Lithium polymer battery

A lithium polymer battery, often abbreviated as LiPo, is a type of rechargeable battery. It is a variation of lithium-ion and lithium-metal technology. The defining feature of a LiPo is its electrolyte. While standard lithium-ion cells use a liquid lithium salt dissolved in an organic solvent, LiPo cells use a polymer electrolyte. This electrolyte can be a solid or a semi-solid material. Common examples include polyethylene glycol (PEG) or poly(methyl methacrylate) (PMMA).

Because they use polymers, these batteries offer excellent flexibility. They also show increased resistance to the volume changes that occur during charging and discharging. This makes them highly efficient for modern electronics.

The working principle of a LiPo cell relies on the movement of lithium ions. This process is known as intercalation and de-intercalation. During operation, lithium ions move between a positive electrode and a negative electrode. To keep the battery safe, a microporous polymer separator sits between these electrodes. This separator prevents the electrodes from touching while still allowing ions to pass through. The electrolyte acts as the conductive medium for this movement.

In a gelled system, the liquid is trapped within a polymer network. This allows for better ion transfer than a completely dry system.

Polymer electrolytes are categorized into two main types. The first is the dry solid polymer electrolyte (SPE). In an SPE, lithium salts are dissolved directly into a polymer matrix. However, SPEs often have poor conductivity at room temperature because ion transfer is difficult. To solve this, scientists developed gel polymer electrolytes (GPE). A GPE incorporates an organic liquid electrolyte into the polymer matrix. This creates a hybrid system that behaves like a mix of a solid and a liquid.

GPEs offer higher thermal stability and lower volatility, which improves safety.

The history of this technology spans several decades of research. Michel Armand developed the first dry SPE prototype around 1978. In 1985, organizations in France and Canada also worked on these prototypes. By 1988, Sony began researching cells that used gelled polymer electrolytes.

While polymer batteries seemed indispensable in the early 1990s, they did not enter the market until 1998. In 1996, Bellcore announced a "plastic lithium-ion cell" known as PLiON. This technology was eventually commercialized in 1999. Since then, companies in the United States and Japan have continued to develop gelled systems.

Understanding the electrical specifications of these cells is vital for safety. A single LiPo cell has a voltage that changes based on its charge. For cells using lithium-metal-oxides, the nominal voltage is typically 3.6 or 3.7 volts. A fully charged cell reaches about 4.2 volts. When the cell is fully discharged, the voltage drops to between 2.7 and 3.0 volts.

If the cells are based on lithium-iron-phosphate (LiFePO4), the voltage range is different. Those cells charge to 3.6–3.8 volts and discharge to 1.8–2.0 volts. Users must use electronic circuits to prevent overcharging or over-discharging.

LiPo batteries are widely used because of their high energy density and light weight. They are common in radio-controlled aircraft, drones, and model trains.

They are also found in smartphones, tablets, and thin laptop computers. Because the polymer casing is flexible, manufacturers can create batteries in almost any shape. This is helpful for fitting power sources into tight spaces. Some portable jump starters use three or six LiPo cells in series to start vehicles. They are even used in electric vehicles by companies like Hyundai and Kia.

Safety is a major consideration when using LiPo technology. Because they lack a rigid metal case, they use a flexible polymer laminate case. This means they can visibly expand or inflate if they are overcharged.

This expansion happens due to slight vaporization of the electrolyte. If the cells are not stored correctly, they can be damaged. For long-time storage, the voltage should be kept between 3.6 and 3.9 volts per cell. Proper management of these cells is essential to prevent risks like fire.