Soft and flexible: core-shell ionic liquid resistive memory for electronic synapses

Resistive switching memory (RSM) devices are promising for next-generation memory due to simple fabrication, stability, and ultrafast low-voltage switching. Since Chua’s memristor concept and HP’s solid-state demonstration, RSMs have been explored for neuromorphic computing to emulate synaptic functions. Biological synapses operate efficiently via ion-mediated neurotransmission, motivating liquid-based soft RSMs that leverage cation/anion transport for neuromorphic behavior. This work targets a soft, flexible electronic synapse by employing an ionic liquid electrolyte where mobile ions enable filamentary conduction via electrochemical metallization of active electrodes (Cu). To achieve controllable, multistate resistive switching akin to synaptic weight modulation, the study introduces an Ag@AgCl core-shell species into the ionic liquid to regulate Cu²⁺ transport, enabling stable multilevel states at low voltages in a Cu/Ag@AgCl/Cu configuration.

Prior studies established memristors/memristive systems as candidates for memory and neuromorphic functions, with reported advantages in simple structure and low-power operation. Liquid-based RSMs using electrolytes, liquid metals, and ionic liquids have been investigated for flexibility, high ionic conductivity, low viscosity, and facile fabrication. These devices often operate via electrochemical metallization, where active metal cations (e.g., Cu²⁺) migrate and reduce to form/dissolve conductive filaments, producing resistive switching. However, precise control of ionic motion and filament dynamics remains challenging for achieving stable multistate synaptic behavior, motivating approaches (such as adding functional ionic species) to modulate ion transport and polarization.

Device concept and structure: A soft ionic-liquid-based memristive synapse with a Cu/Ag@AgCl/Cu configuration was developed. Initially, FeCl₃ in glycerol served as the IL electrolyte; Ag nanoparticles were then introduced to form an Ag@AgCl core-shell species within the electrolyte to control Cu²⁺ transport and enable multistate switching.

Electrolyte preparation: FeCl₃ concentration in glycerol was optimized (0.2 wt% FeCl₃ used) to achieve a high OFF/ON ratio. Ag nanoparticles added to FeCl₃ IL resulted in in situ Ag@AgCl core-shell formation (details referenced in Materials and Methods).

Materials characterization: The Ag@AgCl core shell was characterized by FESEM (average particle size ~100–120 nm), EDS (presence of Ag, Fe, Cl), FTIR (functional group identification with characteristic peaks at 3340, 2971, 2890, 1658, 1398, 1317, 1138, 1087, 1048, 855, 647 cm⁻¹), and XRD. Ag thin films exhibited a strong (111) fcc peak at 2θ = 38.02° (polycrystalline, average crystallite size ~100 nm via Debye–Scherrer). Ag@AgCl showed peaks at 2θ = 28.5°, 32.6°, 45.7°, 55.4°, 63.8° corresponding to (111), (200), (220), (222), (400) planes with minor impurity peaks (Ag, FeCl₂) and average crystallite size ~32 nm.

Electrical measurements: For FeCl₃ IL devices (Cu/FeCl₃/Cu), DC I–V sweeps assessed memristive behavior, including semilog I–V, charge–flux analysis (derived from time-dependent I–V), and endurance over 100 cycles with read at 0.15 V. For Ag@AgCl core-shell IL devices (Cu/Ag@AgCl/Cu), dual sweep I–V tests probed multistate switching in positive and negative regions, and capacitance-like behavior near the origin was noted (memristor‖capacitor equivalent). Pulse protocols were applied to emulate synaptic plasticity: 100 repetitive pulses with width = 1 ms at various amplitudes to assess spike-rate-dependent plasticity; additional tests varied pulse width (200 μs, 400 μs, 600 μs, 800 μs, 1 ms) at 1.5 V for 100 pulses. Triangular voltage sweeps (five cycles, period 2 s) monitored time-varying conduction in both polarities.

• Material characterization: Ag@AgCl core-shell particles average ~100–120 nm (FESEM). FTIR confirmed multiple functional groups with peaks at 3340, 2971, 2890, 1658, 1398, 1317, 1138, 1087, 1048, 855, 647 cm⁻¹. Ag thin films showed a dominant (111) fcc peak at 2θ = 38.02° with ~100 nm crystallite size; Ag@AgCl XRD peaks at 28.5°, 32.6°, 45.7°, 55.4°, 63.8° with ~32 nm crystallites and minor Ag/FeCl₂ impurities.
• FeCl₃ IL memristive behavior: Cu/FeCl₃/Cu devices exhibited pinched hysteresis loops passing through the origin, asymmetric time-domain charge and symmetric flux characteristics, indicating nonideal memristive behavior. Endurance over 100 I–V cycles showed stable performance with HRS ≈ 161.9 kΩ and LRS ≈ 15.4 kΩ at 0.15 V (OFF/ON ≈ 10.5:1).
• Ag@AgCl core-shell IL multistate switching: Introducing Ag@AgCl into the IL enabled controlled Cu²⁺ transport, yielding multistate resistive switching under dual voltage sweeps in both polarities. The device exhibited decreasing current (increasing resistance) with consecutive sweeps, consistent with synaptic weight modulation; capacitance-like features near the origin supported a memristor‖capacitor equivalent.
• Mechanism: Multistate behavior arises from (i) Ag@AgCl core-shell moderation of Cu²⁺ mobility and (ii) electrochemical metallization/demetallization dynamics (Cu²⁺ formation at anode, reduction and filament dynamics at cathode, diffusion-driven saturation and filament rupture).
• Synaptic emulation: Spike-rate-dependent plasticity was demonstrated. With 100 pulses at 1.5 V, increasing pulse width enhanced ionic diffusion and reduced current more strongly: at 1 ms, current decreased from ~136 μA to ~97 μA, whereas at 200 μs little change was observed. Triangular sweeps (2 s period) showed nonlinear time-varying current in both voltage polarities, consistent with gradual conductance modulation.

The study addresses the challenge of achieving controllable, multilevel resistive states in soft ionic-liquid memristive devices for neuromorphic applications. By incorporating an Ag@AgCl core-shell species into the FeCl₃-based IL, the device effectively regulates Cu²⁺ transport, enabling stable multistate resistive switching that mimics synaptic weight changes. The observed nonideal memristive characteristics in the FeCl₃ system become functionally advantageous when combined with the core-shell mechanism, as consecutive voltage sweeps and tailored pulse trains produce gradual conductance modulation analogous to synaptic plasticity and spike-rate-dependent plasticity. The capacitance-like response and memristor‖capacitor equivalence further align the device behavior with biological synapses where ionic dynamics determine signal transmission. These findings demonstrate that soft, flexible IL-based memristive platforms can emulate key neuromorphic functions at low operating voltages, highlighting their relevance for electronic synapses and soft bio-inspired electronics.

A core-shell ionic-liquid-based soft resistive memory device (Cu/Ag@AgCl/Cu) was presented, demonstrating controllable multistate switching suitable for synaptic emulation. The Ag@AgCl core shell effectively limits Cu²⁺ motion, producing stable, low-voltage multilevel states, while material characterizations confirmed the core-shell structure. Electrical tests showed reliable endurance (100 cycles) and synaptic-like modulation under dual sweeps and pulse protocols, including spike-rate-dependent plasticity with pulse-width-dependent conductance change. These results position core-shell IL memristors as promising candidates for soft, flexible neuromorphic systems and electronic synapses, potentially opening a pathway toward brain-inspired computation in soft electronics.

• The FeCl₃-based device exhibited nonideal memristive behavior (asymmetric pinched hysteresis in charge characteristics), which may affect precise analog weight updates.
• Reported endurance was demonstrated over 100 I–V cycles; longer-term cycling, retention, and variability were not detailed in the provided text.
• The OFF/ON resistance ratio (~10.5:1 at 0.15 V) is moderate and may limit noise margins for some applications.
• Detailed fabrication parameters and electrolyte composition optimization (beyond 0.2 wt% FeCl₃ and Ag@AgCl formation) are referenced but not fully disclosed in the provided text, limiting reproducibility assessment.