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Lithium-ion batteries (LIBs) suffering from severe performance degradation because of the unstable solid electrolyte interphase (SEI) on the anode at low temperature restricts their practical applications. Herein, lithium difluorophosphate (LiDFP) as the additive is introduced into lithium bis(fluorosulfonyl) imide (LiFSI) based electrolyte to improve the electrochemical performance of graphite/Li half-cells at low temperature. Contrary to the popular perception, we reveal at low temperature that LiDFP attenuates the decomposition of LiFSI by competing the hydrolysis reaction with LiFSI to generate a SEI film rich in LiF and Li 3 PO 4 during prolonged cycling, rather than preferential decomposition. Additionally, the possible reaction equations and the interaction mechanism between LiDFP and LiFSI are proposed by combining in situ electrochemical impedance electrochemical (PRIs-EIS) tests, spectroscopic characterization techniques and density functional theory (DFT) calculations. It is demonstrated that the strong binding energy between LiF (from the decomposition of LiFSI) and LiDFP makes LiDFP easier deposit on the electrode surface. This work demonstrates the synergistic role combining the complementary advantage of film-forming additives and main lithium salts to improve the interfacial stability of LIBs at low temperature. Additionally, it can also pave the new pathway for the design of low temperature electrolytes in LIBs. [Display omitted] • The synergy effect of additive LiDFP and LiFSI improves the interface stability. • The competitive hydrolysis reaction of LiDFP and LiFSI is elucidated. • The strong binding energy between LiF and LiDFP enables LiDFP easier deposit. • The stable and robust interface can avoid the aggregation of organics. [ABSTRACT FROM AUTHOR]

• Directly introducing LiNO 3 into the Gr slurry overcomes its solubility limit. • Sustained releasing LiNO 3 from the Gr reservoir constitutes a virtuous circle in LCE. • Stronger bind of NO 3 –-Li+ weakens shielding effect, inhibiting solvents decomposition. • Increased interaction between anions and Li+ generates a Li 3 N&LiF-rich SEI. • Expanding the LCEs application by regulating the solvation and stabilizing interface. The high de-solvation energy derived from the solvent-rich Li+ solvation sheath and the resultant porous organic-rich electrode/electrolyte interface has hindered the commercialization of low-concentration electrolytes (LCEs). Here, unlike the common method so far reported, we invent an approach of sustained releasing LiNO 3 from the graphite (Gr) anode reservoir to optimize the solvation structure and form a stable interface in LCEs. The solubility limit can be overcome by introducing LiNO 3 directly in the Gr slurry. Moreover, the stronger binding energy between NO 3 – and Li+ drives NO 3 – enter the Li+ solvation sheath, weakening the shielding effect of solvent on Li+, and surprisingly increasing the coordination of PF 6 - to Li+. Thermodynamically, the corresponding lowest unoccupied molecular orbital energy levels of anions decrease, reducing the de-solvation energy of Li+ and forming a Li 3 N&LiF-rich solid electrolyte interface (SEI). This inorganic-rich SEI demonstrates advantages of inhibiting the decomposition of solvent and facilitating the transmission of Li+. As a result, the cell with Gr anode containing 1 wt% LiNO 3 exhibits superior cycle performance in LCE, almost no degradation within 200 cycles, even higher than that in conventional concentration electrolyte. More importantly, the continuous consumption of NO 3 – in turn promotes its sustained redissolution from the anode, constituting a virtuous circle of regulating solvation structure, stabilizing interface and improving battery performance. This work blazes a new trail to revive lithium salt additives with low solubility, and expands the design strategy and application prospect of LCEs by building the bridges between regulating the solvation structure and constructing favorable electrode/electrolyte interface. [ABSTRACT FROM AUTHOR]

The prevailing account attributes the positive effects of boron-based additives on solid electrolyte interface (SEI) layers to its solid products by prior decomposition. However, this study proves the BOF2− anion, a soluble product of film formation additive of lithium difluoro(oxalato)borate, plays a crucial role in improving the SEI layer. Specifically, BOF2− suppresses the excessive decomposition of salt anions by aggregating on the electrode surface. Meanwhile, it catalyzes the reduction of ethylene carbonate, leading to improving the stability of the interface. In the end, ultrasound shaking is used to generate more BOF2− anion and increase the cycle performance of cells at a high rate as excepted. This work helps to understand the effects of soluble products on the SEI forming process and provides new insights for improving the stability of electrode electrolyte interfaces. [Display omitted] • An effect mechanism of SEI formation is proved from the view of soluble product. • The soluble product of LiDFOB prevents its excessive decomposition. • The soluble product catalyzes the reduction of EC. [ABSTRACT FROM AUTHOR]

While the solid electrolyte interphase (SEI) film in carbonate-based lithium-ion batteries (LIBs) has been extensively studied, less is known about the formation of SEI due to the complexity and instability of interfacial chemical reactions between Li+ and solvent. This study presents new insights into the formation mechanism of SEI film on graphite anode in ethylene carbonate (EC) and dimethyl carbonate (DMC) co-solvent-based electrolytes. The potential-resolved in-situ electrochemical impedance spectroscopy technology and density functional theory calculations combined with multi-sweep cyclic voltammetry are utilized to confirm that EC prefers to solvate with Li+. Then, a large amount of Li+-EC and a small amount of Li+-DMC solvent molecules undergo solvent migration process and de-solvation at the electrode/electrolyte interface. It was found that Li+-DMC is preferentially de-solvation and reduced to form an unstable SEI film, while Li+-EC inhibits the reduction of DMC and turns the reduction process to be diffusion-controlled. This work provides a fresh perspective on the formation of SEI film by Li+-solvent interaction, which could guide the construction of a stable interface by regulating the Li+-solvation kinetic for high-performance LIBs. [ABSTRACT FROM AUTHOR]

A non-carbonaceous titanium-based materials of Na 2 Ti 3 O 7 have been considered as an attractive anode material for sodium ion battery (SIBs), due to its low charge potential and high energy density. However, the selection of preferred sodium salts in liquid electrolyte is still a challenge to improve battery performance. Herein, the compatibility of Na 2 Ti 3 O 7 @C anode material with sodium perchlorate (NaClO 4)-based and sodium trifluoromethyl sulfonate (NaCF 3 SO 3)-based electrolytes are studied, respectively. The results show that solid electrolyte interphase (SEI) film formed by NaCF 3 SO 3 -based electrolyte is thin, compact and uniform, which can effectively reduce the transmission energy barrier of Na+. Different from that, the SEI film derived in NaClO 4 -based electrolyte contains more Na 2 CO 3 and NaF, caused by the continuous decomposition of NaClO 4 salt in carbonate solvent. It makes the SEI film thicker and the interface impedance larger, which results in the decline of battery cycle performance. In addition, more sulfur-containing compounds with good sodium conductivity and stability are generated due to the decomposed of NaCF 3 SO 3 salt in carbonate solvent, which improves interfacial transport performance of materials, and inhibits the increase of the SEI film thickness and thus improves the cycling and rate performance of batteries. This study provides a reference method for designing advanced SIBs. [Display omitted] • NaCF 3 SO 3 improves the initial coulomb efficiency of the material. • NaClO 4 promotes the formation of a thick but soft SEI film containing more Na 2 CO 3. • A thin but hard SEI film derived NaCF 3 SO 3 contains more stable inorganic species. • NaCF 3 SO 3 -based electrolyte is more conducive to performance promotion of Na 2 Ti 3 O 7 @C. • Direct visualization information of the formation of SEI film is provided. [ABSTRACT FROM AUTHOR]

[Display omitted] • A potential resolved in-situ electrochemical impedance technique was built. • The solid electrolyte interface layer still be formed and decomposed after long cycle. • The contribution of the interface layer compounds to impedance was distinguished. An understanding of the formation and evolution of the solid electrolyte interface (SEI) layer is still a challenge for lithium ion batteries due to its complexity and non-uniformity. Herein, an in-situ electrochemical technique, named potential resolved in-situ electrochemical impedance spectroscopy (PRIs-EIS), have developed to correlate the Nyquist and Bode plot changes with the voltammetric scan, which is then used to determine the correspondence between circuit components and SEI layer composition. Moreover, the formation and re-oxidation of organic compounds in the SEI layer are studied in conjunction with the electrochemical quartz crystal microbalance characteristics. In particular, it is shown that more inorganic compounds accumulate in the SEI layer as the cycle continues, repairing the pore structure, but decreasing the toughness of the SEI layer. The PRIs-EIS technique is shown to be a powerful, useful and cost-effective tool to illuminate the interfacial reaction mechanism, and the results from this technique aid in the evaluation and design of electrolyte systems. [ABSTRACT FROM AUTHOR]

The dissolution, migration, and deposition of transition metal (TM) ions are the main reasons for the capacity degradation of lithium-ion batteries, which significantly affect their comprehensive electrochemical performance and safety. In this work, the effects of different TM ions (Mn2+, Co2+, and Ni2+) on the electrochemical performance of lithium/mesocarbon microbeads half-cell were compared, and the mechanism was discussed. It is found that Mn2+ ions inhibit the decomposition of lithium salt and promote the decomposition of solvents, resulting in the crack of electrode material and solid electrolyte interface (SEI) film and the increase of capacity degradation. In contrast, Ni2+ ions promote the decomposition of lithium salts to a certain extent, thus inhibiting the excessive damage to the SEI film and electrode material. This study can provide theoretical support and research direction for the design of the electrolytes that can alleviate the damage of TM ions to the anode and the SEI film. [Display omitted] [ABSTRACT FROM AUTHOR]