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GB/T 42260-2022 English PDF (GBT42260-2022)

GB/T 42260-2022 English PDF (GBT42260-2022)

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GB/T 42260-2022: Electrochemical performance test of lithium iron phosphate -- Test method for cycle life

This document describes the test method for cycle life of lithium iron phosphate, i.e., the cathode material for lithium-ion batteries. This document applies to the test using the winding method for cycle life of lithium iron phosphate, i.e., the cathode material for lithium-ion batteries.
GB/T 42260-2022
GB
NATIONAL STANDARD OF THE
PEOPLE REPUBLIC OF CHINA
ICS 77.160
CCS H 21
GB/T 42260-2022
Electrochemical performance test of lithium iron phosphate
- Test method for cycle life
ISSUED ON: DECEMBER 30, 2022
IMPLEMENTED ON: APRIL 01, 2023
Issued by: State Administration for Market Regulation;
Standardization Administration of the PEOPLE Republic of China.
Table of Contents
Foreword ... 3
1 Scope ... 4
2 Normative references ... 4
3 Terms and definitions ... 4
4 Test conditions ... 4
5 Reagents and materials ... 4
6 Instruments and equipment ... 5
7 Test steps ... 7
8 Data recording and cycle life test ... 16
9 Allowable difference ... 17
10 Test report ... 17
Electrochemical performance test of lithium iron phosphate
- Test method for cycle life
1 Scope
This document describes the test method for cycle life of lithium iron phosphate, i.e., the cathode material for lithium-ion batteries.
This document applies to the test using the winding method for cycle life of lithium iron phosphate, i.e., the cathode material for lithium-ion batteries.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. GB/T 6682 Water for analytical laboratory use - Specification and test methods GB/T 18287 General specification of lithium-ion cells and batteries for mobile phone
3 Terms and definitions
There are no terms or definitions to be defined in this document.
4 Test conditions
Unless otherwise specified, each test step should be carried out at a relative humidity not greater than 40.0 % and an ambient temperature of 20 ??? ~ 30 ???. The rolling process should be carried out at a relative humidity not greater than 30.0 % and an ambient temperature not greater than 30 ??C.
5 Reagents and materials
5.1 Lithium iron phosphate: the particle size characteristic value (D50) is 0.5 ??m ~ 8.0 ??m, and the specific surface area is 6 m2/g ~ 30 m2/g.
5.2 Conductive agent: conductive carbon material.
5.3 Polyvinylidene fluoride (PVDF): battery grade, the weight average molecular weight is not less than 5 ?? 105, and the moisture (mass fraction) is not greater than 0.10 %.
5.4 N-methylpyrrolidone (NMP): battery grade, the purity is not less than 99.9 %, and the moisture (mass fraction) is not greater than 0.02 %.
5.5 Aluminum foil: the thickness is 8 ??m ~ 20 ??m.
5.6 Positive electrode tab (positive terminal): made of aluminum, with tab glue. 5.7 Lithium-ion battery separator: polyolefin porous membrane, the porosity is 35.0 % ~ 60.0 %, the air permeability is 100 s/100 mL ~ 500 s/100 mL, the average pore size is not greater than 1.0 ??m, and the thickness is 9.0 ??m ~ 25.0 ??m.
5.8 Graphite: D50 is 10.0 ??m ~ 22.0 ??m, the initial discharge specific capacity is not less than 340.0 mA ?€? h/g, and the initial charge-discharge efficiency is not less than 90.0 %. 5.9 Sodium carboxymethyl cellulose (CMC): the main content (mass fraction) is not less than 99.5 %, and the relative molecular mass is 6.5 ?? 106.
5.10 Styrene-butadiene rubber emulsion (SBR): water-soluble binder, special for lithium batteries, the solid content is 35 % ~ 52 %, the viscosity is 80 mPa ?€? s ~ 400 mPa ?€? s, and the pH is 6.0 ~ 7.0.
5.11 Deionized water: GB/T 6682, not less than grade three.
5.12 Copper foil: the thickness is 5 ??m ~ 12 ??m.
5.13 Negative electrode tab (negative terminal): made of nickel, with tab glue. 5.14 Aluminum plastic film: special for lithium batteries, and the thickness is 110 ??m ~ 160 ??m.
5.15 Polyimide tape.
5.16 Lithium-ion battery electrolyte: lithium-ion battery electrolyte composed of lithium hexafluorophosphate (LiPF6) and mixed carbonate-based organic solvents [ethylene carbonate (EC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), etc.], the moisture is not greater than 0.002 %, the free acid (HF) is not greater than 0.005 %, and the conductivity (25 ???) is not less than 7.0 mS/cm.
5.17 Nitrogen (or argon): the purity (volume fraction) is not less than 99.99 %. 6 Instruments and equipment
6.1 Balance: the accuracy is 0.01 g.
7 Test steps
7.1 Preprocessing
7.1.1 Place lithium iron phosphate (5.1) and conductive agent (5.2) into a vacuum oven (6.3); during drying, vacuum or circulate in a nitrogen (or argon) (5.17) atmosphere; bake at a temperature of 100 ??? ~ 150 ??? for 2 h ~ 20 h to dry; cool to room temperature and place in a desiccator (6.4).
7.1.2 Place PVDF (5.3) into a vacuum oven (6.3); during drying, vacuum or circulate in a nitrogen (or argon) (5.17) atmosphere; bake at a temperature of 70 ??? ~ 90 ??? for 4 h ~ 6 h to dry; cool to room temperature and place in a desiccator (6.4). 7.2 Preparation of positive electrode sheets
7.2.1 Weighing
Calculate the lithium iron phosphate, conductive agent, and PVDF pretreated in 7.1 according to the mass fractions of 90 % ~ 97 %, 1 % ~ 5 %, and 2 % ~ 5 % respectively, and weigh them with a balance (6.1). Calculate the amount of NMP (5.4) according to the design requirements of solid content (mass fraction) of 40 % ~ 65 %, and weigh with a balance (6.1).
7.2.2 Pulping
The cathode pulping process is as follows:
a) Add the weighed NMP into the mixing tank of a dispersing mixer (6.5); gradually add the weighed PVDF; disperse and stir until completely dissolved; to prepare a transparent glue;
b) Add the weighed conductive agent to the above transparent glue; vacuum, disperse and stir evenly;
c) Gradually add the weighed lithium iron phosphate in portions; vacuum, disperse and stir evenly;
d) Add another NMP according to the designed solid content (mass fraction), to control the slurry viscosity at 4000 mPa ?€? s ~ 8000 mPa ?€? s; vacuum, disperse and stir evenly; complete the pulping process.
NOTE: The solid content in this document is the ratio of the mass of the cathode active material lithium iron phosphate, conductive agent, and PVDF to the mass of the cathode slurry. 7.2.3 Coating
digital thickness gauge (6.11) to measure the mass (mc) and thickness (dc) of the positive electrode sheet, respectively.
Use a punching machine (6.10) to punch out an aluminum foil substrate with an area of Sc, and use an electronic balance (6.2) and a desktop digital thickness gauge (6.11) to measure the mass (mAl) and thickness (dAl) of the aluminum foil substrate, respectively. The compacted density of the positive electrode sheet (??c) is calculated according to formula (1):
where:
??c - the compacted density of the positive electrode sheet, in grams per cubic centimeter (g/cm3);
mc - the mass of the positive electrode sheet, in grams (g);
mAl - the mass of the aluminum foil substrate, in grams (g);
Sc - the area of the positive electrode sheet, in square centimeters (cm2); Dc - the thickness of the positive electrode sheet, in centimeters (cm); dAl - the thickness of the aluminum foil substrate, in centimeters (cm). Design according to the compacted density of 2.1 g/cm3 ~ 2.7 g/cm3, calculate the theoretical thickness of the positive electrode sheet, use a roller machine (6.12) to roll the positive electrode sheet after the secondary baking in 7.2.3 to the target thickness, and operate according to the following steps:
a) Use an edge trimming machine (6.7) to trim the edges of the positive electrode sheet after rolling;
b) Use a soft brush (6.8) to remove abnormal protrusions and edge burrs on the surface of the positive electrode sheet;
c) Use an adjustable slitting machine (6.9) to cut the positive electrode sheet to the designed width (Wc) (see Figure 1);
d) Use a ruler (6.13) to measure the length of the area covered by the active material on both sides of the positive electrode sheet, record it as Lc1 (see Figure 1); e) Use an electronic balance (6.2) to weigh the positive electrode sheet after wiping, and number and record;
f) Use a ruler (6.13) to measure the total length of the positive electrode active material and the exposed foil area of the aluminum foil, record it as Lc0 (see Figure 1).
In the exposed foil area, use an ultrasonic welding machine (6.17) to weld the positive electrode tab (5.13) to the A side of the positive electrode sheet. Randomly inspect to ensure that there are no missing welding, weak welding, or over-welding in the battery core, and then place it in a vacuum oven (6.3) for storage. The positive electrode sheet before assembly is shown in Figure 1.
7.3 Preparation of negative electrode sheets
7.3.1 Weighing
Calculate graphite (5.8), conductive agent (5.2), CMC (5.9), and SBR (5.10) according to the mass fractions of 91.0 % ~ 98.0 %, 0.5 % ~ 3.0 %, 0.5 % ~ 3.0 %, and 1.0 % ~ 3.0 % respectively, and weigh them with a balance (6.1). Calculate the amount of deionized water (5.11) according to the solid content (mass fraction) of 45.0 % ~ 60.0 %, and weigh with a balance (6.1).
7.3.2 Pulping
The negative electrode pulping process is as follows:
a) Add the weighed deionized water into the mixing tank of the dispersing mixer (6.5), gradually add the weighed CMC, and disperse and stir for more than 2 h until uniform;
b) Add the weighed conductive agent; vacuum, disperse and stir evenly;
c) Add the weighed graphite; vacuum, disperse and stir evenly;
d) Add the weighed SBR, vacuum; disperse and stir evenly; control the slurry viscosity at 1500 mPa ?€? s ~ 4500 mPa ?€? s; complete the pulping process. 7.3.3 Coating
Design based on the ratio of negative electrode sheet capacity to positive electrode sheet capacity of 1.10 ~ 1.15. Calculate the single-sided density of the negative electrode sheet. Control the single-sided coating surface density of the negative electrode slurry within the range of 60 g/m2 ~ 110 g/m2, the thickness difference to be not greater than 5 ??m, and the density deviation between the front and back sides to be less than 5.0 g/m2.
Use a coater (6.6) to evenly coat the mixed negative electrode slurry on the front and back sides of the copper foil (5.12). The coating rate parameter of the coater (6.6) is set to 800 mm/min ~ 2000 mm/min, and the baking temperature of the blast is set to 70 ??? ~ 90 ???.
The compacted density of the negative electrode sheet (??a) is calculated according to formula (2):
where:
??a - the compacted density of the negative electrode sheet, in grams per cubic centimeter (g/cm3);
ma - the mass of the negative electrode sheet, in grams (g);
mCu - the mass of the copper foil substrate, in grams (g);
Sa - the area of the negative electrode plate, in square centimeters (cm2); da - the thickness of the negative electrode sheet, in centimeters (cm); dCu - the thickness of the copper foil substrate, in centimeters (cm).
Design according to the compacted density of 1.45 g/cm3 ~ 1.65 g/cm3, calculate the thickness of the negative electrode sheet, use a roller machine (6.12) to roll the negative electrode sheet after the secondary baking in 7.3.3 to the target thickness, and operate according to the following steps:
a) Use an edge trimming machine (6.7) to trim the edges of the negative electrode sheet after rolling;
b) Use a soft brush (6.8) to remove abnormal protrusions and edge burrs on the surface of the negative electrode sheet;
c) Use an adjustable slitting machine (6.9) to cut the negative electrode sheet to the designed width (Wa) (see Figure 2);
d) Use a ruler (6.13) to measure the length of the area covered by the active material on both sides of the negative electrode sheet, record them as lengths La1 and La2, and ensure that Lc1 < La2 < La1;
e) Use an electronic balance (6.2) to weigh the negative electrode sheet after wiping, and number and record;
f) Use a ruler (6.13) to measure the total length of the negative active material and the exposed foil area of the aluminum foil, record it as La0.
In the exposed foil area, use an ultrasonic welding machine (6.17) to weld the negative electrode tab (5.13) to the C side of the negative electrode sheet. Randomly inspect to ensure that there are no missing welding, weak welding, or over-welding in the battery core, and then place it in a vacuum oven (6.3) for storage. The negative electrode sheet before assembly is shown in Figure 2.
7.4 Separator preparation
Take the lithium-ion battery separator (5.7) and use the adjustable slitting machine (6.9) to cut the separator. The length is recorded as Ls and the width is recorded as Ws. It shall meet the requirements of Wc < Wa < Ws and Lc0 < La0 < Ls, where Wc is the width of the positive electrode sheet, Wa is the width of the negative electrode sheet, Ws is the width of the separator, Lc0 is the length of the positive electrode sheet, La0 is the length of the negative electrode sheet, and Ls is the length of the separator.
7.5 Battery assembly
The schematic diagram of how to roll up the upper and lower separators and the positive and negative electrode sheets is shown in Figure 3. To assemble a test battery, it can refer to the following steps:
a) Take the cut separators in 7.4, and put them on the winding needle of a winding machine (6.14);
b) Place the negative electrode sheet in 7.3.4 between the two layers of separators and align them in the center;
c) Take the positive electrode sheet in 7.2.4 and place it on a separator, so that the positive electrode sheet, separators, and negative electrode sheet are centered and wound;
d) Start the winding machine, and automatically wind according to the set program; e) Take out the winding core from the winding needle, and use polyimide tape (5.15) to glue and fix the tail and top of the core respectively;
f) Place the winding core flat on a battery flattening machine (6.15) and press it flat. After the flattening is completed, disassemble the first winding core, and check the status of the electrode sheets to ensure there are no cracks;
g) Put the winding core into an aluminum plastic film (5.14) shell, use a heat-sealing machine (6.16) to seal the top side of the aluminum plastic film shell to make a battery core; place it in a vacuum oven (6.3), and set the oven temperature at 50 ??? ~ 85 ???; vacuum or dry under nitrogen (or argon) (5.17) atmosphere circulation for 15 h ~ 36 h;
h) Transfer the battery core to an inert atmosphere (argon) glove box (6.18); use a pipette gun (6.19) to inject the lithium-ion battery electrolyte (5.16) into the open end of the aluminum plastic film shell; after injection, use a vacuum sealing machine (6.20) to perform initial vacuum sealing in the glove box;
NOTE: Other battery formation conditions are agreed upon between the supply and demand parties. 7.7 Battery capacity division
Take the test battery formed in 7.6, use a lithium-ion battery electrochemical performance tester (6.22) to divide the capacity. In the capacity dividing process, the constant-current constant-voltage charging cut-off voltage is 3.6 V ~ 3.7 V, the constant- voltage charging cut-off current is 0.02 C ~ 0.05 C, and the constant-current discharge end voltage is 2.0 V ~ 2.5 V. It can refer to the following steps for the capacity division system:
a) Use a 0.1 C/0.1 C rate constant-current constant-voltage charging-discharging system for charging and discharging (the initial charge-discharge efficiency can be calculated). That is, at 0.1 C rate, constant-current charge to 3.6 V ~ 3.7 V; then constant-voltage charge, the constant-voltage charging cut-off current is 0.02 C ~ 0.05 C; and then at 0.1 C rate, constant-current discharge to 2.0 V ~ 2.5 V; b) Use a 0.5 C/0.5 C rate constant-current constant-voltage charging-discharging system for charging and discharging. That is, at 0.5 C rate, constant-current charge to 3.6 V ~ 3.7 V; then constant-voltage charge, the constant-voltage charging cut-off current is 0.02 C ~ 0.05 C; and then at 0.5 C rate, constant-current discharge to 2.0 V ~ 2.5 V;
c) Use a 1 C/1 C rate charging-discharging system for charging and discharging. That is, at 1 C rate, constant-current charge to 3.6 V ~ 3.7 V; then constant-voltage charge, the constant-voltage charging cut-off current is 0.02 C ~ 0.05 C; and then at 1 C rate, constant-current discharge to 2.0 V ~ 2.5 V;
d) At 1 C rate, constant-current constant-voltage charge to the end voltage of 3.5 V, and the constant-voltage charging cut-off current is 0.02 C ~ 0.05 C.
Among them, the current value corresponding to the charging-discharging current (It) at t C rate can be calculated by referring to formula (3):
where:
m - the mass of the active material lithium iron phosphate in the test battery, in grams (g);
C - the initial discharge specific capacity in the half-battery of the active material lithium iron phosphate in the test battery, in milliamperes per gram (mA ?€? h/g); t - the rate at which charging or discharging is completed in 1/t h, in per hour (h-1). NOTE: Other battery capacity division systems are agreed upon between the supply and demand parties.
7.8 Battery test
After the formation and capacity division of the test battery, use a lithium-ion battery electrochemical performance tester (6.22) to carry out the cycle life test. The charge and discharge voltage limits are as follows:
a) Charge limit voltage: constant-current constant-voltage charge to 3.6 V ~ 3.7 V, constant-voltage charging cut-off current is 0.02 C ~ 0.05 C;
b) Discharge end voltage: 2.0 V ~ 2.5 V;
c) Charge-discharge system: according to the provisions in GB/T 18287, use 1C/1C to perform charge-discharge cycles at ambient temperatures of (23 ?? 2) ??? and (55 ?? 2) ??? respectively.
8 Data recording and cycle life test
8.1 Data recording
Record the charge-discharge capacity at different number of cycles during the cycle process of the test battery. The discharge capacity of the 1st cycle when it is discharged to the end voltage is recorded as Q1, and the discharge capacity of the nth cycle when it is discharged to the end voltage is recorded as Qn.
8.2 Cycle life test
The ratio of the discharge capacity of the nth cycle to the discharge capacity of the 1st cycle of lithium iron phosphate is calculated according to formula (4): where:
??n - the ratio of the discharge capacity of the nth cycle to the discharge capacity of the 1st cycle;
Qn - the discharge capacity of the nth cycle, in milliampere hour (mA ?€? h); Q1 - the initial discharge capacity, in milliampere hour (mA ?€? h).
Calculation results are rounded to one decimal place.
The cycle life of lithium iron phosphate is determined as follows: the number of cycles n when ??n ??? 80 % and ??n+1 < 80 % is the cycle life of the test sample.

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