1 Introduction
With the development of consumer electronics products, electric vehicles, energy storage power systems and aerospace technology, the demand for batteries is increasing. Batteries are required not only to have excellent electrochemical performance, but also to meet the requirements of safety, cheapness and environmental protection. Since the Voltaic battery ushered in a new era of battery development, inorganic batteries have made great progress. Lithium-ion batteries, in particular, have attracted much attention because of their high energy density, long cycle life, no memory effect and fast charge and discharge. Current commercial lithium-ion batteries use transition metal oxides or phosphates produced from ores as cathode materials, such as LiCoO
2, LiMn
2O
4, LiFePO
4 or LiNi
xMn
yCo
z . Most of the transition metal resources are non-renewable, and the battery recycling technology is complex and costly, which may face difficulties such as resource shortage from the perspective of long-term development
[3,4]. In contrast, organic electrode materials have unique advantages such as green environmental protection, high sustainability, low cost, diverse structures, strong designability and high theoretical specific capacity, which can be potentially used in rechargeable ion batteries, redox flow batteries and supercapacitors, so they have good prospects for development. However, organic electrode materials are still facing difficulties such as high solubility in electrolyte, poor conductivity and low density, and their material characteristics, action mechanism and structure-activity relationship need to be further studied.
2 Redox Mechanism of Organic Electrode Material
The redox reaction of organic electrode materials is actually the change of the charge state of its electroactive sites, and the reversible electrochemical redox reaction is the basic requirement of the electrode of rechargeable devices. Organic electrode materials are generally classified into three types: n-type, p-type, and bipolar organics (Figure 1).
The charging process of n-type organic electrode materials is that the neutral state (N) is reduced to the negatively charged anionic state (N-), which combines with cations (such as lithium ions) for charge compensation, and then returns to the original neutral state through reversible oxidation during discharge. Because of the flexibility and adjustability of the structure of organic materials, the ionic radius of cations is highly compatible, so for many n-type organic compounds, in addition to Li+, Na+, K+ and even polyvalent metal ions with larger ionic radii,For example, Mg2+ and Al3+ can also neutralize the negative charge of N- during discharge, so organic electrode materials are expected to become universal electrode materials for high-performance ion batteries.
The p-type organic electrode material is oxidized from a neutral state (P) to a positively charged state (P+) during charging, the positively charged P+ interacts with anions for charge neutralization, and then is reversibly reduced to a neutral state during discharging. The anion in the whole process can be Cl , P ,B and TFSI- or Cl- and N in aqueous electrolyte, etc. The redox mechanism of p-type materials is to lose electrons first and produce products with relatively low electron energy levels, while n-type materials receive electrons at high electron energy levels in their initial electrochemical response. Therefore, p-type organic electrode materials generally have a higher redox potential than n-type materials.
The neutral bipolar organic substance (B) can be either reduced to a negatively charged state (B-) or oxidized to a positively charged state (B+) depending on the magnitude of the applied voltage.
3 Classification of organic electrode materials
According to the different redox active centers, organic compounds can be divided into: organic sulfides, organic radicals, imine compounds, azo compounds and carbonyl compounds. In this section, the representative organic electrode materials in these five categories will be introduced.
3.1 Organic sulfides
The redox mechanism for disulfides and polysulfides is reversible cleavage and recombination of the S — S bond (Scheme 1). The cycle stability is poor because the small organic disulfide is soluble in the electrolyte. To solve the solubility problem, polymers with S — S bonds in the backbone have been developed, such as poly (2,2 '-dithiodiphenylamine) (PDTDA, fig. 2a)
[5]. Although the solubility problem is improved, the reaction rate of these compounds is slow because the large steric hindrance between the S — S bonds in the main chain hinders the regeneration of the disulfide bonds. Later, researchers modified S — S functional groups to conductive polymers to solve the problem of disulfide bond regeneration
[6].
Inoue et al. First proposed the idea of using polysulfides as electrode materials, and developed poly (2,5-dimercapto-1,3,4-thiadiazole) (PDMcT, fig. 2C) and its derivatives
[7]. Since then, poly (N-methylpyrrole) (S-PMPy, fig. 2D), dimethyl trisulfide (DMTS, fig. 2e) and dipyridyl polysulfide (Py
2S
x, fig. 2F) modified by pentylthiophene ring have been designed and synthesized successively. Fu et al.The theoretical energy density of 1 '- (hexathiobisthio) bispiperidine (PMTH, Fig. 2h) and carbon nanotubes (CNT) as cathode materials for lithium-ion batteries can reach 1300 Wh·kg
-1, and the specific area capacity tested at a low electrolyte to active material ratio of 3μL·mg
-1 is 5.3 mAh·cm
-2, which is equivalent to the actual energy density of active material generated by a high-load cathode of 1156 Wh·kg
-1[8][9][10][11]. Although the specific capacity of electrode materials can be significantly improved by replacing disulfide bonds with polysulfide fragments, their reaction rate is slow due to poor rebonding efficiency during charging, resulting in large electrochemical polarization in metal ion batteries. In addition, they are easy to dissolve in non-protonic electrolytes, resulting in rapid capacity decay and poor stability.
Unlike disulfides and polysulfides (n-type), the redox process of thioethers (p-type) is not accompanied by bond cleavage (Scheme 1). The first reported thioethers were poly (2-phenyl-1,3-dithiolane) (PPDT, Fig. 2i) and poly (1,4-bis (1,3-dithiolan-2-yl) benzene) (PDDTB, Fig. 2j), both of which have an average discharge potential of 2.2 V(vs Li
+/Li)
[12]. The specific capacity of PDDTB was 378 mAh·g
-1 in the third cycle and remained at 300 mAh·g
-1 in 20 cycles. Later, Zhan et al. reported another thioether-containing conjugated polymer, polytetrahydrobenzodithiophene (PTBDT, Fig. 2K), which could maintain a high specific capacity of 560 mAh·g
-1 after 20 charge-discharge cycles due to the multi-electron process of reversible redox reaction on the thioether bond
[13].
3.2 Organic free radical
The organic radical compound is a bipolar organic compound (Scheme 2), and its two-electron reaction is still poorly studied. In most cases, the compounds are only used as p-type electrode materials, and have high discharge potential and stable cycle performance.
Most of the reported organic radicals used as p-type electrode materials are nitroxy-based compounds, the most famous of which is 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO, fig. 3A), and many organic radical polymers contain TEMPO as an electrochemically active center in the side chain.For example, poly (2,2,6,6-tetramethylpiperidinyloxy) methacrylate (PTMA, fig. 3B), a highly studied polymer derivative of TEMPO, has fast reversible redox kinetics with high potential, good power characteristics, and long cycle life
[14][15⇓⇓~18][19⇓~21]. In addition, a typical nitroxyl radical polymer is poly 2,2,5,5-tetramethyl-3-oxiranyl-3-pyrroline-1-oxirane (PTEO, fig. 3C)
[22]. PTEO has 147 mAh·g
-1 theoretical specific capacity and excellent rate capability, and can be cycled 1000 times at a large voltage of 3. 7 V.
There are also a few free radical polymers used as n-type electrode materials. Because of their large molecular weight, their theoretical specific capacity is generally lower than that of nitroxyl free radical polymers. For example, the theoretical specific capacity of polypropoxystyrene (PGVS, fig. 3D) is only 51 mAh·g
-1,Nishide and so on, using PGVS as the negative electrode and polyTEMPO-substituted norbornene as the positive electrode to prepare an all-radical polymer battery.Due to the fast redox kinetics of the free radicals and the efficient charge transport within the polymer layer, the cell has extremely high rate capability, can be quickly charged within 10 s, and also has a large discharge current (360 C) without loss of output voltage
[23].
The greatest advantage of free radical compounds is their high rate capability and good stability, but these materials face the problems of low intrinsic conductivity and self-discharge. In addition, there are many inactive sites in the structure of free radical polymers, which limits their theoretical specific capacity.
3.3 Imine compound
A reversible electrochemical reaction occurs between the nitrogen atom in the imine group (C = N) of the imine compound and a metal ion (such as a lithium ion), the carbon-nitrogen double bond in the imine group is dissociated into a single bond, and intramolecular electron transfer occurs through the conjugated structure connected with the imino group (Scheme 3).
The imine organic electrode materials can be divided into Schiff bases, pteridine derivatives and phenazine derivatives (fig. 4)
[24][25,26][27,28]. Schiff bases usually have a repeating H Hückel unit (— N = CH — Ar — HC = N —), and the unconjugated — CH = N — Ar — N = CH — isomer unit has no redox activity. The conjugated and planar structure plays a key role in the redox activity of Schiff base materials
[29][30]. Generally, Schiff base has a low discharge voltage, so it is a potential negative electrode material, and polymeric Schiff base has excellent thermal/mechanical properties
[31]. Pteridine derivatives, such as alloxazine, can be used as cathode materials for lithium ion batteries due to their relatively high discharge voltage (>2 V vs Li
+/Li)
[32,33]. Phenazine derivatives are also organic molecules with redox-active C = N bonds, such as a quinoxaline heteroaromatic molecule containing six imine groups (3Q, Fig. 4h), which has many electron-deficient phenazine sites where multiple redox reactions take place. Organic anodes based on 3Q molecules retain 70% of their specific capacity after 10,000 cycles at 8 A·g
-1 current
[34]; It has 222 mAh·g
-1 specific capacity at 20 C rate, which is equivalent to 60% of the initial specific capacity.
3.4 Azo compound
Wang et al. First discovered that azo compounds can be obtained by electrochemical reduction of nitro compounds. The azo group (N = N) of this emerging organic electrode material can be used as a redox active center for reversible electrochemical reaction with metal ions. In the redox process, reversible interconversion occurs between single and double bonds (Scheme 4)
[35].
Azo compounds generally have longer cycle life and higher specific capacity
[36]. For example, lithium azobenzene-4,4 '-dicarboxylate (ADALS, fig. 5C) retains 89% of its initial specific capacity after 5000 cycles at 20 C, corresponding to a 0.0023% specific capacity decay per cycle
[37].
3.5 Carbonyl compound
The redox mechanism for carbonyl compounds is the reversible enolization of the carbonyl group to form the radical anion (Scheme 5). These compounds are considered to be the most potential organic electrode materials due to their high theoretical specific capacity, fast reaction kinetics and diverse structures
[38].
Based on the characteristics of the functional groups, carbonyl compounds can be divided into three main groups: quinones and ketones, conjugated carboxylates, and anhydrides and imides. Quinones and ketones generally have higher theoretical specific capacities
[39⇓~41]. The theoretical specific capacity of the simple 1,4-benzoquinone molecule (BQ, fig. 6a) is 496 mAh·g
-1, which is much higher than that of the inorganic
(155 mAh·g
-1)
[42][43]. In addition, quinone and ketone materials generally have higher redox potentials than other types of carbonyl compounds and are the most promising organic cathode materials for high energy density
[44]. However, like other organic small molecule materials, quinone and ketone materials are easily soluble in electrolytes. The carboxyl group of the conjugated carboxylate can form a conjugated system with the aromatic ring, and the carboxylate has higher polarity, lower solubility in the electrolyte, and usually has a lower charge-discharge voltage platform, so it is often used as an anode material
[45⇓⇓~48]. The carbonyl groups of acid anhydrides and imides directly combine with nitrogen or oxygen to form O = C — N — C = O or O = C — O — C = O fragments, which have similar structures and therefore have similar redox mechanisms
[49⇓⇓⇓⇓⇓~55]. They usually have large aromatic conjugated planes, partial enolization of C = O in the redox process, and the aromatic core in the structure can stabilize negative charges, so compared with the first two types of carbonyl compounds, anhydrides and imides have the most advantages in cycle stability. Theoretically, all four carbonyl groups in each anhydride and imide structure can participate in the redox reaction, but in order to avoid charge repulsion and irreversible structural damage during charging and discharging, only two carbonyl groups are usually reduced
[56⇓~58].
4 Optimization Strategy of Carbonyl Compound Electrode Material
Carbonyl compound electrode materials can achieve high energy density, high cycle stability and high power density at the same time, which is the most studied organic electrode materials. However, carbonyl compound electrode materials are still far from the goal of practical application, mainly because of their high solubility and poor conductivity. In this section, we take carbonyl compound electrode materials as an example to elaborate on the design strategies for improving the electrochemical performance of organic electrode materials.
4.1 Molecular Structure Design
A prominent advantage of organic electrode materials is the designability of their molecular structures. In order to solve the inherent problems of organic electrode materials, the physical and chemical properties of organic materials can be controlled by modifying the skeleton structure and electronic structure of organic materials through appropriate synthetic routes.
4.1.1 Molecular Engineering
Molecular engineering strategies aim to effectively improve the electrochemical activity of individual molecules. One is to design molecules with more active sites per molecular weight, or to minimize the inert components in the molecular structure to obtain higher theoretical specific capacity. For example, Chen et al. Designed and synthesized cyclohexanone (C
6O
6) composed of only carbonyl units (Fig. 7A) and used it as a cathode material for lithium-ion batteries
[59]. At a current density of 20 mA·g
-1, the discharge specific capacity at an average discharge voltage of 1.7 V,C
6O
6 can reach 902 mAh·g
-1, close to the theoretical value (957 mAh·g
-1), corresponding to the ultra-high energy density of 1533 Wh·kg
-1 (fig. 7B). After 100 cycles at a current density of 50 mA·g
-1, the specific capacity retention reached 82% (fig. 7c).
Secondly, molecular engineering strategies can also enhance the conductivity and ion mobility of charge carriers by adjusting the planar conjugation of molecules and designing 3D skeletons. For example, Xu et al. designed and synthesized three compounds containing 1,4-benzoquinone (BQ) moieties through near-planar molecular structure: 2,2 '-bis-p-benzoquinone (BBQ), 1,4-bis (p-benzoquinonyl) benzene (BBQB), and 1,3,5-tris (p-benzoquinonyl) benzene (TBQB) (Fig. 8A)
[60]. Compared with BBQ, the benzene rings inserted by BBQB and TBQB not only weaken the charge repulsion interaction between the active units, but also make the molecular structure close to the planar state to reduce their solubility in the electrolyte (Fig. 8B). As a cathode material for lithium-ion batteries, it has a carbonyl utilization rate as high as 100%, and the initial specific capacity of BBQB and TBQB is 367 and 397 mAh·g
-1, respectively. BBQB with better flatness shows significantly improved cycle performance, and still maintains a high specific capacity of 306 mAh·g
-1 after 100 cycles (Figure 8 C).
Stoddart et al. designed and synthesized (-) -NDI-Δ with a triangular rigid structure, which is more stable in organic solvents than the original NDI-Δ. The initial specific capacity of (-) -NDI-Δ is 151 mAh·g
-1, indicating that each molecule receives 5.9 Li
+ (Figure 9 a, B)
[61]. In addition, the three-dimensional structure inside the molecule and the triangular lithium ion channel make the molecule have better fast charge and discharge performance, and can provide 58.1 mAh·g
-1 specific capacity at the ultra-high rate of 100 C (Fig. 9c, d).
Calix [6] quinone (C6Q), which also has a 3D structure, has a larger cavity that allows more electrolyte to enter, which is more conducive to the lithiation/delithiation process
[62]. The prepared lithium ion battery shows an initial specific capacity as high as 423 mAh·g
-1 at 0.1 C (the theoretical specific capacity is 447 mAh·g
-1). After 100 cycles, the specific capacity of C6Q remains at 216 mAh·g
-1, and even after 300 cycles, C6Q still achieves a high specific capacity of 195 mAh·g
-1. Due to its large specific capacity and wide electrochemical window, C6Q can provide energy densities up to 1201 Wh·kg
-1.
To sum up, the molecular structure design and optimization of materials at the molecular level can improve the theoretical specific capacity of materials, reduce the solubility of active materials, and improve the rate performance and cycle stability of batteries.Therefore, it is very important to carry out effective structure-activity analysis and framework design at the molecular level for the development of electrode materials with excellent comprehensive properties.
4.1.2 Substituent group
The introduction of different substituents can effectively adjust the electronic structure of carbonyl compounds, and then affect their properties. The cyclic stability of carbonyl compounds can be enhanced by the introduction of large polar groups, which mainly include — OM (M represents metal, such as Li, Na and K), — COOM and —SO
3M. For example, BQ is highly soluble in organic electrolytes and has poor cycle stability, while the disodium salt of disubstituted 2,5-dihydroxy-1,4-benzoquinone (Na
2DBQ, Fig. 10a) has a long cycle life of up to 300 cycles
[63]. The introduction of two —SO
3Na groups into anthraquinone (AQ) to form anthraquinone-1,5-disulfonic acid sodium salt (AQDS, fig. 10 B) has better cycling performance compared to the original AQ
[64]. In addition, the delocalized polarization of electrons in 2,5-diamino-1,4-benzoquinone (DABQ, fig. 10c) polarizes the amino and carbonyl groups, thus creating strong hydrogen bonds that stabilize the crystal, making it unusually low in solubility in organic electrolytes
[65].
Groups with different electronic properties can adjust the energy level of the lowest unoccupied molecular orbital (LUMO). A low LUMO level means a high electron affinity, which results in a high electrode potential. Therefore, electron-withdrawing groups (such as —CF
3, — CN, — F, — Cl, — Br, and —SO
3Na) and electron-donating groups (such as —NH
2, —CH
3, —OCH
3, — OLi, and — ONa) are often used as voltage increasing and voltage decreasing agents, respectively, to adjust the electrode potential of the carbonyl compound electrode material. The electron-withdrawing group makes the electron cloud of the redox center thinner and makes it more difficult to extract electrons from the redox center, thus increasing the redox potential of the redox center. On the contrary, the electron-donating group can contribute more electron clouds to the redox center, thus facilitating the extraction of electrons from the redox center and lowering the redox potential
[3,66]. For example, Dunn et al systematically studied the effect of a series of substituents on the electrode potential of naphthalimide based materials
[67]. By changing the aryl substituents from electron-donating groups (—NMe
2) to electron-withdrawing groups (— F and — CN), the discharge potential of these materials was increased from 2.3 V to 2.9 V(vs Li
+/Li).
In addition to the intrinsic properties of substituents, the number of substituents can also have a significant effect on the properties of carbonyl compounds. Shaijumon et al. Introduced different numbers of Br atoms into 3,4,9,10-perylenetetracarboxylic diimide (PDI) derivatives to obtain PDI-Br, PDI-Br
2, PDI-Br
3 and PDI-Br
4, and systematically studied the effect of the number of functional groups on the reduction potential
[68]. Increasing the number of substituents leads to the deformation of the molecular structure, which indicates that the angle of the diphenyl nucleus becomes larger with the introduction of substituents, thus stabilizing other electrostatic repulsion forces in the molecule. The study showed that the initial reduction potential of the monobromo to tetrabromo samples increased by 80, 90, 120, and 130 mV, respectively, compared to the reduction potential of the unsubstituted PDI (Fig. 11A).
In addition, the position of the substituent has a certain impact on the electrochemical performance of the organic electrode material. In the dihydroxyterephthalate system, due to the Coulomb interaction, as shown in Figure 11b, the discharge potential of the substituent shows a positive shift of about 300 mV from the para (Li
4-o-DHT) to the ortho (Li
4-p-DHT)
[69].
4.1.3 Heteroatom
The introduction of heteroatoms can change the electron cloud density and molecular polarity of carbonyl compounds. For example, O, N, and S are all more electronegative than C. Introducing these heteroatoms into the 9,10-anthraquinone (AQ) molecule yields the compounds benzo [1,2-b: 4,5-b '] dithiophene-4,8-dione (BDTD), benzofuran [5,6-b] furan-4,8-dione (BFFD), and pyrido [3,4-g] isoquinoline-5,10-dione (PID), respectively (fig. 12A)
[70]. Different heterocyclic benzoquinone molecules have different carbonyl utilization, and the initial specific capacity of BDTD, BFFD and PID is as high as 208~234 mAh·g
-1. After 100 cycles, the specific capacities of BDTD, BFFD and PID were 54% ~ 86% of their initial values. It is clear that the heterocyclic structure can promote the nucleophilic addition reaction of electrons. At the same time, these heterocyclic compounds show better rate capability than AQ. In particular, the BFFD can still provide 181 mAh·g
-1 reversible specific capacity at the ultra-high charge-discharge rate of 100 C.
A similar heteroatom effect was observed in the carboxylic ester of sodium terephthalate, where the oxygen atoms in the carboxylic ester group of sodium terephthalate were gradually replaced by sulfur atoms
[71]. By introducing two sulfur atoms into a carboxylate scaffold, the actual specific capacity reaches a 466 mAh·g
-1 at a current density of 50 mA·g
-1. When four sulfur atoms are introduced, the specific capacity increases to 567 mAh·g
-1 under the same conditions. It can be seen that the heteroatoms improve the electron delocalization range, conductivity, and sodium storage capacity.
The combination of molecular engineering and heteroatom strategy can improve the electrochemical performance of carbonyl compounds. For example, Wang et al. integrated three functional groups (— S —, C = O, C C N) into a molecule to obtain dicyananthraquinone (DTN). The carbonyl and cyano groups in the molecule are used to improve the specific capacity, while the heteroatom (S) is used to improve the redox potential
[72]. When used as a cathode material for lithium ion batteries, it can react with three Li ions in the voltage range of 1.0 to 3.5 V (Figure 6). The reversible specific capacity of the cathode based on DTN is 270.2 mAh·g
-1 after 300 cycles at 0. 5 C rate, and the specific capacity of the cathode based on DTN is 161.5 mAh·g
-1 after 1000 cycles at 5 C rate.
4.1.4 Conjugate extension
Most carbonyl compound electrode materials contain aromatic conjugated systems, which can enhance the π-π intermolecular interaction between materials, achieve better charge transport, inhibit the dissolution of compounds, stabilize the state of charge and discharge, and enhance the cycle performance. For example, polypentene ether ketone sulfide (PPTS, fig. 13) has an actual specific capacity of 290 mAh·g
-1, which is very close to the theoretical specific capacity of PPTS, indicating full utilization of the active site of PPTS and good reversibility
[73]. PPTS also has fast charge-discharge capability (specific capacity of about 160 and 100 mAh·g
-1 at current densities of 10 and 50 A·g
-1) and ultra-long cycle life (10,000 cycles). The rate capability, actual specific capacity and cycle performance of PPTS are better than those of polybenzoquinone sulfide (PBQS, theoretical specific capacity is 388 mAh·g
-1) and polyanthraquinone sulfide (PAQS, theoretical specific capacity is 225 mAh·g
-1), which have similar structure but smaller conjugated system. The results demonstrate the importance of the extension of the π-conjugated system to the electrochemical performance. The enhanced π-π intermolecular interaction and the long distance π-π stacking structure are favorable for the fast charge transport, which contributes to the excellent electrochemical properties of PPTS.
In the process of reaction between O in carbonyl and cation, carbon radical intermediates will be produced, and the increase of conjugation can consume unpaired electrons, thus helping to stabilize the radical intermediates. As shown in Scheme 7A, trichloroisocyanuric acid (DCCA) combines with three Na
+ and electrons to form an α-C · radical intermediate. Because of the poor stability of the free radical intermediates produced, side reactions are very easy to occur, thus irreversibly forming inactive precipitates, so the cycle stability of DCCA is very poor. To solve this problem, Lu et al., through conjugation effects, as well as steric effects of the rigid geometry, obtained a stable α-C · radical intermediate that exhibits weak electron affinities, thus placing the α-C
- at a low potential (vs Na/Na
+<0.5 V), which promotes the "bonding" behavior between the Na
+ and the carbon atom, with the total amount of Na
+ embedded in the carbon atom approaching one ion per C radical (Scheme 7 B)
[74]. Moreover, the prepared organic electrodes showed excellent reversible conversion between C = O and C — O — Na, and the specific capacity of 3-N-salicylanthrylenequinylamine (TSAQ) anode did not decrease significantly after 2500 cycles at high current density of 1 A·g
-1.
Therefore, it is very effective to adopt the method of conjugation extension for organic electrode materials, which can improve the rate capability and cycle stability, but it also has limitations in obtaining high energy density and may lead to a significant increase in molecular weight. The trade-off between the degree of conjugate propagation and energy density should be carefully considered.
4.1.5 Polymerization
The incorporation of electrochemically active segments into the polymer can effectively avoid the dissolution problem of small molecular carbonyl compounds. For example, Xu et al. prepared polymers poly (NBE-BQ) and poly (NBE-NQ) (Scheme 8) by ring-opening metathesis polymerization of norbornene-derived monomers (NBE-BQ and NBE-NQ) containing 1,4-benzoquinone (BQ) and 1,4-naphthoquinone (NQ), which effectively reduced the solubility of BQ and NQ in the electrolyte and improved the cycle stability
[75]. The redox potentials of the BQ and NQ units in the polymers were kept high, and the redox potentials of the poly (NBE-BQ) and poly (NBE-NQ) were 2. 7 V and 2. 5 V, respectively.
The different polymerization sites of the electrochemically active monomers affect the stability and electrochemical performance of the materials. For example, poly (1,4-anthraquinone) (P14AQ) and poly (1,5-anthraquinone) (P15AQ) (fig. 14a) have been reported as cathode materials for lithium-ion batteries with energy densities of 563 Wh·kg
-1 and 502 Wh·kg
-1, respectively
[76]. Among them, P14AQ reached 69% of the maximum specific capacity at 20 C rate, and the specific capacity decreased by only 0.6% after 1000 cycles at 2 C rate (fig. 14 B). However, the specific capacity retention of P15AQ was only 67.6% even after 100 cycles at 1 C rate (fig. 14C). Compared with P15AQ, the distance between the redox active atoms of P14AQ and the adjacent molecules is larger, and the steric hindrance near the redox active site of the connecting unit anthraquinone (AQ) is relatively smaller, so the accessibility of charged cations of P14AQ is better, and the cations are easier to diffuse between molecules, so that the P14 AQ material has high rate capability and better cycle stability.
Compared with chain polymers, two-dimensional (2D) polymers usually have higher specific surface area and porous structure, which can enhance charge transport, promote ion diffusion, inhibit dissolution and improve stability, and they often show high activity in metal ion batteries
[77].
Covalent organic frameworks (COFs) are a new class of porous crystalline polymers, which are two-dimensional or three-dimensional ordered structures formed by organic structural units. They usually have high density of redox sites, permanent and controllable pores, high specific surface area and easily adjustable structure, which can promote the rapid migration of ions and provide higher theoretical specific capacity, while the polymer structure reduces their solubility, so they can be used as new organic electrode materials in rechargeable devices.
For example, when tetraaminobenzoquinone (TABQ) and pyromellitic dianhydride (PMDA) were used to synthesize the microporous COF: poly (imide-benzoquinone) (PIBN), it was found that eight Li
+ on the imide group and two Li
+ in the quinone carbonyl group of each repeat unit in the COF were involved in two consecutive steps of lithiation and delithiation (Fig. 15A)
[78]. Graphene-doped PIBN-G can achieve large reversible specific capacities of 271.0 and 193.1 mAh·g
-1 at rates of 0.1 C and 10 C, respectively (Fig. 15 B, C), with a specific capacity retention of more than 86% after 300 cycles (Fig. 15 d). The excellent electrochemical performance of PIBN-G will stimulate further research on the design and general use of COF-based electrode materials.
From the above studies, it can be found that the polymerization strategy has a significant effect on reducing the solubility of the material and improving the cycle stability, but the introduction of inactive linking sites in the polymerization process will reduce the energy density and electronic conductivity. Therefore, when using polymerization strategy to design electrode materials, it is usually necessary to balance the relationship between dissolution and theoretical specific capacity, and also to consider its impact on energy density.
In the section 4.1 Structural Design Strategies, the application of molecular engineering strategies, substituent, heteroatom and polymerization strategies are described through the analysis of representative examples of carbonyl compound electrode materials, and the effects of corresponding strategies on the functional properties of carbonyl compounds are summarized in detail. In addition to the above strategies to control the electrochemical behavior of materials at the molecular level, nanocrystallization of materials, compounding with other materials and optimization of electrolytes are also important methods to improve the performance.
4.2 Nanocrystallization
Nanocrystallization of materials is a universal method to improve the performance of materials, which has been proved to be effective in improving the utilization of active materials, improving the kinetic performance of electrochemical reactions and inhibiting the pulverization of active materials.
As early as 2013, when Chen et al. Used the organic tetralithium salt of 2,5-dihydroxyterephthalic acid (Li
4C
8H
2O
6) in bulk, nanoparticle and nanosheet forms as the electrode active material for rechargeable lithium-ion batteries, they found that the order of electrochemical performance from high to low was: nanosheet > nanoparticle > bulk
[79]. The initial discharge specific capacity of the nano-sheet Li
4C
8H
2O
6 positive electrode is 223 mAh·g
-1, and the initial discharge specific capacity of the negative electrode is 358 mAh·g
-1. It can provide specific capacities of 145 and 175 mAh·g
-1 as a positive electrode and a negative electrode, respectively, even at a rate of 5 C, which are about 7 and 2 times higher than those of a bulk-like Li
4C
8H
2O
6 electrode.
In 2016, Yu et al. studied the influence of shape control and size effect of disodium rosepalmitate (SR) on sodium ion storage characteristics, and synthesized electrode materials with three morphologies of micron particles, micron rods and nanorods (Fig. 16A)
[80]. Nanorods were found to exhibit the best cycling stability (Fig. 16b). In addition, the small size and large surface area of the nanorods allow sodium ions to be quickly transferred to the interior of the SR, so the specific capacity and rate capability of SR nanorods are also much higher than those of microparticles and microrods (Fig. 16c).
In recent years, porous COFs have been widely studied as electrode materials in lithium ion batteries
[81⇓~83]. Most of the COFs materials are synthesized in one step in solution, and have a close-packed blocky structure, which results in a low effective utilization of the redox active sites. For example, 2,6-diaminoanthraquinone-based COF (DAAQ-COF) has slow ion diffusion due to dense stacking of planar sheets
[84]. After ball milling, the bulk COF can be exfoliated into 2D-COF nanosheets with a thickness of about 4 to 12 nm (fig. 17A). Compared to the bulk DAAQ-COF material, the exfoliated COF has excellent electrochemical properties: high specific capacity (420 mAh·g
-1 at 100 mA·g
-1 current density, fig. 17b), high rate capability (198 mAh·g
-1 at 5 A·g
-1 current density, fig. 17c) and long cycle stability (99% retention after 10,000 cycles at 5 A·g
-1 current density). This is because the exfoliated nanosheets increase the specific surface area of the material, which is conducive to the full penetration of electrolyte and the acceleration of electron and ion diffusion, thus achieving high rate performance. At the same time, stripping into nanosheets improves the effective utilization of redox active sites and shortens the length of ion/electron migration, thus greatly improving the specific capacity of active materials and the storage power of ions.
It can be seen from the above reports on nanocrystallization of carbonyl compound electrode materials that nanocrystallization can further enhance the ion diffusion rate and improve the ion accessibility during cycling on the premise of maintaining the theoretical specific capacity of the material, and at the same time, it can also promote the release of stress and inhibit the pulverization of active materials. However, it is important to note that nanocrystallization also increases the risk of dissolution.
4.3 Composite Material Design
The poor intrinsic conductivity of most carbonyl compounds, coupled with their solubility in organic electrolytes, limits their direct use as electrode materials. An effective improvement strategy is to combine the active material with the conductive matrix. Conductive carbons, such as graphene and carbon nanotubes, are often used as additives to various organic materials, which can provide the necessary electrical pathways for electrochemical reactions in electrodes. At the same time, through the interaction between the carbon material and the carbonyl compound electrode material, the dissolution of the active material is inhibited to a certain extent, and the cycle stability is improved.
For example, poly [5] quinone (P5Q) has a high theoretical specific capacity (446 mAh·g
-1) and a cavity-like structure that facilitates electron transport, but its high solubility in electrolytes prevents it from being used as an electrode material in rechargeable batteries. To solve this problem, Zhang et al. encapsulated P5Q into ordered mesoporous carbon (CMK-3) with large surface area to form a composite material, which was then integrated with single-walled carbon nanotubes (SWCNTs) to produce a thin film, which was used as a positive electrode for sodium-ion batteries (SIBs)
[85]. The initial specific capacity of the modified SIBs is as high as 418 mAh·g
-1, and the specific capacity of the modified SIBs is still 290 mAh·g
-1 after 300 cycles at a rate of 0.1 C. Even at 1 C rate, the specific capacity can still reach the 201 mAh·g
-1.
Researchers have done a lot of work around the development of various conductive additives and composite methods, which have greatly improved the performance of carbonyl electrode materials
[86⇓⇓⇓~90]. For example, Feng et al. integrated a highly stable crystalline 2D-PAI COF on the surface of CNT (Fig. 18a), and synthesized a tubular core-shell polyarylimide hybrid (2D-PAI @ CNT) with rich π-conjugated redox-active naphthalimide units, strong cyclic imide bonds, high surface area and permanent accessible pores, which greatly improved the performance compared with the original 2D-PA anode
[91]. At the current density of 0.1 A·g
-1, the utilization of the redox active sites of the original 2D-PAI cathode was only 22.6%, while the effective utilization of the redox active sites of 2D-PAI @ CNT could reach 82.9%, while maintaining the excellent cycle stability of 2D-PA (Fig. 18b). At 0.5 A·g
-1 current density, 2D-PAI @ CNT achieved high rate performance and 100% specific capacity retention after 8000 cycles (fig. 18c), far exceeding the polyimide electrode of the same period.
The types and characteristics of carbon materials and the techniques used to prepare the composites are also important for improving the electrochemical performance of carbonyl compound electrode materials. Recently, Li et al. Solved the overall problem of organic electrode materials in terms of solubility, conductivity, and active material loading by in-situ coating graphyne (GDY) nanocoatings on small molecule organic positive electrode 3,4,9,10-tetracarboxylic anhydride (PTCDA) and SR particles (Fig. 19 a)
[92]. The GDY-coated PTCDA (PTCDA @ GDY) sample can remain remarkably flat at rates up to 20 C due to reduced polarization, providing a high specific capacity of about 95 mAh·g
-1 and a high coulombic efficiency close to 100%. In contrast, the specific capacity of unmodified PTCDA decays more rapidly as the rate increases, no specific capacity is retained after the rate reaches 10 C, and the coulombic efficiency becomes disordered (Fig. 19b). In addition, as shown in fig. 19c, the PTCDA @ GDY electrode can also provide a higher specific capacity and a more stable specific capacity retention rate, and more importantly, the electrode has an active material loading of up to 93%, and the prepared PTCDA @ GDY has a high energy density of 310 Wh·kg
-1. Compared with the unmodified SR cathode, the GDY-coated SR cathode is also greatly improved in terms of Na
+storage, stability and rate capability.
Carbonyl compounds and carbon materials can inhibit the dissolution of active materials and improve the cycle stability and rate performance, so it is of great significance to develop advanced composite technology or novel carbon materials and improve the loading capacity of active materials.
4.4 Electrolyte optimization
The nature of the electrolyte will directly affect the dissolution behavior of the carbonyl compound
[93]. Optimizing and exploring new electrolytes can alleviate the dissolution problem and enhance the cycle stability of carbonyl compounds. Chen et al. Developed an electrolyte system of CF
3SO
3Na/ triethylene glycol dimethyl ether (TEGDME) with CF
3SO
3Na concentrations of 1, 2, 3 and 4 mol/L, respectively
[94]. As shown in Figure 20 a, lower concentrations of electrolyte (1 to 3 mol/L) turned yellow after 1 d, indicating AQ dissolution. On the contrary, in the electrolyte with high salt concentration (4 mol/L), the color of the solution hardly changed even after 10 days. AQ was encapsulated in CMK-3 (AQ/CMK-3) as the positive electrode of SIBs. The initial specific discharge capacity of AQ/CMK-3 increased with the increase of salt concentration in the electrolyte. The cycle performance of AQ/CMK-3 in 4 mol/L electrolyte is also better than other electrolytes with lower salt concentration. AQ/CMK-3 maintained a high specific capacity of 88% even after 50 cycles (fig. 20 B). Fig. 20c shows the coulombic efficiency of AQ/CMK-3 in electrolytes with concentrations ranging from 1 mol/L to 4 mol/L. In the electrolyte with low sodium salt concentration, the coulombic efficiency is very low due to severe dissolution of AQ/CMK-3; In contrast, 4 mol/L electrolyte results in nearly 100% coulombic efficiency after 5 cycles.
Ionic liquids as electrolytes are also one of the effective ways to solve the dissolution of organic electrode materials. It was found that the polarity of the ionic liquid, the number of donors and the interaction energy had an effect on the inhibition of quinone dissolution
[95]. Compared with 1,2-dimethoxyethane (DME), which is widely used as an electrolyte for lithium ion batteries, the electrochemical performance of quinones in ionic liquids is significantly enhanced. In an N-methyl-N-propylpyrrolidinium bis (trifluoromethanesulfonyl) amide ( [PY13] [TFSI]) -based electrolyte, the positive electrode of calix [4] quinone (C4Q) provides high energy density (863 Wh·kg
-1) with excellent specific capacity retention (about 99.9% after 300 cycles at 130 mA·g
-1 current density). In addition, 5,7,12,14-pentenone (PT) in [PY13] [TFSI] electrolyte showed high rate capability and long cycle stability, and 9,10-anthraquinone (AQ), 1,2-naphthoquinone (NQ) and 9,10-phenanthrenequinone (PQ) also showed high reversible specific capacity.
Solid-state electrolytes can completely solve the problem of dissolution of electroactive materials. In 2014, Chen et al. prepared an all-solid-state lithium organic battery based on polymethacrylate (PMA)/polyethylene glycol (PEG) nanocomposite polymer electrolyte and P5Q as the positive electrode
[96]. The average working voltage of the cathode containing 55 wt% P5Q is 2.6 V at 0.2 C rate, the initial specific capacity is up to 418 mAh·g
-1, and 94.7% of the specific capacity is retained after 50 cycles. In addition to polymer electrolytes, carbonyl electrode materials are also compatible with sulfide-based solid state electrolytes. For example, Yao et al. fabricated all-solid-state SIBs using Na
3PS
4 as the electrolyte and SR as the positive electrode (fig. 21a)
[97]. As shown in fig. 21b, the scanning electron microscope image and the corresponding energy dispersive X-ray (EDX) pattern demonstrate good contact between the organic positive electrode and the electrolyte. The ionic conductivity of the Na
3PS
4 electrolyte was 0.39 mS·cm
-1 at 60 ℃, and the battery showed 70% specific capacity retention after 400 cycles at 0.2 C rate.
Although the dissolution of the active material can be completely prevented by using the solid state electrolyte, the solid state electrolyte causes problems of low ionic conductivity and large interface resistance at ambient temperature. With the development of research, more and more solid electrolyte materials with high ionic conductivity and low activation energy have been developed
[98⇓⇓~101]. Although this is beyond the scope of this paper, if the ionic conductivity, chemical and electrochemical stability, and interface problems of solid-state electrolytes can be properly addressed, the development of all-solid-state electrolytes may be the ultimate way to achieve practical application of carbonyl compound electrode materials.
Based on the strategies of molecular structure design, nanocrystallization, composite and electrolyte optimization, carbonyl compound electrode materials have achieved considerable electrochemical performance. We strongly recommend that more attention be paid to the structural design, especially the strategies of molecular engineering and increasing conjugation, because almost all the properties of carbonyl compound electrode materials, including specific capacity, redox potential, reaction kinetics, electronic conductivity and dissolution, can be reasonably adjusted by this method. In addition, various optimization methods are not isolated. In the process of research, through the combination of different strategies, new electrode materials and electrolyte systems are constantly improved and explored to meet the practical application needs of rechargeable devices.
5 Application of Carbonyl Compounds as Electrode Material
The advantages of excellent electrochemical performance, sustainability, processability and structural diversity provide organic electrode materials with potential applications in various types of rechargeable devices. In this section, carbonyl compounds will be used as an example to introduce the application of organic electrode materials in rechargeable devices other than conventional lithium-ion batteries.
5.1 Organic alkali metal ion battery
Sodium (potassium) ion batteries are potential battery systems for large-scale applications due to the abundance and low cost of sodium and potassium. However, due to the large radii of sodium (potassium) ions (1.02 and 1.3838 Å), the electrode materials that can store Na
+(K
+) reversibly are greatly limited
[102,103]. Carbonyl compounds have flexible skeleton structures, and the carbonyl compound electrode materials for lithium ion batteries can be directly used as electrodes for sodium ion batteries through reasonable design. For example, Lei et al. Used SR as a cathode material for sodium-ion batteries, which showed a high reversible specific capacity of 173.5 mAh·g
-1 at a current density of 50 mA·g
-1 and maintained a specific capacity of 115 mAh·g
-1 at a current density of 5 A·g
-1[104]. After 1500 cycles at the current density of 1 A·g
-1, the specific capacity is still as high as 110 mAh·g
-1, which has excellent cycle stability.
Similarly, carbonyl compounds can also be used as the positive electrode of potassium ion batteries. Lee et al. prepared a series of polymers with different inert alkyl chain lengths (PTCDA-kC) based on the polymerization of PTCDA monomers and chain diamines, and used them as the positive electrode of potassium ion batteries
[105]. The PTCDA-2C electrode achieved a higher potassium storage capacity in a shorter discharge time, with a high energy density of 113 Wh·kg
-1 at a high power density of 35.2 kW·kg
-1, equivalent to a complete discharge in 9.72 s at a high current density of 147.06 C (Fig. 22 a). The material also showed remarkable cycling stability, with almost no change in specific capacity after 1000 cycles at a high current density of 7.35 C (Figure 22b). As shown in fig. 22c, the all-organic potassium ion battery prepared using the PTCDA-2C positive electrode and the dipotassium terephthalate (K
2TP) negative electrode can achieve very high energy density (173.2 and 108.3 Wh·kg
-1 at power densities of 71 and 1330 W·kg
-1, respectively), and the battery also exhibits excellent cycle stability over 350 cycles.
5.2 Organic multivalent metal ion battery
Because that carbonyl compound can not generate volume change and irreversible structural phase change due to metal ion intercalation and deintercalation, the carbonyl compound can be use as a general electrode in an alkali metal ion battery, and can also be applied to a multivalent metal ion battery.
For example, Wang et al. formed polyimide and carbon nanotube composites (PI @ CNT) in situ and used them as cathode materials for Li
+, Mg
2+ and Al
3+.The PI @ CNT positive electrode provided high reversible specific capacities of 180 mAh·g
-1, 130 mAh·g
-1, and 110 mAh·g
-1 for Li, Mg, and Al ion batteries, respectively. The full cell assembled using the PI @ CNT positive electrode and the lithium negative electrode exhibited long cycle life, and no specific capacity fading was observed after 10,000 cycles (fig. 23 a)
[106]. In addition, the PI @ CNT cathode also exhibited a fast kinetic rate, maintaining 55% of the reversible specific capacity at 0.5 C for all Li
+, Mg
2+, and Al
3+,PI@CNT as the charge-discharge current increased to the 3 A·g
-1(20 C) (Fig. 23b, C). Moreover, the PI @ CNT composite can maintain high rate capability and long cycle stability even in a wide operating temperature range (-40 ~ 50 ℃) for the three ion batteries. These results have made low-cost industrial polyimides a versatile anode for different multivalent metal batteries.
5.3 Aqueous rechargeable battery
Water-based rechargeable batteries are low cost, non-toxic, non-flammable, and the water-based electrolyte is highly sustainable. The universal quinone negative electrode in the aqueous battery shows high specific capacity, long cycle life, and rapid reaction power in the aqueous battery
[107]. And such cell can operate under extreme condition such as high pH, low pH, low temperature, and atmospheric (with/without O
2) conditions. The acid battery composed of 4,5,9,10-perylenetetraone (PTO) and PbO
2 can achieve a long cycle life of 1500 cycles and a fast charge capability of up to 20 C, achieving excellent cycle stability and rate capability.
In 2018, Xia et al. coupled PTO with a Zn metal anode to achieve a PTO//Zn aqueous full cell with high energy density (Fig. 24a), and prepared the full cell with stable cycle life (1000 cycles, Fig. 24b) and fast charging capability (20 A·g
-1, Fig. 24c)
[108]. Due to the inherent insolubility of PTO and the corresponding discharge products, the expensive fluorine-containing membrane (Nafion membrane) can be replaced by a conventional porous glass fiber membrane to separate the positive and negative electrodes. Moreover, the manufactured strip-shaped PTO TO Zn battery can provide a specific capacity of 337 mAh·g
-1 at a current density of 0.04 A·g
-1, and can still maintain a specific capacity of 122 mAh·g
-1 after being rapidly fully charged within 44 s, which is close to the high-rate performance of a PTO TO Zn aqueous button battery. Considering the thickness of the ribbon battery, its volumetric energy density and power density can reach 38.72 mWh·cm
-3 and 1.70 W·cm
-3, respectively, and it has excellent mechanical durability and good flexibility.
Water-based rechargeable batteries use non-flammable water-based electrolytes, which are intrinsically safe and largely do not rely on the protection of complex battery management systems, so they have the advantages of safety and low cost over batteries using organic electrolytes. However, the electrochemical stability window of water (1. 23 V) is much narrower than that of organic electrolytes (4. 5 V), which greatly limits the energy density of water-based batteries
[109].
5.4 Redox flow battery
Redox flow battery is a new type of battery construction technology. An electrochemically active substance is dissolved in an electrolyte to form a negative electrode solution and a positive electrode solution, the positive and negative electrode solutions are separated by an ion-selective membrane (e.g., a Nafion membrane), and then charging and discharging are performed by electrochemical oxidation-reduction reactions of the negative electrode solution and the positive electrode solution in the battery. Because water is very abundant, cheap, environmentally friendly and has high ionic conductivity, and water can dissolve high concentrations of metal salts, the aqueous redox flow battery is the most studied type of flow battery.
In 2014, Aziz et al prepared the first metal-free quinone-bromide aqueous redox flow battery using a sulfuric acid solution of 9,10-anthraquinone-2,7-disulfonic acid (AQDS) as the negative electrode material
[110]. In order to reduce the cost of organic redox flow batteries, Aziz et al. designed an alkaline quinone flow battery using 2,6-dihydroxyanthraquinone (DHAQ) and ferrocyanide in alkaline solution
[111]. The use of organic and organometallic complexes in the base can solve the corrosion and safety problems of acid flow batteries. In addition, since many components can be made from inexpensive polyolefin or polyvinyl chloride plastics, the reduction in corrosivity can further greatly reduce material costs. In 2019, they found that DHAQ tends to form anthrone intermediates, which then irreversibly form dimers, resulting in specific capacity loss of DHAQ and ferrocyanide flow batteries. By modifying the working conditions of the battery, they reduced the specific capacity decay rate of the anthraquinone-based flow battery by about two orders of magnitude, from 5.6%/d to 0.042%/d
[112].
Electrodes for redox flow batteries do not need to undergo changes in the morphology of the active material due to ion intercalation or deintercalation. In addition, the redox flow battery system is also scalable, and the specific capacity can be easily and inexpensively increased by adjusting the concentration and volume of the active material without sacrificing any power density. A large number of organic electrode materials with various redox active units will further improve the performance of future redox flow batteries, thus promoting the development of safe and economical rechargeable devices.
5.5 Upercapacitor
Supercapacitors have the characteristics of fast charging and discharging, and also have the energy storage characteristics of batteries. Organic carbonyl compounds used in supercapacitors usually store charge by pseudocapacitance, and their reversible redox reactions mainly occur on the surface of electrode materials, which usually require active materials with higher energy density.
Structurally ordered high surface area materials with tunable pore sizes, such as COFs, are excellent candidates for supercapacitors. For example, Banerjee et al. designed and synthesized a covalent organic framework TpOMe-DAQ with redox activity, and the exfoliated TpOMe-DAQ flakes showed excellent capacitance performance in H
2SO
4 aqueous solution at 2 mol·L
-1 (1280 mF·cm
-2 and 135 F·g
-1, Fig. 25A), and the cycle stability of galvanostatic charge-discharge (GCD) was more than 100,000 times without affecting the capacitance (Fig. 25B)
[113]. In addition, the prepared stand-alone supercapacitor can still maintain about 65% of the initial capacitance after 50000 consecutive charge-discharge cycles even at a current density of 5 mA·cm
-2, and the coulombic efficiency remains unchanged. As a proof-of-concept study, a 1.8 V LED could be lit for 20 s using three solid-state devices connected in series and charged in a 3 V voltage window.
To sum up, compared with rigid inorganic compounds, organic electrode materials have versatility and adjustability, which are suitable for general electrodes of different metal ion batteries, and can realize rapid insertion/extraction of multivalent metal ions through ion coordination charge storage mechanism.It solves the power and cost problems of multivalent metal ion batteries when using inorganic crystalline cathode materials, and provides new opportunities for the design of high power and long life multivalent metal ion rechargeable batteries.
The electrode potential of organic electrode materials is usually compatible with the electrochemical stability window of most electrolytes, and its electrode potential is easy to adjust, so organic electrode materials have lower requirements for the stability of electrolytes
[114]. In particular, the solid electrolyte can mechanically inhibit the growth of lithium dendrites, has better safety and higher energy density, is the most promising substitute for the traditional liquid battery, and the organic electrode material can be compatible with the solid electrolyte (such as Na
3PS
4),In addition, its volume expansion is small, and it can also use chemical interaction to maintain close contact between the active material and the solid electrolyte, so its application in all-solid-state batteries is also a promising research direction
[115][116].
Some organic electrode materials have strong adhesion (such as dopamine polymer), and flexible binderless composite cathode can be prepared by reasonable design, which can reduce irreversible specific capacity loss and improve cycle stability
[117].
In addition, compared with rigid inorganic materials, organic electrode materials have the characteristics of solubility, easy film formation and processability. Thus, organic materials can be easily loaded onto flexible devices by techniques such as casting, printing, vapor deposition, vacuum filtration, and in situ polymerization, which is advantageous in producing flexible or stretchable devices.
6 Conclusion and prospect
In this paper, the reaction mechanism and classification of organic electrode materials are systematically introduced, and then the strategies to optimize the performance of organic carbonyl compounds and their applications beyond traditional lithium-ion batteries are discussed. Fig. 26 summarizes the redox potential and specific capacity of common organic and inorganic electrode materials. The energy density of some organic compounds, such as C6O6, is much higher than that of traditional inorganic materials, even exceeding that of 1500 Wh·kg-1, making them excellent candidates for high energy density and low cost electrodes for rechargeable devices.
Although organic electrode materials have achieved considerable electrochemical performance based on molecular structure design, nanocrystallization, composite and electrolyte optimization strategies, they still face great challenges to achieve practical applications. First of all, improving the actual specific capacity is an aspect of organic electrode materials that needs to be broken through. Although smaller organic compound molecules have higher theoretical specific capacity, in actual batteries, the actual specific capacity of organic electrode materials rarely reaches the theoretical specific capacity calculated according to their molecular formulas due to instability/solubility in organic electrolytes. Second, many organic electrode materials can outperform current commercial inorganic materials in terms of gravimetric energy density/specific capacity, but organic compounds have low tap density,Usually, the volume energy density/specific capacity is relatively small, so in some specific scenarios (such as large-scale energy storage power stations with low volume requirements), organic electrode materials show better application prospects. Thirdly, poor recyclability is also one of the bottlenecks to be overcome by organic electrode materials, which is closely related to their dissolution in organic electrolytes. Fourthly, organic electrode materials usually have poor electronic conductivity. In order to meet the rate performance requirements of fast charging devices, the usual solution is to incorporate a large number of conductive additives, so that the loading of active materials is limited, resulting in limited overall energy density. Finally, most of the research on organic electrode materials is still at the laboratory scale (half-cell and laboratory-scale testing), and there is an urgent need for research on full-cell, especially large-scale cells.
Considering the above problems, we suggest that further research on organic electrode materials should focus on the following aspects. Firstly, it is necessary to explore organic compounds with higher theoretical specific capacity through fine molecular design strategies, and to increase the loading of active materials as much as possible, so as to improve the energy density of batteries to meet the requirements of actual battery systems. Second, there is a need for a breakthrough in increasing the tap density of organic electrode materials and improving the volumetric energy density of devices in order to expand the practical application range of organic electrode materials. Thirdly, polymerization or compounding with insoluble materials can effectively prevent the dissolution of active substances. However, in order to obtain rechargeable devices with high energy density, good safety and high cycle stability, future research should focus on the development of high-performance solid electrolytes. Fourth, although the compounding of organic electrode materials with carbon materials can provide higher rate performance, nano-functionalization can also be used as an alternative method to improve the rate performance without significantly reducing the actual specific capacity. However, the fundamental way to obtain high power electrode materials is to expand the conjugation degree of molecular structure and improve the electronic and ionic conductivity of organic electrode materials from the perspective of molecular design. Finally, for further practical applications, consideration must be given to the mass production of high-performance and stable organic electrode materials at low cost through simple, scalable methods.
To sum up, organic electrode materials are still in the early stage of development. With the in-depth study of organic electrode materials and the development of more and more materials with excellent comprehensive properties, we can see the broad development space of organic electrode materials, and believe that they will become an important force to promote the development of science and technology in the foreseeable future.
National Natural Science Foundation of China(21631003)
National Natural Science Foundation of China(21871024)
National Natural Science Foundation of China(21805005)
Fundamental Research Funds for the Central Universities(FRF-BD-18-017A)
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