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What's the relationship between energy density and size of lithium batteries for electric vehicles

PostTime: 2020-12-28     Views: 802

What's the relationship between energy density and size of lithium batteries for electric vehicles

Lithium ion battery

Step 1: list the related fixed parameters

The positive electrode capacity of Li ion battery is 142mah / g;

Negative electrode capacity: 330mah / g;

The compaction density of positive electrode was 3.8g/cm3;

The compaction density of negative electrode was 1.55g/cm3;

The proportion of lithium hydroxide in positive electrode: 94% and the proportion of graphite powder in negative electrode: 94%

Diaphragm thickness: 0.016;

Ear width: 4mm;

As for a kind of lithium-ion battery system, some parameters are fixed, so we just need to set them in the design!

Step 2: battery design tour

1. Small battery, one core, then the maximum thickness of the core: = battery thickness shell thickness = 5.2-0.2 * 2 = 4.8mm (generally, the thickness of aluminum shell is 0.2-0.3, which may be different in different places, but it doesn't matter much)

2. If the core height is 1 mm except the top cover, 0.5 mm for the bottom cover, 1.5 mm for the plastic spacer and 1 mm for the reserved space, then the core height = 50-1-0.5-1.5-1 = 46 mm, that is to say, the diaphragm can be 44 mm wide. Generally, if we make aluminum shell, we may choose 20um diaphragm. If we make large battery, the diaphragm may be thicker, about 25um to 35um.

3. The width of the negative electrode is definitely narrower than that of the diaphragm, which can be 1 mm or 2 mm. It depends on how you keep it. Assuming 2 mm here, the width of the negative electrode is 42 mm. (it can be widened by 3-4mm when making some large batteries)

4. If the negative pole covers the positive pole, then the positive pole is 1-3mm narrower than the negative pole. Because it is a small battery, we choose 1mm as the width of the positive pole

=44-1=43mm。

5. According to the relevant experience, it is assumed that the density of both sides of the positive electrode is 40.5mg/cm2, the gram capacity of the positive electrode is 140mAh / g, and the active material proportion is 94%; the gram capacity of the negative electrode is 320mah / g, and the active material proportion is 94%. Safety factor = 1.03

(can also be higher, 1.1)

Then we can calculate: density of both sides of negative electrode = 40.5 * 140 * 94% / (320 * 94%)

05 = 18.45mg/cm26, the positive electrode material can be pressed to 3.8g/cm3, the negative electrode can be pressed to 1.6g/cm3, aluminum foil 12um, copper foil 12um.

7. Then the thickness of positive and negative electrodes can be calculated: the thickness of positive electrode is 40.5 / 3.8 * 10 + 16 = 119um, the thickness of negative electrode is 18.45 / 1.6 + 10 = 127um;

8. Then, there are two layers in one winding, and the thickness of single layer = the thickness of positive electrode + the thickness of negative electrode + the thickness of two layers of diaphragm

=119+127+16*2=278um,

9. It can be seen that the total number of coils = core thickness / (single layer thickness * 2)

=4.8 * 90% / (278 / 1000) = 15.5 turns, rounded: 15 turns for negative pole and 16 turns for positive pole

(90% of the space is reserved for the thickness of the battery, and the value of the thickness is also different, which depends on the rebound of the material thickness.)

10. Calculate the needle size, needle width = core width - core thickness - 2=

(34-0.20*2)-4.8)*92%=27mm

11. Calculation of positive plate length: positive plate length = (number of layers - 2) * double-layer winding thickness+

Needle width + needle thickness + winding thickness of one layer + needle width) * (number of layers - 1) / 2+

Needle width thickness / 2 = ((16-2) * 0.. 278 * 2)+

(27 + 27 + 0.5 + 0.262)) * (16-1) / 2 + 27-5.2/2 = 505mm, fine-tuning the size of the test paper.

Calculation of negative electrode length: negative electrode = positive electrode length - needle width - tail scraping position - 10

12. The width and thickness of the polar ear, small battery, polar ear 0.1 * 4mm is basically enough

13. Electrode scraping position:

A=10

B = needle width * 2.7 + 3

C=D+4*2

D = polar ear width + (2-4)

E = needle-c / 2

14. Calculation capacity: design capacity = positive dressing amount * gram capacity * active substance ratio = * 140mAh / g * 7.516 * 94% = 1052mah

15. Nominal capacity: in general design, the design capacity is about 1.05% of the nominal capacity

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