Interfacial Ionic Modulation in a Solid Polymer Electrolyte-Based Ionotronic Memristor for Neuromorphic Learning and...

Solid polymer electrolyte (SPE)-based memristors usually exhibit predictable digital nonvolatile switching with limited conductance tunability, restricting their use in neuromorphic systems. This behavior arises because of mobile cations and anions in SPEs that rapidly form robust conductive filaments via a positive-feedback process, producing sharp set/reset transitions. Here, we demonstrate that by employing a suitable pulse scheme, a nonfilamentary pathway can be accessed through interfacial ionic modulation, leading to gradual conductance modulation relevant for neuromorphic computing. We fabricated CuBr-dissolved polyethylene oxide (Cu-SPE) based memristors that show conventional digital filamentary switching on DC bias sweep mode. The optimized device (2 wt % CuBr) achieved an ON/OFF ratio of >103 with average set and reset voltages of +0.65 V and -0.7 V, respectively, and improved uniformity. The electrochemical characterization reveals a low Cu+ transport number (tCu+ ∼ 5%), with the majority of ionic current carried by anions, leading to concentration polarization during the biasing conditions. By harnessing this polarization effect and modulating Cu+ ion injection via tailored pulse schemes, the Cu-SPE memristor reproducibly emulates synaptic behaviors across temporal scales, including paired-pulse facilitation and depression (PPF/PPD), pulse-amplitude and pulse-width dependent synaptic weight modulation, and Hebbian learning rules. The device also reproduces key nociceptive responses including threshold firing, no-adaptation, and sensitization. It can, further, follow Pavlov's classical conditioning and act as a Morse code generator. A control experiment replacing the copper electrode with gold confirms that interfacial ionic modulation drives the neuromorphic behavior. These results establish that SPE-based memristors can simultaneously support digital filamentary memory and operate in a nonfilamentary, ionically mediated regime similar to fluidic memristors, thereby broadening their applicability in neuromorphic hardware.