Heterostuctured Surface Layers of Ni-based Cathode Materials for Li-ion Batteries

Abstract

In an attempt to overcome the problems associated with LiNiO2, the solid solution series of lithium nickel-metal oxides, Li[Ni1-xMx]O2 (with M = Co, Mn, Al, Ti, Mg, etc.), have been investigated as favorable cathode materials for high-energy and high-power lithium-ion batteries. However, along with the improvement in the electrochemical properties in Ni-based cathode materials, the thermal stability has been a great concern, and thus violent reaction of the cathode with the electrolyte needs to be avoided. Firstly, we report a heterostructured Li[Ni0.54Co0.12Mn0.34]O2 cathode material which has a high energy and safety. The core part of the particle consist of LiNi0.54Co0.12Mn0.34]O2 with a layered phase (R3-m) and shell part with a thickness of < 0.5 μm consists of highly stable Li1+x[CoNixMn2-x]2O4 spinel phase (Fd-3m). The material demonstrates reversible capacity of 200 mAh g-1 and retains 95% capacity retention at the most severe test condition of 60 °C. In addition, amount of oxygen evolution from the lattice in the cathode with two heterostructures is reduced by 70%, compared to the reference sample. All these results suggest that the bulk Li[Ni0.54Co0.12Mn0.34]O2 consisting of two heterostructures satisfy the requirements for hybrid electric vehicles, power tools, and mobile electronics. Secondly, solid solution series of lithium nickel metal oxides, Li[Ni1-xMx]O2 (with M = Co, Mn, and Al) have been investigated intensively to enhance the inherent structural instability of LiNiO2. However, when a voltage range of Ni-based cathode materials was increased up to > 4.5 V, phase transitions occurring above 4.3 V resulted in accelerated formation of trigonal phase (P-3m1) and NiO phases, leading to and pulverization of the cathode during cycling at 60 °C. In an attempt to overcome these problems, LiNi0.62Co0.14Mn0.24O2 cathode material with pillar layers in which Ni2+ ions were resided in Li slabs near the surface having a thickness of ~10 nm was prepared using a PVPfunctionalized Mn precursor coating on Ni0.7Co0.15Mn0.15(OH)2. We confirmed the formation of a pillar layer via various analysis methods (XPS, HRTEM, and STEM). This material showed excellent structural stability due to a pillar layer, corresponding to 85 % capacity retention between 3.0 to 4.5 V at 60 °C after 100 cycles. In addition, amount of heat generation was decreased by 40 %, compared to LiNi0.70Co0.15Mn0.15O2.