A solid-state electrolyte comprising a composite salt mixture comprising: a metal cation selected from the group consisting of Li+, Na+, K+, Mg2+, Ca2+, Zn2+, Al3+, and any combination thereof; and a plurality of weakly coordinating anions (WCAs) including 0.5 mole percent (mol %) to 90 mol % of each WCA present therein, each WCA including a same core group of atom(s) and differing from any other WCA in a number, a type, and/or a position of any substituent of the core group of atom(s), the composite salt mixture has a cationic conductivity that is one to four orders of magnitude greater than a cationic conductivity of a corresponding salt comprising the metal cation and any WCA of the plurality of WCAs alone. An electrochemical cell comprising: an anode, a cathode, and the solid-state electrolyte.