A lithium polymer battery, abbreviated as LiPo, LIP, Li-poly,lithium-poly and others, is a rechargeable battery of lithium-ion technology using a polymer electrolyte instead of a liquid one. High conductivity semisolid (gel) polymers form this electrolyte. These batteries provide a higher specific energy than other lithium-battery types and are being used in applications, where weight is a critical feature - like tablet computers, cellular telephone handsets, radio-controlled aircraft and home-use medical devices such as NebSmart® mesh nebulizer.
Working principle
Just as with other lithium-ion cells, LiPos work on the principle of intercalation and de-intercalation of lithium ions from a positive electrode material and a negative electrode material, with the liquid electrolyte providing a conductive medium. To prevent the electrodes from touching each other directly, a microporous separator is in between which allows only the ions and not the electrode particles to migrate from one side to the other.
Charging
The voltage of a LiPo cell depends on its chemistry and varies from about 2.7-3.0 V (discharged) to about 4.20 V (fully charged), for cells based on lithium-metal-oxides (such as LiCoO2), and around 1.8-2.0 V (discharged) to 3.6-3.8 V (charged) for those based on lithium-iron-phosphate (LiFePO4).
The exact voltage ratings should be specified in product data sheets, with the understanding that the cells should be protected by an electronic circuit that won't allow them to overcharge nor over-discharge under use.
For LiPo battery packs with cells connected in series, a specialized charger may monitor the charge on a per-cell basis so that all cells are brought to the same state of charge (SOC).
Applying pressure on LiPo cells
Unlike lithium-ion cylindrical and prismatic cells, which have a rigid metal case, LiPo cells have a flexible, foil-type (polymer laminate) case, so they are relatively unconstrained. By themselves the cells are over 20% lighter than equivalent cylindrical cells of the same capacity.
Being lightweight is an advantage when the application requires minimum weight, such as in the case of radio controlled models. However, it has been established that moderate pressure on the stack of layers that compose the cell results in increased capacity retention, because the contact between the components is maximized and delamination and deformation is prevented, which is associated with increase of cell impedance and degradation.
Applications
LiPo cells provide manufacturers with compelling advantages. They can easily produce batteries of almost any desired shape. For example, the space and weight requirements of mobile phones, notebook computers and NebSmart® mesh nebulizer can be completely satisfied.
Safety
Plastic packaging applied on LiPo cells makes this kind of battery very safe for users. Internal problems can be immediately through the packaging performance, even if there are security risks, it will not explode, but shape change only.
