Negative thermal expansion and oxygen-redox electrochemistry

Structural disorder within materials gives rise to fascinating phenomena, attributed to the intricate interplay of their thermodynamic and electrochemical properties^1,2. Oxygen-redox (OR) electrochemistry offers a breakthrough in capacity limits, while inducing structural disorder with reduced electrochemical reversibility^3-5. The conventional explanation for the thermal expansion of solids relies on the Grüneisen relationship, linking the expansion coefficient to the anharmonicity of the crystal lattice⁶. However, this paradigm may not be applicable to OR materials due to the unexplored dynamic disorder-order transition in such systems^7,8. Here we reveal the presence of negative thermal expansion with a large coefficient value of -14.4(2) × 10^-6 °C^-1 in OR active materials, attributing this to thermally driven disorder-order transitions. The modulation of OR behaviour not only enables precise control over the thermal expansion coefficient of materials, but also establishes a pragmatic framework for the design of functional materials with zero thermal expansion. Furthermore, we demonstrate that the reinstatement of structural disorder within the material can also be accomplished through the electrochemical driving force. By adjusting the cut-off voltages, evaluation of the discharge voltage change indicates a potential for nearly 100% structure recovery. This finding offers a pathway for restoring OR active materials to their pristine state through operando electrochemical processes, presenting a new mitigation strategy to address the persistent challenge of voltage decay.