Waterproof lithium metal anode enabled by cross-linking encapsulation

Recent Progress on the Air-Stable Battery Materials for Solid-State Lithium Metal Batteries

Solid-state lithium metal batteries (SSLMBs) offer numerous advantages in terms of safety and theoretical specific energy density. However, their main components namely lithium metal anode, solid-state electrolyte, and cathode, show chemical instability when exposed to humid air, which results in low capacities and poor cycling stability. Recent studies have shown that bioinspired hydrophobic materials with low specific surface energies can protect battery components from corrosion caused by humid air. Air-stable inorganic materials that densely cover the surface of battery components can also provide protection, which improves the storage stability of the battery components, broadens their processing conditions, and ultimately decreases their processing costs while enhancing their safety. In this review, the mechanism behind the surface structural degradation of battery components and the resulting consequences are discussed. Subsequently, recent strategies are reviewed to address this issue from the perspectives of lithium metal anodes, solid-state electrolytes, and cathodes. Finally, a brief conclusion is provided on the current strategies and fabrication suggestions for future safe air-stable SSLMBs.

Simultaneous High Ionic Conductivity and Lithium-Ion Transference Number in Single-Ion Conductor Network Polymer Enabling Fast-Charging Solid-State Lithium Battery

Developing solid-state electrolyte with sufficient ionic conduction and flexible-intimate interface is vital to advance fast-charging solid-state lithium batteries. Solid polymer electrolyte yields the promise of interfacial compatibility, yet its critical bottleneck is how to simultaneously achieve high ionic conductivity and lithium-ion transference number. Herein, single-ion conducting network polymer electrolyte (SICNP) enabling fast charging is proposed to positively realize fast lithium-ion locomotion with both high ionic conductivity of 1.1 × 10-3 S cm-1 and lithium-ion transference number of 0.92 at room temperature. Experimental characterization and theoretical simulations demonstrate that the construction of polymer network structure for single-ion conductor not only facilitates fast hopping of lithium ions for boosting ionic kinetics, but also enables a high dissociation level of the negative charge for lithium-ion transference number close to unity. As a result, the solid-state lithium batteries constructed by coupling SICNP with lithium anodes and various cathodes (e.g., LiFePO4 , sulfur, and LiCoO2 ) display impressive high-rate cycling performance (e.g., 95% capacity retention at 5 C for 1000 cycles in LiFePO4 |SICNP|lithium cell) and fast-charging capability (e.g., being charged within 6 min and discharged over than 180 min in LiCoO2 |SICNP|lithium cell). Our study provides a prospective direction for solid-state electrolyte that meets the lithium-ion dynamics for practical fast-charging solid-state lithium batteries.

An artificial interfacial layer with biomimetic ionic channels towards highly stable Li metal anodes

Lithium (Li) metal with low electrochemical potential and high theoretical capacity is a promising anode material for next-generation batteries. However, the low reversibility and safety problems caused by the notorious dendrite growth significantly impede the development of high-energy-density lithium metal batteries (LMBs). Here, to enable a dendrite-free and highly reversible Li metal anode (LMA), we develop a cytomembrane-inspired artificial layer (CAL) with biomimetic ionic channels using a scalable spread coating method. The negatively charged CAL with uniform intraparticle and interparticle ionic channels facilitates the Li-ion transport and redistributes the Li-ion flux, contributing to stable and homogeneous Li stripping and plating. Furthermore, a robust underneath transition layer with abundant lithiophilic inorganic components is in-situ formed through the transformation of CAL during cycling, which promotes Li-ion diffusion and suppresses the continuous side reactions with the electrolyte. Additionally, the resulting cytomembrane-inspired artificial Janus layer (CAJL) displays an ultrahigh Young's modulus (≥10.7 GPa) to inhibit the dendrite growth. Consequently, the CAJL-protected LMA (Li@CAJL) is stably cycled with a high areal capacity of 10 mAh cm^-2 at a high current density of 10 mA cm^-2. More importantly, the effective CAJL modification realizes the stable operation of a practical 429.2 Wh kg^-1 lithium-sulfur (Li-S) pouch cell using a low electrolyte/sulfur (E/S) ratio of 3 μL mg^-1. The facile yet effective protection strategy of LMAs can promote the practical application of LMBs.

Surface engineering toward stable lithium metal anodes

The lithium (Li) metal anode (LMA) is susceptible to failure due to the growth of Li dendrites caused by an unsatisfied solid electrolyte interface (SEI). With this regard, the design of artificial SEIs with improved physicochemical and mechanical properties has been demonstrated to be important to stabilize the LMAs. This review comprehensively summarizes current efficient strategies and key progresses in surface engineering for constructing protective layers to serve as the artificial SEIs, including pretreating the LMAs with the reagents situated in different primary states of matter (solid, liquid, and gas) or using some peculiar pathways (plasma, for example). The fundamental characterization tools for studying the protective layers on the LMAs are also briefly introduced. Last, strategic guidance for the deliberate design of surface engineering is provided, and the current challenges, opportunities, and possible future directions of these strategies for the development of LMAs in practical applications are discussed.

Air-Stable Protective Layers for Lithium Anode Achieving Safe Lithium Metal Batteries

With markedly expansive demand in energy storage devices, rechargeable batteries will concentrate on achieving the high energy density and adequate security, especially under harsh operating conditions. Considering the high capacity (3860 mA h g^-1 ) and low electrochemical potential (-3.04 V vs the standard hydrogen electrode), lithium metal is identified as one of the most promising anode materials, which has sparked a research boom. However, the intrinsically high reactivity triggers a repeating fracture/reconstruction process of the solid electrolyte interphase, side reactions with electrolyte and lithium dendrites, detrimental to the electrochemical performance of lithium metal batteries (LMBs). Even worse, when exposed to air, lithium metal will suffer severe atmospheric corrosion, especially the reaction with moisture, leading to grievous safety hazards. To settle these troubles, constructing air-stable protective layers (ASPLs) is an effective solution. In this review, besides the necessity of ASPLs is highlighted, the modified design criteria, focusing on enhancing chemical/mechanical stability and controlling ion flux, are proposed. Correspondingly, current research progress is comprehensively summarized and discussed. Finally, the perspectives of developing applicable lithium metal anodes (LMAs) are put forward. This review guides the direction for the practical use of LMAs, further pushing the evolution of safe and stable LMBs.

Bioinspired Oil-Infused Slippery Surfaces with Water and Ion Barrier Properties

Encapsulation materials play an important role in many applications including wearable electronics, medical devices, underwater robotics, marine skin tagging system, food packaging, and energy conversation and storage devices. To date, all the encapsulation materials, including polymer layers and inorganic materials, are solid materials. These solid materials suffer from limited barrier lifetimes due to pinholes, cracks, and nanopores or from complicated fabrication processes and limited stretchability for interfacing with complex 3D surfaces. This paper reports a solution to this material challenge by demonstrating bioinspired oil-infused slippery surfaces with excellent waterproof property for the first time. A water vapor transmission test shows that locking a thin layer of oil on the silicone elastomer improves the water vapor barrier performance by three orders of magnitude. Accelerated lifetime tests suggest robust water barrier characteristics that approach 226 days at 37 °C even under severe mechanical damage. A combination of temperature- and thickness-dependent experimental measurements and reaction-diffusion modeling reveals the key waterproof property. In addition to serving as a barrier to water, the oil-infused surface demonstrates an attractive ion barrier property. All these exceptional properties suggest the potential applications of slippery surfaces as encapsulation materials for medical devices, underwater electronics, and many others.

Electrolytes enriched by potassium perfluorinated sulfonates for lithium metal batteries

Lithium (Li) metal is widely considered as a promising anode for next-generation lithium metal batteries (LMBs) due to its high theoretical capacity and lowest electrochemical potential. However, the uncontrollable formation of Li dendrites has prevented its practical application. Herein, we propose a kind of multi-functional electrolyte additives (potassium perfluorinated sulfonates) from the multi-factor principle for electrolyte additive molecular design (EDMD) view to suppress the Li dendrite growth. The effects of these additives are revealed through experimental results, molecular dynamics simulations and first-principles calculations. Firstly, K⁺ can form an electrostatic shield on the surface of Li anode to prevent the growth of Li dendrites. Secondly, potassium perfluorinated sulfonates can improve the activity of electrolytes as co-conductive salts, and lower the electro-potential of Li nucleation. Thirdly, perfluorinated sulfonate anions not only can change the Li⁺ solvation sheath structure to decrease the desolvation energy barrier and increase the ion migration rate, but also can be partly decomposed to form the superior solid electrolyte interphase (SEI). Benefited from the synergistic effects, an outstanding cycle life over 250 h at 1 mA cm^-2 is achieved in symmetric Li||Li cells. In particular, potassium perfluorinated sulfonate additives (e.g., potassium perfluorohexyl sulfonate, denoted as K⁺PFHS) can also contribute to the formation of high-quality cathode electrolyte interphase (CEI). As a result, Li||LiNi_0.6Mn_0.2Co_0.2O₂ full cells exhibit significantly enhanced cycling stability. This multi-factor principle for EDMD offers a unique insight on understanding the electrochemical behavior of ion-type electrolyte additives on both the Li metal anode and high-voltage cathode.

Lithium Metal Electrode with Increased Air Stability and Robust Solid Electrolyte Interphase Realized by Silane Coupling Agent Modification

The quality of the solid electrolyte interphase (SEI) layer is the decisive factor for the electrochemical performance of Li-metal-based batteries. Due to the absence of effective bonding, a natural SEI layer may exfoliate from the Li anode during interfacial fluctuations. Here, a silane coupling agent is introduced to serve as an adhesion promoter to bridge these two dissimilar materials via both chemical bonding and physical intertwining effects. Its inorganic reactive groups can combine with the Li substrate by forming LiOSi bonds, while organic functional groups can take part in the formation of the SEI layer and thereby bond with SEI components. Li metal electrodes with silane coupling agent modification exhibit excellent electrochemical performance, even under extreme testing conditions. This modification layer with dense structure could also protect the Li metal from corrosion by air, evidenced by the comparable electrochemical activity of the modified Li metal electrodes even after being exposed in air for 2 h. This design provides a promising pathway for the development of Li metal electrodes that will be stable both in electrolyte and in air.

A Protective Layer for Lithium Metal Anode: Why and How

Lithium metal is the most promising candidate anode material for high energy density batteries, but its high activity and severe dendrite growth lead to safety concerns and limit its practical use. Constructing a protective layer (PL) on the lithium surface to avoid the side reactions and stabilize the electrode-electrolyte interface is an effective approach to solve these problems. In this review, the recent progress on PLs is summarized, and their desired properties and design principles are discussed from the aspects of materials selection and the corresponding fabrication methods. Advanced PLs with different properties are then highlighted, including a self-adjusting feature to increase structural integrity, the synergistic effect of organic and inorganic hybrids to improve mechanical properties and ionic conductivity, the use of embedded groups and ion diffusion channels to regulate ion distribution and flux, and a protective barrier to suppress corrosion from humid air or water. Finally, the remaining challenges and the possible solutions for PL design in future studies are proposed.

Recent Developments in Dendrite-Free Lithium-Metal Deposition through Tailoring of Micro- and Nanoscale Artificial Coatings

Forty years after the failed introduction of rechargeable lithium-metal batteries and 30 years after the successful commercialization of the lower capacity, graphite-anode-based lithium-ion battery by Sony, demand for higher energy density batteries is leading to reinvestigation of the problem of dendrite growth that makes the metallic lithium anodes unsafe and prevented commercialization to begin with. One strategy to mitigate dendrite growth is to deposit thin, tailored, corrosion-passivating coatings on the metallic lithium, instead of allowing the metal to spontaneously react with the organic electrolyte solution to form its passivating solid electrolyte interface (SEI). The challenge is to find and to deposit a coating that is electronically insulating yet allows uniform permeation of Li⁺ at a high cycling rate, such that Li-metal is electrodeposited uniformly on the nanoscale below the tailored coating. Recently, a number of studies have examined multicomponent films, taking advantage of the properties of two different materials, which can be tuned separately or chosen for their complementary properties. Use of these multicomponent coatings will likely enable future researchers to create rationally designed SEIs capable of effectively suppressing the growth of Li dendrites. This review discusses recent developments in micro- and nanoscale tailored coatings to meet that need.

Shaping Li Deposits from Wild Dendrites to Regular Crystals via the Ferroelectric Effect

Manipulating the Li plating behavior remains a challenging task toward Li-based high-energy batteries. Generally, the Li plating process is kinetically controlled by ion transport, concentration gradient, local electric field, etc. A myriad of strategies have been developed for homogenizing the kinetics; however, such kinetics-controlled Li plating nature is barely changed. Herein, a ferroelectric substrate comprised of homogeneously distributed BaTiO₃ was deployed and the Li plating behavior was transferred from a kinetic-controlled to a thermodynamic-preferred mode via ferroelectric effect. Such Li deposits with uniform hexagonal and cubic shapes are highly in accord with the thermodynamic principle where the body-centered cubic Li is apt to expose more (110) facets as possible to maximally minimize its surface energy. The mechanism was later confirmed due to the spontaneous polarization of BTO particles trigged by an applied electric field. The instantly generated reverse polarized field and charged ends not only neutralized the electric field but also leveled the ion distribution at the interface.