Researchers in China develop high-voltage-resistant electrolyte for ultrahigh voltage Li metal batteries
Researchers in China have developed a high-voltage-resistant (HV electrolyte) for use in ultrahigh-voltage lithium metal batteries. As reported in an open-access paper in the RSC journal Energy & Environmental Science, Li||LiNi0.8Co0.1Mn0.1O2 (NCM811) cells, which can work in a wide operating temperature range from −30 to 70 °C, have capacity retentions of 95.1 % after 160 cycles and 85.7 % after 100 cycles at ultrahigh cut−off voltages of 4.7 and 4.8 V, respectively, with the new electrolyte.
Li||NCM811 cells with a thin (50 μm) lithium metal anode and a lean electrolyte were also constructed, and had a capacity retention of 89.2 % after 150 cycles, demonstrating high potential in practical use as high-energy-density batteries.
With the increasing demand for rechargeable batteries with a high energy density (≥ 350 Wh kg-1) for electric vehicles, energy storage, and portable electronics, developing novel electrochemical systems is indispensable to overcome the disadvantages of the commercial lithium−ion batteries (LIBs), although it is challenging. Because energy density is highly related to the specific capacity and working potential, increasing the capacity of electrodes and/or improving the working voltage are the most promising strategies to obtain rechargeable batteries with improved energy densities.
On the anode side, lithium metal with an ultrahigh specific capacity (3860 mAh g-1) and ultralow redox potential (−3.04 V vs standard hydrogen electrode), is one of the ideal anodes to replace commercial graphite (372 mAh g-1). On the cathode side, both increasing the cut−off voltage (> 4.5 V) of the prevailing cathode materials (such as LiNixCoyMnzO2, x+y+z=1) and developing novel materials such as Li−rich layer oxides and high−voltage spinel oxides with improved capacities (> 250 mAh g-1) can significantly increase the energy density.
However, compared with developing novel materials, improving the cut−off voltages of commercial cathodes is easier and more effective. Therefore, developing high-voltage lithium metal batteries (LMBs) has attracted overwhelming attention recently. However, commercial ethylene carbonate (EC)-based electrolytes show poor compatibility with both cathodes at ultrahigh voltages and lithium metal anodes.
For example, with increasing nickel content, Ni−rich layer oxides (such as LiNi0.8Co0.1Mn0.1O2 (NCM811), show more severe structural instability including transition metal dissolution, phase transformation, and ion mixing, especially under ultrahigh voltages. The aggressive Ni4+ on the surface of the cathode under high voltages reacts with EC-based electrolytes, leading to unstable and over-growth of a cathode−electrolyte interphase (CEI), steadily decreasing the performance of the cathodes. Meanwhile, EC-based electrolytes are prone to being reduced on lithium metal, resulting in the formation of an inhomogeneous and unstable solid−electrolyte interphase (SEI), which leads to the formation of lithium dendrites, capacity fade and a low Coulombic efficiency (CE).—Xiao et al.
The researchers’ HV electrolyte is composed of 1 M LiPF6 in a mixture of fluorethylene carbonate (FEC) and bis(2,2,2-trifluorethyl) carbonate (BTC), produced by fluorination of the solvents in commercial EC-based electrolytes.
Compared with a base electrolyte (1M LiPF6 in a mixture of EC and diethyl carbonate (DEC), the HV electrolyte shows better oxidation stability toward cathodes, improved compatibility with lithium metal anodes, and superior electrochemical kinetics in high−voltage LMBs. The solvents in the HV electrolyte are easily reduced on the lithium metal anode, forming a LiF−rich SEI, which suppresses the formation of lithium dendrites.
The HV electrolyte is also nonflammable, implying its stability at high temperatures and safety in practical use.
P. Xiao, Y. Zhao, Z. Piao, B. Li, G. Zhou and H. Cheng (2022) “A Nonflammable Electrolyte for Ultrahigh−Voltage (4.8 V−Class) Li||NCM811 Cells with A Wide Temperature Range of 100 °C” Energy Environ. Sci. doi: 10.1039/D1EE02959B