Na0.44MnO2 Positive Electrode for Advanced Na-Ion Batteries: From Nanoplatelets to Nanofibers
Authors: Zhou Xuan , Wang Yaping , Fu Bi 
Published: February 2020

Room-temperature sodium-ion battery (NIB) has re-attracted increasing attention in recent years because of its unlimited raw material resources and physical and chemical similarity to Lithium-ion battery. Recently, the world’s first Na-ion battery powered vehicle has been demonstrated by British company-Faradion, which further proves the capability of this new type of battery technology.  Research on appropriate cathode materials for NIB started in the 1970s along with Li-ion batteries, and many intercalation compounds have been studied including layered oxides (NaxMO2 (M=3d transition metal such as Mn, Fe, Co, Cr, Ni, V, Ti etc.)), polyanion compounds (NaxMPO4, NaxMPO4F, (M= Mn, Fe, V, Ti etc.) and other compounds such as TiS2 , TaS2, MoSe2, and SnSeyS2−y etc. Among these, Na0.44MnO2 has attracted lots of attention as a cathode material because of its wide tunnel structure with high theoretical specific capacity (122 mAhg-1) and good cyclability. Na0.44MnO2 has an orthorhombic crystal structure with MnO6 octahedra and MnO5square pyramid, and the latter forms edge-linked chains linked to two double and one triple octahedral chain by the vertices forming two types of channels. The wide tunnel structure helps accommodate some of the stresses associated with structural changes during cycling.

The objective of this research work is an extension of our previous work. We have developed a simple, straightforward, and less time consuming thermo-chemical conversion process to manufacture submicron to micron size platelet/bar shaped single crystal powders of Na0.44MnO2 cathode material from an aqueous based solution precursor. Electrochemical characterization indicated specific capacity close to the theoretical value (122 mAhg-1) with good cyclability. However, the rate performance of Na0.44MnO2 is still not satisfied. The Na0.44MnO2 submicron to micron size platelet/bars exhibited a discharge specific capacity of 20 mAhg-1 at 12.3 C. Here we report a unique Na0.44MnO2 nanofibrous one dimensional (1D) structure manufactured by electrospinning technique. The phase, crystallinity and microstructures of the synthesized materials were investigated by X-ray diffraction, Scanning electron microscopy and Transmission electron microscopy. The electrochemical behavior was evaluated using electrochemical techniques including galvanostatic charge/discharge cycling, cyclic voltammetry and electrochemical impedance spectroscopy etc. The synthesized nanofiber (NF) showed a hierarchal structure consisted of many nanograins stacked along the fibers’ axial direction and demonstrated a superior rate performance with reversible specific capacity of 69.5 mAhg-1 at 10 C. It is believed that this outstanding rate performance of thus obtained Na0.44MnO2 nanofiber is resulted from the unique functional properties of 1D nanostructure with direct current pathways, shortened ion diffusion length and large electrolyte−electrode contact area and will pave a way for promoting the substantial use of sodium-ion storage systems in the future.