Evidence of Solid-Solution Reaction upon Lithium Insertion into Cryptomelane K0.25Mn₂O₄ Material

Cryptomelane-type K 0.25 Mn 2 O 4 material is prepared via a template-free, one-step hydrothermal method. Cryptomelane K 0.25 Mn 2 O 4 adopts an I 4/ m tetragonal structure with a distinct tunnel feature built from MnO 6 units. Its structural stability arises from the inherent stability of the MnO 6 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 Mn 3+ and Mn 4+ 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 0.25 Mn 2 O 4 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 Mn 3+ . 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.