As we all know, ternary lithium batteries come in three packaging forms: cylindrical lithium-ion batteries, prismatic lithium-ion batteries, and pouch lithium-ion batteries. Cylindrical and prismatic batteries mostly use steel or aluminum casings, while pouch batteries use aluminum-plastic film. Today, we'll discuss the differences between cylindrical and prismatic ternary lithium batteries.
1. Energy density ratio
Energy density ratio refers to the capacity per unit weight of battery. For cylindrical cells, based on the current mainstream 18650 (1.75AH) in China, the energy density ratio can reach 215Wh/kg, while for prismatic cells (50AH), it can reach 205Wh/kg. The system assembly rate is around 60% for 18650 cells and around 70% for prismatic cells. (The system assembly rate can be imagined as placing hams in a box; square hams have smaller gaps between them, resulting in a higher system assembly rate.)
Based on these calculations, the energy density ratio of an 18650 battery pack system is approximately 129 Wh/kg, while that of a prismatic battery pack system is approximately 143 Wh/kg. In the future, when the energy density ratios of 18650 and prismatic cells reach the same level, prismatic lithium battery packs, with their higher assembly ratio, will have a more significant advantage.
2. Multiplier
Charge/discharge rate = charge/discharge current / rated capacity. The higher the charge/discharge rate, the faster the charging speed supported by the battery. Domestically manufactured mainstream energy-type battery cells, such as the 18650, operate at around 1C, while prismatic cells can reach around 1.5-2C (with proper thermal management). Both are still some distance from the policy target of 3C. However, with the continuous improvement of prismatic cell manufacturing processes, achieving the established 3C target is entirely possible.
3. Cycle life
After 2000 cycles of 1C charge and 1C discharge, the best square battery cell still retains 80% of its capacity, while the best 18650 battery cell only retains 70%. This shows that the cycle life of the square battery is superior to that of the 18650 battery.
4. Consistency
Cell consistency is a major headache for cell manufacturers, and for users, the more cells there are, the harder it is to control consistency. Assuming a defect rate of one in ten thousand for a single cell, a round cell battery pack consists of 7,000 18650 cells, and a square cell battery pack consists of 250 cells. A battery pack without any defective cells is considered合格 (qualified). Therefore, the合格率 (qualification rate) for a round cell battery pack is 0.99999^7000 = 93.2%, and the合格率 (qualification rate) for a square cell battery pack is 0.99999^245 = 99.7%. Of course, the theoretical knowledge about battery consistency is much more complex than this; this is just an example to illustrate that using square cell battery packs with larger individual cell capacity and fewer cells is more conducive to controlling consistency.
5. Manufacturing costs
Manufacturing costs include material costs, equipment costs, labor, utilities, site costs, and hidden auxiliary material costs. Currently, mainstream companies can control the overall cost of 18650 at around 0.5 yuan/WH, while the overall cost of square-shaped ones is approximately 0.8 yuan/WH.
Before the rise of the new energy vehicle industry, 18650 battery cells were already widely used in various fields such as electronic products and industrial equipment. However, the single-cell capacity of 18650 battery cells is relatively low, which is insufficient to meet the requirements of electric vehicles, so Tesla developed the 21700 model. Currently, the high degree of automation in equipment is where cylindrical battery manufacturers can save the most on labor costs, but the production and sales chain of cylindrical batteries is already very mature, and there is almost no room for further compression.
There is still significant room for improvement in the size and electrode thickness of prismatic cells. In terms of size, they are becoming increasingly larger, from the initial 20AH to the current mass production of 50AH products. Larger capacity means lower cost per watt (WH), so we believe it is entirely possible for the cost of large-capacity prismatic cells to reach parity with cylindrical cells when mass production begins.
