Lithium Polymer Battery

Lithium polymer batteries (also known as LiPo batteries) have become an increasingly popular choice in radio control world. Offering long run times and superior power capacity, these lightweight lithium polymer cells offer greater convenience.

They feature an excellent C rating, making them suitable for applications that demand fast bursts of speed such as drone racing.

Non-aqueous solvent-filled ionomers that bind anions to polymer host membranes have demonstrated improved performance of Li-polymer batteries by mitigating dendritic deposition.

The Electrolyte

Lithium-ion batteries are an indispensable power source for electronic devices like mobile phones, computers and tablet PCs. Although lithium-ion batteries do an admirable job at providing energy to these devices, they also present various limitations that often impact performance negatively. Lithium polymer batteries offer an alternative by forgoing liquid electrolyte use altogether in favor of solid-state electrolytes; eliminating physical damage or thermal runaway from compromiseing these liquid ones and offering solid electrolyte solutions instead.

Solid polymer electrolytes (SPEs) offer lower boiling points than organic liquid electrolytes and can be made more flexible through incorporation of nanofibers into them, giving lithium polymer batteries the potential to fit within credit cards more compactly than usual. Plus, thanks to thinner SPEs used in polymer batteries compared with regular lithium-ion ones, their capacity can even double.

Solid-state electrolytes offer many advantages over batteries with liquid electrolytes, particularly manufacturing time. Conventional batteries typically involve several steps such as customizing positive and negative electrode materials before assembling with separators and adding liquid electrolyte; by contrast, lithium polymer batteries can often be assembled all in one step.

Lithium polymer batteries offer additional safety. Because they do not use liquid electrolytes, lithium polymers are less vulnerable to problems caused by high State of Charge (SOC) or overcharge. A typical liquid battery, when exposed to SOC/overcharge cycles, will expand due to slight vaporisation of its electrolyte; this expansion often leads to cracks at electrode|electrolyte interface and interphases and mechanical stresses caused by this expansion process that lead to degradation of internal cell layers or unfavorable metallic electrodepositions such as dendrites.

Shi et al. have focused their research efforts on creating SPEs that can overcome such issues, creating a 3D self-assembled SPE using an ionic liquid compounded with cellulose that has an impressive 2.0×10-4 S/cm at 25degC ionic conductivity and excellent mechanical properties. Furthermore, the cellulose also strengthens structural integrity of the SPE while forming strong lithium coordination to promote lithium salt dissolution thereby improving overall ion transport while preventing leakage of IL leakage.

The Cathode

The cathode of a battery serves as its negative electrode, drawing electrons away from its positive electrode during discharge. Cathode materials tend to be carbon-based such as graphite or silicon; graphite tends to be used more due to its lower cost, higher electrical conductivity and stability while silicon offers higher energy density. There are various iterations of cathode materials used today which aim for balance between energy density and cycle life.

An anode is the positive electrode in a battery, serving to absorb positively charged lithium ions during charging. Common anode materials include nickel-cobalt alloys that offer good balance between energy density and cycle life; less expensive than pure nickel options with lower melting point/boiling point profiles that make it suitable for automotive batteries’ high temperatures.

Charging occurs by applying an overvoltage from an external power source to each cell, forcing electrons from its positive to its negative electrode and intercalating lithium ions with anode material for intercalation. Once charged, battery capacity increases slowly until its top-of-charge voltage limit per cell is met.

Lithium-ion batteries have received some press for their tendency to spontaneously catch fire; however, their risk remains relatively low. More commonly seen among these batteries is thermal runaway, in which their negative electrode heats up before beginning to oxidize and potentially ignite, eventually leading to explosion or fire in an explosive chain reaction that leads to explosion or fire in a battery pack.

Lithium-ion cells with solid polymer electrolytes may help reduce the risk of thermal runaway. Such electrolytes consist of plastic-like films which replace traditional porous separators in organic solvent-soaked pores; usually comprising one of several polymers like acrylonitrile, butyrate or polyvinylidene fluoride (PVdF).

The Anode

Lithium in its pure elemental form is highly reactive. To avoid interaction with water and hydrogen gas production that would cause battery explosion, lithium-ion batteries use non-aqueous electrolytes made up of lithium salts in polymers such as ethylene carbonate or propylene carbonate – four key components making up one battery cell in this case.

Cathodes in lithium batteries consist of metal oxide, while anodes consist of porous carbon. When discharged, lithium ions move from the anode through electrolyte and separator to cathode via electrolyte for electrical energy production. Conversely, during charging they return through anode release back into cathode which provides more power.

Both cathodes and anodes of battery packs are made from synthetic compounds which are ground into fine particles before mixing with binders and solvents before being coated onto metal foil (usually aluminum for anode and copper for cathode). After being spread across this metal foil surface, these mixtures are spread on top and dried in an oven to secure their structure, remove binder/solvent residue, and create microporous film permeable to lithium ions.

An anode typically consists of nickel-cobalt oxide (NiCoO2) or lithium-manganese dioxide (LiMn2O4); both materials offer lower costs than cobalt while being easily reversibly oxidized back to their natural states of nickel and manganese. Li-LiMn2O4’s cubic lattice structure facilitates three-dimensional diffusion of lithium ions into and out of crystals; during discharge one lithium ion per two Mn ions are inserted into these crystals while charging removes one lithium ion from one per two Mn ions then extracted reversibly during charging.

Some batteries use plastic conducting carbon anodes, which are much cheaper and have reduced cycling capability and capacity than metallic lithium anodes. Still, they make for an attractive recycling option since you don’t need to undergo expensive and time-consuming purification processes afterwards; however, this does lead to the buildup of metallic lithium on its surface which reduces capacity over time.

Thermal Runaway

Thermal runaway of a lithium polymer battery occurs when its internal chemical reactions become self-sustaining, producing heat and releasing energy at an exponentially increasing rate. At this stage, pressure rapidly increases within the battery as gases form which cause expansion within cells as they expand causing foil packaging of the battery to burst and releases large volumes of flammable gasses that can be hazardous for human life.

Thermal runaway can be caused by multiple factors, including overcharging the battery, exposing it to high temperatures, mechanical failure or internal/external short circuiting. It is a sudden event which is difficult to control, eventually leading to its destruction due to lithium’s organic materials destabilising and breaking down, creating dangerously rapid increases in temperature that creates internal heating within battery cells that leads to thermal runaway processes.

All Lithium-ion batteries generate some heat, but if the rate at which this heat is generated exceeds its rate of dissipation then its internal temperature will increase exponentially until stability is lost. At that point the internal cell will swell as its separator structure collapses, leading to electrode contact between electrodes and sparking exothermic chemical reactions that rapidly accelerate temperature rise while producing massive amounts of heat that further raise the cell temperature until thermal runaway occurs.

Reducing thermal runaway can be an almost insurmountable challenge, which is where failsafes like separator shutdown come into play – they work by shutting off lithium transport which stops thermal runaway process, although they may take some time to work their magic; until that time arrives, battery temperature continues to increase steadily.

Therefore, to minimise the risk of this event and protect batteries more effectively from thermal runaway, one effective strategy is storing them in a battery cabinet equipped with temperature sensors that monitor internal cell temperatures and can intervene before they reach thermal runaway threshold. These cabinets also come equipped with communication capabilities so as to enable data-driven decision making when it comes to charging, storage and maintenance needs.