Between electric cars, cell phones, and laptops it seems as if batteries are everywhere. This is not going to change any time soon. Global electricity use is skyrocketing and smartphones, tablets, and e-readers are all becoming more common. In addition, batteries are finding applications in energy storage as the renewable energy sector continues to grow. Engineers and scientists have developed many novel technologies to supply our storage needs, but none seems to have established itself as the ultimate technology. Flywheel, compressed air, and thermal storage are all strong contenders for grid-scale storage while lithium-ion, nickel-cadmium and nickel-metal-hydride batteries compete for portable electricity storage. What it all comes down to is that we still have not found an optimal way to store our electricity. This article will discuss the technology and potential of lithium batteries.
Until the 1990s nickel-cadmium (NiCad) batteries were practically the only choice in rechargeable batteries. The major problem with these devices was that they had a high-temperature coefficient. This meant that the cells' performance would plummet when they heated up. In addition, cadmium, one of the cell's main elements, is costly and environmentally unfriendly (it is also used in thin film panels). Nickel-metal-hydride (NiMH) and lithium-ion emerged as competitors to NiCad in the 90s. Since then a mind numbing number of technologies have appeared on the market. Amongst these lithium-ion batteries stand out as a promising candidate for a wide range of uses.
Lithium-ion cells have been used in hundreds of applications including electric cars, pacemakers, laptops and military microgrids. They are extremely low maintenance and energy dense. Unfortunately, commercial lithium-ion cells have some serious drawbacks. They are very expensive, fragile and have short lifespans in deep-cycle applications. The future of many budding technologies, including electric vehicles, depends on improvements in cell performance.
A battery is an electrochemical device. This means that it converts chemical energy into electrical energy. Rechargeable batteries can convert in the opposite direction because they use reversible reactions. Every cell is composed of a positive electrode called a cathode and a negative electrode called an anode. The electrodes are placed in an electrolyte and connected via an external circuit that allows electron flow.
Early lithium batteries were high temperature cells with molten lithium cathodes and molten sulfur anodes. Operating at around 400 degrees celcius, these thermal rechargeable batteries were first sold commercially in the 1980s. However, electrode containment proved a serious problem due to lithium's instability. At the end temperature issues, corrosion and improving ambient temperature batteries slowed the adoption of molten lithium-sulfur cells. Though this is still theoretically a very powerful battery, scientists found that trading some energy density for stability was necessary. This lead to lithium-ion technology.
A lithium-ion battery generally has a graphitic carbon anode, which hosts Li+ ions, and a metal oxide cathode. The electrolyte consists of a lithium salt (LiPF6, LiBF4, LiClO4) dissolved in an organic solvent such as ether. Since lithium would react very violently with water vapor the cell is always sealed. Also, to prevent a short circuit, the electrodes are separated by a porous materials that prevents physical contact. When the cell is charging, lithium ions intercalate between carbon molecules in the anode. Meanwhile at the cathode lithium ions and electrons are released. During discharge the opposite happens: Li ions leave the anode and travel to the cathode. Since the cell involves the flow of ions and electrons, the system must be both a good electrical and ionic conductor. Sony developed the first Li+ battery in 1990 which had a lithium cobalt oxide cathode and a carbon anode.
Overall lithium-ion cells have important benefits that have made them the leading choice in many applications. Lithium is the metal with both the lowest molar mass and the greatest electrochemical potential. This means that Li-ion batteries can have very high energy density. A typical lithium cell potential is 3.6V (lithium cobalt oxide-carbon). Also, they have a much lower self discharge rate at 5% than that of NiCad batteries which usually self-discharge at 20%. In addition, these cells don't contain dangerous heavy metals such as cadmium and lead. Finally, Li+ batteries do not have any memory effects and do not need to refilled. This makes them low maintenance compared to other batteries.
Unfortunately lithium ion technology has several restricting issues. First and foremost it is expensive. The average cost of a Li-ion cell is 40% higher than that of a NiCad cell. Also, these devices require a protection circuit to maintain discharge rates between 1C and 2C. This is the source of most static charge loss. In addition, though lithium ion batteries are powerful and stable, they have a lower theoretical charge density than other kinds of batteries. Therefore improvements of other technologies may make them obsolete. Finally, they have a much shorter cycle life and a longer charging time than NiCad batteries and are also very sensitive to high temperatures.
These issues have sparked interest in other chemistries, such as lithium-air, lithium-polymer and lithium-iron. Since I do not have time to go through all these devices, we'll briefly look at lithium-air batteries. In these systems, Li is oxidized at the anode, releasing electrons that travel through an external circuit. Li+ ions then flow to the cathode where they reduce oxygen, forming the intermediary compound lithium peroxide. In theory, this allows for a truly reversible reaction to take place, improving the performance of lithium-air batteries in deep-cycle applications. However, much like Li+ cells, these batteries suffer from short lives. This is due to the formation of oxygen radicals that decompose the cell's organic electrolyte. Fortunately two lithium-air batteries developed independently in 2012 by Jung et al., a team of researchers from Rome and Seoul, and Peter Bruce, who led a group at St. Andrews, seem to have solved this problem. Both the groups' batteries underwent approximately 100 charging and discharging cycles without losing much of their capacity. Bruce's device lost only 5% capacity during tests. The batteries also have higher energy density than their lithium ion counterparts. This is a sign that the future of energy storage may reside with powerful, resilient lithium-air chemistry. However we will first have to overcome durability, cost and weight problems.
Though novel lithium battery chemistries are being developed and marketed, Li+ batteries remain near the top of the food chain for now. As we mentioned previously, this technology is often considered the first choice for electric vehicles and electronic devices due to its energy density. Tesla's Roadster contains no less than 6831 lithium ion batteries. Arranged into packs of 69, the cells are capable of taking the vehicle from 0 to 60 mph in just 3.9 seconds. Just in case you were wondering, 69 goes into 6831 exactly 99 times. Also, if you are reading this article on your laptop, it is likely that it is powered by a lithium cell.
The major drawback to current Li batteries is their susceptibility to aging effects, especially when heated. You may have noticed that laptop and cell phone life deteriorates dramatically after a few years. This is largely due to aging. This issue has made the technology ill suited for backup and grid-scale power. Despite this, Li-ion batteries have competed for energy storage projects with alternative technologies such as thermal, flywheels and compressed air storage. Most of these installations have been in California. Silent Power's Li+ cells are being used to dampen power fluctuations in Sacramento and Greensmith has installed 1.5 megawatts of grid-balancing lithium-ion batteries throughout the state. In addition, AES Energy Storage has installed, or is in the process of installing, 76MW of Li+ battery capacity worldwide with 500MW in development. The main benefit of this technology is the fact that we understand it well and have the immediate resources for it to work. In large scale projects lithium-ion batteries have been most successful in sites where there are severe space restrictions or minimal maintenance capabilities.
In the near future it seems as if lithium ion technology is set to continue to dominate many applications. Li+ batteries are a proven concept, unlike some other technologies that have remained cloistered in the lab. The possible emergence of electric vehicles and the booming demand for electronics will undoubtedly have positive effects on the industry. Unfortunately, all good things come to an end. Analysts forecast that the technology will lose some of its competitive edge once infant technologies such as aluminium-ion, zinc-bromine and lead-carbon come on the market. For example on the topic of lithium ion batteries in storage applications, Lux Research said the following:
"Li-ion batteries developed for transportation applications are energy-dense storage devices. Stationary storage projects rarely value this metric, resulting in wasted value for grid-tied Li- ion battery systems. Rapidly evolving technologies with equivalent or superior performance metrics and substantially lower costs and higher resource availability will take over the majority of the grid storage market in the coming years."
Though they are unlikely to be used in many grid-scale storage projects, Li-ion batteries will certainly play a large role in our future. Their high cost will probably drop as the concept continues to mature and the devices become more widespread. A study by Mckinsey research found that 1/3 price reductions could be achieved through economies of scale alone. In any case lithium ion batteries are going to have to fight to keep the advantage they currently have.
Lithium is just a small part of the global picture. There are currently many competing concepts in the world of energy storage, each with their own pros, cons and background.