Analysis of New Design Cathode Material of Li Ion Battery Using Ab-Initio Study and Experiment
Authors: Yi Sangkoan , Moon Janghyuk , Cho Kyeongjae , Cho Maenghyo 
Published: February 2020

Li-ion batteries are widely used in portable electronic devices because of their higher specific capacity, higher energy density and good cycle life compared to previous batteries. However, they are limited to use for HEV(Hybrid Electronic Vehicle), EV(Electronic Vehicle) and other big electronic devices because most commonly used battery cathode material like the LiCoO2 (LCO) has insufficient performance. Thus, need to research continue to be directed finding new cathode materials for use large-scale electronic devices. Until now, several different materials have been investigated for alternative cathode materials in order to improve the performance of Li-ion batteries. In various cathode materials, poly-oxyanion compounds with strong binding with oxygen have excellent thermal stability than other cathode materials. Among these, phosphate (LiTMPO4(TM=Fe, Mn, Co and so on)) is interested in many researchers because this material is show the higher capacity (~170mAh/g) than existing material like LCO (~120mAh/g), and has very safety. And this cathode material has stable structure and was show almost theoretical capacity because phosphate has octahedral structure that six oxygen atoms are formed octahedron with transition metal (TM). The octahedral structure is very stable, so cannot occur structural problems when Li ion is extracted. However, phosphate has poor ionic conductivity and only one dimensional Li diffusion path so if this path is blocked, diffusivity is many decreased. And iron phosphate LiFePO4 has lower voltage (~3.4V), hence has low energy density and power density. Recently, orthosilicates materials (Li2TMSiO4(TM=Fe, Mn, Co and so on)) receive huge interest because have higher capacity (~330mAh/g) than other cathode materials and safer than commercialized cathode material. However in previous studies, silicate cannot show theoretical capacity and has poor electrochemical properties. For example, Li2FeSiO4 (LFS) only show more than 160mAh/g with reversible capacities of 120-140mAh/g cannot show theoretical capacity 330mAh/g. Silicates are polymorphism through synthesis method, so have many space groups like Pmn21, P21n, Pmnb and so on. These structures are all tetrahedral structures that four oxygen atoms are formed tetrahedron with transition metal (TM). The tetrahedral structure has structural problems when Li ion is extracted because relatively few oxygen atoms are formed. Therefore, polyhedrons are changed when Li ion is extracted, so this change is occurred structural problems and cannot shows theoretical capacity. If transition metal of phosphate and silicate is Fe, the materials have strong point at price because Fe is cheaper than other transition metals and the Fe is very safer than other transition metals. In this research, we suggest new design polyoxy-anion cathode material structure for develop battery performance. This material is that substituted from phosphorus(P) to silicon(Si) step by step, next we show that substituted structures have developed properties and structural stability when occurred charge and discharge process at Li ion battery. We named that this new design cathode material is phospho-silicate LiFeP1-xSixO4(LFPS).  However, phosphate has olivine structure and silicates have all tetrahedral structures, so we don’t know the structure when mixed P and Si. Therefore we calculated the most stable structure when mixed P and Si based on first principles calculation among possible structures, and found that most stable structure is olivine structure. So we calculated several properties the olivine structure LFPS. As a result, the LFPS is controlled voltage because voltage is increased linearly following Si ratio. And this result is very positive because increased voltage is meant that increase energy density and power density. This point was critical disadvantage of olivine iron phosphate. Next when mixed Si and P, Li-O bonding length is increased linearly, this result means that diffusivity is better than original material’s kinetically. And Fe-O bonding length is decreased, that is Fe-O bonding strength is increased, so anti-site defect that induced Li path blocked is prevented. Finally, we validated this research result through experiment. This research of the new polyoxy-anion compound cathode material that used ab-initio study and experiment show the new promising cathode material compared with existing cathode material like LCO. Furthermore, the research provides the different directions that can develop the Li ion battery, so advanced for using HEV(Hybrid Electronic Vehicle), EV(Electronic Vehicle) and other big electronic devices.