Appraisal of calcium ferrites as cathodes for calcium rechargeable batteries: DFT, synthesis, characterization and electrochemistry of Ca₄Fe₉O₁₇

Abstract

Ca4Fe9O17 shows appealing characteristics as a cathode for Ca batteries. A novel synthesis approach is reported and hints of electrochemical activity are observed.

The Promise of Calcium Batteries: Open Perspectives and Fair Comparisons

“…While cell-level assessments of Ca metal batteries need to be thoroughly pursued, recent achievements may be initially compared to the energy densities mentioned above. Thus far, several promising Ca-metal battery configurations have been realized: Ca//V 2 O 5 , Ca//Ca 4 Fe 9 O 17 , Ca//LiTiO 2 , Ca/C-Fiber, Ca//TiS 2 , Ca//FePO 4 , Ca//Ca 3 Co 2 O 6 , Ca//PAQ­(organic), and Ca//S. − Figure b,c summarizes their capacities, operating voltages (average), and energy densities, including those for Li cells as discussed above. Thus far, Ca battery energy densities can reach as high as 250 Wh/kg, with discharge charge capacities between 60 and 250 mAh/g and operating voltages between 1 and 4 V. Current densities achieved thus far are in the range of 20–500 mA/g, and C-rates from 0.2 C to upwards of >5 C with slightly lower capacities at these rates than Li batteries. ,,, Promising rechargeable batteries have been realized with Ca//LiTiO 2 and Ca//C-fiber, specifically owing to successful electrolyte formulations (Ca­(BH 4 ) 2 + LiBH 4 in THF, and NaPF 6 in 1:1:1 by v/v EC-EMC-DMC, respectively) that help sustain Ca redox activity by providing a stable Ca 2+ -permeable artificial solid electrolyte interface (SEI) to enable long-term cycling or increase cation mobility in the electrolyte.…”

Searching Ternary Oxides and Chalcogenides as Positive Electrodes for Calcium Batteries

“…While TiS 2 has been demonstrated to intercalate reversibly Ca 2+ , , our screening criteria could not reveal this compound, as a stoichiometric calciated structure, i.e., Ca x TiS 2 , is presently unknown. However, the robustness of our strategy is validated by the rediscovery of chevrel-Mo 6 S 8 and CaMoO 3 phases that have received attention as Ca electrodes. ,, Our screening procedure did not identify the following compounds which have been studied computationally before: Ca 4 Fe 9 O 17 has a low E m ∼720 meV but does not have a matching, ordered, charged structure, and Ca 3 Co 4 O 9 exhibits E m ∼900 meV but is significantly unstable ( E hull ∼73 meV/atom). Furthermore, we did not consider Ca 3 Co 2 O 6 as a possible candidate since previous experimental studies have reported the structure to undergo an irreversible phase transformation upon Ca extraction …”

“…On the computational front, using density functional theory (DFT , ) Arroyo-de Dompablo et al spearheaded the investigation (and discovery) of alternative Ca-intercalation chemistries, including oxide (i.e., CaMn 2 O 4 , Ca 2 Mn 2 O 5 , CaMn 4 O 8 , Ca 2 Fe 2 O 5 , Ca 4 Fe 9 O 17 , Ca 3 Co 4 O 9 , and Ca 3 Co 2 O 6 ) and mixed anion (i.e., CaFeSO, CaCoSO, CaNiN, Ca 3 MnN 3 , Ca 2 Fe­(Si 2 O 7 ), CaM­(P 2 O 7 ) with M = V, Cr, Mn Fe and Co, CaV 2 (P 2 O 7 ) 2 , Ca­(VO) 2 (PO 4 ) 2 , and α-VOPO 4 ) frameworks. − For example, they reported a low Ca E m of ∼650 meV in α-VOPO 4 , along with the high theoretical gravimetric capacity (∼312 mAh/g) and average (theoretical) intercalation voltage of ∼2.8 V vs Ca/Ca 2+ , making VOPO 4 a promising candidate for further experimental investigation.…”

“…Our screening procedure did not identify the following compounds which have been studied computationally before: Ca 4 Fe 9 O 17 has a low E m ∼720 meV52 but does not have a matching, ordered, charged structure, and Ca 3 Co 4 O 9 exhibits E m ∼900 meV51 but is significantly unstable (E hull ∼73 meV/atom58 ). Furthermore, we did not consider Ca 3 Co 2 O 6 as a possible candidate since previous experimental studies have reported the structure to undergo an irreversible phase transformation upon Ca extraction.…”

Searching Ternary Oxides and Chalcogenides as Positive Electrodes for Calcium Batteries

“…and α-VOPO 4 ) frameworks. [47][48][49][50][51][52] For example, they reported a low Ca E m of ∼650 meV in α-VOPO 4 , along with the high theoretical gravimetric capacity (∼312 mAh/g) and average (theoretical) intercalation voltage of ∼2.8 V vs. Ca/Ca 2+ , making VOPO 4 a promising candidate for further experimental investigation. Undoubtedly, the above experimental and theoretical studies have made sizeable progress towards the development of practical Ca-battery prototypes.…”