CN118263539A - A lithium ion secondary battery and its preparation process - Google Patents

Abstract

The present invention provides a lithium ion secondary battery and a preparation process thereof, comprising: (1) preparation of positive and negative electrode sheets; (2) preparation of a core; (3) heat sealing and liquid injection; (4) formation treatment: the battery cell is subjected to a pre-charge treatment of one charge and discharge within the range of a lower limit voltage I and an upper limit voltage I, the upper limit voltage I is increased, and one charge and discharge is performed within the range of the lower limit voltage I and the increased upper limit voltage I, until the upper limit voltage I is increased to the upper limit voltage II; (5) capacity division treatment: the battery after the formation treatment is subjected to 20 to 100 charge and discharge within the range of the lower limit voltage II and the upper limit voltage III, the upper limit voltage III is increased, and 20 to 100 charge and discharge are performed within the range of the lower limit voltage II and the increased upper limit voltage III, until the upper limit voltage III is increased to the upper limit voltage IV, to obtain a lithium ion secondary battery. The process of the present invention optimizes the formation treatment and capacity division treatment processes, maintains the late cycle stability, and realizes the long cycle of the battery.

Description

A lithium ion secondary battery and its preparation process

Technical Field

The present invention belongs to the technical field of lithium ion batteries, and in particular relates to a lithium ion secondary battery and a preparation process thereof.

Background technique

At present, with the rapid development of new energy fields, the demand for lithium batteries is getting higher and higher. Power batteries are the main core structure of electric vehicles, and their performance mainly determines the performance of the entire vehicle. However, the current battery cost, energy density and cycle life are the core concerns of lithium batteries. How to improve the energy density and cycle life of batteries is not only a matter of battery materials, but also closely related to battery design, processing technology, and later formation and capacity separation.

In the battery design process, in order to prevent lithium deposition at the negative electrode, the negative electrode/positive electrode (N/P>1) is usually designed, and the negative electrode is generally excessive. The formation process is the process of forming an effective passivation film or SEI film at the negative electrode. Since a large amount of active Li is consumed in the process of forming the SEI film on the negative electrode surface, the activity of the positive electrode is further reduced, and the N/P is further expanded, resulting in a waste of the negative electrode.

In order to ensure the full play of battery energy density and reduce battery costs, battery design and subsequent formation and capacity division have become breakthroughs in improving battery performance. There are some related technologies reported in the prior art:

Patent CN109216806A introduces a lithium-ion battery formation method that uses different current and potential pulse stimulations at different stages to activate the lithium insertion and removal activity of the negative electrode, increase the stability of the SEI film, and improve the battery's cycle performance. However, this technology does not indicate how to increase battery capacity and improve battery cycle design.

Patent CN109216810A provides a lithium-ion battery charging method and its cycle performance test based on the discharge capacity of the lithium battery, combined with the charging rate coefficient of the lithium-ion battery under fast charging conditions to charge the battery to the required voltage value, and the lithium battery is cycled using the static-discharge-static-charge process steps to test the cycle performance of the lithium battery. The charging current is set based on the discharge capacity of the lithium battery in the previous step. Since this charging method is not easy to maintain in the later stage, it is easy to form free lithium metal sheets, making the battery unsafe.

Patent CN109037813A provides a formation process for high energy density soft-pack lithium-ion batteries. Specifically, the formation process is completed by pressurizing and heating the battery cells multiple times, pre-charging the battery cells, evacuating the batteries, and venting the air bags. However, there are unsafe factors in pressurizing and heating the battery cells multiple times, and the operation steps are relatively complicated.

Summary of the invention

In order to overcome the problems existing in the prior art, the present invention provides a lithium-ion secondary battery and a preparation process thereof, which improves the capacity of the battery during the cycle process and improves the battery life by controlling the negative electrode surface capacity/positive electrode surface capacity (N/P) ratio and optimizing the formation treatment and capacity separation treatment processes.

In order to achieve the purpose of the present invention, the present invention adopts the following technical solutions:

In a first aspect, the present invention provides a process for preparing a lithium ion secondary battery, comprising the following steps:

(1) Preparation of positive and negative electrode sheets: The positive and negative electrode slurries are coated on the negative electrode sheet and the positive electrode sheet respectively at a ratio of negative electrode surface capacity to positive electrode surface capacity (N/P) of 0.8 to 1; then, the coated electrode sheets are rolled and cut to form negative electrode layers and positive electrode layers;

(2) Preparation of the electrode core: separating the positive electrode layer, the negative electrode layer and the separator layer in the order of negative electrode layer, separator layer, positive electrode layer and separator layer from the inside to the outside, and winding them with the separator to obtain the electrode core;

(3) Heat sealing and liquid injection: placing the electrode core into an aluminum-plastic film packaging bag and heat sealing it to make a battery core, injecting electrolyte into the battery core and letting it stand;

(4) Formation treatment: performing a pre-charging treatment of charging and discharging the battery cell once within the range of a lower voltage limit I and an upper voltage limit I, then gradually increasing the upper voltage limit I, and performing a charging and discharging treatment within the range of the lower voltage limit I and the increased upper voltage limit I, until the upper voltage limit I is increased to the upper voltage limit II;

(5) Capacity division treatment: the battery after formation treatment is charged and discharged for 20 to 100 times within the range of lower limit voltage II and upper limit voltage III, then the upper limit voltage III is gradually increased, and 20 to 100 times of charge and discharge are completed within the range of lower limit voltage II and increased upper limit voltage III, until the upper limit voltage III is increased to upper limit voltage IV, and after being left for 5 to 30 minutes, a lithium ion secondary battery is obtained.

In step (1) of the preparation method of the present invention, the positive and negative electrode slurries are coated on the negative electrode sheet and the positive electrode sheet in a ratio of negative electrode surface capacity/positive electrode surface capacity (N/P, N/P=negative electrode active material gram capacity×negative electrode surface density×negative electrode active material content ratio÷(positive electrode active material gram capacity×positive electrode surface density×positive electrode active material content ratio)) of 0.85 to 1. The method of the present invention performs N/P design on the actual capacity of the battery material (the first-week capacity of the material is less than the actual capacity). When N/P is less than 1, the internal resistance of the battery is effectively reduced, polarization is reduced, and the cycle life of the battery is further improved; at the same time, the negative electrode material is saved, the capacity of the negative electrode material is fully utilized, and the cost is saved.

In step (1) of some specific preparation methods, graphite, styrene-butadiene rubber, conductive carbon black and carboxymethyl cellulose are dispersed in water in a weight ratio of (92-96):(0.5-4):(1-5):(0.5-4) to obtain a solid content of 45%-55%, and the negative electrode slurry is obtained after stirring for 4-8 hours; the ternary positive electrode material, polyvinylidene fluoride and conductive carbon black are dispersed in N-methylpyrrolidone in a weight ratio of (92-96):(1-5):(1-5) to obtain a solid content of 55%-70%, and the positive electrode slurry is obtained after stirring for 4-8 hours.

In the specific process of preparing positive and negative electrode sheets according to the method of the present invention, the coated positive electrode sheets and negative electrode sheets are baked at 80-120°C for 8-24 hours and then roll-pressed. The compaction density of the positive electrode sheets after rolling is 2.8-3.5 g/cc, and the compaction density of the negative electrode sheets after rolling is 1.4-1.8 g/cc.

In step (2) of the method of the present invention, the negative electrode material is selected from natural graphite negative electrode, artificial graphite negative electrode or silicon carbon negative electrode; the positive electrode material is selected from ternary positive electrode, lithium iron phosphate positive electrode or lithium titanate; the diaphragm layer is selected from PP and PE composite film or ceramic diaphragm.

In step (3) of the method of the present invention, electrolyte is injected into the battery core at an injection volume of 2 g/Ah to 8 g/Ah; in some preferred embodiments, the battery core is left to stand for 8 to 24 hours after the electrolyte is injected.

In step (4) of the method of the present invention, the battery cell completes the pre-charging treatment and one charge-discharge treatment at a current density of 0.02 to 0.5C, preferably 0.05 to 0.3C, within the range of the lower limit voltage I and the upper limit voltage I; more preferably, the upper limit voltage I is increased to the upper limit voltage II by an increase of 0.05 to 0.2V. The above-mentioned formation treatment process optimized by the present invention can form a denser SEI film, improve the quality of SEI, maintain the later cycle stability, reduce the occurrence of later side reactions, and improve the battery cycle life.

In some specific embodiments, the voltage ranges of the lower limit voltage I, the upper limit voltage I and the upper limit voltage II in the formation treatment process of the method of the present invention are different for different positive electrode materials; if a ternary positive electrode is selected as the positive electrode material, the formation treatment process can be carried out as follows: the battery cell is pre-charged within the range of the lower limit voltage I (2V) and the upper limit voltage I (3.5-3.6V) at a current density of 0.02-0.5C for one charge and discharge; then, the upper limit voltage I is increased by 0.05-0.2V each time, and one charge and discharge is completed within the range of the lower limit voltage I (2V) and the increased upper limit voltage at a current density of 0.02-0.5C; and so on, until the upper limit voltage I is increased to the upper limit voltage II (3.8V), the formation treatment process is completed.

If lithium iron phosphate is selected as the positive electrode material, the formation treatment process can be carried out as follows: the battery cell is pre-charged within the range of the lower limit voltage Ⅰ (2V) and the upper limit voltage Ⅰ (3.2~3.4V) at a current density of 0.02~0.5C to complete one charge and discharge. Then, the upper limit voltage Ⅰ is increased by 0.05~0.2V each time, and one charge and discharge is completed at a current density of 0.02~0.5C within the range of the lower limit voltage Ⅰ (2V) and the increased upper limit voltage; and so on, until the upper limit voltage Ⅰ is increased to the upper limit voltage Ⅱ (3.5V), the formation treatment process is completed.

If lithium titanate is selected as the positive electrode material, the formation treatment process can be carried out as follows: the battery cell is pre-charged within the range of the lower limit voltage Ⅰ (1.2V) and the upper limit voltage Ⅰ (2.2~2.4V) at a current density of 0.02~0.5C to complete one charge and discharge. The upper limit voltage Ⅰ is then increased by 0.05~0.2V each time, and one charge and discharge is completed at a current density of 0.02~0.5C within the range of the lower limit voltage Ⅰ (1.2V) and the increased upper limit voltage; and so on, until the upper limit voltage Ⅰ is increased to the upper limit voltage Ⅱ (2.5V), the formation treatment process is completed.

In step (5) of the method of the present invention, the battery after the formation treatment completes 20 to 100 charge and discharge cycles at a current density of 0.1 to 2C, preferably 0.3 to 2C, within the range of the lower limit voltage II and the upper limit voltage III; more preferably, the upper limit voltage III is increased to the upper limit voltage IV by an increase of 0.05 to 0.2V. In the optimized capacity division process of the present invention, lithium deposition on the surface of the negative electrode material caused by overcharging of the material is effectively prevented, the negative electrode structure is protected from being destroyed, the structure is made more stable, and finally a long cycle of the battery is achieved, thereby improving the battery life.

In some specific embodiments, the voltage ranges of the lower limit voltage II, the upper limit voltage III and the upper limit voltage IV in the capacity division process of the method of the present invention are different for different positive electrode materials; if a ternary positive electrode is selected as the positive electrode material, the capacity division process can be carried out as follows: the battery after the formation treatment is charged and discharged 20 to 100 times at a current density of 0.1 to 2C within the range of the lower limit voltage II (2.5V) and the upper limit voltage III (3.8 to 3.9V); then, the upper limit voltage III is increased by 0.05 to 0.2V each time, and 20 to 100 times of charge and discharge are completed at a current density of 0.1 to 2C within the range of the lower limit voltage II (2.5V) and the increased upper limit voltage III; and so on, until the upper limit voltage III is increased to the upper limit voltage IV (4.2V), and the capacity division process is completed after being left for 5 to 30 minutes.

If lithium iron phosphate is selected as the positive electrode material, the capacity division treatment process can be carried out as follows: the battery after formation treatment is charged and discharged 20 to 100 times at a current density of 0.1 to 2C within the range of the lower limit voltage II (2.5V) and the upper limit voltage III (3.4 to 3.5V); then, the upper limit voltage III is increased by 0.05 to 0.2V each time, and 20 to 100 times of charge and discharge are completed at a current density of 0.02 to 0.5C within the range of the lower limit voltage II (2.5V) and the increased upper limit voltage III; and so on, until the upper limit voltage III is increased to the upper limit voltage IV (3.65V), and the capacity division treatment process is completed after being left for 5 to 30 minutes.

If lithium titanate is selected as the positive electrode material, the capacity division treatment process can be carried out as follows: the battery after the formation treatment is charged and discharged 20 to 100 times at a current density of 0.1 to 2C within the range of the lower limit voltage II (1.5V) and the upper limit voltage III (2.5 to 2.6V); then, the upper limit voltage III is increased by 0.05 to 0.2V each time, and 20 to 100 times of charge and discharge are completed at a current density of 0.02 to 0.5C within the range of the lower limit voltage II (1.5V) and the increased upper limit voltage III; and so on, until the upper limit voltage III is increased to the upper limit voltage IV (2.8V), and the capacity division treatment process is completed after being left for 5 to 30 minutes.

In a second aspect, the present invention provides a lithium ion secondary battery, which is prepared by the above-mentioned preparation process;

The first sub-capacity of the lithium-ion secondary battery is 850-950 mAh, and the first coulombic efficiency is 86-98%;

In some specific embodiments, after the lithium ion secondary battery is cycled for 300 cycles, the battery reversible capacity retention rate is 96-100%.

The above technical solution has the following technical effects:

The method of the present invention reduces N/P to below 1, thereby effectively reducing the internal resistance of the battery, reducing polarization generation, and further improving the battery cycle life; at the same time, it saves negative electrode materials, fully utilizes the capacity of the negative electrode materials, and saves costs.

The preparation process of the present invention optimizes the formation treatment and volume fractionation treatment processes, maintains the late cycle stability, reduces the occurrence of late side reactions, effectively avoids lithium deposition on the surface of the negative electrode material caused by overcharging of the material, protects the negative electrode structure from being destroyed, and ultimately achieves a long cycle of the battery.

Detailed ways

In order to facilitate the understanding of the present invention, the present invention will be further described below in conjunction with examples. It should be understood that the following examples are only for a better understanding of the present invention and do not mean that the present invention is limited to the following examples.

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.

The following methods were used to test the electrochemical properties of the prepared batteries in the following examples:

(1) Initial efficiency: The ratio of the first discharge of the battery to the first charge of the battery is the initial efficiency;

(2) Capacity loss after 100, 200 and 300 cycles: GB31241-2014.

Example 1

(1) Preparation of negative electrode sheet: Graphite, SBR (styrene butadiene rubber), conductive carbon black, and CMC (carboxymethyl cellulose) were dispersed in deionized water at a weight ratio of 93:2:3: ² , wherein the solid content was adjusted to 45%, and stirred for 6 hours to obtain a negative electrode slurry; then, the slurry was coated on a 12 μm thick copper foil at a double-sided surface density of 11 mg/cm2, baked at 110°C for 12 hours, and then rolled to obtain a compacted density of 1.55 g/cc. After cutting, a negative electrode layer with a length of 8.8 cm and a width of 6.4 cm was obtained;

Preparation of positive electrode sheet: The ternary positive electrode material (Umicore 111), PVDF (polyvinylidene fluoride), and SP (conductive carbon black) were dispersed in NMP (N-methylpyrrolidone) according to a weight ratio of 93:3:4, wherein the solid content was adjusted to 65%, and stirred for 8 hours to obtain a positive electrode slurry; then, it was coated on a 10-micron thick aluminum foil according to a double-sided surface density of 21 mg/ ^cm2 , baked at 110°C for 12 hours, and then rolled to obtain a compacted density of 3.35 g/cc. After cutting, a positive electrode layer with a length of 8.8 cm and a width of 6 cm was obtained;

Wherein, the N/P ratio is 0.95; N/P = negative electrode active material gram capacity × negative electrode surface density × negative electrode active material content ratio ÷ (positive electrode active material gram capacity × positive electrode surface density × positive electrode active material content ratio);

(2) Preparation of the electrode core: The 7 positive electrode layers, 8 negative electrode layers and lithium battery separator (Celgard, USA, 12 μm) obtained above are separated in the order of negative electrode layer, separator layer, positive electrode layer and separator layer from the inside to the outside, and the entire battery core is wrapped with the separator, and then the tabs are welded to obtain the electrode core;

(3) placing the electrode core into an aluminum-plastic film packaging bag and heat-sealing it to make a battery core, injecting 6 g of lithium battery electrolyte into the battery core and leaving it to stand for 12 hours;

(4) The lower limit voltage I of the formation charge voltage is set to 2V, the upper limit voltage I is set to 3.6V, and the battery cell is pre-charged within the range of 2 to 3.6V at 0.1C to complete one charge and discharge; then, the upper limit voltage I is increased to 3.7V, and the charge voltage is controlled within the range of 2 to 3.7V at 0.1C to complete one charge and discharge; thereafter, the upper limit voltage I is set to 3.8V (i.e., the upper limit voltage II), and the charge voltage is controlled within the range of 2 to 3.8V at 0.1C to complete one charge and discharge;

(5) The battery after the formation treatment is charged and discharged 20 times at a current density of 0.1 to 2C within the range of a lower voltage limit II (2.5V) and an upper voltage limit III (3.9V); then, the upper voltage limit III is increased by 0.05V each time, and 20 times of charge and discharge are performed at a current density of 0.1 to 2C within the range of the lower voltage limit II (2.5V) and the increased upper voltage limit III (3.95V, 4.0V, 4.05V, 4.1V, 4.15V, and 4.2V, respectively), to complete the battery capacity division treatment, and after standing for 10 minutes between tests, a lithium ion secondary battery 1 is obtained.

Example 2

This embodiment adopts the same method as steps (1) to (3) of embodiment 1 to obtain a battery cell, except that NP is 0.92;

(4) The lower limit voltage I of the formation charge voltage is set to 2V, the upper limit voltage I is set to 3.5V, and the battery cell is pre-charged within the range of 2 to 3.5V at 0.2C to complete one charge and discharge; then, the upper limit voltage I is increased to 3.6V, and the charge voltage is controlled within the range of 2 to 3.6V at 0.2C to complete one charge and discharge; thereafter, the upper limit voltage I is set to 3.7V, and the charge voltage is controlled within the range of 2 to 3.7V at 0.2C to complete one charge and discharge; and the upper limit voltage I is set to 3.8V, and the charge voltage is controlled within the range of 2 to 3.8V at 0.2C to complete one charge and discharge;

(5) The battery after formation treatment is charged and discharged 20 times at a current density of 0.1 to 2C within the range of (2.5V) and upper limit voltage III (3.9V); then, the upper limit voltage III is increased by 0.05V each time, and 20 times of charge and discharge are performed within the range of lower limit voltage II (2.5V) and increased upper limit voltage III (3.95V, 4.00V, 4.05V, 4.10V, 4.15V, 4.20V, respectively) at a current density of 0.1 to 2C to complete the battery capacity division treatment. After 15 minutes of stabilization between tests, a lithium ion secondary battery 2 is obtained.

Example 3

This embodiment adopts the same method as steps (1) to (3) of embodiment 1 to obtain a battery cell, except that NP is 0.85;

(4) The lower limit voltage I of the formation charge voltage is set to 2V, the upper limit voltage I is set to 3.6V, and the battery cell is pre-charged within the range of 2 to 3.6V at 0.1C to complete one charge and discharge; then, the upper limit voltage I is increased to 3.7V, and the charge voltage is controlled within the range of 2 to 3.7V at 0.1C to complete one charge and discharge; thereafter, the upper limit voltage I is set to 3.8V (i.e., the upper limit voltage II), and the charge voltage is controlled within the range of 2 to 3.8V at 0.1C to complete one charge and discharge;

(5) The battery after the formation treatment is charged and discharged 60 times at a current density of 0.1 to 2C within the range of lower voltage limit II (2.5V) and upper voltage limit III (3.8V); then, the upper voltage limit III is increased by 0.1V each time, and 60 times of charge and discharge are performed at a current density of 0.1 to 2C within the range of lower voltage limit II (2.5V) and the increased upper voltage limit III (3.90V, 4.00V, 4.10V, and 4.20V, respectively), to complete the battery capacity division treatment. After 5 minutes of stagnant time between tests, a lithium ion secondary battery 3 is obtained.

Example 4

This embodiment adopts the same method as steps (1) to (3) of embodiment 1 to obtain a battery cell, except that NP is 0.80;

(4) The lower limit voltage I of the formation charge voltage is set to 2V, the upper limit voltage I is set to 3.5V, and the battery cell is pre-charged within the range of 2 to 3.5V at 0.1C to complete one charge and discharge; then, the upper limit voltage I is increased to 3.6V, and the charge voltage is controlled within the range of 2 to 3.6V at 0.1C to complete one charge and discharge; thereafter, the upper limit voltage I is set to 3.7V, and the charge voltage is controlled within the range of 2 to 3.7V at 0.2C to complete one charge and discharge; and the upper limit voltage I is set to 3.8V, and the charge voltage is controlled within the range of 2 to 3.8V at 0.1C to complete one charge and discharge;

(5) The battery after the formation treatment is charged and discharged 80 times at a current density of 0.1 to 2C within the range of a lower voltage limit II (2.5V) and an upper voltage limit III (3.8V); then, the upper voltage limit III is increased by 0.2V each time, and 80 times of charge and discharge are performed at a current density of 0.1 to 2C within the range of the lower voltage limit II (2.5V) and the increased upper voltage limit III (4.0V and 4.2V, respectively), to complete the battery capacity division treatment. After 20 minutes of stagnating between tests, a lithium ion secondary battery 4 is obtained.

Example 5

This embodiment adopts the same method as steps (1) to (3) of embodiment 1 to obtain a battery cell, except that NP is 0.88;

(4) The lower limit voltage I of the formation charge voltage is set to 2V, the upper limit voltage I is set to 3.6V, and the battery cell is pre-charged within the range of 2 to 3.6V at 0.1C to complete one charge and discharge; then, the upper limit voltage I is increased to 3.7V, and the charge voltage is controlled within the range of 2 to 3.7V at 0.1C to complete one charge and discharge; thereafter, the upper limit voltage I is set to 3.8V (i.e., the upper limit voltage II), and the charge voltage is controlled within the range of 2 to 3.8V at 0.1C to complete one charge and discharge;

(5) The battery after the formation treatment is charged and discharged 50 times at a current density of 0.1 to 2C within the range of a lower voltage limit II (2.5V) and an upper voltage limit III (3.8V); then, the upper voltage limit III is increased by 0.05V each time, and 50 times of charge and discharge are performed at a current density of 0.1 to 2C within the range of the lower voltage limit II (2.5V) and the increased upper voltage limit III (respectively 3.85V, 3.90V, 3.95V, 4.00V, 4.05V, 4.10V, 4.15V, 4.20V), to complete the battery capacity division treatment, and after 20 minutes of stagnating between tests, a lithium ion secondary battery 5 is obtained.

Example 6

This embodiment adopts the same method as steps (1) to (3) of embodiment 1 to obtain a battery cell, and the positive electrode material is selected from lithium iron phosphate of Defang Nano, the difference is that the NP is 0.92;

(4) The lower limit voltage I of the formation charge voltage is set to 2V, the upper limit voltage I is set to 3.3V, and the battery cell is pre-charged within the range of 2 to 3.3V at 0.1C to complete one charge and discharge; then, the upper limit voltage I is increased to 3.4V, and the charge voltage is controlled within the range of 2 to 3.4V at 0.1C to complete one charge and discharge; thereafter, the upper limit voltage I is set to 3.5V (i.e., the upper limit voltage II), and the charge voltage is controlled within the range of 2 to 3.5V at 0.1C to complete one charge and discharge;

(5) The battery after the formation treatment is charged and discharged 50 times at a current density of 0.1 to 2C within the range of a lower voltage limit II (2.5V) and an upper voltage limit III (3.5V); then, the upper voltage limit III is increased by 0.05V each time, and 50 times of charge and discharge are performed at a current density of 0.1 to 2C within the range of the lower voltage limit II (2.5V) and the increased upper voltage limit III (3.55V, 3.60V, and 3.65V, respectively), to complete the battery capacity division treatment. After 20 minutes of stagnating between tests, a lithium ion secondary battery 6 is obtained.

Example 7

This embodiment adopts the same method as steps (1) to (3) of embodiment 1 to obtain a battery cell, and the positive electrode material is selected from lithium titanate of Bamo Technology, the difference being that the NP is 0.92;

(4) The lower limit voltage I of the formation charge voltage is set to 1.2V, the upper limit voltage I is set to 2.3V, and the battery cell is pre-charged within the range of 1.2 to 2.3V at 0.1C to complete one charge and discharge; then, the upper limit voltage I is increased to 2.4V, and the charge voltage is controlled within the range of 2 to 2.4V at 0.1C to complete one charge and discharge; thereafter, the upper limit voltage I is set to 2.5V (i.e., the upper limit voltage II), and the charge voltage is controlled within the range of 1.2 to 2.5V at 0.1C to complete one charge and discharge;

(5) The battery after the formation treatment is charged and discharged 50 times at a current density of 0.1 to 2C within the range of a lower voltage limit II (1.5V) and an upper voltage limit III (2.6V); then, the upper voltage limit III is increased by 0.05V each time, and 50 times of charge and discharge are performed at a current density of 0.1 to 2C within the range of the lower voltage limit II (1.5V) and the increased upper voltage limit III (2.65V, 2.70V, 2.75V, and 2.80V, respectively), to complete the battery capacity division treatment. After 20 minutes of stagnating between tests, a lithium ion secondary battery 7 is obtained.

Comparative Example 1

In this comparative example, a battery cell was obtained by the same method as steps (1) to (3) of Example 1, except that NP was 1.1, to obtain a lithium ion secondary battery 1-1.

Comparative Example 2

This comparative example differs from Example 1 in that step (4) is performed in the following manner;

(4) After the battery is pre-charged, the formation charge voltage is set to be controlled within the range of 2 to 4.2 V and one charge and discharge is completed at 0.2 C to obtain a lithium ion secondary battery 1-2.

Comparative Example 3

This comparative example differs from Example 1 in that step (5) is performed in the following manner:

(5) The battery after the formation treatment was charged and discharged for 100 times at a cycle voltage of 2.75 to 4.2 V, and finally a battery cycle test was completed. After being left aside for 10 minutes, a lithium ion secondary battery 1-3 was obtained.

The electrochemical performance of the lithium-ion battery prepared above was tested, and the data are shown in the following table;

It can be seen from the data in the above table that the method of the present invention effectively reduces the internal resistance of the battery, improves the initial efficiency of the whole battery, and increases the cycle life of the battery by adjusting the battery NP ratio formula and optimizing the formation treatment and capacity separation treatment.

Claims (10)

1. A process for preparing a lithium ion secondary battery, characterized in that it comprises the following steps: (1) Preparation of positive and negative electrode sheets: The positive and negative electrode slurries are coated on the negative electrode sheet and the positive electrode sheet respectively at a ratio of negative electrode surface capacity to positive electrode surface capacity (N/P) of 0.8 to 1; then, the coated electrode sheets are rolled and cut to form negative electrode layers and positive electrode layers; (2) Preparation of the electrode core: separating the positive electrode layer, the negative electrode layer and the separator layer in the order of negative electrode layer, separator layer, positive electrode layer and separator layer from the inside to the outside, and winding them with the separator to obtain the electrode core; (3) Heat sealing and liquid injection: placing the electrode core into an aluminum-plastic film packaging bag and heat sealing it to make a battery core, injecting electrolyte into the battery core and letting it stand; (4) Formation treatment: performing a pre-charging treatment of charging and discharging the battery cell once within the range of a lower voltage limit I and an upper voltage limit I, then gradually increasing the upper voltage limit I, and performing a charging and discharging treatment within the range of the lower voltage limit I and the increased upper voltage limit I, until the upper voltage limit I is increased to the upper voltage limit II; (5) Capacity division treatment: the battery after formation treatment is charged and discharged for 20 to 100 times within the range of lower limit voltage II and upper limit voltage III, then the upper limit voltage III is gradually increased, and 20 to 100 times of charge and discharge are completed within the range of lower limit voltage II and increased upper limit voltage III, until the upper limit voltage III is increased to upper limit voltage IV, and after being left for 5 to 30 minutes, a lithium ion secondary battery is obtained. 2. The preparation process according to claim 1 is characterized in that in step (1), the positive and negative electrode slurries are coated on the negative electrode sheet and the positive electrode sheet in a ratio of negative electrode surface capacity to positive electrode surface capacity (N/P) of 0.85 to 1. 3. The preparation process according to claim 1 or 2, characterized in that in step (1), graphite, styrene-butadiene rubber, conductive carbon black and carboxymethyl cellulose are dispersed in water in a weight ratio of (92-96):(0.5-4):(1-5):(0.5-4) to obtain a negative electrode slurry with a solid content of 45%-55%, and stirred for 4-8 hours; The ternary positive electrode material, polyvinylidene fluoride and conductive carbon black are dispersed in N-methylpyrrolidone in a weight ratio of (92-96):(1-5):(1-5) to obtain a solid content of 55%-70%, and the positive electrode slurry is obtained after stirring for 4-8 hours. 4. The preparation process according to claim 3 is characterized in that the coated positive electrode sheet and negative electrode sheet are baked at 80-120°C for 8-24 hours and then roll-pressed, and the compaction density of the positive electrode sheet after rolling is 2.8-3.5 g/cc, and the compaction density of the negative electrode sheet after rolling is 1.4-1.8 g/cc. 5. The preparation process according to any one of claims 1 to 4, characterized in that in step (2), the negative electrode material is selected from natural graphite negative electrode, artificial graphite negative electrode or silicon-carbon negative electrode; The positive electrode material is selected from a ternary positive electrode, a lithium iron phosphate positive electrode or lithium titanate; The diaphragm layer is selected from PP and PE composite films or ceramic diaphragms. 6. The preparation process according to any one of claims 1 to 5, characterized in that, in step (3), the electrolyte is injected into the battery cell at an injection volume of 2 g/Ah to 8 g/Ah; Preferably, the electrolyte is injected and then left to stand for 8 to 24 hours. 7. The preparation process according to any one of claims 1 to 6, characterized in that in step (4), the battery cell completes the pre-charging process and the one-time charging and discharging process at a current density of 0.02 to 0.5C, preferably 0.05 to 0.3C, within the range of the lower limit voltage I and the upper limit voltage I; More preferably, the upper limit voltage I is increased to the upper limit voltage II by an increase range of 0.05 to 0.2V. 8. The preparation process according to any one of claims 1 to 7, characterized in that, in step (5), the battery after the formation treatment is charged and discharged 20 to 100 times at a current density of 0.1 to 2C, preferably 0.3 to 2C, within the range of a lower limit voltage II and an upper limit voltage III; More preferably, the upper limit voltage III is increased to the upper limit voltage IV by an increase range of 0.05 to 0.2V. 9. A lithium ion secondary battery, characterized in that it is prepared by the preparation process described in any one of claims 1 to 8; The first sub-capacity of the lithium-ion secondary battery is 850-985 mAh, and the first coulomb efficiency is 86-98%. 10 . The lithium-ion secondary battery according to claim 9 , wherein after the lithium-ion secondary battery has been cycled for 300 cycles, the reversible capacity retention rate of the battery is 96-100%.