Localized concentrated high-concentration electrolyte enhanced stability and safety for high voltage Li-ion batteries

Screening of F-containing electrolyte additives and clarifying their decomposition routes for stable Li metal anodes

Constructing a LiF-rich solid electrolyte interphase (SEI) is a feasible strategy for inhibiting lithium (Li) dendrites of Li metal anodes (LMAs). However, selecting appropriate F-containing additives with efficient LiF contribution is still under active research. Herein, a series of fluorinated additives with diverse F/C molar ratios are investigated, and we demonstrate that the hexafluoroglutaric anhydride (F6-0) holds the best capability to derive the LiF-rich SEI in regular carbonate electrolytes (RCEs). To ameliorate the decomposition kinetics of the F6-0, LiNO3 (LNO) as an adjuvant is further introduced in the system. As a result, the reduction efficiency of F6-0 is increased to 91% under the F6-0/LNO synergistic effect, enabling the LMA with a uniform LiF-rich SEI in the RCE with merely 4 vol. % F6-0/LNO (F6L) addition. The LiNi0.8Co0.1Mn0.1O2||Li-20μm full-cell with the F6L also showcases better cycling and rate performances than the cases with other F-containing additives.

Nonsolvating Fluoroaromatic Cosolvent Enabled Long-Term Cycling of High-Voltage Lithium-Ion Batteries with Organosulfur Electrolytes

The structure-activity relationships of nonsolvating cosolvents for organosulfur-based electrolyte systems were revealed. The performance of nonsolvating dilutant fluorobenzene (FB) was compared to various fluorinated ether dilutants in high-voltage electrolytes containing a concentration of 1.2 M LiPF6 dissolved in fluoroethylene carbonate (FEC), ethyl methyl sulfone (EMS), and the dilutant. In a high-voltage and high-loading LiNi0.8Mn0.1Co0.1O2 (NMC811) full cell configuration, the organosulfur-based electrolyte containing FB dilutant enabled superior electrochemical performance compared to the electrolytes using other nonsolvating fluorinated ether formulations. Moreover, the FB-containing electrolyte exhibited the highest ionic conductivity and lowest viscosity among all organosulfur-based electrolytes containing nonsolvating dilutant. These improvements are attributed to the enhanced physical properties of electrolyte and lithium-ion mobility. Furthermore, by employing first-principles simulations, the observed suppression of side reactions at high voltage is linked to FB's lower reactivity toward singlet dioxygen, which is likely produced at the NMC interface. Overall, FB is considered an excellent diluent that does not impede cell operation by mass decomposition at the cathode.

570 Wh kg⁻1-Grade Lithium Metal Pouch Cell with 4.9V Highly Li+ Conductive Armor-Like Cathode Electrolyte Interphase via Partially Fluorinated Electrolyte Engineering

Lithium-rich manganese-based layered oxides (LRMOs) are promisingly used in high-energy lithium metal pouch cells due to high specific capacity/working voltage. However, the interfacial stability of LRMOs remains challenging. To address this question, a novel armor-like cathode electrolyte interphase (CEI) model is proposed for stabilizing LRMO cathode at 4.9 V by exploring partially fluorinated electrolyte formulation. The fluoroethylene carbonate (FEC) and tris (trimethylsilyl) borate (TMSB) in formulated electrolyte largely contribute to the formation of 4.9 V armor-like CEI with LiBxOy and LixPOyFz outer layer and LiF- and Li3PO4-rich inner part. Such CEI effectively inhibits lattice oxygen loss and facilitates the Li+ migration smoothly for guaranteeing LRMO cathode to deliver superior cycling and rate performance. As expected, Li||LRMO batteries with such electrolyte achieve capacity retention of 85.7% with high average Coulomb efficiency (CE) of 99.64% after 300 cycles at 4.8 V/0.5 C, and even obtain capacity retention of 87.4% after 100 cycles at higher cut-off voltage of 4.9 V. Meanwhile, the 9 Ah-class Li||LRMO pouch cells with formulated electrolyte show over thirty-eight stable cycling life with high energy density of 576 Wh kg-1 at 4.8 V.

Nonflammable Localized High-Concentration Electrolytes with Long-Term Cycling Stability for High-Performance Li Metal Batteries

High-concentration electrolytes (HCEs) can effectively enhance interface stability and cycle performance of Li metal batteries (LMBs). However, HCEs suffer from low ionic conductivity, high viscosity, high cost, and high density. Herein, fluorobenzene (FB) diluted localized high-concentration electrolytes (LHCEs) consisting of lithium bis(fluorosulfonyl)imide (LiFSI)/triethyl phosphate (TEP)/FB are developed. 2.3 M LHCE can reserve concentrated Li⁺-FSI^--TEP solvation structures. Diluent FB possesses low density, low viscosity, low cost, low dielectric constant, low LUMO, and a good fluorine-donating property, which can significantly reduce viscosity, improve ionic conductivity, promote the formation of LiF-rich SEI, and enhance interaction of Li⁺-TEP and Li⁺-FSI^- ion-pairs of the electrolytes. 2.3 M LHCE is a highly safe nonflammable electrolyte. 2.3 M LHCE can effectively inhibit dendrite growth on Li metal anode. 2.3 M LHCE endows LiFePO₄ cells with good rate capability (discharge capacity of 112.7 mAh g^-1 at 5 C rate) and excellent cycling performance (capacity retention of 95.4% after 1000 cycles). 2.3 M LHCE also shows good compatibility with LiNi_0.8Co_0.1Mn_0.1O₂ and exhibits outstanding cycle stability (capacity retention of 86.4% after 500 cycles). Therefore, 2.3 M LHCE is a promising electrolyte for practical applications in LMBs.

Antioxidation Mechanism of Highly Concentrated Electrolytes at High Voltage

It has been researched that highly concentrated electrolytes (HCEs) can solve the problem of the excessive decomposition of dilute electrolytes at a high voltage, but the mechanism is not clear. In this work, the antioxidation mechanism of HCE at a high voltage was investigated by in situ electrochemical tests and theoretical calculations from the perspective of the solvation structure and physicochemical property. The results indicate that compared with the dilute electrolyte, the change of solvation structures in HCE makes more PF₆^- anions easier to be oxidized prior to the dimethyl carbonate solvents, resulting in a more stable cathode-electrolyte interphase (CEI) film. First, the lower oxidation potential of the solvation structure with more PF₆^- anions inhibits the side effects of the electrolyte effectively. Second, the CEI film, consisted of LiF and Li_xPO_yF_z generated from the oxidation of PF₆^- and Li₃PO₄ generated from the hydrolysis of LiPF₆ via the soluble PO₂F₂^- intermediate, can reduce the interface impedance and improve the conductivity. Intriguingly, the high viscosity of HCEs and the hydrolysis of LiPF₆ are proven to play a positive role in enhancing the interfacial stability of the electrolyte/electrode at a high voltage. This study builds a deep understanding of the bulk and interface properties of HCEs and provides theoretical support for their large-scale application in high-voltage battery materials.

High-voltage liquid electrolytes for Li batteries: progress and perspectives

Since the advent of the Li ion batteries (LIBs), the energy density has been tripled, mainly attributed to the increase of the electrode capacities. Now, the capacity of transition metal oxide cathodes is approaching the limit due to the stability limitation of the electrolytes. To further promote the energy density of LIBs, the most promising strategies are to enhance the cut-off voltage of the prevailing cathodes or explore novel high-capacity and high-voltage cathode materials, and also replacing the graphite anode with Si/Si-C or Li metal. However, the commercial ethylene carbonate (EC)-based electrolytes with relatively low anodic stability of ∼4.3 V vs. Li⁺/Li cannot sustain high-voltage cathodes. The bottleneck restricting the electrochemical performance in Li batteries has veered towards new electrolyte compositions catering for aggressive next-generation cathodes and Si/Si-C or Li metal anodes, since the oxidation-resistance of the electrolytes and the in situ formed cathode electrolyte interphase (CEI) layers at the high-voltage cathodes and solid electrolyte interphase (SEI) layers on anodes critically control the electrochemical performance of these high-voltage Li batteries. In this review, we present a comprehensive and in-depth overview on the recent advances, fundamental mechanisms, scientific challenges, and design strategies for the novel high-voltage electrolyte systems, especially focused on stability issues of the electrolytes, the compatibility and interactions between the electrolytes and the electrodes, and reaction mechanisms. Finally, novel insights, promising directions and potential solutions for high voltage electrolytes associated with effective SEI/CEI layers are proposed to motivate revolutionary next-generation high-voltage Li battery chemistries.

Effect of temperature on concentrated electrolytes for advanced lithium ion batteries

Salt-concentrated electrolytes are emerging as promising electrolytes for advanced lithium ion batteries (LIBs) that can offer high energy density and improved cycle life. To further improve these electrolytes, it is essential to understand their inherent behavior at various operating conditions of LIBs. Molecular dynamics (MD) simulations are extensively used to study various properties of electrolytes and explain the associated molecular-level phenomena. In this study, we use classical MD simulations to probe the properties of the concentrated electrolyte solution of 3 mol/kg lithium hexafluorophosphate (LiPF₆) salt in the propylene carbonate solvent at various temperatures ranging from 298 to 378 K. Our results reveal that the properties such as ionic diffusivity and molar conductivity of a concentrated electrolyte are more sensitive to temperature compared to that of dilute electrolytes. The residence time analysis shows that temperature affects the Li⁺ ion solvation shell dynamics significantly. The effect of temperature on the transport and dynamic properties needs to be accounted carefully while designing better thermal management systems for batteries made with concentrated electrolytes to garner the advantages of these electrolytes.

Localized high concentration electrolytes decomposition under electron-rich environments

Localized high concentration electrolytes have been proposed as an effective route to construct stable solid-electrolyte interphase (SEI) layers near Li-metal anodes. However, there is still a limited understanding of the decomposition mechanisms of electrolyte components during SEI formation. In this work, we investigate reactivities of lithium bis(fluorosulfonyl)imide (LiFSI, salt), 1,2-dimethoxyethane (DME, solvent), and tris(2,2,2-trifluoroethyl)orthoformate (TFEO, diluent) species in DME + TFEO mixed solvents and 1M LiFSI/DME/TFEO solutions. By supplying an excess of electrons into the simulation cell, LiFSI is initially reduced via a four-electron charge transfer reaction yielding F^- and N(SO₂)₂ ^3-. The local solvation environment has little effect on the subsequent TFEO reaction, which typically requires 6 |e| to decompose into F^-, HCOO^-, CH₂CF^-, and ^-OCH₂CF₃. Besides, the TFEO dehydrogenation reaction mechanism under an attack of anions is also identified. Unlike salt and diluent, DME shows good stability with any excess of electrons. The energetics of most relevant reactions are characterized. Most reactions are thermodynamically favorable with low activation barriers.

Electrolyte Therapy for Improving the Performance of LiNi0.5Mn1.5O4 Cathodes Assembled Lithium-Ion Batteries

High voltage spinel manganese oxide LiNi₀.₅Mn_1.5O₄ (LNMO) cathodes are promising for practical applications owing to several strengths including high working voltages, excellent operating safety, low costs, and so on. However, LNMO-based lithium-ion batteries (LIBs) fade rapidly mainly owing to unqualified electrolytes, hence becoming a big obstacle toward practical applications. To tackle this roadblock, substantial progress has been made thus far, and yet challenges still remain, while rare reviews have systematically discussed the status quo and future development of electrolyte optimization coupling with LNMO cathodes. Here, we discuss cycling degradation mechanisms at the cathode/electrolyte interface and ideal requirements of electrolytes for LNMO cathode-equipped LIBs, as well as review the recent advance of electrolyte optimization for LNMO cathode-equipped LIBs in detail. And then, the perspectives regarding the future research opportunities in developing state-of-the-art electrolytes are also presented. The authors hope to shed light on the rational optimization of advanced organic electrolytes in order to boost the large-scale practical applications of high voltage LNMO cathode-based LIBs.