Lithium-carbon materials

Lithium-carbon materials are, in principle, no different from other lithium-containing alloys. However, since this topic is treated in more detail in Chapter III, Sec. 5, only a few points will be briefly discussed here. 

TM oxide insertion 293-321 lithium carbon batteries, secondary 45 lithium carbon materials 361... 

Carbon Materials. In the lithium-ion cell, carbon materials, which can reversibly accept and donate significant amounts of lithium (Li C = 1 6) without affecting their mechanical and electrical properties, can be used for the anode instead of metallic lithium. Carbon material is used as an anode in lithium-ion cells since the chemical potential of lithiated carbon material is close to that of metallic lithium, as shown in Fig. 34.2. Thus an electrochemical cell made with a lithiated carbon material will have almost the same open-circuit voltage as one made with metallic lithium. In practice, the lithium-ion cell is manufactured in a fully discharged state. Instead of using lithiated carbon material, which is air-sensitive, the anode is made with carbon and lithiation is carried out by subsequent formation of the cell. 

Manufacture. Lithium fluoride is manufactured by the reaction of lithium carbonate or lithium hydroxide with dilute hydrofluoric acid. If the lithium carbonate is converted to the soluble bicarbonate, insolubles can be removed by filtration and a purer lithium fluoride can be made on addition of hydrofluoric acid (12). High purity material can also be made from other soluble lithium salts such as the chloride or nitrate with hydrofluoric acid or ammonium bifluoride (13). 

Lithium Carbonate. Lithium carbonate [554-13-2], Li2C02, is produced in industrial processes from the reaction of sodium carbonate and Hthium sulfate or Hthium chloride solutions. The reaction is usually performed at higher temperatures because aqueous Hthium carbonate solubiHty decreases with increasing temperatures. The solubiHty (wt %) is 1.52% at 0°C, 1.31% at 20°C, 1.16% at 40°C, 1.00% at 60°C, 0.84% at 80°C, and 0.71% at 100°C. Lithium carbonate is the starting material for reactions to produce many other Hthium salts, including the hydroxide. Decomposition of the carbonate occurs above the 726°C melting point. 

Figure 3 shows the voltage-capacity relation for lithium/carbon electrochemieal cells made from representative materials from each of the three regions of Fig. 2. 

For convenience and simplicity, the electrochemical study of electrode materials is normally made in lithium/(eleetrode material) eells. For earbonaeeous materials, a hthium/carbon eell is made to study electroehemical properties, sueh as eapaeity, voltage, eyeling life, etc.. Lithium/carbon coin cells use metallie lithium foil as the anode and a partieular carbonaceous material as the... 

Lithium/carbon cells are typically made as coin cells. The lithium/carbon coin cell consists of several parts, including electrodes, separator, electrolyte and cell hardware. To construct a coin cell, we first must prepare each part separately. Successful cells will lead to meaningful results. The lithium/carbon coin cells used metallic lithium foil as the anode and a carbonaceous material as the cathode. The metallic lithium foil, with a thickness of 125 pm, was provided by Moli Energy (1990) Ltd.. Idie lithium foil is stored in a glove-box under an argon atmosphere to avoid oxidation. 

Two electrochemical lithium/carbon cells were made for each of the pyrolyzed materials. We used currents of 18.5 mA/g (20-hour rate) for the fust three charge-discharge cycles and 37 mA/g (10-hour rate) for the extended cycling test. 

Chapter 11 reports the use of carbon materials in the fast growing consumer eleetronies applieation of lithium-ion batteries. The principles of operation of a lithium-ion battery and the mechanism of Li insertion are reviewed. The influence of the structure of carbon materials on anode performance is described. An extensive study of the behavior of various carbons as anodes in Li-ion batteries is reported. Carbons used in commereial Li-ion batteries are briefly reviewed. 

Carbon materials which have the closest-packed hexagonal structures are used as the negative electrode for lithium-ion batteries carbon atoms on the (0 0 2) plane are linked by conjugated bonds, and these planes (graphite planes) are layered. The layer interdistance is more than 3.35 A and lithium ions can be intercalated and dein-tercalated. As the potential of carbon materials with intercalated lithium ions is low,... 

There are many kinds of carbon materials, with different crystallinity. Their crystallinity generally develops due to heat-treatment in a gas atmosphere ("soft" carbon). However, there are some kinds of carbon ("hard" carbon) in which it is difficult to develop this cristallinity by the heat-treatment method. Both kinds of carbon materials are used as the negative electrode for lithium-ion batteries. 

There are some other matters that should be considered when comparing metallic lithium alloys with the lithium-carbons. The specific volume of some of the metallic alloys can be considerably lower than that of the carbonaceous materials. As will be seen later, it is possible by selection among the metallic materials to find good kinetics and electrode potentials that are sufficiently far from that of pure lithium for there to be a much lower possibility of the potentially dangerous forma-... 

In batteries of this type, solntions of lithium salts in thionyl chloride, SOCI2, are nsed as the electrolyte. Exceptionally, this strongly oxidizing solvent also serves as the active material for the cathodic reaction. Thus, during discharge thionyl chloride is electrochemicaUy reduced at a cathode made of carbon materials ... 

Suitable polymer anodes have not been found instead, in the search for improved lithium hosts, lithium-inserting carbon materials have been developed and batteries produced, usually with a metal oxide or sulfide anode (hence they are not considered fnrther here). 

As to anodes, in most of the research work a generously dimensioned sheet of lithium metal has been used. Such an electrode is rather irreversible, but this is not noticed when a large excess of lithium is employed. Li-Al alloys and carbon materials inserting lithium cathodically during recharging can be used as anodes in nonaqueous solutions. Zinc has been used in polymer batteries with aqueous electrolyte (on the basis of polyaniline). 

Fichter and Kern O first reported that uric acid could be electrochemically oxidized. The reaction was studied at a lead oxide electrode but without control of the anode potential. Under such uncontrolled conditions these workers found that in lithium carbonate solution at 40-60 °C a yield of approximately 70% of allantoin was obtained. In sulfuric acid solution a 63% yield of urea was obtained. A complete material balance was not obtained nor were any mechanistic details developed. In 1962 Smith and Elving 2) reported that uric acid gave a voltammetric oxidation peak at a wax-impregnated spectroscopic graphite electrode. Subsequently, Struck and Elving 3> examined the products of this oxidation and reported that in 1 M HOAc complete electrochemical oxidation required about 2.2 electrons per molecule of uric acid. The products formed were 0.25 mole C02,0.25 mole of allantoin or an allantoin precursor, 0.75 mole of urea, 0.3 mole of parabanic acid and 0.30 mole of alloxan per mole of uric acid oxidized. On the basis of these products a scheme was developed whereby uric acid (I, Fig. 1) is oxidized in a primary 2e process to a shortlived dicarbonium ion (Ha, lib, Fig. 1) which, being unstable, under-... 

Practically every battery system uses carbon in one form or another. The purity, morphology and physical form are very important factors in its effective use in all these applications. Its use in lithium-ion batteries (Li-Ion), fuel cells and other battery systems has been reviewed previously [1 -8]. Two recent applications in alkaline cells and Li-Ion cells will be discussed in more detail. Table 1 contains a partial listing of the use of carbon materials in batteries that stretch across a wide spectrum of battery technologies and materials. Materials stretch from bituminous materials used to seal carbon-zinc and lead acid batteries to synthetic graphites used as active materials in lithium ion cells. 

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. 

In this paper, new highlights are proposed to interpret the reversible and irreversible capacities of nanostructured carbons in lithium batteries. A proportional dependence between the irreversible capacity and the active surface area (ASA) of carbon materials will be demonstrated, showing the ASA concept more universal than any of the other parameters which were previously considered. In-situ 7Li NMR will be also presented as a means to... 

In this paper, we presented new information, which should help in optimising disordered carbon materials for anodes of lithium-ion batteries. We clearly proved that the irreversible capacity is essentially due to the presence of active sites at the surface of carbon, which cause the electrolyte decomposition. A perfect linear relationship was shown between the irreversible capacity and the active surface area, i.e. the area corresponding to the sites located at the edge planes. It definitely proves that the BET specific surface area, which represents the surface area of the basal planes, is not a relevant parameter to explain the irreversible capacity, even if some papers showed some correlation with this parameter for rather low BET surface area carbons. The electrolyte may be decomposed by surface functional groups or by dangling bonds. Coating by a thin layer of pyrolytic carbon allows these sites to be efficiently blocked, without reducing the value of reversible capacity. 

Zheng T., Dahn J.R., Applications of carbon in Lithium-ion batteries, In Carbon materials for advanced technologies, Burchell T.D. Ed., Elsevier, Oxford, 1999, pp 341-388. 

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