The energy density which can be stored in an intercalation battery is fundamentally limited by the electrochemical capacity of the cathode material. While current state-of-the-art systems rely on transition metal redox, moving past the conventional limits of transition metal capacity is a direct route towards dramatically increasing the achievable energy densities of Li-ion and Na-ion systems. Unfortunately, most reported materials operating beyond the transition metal limit are plagued by a substantial hysteresis between their charge and discharge voltages. While two model materials for non-hysteretic post-transition-metal redox are known, Na2Mn3O7 and Li2IrO3, the mechanism underlying their behavior remains controversial, with inconsistencies between proposed theory and experimental observation. We use high-accuracy RPA electronic structure calculations to reconcile all available experimental data for these materials and propose a new mechanism to explain their electrochemical behavior [1]. We show that the their unique reversibility arises from a delocalized, π-bonded network of metal-d and O-p orbitals that are typically assumed to be non-bonding, as well as an exceptional resistance to transition metal migration. The delocalized redox center, which we term π-redox, leads to a range of characteristic properties, such as a dependence of oxidation voltage on the global size of the π-bonded network, unusual magnetic couplings, and collective structural distortions. Our analysis leads to specific design criteria for realizing similarly reversible behavior in other systems and represents the first quantitatively-predictive theory of non-hysteretic, post-transition-metal electrochemistry.

Non-hysteretic electrochemistry beyond the transition-metal limit in Li- and Na-ion cathode materials enabled by delocalized π-redox

ACS Spring Meeting 2021

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