Lithium-ion capacitor

Lithium-ion capacitors (English lithium ion capacitor, LIC) are so-called " hybrid capacitors " with asymmetrical electrodes, that is, special double -layer capacitors ( EDLCs ) with two differently constructed electrodes. Its electrical capacity is made up of the series circuit of a positive electrode of a conventional electric double layer capacitor with a static double layer capacitance of a battery and the like, doped with lithium negative electrode having an electrochemical pseudocapacitance.

The doping of the negative electrode will cause the voltage resistance of the capacitor is about 3.8 V. Since the stored energy of a capacitor increases with the square of voltage, power density ( storage capacity ) of lithium ion capacitors at about 3.8 V is significantly higher than that of the conventional double-layer capacitors 2.5 V. At the same time keep the lithium -ion capacitors the very high power density of double-layer capacitors, so have the capacity for rapid loading and unloading coupled with a great strength and a long life cycle, thereby having significant advantages over lithium - ion batteries.

Your property fast charge and discharge capability coupled with a relatively high energy density makes lithium -ion capacitors for use in new concepts of electric mobility attractive, such as a memory for the recovery of braking energy (recuperation ) and to supply energy at peak load demand in trains, buses and in motor vehicles come into question.

  • 2.1 Functionality
  • 2.2 Features
  • 3.1 ingredients
  • 3.2 electrodes
  • 3.3 electrolyte
  • 3.4 separators
  • 3.5 collectors and housing
  • 4.1 capacity
  • 4.2 Dielectric strength and polarity
  • 4.3 Internal resistance
  • 4.4 power density and energy density
  • 4.5 Service life
  • 4.6 cycle strength
  • 4.7 Leakage current and self-discharge
  • 4.8 Characteristic values ​​compared
  • 4.9 Note to indicate different polarity
  • 5.1 Advantages and disadvantages compared to LI batteries
  • 5.2 Advantages and disadvantages compared to EDLCs

Basic structure, memory allocation principles and Family

Basic structure

Lithium-ion capacitors are similar to the basic construction produces the double layer capacitors. They also consist of two large-area electrodes which are electrically connected by a conductive electrolyte, an ionic conductor. The electrodes are separated by an electrically permeable membrane (separator) and against direct contact against each other and thus protected against a short circuit. Scale current collector (collectors ) contact the respective electrode and connect it to the external terminals. These bases can be wound into a coil or can be processed in several layers in a stack. Then they are incorporated into a common housing ( cell) and more or less closed off hermetically.

Lithium -ion capacitors, however, differ in one respect from conventional double-layer capacitors that have two symmetric static double layer electrodes. They combine a bilayer electrode with a battery -like electrode, thus becoming hybrid capacitors. Of the lithium - ion batteries, the lithium -ion capacitors differ in the structure of the positive electrode is an electrochemical metal - oxide electrode in Li -ion batteries.

As hybrid capacitors, lithium ion capacitors have two different electrodes that have different capabilities. The positive electrode of a LIC usually consists of activated carbon, and thus corresponds to a conventional electrode of a double layer capacitor that stores electric energy in a static electrical Helmholtz double layer.

The special feature of the lithium-ion capacitor, the negative electrode electrochemical redox intercalated with lithium ions. It consists, depending on the manufacturer, either also from specially suitable activated carbon or graphite, of a conductive polymer or graphs in the form of carbon nanotubes and stores the electrical energy electrochemical in a so-called pseudo- capacitance on redox reactions associated with faradaic charge -transfer transitions. Both types of memories, such as static, the Faraday, have a linear dependence on the stored electrical energy to the voltage on the capacitor.

Static and electrochemical storage principle

Static double layer capacitance

The physical effect that occurs in a Helmholtz double- layer of an electrode, causes the application of a voltage in both the surface area of ​​an electrode and the electrolyte in each case an electrically isolating layer, a double layer is formed, the symmetrical mirror images and on the second electrode of the capacitor is later on. The " thickness " of a layer in the electrolyte is in the range of the diameter of a solvent molecule, ie about 0.1 to 10 nm in these boundary layers, the anionic or cationic charges accumulate during charging of the capacitor voltage-dependent mirror image with an adsorption statically. The charges of the adsorbed ions in the electrolyte are compensated by counter- charges in the electrodes.

Between the accumulated charges, the ions in the electrolyte and the ions in the electrode within the phase boundaries, there is a charge separation with the formation of an electric field the strength of which corresponds to the applied voltage. Therefore, a static condenser is formed by the Helmholtz double layer. When discharging, the ions are distributed according to a desorption reaction again in the electrolyte.

The electrochemical pseudocapacitance

Storing electrical energy in a floating capacity is effected by means of a simple reversible redox reaction ( reduction-oxidation reaction) between the electrode and the cation in the electrolyte, which occurs at the surface of the electrode. Enter when unloading the cations to the negative electrode (cathode), respectively, an electron from flowing through the external circuit to the positive electrode (anode). At the same time an equal number of anions migrate through the electrolyte from the negative to the positive electrode. At the positive electrode but not the ions occupy the electron back on, but the present there and strongly ionized in the charged state and therefore quite " electron hungry " transition metal ions.

The redox reactions are within narrow voltage ranges, such as a capacitance, and can be effectively measured as well, although in contrast to batteries no material change occurs in the electrodes. The ability of the electrode, oxidation-reduction reactions, which results in a floating capacity to effect depends on the material of the electrodes. Suitable, as they allow the electrochemical redox reactions, electrodes of particular activated carbon, conductive polymers, or certain metals or metal oxides, which are introduced into the electrode material by means of doping, and with the aid of an intercalation, i.e., deposition of impurities or compounds into the interstices inserted layer of levels, for example from graphite, in which then the oxidation-reduction reactions take place with the appropriate cations. A pseudo- capacitance may, for the same volume and the same weight, is up to 100 times greater capacity than a static capacitance form a Helmholtz double layer. That depends on the size of the atoms involved, which are usually much smaller than the ions in the electrolyte. In addition, the electrochemical redox reactions are very fast. This lithium -ion capacitors have a significant advantage over batteries: The loading and unloading process is much faster than batteries.

Family assignment

By the combination of two different types of electrical energy storage in a device, an electrostatic capacitance of the Helmholtz double layer and a electrochemical redox pseudo capacitance, it is necessary to make a classification of the resulting devices. This assignment leads to a family pedigree that, the hybrid capacitors under the generic term " supercapacitors " is the separation into pure double-layer capacitors, in pseudo capacitors and capacitors in which carry both effects in itself. Based on this division is the construction of the respective electrode, which determines the function properties of the capacitors. In industry, supercapacitors are also often called ultracapacitors.

Functions and properties

Operation

  • Operation of lithium-ion capacitors

Distribution of ions in the discharged capacitor

Distribution of ions in a charged capacitor and the derived from the structure of the equivalent circuit diagram

Lithium-ion capacitors ( LIC) belong to the family of the supercapacitors and capacitors in hybrid with asymmetric electrodes. Combining a static bilayer electrode of an electrochemical redox electrode is doped with lithium ions intercalated and which are both connected by a lithium containing electrolyte electrically interconnected.

When a voltage of the capacitor the charge carrier in the electrolyte dissociates into their positive (cations) and negative charges (anions). At the positive electrode of a LIC, usually consisting of activated carbon, then the anionic charges accumulate during charging of the capacitor voltage dependent with a static adsorption reaction at the double layer at. Thereby forming on the positive electrode of the LIC is an internal capacitor of the capacitance, which arises from the surface of the electrode and the distance represented by the double layer.

The special feature of the lithium-ion capacitor, the negative electrochemical redox electrode. It consists, depending on the manufacturer, either also of activated carbon or graphite, of a conductive polymer or graphs in the form of carbon nanotubes. In the negative electrode of the lithium-ion capacitor positively charged lithium atoms are intercalated by doping into the interstices of the electrode material layer levels ( intercalates ) during the manufacturing process. There they give a charge in the form of an electron to the material of the electrode further, creating a surplus of electrons is generated and the potential of this electrode is lowered. This effect has an effect as if in the negative electrode of a galvanic element installed, that a bias voltage (clamp voltage ) is generated from about 2.2 to 3 volts. Of the lithium -ion capacitor is only charged when the voltage present at the capacitor voltage is greater than the clamp voltage. Then wander distributed in the electrolyte positive lithium ions to the negative electrode, may also undergo intercalation and leave a redox reaction with their cargo there. The negatively charged electrode, thereby forming an internal capacitor, the capacitance resulting from the capacity of the pseudo- intercalated lithium ions.

Since both the electrostatic energy stored in the double layer and the redox reactions of the pseudo capacitance linearly related to the charge stored in the capacitor corresponds to the voltage waveform across the capacitor of the stored energy. This distinguishes the capacitors from the accumulators whose voltage regardless of the charge remains largely constant at the terminals.

The negatively and positively charged electrodes thus forming two internal capacitors whose capacitances are connected in series across the electrolyte. The total capacitance of the capacitor is given by:

With the capacity of the doped electrode, the capacitance of the double layer electrode.

Due to the very small size of the lithium ions is formed on the doped a negative electrode having a high carrier concentration, which has the result that the floating capacity of this electrode is very large; very much greater than the double-layer capacity of the positive electrode. The value of the floating capacity of this electrode is often an order of magnitude greater than that of the static double layer capacitance.

However, if, in a series circuit of two capacitors, the value of much greater than the value of is, the smaller capacitance value is determined, the total capacitance of the capacitor.

Thus, the total capacity of a lithium ion capacitor of ( static ) double layer capacity of the positive electrode.

As described above, the doping of the anode causes a lowering of the potential in the medium of about 2.5 V. The cathode, in its double-layer, a withstand voltage of about 1.3 volts so that the total power strength of the lithium-ion capacitor with 3, 8 V can be specified. Because the energy stored in a capacitor increases with the square of the voltage

Can be stored in a lithium -ion capacitor by the increased to 3.8 V voltage strength significantly more electrical energy than in a conventional EDLC with only 2.5 V dielectric strength.

The reduction produced by doping the negative electrode potential may be utilized during operation of the capacitor, however, is not. That is, in the anode "effective galvanic element " is not suitable to be able to deliver electric energy by means of chemical processes such as in a lithium -ion battery. The LIC is thus not to be discharged to 0 V or shorted at the terminals. Due to the structure of the lithium -ion capacitors as a hybrid capacitors with a Li -doped negative electrode lithium -ion capacitors are also polarized capacitors may be operated only with the correct polarity.

Properties

Lithium -ion capacitors are only in a few standard types of technical descriptions thereof are still partly very incomplete on the market. Nevertheless, you can filter the available data properties of the LICs, with which they can be compared with other components.

The withstand voltage of the lithium -ion capacitor of about 3.8 V the energy density ( storage capacity ) is much higher than that of conventional double layer and super capacitors with 2.5 V, however, significantly lower than that of lithium-ion batteries. But keep lithium -ion capacitors, the high power density of double-layer capacitors, so have the capacity for rapid loading and unloading, with a large number of charge cycles, typical data already go up to 1 million cycles. This large number of charge cycles associated with a greater temperature range means that LICs have a significantly longer life than the Li- batteries. Life times of 12 years at 30 ° C with a capacitance change of only 15% are certainly achievable and thus offer the LI- capacitors of the industry and the consumer an important argument for use in stand-alone, off-grid systems or for recuperation of braking energy. Also the efficiency of the electrical energy storage and in the self-discharge rate (<5 % at 25 ° C for 3 months storage) LICs offer advantages over the LI batteries. Lithium -Ion Capacitors are also classified as "environmentally friendly " because no banned heavy metals are used in them.

Construction

  • Types of lithium -ion capacitors

Schematic structure of a double layer capacitor with stacked electrodes, 1 positive electrode, negative electrode 2, 3 Separator

Basically, there are all the lithium-ion capacitors from two different electrodes that are separated by a separator, and the collectors are connected via the terminals to the outside world, and which are fitted together in a tight enclosure as possible. The inner construction can be layered in, or prepared as a wrap. Layering or winding includes not only a separator for the mechanical separation of the electrodes but also one or two more separator layers to protect the cells from direct metallic contact with the metal housing. This construction of a lithium -ion capacitor can be generally loaded only with the maximum cell voltage. Higher voltage strengths are achieved by connecting a plurality of capacitors.

Ingredients

Lithium -ion capacitors come entirely from heavy metals such as cadmium, lead and mercury. They are therefore classified as environmentally friendly.

Lithium -ion capacitors differ in the amount of processing lithium significantly from lithium - ion batteries. Although, in the negative electrode is incorporated, a large number of lithium atoms, the total amount of lithium in the condenser is relatively small. For example, in a 2000 F capacitor with a total weight of about 200 g only a total mass of 0.3 g of lithium installed. This low proportion of lithium accounts for legal restrictions with respect to possible danger, danger of fire or explosion in the lithium -ion capacitors.

Electrodes

Lithium-ion capacitors have a hybrid capacitor, two electrodes of different design with different properties. For the positive electrode, they should have particularly good properties for forming a static double layer capacitance. For the negative electrode it is to have a large capacity for intercalation of ions to generate a large pseudocapacitance.

Electrodes for capacitors are initially always have good conductivity as possible. In addition, they are for the static double layer capacitance have a large surface area as possible with the smallest volume and weight. This requirement is met by the electrode of activated carbon, graphite or graphene.

Active carbon, graphite or graphene consists mainly of carbon, and has a very large surface area. It is up to 2000 m² / g Along the crystal planes of carbon is electrically conductive and therefore very well suited as electrode material.

In the simplest form these carbon electrodes are used in pressed powder activated with highly porous structure. The pores are like a sponge and are interconnected ( open pore ) and together form the very large internal surface area. For an electrode of activated carbon by 1000 m² / g results in a typical double-layer capacitance of 10 microfarads / cm ² a specific capacitance of 100 F / g In another form of activated carbon may be carbon fibers (English Activated Carbon Fiber, ACF), to be spun, can be processed to fabrics for flexible electrodes. The surface of such fabrics is usually greater than that of the sponge-like powder and may reach about 1500 m² / g. Electrodes made of activated carbon or graphite are quite inexpensive, non-toxic and do not contain substances harmful to the environment. You can also from inexpensive, natural raw materials such as coconut shells, sugar or algae, are produced.

The negative electrode of the lithium ion capacitors has to be made of a suitable especially for generating a pseudo- capacitance material. It must therefore have the ability to intercalate ions. This "fit " of lithium atoms is performed both as part of the manufacturing process by doping as well as during the operation of loading the capacitor.

Also the materials described above, activated carbon, graphite and graphene can have a large pseudocapacitance, when the pore size in the material has a very small diameter. The ability of the activated carbon to be able to store an impurity, such as lithium in the intermediate layers of its crystal planes increases significantly, when the activated carbon is covered with a conductive polymer.

Quite suitable for intercalation are electrodes made of a conductive polymer such as polypyrrole, polyaniline, polythiophene, or pentacene, a polyacenes ( PAS polyacenic semiconductor). They are inexpensive to produce and have a large charge carrier mobility of up to 5 cm ² / Vs. The material is amorphous in a very porous structure. This structure makes it possible to store a large number of lithium ions with great stability. Li -doped PAS has however the same volumetric energy density, such as metallic lithium, a conductive polymer electrodes have due to chemical instability in their electrochemical reactions to have a shorter life cycle, and reduced resistance to the activated carbon electrodes. Compared to Li -Ion batteries but the life cycle and the strength of the LIC is still very much greater.

Recent developments using electrodes in the form of carbon nanotubes or graphene. Graph has a very large surface area, a gram of which has an area of ​​2675 square meters. Researchers at MIT developed electrodes for ultracapacitors with mats of carbon nanotubes, which have a diameter of between 0.7 to 2 nm with a length of several tens of microns and can reach a theoretical capacity of 550 F / g. Law clearly this electrode is shown during loading with ions of J. Schindall. The two-dimensional structure of the graphene sheet also improves the charging and discharging of a capacitor therewith. The charge carriers in vertically oriented graphene nanosheets can migrate faster into the deeper structures of the electrode or come out and thus accelerate the switching speed.

Electrolyte

The electrolyte of lithium ion capacitors, the electrically conductive connection of the two electrodes in the capacitor. It aims to provide the required loading of the capacitor anions for the double layer capacitance and the cations of the redox reactions of the pseudo capacitance. Its characteristics determine the voltage window in which the capacitor can be operated, its temperature, the internal resistance (ESR) and on its stability, even the long-term behavior of the capacitor.

An electrolyte always consists of a solvent in the conductive salts are dissolved. The salts dissociate in the solvent to positive cations and negative anions and make the electrolyte conductivity. The electrolyte must the porous, sponge-like or cross-linked structure of the electrodes can penetrate, its viscosity must be small enough to the electrode surface to fully wet. He must also be chemically inert and must not attack the materials of the capacitor chemically. The electrolyte used in the lithium ion capacitor is usually anhydrous and comprises aprotic organic solvents such as ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate or 1,2-dimethoxyethane and the dissolved lithium salts such as LiPF 6 or triazolates. Electrolytes with organic solvents are more expensive than aqueous electrolytes, but have a higher dissociation voltage of up to about 4 V, and a wider temperature range. Their slightly lower conductivity to aqueous electrolyte does have the effect, but since the energy density of a lower power density increases with the square of voltage, have LICs with organic solvent electrolyte, a higher energy density than those having aqueous electrolytes. The peculiarity of the electrolyte for lithium -ion capacitors is that the cations for intercalation into the negative electrode must be very small. The lithium salt dissociates consequently so that lithium is in the form of individual atoms in the electrolyte. Only then will the necessary Faraday charge transfer transitions of the pseudocapacitance can also take place.

Separators

Separators are the two electrodes mechanically separated to prevent a short circuit. You can be very thin ( a few hundredths of a millimeter ) and must be highly porous in order to contribute as little as possible to the internal resistance (ESR ) of the capacitor. Furthermore, they must be chemically inert to keep the impact on the long-term stability and the low conductivity of the electrolyte. Low cost lithium-ion capacitor using as the separator open capacitor papers, professional LICs employ porous plastic film, glass fiber fabric or porous ceramic fabric as separators.

Collectors and housing

The panels ( current collector ) are used for making electrical contact with the electrode material and connect them to the terminals of the capacitor. You must have good conductivity, after all, should peak currents of up to 100A without causing problems to the capacitor cell or be removed from it. If the housing as usual consists of a metal collector and the housing should be made of the same material, usually aluminum, because otherwise would form a galvanic cell in the presence of an electrolyte, which could cause corrosion. The collectors are either sprayed by a spraying process onto the electrodes or consist of a metal foil on which the electrode is mounted.

Electrical characteristics

Capacity

Measurable from the outside to the terminals of a lithium ion capacitor of the capacitance results from the energy content of a capacitor charged by the charging voltage:

This capacity is also called " DC voltage capacity ". It is measured in accordance with applicable standards (DIN EN 62391-1 ) by the capacitor is initially charged with a constant current source to its nominal voltage. Thereafter, the capacitor is held for 30 minutes on this voltage value, and then discharged with a discharge current defined, wherein the time is detected which elapses in which the voltage of 80 % drops to 40% of the rated voltage. The capacity is then obtained according to the definition of s in the picture from the formula:

The measurement methods that are specified by the individual manufacturers may differ in some details from the standardized method, see for example:

The standardized but very time-consuming to use to measure the capacity can be calculated from the energy content and by measuring the voltage drop from 90% to 70 % of the nominal voltage value according to the following formula

The capacity of a lithium -ion capacitor is strongly frequency dependent. Even at a measurement frequency of 10 Hz, the measured value drops to only about 20% of the DC voltage value. This behavior is due to the limited mobility of the ions in the electrolyte, especially in the porous structure of the electrodes. The properties that result from this can be described serially connected RC circuits electrically quite well to a series circuit. In order to utilize the full capacity of a pore until the end of the pore, every individual capacities must be reached via the serial RC time constants, while the flowing current has to overcome a growing line resistance. Thus, the entire capacitance of the capacitor is achieved only after prolonged operating times. When an alternating voltage with very low frequency, only the advantage of the greatly reduced capacitance at the pore entrance. The frequency dependence of the capacitance also has impact on the operation of the capacitors. If the LICs to be operated with high-speed charging and discharging, then the application is no longer the full value of the DC voltage capacity. The usable capacity is smaller and has to be adjusted by appropriate selection of the capacitor in the individual case of the application.

Withstand voltage and polarity

The withstand voltage of the lithium -ion capacitors resulting from the sum of the dielectric strengths of the two electrodes. By the doping with lithium is formed in the negative electrode when it is connected to ground, a voltage of about 2.2 V. Together with the in-line double-layer positive electrode, which can be loaded with about 1.3 volts and a small further voltage reduction of the negative electrode of about 0.5 V results in the total withstand voltage of a lithium -ion capacitor to about 3.8 V. the lower potential difference between uncharged and charged state of the two electrodes can be explained by the fact that the capacity of the negative electrodes is very much larger than that of the positive electrode and therefore, the influence of the charge on the voltage is lower. Manufacturer-specific value of the total voltage strength may be exceeded or. If a higher voltage is applied as the specified value from the respective manufacturer to the capacitor and a maximum dissociation, wherein the Helmholtz effect remains stable in the bilayer is exceeded, an electrolytic decomposition of the electrolyte. This can lead to chemical reactions that lead to the formation of gas and thus can destroy the capacitor.

Lithium-ion capacitors are polarized capacitors. The doping of the negative electrode can be destroyed when the capacitor is placed with its positive terminal to ground is shorted, is operated with AC voltage or wrong polarity. A lower voltage limit of about 2.2 V must not be exceeded in lithium -ion capacitors.

Internal resistance

The loading or unloading of a double layer capacitor is connected to a polarization of ions in the electrolyte and a movement of the charge carriers through the separator deep into the pores of the electrodes inside. During this movement of the ions in the electrolyte losses occur which can be measured as the internal resistance of the capacitor. With the electric model of series-connected RC elements in the pores of the electrodes in the image can thereby easily explain that the internal resistance of double-layer capacitors with increasing penetration of the charge carriers delayed increases in the pores of the electrodes. Since the charge carrier mobility is still too limited, not only the capacity but also the internal resistance is highly dependent on frequency. During charging and discharging of a capacitor, the current flow is a direct current. The effective internal resistance, sometimes called ESRDC, therefore, a direct-current resistance. It is calculated from the voltage drop which results from the extension of the straight section of the discharge voltage at the time of Entladebeginns as the intersection with the discharge curve according to the following formula:

The discharge current for the measurement of the internal resistance is specified in LICs by the manufacturer. A faster alternative to use to measure an internal resistance is the measurement of an AC resistance. This AC resistance is called ESR ( en: Equivalent Series Resistance). It is measured at 1 kHz, and has a much smaller resistance value.

The internal resistance determines several properties of double-layer capacitors. It limits to the charging and discharging a capacitor. Together with the capacitance of the capacitor results in the time constant of

This time constant determines the time limit by which a capacitor can be charged and discharged. A 100 F capacitor to the internal resistance of 30 milliohms for example, has a time constant of 0.03 x 100 = 3 s, i.e., after 3 seconds only store a limited by the internal resistance of power, the capacitor 62 3% of the charging voltage reached. Since up to the full charging of the capacitor for a time period of about 5 is required, the voltage is then S reaches the charge voltage is around 15.

However, the internal resistance is also the limiting factor if the advantage of fast Lade-/Entladefähigkeit is to be utilized towards batteries with lithium -ion capacitors. Because at the very high charge and discharge currents that occur when power applications with these capacitors, occur internal losses,

The lead via the internal resistance to heating of the capacitor. This heating is the main cause for the size limitation on the charge and discharge, in particular in frequent Lade-/Entladevorgängen.

Since both the charge carrier mobility of the ions in the electrolyte, as well as the conductivity of the electrolyte compared to electrons is significantly lower in the metallic conductors, but the internal resistance of EDLCs is higher than that of other capacitor technologies significantly smaller than batteries, and also shows a significantly better low-temperature performance. However, both properties depend strongly on the composition of the electrolyte and differ significantly in the different ranges of the various manufacturers.

Power density and energy density

Lithium -ion capacitors much faster to be used or be discharged compared to batteries and thus increase the availability time of the devices. This is a crucial application criterion of LICs compared to batteries and can be found in the concept of power density again, a performance data that is either based on their mass and is then expressed as gravimetric power density in kW / kg or as a volumetric power density in kW/cm3. It is determined by the heat generation in the current load on the internal resistance. High power densities allow applications to buffer consumers ( energy storage ), the short-term need a high current or release (eg regenerative braking ).

The energy density on the other hand is the measure for the storable electric energy in a capacitor. It is an important parameter for comparison with batteries and is reported as gravimetric energy density in Wh / kg or kWh / kg. Sometimes the energy density is also related to the volume of construction, then it is entered as volumetric energy density in Wh/cm3 or kWh/cm3.

Power density and energy density are usually represented in a so-called Ragone plot. With such a graph, the classification of a particular memory technology compared with other technologies is visually represented graphically. The Ragone plot of energy density versus power density clearly shows that lithium -ion capacitors have an approximately four times higher storage capacity of electrical energy to EDLCs, without sacrificing the ability of rapid charging and discharging to lose with high charge and discharge currents.

Life

In general, the service life of lithium -ion capacitors is highly dependent on the purity and quality of the parts used. In addition, the lifetime depends on the double layer capacitors of the operating voltage and the operating temperature similar. However, this new capacitor technology is today (2011) still in the starting phase, so that are not yet further information is available.

Cycle strength

The ability of capacitors to be able to survive a low charging and discharging is described by the term " cycle strength ". The currents that occur when loading or unloading of lithium -ion capacitors can be very large. For example, the capacitors were tested with 50, 100 and 150 A in the cycle mode with 60 s cycle time for checking the suitability of LICs with the capacity of 2000 F for energy recovery in buses or large industrial machines.

With such high currents occur not only a strong internal heating of the capacitors, wherein the thermal expansion provides an additional stress factor, but it also caused more severe electromagnetic forces affecting the strength of the electrode - collector junction. A large cycle strength of lithium -ion capacitors with up to 500,000 cycles, the capacitance change compared with the initial value of less than ± 10%, so is not only a question of the chemical stability of the lower parts, but also the result of a mechanically robust and stable construction.

Residual current and self-discharge

In charged lithium -ion capacitors, as in all high-value capacitors, as a so-called residual current, also known as leakage current occur. This residual current is temperature - and voltage-dependent. The higher the temperature and the higher the voltage the higher the leakage current.

The leakage current is specified, the term " self-discharge ". In this case, the voltage loss is indicated within a defined time. The self-discharge is less in the lithium -ion capacitors than standard double-layer condensates and about as large as in batteries and is about 5% per month at room temperature.

Characteristics in comparison

Lithium -ion capacitors are only in a few standard types of technical descriptions thereof are still partly very incomplete on the market. This new technology further developed by customer requirements types will extend the offer in a few years. The characteristic values ​​of the time (2011) offered lithium -ion capacitors of different manufacturers are also a mirror of the respective levels of development.

Note to indicate different polarity

Lithium-ion capacitors to store the electrical energy with an electrochemical process. Thus, they are similar in their effect to the accumulators. In the labeling of the electrodes by the terms anode and cathode, it may, depending on whether a component is considered as a producer or as a consumer, so to get confused. As in an electric generator for direct current ( battery ), the cathode of positive polarity ( ). In contrast, when an electrical load is - capacitors consumer - the cathode of the negative polarity (-). In the following, therefore, the electrodes are named only with their polarity.

Pros and Cons

Advantages and disadvantages compared to LI batteries

The hybrid design of lithium -ion capacitors with a negative, doped with lithium-ion battery electrode and a positive electrode of lithium - free double layer of activated carbon provides some fundamental advantages over lithium - ion batteries.

  • The risk of fire is significantly lower. For in lithium - ion batteries may result in overcharging or overloading at the positive electrode to chemical reactions with fire when lithium metal oxides react spinel type with the electrolyte. However, since lithium-ion capacitors have a lithium - free double -layer electrode may be no chemical reactions with the electrolyte. You can possibly even a " nail test " survive.
  • They also have a much reduced demand for lithium
  • And only use eco-friendly materials, banned heavy metals are not used

The storage of electrical energy takes place at the LI- capacitors by two physical processes, static in the bilayer and faradaysch in the pseudo capacity and not through a chemical process as in Li -ion batteries. Characterized LICs have the following advantages:

  • LICs can not be overloaded when the applied voltage at the rated voltage remains.
  • They have a much higher power density with the ability of rapid charge and discharge
  • LICs are fixed cycle and they have a significantly higher Lade-/Entladezyklenzahl of at least 500,000 cycles within the lifetime
  • LICs have a significantly longer lifetime (> 10 years ) because the parameters are not affected by the virtual absence of chemical processes.
  • A smaller internal resistance, which enables very high peak currents with lower self-heating at high currents, can be explained also by the type of storage without chemical processes,
  • LICs have a wider temperature range, -20 to 70 ° C over -20 to 60 ° C
  • They are maintenance free

In contrast, the following disadvantages compared to LI batteries are:

  • The price is significantly higher
  • The energy density is much lower, that is, a LIC stores significantly less electrical energy per unit volume as a battery LI
  • The number of providers is not (yet ) quite limited

Advantages and disadvantages compared to EDLCs

Even compared to double layer capacitors have lithium -ion capacitors advantages:

  • They have a higher energy density with about four times higher storage capacity based on the same physical volume
  • And the higher nominal voltage of about 3.8 V facilitated by reducing switching losses, the design of the electronic control.

In contrast, the following disadvantages are:

  • A lower cycle strength with a lower Lade-/Entladezyklenzahl
  • A minimum voltage of 2.2 V must not be exceeded
  • LI capacitors are also not short-circuit proof
  • The price of lithium capacitors is much higher than that of EDLCs

Applications and market

Lithium -ion capacitors as relatively new capacitor technology is located in the years 2010 and 2011 in the phase of industrialization. That is, the capacitor manufacturers are preparing their production on a future mass production and electronics manufacturers are developing new circuits and concepts. Offers the capacitors from several manufacturers under different names: " Premlis " ACT, " EneCapTen " FDK " Ultimo ", JM Energy, "nano -hybrid capacitor", NCC, "LIC", NEC Tokin "Lithium Ion Capacitor " Taiyo Yuden.

Lithium -ion capacitors are characterized by a higher energy density compared with double-layer supercapacitors and have the same high power density, ie, the same ability to rapidly charge and discharge. Therefore, initially already existing applications where double-layer or super capacitors are used, replaced by new LICs, because the solution with lithium -ion capacitors results in either a smaller footprint or use of the available space to the higher energy density. Here, the following applications are:

  • Intermediate energy storage in wind power plants and in the photovoltaic power generation (solar ) with fluctuating loads
  • Uninterruptible power systems (UPS),
  • Energy storage in self-sufficient street lighting.
  • Energy recovery in braking systems of industrial equipment, such as forklifts and cranes,
  • Energy recovery in braking systems of cars, trains and buses ( recuperation)
  • Battery replacement for electric screwdriver, fast loading possible

The market for lithium -ion capacitors is estimated for 2020 to approximately 60 million euros with around 40 million units.

Trends

Lithium -ion capacitors were developed at the beginning of the new 21st century. The global marketing started in 2005 by FDK, Asahi Chemical Industry (ACT ) followed in 2006, then JMEnergy. The first of these hybrid capacitors that connected the storage principle of double-layer capacitors with the battery storage technology of the lithium - ion batteries in a housing together, worked together with electrodes of activated carbon. The negative electrode was doped with lithium in these capacitors ions, the positive electrode corresponded to that of a conventional EDLCs. As the electrolyte was an aqueous solution of conductive salts to bear.

In order to improve the temperature behavior and to make the capacitors specially designed for use in the automotive sector capable of water-containing electrolyte was replaced by a lithiated salts electrolytes based on organic solvents in the development of LICs.

Another development took place at the electrodes. Although the activated carbon having an extremely large surface area materials have been found, whose structure do not have major surfaces. Conductive polymers, such as pentacene or polythiophene have such large internal surface areas. By doping the negative electrode with lithium or lithium titanate ( Li4Ti5O12 ) can be achieved very good conductivity values ​​. Studies at the Los Alamos National Laboratory showed that in LICs energy densities of 39 Wh / kg and power densities of 35 kW / kg can be achieved with such electrodes.

Inspired by the ever stronger mounting pressure from the automotive field by storing electrical energy with high capacity and fast reaction time dealing with the lithium -ion capacitors a number of research projects. The relatively new technology to be able to produce carbon nanotubes at high precision with a larger size, was studied in terms of the capacitive properties. At the University of Tokyo, Graduate School of Agriculture, nanocrystalline lithium titanate were incorporated into the carbon nanotubes. The resulting electrodes can reach specific capacitance values ​​up to 180 F / g. Corresponding composition capacitors are called " nano-hybrid capacitors ", achieve energy densities of 40 Wh / l and power density of 7.5 kW / l and can thus just as fast as conventional EDLCs are loaded or unloaded.

Hermann von Helmholtz Intercalation (chemistry) Electrochemistry Electric power Solar plant Digital Object Identifier
525100
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