Self-terminating, heterogeneous solid-electrolyte interphase enables reversible Li-ether cointercalation in graphite anodes.

Ether solvents are suitable for formulating solid-electrolyte interphase (SEI)-less ion-solvent cointercalation electrolytes in graphite for Na-ion and K-ion batteries. However, ether-based electrolytes have been historically perceived to cause exfoliation of graphite and cell failure in Li-ion batteries. In this study, we develop strategies to achieve reversible Li-solvent cointercalation in graphite through combining appropriate Li salts and ether solvents. Specifically, we design 1M LiBF4 1,2-dimethoxyethane (G1), which enables natural graphite to deliver ~91% initial Coulombic efficiency and >88% capacity retention after 400 cycles. We captured the spatial distribution of LiF at various length scales and quantified its heterogeneity. The electrolyte shows self-terminated reactivity on graphite edge planes and results in a grainy, fluorinated pseudo-SEI. The molecular origin of the pseudo-SEI is elucidated by ab initio molecular dynamics (AIMD) simulations. The operando synchrotron analyses further demonstrate the reversible and monotonous phase transformation of cointercalated graphite. Our findings demonstrate the feasibility of Li cointercalation chemistry in graphite for extreme-condition batteries. The work also paves the foundation for understanding and modulating the interphase generated by ether electrolytes in a broad range of electrodes and batteries.

Uncovering the Redox Shuttle Degradation Mechanism of Ether Electrolytes in Sodium-Ion Batteries and its Inhibition Strategy.

Ether-based electrolytes exhibit excellent performance when applied in different anode materials of sodium ion batteries (SIBs), but their exploration on cathode material is deficient and the degradation mechanism is still undiscovered. Herein, various battery systems with different operation voltage ranges are designed to explore the electrochemical performance of ether electrolyte. It is found for the first time that the deterioration mechanism of ether electrolyte is closely related to the "redox shuttle" between cathode and low-potential anode. The "shuttle" is discovered to occur when the potential of anodes is below 0.57 V, and the gas products coming from "shuttle" intermediates are revealed by differential electrochemical mass spectrometry (DEMS). Moreover, effective inhibition strategies by protecting low-potential anodes are proposed and verified; ethylene carbonate (EC) is found to be very effective as an additive by forming an inorganics-rich solid electrolyte interphase (SEI) on low-potential anodes, thereby suppressing the deterioration of ether electrolytes. This work reveals the failure mechanism of ether-based electrolytes applied in SIBs and proposes effective strategies to suppress the "shuttle," which provides a valuable guidance for advancing the application of ether-based electrolytes in SIBs.

Unveiling the Critical Role of Ion Coordination Configuration of Ether Electrolytes for High Voltage Lithium Metal Batteries.

Albeit ethers are favorable electrolyte solvents for lithium (Li) metal anode, their inferior oxidation stability (<4.0 V vs. Li/Li+ ) is problematic for high-voltage cathodes. Studies of ether electrolytes have been focusing on the archetype glyme structure with ethylene oxide moieties. Herein, we unveil the crucial effect of ion coordination configuration on oxidation stability by varying the ether backbone structure. The designed 1,3-dimethoxypropane (DMP, C3) forms a unique six-membered chelating complex with Li+ , whose stronger solvating ability suppresses oxidation side reactions. In addition, the favored hydrogen transfer reaction between C3 and anion induces a dramatic enrichment of LiF (a total atomic ratio of 76.7 %) on the cathode surface. As a result, the C3-based electrolyte enables greatly improved cycling of nickel-rich cathodes under 4.7 V. This study offers fundamental insights into rational electrolyte design for developing high-energy-density batteries.

Stabilizing Interfaces in High-Temperature NCM811-Li Batteries via Tuning Terminal Alkyl Chains of Ether Solvents.

Ether electrolytes are promising for lithium metal batteries. Despite the intensive research in recent years, most state-of-the-art ether electrolytes still cannot form reliable electrode-electrolyte interfaces in NCM811-Li batteries at diluted concentrations, especially in those operating at elevated temperatures. We report a simple but effective strategy to break this bottleneck and stabilize interfaces in high-temperature NCM811-Li batteries in ether electrolytes. We propose that by gradually extending the terminal groups of glycol diethers from methyl groups to n-butyl groups, the comprehensive stability of ether electrolytes is improved. An anion-dominated solvation structure is realized at a concentration of 1 M. Accordingly, the electrode-electrolyte interactions are suppressed, and a thinner, denser, and more inorganic-rich solid- /cathode-electrolyte interface is achieved. Additionally, the surface phase transition and structural degradation of NCM811 cathode are alleviated. Consequently, in the ethylene glycol dibutyl ether-based electrolyte, the Coulombic efficiency for Li-Cu cells working at 60 °C is boosted to 99.41 % with a cycling life of over 200 cycles. The lifespan of high-temperature NCM811-Li cells is prolonged by more than 400 % with a stable average Coulombic efficiency of 99.77 % at quasi-practical conditions of 50 µm Li, lean electrolyte of 10 µL mAh-1 , and medium-high cathode loading of >2.2 mAh cm-2 .

Building a Beyond Concentrated Electrolyte for High-Voltage Anode-Free Rechargeable Sodium Batteries.

Low-cost and scalable sodium ion (Na-ion) batteries serve as an ideal alternative to the current lithium-ion batteries. To compensate for the shortage of energy density, the most accessible solution is developing a high-voltage anode-free configuration comprising a lightweight Al current collector on the anode and a high-voltage sodiumized cathode. However, it imposes stringent Na reversibility and high-voltage stability requirements on the electrolyte. A 3A zeolite molecular sieve film is rationally designed, and a highly aggregated solvation structure is constructed through the size effect. It suppresses the trace but continuous oxidative decomposition and extends the oxidative stability to 4.5 V without sacrificing the Na reversibility of the anode (99.91 %). Thus, we can make anode-free cells with high energy density of 369 and 372 W h kg-1 for 4.0 and 4.25 V class cells, respectively. Furthermore, this strategy enables a long lifespan (250 cycles) for 4.0 V-class anode-free cells.

Intrinsic Nonflammable Ether Electrolytes for Ultrahigh-Voltage Lithium Metal Batteries Enabled by Chlorine Functionality.

The issues of inherent low anodic stability and high flammability hinder the deployment of the ether-based electrolytes in practical high-voltage lithium metal batteries. Here, we report a rationally designed ether-based electrolyte with chlorine functionality on ether molecular structure to address these critical challenges. The chloroether-based electrolyte demonstrates a high Li Coulombic efficiency of 99.2 % and a high capacity retention >88 % over 200 cycles for Ni-rich cathodes at an ultrahigh cut-off voltage of 4.6 V (stable even up to 4.7 V). The chloroether-based electrolyte not only greatly improves electrochemical stabilities of Ni-rich cathodes under ultrahigh voltages with interphases riched in LiF and LiCl, but possesses the intrinsic nonflammable safety feature owing to the flame-retarding ability of chlorine functional groups. This study offers a new approach to enable ether-based electrolytes for high energy density, long-life and safe Li metal batteries.

BF4- Tailoring Solvation Chemistry of Ether-Based Electrolytes to Construct Stable Electrode/Electrolyte Interfaces for Sodium-Ion Full Batteries.

The ether-based electrolytes show excellent performance on anodes in sodium-ion batteries (SIBs), but they still show poor compatibility with the cathodes. Here, ether electrolytes with NaBF4 as the main salt or additive were applied in NFM//HC full cells and showed enhanced performance than the electrolyte with NaPF6. Then, BF4- was found to have a stronger interaction with Na+, which could reduce the solvation of Na+ with the solvent, thus inducing the formation of the cathode electrolyte interface (CEI) and solid electrolyte interface (SEI) layers rich in inorganic species. Moreover, the morphology, structure, composition, and solubility of CEI and SEI were explored, concluding that NaBF4 could induce more stable CEI and SEI layers rich in B-containing species and inorganics. This work proposes using NaBF4 as the main salt or additive to improve the performance of ether electrolytes in NFM//HC full cells, which provides a strategy to improve the compatibility of ether-based electrolytes and cathodes.

Long-Life Lithium-Ion Sulfur Pouch Battery Enabled by Regulating Solvent Molecules and Using Lithiated Graphite Anode.

The development of lithium-sulfur (Li-S) batteries is severely limited by the shuttle effect and instability of Li-metal anode. Constructing Li-ion S batteries (LISBs), by using more stable commercial graphite (Gr) anode instead of Li-metal, is an effective way to realize long-cycle-life Li-S batteries. However, Gr electrode is usually incompatible with the ether-based electrolytes commonly used for Li-S batteries due to the Li+ -ether complex co-intercalation into Gr interlayers. Herein, a solvent molecule structure regulation strategy is provided to weaken the Li+ -solvent binding by increasing steric hindrance and electronegativity, to accelerate Li+ de-solvation process and prevent Li+ -ether complex co-intercalation into Gr anode. Meanwhile, the weakly solvating power of solvent can suppress the shuttle effect of lithium polysulfides and makes more anions participate in Li+ solvation structure to generate a stable anion-derived solid electrolyte interface on Gr surface. Therefore, a LISB coin-cell consisting of lithiated graphite anode and S@C cathode displays a stable capacity of ≈770 mAh g-1 within 200 cycles. Furthermore, an unprecedented practical LISB pouch-cell with a high Gr loading (≈10.5 mg cm-2 ) also delivers a high initial capacity of 802.3 mAh g-1 and releases a stable capacity of 499.1 mAh g-1 with a high Coulombic efficiency (≈95.9%) after 120 cycles.

Elucidating the Mechanism of Fast Na Storage Kinetics in Ether Electrolytes for Hard Carbon Anodes.

The sodium storage performance of a hard carbon (HC) anode in ether electrolytes exhibits a higher initial Coulombic efficiency (ICE) and better rate performance compared to conventional ester electrolytes. However, the mechanism behind faster Na storage kinetics for HC in ether electrolytes remains unclear. Herein, a unique solvated Na+ and Na+ co-intercalation mechanism in ether electrolytes is reported using designed monodispersed HC nanospheres. In addition, a thin solid electrolyte interphase film with a high inorganic proportion formed in an ether electrolyte is visualized by cryo transmission electron microscopy and depth-profiling X-ray photoelectron spectroscopy, which facilitates Na+ transportation, and results in a high ICE. Furthermore, the fast solvated Na+ diffusion kinetics in ether electrolytes are also revealed via molecular dynamics simulation. Owing to the contribution of the ether electrolytes, an excellent rate performance (214 mAh g-1 at 10 A g-1 with an ultrahigh plateau capacity of 120 mAh g-1 ) and a high ICE (84.93% at 1 A g-1 ) are observed in a half cell; in a full cell, an attractive specific capacity of 110.3 mAh g-1 is achieved after 1000 cycles at 1 A g-1 .

Designing Advanced In Situ Electrode/Electrolyte Interphases for Wide Temperature Operation of 4.5 V Li||LiCoO2 Batteries.

High-energy-density batteries with a LiCoO2 (LCO) cathode are of significant importance to the energy-storage market, especially for portable electronics. However, their development is greatly limited by the inferior performance under high voltages and challenging temperatures. Here, highly stable lithium (Li) metal batteries with LCO cathode, through the design of in situ formed, stable electrode/electrolyte interphases on both the Li anode and the LCO cathode, with an advanced electrolyte, are reported. The LCO cathode can deliver a high specific capacity of ≈190 mAh g-1 and show greatly improved cell performances under a high charge voltage of 4.5 V (even up to 4.55 V) and a wide temperature range from -30 to 55 °C. This work points out a promising approach for developing Li||LCO batteries for practical applications. This approach can also be used to improve the high-voltage performance of other batteries in a broad temperature range.

Cycling Performance and Kinetic Mechanism Analysis of a Li Metal Anode in Series-Concentrated Ether Electrolytes.

Li metal batteries are facing unprecedented opportunities because of the urgent demand for high-energy-density rechargeable batteries as well as enormous challenges due to their low Coulombic efficiency and notorious safety issues. Recently, high-concentration electrolytes have attracted much attention owing to their superior properties. However, Li metal plating is an exceedingly complicated process including several elementary steps; it is not comprehensive enough to uncover the mystery of concentrated electrolytes by studying the structure of electrolytes, the existence of anions, cations, and solvent molecules using only conventional means. Here, we first report the apparent cycling performance of Li metal anode in series-concentrated LiTFSI/DOL-DME electrolytes. It shows that the Li metal anode in 4 M LiTFSI/DOL-DME electrolyte can operate 180 cycles stably at 2 mA cm-2 with an average Coulombic efficiency of 98%, in which the Li plating layer is more compact and without dendrites. Subsequently, the kinetic parameters were obtained by the microelectrode technique. We found that appropriate mass transfer, interface, and surface step kinetic parameters (migration number, exchange current density, nucleation rate, and corrosion rate) and their good matching degree are favorable for the cycling of Li metal anodes. This work reveals the relationship between the elementary steps and the apparent cycling performance during the Li plating process under certain operating conditions; it is a major breakthrough in the study of high-concentration electrolytes and can provide a new perspective for future research.