Hybrid Solid Electrolyte Interphases Enabled Ultralong Life Ca‐Metal Batteries Working at Room Temperature

Abstract

Currently, the application of calcium metal anodes is challenged by rapidly degenerated plating/stripping electrochemistry without suitable solid electrolyte interphases (SEIs) capable of fast Ca2+ transport kinetics and superior ability to resist anion oxidation. Here, through in situ evolved Na/Ca hybrid SEIs, symmetrical Ca//Ca batteries readily remain stable for more than 1000 h deposition–dissolution cycles (versus less than 60 h for those with pure Ca SEIs under the same condition). Coupled with a specially designed freestanding lattice‐expanded graphitic carbon fiber membrane and tailored operation voltages, the proof‐of‐concept Ca‐metal batteries reversibly run for almost 1900 cycles with ≈83% capacity retention and a high average discharge voltage of 3.16 V. The good performance not only benefits from the stable SEIs at the Ca metal surface which affords free Ca2+ transports and prohibits out‐of‐control fluridation of Ca (forming CaF2 ion‐/electron‐insulating layer) but is also attributed to reversible relay insertion/extraction electrochemistry in the cathode. This work sheds new light on durable metal battery technology.

A Computational Study on the Ca2+ Solvation, Coordination Environment, and Mobility in Electrolytes for Calcium Ion Batteries

“…Ca(ClO4)2 was previously attempted, and showed some very low levels of plating/stripping kinetics, 12 which may be associated to passivation by the decomposition products of the perchlorate, as observed in Li systems. This might be mitigated through the use of prepassivation layers, 23,25 to decouple the conductivity of Ca(ClO4)2 from the decomposition to form the solid electrolyte interface, yet such experimental studies remain to be pursued. Likewise, TFSI has been previously used, but is preferable as the electrolyte in which a pre-passivated layer using another salt has been performed, as the decomposition products of TFSI appear unfavorable for Ca 2+ transport across the SEI.…”

“…[20][21] Salts that have been explored with these solvents have included Ca(BF4)2, Ca(TFSI)2, Ca(TFS)2, Ca(ClO4)2, Ca(BH4)2, Ca[B(hfip)4]2, as well as Ca(PF6)2. [22][23] Mixed cation salts (Ca+Li or Ca+Na) in carbonate or THF solvents have also been proposed towards engineering the solid electrolyte interface (SEI), [24][25] namely the solid layer formed at the electrode/electrolyte interface from the decomposition of electrolyte (both solvent and salt) and is permeable to ions to allow for redox activity to proceed. Additionally, a range of commercial salts have been examined experimentally for their solvation properties in solvents such as PC, EC, DMF, THF, DME, and glymes.…”

Current Status and Challenges of Calcium Metal Batteries

“…[ 58 ] Differently, a specially designed freestanding lattice‐expanded graphitic carbon fiber membrane cathode reversibly runs for almost 1900 cycles with 83% capacity retention and a high average discharge voltage of 3.16 V in CMBs. [ 59 ] By using an artificial hybrid solid electrolyte layer modified Ca as the anode, the carbon membrane cathode offered a high discharge voltage (>3.3 V) and a large capacity of ≈80 mAh g −1 at 200 mA g −1 and a high window voltage of 2.0–5.0 V. [ 60 ] Similarly, a voltage tailorable Ca‐metal battery with cellulose waste paper derived graphitic carbon as the cathode delivered a high working voltage of 3.2 V and reversible capacity of 101 mAh g −1 at 2.0–4.7 V. [ 61 ]…”

“…By contrast, the process was activated by in situ evolved Na/Ca hybrid SEIs in NaPF 6 electrolyte, as illustrated in Figure a. [ 59 ] Largely reduced charge transfer resistance indicated the enhanced solavtion/desolvation kinetics, and significantly lowered overpotentials in deposition and dissolution processes demonstrated the weakened diffusion energy barrier (Figure 5b,c). They together enabled a stable plating/stripping cycling performance of Ca‐metal in hexafluorophosphate electrolyte (Figure 5d).…”

“…All the data are adapted from publications reported previously. [4,[10][11][12][13][14][15][16][17]41,43,44,[46][47][48][49][50][52][53][54]56,59,61,63,64,66,[79][80][81][82] polyanionic phosphates but also developing multicomponent high-voltage or Ca-rich large capacity cathodes such as Ni/Mnbased Li/Na-counterparts and defect/interface-engineered fast kinetics and cycling-stable cathodes. [31][32][33][34][35][36]…”