Structural and Electrochemical Investigation of 3D Tunnel Type Li0.44MnO2 Cathode Material for Secondary Li-Ion Batteries

Abstract

The design of new cathode materials remains a key goal in the development of (post) Li-ion batteries. In that regard, densely packed structures like Li0.44MnO2 are judicious for study. The presence of Mn as a transition metal makes it more attractive due to its sustainable nature, low cost, elemental abundance, structural diversity/polymorphism, and rich oxidation states (2+ to 7+) offering tunable redox potential [1]. Here, we have investigated Mn-based tunnel-type Li0.44MnO2 oxide for secondary LIBs. i) Li44MnO2 (LMO) was prepared by a soft chemical method using the corresponding sodium manganese oxide (Na0.44MnO2) as a parent compound [2]. The crystal structure of Li0.44MnO2 maintains the parent Na0.44MnO2-type tunnel structure which is analogous to Na4Mn4Ti5O18 having an orthorhombic (s.g. Pbam) crystal system [3]. To understand redox chemistry, this material was studied for both cationic (Mn) and anionic (O) redox reactions using ex-situ X-ray photoelectron spectroscopy. The lithium (de)insertion mechanism of Li0.44MnO2 was explored involving ab initio calculations and in-situ X-ray diffraction experiments. Combined with electrochemical measurements, structural characterizations, and first principles DFT calculations revealed the capacity correlated strongly with both cationic and anionic redox reactions. ii) We have studied the phase transition of Li44MnO2 from a 3D tunnel-type structure to a cubic spinel structure by in-situ X-ray diffraction and Raman spectroscopy. The mechanism of transformation was elucidated by employing transmission electron microscopy. In this case, diffusion-less transformation occurs involving the rearrangement of atoms during heating [4]. An increase in the average redox potential from the pristine orthorhombic phase to the final spinel phase was observed by electrochemical studies. iii) Further to improve the electrochemical performance of Li0.44MnO2, Ti was used as a dopant. The Li0.44[Mn1-xTix]O2 (x=0, 0.11, 0.22, 0.33, 0.44, 0.56) series were obtained with chemical ion exchange from Na precursors. The Na precursors (Na0.44[Mn1-xTix]O2) were prepared via a simple solid-state route. Ti-substituted compositions exhibited similar crystal structures to their parent LMO i.e. orthorhombic framework with Pbam symmetry. A tunnel-type morphology was observed in all cases. The electrochemical performance of Ti-substituted LMO as cathode material for LIBs was studied in a half-cell configuration. The structure and electrochemical properties of Ti-doped LMO, reaching the highest capacity of 128 mAh/g, will be described. Keywords: batteries; cathode; tunnel type manganese oxides; phase transformation; doping