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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]

The surficial residual alkali is a key factor that leads to aggravated phase transition and decay in cobalt-free high-nickel cathode. In this paper, we develop a one-step in-situ modification technique to convert the residual alkali on the LiNi 0.9 Mn 0.1 O 2 (NM91) surface into Li 2 MoO 4 coating. As a fast ionic conductor, Li 2 MoO 4 coating not only facilitate Li+ diffusion, but also inhibits the transition from layered to rock salt phase on the cathode surface. Moreover, the multi-aperture architecture formed in the conversion promotes the high-valent Mo enter-into the lattice and realizes the gradient doping of Mo4+ through thermodynamic diffusion. Due to the pillar effect, Mo doping increases c-axis spacing, mitigates cation mixing, and reduces the irreversible H2-H3 phase transition. As a result, both the Li+ diffusion kinetics and thermodynamic stability are improved. Consequently, the as-prepared Mo modified NM91 exhibits an increased capacity retention from original 62.3–75.6 % (100 cycles, 0.2 C) and enhanced rate capability of 131.96 mAh g−1 at 5.0 C. This work provides a facile "reducing alkali" technological process, and lays foundation for the material design and performance optimization of high energy density cathodes in lithium-ion batteries. • Cobalt-free high nickel LiNi 0.9 Mn 0.1 O 2 cathode material was synthesized, and LiNi 0.9 Mn 0.1 O 2 was modified by doping and coating synergistically by one-step method to achieve a discharge capacity of 131.91 mAh g−1 at 5 C rate (LiNi 0.9 Mn 0.1 O 2 is only 77.7 mAh g−1) and a capacity retention rate of up to 75.6 % after 100 cycles at 0.2 C (LiNi 0.9 Mn 0.1 O 2 is only 62.3 %). • The mechanism of synergistic modification on the interfacial side reaction and structural stability of cobalt-free high nickel materials was clarified by characterization and DFT calculation. • The help of in-situ alkali reduction process to improve the stability of cobalt-free high nickel cathode materials was studied in a pioneering way, which laid a foundation for the correlation of interface stability and performance optimization of cobalt-free high nickel cathode materials, and had potential industrial application prospects. [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]

High nickel LiNi 0.8 Co 0.15 Al 0.05 O 2 (NCA) cathode material has become one of the most promising cathode materials in the field of power batteries, but the side reaction between the conventional LiPF 6 -based electrolyte and the surface of the NCA material results in poor material interface stability. In this work, a compatible lithium difluoro(oxalate)borate (LiDFOB)-based electrolyte by the employment of sulfolane (SL) as a representative sulfur-containing solvent has been built to enhance the interfacial stability of NCA electrode. The composition, morphology and electrochemical properties are studied by X-ray photoelectron spectroscopy, scanning electron microscopy and transmission electron microscopy et al. Results show that the decomposition of LiDFOB on the surface of the positive electrode material contributes to forming a uniform cathode electrolyte interface (CEI) film, which subsequently reduces the dissolution of metal ions and hinders the structure transition of the material from the layered structure to spinel or rock-salt phases. Besides, the sulfur-containing products resulted from the decomposition of SL promote the conductivity of Li+ ions for CEI film, which makes up for shortcomings of LiDFOB. Moreover, more LiF has been formed due to the synergistic action between LiDFOB salt and SL solvent, improving the stability and density of CEI film, reducing the thickness of CEI film, and further improving the cycle performance of the battery. [ABSTRACT FROM AUTHOR]