1 Introduction

The energy crisis is becoming a global problem that mankind must face but has not yet been solved [1,2,3,4,5]. It is well known that energy collection and storage are two fundamental technologies in energy systems [6, 7]. Capacitors are widely used electronic components to store electrical energy in an electric field [8,9,10] and have many advantages [11, 12], including fast charging, long life, and high efficiency [13]. Capacitors, as one of the three basic passive electronic components (resistors, capacitors, and inductors) [14], occupy an important position in the electronic components industry and are one of the essential components in electronic circuits, accounting for about 56% of the global passive electronic components market. The capacitor is a component made of two pieces of conductors close to and insulated from each other to store charge, and it mainly plays the role of tuning, filtering, coupling, bypassing, and energy conversion in circuits [15,16,17]. Aluminum electrolytic capacitors have been widely used in communication products, industrial control, and aviation and military equipment because of their small size, large storage capacity, and high-cost performance [18,19,20,21]. The development of new industries, such as the new energy industry and frequency conversion technology and the continuous improvement of capacitor performance, leads to a sharp increase in the demand for aluminum electrolytic capacitors (Fig. 1).

Fig. 1
figure 1

Aluminum electrolytic capacitor structure schematic. [22]

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Electrolyte is the conductive medium of aluminum electrolytic capacitors and provides the conductive ions needed for the capacitor to work. The electrolyte largely determines the characteristics of the capacitor, such as temperature characteristics, frequency characteristics, lifetime, and voltage tolerance [23]. The electrolyte needs to have suitable physical and chemical properties, such as high chemical stability, non-toxicity, high electrical conductivity, high spark voltage, and electrochemical ability to repair the dielectric oxide film [24]. With the development of research, the state of development of electrolytes can be divided into three stages. The first stage of boric acid, the ethylene glycol system, more than 50 years ago is the most widely used, but the boric acid and ethylene glycol esterification reaction occurs to generate water, capacitors in the water content are high, resulting in conductivity, sparking voltage reduction, resulting in high-voltage capacitor is limited; and once the water content is too high, the water ionization produces hydrogen, high-temperature water vaporization of water vapor, so that the internal pressure increases, which can easily lead to capacitor bursting; at the same time, a large amount of water will cause corrosion of the electrode foil, affecting the life of the capacitor. Therefore, the electrolyte of boric acid and ethylene glycol system cannot be used in high-temperature environments and has been basically eliminated. The second phase of straight-chain carboxylate (ammonium adipate, ammonium sebacate, etc.), boric acid, and ethylene glycol system, which is currently widely used. However, straight-chain carboxylates at low temperatures by the melting point prone to solid crystallization precipitation, affecting the low-temperature performance of the capacitor; and with the growth of straight-chain carboxylate carbon chain, resulting in a decrease in solubility in the solvent, conductivity decline; straight-chain carboxylate due to the carboxyl group at both ends of the no protective groups, with the use of the temperature and time, serious esterification of the electrolyte, a sharp decrease in conductivity, resulting in a decrease in equivalent series resistance value increases. The resistance value increases, the capacitor heats up seriously, the electrolyte volatilization causes the capacitor to open the valve, and the capacitor fails prematurely. This leads to the performance and application range of capacitors being limited in practical applications. The third development stage is the multi-branched carboxylate and ethylene glycol system. Compared with linear carboxylates, the introduction of side-chain groups in multi-branched carboxylates enhances their ability to inhibit esterification under high-temperature conditions, thereby improving high-temperature stability. Moreover, due to the steric hindrance of the groups on the side chain and the polarization of the alkoxy group, its solubility in ethylene glycol is increased, thereby improving its low-temperature performance. The solubility of linear carboxylate in ethylene glycol is 5–10%. In a low-temperature environment, the content of linear carboxylate in electrolyte is more than 10%, which is easy to precipitate crystals and affect the performance of the capacitor. The solubility of multi-branched carboxylates in ethylene glycol is as high as about 20%. The solubility and thermal stability of branched carboxylates are better than those of linear carboxylates. In practical applications, branched carboxylates show higher breakdown voltage, lower effective resistance, and better thermal stability (Fig. 2).

Fig. 2
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Schematic representation of the main components at each stage of the development of the electrolyte for aluminum electrolytic capacitors

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Due to the complex terrain and diverse environments on the earth (e.g., extremely cold weather in high latitude and plateau regions), new challenges have been posed to the development of energy storage devices [25], in addition, human exploration missions to Mars and the moon also require the use of a new type of energy storage system that can withstand extreme temperatures [26, 27]. Aluminum electrolytic capacitors can meet the needs of normal operation under most environmental conditions, so they are widely used, but normal operation under extreme conditions requires more detailed optimization and improvement of aluminum electrolytic capacitors. Several studies have been conducted to investigate the electrochemical performance of various electrolyte compositions at ambient temperatures; however, little is known about the factors that affect the wide temperature of capacitors. In this review, the factors affecting the wide temperature of electrolytes are discussed systematically.

2 Factors affecting capacitor wide temperature

The electrolyte is a key component of capacitors, which play the role of providing oxygen-negative ions to repair the defects in the oxide film of anode foil [28]. The electrolyte is composed of solvents, solutes, and additives, and the additives include sparking voltage enhancers, corrosion inhibitors, hydrogen eliminators, hydration-proofing agents, and stabilizers, which affect the service life of aluminum electrolytic capacitors, and the service temperature. In this section, the wide-temperature range that can be achieved by various components of the electrolyte and the factors affecting them are discussed.

Ionic conductivity and viscosity are two important characteristics of the electrolyte, both of which determine the wide-temperature range of aluminum electrolytic capacitors. According to the Arrhenius equation, the temperature and ionic conductivity (σ) of the electrolyte are proportional to each other. This means that the ionic conductivity of the electrolyte increases with increasing temperature according to the following equation:

$$\begin{array}{c}\sigma ={\sigma }_{0}{e}^{-\frac{{E}_{a}}{RT}}\end{array}$$
(1)

where Ea is the activation energy, R is the gas constant, and σ is a constant derived from fitting the experimental results. Usually, low activation energy represents high conductivity at high temperatures [29]. In addition, ionic conductivity and viscosity according to Walden’s rule

$$\begin{array}{c}\Delta n=\frac{{z}^{2}eF}{Ar}\end{array}$$
(2)

where Δ is the infinite dilution molar conductivity, n is the viscosity of the solvent, z is the ionic valence, e is the charge, F is Faraday’s constant, A is a constant whose value depends on the friction conditions, and r is the average radius of the ion. Δ is related to the ionic conductivity as follows:

$$\begin{array}{c}\sigma =\Delta c\end{array}$$
(3)

where c is the concentration of the electrolyte. In addition, Eq. (4), according to the viscosity is also temperature dependent. In general, the viscosity of the electrolyte decreases with increasing temperature and it is inversely proportional to the ionic conductivity [30]

$$\begin{array}{c}{n=Ae}^{\frac{{E}_{a}}{RT}}\end{array}$$
(4)

From the data in Table 1, it can be seen that ethylene glycol, diethylene glycol, ionic conductivity at room temperature are large, viscosity is small, ionic conductivity is proportional to the temperature, viscosity is inversely proportional to the temperature, and at low temperatures than other solvents, electrochemical performance is superior. Yang [31] developed a working electrolyte with a conductivity of 2.6 × 10–3 S/cm, a spark voltage of 500 V or more, and stable high-temperature performance using ethylene glycol and diethylene glycol as solvents. Huang [32] used ethylene glycol, diethylene glycol, and polyethylene glycol as solvents to prepare the electrolyte. The experimental results showed that the electrolyte, the flash fire voltage is relatively stabilized, and the performance is superior.

Table 1 Common solvent ionic conductivity and viscosity
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2.1 Solvent

Solvent as the main part of electrolytes is the basis of electrolytes, accounting for 60–75%, mainly used as the medium for dissolving solutes, additives, and other substances. The solvent should have good solubility and high polarity, with small medium loss to meet the electrolyte’s wide-temperature use. The melting point and boiling point of the solvent must be within a certain temperature range, especially at high temperatures, the solvent viscosity–temperature change and vapor pressure to be small, so that the solvent can effectively improve the heat resistance and thermal stability of the electrolyte. Aluminum electrolytic capacitors’ operating temperature and use of the field are largely limited by the solvent. The solvent of the electrolyte determines the thermal stability of the electrolyte, and for a single solvent, the range of melting and boiling point values is usually small. For working electrolytes with different requirements, one or more of the above solvents can be used for compounding to meet the dissolution requirements of different solutes and to expand the wide-temperature range of electrolytes. Liu [33] used ethylene glycol and water as solvents, and adipic acid as a solvent electrolyte, the conductivity can reach 45 × 10–3 S/cm and has excellent performance of high-temperature resistance (Table 2).

Table 2 Common solvent and melting and boiling points
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2.2 Solutes

Solutes, as the core part of the electrolyte, maintain and repair the oxide film, improve the conductivity of the electrolyte, and at the same time cannot be corrosive to the metal and its oxide film and are easily dissolved, and have a strong ionization ability—the easier it is to ionize positive and negative ions or ion groups, the stronger the ability to repair. In electrolyte formulations, the solute percentage is 25–35%. The glycol system alone, despite its large sparking voltage value, has a solvent conductivity of 1 ~ 2 S/cm, which cannot provide electronegative ions to migrate to the anode foil to repair defects and cannot be used as an electrolyte. Since the electrolyte generally requires a neutral or acidic pH, the ammonium carboxylate has a higher solubility and ionization effect in solvent systems such as ethylene glycol. At present, the solutes commonly used in aluminum electrolytic capacitors are organic weak acids and their ammonium salts, inorganic weak acids, and their ammonium salts, and the commonly used electrolyte solutes are ammonium adipate, ammonium heptanedioate, ammonium azelate, ammonium hydrogen azelate, ammonium sebacate, ammonium dodecanedioate, boric acid, and its salts. For aluminum electrolytic capacitors with different temperature requirements, one or more of the above solutes are selected for compounding, but most of the above solutes are ammonium salts of straight-chain carboxylic acids, which have poor solubility in solvents, and the main scope of use is the electrolyte for normal-temperature aluminum electrolytic capacitors, and it is difficult to meet the performance needs of wide-temperature aluminum electrolytic capacitors, so there is an urgent need for the development of a new type of electrolyte solutes with targeted.

The solubility of branched polycarboxylic acid ammonium salts in ethylene glycol is greatly increased due to the spatial site resistance effect of the branched group and the polarization of the alkoxy group, and the solubility can be as high as 20% [34], which can be applied to a wider temperature range. The reason is that the alkane group in the α position has a large spatial site resistance, and its neighboring carboxylic acid group is not easy to esterify in the high-temperature environment and maintains stable group properties. Compared with the β or γ position branched group, the α position branched chain is more advantageous. On the other hand, the larger the size of the α-position side-chain moiety, the larger its spatial site resistance, and the stronger the protection of the side-chain carboxylic acid moiety. According to the induced effect, the charged groups of the side chain have a greater influence on the reducing properties of the branched organic matter. In the process of repairing the oxide film, the stronger the ability of oxygen-negative ions to provide electrons, the higher its oxidation efficiency, but the lower the sparking voltage of the corresponding electrolyte, therefore, the comprehensive consideration of the electrolyte branched type of long-chain α-position alkane groups of the type and size. If the alkyl group is not branched at the α position, the site resistance effect is much smaller, e.g., for ammonium 5,6-decanedioate. For dibasic carboxylates, if one carboxyl group is protected by the site-blocking effect of the branched hydrocarbon group, its performance is superior to that of straight-chain carboxylates, and if both carboxyl groups are protected by the branched hydrocarbon group, its performance is even better [35]. Most of the literature shows that the solubility and thermal stability of branched polycarboxylates in ethylene glycol solution are better than that of straight-chain carboxylates, and the branched polycarboxylates have the ability of chemical self-repair, which can be applied to a wider temperature range, so the use of branched polycarboxylate ammonium salt and ethylene glycol working electrolyte system can produce low-impedance, high-voltage, wide-temperature, long-life high-end capacitor products (Tables 3, 4) [36].

Table 3 Fig. 3a Electrolyte for capacitors with different operating temperatures
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Table 4 Fig. 3b Electrolytes for capacitors with different lifetime
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As shown in Fig. 3 through induction and summarization, it is found that the high-boiling point substances of conductive polymers help to improve the high-temperature resistance of electrolytes. Low melting point substances such as sulfoxide and ethylamine are favorable for lowering the melting point of the electrolyte, so that the electrolyte can work at lower temperatures. Suitable additives can extend the life of capacitors.

Fig. 3
figure 3

a Aluminum electrolytic capacitor operating temperature graph. b Aluminum electrolytic capacitor. Lifetime graph

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2.3 Additive

2.3.1 Sparking voltage booster

Under the action of applied current, the thickness of the oxide film of the anode electrode foil increases, when the oxide film increases to a certain thickness, the film defects occur at the micro-flash fire, with the polarization voltage increasing, the flash fire frequency and amplitude are greater and greater, until there is a spark visible to the naked eye, the performance of the curve on the curve of the boost for the curve is constantly oscillating, and the subsequent increase in the voltage with the continuation of the polarization voltage, the voltage of the electrolyte is basically unchanged, and the voltage corresponding to this time is sparking voltage. The working electrolyte flashover voltage (Us) and working voltage (Uw) have the following requirements: Us = 1.2 UW. For electrolytic capacitors with a working voltage of 400 V, a flashover voltage of at least 480 V is required.

Sparking voltage is one of the important parameters of the working electrolyte of aluminum electrolytic capacitors [47], which directly determines the rated operating voltage of the capacitor. The flashover voltage is inversely proportional to the ionic conductivity, which is also closely related to the service life and positioning requirements of the capacitor. Adjusting the sparking voltage in a suitable range allows the capacitor to operate under a wide range of temperatures. Deng [48] found that polyboronic acid glycol esters increase the sparking voltage of electrolytes (Table 5).

Table 5 Common sparking voltage boosters
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2.3.2 Anticorrosive

Anode corrosion in capacitors is an important factor in determining the length of life. There are many ways to prevent corrosion in the electrolyte adding a corrosion inhibitor is a simple and very effective method. Adding corrosion inhibitors in the electrolyte can greatly reduce the corrosion rate, thereby improving the performance of the capacitor to extend the life of electrolytic capacitors. The capacitor production process into the impurities and the electrolyte itself contains impurities is the main cause of capacitor anode corrosion. These impurities contain ionic radii of small Cl− and SO42−; on the one hand, they can hinder the formation of metal passivation film, and on the other hand, because of their small radius, penetration ability, ease of destruction of the metal surface has been formed by the protective film. In the process of capacitor use, it is easy to local corrosion of anode lead strip and aluminum foil. In the early stage of corrosion, these two ions are the most destructive and are the cause of corrosion. Excessive moisture in the electrolyte and capacitors containing too much electrolyte is also an important factor leading to anode corrosion. Excessive moisture is easy to destroy the oxide film structure, while corrosion of salt will be hydrolyzed to generate Al(OH)3. Too much electrolyte makes it easy to make the initial electrochemical corrosion pure chemical corrosion, so that the capacitor’s anodic corrosion is aggravated.

Anti-corrosion agents are generally of the following types: (1) Amine and amide anti-corrosion agents. Nitroaniline (including neighboring, m, p and dinitroaniline, 4-amino-3-nitroanisole, nitrobenzamide, azoline derivatives. The common characteristic of amine compounds is that they contain a polar amine group and a non-polar benzene ring, and are soluble in solvents such as ethylene glycol. Polar amino nitrogen atoms have unbonded lone pairs of electrons, which can form coordination bonds with metals and chemisorb on metal surfaces. Formation of membrane, thereby inhibiting the invasion of small ions such as (Cl−), plays a role in corrosion prevention. (2) Ketone and carboxylic acid corrosion inhibitors. Ketone and carboxylic acid corrosion inhibitors have an anti-corrosion effect, because they contain carbonyl in the molecule. The carbonyl oxygen atom in the high-density of electron cloud can provide electrons to the metal, the formation of coordination bonds or suction in the metal surface, and the formation of a cover film, thus playing a role in corrosion prevention. Commonly used ketone and carboxylic acid corrosion inhibitors are mainly nitroacetophenone (neighboring, between, on); humic acid and derivatives; benzoic acid and derivatives, and adipic acid and derivatives. (3) Quinoline anti-corrosion agents. Quinoline anti-corrosion agents for capacitors are mainly 8-carboxyquinoline and 2,3-dihydroxyquinoline. Because the quinoline molecule contains N and O atoms two polar groups, can act with aluminum ions, the formation of a complex film on the surface of aluminum ions, thus playing a role in corrosion prevention. They are effective anti-corrosion agents for metals in chloride and sulfate ion environments.

2.3.3 Hydrogen eliminators

Hydrogen eliminators are also known as gas absorbers, depolarizers, and explosion-proofing agents. Its purpose is to eliminate the gas (mainly hydrogen, followed by water vapor) released from the working electrolyte to reduce the pressure inside the capacitor. The oxygen needed for the repair of the anodic oxide film of the capacitor is actually provided by water electrolysis, so hydrogen is inevitably precipitated at the cathode. Excessive hydrogen precipitation can lead to increased internal pressure, which in severe cases can lead to bottoming out or even the opening of the explosion-proof valve. Hydrogen eliminators in the working electrolyte can effectively inhibit or eliminate hydrogen. There are many types of such additives, such as compounds containing nitro, aromatic rings, and unsaturated hydrocarbon groups. Commonly used hydrogen eliminators include resorcinol, p-nitrophenol, p-nitrobenzyl alcohol, p-benzoquinone, and so on. From the organic molecular structure, these compounds have a “nitro effect”, and they all contain one or more nitro aromatic rings. Nitro and aromatic ring groups have strong electron-absorbing ability, and the π electrons on the nitro and benzene ring form a ππ conjugation system caused by the conjugation effect. Such compounds have strong reducing properties, which can effectively reduce the rise in internal pressure of the capacitor caused by the release of hydrogen from the cathode when the capacitor is in operation. In addition to hydrogen, the working electrolyte also releases water vapor at high temperatures. Inhibit or eliminate water vapor caused by the rise in internal pressure additives are sucrose, glucose, mannitol, and other polysaccharides, as well as polypropylene amine, polyvinyl alcohol, etc. As they are water-soluble polymers, have a strong hydrophilicity can make the water molecules fixed, can prevent water and other harmful substances from directly in contact with the dielectric membrane, inhibit the increase in saturation vapor pressure, and prevent hydration and erosion.

2.3.4 Hydration-proofing additives

Since the electrolyte is generally an organic polycarboxylic acid ammonium salt, esterification reaction with ethylene glycol will occur at high temperatures to produce a certain amount of water, and the hydrolysis reaction between water and aluminum foil at high temperatures will gradually destroy the oxide film structure on the surface of the aluminum anode foil [49], and problems such as capacitance decrease, change in the relative permittivity of the aluminum oxide film, and fracture of the aluminum bar will occur, leading to deterioration or even failure of the capacitor performance, so it is necessary to add an additive to the electrolyte to prevent the Therefore, it is necessary to add additives to the electrolyte to prevent hydration from occurring, so that the failure of capacitor performance can be improved. Adding phosphoric acid and its salts to the electrolyte can be used to form a chained aluminum phosphate film [50] (Al3+ + PO43− = AlPO4) on the surface of the Al2PO3 dielectric film, preventing water molecules from contacting the aluminum trioxide film. However, due to the strong acidity of phosphoric acid, the addition of excessive amounts of corrosive effect on the oxide film; and the amount of small and cannot achieve the desired effect, and hydration-proofing cooperation is not persistent, the aluminum phosphate layer is not stable at temperatures of 90 °C and above, and is prone to lose the ability of hydration-proofing cooperation [51]. Hang [52] found that organic acids could remove poor-quality dielectric film precursors (hydrated films). Phosphoric acid inhibited ionic transport in the dielectric film, and the hydrated film thickness was reduced by about 20% and 23.7% for organic acid and phosphoric acid, respectively, compared with boric acid, which enhanced the withstand voltage and specific capacitance of the anode foils and facilitated the development of aluminum electrolytic capacitors in the direction of high specific capacitance and miniaturization. Adding ethylene glycol and phosphorus pentoxide in the electrolyte to form an esterified substance to act as a hydration-proofing agent. This esterification does not corrode the aluminum foil and the protective film formed is extremely stable at high temperatures, which can effectively extend the life of aluminum electrolytic capacitors [51]. Hydrogen atoms in the hydroxyl-carboxy groups or other groups in these additives form strong electron-pair adsorption with oxygen atoms on the oxide film and “adhere to the surface of the oxide film” preventing the erosion of the oxide film by water (Fig. 4) [53, 54].

Fig. 4
figure 4

Schematic diagram of dielectric film growth process. [52]

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Other hydration-proofing additives are silicate compounds, because silicate ions are effective in inhibiting the hydration of the aluminum oxide film. Aluminum salts that provide a large amount of aluminum ions can also be used, and when the electrolyte has sufficient aluminum ions, aluminum is difficult to dissolve and does not undergo a hydration reaction. In the electrolyte, add polyvinyl alcohol, mannitol, lactose ribose, and other polysaccharide organic polymer substances, because they are water-soluble macromolecules that have a strong hydrophilicity and can make water molecules fixed can prevent water and other harmful substances in direct contact with the dielectric membrane inhibit saturated steam [51].

2.3.5 Stabilizers

Stabilizers are mainly phosphoric acid organic and inorganic compounds. Phosphate ions can form aluminum phosphate with aluminum ions or form more stable aluminum phosphate complex structures. Phosphorus is a pro-oxidant, phosphorus atoms are usually combined with oxygen atoms and then combined with other atoms or groups, and therefore, the addition of phosphate or phosphite ions leads to the aluminum phosphate compounds not only existing on the surface of the oxide film but also may be formed in aluminum phosphate embedded in the lattice of aluminum trioxide, so that the alumina film has good stability.

3 Low-temperature electrolyte for capacitors

Dimethyl sulfoxide (DMSO) has a significant freezing point lowering effect in water (low melting point to −73 °C at about 60 wt% mixing with water) [55] and has high thermochemical and electrochemical stability, good miscibility with water, low toxicity, and the ability to compete with water molecules for the formation of hydrogen bonds, which breaks the hydrogen bonds between water molecules and lowers the freezing temperature of the electrolyte [56]. These make DMSO a suitable low-temperature additive for electrolytes. Alvin Virya et al. [57] added dimethyl sulfoxide (DMSO) to an aqueous Na2SO4–polyacrylamide (PAM) electrolyte and showed a significant improvement in capacitor performance at  −20 °C without affecting the good performance at ambient temperature. Tao [58] et al. proposed a DMSO–H2O electrolyte (2 M NaClO4) with an ionic conductivity of 0.11 mS/cm even at −50 °C. Lu [59] et al. proposed 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4) as a solvent electrolyte in a ternary liquid mixture of water, dimethylsulfoxide, and acetonitrile, which can be used for high-performance supercapacitors operating at −70 °C.

Poly(vinyl alcohol) and acetonitrile also showed excellent low-temperature performance. Xu [60] et al. prepared to antifreeze poly(vinyl alcohol) solution (with the addition of ethylene glycol and propylene tritanol), which can make the full-gel capacitor work properly at −20 °C. Zuo [61] et al. tetrahydrofuran (THF), 2-methyltetrahydrofuran (MeTHF) was used as a low-temperature solvent and added to the organic electrolyte spiro quaternary ammonium tetrafluoroborate (SBP-BF4)/acetonitrile (AN) system to formulate a novel low-temperature electrolyte, which possesses good electrochemical performance at −70 °C (Table 6).

Table 6 Electrolyte and operating temperature for different types of capacitors
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4 Summary and outlook

Aluminum electrolytic capacitor electrolyte at this stage in the multi-branched chain carboxylic acid ammonium salt + ethylene glycol system, the system-wide-temperature range up to −55–135 °C.

  • Electrolyte solutes, solvents, additives, ionic conductivity, and temperature are proportional to the viscosity and temperature is inversely proportional to ethylene glycol, diethylene glycol ionic conductivity and viscosity can be used at low temperatures −55 °C, −70 °C when the electrochemical properties are maintained well.

  • The matching of components also affects the wide-temperature range of capacitors. The summary of the electrolyte formula found in the aluminum electrolytic capacitor uses temperature as low as −55 °C, sulfoxide substances are mostly used as a co-solvent, and ethylamine, and glycol as a solvent appeared, which is related to their good low-temperature performance.

  • When aluminum electrolytic capacitors are used at higher temperatures 135 °C, the main solvents and auxiliary solvents generally choose substances with higher boiling points, such as glycerol, γ-butyrolactone, etc.

  • By adding appropriate additives, the use temperature of aluminum electrolytic capacitors can be expanded to extend the life of the capacitor.

  • Acetonitrile, polyvinyl alcohol, and other substances can be used in supercapacitor electrolytes at −70 °C, and the same effect can be achieved by adding these substances to aluminum electrolytic capacitor electrolytes.

At present, there are few studies on additives to improve the performance of electrolytes. It is necessary to carry out more in-depth research, so that the performance of aluminum electrolytic capacitors can be more clearly regulated, to give full play to the role of aluminum electrolytic capacitors as basic electronic components (Fig. 5).

Fig. 5
figure 5

Main components of electrolyte solution

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