A new high-voltage calcium intercalation host for ultra-stable and high-power calcium rechargeable batteries

Zheng-Long Xu,Zheng-Long Xu,Yoon-Joo Ko,Sang-Young Lee,Gabin Yoon,Sung-Pyo Cho,Jongwoo Lim,Kisuk Kang,Kisuk Kang,Jooha Park,Jian Wang,Hyunseok Moon

doi:10.1038/s41467-021-23703-x

Abstract

Rechargeable calcium batteries have attracted increasing attention as promising multivalent ion battery systems due to the high abundance of calcium. However, the development has been hampered by the lack of suitable cathodes to accommodate the large and divalent Ca2+ ions at a high redox potential with sufficiently fast ionic conduction. Herein, we report a new intercalation host which presents 500 cycles with a capacity retention of 90% and a remarkable power capability at ~3.2 V (vs. Ca/Ca2+) in a calcium battery. The cathode material derived from Na0.5VPO4.8F0.7 is demonstrated to reversibly accommodate a large amount of Ca2+ ions, forming a series of CaxNa0.5VPO4.8F0.7 (0 < x < 0.5) phases without any noticeable structural degradation. The robust framework enables one of the smallest volume changes (1.4%) and the lowest diffusion barriers for Ca2+ among the cathodes reported to date, offering the basis for the outstanding cycle life and power capability.

Highlights

Rechargeable calcium batteries have attracted increasing attention as promising multivalent ion battery systems due to the high abundance of calcium
Among the multivalent battery systems, calcium ion batteries (CIBs) are capable of offering the highest voltage due to the low reduction potential of Ca/Ca2+ with −2.9 V, which is lower than −2.4 V for magnesium, −1.7 V for aluminum, and −0.76 V for zinc[8,9], enabling a high energy density with the voltage comparable to that of lithium ion batteries (LIBs)
In order to verify the intercalation capability of Ca2+ ion into NVPF framework, we constructed an electrochemical cell as illustrated in Fig. 1a, which consists of desodiated NVPF as the working electrode, activated carbon as a counter electrode, and an electrolyte of 1 M Ca(PF6)[2] in EC/PC19

Summary

Introduction

Rechargeable calcium batteries have attracted increasing attention as promising multivalent ion battery systems due to the high abundance of calcium. In our study exploring a new CIB cathode here, we attempted to search for a material chemistry with a rigid open framework that is less sensitive to the volume change arising from the large guest ion insertion/extraction In this regard, a success-proven polyanion-based cathode for sodium ion battery, Na1.5VPO4.8F0.7 (NVPF)[21,22,23], attracted our immediate attention. Exploiting the structurally robust and open-framework NVPF host, we show that the desodiated NVPF can function as a stable and fast-kinetic calcium ion intercalation host and be a strong contender to the currently available cathodes for rechargeable CIBs. It is demonstrated that the NVPF-based host allows reversible Ca2+ ion intercalation and deintercalation at ~3.2 V (vs Ca/Ca2+) in calcium cells with the capacity fading rate of 0.02% per cycle over 500 cycles, which records one of the lowest values reported to date for CIB electrodes. The origin of the outstanding performance is elucidated through comprehensive experimental and theoretical investigations

Methods

Results

Conclusion