In the early research stage of carbon as anode material for lithium-ion batteries, most of the work is to find anode materials suitable for lithium-ion batteries, which is mainly due to the diversity and multifaceted nature of carbon materials. With the development of research, researchers have a deeper and more comprehensive grasp of the key factors that control the electrochemical performance of carbon. A full understanding of lithium-intercalation carbon helps battery developers to design and modify carbon to meet the requirements of lithium-ion battery applications, and then in the process of carbon processing, carbon cognition has been further tested and adjusted. Therefore, in repeated studies, the electrochemical performance of carbon has been improved step by step to approach the application target in Li-ion batteries. To learn more about lithium-ion battery anode materials, please click to enter the website.
A series of modifications to carbon is the doping of heteroatoms (carbon alloys). For example, Dahn et al. adsorbed silicon into carbon by cracking silicon-containing resin [131~135]. Since each silicon can have four lithium bond stations, the silicon-containing carbon negative electrode obtained a high reversible capacity. . The effects of phosphorus, sulfur, and nitrogen doping have also been extensively studied [136–141]. The most prominent effect on carbon-based doping is boron, because boron atoms can enter the graphite layer in large quantities and reduce structural stress. Doping of boron element can promote the diffusion of carbon atoms, increase the crystal size, and improve the crystallinity of carbon during heat treatment. These factors can improve the reversible capacity of lithium storage. Several research groups have also continued to study the doping of boron in different carbon materials [142~148].
In addition, a series of studies on the modification of carbon surfaces have also been carried out. Takamura and his co-workers have done extensive research in this regard since the realization that Li+ intercalation into carbon surfaces for the first time is a prerequisite in all intercalation processes [148~154]. The simplest carbon surface modification methods focus on functional groups, such as removing some unfavorable functional groups on the carbon surface (such as -OH[155] on the carbon surface or introducing -COOH[104], -I[156] and so on. Certain functional groups are prone to forming SEI films and intercalating Li+. Notably, Peled et al. [157] observed mild oxidation, which can form nanochannels or micropores and form a dense layer of oxide on the graphite surface, thus Both the reversible capacity and the Coulombic efficiency are improved. In addition, mild oxidation on the graphitized MCMB surface can remove the thin carbon layer on the particle surface, increasing the rate capacity [158,159].
The researchers also demonstrated that mild oxidation can suppress the exfoliation of the graphite layer in PC-based electrolytes [69]. In fact, a more popular trend in surface modification is to coat the carbon surface with a film, called a “shell-core” structure. The core material is usually graphite because it is more sensitive to the electrolyte, while the shell material can vary, including metals or alloys, metal oxides, conducting polymers, and other types of carbon. The material of the shell can be active or inert to the storage of Li+. The former type seems to be more valuable for reversible capacity. Carbon, similar to nuclear materials to a certain extent, is one of the most promising cladding materials [160,161].
Another interesting case is the mixing effect of graphite-coke layers and graphite-hard carbon mixtures [162~164]. The most attractive advantage of graphite anode material is that its operating voltage is very low and flat, but this can also be a disadvantage from another perspective: due to the end of Li+ deintercalation from graphite (i.e. the discharge process of Li-ion batteries), graphite The working voltage of the negative electrode will suddenly increase after three plateaus, which will cause the overall working voltage of the lithium-ion battery to drop sharply. For users of lithium-ion batteries, it is very inconvenient to use if they do not know how much capacity is left in the battery. The Sanyo research team believes that the rapid change of the charge and discharge voltage of the graphite electrode in the lithium-ion battery in the lower voltage region will cause side reactions such as electrolyte decomposition, which is due to the uneven voltage distribution across the graphite electrode. , from this point of view, disordered carbon with inclined operating voltage is superior to graphite. One solution is to mix graphite and disordered carbon in an appropriate ratio to suppress the generation of inactive lithium due to electrolyte decomposition during long-term charge-discharge cycles in large lithium-ion batteries.
Read more: What is the charge-discharge mechanism of cathode materials?
