Researchers from the Hong Kong University of Science and Technology have recently reported on the performance of a new commercial ion exchange membrane for Vanadium flow batteries. The new composite membrane consists of two layers: a microporous layer and a Nafion layer which is much thinner than conventional membranes made from Nafion only. The new membrane is substantially cheaper than the conventional Nafion membrane and at the same time improve the batter efficiency (71.2% ==> 76.3%) and the electrolyte energy density (54.1% ==> 68.4% electrolyte utilization).
A Bergen-based Norwegian shipowner Rederiet Stenersen has ordered a pair of 17,500 metric ton chemical tankers outfitted with lithium ion batteries from a Finnish company called WE Tech Solutions to improve the efficiency of its diesel generator set which provides on board electric power. The batteries will act as a spinning reserve to handle peak loads, thus allowing the generators to run for most of the time at steady high efficiency operating point. Of course more efficient use of diesel fuel is not really a world changing innovation if you are concerned about lowering GHG emissions to zero, but this use of lithium ion batteries is another sign of the progress being made in performance and cost of these high energy density long-lived batteries.
Recently the website insideevs.com published a story about an energy storage innovation award won by the company EVgo which is creating a network of fast charging stations for electric vehicles. In cooperation with UC San Diego they set up an electric charging station powered by PV panels on the station sunroof connected to used lithium ion batteries from BMW i3 electric cars.
The story claims that the solar energy falling on the station roof can charge approximately 15 cars per day, but does not say whether this is the yearly average or whether this represents the peak Summer charging rate.
EVgo is planning to open its first 350kW ultra-fast public charging station in June of 2017 in Bakersfield, California. This station will have four 350kW chargers and will also feature a sunroof connected to lithium ion storage batteries.
A casual reader might get the impression that this station will charge vehicles using only the solar energy which falls on the sunroof. However, it is pretty clear that such is not the case. The capital expense of these high power chargers will not be justified by charging a few tens of cars a day. Nor will customers be happy about their ability to charge up depending upon how many cars happened to use the station in the previous few hours. Including solar panels and used EV batteries may save some amount carbon emissions, but clearly the intention is to hook up this station to the grid and have year round day and night service.
A 100% solar powered transportation system will require a lot more solar panels than can be included on the sunroofs of charging stations. It will also require some means of dealing with long term variations (e.g. seasonal variations due to the the tilt of the earth’s axis and long term variations in cloud cover) in the solar energy flux. Insofar as the costs of lithium ion battery storage are driven down by a cycle life measured in the thousands, they are not a practical tool for dealing solar flux variations on these long time scales.
rogerkb at energyevolutionjournal dot com
The most common anodes in commercial lithium ion batteries are graphite intercalation anodes. Graphite is a form of solid carbon which consists of stacked layers with each layer taking the form of a 2D honeycomb lattice. In the process of intercalation, positively charged lithium ions slip between the graphite layers while the carbon layers gains electrons to balance the overall charge in the intercalation compound. Graphite anodes are comparatively cheap to manufacture and they have very long cycle life. However, there energy density is relative modest.
Silicon has been frequently explored as an alternative to graphite for lithium ion battery anodes. When graphite has has absorbed a maximum capacity of lithium ions it contains one lithium atom for each 6 carbon atoms. When silicon as absorbed a maxim capacity of lithium it contains 15 lithium atoms for each 4 atoms of silicon. As a result the theoretical maximum energy density of a silicon anode is much higher (nearly ten times) than that of a graphite anode.
However, the high ratio of lithium to silicon in the fully charged state leads to a large increase in the anode volume (nearly 300%) which leads to silicon particle cracking and degradation of the anode performance after a relative small number of cycles.
An article published by chemical engineering researchers at the University of Waterloo in Ontario, Canada entitled Evidence of covalent synergy in silicon–sulfur–graphene yielding highly efficient and long-life lithium-ion batteries points the way to a possible silicon anode design which overcomes the cycle life problem.
This anode design embeds silicon nano-particles in a matrix of sulfur doped graphene and polyacrylonitrile (PAN). Graphene is a single atom thick two dimensional hexagonal lattice of carbon atoms. The hexagonal layers of carbon mention above in the discussion of graphite anodes are in fact graphene if they can be separated from the other layers of carbon which from graphite particles are composed. This separation can be achieved by various methods. One method is to disperse graphite particles in an organic solvent which separates the hexagonal layers.
Doping the graphene with sulfur helps to create strong chemical bonds with the silicon nano-particles. The precise role played by PAN, which is a synthetic, semi-crystalline organic polymer resin, with the linear formula (C3H3N)n is not clear to me, but apparently it is needed to stabilize the overall electrode structure.
The Waterloo researchers claim to have build an anode that maintain a specific current capacity of >1000mah/gm (the theoretical maximum for graphite is 372mah/gm) for 2275 cycles. The incredibly precise number of 2275 cycles probably means that they had a single sample which failed after exactly this many cycles. This sample provides an existence theorem for a long cycle life silicon anode, but does not prove that such results can be achieved on a consistent basis. Furthermore even longer cycle life is desirable that was demonstrated in the experiment are desirable for certain applications.
Interestingly this anode configuration does not prevent the fracturing of the silicon particles as they repeatedly expand while absorbing lithium ions. Electron microscopy of the the anode at the end of life shows that the particles have fractured into small pieces, but the pieces remain bound to the sulfur doped graphene and apparently continue to function as effective absorbers of lithium ions over many charge cycles of the battery.
Fabrication of the electrodes sounds comparatively easy. The silicon particles, sulfur doped graphene, and PAN are mixed together with graphite oxide and the organic solvent dimethylformamide under ultrasonic radiation. The resulting slurry is coated onto copper sheets and dried by a heat treatment.
If this research ultimately results in a practical battery design one of the key cost issues will be the production of the graphene itself. Graphene is being intensively researched for a variety of applications of because of its unique chemical, electronic and mechanical properties. However, economic methods for the large scale production of graphene have not yet been developed. When and if such processes are developed then this anode design may be a candidate for improved energy density lithium ion batteries.
Lithium ion battery manufacturer Electrovaya and its whollly owned subsidiary Litarion have announced the availability of a new lithium ion battery with very long cycle life (>9000 at 100% depth of discharge). They also claim that this cell offers superior safety characteristics and potentially lower manufacturing costs (ultimately as much as 50% lower) than conventional automotive lithium ion batteries. The Electrovaya batteries use the typical EV battery (e.g. Tesla and Nissan Leaf) chemistry of a Lithium/Nickel/Manganese/Cobalt (NMC) cathode and a graphite anode. The feature which distinguishes the new Electrovaya/Litarion battery is the ceramic coated separator. The separator is a thin porous membrane which allows ions to to move from one electrode to the other but prevents contact between the electrodes. The separators have a strong influence on the battery safety and on battery cycle life. Apparently they are also very expensive if a new separator can really drive battery cost down by 50% as claimed in the Electrovaya/Litarion press release. A cycle life of 9000 also raises the possibility that an automotive battery back could last the lifetime of the car. The avoidance of buying an expensive battery replace midway through the life of an EV is an attractive possibility. However, since the electrode chemistry is not changing this battery does not represent an improvement in energy density or EV range.
Apart from automotive applications, the very long cycle life of these batteries may make them potentially attractive for grid energy storage applications which require multiple cycles per day. At one cycle per day it would take 24 years to realize the full value of the battery, which is a long time scale in our current system of economic production. For short time scale, high value grid balancing applications such as frequency regulation, which requires multiple cycles per day, these batteries might be an attractive technology in spite of their relatively high cost per kWh. For example the Tesla Power Wall has an installed price of $714/kWh. If you could reduce this cost to $500/kWh then 9000 cycles would result in a cost of 5.5¢/kWh plus interest. For a high value application like frequency regulation this might be an attractive proposition. Batteries which use LiFePO4 cathodes have often been promoted for grid storage application over batteries using the NMC cathode because of superior cycle life and safety, even though their energy density is lower. This new NMC design may take away these advantages from the LiFePO4 chemistry.
NGK Insulators has recently announced that it will sell a 50MW/300MWh sodium sulfur (NAS) battery system to Mitsubishi Electric Corporation. When this battery storage system becomes operational in 2016 it will be the largest in the world eclipsing NGK insulator’s previous record of a 34MW/238MWh battery storage system for a Japanese wind farm. I have written about NAS batteries previously here.
Installed costs are about $400/kwh with an expected cycle life of 4500 at 80% depth of discharge. By contrast the much ballyhooed 10kwh Tesla Powerwall will set you back $7140 dollars or $714/khw, and I believe the cycle life is significantly less than that of NAS batteries.
NGK batteries have more capacity deployed on he grid than any other battery type. The sales of this technology suffered a blow in 2011 when a battery fire broke out at one installation site. NGK shut down manufacturing operations as well as existing installations until they had analyzed the cause of the fire and developed a technical fix. Although they resumed manufacturing operations in 2012 and have rebuilt a number of existing sites with with enhanced safety features, I had the impression that sales of their batteries had slowed down. This latest sale indicates that this technology still has life left in it. NGK’s batteries are probably too expensive to truly revolutionize grid operations, but the the current generation of lithium ion batteries are even more expensive and therefore even less likely to bring about a revolution in grid operations.
Sion Power has developed a lithium sulfur battery with and energy density of 350Wh/kg, and they claim that a density of 600Wh/hg will be achievable relatively soon. They do not specifically cite a cycle life for this battery but if I attempt to interpret the cycle life information given in this presentation I conclude that Sion Power’s Current lithium sulfur battery has a cycle life at full depth of discharge considerably less than 1000 cycles. Cycle life is currently the Achilles heel of lithium sulfur batteries. Unwanted side reaction of the lithium sulfide formed in the sulfur cathode as well as mechanical damage due the the growing and shrinking of the sulfur cathode conspire to limit the cycle life of this battery technology. Like Oxis Energy Sion Power’s applications page does not mention the obvious market of consumer electronics, making be suspect that the cost of the batteries is fairly high compare to lithium ion batteries. The high theoretical energy density of lithium sulfur batteries makes them an attractive alternative to lithium ion batteries, but I do not think that this technology is ready for prime time.
Lithium sulfur batteries have a much higher theoretical maximum energy density than lithium ion batteries (3 to 5 time according to this article). To date, however, practical implementation of this battery chemistry have have achieve only a fraction of the theoretical maximum energy density and have relatively short cycle life because of undesirable side reaction of the lithium sulfide formed in the sulfur cathode and because of physical degradation of the cathode due to swelling and shrinking during the charge/discharge cycle. Lots of fundamental research is being done trying to overcome both of these problems, but commercialization still seems to be in the indefinite future.
It is not clear whether Oxis’ technology represent to leading edge of commercialization for this technology or not. Cost is a key in addition to performance. On their applications web page Oxis mentions electric vehicles, grid energy storage for renewable, and mobile power for military applications. The fact that they do not mention consumer electronics makes me suspect that their technology is probably expensive. Why would they neglect this large existing, performance hungry market if they had costs competitive with lithium ion batteries? I think that I will not hold my breath waiting for Oxis’ batteries to revolutionize energy storage markets.