Improving cycling stability and suppressing voltage fade of layered lithium-rich cathode materials via SiO2 shell coating

Li-rich layered metal oxide (Li1.2Mn0.56Ni0.16Co0.08O2, denoted as LLMO) cathode materials with SiO2 coating have been successfully synthesized by using a wet chemical method combined with high temperature annealing. Scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) analyses revealed that an amorphous SiO2 coating layer has formed on the surface of the LLMO electrode. Electrochemical analysis demonstrated that an optimal SiO2 coating amount on the surface of LLMO particles can effectively improve the cycling stability, suppress the voltage and capacity decay, and enhance the thermal stability of these electrodes. An optimal SiO2 coating amount of 0.5wt% on an LLMO electrode (S-LLMO-0.5wt%) improved the capacity retention to 82.6% after 200cycles at 25 degrees C (67.0% for the pristine LLMO electrode) and 68.8% after 100cycles at 60 degrees C (56.3% for the pristine LLMO electrode), respectively. Besides, the voltage drop of the S-LLMO-0.5wt% sample (46.8mV/53.6mV) was lower than that of the pristine LLMO sample (366.1mV/60.3mV) after 200cycles at 3.2V/3.8V, confirming that 0.5wt% SiO2 coating can reduce the capacity loss and slower the voltage degradation rate of LLMO electrodes. In addition, the exothermic reaction of the pristine LLMO (denoted as P-LLMO from now on) electrode can be delayed by SiO2 coating while the decomposition temperature of the S-LLMO-0.5wt% sample is significantly higher than that of the P-LLMO. These results are attributed to a uniform amorphous SiO2 coating layer that not only blocks the highly active oxygen release from the bulk material but also prevents the direct contact of the electrolyte to the positive electrodes, suppressing structural transition from a layered phase to a spinel-like one.