The material stability of ramsdellite type Li2+x Ti3O7 has been investigated from the view of the correlation between structural and electrochemical behaviors during Li insertion and extraction. The interfacial reaction of Li2+x Ti3O7 is identified by using AC-impedance spectroscopy and various electrochemical measurements are carefully conducted to interpret each elemental reaction of the complicated interfacial reaction. The impedance spectra indicate three semi-circles in the frequency range from 1MHz to 5mHz which fit expectations well. Li+ insertion at the interface is explained by applying the adatom model that includes desolvation and absorption of lithium ions through electron transfer and lattice incorporation.