Lithium-ion batteries Can and Casing

Lithium-ion batteries are very sensitive to atmospheric effects like the presence of moisture and interaction with oxygen. Moisture and oxygen render the electrolyte useless and may also induce effects (like increasing flammability) which are unwanted. Also, the battery has to maintain a pressure which will prevent its collapse in environmental conditions.

Aluminium was used in batteries initially due to its high resistance to environmental factors of moisture and air. With time polymers, such as polypropylene, have replaced Aluminium as the material of choice for casings due to the easy moldability of polymers into various shapes along with excellent chemical resistance. The shapes in which batteries are produced are not restricted, although cuboidal (in mobile phones and hybrid vehicles) and cylindrical shapes are very common. In addition to the outer casing polymeric foams are used to prevent damage to the cell from accidental shock.

Ni-coated steels are used as a casing material for lithium ion batteries due to the excellent chemical resistance and corrosion protection provided by nickel to the steel. There is a development of a material (Supernickel) which apart from providing the properties of Nickel coated steel, also provides galvanic resistance.

The cylindrical cell continues to be one of the most widely used packaging styles for primary and secondary batteries. The advantages are ease of manufacture and good mechanical stability. The tubular cylinder can withstand high internal pressures without deforming.Typical applications for the cylindrical cell are power tools, medical instruments, laptops and e-bikes. To allow variations within a given size, manufacturers use partial cell lengths, such as half and three-quarter formats. The most popular design, the 18650 Li-ion, is used in power tools, medical devices, laptops and e-bikes. The 3Ah 18650 delivers 248Ah/kg.

There are other cylindrical Li-ion formats with dimensions of 20700, 21700 and 22700. While the 18650 has a volume of 66cm3 with a capacity of around 3000mAh, the 97cm3 volume of the 21700 is said to produce a capacity of up to 6000mAh, essentially doubling the capacity with a 50% increase in volume.

The larger 26650 cell with a diameter of 26mm does not enjoy the same popularity as the 18650. The 26650 is commonly used in load-leveling systems. A thicker cell is said to be harder to build than a thinner one. Making the cell longer is preferred.The 32650 cell has a capacity of 6000mA and the dimensions are 32mm in diameter and 65mm in height but weighs about 370g.

The plastic parts used in batteries vary depending on the battery usage. Some batteries have entirely plastic formulations ranging from the electrolyte (polymeric electrolytes) to the casing. Most batteries intended for vehicle usage have higher proportions of plastic materials. This is due to the very low weight of plastics as compared to metals. Plastic incorporation in batteries increases the electrolyte efficiency if used in polymeric electrolytes.

Commercially available lithium-ion batteries have limited plastic usage. Plastic components incorporated in batteries include separators and casing components. Plastics have good shock absorbing characteristics (Polypropylene has good shock absorbing characteristics) and prevent damage to the basic cell unit from minor accidental shocks. Also, as plastics are insulators their formulations (like Polyethylene Terephthalate) are excellent at preventing short-circuit and act as separators.

Plastic parts also lower the cost of production of batteries and will eventually replace most metallic components of batteries.

Separator
Most conventional batteries use separators, which are essentially membranes made up of a gelatinous material, to prevent short-circuiting of the electrodes. Micro-porous, polymer-based separators are used to block the physical contact between the anode and cathode laminates in a cell. Polyethylene (PE) or polypropylene (PP) membranes are used as separators. The desired characteristics of a separator include its thickness uniformity (in micron scale), adequate mechanical strength during cell fabrication, and chemical as well as electrochemical stability when in contact with the electrolyte and during charge-discharge cycling respectively.

Smart separator materials have recently been developed which melts in situations like an accidental short circuit, over-charging/discharging or thermal runaway (all these leads to the generation of excessive heat), only to block the physical contact between the electrodes.

In lithium-ion batteries, dendritic formations are unavoidable and this leads to the phenomenon of growing electrodes. When the dendrites from the electrodes (after growing considerably) meet each other the electrodes become essentially short-circuited. This leads to sparking which further ignites the flammable electrolyte. To avoid this situation most batteries incorporate a separator, which has small openings to allow for the diffusion of ions. This separator acts as a barrier for dendrites and hinders their growth in unwanted directions. The downside of the use of a separator is the hindrance created for diffusion of ions between the electrodes. This reduces the response time and increases the charging time. Efforts are on the way to eliminate the use of separators by using solid-state systems (like solid electrolytes) to create more efficient batteries.

All commercial separators so far have been made of polyolefins but they provide only limited heat resistance. Research is now focusing on separators made of different materials which would offer superior heat resistance. These include heat-resistant rubber such as silicone rubber and fluororubber, aromatic polyamide resin, liquid crystalline polyester resin, heat-resistant resin containing polyoxyalkylene, and resin with cross-linked groups. Separators made of such materials are expected to demonstrate not only high temperature stability and safety but also superior ion transportation for better rate capability at high current discharge.

The total market for separators for all applications of Li-ion batteries was approximately 900 mm2 in 2015 and the compound annual growth rate (CAGR) amounted to 15% in the period between 2005 and 2015. It is expected that the separator market for Li-ion batteries will continue to grow steadily with CAGR of 12% reaching ca. 2700 mm2> in 2025 and a major contribution to this growth is expected to come from the needs of electric vehicles and buses.