Synthesis and characterization of sodium vanadium oxide gels: the effects of water (n) and sodium (x) content on the electrochemistry of NaxV2O5·nH2O

Abstract

Sodium vanadium oxide gels, Na_xV₂O₅·nH₂O, of varying sodium content (0.12 < x < 0.32) were prepared by careful control of an ion exchange process. The water content (0.23 > n > 0.01) and interlayer spacing were found to be inversely proportional to the sodium level (x), thus control of sodium (x) content provided a direct, chimie douce approach for control of hydration level (n) and interlayer spacing, without the need for high temperature treatment to affect dehydration. Notably, the use of high temperatures to modify hydration levels can result in crystallization and collapse of the interlayer structure, highlighting the distinct advantage of our novel chimie douce synthesis strategy. Subsequent to synthesis and characterization, results from an electrochemical study of a series of Na_xV₂O₅·nH₂O samples highlight the significant impact of interlayer water on delivered capacity of the layered materials. Specifically, the sodium vanadium oxide gels with higher sodium content and lower water content provided higher capacities in lithium based cells, where capacity delivered to 2.0 V under C/20 discharge ranged from 170 mAh/g for Na_0.12V₂O₅·0.23H₂O to 300 mAh/g for Na_0.32V₂O₅·0.01H₂O. The capacity differences were maintained as the cells were cycled.