Zn2GeO4 and Zn2SnO4 nanowires for high-capacity lithium- and sodium-ion batteries

Abstract

Germanium (Ge) and tin (Sn) are considered to be the most promising alternatives to commercial carbon materials in lithium- and sodium-ion batteries. High-purity zinc germanium oxide (Zn₂GeO₄) and zinc tin oxide (Zn₂SnO₄) nanowires were synthesized using a hydrothermal method, and their electrochemical properties as anode materials in lithium- and sodium-ion batteries were comparatively investigated. The nanowires had a uniform morphology and consisted of single-crystalline rhombohedral (Zn₂GeO₄) and cubic (Zn₂SnO₄) phases. For lithium ion batteries, Zn₂GeO₄ and Zn₂SnO₄ showed an excellent cycling performance, with a capacity of 1220 and 983 mA h g⁻¹ after 100 cycles, respectively. Their high capacities are attributed to a combination of the alloy formation reaction of Zn and Ge (or Sn) with Li, and the conversion reactions: ZnO + 2Li⁺ + 2e⁻ ↔ Zn + Li₂O and GeO₂ (or SnO₂) + 4Li⁺ + 4e⁻ ↔ Ge (or Sn) + 2Li₂O. For the first time, we examined the cycling performance of Zn₂GeO₄ and Zn₂SnO₄ in sodium ion batteries; their capacities were 342 mA h g⁻¹ and 306 mA h g⁻¹ after 100 cycles, respectively. The capacity of Zn₂SnO₄ is much higher than the theoretical capacity (100 mA h g⁻¹), while that of Zn₂SnO₄ is close to the theoretical capacity (320 mA h g⁻¹). We suggest a contribution of the conversion reaction in increasing the capacities, which is similar to the case of lithium ion batteries. The present systematic comparison between the lithiation and sodiation will provide valuable information for the development of high-performance lithium- and sodium-ion batteries.