The arrival of Lithium Ion batteries have been nothing short of a blessing in the world of technology and gadgets. These batteries are more energy dense compared to both Lead Acid and Nickel Metal hydride. They are extremely versatile and have been powering airplanes to pocket devices (e.g. cell phones). They hardly require any maintenance. Such has been the impact of Li ion batteries that they have pushed electric vehicles from mere concept to a reality. Their use in handheld devices in particular, has revolutionized communication technology.
Today energy storage solutions at grid level are being implemented courtesy Lithium Ion (Li-ion). However, continual conquest in human ingenuity means Lithium Ion battery technology will have to cope with ever rising expectations. It is both a race against fossil fuels and a race against time. One fact in favour of battery technology is that expectations from fossil fuels have plateaued while for Li-ion technology the prospects are still improving. This article looks at the developments that will shape the future of lithium ion batteries.
The expectations from almost every new technology can be represented graphically by a curve called the “Hype cycle”. The curve can be separated into five regions namely: Technology trigger, Peak of inflated expectation, Trough of disillusionment, slope of enlightenment and finally plateau of productivity. At present, the locus of Li-ion technology is passing through the plateau of productivity. This means expectations from the technology are seeing a gradual increase.
It is interesting to note that several other battery chemistries are also blossoming in the wake of Li ion technology. As the appetite for energy storage solutions is wetted and as the transport infrastructure is electrified, there will be several contenders challenging the perch currently occupied by Li-ion. Among them are Lithium-Sulphur, Lithium-Air, Magnesium Ion and Sodium- Air batteries. These technologies are still in the region of undulating expectations on the Hype curve (see image on the right).
Even within the Li- ion umbrella, there are several battery chemistries. Improvements in each of these sub-classes has been driven by specific applications. For example, if system safety is a priority, one type of Li ion battery is used (LiFePo4). If more power is required than another type of chemistry is used (LiMnO). There is no single Li-ion battery that can serve every high performance application. However through continual research, customer feedback, improvement in battery internal architecture and advance manufacturing techniques, nearly all batteries of Li-ion classification are better today than they were at the time of their first release.
The following are the developments that are being carried out in Li-ion battery configuration to further improve their utility:
One of the pertinent issues of Li-ion battery is the flammability of electrolyte. For this reason there are certain restrictions over their transport as air cargo. It is a requirement that Equivalent Lithium Content (ELC) must not exceed 8 gms per battery for air freight. As of 2014, their flammability has caused at least 4 major incidents (lithium ion battery fires). This has put a question mark on their safety. Projects like Solar Impulse have to a certain degree quashed these concerns. There are developments both at the cell level and the pack level for improving Li-ion safety record. At the cell level, the chemistry is being altered and Non-flammable materials are being explored. At the pack level heat isolation techniques are being implemented.
Researchers in 2014 found a range of non-flammable electrolytes using perfluoropolyethers (PFPEs). These electrolytes show thermal stability even for temperatures beyond 200° C, meaning they are at least twice as stable compared to conventional Li-ion electrolytes.
In March 2016, Microvast Inc, launched their non-flammable battery. This technology is a breakthrough for the industry as it resolves battery safety issues through a multi-level approach from materials to the system level. The high heat handling capability may have reduced energy density but it has increased battery life significantly.
The chronological life of battery is one specification manufacturers are reluctant on releasing. Normally a battery life of 5 year is routinely cited by most battery developers. After 5 years, the battery capacity is reduced to 70% of the original value. It has been mentioned by US Advanced Battery Consortium (USABC) that the life of the battery should be at least 10 years to help in the shifting of technology from fossil fuel to electrification.
The degradation of the battery starts immediately after it is manufactured. Part of it occurs because of its usage, part of it occurs because of its natural aging. Li-ion has a much lower degradation compared to other batteries. Further retardation of natural degradation is being explored by many manufacturers. Normally battery life is specified in charge cycles and the average lifetime of the cells is around 1000 cycles. However the use of certain Carbon anodes has increased the number of cycles to 10,000 cycles.
Two things that drastically decrease the life of the battery are exposure to temperatures beyond the recommended range and a duty cycle that is heavy (i.e. draining battery completely each time). Similarly overcharging is also dangerous for longevity of the battery. Therefore at the system level, a robust cooling system improves the usable age of the batteries. Accurate charge control mechanisms and pack sizing can alleviate the damage done by overcharging and over draining.
One of the reservation against charging of large Lithium ion battery packs is the longer time duration. Although there is technology that allows us to charge rapidly (flash charging). However, the quick charging method is prone to imbalance of charge at the cells level. Therefore the recommended practice is to balance the cells every once in a while with slow charging (at least once every fifth charge). Quick charging can also reduce battery life because heat is released during the charge process. As mentioned earlier heat degrades battery life. There are numerous methods that are being explored for reducing the charge time, including the replacement of battery pack altogether.
Nanyang Technical University has produced a battery that reaches 70% of its capacity within two minutes without compromising the life of the battery. In fact they deem that with their Titanium dioxide nanotubes for anode, the battery could last more than 20 years.
At present, Lithium/ Lithium carbonate inside the batteries is not economical to recycle. Research by Tru Group indicates that Lithium in the market was in surplus because of low demand before 2013. The demand and supply reached parity in 2013. With large scale manufacturing coming online like the Gigafactory in Nevada (that will churn out 35 Gigawatt hour of cells per year), it is anticipated that price of Lithium will rise. It is further anticipated that a production crunch will occur by 2017 after which it would make economic sense to reclaim Lithium from the battery. At present, the salvage of metals like Cobalt and Nickel are more lucrative to the recycling business. Any call for Lithium recycling at present is only based on a sustainability point of view.
In Li-ion batteries, the 18650 has been an extremely popular size. The size refers to a cylindrical volume with diameter of 18mm and length of 65 mm. This size provides versatility that allows it to be used in Laptops as well as Electric cars (Tesla Roadster). The industry however is gradually moving towards larger 26650 ( Over 2,600 W/kg and 5,800 W/L). The 18650 size provided large surface area compared to volume which helped in cooling of the batteries. On the other hand, 26650 packs more energy and therefore can meet larger energy requirements. With the development of better cooling systems and techniques at the pack level, the thermal profile of 26650 can now be managed.
The 26650 also has a lower cell impedance compared to 18650. It is extremely likely that Tesla will replace 18650 with the 26650 in the near future. The transition to larger cells is not limited to cylindrical cells. Larger pouch cells (dimensions above 300 x 200 mm) are likely to overtake many smaller pouch and prismatic cells in the future particularly for large scale applications (Electric cars, Grid level storage).
The costs for Li-ion have continuously decreased over the last decade and are bound to decrease further due to economy of scale. Bulk of the Lithium reserves are present in countries that are not friendly (China and Bolivia) to open trade and sourcing can become difficult in the future. However, if the price reaches a certain point, recycling of Lithium will kick in to stabilize the price.
Li ion prices have dropped significantly over the years. As can be seen from the graph, the price per kWh in 2010 was $750 while it dropped to only $145 in 2016. A report by Citigroup in 2014 highlighted that the point when battery price drops below $230/kWh will mark a terminal decline in fossil fuels usage. UBS ( a global financial services company) similarly researched that the price of the battery may drop below $100 per kWh before its eventual stabilization.
The rate of incidents in 18650 cells is quoted as 1 in 5 million. This may seem very low but given that large number of cells are used in EV (in hundreds), with thousands of EVs being produced, this indecent rate can translate into 16 fires per year. In light of the modern safety standards, this is totally unacceptable.
With better manufacturing techniques and insight, reliability of the Lithium ion battery at cell level has been improving consistently. Reliability of cells is closely related to manufacturing tolerances and implementation of quality control methods. Being almost solid state, devoid of sloshing liquid electrolytes, Li-ion by design are far more robust than Lead- Acid counterpart. But it has to be said that because of decades of production, Lead Acid batteries enjoy unparalleled manufacturing and fault rectifying experience. Therefore with time, the fault incidences in Li-ion are bound to go down.
At the system level, reliability can be improved by identification, critical design and analysis. As mentioned earlier, many battery packs are made up of hundreds of cells. More elements in any system can decrease the overall reliability. On the other hand failure of a single element if replaceable by a redundant element can improve reliability. Therefore a balance has to be struck. The system should be designed with identified critical elements and non critical elements. The critical elements should have backup/ spares while any redundancy in non critical elements should be eliminated. The Pacadu Technology is one such system level technology that not only allows integration but also isolation and replacement of faulty components. Similar developments to improve pack level reliability are also underway.
The rapid and diverse improvement in Lithium Ion batteries is heartening for renewable energy propagation.
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