Towards Achieving Stable Lithium Metal Cycling

Abstract

The ability to directly utilize Lithium (Li) metal anodes in rechargeable batteries presents itself as an ideal situation, albeit a challenging one to attain. The use of Li metal as an anode would provide all Li-based batteries with the maximum possible specific capacity (3860 mAh/g) in comparison to existing commercial anode selections (e.g. graphite – 380 mAh/g)1. Nonetheless, Li metal anodes remain absent in commercial devices due to inherent safety concerns associated with the formation of Li dendrites during practical rate cycling, as well as Li metals’ susceptibility to exhibit high reactivity towards commercially available organic electrolytes1-3. This coupled use of flammable commercial electrolytes and dendrite forming Li metal can adversely feed potential safety concerns, due to possibilities of thermal runaway1-3. Additionally, Li metal is known to react violently with water which is easily found as an impurity in commercially available organic solvents; or passivate in the presence of small quantities of moisture rendering it un-rechargeable. This property of Li metal has also hindered its use in battery systems such as Li-air, where ideally one would utilize ambient (moisture containing) air for the oxygen (O2) reduction reaction at the cathode, confining majority of research on this battery system to Li-O2 4. Hence, significant efforts in recent rechargeable battery literature have targeted the development of robust electrolytes, capable of better stability in the presence of Li metal, or Li+ ions with which the electrolyte must not negatively react. With respect towards Li dendrite suppression, multiple approaches have been utilized in literature. The use of solid electrolytes as a mechanical barrier, or the use of specific organic solvent-based electrolytes (inclusive of alterations in cations/anions of salts) which control the properties of the solid-electrolyte interface (SEI) are noted observations5. Amongst the various classes of Li battery electrolytes developed to date, ionic liquids (ILs) have been utilized as electrolytes which can facilitate enhanced Li cycling efficiencies and favorable Li plating morphologies while being inherently non-volatile/non-flammable alternatives to commercially available organic electrolytes. Hence, ILs offer a potentially safer alternative to commercially available organic electrolytes1-3,5. Apart from non-volatility, ILs have demonstrated relatively higher decomposition temperatures to organic-solvent based electrolytes, high ionic conductivities and wider electrochemical windows6. Nonetheless, despite their noted advantages, ionic liquids do also display low conductivity and limited material options which exist as liquids at room temperature7. Within the ILs published to date, combinations of various cations (Imidazolium, Pyrrolidinium, Piperidinium, Ammonium, etc.) and anions (TFSI, FSI, BF4, DCA, etc.) have been presented – each with its own distinct advantage1. In all, through the capability to combine various cations and anions; the use of such ILs could produce a simpler and perhaps more uniform SEI, resulting in the improved cycling behaviors reported to date3,8-10. However, to our knowledge, none of these reports have shown the capability to sustain dendrite-free Li growth upon application of practical cycling rates, and allow for stable cycling in the presence of water11,12. Here, we introduce a new class of ILs capable of sustaining dendrite-free Li morphologies at practical cycling rates. Further, we study the specific interactions of these ILs with Li metal, in the absence and presence of water in the electrolyte. Electrochemical results, along with fundamental analytical analyses will be presented and discussed. Best et. al., Energy Environ. Sci., 2013, 6, 979-986.Forsyth et. al., Phys. Chem. Chem. Phys., 2011, 13, 4632-4640.Howlett et. al., Electrochem. Solid-State Lett., 7 (5), A97-A101, 2004.Bruce et. al., Nat. Mater., 11, 19-29, 2012.Forsyth et. al., Adv. Mater., 2005, 17, 2497-2501.Best et. al., J. Electrochem. Soc., 160 (10), A1629-A1637, 2013.Bayley et. al., Phys. Chem. Chem. Phys., 2009, 11, 7202-7208.Forsyth et. al., J. Electrochem. Soc., 153 (3), A595-A606, 2006.Best et. al., J. Phys. Chem. C, 2010, 114, 21775-21785.Zhao et. al., J. Pow. Sources, 174, 2007, 352-358.Simka et. al., Electrochim. Acta, 54, 2009, 5307-5319.Novak et. al., Electrochim. Acta, 55, 2010, 6332-6341.