The reaction mechanism of SnSb and Sb thin film anodes for Na-ion batteries studied by X-ray diffraction, 119Sn and 121Sb Mössbauer spectroscopies

• SnSb and Sb thin films are studied as anode materials for Na-ion batteries.
• Local environments are compared to powder model compounds.
• Crystallization into Na3Sb occurs at higher temperatures (65 and 95 °C).

The electrochemical reaction of Sb and SnSb anodes with Na results in the formation of amorphous materials. To understand the resulting phases and electrochemical capacities we studied the local order using 119Sn and 121Sb Mössbauer spectroscopies in conjunction with measurements performed on model powder compounds of Na–Sn and Na–Sb to further clarify the reactions steps. For pure Sb the sodiation starts with the formation of an amorphous phase composed of atomic environments similar to those found in NaSb, and proceeds further by the formation of crystalline Na3Sb. The reversible reaction takes place during a large portion of the charge process. At full charge the anode material still contains a substantial fraction of Na, explaining the lack of recrystallization into crystalline Sb. The reaction of SnSb yields Na3Sb at full discharge at higher temperatures (65 and 95 °C) while the RT reaction yields amorphous compounds. The electrochemically-driven, solid-state amorphization reaction occurring at RT is governed by the simultaneous formation of Na-coordinated Sn and Sb environments, as monitored by the decrease (increase) of the 119Sn (121Sb) Mössbauer isomer shifts. Overall, the monitoring of the hyperfine parameters enables to correlate changes in Na content to the local chemical environments.