Rechargeable Solid-State Na-Metal Battery Operating at -20 °C

Achieving satisfactory performance for a solid-state Na-metal battery (SSNMB) with an inorganic solid electrolyte (SE), especially under freezing temperatures, poses a challenge for stabilizing a Na-metal anode. Herein, this challenge is addressed by utilizing a Natrium super ionic conductor (NASICON) NASICON-type solid electrolyte, enabling the operation of a rechargeable SSNMB over a wide temperature range from -20 to 45 °C. The interfacial resistance at the Na metal/SE interface is only 0.4 Ω cm2 at 45 °C and remains below 110 Ω cm2 even at -20 °C. Remarkably, long-term Na-metal plating/stripping cycles lasting over 2000 h at -20 °C are achieved with minimal polarization voltages at 0.1 mA cm-2 . Further analysis reveals the formation of a uniform Na3- x Cax PO4 interphase layer at the interface, which significantly contributes to the exceptional interfacial performance observed. By employing a Na3 V1.5 Al0.5 (PO4 )3 cathode, the full battery system demonstrates excellent adaptability to low temperatures, exhibiting a capacity of 80 mA h g-1 at -20 °C over 50 cycles and retaining a capacity of 108 mAh g-1 (88.5% of the capacity at 45 °C) at 0 °C over 275 cycles. This research significantly reduces the temperature threshold for SSNMB operation and paves the way toward solid-state batteries suitable for all-season applications.

A Sodiophilic Amyloid Fibril Modified Separator for Dendrite-Free Sodium-Metal Batteries

Sodium (Na) batteries are being considered as prospective candidates for the next generation of secondary batteries in contrast to lithium-based batteries, due to their high raw-material abundance, low cost, and sustainability. However, the unfavorable growth of Na-metal deposition and severe interfacial reactions have prevented their large-scale applications. Here, a vacuum filtration strategy, through amyloid-fibril-modified glass-fiber separators, is proposed to address these issues. The modified symmetric cell can be cycled for 1800 h, surpassing the performance of previously reported Na-based electrodes under an ester-based electrolyte. Moreover, the Na/Na3 V2 (PO4 )3 full cell with a sodiophilic amyloid-fibril-modified separator exhibits a capacity retention of 87.13% even after 1000 cycles. Both the experimental and the theoretical results show that the sodiophilic amyloid fibril homogenizes the electric field and Na-ion concentration, fundamentally inhibiting dendrite formation. Simultaneously, the glutamine amino acids in the amyloid fibril have the highest adsorption energy for Na, resulting in the formation of a stable Na3 N- and NaNx Oy -rich solid-electrolyte-interface film on the anode during cycling. This work provides not only a possible pathway to solve the dendrite problem in metal batteries using environmentally friendly biomacromolecular materials, but also a new direction for expanding biomaterial applications.

From Nanoalloy to Nano-Laminated Interfaces for Highly Stable Alkali-Metal Anodes

Metal anodes are considered the holy grail for next-generation batteries because of their high gravimetric/volumetric specific capacity and low electrochemical potential. However, several unsolved challenges have impeded their practical applications, such as dendrite growth, interfacial side reactions, dead layer formation, and volume change. An electrochemically, chemically, and mechanically stable artificial solid electrolyte interphase is key to addressing the aforementioned issue with metal anodes. This study demonstrates a new concept of organic and inorganic hybrid interfaces for both Li- and Na-metal anodes. Through tailoring the compositions of the hybrid interfaces, a nanoalloy structure to nano-laminated structure is realized. As a result, the nanoalloy interface (1Al₂ O₃ -1alucone or 2Al₂ O₃ -2alucone) presents the most stable electrochemical performances for both Li-and Na-metal anodes. The optimized thicknesses required for the nanoalloy interfaces for Li- and Na-metal anodes are different. A cohesive zone model is applied to interpret the underlying mechanism. Furthermore, the influence of the mechanical stabilities of the different interfaces on the electrochemical performances is investigated experimentally and theoretically. This approach provides a fundamental understanding and establishes the bridge between mechanical properties and electrochemical performance for alkali-metal anodes.

Fluorinated Ether Based Electrolyte Enabling Sodium-Metal Batteries with Exceptional Cycling Stability

Sodium-metal batteries with conventional organic liquid electrolytes have disadvantages including dendrite deposition and safety concern. In this work, we report a low-flammable electrolyte (NaPF₆-FRE) consisting of 1 M NaPF₆ in 1,2-dimethoxyethane (DME), fluoroethylene carbonate (FEC), and 1,1,1,3,3,3-hexafluoroisopropylmethyl ether (HFPM) (2:1:2, in volume ratio). The symmetric Na and Na||Cu cells with a 1 M NaPF₆-DME electrolyte absorbed in a porous separator, such as the porous glass-fiber, show very poor cycling performance. In addition, the cell with a Na₃V₂(PO₄)₃ (NVP) cathode and 1 M NaPF₆-DME electrolyte shows low Coulombic efficiency. FEC was added into the NaPF₆-DME-based electrolyte to reduce the irreversible capacity of the NVP cathode and improve the Coulombic efficiency of the cell. However, the high reactivity of FEC with the Na electrode leads to formation of an unstable solid electrolyte interphase (SEI) and large interfacial resistance, and HFPM was further added to stabilize the Na electrode surface by forming a new fluorine-containing organic layer. The new prepared low-flammable electrolyte (NaPF₆-FRE) with 1 M NaPF₆ in DME, FEC, and HFPM (2:1:2, in volume ratio) shows a wide electrochemical window of 5.2 V. The Na symmetric cells with this low-flammable electrolyte show superior cycling performance for 800 h with a stable voltage profile at 0.5 mA cm^-2, 0.5 mA h cm^-2 and 1 mA cm^-2, 1 mA h cm^-2, respectively. The NVP||Na cells show an excellent capacity retention of 94% after 2000 cycles and superior Coulombic efficiency of 99.9% on average at 5 C.