Lithium electronic conductivity

A second type of soHd ionic conductors based around polyether compounds such as poly(ethylene oxide) [25322-68-3] (PEO) has been discovered (24) and characterized. These materials foUow equations 23—31 as opposed to the electronically conducting polyacetylene [26571-64-2] and polyaniline type materials. The polyethers can complex and stabilize lithium ions in organic media. They also dissolve salts such as LiClO to produce conducting soHd solutions. The use of these materials in rechargeable lithium batteries has been proposed (25). 

The situation in beryllium metal is more complex. We might expect all of the 2s molecular orbitals to be filled because beryllium has the electron configuration ls22s2. However, in a crystal of beryllium, the 2p MO band overlaps the 2s (Figure 5). This means that, once again, there are vacant MOs that differ only infinitesimally in energy from filled MOs below them. This is indeed the basic requirement for electron conductivity it is characteristic of all metals, including lithium and beryllium. 

There is a difference in the behavior of benzenediolatoborate and naphthalenedio-latoborate solutions on the one hand, and lithium bis[2,2 -biphenyldiolato(2-)-0,0 ] borate (point 5 in fig. 8) lithium bis[ sali-cylato (2-) Jborate (point 6) or benzene-diolatoborate/phenolate mixed solutions on the other (Fig.8). This can be tentatively explained by the assumption of different decomposition mechanisms due to different structures, which entail the formation of soluble colored quinones from benzenediolatoborate anions and lithium-ion conducting films from solutions of the latter compounds (points 5 and 6) [80], The assumption of a different mechanism and the formation of a lithium-ion conducting, electronically insulating film is supported by... 

An example of a layer structure mixed conductor is provided by the cathode material L CoC used in lithium batteries. In this solid the ionic conductivity component is due to the migration of Li+ ions between sheets of electronically conducting C0O2. The production of a successful mixed conductor by doping can be illustrated by the oxide Cei-jPxx02- Reduction of this solid produces oxygen vacancies and Pr3+ ions. The electronic conductivity mechanism in these oxides is believed to be by way of electron hopping between Pr4+ and Pr3+, and the ionic conductivity is essentially vacancy diffusion of O2- ions. 

Fig. 8.11 Partial lithium ion conductivity of the predominantly electronically conducting compound Lij+jSb as a function of stoichiometry. The variation in the conductivity is due to changes of the transport mechanism.

As noted above, the lithium ions flow through the electrolyte whereas the electrons generated from the reaction, Li = Li+ + e, go through the external circuit to do work. Thus, the electrode system must allow for the flow of both lithium ions and electrons. That is, it must be both a good ionic conductor and an electronic conductor. As discussed below, many electrochemically active materials are not good electronic conductors, so it is necessary to add an electronically conductive material such as carbon... 

By contrast, the organic polymers used in lithium batteries have negligible electronic conductivity but are ionic conductors. 

It is perhaps useful to mention that the use of electronically conducting polymers, such as poly(acetylene), (CH) poly(pyrrole), (C4H5N)X, and poly(aniline), (CeHgNf ), has been proposed for positives for lithium batteries. The electrochemical process of these lithium-polymer positives is somewhat similar to an intercalation reaction. On charging, the polymer (P) is oxidized by acquiring a positive charge to form a polaron, and this is... 

The lithium polymer battery (LPB), shown schematically in Fig. 7.21, is an all-solid-state system which in its most common form combines a lithium ion conducting polymer separator with two lithium-reversible electrodes. The key component of these LPBs is the polymer electrolyte and extensive work has been devoted to its development. A polymer electrolyte should have (1) a high ionic conductivity (2) a lithium ion transport number approaching unity (to avoid concentration polarization) (3) negligible electronic conductivity (4) high chemical and electrochemical stability with respect to the electrode materials (5) good mechanical stability (6) low cost and (7) a benign chemical composition. 

The isopiestic and manometric methods (units A2j A2.4) for determination of water activity have the limitation of being dependent on fixed laboratory equipment. The electronic-type sensors have advantages of portability, speed, and simplicity of measurement. The characteristics of a sensor depend upon the manufacturer and each instrument must be calibrated separately. The anodized sensors have advantages of ruggedness, small dimensions, and fast response, as well as freedom from large temperature coefficients and less susceptibility to contamination of the lithium chloride conductivity sensors (Smith, 1971). 

The topic of this book is focused on active masses containing carbon, either as an active mass (e.g., negative mass of lithium-ion battery or electrical double layer capacitors), as an electronically conducting additive, or as an electronically conductive support for catalysts. In some cases, carbon can also be used as a current collector (e.g., Leclanche cell). This chapter presents the basic electrochemical characterization methods, as applicable to carbon-based active materials used in energy storage and laboratory scale devices. 

Carbon is used in lithium-ion cells for different functions conductive carbon black and/or graphite additives are applied in both the negative and the positive electrode to improve the electronic conductivity of the electrodes. These conductive additives constitute a fraction of up to about 10% of the total carbon consumption. The major fraction is represented by the active carbon materials which are electrochemically reduced and oxidized in the negative electrode during the battery charge and discharge process, respectively. 

The electrolyte must be a pure ionic conductor, preferably with a high transport number for lithium ions, as an electronic conductivity of the electrolyte would create short-circuit ( leakage ) currents between the electrodes. Both electrodes must have a high electronic conductivity and a sufficient ionic conductivity for lithium. The metal current collectors foils (current collectors) are pure electron conductors that allow only electrons to migrate to the external electric leads to the consumer or charger unit. 

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