Rechargeable calcium batteries have attracted increasing attention as promising multivalent ion battery systems due to the high abundance of calcium. However, the development has been hampered by the lack of suitable cathodes to accommodate the large and divalent Ca2+ ions at a high redox potential with sufficiently fast ionic conduction. Herein, we report a new intercalation host which presents 500 cycles with a capacity retention of 90% and a remarkable power capability at ~3.2 V (vs. Ca/Ca2+) in a calcium battery. The cathode material derived from Na0.5VPO4.8F0.7 is demonstrated to reversibly accommodate a large amount of Ca2+ ions, forming a series of CaxNa0.5VPO4.8F0.7 (0 < x < 0.5) phases without any noticeable structural degradation. The robust framework enables one of the smallest volume changes (1.4%) and the lowest diffusion barriers for Ca2+ among the cathodes reported to date, offering the basis for the outstanding cycle life and power capability.
Coupling thin Li metal anodes with high-capacity/high-voltage cathodes such as LiNi 0.8 Co 0.1 Mn 0.1 O 2 (NCM811) is a promising way to increase lithium battery energy density. Yet, the realization of high-performance full cells remains a formidable challenge. Here, we demonstrate a new class of highly coordinated, nonflammable carbonate electrolytes based on lithium bis(fluorosulfonyl)imide (LiFSI) in propylene carbonate/fluoroethylene carbonate mixtures. Utilizing an optimal salt concentration (4 M LiFSI) of the electrolyte results in a unique coordination structure of Li +-FSI À-solvent cluster, which is critical for enabling the formation of stable interfaces on both the thin Li metal anode and high-voltage NCM811 cathode. Under highly demanding cell configuration and operating conditions (Li metal anode = 35 mm, areal capacity/charge voltage of NCM811 cathode = 4.8 mAh cm À2 /4.6 V, and anode excess capacity [relative to the cathode] = 0.83), the Li metal-based full cell provides exceptional electrochemical performance (energy densities = 679 Wh kg cell À1 /1,024 Wh L cell À1) coupled with nonflammability.
Lithium (Li)-metal batteries have garnered considerable attention as a promising high-energy-density power source beyond commercial Liion batteries. [1][2][3][4] In order to achieve the ultimate goal of high-energydensity, Li-metal anodes should be preferentially coupled with highvoltage/high-capacity cathodes. [5,6] However, the catalytic reactivity of high-voltage cathode materials leads to undesired interfacial side reactions with electrolytes, resulting in electrolyte decomposition and capacity decay upon cycling. [7,8] This issue of the high-voltage cathodes, together with strong reducing nature of Li-metal anodes, poses an obstacle to practical applications of Li-metal batteries. Besides these energy-density and longevity issues, another formidable problem of Li-metal batteries is their safety; Li dendrites grown from Li-metal anodes vigorously react with liquid electrolytes and can trigger internal short-circuit failures, leading to cell fire or explosion through uncontrolled thermal runaway. [9][10][11] The aforementioned challenges of Limetal batteries often stem from liquid electrolytes and their interfacial instability with electrodes. Conventional carbonate-based liquid electrolytes, widely used in commercial Li-ion batteries, are not stable with Limetal anodes [12] and show a limited electrochemical stability window (<4.4 V). [13,14] Ether-based (localized) high-concentration electrolytes were suggested to improve cycling performance of Li-metal batteries. However, their insufficient oxidation stability limited a cell voltage to 4.5 V. [15,16] Recently, nonflammable fluorinated electrolytes have been explored to widen electrochemical stability window and mitigate reactivity of Li-metal anodes. [17,18] However, these electrolytes contain volatile solvents with low boiling points, which thus hinders the application of resulting Li-metal batteries in hightemperature environments.In contrast to the previous studies mostly focusing on carbonate-or ether-based electrolytes that have shown the limitations in thermal tolerance and operating voltage window, herein, we propose a new class of nitrile electrolytes based on concentrated nitrile electrolytes (CNEs) containing co-additives (Li nitrate (LiNO 3 ) and indium fluoride (InF 3 )). This new nitrile electrolyte enables the stable operation of high-voltage (4.9 V-class) Li-metal cells over wide range of temperatures and even under exposure to flame. The CNE is composed of high-concentration lithium bis(fluorosulfonyl) imide (LiFSI) in a solvent mixture of succinonitrile (SN)/acetonitrile (AN). SN is known to show high oxidation stability (a prerequisite for high-voltage cells), along with ion-dissociating capability, thermal stability (boiling point of 266 °C), and nonflammability. [19][20][21] However, its poor reduction stability against Li metals leads to undesired electrochemical polymerization that forms a thick passivation layer, thereby hampering ion transport toward Li metals. [22,23] To resolve this issue, we exploit a concept of concentrated elect...
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