Rechargeable lithium-metal batteries (LMBs) are regarded as the "holy grail" of energy-storage systems, but the electrolytes that are highly stable with both a lithium-metal anode and high-voltage cathodes still remain a great challenge. Here a novel "localized high-concentration electrolyte" (HCE; 1.2 m lithium bis(fluorosulfonyl)imide in a mixture of dimethyl carbonate/bis(2,2,2-trifluoroethyl) ether (1:2 by mol)) is reported that enables dendrite-free cycling of lithium-metal anodes with high Coulombic efficiency (99.5%) and excellent capacity retention (>80% after 700 cycles) of Li||LiNi Mn Co O batteries. Unlike the HCEs reported before, the electrolyte reported in this work exhibits low concentration, low cost, low viscosity, improved conductivity, and good wettability that make LMBs closer to practical applications. The fundamental concept of "localized HCEs" developed in this work can also be applied to other battery systems, sensors, supercapacitors, and other electrochemical systems.
High-voltage batteries with Li-metal anodes can offer desirable high energy densities. Despite their excellent oxidative stability, sulfones have various limitations to be useful in Li-metal batteries, in particular their instability with Li metal. Here, we achieved a high Li Coulombic efficiency of nearly 99% in a sulfonebased localized high-concentration electrolyte (LHCE) with the addition of a nonsolvating co-solvent. In addition, this co-solvent is highly beneficial for realizing stable battery cycling up to 4.9 V.
Sodium (Na) metal is a promising anode for Na-ion batteries. However, the high reactivity of Na metal with electrolytes and the low Na metal cycling efficiency have limited its practical application in rechargeable Na metal batteries. High-concentration electrolytes (HCE, ≥4 M) consisting of sodium bis(fluorosulfonyl)imide (NaFSI) and ether solvent could ensure the stable cycling of Na metal with high Coulombic efficiency but at the cost of high viscosity, poor wettability, and high salt cost. Here, we report that the salt concentration could be significantly reduced (≤1.5 M) by a hydrofluoroether as an "inert" diluent, which maintains the solvation structures of HCE, thereby forming a localized high-concentration electrolyte (LHCE). A LHCE [2.1 M NaFSI/1,2-dimethoxyethane (DME)−bis(2,2,2-trifluoroethyl) ether (BTFE) (solvent molar ratio 1:2)] enables dendrite-free Na deposition with a high Coulombic efficiency of >99%, fast charging (20C), and stable cycling (90.8% retention after 40 000 cycles) of Na∥Na 3 V 2 (PO 4 ) 3 batteries.
The lithium (Li) metal battery (LMB) is one of the most promising candidates for next-generation energy storage systems. However, it is still a significant challenge to operate LMBs with high voltage cathodes under high rate conditions. In this work, an LMB using a nickel-rich layered cathode of LiNi 0.76 Mn 0.14 Co 0.10 O 2 (NMC76) and an optimized electrolyte [0.6 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) + 0.4 M lithium bis(oxalato)borate (LiBOB) + 0.05 M LiPF 6 dissolved in ethylene carbonate and ethyl methyl carbonate (EC:EMC, 4:6 by weight)] demonstratesexcellent stability at a high charge cutoff voltage of 4.5 V. Remarkably, these Li||NMC76 cells can deliver a high discharge capacity of >220 mAh g -1 (846 Wh kg -1 ) and retain more than 80% capacity after 1000 cycles at high charge/discharge current rates of 2C/2C (1C = 200 mA g -1 ). This excellent electrochemical performance can be attributed to the greatly enhanced structural/interfacial stability of both the Ni-rich NMC76 cathode material and the Li metal anode using the optimized electrolyte.
electric vehicles. [1] LIBs with the conventional carbonaceous anode materials such as graphite have played a dominant role in the current market of customer electronics and electrical transportation. However, low capacity of carbonaceous anode materials (372 mAh g −1 ) also limits the further increase in energy densities of LIBs. [2] In this regard, silicon (Si)-based anode is considered as one of the most promising anode candidates in further boosting the specific energy of LIBs because Si has one of the highest practical capacity of 3579 mAh g −1 among various anode materials and a relatively low lithiation potential of 0.2 V versus Li/Li + . However, fast capacity fade and large swelling of Si anodes related to their large volume expansion (>300%) upon lithiation greatly hindered their deployment in practical applications. [3] There has been significant progress toward understanding and mitigating the capacity fade in Si-based anodes, including exploiting nanostructured Si materials, [4] porous structures, [5] surface coatings, [6] core-shell structures, [7] and novel binders. [8] However, the development of novel electrolytes for Si-based anodes is relatively slow because most researches have been focused on the structure development of Si electrodes. The conventional electrolytes for Si anodes are LiPF 6 /carbonate-based electrolytes with a certain amount of fluoroethylene carbonate (FEC) as an additive or cosolvent (from 5% to 10% by weight in the electrolytes). [9] Linear carbonate solvents usually have relatively low flashpoints, so they are easily ignited and may lead to safety problems under certain extreme conditions. [10] In addition, the formed solid electrolyte interphase (SEI) on anode surface in conventional carbonate electrolytes is unstable and cannot withstand the large volume changes of Si during cycling. Although the introduction of FEC in the conventional LiPF 6 / carbonate electrolytes can improve the cycling performance of Si anodes, increased amount of FEC may lead to increased gassing in full cells. Because such high content of FEC in the electrolytes may form a detrimental cathode electrolyte interface (CEI) on cathode surface and generate a serious gassing issue especially at high charge cutoff voltages and elevated temperatures, which lead to impedance increase, capacity fading and safety issue of the Si-based full cells. Therefore, the Silicon anodes are regarded as one of the most promising alternatives to graphite for high energy-density lithium-ion batteries (LIBs), but their practical applications have been hindered by high volume change, limited cycle life, and safety concerns. In this work, nonflammable localized highconcentration electrolytes (LHCEs) are developed for Si-based anodes. The LHCEs enable the Si anodes with significantly enhanced electrochemical performances comparing to conventional carbonate electrolytes with a high content of fluoroethylene carbonate (FEC). The LHCE with only 1.2 wt% FEC can further improve the long-term cycling stability of Si-base...
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