High Conductivity in Na⁺-Ion Conducting Solid Electrolytes for Na-Ion Batteries Through a Co-Doping Strategy

Rechargeable batteries are regarded as the most promising energy storage technology because of their reliability and high energy conversion efficiency. Although Li-ion batteries (LIBs) have been widely used in portable electronic devices, Na-ion batteries (NIBs) are considered as a potential candidate to replace LIBs because of their non-toxicity, low cost, and elemental abundance.<br/>Solid state sodium-ion batteries are seen as potentially cheaper and safer alternatives to current lithium-ion battery systems. The replacement of the presently used liquid electrolytes by non-flammable solid electrolytes is an important avenue to create safer batteries, while the high natural abundance of sodium compared to lithium would allow for significant cost reduction. The sodium superionic conductor, NASICON, first reported by Hong, is one of the best-known sodium-ion conducting solid electrolytes, displaying high bulk ionic conductivity and good stability toward NASICON -based electrodes. However, the practical use of NASICON has been impeded by low ion mobility at room temperature and poor interfacial connectivity.<br/>Here, the improvement of total conductivity has been achieved via La and Zn co-doping of NASICON. Total conductivity values of 4.68 × 10⁻³ S cm⁻¹ at room temperature and 2.89 × 10^–2 S cm^–1 at 100 °C were obtained for NZSP-LZ-0.2, which are amongst the highest values recorded for a NASICON based system. Furthermore, the system shows good air stability and could represent a suitable material for application in the field of Na-ion batteries. Symmetric cells with sodium metal as electrodes (Na| NZSP-LZ-0.2 |Na) are assembled and cycled stably for over 3000 cycles at a current density of 0.2 mA/cm².