Cathode materials and lithium primary cells

In practical terms, the twin objectives of protecting the lithium from corrosion while avoiding unacceptable levels of voltage delay can be considered to have been met, However, the detailed mechanisms of film formation and disruption are still matters of some controversy. In particular, the interaction of thin films formed rapidly on lithium surfaces exposed to the atmosphere with the thicker films formed by subsequent reaction with the cathodic reagent is not well understood, 

Four main groups of compounds may be distinguished polycarbon fluorides, oxosalts, oxides and sulphides. 

10 Lithium-(CFT)ri cylindrical cell (based on the design of Matsushita (BR435), by permission.) 

Li(s)IUBF4.BL-DMEI(CFJ() (s) with associated cell reaction 

It is believed that the discharge mechanism involves the formation of an intermediate lithium intercalation compound in which both lithium and fluorine are situated between the carbon layers of the graphitic structure. The carbon formed is graphitic and improves the cell performance as the discharge progresses, leading to a high cathode utilization - close to 100% for low currents. The lithium fluoride precipitates. 

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Lithium primary batteries with liquid cathodes are a relatively mature technology. Incremental improvements in capacity and performance may occur through design modifications and the use of new materials such as improved carbons in the passive cathode. The U.S. Army is adopting Lithium/Manganese Dioxide replacements for some of the Lithium/Sulfur Dioxide Batteries listed in Table 1 in certain applications. These replacements provide higher capacity and energy at room temperature but not at lower temperature. See the chapter on Lithium Primary Cells Solid Cathodes in this work. 

Preliminary electrochemical tests of materials obtained have been performed in two types of cells. Primary discharge measurements have been executed in standard 2325 coin-type cells (23 mm diameter and 2.5 mm height) with an electrolyte based on propylene carbonate - dimethoxyethane solution of LiC104. Cathode materials have been prepared from thermally treated amorphous manganese oxide in question (0.70 0.02g, 85wt%.) mixed with a conductive additive (10 % wt.) and a binder (5wt%). Lithium anodes of 0.45 mm thickness have been of slightly excess mass if compared to the stoichiometric amount, so as to ensure maximal possible capacity of a cell and full consumption of the cathode material. 

Electrolytes are generally lithium salts dissolved in aprotic solvents or added to the liquid electroactive molecules as supporting electrolytes to carry the current through the cell. Before describing a number of primary cells of technical and commercial importance, the general properties of the electrolytes, the lithium anode and the more common cathode materials will be discussed. Secondary cells are discussed in Chapter 7. 

While the development of primary cells with a lithium anode has been crowned by relatively fast success and such cells have filled their secure rank as power sources for portable devices for public and special purposes, the history of development of lithium rechargeable batteries was full of drama. Generally, the chemistry of secondary batteries in aprotic electrolytes is very close to the chemistry of primary ones. The same processes occur under discharge in both types of batteries anodic dissolution of lithium on the negative electrode and cathodic lithium insertion into the crystalline lattice of the positive electrode material. Electrode processes must occur in the reverse direction under charge of the secondary battery with a negative electrode of metallic lithium. Already at the end of the 1970s, positive electrode materials were found, on which cathodic insertion and anodic extraction of lithium occur practically reversibly. Examples of such compounds are titanium and molybdenum disulfides. 

Typically, a lithium-ion cell electrode is a composite, also called a laminate, consisting of the following agglomerated components (1) primary particles of active intercalation materials and (2) inactive materials coated and pressed (calendered) onto the metalhc current collecting foil (Cu for the anode, A1 for the cathode). The group of inactive materials typically consists of conductive additive(s) and polymeric binder. 

When discussing cathodes for primary cells, it is important to make a distinction between solid materials, mentioned above, and those in which the cathode electrochemical component is not in the physical electrode structure, but rather exist in a liquid or gas state. Examples include zinc air cells, lithium thionyl chloride cells, and lithium sulfur dioxide cells. In each case, the physical cathode stmcture serves as a catalytic reaction... 

Lithium batteries use nonaqueous solvents for the electrolyte because of the reactivity of lithium in aqueous solutions. Organic solvents such as acetonitrile, propylene carbonate, and dimethoxyethane and inorganic solvents such as thionyl chloride are typically employed. A compatible solute is added to provide the necessary electrolyte conductivity. (Solid-state and molten-salt electrolytes are also used in some other primary and reserve lithium cells see Chaps. 15, 20, and 21.) Many different materials were considered for the active cathode material sulfur dioxide, manganese dioxide, iron disulfide, and carbon monofluoride are now in common use. The term lithium battery, therefore, applies to many different types of chemistries, each using lithium as the anode but differing in cathode material, electrolyte, and chemistry as well as in design and other physical and mechanical features. 

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