Doping Enhanced Charge Transportation in NASICONs for Energy Storage

With 3D framework structure and special corner-sharing characteristics, natrium superionic conductors (NASICONs) can vary their crystal structure with temperature change or on cation insertion/desertion, serve as both electrode and electrolyte materials, which make them fascinating in energy storage applications. However, those structure characteristics also bring about challenges on the understandings of charge transport mechanism, such as the precise cation/electron diffusion passage and the means to enhance or adjust it. Herein this review, we take typical NASICONs, the single ion conductor, Na3Zr2Si2PO12(NZSP) and ion/electron mixed conductor, Na3V2(PO4)3(NVP) as the examples, and briefly review the studies on charge transport mechanism as well as the strategy to enhance it. We find that doping is the mostly viable method to increase both ionic and electronic conductivity, while, considering the different roles that NASICON may act in energy storage, special attention should be paid on the doping effect. Typical doping elements were classified by their functions, such as working on the SE bulk, grain boundary, or enrich the vacancies. We found that some elements could enhance both σiof NZSP and σeof NVP, while most of the σeresults were obtained on pallet level, therefore, advanced particle-level electronic conductivity testing method or instrument is required.