Preparation And Characterization Of P2-NaCo_0.5Mn_0.5O₂ Cathode Materials For Sodium-ion Batteries

With the widely application of rechargeable lithium-ion batteries（LIBs） in portable electronic devices, electric vehicles and other power system, the demand for lithium source is increasing. However, the limited resource and uneven distribution of lithium on earth strongly restrict the application of LIBs in future. In contrast, the natural resources of sodium is rich and easy to obtain. In particular, rechargeable sodium-ion batteries（SIBs）, share the same working principle as LIBs, which enables SIBs as the promising alternative of LIBs. In the research on the SIBs, searching for the new electrode materials with excellent electrochemical performance is highly desired. P2-type layered transition metal oxide, as the cathode material of SIBs, has attracted widespread attentions for its stable structure and excellent cycle and rate performance. Whereas, they suffers from the low initial coulumbic efficiency, low working voltage and so on. Hence, it is of great significance to explore new P2-type layered cathode materials. To solve the existing problems, we have launched a series of detailed studies about P2- type NaCo_0.5Mn_0.5O₂ cathode materials and obtained the following research results:Firstly, by employing the simple solid-state method, we successfully prepared the stoichiometry and pure P2 type NaCo_0.5Mn_0.5O₂ materials. The material showed the block-type morphology, with the particle sizes of 0.5- 4 μm. The optimum voltage range of this cathode was 1.5- 4.15 V, in which NaCo_0.5Mn_0.5O₂ samples not only show a nearly 100% initial coulumbic efficiency and a high reversible discharge capacity of 124.3m A h g-1, but also have the excellent cycle stability, rate performance and long cycle life. In our opinion, the beginning sodium ion content is more than that of traditional P2 layered materials and sodium could be released in the charging process and inserted into the structure in the discharging process, leading to higher reversible specific capacity. Ex-situ XRD results showed that the layered P2 structure of materials remained in the process of charge and discharge, providing favorable conditions for the insertion and exertion of sodium ion. In view of the excellent electrochemical performance, P2-NaCo_0.5Mn_0.5O₂ has a very good development prospect as cathode material for SIBs.Secondly, we studied the application of sodium-based material NaCo_0.5Mn_0.5O₂ as the cathode for LIBs, which constructed a new hybrid battery system. The test used the glass fiber filter as the separator, Li PF6 as electrolyte and lithium sheet as anode for the electrochemical measurements. The charge-discharge mechanism of hybrid battery was that sodium ion was released from NaCo_0.5Mn_0.5O₂ in the first charging process which leaded to phase transition. Then, lithium ion becomes the dominant carriers in the subsequent electrochemical cycles. In this chapter, the feasibility of P2-NaCo_0.5Mn_0.5O₂ material as cathode materials for lithium ion batteries were comprehensively studied in terms of the electrochemical performance.Finally, to further improve the electrochemical performance of NaCo_0.5Mn_0.5O₂, we tried to synthesize the spherical NaCo_0.5Mn_0.5O₂. We first prepared the spherical precursor Co1.5Mn1.5O4 by the solvent thermal method, and then spherical NaCo_0.5Mn_0.5O₂ was further prepared by solution impregnation method. Manganese nitrate and cobalt nitrate were optimized as the starting materials with the obtained precursor Co1.5Mn1.5O4, which exhibited the particle size of around 500 nm. The size of the prepared spherical NaCo_0.5Mn_0.5O₂ is between 2.5- 3 μm. Compared with NaCo_0.5Mn_0.5O₂ samples prepared by solid state method, spherical NaCo_0.5Mn_0.5O₂ own the high specific capacity of 139.3 m A h g-1 and excellent cycle performance, but the low initial coulumbic efficiency and poor rate performance. In the following works, we will work hard to overcome the shortcomings, hope to find out the effective methods to improve the electrochemical performance of spherical NaCo_0.5Mn_0.5O₂ materials.In this paper we prepared layered cathode material P2- NaCo_0.5Mn_0.5O₂ with different morphologies by two different methods. Then we analyze and discuss the structure, morphology and electrochemical performance of the materials in detail. Although there are still a lot of work and exploration to be done and improved, we have in-depth understanding about the application in SIBs and hybrid battery system for P2 type layered materials, which provides reference and thoughtful for the future research.