The binding mechanism of 11Li is investigated on the basis of the 9Li+n+n three-cluster model. To understand the weakly-bound structure creating the neutron-halo structure, we solve the 9Li + n + n three-cluster motion using the reliable 9Li-n interaction constructed on the basis of the recent experimental data for 10Li of Bohlen et al. The neutron-neutron correlation is carefully investigated by employing the extended basis functions described by the hybrid-VT model. The result for the binding energy exhibits considerable improvement but is still small in comparison with experiments. To address this shortcoming, an additional correlation accompanied by the pairing excitation of the 9Li-core is discussed.