GB/T 39861-2021Electrochemical performance test of lithium manganese oxide—Test method for discharge plateau capacity ratio and cycle life (English PDF)

3.1 Solid Content

Solid content refers to the mass fraction (of the total amount) of the remaining part of emulsion or paint after drying under the specified conditions.

6.11 Adjustable slitting machine

Add the weighed NMP (5.4) into the stirring tank in the sanding machine (6.5); gradually add the weighed polyvinylidene fluoride (5.3) and perform dispersive stirring, until it is completely dissolved. Afterwards, add the weighed conductive agent (5.2) and perform dispersive stirring. After the dispersion is uniform, add the weighed lithium manganese oxide (5.1) and perform dispersive stirring, until the dispersion is uniform.

7.2.3 Coating

Use the coating machine (6.7) to evenly coat the stirred cathode paste on both sides of the aluminum foil (5.5). The coating speed parameter of the coating machine (6.7) is set to 800 mm/min; the blast baking temperature is set to 130 °C. Control the single surface areal density within the range of 200 g/m² ~ 230 g/m², the thickness difference <= 3 µm and the areal density deviation of the front and back surfaces < 5.0 g/m². After the coating is completed, transfer the preliminarily dried and winded cathode sheet to the vacuum oven (6.8) for secondary drying. The baking temperature is controlled at 110 °C ~ 120 °C; bake for 12 h; cool down under nitrogen (5.18) atmosphere.

7.2.4 Preparation

Take the cathode sheet in 7.2.3, which is dried and reaches the requirements of being processed; use the sheet-punching machine (6.12) to punch out a circular or square cathode sheet with an area of Sc. Use the electronic balance (6.2) and the desktop digital thickness gauge (6.13) to perform mass measurement mc and thickness measurement dc on the punched cathode sheet.

Use the sheet-punching machine (6.12) to punch out an aluminum foil substrate with an area of Sc. Use the electronic balance (6.2) and the desktop digital thickness gauge (6.13) to perform mass measurement mAl and thickness measurement dAl on the punched aluminum foil substrate.

The compaction density rhoc of the cathode sheet shall be calculated in accordance with Formula (1):

Where, rhoc---the compaction density of the cathode sheet, expressed in (g/cm3); mc---the mass of the cathode sheet, expressed in (g); mAl---the mass of the aluminum foil substrate, expressed in (g);

Sc---the area of the cathode sheet, expressed in (cm2); dc---the thickness of the cathode sheet, expressed in (µm); graphite anode (5.9), conductive agent (5.2), sodium carboxymethyl cellulose (5.10) and styrene butadiene rubber emulsion (5.11); use the electronic balance (6.1) for weighing. In accordance with the solid content of 50.0% ~ 60.0%, calculate deionized water (5.12); use the electronic balance (6.1) for weighing.

7.3.2 Pulping

Add the weighed deionized water (5.12) into the stirring tank in the planetary stirrer (6.6); gradually add the weighed sodium carboxymethyl cellulose (5.10) and perform dispersive stirring for over 2 h, until it is uniform. Add the weighed conductive agent (5.2) and perform dispersive stirring, until the dispersion is uniform. Add the weighed artificial graphite anode (5.9) and perform dispersive stirring. Finally, add the weighed styrene butadiene rubber emulsion (5.11) and perform dispersive stirring, until the dispersion is uniform.

7.3.3 Coating

In accordance with the areal capacity of the anode sheet: when the areal capacity of the cathode sheet is >= 1.1, calculate the single surface areal density of the anode sheet; control the areal density deviation of the front and back surfaces < 5.0 g/cm².

Use the coating machine (6.7) to evenly coat the 7.3.2 stirred anode paste on both sides of the copper foil (5.13). The coating speed parameter of the coating machine (6.7) is set to 800 mm/min; the blast baking temperature is set to 90 °C.

After the coating is completed, transfer the preliminarily dried and winded anode sheet to the vacuum oven (6.8) for secondary drying. The baking temperature is controlled at 100 °C ~ 110 °C; bake for 12 h; cool down under nitrogen (5.18) atmosphere.

7.3.4 Preparation

Take the anode sheet in 7.3.3, which is dried and reaches the requirements of being processed; use the sheet-punching machine (6.12) to punch out a circular or square anode sheet with an area of Sa. Use the electronic balance (6.2) and the desktop digital thickness gauge (6.13) to perform mass measurement ma and thickness measurement da on the punched anode sheet.

Use the sheet-punching machine (6.12) to punch out a copper foil substrate with an area of Sa. Use the electronic balance (6.2) and the desktop digital thickness gauge (6.13) to perform mass measurement mCu and thickness measurement dCu on the punched copper foil substrate.

The compaction density rhoa of the anode sheet shall be calculated in accordance with Formula (2):

c) Take the cathode sheet in 7.2.4 and place it on the above-mentioned half-folded separator; align the cathode sheet, the separator and the anode sheet in the center; make the active substance coverage area of the cathode sheet completely aligned with the active substance coverage area of the anode sheet, and then, perform winding;

d) Take out the winding core, which is winded in a certain shape, from the winding needle. Use the insulating glue (5.17) to paste the tail of the winding core and the battery body together;

e) Place the winding core flatwise on the battery flat press (6.17) to press and level it;

f) Number and record the leveled winding cores one by one; place them in the vacuum oven (6.8); set the oven temperature to 80 °C; vacuumize and dry for 24 h. During the baking process, in every 4 h, vacuumize and supplement nitrogen once;

g) Use the aluminum-plastic film heat sealing machine (6.18) to seal the pack the winding core in the aluminum-plastic film (5.16), with an air bag and liquid injection tube (6.21) on one side;

h) Place the plastic-sealed battery in the drying oven (6.3) and bake it for 10 h.

Set the oven temperature to 80 °C. After baking, transfer it to the inert atmosphere (argon) glove box (6.20). At the air bag of the battery, use the liquid injector (6.22) to inject the electrolyte for lithium ion batteries (5.19) into the liquid injection tube (6.21); use the insulating glue (5.17) to seal it;

i) Place the test battery (with the air bag facing upwards) with the injected electrolyte for lithium ion batteries (5.19) aside for 24 h ~ 36 h.

7.6 Battery Formation

Take the test battery, which is placed aside in 7.5; place it in insulating splints; use an appropriate mode to fix it. Afterwards, place the test battery with the insulating splints into the thermostat (6.23). Set the thermostat temperature to 45 °C ± 1 °C; preserve the heat for 2 h ~ 3 h.

Use the lithium-ion battery electrochemical performance testing machine (6.24) for charging and discharging. The charging and discharging system is as follows:

a) Charging cut-off voltage: 4.20 V;

b) Discharging cut-off voltage: 3.00 V;

In the first cycle, charge in accordance with the requirements: 0.02 C for 4 h, 0.05 C for 4 h, 0.1 C for 2 h, and 0.2 C, until it reaches the cut-off voltage; perform constant-voltage in the nth cycle is recorded as Qn.

8.1 Calculation of Discharge Plateau Capacity Ratio

The discharge plateau capacity ratio in the nth cycle of lithium manganese oxide is calculated in accordance with Formula (4): Where, Pn---the discharge plateau capacity ratio in the nth cycle;

Qn.U---the discharge capacity when discharged to the voltage U (3.60 V) requested by the test in the nth cycle, expressed in (mA - h);

Qn---the discharge capacity when discharged to the cut-off voltage in the nth cycle, expressed in (mA - h). The calculation result shall retain one decimal place.

8.2 Calculation of Cycle Life

The ratio of the discharge capacity of lithium manganese oxide in the nth cycle to the discharge capacity in the first cycle is calculated in accordance with Formula (5):

Where, etan---the ratio of the discharge capacity in the nth cycle to the discharge capacity in the first cycle;

Qn---the discharge capacity in the nth cycle, expressed in (mA - h);

Q1---the discharge capacity in the first cycle, expressed in (mA - h).

The cycle life of lithium manganese oxide shall be determined in accordance with the following method: when etan >= 80%, etan+1 < 80%, the number of cycles (n) is the cycle life of the test sample.