(Digital Presentation) Recent Development of the Cobalt Free and Lithium Rich Manganese Based Disordered Rocksalt Oxyfluorides As a Cathode Material for Lithium Ion Batteries

Abstract

Recently, new types of cation disordered rocksalt (DRS) have been reported which show good reversibility. In our study we combined the strategy of using high-valent cations with partial substitution of fluorine for oxygen anions in disordered rocksalt-structure phase to achieve optimal Mn2+/Mn4+ double-redox reaction in the composition system Li2MnxTi1-xO2F (1/3 ≤ x ≤ 1). we synthesized 4 different compositions (Li2MnIIIO2F, Li2MnII 1/3MnIII 1/3TiIV 1/3O2F, Li2MnII 1/2TiIV 1/2O2F and Li2MnII 1/3TiIII 1/3TiIV 1/3O2F). Two of them were synthesized for the first time, Li2MnII 1/3MnIII 1/3TiIV 1/3O2F and Li2Mn II 1/3TiIII 1/3TiIV 1/3O2F. By studying the electrochemical properties of different compounds we found that Ti+4 in the structure keeps Mn at the second state of charge, thus enabling a double redox reaction of Mn2+/Mn4+. By investigating the electrochemical properties of all samples we found that the sample with the composition Li2Mn2/3Ti1/3O2F showed the best electrochemical properties with initial high discharge capacity of 227 mAh g-1 in the voltage window of 1.5-4.3 V and 82% of capacity retentionafter 100 cycles. However, fluorination might lead to several issues such as synthesis limitation, lithium diffusion issues due to preferable strong Li-F bonds, etc. thus, two more different samples based on the Li2Mn2/3Ti1/3O2F composition were synthesized and their properties were investigated (Li1.5MnII 1/3MnIII 1/3TiIV 1/3O2F0.5 and Li1.25MnII 1/3MnIII 1/3TiIV 1/3O2F0.25) in order to find the proper amount of fluorine in the structure which promises the electrochemical behavior. In the following the effect of fluorine on lithium diffusion was investigated by ex-situ Raman studies. These studies shed light on the diffusion pathways of lithium ions during charge and discharge process. The structural characteristics are examined using X-ray diffraction patterns, Rietveld refinement, energy-dispersive X-ray spectroscopy and scanning electron microscopy, transmission electron microscopy and Raman spectroscopy. The oxidation states and charge transfer mechanism are also studied further using extended X-ray absorption fine structure and X-ray photoelectron spectroscopy in which the results approve the double redox mechanism of Mn2+/Mn4+ in agreement with Mn-Ti structural charge compensation. The findings pave the way for designing high capacity electrode materials with multi-electron redox reactions.