Lithium-ion Cell Materials in Practice

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. 

In addition to carbon-based systems, other intercalation compounds are also currently being proposed as alternative lithium ion cell negative plates. Examples include LiJtTiS2, Li/TiC, L /H-sO and, more recently, a family of Li SnOv compounds. However, the applicability of these materials in practical batteries has not yet been established, and coke and graphite are still the materials used in all commercial lithium ion cells. 

The rheological phase reaction method offers a new and simple way to prepare compounds and materials. The prepared compounds LiCo YyMn2.x.y04 show excellent capacity and cycleability. The cathode behavior of the LiMn204 can be greatly improved by simultaneous doping of cobalt and yttrium, and our results on LiCo, YyMn2.x-y04 show that the composition with x=0.02, y=0.015 exhibits the best high current rate (IC) capacity up to 60 cycles. The compound after optimization may find potential application in practical lithium-ion cells. 

The rate of this process in aprotic electrolytes is rather high the exchange current density is fractions to several mA/cm. As pointed out already, the first contact of metallic lithium with electrolyte results in practically the instantaneous formation of a passive film on its surface conventionally denoted as solid electrolyte interphase (SEI). The SEI concept was formulated yet in 1979 and this film still forms the subject of intensive research. The SEI composition and structure depend on the composition of electrolyte, prehistory of the lithium electrode (presence of a passive film formed on it even before contact with electrode), time of contact between lithium and electrolyte. On the whole, SEI consists of the products of reduction of the components of electrolyte. In lithium thionyl chloride cells, the major part of SEI consists of lithium chloride. In cells with organic electrolyte, SEI represents a heterogeneous (mosaic) composition of polymer and salt components lithium carbonates and alkyl carbonates. It is essential that SEI features conductivity by lithium ions, that is, it is solid electrolyte. The SEI thickness is several to tens of nanometers and its composition is often nonuniform a relatively thin compact primary film consisting of mineral material is directly adjacent to the lithium surface and a thicker loose secondary film containing organic components is turned to electrolyte. It is the ohmic resistance of SEI that often determines polarization of the lithium electrode. 

To be able to understand how computational approaches can and should be used for electrochemical prediction we first of all need to have a correct description of the precise aims. We start from the very basic lithium-ion cell operation that ideally involves two well-defined and reversible reduction and oxidation redox) reactions - one at each electrode/electrolyte interface - coordinated with the outer transport of electrons and internal transport of lithium ions between the positive and negative electrodes. However, in practice many other chemical and physical phenomena take place simultaneously, such as anion diffusion in the electrolyte and additional redox processes at the interfaces due to reduction and/or oxidation of electrolyte components (Fig. 9.1). Control of these additional phenomena is crucial to ensure safe and stable ceU operation and to optimize the overall cell performance. In general, computations can thus be used (1) to predict wanted redox reactions, for example the reduction potential E ) of a film-forming additive intended for a protective solid electrolyte interface (SEI) and (2) to predict unwanted redox reactions, for example the oxidation potential (Eox) limit of electrolyte solvents or anions. As outlined above, the additional redox reactions involve components of the electrolyte, which thus is a prime aim of the modelling. The working agenda of different electrolyte materials in the cell -and often the unwanted reactions - are addressed to be able to mitigate the limitations posed in a rational way. 

The practical electrochemical parameters (actual cell capacity, cell voltage, etc.) are strongly related to the theoretical thermodynamic calcnlalions and are usually diminished by a certain factor because of the occurrence of various real-life usage losses. The most important theoretical properties of battery materials (electrochemical potential of the cell, cell s theoretical capacity, and energy) are derived from thermodynamics of the electrode reactions in lithium-ion cell (Table 1.1). A comprehensive, in-depth discussion of thermodynamics of the processes occurring in a lithium-ion cell can be found elsewhere [4]. Some of the most crucial formulas are Usted below. 

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. 

In the lithium-ion approach, the metallic lithium anode is replaced by a lithium intercalation material. Then, tw O intercalation compound hosts, with high reversibility, are used as electrodes. The structures of the two electrode hosts are not significantly altered as the cell is cycled. Therefore the surface area of both elecftodes can be kept small and constant. In a practical cell, the surface area of the powders used to make up the elecftodes is nomrally in the 1 m /g range and does not increase with cycle number [4]. This means the safety problems of AA and larger size cells can be solved. 

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. 

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