Moisture-Driven Degradation Pathways in Prussian White Cathode Material for Sodium-Ion Batteries.

Dickson O Ojwang,William R Brant,Lennart Häggström,Ronnie Mogensen,Tore Ericsson,Ashok S Menon,Christian Njel,Julia Maibach,Mikael Svensson

doi:10.1021/acsami.0c22032

Dickson O Ojwang, William R Brant + Show 7 more

Open Access

https://doi.org/10.1021/acsami.0c22032

Copy DOI

Abstract

The high-theoretical-capacity (∼170 mAh/g) Prussian white (PW), NaxFe[Fe(CN)6]y·nH2O, is one of the most promising candidates for Na-ion batteries on the cusp of commercialization. However, it has limitations such as high variability of reported stable practical capacity and cycling stability. A key factor that has been identified to affect the performance of PW is water content in the structure. However, the impact of airborne moisture exposure on the electrochemical performance of PW and the chemical mechanisms leading to performance decay have not yet been explored. Herein, we for the first time systematically studied the influence of humidity on the structural and electrochemical properties of monoclinic hydrated (M-PW) and rhombohedral dehydrated (R-PW) Prussian white. It is identified that moisture-driven capacity fading proceeds via two steps, first by sodium from the bulk material reacting with moisture at the surface to form sodium hydroxide and partial oxidation of Fe2+ to Fe3+. The sodium hydroxide creates a basic environment at the surface of the PW particles, leading to decomposition to Na4[Fe(CN)6] and iron oxides. Although the first process leads to loss of capacity, which can be reversed, the second stage of degradation is irreversible. Over time, both processes lead to the formation of a passivating surface layer, which prevents both reversible and irreversible capacity losses. This study thus presents a significant step toward understanding the large performance variations presented in the literature for PW. From this study, strategies aimed at limiting moisture-driven degradation can be designed and their efficacy assessed.

Highlights

Lithium-ion batteries (LIBs) have been instrumental in enabling our portable society due to their superior specific energy density, energy efficiency, and long cycle life.[1]
To enable the advancement of Na-ion technology, electrode materials based on transition-metal oxides, organic compounds, and polyanionic compounds based on phosphates or sulfates have been investigated as insertion electrodes.[3−5] they usually suffer from insufficient cycling and rate performance mainly from structural instability caused by multiple phase transitions and substantial volume changes during cycling.[6]
Comparison, the R-Prussian white (PW) is thermally stable to ∼300 °C with a negligible weight loss, ∼0.3 wt % (Figure 1b)

Summary

Introduction

Lithium-ion batteries (LIBs) have been instrumental in enabling our portable society due to their superior specific energy density, energy efficiency, and long cycle life.[1] Today they are set to support the energy revolution powering electric and low emission plug-in hybrid vehicles in addition to stationary energy storage applications.[2]. Most Prussian blue analogues (PBAs) exist with a range of different compositions in addition to variations of x, y, and n contents, leading to highly variable electrochemical performance even for the same compositional family.[7−10] Further variability originates from a wide spectrum of synthesis routes, evaluation procedures, and storage conditions.[11] In particular, the electrochemical performance is broadly accepted to be heavily affected by the presence of water.

Results

Discussion

Conclusion