More oxidative than carbonate solvents such as EC Oxidative additive with high HOMO energy HOMO energy of carbonate solvents such as EC e -Stable SEI layer Energy HOMO energy of high-voltage solvents such as sulfone, fluorinated solvents Highly stable electrolytes at high voltages (without the SEI formation on the cathode) Search for high-voltage electrolytes using HOMO energy calculation Possible candidates by theoretical molecular orbital calculation Screening oxidative additives or high-voltage solvents using electrochemical floating test and LSV/CV Finding the optimized electrolytes for high-voltage cathodes through electrochemical test of cellsWe present the useful processes in the research of functional electrolytes for interfacial stability of high-voltage cathodes in Li-ion batteries.Advanced electrolytes with unique functions such as in-situ formation of a stable artificial solid electrolyte interphase (SEI) layer on the anode and the cathode, and the improvement in oxidation stability of the electrolyte have recently gained recognition as a promising means for highly reliable lithium-ion batteries with high energy density. In this review, we describe several challenges of the cathode (spinel lithium manganese oxide (LMO), lithium cobalt oxide (LCO), lithium nickel cobalt manganese oxide (NCM), spinel lithium manganese nickel oxide (LNMO), and lithium-rich layered oxide (Li-rich cathode))-electrolyte interfaces and highlight the recent 10 progress in the use of oxidative additives and high-voltage solvents in high-performance cells. 75 solvents with high anodic stability, researchers have also investigated the use of functional oxidative additives to modify the surface chemistry of the cathode and attain high-performance cathodes for use in LIBs. [25][26][27] In this review, we present the problematic issues regarding 80
Lithium bis(oxalato)borate (LiBOB) is utilized as an oxidative additive to prevent the unwanted electrolyte decomposition on the surface of Li 1.17 Ni 0.17 Mn 0.5 Co 0.17 O 2 cathodes. Our investigation reveals that the LiBOB additive forms a protective layer on the cathode surface and effectively mitigates severe oxidative decomposition of LiPF 6 -based electrolytes. Noticeable improvements in the cycling stability and rate capability of Li 1.17 Ni 0.17 Mn 0.5 Co 0.17 O 2 cathodes are achieved in the LiBOB-added electrolyte. After 100 cycles at 60 • C, the discharge capacity retention of the Li 1.17 Ni 0.17 Mn 0.5 Co 0.17 O 2 cathode was 28.6% in the reference electrolyte, whereas the LiBOB-containing electrolyte maintained 77.6% of its initial discharge capacity. Moreover, the Li 1.17 Ni 0.17 Mn 0.5 Co 0.17 O 2 cathode with LiBOB additive delivered a superior discharge capacity of 115 mAh g −1 at a high rate of 2 C compared with the reference electrolyte. The OCV of a full cell charged in the reference electrolyte drastically decreased from 4.22 V to 3.52 V during storage at 60 • C, whereas a full cell charged in the LiBOB-added electrolyte exhibited superior retention of the OCV. . [1][2][3][4] Because a large reversible capacity of lithium-rich cathodes is attained with the condition of charging to the voltage range of 4.6-4.8 V at the first charge, 1 the oxidative decomposition of LiPF 6 /carbonate-based electrolytes occurring above 4.5 V vs. Li/Li + is inevitable. 5,6 The anodic limit of current electrolytes is not high enough to prevent such side reactions, which results in the formation of a resistive surface film on the cathode and continuous electrolyte decomposition at high voltages. Therefore, these undesired reactions limit the practical application of lithium-rich cathode materials. From this viewpoint, the formation of surface films through the use of oxidative additives in the electrolytes is thought to be one of the most effective strategies to stabilize the cathode-electrolyte interface in lithium-ion batteries (LIBs). 7,8 Recently, many research groups have reported the effect of electrolyte additives preventing significant electrolyte decomposition at lithium-rich cathodes that are operated above 4.5 V.9-13 Tri(hexafluoro-iso-propyl)phosphate (HFiP) was proposed as a oxidative additive (OA) to improve the cycling performance of the lithium-rich cathode Li 8 These authors also reported that the salt-type additive, LiFOB, can serve as a bi-functional additive, modifying the cathode and anode interfaces, in a subsequent paper.14 In addition, it has been reported that a LiFOB-lithium bis(oxalato)borate (LiBOB) combination leads to an improvement in the electrochemical performances of graphite/Li 1.2 Mn 0.55 Ni 0.15 Co 0.1 O 2 full cells 30• C. Lu et al. reported that the LiFOB additive improved the cycling stability of graphite/xLi 2 MnO 3 · (1-x)LiMO 2 full cells at room temperature. 4 These authors mentioned that the LiFOB-originated solid electrolyte interphase (SEI) on the anode effectively ...
Lithium difluoro(bisoxalato)phosphate (LiDFBP) is introduced as a novel lithium‐salt‐type electrolyte additive for lithium‐rich cathodes in lithium‐ion batteries. The investigation reveals that LiDFBP is oxidized to form a uniform and electrochemically stable solid electrolyte interphase (SEI) on the lithium‐rich cathode. The LiDFBP‐derived SEI layer effectively suppresses severe electrolyte decomposition at high voltages and mitigates the voltage decay of the lithium‐rich cathodes caused by undesirable phase transformation to spinel‐like phases during cycling. Furthermore, the cell with electrolyte containing LiDFBP achieves substantially improved cycling performance and delivers a high discharge capacity of 116 mA h g−1 at a high C rate (20 C). The unique function of the LiDFBP additive on the surface chemistry of lithium‐rich cathodes is confirmed through X‐ray photoelectron spectroscopy, SEM, and TEM analyses.
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