The performance improvement of lithium-ion batteries largely depends on the characteristics of the cathode material. Currently, inorganic cathode materials are widely used, but they suffer from various drawbacks. Compared with inorganic cathode materials, organic cathode materials have advantages such as high theoretical specific capacity, abundant raw materials, environmental friendliness, strong structural designability, and system safety, making them a promising energy storage material. This article mainly introduces several organic compounds used as lithium-ion cathode materials, and compares and analyzes their electrochemical performance and electrochemical reaction mechanisms.
• Conductive organic polymer cathode materials
Early research on organic cathode materials focused primarily on conductive polymers. However, single-state conductive polymer cathode materials have many drawbacks and cannot meet the needs of practical applications. Therefore, research began on various composite materials based on conductive polymers. Researchers prepared PPy/V2O5 composites by doping V2O5 into polypyrrole. After charge and discharge, the PPy/V2O5 composites underwent anion doping/dedoping and Li+ insertion/deintercalation reactions, resulting in changes in the percentage of elements and the internal morphology of the cathode material, leading to poor cycle stability.
Figure 1. Electrochemical properties and surface morphology of the PPy/V2O5 composite material after charge and discharge.
These conductive polymers, used as cathode materials in lithium-ion batteries, achieve their electrochemical function through anion doping/dedoping. They typically suffer from the following drawbacks: a large electrolyte volume is required in the reaction system, making it difficult to improve the battery's energy density and conductivity; the electrochemical reaction rate is slow, requiring the doping of large amounts of conductive agents; organic polymers still exhibit slow dissolution in the electrolyte; long-term cycle stability is low; and the theoretical capacity is not high. Significant room for improvement exists.
• Organic sulfide cathode materials
Researchers then turned their attention to organosulfur compounds that release and store energy through the breaking and bonding of SS bonds. They found that increasing the sulfur chain length could increase the specific capacity, but due to the insulating nature of sulfur itself, and the fact that the intermediate product Li₂SX generated by the electrode reaction is easily dissolved in the electrolyte and deposited on the lithium anode surface, the charge-discharge power and cycle performance of the battery were severely affected. Therefore, they introduced SS bonds into organic molecules to form various linear, ladder-like, or network-like multi-crosslinked sulfur polymers, with representative compounds shown in Table 1.
Table 1 Typical organosulfur compound cathode materials
While organosulfur compound cathode materials have improved the electrochemical activity and cycle stability of batteries to some extent, they generally suffer from the following problems: rapid capacity decay and easy degradation; difficulty in overcoming solubility in electrolytes, resulting in low cycle stability; the problem of sulfur ions generated during discharge transferring to the negative electrode; poor conductivity and slow electrochemical reaction rate at room temperature; and the cycle performance of organosulfur compound cathode active materials still falls short of practical application requirements.
• Oxygen-containing conjugated organic cathode materials
Organic conjugated oxygen-containing compound electrode materials have become a research hotspot for lithium-ion battery cathode materials due to their advantages such as high specific capacity, structural diversity, and fast reaction kinetics. Carbonyl compounds, represented by anthraquinone and its polymers, and acid anhydrides containing conjugated structures, are gradually attracting attention as an emerging cathode material. Their electrochemical reaction mechanism is as follows: during discharge, the oxygen atom on each carbonyl group gains an electron and simultaneously inserts a lithium ion to form an enol lithium salt; during charging, lithium ions are extracted, the carbonyl group is reduced, and the reversible insertion and extraction of lithium ions is achieved through the conversion between the carbonyl group and the enol structure.
Researchers studied the electrochemical performance of a novel organic quinone compound, 1,4,5,8-tetrahydroxy-9,10-anthraquinone (THAQ, Figure 2), and its oxidation product (O-THAQ), which showed high initial charge-discharge capacity and cycling performance.
Figure 2 Electrochemical reaction mechanism of THAQ
Based on this, the dimer tetrahydrohexaquinone (THHQ) of THAQ was prepared through a one-step oxidation reaction. The quinone content in this material was further increased, and the electrochemical performance was improved. This is because the solubility of the molecule decreased and the stability increased after the dimer was formed.
Figure 3. Molecular structure of THHQ
Subsequently, various scientists improved upon it, producing formulations based on pyromellitic dianhydride, quinone, and AlCl3.
Nonobenzohexaquinone (DBHQ) and 2,4-trinitro-9-fluorenone (TNF) compounds were synthesized using these as the main raw materials, resulting in increased average energy density and higher capacity.
Figure 4. Molecular structure of DBHQ
Figure 6. Charge-discharge mechanism of TNF electrode
In general, oxygen-containing conjugated organic cathode materials with carbonyl and nitro groups as their main functional groups exhibit high discharge capacity but poor cycle life and rate performance. Therefore, scientists have conducted modification studies on these materials, and some modification schemes and research results are as follows:
(1) Increasing the content of conductive carbon. This not only inhibits the dissolution of active materials to a certain extent, but also improves the conductivity of the electrode. For example, organic active materials (calix, aromatic hydrocarbons, CQ, derivatives of benzoquinone) can be covalently grafted onto the surface of conductive carbon particles (carbon black CB) or inorganic nano-SiO2 to obtain CB/CQ and SiO2/CQ composite active materials. Better cycle performance and rate performance are achieved by sacrificing the specific capacity of the electrode material.
Figure 7 Cyclic stability performance of CB/CQ (red) and SiO2/CQ (black) composite active materials
(2) The solubility of organic matter in electrolyte is reduced by lithium/sodium salting, and the contact area between organic matter and conductive material is increased by nano-sized organic carbonyl lithium/sodium salt compounds, shortening the Li+ diffusion channel and improving energy density.
Scientists studied the electrochemical performance of disodium cyclopentenetriketoate at different particle sizes. The results showed that particles with a diameter of 150 nm have better electrochemical performance and better cycle stability as cathode materials.
Figure 8. Schematic diagram of the charge-discharge mechanism of disodium cyclopentenyltriketoate.
(3) The solubility of oxygen-containing compounds in the electrolyte solution is reduced by the polymerization process, thereby improving the cycle life of the battery.
The designed and synthesized porous pyrene-4.5.9.10-tetraketone polymer (PPYT) has an initial discharge capacity of 231 mAh/g, and its specific capacity is still maintained at 83% of the initial capacity after 500 cycles. The specific capacity at 30C can be maintained at 90% at 1C.
Figure 9. Electrochemical reaction mechanism and cycle life of PPYT
• Outlook
Future research should build upon existing findings by designing organic compounds with unique functional group structures. For example, substituting the aforementioned oxygen-containing conjugated groups into macrocyclic conjugated structures can achieve lithium-ion insertion and extraction during charging and discharging, while utilizing multiple substituted active sites to achieve higher theoretical specific capacity. Macrocyclic conjugated systems can reduce solubility in electrolytes, further improving the discharge capacity and cycle stability of lithium-ion batteries, and also enhance conductivity. Green and sustainable energy is the future direction of development.
