polarity for charge and discharge. (The term ‘cathode’ is often mistakenly used to identify the positive electrode in the rechargeable lithium cell. The terminology was established for primary cells where discharge is the only process. For rechargeable cells the terminology is correct for discharge only. During discharge, the positive electrode accepts electrons (is reduced) and therefore is correctly identified as the cathode. The negative (lithium) electrode where Li+ is formed is the anode. During charge the process is reversed. The positive electrode donates electrons (is oxidized) and therefore becomes the anode. The negative lithim electrode accepts electrons (is reduced) and therefore is the cathode.) Types of Rechargeable Lithium Cells Rechargeable lithium cells can be classified into four types; organic, polymeric, inorganic, and molten salt electrolyte systems (See Fig. 1). The organic and inorganic electrolyte systems operate at ambient temperature, the polymeric electrolyte system operates somewhat above ambient temperature and the molten salt system operates at high temperature. The organic electrolyte rechargeable lithium cell utilizes a solid positive electrode (solid cathode—see explanation above). Positive electrodes of this type include titanium disulfide (TiS2), molybdenum disulfide (MoS2), niobium triselenide (NbSe3), manganese dioxide (MnO2), vanadium pentoxide (V2O5) and cobalt oxide (CoO2). The TiS2, MoS2 and NbSe3 are known as transition metal chalcogenides. The first two compounds have a two-dimensional layered structure, the latter has a onedimensional chain structure. MnO2, V2O5 and CoO2 are transition metal oxides and they exist in a three-dimensional network structure. Lithium ions (Li+) are inserted into a host structure electrochemically during discharge. The host lattice undergoes only a minor structural change during the lithium intercalation (insertion) process (discharge) and it returns to its original state on charge when the Li+ deintercalates.
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