Date: 2025-07-31 hits: 106
A. Introduction
As application demands continue to increase, the capacity requirements for lithium battery cells are also increasing. Increasing the cell diameter has become a common method for increasing capacity. However, excessive cell diameter also presents a series of problems. In-depth research into the causes of excessive diameter is crucial for optimizing lithium battery performance and promoting industry development.
B. Production Process Limitations
(I) Space Utilization and Deformation Issues in the Winding Process
Currently, mainstream winding processes are a key step in battery cell manufacturing, but they suffer from inherent structural limitations.
1. Internal Space Utilization Issues
During the winding process, the electrode sheets and separators are continuously wound around a winding needle, forming a "multi-layered" structure. Because the bending areas of the winding needle (such as the corners of a square winding needle) and the welding areas (where the tabs meet) cannot be fully filled with active material, internal space utilization is low. To compensate for this capacity loss, the cell diameter must be increased to accommodate more electrode sheets, thereby increasing the overall cell capacity.
2. Electrode Deformation Issues
After winding, the battery cell needs to be shaped by needle withdrawal (retraction). However, the shape of the needle (e.g., flattened) can cause fluctuations in electrode tension (linear speed can vary by more than 10 times), leading to "S" deformation (electrode twisting) or internal wrinkles. To avoid these problems, the diameter should be appropriately increased to reduce the impact of tension on the edges, ensuring electrode integrity and battery cell performance.
C. Material Properties
(I) Expansion and Thermal Runaway Risk
1. Electrode Expansion
During charging, the formation of the SE1 film (solid electrolyte interface film) generates gas. Simultaneously, the electrode materials (such as graphite and silicon-based materials) experience lattice expansion due to lithium ion intercalation. The flat structure of the prismatic cell has weak pressure resistance. As the diameter increases, the stress on the shell wall increases, leading to increased swelling. This not only affects the appearance and dimensional stability of the cell, but may also adversely affect its performance and safety.
2. Thermal Management Challenges
Larger-diameter cells have increased internal thermal resistance, making it difficult for heat to dissipate during charging and discharging, leading to larger internal temperature differences (e.g., the top-bottom temperature difference can reach 5-10°C). Uneven temperatures accelerate electrode material aging and increase the risk of thermal runaway (in thermal runaway, the gas exhaust path is extended, and pressure builds more rapidly). Thermal management issues are a key factor limiting further increases in cell diameter.
D. Safety and Structural Design
(I) Electrode Alignment and Separator Integrity
Larger-diameter cells have more winding layers, requiring higher precision alignment between the electrode and separator. Improper tension control can easily lead to pole piece misalignment and separator wrinkling, resulting in direct contact between the positive and negative electrodes (short circuit). Short circuits are a major safety hazard for lithium batteries, potentially causing serious consequences such as thermal runaway, fire, and even explosion. Therefore, strict requirements for pole piece alignment and separator integrity limit the expansion of battery cell diameter.
(2) Thermal Runaway Protection
Increasing the cell diameter increases the thermal runaway exhaust path (the distance from the interior to the outer casing), preventing the timely discharge of high-temperature gases, exacerbating internal pressure buildup and increasing the risk of explosion.
To ensure cell safety, more effective thermal runaway protection measures are needed, but this also imposes certain constraints on increasing cell diameter.
E. Equipment and Process Compatibility
(I) Limitations of Winding Needles and Tension Control
1. Winding Needle Shape Limitations
A round winding needle can cause tab deformation. Flattened winding needles require variable tension winding (tension decreases with increasing layer count) or variable speed winding (reducing line speed fluctuations) to reduce deformation. However, these process adjustments require a certain sacrifice in production efficiency and their adaptability to larger-diameter battery cells remains limited. Different winding needle shapes can hinder the expansion of battery cell diameters.
2. Equipment Accuracy Requirements
The winding of larger-diameter battery cells requires higher tension control accuracy (error must be less than ±5%) and winding needle concentricity (deviation must be less than 0.1mm). The accuracy of existing equipment limits further expansion of battery cell diameter. Equipment accuracy has become a hardware factor restricting the expansion of battery cell diameter.
F. Performance Requirements
(I) Balancing Capacity and Power
1. Capacity Increase Demand
To meet the high-capacity demands of energy storage and power batteries (e.g., 500Ah+ and 600Ah+ cells), the cell diameter must be increased to accommodate more active material. However, as the diameter increases, the electron migration path (the distance from the electrode edge to the tab) becomes longer, leading to increased internal resistance (and a decrease in efficiency of approximately 2-5%). This must be offset by a full-tab design (shortening the current path). While pursuing high capacity, the efficiency issues associated with increased internal resistance must be addressed.
2. Power Performance Limitations
Rate performance (fast charge/discharge capability) of large-diameter cells decreases due to the increased internal ion diffusion distance. To balance capacity and power, optimized electrode materials (e.g., high-rate graphite) or structures (e.g., multi-tab designs) are necessary, but this also increases manufacturing complexity. This balance between capacity and power is one of the key factors contributing to the challenges of increasing cell diameter.
G. Summary
The excessively large diameter of lithium battery cells is a result of a combination of production process limitations, material properties, safety design, equipment compatibility, and performance requirements. Although increasing the diameter can increase capacity, issues such as deformation, thermal management, safety and power performance need to be addressed.
