The Future of Portable Power
Lithium-ion technology is the best power source yet devised for today's consumer electronics. But contemporary batteries have their limitations and superior solutions are under development in labs around the world.
Smartphones represent the peak of portable electronic design. These powerful cellular- and Internet-enabled devices boast computing power and memory capacity that matches the spec of desktop PCs and Macs from only a few years ago. An Apple iPhone 6S, for example, sports a dual-core, 1.8-GHz, 64-bit processor, plus 2 GByte RAM and 128 GByte Flash.
But today's smartphones have an Achilles Heel; their batteries. Lithium-ion (Li-ion) batteries have struggled to maintain an energy density (Wh/kg) improvement of around seven percent per year.
For example, the original iPhone weighed in with a 620 MHz 32-bit processor with 128MByte RAM, 16GByte Flash and a 5.18Wh battery, while today's iPhone 6S sports a 6.55Wh cell. The electronics of the latest Apple smartphone represent a dramatic leap in performance compared with the first model while the battery energy density has improved by only around 26 percent in eight years.
According to Apple's specifications, the Li-ion battery in the iPhone 6S has a capacity of 1715 mAh and is capable of around 11 hours of Internet browsing or high definition video playback. It's an impressive level of performance, but still not sufficient to stop travelers, for example, diving for the charger at the first sight of an airport terminal mains socket in order to top up.
Is the big leap in LI-ion battery technology just around the corner or does the technology represent only a waypoint on the journey to a power source offering weeks or even months of service between recharges?
Developing the Li-ion battery
It's taken over 40 years to develop the Li-ion technology that powers today's portable products. Lithium-based batteries are successful because they combine high capacity with low weight, resulting in more energy per kilogram than any other metal.
During charging, lithium ions are energized and move from the LiCoO2 to the carbon. When the battery is in use, the ions move back the other way causing liberated electrons to travel in the opposite direction round the circuit to power the load. (See Fig. 1.)
Figure 1: In a conventional Li-ion battery, lithium ions (green) move between the electrodes while liberated electrons power the load. Charging moves the ions back to the negative electrode.
However, a key weakness of Li-ion batteries is their fragility. Each time ions are shifted, some react with the electrodes and remain forever embedded in the material. Eventually the supply of free ions is depleted and the battery fails. Each charging cycle also causes some volumetric expansion of the electrodes which stresses the structure and causes microscopic damage, diminishing its ability to 'store' ions. Consequently, Li-ion batteries can only be recharged a limited number of times. Moreover, overcharging can 'force' so many ions into the electrode that disintegration of the material can occur. It's important to properly manage the charging and discharge rate of Li-Ion batteries in portable devices using a power management IC such as the bq40Z50-R1 Li-Ion Battery Pack Manager from Texas Instruments.
Early versions of Li-ion batteries employed a liquid electrolyte to separate the electrodes, later using a porous separator soaked in an electrolytic gel. This allowed the batteries to have a sandwich construction leading to the thin designs common to today's mobile handsets. Further development led to Lithium polymer (Li-Pol) cells that used a solid polymer as the separator. One downside of Li-Pol batteries is that the ions travel more slowly through the solid polymer than liquid electrolyte, so charging takes longer.
Building a Better Battery
Millions of research dollars continue to be spent to improve Li-ion batteries. Scientists focus their efforts on enhancing characteristics such as energy density, self-discharge rate, peak demand and pulse performance, charging time, and tolerance to deep discharge, together with improving device safety.
Developments have primarily targeted two areas: alternative materials for positive electrodes, negative electrodes and electrolytes--with a view to packing more lithium ions into the electrodes, making it easier for the ions to move in and out, and easing the passage of the ions through the electrolyte--and overcoming the technology's inherent safety challenges.
Positive electrode materials nearing commercialization include lithium nickel manganese cobalt oxide (LiNixMnyCozO2), which has an energy density about 20% greater than LiCoO2 but at higher cost, and lithium nickel cobalt aluminum oxide (LiNixCoyAlzO2), which has an energy density about 35% greater than LiCoO2. Experimental negative electrode materials include lithium titanate (Li4Ti5O12) (which has low energy density but higher recharge cycles), hard carbon (greater storage capacity), tin/cobalt (energy density), and silicon/carbon or pure silicon (energy density).
There are also several interesting initiatives for improving the mobilityof the ions. One example comes from the University of Illinois at Chicago (UIC) and replaces the thin, almost two-dimensional, positive- and graphite negative-electrodes of a conventional Li-ion battery with three-dimensional porous nickel structures. LiMnO2 and nickel tin (NiSn) are plated onto the structures to form the positive- and negative-electrodes, respectively. The result is electrodes that can hold many more lithium ions than a conventional device with greater freedom of movement. The university claims this battery would be 30 times smaller than a device of the same capacity and could be charged 1000 times quicker.
UIC is also doing some pioneering work replacing lithium ions (which carry a +1 charge) with magnesium ions (which have a +2 charge). The result could be a battery with a considerably higher energy density than Li-ion cells and can withstand many more recharging cycles.
Researchers have also concentrated their efforts on employing nanoscale (10-9 m or nm) materials to improve the mobility of lithium ions through electrodes and electrolytes. For example, scientists at South Korea's Pohang University have built a prototype battery from pumpkin-shaped molecules organized in a honeycomb-like structure which can be used as a solid electrolyte. The molecules have a thin channel (measuring 75 nm in diameter) running through them which enables lithium ions to diffuse far more freely than in a conventional electrolyte. (See Fig. 2.) In tests, the porous electrolyte demonstrated lithium ion conductivity of around three times that of conventional commercial solid electrolytes.
Figure 2: South Korea's Pohang University's electrolyte enables lithium ions to diffuse far more freely than a conventional electrolyte. (Credit: Pohang University)
Another example of nanomaterials at work to improve Li-ion batteries comes from Massachusetts Institute of Technology (MIT). Researchers Byoungwoo Kang and Gerbrand Ceder at the institute claimed that by using nanoball electrodes, batteries could be charged about 100 times as fast as normal Li-ion batteries, resulting in a smartphone that could charge in 10 seconds. The 50-nm balls of lithium iron phosphate dramatically improved ion mobility and the MIT researchers further accelerated the process by coating the balls with a thin layer of lithium phosphate.
Carbon nanotubes might already be at work inside smartphone batteries. The exact composition of most positive- and negative-electrodes are currently held as trade secrets, but the level of commercial production of carbon nanotubes hints that Li-ion batteries are already taking advantage of their properties. Carbon nanotubes exhibit greater surface area, higher conductivity and better mechanical stability than bulk carbon. Other developments include eliminating the carbon altogether and replacing it with silicon or germanium nanowires to further increase the surface area of the negative electrode. This again boosts the mobility of the lithium ions and allows more to be absorbed during charging without volumetrically stressing the material (enabling more recharge cycles).
'Nanostructuring' generally increases the surface area-to-volume ratio, which improves both energy- and power-density due to an increase in electrochemically active surface area and a reduction in ion transport lengths. The downside is an increase in side reactions between the electrode and the electrolyte, causing higher self-discharge, fewer recharging cycles and shorter shelf life.
Prototype batteries using nanomaterials exhibit much higher energy densities than today's commercial batteries. But at present materials are expensive and the manufacturing process is difficult to scale to industrial levels.
The Next Generation of Lithium Battery
One development that brings together all the strands of current Li-ion battery development is the Lithium-sulfur (Li-S) battery. This device takes advantage of developments in materials, 'three-dimensional' electrodes and nanomaterials to improve on today's Li-ion products. Current developments target electric vehicles, but the hope is the technology can be shrunk such that the battery is suitable for portable products like smartphones.
The negative electrode is a thin sliver of lithium while the cathode is lithium oxide (Li₂O₂) in contact with active sulfur. The reason for such keen interest in the technology is the predicted maximum energy density. The best contemporary Li-ion batteries produce about 200 Wh/kg and the technology has a theoretical limit of around 320 Wh/kg. The theoretical limit for Li-S is around 500 Wh/kg. The key advantage for these types is that the sulfur can 'host' two lithium ions compared to the 0.5 to 0.7 for conventional intercalation materials - resulting in the superior energy density.
Beyond the Battery
Other power sources for portable power include supercapacitors and fuel cells. A supercapacitor is a high-capacity capacitor that bridges the gap between electrolytic capacitors and rechargeable batteries, such as the EDLC 5.5V EDLC Supercapacitor from TDK. Supercapacitors offer higher energy storage and power density than conventional capacitors, making them excellent for burst or pulse load applications like an LED flash, power amplifiers, or certain audio circuits. SuperCapacitors can also provide power for devices that draw very little current over a long time, such as the Real-Time Clock (RTC) and Watchdog FRAM Supervisory ICs from Cypress Semiconductor.
Although very promising, supercapacitors have two main disadvantages over batteries. First is a voltage range of 2.5 to 2.7V (compared to 3.5 to 3.7V for Li-ion batteries). To achieve higher voltages, several supercapacitors are connected in series which increases complexity by demanding careful voltage balancing. Moreover, the voltage of a supercapacitor decreases on a linear scale from full to zero which results in some stored energy remaining in the device once the voltage drops below a usable threshold. The second drawback is the supercapacitor's low energy density. Compared with the Li-ion battery's 200 Wh/kg, even the best supercapacitors struggle to exceed 10 Wh/kg. That means that a bank of supercapacitors will take up much more space than an equivalent Li-ion battery.
Fuel cells represent perhaps the most esoteric attempt to steer portable power sources away from conventional batteries. Invented in 1838 and cemented into popular perception during the Apollo 13 crisis, fuel cells have long been used as a method of converting the chemical energy of fuel into electricity, and are considered a good option for electric vehicles. However, due to their size portable fuel cells are inappropriate for mobile electronic products at this time.
Li-ion batteries are a rapidly maturing technology that lie at the heart of the most capable portable consumer electronic devices. But while providing satisfactory service, consumers crave longer life from batteries. That's stimulating ongoing research to develop and refine the chemistry and physics to ensure that Li-ion technology continues to evolve. Some of that research promises to yield lithium-based batteries with double the run time of existing cells in like-for-like applications. However, even that might not be enough to satisfy the demands of future consumer electronic products, so expect to see alternative technologies like supercapacitors, fuel cells, energy harvesting, and yet-to-be-invented power storage devices enter the fray.