Alkaline earth metal vanadates as sodium-ion battery anodes

Abstract

The abundance of sodium resources indicates the potential of sodium-ion batteries as emerging energy storage devices. However, the practical application of sodium-ion batteries is hindered by the limited electrochemical performance of electrode materials, especially at the anode side. Here, we identify alkaline earth metal vanadates as promising anodes for sodium-ion batteries. The prepared calcium vanadate nanowires possess intrinsically high electronic conductivity (> 100 S cm^-1), small volume change (< 10%), and a self-preserving effect, which results in a superior cycling and rate performance and an applicable reversible capacity (> 300 mAh g^-1), with an average voltage of ∼1.0 V. The specific sodium-storage mechanism, beyond the conventional intercalation or conversion reaction, is demonstrated through in situ and ex situ characterizations and theoretical calculations. This work explores alkaline earth metal vanadates for sodium-ion battery anodes and may open a direction for energy storage.The development of suitable anode materials is essential to advance sodium-ion battery technologies. Here the authors report that alkaline earth metal vanadates are promising candidates due to the favorable electrochemical properties and interesting sodium-storage mechanism.

Fig. 1

Comparison of the CaV₄O₉ nanowires in this work and previously reported SIB anodes. a Average voltage vs. reversible capacity of the anodes for SIBs. b Volume change of the reported alloying or conversion reaction anodes for SIBs

Fig. 2

Characterization of CVO-450 and CVO-550 nanowires. a XRD patterns of CVO-450 and CVO-550. b, c Crystal structures of the CaV₄O₉; the green and red balls represent Ca and O ions, respectively, and the grey polyhedrals represent the V–O pyramids. d, e EDS mapping of CVO-450 and CVO-550, respectively. The scale bar is 6 µm for d and 3 µm for e. f–h TEM and HRTEM images of CVO-450; the inset of h is the SAED pattern of CVO-450. The scale bars of f–h and the inset of h are 200 nm, 50 nm, 5 nm, and 2 nm^–1, respectively. i–k TEM and HRTEM images of CVO-550; the inset of k is the SAED pattern of CVO-550. The scale bars of i–k and the inset of k are 500 nm, 100 nm, 5 nm, and 2 nm^–1, respectively

Fig. 3

Electrochemical properties of CaV₄O₉ nanowires and VO₂ nanowires as SIB anodes. a Discharge/charge profiles of CVO-450 after different numbers of cycles at 100 mA g⁻¹. b I–V curves of CVO-450 and VO₂-450 nanowires. Inset is the SEM image of the CVO-450 single nanowire device. Scale bar, 10 µm. c Rate performances of CVO-450, CVO-550, and VO₂-450 at different current densities of 100, 300, 500, 1000, 2000 and 5000 mA g⁻¹. d Cycling performances and Coulombic efficiency of CVO-450, CVO-550, and VO₂-450 at a current density of 1000 mA g⁻¹

Fig. 4

Cyclic voltammetry and in situ XRD results of CVO-450. a CV curves of CVO-450 at scan rates from 0.5 to 50 mV s⁻¹. b The relation between the square root of the scan rate (v ^1/2) and the corresponding currents at 0.01 V. c In situ XRD results of CVO-450 during the initial discharge and charge process

Fig. 5

Ex situ TEM characterization of CVO-450 at the discharge and charge states. a–d TEM image, SAED patterns, and HRTEM image of CVO-450 at the sodiation state after 5 cycles at 1000 mA g⁻¹. The SAED patterns of b and c were collected from region 1 and region 2 in a, respectively. e–h TEM image, SAED patterns, and HRTEM image of CVO-450 at the sodiation state after 300 cycles at 1000 mA g⁻¹. The SAED patterns of f and g were collected from region 3 and region 4 in e, respectively. i–l TEM image, SAED patterns and HRTEM image of CVO-450 at the desodiation state after 300 cycles at 1000 mA g⁻¹. SAED patterns of j and k were collected from region 5 and region 6 in i, respectively. Scale bars: a 200 nm; e, i 500 nm; b, c, f, g, j, k 5 nm⁻¹; d, h, l 5 nm

Fig. 6

In situ TEM results of CVO-450. a Configuration of the in situ TEM device. Scale bar, 500 nm. b Time-lapse TEM images after applying a voltage bias. Scale bar, 500 nm. c EELS data collected from the red circle area at 0 and 35 min

Fig. 7

Illustration of the reaction mechanism during initial sodiation and subsequent cycles. a Sodium-storage mechanism of CaV₄O₉. b Sodium-storage mechanism of VO₂