The invention relates to a compound of the formula Li.sub.7-xPS.sub.6-xX.sub.x-z(BH.sub.4).sub.z, in which x is selected from the group comprising Cl, Br, I, F and CN, 0≤x≤2, 0≤z≤0.50. This compound can be used as a solid electrolyte of a lithium-ion electrochemical element.

The invention relates to a compound of the formula Li.sub.7xPS.sub.6xX.sub.xx(BH.sub.4).sub.x in which x is selected from the group comprising Cl, Br, I, F and CN, 0x2, 0z0.50. This compound can be used as a solid electrolyte of a lithium-ion electrochemical element.

Thermal energy storage (TES) systems based on metal hydride pairs using new class of high efficiency materials are evaluated. The use of low temperature metal cost effective material such hydrides NaAlH4 and Na3AlH6 became possible. In order to obtain high efficiency at reasonable cost high temperature materials were altered by the addition of materials to form reversible alloys and hydrides. The compounds were cycled to determine stability of hydrogen capacity over extended number of cycling. A thermal energy storage system based on two metal hydride pairs such as CaAl/CaH2/Al:NaAlH.sub.4, Ca.sub.2Si/CaH.sub.2/Si:Na.sub.3AlH.sub.6 and NaMgH.sub.2FSi/Mg2SiNaF:Na.sub.3AlH.sub.6 allows low cost and high efficiency performance.

A hydride ion conductor is represented by:

MAMBH.sub.4-xF.sub.xFormula (1), where MA is selected from the group consisting of Ca, Sr, and Ba, MB is selected from the group consisting of Mg and Ca, and is different from MA, and x is 0<x<4.