A bidirectional growth mechanism for a stable lithium anode by a platinum nanolayer sputtered on a polypropylene separator

Tailoring Lithium Horizontal Deposition for Long-Lasting High-Loading NCA (≥5 mA h cm-2)||Lithium-Metal Full Cells in Carbonate Electrolytes

We report a design for a synergistic lithium (Li) metal hosting layer for high-loading Li(Ni,Co,Al)O2 (NCA) (≥5 mA h cm-2)||Li-metal full cells in carbonate electrolytes. Based on density functional theory calculations, the hosting layer was designed as a three-dimensional silver/carbon composite nanofiber (Ag/CNF) network with high Li affinity and a platinum (Pt)-coated polypropylene separator with low Li affinity. This design enabled the tailoring of horizontal Li deposition on the Ag/CNF hosting layer. The Li deposition behavior modulated by the hosting layer was thoroughly examined based on the initial Li deposition and cycling behaviors of the Li||Li symmetric cell configuration. Cryogenic focused-ion beam cross-sectional images of the cycled Li anodes clearly demonstrated that dense lithium deposition was enabled by the synergistic hosting layer high-loading NCA (≥5 mA h cm-2)||Li-metal full cells. When the hosting layer was used, the average cycling performance improved by 78.27% under various cycling conditions. Our work demonstrates that the synergistic hosting layer design is a fruitful pathway to accelerate the commercialization of high-energy-density Li-metal batteries in carbonate electrolytes.

Recent Progress of Advanced Functional Separators in Lithium Metal Batteries

As a representative in the post-lithium-ion batteries (LIBs) landscape, lithium metal batteries (LMBs) exhibit high-energy densities but suffer from low coulombic efficiencies and short cycling lifetimes due to dendrite formation and complex side reactions. Separator modification holds the most promise in overcoming these challenges because it utilizes the original elements of LMBs. In this review, separators designed to address critical issues in LMBs that are fatal to their destiny according to the target electrodes are focused on. On the lithium anode side, functional separators reduce dendrite propagation with a conductive lithiophilic layer and a uniform Li-ion channel or form a stable solid electrolyte interphase layer through the continuous release of active agents. The classification of functional separators solving the degradation stemming from the cathodes, which has often been overlooked, is summarized. Structural deterioration and the resulting leakage from cathode materials are suppressed by acidic impurity scavenging, transition metal ion capture, and polysulfide shuttle effect inhibition from functional separators. Furthermore, flame-retardant separators for preventing LMB safety issues and multifunctional separators are discussed. Further expansion of functional separators can be effectively utilized in other types of batteries, indicating that intensive and extensive research on functional separators is expected to continue in LIBs.

Plating and Stripping Calcium Metal in Potassium Hexafluorophosphate Electrolyte toward a Stable Hybrid Solid Electrolyte Interphase

The use of calcium (Ca) metal anodes in batteries is currently challenged by the development of a suitable solid electrolyte interface (SEI) that enables effective Ca²⁺ ion transport. Native calcium electrolytes produce a passivation layer on the surface of the calcium electrodes during cycling, causing a decrease in capacity during cycling and the need for large overpotentials. The use of a hybrid SEI is a strategy to mitigate the uncontrolled production of a passivation layer and reduce the overpotentials needed for the plating and stripping of calcium. Here, we report the development of a hybrid potassium (K)/Ca SEI layer investigated in symmetric Ca//Ca cell configurations. Using KPF₆ salt in a ternary mixture of carbonate solvent (EC/EMC/DMC), Ca//Ca cells can be cycled up to 200 h at a capacity of 0.15 mAh/cm² with a current density of 0.025 mA/cm². The symmetrical cells consistently cycle at overpotentials of 1.8 V. Ex-situ X-ray diffraction (XRD) of cycled electrodes reveals plating and stripping of both calcium and potassium. Energy dispersive X-ray (EDX) maps confirm the plating of calcium and potassium during galvanostatic cycling. Scanning electron microscopy (SEM) cross-sectional views of the calcium electrodes reveal a continuous SEI layer formed over the calcium metal. XRD analysis reveals the SEI layer consists of K-based inorganics along with the identification of permanent and transient phases. FTIR outlines the parallel plating of both calcium and potassium at both regions of redox activity. Raman spectroscopy of the electrolyte reveals compositional changes over the course of cycling that promote increased plating and stripping. The results indicate that potassium electrolytes are a possible route for tuning the SEI to enable reversible calcium electrochemical cycling.

Review of Multifunctional Separators: Stabilizing the Cathode and the Anode for Alkali (Li, Na, and K) Metal-Sulfur and Selenium Batteries

Alkali metal batteries based on lithium, sodium, and potassium anodes and sulfur-based cathodes are regarded as key for next-generation energy storage due to their high theoretical energy and potential cost effectiveness. However, metal-sulfur batteries remain challenged by several factors, including polysulfides' (PSs) dissolution, sluggish sulfur redox kinetics at the cathode, and metallic dendrite growth at the anode. Functional separators and interlayers are an innovative approach to remedying these drawbacks. Here we critically review the state-of-the-art in separators/interlayers for cathode and anode protection, covering the Li-S and the emerging Na-S and K-S systems. The approaches for improving electrochemical performance may be categorized as one or a combination of the following: Immobilization of polysulfides (cathode); catalyzing sulfur redox kinetics (cathode); introduction of protective layers to serve as an artificial solid electrolyte interphase (SEI) (anode); and combined improvement in electrolyte wetting and homogenization of ion flux (anode and cathode). It is demonstrated that while the advances in Li-S are relatively mature, less progress has been made with Na-S and K-S due to the more challenging redox chemistry at the cathode and increased electrochemical instability at the anode. Throughout these sections there is a complementary discussion of functional separators for emerging alkali metal systems based on metal-selenium and the metal-selenium sulfide. The focus then shifts to interlayers and artificial SEI/cathode electrolyte interphase (CEI) layers employed to stabilize solid-state electrolytes (SSEs) in metal-sulfur solid-state batteries (SSBs). The discussion of SSEs focuses on inorganic electrolytes based on Li- and Na-based oxides and sulfides but also touches on some hybrid systems with an inorganic matrix and a minority polymer phase. The review then moves to practical considerations for functional separators, including scaleup issues and Li-S technoeconomics. The review concludes with an outlook section, where we discuss emerging mechanics, spectroscopy, and advanced electron microscopy (e.g. cryo-transmission electron microscopy (cryo-TEM) and cryo-focused ion beam (cryo-FIB))-based approaches for analysis of functional separator structure-battery electrochemical performance interrelations. Throughout the review we identify the outstanding open scientific and technological questions while providing recommendations for future research topics.

Tutorial review on structure – dendrite growth relations in metal battery anode supports

This tutorial review explains surface and bulk chemistry – electrochemical performance relations of lithium, sodium and potassium metal anodes.

Solid-Liquid Electrolyte as a Nanoion Modulator for Dendrite-Free Lithium Anodes

Rechargeable lithium (Li) metal batteries are considered the most promising of Li-based energy storage technologies. However, tree-like dendrite produced by irregular Li⁺ electrodeposition restricts it wide applications. Herein, based on a cation-microphase-regulation strategy, we create solid-liquid electrolytes (SLEs) by absorbing commercial liquid electrolytes into polyethylene glycol (PEG) engineered nanoporous Al₂O₃ ceramic membranes. By means of molecular dynamics simulations and comprehensive experiments, we show that Li ions are regulated and promoted in the two microphases, the channel phase and nonchannel phase, respectively. The channel phase can achieve homogeneous Li⁺ flux distribution by multiple mechanisms, including its uniform array of nanochannels and ability to suppress lateral dendrite growth by its high modulus. In the nonchannel phase, PEG chains swollen by electrolyte facilitate desolvation and fast conduction of Li⁺. As a result, the studied SLEs exhibit high ionic conductivity, low interfacial resistance, and the unique ability to stabilize deposition at the Li anode. By means of galvanostatic cycling studies in symmetric Li cells and Li/Li₄Ti₅O₁₂ cells, we further show that the materials open a path to Li metal batteries with excellent cycling performance.