Synthesis and Electrochemical Cycling Characteristics of Nanostructured Antimony Alloying Electrodes for Energy Storage Applications

Abstract

Nanostructured antimony is a highly promising alloying electrode material for both Li-ion and Na-ion battery systems, possessing a large gravimetric charge storage capacity (660 mAh g-1) combined with extraordinary rate capability (cycling at a 20C rate results in only a ~15% decrease in capacity, relative to 1C). Herein, we elucidate the fundamental drivers for the performance of antimony nanomaterial-based Na-ion battery negative electrodes with a particular focus on the effects of morphology, temperature and oxidation. In support of these studies, we report the supercritical-fluid-based synthesis of highly anisotropic, hexagonal antimony nanoplatelets with 1000:1 aspect ratios and average thicknesses of 50±10 nm and use the nanoplatelets as a model system to investigate how morphology and strain impact the phases that are encountered during the electrochemical alloying of nanostructured antimony in Na-ion battery electrodes, the thermodynamics and kinetics of battery cycling and the electrode morphology and conductivity. For these investigations we study the electrodes via capacity and differential capacity analysis, a high-fidelity galvanostatic intermittent titration technique, conductive atomic force microscopy and in situ X-ray diffraction. We find that active material anisotropy results in increased ordering and crystallinity during cycling for the antimony nanomaterial-based electrodes and increased composite heterogeneity. We also identify c-NaSb a previously unobserved phase for antimony-based Na-ion battery electrodes that occurs primarily during degraded cycling. We then utilize electrochemical impedance spectroscopy to investigate the source of temperature-based performance changes in antimony nanocrystal-based Na-ion battery electrode materials. We find that increased charge transfer resistance is predominantly responsible for the observed 100 mAh g-1 capacity reduction (~20%) that occurs upon changing the cycling temperature from 50 to 5°C. Furthermore, we determine that the observed decrease in capacity at low temperature is almost entirely caused by increased charge transfer resistance due to less facile Na-ion transport across the solid-electrolyte interphase layer-electrode interface. Additionally, we systematically study the effects of oxidation on antimony-based electrode performance. Most antimony-based and antimony oxide-based electrodes are typically fabricated into electrodes in a water-based slurry under atmospheric conditions. While this fabrication approach is lower cost and much safer than the methods used for other battery electrode materials that must be processed using toxic organic solvents under inert gas, the level of oxide formation that occurs during water-based slurry processing of nanostructured antimony has largely been undiscussed and left as an uncontrolled variable during electrode fabrication. Here, we systematically investigate the impact of oxide formation on changes in the electrochemical performance of antimony nanocrystal-based Na-ion battery negative electrodes, providing insight as to why small amounts of oxide result in enhanced gravimetric capacity and capacity retention, while more extensive oxidation results in reduced rate capability. We find that barriers to the sodiation of highly oxidized antimony are reduced by amorphization of the crystalline antimony oxide (c-Sb2O3) during extended cycling, while sodium sequestration leads to low reversible sodium storage utilization upon extended cycling.