Structural effects of anomalous current densities on manganese hexacyanoferrate for Li-ion batteries

A battery management system (BMS) plays a pivotal role in providing optimal performance of lithium-ion batteries (LIBs). However, the eventual malfunction of the BMS may lead to safety hazards or reduce the remaining useful life of LIBs. Manganese hexacyanoferrate (MnHCF) was employed as the positive electrode material in a Li-ion half-cell and subjected to five cycles at high current densities (10 A gMnHCF−1) and to discharge at 0.1 A gMnHCF−1, instead of classical charge/discharge cycling with initial positive polarization at 0.01 A gMnHCF−1, to simulate a current sensor malfunctioning and to evaluate the electrochemical and structural effects on MnHCF. The operando set of spectra at the Mn and Fe K-edges was further analyzed through multivariate curve resolution analysis with an alternating least squares algorithm (MCR–ALS) and extended X-ray absorption fine structure (EXAFS) spectroscopy to investigate the structural modifications arising during cycling after the applied electrochemical protocol. The coulombic efficiency in the first cycle was dramatically affected; however, the local structural environment around each photo absorber recovered during charging. The identification of an additional spectral contribution in the electrochemical process was achieved through MCR-ALS analysis, and the Mn-local asymmetry was thoroughly explored via EXAFS analysis

Synergistic Effect of Co and Mn Co-Doping on SnO2 Lithium-Ion Anodes

The incorporation of transition metals (TMs) such as Co, Fe, and Mn into SnO2 substantially improves the reversibility of the conversion and the alloying reaction when used as a negative electrode active material in lithium-ion batteries. Moreover, it was shown that the specific benefits of different TM dopants can be combined when introducing more than one dopant into the SnO2 lattice. Herein, a careful characterization of Co and Mn co-doped SnO2 via transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy and X-ray diffraction including Rietveld refinement is reported. Based on this in-depth investigation of the crystal structure and the distribution of the two TM dopants within the lattice, an ex situ X-ray photoelectron spectroscopy and ex situ X-ray absorption spectroscopy were performed to better understand the de-/lithiation mechanism and the synergistic impact of the Co and Mn co-doping. The results specifically suggest that the antithetical redox behaviour of the two dopants might play a decisive role for the enhanced reversibility of the de-/lithiation reaction

Multi-edge and Multiple Scattering EXAFS Analysis of Metal Hexacyanoferrates: Application in Battery Materials

The occurrence of multiple scattering (MS) phenomena in the extended portion of the X-ray absorption spectrum permits to disclose details of the local atomic structure of the target atom. The high order MS paths become relevant only in peculiar structures, regardless the crystallinity of the investigated sample, such as in metal hexacyanoferrates. General guidelines and prescriptions are presented in order to give the authors a useful guide for a correct interpretation of the EXAFS spectra of this class of compounds, which is characterized by close, but sufficiently separated, discontinuities of the absorption coefficient due to the probed contiguous transition metal K-edges. The large number of experimental points, the occurrence of large four-body MS terms, and the presence of multiple probes are discussed in the frame of the renewed attention of these compounds for their use in the battery community

Effect of Water and Alkali-Ion Content on the Structure of Manganese(II) Hexacyanoferrate(II) by a Joint Operando X-ray Absorption Spectroscopy and Chemometric Approach

Manganese hexacyanoferrate (MnHCF) is made of earth-abundant elements by a safe and easy synthesis. The material features a higher specific capacity at a higher potential than other Prussian blue analogs. However, the effect of hydration is critical to determine the electrochemical performance as both the electrochemical behavior and the reaction dynamics are affected by interstitial/structural water and adsorbed water. In this study, the electrochemical activity of MnHCF is investigated by varying the interstitial ion content through a joint operando X-ray absorption spectroscopy and chemometric approach, with the intent to assess the structural and electronic modifications that occur during Na release and Li insertion, as well as the overall dynamic evolution of the system. In MnHCF, both the Fe and Mn centers are electrochemically active and undergo reversible oxidation during the interstitial ion extraction (Fe2+/Fe3+ and Mn2+/Mn3+). The adsorption of water results in irreversible capacity during charge but only on the Fe site, which is suggested by our chemometric analysis. The local environment of Mn experiences a substantial yet reversible Jahn\u2013Teller effect upon interstitial ion removal because of the formation of trivalent Mn, which is associated with a decrease of the equatorial Mn 12N bond lengths by 10 %

Beyond the oxygen redox strategy in designing cathode material for batteries: Dynamics of a prussian blue-like cathode revealed by operando X-ray diffraction and X-ray absorption fine structure and by a theoretical approach

The dynamics of the lithiation/delithiation process and the nitrosyl electroactivity in copper nitroprusside were studied by operando X-ray diffraction and operando X-ray absorption fine structure (XAFS). Data were interpreted based on a joint study performed by means of density functional theory calculations. This approach allows the relevant structural and electronic information to be retrieved from the measured scattering and absorption data and therefore the lithiation mechanism in copper nitroprusside to be untangled, which occurs with the reduction of both metals generating a lattice basal plane contraction and an axial elongation. An increase in the Debye-Waller factors for Cu-N bonds and a decreasing trend for the Cu-NC-Fe linear chains along with lithium insertion reveal a general increase in the Cu local disorder, which is thought to be the main cause of the rapid capacity fading observed during cycling. The ligand electroactivity of the nitrogen atom, detected by following vibrational frequencies, delivers an extra capacity and represents an alternative path to cationic and oxygen redox

Soft X-Ray Microscopy of Manganese Hexacyanoferrate Cathode Materials for Lithium and Sodium Batteries" (ise202741)

A very interesting, durable and cheap class of active intercalation cathode materials is represented by open-framework structures such as Prussian blue analogues (PBAs). Manganese hexacyanoferrate (MnHCF) with nominal formula Na2Mn[Fe(CN)6]¿zH2O, only includes abundant elements and is safely and easily synthesized. The material features high specific capacities at high potentials when compared to other PBAs [1]. A combined XAS and XRD study has demonstrated that both Fe and Mn sites are involved in the electrochemical process but a capacity fading is observed during the charge and discharge steps, due to a Jahn-Teller (J-T) effect [2]. Using energy-dependent soft x-ray microscopy [3,4] in this study we reveal and visualize the spatial distribution of the oxidation state of Mn, Fe and N at different electrode composition and after 50 cycles in a Li and in a Na battery. The high-resolution access to the metal L edge provided complementary details on the metal-ligand characteristic to the one already available by hard x-ray absorption, while the images demonstrate particles at different state of charge, likely caused by J-T-related passivations, in particular in the cycled lithiated samples. This confirms the superior stability to sodiation and provides an important support for strategies aimed to the suppression of fading

Detailing the self-discharge of a cathode based on a prussian blue analogue

Prussian Blue analogues (PBAs) are a promising class of electrode active materials for batteries. Among them, copper nitroprusside, Cu[Fe(CN)5NO], has recently been investigated for its peculiar redox system, which also involves the nitrosyl ligand as a non-innocent ligand, in addition to the electroactivity of the metal sites, Cu and Fe. This paper studies the dynamics of the electrode, employing surface sensitive X-ray Photoelectron spectroscopy (XPS) and bulk sensitive X-ray absorption spectroscopy (XAS) techniques. XPS provided chemical information on the layers formed on electrode surfaces following the self-discharge process of the cathode material in the presence of the electrolyte. These layers consist mainly of electrolyte degradation products, such as LiF, LixPOyFz and LixPFy. Moreover, as evidenced by XAS and XPS, reduction at both metal sites takes place in the bulk and in the surface of the material, clearly evidencing that a self-discharge process is occurring. We observed faster processes and higher amounts of reduced species and decomposition products in the case of samples with a higher amount of coordination water

Local interactions governing the performances of Lithium- and Manganese-rich cathodes

The local structural and electronical transformations occurring along the first charge and discharge cycle of Li- and Mn-rich Li[Li0.2Ni0.16Mn0.56Co0.08]O2 cathode material have been characterized by X-ray absorption spectroscopy at several complementary edges. The irreversible spinel formation, occurring at the expenses of the cycling layered phase during the first charge, is quantified (about 10%) and spatially localized. The local strains induced by the Ni oxidation have been evaluated. They induce the formation of a low spin Mn3+ in the layered structure in parallel to the irreversible formation of the spinel phase in the particles bulk. The charge balance has been quantified for all the elements along the first charging cycle, confirming a reversible oxygen oxidation along the charge. Overall, these quantitative results provide an experimental basis for modeling aimed to control the structure and its evolution, for instance, hindering the spinel formation for the benefit of the material cycle life. © 2021 American Chemical Society

Role of Manganese in Lithium- and Manganese-Rich Layered Oxides Cathodes

Lithium-rich transition-metal-oxide cathodes are among the most promising materials for next generation lithium-ion-batteries because they operate at high voltages and deliver high capacities. However, their cycle-life remains limited, and individual roles of the transition-metals are still not fully understood. Using bulk-sensitive X-ray absorption and emission spectroscopy on Li[Li0.2Ni0.16Mn0.56Co0.08]O2, we inspect the behavior of Mn, generally considered inert upon the electrochemical process. During the first charge Mn appears to be redox-active showing a partial transformation from high-spin Mn4+ to Mn3+ in both high and low spin configurations, where the latter is expected to favor reversible cycling. The Mn redox-state with cycling continues changing in opposition to the expected charge compensation and is correlated with Ni oxidation/reduction, also spatially. The findings suggest that strain induced on the Mn-O sublattice by Ni oxidation triggers Mn reduction. These results unravel the Mn role in controlling the electrochemistry of Li-rich cathodes