Investigation of electronic properties and sodium ion diffusion in Na4Ni5(PO4)2(P2O7)2 polyanionic compound for Na-ion batteries

Researchers are exploring sodium-ion batteries as a potential alternative for storing energy. These batteries are attractive because sodium is plentiful, inexpensive, and safer than some other materials used in batteries. Employing the first principles Density Functional Theory, the electronic properties of sodiated Na 4 Ni 5 (PO 4 ) 2 (P 2 O 7 ) 2 (NNPO) and desodiated Ni 5 (PO 4 ) 2 (P 2 O 7 ) 2 (NPO) have been investigated. Sodiation refers to the process of inserting sodium ions into the material. The complete linear synchronous transit (LST)/quadratic synchronous transit (QST) algorithm was considered to investigate the migration of the Na-ion in Na 4 Ni 5 (PO 4 ) 2 (P 2 O 7 ) 2 . The calculations of the Na-ion diffusion barrier, ionic conductivity, and diffusion coefficient were also carried out. As the main results, both Na 4 Ni 5 (PO 4 ) 2 (P 2 O 7 ) 2 and Ni 5 (PO 4 ) 2 (P 2 O 7 ) 2 as a host material exhibit metallic behavior. As a novelty, we report a high sodium ionic conductivity estimated to 4.74 × 10 −5 S/cm and a high open circuit voltage equal to 4.6 V. Both values make this material a suitable cathode for Na-ion batteries. Discussion in terms of challenges and efficiency improvement is addressed as well. • E a is equal to 0.97 eV, which is in good agreement with the experimental data. • The high open circuit voltage corresponds to the redox potential of Ni 3+ /Ni 2+ . • The presence of pyrophosphates (P 2 O 7 ) stabilizes the structure upon sodiation. • Both NNPO and NPO have stable structures, ensuring good reversibility of NNPO.