GB/T 42260-2022Electrochemical performance test of lithium iron phosphate - Test method for cycle life (English PDF)

Foreword

This document was issued on 30 December 2022 by the State Administration for Market Regulation; Standardization Administration of the PRC and takes effect on 1 April 2023.

It is a GB/T standard: recommended rather than compulsory, but it is the text a Chinese reviewer applies when assessing a submission.

1 Scope

GB/T 42260-2022 covers the cycle life test for lithium iron phosphate as a cathode material, not as a finished cell - the ambient conditions the work is carried out under, the reagents and materials, the instruments and equipment, the test steps, the data recording and the cycle life determination, the allowable difference between results, and the test report. The route is fixed from end to end: the powder, characterised by its particle size and specific surface area, is dried in a vacuum oven under nitrogen or argon, combined with a conductive carbon, made into an electrode, rolled in an atmosphere drier than the rest of the work demands, and built into a wound test battery that is then cycled while the discharge capacity of each cycle is recorded against that of the first. Fixing the build is the whole point of the document. Cycle life is the figure a cathode supplier quotes and a cell maker buys on, and it moves with moisture, electrode loading and press density at least as much as with the powder, so two suppliers' numbers mean nothing side by side unless the battery around the material was made the same way. Written for cathode material producers, cell manufacturers and qualification laboratories.

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 degrees C ~ 30 degrees C. 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 um ~ 8.0

um, 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 x 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 um ~ 20 um.

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 um, and the thickness is 9.0 um ~ 25.0 um.

5.8 Graphite: D50 is 10.0 um ~ 22.0 um, 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 x 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 um ~ 12 um.

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 um

~ 160 um.

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 degrees C) 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 degrees C ~ 150 degrees C 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 degrees C ~ 90 degrees C 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 (rhoc) is calculated according to

formula (1):

where:

rhoc - 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 um, 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 degrees C

~ 90 degrees C.

The compacted density of the negative electrode sheet (rhoa) is calculated according to

formula (2):

where:

rhoa - 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

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:

etan - 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 etan >= 80 % and etan+1