Sodium (Na) ion batteries have been a promising alternative to Lithium (Li) ion batteries due to earth-abundance and low cost of sodium material. Especially, anode materials for Na-ion batteries have been received a great deal of interests because graphite anode based on Li-ion reaction chemistry is not sufficient for Na-ion batteries. Unfortunately, most of anode materials for Na-ion batteries, which have been reported so far, have limitations such as low reversible specific capacity, poor rate capability, and shorter cycle life, etc.

Tin oxide have had potentials as an anode for Na-ion batteries due to earth-abundant and low cost material as well as its high theoretical reversible specific capacity, lower redox potentials, and so on. A lot of research for tin oxide as an anode have been reported, but there are some problems to overcome. First, the degree of crystallinity needs to be optimized to reduce kinetic barriers for electrochemical reaction with Na ions. To control the degree of crystallinity, solution-free and binder-free fabrication process should be necessary because current fabrication processes such as solution-based and binder-combined method is not effective which need thermal annealing process to remove solution and binder additives. Second, the morphological design needs to be optimized to become more stable from volume expansion. Even though various nanostructures like randomly distributed nanoparticles or nanowires have been studied, morphological design for volume expansion should be considered to significantly improve reversible specific capacity, rate capability, and, especially, cycle life.

In this study, amorphous tin oxide (a-SnO_x) nanohelix (NH) structures is fabricated by oblique angle deposition (OAD) method using electron beam evaporation. Solution-free and binder-free fabrication process can be realized by electron-beam evaporation which is a physical vapor deposition process and then tin oxide can be directly deposited on copper foil without any additives. The degree of crystallinity can be freely controlled because as-deposited tin oxide has amorphous phase. Moreover, three-dimensional NH structures, which are aligned with the same direction, have high porosity, high aspect ratio and large surface area to result in remarkable reversible specific capacity and stability for volume expansion. To the best of our knowledge, a-SnO_x NH structures show the world best electrochemical performance for tin oxide based anode of Na-ion batteries with significantly excellent reversible specific capacity (more than 900mAhg^-1), rate capability and stability for cycle life, even without the use of any carbon additives. Unlike to crystalline SnO and SnO₂ nanoparticles based anodes, a-SnO_x helix nanostructures based anode clearly shows different electrochemical Na-ion storage behavior considering two separated reaction mechanism: conversion reaction and alloying reaction. The reaction mechanism issues as well as more fundamental advantages of a-SnO_x NH structures as an anode for efficient Na-ion batteries will be discussed in detail based on transmission electron microscopy, ex-situ X-ray diffraction, ex-situ X-ray absorption spectroscopy, and a variety of electrochemical analysis, etc.