Positive electrode reactions

Fig. 2. Standard potentials of battery (a) negative electrode and (b) positive electrode reactions (13). Potentials are given in volts.

The mechanism of the positive electrode reactions is very complex and even for the lower plateau operation involves numerous electrochemical and chemical steps. One of the intermediates, the so called phase, seems to inhibit the reduction kinetics and limit the cell performance. Measures which eliminate this intermediate bring about some undesirable effects, such as higher corrosion rates, narrowing of operating temperature range, increased cost, and—in the case of the use of CU2S as an additive— possibility of short-circuiting due to precipitation of Cu in the separator. 

A great diversity of positive electrode reactions have also been described and/or used in commercial cells. These include the following ... 

The second molten salt battery to have received detailed attention is the lithium/ iron sulphide battery. During discharge, the negative electrode reaction is the dissolution of Hthium from a lithium/aluminium alloy (10—20% Li), while the positive electrode reaction is the reduction of iron disulphide which occurs in stages... 

Reaction (10.15) is not totally reversible and after the first discharge the positive electrode reaction is better represented by the equation... 

The positive electrode reaction involves the reduction of O2 and proceeds in two basic steps ... 

More interesting is the commonly encountered situation where an ion diffuses in a majority electronic conductor. Thus, diffusion in metallic and semiconducting alloys or of inserted species in transition metal oxides and chalcogenides fall into this category. Many electrode reactions are of this type. Lithium diffusion in j5-LiAl and other alloys is of interest in negative electrode reactions for advanced hthium batteries hydrogen and lithium diffusion in oxides (e.g. VeOn) and sulfides (e.g. TiSa) are of importance as positive electrode reactions for batteries and elec-trochromic devices. 

Battery chemistry Solvent/el (rolyte Positive electrode reaction Ceil voltagc/V Energy den ity/W i kg Power density... 

Currently, two mechanisms are used for positive electrode reactions a liquid phase mechanism and a solid phase mechanism. 

Battery chemistry Solvent/electrolyte Positive electrode reaction Cell voltage/V Energy density/Wh kg Power density... 

Lithium-polysulfide (Li-PS) flow batteries originated from Li-S batteries. Still using lithium metal as the negative electrode, Li-PS flow cells utilize a porous carbon electrode and soluble lithium polysulfides (Li2Sx where 8 x 4) as the positive electrode and positive electrolyte, respectively. The positive electrode reactions can be described as ... 

Alkaline gel electrolytes based on potassium-salt poly (acrylic acid) (PAAK), typically, PAAK-KOH-H2O was used in metal hydride reactions to enhance the charge-discharge reactions and capacity retention [62]. A silica-based gel electrolyte was used in lead dioxide-positive electrode reaction to give superior discharge capacity and greatly reduced the rate of electrode corrosion [63]. The Na2S04-PAAK gel electrolyte was employed in the middle to separate acid and alkaline chambers with an AEM and a CEM. Detailed information on preparation of gel electrolytes and setup is reported in Ref. [56]. 

The current is conducted by migration of ions - positive ones (cations) to the cathode (negative electrode), and negative ones (anions) to the anode (positive electrode). Reactions take place at the electrodes by transfer of electrons to or from them. 

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