Synthesis of Spinel-Structured Core-Shell Nanocrystals for Lithium Ion Batteries
Authors: Kwon Bob Jin , Kim Chunjoong , Cabana Jordi 
Published: February 2020

High power density and excellent cycling stability in lithium ion battery (LIBs) are required to meet the criteria for the power source such as electric vehicles. Higher power density is generally hindered by sluggish diffusion of Li ions in micrometric materials commonly adopted in LIBs.[1] Size reduction of active materials can significantly improve the rate of lithium ions because of the shortened length of diffusion path. In the meantime, high surface area enables facile access of charges via large contact area with the electrolyte or electronic conducting additives. In this perspective, much effort have been devoted to the nanosized particles, which report the high power density that cannot be achieved from the micrometric counterparts. However, in return, increased surface area of particles can lead to the vigorous unfavorable interfacial reaction that mainly occurs through the energetically unstable surface such as dissolution of active materials and decomposition of electrolyte.[2] Reducing the active transition metal in the vicinity of the surface by partially replacing inactive ions can improve the interfacial stability, but must be prepared as the form of thin passivating barriers to preserve storage capacity. Post-synthetic coating procedure with stable phases such as Al2O3 and MgO, has been performed on the surface of particles as the most common approach,[3] however, it is difficult for coating phases to access buried interfaces because of agglomerated feature of the particles.
Herein, Mn3O4 nanocrystals (NCs) were synthesized by thermal decomposition of manganese acetate in oleylamine and an ultra-thin shell layers that are enriched in aluminum were introduced, finally leading to the formation of core-shell (C-S) NCs. The particle size of Mn3O4 and C-S NCs were approximately 21 nm and 24 nm, respectively, as shown in Figure 1. The atomic ratio of Al/Mn in C-S NCs was found to be ~0.2 by energy dispersive X-ray spectroscopy (EDX). The crystal structures of Mn3O4 and C-S NCs were studied using X-ray diffraction (XRD) in Figure 2, both of which matched with a tetragonal spinel structures (I41amd, JCPDS card # 55492) without any peaks related to Al2O3. In order to study composition and microstructure of the shell layer, the scanning transmission electron microscopy (STEM) will be carried out.
C-S LiMn2O4 NCs are prepared by mixing precursor C-S NCs and Li sources followed by thermal treatment. The electrochemical properties will be evaluated in Li metal half cells compared to the counterpart without shell layer. The effect of the shell layer on the electrochemical stability will be discussed.
Fig. 1. TEM images of (a) Mn3O4 and (b) C-S NCs. Magnified images are shown in the insets (Scale bars of (a) and (b) are 5 nm and 10 nm, respectively).
Fig. 2. XRD patterns of (a) Mn3O4 and (b) C-S NCs, which match well with a tetragonal phase

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Figure 1