A rechargeable electrochemical cell (10) comprises an anode compartment (14) and a cathode compartment (15), the compartments being enclosed in part by metal plates (11, 12) that define end faces of the cell (10), the two compartments (14, 15) being separated by an impermeable, ion-conducting electrolyte element (13). The cell (10) is a molten sodium/metal chloride cell, and the plate (12) enclosing the cathode compartment defines a slightly concave end face surrounded by a flat rim. The volume change of the cathodic materials as the cell is heated up to its operating temperature are accommodated by flattening of the concave end face, so the overall thickness of the cell (10) does not change.

A battery cell capable of self-priming with molten metal produced within the battery cell includes a cathode compartment configured to contain a catholyte that releases metal ions, an anode compartment configured to receive electrons from an external power supply, an ion-selective membrane positioned between the cathode compartment and the anode compartment and configured to selectively transport the metal ions from the cathode compartment to the anode compartment when self-priming the battery cell, and an electrically conductive coating on a surface of the ion-selective membrane facing the anode compartment and configured to distribute the electrons received from the external power supply across the ion-selective membrane when self-priming the battery cell. Self-priming includes combining the electrons with the metal ions arriving at an interface between the electrically conductive coating and the ion-selective membrane to produce the molten metal within the anode compartment.

An electrolyte includes a composite salt mixture formed from a halogen-free closo-borate salt and a halogenated closo-borate salt. The halogen-free closo-borate salt includes a first cation selected from Li.sup.+, Na.sup.+, Mg.sup.2+, or Ca.sup.2+, and a closo-borate anion with the structure [B.sub.yH.sub.(yz)R.sub.z].sup.2, [CB.sub.(y1)H.sub.(yz)R.sub.z].sup., [C.sub.2B.sub.(y2)H.sub.(yt1)R.sub.t].sup., [C.sub.2B.sub.(y3)H.sub.(yt)R.sub.t].sup., or [C.sub.2B.sub.(y3)H.sub.(yt1)R.sub.t].sup.2, and a second cation selected from Li.sup.+, Na.sup.+, Mg.sup.2+, or Ca.sup.2+, and a halogenated closo-borate anion with the structure [B.sub.yH.sub.(yzi)R.sub.zX.sub.i].sup.2, [CB.sub.(y1)H.sub.(yzi)R.sub.zX.sub.i].sup., [C.sub.2B.sub.(y2)H.sub.(ytj1)R.sub.tX.sub.j].sup., [C.sub.2B.sub.(y3)H.sub.(ytj)R.sub.tX.sub.j].sup., or [C.sub.2B.sub.(y3)H.sub.(ytj1)R.sub.tX.sub.j].sup.2. The parameter y is an integer within a range of 6 to 12, z is an integer within a range of 0 to y, t is an integer within a range of 0 to (y1), z+i is an integer within a range of 0 to y, t+j is an integer within a range of 0 to y1, R is a linear, branched-chain, or cyclic C1-C18 alkyl or fluoroalkyl group, and X is F, Cl, Br, and/or I.

A battery cell includes a cathode compartment configured to contain a catholyte that releases metal ions, an anode compartment configured to receive electrons from an external power supply, an ion-selective membrane positioned between the cathode compartment and the anode compartment and configured to selectively transport the metal ions from the cathode compartment to the anode compartment when self-priming the battery cell, and an electron transport structure configured to provide electrons to at least one of the ion-selective membrane or an electrically conductive coating on the ion-selective membrane without a molten metal within the anode compartment when self-priming the battery cell, such that the electrons are combined with the metal ions arriving at an interface between the electron transport structure and the ion-selective membrane when self-priming the battery cell to produce the molten metal within the anode compartment.

An electrolyte includes a composite salt mixture with between 0.975 and 0.05 mole fraction of a halogen-free closo-borate salt and between 0.025 and 0.95 mole fraction of a halogenated closo-borate salt, and a first activation energy before a heat treatment and a second activation energy after the heat treatment, the second activation energy being less than the first activation energy.

An electrochemical cell includes electrolyte includes a cathode, an anode, and an inorganic boron cluster solid state electrolyte that has a metal cation selected from the group consisting of Li.sup.+, Na.sup.+, K.sup.+, Mg.sup.2+, Ca.sup.2+, Zn.sup.2+, and Al.sup.3+, a composite salt mixture includes between 0.975 and 0.05 mole fraction of a first boron cluster salt and between 0.025 and 0.95 mole fraction of a second boron cluster salt. In some variations the composite salt mixture includes a third boron cluster salt and can have an activation energy less than 0.65 eV at one or more temperatures above -30.sup.oC. The electrochemical cell can also include a catholyte, an anolyte, and/or a separator, and the catholyte, the anolyte, and/or the separator can include the inorganic boron cluster solid state electrolyte.