Abstract: A new composition of matter corresponds to the formula A.sub.x M(PS.sub.3).sub.y wherein A is at least one Group Ia metal, x is a numerical value greater than zero but equal to or less than 6 divided by the valency of the element, M is at least one transition metal selected from the group consisting of nickel or iron, P is phosphorus, S is sulfur and y is a numerical value between about 0.9 and 1.0. These materials are useful as cathode materials in electrochemical cells and as thermal collectors from solar radiation.Compositions of the present invention are used in fabricating rechargeable electrochemical cells in the discharged state which comprise an anode containing as the anode-active material at least one Group Ia metal; a cathode containing as the cathode-active material a non stoichiometric trisulfide of the formula A.sub.x M(PS.sub.3).sub.