Anode materials carbonaceous

One criterion for the anode material is that the chemical potential of lithium in the anode host should be close to that of lithium metal. Carbonaceous materials are therefore good candidates for replacing metallic lithium because of their low cost, low potential versus lithium, and wonderful cycling performance. Practical cells with LiCoOj and carbon electrodes are now commercially available. Finding the best carbon for the anode material in the lithium-ion battery remains an active research topic. 

The Li-Ion system was developed to eliminate problems of lithium metal deposition. On charge, lithium metal electrodes deposit moss-like or dendrite-like metallic lithium on the surface of the metal anode. Once such metallic lithium is deposited, the battery is vulnerable to internal shorting, which may cause dangerous thermal run away. The use of carbonaceous material as the anode active material can completely prevent such dangerous phenomenon. Carbon materials can intercalate lithium into their structure (up to LiCe). The intercalation reaction is very reversible and the intercalated carbons have a potential about 50mV from the lithium metal potential. As a result, no lithium metal is found in the Li-Ion cell. The electrochemical reactions at the surface insert the lithium atoms formed at the electrode surface directly into the carbon anode matrix (Li insertion). There is no lithium metal, only lithium ions in the cell (this is the reason why Li-Ion batteries are named). Therefore, carbonaceous material is the key material for Li-Ion batteries. Carbonaceous anode materials are the key to their ever-increasing capacity. No other proposed anode material has proven to perform as well. The carbon materials have demonstrated lower initial irreversible capacities, higher cycle-ability and faster mobility of Li in the solid phase. 

This new formulation of electrolytes based on a mixture of EC with a linear carbonate set the main theme for the state-of-the-art lithium ion electrolytes and was quickly adopted by the researchers and manufacturers. Other linear carbonates were also explored, including DEC, ° ethylmethyl carbonate (EMC), ° and propylmethyl carbonate (PMC), ° ° and no significant differences were found between them and DMC in terms of electrochemical characteristics. The direct impact of this electrolyte innovation is that the first generation carbonaceous anode petroleum coke was soon replaced by graphitic anode materials in essentially all of the lithium ion cells manufactured after 1993. At present, the electrolyte solvents used in the over one billion lithium ion cells manufactured each year are almost exclusively based on the mixture of EC with one or more of these linear carbonates, although each individual manufacture may have its own proprietary electrolyte formulation. 

As a result, the acid strength of the proton is approximately equivalent to that of sulfuric acid in nonaqueous media. In view of the excellent miscibility of this anion with organic nonpolar materials, Armand et al. proposed using its lithium salt (later nicknamed lithium imide , or Lilm) in solid polymer electrolytes, based mainly on oligomeric or macro-molecular ethers. In no time, researchers adopted its use in liquid electrolytes as well, and initial results with the carbonaceous anode materials seemed promising. The commercialization of this new salt by 3M Corporation in the early 1990s sparked considerable hope that it might replace the poorly... 

Because of the similar potentials between fully lithiated graphite and lithium metal, it has been suggested that the chemical nature of the SEIs in both cases should be similar. On the other hand, it has also been realized that for carbonaceous anodes this formation process is not expected to start until the potential of this anode is cathodically polarized (the discharge process in Figure 11) to a certain level, because the intrinsic potentials of such anode materials are much higher than the reduction potential for most of the solvents and salts. Indeed, this potential polarization process causes one of the most fundamental differences between the SEI on lithium metal and that on a carbonaceous anode. For lithium metal, the SEI forms instantaneously upon its contact with electrolytes, and the reduction of electrolyte components should be indiscriminate to all species possible,while, on a carbonaceous anode, the formation of the SEI should be stepwise and preferential reduction of certain electrolyte components is possible. 

Endo et al. investigated the reductive decomposition of various electrolytes on graphite anode materials by electron spin resonance (ESR). In all of the electrolyte compositions investigated, which included LiC104, LiBF4, and LiPFe as salts and PC, DMC, and other esters or ethers as solvents, the solvent-related radical species, which were considered to be the intermediates of reductive decomposition, were detected only after prolonged cathodic electrolysis. With the aid of molecular orbital calculation, they found that the reduction of salt anion species is very difficult, as indicated by their positive reduction enthalpy and that of free solvent (A/4 — 1 kcal mol ). However, the coordination of lithium ions with these solvents dramatically reduces the corresponding reduction enthalpy (A/ —10 kcal mol ) and renders the reaction thermodynamically favored. In other words, if no kinetic factors were to be considered, the SEI formed on carbonaceous anodes... 

A wide variety of carbonaceous materials can intercalate or insert lithium reversibly and thus may be candidates for anodes for lithium ion batteries. In recent years, many types of carbons have been tested as alternative anodes for rechargeable lithium batteries, part of which have found use as anodes in practical, commercial lithium ion batteries. The most straightforward way of classifying these electrodes is according to the type of the carbon, which determines their capacity and basic electrochemical behavior. The major types of carbons tested in recent years as anode materials for Li ion batteries are listed below ... 

A land-based arrangement for CP in soil is shown in Figure 10.18. A backfill of a carbonaceous material such as coke breeze or graphite particles is used around the anode. The backfill is electron-conducting, which makes the effective size of the anode larger. This in turn reduces the resistance between the anode and the structure, and it reduces the consumption of anode material. If the resistivity of the soil is very high, buried pipelines may be protected by means of a continuous anode along the pipeline at a suitable distance from it. 

In this case a potassium-graphite (KCg) electrode has been used as the carbonaceous anode material. Upon anodic polarisation this electrode irreversibly deintercalates potassimn resulting in a graphite-like compound, which on subsequent cycles performs with fast kinetics of its lithium intercalation-deintercalation process [89, 90]. Accordingly, the first charging process of the battery may be written as shown in Equation 7.7 at the anode. 

Among so many kinds of carbonaceous materials, the practical anode materials most widely used in commercialized lithium-ion batteries can be roughly classified into three categories hard carbon, and natural and synthetic graphite. All these types of carbons have different advantages and disadvantages, which will be explained in the following concrete examples. 

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