Development of LVP cathode materials and its effect on LFP in lithium ion batteries

Abstract

Nowadays, with the development of technology, there is an increasing demand for energy storage solutions. This need is met through both primary and secondary battery technologies. Secondary batteries, distinguishable by their rechargable nature, encompass a variety of types including lithium ion batteries (LIB), lithium polymer batteries (LPB), Ni-MH, Ni-Cd and Pb-Acid are examples of this type of batteries. LIB show both higher gravimetric energy and volumetric energy density compare to other secondary batteries. The fundamental components of LIB include the positive and negative electrode, seperator and electrolyte. Within the battery, electrolyte allows lithium ions transfer between negative and positive electrode thanks to medium is provides. A non-conductive separator is placed between positive and negative electrode to prevent short circuit formation. The movement of lithium ions between the electrodes and the formation of a lithiated structure constitutes the basis of LIB. During discharge, lithium ions migrate from the negative electrode to the positive electrode, and the opposite migration of lithium ions occurs in the charging state. While electron transition is observed in the external circuit in the same direction as the movement of lithium ions, a current flows in the opposite direction to the electrons. The cathode material is the primary factor influencing the electrochemical performance of LIB. The charactericties of cathode materials are determined by their crystal structures. Layered structures have ahigh energy density, olivine structures are structurally stable, and spinel structures, with their three-dimensional structural network, provide good lithium ion conductivity. LiCoO2, LiMn2O4, LiMnPO4, LiTi2(PO4)3, LiFePO4 (LFP), Li3V2(PO4)3 (LVP), LiNiMnCoO2, LiNiCoAlO2 and LiVOPO4 are examples of cathode materials studied in recent years. LVP, lithium vanadium phosphate, has high theoretical specific capacity (197 mAh/g), thermal stability, and high ionic and electrical conductivity. However, high voltage values (~ 4.55 V) are required in order to obtain these capacity values, which then might cause decomposition of the electrolyte. Furthermore, when the LVP cathode material is charged above 4.55 V, three lithium ions in the cathode material are deintercalated. It also generates a solid solution during discharge since the cathode material does not undergo a two-phase reaction. Unlike in a two-phase transition reaction, a solid solution formation results in increased volumetric change within the cell and decreased electrostatic attraction. When the cathode material is charged below 4.55 V, two lithium ions undergo intercalation and deintercalation, and just 132 mAh/g theoretical capacity is obtained. A two-phase reaction takes place during discharge below 4.55 V, with no solid solution formation observed. Within the scope of this thesis, primarily LVP cathode active material was obtained thanks to the sol-gel in order to examine electrochemical characteristics of batteries. Then, in order to examine the effect of the high electrical and ionic conductivity of LVP on the LFP cathode, LVP surface modification was applied and its morphological, structural and electrochemical properties were examined. Two distinct techniques were used to modify the surface: synthesis of LVP@LFP cathode utilizing the sol-gel method on LFP cathode material and LVP cathode lamination on LFP cathode lamination. In this work, the sol was made using the sol-gel method, and the gel structure was produced by mixing for 4 hours at 80 oC with a rate of 300 rpm. The gel was left to dry at 80 oC for 15 hours, and heat treated at 700 oC in a N2 atmosphere for 4 hours. After the heat treatment, when the tube furnace was cooled to 300 oC and 200 oC, nitrogen feeding was stopped, and LVP-1, LVP-2 samples were obtained, respectively. Then, using the same method, the LVP material was nucleated on the LFP surface and coated on the surface. For the double layer lamination process, battery slurry was prepared separately for LFP and LVP powders, and first LFP lamination was performed, followed by LVP lamination. The synthesized active materials were analyzed using XRD and Rietveld methods. According to the Rietveld analysis, the LVP-1 cathode material consists of 75% monoclinic Li3V2(PO4)3, 13% LiVOPO4, 6% Li3PO4, and 6% V2O5 phases. The LVP-2 cathode powder is composed of 90.3% Li3V2(PO4)3, 4.6 Li3PO4, and 5.2% V2O3 phases. The increased contact with oxygen at lower temperatures reduced the decomposition of the Li3V2(PO4)3 phase, resulting in fewer impurity phases. The LVP@LFP cathode contains 83.8% LFP and 16.2% LVP, as determined by Rietveld analysis. The double-laminated sample's coating thickness was examined using optical microscopy, revealing a 200-micron LFP layer and a 50-micron LVP layer. All samples exhibit a heterogeneous porous structure without a specific morphology. The electrochemical performance of the cathode materials was investigated over different voltage ranges. In the 3.0-4.6 V range, the initial discharge capacities for LVP-1, LVP-2, LVP@LFP, and LVP@LFP double coating were 99.67 mAh/g, 152.76 mAh/g, 163.8 mAh/g, and 149.46 mAh/g, respectively. In the 3.0-4.2 V range, the capacities were 76.45 mAh/g, 96.36 mAh/g, 153.97 mAh/g, and 144.44 mAh/g, respectively. Higher voltage values resulted in increased capacity, but with poorer capacity retention.