit of the charging phase in several of the charging cycles is the same, and the discharging voltage limit of the discharging phase is the same. After the charging phase reaches the charging voltage limit, the charging phase enters the discharging phase, and after reaching the discharging voltage limit, the charging phase enters the rest phase.

[0057] In the same charging cycle, the charging stage and the discharging stage are both constant current charging or constant current discharging, and the current value of the discharging stage is greater than the current value of the charging stage, and the discharge time of the discharging stage is less than the charging time of the charging stage. This process increases the activity of the active materials between the positive and negative electrodes by forming negative pulse charging and discharging through short-time rapid discharge.

[0058] During the charging process of lithium-ion batteries, the battery's ability to accept current can be improved by discharging the battery to a certain extent; then the ohmic polarization is eliminated by shelving, and the concentration polarization and electrochemical polarization are alleviated by negative pulses. On this basis, the step charging method is combined to achieve fast charging while ensuring that the negative electrode does not deposit lithium.

[0059] After eliminating polarization through negative pulses, the current of step charging can be increased, shortening the overall charging time and improving the charging efficiency in the formation and capacity division stages.

[0060] During the formation or capacity separation process, the charging current values ​​in several charging stages first increase from low to high and then decrease from high to low. The process of increasing the current from low to high increases the charging current, shortens the overall charging time, and improves the charging efficiency of the formation and capacity separation stages. The process of decreasing the current from high to low stabilizes the active material in the later stages of charge and discharge.

[0061] The discharge current range of the discharge stage in the formation process is 0.5C-10C, preferably 1C-3C, to reduce the variation range and ensure stable charging; the discharge current range of the discharge stage in the capacity division process is 0.5C-10C, preferably 2C-5C, to ensure stable charging and rapid discharge.

[0062] The shelf time of the shelf stage in the formation process or the fractionation process ranges from 1s to 5min, preferably from 5s to 20s. Within this time range, a more obvious effect can be achieved. If the shelf time is too long, not only the effect is not significantly increased, but also the overall formation or fractionation time is significantly increased.

[0063] The formation temperature range in the formation process is 40° C.-50° C. In a high temperature environment, the chemical activity of the internal materials of the battery is improved, the reaction and activation process are accelerated, and the formation efficiency can be increased by 20-50%.

[0064] The temperature range of the volume separation process is 20° C.-30° C., which is a normal temperature environment.

[0065] The following are the test data of Example 1 using the method of this scheme and Comparative Example 1 in the prior art:

[0066] Example 1:

[0067] The battery cell model is 32135-15Ah, with a lithium iron phosphate positive electrode and graphite negative electrode in a cylindrical wound structure. After the battery is filled and stored, it is formed and capacity-separated according to the following steps. The capacity-separated discharge capacity, initial efficiency, self-discharge after 30 days at room temperature, disassembly to confirm lithium deposition, and room temperature cycling performance are tested. Furthermore, the negative electrode potential to lithium is greater than 0V during charging.

[0068] The chemical process steps are as follows:

[0069] step temperature Steps Step 1 45±3℃ Set aside for 3 minutes Step 2 45±3℃ 0.05C constant current charging, time limit 120min, voltage limit 3.65V; Step 3 45±3℃ Set aside for 10 minutes Step 4 45±3℃ 0.2C constant current charging for 22 minutes, voltage limit 3.65V Step 5 45±3℃ 1.5C constant current discharge 10s, voltage limit 2.0V Step 6 45±3℃ Wait 10 seconds Step 7 45±3℃ 0.5C constant current charging for 12 minutes, voltage limit 3.65V Step 8 45±3℃ 2C constant current discharge 10s, voltage limit 2.0V Step 9 45±3℃ Wait 10 seconds Step 10 45±3℃ 1.0C constant current charging for 9 minutes, voltage limit 3.65V Step 11 45±3℃ 2.5C constant current discharge 10s, voltage limit 2.0V Step 12 45±3℃ Wait 10 seconds Step 13 45±3℃ 0.5C constant current charging for 12 minutes, voltage limit 3.65V Step 14 45±3℃ 2C constant current discharge 10s, voltage limit 2.0V Step 15 45±3℃ Set aside for 10 minutes Step 16 25±3℃ End of transformation

[0070] Total formation time: 199min10s, formation capacity about 52% SOC;

[0071] The steps of volume separation are as follows:

[0072]

[0073]

[0074] Total time for separation: 368 minutes;

[0075] Comparative Example 1: Cell model 32135-15Ah, lithium iron phosphate cathode, graphite cathode, cylindrical wound structure. After battery filling and storage, the following steps were followed for formation and capacity separation. The cells were tested for capacity separation discharge capacity, initial efficiency, self-discharge after 30 days at room temperature, disassembly to confirm lithium deposition, and room temperature cycling performance. Furthermore, the negative electrode potential to lithium was tested to ensure it was greater than 0V during charging.

[0076] The chemical process steps are as follows:

[0077] step temperature Steps Step 1 45±3℃ Set aside for 3 minutes Step 2 45±3℃ 0.05C constant current charging, time limit 120min, voltage limit 3.65V; Step 3 45±3℃ Set aside for 10 minutes Step 4 45±3℃ 0.10C constant current charging, time limit 240min, voltage limit 3.65V Step 5 45±3℃ Set aside for 10 minutes Step 6 25±3℃ End of transformation

[0078] Total duration: 383min;

[0079] The steps of capacity separation are as follows:

[0080] step temperature Steps Step 1 25±3℃ Set aside for 3 minutes Step 2 25±3℃ 0.33C constant current and constant voltage charging, cut-off current 0.05C, time limit 240min, voltage limit 3.65V; Step 3 25±3℃ Set aside for 10 minutes Step 4 25±3℃ 0.33C constant current discharge, time limit 200min, voltage limit 2.0V Step 5 25±3℃ Set aside for 10 minutes Step 6 25±3℃ 0.33C constant current charging, cut-off current 0.01C, time limit 30min, voltage limit 3.3V; Step 7 25±3℃ Set aside for 3 minutes Step 8 25±3℃ End of capacity division

[0081] Total time for separation: 496 minutes;

[0082] The data comparison of Example 1 and Comparative Example 1 is as follows:

[0083]

[0084] The table above shows that compared to Comparative Example 1, Example 1 has shorter formation and capacity separation times and higher charging efficiency. However, there are no significant differences in their electrical performance data. Disassembly of the electrode after formation and capacity separation revealed no lithium deposition, and the three-electrode test also showed that the negative electrode potential to lithium was greater than 0V.

[0085] In summary, the formation and capacity separation method of the present invention is used on a cylindrical lithium iron phosphate battery. A negative pulse is introduced to alleviate concentration polarization and electrochemical polarization, and the ohmic polarization is eliminated by leaving the battery for a while after the negative pulse. This method can improve the charging efficiency in the formation or capacity separation stage.

[0086] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A negative pulse capacity division method for lithium iron phosphate cylindrical batteries, characterized by: Including the chemical formation process and the volume separation process, during the chemical formation process and the volume separation process: The battery cell is placed in the corresponding formation temperature and capacity temperature environment, and the charging process is divided into multiple charging cycles. In two adjacent charging cycles, the charging current value changes in a step-by-step manner; Each charging cycle includes a charging phase, a discharging phase, and a rest phase. The charging voltage limit of the charging phase in several of the charging cycles is the same, and the discharging voltage limit of the discharging phase is the same. After the charging phase reaches the charging voltage limit, the charging phase enters the discharging phase, and after reaching the discharging voltage limit, the charging phase enters the rest phase. In the same charging cycle, the charging stage and the discharging stage are both constant current charging or constant current discharging, and the current value of the discharging stage is greater than the current value of the charging stage, and the discharge duration of the discharging stage is less than the charging duration of the charging stage; During the formation process and the capacity separation process, the charging current values ​​in several charging stages first increase from small to large, and then decrease from large to small; The discharge current range of the discharge stage in the formation process is 1C-3C; the discharge current range of the discharge stage in the capacity separation process is 2C-5C; The duration of the holding phase in the formation process and the volume separation process ranges from 5s to 20s.

2. The negative pulse capacity division method for lithium iron phosphate cylindrical batteries according to claim 1, characterized in that: The formation temperature range in the formation process is 40°C-50°C; the separation temperature range in the separation process is 20°C-30°C.

3. The negative pulse capacity division method for lithium iron phosphate cylindrical batteries according to claim 1, characterized in that: The chemical formation process includes the following steps: Step 1: Before charging, let it sit for 3 minutes; Step 2: Charge at a constant current of 0.05C, with a time limit of 120 minutes and a voltage limit of 3.65V; Step 3: Set aside for 10 minutes; Step 4: Charge at 0.2C constant current for 22 minutes, with a voltage limit of 3.65V; Step 5: Discharge at 1.5C constant current for 10s, voltage limit 2.0V; Step 6: Wait for 10 seconds; Step 7: Charge at 0.5C constant current for 12 minutes, with a voltage limit of 3.65V; Step 8: Discharge at 2C constant current for 10s, voltage limit 2.0V; Step 9: Wait for 10 seconds; Step 10: Charge at 1.0C constant current for 9 minutes, with a voltage limit of 3.65V. Step 11: Discharge at 2.5C constant current for 10s, voltage limit 2.0V; Step 12: Wait for 10 seconds; Step 13: Charge at 0.5C constant current for 12 minutes, with a voltage limit of 3.65V; Step 14: Discharge at 2C constant current for 10s, voltage limit 2.0V; Step 15: Set aside for 10 minutes; Step 16: Finishing.

4. The negative pulse capacity division method for lithium iron phosphate cylindrical batteries according to claim 1, characterized in that: The volume separation process includes the following steps: Step 1: Before charging, let it sit for 3 minutes; Step 2: Charge at 1.5C constant current for 6 minutes, with a voltage limit of 3.65V; Step 3: Discharge at 3C constant current for 10s, voltage limit 2.0V; Step 4: Wait for 10 seconds; Step 5: Charge at 1C constant current for 9 minutes, with a voltage limit of 3.65V; Step 6: Discharge at 3C constant current for 10s, voltage limit 2.0V; Step 7: Wait for 10 seconds; Step 8: Charge at 0.5C constant current for 6 minutes, with a voltage limit of 3.65V; Step 9: Discharge at 3C constant current for 10s, voltage limit 2.0V; Step 10: Wait for 10 seconds; Step 11: Charge at 0.33C constant current and constant voltage, cut-off current 0.05C, time limit 90min, and voltage limit 3.65V; Step 12: Set aside for 10 minutes; Step 13: Discharge at a constant current of 0.33C, with a time limit of 200 minutes and a voltage limit of 2.0V; Step 14: Set aside for 10 minutes; Step 15: Charge at a constant current of 0.33C, a cut-off current of 0.01C, a time limit of 30 minutes, and a voltage limit of 3.3V; Step 16: Set aside for 3 minutes; Step 17: The separation is completed.

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