CN113517481B

CN113517481B - Capacity grading method for lithium battery

Info

Publication number: CN113517481B
Application number: CN202110378888.2A
Authority: CN
Inventors: 苏锋; 常林荣
Original assignee: Zhejiang Chaowei Chuangyuan Industrial Co Ltd
Current assignee: Zhejiang Chaowei Chuangyuan Industrial Co Ltd
Priority date: 2021-04-08
Filing date: 2021-04-08
Publication date: 2022-12-27
Anticipated expiration: 2041-04-08
Also published as: CN113517481A

Abstract

The invention relates to the field of lithium battery manufacturing, and provides a capacity grading method of a lithium battery aiming at the problem that the capacity of a battery core cannot be graded in the prior art, which comprises the following steps that 1, the battery core of the lithium battery is charged and discharged to obtain a charging and discharging curve; 2, segmenting the charge-discharge curve according to the voltage range of the SOC to obtain the slope of each segment; finding a section with a large tangent slope and a strong linear relation as a screening section; controlling the charging capacity of the battery cell within the voltage range of the screening section obtained by the step 2 during formation, and laying the battery cell after formation to obtain the steady-state voltage of the battery cell in the screening section; 4, performing linear fitting on the voltage and the capacity on the screening interval curve selected in the step 2 to obtain a formula y = ax + b; and 5, substituting the voltage obtained by the step 3 into a formula of the step 4 to obtain the actual capacity of the battery cell, and then grouping the capacities of the battery cells. The invention matches the actual capacity of the battery through the sampling point of the pre-charging voltage, cancels the capacity grading process and improves the production efficiency.

Description

Capacity grading method for lithium battery

Technical Field

The invention belongs to the field of lithium battery manufacturing, and particularly relates to a capacity grading method of a lithium battery.

Background

At present, the lithium ion battery has the advantages of excellent cycle performance, rate performance, high power, high energy density and the like, and is widely applied to the aspects of electric automobiles, buses, mobile phones, notebooks, electric bicycles, electric tools, energy storage and the like. Although the voltage of a lithium ion battery cell is high, generally about 3.0-4.2V, for the application field of electric vehicles, energy storage or buses, the lithium battery cell needs to be connected in series and in parallel to reach a certain voltage and capacity for use, and the series and parallel connection needs to be used only when each battery cell has good consistency, such as parameters of capacity, voltage, internal resistance, discharge platform, self-discharge rate and the like, which need to be controlled within a certain range.

In the actual production process, the consistency of the batteries is controlled from the processes of batching, coating, tabletting, assembling, forming and battery sorting, and particularly, the sorting of the battery capacity is very important. At present, each manufacturer can charge and discharge the battery cell after the battery cell is formed, and select the capacity. However, the capacity sorting procedure is added during mass production, a large amount of charging and discharging cabinet equipment, manpower, energy consumption and the like are needed, and the time for discharging the battery cells is prolonged. Patent CN201610679638.1 discloses a method for selecting the voltage of a lithium ion battery and its application, wherein an equation is established between the voltage during precharging and the charging capacity and the rated capacity, and the voltage value is selected according to the rated capacity required by the battery. However, this method cannot step the cell capacity, and the capacity and voltage are not in a linear relationship, so that the actual capacity of the battery cannot be accurately estimated. Accordingly, an ideal solution is needed.

Disclosure of Invention

Aiming at the problem that the capacity of a battery core cannot be graded in the prior art, the invention provides the capacity grading method of the lithium battery.

In order to achieve the above purpose, the invention adopts the following technical scheme,

a lithium battery capacity grading method and application thereof comprise the following steps:

(1) Charging and discharging a battery core of a lithium battery to obtain a charging and discharging curve;

preferably, the charging and discharging operations are as follows:

(2) Segmenting the charge-discharge curve according to the voltage range of the SOC to obtain the slope of each segment; finding a section with a large tangent slope and a strong linear relation as a screening section;

(3) During formation, controlling the charging capacity of the battery cell within the voltage range of the screening section obtained in the step (2), and after formation, laying the battery cell to obtain the steady-state voltage of the battery cell in the screening section;

(4) Performing linear fitting on the voltage and the capacity on the screening section curve selected in the step (2) to obtain a formula y = ax + b, wherein y represents the actual capacity of the battery cell, x represents the steady-state voltage of the battery cell in the section, and a and b are constants;

(5) And (4) substituting the voltage obtained in the step (3) into the formula in the step (4) to obtain the actual capacity of the battery cell, and then grouping the capacities of the battery cells.

The invention provides a capacity grading method of a lithium battery, which is characterized in that a linear equation is established between the voltage of a battery core during pre-charging and the actual capacity of the battery, and the actual capacity of the battery is fitted through sampling points of pre-charging voltage. The slope of each segment is higher by using a tangent method. The relation between the actual capacity of the battery cell and the voltage is linear and strongly correlated, and the voltage is the voltage in a steady state. The method can cancel the capacity grading process, only charges certain electric quantity into the battery during the pre-preparation, can save the use of a capacity grading detection cabinet, reduces the production period of the battery, greatly improves the production efficiency, has small deviation between the fitted capacity and the actual capacity and high precision, and is suitable for mass production.

Preferably, the system of the battery core is one of lithium iron phosphate-graphite, ternary-graphite, lithium manganate-graphite, ternary composite-graphite, lithium manganate composite-graphite, lithium manganese-rich base-graphite, lithium iron manganese phosphate-graphite, ternary-lithium titanate, lithium manganate-lithium titanate, ternary composite-lithium titanate, lithium manganese-rich base-lithium titanate, lithium iron manganese phosphate-lithium titanate and lithium manganate composite-lithium titanate system.

Preferably, the segmentation of step (2) is performed by segmenting every 10% of the voltage range, and dividing into 10 segments.

Preferably, when the core is a lithium iron phosphate-graphite system, the screening section is 0% -10% and 90% -100% of the SOC section. When the core system is a lithium iron phosphate-graphite system, the tangent slope of the SOC section is larger and the linear relation is stronger in the ranges of 0% -10% and 90% -100%.

Preferably, the screening interval of the lithium iron phosphate-graphite system is 90% -100% of the SOC interval. In the 0-10% section, the film formation of the negative electrode of the battery cell is incomplete, and the activation is incomplete, so that the formation voltage of the lithium iron phosphate-graphite system is controlled to be 90-100% of the SOC section.

Preferably, when the core is a ternary composite-graphite system, the screening interval is 0% -10%, 40% -60% of the SOC interval. When the core system is a ternary composite-graphite system, 0% -10% and 40% -60% of the tangent slope of the SOC section is large.

Preferably, the screening section of the ternary complex-graphite system is 40% -60% SOC section. Similarly, the formation voltage of the ternary composite-graphite system is controlled to be 40-60% of the SOC section because the film formation of the negative electrode of the battery cell is incomplete and the activation is incomplete in the 0-10% section.

Preferably, the shelf of the step (3) is a normal-temperature shelf of 12-72 h.

Therefore, the invention has the following beneficial effects: the method can cancel the capacity grading process, only charge the battery with certain electric quantity during the pre-preparation, save the use of a capacity grading detection cabinet, reduce the production period of the battery, greatly improve the production efficiency, have small deviation between the fitted capacity and the actual capacity and high precision, and is suitable for mass production

Drawings

FIG. 1 is a charge-discharge curve diagram of a lithium iron phosphate-graphite system;

fig. 2 is a capacity-voltage diagram of a lithium iron phosphate-graphite system;

FIG. 3 is a graph of charge and discharge curves for a ternary composite-graphite system;

fig. 4 is a graph of capacity versus voltage for the ternary composite-graphite system.

Detailed Description

The invention is further described below with reference to specific embodiments.

In the present invention, unless otherwise specified, all the raw materials and equipment used are commercially available or commonly used in the art, and the methods in the examples are conventional in the art unless otherwise specified.

Example 1

1. Production charge-discharge curve of lithium iron phosphate-graphite system

Taking 50-100 mass-produced 20Ah battery cells, repeating the following process for 3 times to perform charging and discharging:

working steps	Working mode	Time (min)	Current (A)	Voltage (V)	Cutoff current (A)
						1	Constant current and constant voltage charging	10	3.65	1
2	Lay aside	10
						3	Constant current discharge	20	2.0
4	Lay aside	10

Obtaining a standard charge-discharge curve through the charge-discharge process;

2. taking the slope of a standard charging and discharging curve: segmenting the curve according to the 10% SOC interval, and taking the slope of the curve;

3. according to step 2, taking the voltage range 90-100% thereof, in this example 98% SOC;

4. pressurizing and forming the battery cores which are injected with liquid in batches and aged, wherein the formation parameters are as follows:

the formation charge is just 98% SOC;

5. standing the formed battery cell for 36 hours at normal temperature, and measuring the steady-state voltage of the battery cell;

6. taking 50 batteries under a steady state voltage for normal capacity grading to obtain the actual capacity of the 50 batteries;

7. fitting the actual capacity of the battery cell subjected to capacity division in the step 6 with the corresponding steady-state voltage to obtain a formula y = -114.14x +401.7, wherein y is the actual capacity, and x is the steady-state voltage;

8. and (4) substituting the steady-state voltages of the battery cells in the batch obtained in the step (5) into the formula in the step (7), obtaining the actual capacity of the battery cells through fitting, and then performing capacity grading according to the capacity sorting standard.

After normal batch production (electrical core system and capacity unchanged), steps 1, 2, 3 and 6, 7 can be omitted. The capacity of the battery cell can be obtained by forming the production battery cell, measuring the steady-state voltage of the production battery cell and substituting the steady-state voltage into the formula, and then capacity grading is carried out according to the capacity sorting standard.

Example 2

1. Battery cell charging and discharging curve produced by ternary composite-graphite system

working steps	Working mode	Time (min)	Current (A)	Voltage (V)	Cutoff current (A)
						1	Constant current and constant voltage charging	10	4.2	1
2	Lay aside	10
						3	Constant current discharge	20	3.0
4	Lay aside	10

Obtaining a standard charging and discharging curve through the charging and discharging process;

2. taking the slope of a standard charging and discharging curve: segmenting the curve according to 10% SOC segment, taking its slope;

3. according to step 2, taking the voltage range from 40% to 60% SOC, in this example, 55% SOC;

the formation charge is just 45% SOC;

7. fitting the actual capacity of the battery cell subjected to capacity division in the step 6 with the corresponding steady-state voltage to obtain a formula y = -86.425x +363.51, wherein y is the actual capacity, and x is the steady-state voltage;

8. and (4) substituting the steady-state voltages of the battery cores in the batch obtained in the step (5) into the formula in the step (7), obtaining the actual capacity through fitting, and grading the capacity according to the capacity sorting standard.

After normal batch production (the electrical core system and the capacity are unchanged), steps 1, 2, 3, 6 and 7 can be omitted. The capacity of the battery cell can be obtained by forming the production battery cell, measuring the steady-state voltage of the production battery cell and substituting the steady-state voltage into the formula, and then the capacity is graded according to the capacity sorting standard.

It can be seen from the above embodiments that the capacity grading method according to the present invention can quickly, efficiently and accurately grade the capacity of the battery cell.

Although the present invention has been described with reference to the preferred embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the present invention.

Claims

1. A capacity grading method of a lithium battery is characterized by comprising the following steps:

(1) Charging and discharging the battery core of the lithium battery to obtain a charging and discharging curve;

(2) Segmenting the charge-discharge curve according to the voltage range of the SOC to obtain the slope of each segment; finding a section with a large tangent slope and a strong linear relation as a screening section; the segments are segmented according to every 10% of the voltage range and are divided into 10 segments; when the electric core system is a lithium iron phosphate-graphite system, the screening section is 0% -10% and 90% -100% of the SOC section; when the electric core system is a ternary composite-graphite system, the screening interval is 0-10%, 40-60% of the SOC interval;

(3) During formation, the battery cell charging capacity is controlled within the voltage range of the screening section obtained in the step (2), and after formation, the battery cell is laid aside to obtain the steady-state voltage of the battery cell in the screening section; the shelf is a normal-temperature shelf of 12-72 h;

(5) Substituting the voltage obtained in the step (3) into the formula in the step (4) to obtain the actual capacity of the battery cell, and then grouping the capacity of the battery cell;

and (4) after normal batch production, the electric core system and the capacity are not changed, and only the step (3) is needed in the steps.

2. The method of claim 1, wherein the screening segment of the lithium iron phosphate-graphite system is 90% -100% soc segment.

3. The capacity grading method of a lithium battery according to claim 1, characterized in that the screening section of the ternary complex-graphite system is 40% -60% soc section.