Precursor Ratio-Controlled Chloride Vacancies in Lead-Free KCuCl3-x Memristors for Forming-Free Gradual Resistive Switching and Neuromorphic Learning

Defect engineering of memristive materials offers a powerful strategy for regulating ionic transport and analog resistive switching required for neuromorphic computing. Here, we report a wet chemical synthesis route for chloride-vacancy (VCl) engineering in lead-free all-inorganic KCuCl3-x thin films and establish a direct correlation between defect concentration, electronic structure, transport dynamics, and neuromorphic performance. Controlled precursor-ratio variation systematically tunes the VCl concentration in KCuCl3-x thin films, driving preferential vacancy formation at the terminal chloride sites, band-gap narrowing, and enhanced electronic disorder. W/KCuCl3-x/FTO memristors exhibit stable bipolar VCM-type resistive switching, with increasing VCl concentration promoting forming-free operation. An optimal VCl concentration (~ 1.3*10^21 cm^-3) provides a balance between ionic mobility and defect controllability, resulting in highly linear potentiation/depression characteristics and superior pulse endurance (>104 cycles). When the experimentally measured conductance characteristics of the optimized memristor are implemented as synaptic weights in artificial neural network simulations, recognition accuracies of 98.96% and 91.82% are achieved on the MNIST and Fashion-MNIST datasets, respectively. These findings establish precursor ratio-controlled VCl engineering as an effective strategy for simultaneously optimizing ionic transport, analog switching, and neuromorphic learning in lead-free halide perovskite memristors, providing a general materials design principle for energy-efficient neuromorphic electronics.