A method of forming a crosslinked polymer and an anion, the method comprising the step of reacting a first polymeric substituent comprising a nucleophilic group and a second polymeric substituent comprising an electrophilic group wherein the first polymeric substituent is a substituent of a first polymer and the second polymeric substituent is a substituent of the first polymer or a second polymer. The first and second polymers may be non-conjugated or conjugated. The crosslinked polymers may be used in electrochemical devices, for example battery cells.