Li-ion batteries are widely used for variety of applications ranging from small-scale portable electronics to large-scale grid storage. After conquering the consumer electronics sector, the current focus is on growing implementation of Li-ion batteries in battery electric vehicles (BEVs) and electric stationary storage (ESS). While the application base is exponentially growing, the cost and supply for constituent metals (e.g. Li, Co, Ni) have raised a concern. While various monovalent (Na⁺, K⁺) and multivalent (Mg²⁺, Ca²⁺, Al³⁺) alkali-based systems are pursued as alternatives for Li-ion batteries, there is steady effort to replace toxic and expensive Co from battery cathodes. In the quest to design Co free cathodes, various Ni-rich cathodes have achieved great success. In parallel, it is also worthwhile to explore Mn-based cathodes for Li-ion batteries, special for application not demanding high energy density. If realised, Mn-based cathodes can offer low-cost Li-ion batteries for real-life applications.

Mn-based oxides offer rich structural diversity (e.g. tunnel, spinel, one-to-three dimensional frameworks), ease of synthesis, economy and adequate electrochemical performance. The current work attempts to unveil the relationship of phase transition and electrochemical activity in Mn-based oxide cathodes A_0.44MnO₂ (A= Na, Li). Two stories will be presented.

(1) Phase pure Na₄₄MnO₂ compound was prepared by using facile solution combustion method taking low-cost nitrates and urea as precursors. This rod-shaped tunnel type sodium insertion material has a 3D tunnel structure with orthorhombic framework (s.g. Pbam). It was found to work as insertion host for Li, Na, and K-ions batteries. This versatile cathode works as a robust cathode for Li-ion batteries via facile electrochemical ion exchange exhibiting capacity over 141 mAh/g (at rate of C/20). Solid-solution redox mechanism was revealed by using PITT and ex situ X-ray diffraction study (Vanam et al, Inorg. Chem. 2022, 61, 3959-69).

(2) In the second part, phase-pure Li₄₄MnO₂ was prepared by chemical ion exchange of Na_0.44MnO₂ by lithium using a molten-salt method. This process allowed complete replacement of Na-ions by Li-ions keeping the original 3D tunnel-type structure intact. It delivered a reversible capacity of 140 mAh/g involving Mn-redox at (3V vs Li). The electrochemical cycling stability was improved by forming thin layer of Al₂O₃ to circumvent Mn dissolution into electrolyte. The underlying redox mechanism was investigated combining ex situ diffraction and DFT computation (Figure) (manuscript submitted).

Further, polymorphic phase transition was observed in Li_0.44MnO₂ cathode. Upon annealing at high temperature, Li_0.44MnO₂ was found to undergo irreversible phase transition from tunnel to spinel structure around 465 C. It was captured by in-situ high-temperature X-ray diffraction, in-situ Raman spectroscopy and ex situ transmission electron microscopy. Upon cooling, structural ordering was observed below 20 K, which was probed by low-temperature neutron powder diffraction. The effect of structure on electrochemical activity will be described.