In the rolling process, why does the positive electrode use hot rollers and the negative electrode uses cold pressing?

This is mainly due to three reasons: differences in material characteristics between the cathode and anode, varying process effects and performance requirements, and different temperature sensitivities of binders.

1.Differences in Material Characteristics Between Cathode and Anode

The cathode materials (such as LiFePO4, NCM) are hard and poorly conductive, and hot rolling can effectively enhance the compaction effect:

the high hardness of particles leads to high compaction resistance (the compaction resistance of the cathode is four times that of the anode), and hot rolling softens the PVDF binder to enhance the bonding force between the active material and the current collector.

Hot rolling can reduce pole piece rebound by about 50%, reduce rolling force by up to 62% (depending on the specific material system and process capabilities), and simultaneously improve the distribution of conductive agents, enhancing electron conduction efficiency.

The anode's graphite is low in hardness and prone to plastic deformation, but excessive compaction can easily lead to particle crushing:

secondary cold rolling adjusts thickness and pore structure in stages, reducing stress concentration and avoiding particle fracture caused by a single high pressure.

Secondary rolling can make the pore distribution more uniform, reducing the expansion rate from 5.00% to 4.47% after cycling and improving cycle stability.

2.Process Effects and Performance Requirements

Optimization of cathode hot rolling:

Hot rolling at 100°C significantly reduces pole piece resistance (by 2.1%) and thickness rebound rate (by 50%), while increasing the peak peel strength.

Hot rolling requires less rolling force when thinning pole pieces, and thickness uniformity is easier to control (uniformity of roller surface temperature is required to be high, as it deteriorates at 120°C).

Advantages of anode secondary cold rolling:

Secondary cold rolling gradually increases compaction density, avoiding a decrease in peel strength caused by a single high pressure (e.g., peel strength after one-time rolling is 0.298N vs. remaining stable at 0.298N after secondary rolling).

The lateral and longitudinal elongation rates stabilize at 0.27% and 1.17%, respectively, reducing the risk of pole piece cracking.

3.Binder and Temperature Sensitivity

The cathode's PVDF maintains good viscosity at high temperatures (40~150°C), and hot rolling promotes crosslinking with active substances, enhancing bonding strength.

The anode's aqueous binder (such as CMC/SBR) is heat-sensitive, and high temperatures may cause degradation.

Cold rolling maintains chemical stability, avoiding a decrease in peel strength. Due to the cathode's hardness and poor conductivity, hot rolling is required to improve compaction and electrical performance; anode secondary cold rolling balances the need for plastic deformation with structural integrity, avoiding particle crushing and stabilizing peel strength. The differences in processes between the two are determined by material characteristics and performance optimization goals.

References:

Fundamental Research on Microstructure Evolution and Performance Regulation of Lithium-ion Battery Pole Piece During Rolling Deformation, Zhang Junpeng

Impact of Secondary Rolling Ratio on Anode Pole Piece Characteristics, Liu Zhongkui

Impact of Hot Rolling on the Performance of Lithium-ion Battery Cathode Pole Piece, Lv Zhaocai

Study on Mechanics and Deformation Characteristics During Lithium-ion Battery Pole Piece Rolling Process, Xu Chengjie