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||LiNi_0.8Co_0.1Mn_0.1O₂ (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 LiNi_xCo_yMn_zO₂, 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 LiNi_0.8Co_0.1Mn_0.1O₂ (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.

Resources

• 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