Microwave-Assisted Hydrothermal Synthesis of Space Fillers to Enhance Volumetric Energy Density of NMC811 Cathode Material for Li-Ion Batteries

Abstract

Ni-rich layered transition metal (TM) oxides are considered to be the most promising cathode materials for lithium-ion batteries because of their high electrochemical capacity, high Li+ ion (de)intercalation potential, and low cobalt content. However, such materials possess several drawbacks including relatively low volumetric energy density caused by insufficient values of tap density. Herein, we demonstrate an exceptionally rapid and energy-saving synthesis of the mixed hydroxide precursor for the LiNi0.8Mn0.1Co0.1O2 (NMC811) positive electrode (cathode) material through a microwave-assisted hydrothermal technique. The obtained material further serves as a space-filler to fill the voids between spherical agglomerates in the cathode powder prepared via a conventional co-precipitation technique boosting the tap density of the resulting mixed NMC811 by 30% up to 2.9 g/cm3. Owing to increased tap density, the volumetric energy density of the composite cathode exceeds 2100 mWh/cm3 vs. 1690 mWh/cm3 for co-precipitated samples. The crystal structure of the obtained materials was scrutinized by powder X-ray diffraction and high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM); the cation composition and homogeneity of TM spatial distribution were investigated using energy-dispersive X-ray spectroscopy in a STEM mode (STEM-EDX). Well-crystallized NMC811 with a relatively low degree of anti-site disorder and homogeneous TM distribution in a combination with the co-precipitated material delivers a reversible discharge capacity as high as ~200 mAh/g at 0.1C current density and capacity retention of 78% after 300 charge/discharge cycles (current density 1C) within the voltage region of 2.7–4.3 V vs. Li/Li+.