Topological dual double node-line semimetals NaAlSi(Ge) and their potential as cathode material for sodium ion batteries†
Using a first-principles method and an effective model, we propose that the real material family NaAlSi(Ge) behaves as dual double node-line semimetals (DNLSs) with unconventional topological surface states. The dual DNLSs locate in kz = 0 and kz = π/a planes, respectively. Both of the DNLSs consist of homocentric two nodal lines: one is a type-I nodal line and the other one is a type-I nodal line embedded with four type-III nodal points. The type-III nodal points accompanied by a single-direction flat band are proposed to be a new platform for studying correlated phases. More intriguingly, because of the large proportion of Na ions, the NaAlSi(Ge) family is a potential candidate for topological sodium ion batteries (SIBs) with a high theoretical reversible capacity (476(238) mA h g−1) and suitable migration barrier.