Compound

A compound of formula (I): wherein X is aluminium or boron; R1 in each occurrence is independently a monovalent substituent; R2 is a divalent organic group; and M+ is a cation. Preferably, X is boron, M+ is a lithium ion, each R1 is independently selected from C1-20 alkyl wherein one or more C atoms other than the C atom bound to O of OR1 or a terminal C atom may be replaced with O, and one or more H atoms may be replaced by F, and R2 is a divalent organic group of formula (II): wherein R3 in each occurrence is independently H or a substituent and Ar1 is a C6-20 arylene group or heteroarylene group. More preferably, X is boron, M+ is a lithium ion, Ar1 is unsubstituted or substituted 1,2-phenylene, each R1 is 2,2,3,3,4,4,5,5-octafluoropentyl, Ar1 is unsubstituted 1,2-phenylene, and each R3 is trifluoromethyl. The compound of formula (I) may be used as an electrolyte in a metal battery or a metal ion battery.