A Novel Superconcentrated Aqueous Electrolyte to Improve the Electrochemical Performance of Calcium-ion Batteries

Abstract

Herein, we applied a novel superconcentrated aqueous electrolyte in calcium-ion batteries for the first time to improve electrochemical performance by decreasing the hydration number and radius of calcium ions. Their charge/discharge capacities with the superconcentrated electrolyte were ca. 13% higher than that with the dilute electrolyte. The cycling performance improved quite remarkably in the superconcentrated electrolyte, because of the suppression of structural collapse of copper hexacyanoferrate used as the electrode. The results have been confirmed through X-ray diffraction results.Keywords: Calcium-ion battery | Superconcentrated aqueous electrolyte | CuHCF electrodePolyvalent ion batteries (PIBs) have been suggested as one of the alternatives for lithium ion batteries, because the former are expected to have high energy density. 17 However, polyvalent cations with large charge densities, such as Mg 2+ , Ca 2+, and Al 3+ , are much slowly diffused in an electrode than lithium because they are seriously affected by cationcation repulsive and cationanion attractive forces. 510 This results in poor electrochemical performances such as low charge/ discharge capacities, poor rate capability, and cycling ability. Therefore, the first step to develop PIBs is to resolve this problem.Recently, it has been demonstrated that the problem of the slow diffusion of polyvalent cations can be resolved by using aqueous electrolytes. 11,12 Since polyvalent cations are hydrated by water molecules in an aqueous electrolyte, their charge density is weakened by the shielding effects of water, which enables rapid diffusion of polyvalent cations in an electrode during coinsertion/extraction reactions. 11,12 In such a case, an active material with large channels that allow the passage of hydrated polyvalent cations is necessary. Wang et al. 13,14 observed that various polyvalent cations could rapidly diffuse into Prussian Blue analogue (PBA) electrodes, which contain large open interstitial sites (4.6 ¡ in diameter) and ©100ª channels (3.2 ¡ in diameter) in polyvalent cation-based aqueous electrolytes. However, the low charge/discharge capacities and capacity reduction during the initial cycles must be improved.Therefore, to improve the electrochemical performance, such as charge/discharge capacities and cycle ability, in a PBA electrode and polyvalent cation-based aqueous electrolyte system, we applied a novel superconcentrated aqueous electrolyte in this study. The hydration number and radius of polyvalent cations are changed by applying a superconcentrated electrolyte, which, in turn, have a positive influence on the electrochemical performance.Calcium-ion batteries, which are a type of PIBs, have attracted much attention because they are expected to have high energy density (low reduction potential, similar to that of the lithium ion, and high theoretical capacities) and low cost (abundant resource). 1519 In this research, therefore, we studied a calcium-based aqueous electrolyte as the electrolyte for PIBs.The electrochem...

Recent Progress of Rechargeable Batteries Using Mild Aqueous Electrolytes

“…Recently, some works demonstrated that the existence of water can effectively facilitate the Ca 2+ ion diffusion because of the weakened charge density derived from the shielding effect of water, which is similar to Mg‐ion battery mentioned above. Jeong and Lee investigated the effect of electrolyte concentration on the storage performance of Ca 2+ ion in CuHCF host. They found that the superconcentrated aqueous electrolyte, 8.37 mol L −1 Ca(NO 3 ) 2 , is beneficial to the improvement of discharge capacity and cycle life compared to dilute electrolyte (1 mol L −1 Ca(NO 3 ) 2 ).…”

Water in Rechargeable Multivalent‐Ion Batteries: An Electrochemical Pandora's Box

“…Bulk water has an electrochemical stability window of about 1.23 V. Recently, however, reports of high‐voltage aqueous batteries (2.5 to 4 V) have claimed the successful use of interfacial controls to isolate bulk water from electrochemical contact ,,. This was achieved through the creation of an artificial solid–electrolyte interface (SEI) on the electrode surface, which passivates against proton/hydroxyl electrolytic decomposition, but allows cation‐specific diffusion.…”

A concentrated electrolyte for zinc hexacyanoferrate electrodes in aqueous rechargeable zinc-ion batteries

“…5a and 5b, improved cycling performance is observed in the concentrated electrolyte. In our previous study, it was demonstrated that the crystalline lattice structure of an electrode could be maintained after cycling if the hydration number of the cations were decreased, leading to improved cycling performance [22]. Therefore, in this study, the reason for the improved cycling performance in the concentrated electrolyte may be the decreased hydration number of the zinc ions.…”

“…In our previous study, it was demonstrated that a PBA electrode with a collapsed crystalline structure realizes relatively poor electrochemical performances in an aqueous electrolyte [22]. As indicated by the XRD results, the crystallinity of ZnHCF synthesized using 0.1 mol dm -3 Zn(NO 3 ) 2 is to that of ZnHCF synthesized using 0.05 mol dm -3 Zn(NO 3 ) 2 , which likely accounts for the differences in the charge/discharge properties of the two ZnHCF electrodes.…”

“…In this work, we applied a concentrated electrolyte in ARZIBs with PBA electrodes to improve the electrochemical performance by changing the hydration number of zinc ions. Our previous study demonstrated that the electrochemical performance of aqueous rechargeable calcium-ion batteries could be improved by applying a superconcentrated electrolyte, and the improved performance was attributed to a decrease of the hydration number and radius of calcium ions [22]. Therefore, it was expected that the hydration number and radius of zinc ions could be changed by applying a concentrated electrolyte in ARZIB systems, which, in turn, would positively influence the electrochemical performance.…”