Ca 2+ (ionic radius 1.18 Å) and its multivalent nature. [7] Several electrode materials with large lattice spacing (e.g., V 2 O 5 and CaCo 2 O 4 ) or open frameworks (e.g., Prussian blue analogs) have been explored for CIBs. [8] Recently, a rechargeable CIB has been developed based on a dual-ion configuration, however, the electrochemical performances, particularly the rate capability (55% capacity retention at 0.4 A g −1 ) were still far from actual demands. [4c] A hybrid-ion strategy combining the respective advantages of different ions has been confirmed to be a feasible approach to enhance reaction kinetics of sluggish ions with large radius. [9] For example, the rate performance and cycling life of magnesium-ion batteries have been optimized significantly via a hybrid Li + /Mg 2+ design. [9c,e] In this report, we present a calcium-ion based energy storage device with a triion configuration (Ca 2+ /Li + /PF 6 − ). It was found that hybridizing calcium-ion based electrolyte with a small amount of Li + ions could significantly reduce the charge transfer resistance due to its small ionic radius and fast diffusion kinetics. After optimization, the as-fabricated CIBs delivered high rate capability of 15 C and superior cycling performance over 1500 cycles with 86% capacity retention at 5 C at room temperature, which is the best result of reported calcium-ion based full batteries.The configuration of the calcium-ion based tri-ion battery (CTIB) is featured with a rationally designed electrolyte with appropriate ratios of Ca 2+ , Li + , and PF 6 − ions, as shown in Figure 1a. Several metals that can alloy with Ca 2+ and Li + ions were selected as both the active anode materials and current collectors, while the expanded graphite was chosen as the cathode. During charging, Ca 2+ and Li + transport through the electrolyte to the metal anode and form alloys. Meanwhile, the intercalation of PF 6 − ions into the graphite occurs on the cathode side. The discharging process occurs reversibly.Owing to the high intercalation potential of PF 6 − ions, the stability of the hybrid electrolyte was evaluated with EC+EMC+PC+DMC (3:3:1:1 v/v/v/v) as the solvent. Linear sweep voltammetry (LSV) measurements of the pure solvent and electrolyte (0.6 m) with different Li contents (0 at.%, 15 at.%, 35 at.%, 50 at.%, 75 at.%, and 100 at.%) were conducted in the potential range of 3-5 V using stainless steel plates as the working electrode and the counter electrode. As shown in Figure S1, Supporting Information, oxidative current densities of the pure solvent and electrolyte with different Li contents are limited to 0.03-0.17 mA cm −2 , which are much lower than the standard cutoff value 0.5 mA cm −2 , suggesting high stability of the prepared electrolyte. Electrochemical Calcium-ion batteries (CIBs) are promising energy storage devices due to the merits of natural abundance, similar standard reduction potential to lithium, and bivalent-ion characteristic of calcium. However, the development of CIBs is hindered by the low rate capabilit...