The Electrochemical Performance of The Blended Cathode of LMFP and NMC811

Blended cathodes are one of new approach for enhancing both characteristics of different cathode active materials. In previous study, a blended cathode between olivine structural cathode active materials and layered lock salt cathode active materials are applied to lithium ion cell to realize high thermal stability derived from olivine and large capacity derived from layered rock salt type. The blended cathode between lithium iron phosphate (LFP: LiFePO 4 ) and lithium nickel cobalt manganese oxide (NMC 1-x-y, x, y : LiNi 1-x-y Mn x Co y O 2 ) shows a high stability of crystal structure and large specific capacity. However, an operation voltage of LFP and NMC is 3.3 V and 3.8 V, respectively. The difference in their operation voltage leads to a poor performance of cathode. Recently, lithium manganese iron phosphate (LMFP: LiMn 1-x Fe x PO 4 /C) has been expected as a excellent cathode active material due to higher operation voltage and higher energy density than those of LFP. LMFP is prepared by a replacing of Fe by Mn in LFP. The Mn 2+ /Mn 3+ redox can be used, leading to higher operating average voltage of 3.8V with high stability of ordinary olivine structure features (high stability, capacity:170 mA h g -1 , long cycle performance). Therefore, the blended cathode between LMFP and NMC has been expected as a new cathode with high performances. The cathode performance depends on the blending ratio between LMFP and NMC. The effect of blending ratio on cathode performance have been reported in previous study. Various NMCs has been widely used as a cathode active material. Recently, a lithium ion battery using high nickel NMC811 cathode have been actively studied, due to its high specific capacity. However, NMC811 cathode has a low thermal stability due to weak chemical bond between Ni and O, elution of Ni and Mn in an electrolyte and a crack formation due to volume expansion and shrinkage during charge and discharge cycles. These problems have to be solved to realize lithium ion battery with NMC811 cathode. In previous study, Both LMFP and NMC cathodes have been made by solid-state process. The blended cathode using cathode active materials made by solid-state reactions shows a concern in a sense of a cathode layer density. In this study, physical properties and electrochemical performance of the blended cathode between LMFP made by hydrothermal-synthesis and NMC811 made by solid-state reaction were investigated by using laminated cells at room and high temperatures. Slurry of the blended cathode was prepared by mixing of PVdF, conductive additive, and two cathode active materials into NMP. The making slurry was coated on carbon coated Al foil and dried at 80 °C for 3hr under vacuum. The fabricated electrodes were evaluated by half-cells and full-cells. The sample name was added based on the blending ratio of LMFP. The morphology of LMFP particles in the blended cathodes was changed by a press process. The specific capacity of the blended cathode of LMFP 20 wt.% was 182 mA h g -1 which was a larger specific capacity than 180 mA h g -1 of NMC811 cathode. The blended cathode of LMFP 20 wt.% exhibited the lowest porosity among various blended cathodes, suggesting that LMFP may play a role in the formation of conductive pathway. In addition, the blended cathode of LMFP 20wt.% exhibited a good cycle performance 80% discharge capacity retention (based on 0.2C)) at room temperature, which was better than that of NMC811 (65% discharge capacity retention). After disassembling the full-cell, the cross-section of the cathode was observed with SEM. Many cracks were observed in the NMC811 particles in the NMC811 cathode. On the other hand, no cracks were observed in the NMC811 particles of the blended cathode, indicating that LMFP could suppress the crack formation in NMC811 particles. This is one of reasons for the high cycle ability of the blended cathode. A cycle performance at 60 °C showed that the discharge capacity of NMC811 cathode decreased during 300 cycles, whereas that of the blended cathode of LMFP 20wt.% also decreased with much better discharge retention. From the cross-sectional SEM images, may cracks in NMC811 were observed in both pure NMC cathode and LMFP 20wt.% blended cathode. However, a less crack formation of NMC811 was confirmed in the blended cathode of LMFP 20wt.%. Based on these results, the effect of the blended cathode can be explained. Firstly, the improvement in specific capacity is due to the conductive pathway of the LMFP. Secondly, the improvement in cycle performance is due to the suppression of crack formation in the NMC811. Figure 1