In recent years, many new types of Lithium-ion batteries have emerged. Their basic operating principals are similar (see Lithium-Ion Batteries for the Most Concentrated Power), but each new type offers distinct advantages and disadvantages, making them more suitable to specific uses that exploit their properties.
The Lithium-Iron-Phosphate Battery, a.k.a. The Beltway Battery
This type of battery is categorized by its unique ability to be fully charged in a very short period of time. The chemical formula for the positive terminal, or cathode, is LiFePO4, lithium ferrophosphate; thus, it is often referred to as LFP battery. The lithium-ion-phosphate battery is largely immune to the problem of thermal runaway, which can lead to explosions and other mishaps in some of the other variations. The reason for this is the strength of the chemical bond between the iron, phosphorus and oxygen in the cathode. If the battery is subjected to a destabilizing condition such as a short-circuit, the oxygen is locked more tightly in place, and far less likely to revert to a gas and feed into any nascent spark, nurturing it into a destructive event.
The enabling factor for the speed at which this type of Li-ion can charge is the rate that lithium ions can enter the cathode, which itself has “tunnels” into its exterior that lead deep into the body. The trick is to create a structure on the surface of the electrode—a “beltway” that leads the lithium ion straight into a tunnel, which quickly ferries the ion deep into the electrode, greatly speeding up the charging process.
The “beltway” is created by coating the electrode with a compound of lithium, iron and phosphorous in different proportion than that composing the electrode itself. Heating causes the formation of a glass-like skin, which is covered with the beltways that lead the ions directly into the tunnels.
Along with greater charging speed, the Li-ion can also deliver power more quickly (power density). It can’t hold quite as much charge per pound (energy density) as the ubiquitous lithium-cobalt-oxide battery found in many mobile consumer devices.
Its output is a steady 3.2 V until it is nearly exhausted, which, in some applications, can greatly simplify the control circuitry necessary for all Li-ions. Indeed, in some cases, it is even possible to string four of them together in a series, and drop the package in as a direct replacement for a 12-V lead-acid battery. Because it doesn’t contain cobalt, end-life recycling is greatly simplified, making this type of Li-ion quite stable compared to other Li-ions, and generating much less extraneous heat. This, and the quick charging and discharging ability of this type of Li-ion, may be the reason for the Navy’s heavy investment in this technology for its shipboard railgun-kinetic-weapon project.
Lithium-Titanate
While the cathode may steal the show in the “beltway” battery, it’s the anode that is the noteworthy feature of the lithium-titanate (LTO) battery, which is devoid of carbon and is coated with nanocrystals of lithium-titanate (Li4Ti 5O12). This gives the anode over thirty times the effective surface area as compared to that of a more typical carbon electrode. More surface area means faster transit of electrons entering or leaving the electrode, which translates into both faster charging times and to higher peak current, if needed, during discharge. In addition, as carbon is a factor in the overheating and thermal runaway that plagues many other types of Li-ions, the LTO makes for a very safe choice.
Aside from the faster charging time, another important feature of this type of Li-ion battery is that it can go through over 1,500 discharge-recharge cycles. After its useful life is completed, these Li-ions are easier to dispose of because titanium is decidedly nontoxic. Another noteworthy feature is their remarkable efficiency; for every 100 watt-hours spent charging the device, over 95 watt-hours will be returned during discharge.
However, an important disadvantage to LTO batteries is that their chemistry yields a voltage of 2.4 V. This is not only too low for many types of semiconductor devices, but also contributes to the LTO’s somewhat depressed energy density.
Titanium-Dioxide Anode
A novel new battery based on a titanium-dioxide anode can be charged to over two-thirds of full capacity in less than two minutes. Expected to be commercially available within two years, this prototype—developed by Chen Xiaodong of Nanyang University in Singapore—is also touted as being able to be charged and depleted 10,000 times. If this is indeed the case, this could mean that Electric Vehicle users will never need to replace their car’s battery!The key to this technology is, once again, nanotechnology. The titanium dioxide of the anode is present in the form of nanotubes, which speed up the chemical transport deep into the anode, and thereby hasten charging. One of the reasons that this technology is expected to quickly move forward to commercialization is the ease with which the anode, essentially a nanotube gel, can be produced. It involves little more than mixing titanium-dioxide nanoparticles with sodium hydroxide at the right temperature.
Developers of this battery type point out that its eventual adoption by EV manufacturers will pose an interesting challenge. Battery-charging stations now in place to charge EVs only need to supply enough current to fully charge the vehicle in hours. An EV equipped with this proposed new battery can absorb the same amount of charge in minutes, requiring charging stations that can provide approximately 100 times the currently required amperage.
Solid-State Li-Ions
This class of Li-ion features a solid-state, inorganic electrolyte, as opposed to the liquid, organic electrolytes of present-day commercial batteries. With a solid-state electrolyte, there is no possibility of leakage, and fire risk is considerably abated, as the solid-state electrolytes aren’t especially flammable. This eliminates a major danger associated with Li-ion batteries, and for that reason alone, it is of great interest to all present and potential users.
Figure 1: Solid-state electrolyte eliminates the possibility of flammable liquids. (Source: Charged)
The main barrier to success is the slow movement of ions through the solid electrolyte. At the University of Tokyo, some success has been achieved with a solid-state, lithium-germanium-phosphorous-sulfide electrolyte. Here, the rate of ion flow is equal to what can usually be observed in liquid electrolytes. But at this point, the new electrolyte has not been able to function well with the battery’s anode.
Other researchers have developed solid-state electrolytes that function well with the anode, but at a cost of a slower permeability to ions. Problems have also been noted at the cathode, as an insulating layer that forms between it and the solid-state electrolyte has proved troublesome.
All observers agree that even after the right combination of electrode, cathode and electrolyte are found, the manufacturing challenges that would need to be surmounted are formidable. Dr. Chihiro Yada of Toyota is investigating ion transport down to a “nano-structural” level, and although progress is being made, doesn’t foresee a commercial product until 2020.
Despite the fact that Li-ion batteries have been with us since the 1970s, the technology is still benefiting from continual innovations in chemistry and nanotechnology. Li-ions are now, and will continue to be, at the nexus of progress in mobile electronic devices, transportation and energy generation.