A "tug-of-war" effect tunes Li-ion transport and enhances the rate capability of lithium metal batteries

Geometric Design of Interface Structures and Electrolyte Solvation Chemistry for Fast Charging Lithium-Ion Batteries

The grain sizes of solid electrolyte interphase (SEI) and solvation structure of electrolytes can affect Li+ ion transport across SEI and control the desolvation kinetics of solvated Li+ ions during fast-charging of Li-ion batteries (LIBs). However, the impact of the geometric structure of SEI grains on the fast charging capability of LIBs is rarely examined. Here, the correlation between the SEI grain size and fast charging characteristics of cells is explored, and the desolvation kinetics is controlled by replacing the strongly binding ethylene carbonate (EC) solvent with a weakly binding nitrile-based solvent under fast charging conditions. The evolution of small grains of SEI to provide sufficient paths for Li+ ion supply can be achieved by the modification of solvation structure in the electrolyte. Additionally, the less resistive SEI composition and low viscosity of isoBN-containing electrolyte enable a more rapid charging of LiNi0.8Co0.1Mn0.1O2/graphite full cells by facilitating the SEI crossing of Li+ ions with less Li plating at a charging rate of 4 C at 25 °C. This work sheds light on solvation structure and interface engineering to enhance the fast charging cycle stability of LIBs for tailorable adoption in transportation sectors.

Ultrafast Lithium-Ion Transport Engineered by Nanoconfinement Effect

Amid the burgeoning demand for electrochemical energy storage and neuromorphic computing, fast ion transport behavior has attracted widespread attention at both fundamental and practical levels. Here, based on the nanoconfined channel of graphene oxide laminar membranes (GOLMs), the lithium ionic conductivity typically exceeding 102 mS cm-1 is realized, one to three orders of magnitude higher than traditional liquid or solid lithium-ion electrolyte. Specifically, the nanoconfined lithium hexafluorophosphate (LiPF6)-ethylene carbonate (EC)/ dimethyl carbonate (DMC) electrolyte demonstrates the ionic conductivity of 170 mS cm-1, outperforming the bulk counterpart by ≈16 fold. At the ultralow temperature of -60 °C, the nanoconfined electrolyte also maintains a practically useful conductivity of 11 mS cm-1. Furthermore, the in situ experimental and theoretical framework enables to attribute the enhanced ionic conductivity to the layer-by-layer cations and anions distribution induced by high surface charge and nanoconfinement effects in GO nanochannels. More importantly, integrating such rapid lithium-ion transport nanochannel into the LiFePO4 (LFP) cathode significantly improves the high-rate and long-cycle performance of lithium batteries. These results exhibit the convention-breaking ionic conductivity of nanoconfined electrolytes, inspiring the development of ultrafast ion diffusion pathways based on 2D nanoconfined channels for efficient energy storage applications.

A Self-Healing, Flowable, Yet Solid Electrolyte Suppresses Li-Metal Morphological Instabilities

Lithium metal (Li0) solid-state batteries encounter implementation challenges due to dendrite formation, side reactions, and movement of the electrode-electrolyte interface in cycling. Notably, voids and cracks formed during battery fabrication/operation are hot spots for failure. Here, a self-healing, flowable yet solid electrolyte composed of mobile ceramic crystals embedded in a reconfigurable polymer network is reported. This electrolyte can auto-repair voids and cracks through a two-step self-healing process that occurs at a fast rate of 5.6 µm h-1. A dynamical phase diagram is generated, showing the material can switch between liquid and solid forms in response to external strain rates. The flowability of the electrolyte allows it to accommodate the electrode volume change during Li0 stripping. Simultaneously, the electrolyte maintains a solid form with high tensile strength (0.28 MPa), facilitating the regulation of mossy Li0 deposition. The chemistries and kinetics are studied by operando synchrotron X-ray and in situ transmission electron microscopy (TEM). Solid-state NMR reveals a dual-phase ion conduction pathway and rapid Li+ diffusion through the stable polymer-ceramic interphase. This designed electrolyte exhibits extended cycling life in Li0-Li0 cells, reaching 12 000 h at 0.2 mA cm-2 and 5000 h at 0.5 mA cm-2. Furthermore, owing to its high critical current density of 9 mA cm-2, the Li0-LiNi0.8Mn0.1Co0.1O2 (NMC811) full cell demonstrates stable cycling at 5 mA cm-2 for 1100 cycles, retaining 88% of its capacity, even under near-zero stack pressure conditions.

Design of Localized High-Concentration Electrolytes from the Perspective of Physicochemical Properties

The physicochemical properties of electrolytes profoundly impact the energy density, rate performance, and manufacturability of rechargeable lithium batteries. Localized high-concentration electrolytes (LHCEs), a novel electrolyte class, have attracted considerable interest, yet the impact of diluents on their physicochemical properties remains unclear, as most reports involve only a few samples. Here we prepared 345 electrolyte samples using 21 diluents and systematically investigated the effect of diluent type and content on the miscibility, density, viscosity, and ion conductivity of LHCEs. We found that the physicochemical properties of LHCEs are mainly affected by the diluents' density and viscosity, regardless of type. Notably, the ionic conductivity exhibits two typical variance trends, "volcano" and "descending," both correlating strongly with diluents' viscosity rather than dielectric constant, a parameter commonly employed in electrolyte design. This anomaly can be explained by the "plum pudding" solvation model, providing essential insights for developing lightweight, highly fluid, and conductive LHCEs.

4-Fluorobenzyl cyanide, a sterically-hindered solvent expediting interfacial kinetics in lithium-ion batteries

The electrochemical performance of lithium-ion batteries (LIBs) is plagued by sluggish interfacial kinetics. Fortunately, the Li+ solvation structure bridges the bulk electrolyte and interfacial chemistry, providing a pathway for promoting electrochemical kinetics in LIBs. Herein, we improve the interfacial kinetics by tuning the Li+ coordination chemistry based on solvent molecular engineering. Specifically, 4-fluorobenzyl cyanide (FBCN), featuring steric hindrance and a weak Lewis basic center, is designed to construct a bulky coordination structure with Li+, weakening ion-dipole interaction (Li+-solvents) but promoting coulombic attraction (Li+-anions) at a normal Li salt concentration. This sterically-controlled solvation chemistry reduces the interfacial barrier and thus contributes to improved rate performance, as demonstrated practically in LiFePO4//graphite pouch cells. This study provides fresh insights into solvent steric control and coordination chemistry engineering, opening a new avenue for enhancing electrochemical kinetics in LIBs.

Two-Dimensional Electrolyte Design: Broadening the Horizons of Functional Electrolytes in Lithium Batteries

ConspectusSince their commercialization in the 1990s, lithium-ion batteries (LIBs) have been increasingly used in applications such as portable electronics, electric vehicles, and large-scale energy storage. The increasing use of LIBs in modern society has necessitated superior-performance LIB development, including electrochemical reversibility, interfacial stability, efficient kinetics, environmental adaptability, and intrinsic safety, which is difficult to simultaneously achieve in commercialized electrolytes. Current electrolyte systems contain a solution with Li salts (e.g., LiPF6) and solvents (e.g., ethylene carbonate and dimethyl carbonate), in which the latter dissolves Li salts and strongly interacts with Li+ (lithiophilic feature). Only lithiophilic agents can be functionally modified (e.g., additives and solvents), altering the bulk and interfacial behaviors of Li+ solvates. However, such approaches alter pristine Li+ solvation and electrochemical processes, making it difficult to strike a balance between the electrochemical performance and other desired electrolyte functions. This common electrolyte design in lithiophilic solvents shows strong coupling among formulation, coordination, electrochemistry, and electrolyte function. The invention of lithiophobic cosolvents (e.g., multifluorinated ether and fluoroaromatic hydrocarbons) has expanded the electrolyte design space to lithiophilic (interacts with Li+) and lithiophobic (interacts with solvents but not with Li+) dimensions. Functional modifications switch to lithiophobic cosolvents, affording superior properties (carried by lithiophobic cosolvents) with little impact on primary Li+ solvation (dictated by lithiophilic solvents). This electrolyte engineering technique based on lithiophobic cosolvents is the 2D electrolyte (TDE) principle, which decouples formulation, coordination, electrochemistry, and function. The molecular-scale understanding of TDEs is expected to accelerate electrolyte innovations in next-generation LIBs.This Account provides insights into recent advancements in electrolytes for superior LIBs from the perspective of lithiophobic agents (i.e., lithiophobic additives and cosolvents), establishing a generalized TDE principle for functional electrolyte design. In bulk electrolytes, a microsolvating competition emerges because of cosolvent-induced dipole-dipole and ion-dipole interactions, forming a loose solvation shell and a kinetically favorable electrolyte. At the electrode/electrolyte interface, the lithiophobic cosolvent affords reliable passivation and efficient desolvation, with interfacial compatibility and electrochemical reversibility even under harsh conditions. Based on this unique coordination chemistry, functional electrolytes are formulated without significantly sacrificing their electrochemical performance. First, lithiophobic cosolvents are used to tune Li+-solvent affinity and anion mobility, promoting Li+ diffusion and electrochemical kinetics of the electrolyte to benefit high-rate and low-temperature applications. Second, the lithiophobic cosolvent undergoes less thermally induced decomposition and constructs a thermally stable interphase in TDEs, affording electrolytes with high-temperature adaptability and cycling stability. Third, the lithiophobic cosolvent modifies the local Li+-solvent-anion topography, controlling electrolyte electrochemical reversibility to afford numerous promising solvents that cannot be used in common electrolyte design. Finally, the lithiophobic cosolvent mitigates detrimental crosstalk between flame retardants and carbonate solvents, improving the intrinsic electrolyte safety without compromising electrochemical performance, which broadens the horizons of electrolyte design by optimizing versatile cosolvents and solvents, inspiring new ideas in liquid electrochemistry in other battery systems.

Constructing Ag@SiO2-TiO2 Nanofiber Interlayers with a Three-Dimensional Lithiophilic Gradient Framework for an Ultrastable Lithium Metal Anode

Lithium metal batteries have become one of the most promising rechargeable batteries due to the ultrahigh theoretical specific capacity of the Li metal anode. However, the Li dendrite growth and volume change of the Li metal anode during repeated Li plating-stripping cycles restrict the practical viability. Herein, a unique lithiophilic gradient structure of uniformly incorporating Ag nanoparticles into a three-dimensional (3D) nanofiber framework with amorphous SiO2 and TiO2 hybrids was prepared by an electrospinning process and used as a multifunctional interlayer between the pristine separator and Li metal foil. The 3D framework not only possesses excellent flexibility but also alleviates volume changes, which can withstand massive Li loading and promote uniform Li+ distribution. In addition, the 3D lithiophilic gradient structure allows for regulable Li+ flux and suppresses Li dendrite growth. Impressively, the Li||Li symmetric batteries with Ag@SiO2-TiO2 interlayers exhibit a prolonged lifespan of 1500 h at 0.5 mA cm-2 for 0.5 mAh cm-2. The full cells coupled with the Ag@SiO2-TiO2 interlayer show a capacity retention rate of 94.6% after 1000 cycles and a high rate capability. This work provides promising guidance for the design of a gradient-distributed lithiophilic structure toward an ultrastable Li metal anode.