P2-NaxNi0.2Co0.1Mn0.7O2 as a Cathode Material for Sodium-ion Batteries

Abstract

Nowadays, we can enjoy the internet and call anywhere we want with electrical devices including mobile phone and laptop computer. These electrical devices needs power to supply their services. So, the importance of batteries as power sources for these is getting larger and larger. Up to now, lithium ion batteries(LIBs) have been in charge of power sources for these devices. Therefore, the lithium ion battery market enlarged for the last decades. At this moment in time, Sodium-ion batteries (NIBs) deserve greater attention for replacing the LIBs as energy storage system. The NIBs have the merit for cost. Sodium resources are distributed everywhere, and sodium is one of the most rich elements in the Earth’s crust. It leads to low cost for sodium sources. Additionally, sodium do not alloy with aluminum at low voltage in contrast to lithium. It means that aluminum current collector can replace the copper current collector. So, Sodium batteries have lower price than current lithium ion batteries. However, there are limitations for commercializing NIBs just now. On the cathode materials, the cathode materials for NIBs have relatively low operating voltage and low specific capacity. So, developing large specific capacity & high voltage cathode materials for NIB(sodium ion battery) is required for commercializing the Na-ion battery technology for large scale and low cost energy storage system. In this situation, layered NaxNCMO2, a sodium nickel cobalt manganese oxide, a ternary system, was adopted as a promising cathode material for NIBs. Because, regarding that the layered LiNixCoyMnzO2 cathode materials are already commercialized, it will be easier to apply layered sodium cathode materials than other kinds of materials when commercializing NIBs. But the NaxNCMO2 has lower specific capacity than that I expected. We try to get higher specific capacity by controlling temperature and sodium ratio. Therefore, to improve specific capacity for NaxNi0.2Co0.1Mn0.7O2 material, high crystalline NaxNi0.2Co0.1Mn0.7O2 material is synthesized at different temperature condition and with different sodium ratio. Firstly, a NaxNCMO2 material is used as a cathode material for NIBs. The compound is heated at 800℃ for 10h, but it has low crystallinity and low specific capacity. We changed the synthesis temperature at 600℃ for 6h, 25℃ and 900℃ 10h. Crystallinity is improved and specific capacity also increase. Additionally, good cycle retention is achieved at the same time. Hence, optimizing temperature for higher crystallinity, larger specific capacity and good cycle retention can be achieved. Secondly, sodium ratio is used as a factor that can affect a specific capacity. When ICP(inductively coupled plasma) analysis is conducted, sodium ratio to transition metals is just about 0.55. So, 0.8 and 0.9 sodium ratios to transition metals is used to optimize. However, when 0.8 and 0.9 sodium ratio is added, impurity XRD(X-ray Diffraction) peaks are detected. Also, a compound of 0.9 sodium ratio shows lower specific capacity than other compounds. Therefore, sodium ratio V.S. transition metals above 0.7 can affect a decline of specific capacity. When optimized NaxNi0.2Co0.1Mn0.7O2 was ball milled, this material showed amorphous and crystalline characteristics at the same time in the XRD pattern. And, SEM image showed crushed particle. For electrochemical test, amorphous NaxNi0.2Co0.1Mn0.7O2 showed larger specific reversible capacity than crystalline NaxNi0.2Co0.1Mn0.7O2. It is not sure but we can explain that Na stored in grain boundary affected the additional reversible capacity by acting capacitor.