Electroreduction of nickel (II) chloride, nickel (II) fluoride and tungsten (VI) oxide mixtures in a heat activated battery

The paper presents the results of studies of the discharge characteristics of elements of a heat activated battery (HAB) containing mixtures of NiCl₂-NiF₂-WO₃ as a positive electrode. It is shown that the addition of tungsten oxide to a mixture of lithium halides makes it possible to increase the removable current density and discharge voltage. It has been established that the reason for the increase in these electrical characteristics is the presence of tungstate compounds (nickel tungstate, lithium tungstate) in the reduction products, which are formed during the operation of the studied HAB elements in a stationary mode. These compounds have a sufficiently high conductivity, which makes it possible to reduce the passivation of the positive electrode and reduce the internal resistance of the HAB element. The optimal composition of the cathode mixture for the studied discharge conditions of HAB elements has been determined. The maximum capacity of the discharge plateau is 0.4 A×h×g^–1, the voltage of the discharge plateau varies from 2.40 to 1.65 V, depending on the density of the discharge current. The reduction products of NiCl₂–NiF₂–WO₃ cathode mixtures were studied using XRD, SEM and STA methods. It has been established that nickel halides, which are part of the studied cathode mixtures, are reduced to metal and lithium halides by a two-electron mechanism, according to the electrochemical reaction: NiX₂+Li⁺+2e^–→Ni+2LiX, where X is Cl, F. The reduced nickel forms a metallic dendritic sponge, the density of which, under equal discharge conditions, is determined by the ratio of the components in the initial cathode mixture. The pores of the dendritic sponge are partially filled with a salt fraction based on lithium halides. The reduction of tungsten oxide to metal has an intermediate stage of formation in tungstate compounds occurring during the stationary operation of the HAB element. In the reduction products of cathode mixtures with a content of up to 5 wt.% of tungsten oxides, the formation of LiCl–Li₂O solid solutions is observed. At higher concentrations of the oxide component in the cathode mixtures, zones containing pure lithium oxide are formed in the salt fraction of the reduction products. Tungsten is deposited on the surface of nickel dendrites, forming areas in the form of dotted inclusions. The STA curves of the salt fraction formed during electrochemical reactions have a single thermal effect corresponding to the temperatures of co–melting of a solid solution of LiCl–Li₂O and a triple mixture of lithium halides LiF–LiCl–LiBr.