Lithium Polymer Batteries

• Lightweight, flat, powerful, long-lasting. And astonishingly variable in design and capacity. These are the advantages that set lithium polymer batteries apart. They stand out from other types of lithium batteries in a whole range of other factors. They are also a recommended alternative to conventional lithium-ion batteries in countless applications. In small, flat, mobile devices, in particular, they can guarantee constant energy supply. This white paper provides an introduction to lithium polymer battery technology. It contains some important information on the design of housings and on how to handle these energy accumulators.

Significant criteria that distinguish Li-polymer batteries from other types of cell:

Li-ion cells have a fixed housing in stainless steel or aluminium. The housing is usually cylindrical (‘round cells’). Rectangular shapes are, however, also available.

Disadvantages: relatively high tool costs for housing manufacture; restricted dimensions.

Advantages: robust, mechanically strong housing, making the battery difficult to damage. A laser welding process seals the cells.

Li-polymer cells, also known as soft or pouch cells, have a thin and somewhat ‘soft’ housing –like a pouch – made from deep-drawn aluminium foil. The most prismatic housing can be produced more easily and cheaply than the hard cases of Li-ion cells. The other components, in wafer-thin layer foils (< 100 µm) can also be mass-produced at relatively low cost.

The cells are lightweight, thin and can be made in a wide range of shapes and sizes. Both large formats and heights of less than 1 mm can be achieved. The cells do, however, require careful mechanical handling.

The housing foil is coated on both sides with plastic. Inside: polyolefins, resistant to the cell components. Outside: polyamide, resistant to the outer environment. This waterproof laminate is welded and surrounds the cell comprising cathode, anode, and separator.

The execution of the deflector in the area of the terrace was a critical point. An additional foil welded to the deflector increases the sealing in this area of the welding of the ‘housing’.

Electrode set: In Li-polymer batteries, the electrode set comprises a carbon-based substance (graphite+additives) pasted onto a metallic substrate. The cathode consists of three dimensional, lithiated cobalt oxides or nickel/manganese/cobalt (NMC) mixed oxides, also pasted onto a metallic substrate.

Deflectors are present on both electrodes. They are wound around the core together with the separator, normally a three-layered polyolefin. The core generally consists of a flat pin to create the rectangular winding. The winding sits in the bottom of the pouch foil, which is partly folded and laid over the winding. The seal is created by welding the foil.

Design: An advantage is the almost limitless range of sizes and formats thanks to the lack of a rigid steel housing and the compact construction. In particular, the possibility of designing very flat cells sets Li-polymer battery technology apart. Such batteries can be thinner than 1 mm.

This results in significant design freedom for the end product. Individual dimensions can be realized even for small batch sizes, while the space reserved for the battery can be used to its full potential.

Energy density: The energy density of these cells is higher than that of other types. Relative to their overall weight, Li-polymer cells have a slightly higher energy density than Li-ion cells. Like Li-ion batteries, they can be easily connected in parallel to allow higher capacities.

Self-discharge: A further advantage of LiPo cells is their relatively low rate of self-discharge.They should nevertheless be protected from overcharging, deep discharge and extreme temperatures.

Approval: The spread of Li-polymer cells on the market confirms the advantages and acceptance of this technology. Many of the cells on the market are certified to UL 1642. Before a particular cell is used, it should be verified whether it has a UL approval and whether the manufacturer has the tools required for production.