Current-Density-Dependent Electroplating in Ca Electrolytes: From Globules to Dendrites

Abstract

Multivalent cation rechargeable batteries, including those based on Ca, Mg, Al, etc., have attracted considerable interest as candidates for beyond Li-ion. Recent developments have realized promising electrolyte compositions for rechargeable Ca batteries; however, an in-depth understanding of the Ca plating and stripping behavior, and the mechanisms by which adverse dendritic growth may occur, remains underdeveloped. In this work, via in-situ transmission electron microscopy, we have captured the real-time nucleation, growth, and dissolution of Ca, the formation of dead Ca, and demonstrated the critical role of current density and the solid-electrolyte interphase layer in controlling the plating morphology. In particular, the interface was found to influence Ca deposition morphology, and can lead 2 to Ca dendrite growth under unexpected conditions. These observations allow us to propose a model explaining the preferred conditions for reversible and efficient Ca plating.Multivalent cation batteries based on Mg, Ca, Al, etc. have attracted significant interest as potential candidates to replace Li-ion batteries in recent years. [1][2][3][4][5] These metallic anodes have much higher natural metal abundancy, and are reported to be much less prone to dendrite formation compared with metallic Li anode, [3][4][5][6][7][8][9][10][11] potentially due to their lower self-diffusion barriers. 1,12,13 The Ca-ion system has demonstrated significant potential. It has a comparable volumetric capacity to Li, and compared with other multivalent systems like Mg, it also has the advantages of higher earth abundance, lower reductive potential and lower charge density. 1 Despite this, the development of Ca-ion batteries has been slow in part due to issues with the anode, where most studied electrolytes react with metallic Ca, rapidly forming surface passivation layers comprised of CaCl2, Ca(OH)2, or CaCO3, that block Ca ion diffusion and make further plating impossible. [14][15][16][17] However, recent breakthroughs in electrolyte research have brought renewed interest in Ca-ion batteries. [18][19][20][21] These works have demonstrated promising Ca-based electrolytes that are capable of continuous plating and stripping with relatively high efficiency at moderately elevated 17 or room-temperatures. 4,11,22 While most previous studies demonstrated fairly smooth plating morphology, [3][4][5][6][7][8][9][10][11] a recent paper by Davidson et al. 23 showed that dendrites do grow in Mg-ion electrolyte. This challenges the widely accepted belief that multivalent systems do not form dendrites easily. Since research into Ca-ion electrolytes is at an early stage, little work has been done to systematically study their plating and stripping processes. This study explores the electroplating morphology and mechanism within the Ca-ion system via in situ transmission electron microscopy (TEM) to evaluate the feasibility of employing metallic Ca anodes, and to provide a deeper understanding of this system for future optimization.

Quantifying Species Populations in Multivalent Borohydride Electrolytes

“…Indeed, a recent study highlighted that such rate-dependent calcium plating problems are encountered in Ca(BH4)2/THF electrolytes. 46 Therefore, we propose the relatively small, concentration-dependent population of CaBH4 + as the underlying cause. Our findings thus inform both recent and ongoing efforts at studying interfacial phenomena in these unique electrolyte environments.…”

Monocarborane cluster as a stable fluorine-free calcium battery electrolyte

“…Ca electrolytes were prepared by dissolving CMC into the weakly coordinating solvents, which enabled the dissociation of Ca salts 37 – 39 . As the weakly coordinating solvents, THF, DME, and a mixture of DME/THF (1:1, v/v) were selected, and the solubilities of CMC were evaluated by ICP-OES using saturated solutions.…”

Recent Advances and Perspectives on Calcium‐Ion Storage: Key Materials and Devices

“…with a 1 m Ca(BH 4 ) 2 in THF electrolyte. [ 28 ] Via in situ TEM, it showed the morphology of plating Ca transited from globular to dendritic growth with an increase of current density and isolated dead Ca deposits electrochemically formed during the stripping process. The study also revealed the relationship between the SEI layer and dendritic deposition, providing important guidance for the optimization of reversible and efficient Ca plating.…”

“…Following the difficulty of reversible Ca metal anode, whether Ca dendrite will occur as serious as lithium dendrite is waiting to be inspected, which may limit the practical application of Ca metal even if its high reversibility is achieved. [ 28 ] In addition, the irreversible Ca metal anode has caused the difficulty in constructing Ca 2+ ‐half‐cells, which has added great complexity in exploring CIBs electrode materials. One alternative is using an activated carbon capacitive electrode as a replacement.…”