Polymer lithium batteries refer to lithium-ion batteries that use polymers as electrolytes, and can be further divided into "semi-polymer" and "full polymer" types.
"Semi-polymer" refers to coating a barrier film with a polymer (usually PVDF) layer, which strengthens the cell's adhesion and allows for a more rigid battery; the electrolyte remains a liquid. "Full polymer," on the other hand, uses a polymer to form a gel network inside the cell before injecting the electrolyte. While "full polymer" batteries still use liquid electrolyte, the amount used is significantly less, greatly improving the safety performance of lithium-ion batteries. To the author's knowledge, only Sony is currently mass-producing "full polymer" lithium-ion batteries.
In other words, polymer batteries refer to lithium-ion batteries that use aluminum-plastic packaging film as their outer packaging, also known as soft-pack batteries. This packaging film consists of three layers: a PP layer, an Al layer, and a nylon layer. Because PP and nylon are polymers, this type of battery cell is called a polymer battery.
I. The difference between polymer lithium batteries and lithium batteries
1. The raw materials are different. The raw material of lithium-ion batteries is electrolyte (liquid or colloid); the raw material of polymer lithium batteries is electrolyte, which can be polymer electrolyte (solid or colloid) or organic electrolyte.
2. In terms of safety, lithium-ion batteries are prone to explosion in high temperature and high pressure environments; polymer lithium batteries use aluminum-plastic film as the outer shell, and when organic electrolyte is used inside, they will not explode even when the liquid is very hot.
3. Different shaping: Polymer batteries can be made thin, with arbitrary area and shape because their electrolyte can be solid or gel-like rather than liquid. Lithium batteries, on the other hand, use electrolyte and require a sturdy outer shell as a secondary packaging to contain the electrolyte.
4. The cell voltages are different. Because polymer batteries use high-molecular materials, they can be made into multiple layers in the cell to achieve high voltage. However, the nominal capacity of a lithium battery cell is 3.6V. To achieve high voltage in practical applications, multiple cells need to be connected in series to form an ideal high-voltage operating platform.
5. Due to different manufacturing processes, polymer batteries are easier to produce when they are thinner, while lithium batteries are easier to produce when they are thicker. This allows lithium batteries to be used in a wider range of fields.
6. Capacity: The capacity of polymer batteries has not been effectively improved, and is even reduced compared to the standard capacity of lithium batteries.
II. Advantages and disadvantages of polymer lithium batteries
(I) Advantages of polymer lithium batteries
1. Good safety performance. Polymer lithium batteries use aluminum-plastic soft packaging in their structure, which is different from the metal shell of liquid lithium cells. In the event of a safety hazard, lithium-ion cells will simply explode, while polymer cells will only bulge or at most burn.
2. Thinner profiles allow for even thinner and ultra-thinner designs, with thicknesses potentially below 1mm, enabling integration into credit cards. While conventional liquid lithium batteries face technical limitations in achieving a thickness below 3.6mm, 18650 batteries have even more stringent volume requirements.
3. Lightweight and high-capacity: Polymer electrolyte batteries do not require a metal casing for protection, so for the same capacity, they are 40% lighter than steel-cased lithium batteries and 20% lighter than aluminum-cased batteries. For larger volumes, polymer batteries have a higher capacity, approximately 30% greater.
4. The shape can be customized. The thickness of polymer batteries can be increased or decreased according to practical needs. For example, a new laptop from a well-known brand uses trapezoidal polymer batteries to make full use of the internal space.
(II) Advantages of polymer lithium batteries
1. The main reason is the higher cost, because the design can be customized according to customer needs, and the research and development costs must be taken into account. In addition, the variety of shapes and types leads to various tooling fixtures and jigs used in the manufacturing process, which also increases the cost.
2. Polymer batteries themselves have poor versatility, which is also a result of flexible design. Often, for a difference of just 1mm, it is necessary to design a new battery for the customer.
3. Once it breaks, the battery is completely ruined, and it requires protection circuit control. Overcharging or over-discharging will damage the reversibility of the chemical substances inside the battery, thus seriously affecting the battery's lifespan.
4. The lifespan is shorter than that of the 18650 because different designs and materials are used. Some contain liquid, while others are dry or gel-like. Furthermore, their performance during high-current discharge is not as good as that of the cylindrical 18650 battery.
