Neuromorphic electronics has received considerable attention recent years, and its basic functional units are synaptic electronic devices. A two-terminal artificial synapse with sandwiched structure emulates plasticity of the biological synapses under the action of nerve-like electrical impulse signals. In this paper, P3 phase Na_2/3Ni_1/3Mn_2/3O₂ multi-element metal oxides with layered structure are synthesized by sol-gel process. Owing to the fact that Na⁺ is easy to embed/eject into its crystal structure, an ion-migrating artificial synapse based on Na_2/3Ni_1/3Mn_2/3O₂ is designed and fabricated. The device emulates important synaptic plasticity, such as excitatory postsynaptic current, paired-pulse facilitation, spike-number dependent plasticity, spike-frequency dependent plasticity, spike-voltage amplitude dependent plasticity and spike-duration dependent plasticity. The device realizes the identification and response to Morse code commands.