Search

The energy density of rechargeable batteries utilizing metals as anodes surpasses that of Li ion batteries, which employ carbon instead. Among possible metals, magnesium represents a potential alternative to the conventional choice, lithium, in terms of storage density, safety, stability, and cost. However, a major obstacle for metal-based batteries is the identification of electrolytes that show reversible deposition/dissolution of the metal anode and support reversible intercalation of ions into a cathode. Traditional Grignard-based Mg electrolytes are excellent with respect to the reversible deposition of Mg, but their limited anodic stability and compatibility with oxide cathodes hinder their applicability in Mg batteries with higher voltage. Non-Grignard electrolytes, which consist of ethereal solutions of magnesium(II) bis(trifluoromethanesulfonyl)imide (Mg(TFSI)2), remain fairly stable near the potential of Mg deposition. The slight reactivity of these electrolytes toward Mg metal can be remedied b...

Unlike the more established lithium-ion based energy storage chemistries, the complex intercalation chemistry of multivalent cations in a host lattice is not well understood, especially the relationship between the intercalating species solution chemistry and the prevalence and type of side reactions. Among multivalent metals, a promising model system can be based on nonaqueous Zn2+ ion chemistry. Several examples of these systems support the use of a Zn metal anode, and reversible intercalation cathodes have been reported. This study utilizes a combination of analytical tools to probe the chemistry of a nanostructured δ-MnO2 cathode in association with a nonaqueous acetonitrile–Zn(TFSI)2 electrolyte and a Zn metal anode. As many of the issues related to understanding a multivalent battery relate to the electrolyte–electrode interface, the high surface area of a nanostructured cathode provides a significant interface between the electrolyte and cathode host that maximizes the spectroscopic signal of any s...

Unlocking the full performance capabilities of battery materials will require a thorough understanding of the underlying electrochemical mechanisms at a variety of length scales. A broad arsenal of X-ray microscopy and mapping techniques is now available to probe these processes down to the nanoscale. The tunable nature of X-ray sources allows for the extraction of chemical states through spectromicroscopy. The addition of phase contrast imaging can retrieve the complex-valued refraction of the material, giving an even more nuanced chemical picture. Tomography and coherent Bragg diffraction imaging provide a reconstructed three-dimensional volume of the specimen, as well as internal strain information from the latter. Many recent insights into battery materials have been achieved through the creative use of these, and similar, methods. Experiments performed while the battery is being actively cycled reveal behavior that differs significantly from what is observed at equilibrium and metastable conditions. ...

Reversible intercalation reactions provide the basis for modern battery electrodes. Despite decades of exploration of electrode materials, the potential for materials in the nonoxide chemical space with regards to intercalation chemistry is vast and rather untested. Transition metal fluorides stand out as an obvious target. To this end, we report herein a new family of iron fluoride-based perovskite cathode materials AxK1–xFeF3 (A = Li, Na). By starting with KFeF3, approximately 75% of K+ ions were subsequently replaced by Li+ and Na+ through electrochemical means. X-ray diffraction and Fe X-ray absorption spectroscopy confirmed the existence of intercalation of alkali metal ions in the perovskite structure, which is associated with the Fe2+/3+ redox couple. A computational study by density functional theory showed agreement with the structural and electrochemical data obtained experimentally, which suggested the possibility of fluoride-based materials as potential intercalation electrodes. This study inc...

International audience; Anionic redox chemistry has emerged as a new paradigm to design higher-energy lithium ion-battery cathode materials such as Li-rich layered oxides. However, they suffer from voltage fade, large hysteresis and sluggish kinetics, which originate intriguingly from the anionic redox activity itself. To fundamentally understand these issues, we decided to act on the ligand by designing new Li-rich layered sulfides Li1.33 – 2y/3Ti0.67 – y/3FeyS2, among which the y = 0.3 member shows sustained reversible capacities of ~245 mAh g−1 due to cumulated cationic (Fe2+/3+) and anionic (S2−/Sn−, n < 2) redox processes. Moreover, its negligible initial cycle irreversibility, mitigated voltage fade upon long cycling, low voltage hysteresis and fast kinetics compare positively with its Li-rich oxide analogues. Moving from the oxygen ligand to the sulfur ligand thus partially alleviates the practical bottlenecks affecting anionic redox, although it penalizes the redox potential and energy density. Overall, these sulfides provide chemical clues to improve the holistic performance of anionic redox electrodes, which may guide us to ultimately exploit the energy benefits of oxygen redox.

International audience; The limits of intercalation electrochemistry continue to be tested in the quest for ever increasing gains in the storage capability of Li-ion cathodes. The subsequent push for multi-electron reactivity has led to the recognition of the extremely versatile role of oxide ligands in charge compensation when there is a large redox swing. Li 3 IrO 4 is a unique model of such activity because it can reversibly cycle between Li 1 IrO 4 and Li 4.7 IrO 4. Here, X-ray spectroscopy, magnetic measurements and computational simulations uncover the evolution of O states in the different steps, compared to the involvement of Ir. While the process between Li 1 IrO 4 and Li 3 IrO 4 is dominated by the unconventional lattice oxygen redox, the process between Li 3 IrO 4 and Li 4.7 IrO 4 involves a conventional change of the formal oxidation state of Ir, which affects O due to the high covalency. The O states of Li 3 IrO 4 exhibit a very high reversibility after the whole 3.7-electron process, completely restoring the pristine state.

Spinel vanadium oxides are a promising candidate material for multivalent battery cathodes due to the various possible valence states of V within the spinel framework, which allows for pathways in Mg2+ ion deintercalation [1]. In recent work, Hu et al. experimentally demonstrated that spinel vanadium oxides nanocrystals have a high capacity for Mg2+ electrochemical cycling [2]. While they reported capacities above 200mAh/g at 110 °C, thus outperforming counterparts such as Cr and Mn oxide spinels, it was noted that these capacities are limited by complex defect structures, as suggested by peak pair distances that cannot be explained in the spinel framework in X-ray diffraction measurements. Here, we will utilize atomic-resolution scanning transmission electron microscopy to characterize the structural defects in spinel V2O4 nanocrystals and their possible effect on Mg2+ intercalation. Additionally, we will report the effects of thermal annealing on the structure as a function of temperature and explore the stability of the MgV2O4 nanocrystals. We find that the effects of electron beam exposure can be used as a proxy for damage incurred on the crystals during electrochemical cycling, on the structure of the crystals. The atomic-resolution characterization will be conducted with an aberration-corrected cold field emission JEOL ARM200CF operated at 200kV primary electron energy and emission current of 15μA. The electron probe will be operated at 24mrad convergence semi-angle. High angle annular dark field, low angle annular dark field and annular bright field detectors will be set to the following inner angles: 75 mrad, 30 mrad, 11mrad. X-ray energy dispersive spectroscopy measurements will be conducted using a large solid-angle Oxford XMAX100TLE detector. Additionally, the ARM200CF is equipped with a post-column Gatan Continuum GIF spectrometer providing capabilities for electron energy loss spectroscopy measurements. In this contribution, we will demonstrate that thermal annealing at higher temperatures increases the crystal size of the MgV2O4 crystals while simultaneously decreasing the presence of defect structures. Additionally, we will characterize a variety of defect structures, similar to those shown in Figure 1, present in crystals annealed at lower temperatures using atomic resolved images and electron spectroscopy techniques. [3] Figure 1. LAADF Image showing a grain boundary defect in a spinel MgV2O4 nanocrystal thermally anneal at 400 degrees Celsius. [1] Computer Modeling Investigation of MgV2O4 for Mg-ion Batteries. Navaratnarajah Kuganathan, Konstantinos Davazolglou, Alexander Chroneos; Journal of Applied Physics 2020 127, 035106. [2] High Capacity for Mg2+ Deintercalation in Spinel Vanadium Oxide Nanocrystals. Linhua Hu, Jacob R. Jokisaari, Bob Jin Kwon, Liang Yin, Soojeong Kim, Haesun Park, Saul H. Lapidus, Robert F. Klie, Baris Key,Peter Zapol, Brian J. Ingram, John T. Vaughey, and Jordi Cabana; ACS Energy Letters 2020 5 (8), 2721-2727. [3] This work is solely supported by the Joint Center for Energy Storage Research (JCESR) and Energy Innovation Hub funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences. Figure 1

Transformational increases in the storage capacity of battery cathodes could be achieved by tapping into the redox activity at oxide ligands in addition to conventional transition metal couples. Yet the key signatures that govern such lattice oxygen redox (LOR) have not been ascertained. Li3IrO4 has the largest reversible LOR, rendering it a unique model system. Here, X-ray spectroscopy and computational simulations reveal that LOR in Li3IrO4 is selectively compensated via O sites with 3 lone pairs, which are activated by Li/Ir disorder. The 2-electron LOR can be reversed to regenerate the initial state without unlocking competing bulk reactions observed in many other compounds. We uncover an intricate interplay between stoichiometry, O coordination and non-bonding states in LOR and pinpoint spectroscopic signatures. This interplay is indispensable to design materials with 3d metals that fulfill the promise of LOR to overcome the bottlenecks of current cathodes for future implementation in practical batteries.

Li-ion batteries are one of the most promising energy storage sources because of excellent electrochemical performance and has dominated the field of portable devices for the past 20 years. It has also been widely used in vehicle electrification and grid storage. Thus, the demand of Li-ion batteries with high energy density increased dramatically. The electrochemical behavior of current Li-ion batteries mainly relies on the transition metal oxide cathodes1,2,3. Mn-based cathode materials have become highly desirable owing to the low cost and high earth abundance. Recently, a nanostructured disordered rocksalt-type Li4Mn2O5 was synthesized, which displays the highest reported capacity till date up to 355 mAh/g with relative reversibility as compared to other contemporary Li-Mn-O electrodes4,5. In spite of such intriguing electrochemical performance, the detailed charge compensation mechanism associated with (de)lithiation is still unclear. Previous study predicted that the electrochemistry of Li4Mn2O5 was driven by Oxygen/Mn redox. Further study in Mn via X-ray absorption spectroscopy (XAS) revealed that Mn4+/Mn5+ redox couple contributes a small partial capacity in the redox mechanism, 4,5 indicating the significant involvement of oxygen during cycling. However, the mechanism of oxygen participation in the electrochemistry was not studied. In pursuit of understanding the detailed electrochemical mechanism, in this study, X-ray absorption spectroscopic study was performed in both Mn and O K-edge at various states of charge-discharge cycle. XAS measurements were conducted at different charged and discharged states of Li4Mn2O5 cycled between 4.8V and 1.2V voltage window. The obtained Mn K-edge XAS spectra clearly unravels the change of oxidation state of Mn upon charging and discharging, with a quantitative analysis linking the oxidation state and absorption energy via integral method. As a complementary probe to Mn K-edge XAS, O K-edge XAS unambiguously demonstrates the involvement of oxygen upon cycling and uncovers the dominant contribution of oxygen to the electrochemical performance at high voltage charging. This finding could shed light on the design of Oxygen-involved Mn-based cathodes with high capacity and reversibility. References: Lu, Zhonghua, et. al. "Layered Li [Nix Co1− 2x Mnx]O2 Cathode materials for lithium-ion batteries." Electrochemical and Solid-State Letters 4, no. 12 (2001): A200-A203. Lee, Jinhyuk, et al. "Reversible Mn 2+/Mn 4+ double redox in lithium-excess cathode materials." Nature 556, no. 7700 (2018): 185-190. Lun, Zhengyan, et al. "Design Principles for High-Capacity Mn-Based Cation-Disordered Rocksalt Cathodes." Chem 6, no. 1 (2020): 153-168. Diaz-Lopez, Maria, et al. "Operando X-ray Absorption Spectroscopy and Emission Kβ1, 3 Study of the Manganese Redox Activity in High-Capacity Li4Mn2O5 Cathode." The Journal of Physical Chemistry C51 (2018): 29586-29597. Diaz-Lopez, Maria, et al. "Local structure and lithium diffusion pathways in Li4Mn2O5 high capacity cathode probed by total scattering and XANES." Chemistry of Materials9 (2018): 3060-3070.

The US Advanced Battery Consortium goals for 2023 call for low cost, fast charge, electric vehicle (EV) batteries with a 15-minute charge time at 80% pack capacity.[1] This milestone would have a huge impact on the universal market adoption of EVs. Unfortunately, fast charging at rates above 1C, defined as the current at which it takes one hour for a battery to charge/discharge to full capacity, imposes extensive deleterious effects on current lithium-ion battery (LiB) chemistries. High charging rates aggressively accelerate degradation mechanisms owed to structural damage due to both increased cycling temperature and inhomogeneous Li ion mass transport properties. The induced electrode damage can cause void formation between the active material and conductive-binder matrix and delamination from the current collecting foil. The now electronically isolated active material results in large costs on the capacity retention of the battery. In this study, we report measurements via operando X-ray microtomography of cylindrical cells used in EVs under simulated fast charge and drive cycles. Three batteries were measured during cycling after antecedent fast charging cycles, the 3rd, 5th, and 81st - 82nd cycles were analyzed to track morphological damage at different battery life points. To track the spatial and morphological degradation of both the anode and cathode structures, we employed a deep learning segmentation method using the U-Net convolutional neural network (CNN). Using a Euclidean Distance Mapping method, void formation is tracked spatially in 3-dimensions within the electrode coating. Insight into how fast charging induces structural damage will better inform research into fast-charge protocols and new battery chemistries for electrolytes, electrolyte additives, and novel electrode architectures. [1]Liu, Y.; Zhu, Y.; Cui, Y. Challenges and Opportunities towards Fast-Charging Battery Materials. Nat. Energy 2019, 4 (7), 540–550. https://doi.org/10.1038/s41560-019-0405-3.

Electrocatalytic reactions offer alternative routes for the conversion and storage of energy. For instance, the electrocatalytic reduction of carbon dioxide could lead to energy-rich syngas, methanol, or even hydrocarbons. Layered transition metal dichalcogenide nanomaterials, such as MoS2, have emerged as electrocatalysts towards CO2 reduction with high efficiency and low overpotentials. However, the underlying chemical and electronic states defining the catalytic activity have not yet been completely defined. These key states can be probed for both metals and ligands in selected MX2 (M=W, Mo; X=S, Se) using synchrotron-based X-ray absorption spectroscopy (XAS). In recent work we performed in-situ XAS measurements at Mo K-edge and S K-edge for MoS2 nanosheets. Measurements at the S K-edge showed drastic changes at the pre-edge and edge indicating changes in number of available unoccupied states and Zeff respectively. On the other hand, only a small shift of the Mo K-edge was observed. Metal d-states are found to be the active site for catalysis, yet the ligand K-edge presents a better method to examine catalytic processes. Since the pre-edge reflects a transition to metal-ligand hybridized states, its intensity and position gives direct insight into the activity and changes of d-states. Given the vast compositional space of metal and ligand for MX2, the knowledge of how d-state activity will serve to pave the way to a better study of metal and chalcogenide combinations to improve the electrocatalytic performance. Figure 1

Current industrial methodologies for lithium-ion battery cathode materials are suited to the production of well-defined, dense, spherical secondary particle structures assembled from nanocrystalline primary particles. Specifically, LiNixMnyCozO2 (LNMC) cathode material is synthesized by first growing the hydroxide precursor, NixMnyCoz(OH)2 (NMC), through a coprecipitation method using a stoichiometric ratio of mixed transition metal sulfate solutions, sodium hydroxide, and ammonia hydroxide pumped into a continuous stirred tank reactor (CSTR). Each new composition of cathode material requires an extensive synthesis parameter optimization study to refine the even formation of metal species, density, and uniformity of size and shape of the final cathode material. The characterization and visualization of the three-dimensional secondary cathode structure, internal morphology, and void network is an elusive task using traditional scanning electron microscopy (SEM) because it is 2D technique that requires the use of mechanical cross-sectioning which can cause the deformation of the internal structure. Yet the variation of the internal structure and density changes within LNMC secondary particles can have large effects on both the cycling kinetics, capacity, and lifetime of the final cathode material. In this study, we present a five-parameter optimization study of Ni0.25Mn0.25Co0.50(OH)2 (NMC112) using our lab-built CSTR. The optimization and morphological changes of the primary and secondary particles during high temperature lithiation is investigated further. Using synchrotron-based transmission X-ray microscopy tomography (TXM) in combination with a novel deep learning semantic segmentation process, the three-dimensional cathode structure and internal pore network of LNMC112 particles was mapped across different lithiation temperature and reaction times. The deep learning semantic segmentation allowed for the quantification of void volume, shape, size, and final overall density of the cathode secondary particles. The results of these morphological differences show effects in both the kinetics and cyclability through galvanostatic cycling. The quantification of the three-dimensional cathode microstructure shown in this study is agnostic of the specific cathode composition. Therefore, it is highly suited to better inform optimization targets of the coprecipitation synthesis, as well as illuminating the importance the internal pore network of secondary particles.

LiNi0.80Co0.15Al0.05O2 (NCA) is one of the most promising cathode material for Lithium ion batteries which has relatively high theoretical specific capacity. Achieving a thorough understanding of reaction processes at the single particle level will help to close the gap between practical and theoretical capacity of NCA. AMPIX cells with diluted NCA cathode (20% active material) were studied using operando μX-Ray transmission. The single NCA particles were located successfully. The electrochemical performance of single particles were studied at different points in the lifetime of the battery as well as at different charge/discharge rates (C/20, C/8, C/3). The XRD patterns sets of each particle were compared as well as the unit cell parameters of each individual particles. The rates of (de)lithiation of each particle were quantified for analysis of inter-particle dynamics in this work. The kinetic effect on the homogeneity of each particles at different rates was also analyzed by phase analysis in this work.

Metal-organic frameworks (MOFs) are promising materials for electrocatalysis; however, lack of electrical conductivity in the majority of existing MOFs limits their effective utilization in the field. Herein, an excellent catalytic activity of a 2D copper (Cu)-based conductive MOF, copper tetrahydroxyquinone (CuTHQ), is reported for aqueous CO2 reduction reaction (CO2 RR) at low overpotentials. It is revealed that CuTHQ nanoflakes (NFs) with an average lateral size of 140 nm exhibit a negligible overpotential of 16 mV for the activation of this reaction, a high current density of ≈173 mA cm-2 at -0.45 V versus RHE, an average Faradaic efficiency (F.E.) of ≈91% toward CO production, and a remarkable turnover frequency as high as ≈20.82 s-1 . In the low overpotential range, the obtained CO formation current density is more than 35 and 25 times higher compared to state-of-the-art MOF and MOF-derived catalysts, respectively. The operando Cu K-edge X-ray absorption near edge spectroscopy and density functional theory calculations reveal the existence of reduced Cu (Cu+ ) during CO2 RR which reversibly returns to Cu2+ after the reaction. The outstanding CO2 catalytic functionality of conductive MOFs (c-MOFs) can open a way toward high-energy-density electrochemical systems.

The electrochemical reactions occurring in batteries involve the transport of ions and electrons among the electrodes, the electrolyte, and the current collector. In Li-ion battery electrodes, this dual functionality is attained with porous composite electrode structures that contain electronically conductive additives. Recently, the ability to extensively cycle composite electrodes of Li4Ti5O12 without any conductive additives generated questions about how these structures operate, the answers to which could be used to design architectures with other materials that reduce the amount of additives that do not directly store energy. Here, the changes occurring in carbon-free Li4Ti5O12 electrodes during lithiation were studied by a combination of ex situ and operando optical microscopy and microbeam X-ray absorption spectroscopy (μ-XAS). The measurements provide visualizations of the percolation of lithiated domains through the thick (∼40-μm) structure after a depth of discharge of only 1%, followed by a sec...

Li-rich cathode materials are of significant interest for coupling anionic redox with cationic redox chemistry to achieve high-energy-density batteries. However, lattice oxygen loss and derived structure distortion would induce serious capacity loss and voltage decay, further hindering its practical application. Herein, a novel Li-rich cathode material, O3-type Li0.6 [Li0.2 Mn0.8 ]O2 , is developed with the pristine state displaying both a Li excess in the transition metal layer and a deficiency in the alkali metal layer. Benefiting from stable structure evolution and Li migration processes, not only can high reversible capacity (≈329 mAh g-1 ) be harvested but also irreversible/reversible anionic/cationic redox reactions are comprehensively assigned via the combination of in/ex situ spectroscopies. Furthermore, irreversible lattice oxygen loss and structure distortion are effectively restrained, resulting in long-term cycle stability (capacity drop of 0.045% per cycle, 500 cycles). Altogether, tuning the Li state in the alkali metal layer presents a promising way for modification of high-capacity Li-rich cathode candidates.

Recent progress in the field of scanning/transmission electron microscopy (S/TEM) has resulted in intense electron probes providing an ultra-high spatial (e.g. sub-Å) and energy (3 meV) resolution, with a previously unattainable signal-to-noise ratio. Furthermore, the STEM-based methods: direct imaging of both heavy and light elements, and both the energy-dispersive X-ray (EDX) and electron energy loss (EEL) spectroscopies are becoming the most promising characterization tools for battery materials, providing an unprecedented opportunity to probe the evolution of structure and electronic state of the cathodes following their electrochemical activity. The current talk will focus on the electron microscopy characterization of various materials that are considered as potential cathodes for the multivalent batteries, including transition metal (TM) oxides and polyanionic compounds. Materials were characterized using aberration-corrected JEOL JEM-ARM200CF, equipped with a cold field emission and operated at 200 kV, in pristine, cycled, and in-situ irradiated phases exploring their structure, chemistry, and electronic states. The in-situ technique mimics the electrochemical cell and allows for single-particle tracking of the dynamic processes occurring upon oxygen loss from the material providing helpful knowledge to further strengthen the understanding of the electrochemical activity of the cathodes. Several imaging modes, including high/low angle annular dark field (H/LAADF) and annular bright field (ABF), in conjunction with EELS/EDX, were used extensively for the analysis. Various parameters such as TM valence, phase transition, oxygen deficiency, and multivalent ion intercalation will be discussed.

The capability of the tailored solid-solution spinel, MgCrMnO4, is evaluated by theoretical and experimental approaches. Lattice Mg2+ in the designed oxide is electrochemically utilized at high potentials in a non-aqueous electrolyte. Complementary evidence supports bulk Mg2+ (de)intercalation throughout the designed oxide frame where strong electrostatic interaction between Mg2+ and O2- exists. Mg/Mn antisite inversion in the spinel is lowered via post-annealing to further improve Mg+2 mobility. Spinel lattice is preserved upon removal of Mg2+ without any phase transformations, denoting structural stability at the charged state at a high potential. In the remagnesiated state, insertion of Mg2+ into interstitial sites in the spinel is detected possibly resulting in partial reversibility which needs to be addressed for structural stability. The observations constitute a first clear path to the development of a practical high voltage Mg-ion cathode using a spinel oxide.

Multivalent-ion batteries, based on the reversible reactivity of metal cations Mn+ (where n ≥ 2), offer great promise to supersede the current state-of-the-art Li-ion battery and provide a step change in energy storage capability. Oxide materials based on the spinel structure, AB2O4 (where A is the diffusing ion, M2+) have previously demonstrated potential to be high capacity, high voltage cathode materials for such batteries, with a 3D network of M2+ diffusion channels to facilitate ion transport.1–3 In particular, the MgCr2O4 spinel has been of interest due the predicted high electrochemical potential (~3.5 V) of Cr3+/Cr4+ vs. Mg/Mg2+.2 However, reversible Mg2+ intercalation kinetics has often proven to be poor in such systems, with large overpotentials observed on charge and discharge.3,4 The origin of these kinetic barriers is still poorly understood, where it is postulated that poor Mg2+ diffusion through the oxide lattice, sluggish ion transfer between the electrode and electrolyte, or resistive surface layers from electrolyte decomposition products may be responsible. In this presentation, we discuss the application of solid-state impedance spectroscopy to a library of doped spinel compounds, M1−x Cr2−2x Ti2x O4, where M = Mg or Zn, to elucidate the diffusion behavior of Mg2+ and Zn2+ in these systems. We reveal that incorporation of Ti had significant impact on the impedance behavior of the materials. As both M2+ and e− were found to diffuse in these materials, they displayed mixed ionic and electronic conductor (MIEC) behavior.5,6 This presentation includes the development and interpretation of a new physical model to describe the motion of M2+ and e− in the M1−x Cr2−2x Ti2x O4 spinel, which was necessary to accurately describe their diffusive behavior and extract their conductivities. This study therefore represents a breakthrough in the understanding of M2+ motion in these systems, and provides a methodological framework for further studies of other electrode chemistries. References: (1) Canepa, P.; Sai Gautam, G.; Hannah, D. C.; Malik, R.; Liu, M.; Gallagher, K. G.; Persson, K. A.; Ceder, G. Odyssey of Multivalent Cathode Materials: Open Questions and Future Challenges. Chem. Rev. 2017, 117 (5), 4287–4341. (2) Liu, M.; Rong, Z.; Malik, R.; Canepa, P.; Jain, A.; Ceder, G.; Persson, K. A.; Liu, M. Spinel Compounds as Multivalent Battery Cathodes: A Systematic Evaluation Based on Ab Initio Calculations. Energy Environ. Sci 2015, 8 (3), 964–974. (3) Hu, L.; Johnson, I. D.; Kim, S.; Nolis, G. M.; Freeland, J.; Yoo, H. D.; Fister, T. T.; McCafferty, L.; Ashton, T. E.; Darr, J. A.; et al. Tailoring the Electrochemical Activity of Magnesium Chromium Oxide Towards Mg Batteries Through Control of Size and Crystal Structure. Nanoscale 2019, 11, 639–646. (4) Shimokawa, K.; Ichitsubo, T. Spinel–Rocksalt Transition as a Key Cathode Reaction toward High-Energy-Density Magnesium Rechargeable Batteries. Current Opinion in Electrochemistry. Elsevier B.V. 2020, pp 93–99. (5) Lai, W.; Haile, S. M. Impedance Spectroscopy as a Tool for Chemical and Electrochemical Analysis of Mixed Conductors: A Case Study of Ceria. J. Am. Ceram. Soc. 2005, 88 (11), 2979–2997. (6) Jamnik, J.; Maier, J. Treatment of the Impedance of Mixed Conductors. Equivalent Circuit Model and Explicit Approximate Solutions. J. Electrochem. Soc. 1999, 146 (11), 4183–4188.

Multinary transition metal compounds are important cathode materials in Li-ion batteries. The most representative compounds can be catalogued under two main structural families: layered and spinel, with the former having high capacity, but the latter showing greater stability. Synergies are frequently sought by integrate both structures into a composite. The combination of Co, Mn and Ni in these structures leads to the large capacity and operation at high voltage. However, these mixed compounds are mainly synthesized at high temperature and relatively long times. The resulting thermodynamic control makes it difficult to tailor structure, morphology and surface chemistry through metastability. In this context, hydrothermal methods emerge as an attractive pathway of synthesis that could open new up new material permutations, but their versatility toward complex oxide cathodes has not been fully exploited. As a step in this direction, we demonstrate the synthesis of Li1.1[Mn0.6Co0.8Ni0.6]O4-σ with a spinel-layer structure and investigated its electrochemistry and redox mechanisms at high potentials. This strategy expands the current library of cathode materials and brings new opportunities for materials design.

Transition metal dichalcogenide (TMDCs) alloys could have a wide range of physical and chemical properties, ranging from charge density waves to superconductivity and electrochemical activities. While many exciting behaviors of unary TMDCs have been demonstrated, the vast compositional space of TMDC alloys has remained largely unexplored due to the lack of understanding regarding their stability when accommodating different cations or chalcogens in a single-phase. Here, a theory-guided synthesis approach is reported to achieve unexplored quasi-binary TMDC alloys through computationally predicted stability maps. Equilibrium temperature-composition phase diagrams using first-principles calculations are generated to identify the stability of 25 quasi-binary TMDC alloys, including some involving non-isovalent cations and are verified experimentally through the synthesis of a subset of 12 predicted alloys using a scalable chemical vapor transport method. It is demonstrated that the synthesized alloys can be exfoliated into 2D structures, and some of them exhibit: i) outstanding thermal stability tested up to 1230 K, ii) exceptionally high electrochemical activity for the CO2 reduction reaction in a kinetically limited regime with near zero overpotential for CO formation, iii) excellent energy efficiency in a high rate Li-air battery, and iv) high break-down current density for interconnect applications. This framework can be extended to accelerate the discovery of other TMDC alloys for various applications.

Rechargeable batteries based on the intercalation of multivalent cations are a promising energy storage technology to overcome the limitations of safety, cost, and energy density of state-of-the-art Li-ion batteries. However, functionality of cathodes in Mg batteries has only been realized with a few of S-containing compounds, which possess limited capacity and operate at low potential, hindering competitive energy densities. Spinel oxides have been predicted to improve both voltage and capacity. However, the strong electronic interaction between these cations and the corresponding frameworks impose the challenge of Mg2+ low mobility and sluggish kinetics. Herein, a series of spinel oxides with designed sizes and defects have been evaluated as possible cathodes, studying their fundamental ability to intercalate Mg2+ by using different temperatures. Mg2+ deintercalation at high voltage was achieved at 110oC, and, under certain conditions, even at room temperature. In addition, the capabilities of Mg2+ reversible intercalation are correlated to particle size. Our results demonstrate that the design of nanosized spinel oxides could enable an efficient pathway to improve Mg2+ reaction kinetics, thus advancing the ability to develop new multivalent cathode protypes with high energy density. References: 1. Liu, M., Rong Z., Malik, R., Canepa P., Jain A., Ceder G., Persson K. Spinel compounds as multivalent battery cathodes: a systematic evaluation based on ab initio calculations. Energy Environ. Sci. 2015, 8, 964. 2. Kim C., Phillips P. J., Key B., Yi T., Nordlund D., Yu Y.-S., Bayliss R. D., Han S., He M., Zhang Z., Burrell A. K., Klie R. F., Cabana J. Direct Observation of Reversible Magnesium Ion Intercalation into a Spinel Oxide Host. Adv. Mater. 2015, 27, 3377. 3. Hu, L., Johnson I.D., Kim S., Nolis G.M., Freeland J.W., Yoo H.D., Fister T., McCafferty L., Ashton T., Darr J., Cabana J. Tailoring the electrochemical activity of magnesium chromium oxide towards Mg batteries through control of size and crystal structure. Nanoscale 2019, 11, 639.

The market for electric vehicles has increased significantly in the recent decade. Thus lithium-ion batteries with high energy and power density, with the ability to withstand extreme cycling environments are necessary to compete with the flexibility of the fossil fuel reliant combustion engine. The main instabilities in the Li-ion battery can be traced to the electrode-electrolyte interface, here electroactive transition metals react with the electrolyte resulting in irreversible transitions forming insulating layers, or solid cathode electrolyte interfaces (CEI), that inhibit ion transport. Cathode material dissolution may also occur at the surface due to acidic attack from the HF byproduct from the decomposition of the LiPF6 electrolyte. During electrochemical cycling, uneven lithium diffusion causes the surface to become more reduced than the core which leads to structural changes which impedes ion transport. These structural changes cause nonuniform contraction and expansion of the lattice loosening connections between the transition metal oxide layers eventually leading to severe macroscopic cracking. This effect has been seen at lower cycling rates when particle size is increased. The cracks allow the electrolyte to permeate the particle allowing CEI formation within the particle greatly decreasing the cathode life. A solution to these detrimental surface interactions is by replacing the electroactive transition metal content at the surface with inactive ions improving the interfacial stability. To prevent significant decreases in storage capacity the coating is a thin passivating barrier. Previously our group has introduced a strategy toward stabilizing the electrode-electrolyte interface using Ni0.25Mn0.25Co0.50O nanocrystals (NC) of active material. Each individual NC is passivated by an epitaxial grown conformal ultrathin Al2O3 shell with a final composition of LiCo0.5Ni0.25Mn0.25O2 with a concentration gradient of Al3+ ions toward the outer layers after high temperature lithiation. This material showed increased cycling stability under harsh conditions of increased cycle rate and temperature. While this research had promising results, industrial methods are currently better suited to produce well defined dense secondary particle structures with a spherical morphology, made up of agglomerated nanoparticles. While many coating methods being researched on these secondary structures require coating materials to be deposited on as-made, pre-lithiated cathode material, this method makes it difficult to completely coat all surfaces, while losing the ability to control surface chemistry and homogeneity. Here an optimized systematic study was performed to form an aluminum shell in the same process of growth of NixMnzCoy(OH)2 (NMC) precursors. This procedure affords precise control over shell thickness and concentration gradient architectures. The core-shell precursors are reacted with LiOH in a high-temperature solid-state reaction, allowing the aluminum shell to diffuse into the surface lattice which will further suppress phase distortion in the bulk. The higher concentration of Al3+ at the surface protects against side reactions with the electrolyte. These secondary structured LiNi0.25Mn0.25Co0.50O2 were electrochemically tested using both Li-metal half cells and graphite anode full cells and the results were compared to nanocrystalline primary particles of a similar composition. While aluminum coatings have been studied on cathode material previously, it is still a challenge to determine finite spatial changes in distribution of elements within individual particles in 3D with current conventional methods available, such as SEM-EDS analysis. In this study, synchrotron-based imaging methods of X-ray fluorescence (XRF) nanoprobe cross-sectional mapping, XRF tomography, and transmission X-ray elemental tomography were used to evaluate aluminum coated gradient NMC material and other stabilized architectures of interest such as full concentration gradient NMC material. For tomographic measurements particles were loaded into 50-micron capillaries allowing for high throughput imaging. These techniques allowed for little sample manipulation which preserved sample morphology in pristine state. Increased resolution offered further insight into elemental distribution and extent of aluminum migration after high temperature lithiation. Advancements in these imaging techniques can better inform the future of stabilized cathode architecture design. Figure 1

Building a stable chemical environment at the cathode/electrolyte interface is directly linked to the durability of Li-ion batteries with high energy density. Recently, colloidal chemistry methods have enabled the design of core-shell nanocrystals of Li

International audience; In the version of this Article originally published, the tickmarks on the x axis in Fig. 5e were labelled incorrectly; the values shown were 160, 164, 168 and 172 eV, but they should have been 162, 167 and 172 eV and on the major tickmarks only. The corrected Fig. 5e is shown below and has been updated in all versions of the Article.

Journal of the Electrochemical Society 166(6), A1257 - A1265 (2019). doi:10.1149/2.1311906jes, Olivine-type LiMnPO4 was synthesized via a citric-acid assisted sol-gel method. The structural evolution of the as-prepared material during calcination and the crystallization of $LiMnPO_{4}$ was investigated by XRD measurements. A reaction mechanism including two intermediate phases was derived for the synthesis. Furthermore, the electrochemical Li extraction and insertion processes were studied by in situ/ex situ XRD and MAS NMR measurements. The phase transition from $LiMnPO_{4}$ to $MnPO_{4}$ and vice versa proceeds via a two-phase mechanism. The 31P MAS NMR spectra of $MnPO_{4}$, which forms upon delithiation as confirmed by XRD patterns, shows a very broad signal. Possible explanations for the large width of this signal are given and evaluated., Published by Electrochemical Soc., Pennington, NJ

A comprehensive study on the effects of experimental parameters on the composition and size of manganese oxide nanocrystals was completed using colloidal chemistry. The reactions studied involved the thermolysis of Mn2+ acetate and Mn3+ acetylacetonate in oleylamine. Temperature was found to be the dominant factor affecting the composition and size of the products. Reactions completed below 200 °C favored the formation of nanocrystals smaller than 20 nm, with the presence of even impurity amounts of oxidizing agents leading to the formation of Mn3O4. Nanocrystals of MnO could only be synthesized below 200 °C if Mn2+ acetate was used, and the reaction was carefully controlled to have no O2 and H2O contamination. In turn, particle growth was rapid above this temperature. In this case, regardless of the oxidizing agents used or oxidation state of the Mn precursor, nanocrystals of MnO formed after annealing for at least 1 h at temperatures higher than 200 °C. This finding suggests the role of oleylamine as solvent, surfactant, and reducing agent at sufficiently high annealing temperatures. These results increase the understanding of redox stability of manganese during the colloidal synthesis of semiconductor metal oxide nanocrystals.

The finding that triggering the redox activity of oxygen ions within the lattice of transition metal oxides can boost the performances of materials used in energy storage and conversion devices such as Li-ion batteries or oxygen evolution electrocatalysts has recently spurred intensive and innovative research in the field of energy. While experimental and theoretical efforts have been critical in understanding the role of oxygen nonbonding states in the redox activity of oxygen ions, a clear picture of the redox chemistry of the oxygen species formed upon this oxidation process is still missing. This can be, in part, explained by the complexity in stabilizing and studying these species once electrochemically formed. In this work, we alleviate this difficulty by studying the phase Ba5Ru2O11, which contains peroxide O-2(2-) groups, as oxygen evolution reaction electrocatalyst and Li-ion battery material. Combining physical characterization and electrochemical measurements, we demonstrate that peroxide groups can easily be oxidized at relatively low potential, leading to the formation of gaseous dioxygen and to the instability of the oxide. Furthermore, we demonstrate that, owing to the stabilization at high energy of peroxide, the high-lying energy of the empty sigma* antibonding O-O states limits the reversibility of the electrochemical reactions when the O-2(2-)/O2- redox couple is used as redox center for Li-ion battery materials or as OER redox active sites. Overall, this work suggests that the formation of true peroxide O-2(2-) states are detrimental for transition metal oxides used as OER catalysts and Li-ion battery materials. Rather, oxygen species with O-O bond order lower than 1 would be preferred for these applications.

Stabilization of electrode-electrolyte interfaces is required to increase the energy stored in battery electrodes. Introducing redox-inactive ions on the electrode surface minimizes deleterious side reactions without affecting the bulk properties. A synthetic challenge exists to grow such layers conformally at each primary particle, to fully passivate interfaces that are buried in the final electrode architecture. The development of methods of sequential colloidal growth of complex oxides and overlayers, enabled by surfactant interactions, would provide novel means to advance toward this goal. Here, nanocrystals composed of LiCoO2, a commercially relevant material for high energy devices, were grown with a shell enriched in Al3+, deposited conformally through a one-pot colloidal synthetic method. The effects of synthetic conditions on the composition of the Al-rich shell and the corresponding electrochemical performance were investigated. The modified nanocrystals showed enhanced electrochemical properties, while maintaining carrier transport.

Author(s): Devaux, D; Wang, X; Thelen, JL; Parkinson, DY; Cabana, J; Wang, F; Balsara, NP | Abstract: Lithium (Li) batteries comprising multivalent positive active materials such as copper vanadium oxide have high theoretical capacity. These batteries with a conventional liquid electrolyte exhibit limited cycle life because of copper dissolution into the electrolyte. We report here on the characterization of solid-state Li metal batteries with a positive electrode based on α-Cu6.9V6O18.9 (α-CuVO3). We replaced the liquid electrolyte by a nanostructured solid block copolymer electrolyte comprising of a mixture of polystyrene-b-poly(ethylene oxide) (SEO) and lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) salt. In situ X-ray diffraction was used to follow the Li insertion/de-insertion mechanism into the α-CuVO3 host material and its reversibility. In situ X-ray scattering revealed that the multistep electrochemical reactions involved are similar in the presence of liquid or solid electrolyte. The capacity fade of the solid-state batteries is less rapid than that of α-CuVO3-Li metal batteries with a conventional liquid electrolyte. Hard X-ray microtomography revealed that upon cycling, voids and Cu-rich agglomerates were formed at the interface between the Li metal and the SEO electrolyte. The void volume and the volume occupied by the Cu-rich agglomerates were independent of C-rate and cycle number.

Decoupling the relevant parameters determining the electrochemical performance of spinel-type LiNi0.5Mn1.5O4 would contribute to promote its commercialization as cathode material for Li-ion batteries with high energy density. These parameters mainly comprise Ni/Mn ordering and non-stoichiometry, but their drivers and individual contribution to electrochemical performance remain to be fully ascertained. A series of samples annealed at different temperatures in the vicinity of an ordering transition have been thoroughly characterized by means of neutron powder diffraction to accurately establish composition–structure–property relationships in this material. The analysis revealed that deviations from a perfectly ordered crystal are possible through two different types of defects with significantly different effects on properties. These structural defects are in addition to previously described compositional defects, involving the creation of Mn3+ in the spinel lattice and Ni-rich rock salt secondary phases. Among the two types, the formation of antiphase boundaries is detrimental to transport, leading to poor rate performance of the electrode. In contrast, Ni/Mn mixing in an ordered framework can lead to behavior competitive with fully disordered samples, even at much lower Mn3+ contents that theoretically impart enhanced electronic conductivity. This work establishes design guidelines for fast transport in materials close to full stoichiometry, avoiding deleterious effects of rock salt impurities and Mn3+ dissolution.

The performance of battery electrode materials is strongly affected by inefficiencies in utilization kinetics and cycle life as well as size effects. Observations of phase transformations in these materials with high chemical and spatial resolution can elucidate the relationship between chemical processes and mechanical degradation. Soft X-ray ptychographic microscopy combined with X-ray absorption spectroscopy and electron microscopy creates a powerful suite of tools that we use to assess the chemical and morphological changes in lithium iron phosphate (LiFePO4) micro- and nanocrystals that occur upon delithiation. All sizes of partly delithiated crystals were found to contain two phases with a complex correlation between crystallographic orientation and phase distribution. However, the lattice mismatch between LiFePO4 and FePO4 led to severe fracturing on microcrystals, whereas no mechanical damage was observed in nanoplates, indicating that mechanics are a principal driver in the outstanding electrode ...

This is the author's version of a work that was accepted for publication in Thin Solid Films. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definative version wass subsequently published in Thin Solid Films, 2015. 576: 55-60. DOI:10.1016/j.tsf.2014.11.019.

Redox-driven phase transformations in solids determine the performance of lithium-ion batteries, crucial in the technological transition from fossil fuels. Couplings between chemistry and strain define reversibility and fatigue of an electrode. The accurate definition of all phases in the transformation, their energetics, and nanoscale location within a particle produces fundamental understanding of these couplings needed to design materials with ultimate performance. Here we demonstrate that scanning X-ray diffraction microscopy (SXDM) extends our ability to image battery processes in single particles. In LiFePO

The energy density of rechargeable batteries utilizing metals as anodes surpasses that of Li ion batteries, which employ carbon instead. Among possible metals, magnesium represents a potential alternative to the conventional choice, lithium, in terms of storage density, safety, stability, and cost. However, a major obstacle for metal-based batteries is the identification of electrolytes that show reversible deposition/dissolution of the metal anode and support reversible intercalation of ions into a cathode. Traditional Grignard-based Mg electrolytes are excellent with respect to the reversible deposition of Mg, but their limited anodic stability and compatibility with oxide cathodes hinder their applicability in Mg batteries with higher voltage. Non-Grignard electrolytes, which consist of ethereal solutions of magnesium(II) bis(trifluoromethanesulfonyl)imide (Mg(TFSI)

Capacity fading has limited commercial layered Li-ion battery electrodes to

The hierarchical nature of the cathode in Li-ion batteries can result in phenomena determining electrochemical performance occurring at different length-scales, from individual atoms to the whole electrode. In architectures designed for high density of charge storage, transport limitations can emerge at the microscale that compromise effective utilization and accelerate degradation. These limitations manifest as chemical heterogeneity within the electrode. Micro-focused diffraction mapping using a laboratory X-ray source provides maps with sub-mm resolution of a whole electrode composed of commercial LiMn2O4. Evidence of disparate local utilization both laterally and along the depth of the electrode was obtained, especially at high rates and after multiple charge-discharge cycles. As a model to study the persistence of heterogeneity due to transport limitations, lateral gradients to lithium transport were introduced by cycling against Li anodes of small diameter. The resulting maps revealed the effects of anisotropic electric migration and diffusion to be separated, confirming that diffusion is the primary limitation for long-range kinetics. Tracking of subsequent relaxation revealed that the heterogeneity was metastable despite a strong thermodynamic driving force, maintained by poor lithium transport through the solid electrode matrix. This study enriches our understanding of transport across thick electrode architectures and the imposition of unique frustrated states, away from equilibrium.

Recent reports of remarkably high oxide ion conduction in a new family of strontium silicates have been challenged. It has recently been demonstrated that, in the nominally potassium substituted strontium germanium silicate material, the dominant charge carrier was not the oxygen ion, and furthermore that the material was not single phase (R. D. Bayliss et. al., Energy Environ. Sci., 2014, DOI: 10.1039/c4ee00734d). In this work we re-investigate the sodium-doped strontium silicate material that was reported to exhibit the highest oxide ion conductivity in the solid solution, nominally Sr0.55Na0.45SiO2.775. The results show lower levels of total conductivity than previously reported and sub-micron elemental mapping demonstrates, in a similar manner to that reported for the Sr0.8K0.2Si0.5Ge0.5O2.9 composition, an inhomogeneous chemical distribution correlating with a multiphase material. It is also shown that the conductivity is not related to protonic mobility. A density functional theory computational approach provides a theoretical justification for these new results, related to the high energetic costs associated with oxygen vacancy formation.

LiNi0.5Mn1.5O4 epitomizes the challenges imposed by high electrochemical potential reactivity on the durability of high energy density Li-ion batteries. Postsynthesis coatings have been explored as a solution to these challenges, but the fundamentals of their function have not been ascertained. To contribute to this understanding, the surface of LiNi0.5Mn1.5O4 microparticles was modified with Mg2+, a coating component of literature relevance, using two different heat treatment temperatures, 500 and 800 °C. A combination of characterization tools revealed that Mg2+ was introduced mainly as an inhomogeneous MgO coating in the sample treated at 500 °C, and into the spinel lattice at the subsurface of the particles at 800 °C. Comparing the properties of these two different materials with an unmodified baseline afforded the opportunity to evaluate the effect of varying surface chemistries. Coulometry in Li metal half cells was used as a macroscopic measure of side reactions at the electrode–electrolyte interfa...

Ultrathin conformal coatings of the lithium ion conductor, lithium aluminum oxide (LiAlO2), were evaluated for their ability to improve the electrochemical stability of LiNi0.5Mn1.5O4/graphite Li-ion batteries. Electrochemical impedance spectroscopy confirmed the ion conducting character of the LiAlO2 films. Complementary simulations of the activation barriers in these layers match experimental results very well. LiAlO2 films were subsequently separately deposited onto LiNi0.5Mn1.5O4 and graphite electrodes. Increased electrochemical stability was observed, especially in the full cells, which was attributed to the role of the coatings as physical barriers against side reactions at the electrode–electrolyte interface. By comparing data from full cells where the coatings were applied to either electrode, the dominating failure mechanism was found to be the diffusion of transition metal ions from the cathode to the anode. The LiNi0.5Mn1.5O4/graphite full cell with less than 1 nm LiAlO2 on the positive electr...

The electrochemical characteristics of lepidocrocite-type titanates derived from K0.8Ti1.73Li0.27O4 are presented for the first time. By exchanging sodium ions for potassium, the practical specific capacity of the titanate in both sodium and lithium half cells is considerably enhanced. Although the gross structural features of the titanate framework are maintained during the ion exchange process, the symmetry changes because sodium occupies different sites from potassium. The smaller size of the sodium ion as compared to potassium and the change in site symmetry allow more alkali metal cations to be inserted reversibly into the structure during discharge in sodium and lithium cells than in the parent compound. Insertion of lithium cations takes place at an average of about 0.8 V vs Li+/Li while sodium intercalation occurs at 0.5 V vs Na+/Na, with sloping voltage profiles exhibited for both cell configurations, implying single-phase processes. Ex situ synchrotron X-ray diffraction measurements show that a ...

Using transmission electron microscopy, we have analyzed the structure of nanoporous NiO formed by thermal decomposition of Ni(OH)2. It was found that a tabular single crystal of Ni(OH)2 transforms pseudomorphically to an array of topotaxially aligned NiO nanocubes separated by nanoscale pores at a volume fraction of about 50%. High-resolution electron tomography on NiO produced by ex situ heating combined with direct in situ observation of the transformation under the electron beam revealed a dramatic redirection of the reaction front when pores are closed by consolidation of NiO nanocubes around the perimeter of the tabular crystals. This causes local changes in the density, structure, and connectivity of the pores. Insights from this study are of broader interest for the class of decomposition reactions that result in nanocrystalline materials with highly tailored porosity and are significant for the development of devices with impact in fields as varied as energy storage, catalysis, and sensing.

The functionality of several titanium oxides as active materials in electrodes free of any conductive additive was evaluated and rationalized. Correlations were established between the phase transformation during lithiation and both utilization and durability. Ramsdellite-type Li2Ti3O7 was found to perform well, enabled by the small, isotropic volume change that accompanies lithium intercalation. In contrast, the anatase and rutile polymorphs of TiO2 suffered losses, to different extents. Rutile could be fully lithiated, which resulted in a significant reduction of crystallinity. Changes in the voltage profile suggest that an amorphous fraction existed that was mainly responsible for a very stable activity upon subsequent cycling. In contrast, anatase electrodes could not be fully lithiated, probably due to kinetic limitations imposed by the transformation, and underwent severe decay upon cycling. Both utilization and stability could be improved to some extent by mixing this phase with Li4Ti5O12, which could provide a mechanically stable scaffold during operation. Our work identifies rules and suggests pathways for the design of high-energy-density electrodes based on these phases through the elimination of additives such as high-surface-area carbons.

Reactivity at interfaces between Li-ion battery cathodes and electrolytes imposes limitations to activity and durability. Before solutions can be devised, it is indispensable to precisely define the underlying chemical mechanisms. Interfacial instability is driven by the formation of highly oxidized species on the surface of the cathode particles, but these mechanisms are inherently local and they depend on their morphology and chemistry. A combination of X-ray absorption spectroscopy and X-ray microscopy will be presented to establish the changes undergone by cathode particles at different points of the charge, revealing the consequences of interfacial reactions at very high spatial resolution. Modifying the surface of the cathode particles with redox-inactive cations is an established strategy to prevent these undesired reactions and increase electrode durability generally. Developments will be discussed in the manipulation of chemical assembly to achieve the highest level of sophistication in this approach, through core-(epitaxial) shell architectures at the level of primary particles. The nanostructures are produced via colloidal methods that generate objects with high chemical and morphological definition. As a consequence, they constitute valuable models to evaluate effect of surface modifications on cathode-electrolyte interfacial interactions. The presentation will establish comparisons of the changes at the surface of cathode particles depending on the presence and identity of a passivating shell. These comparisons inform the definition of mechanisms of stabilization, which will be subsequently used to propose further avenues of design towards cathodes with high capability for energy storage with long life.

Chromium oxides with the spinel structure have been predicted to be promising high voltage cathode materials in magnesium batteries. Perennial challenges involving the mobility of Mg2+ and reaction kinetics can be circumvented by nano-sizing the materials in order to reduce diffusion distances, and by using elevated temperatures to overcome activation energy barriers. Herein, ordered 7 nm crystals of spinel-type MgCr2O4 were synthesized by a conventional batch hydrothermal method. In comparison, the relatively underexplored Continuous Hydrothermal Flow Synthesis (CHFS) method was used to make highly defective sub-5 nm MgCr2O4 crystals. When these materials were made into electrodes, they were shown to possess markedly different electrochemical behavior in a Mg2+ ionic liquid electrolyte, at moderate temperature (110 °C). The anodic activity of the ordered nanocrystals was attributed to surface reactions, most likely involving the electrolyte. In contrast, evidence was gathered regarding the reversible bulk deintercalation of Mg2+ from the nanocrystals made by CHFS. This work highlights the impact on electrochemical behavior of a precise control of size and crystal structure of MgCr2O4. It advances the understanding and design of new cathode materials for Mg-based batteries.