On the Dynamics of Battery Materials Revealed by Operando XRD and XAS

Affordable energy is a crucial factor for sustainable economic growth. The electrochemical energy storage plays a major societal role due to its widespread technological applications. Host nanostructured materials have a crystal structure with insertion sites, channels and/or interlayer spacings allowing the rapid insertion and extraction of lithium ions with generally little lattice strain, making them useful as electrode materials for batteries. Dynamic processes occurring in batteries can be studied by operando modality, which provide a realistic representation of the electrochemical reactions occurring at the electrodes, allowing to check the structural and electronic reversibility of a battery system while at least one full cycle is performed. For all these reasons, ex situ studies, which reflect a given state of charge (SOC) of electrode materials are complemented by operando measurements using complementary techniques such as X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). X-ray absorption spectroscopy is a synchrotron radiation based technique that provide both electronic and structural information on a selected atom. Operando synchrotron radiation x-ray powder diffraction (SR-XRPD) experiments allow monitoring the periodic structure of a material during the intercalation/release process of ions. The potentiality of the joint XAS-XRD approach in the newly proposed Prussian blue analogues (PBA) cathodes materials for rechargeable batteries is here highlighted, giving emphases on the copper hexacyanoferrate, copper nitroprusside, and manganese hexacyanoferrate. As an example, manganese hexacyanoferrate (MnHCF) is made of earth-abundant elements by a safe and easy synthesis and features high specific capacities at a higher potential than other PBAs. Na-rich MnHCF was tested in both Li- and Na-ion organic electrolytes in a post-Li strategy perspective and investigated using XAS and XRPD. Both Fe and Mn sites are involved in the electrochemical process, and the high delivered capacity (>130 mAh g-1) results from a reversible evolution in the metallic centers’ oxidation states (Fe3+/Fe2+ and Mn2+/Mn3+). Along with the Mn2+/Mn3+ oxidation, the Mn local environment experiences a substantial yet reversible Jahn–Teller (JT) effect, detected by the spectroscopic technique: the EXAFS shows a substantial and reversible basal contraction (10%) in the charged states due to the equatorial Mn-N bonds’ shortening (2.18 Å → 1.96 Å). Furthermore, operando XRD highlights a non-cooperative JT (NCJT) distortion. The lattice volume expands only by 2% upon electrochemical alkali-ion removal and insertion, which contrasts with EXAFS-retrieved Mn−N distances. This apparent disagreement underlines the capability of the PBA open framework to mitigate and disperse the pronounced variation of the JT-active Mn3+, granting overall stability to the structure. XANES. The dynamic of the ions insertion/release electrochemical reaction was further investigated by analysing the full XAS dataset by a multivariate curve resolution with alternating least squares algorithm (MCR-ALS), with the intent to assess the number of species involved and their evolutions during the electrochemical process.