Structural water and disordered structure promote aqueous sodium-ion energy storage in sodium-birnessite

Xiaoqiang Shan,Katharine L Page,Daniel S Charles,Fenghua Guo,Jinpeng Wu,Zachary Lebens-Higgins,Wanli Yang,Xiaowei Teng,Wenqian Xu,Joerg C Neuefeind,Louis F J Piper,Mikhail Feygenson,Sara Abdel Razek

doi:10.1038/s41467-019-12939-3

Abstract

Birnessite is a low-cost and environmentally friendly layered material for aqueous electrochemical energy storage; however, its storage capacity is poor due to its narrow potential window in aqueous electrolyte and low redox activity. Herein we report a sodium rich disordered birnessite (Na0.27MnO2) for aqueous sodium-ion electrochemical storage with a much-enhanced capacity and cycling life (83 mAh g−1 after 5000 cycles in full-cell). Neutron total scattering and in situ X-ray diffraction measurements show that both structural water and the Na-rich disordered structure contribute to the improved electrochemical performance of current cathode material. Particularly, the co-deintercalation of the hydrated water and sodium-ion during the high potential charging process results in the shrinkage of interlayer distance and thus stabilizes the layered structure. Our results provide a genuine insight into how structural disordering and structural water improve sodium-ion storage in a layered electrode and open up an exciting direction for improving aqueous batteries.

Highlights

Birnessite is a low-cost and environmentally friendly layered material for aqueous electrochemical energy storage; its storage capacity is poor due to its narrow potential window in aqueous electrolyte and low redox activity
Our results demonstrate that Na-rich and disordered birnessite structure can afford a stable potential window of 2.5 V in an aqueous electrolyte with high overpotential towards the gas evolution reactions
In situ XRD has revealed the role of water co-deintercalation in mitigating interlayer expansion during the high potential charging

Summary

Introduction

Birnessite is a low-cost and environmentally friendly layered material for aqueous electrochemical energy storage; its storage capacity is poor due to its narrow potential window in aqueous electrolyte and low redox activity. The co-deintercalation of the hydrated water and sodium-ion during the high potential charging process results in the shrinkage of interlayer distance and stabilizes the layered structure. Sodium-manganese hexacyanoferrate reported by Goodenough’s group showed good energy performance and cycling life in a non-aqueous electrolyte[6,7,8] Another approach is to use a large interstitial host framework, especially layered structure. Birnessite has a large interlayer distance (~7 Å), its storage capacities for Na-ion were low due to the limited thermodynamically stable potential window (~1.23 V) of an aqueous electrolyte and ineffective redox process[26,27,28]. Co-deintercalation of water molecules along with Na-ion at the high potential charging, evidenced by in situ XRD, can stabilize the layered structure from over-expansion of the interlayer distance

Methods

Results

Conclusion