In-Situ Synthesis of High Voltage Cathode Li2FeMn3O8 in Garnet Scaffold for Solid State Battery Application

Introduction

The safety concern of Li-ion batteries due to the potential leakage and the flammability of the electrolyte severely hinders applications. The emergence of all-solid-state batteries, consisting of solid state electrodes and solid state ionic conductor electrolyte, will solve this safety problem because it eliminates the problematic liquid electrolytes.

One key issue that needs to be addressed in constructing such a battery is the contact between electrode and electrolyte. Different from liquid electrolytes that can submerge the cathodes, it is difficult for solid state electrolyte to have a sufficient touch with electrodes. A common cathode-side fabrication method is to mix the electrolyte powder with cathode powder and electronically conductive materials, yielding a point-to-point contact. This contact configuration limits the number of effective pathways of li-ion diffusion, therefore hurts the performance of the battery. 

solves the aforementioned problems. Starting with precursor solutions, reactants are homogenously mixed at a molecular level, and are much easier to get into the scaffold than powders. Additionally, in-situ synthesis allows the high voltage cathode material to adhere to the surface of the electrolyte scaffold, yielding a face-to-face contact. This enlarges contact area, creating more pathways for Li-ion diffusion and improves the conductivity between the cathode and the electrolyte.

Experimental Methods

The high voltage cathode Li₂FeMn₃O₈ (LFMO) is synthesized by glycine-nitrate combustion. Precursor solution containing metal nitrates and glycine with an optimized ratio is infiltrated into the porous garnet electrolyte scaffold. After drying at 300℃ combustion takes place. The pellet is subsequently annealed at 700℃ to achieve the desired LFMO phase. Finally, carbon nanotube ink is infiltrated to the scaffold to form a conductive network over the cathode.

Results

Fig. 1a shows a schematic of the cathode. With the conductive network and improved contact of cathode and electrolyte, it is expected to have an improved performance. As is shown in Fig. 1b, the cathode adheres to the surface of the scaffold, forming a face-to-face contact.

The LFMO cathode has been tested in LFMO/Li coin cell with LiPF₆/ FEC electrolyte. Fig.2 a) gives the XRD spectrum of the cathode prepared by glycine-nitride combustion method. The spectrum matches with JCPDS#48-0258, indicating a Li₂FeMn₃O₈ phase. Fig.2 b) shows the cycling performance of the cathode. It can be seen from the voltage profile of the tested cell that LFMO has two plateaus at around 4.1V and 4.9V, with capacity being 104mAh/g.