Lithium Fluoride Additives for Stable Cycling of Lithium Batteries at High Current Densities

Abstract

reactions at the roughened electrode-electrolyte interface. Indeed while such reactions always occur when liquids are in conformal contact with reactive metals such as Li, during each cycle of deposition a roughened metal surface exposes new Li metal to the electrolyte, which leads to continuous formation of a new so-called solid electrolyte interphase (SEI) layer that ultimately depletes the electrolyte. [ 14 ] All of these situations are exacerbated by the common use of fl ammable organic liquids as electrolyte solvents in rechargeable batteries to improve ionic conductivity of the electrolyte, which now add the threat of fi re or even explosion to the list of potential failure modes. [ 4,[14][15][16] Over the years, several efforts have been made to eliminate the possibility of dendrite-induced short-circuits in batteries by designing high modulus electrolytes through which a growing metal dendrite cannot penetrate. [17][18][19] These efforts have largely met with, at most, limited success because of the fundamental diffi culty in designing materials that are simultaneously mechanically strong-enough to stop dendrites, but in which fast ionic transport needed to sustain battery performance can be achieved at moderate temperatures. A notable exception is the work of Tu et al. [ 20,21 ] which shows that a Al 2 O 3 ceramic separator with uniform, nanometer-sized pores that hosts a liquid electrolyte in its pores is able to perform both functions. As important are recent examples of liquid electrolyte-swollen cross-linked polymer networks in which anions are either free [ 22 ] or permanently anchored to a rigid support, [ 23,24 ] which have been reported impressive abilities to extend the lifetime of metallic lithium anodes. However, as none of these approaches address the root cause of the electrodeposition instabilities that trigger dendrite formation, more elegant solutions in the form of SEI additives have been sought to stop dendrites at the nucleation stage. There are generally two approaches that have been investigated in the previous literature: (1) Direct addition, wherein specifi c chemical agents are used as electrolyte additives to promote formation of a stable, Li-ion conducting SEI, but do not take part in the bulk ion transfer. Hydrofl uoric acid, [ 25 ] vinylene carbonate, [26][27][28] lithium bis(oxalato)borate, [ 29,30 ] lithium nitrate, [ 31 ] and organic sultones [ 32,33 ] have all been reported to function in this way. While each of these additives have been shown to improve the cell stability to an extent, wider use of all are challenged by the associated decrease in ionic conductivity and gradual loss of effi cacy due to decomposition. [ 31 ] (2) Indirect formation of stabilizing layers. This approach involves the formation of a stable layer by internal reactions of two or more components added to the electrolyte. A recent example by Miao et al. [ 34 ] showed that a binary mixture of LiTFSI and LiFSI can signifi cantly improve the cyclability of a LMB due to the formation of a LiF layer by d...

Designing solid-liquid interphases for sodium batteries

“…It is notable that the diffusion barrier for NaBr adatoms is substantially lower than for NaF, and even in a liquid electrolyte is comparable to those computed for Mg in vacuum. On the basis of earlier reports that LiF coatings on Li metal dramatically stabilize electrodeposition of Li 13, 22 , and that NaF coatings on Na has a similar large-stabilizing effect on Na deposition 18 , we hypothesize that a Na anode protected by a coating of NaBr would be particularly attractive for room-temperature sodium batteries employing liquid electrolytes.…”

“…Unlike previous experiments, where the source of halide salts in SEI layer of anode is degradation of active materials 18, 39, 40 or precipitation of a poorly soluble electrolyte salt additive 13, 22 , we here employ a well-known chemical reaction to create a layer of NaBr at the interface. Specifically, we carried out a reaction of the sodium metal anode with bromopropane to undergo Wurtz reaction as illustrated in the Fig.…”

“…Instead, dendrites on Na (and Li) arise from inhomogeneities in the resistance of the solid–electrolyte interphase (SEI), formed spontaneously on the anode surface when in contact with an electrolyte. The resultant concentration of electric field lines on faster growing regions of the interface drives the morphological instability loosely termed dendritites 12, 13 . At later stages, uncontrolled dendritic deposition leads to metallic structures able to bridge the inter-electrode space, ultimately short-circuiting the cell.…”

“…In contrast, several approaches have been reported for preventing/retarding Li dendrite proliferation in Li metal batteries 11, 12 . Some of the approaches include using high modulus electrolyte or nanoporous/tortuous separator 14, 20–22 , modifying the ion transport in electrolytes by using single ion conductors and ionic liquids 23–27 , or forming a stable electrode-electrolyte interface to suppress the nucleation of dendrites 4, 13, 28–30 . In addition to preventing dendrite induced short circuits, the last approach may impede unwanted parasitic reactions between the electrode and electrolyte that lead to formation of insulating products and loss of electrochemically active material, causing decay in the battery capacity with increasing charge-discharge cycles 12 .…”

“…A common feature of these methods is that they produce lithium fluoride (LiF) in the SEI. In recent studies 13, 22 , direct incorporation of LiF as an additive in liquid electrolytes was reported to yield dramatic enhancements to battery lifetime in Li metal cells at both high and low current densities. Cui and co-workers 18 were among the first to show that application of this concept to Na metal batteries, through electrolyte additives that generate sodium fluoride (NaF), leads to markedly higher coulombic efficiencies (as high as 99%) in Na||Cu cells.…”