Researchers around the world are striving to develop new materials for energy-efficient and high energy density lithium-ion batteries [1]. Lithium metal with a theoretical specific capacity of 3860 mAh/g, low density (0.534 g/cm³), and the lowest potential (−3.040 V vs. standard hydrogen electrode) is consider the ultimate anode material for high specific energy batteries [2]. However, various issues remain to be address that hinder its use in commercial batteries, namely, cycling stability, Coulombic efficiency, and safety aspects associated with dendritic growth [3]. Inactive lithium, also known as “dead lithium,” originating from the dendrites that become separated from the surface over prolonged cycling contribute to anode capacity loss and require high negative to positive electrode capacity ratio (N/P). In addition, due to the extremely low standard redox potential of lithium, electrolytes readily react with the lithium metal surface even without any potential polarization. These reactions lead to the formation of mostly insoluble species in a layer often referred to as solid electrolyte interface, SEI. Ideally, the SEI layer is self-terminating; however, as fresh lithium gets exposed via dendritic growth, SEI formation continues. The steady and uncontrollable growth of SEI throughout the functional life of the battery leads to gradual resistant growth responsible for the capacity fade and eventual “death” of the battery.

In previous art, alternative electrolytes, electrolyte additives, and artificial SEIs were studied [4] [5]. For example, the electrolyte additive lithium fluoride (LiF) was used in carbonate electrolytes and provided a strong protective layer that reduced side reactions and improved the life capacity of the battery [6]. Recently, 3-dimensional design of the anode’s current collector was shown to accommodate Li deposition resulting in suppressed SEI growth and volume expansion during cycling [7].

In the present work, we use sulfur-containing compounds as additives at a very low concentration (1 – 50 mM) in standard 1M LiPF₆ EC:DMC (v:v = 1:1). Coin cells (2032) were assembled using lithium foil (100 mm thick), separator (Celgard), and NMC811 cathode (> 10 mg/cm²). Cells were first rested and activated at a slow rate then cycled at C/3 and 1C for charge and discharge respectively in prescribed voltage cutoff window. As shown in Figure 1, the sulfur-containing cell had more than 300 cycles before 90% capacity retention relative to the beginning of life (BOL) capacity. The sulfur-free control cell lasted less than 150 cycles above the 90% retention line. Electrochemical impedance spectroscopy (EIS) measurements for cycled cells showed lower interfacial resistance for cells with sulfur-containing additives compared to control cells. The reason for the improved cycle stability can be attributed to the stability afforded by the additives to the SEI layer.

Figure 1: A comparison of cell performance between control (black) and sulfur-containing additive (green). The Red line indicates the 90% retention of the battery.

References

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