Abstract
Lithium ion batteries for electric vehicle (EV) applications require high energy density and high safety. For this reason, layered oxides such as LiNiCoMnO2 (NCM) and LiNiCoAlO2 (NCA) are selected as positive electrode materials because of their large charge / discharge capacity. In recent years, capacity and energy density have been increasing due to an increase in the Ni content. However, an increase in the Ni content tends to cause heat generation and oxygen release from the crystal structure during charge / discharge, which leads to a decrease in battery safety such as the battery being easily ignited.
On the other hand, olivine-type cathode materials such as LiFePO4 (LFP) and LiFeMnPO4 (LFMP) are widely known as highly safe cathode materials that generate less heat and do not cause oxygen release from the crystal structure during charge / discharge. However, these olivine-type materials have smaller charge / discharge capacities and powder densities than layered oxides, and thus have low energy densities and are therefore difficult to apply to batteries for electric vehicles.
In this study, we investigated the effects of mixing layered oxides and olivine-type materials to compensate for the advantages and disadvantages of layered oxides and olivine-type materials, and to achieve cathode materials with both high energy density and high safety.
As the cathode electrode material, NCM532 was used as a layered oxide, and LFP or LFMP produced by a hydrothermal synthesis method were used as an olivine type material. These materials were mixed at a predetermined weight ratio when preparing the electrode paste. The prepared paste was applied to an Al foil and dried to obtain a mixed electrode of NCM / olivine type material.
The mixed state of NCM / olivine type material in the electrode was observed by SEM. As a result, a high dispersion state was obtained regardless of the mixing ratio of both materials.
For charge / discharge tests, 50 mAh Laminate cells were prepared using the NCM / olivine type material electrode for cathode and natural graphite for anode. As a result, NCM / LFP mixed electrode showed a slight decrease in energy density due to an increase in the capacity of the 3.1 V region derived from the change in Fe valence as the amount of LFP mixed increased.
On the other hand, in the NCM / LFMP mixed electrode, the capacity in the 3.9 V region derived from the change in the Mn valence in LFMP increased as the amount of LFMP mixed increased. Since the Fe content of LFMP was small, the capacity in the 3.1 V region was hardly observed due to the increase in the amount of LFMP.
For a safety test, a 2 Ah laminate cells were prepared and an overcharge test was performed. Overcharging was performed up to a voltage of 30 V by applying a current of 20 A. As a result of evaluating the cell using the NCM single electrode, the surface temperature rose to 195.2 oC and the cell was flamed.
On the other hand, in the cell using the NCM / LFP electrode, the surface temperature during overcharging was about 50 oC, and although the cell swelled, no leakage or flame occurred.
In the case of the cell using the NCM / LFMP electrode, the cell surface temperature increased to 155.4 oC and leakage occured at a weight ratio of 80/20. At the weight ratio of 70/30 or less, the cell surface temperature decreased to around 50 oC, and the cells only swelled and did not flame.
In conclusion, we were able to obtain a positive electrode material that has both high safety and high energy density by mixing NCM with an olivine type positive electrode material. In the future, we will consider mixing olivine-type cathode materials with higher energy density oxides such as NCA.
Figure 1
