Overcharging electrode potential

The applied condition represents a relatively large positive deviation of the single-electrode potential for a cathode from the oxidation potential of the redox couple [R]/[0], For a single-electron reaction at room temperature, the above criterion for the deviation Ec — E corresponds to RJInF = 0.026 V, and one would therefore expect the simplification that leads to eq 13 to hold true for most of the overcharge situations encountered in practical applications. 

When the cell is charged galvanostatically, the potentials of the two electrodes increase. When the electrode potentials reach the potentials of H2 and O2 evolution, gassing starts. During overcharge, the basic electrochemical reaction that occurs is that of water decomposition and evolution of H2 and O2. When 1 Ah of electricity passes through the electrode, 0.3661 g of H2O is decomposed to 0.0367 g of H2 and 0.2985 g of O2. 

The redox shuttle molecule then diffuses back to the positive electrode for the next redox cycle, and the electrons move from the positive electrode to the negative electrode through the external circuit. During normal operation, the redox potential of the redox shuttle is not reached and the molecules stay inactive. When the cell is overcharged, the potential of the positive electrode increases, and the redox cycle of the redox shuttle molecules is activated. The net reaction of the redox ( cle is to shuttle the charge forced by the external circuit through the lithium-ion cell without also forcing intercalation/ deintercalation of lithium in the electrodes of the cell. 

The hydrogen evolution reaction occurs because the electrode potential for this reaction is positive to that of the iron electrode reaction (Eq. 2.4). Therefore, these batteries will have to be overcharged by 60-100% to achieve their full capacity. The hydrogen evolution that occurs during charging is undesirable because it lowers the round-trip energy efficiency and results in loss of water from the electrolyte. 

It has been shown [15] that the insulating a-PbO layer forms on lead-sheet electrodes at high positive potentials via an oxidation process which is the equivalent of overcharge. Furthermore, a-PbO can be produced at the grid interface during selfdischarge of an acid-starved battery at low states-of-charge [16]. There is evidence... 

An intercalation potential very close to the reduction of Li ions may also lead to metallic Li electrodeposition on the graphite electrode. The metallic lithium formed in this way is a finely divided powder that, unsurprisingly, is highly reactive, making the batteiy veiy unstable. This situation may occur in the event of an accidental overcharge of the battery. Practically, today all the commercial battery packs include electronics that monitor the batteiy and prevent overchai ng. 

As described in the relevant report, the tert-aUcylbenzene compound decomposes by oxidation at a potential of +4.6 to +5.0 V (relative value to that of lithium), and cobalt or nickel in the positive electrode rapidly dissolves and deposits on the negative electrode to inhibit a reaction of a carbonate in the non-aqueous electrolytic solution with a lithium metal deposited on the negative electrode. Further, in the invention, the internal short circuit may be formed in the battery by the deposition of cobalt and nickel, whereby the overcharge inhibitive effect can be attained and the safety of battery can be assured [133]. 

To prevent overcharge and lithium plating, batteries are typically manufactured with an excess capacity of the negative electrode. However, even with excess negative capacity, lithium can deposit if the potential drop between negative electrode and electrolyte reaches 0 V (vs Li- -/Li). This condition occurs in graphite electrodes upon fast charge, or... 

The redox shuttle [13-15, 17] is an electrolyte additive that can be reversibly oxidized/reduced at a characteristic potential and provides an intrinsic overcharge protection for lithium-ion batteries that neither increases the complexity and weight of control circuitry nor permanently disables the cell when activated. The redox shuttle molecule (S) has its defined redox potential, at which it can be oxidized on the positive electrode and form a radical cation (S ) (see Equation 1). 

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