Spherical Shaped Particle of Randomly Packed LiFePO4 Nanoplates for High-Rate of Li-Ion Batteries

Abstract

Energy storage is the key factor for saving and using renewable energy. Among the various energy storage devices, Li-ion battery is the most promising storage device owing to its high energy density, efficiency and cycle life. Not only for the small application like mobile phone and laptop computer but also electric vehicles (EVs) and massive energy storage system (ESS), numerous applications have been used Li-ion battery. However its high energy density is like a double-edged sword, the safety issues have been a problem for the practical market. The safety problems from the Li-ion battery usually come from thermal runaway, which is mainly come from oxygen release from the cathode material. Lithium iron phosphate (LFP) has PO4 polyanion that oxygen atom is strongly bonded with phosphorus and prevents oxygen release, therefore, it is well known as the safe cathode material for the Li-ion battery. Also for the high power battery applications, LFP is a promising material owing to the ability of high-rate charge and discharge performance. For the high rate availability on LFP material, two important key factors should be satisfied; one is fast ion diffusion inside the material and the other is high electric conductivity. The Li-ion diffusivity coefficient and electron conductivity are intrinsic properties of the material, however, particle size or morphology and conducting agent coating can raise the speed of diffusion and conduction. Here, we demonstrated those factors by solvo-thermally synthesized plate shape nanoparticle and spherical secondary particle with carbon coating using spray-drying. The nanoparticle has a flat shape in b-axis, therefore, diffusion speed increase because the diffusion distance of lithium is greatly reduced. The nanoparticle without spray-drying showed low capacity and also poor C-rate performance even with the carbon coating. Besides, after forming the spherical particles, it showed 162mAh/g of initial capacity at the 0.1C rate and maintains more than 80% of initial capacity (~130.91mAh/g) at the 50C rate. Moreover, electrode density which is the intrinsic weakness of nanoscale electrode material has been improved and the spheres carried out excellent cycling ability which shows 98% capacity retention over 500cycles at 1C after the high C-rate test. In order to analyze the causes of such good characteristics, morphology and crystallographic directions were confirmed by SEM and TEM image, and electrical conductivity and diffusion coefficient were calculated by using Cyclic Voltammetry (CV), Galvanostatic Intermittent Titration Technique (GITT) and Electrochemical Impedance Spectroscopy (EIS). More details will be discussed at the meeting.