Tantalum, MLCC, and supercapacitor technologies are ideal for many energy storage applications because of their high capacitance capability. These capacitors have drastically different electrical and environmental responses that are sometimes not explicit on datasheets or requires additional knowledge of the properties of materials used, to select the best solution for a given design.
This paper compares the performance of these technologies over energy density, frequency response, ESR, leakage, size, reliability, efficiency, and ease of implementation for energy harvesting/scavenging/hold-up applications. A brief, material properties benefits and considerations of X5R, Tantalum, Tantalum polymer, and electrochemical double-layer capacitors is provided. An example of an energy storage circuit problem is provided that has a capacitance and voltage requirement that is not achieved with a single, maximum CV capacitor for any of the relevant technologies. Capacitor banks are built with each technology that are viable solutions.
Design considerations are discussed for optimization of each capacitor bank and analyzed. Results of the analysis will show where each technology excels. This paper should be of interest to component engineers, program managers, and power electronics engineers working on energy harvesting, scavenging, and hold-up applications, due to its impact on system design and performance.
The paper was presented by Daniel West, AVX USA at the 3rdPCNS 7-10th September 2021, Milano, Italy as paper No.4.4.
Energy storage capacitors can typically be found in remote or battery powered applications. Capacitors can be used to deliver peak power, reducing depth of discharge on batteries, or provide hold-up energy for memory read/write during an unexpected shut-off. Capacitors also charge/discharge very quickly compared to battery technology and are optimal for energy harvesting/scavenging applications, and depending on power requirements, can replace batteries altogether.
Combining the superior power density of capacitors with a wide operating temperature range, high reliability, low weight, and high efficiency, it is easy to see how capacitor technology is ideal for energy storage applications, but sometimes it is not easy to see which capacitor technology should be selected for energy storage. Capacitor performance across temperature, voltage, frequency, and time should be considered, but this data is not always prevalent on a datasheet. Capacitor specifications of capacitance, DC leakage current (DCL), equivalent series resistance (ESR), size, etc. are typically room temperature measurements under a very specific test condition. Furthermore, energy storage capacitors will often be set up in some parallel/series combination that can pose unique challenges or unexpected behaviour. In short, without enough knowledge of the specific capacitor technology used, there will likely be many design challenges requiring lots of trial and error, to achieve the optimal energy storage capacitor bank.
Capacitor Technology & Selection
Only ceramic, Tantalum (solid electrolytic), and supercapacitor technologies are reviewed in this paper to be concise, but also to present information on energy storage capacitor technologies that may not be as prolific as aluminium electrolytics, and yet not so obscure that it would be unlikely considered for a general energy storage application. Ceramics are ubiquitous and widely used for decoupling and filtering applications, but there are dielectric formulations that can achieve very high capacitance per unit volume (CV), that make them viable for energy storage in addition to their small size and low costs.
Tantalum and Tantalum Polymer (TaPoly) capacitors are also high CV devices, but extremely stable across temperature and voltage. Electrochemical Double Layer Capacitors (EDLC), commonly known as supercapacitors, are peerless when it comes to bulk capacitance value, easily achieving 3000F in a single element discrete capacitor. However, these technologies perform differently based on application details and knowing even an introductory level of materials and construction helps in selecting the optimal capacitor for a given set of design constraints.
Multilayer Ceramic Capacitors (MLCC) MLCC dielectrics are organized into 3 main classes by the Electronics Industry Alliance (EIA) and the International Electrotechnical Commission (IEC) because there are many ceramic formulas that could realize a capacitor. The classification system is intended to organize the performance of many dielectric formulas by providing its maximum capacitance change within a specific temperature range. With this information, a designer is more prepared to select a ceramic capacitor based on temperature stability, but there is more to consider if the impact of Barium Titanate composition is understood.
Class 2 and class 3 MLCCs have a much higher BaTiO3 content than Class 1 (see table 1). High concentrations of BaTiO3 contributes to a much higher dielectric constant, therefore higher capacitance values within a given volume, which is great for high CV devices. However, the crystalline shape of BaTiO3 (see figure 1) deforms in the presence of voltage, temperature, and naturally deforms over time. When deformed, the dipole moment of a BaTiO3 crystal is reduced, limiting the amount in which it can reinforce an electric field, and effectively reduces the capacitance value of the device.
This means for any Class 2 or Class 3 MLCC, the actual capacitance that can be achieved with applied voltage is lower than the specified capacitance value found on datasheets (see table 2). It should be noted that capacitance change of Class 1 ceramics due to voltage bias or aging is virtually zero. The loss or change in capacitance due to temperature, time, and voltage are additive for MLCCs, and must be considered to select the optimal energy storage capacitor, especially if it is a long life or high temperature project.
Tantalum & Tantalum Polymer Tantalum and Tantalum Polymer capacitors are suitable for energy storage applications because they are very efficient in achieving high CV. For example, for case sizes ranging from EIA 1206 (3.2mm x 1.6mm) to an EIA 2924 (7.3mm x 6.1mm), it is quite easy to achieve capacitance ratings from 100μF to 2.2mF, respectively. In addition, capacitance values are extremely stable across voltage and temperature range when compared to Class 2 and Class 3 MLCC dielectrics, but an energy storage capacitor selection should not be based on these parameters alone.
Tantalum and TaPoly capacitor dielectrics are formed by dipping a very porous pellet of sintered Tantalum grains (anode) in an acid bath followed by a process of electrolysis (see figure 2). The oxide (Ta2O5) layer thickness contributes a great amount to the device voltage handling and its overall reliability. It should be noted that the dielectric thickness does have a correlation to the device rated voltage, but this correlation is not standardized and will change based on manufacturer or the intended application that the capacitor is designed for. The cathode is formed by a second process of electrolysis to form either a Manganese oxide (MnO2) layer or conductive polymer layer. From this point, energy storage capacitor benefits diverge toward either high temperature, high reliability devices, or low ESR (equivalent series resistance), high voltage devices.
Standard Tantalum, that is MnO2 cathode devices have low leakage characteristics and an indefinite lifetime2, showing improved reliability the longer it is used, but care should be taken to follow voltage derating recommendations (50%) and manage polarity to preserve the dielectric layer. The conductive polymer cathode systems contribute to very low ESR values that can be up to 1/8 of an equivalent rated MnO2 device, which means 8x power handling capability, but at the same time conductive cathode leakage currents are inherently higher, as seen in the study included in this paper.
The polymer material is hygroscopic and will wear out over time, although much slower than that of Aluminium electrolytics3, but the polymer material also contributes to a benign failure mechanism, and lower voltage derating recommendations (80%), which account for the higher rated voltage devices.
Electrochemical Double Layer Capacitor (EDLC) Supercapacitors rely on an electrochemical and a double layer of highly dense, yet porous activated carbon to achieve their extremely high capacitance values. The electrochemical has salt ions that will polarize in the presence of an electric field, providing the bulk charge storage mechanism, and the ions have a very large surface area to be distributed via the activated carbon layers (see figure 3). A typical activated carbon electrode layer will have a surface area of hundreds to thousands of m2/g depending on the design. A pair of activated carbon electrode systems, with a separator in between, would be wound and inserted into can, filled with an electrochemical and sealed and terminated to complete a discrete supercapacitor package. The capacitance values of a discrete supercapacitor can range from a single Farad to thousands of Farads, and the voltage rating would be based on electrochemical properties, as opposed to dielectric thickness like that of ceramic or Tantalum technology.
The unique material properties of a supercapacitor give it energy and power characteristics that do not fall under battery technology nor solid-state capacitor technology e.g. MLCCs (see table 3). Compared to batteries, supercapacitors retain much lower levels of energy, but can deliver an enormous amount of power with significantly increased number of charge/discharge cycles than that of batteries. This property makes it ideal for many peak power, remote, battery replacement/supplement, and energy harvesting/scavenging applications. However, typical electrochemical voltage capability is relatively low (compared to traditional capacitors), meaning series/parallel combinations of discrete capacitors would need to be implemented, in addition to considering the environmental effects on supercapacitor life and reliability.
Table 4 shows electrical performance and lifetime at temperature, for three different electrochemical systems. Although aqueous systems have the lowest voltage breakdown specification per cell, the cells are easily stacked into series configurations to achieve higher voltage ratings (up to 20V) without diminished electrical performance and is therefore presented as a discrete device voltage rating, and boasts the lowest leakage currents. Propylene carbonate devices have the widest temperature range, lower derating requirements, and superior expected lifetime performance. Acetonitrile devices are currently the most common technology used for their ability to achieve high capacitance and low ESR. In addition, it has been found that for roughly every 10°C or 0.2V derating that is applied, the expected lifetime effectively doubles4,5, which makes this technology viable for long term applications.