Sodium　titanate　(Na2Ti7O15)　with　tunnel　structure　has　been　experimentally　proposed　to　be　a　good　candidate　for　anode　material　of　sodium　ion　batteries　because　of　its　high　reversible　capacity　and　excellent　stability.　In　this　work,　the　first　principle　calculations　have　been　performed　to　investigate　the　variations　of　the　structures　and　electronic　properties　of　Na2Ti7O15after　inserting　different　amount　of　Na　atoms.　Based　upon　ab　initio　molecular　dynamics　simulations,　the　most　stable　structures　of　various　Na2+xTi7O15compounds　have　been　determined.　From　a　thermodynamical　point　of　view,　Na2Ti7O15can　be　sodiated　to　Na3.5Ti7O15with　a　theoretical　capacity　of　64.6　mA　h/g,　while　further　addition　of　sodium　results　in　a　negative　intercalation　potential　due　to　the　significant　destruction　of　the　tunnel　structure.　During　the　sodium　insertion,　the　small　lattice　expansion　indicates　the　good　structural　retaining　ability　of　Na2+xTi7O15compounds,　however,　the　configurations　of　four　bridging　TiO6octahedra　in　the　unit　cell　that　connect　different　zigzag　edge-sharing　octahedra　are　distorted　obviously.　Consequently,　the　intercalation　of　Na　atoms　has　remarkable　influences　on　the　distributions　of　3d　states　of　those　Ti　atoms　belonged　to　above　bridging　octahedra.　Furthermore,　compared　to　Na2Ti7O15,　the　introducing　of　Na　atoms　leads　to　the　upward　movement　of　the　Fermi　level,　and　correspondingly　the　metallic　state　is　predicted　for　the　sodiated　Na2+xTi7O15,　suggesting　the　enhancement　of　the　electronic　conductivity　of　the　system.　©　2016　Elsevier　B.V.