Novel Nanoscale Electrode Materials for Sodium-Ion Batteries
Authors: Bodnarchuk Maryna , Kravchyk Kostiantyn , Walter Marc , Kovalenko Maksym V. 
Published: February 2020

Colloidal nanocrystals can serve as ideal model electrode materials for studying the effects of downsizing on electrode performance. We will present two examples of monodisperse nanocrystals as anode and cathode materials for Li-ion and Na-ion batteries (LiBs and NiBs).

       Fluoroperovskites of the form NaMF3 and LiMF3 (M = Ni, Mn, Fe) are identified as perspective oxygen-free cathode materials for LiBs and NiBs. Among them, NaFeF3 would have a clear priority due to its unbeatable combination of the high natural abundance of Fe and its non-toxicity. At the same time, mixed-metal fluorides exhibit a poor electronic conductivity. Therefore, fine particles of complex fluorides that provide short conductive path for electrons are required for high ion diffusion and improving the capacity of the cathodes. We have developed colloidal synthesis of NaFeF3 nanocrystals using decomposition of heterometallic fluorinated β-diketonates NaM(hfac)3  (M = Mn, Fe, Co and Ni; hfac=hexafluoroacetylacetonate) at 200-300 °C in a medium of high-boiling apolar solvent. NaM(hfac)3  compounds are stable in moist air and can be obtained by simple synthetic procedure from cheap, commercially available starting reagents. Such crystalline NaFeF3 nanoplates are capable of fast Na and Li intercalation/deintercalation. The use of NaFeF3 cathode material in form of nanoplates allowed to increase its cyclic stability and rate capability performance. In addition, electrochemical performance of NaFeF3 nanoplates was measured in a full cell configuration with antimony anode. The charge capacity of NaFeF3 nanoparticles after 20 cycles at 20mA/g was equal to 150mAh/g.

     Antimony sulfide has high potential as anode material for NiBs with are high theoretical capacity (946 mA h g-1). Nanostructured Sb2S3 can improve kinetic performance of NiB due to reducing the transport length of Na ions and electrons as well as accommodating the volume changes. We have developed hot-injection colloidal synthesis of monodisperse Sb2S3 nanoparticles of various shapes and sizes via reaction SbCl3 and bis(trimethylsilyl) sulfide in hot-boiling solvent. Crystalline and amorphous nanoparticles were tested as potential anode materials of LiBs and NiBs. In comparison with bulk Sb2S3, we show that monodisperse crystalline or amorphous Sb2S3 nanoparticles have enhanced the cycling stability. 20 nm amorphous Sb2S3 nanospheres exhibited higher capacity and excellent cycle performance. In particular, they showed Lion and Na-ion capacities of 1100 and 950 mAh/g, respectively, at a current density 330 mA/g. Besides good cycling performance, nanoscale Sb2S3 showed an excellent rate capability.

     Metal phosphides such as FeP, CoP, NiP2, and CuP2 remain essentially unexplored as electrode materials for SIBs, despite their high theoretical charge storage capacities of 900-1300 mAh g-1. We have developed the synthesis of metal phosphide NCs and assessed their electrochemical properties as anode materials for SIBs, as well as for LIBs. We also compared the electrochemical characteristics of phosphides with their corresponding sulfides, using the environmentally benign iron compounds, FeP and FeS2, as a case study. We show that despite the appealing initial charge storage capacities of up to 1200 mAh g-1, enabled by effective nanosizing of the active electrode materials, further work toward optimization of the electrode/electrolyte pair is needed to improve the cycling performance of FeP and other phosphides.