(Invited) Anode-Free Sodium Metal Batteries

Abstract

Sodium-ion batteries (SIBs) have been regarded as one of the most promising alternatives to lithium-ion batteries (LIBs) due to their analogous working mechanism but greater abundance. The cost-effectiveness of Na batteries is expected to overtake that of LIBs in terms of electromobile applications if their energy densities can reach 200 Wh kg-1. In an anode-free configuration, all active sodium is stored in the cathode while anode only contains current collector with zero excess sodium, therefore achieving maximized energy density, significantly reduced cost, and simplified manufacture procedures. However, the anode-free batteries often suffer rapid capacity decay due to the absence of a reservoir to replenish the Na loss during cycling. In this work, we employed repeated cold rolling and folding to fabricate a metallurgical composite of sodium-antimony-telluride Na2(Sb2/6Te3/6Vac1/6) dispersed in electrochemically active sodium metal, termed “NST-Na”. This new intermetallic has a vacancy-rich thermodynamically stable fcc structure and enables state-of-the-art electrochemical performance in widely employed carbonate and ether electrolytes. NST-Na achieves 100% depth-of-discharge (DOD) in 1 M NaPF6 in G2, with 15 mAh cm-2 at 1 mA cm-2 and CE of 99.4%, for 1000 hours of plating/stripping. Sodium metal batteries (SMB, NMB) with NST-Na and Na3V2(PO4)3 (NVP) or sulfur cathodes give significantly improved energy, cycling and CE (> 99%). Anode-Free battery with NST collector and NVP obtains 0.23% capacity decay per cycle. Imaging and tomography using (cryo-)FIB sectioning, (cryo-)SEM, and (cryo-)TEM imaging indicate that sodium metal fills the open space inside the self-supporting sodiophilic NST skeleton, resulting in dense (pore-free and SEI-free) metal deposits with flat surfaces. The baseline Na deposit consists of filament-like dendrites and Dead Metal, intermixed with pores and SEI. Density functional theory (DFT) calculations show that uniqueness of NST lies in the thermodynamic stability of Na atoms (rather than clusters) on its surface that leads to planar wetting, and in its own stability that prevents decomposition during cycling. Figure 1