Refined Electrolyte and Interfacial Chemistry toward Realization of High-Energy Anode-Free Rechargeable Sodium Batteries

Abstract

Anode-free rechargeable sodium batteries represent one of the ultimate choices for the ‘beyond-lithium’ electrochemical storage technology with high energy. Operated based on the sole use of active Na ions from the cathode, the anode-free battery is usually reported with quite a limited cycle life due to unstable electrolyte chemistry that hinders efficient Na plating/stripping at the anode and high-voltage operation of the layered oxide cathode. A rational design of the electrolyte toward improving its compatibility with the electrodes is key to realize the battery. Here, we show that by refining the volume ratio of two conventional linear ether solvents, a binary electrolyte forms a cation solvation structure that facilitates flat, dendrite-free, planar growth of Na metal on the anode current collector and that is adaptive to high-voltage Na (de)­intercalation of P2-/O3-type layered oxide cathodes and oxidative decomposition of the Na2C2O4 supplement. Inorganic fluorides, such as NaF, show a major influence on the electroplating pattern of Na metal and effective passivation of plated metal at the anode–electrolyte interface. Anode-free batteries based on the refined electrolyte have demonstrated high coulombic efficiency, long cycle life, and the ability to claim a cell-level specific energy of >300 Wh/kg.

Development of High-Performance Iron-Based Phosphate Cathodes toward Practical Na-Ion Batteries

“…Na-ion batteries (NIBs) have received great attention in large-scale energy storage systems and smart grids because of huge abundance and low cost of sodium sources. − Substantial efforts have been devoted to developing cost-effective and high-performance cathodes these years toward commercially available NIBs. − Particularly, the benign and cheap iron-based phosphates were regarded as one of the best choices for NIBs as parallel LiFePO 4 cathodes have gained huge commercial success in lithium-ion batteries. However, the olivine-NaFePO 4 candidates could not be directly obtained by the conventional ceramic method due to the unfavorable thermodynamics feature and trend to transform into the more stable maricite phase. , Unfortunately, the maricite-type NaFePO 4 is usually believed to be of electrochemical inertness due to absence of effective Na + ion diffusion channels .…”

Restructuring Electrolyte Solvation by a Partially and Weakly Solvating Cosolvent toward High-Performance Potassium-Ion Batteries

“…However, most fluorinated diluents cannot dissociate potassium salts, preventing an effective increase in the K + transference number. Recently, several different strategies have been reported to improve the solvated structure of electrolytes. − Several key studies illustrate the development of the solvated structure of electrolytes, demonstrate the realization of WSEs, and improve the electrochemical properties. , It has been shown that the interaction between solvent molecules can regulate solvated structures. − Furthermore, systematic changes in the molecular skeleton of ethers have a profound effect on the chelation behavior of [K + –solvent] and the electrochemical stability of electrolytes . For example, shortening the alkyl chain in the middle of DME reduces the chelating ability, while extending it at its end increases the spatial site resistance. , These strategies share a common core of altering the coordinated environment around K + to promote the formation of WSEs .…”

Progressive Self-Leveling Deposition Improves the Cyclability of Anode-less Sodium Metal Batteries Revealed by In Situ EPR Imaging

“…Severe side reactions of Na metal with electrolyte and uncontrolled Na dendrites stemming from uneven Na deposition lead to fast depletion of active Na and electrolyte, resulting in inferior cycling stability, and even safety hazards . Many methods have been tried to resolve these issues from multiple aspects, such as adjusting the components of electrolyte solvent from ester to ether to inhibit side reactions, , developing functional electrolyte additives to build stable SEI, , and modifying the current collector to regulate the nucleation and growth of Na metal deposits. , Among these methods, multidimensional current collectors with large surface areas can provide enough nucleation sites for uniform electrodeposition and limit the volume change stemming from sodium plating and stripping . Wu et al devised core–shell C@Sb nanoparticles to compose a nucleation buffer layer for homogeneous electrochemical deposition of sodium metal .…”