Research Progress and Perspective on Lithium/Sodium Metal Anodes for Next-Generation Rechargeable Batteries

Study of Na Deposition Formation in Mixed Ethylene: Propylene Carbonate Electrolytes by Inert/Cryoelectron Microscopy

The sodium anode-free combines low-cost and high energy density, demonstrating a promising alternative to the Li battery counterpart. Nevertheless, the uptake of a sodium anode-free battery is greatly impeded by the uncontrollable dendrite proliferation upon the chemically active metallic Na. An insightful mechanistic understanding of Na deposition nucleation and growth behavior in ethylene carbonate and propylene carbonate (EC/PC, 1:1) is revealed via various inert and/or cryo-electron microscopy characterization techniques. The deposit morphology, size, and distribution were studied with different current densities and areal capacity. The Na deposit distribution changes from nonparametric distribution to normal distribution which can be attributed to the effect of interparticle diffusion coupling (IDP). The atomic information on the Na deposit was revealed via cryogenic transmission electron microscopy.

Molecule Isolation Protective Interface Formed by Planar Additive for Stable Sodium Metal Anodes

Sodium (Na) metal is an ideal anode for Na-based batteries because of its high specific capacity and low potential. However, interface issues such as side reactions with the electrolyte and uneven deposition severely hinder its practical application. Here, we report a zinc phthalocyanine (ZnPc) electrolyte additive with a planar molecular structure that can form a dense molecular layer when tightly adsorbed on the Na metal anode surface. Such a planar molecular layer can suppress side reactions between the anode and the electrolyte as well as homogenize Na+ flux to reduce dendrite growth. As a result, the molecular isolation interface formed by ZnPc adsorption on the surface of the Na metal anode enhances the interface stability and the cycling performance of the Na metal anode, with the average Coulombic efficiency of the half-cell of 99.95% after 350 stable cycles at 1 mA cm-2 for 1 mAh cm-2. Moreover, the assembled Na||Na3V2(PO4)3 full-cell with this additive delivers excellent stability over 120 cycles, proving the effectiveness of the ZnPc additive in practical application.

Few-layer Graphene Lithiophilic and Sodiophilic Diffusion Layer on Porous Stainless Steel as Lithium and Sodium Metal Anodes

In order to subdue the obvious problem of uneven electric field distribution on regularly used copper/aluminum current collectors for alkali metal batteries, graphene on porous stainless steel (pSS_Gr) was fabricated using the ion etching technique that is employed as an effective host for lithium and sodium metal anodes. The binder-free pSS_Gr demonstrated stable Li plating and stripping at areal current and capacity of 6 mA cm^-2 and 2.54 mAh cm^-2 , respectively, for over 1000 cycles with 98% coulombic efficiency (C.E.). Also, in the case of Na metal anode, the host has shown stable performance at 4 mA cm^-2 and 1 mAh cm^-2 over 1000 cycles with ∼100% C.E.. Further, a full cell composed of Li-plated pSS_Gr as an anode and LiFePO₄ as a cathode is electrochemically tested at 50 mA g^-1 current density with stable 100 cycles.

Polystyrene-Based Single-Ion Conducting Polymer Electrolyte for Lithium Metal Batteries

Lithium metal batteries are one of the more promising replacements for lithium-ion batteries owing to their ability to reach high energy densities. The main problem limiting their commercial application is the formation of dendrites, which significantly reduces their durability and renders the batteries unsafe. In the present work, we used a single-ion conducting gel polymer electrolyte based on a poly(ethylene-ran-butylene)-block-polystyrene (SEBS) block copolymer, which was functionalized with benzenesulfonylimide anions and plasticized by a mixture of ethylene carbonate and dimethylacetamide (SSEBS-Ph-EC-DMA), with a solvent uptake of 160% (~12 solvent molecules per one functional group of the membrane). The SSEBS-Ph-EC-DMA electrolyte exhibits an ionic conductivity of 0.6 mSm∙cm−1 at 25 °C and appears to be a cationic conductor (TLi+ = 0.72). SSEBS-Ph-EC-DMA is electrochemically stable up to 4.1 V. Symmetrical Li|Li cells; further, with regard to SSEBS-Ph-EC-DMA membrane electrolytes, it showed a good performance (~0.10 V at first cycles and <0.23 V after 700 h of cycling at ±0.1 mA∙cm−2 and ±0.05 mAh∙cm−2). The LiFePO4|SSEBS-Ph-EC-DMA|Li battery showed discharge capacity values of 100 mAh∙g−1 and a 100% Coulomb efficiency, at a cycling rate of 0.1C.

Dual Structure-Material Design of Separators toward Dendrite-Free Lithium Metal Anodes

The practical applications of lithium metal anodes have been severely hindered by the Li dendrite issue. Herein, a dual structure-material design strategy was developed to fabricate a new type of separator using interconnected hollow porous polyacrylonitrile (PAN) nanofibers (HPPANF), which delivered controllable and dendrite-free Li depositions. The interconnected mesopores on HPPANF bridged the hollow interiors with the outside voids among the fibers, enabling outstanding electrolyte uptake capabilities for high ion conductivity, and nano-level wetted electrolyte/anode interface for uniform Li plating/stripping. In parallel, the HPPANF separator enriched with polar groups acted as an exceptional polymer-based solid-state electrolyte, providing 3D ion channels for the transport of Li ions. Benefiting from the dual structure-material design, the HPPANF separator induced uniform Li ion flux for dendrite-free Li depositions, which caused enhanced cycling stability (1300 h, 3 mA cm^-2 ). This work demonstrates a new method to stabilize Li metal anodes through rational separator design.