The battery systems: normalAl/normalNaCl‐AlCl3‐Al2X3/normalNi‐normalfelt false(X=S,normalSe,normalTefalse) and the corresponding system without chalcogen have been studied experimentally at 175°C. Charge/discharge experiments, performed on cells with normalNaCl saturated melts, show that advantages with regard to rate capability and cyclability can be obtained with systems containing dissolved chalcogen compared with the chalcogen‐free system. Exchange of chalcogen between cathode and electrolyte during cycling was confirmed by performing gravimetric analysis and Raman spectroscopy of the electrolytes. Cathode reactions were studied by coulometric titrations (performed on cells with slightly acidic normalNaCl‐AlCl3 melts and small amounts of chalcogen) and compared with model calculations. Cells containing chalcogen revealed at least three voltage plateaus during cycling. The lowest plateau is associated with formation/decomposition of essentially NiySz and NiySez in the sulfide and selenide system, respectively. Cells containing selenide revealed extra capacity below the NiySez‐normalplateau , most probably associated with a AlvNiySez compound. On the second plateau of sulfide systems NiCl2 or a NiySzCl2y−2z compound with y>false(4.4±0.2false)·z is formed during charging. Reduction of the formed compound to Ni takes place via consumption of sodium chloride. For acidic melts, sulfide at the cathode was found to be present asnormalClSnormalCl/\Al\//\Al\/normalClnormalClnormalCl−species.

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A theoretical description of the thermodynamic properties of the battery systems: normalAl/normalNaCl‐AlCl3‐Al2X3/normalNi‐normalfelt false(X=normalthinsp;S,normalSe,normalTefalse) and the corresponding system without chalcogen has been provided for cells with basic to slightly acidic normalNaCl‐AlCl3 melts containing small amounts of chalcogen. The model developed describes the equilibrium concentrations of constituent species in the electrolyte and equilibrium potentials of the electrodes vs. number of coulombs passed through the cells. For cells without chalcogen curves were calculated under the assumption of NiCl2 formation showing corresponding variation of anode potential, cathode potential, and cell voltage as a function of electrolyte composition. For sulfide containing cells the plateau of lowest potential has been found to be associated with essentially pure nickel sulfide, NiySz . A procedure for model fitting to the cathode potential curve‐form of the NiySz‐normalplateau is presented. A general equation is proposed for the reaction taking place along the second plateau.

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