Robust-Dense Composite Cathode with Improved Three-Dimensional Ionic Percolation Network and Electrode/Electrolyte Interface for the Development of All-Solid-State Sodium Batteries

Abstract

All solid-state sodium batteries (AS3Bs) attract immense attention due to their excellent energy and power density, cycle life and thermal security [1]. However, the practical realization of AS3Bs is greatly affected by low to moderate ionic conductivity of the solid electrolyte, unstable electrode/electrolyte interface and poor ion and electron transport between the cathode active materials [2,3]. The unstable electrode/electrolyte interface with point contact leads to huge charge transfer resistance, stress generation and the delamination of the electrode/electrolyte interface during electrochemical reactions. The ion percolation network in the cathode structure is also a critical part. Liquid electrolyte fills the pores present in the cathode and leads to a good Na+ percolation network; however, Na+ percolation cannot be possible in the case of AS3Bs due to the non-fluidity nature of solid electrolyte. This results in low capacity contribution from the cathode owing to fewer cathode active material utilization during electrochemical cycling [4]. Due to the above mentioned issues, the long-cycle stability of AS3Bs is poor.Here we have first prepared Mg doped Na1+3xZr2(SixP1-xO4)3 (NZSP-0.05Mg) with improved ionic conductivity (4.1 mS cm-1 25 ℃) by minimizing the secondary impurity phase. Further, we have utilized an innovative approach to develop a composite cathode by directly firing sodium vanadium fluoro phosphates (NVPF) with Na3.1Zr1.95Mg0.05Si2PO12 (NZSP-0.05Mg) solid electrolyte in the optimized size and weight ratio leading to the robust and dense structure with 3D ionic and electronic percolation network. Additionally, the pores present in the composite cathode is addressed by incorporating polymer electrolyte which further improves the ionic percolation network. The intimate contact between the solid electrolyte and the electrodes was further improved by using a composite solid electrolyte, and the full cell is fabricated by using NVPF composite cathode, composite solid electrolyte and Na metal. The cell delivers a discharge capacity of 103 mAh g-1 after 500 cycles with a Coulombic efficiency of 98 % at 0.1 C and shows excellent capacity retention of 67 mAh g-1 at 2C. The post-mortem XPS analysis confirms the electrochemical stability between the NVPF composite cathode and CSE as well as the Na metal and CSE. Our research provides a new prospective for the preparation of composite cathode and full cell design which can be utilized in solid state sodium and lithium batteries. This work can contribute to a safer battery with long cycle life.