Evidence of solid-solution reaction upon lithium insertion into cryptomelane K_0.25Mn₂O₄ material

Cryptomelane-type K_0.25Mn₂O₄ material is prepared via a template-free, one-step hydrothermal method. Cryptomelane K _0.25Mn₂O₄ adopts an I4/m tetragonal structure with a distinct tunnel feature built from MnO₆ units. Its structural stability arises from the inherent stability of the MnO₆ framework which hosts potassium ions, which in turn permits faster ionic diffusion, making the material attractive for application as a cathode in lithium-ion batteries. Despite this potential use, the phase transitions and structural evolution of cryptomelane during lithiation and delithiation remain unclear. The coexistence of Mn³⁺ and Mn⁴⁺ in the compound during lithiation and delithiation processes induces different levels of Jahn-Teller distortion, further complicating the lattice evolution. In this work, the lattice evolution of the cryptomelane K_0.25Mn₂O₄ during its function as a cathode within a lithium-ion battery is measured in a customized coin cell using in situ synchrotron X-ray diffraction. We find that the lithiation-delithiation of cryptomelane cathode proceeds through a solid-solution reaction, associated with variations of the a and c lattice parameters and a reversible strain effect induced by Jahn-Teller distortion of Mn³⁺. The lattice parameter changes and the strain are quantified in this work, with the results demonstrating that cryptomelane is a relatively good candidate cathode material for lithium-ion battery use. © 2014 American Chemical Society.