short cycle life, high manufacturing cost, poor physical stability, and the tendency to expand and even catch fire.

Polymer batteries (another name for it is  lithium polymer batteries or Lipo Batteries) are highly popular due to their lightweight and flexible design. However, they also have some drawbacks, such as shorter cycle life, higher manufacturing costs, and poor physical stability. Since they do not have a sturdy metal casing, improper handling can cause them to expand or catch fire.

Here are the main disadvantages of polymer batteries categorized by type:

1. Expensive, truly expensive

The production process of polymer batteries is complex, and the purification of the electrolyte system is difficult. Additionally, to fit into various irregular-shaped devices, they need to be custom-molded separately, resulting in a high overall cost. A reliable polymer battery cell on the market costs about $0.75-$1.5/Ah, while an 18650 cylindrical battery of the same capacity costs only about $0.45/Ah. The price difference can be more than double. 

2. Energy density is not superior

Many people believe that polymer batteries are more advanced and have higher energy density. However, this is actually the opposite. The specific energy density of polymer batteries is usually between 200-250Wh/kg , while excellent cylindrical lithium batteries have already surpassed 300Wh/kg. Under the same weight, the amount of electricity that polymer batteries can store is actually less. 

3. Shorter cycle life

The complete charge and discharge cycle of consumer-grade polymer batteries typically lasts only 300-500 times, after which their capacity will drop to less than 80% of the initial level. In contrast, high-quality 18650 cells have a cycle life of over a thousand times, and the difference is even more significant when used in power tools or energy storage applications. 

4. Fast charging and high current discharge are difficult

The internal electrolyte of polymer batteries is a gel, and lithium ions move less smoothly in it compared to liquid electrolytes. Therefore, they are inherently not suitable for high-rate charging and discharging. Once high-rate fast charging is applied, lithium is prone to be deposited inside the battery, and the electrodes may crack. The cycle life will drop sharply from the thousands of times of normal slow charging to less than a thousand times.

5. Completely unusable after one failure

Polymer batteries do not have a standard shape and are almost tailor-made for specific devices, with extremely poor versatility. Once a battery fails, the entire cell is directly scrapped and cannot be repaired. Unlike cylindrical batteries, which can still have individual cells replaced, the maintenance cost is relatively high. 

6. Extreme reliance on protection circuit

Although polymer batteries themselves are not prone to catching fire or exploding, they are extremely sensitive to overcharging and overdischarging. As long as the protection circuit design is inadequate or fails, the chemical activity inside the battery will be irreversibly damaged, significantly reducing its lifespan or even causing it to "starve", so a high-quality protection board must be used. This, in turn, indirectly increases the complexity and cost of the entire solution.