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AQUEOUS ELECTROLYTE HYBRID SUPERCAPACITOR
[Category : - RENEWABLE ENERGY]
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An Innovative Hybrid Supercapacitor
Abstract An innovative aqueous electrolyte hybrid supercapacitor can replace most of current supercapacitors and can open the gate for popularization of hybrid technology; as cost is only one third of the current supercapacitors and working life is more than 100,000 cycles, it has broken the technological bottleneck for high power energy storage. It can also be used in wind and solar energy and power grid buffering as the cost and cycle life hit the future targets in large scale energy storage.
Background of the Technology
1 present electrochemical energy storage technology
Energy storage technology has great market potentials in fields of transportation and energy resources as well as the like；Applying technologies of hybrid，braking energy recovery and start-stop on vehicles can greatly save fossil fuel and decrease pollution, popularizing of them require high power energy storage technology. Renewable energy sources such as wind energy and solar energy are intermittent and random, their applications in large-scale require economical and endurable energy storage technology; in the power grid, as load of usage fluctuates greatly, the installed capacity can be decreased greatly when large-scale energy storage technology is applied to balance the peak and trough of load. It is a pity that all kinds of energy storage technologies nowadays cannot be widely used as having bottlenecks of cost and cyclic life.
Electrochemical energy storage technology mainly utilizes secondary batteries to realize the transformation between electrical energy and chemical energy, with high specific energy, quickly responding, without limitation on geographic position and scale. Commonly used secondary batteries include lead acid battery, nickel hydride battery, lithium ion battery, generally ranging from several tens to several thousands cycles in cyclic life, several tens to several hundreds watt hour/kilogram（Wh/kg）in energy density and below 300 watt / kilogram（W/kg）in power density. Lead acid battery is low cost, widely used in area of starting and backup power as well as the like, the disadvantages are low energy density, bad ability in deep discharging, limited cyclic life and contaminating environment; Nickel hydride battery has relatively high specific power, with bad economy characteristic, has memory effect to some extent. Lithium ion battery has high energy density, without memory effect, with long cyclic life, extremely widely used in area of portable electrical device; however, the large scale application is hindered by drawbacks of high cost, bad consistence in battery pack, and safety risk which bought by organic electrolyte and lithium as well as the like.
Flow battery is a special form of secondary battery, generally storing soluble reactants of cathode and anode in tanks, circulating by means of pump, charged and discharged on inert electrodes, with features of strong ability of deep discharging, long cyclic life, having several forms as all vanadium, zinc bromine, poly sulfur sodium-bromine, iron chrome and so on; After decades of research and development, flow battery of all vanadium and flow battery of zinc bromine nowadays are still being tried large scale commercial applications. All vanadium flow battery requires expensive ion exchanger membrane, the electrolyte containing vanadium costs high. Zinc bromine flow battery is good in economical characteristic, but the fluid of bromine is strong in corrosion and do harm to environment.
Sodium sulfur secondary battery utilized solid electrolyte, using sulfur and sodium as the reactants in positive and negative electrodes, with relatively low cost, high energy density, big power density, working temperature is 350 degrees centigrade about, there is a problem that solid electrolyte is bad in stability as long-term running under high temperature.
In high power energy storage area, starting lead acid batteries for vehicles generally require replacement within several years, often face difficulty of starting under severe cold conditions; in hybrid generally nickel hydride battery is applied, but its economy characteristic hinders the popularization. For start-stop system on vehicles, lead acid batteries in forms of Enhanced Flooded Battery （EFB）and Absorbed Glass Mat（AGM）are used, although their cycle lives individually are 2 and 4 times higher than that of ordinary starting batteries, the costs are higher 50% and 2 times, usually require replacement in only 2-3 years.
Supercapacitor is a kind of special form of electrochemical energy storage technology, based on “electrical double layer mechanism” or “pseudo electric capacitor mechanism”, using large specific surface carbon or large specific surface metal oxide and so on as electrode materials; symmetric supercapacitor has same materials in its two electrodes, it can reach 10-1000 times of that of battery in power density, with cyclic life over 100,000 cycles, without memory effect, enduring deep charging and discharging. However, current supercapacitor is of bad economical feature, pricing about 5 times about that of lithium-ion battery if in same capacity, with energy density of only 1 Wh/kg to 5 Wh/kg about, is hard to be applied in large scale.
Hybrid supercapacitor is constructed with a single electrode of supercapacitor and a single electrode of battery cooperatively, combining features of high power density and very long cyclic life that is from electrode of supercapacitor, with high energy density that from electrode of battery, provides a new direction for large scale energy storage and high power energy storage.
Lithium ion hybrid supercapacitor is constructed with a positive electrode of active carbon and a negative electrode of graphite in organic electrolyte contained lithium ion, while the electrical double layer capacitor charging and discharging in positive electrode, lithium ion intercalating reaction occurs on graphite in negative electrode, the energy density is much higher than that of supercapacitor in symmetric form; however, bad performance on economy has made the commercialization of the hybrid supercapacitor with organic electrolyte difficult.
Aqueous electrolyte is low cost, safe and reliable; current aqueous electrolyte hybrid supercapacitors still have many problems; With nickel electrode as positive electrode and active carbon as negative electrode, the nickel carbon electric capacitor battery costs high; With lead oxide electrode as positive electrode and active carbon electrode as negative electrode, the lead carbon hybrid supercapacitor is easily detached on positive electrode, has contamination of lead and sulfur acid.
Present hybrid supercapacitors have obviously higher energy density over 10Wh/kg compared with symmetric supercapacitors, but the power density is about 0.4-2 kW/kg, lower than symmetric one; cyclic life can reach several ten thousand times; it is relatively fit for requirement for large-scale energy storage; however, the problem of high cost is still not overcome.
2 Practical Demands and Current Situations of Energy Storage Applications
The popularization of large scale energy storage and high power energy storage require energy system have relatively high power density and relatively high energy density, moreover, long life and low cost that are similar with mechanic system are also required. United States Advanced Battery Consortium USABC and Department of Energy DOE have set up standard demands for hybrid vehicle auxiliary energy system that are working with power over 625W/kg lasting 10 second, with total energy of 7.5Wh/kg. Large scale energy storage ordinarily requires that the energy storage devices should reach about 4000 times in cyclic life, the cost should be less than 500 USD per kilowatt hours in near future, reach 125-250 per kilowatt hours in far future.
On area of high power applications such as bus, new energy vehicles, rail transportation, port mechanics, mine mechanics and vehicles, wind turbine and so on, current asymmetric supercapacitor can cost about $60 to $80 USD per kilowatt that is the lowest power cost among all kind of energy storage technologies; however, it is hard to be popularized for a fundamental reason that the cost is still high and the energy density is not enough.
On large-scale energy storage area, lead-carbon battery, lithium ion battery, all vanadium flow battery and sodium sulfur battery have been tried commercialization in difficulty as their present performances on cost and cyclic life are hard to meet the requirements.
Replacing internal combustion engine (ICE) vehicles with electric vehicles (EVs) is a hot point on research and development worldwide; hundred years ago, the EVs used lead acid battery and nickel iron battery had emerged earlier than that of vehicles with ICE, but they had been eliminated by ICE quickly. In recent twenty to thirty years, boom of research and development on EVs have been motivated by the success of lithium ion battery on area of portable electric devices and pressure of environment caused by ICE vehicles as well as promotion of capital market, luxury EVs made by Tesla corporation, used over several thousands 18650 lithium ion batteries, have made advances in market and attracted focus of investors all over the world.
In fact, present technology of battery is far away from mature enough for supporting the dream of replacing ICE vehicles with EVs. Lithium ion battery and fuel cell are two most attractive technologies nowadays; compared with ICE technology supported by cheap metal material and petroleum fuel, lithium ion battery is lower in power density, far less in energy density, with high cost and short life span; pure EVs and plug-in vehicles need large capacity battery packs, they are not only face four difficulties of slow charging, short coverage, high cost and short life span that are very hard to overcome, but also have problems on safe and protection of environment that is not completely solved; fuel cell technology at present has puzzles of high cost and immature hydrogen supply, its distance from application is further than that of lithium ion battery. It is unpractical to popularize EVs and replace ICE vehicles without major breakthrough on battery technology, but only rely on propulsion of capital and government policy.
Popularizing hybrid vehicle that cooperating battery and ICE is more promising than EV; The technology of hybrid vehicle is matured, fuel saving can attained 40%, Toyota hybrid cars Prius had been produced ten millions in last two decades. Hybrid vehicles mainly utilize nickel hydride batteries, storing energy is several ten times fewer than that of pure EV, without necessity of charging pile in addition; However, the high cost and short life of battery are still the bottlenecks for its popularization; Nowadays the price of a battery pack of Prius is $3000 USD, customers demand working life of battery over ten years, but Toyota can only provide eight years guarantee of working. Such price and working life have reached the limitation for nickel hydride battery, it still cannot be popularized.
Introduction of the Innovative Hybrid Supercapacitor
1Breakthrough on materials and principle
With special materials and unique mechanism, the innovative aqueous electrolyte hybrid supercapacitor has broken the bottleneck of cost and cyclic life for electrochemical energy storage technology; the drawbacks of high cost in supercapacitor electrode and low power density in electrode of battery have been overcome.
2 technological features
Applied in high power energy storage area, the most significant advantage of this patented technology is that the cost is far less than that of symmetric supercapacitor; it is only $20USD/kilowatt in power cost, about one third that of symmetric supercapacitor according to energy density, and one fifth when according to energy density. When its power density is above 1kW/kg that is more than 2-3 times than nickel-carbon supercapacitor battery, its energy density reached about 10Wh/kg; meeting for most high power requirements; cycling life is above 100 thousands, similar with that of symmetric supercapacitor; it endure charged and discharged 100% in depth, working rang is -20 to +60 degrees Celsius with good low temperature character; using aqueous electrolyte and green materials, it has no risk of fire and problem of environmental contamination.
According to the same principle, designed in form of lower power density but higher energy density, the energetic form of the innovative hybrid supercapacitor can be 10Wh/kg-20Wh/kg in energy density, cycling life above 10000 cycles, with cost lower than $125 USD per kilowatt-hour, that reached far period target for large scale energy storage applications, be able to be used for renewable energy sources such as wind energy and solar energy.
The producing and assembling for device of the patented technology is easy and only require lower investment; different from producing lithium ion battery and organic electrolyte supercapacitor, it is unnecessary of expensive producing line and storing places of high safety standards.
3 Applications
Breaking the hinder of high cost and low energy density for supercapacitor as illustrated by these features, the technology can bring a revolution on high power energy storage applications, opening the wide market of hundred billion values. For instance, supercapacitor bus utilize supercapacitor and electric motor to drive, quick charging at bus station, without necessity of ICE and battery pack; the price of supercapacitors in present bus is $44,100 USD, if use the innovative hybrid supercapacitor to replace it, the price will be within $14,700 USD that is completely acceptable in market.
The hybrid supercapacitor has lower specific energy than that of lithium ion battery, it cannot be used as the main dynamic source for EV; As it meet popularization requirements in high power energy storage area, it can be used as cheap auxiliary driving source to improve accelerating and collect braking energy as well as reduce load capacity of battery. More important, it can open the gate for popularization of hybrid technology without government subsidies, to create a wider market than that of application of supercapacitor.
When it is utilized in hybrid, the cost of energy storage is merely one third of that of prior art; for instance, replacing the battery pack on present PRIUS of Toyota that is $3000 USD, it only need about $1000 USD, cycle life is surpassed working life of the whole car, this completely broke the bottle neck of hybrid technology; when used in light hybrid and start-stop system in vehicle, the cost is more less.
The technology can also used in large scale energy storage, widely utilized in fields of wind and solar renewable energy resources as well as power grid peak buffering and so on.
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