IntroductionLithium (Li)-ion batteries (LIBs) have now been the indispensable power sources for portable electronic devices, electric vehicles, stationary or grid applications, etc. [1] However, further efforts on extending the cycle life, rate capability, energy density and working temperature range and improving the safety of LIBs are still facing significant challenges for their large-scale applications. Focusing on the increase in energy density of a battery, the possible approach is to use the high capacity electrode (cathode or anode) material and the high voltage cathode material. Ni-rich layered oxides LiNi x Mn y Co 1−x−y O 2 (NMC) with Ni content ≥80% (e.g., NMC811) are regarded as one of the most potential candidates to usher in the new stage of ultra-high energy density LIBs due to their increased specific capacities at higher voltages and the low cost with less Co content. However, the practical applications of these Ni-rich NMC cathode materials are greatly hindered by the poor cathode-electrolyte interface (CEI) layer formed on such cathode surface in the state-of-the-art electrolytes comprised of lithium hexafluorophosphate (LiPF 6 ) in carbonate solvents, especially at voltages higher than 4.3 V versus Li/Li + , [2] causing continuous electrolyte oxidative decomposition and other related side reactions such as transition metal dissolution from the cathode surface, thus leading to poor cycling stability, especially at elevated temperatures and high operating voltages. [3] Therefore, advanced electrolytes with better oxidative protection to Ni-rich NMC cathode materials, especially under high voltages are critically important for enabling application of Ni-rich NMCs in LIBs.Significant efforts have been made to develop novel electrolytes for high voltage cathode materials, mainly through using high anodic solvents to substitute carbonate solvents, the increase of salt concentration and the utilization of film-forming additives. Fan et al. developed an all-fluorinated electrolyte of 1 m LiPF 6 in fluoroethylene carbonate/3,3,3-fluoroethylmethyl carbonate/1,1,2,2-tetrafluoroethyl-2′,2′,2′-trifluoroethyl ether (FEC/FEMC/HFE, 2:6:2 by wt), which significantly enhanced the cycling stability of Li||NMC811 (2.7-4.4 V) and Li||LiCoPO 4 (3.5-5.0 V) in high voltages by effectively inhibiting electrolyte LiNi x Mn y Co 1−x−y O 2 (NMC) cathode materials with Ni ≥ 0.8 have attracted great interest for high energy-density lithium-ion batteries (LIBs) but their practical applications under high charge voltages (e.g., 4.4 V and above) still face significant challenges due to severe capacity fading by the unstable cathode/electrolyte interface. Here, an advanced electrolyte is developed that has a high oxidation potential over 4.9 V and enables NMC811-based LIBs to achieve excellent cycling stability in 2.5-4.4 V at room temperature and 60 °C, good rate capabilities under fast charging and discharging up to 3C rate (1C = 2.8 mA cm −2 ), and superior low-temperature discharge performance down to −30 °C with a ca...

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The approaches enhancing the energy density of lithium (Li)-ion batteries (LIBs) often push the batteries to their safety limit. Therefore, electrolytes that not only enhance electrochemical performances but also improve safety properties of LIBs are urgently needed for further development of LIBs. Although organic phosphorus-containing solvents have been used with organic carbonates to form flame-retarding or nonflammable electrolytes, their poor compatibility with the graphite anodes leads to fast capacity fade of LIBs. Herein we report a promising approach by formulating a nonflammable localized high-concentration electrolyte based on triethyl phosphate, ethylene carbonate, and a partially fluorinated ether. This electrolyte readily generates a robust solid electrolyte interphase on the graphite anode and a cathode electrolyte interphase on the LiNi0.8Mn0.1Co0.1O2 (NMC811) cathode. As a result, this electrolyte shows good compatibility with both the graphite anode and the high-voltage cathode and enables long-term cycling stability of the graphite||NMC811 cells. This work sheds new light in the development of nonflammable electrolytes for high-energy-density LIBs.

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Despite being an effective flame retardant, trimethyl phosphate (TMP a )i sg enerally considered as an unqualified solvent for fabricating electrolytes used in graphite (Gr)-based lithium-ion batteries as it readily leads to Gr exfoliation and cell failure.I nt his work, by adopting the unique solvation structure of localized high-concentration electrolyte (LHCE) to TMP a and tuning the composition of the solvation sheaths via electrolyte additives,excellent electrochemical performance can be achieved with TMP a -based electrolytes in Gr k Li-Ni 0.8 Mn 0.1 Co 0.1 O 2 cells.After 500 charge/discharge cycles within the voltage range of 2.5-4.4 V, the batteries containing the TMP a -based LHCE with ap roper additive can achieve ac apacity retention of 85.4 %, being significantly higher than cells using aL iPF 6 -organocarbonates baseline electrolyte (75.2 %). Meanwhile,d ue to the flame retarding effect of TMP a ,T MP a -based LHCEs exhibit significantly reduced flammability compared with the conventional LiPF 6 -organocarbonates electrolyte.