Литий-полимерные аккумуляторы: технология и характеристики.

Lithium ion battery using a polymer electrolyte

Lithium polymer cells follow the history of lithium ion and lithium metal cells which underwent extensive research during the 1980s, reaching a significant milestone with Sony's first commercial cylindrical lithium ion cell in 1991. After that, other packaging forms evolved, including the flat pouch format.

Lithium polymer cells have evolved from lithium ion and lithium metal batteries. The primary difference is that instead of using a liquid lithium salt electrolyte (such as lithium hexafluorophosphate, LiPF6) held in an organic solvent (such as EC/DMC/DEC), the battery uses a solid polymer electrolyte (SPE) such as polyethylene glycol (PEG), polyacrylonitrile (PAN), poly(methyl methacrylate) (PMMA) or poly(vinylidene fluoride) (PVdF). In the 1970s the original polymer design used a solid dry polymer electrolyte resembling a plastic like film, replacing the traditional porous separator that is soaked with electrolyte. The solid electrolyte can typically be classified as one of three types: dry SPE, gelled SPE and porous SPE. The dry SPE was the first used in prototype batteries, around 1978 by Michel Armand, and 1985 by ANVAR and Elf Aquitaine of France, and Hydro Québec of Canada. From 1990 several organisations like Mead and Valence in the United States and GS Yuasa in Japan developed batteries using gelled SPEs. The main difference between lithium ion polymer cells and lithium ion cells is the physical phase of the electrolyte, such that LiPo cells use dry solid, gel like electrolytes whereas Li ion cells use liquid electrolytes.

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.

The voltage of a single LiPo cell depends on its chemistry and varies from about 4.2 V (fully charged) to about 2.7–3.0 V (fully discharged), where the nominal voltage is 3.6 or 3.7 volts (about the middle value of highest and lowest value) for cells based on lithium metal oxides (such as LiCoO2). This compares to 3.6–3.8 V (charged) to 1.8–2.0 V (discharged) 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. LiPo battery packs, with cells connected in series and parallel, have separate pin outs for every cell. 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).

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. Moderate pressure on the stack of layers that compose the cell results in increased capacity retention, because the contact between the components is maximised and delamination and deformation is prevented, which is associated with increase of cell impedance and degradation.

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 devices and notebook computers can be met. They also have a low self discharge rate, which is about 5% per month.

LiPo batteries are now almost ubiquitous when used to power commercial and hobby drones (unmanned aerial vehicles), radio controlled aircraft, radio controlled cars and large scale model trains, where the advantages of lower weight and increased capacity and power delivery justify the price. Test reports warn of the risk of fire when the batteries are not used in accordance with the instructions. The voltage for long time storage of LiPo battery used in the R/C model should be 3.6~3.9V range per cell, otherwise it may cause damage to the battery. LiPo packs also see widespread use in airsoft, where their higher discharge currents and better energy density compared to more traditional NiMH batteries has very noticeable performance gain (higher rate of fire).

LiPo batteries are pervasive in mobile devices, power banks, very thin laptop computers, portable media players, wireless controllers for video game consoles, wireless PC peripherals, electronic cigarettes, and other applications where small form factors are sought and the high energy density outweighs cost considerations.

Hyundai Motor Company uses this type of battery in some of its battery electric and hybrid vehicles, as well as Kia Motors in their battery electric Kia Soul. The Bolloré Bluecar, which is used in car sharing schemes in several cities, also uses this type of battery.

Lithium ion batteries are becoming increasingly more commonplace in Uninterruptible power supply (UPS) systems. They offer numerous benefits over the traditional VRLA battery and with stability and safety improvements confidence in the technology is growing. Their power to size and weight ratio is seen as a major benefit in many industries requiring critical power back up including data centers where space is often at a premium. The longer cycle life, usable energy (Depth of discharge), and thermal runaway are also seen as a benefit for using Li po batteries over VRLA batteries.

The battery used to start a vehicle engine is typically 12V or 24V, so a portable jump starter or battery booster uses three or six LiPo batteries in series (3S1P/6S1P) to start the vehicle in an emergency, instead of the other jump start methods. The price of a lead acid jump starter is less but they are bigger and heavier than comparable lithium batteries, and so such products have mostly switched to LiPo batteries or sometimes lithium iron phosphate batteries.

All Li ion cells expand at high levels of state of charge (SOC) or over charge, due to slight vaporisation of the electrolyte. This may result in delamination, and thus bad contact of the internal layers of the cell, which in turn brings diminished reliability and overall cycle life of the cell. In comparison to liquid electrolytes and solid organic electrolytes, polymer electrolyte offer advantages such as increased resistance to variations in the volume of the electrodes throughout the charge and discharge processes, improved safety features. excellent flexibility and processability. Solid polymer electrolyte is initially defined as a polymer matrix swollen with lithium salts, which is now referred to as dry solid polymer electrolyte. The conduction mechanism is similar for liquid electrolytes and polymer gels, but GPEs have higher thermal stability and low volatile nature which also further contribute to safety.

Cells with solid polymer electrolytes have not reached full commercialization and are still a topic of research. Prototype cells of this type could be considered to be between a traditional lithium ion battery (with liquid electrolyte) and a completely plastic, solid state lithium ion battery. At that time polymer batteries were promising and it seemed polymer electrolytes would become indispensable. Eventually, this type of cell went into the market in 1998. Although these polymer electrolytes may be dry to the touch, they can still contain 30% to 50% liquid solvent. In this regard, how to really define what a "polymer battery" is remains an open question. Other terms used in the literature for this system include hybrid polymer electrolyte (HPE), where "hybrid" denotes the combination of the polymer matrix, the liquid solvent and the salt. It was a system like this that Bellcore used to develop an early lithium polymer cell in 1996, which was called "plastic" lithium ion cell (PLiON), and subsequently commercialised in 1999. a high molecular weight poly(trimethylene carbonate) (PTMC), polypropylene oxide (PPO), poly[bis(methoxy ethoxy ethoxy)phosphazene] (MEEP), etc. PEO exhibits most promising performance as a solid solvent for lithium salts, mainly due to its flexible ethylene oxide segments and other oxygen atoms that comprise strong donor character, readily solvating Li+ cations. PEO is also commercially available at a very reasonable cost.