Salt-in-Salt Reinforced Carbonate Electrolyte for Li Metal Batteries

Modulating Interfacial Solvation via Ion Dipole Interactions for Low-Temperature and High-Voltage Lithium Batteries

Extending the stability of ether solvents is pivotal for developing low-temperature and high-voltage lithium batteries. Herein, we elucidate the oxidation behavior of tetrahydrofuran with ternary BF4 -, PF6 - and difluoro (oxalato) borate anions and the evolution of interfacial solvation environment. Combined in situ analyses and computations illustrate that the ion dipole interactions and the subsequent formation of ether-Li+-anion complexes in electrolyte rearrange the oxidation order of solvated species, which enhances the electrochemical stability of ether solvent. Furthermore, preferential absorption of anions on the surface of high-voltage cathode favors the formation of a solvent-deficient electric double layer and an anti-oxidation cathode electrolyte interphase, inhibiting the decomposition of tetrahydrofuran. Remarkably, the formulated electrolyte based on ternary anion and tetrahydrofuran solvent endows the LiNi0.8Co0.1Mn0.1O2 cathode with considerable rate capability of 5.0 C and high capacity retention of 93.12 % after 200 cycles. At a charging voltage of 4.5 V, the Li||LiNi0.8Co0.1Mn0.1O2 cells deliver Coulombic efficiency above 99 % at both 25 and -30 °C.

Tri-Anion Solvation Structure Electrolyte Improves the Electrochemical Performance of Li||LiNi0.8Co0.1Mn0.1O2 Batteries

Li||LiNi0.8Co0.1Mn0.1O2 batteries, which consist of lithium metal anode (LMA) matched with NCM811 cathode, have an energy density more than twice that of lithium ion battery (LIB). However, the unstable electrode/electrolyte interface still hinders its practical application. Ether electrolytes show promise in improving the stability of LMA and NCM811 cathodes. However, a robust and stable electrode/electrolyte interface in Li||NCM811 batteries cannot be easily and efficiently achieved with most of the ether electrolytes reported in present studies. Herein, we present a straightforward and efficient tri-anion synergistic strategy to overcome this bottleneck. The addition of ClO4 - and NO3 - anions to LiFSI-based ether electrolytes forms a unique solvation structure with tri-anion (FSI-/ClO4 -/NO3 -) participation (LB511). This structure not only enhances the electrochemical window of the ether electrolytes but also achieves a stable Li||NCM811 batteries interface. The interaction between electrode and electrolyte is suppressed and an inorganic-rich (LiF/Li3N/LiCl) SEI/CEI layer is formed. Meanwhile, the coordination structure in the LB511 electrolyte increases the overpotential for Li deposition, resulting in a uniform and dense layer of Li deposition. Therefore, the Li||Cu cells using the LB511 electrolyte have an average CE of 99.6 %. The Li||NCM811 batteries was cycled stably for 250 cycles with a capacity retention of 81 % in the LB511 electrolyte (N/P=2.5, 0.5 C).

Boosting stable lithium deposition via Li3N-Enriched inorganic SEI induced by a polycationic polymer layer

Lithium (Li) metal is a promising anode material for future high-energy rechargeable batteries due to its remarkable properties. Nevertheless, excess Li in traditional lithium metal anodes (LMAs) reduces the energy density of batteries and increases safety risks. Electrochemical pre-lithiation is an effective technique for regulating the lithium content of the anodes. However, Cu foil or other non-Li based substrates used for pre-lithiation often have inhomogeneous surfaces and high nucleation barrier, leading to uneven tip deposition of lithium metal and fragile SEI. Herein, we have designed an interfacial layer composed of nano-Si particles and cationic polymer (poly (diallyldimethylammonium chloride)) (denoted as Si@PDDA) to induce the formation of Li3N-rich inorganic SEI and regulate the homogeneous plating/stripping of lithium. The uniformly dispersed nano-Si particles can decrease the Li+ nucleation overpotential through alloying reaction with lithium. The surface of Si nano-particles modified by PDDA contains numerous cationic sites, providing an electrostatic shielding layer to seeding the growth of Li metal and inhibiting dendrites formation. More promisingly, PDDA adsorbs electrolyte anions while transporting Li+, significantly accelerating the decomposition kinetics of inorganic salts within the electrolyte. Therefore, a SEI film rich in Li3N was formed on the anodes, ensuring the excellent interfacial stability and electrochemical cycling performance of LMAs. The symmetrical cells exhibit a cycle life of 900 h at 1 mA cm-2. Moreover, the practical full cells operate at a low negative/positive (N/P) capacity ratio (∼3) for over 160 cycles.

Reaction Center Shifting in Partially Fluorinated Electrolytes for Robust Lithium Metal Battery

The strategic formulation of a compatible electrolyte plays a pivotal role in extending the longevity of lithium-metal batteries (LMBs). Here, we present findings on a partially fluorinated electrolyte distinguished by a subdued solvation affinity towards Li+ ions and a concentrated anion presence within the primary solvation layer. This distinctive solvation arrangement redirects the focal points of reactions from solvent molecules to anions, facilitating the predominant involvement of anions in the creation of a LiF-enriched solid-electrolyte interphase (SEI). Electrochemical assessments showcase effective Li+ transport kinetics, diminished overpotential polarization for Li nucleation (28 mV), and prolonged cycling durability in Li||Li cells employing the partially fluorinated electrolyte. When tested in Li||NCM811 cells, the designed electrolyte delivers a capacity retention of 89.30 % and exhibits a high average Coulombic efficiency of 99.80 % over 100 cycles with a charge-potential cut-off of 4.6 V vs. Li/Li+ under the current density of 0.4C. Furthermore, even at a current density of 1C, the cells maintain 81.90 % capacity retention and a high average Coulombic efficiency of 99.40 % after 180 cycles. This work underscores the significance of weak-solvation interaction in partially fluorinated electrolytes and highlights the crucial role of solvent structure in enabling the long-term stability and high-energy density of LMBs.

Enhanced Long-Term Cycling Life of Ni-Rich NMC Cathodes in High-Voltage Lithium-Metal Batteries

Li metal batteries (LMBs) have revived people's interest due to their high energy density. This work compares the cycling stability, structure stability, and thermal stability of Li||0.7Nb-NMC 9055 (0.7% Nb-modified LiNi0.9Co0.05Mn0.05O2) system in commercial carbonate electrolyte (1.0 M LiPF6 in EC/DMC) and designed carbonate electrolyte (1.0 M LiPF6-0.125 M LiNO3-0.025 M Mg(TFSI)2 in FEC-EMC). Li||0.7Nb-NMC 9055 battery with designed carbonate electrolyte exhibited superior capacity retention, 80% after ∼500 cycles. This can be explained by the improved mechanical integrity of the secondary particles and large reduced charge transfer resistance. Further, the real-time thermal monitoring of full cell via a high-precision, multimode calorimeter TAM IV Micro XL shows that the designed carbonate electrolyte with multisalt additive and FEC cosolvent has less heat release during the charging and discharge process, allowing these high-nickel (Ni) cathodes to reach closer to their full potential.

Lightweight Electrolyte Design for Li/Sulfurized Polyacrylonitrile (SPAN) Batteries

Sulfurized polyacrylonitrile (SPAN) recently emerges as a promising cathode for high-energy lithium (Li) metal batteries owing to its high capacity, extended cycle life, and liberty from costly transition metals. As the high capacities of both Li metal and SPAN lead to relatively small electrode weights, the weight and specific energy density of Li/SPAN batteries are particularly sensitive to electrolyte weight, highlighting the importance of minimizing electrolyte density. Besides, the large volume changes of Li metal anode and SPAN cathode require inorganic-rich interphases that can guarantee intactness and protectivity throughout long cycles. This work addresses these crucial aspects with an electrolyte design where lightweight dibutyl ether (DBE) is used as a diluent for concentrated lithium bis(fluorosulfonyl)imide (LiFSI)-triethyl phosphate (TEP) solution. The designed electrolyte (d = 1.04 g mL-1) is 40%-50% lighter than conventional localized high-concentration electrolytes (LHCEs), leading to 12%-20% extra energy density at the cell level. Besides, the use of DBE introduces substantial solvent-diluent affinity, resulting in a unique solvation structure with strengthened capability to form favorable anion-derived inorganic-rich interphases, minimize electrolyte consumption, and improve cell cyclability. The electrolyte also exhibits low volatility and offers good protection to both Li metal anode and SPAN cathode under thermal abuse.

High-Concentrated Binary-Salt Ether Electrolytes for High-Voltage Lithium Metal Batteries with Ni-Rich Cathode

The incompatibility of ether electrolytes with a cathode dramatically limits its application in high-voltage Li metal batteries. Herein, we report a new highly concentrated binary salt ether-based electrolyte (HCBE, 1.25 M LiTFSI + 2.5 M LiFSI in DME) that enables stable cycling of high-voltage lithium metal batteries with the Ni-rich (NCM83, LiNi0.83Co0.12Mn0.05O2) cathode. Experimental characterizations and density functional theory (DFT) calculations reveal the special solvation structure in HCBE. A solvation structure rich in aggregates (AGGs) can effectively broaden the electrochemical window of the ether electrolyte. The anions in HCBE preferentially decompose under high voltage, forming a CEI film rich in inorganic components to protect the electrolyte from degradation. Thus, the high-energy-density Li||NCM83 cell has a capacity retention of ≈95% after 150 cycles. Significantly, the cells in HCBE have a high and stable average Coulombic efficiency of over 99.9%, much larger than that of 1 M LiPF6 + EC + EMC + DMC (99%). The result emphasizes that the anionic-driven formation of a cathode electrolyte interface (CEI) can reduce the number of interface side reactions and effectively protect the cathode. Furthermore, the Coulombic efficiency of Li||Cu using the HCBE is 98.5%, underscoring the advantages of using ether-based electrolytes. This work offers novel insights and approaches for the design of high-performance electrolytes for lithium metal batteries.

Entropy-Driven Hybrid Gel Electrolyte Enables Practical High-Voltage Lithium Metal Batteries

Electrolyte engineering plays a crucial role in enhancing the performance of lithium metal batteries (LMBs) featuring high-voltage cathodes and limited lithium anodes, thereby unlocking their potential for high-energy electrochemical storage. Herein, an entropy-driven hybrid gel electrolyte with enhanced diversity in Li-ion solvation structures is designed by incorporating substantial amounts of insoluble LiPO2F2 and LiNO3 salts into LiPF6-based carbonate electrolytes, followed by in situ thermal polymerization. Specifically, the Li+ solvation structures are modulated via ionophilic NO3- and PO2F2- to generate an anion-rich solvation sheath and thus promote anion reduction at the electrode-electrolyte interface. The interfaces enriched in anion-derived inorganic components facilitate rapid ionic transport, thus enabling smooth and dense Li morphology and ultimately enhancing the electrochemical performance of LMBs. As a result, this high-hybrid gel electrolyte confers LMBs employing high-voltage NCM cathodes, as demonstrated by sustained performance in both coin-cell (500 cycles at 4.5 V) and Ah-level pouch cell configurations under practical conditions (60 cycles, N/P: 1.92, and E/C: 2.0 g Ah -1).

Rational Molecular Engineering via Electron Reconfiguration toward Robust Dual-Electrode/Electrolyte Interphases for High-Performance Lithium Metal Batteries

High-energy-density lithium-metal batteries (LMBs) coupling lithium-metal anodes and high-voltage cathodes are hindered by unstable electrode/electrolyte interphases (EEIs), which calls for the rational design of efficient additives. Herein, we analyze the effect of electron structure on the coordination ability and energy levels of the additive, from the aspects of intramolecular electron cloud density and electron delocalization, to reveal its mechanism on solvation structure, redox stability, as-formed EEI chemistry, and electrochemical performances. Furthermore, we propose an electron reconfiguration strategy for molecular engineering of additives, by taking sorbide nitrate (SN) additive as an example. The lone pair electron-rich group enables strong interaction with the Li ion to regulate solvation structure, and intramolecular electron delocalization yields further positive synergistic effects. The strong electron-withdrawing nitrate moiety decreases the electron cloud density of the ether-based backbone, improving the overall oxidation stability and cathode compatibility, anchoring it as a reliable cathode/electrolyte interface (CEI) framework for cathode integrity. In turn, the electron-donating bicyclic-ring-ether backbone breaks the inherent resonance structure of nitrate, facilitating its reducibility to form a N-contained and inorganic Li2O-rich solid electrolyte interface (SEI) for uniform Li deposition. Optimized physicochemical properties and interfacial biaffinity enable significantly improved electrochemical performance. High rate (10 C), low temperature (-25 °C), and long-term stability (2700 h) are achieved, and a 4.5 Ah level Li||NCM811 multilayer pouch cell under harsh conditions is realized with high energy density (462 W h/kg). The proof of concept of this work highlights that the rational ingenious molecular design based on electron structure regulation represents an energetic strategy to modulate the electrolyte and interphase stability, providing a realistic reference for electrolyte innovations and practical LMBs.

Dual-Salt Electrolyte Additive Enables High Moisture Tolerance and Favorable Electric Double Layer for Lithium Metal Battery

The carbonate electrolyte chemistry is a primary determinant for the development of high-voltage lithium metal batteries (LMBs). Unfortunately, their implementation is greatly plagued by sluggish electrode interfacial dynamics and insufficient electrolyte thermodynamic stability. Herein, lithium trifluoroacetate-lithium nitrate (LiTFA-LiNO3 ) dual-salt additive-reinforced carbonate electrolyte (LTFAN) is proposed for stabilizing high-voltage LMBs. We reveal that 1) the in situ generated inorganic-rich electrode-electrolyte interphase (EEI) enables rapid interfacial dynamics, 2) TFA- preferentially interacts with moisture over PF6 - to strengthen the moisture tolerance of designed electrolyte, and 3) NO3 - is found to be noticeably enriched at the cathode interface on charging, thus constructing Li+ -enriched, solvent-coordinated, thermodynamically favorable electric double layer (EDL). The superior moisture tolerance of LTFAN and the thermodynamically stable EDL constructed at cathode interface play a decisive role in upgrading the compatibility of carbonate electrolyte with high-voltage cathode. The LMBs with LTFAN realize 4.3 V-NCM523/4.4 V-NCM622 superior cycling reversibility and excellent rate capability, which is the leading level of documented records for carbonate electrode.

Intrinsic Solubilization of Lithium Nitrate in Ester Electrolyte by Multivalent Low-Entropy-Penalty Design for Stable Lithium-Metal Batteries

LiNO3 is a remarkable additive that can dramatically enhance the stability of ether-based electrolytes at lithium metal anodes. However, it has long been constrained by its incompatibility with commercially used ester electrolytes. Herein, we correlated the fundamental role of entropy with the limited LiNO3 solubility and proposed a new low-entropy-penalty design that achieves high intrinsic LiNO3 solubility in ester solvents by employing multivalent linear esters. This strategy is conceptually different from the conventional enthalpic methods that relies on extrinsic high-polarity carriers. In this way, LiNO3 can directly interact with the primary ester solvents and fundamentally alters the electrolyte properties, resulting in substantial improvements in lithium-metal batteries with high Coulombic efficiency and cycling stability. This work illustrates the significance of regulating the solvation entropy for high-performance electrolyte design.

Crosslinked solubilizer enables nitrate-enriched carbonate polymer electrolytes for stable, high-voltage lithium metal batteries

High-voltage lithium metal batteries (LMBs) have been considered promising next-generation high-energy-density batteries. However, commercial carbonate electrolytes can scarcely be employed in LMBs owing to their poor compatibility with metallic lithium. N,N-dimethylacrylamide (DMAA)-a crosslinkable solubilizer with a high Gutmann donor number-is employed to facilitate the dissolution of insoluble lithium nitrate (LiNO3) in carbonate-based electrolytes and to form gel polymer electrolytes (GPEs) through in situ polymerization. The Li+ solvation structure of the GPEs is regulated using LiNO3 and DMAA, which suppresses the decomposition of LiPF6 and facilitates the formation of an inorganic-rich solid electrolyte interface. Consequently, the Coulombic efficiency (CE) of the Li||Cu cell assembled with a GPE increases to 98.5% at room temperature, and the high-voltage Li||NCM622 cell achieves a capacity retention of 80.1% with a high CE of 99.5% after 400 cycles. The bifunctional polymer electrolytes are anticipated to pave the way for next-generation high-voltage LMBs.

Demystifying the Salt-Induced Li Loss: A Universal Procedure for the Electrolyte Design of Lithium-Metal Batteries

Lithium (Li) metal electrodes show significantly different reversibility in the electrolytes with different salts. However, the understanding on how the salts impact on the Li loss remains unclear. Herein, using the electrolytes with different salts (e.g., lithium hexafluorophosphate (LiPF6), lithium difluoro(oxalato)borate (LiDFOB), and lithium bis(fluorosulfonyl)amide (LiFSI)) as examples, we decouple the irreversible Li loss (SEI Li+ and "dead" Li) during cycling. It is found that the accumulation of both SEI Li+ and "dead" Li may be responsible to the irreversible Li loss for the Li metal in the electrolyte with LiPF6 salt. While for the electrolytes with LiDFOB and LiFSI salts, the accumulation of "dead" Li predominates the Li loss. We also demonstrate that lithium nitrate and fluoroethylene carbonate additives could, respectively, function as the "dead" Li and SEI Li+ inhibitors. Inspired by the above understandings, we propose a universal procedure for the electrolyte design of Li metal batteries (LMBs): (i) decouple and find the main reason for the irreversible Li loss; (ii) add the corresponding electrolyte additive. With such a Li-loss-targeted strategy, the Li reversibility was significantly enhanced in the electrolytes with 1,2-dimethoxyethane, triethyl phosphate, and tetrahydrofuran solvents. Our strategy may broaden the scope of electrolyte design toward practical LMBs.

Highly Oxidative-Resistant Cyano-Functionalized Lithium Borate Salt for Enhanced Cycling Performance of Practical Lithium-Ion Batteries

Lithium difluoro(oxalato) borate (LiDFOB) has been widely investigated in lithium-ion batteries (LIBs) owing to its advantageous thermal stability and excellent aluminum passivation property. However, LiDFOB tends to suffer from severe decomposition and generate a lot of gas species (e.g., CO2 ). Herein, a novel cyano-functionalized lithium borate salt, namely lithium difluoro(1,2-dihydroxyethane-1,1,2,2-tetracarbonitrile) borate (LiDFTCB), is innovatively synthesized as a highly oxidative-resistant salt to alleviate above dilemma. It is revealed that the LiDFTCB-based electrolyte enables LiCoO2 /graphite cells with superior capacity retention at both room and elevated temperatures (e.g., 80 % after 600 cycles) with barely any CO2 gas evolution. Systematic studies reveal that LiDFTCB tends to form thin and robust interfacial layers at both electrodes. This work emphasizes the crucial role of cyano-functionalized anions in improving cycle lifespan and safety of practical LIBs.

Designing a Stable Solid Electrolyte Interphase on Lithium Metal Anodes by Tailoring a Mg Atom Center and the Inner Helmholtz Plane for Lithium-Sulfur Batteries

Arising from the extraordinary theoretical energy density, rechargeable lithium-sulfur (Li-S) batteries have been reputed as one of the most appealing options for next-generation high-performance energy storage and conversion devices. Unfortunately, their industrial implementation has been strongly governed by the formation of Li dendrites caused by the unstable solid electrolyte interphase (SEI) film. Herein, we report a novel electrolyte by introducing the Mg(NO₃)₂ additive to suppress the growth of Li dendrites, further improving the cycling lifetime of Li-S batteries. On the one hand, Mg²⁺ can rapidly react with Li atoms to generate Mg atoms, replacing the Li atoms on the top surface of Li metal and forming the Mg center simultaneously. On the other hand, NO₃^- can be adsorbed in the inner Helmholtz plane and reduced as an inorganic-rich SEI film for stabilizing the Li metal anode when the electrolyte comes in contact with Li metal, effectively mitigating the formation of Li dendrites. Combining the experimental results and theoretical calculations, we confirm that the Mg atom center and the inorganic-rich SEI film are both beneficial for enhancing the electrochemical performance of Li-S batteries. This work provides a new insight into the electrolyte additive and a possible alternative for the design of high-performance Li-S batteries beyond the LiNO₃ additive.

A Novel Potassium Salt Regulated Solvation Chemistry Enabling Excellent Li-Anode Protection in Carbonate Electrolytes

In lithium-metal batteries (LMBs), the compatibility of Li anode and conventional lithium hexafluorophosphate-(LiPF₆ ) carbonate electrolyte is poor owing to the severe parasitic reactions. Herein, to resolve this issue, a delicately designed additive of potassium perfluoropinacolatoborate (KFPB) is unprecedentedly synthesized. On the one hand, KFPB additive can regulate the solvation structure of the carbonate electrolyte, promoting the formation of Li⁺ FPB^- and K⁺ PF₆ ^- ion pairs with lower lowest unoccupied molecular orbital (LUMO) energy levels. On the other hand, FPB^- anion possesses strong adsorption ability on Li anode. Thus, anions can preferentially adsorb and decompose on the Li-anode surface to form a conductive and robust solid-electrolyte interphase (SEI) layer. Only with a trace amount of KFPB additive (0.03 m) in the carbonate electrolyte, Li dendrites' growth can be totally suppressed, and Li||Cu and Li||Li half cells exhibit excellent Li-plating/stripping stability upon cycling. Encouragingly, KFPB-assisted carbonate electrolyte enables high areal capacity LiCoO₂ ||Li, LiNi_0.8 Co_0.1 Mn_0.1 O₂ (NCM811)||Li, and LiNi_0.8 Co_0.05 Al_0.15 O₂ (NCA)||Li LMBs with superior cycling stability, showing its excellent universality. This work reveals the importance of designing novel additives to regulate the solvation structure of carbonate electrolytes in improving its interface compatibility with the Li anode.

Electrochemical Atomic Force Microscopy Study on the Dynamic Evolution of Lithium Deposition

Lithium metal is one of the most promising anode materials for lithium-ion batteries; however, lithium dendrite growth hinders its large-scale development. So far, the dendrite formation mechanism is unclear. Herein, the dynamic evolution of lithium deposition in etheryl-based and ethylene carbonate (EC)-based electrolytes was obtained by combining an in situ electrochemical atomic force microscope (EC-AFM) with an electrochemical workstation. Three growth modes of lithium particles are proposed: preferential, merged, and independent growth. In addition, a lithium deposition schematic is proposed to clearly describe the morphological changes in lithium deposition. This schematic shows the process of lithium deposition, thus providing a theoretical basis for solving the problem of lithium dendrite growth.

Battery Performance Amelioration by Introducing a Conducive Mixed Electrolyte in Rechargeable Mg-O2 Batteries

With magnesium being a cost-effective anode metal compared to the other conventional Li-based anodes in the energy market, it could be a capable source of energy storage. However, Mg-O2 batteries have struggled its way to overcome the poor cycling stability and sluggish reaction kinetics. Therefore, Ru metallic nanoparticles on carbon nanotubes (CNTs) were introduced as a cathode for Mg-O2 batteries, which are known for their inherent electronic properties, large surface area, and increased crystallinity to favor remarkable oxygen reduction reactions and oxygen evolution reactions (ORR and OER). Also, we deployed a first-of-its-kind, conducive mixed electrolyte (CME) (2 M Mg(NO3)2:1 M Mg(TFSI)2/diglyme). Hence, this synergistic incorporation of CME-based Ru/CNT Mg-O2 batteries could unleash long cycle life with low overpotential, excellent reversibility, and high ionic conductivity and also reduces the intrinsic corrosion behavior of Mg anodes. Correspondingly, this novel amalgamation of CME with Ru/CNT cathode has displayed superior cyclic stability of 65 cycles and a maximum discharge potential of 25 793 mAh g-1 with a small overvoltage plateau of 1.4 V, noticeably subjugating the findings of conventional single electrolyte (CSE) (1 M Mg(TFSI)2/diglyme). This CME-based Ru/CNT Mg-O2 battery design could have a significant outcome as a future battery technology.