Users of the PIRX beamline have obtained groundbreaking results showing this material's electron structure plays a fundamental role in the alkaline electrochemical intercalation process, determining the shape of the discharge curve (OCV) and impacts important performance parameters of Li-ion and Na-ion cells, such as energy density and power density. The discovery is universal in nature and has great significance for the design and search for new electrode materials for Li-ion and Na-ion cells.

Lithium cell technologies are currently the most dynamically developing area related to the storage and processing of electricity for the needs of portable electronics, electric cars and energy storage from renewable sources. The working mechanism of Li-ion cells is based on a reversible reaction of introducing a significant amount of lithium ions together with an equivalent amount of electrons into the structure of a transition metal compound MaXb (M - transition metal, X= O,S):
 xLi⁺ + xe^- + M_aX_b ↔ Li_xM_aX_b

Fig.1. The work scheme of the Li-ion cell. 

This process, called the intercalation process, takes place at room temperature without destroying the structure of the material, even for several thousand cycles of lithium intercalation/deintercalation. The Na-ion cells show the same working mechanism. Indispensable for the process effectiveness is high conductivity of lithium (sodium) ions and electrons in the base material. Layered transition metal oxides used as electrode materials in commercial cells (LiCoO2) are characterized by instability of the crystal structure at lower alkali content, which leads to the limitation of their practical capacity to 50% of the theoretical capacity. This problem also occurs in sodium electrode materials. The starting point for improving the structural stability of layered transition metal oxides was the concept of high-entropy configuration, which can be achieved by introducing several different transition metal cations randomly distributed in one position of the transition metal in the crystal structure of the oxide. The increase in configurational entropy in the proposed high-entropy oxide with the composition NaMn0.2Fe0.2Co0.2Ni0.2Ti0.2O2 contributes to lowering system energy, which led to an increase in its chemical stability, increase in the density of stored energy and improved safety of battery use. Comprehensive studies, both experimental and theoretical, have shown a strong correlation between the structural, transport and electrochemical properties of this oxide. The modification of the crystalline structure occurring in the course of sodium deintercalation leads to metallic conductivity and better kinetics of the NaxMn0.2Fe0.2Co0.2Ni0.2Ti0.2O2 electrode material, which has a high capacity of 180 mAh·g-1, much higher than the commercial LiCoO2 material, which can lead to the development of sodium battery technology, especially for large-scale energy storage.

Link to the full publication: 

Walczak K, Plewa A, Ghica C, et al. NaMn0.2Fe0.2Co0.2Ni0.2Ti0.2O2 high-entropy layered oxide - experimental and theoretical evidence of high electrochemical performance in sodium batteries. Energy Storage Mater. 2022;47:500-514. doi:10.1016/j.ensm.2022.02.038

Written by: prof. Janina Molenda