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Manganese hexacyanoferrate is a promising cathode material for lithium and sodium ion batteries, however, it suffers of capacity fading during the cycling process. To access the structural and functional characteristics at the nanometer scale, fresh and cycled electrodes are extracted and investigated by transmission soft X-ray microscopy, which allows chemical characterization with spatial resolution from position-dependent x-ray spectra at the Mn L-, Fe L- and N K-edges. Furthermore, soft X-rays prove to show superior sensitivity toward Fe, compare to hard X-rays. Inhomogeneities within the samples are identified, increasing in the aged electrodes, more dramatically in the Li-ion system, which explains the poorer cycle life as Li-ion cathode material. Local spectra, revealing different oxidation states over the sample with strong correlation between the Fe L-edge, Mn L-edge, and N K-edge, imply a coupling between redox centers and an electron delocalization over the host framework.

Rechargeable aqueous Zn-ion batteries (ZIBs) have obtained extensive attention owing to their high safety, low-cost, environmental friendliness and high energy density. Nevertheless, developing suitable cathode materials remains challenging due to requirements for appropriate microstructure. We present a porous N-doped carbon nanosheet/MnO2 (NCS/MnO2) derived from bacterial cellulose (BC) by a simple route. BC chunks were soaked in urea solution and then carbonized under Ar flow at 900 °C. N-doped carbon nanosheets were obtained and MnO2 was added by reaction with NaMnO4. Benefiting from both the conductivity and porosity of the NCS support, the NCS/MnO2 composite delivers a high capacity and long cycling stability (114 mA h g−1 at 2 A g−1 after 1800 cycles). The electrode reaction mechanism was further investigated and the MnO2 dissolution/deposition mechanism was confirmed, with a critical role of zinc sulfate hydroxide (ZSH) to assist the deposition of MnO2

In operando experiments aim to perform in situ measurements of an electrochemical cell on a chip using transmission electron microscopy (TEM) during operation, which is a crucial requirement for understanding the fundamental physical and electrochemical phenomena taking place in a battery. These findings can enhance the understanding of electrochemical cells' behaviour and pave the way for designing more efficient and effective electrochemical energy-storage devices. Hence, the aim of this project is manufacturing a thin-film cell suitable for in operando TEM measurements by testing different chip designs and liquid electrolytes. This kind of cells require electrodes thinner than a few hundred nanometers, but typical commercial active material powders mainly consist of micrometric particles. So, this dissertation is focused on designing a process to prepare suitable cells from commercial materials. To accomplish this, particle size selection of LiMn2O4 was performed using separation methods. Having the ability to both deintercalate and intercalate, LiMn2O4 is used as active material for all electrodes in the thin-film cell. Therefore, nanoparticles are selected by centrifugation suspended in ethanol. Dynamic Light Scattering is employed to characterize their size and concentration to study the effects of the solvent, the centrifugation speed, and the use of filters. Then, adequate particles are drop casted on the chip with Polyvinylidene Fluoride (PVDF) in N-Methyl-2-pyrrolidone (NMP) to adhere them onto the surface. This particle distribution on the chip can be observed in Scanning Electron Microscopy (SEM). It was possible to demonstrate with planar microcells on chips electrochemical features that reproduce those typically shown by macroscopic cells. For example, 5 M LiNO3 had a good CV performance with 70 nA for the oxidation peak and -55 nA for the reduction one when selected depositing LiMn2O4 particles with 500 rpm centrifugation. The cell collectors represen

This thesis is developed at the Nanostructured Interfaces for Electrochemical Energy Storage research line of the Institute of Materials Science of Barcelona (ICMAB, CSIC) and focuses on experimental investigation of activation carbon felts (CF) as vanadium redox flow battery (VRFB) electrodes by electrochemical treatments. Because of the environmental impact of the present days, the Redox Flow Battery has emerged as an energy storage system. In this work, a series of electrochemical treatments were carried out using either sulphuric or phosphoric acids to activate the carbon felts that act as electrodes in these batteries. Following treatments, these were tested using a VRFB, and characterised by SEM and XPS. The felts treated in sulphuric acid slightly improve the thermally activated felt benchmark when placed in the positive electrode. Conversely, treatments in phosphoric acid demonstrated capable of improving results provided by the benchmark at both the negative and positive electrode. Overall, the work shows that electrodes activated by electrochemical treatments with low environmental impact and proper performance can be produced.

Synchrotron radiation based techniques are powerful tools for battery research and allow probing a wide range of length scales, with different depth sensitivities and spatial/temporal resolutions. Operando experiments enable characterization during functioning of the cell and are thus a precious tool to elucidate the reaction mechanisms taking place. In this perspective, the current state of the art for the most relevant techniques (scattering, spectroscopy, and imaging) is discussed together with the bottlenecks to address, either specific for application in the battery field or more generic. The former includes the improvement of cell designs, multi-modal characterization and development of protocols for automated or at least semi-automated data analysis to quickly process the huge amount of data resulting from operando experiments. Given the recent evolution in these areas, accelerated progress is expected in the years to come, which should in turn foster battery performance improvements.

A single-step electrochemical liquid-liquid-solid (ec-LLS) deposition route is developed for growing nanowire-based Ge-Sn nanostructures with tailored morphologies and compositions. Composite films, submicron fibers, nanowires, and also three-dimensional (3D) hierarchical structures are fabricated in the same electrolyte, simply by varying the electrodeposition current density. COMSOL simulations indicate that the current density is able to generate sufficient Joule heating to activate the ec-LLS growth mode. A concomitant reduction of Ge and Sn ions during Sn-doped Ge nanowire growth results in the formation of multicomponent heterostructures comprising non-equilibrium GexSn1-x, pure Sn, and Sn-rich amorphous phases. The proposed deposition technique enables the fabrication of high mass loading electrodes with enhanced Li storage properties. In particular, a 3D hierarchical structure composed of 38 wt % Ge delivers a specific capacity of 938 mA h g-1 after 400 cycles, corresponding to a high areal capacity of 4.95 mA h cm-2. We believe that the present study could be considered as a low-cost procedure for the industrial fabrication of anode materials for high-performance Li-ion battery applications.

Developing low-cost and efficient electrodes for hydrogen evolution reaction (HER) under neutral electrolytes remains an unattained milestone. We report a highly performing binder-free electrode through electroless deposition of Ni–P nanoparticles on bacterial cellulose (BC). Not needing carbonization to provide the electric conductivity, BC can maintain its excellent mechanical properties and thin fiber microstructure. The nanometric cellulose fibers facilitate the formation of small Ni–P nanoparticles, leading to more catalytic active sites. The obtained Ni–P/BC electrode presents remarkable HER activity with an overpotential of only 161 mV at 10 mA cm−2 and a low Tafel slope (141 mV dec−1) in 1 M potassium phosphate-buffered saline (pH = 7) electrolyte. Besides, Ni–P/BC also exhibits good stability for 24 h at 10 mA cm−2. This binder-free, low-cost, and easily fabricated electrode holds excellent promise for HER applications in benign neutral environments.

A simple route has been developed for the preparation of mesoporous graphitic carbons with embedded cobalt nanoparticles just using glycine as a nitrogen source, cobalt nitrate and distilled water. After heating the mixture to 300 °C under magnetic stirring, a dry solid product was obtained, which was then carbonized at 900 ºC under argon atmosphere. Changing the glycine/Co molar ratio allowed controlling the size of the cobalt particles and their dispersion in the carbon matrix, the porosity of the carbon and its graphitic character. The carbon–metal composites obtained were tested as oxygen cathodes in Li–O batteries. Cells assembled exhibited a full discharge capacity up to 2.19 mAh cm at a current of 0.05 mA cm and over 39 cycles at a cutoff capacity of 0.5 mAh cm. This work provides a green, feasible and simple way to prepare mesoporous graphitic carbons with embedded cobalt nanoparticles without involving templates. Graphical abstract: [Figure not available: see fulltext.].

The reversible and irreversible cationic and anionic charge compensation mechanisms occurring along the first charge and discharge cycle of Li- and Mn-rich Li[Li0.2Ni0.16Mn0.56Co0.08]O2 cathode material at (dis)charge rates of 0.1C and 5C have been identified and quantified by X-ray absorption and emission spectroscopy. The analysis provided the oxidation states of the transition metals, the Mn local coordination, and the lattice elastic constants. Lattice softening occurs along the first charge, while a minor spinel phase forms irreversibly at the expense of the layered phase. Higher charge rate increases the spinel formation and induces an increased softening in the delithiated lattice, which is expected to correspond to a reduced reversible anionic redox. The results evidence a comparable cationic and anionic oxidation since the beginning of the charge, while only anions are contributing towards its end, equivalent to roughly 10% of structural oxygen irreversibly lost. Higher charge rates resulted in a decreased reversible anionic redox, anticipating the cationic oxidation. The reported results provide a reliable experimental approach to characterize the key parameters controlling the reversible and irreversible cationic and anionic contributions to the charge compensation mechanism.

The use of slurries of conducting particles has been considered a way to extend the electrode area in some energy storage electrochemical cells. When suspensions of conducting particles are used in electrolytes a decreased impedance is observed, even for concentrations much lower than the theoretical percolation limits. Indeed, it is known that polarization occurs when a conducting material is immersed in an electrolyte in presence of electric fields, and bipolar electrochemistry processes may occur. This work demonstrates the dramatic drop in resistance for electrochemical cells with just a few macroscopic conducting pieces immersed in the electrolyte, in the absence of any electrical contact, through bipolar induction. Furthermore, mediation of soluble redox species between adjacent induced poles of opposite charge results in an additional mechanism for charge transfer, contributing further to the decrease in impedance. Relevant parameters like size, geometry, and spatial occupation of inducible pieces within the electric field, are relevant. Remarkably, the effects observed can explain some empirical observations previously reported for carbon suspensions and slurries. Thus, no electronic percolation requiring particle contact, nor ordering, are needed to explain the good performance associated to lowered impedance These results suggest new engineering designs for electrochemical cells with enhanced currents.

Porous carbons are important cathode materials for metal-air batteries, but the most usual methods to prepare these porous structures are complex and of high cost. We have prepared porous carbons from bacterial cellulose (BC) hydrogels by a simple water-alcohol solvent exchange before carbonization. Alcohol treatment facilitates looser and more open structures than untreated BC, resulting in porous carbon structures with high surface area, appropriate for electrochemical applications. Used as cathodes in lithium-oxygen batteries, the carbon derived from 1-butanol treated BC has excellent discharge capacity (5.6 mA h cm−2) and good cycle life. This work presents a sustainable, straightforward and fast way to prepare porous carbon materials from BC.

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.

Energy-dependent full field transmission soft X-ray microscopy (TXM) is able to give a full picture at the nanometer scale of the chemical state and spatial distribution of oxygen and other elements relevant for battery materials, providing pixel-by-pixel absorption spectrum. We show different methods to localize chemical inhomogeneities in Li1.2Mn0.56Ni0.16Co0.08O2 particles with and without VOx coating extracted from electrodes at different states of charge. Considering the 3d(Mn,Ni)-2p(O) hybridization, it has been possible to discriminate the chemical state of Mn and Ni in addition to the one of O. Different oxidation states correspond to specific features in the O-K spectra. To localize sample regions with specific compositions we apply two different methods. In the first, the pixel-by-pixel ratios of images collected at different key energies clearly highlight local inhomogeneities. In the second, introduced here for the first time, we directly correlate corresponding pixels of the two images on a xy scatter plot that we call phase map, where we can visualize the distributions as function of thickness as well as absorption artifacts. We can select groups of pixels, and then map regions with similar spectral features. Core-shell distributions of composition are clearly shown in these samples. The coating appears in part to frustrate some of the usual chemical evolution. In addition, we could directly observe several further aspects, such as: distribution of conducting carbon; inhomogeneous state of charge within the electrode; molecular oxygen profiles within a particle. The latter suggests a surface loss with respect to the bulk but an accumulation layer at intermediate depth that could be assigned to retained O2.

Electrodeposited iridium oxide (K1.7IrO0.8 (OH)2.2 × 1.8 H2O; also called IrOx) is among the best substrates for neural growth, decreasing impedance and stimulating cell growth, when used as a connected electrode. Without direct contact, it has been proven to stimulate neurons through a bipolar mechanism related to the conducting character of the material in the presence of remote electric fields. The remote wireless electrostimulation that arises from it is of large significance in clinical applications. Ionic intercalation simultaneous with iridium oxidation state changes at the induced IrOx cathode and the formation of a redox and ionic gradient at the IrOx substrate is envisaged as the most probable explanation for the observed effects on neural cell growth. This work shows the iridium state gradient using X-ray absorption spectroscopy (XAS) with significant electrochemical features and relaxation times that allow for a persistent effect in the material even after the electric field creating the induced dipole is switched off. It also shows correlated intercalated sodium gradients observed by semiquantitative energy-dispersive X-ray (EDX) analysis data. The bipolar effect is proven and yields new evidence for the behavior of other biocompatible neural growth substrates.

A template free, single-step process is developed for fabrication two-dimensional tin nano-platelets by electrochemical deposition in the presence of Triton X100 (TX100). Electrochemical studies combined with structural characterization revealed that during electrodeposition, TX100 molecules adsorb preferentially on {022} planes of Sn and highly anisotropic growth promotes in [200] direction which results in the formation of platelet morphology. The deposited platelets exhibit a high aspect ratio of 30 (width to thickness) and thickness of 25 ± 5 nm that uniformly covered the substrate with a high platelet density of 9 × 108 cm−2. The electrochemical performance of nano-platelets for lithium storage was studied in detail and compared with other morphologies of tin. Tin nano-platelets exhibited high reversible capacity and excellent cycling performance, the capacity was maintained at 820 mA h g−1 for 100 cycles and more, far superior to the other structures. Excellent rate capability was also observed for nano-platelets up to 5 C, with the ability to be operated at 20 C without damage. The superior electrochemical performance of tin platelets is mainly attributed to its two-dimensional structure that efficiently distributes strain, allowing high mechanical stability even after 100 cycles, as confirmed by Scanning Electron Microscopy (SEM).

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.

Li-O2 batteries can offer large discharge capacities, but this depends on the morphology of the discharged Li2O2, which in turn is strongly affected by the nanostructured carbon used as support in the air cathode. However, the relation with the textural parameters is complex. To investigate the combined effect of channels of different sizes, meso-macroporous carbons with similar mesopore volume but different pore size distribution were prepared from the polymerization of resorcinol-formaldehyde (RF) in the presence of surfactants and micro-CaCO3 particles. The carbon materials were used as active materials of air cathodes flooded by ionic liquid-based electrolytes in Li-O2 cells with two different configurations, one with a static electrolyte and the other with a stirred electrolyte, which favor a film-like and large particle deposition, respectively. The presence of large pores enhances the discharge capacity with both mechanisms. Conversely, with respect to the reversible capacity, the trend depends on the cell configuration, with macroporosity favoring better performance with static, but poorer with stirred electrolytes. However, all mesoporous carbons demonstrated larger reversible capacity than a purely macroporous electrode made of carbon black. These results indicate that in addition to pore volume, a proper arrangement of large and small pores is important for discharge capacity, while an extended interface can enhance reversibility in Li–O2 battery cathodes.