Lithium redox potential

Beginning in the early 1980s [20, 21] metallic lithium was replaced by lithium insertion materials having a lower standard redox potential than the positive insertion electrode this resulted in a "Li-ion" or "rocking-chair" cell with both negative and positive electrodes capable of reversible lithium insertion (see recommended papers and review papers [7, 10, 22-28]). Various insertion materials have been proposed for the anode of rechargeable lithium batteries,... 

From a thermodynamic point of view, apart from charge density and specific charge, the redox potential of lithium insertion into/removal from the electrode materials has to be considered also. For instance, the redox potential of many Li alloys is between -0.3 and -1.0 V vs. Li/Li+, whereas it is only -0.1 V vs. 

Figure 2. Redox potentials for lithium insertion into/removal from several anode materials for lithium cells.

The cathode materials employed for the early lithium-based systems were 3.0 V class oxides or sulfides thus, the redox potential for the additive should be located in the neighborhood of 3.2—3.5 V. Accordingly, the first generation redox additive proposed by Abraham et al. was based on the iodine/ iodide couple, which could be oxidatively activated at the cathode surface at 3.20 V and then reduced at the lithium surface. " " " 2° For most of the ether-based solvents such as THF or DME that were used at the time, the oxidation potential of iodide or triiodide occurred below that of their major decompositions, while the high diffusion coefficients of both iodine and iodide in these electrolyte systems ( 3 x 10 cm s ) offered rapid kinetics to shuttle the overcharge current. Similarly, bromides were also proposed.Flowever, this class of halide-based additives were deemed impractical due to the volatility and reactivity of their oxidized forms (halogen). 

The effect of these ferrocene-based additives on overcharge protection is shown in Figure 44, where AA cells based on lithium, LhMn02, and electrolytes with or without additives were overcharged. In the absence of these redox shuttles (A), the cell voltage continues to rise, indicating the occurrence of major irreversible decompositions within the cell whereas the presence of shuttle agents (B—E) locks the cell potential in the vicinity of their redox potentials... 

Redox shuttles based on aromatic species were also tested. Halpert et al. reported the use of tetracyano-ethylene and tetramethylphenylenediamine as shuttle additives to prevent overcharge in TiS2-based lithium cells and stated that the concept of these built-in overcharge prevention mechanisms was feasible. Richardson and Ross investigated a series of substituted aromatic or heterocyclic compounds as redox shuttle additives (Table 11) for polymer electrolytes that operated on a Li2Mn40g cathode at elevated temperatures (85 The redox potentials of these... 

Overcharge tests were carried out in LiCo02 cathode half-cells that contained these additives, and a redox shuttle effect was observed between 4.20 and 4.30 V, close to the redox potentials of these additives. The same shuttling effect was observed even after 2 months of storage for these cells, indicating the stability and redox reversibility of these additives. A closer examination of the capacity retention revealed that 4-bromo-l,2-dimethoxybenzene seemed to have the best shuttle-voltage performance for the 4.0 V lithium cell used." The stability of these additives against reductive decomposition was also tested by the authors on metallic lithium as well as on carbonaceous anodes, and no deterioration was detected. 

Lithium insertion negative electrodes — (i) Some transition-metal oxides or chalcogenides insert Li ion reversibly at low redox potentials, for example, TiC>2, LL I iOy, M0S2, M0O2. (ii) Lithium alloys - in this case lithium ions, react with other elements polarized to low potentials to reversibly form Li alloys. The reaction usually proceeds reversibly according to the... 

The high theoretical specific capacity (3862 Ah kg ) and capacity density (2047 Ah L" ) of the lithium-metal electrode, together with its promising redox potential, give the battery unique advantages in terms of specific energy and energy density. 

In general, the thermodynamics of free radicals has been, up to now, almost unknown due to their extremely high reactivity and electroactivity. For clarification, several methods were proposed [225,226] from anodic half-wave potentials of carbanions and organo-lithium compounds. Redox catalysis may strongly help to estimate redox potential of a certain number of radicals generated electrochemically. Thus, as seen previously in Scheme 4 and in Scheme 6, in the absence of other side reactions, there are two main different ways... 

The (C4p ) compound can be cathodically reduced in dipolar aprotic solutions of lithium salts. The redox potential is about the same as that of (CF,) however, the reduction is kinetically more hindered . 

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