Adiponitrile (ADN) is studied as an additive of commercial carbonate electrolyte for a graphite/NCM811 Li-ion cell in this work. Compared with the cell using a baseline electrolyte, the cycling stability of the cells using 0.2, 0.5, 1, and 2 wt % ADN-added electrolytes is significantly improved. Among them, the effect is most prominent when the ADN content is 0.5 wt %, and the capacity retention rate of the cell after 200 cycles at 0.3 C is increased by 17.6%. Scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy analyses confirm that ADN can make the coating layer formed on the cathode and anode surfaces of the cell denser and uniform. The formation of the cathodic coating layer does not obey the usual mechanism of adsorption between the nitrile group and metal ions, but it may be due to the polymerization of ADN initiated by carbocations from the oxidation of solvent molecules (ethylene carbonate/ethyl methyl carbonate) in the electrolyte. The cathodic coating layer greatly decreases the oxidation of the electrolyte so that a stable coating layer (also called solid electrolyte interface/SEI) can be formed on the anode, which is the reason for improvement of the cycling performance of the cell. However, the formation of the anode SEI layer is almost independent of ADN, and it still depends on the common reduction reaction of the electrolyte.
Flexible solid-state batteries (FSSBs) are indispensable energy storage devices to fulfil the energy and safety requirements for future flexible applications. The bottlenecks of FSSBs are how to realize high energy density with competent ionic conductivity for room-temperature (RT) flexible applications. Here, the first fabrication of RT FSSB with high energy density is reported, which is realized by in situ integration of a 20-µm-thick hybrid polymer/ceramic/ionic liquid solid-state electrolyte (SSE) between the high energy combination of anode/cathode electrodes. The in situ electrode/electrolyte interfacial integration strategy provides an ultrathin SSE layer, ultralow resistance and superior flexibility, and the SSE guarantees both high ionic conductivity and good compatibility with high-energy cathode LiNi 0.8 Co 0.1 Mn 0.1 O 2 (NCM811). The fabricated Li 4 Ti 5 O 12 /NCM811 FSSB delivers super-low resistance approaching conventional liquid cells and excellent cycling stability up to 600 cycles at RT. The extension of anode to SiO x @graphite leads to a high theoretical energy density of 489.6 Wh kg −1 at material’s level, times higher than current options. In addition, the RT FSSB shows great flexibility, indicating a high performance application in future flexible electronics.
The electrochemical stability window of the electrolyte solution limits the energy content of non-aqueous lithium metal batteries. In particular, although electrolytes comprising fluorinated solvents show good oxidation stability against high-voltage positive electrode active materials such as LiNi 0.8 Co 0.1 Mn 0.1 O 2 (NCM811), the ionic conductivity is adversely affected and, thus, the battery cycling performance at high current rates and low temperatures. To address these issues, here we report the design and synthesis of a monofluoride ether as an electrolyte solvent with Li-F and Li-O tridentate coordination chemistries. The monofluoro substituent (-CH 2 F) in the solvent molecule, differently from the difluoro (-CHF 2 ) and trifluoro (-CF 3 ) counterparts, improves the electrolyte ionic conductivity without narrowing the oxidation stability. Indeed, the electrolyte solution with the monofluoride ether solvent demonstrates good compatibility with positive and negative electrodes in a wide range of temperatures (i.e., from −60 °C to +60 °C) and at high charge/discharge rates (e.g., at 17.5 mA cm −2 ). Using this electrolyte solution, we assemble and test a 320 mAh Li||NCM811 multi-layer pouch cell, which delivers a specific energy of 426 Wh kg −1 (based on the weight of the entire cell) and capacity retention of 80% after 200 cycles at 0.8/8 mA cm −2 charge/discharge rate and 30 °C.
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