New-type Hf-based NASICON electrolyte for solid-state Na-ion batteries with superior long-cycling stability and rate capability

The NASICON electrolyte with good chemical stability, suitable three-dimensional ion channel, high mechanical strength and wide electrochemical window, has been considered to be the one of most promising electrolytes for solid-state Na-ion batteries. However, the unsatisfactory ionic conductivity and large interface resistance severely limit its practical application. Herein, a new-type Hf-based NASICON electrolyte Na3.2Hf1.9Ca0.1Si2PO12 is synthesized, the conductivity of which can reach 1.07 × 10−3 S cm−1 at room temperature and the stability electrochemical window can reach 8.80 V. In addition, a mixed-ion-electron interphase was constructed on the surface of Na3.2Hf1.9Ca0.1Si2PO12 which can guarantee a close contact and fast charge transfer between sodium and the electrolyte. The batteries with this new-type electrolyte can cycle stably more than 2400 h at room temperature and the critical current density can reach 1.9 mA cm−2. Even the current density reaches 2.0 mA cm−2, there was still no obvious short circuit, which was the highest value in similar works. The full cell of Na/Na3V2(PO4)3 with this electrolyte can maintain a capacity of 103.1 mAh g−1 at 0.5 C without capacity decaying even after more than 300 cycles.