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 by the addition of surface-protecting agents, such as MgCl2. Hence, ethereal solutions of Mg(TFSI)2 salt with MgCl2 as an additive have been suggested as a representative non-Grignard Mg electrolyte. In this work, the degradation mechanisms of a Mg metal anode in the TFSI-based electrolyte were studied using a current density of 1 mA cm–2 and an areal capacity of ∼0.4 mAh cm–2, which is close to those used in practical applications. The degradation mechanisms identified include the corrosion of Mg metal, which causes the loss of electronic pathways and mechanical integrity, the nonuniform deposition of Mg, and the decomposition of TFSI– anions. This study not only represents an assessment of the behavior of Mg metal anodes at practical current density and areal capacity but also details the outcomes of interfacial passivation, which was detected by simple cyclic voltammetry experiments. This study also points out the absolute absence of any passivation at the electrode–electrolyte interface for the premise of developing electrolytes compatible with a metal anode.

Undesired reactions at electrode/electrolyte interfaces impose challenges in the durability of Li-ion battery. Traditional strategies of interfacial stabilization involve coating with inactive oxide films on aggregated powders of active cathode oxides. Despite generating gains in electrode performance, the lack of control of film growth of existing methods limits the ability to design its chemical structure and enhance functionality. The complexity of these coated materials also complicates efforts to define the specific chemical and structural features that determine function. Core–shell heterostructures at the nanocrystal level offer opportunities for precise control of chemistry and homogeneity. This ability is demonstrated with the compositional and structural tailoring of passivating layers based on Al3+, grown conformally onto LiCoO2 nanoplates, using thermal treatments. They result in heterostructures from core–shell (LiCoO2 nanoplates@2 nm aluminum oxide) to LiCo1–xAlxO2 gradient structures composed by an Al-rich outer layer on a Co-rich core. While all samples presented improvements in electrochemical performance compared to the bare material, the LiCo1–xAlxO2 gradient heterostructure presented the greatest advantage compared to pure aluminum oxide shells. The presence of a high Al/Co ratio at the surface, combined with the structural epitaxy and presence of Li throughout the particle, was considered to be critical to the best electrode properties and electrode/electrolyte interface stabilization. This work advances our ability to build complex heterostructures that both offer engineering solutions and create novel fundamental insight into the origins of battery durability.

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 LiFePO4 crystals equilibrated after delithiation, SXDM revealed the existence of domains of miscibility between LiFePO4 and Li0.6FePO4. These solid solutions are conventionally thought to be metastable, and were previously undetected by spectromicroscopy. The observation provides experimental verification of predictions that the LiFePO4–FePO4 phase diagram can be altered by coherency strain under certain interfacial orientations. It enriches our understanding of the interaction between diffusion, chemistry, and mechanics in solid state transformations.Keywords: Li-ion battery materials; LiFePO4; nanoscale chemical imaging; redox phase transitions;

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 increases our understanding of the intercalation chemistry of ternary fluorides, which could inform efforts toward the exploration of new electrode materials.

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.

Hybrid supercapacitors that follow a “rocking-chair”-type mechanism were developed by coupling divalent metal and activated carbon electrodes in nonaqueous electrolytes. Conventional supercapacitors require a large amount of electrolyte to provide a sufficient quantity of ions to the electrodes, due to their Daniell-type mechanism that depletes the ions from the electrolyte while charging. The alternative “rocking-chair”-type mechanism effectively enhances the energy density of supercapacitors by minimizing the necessary amount of electrolyte, because the ion is replenished from the metal anode while it is adsorbed to the cathode. Newly developed nonaqueous electrolytes for Mg and Zn electrochemistry, based on bis(trifluoromethylsulfonyl)imide (TFSI) salts, made the metal hybrid supercapacitors possible by enabling reversible deposition on the metal anodes and reversible adsorption on an activated carbon cathode. Factoring in gains through the cell design, the energy density of the metal hybrid supercapacitors is projected to be a factor of 7 higher than conventional devices thanks to both the “rocking-chair”-type mechanism that minimizes total electrolyte volume and the use of metal anodes, which have substantial merits in capacity and voltage. Self-discharge was also substantially alleviated compared to conventional supercapacitors. This concept offers a route to build supercapacitors that meet dual criteria of power and energy densities with a simple cell design.Keywords: energy density; metal anodes; self-discharge; supercapacitors; volume of electrolyte; “rocking chair”-type mechanism

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 second wave of propagation starting with regions in closest contact with the current collector and progressing toward regions in contact with the bulk electrode. These results emphasize the interplay between the electronic and ionic conductivities of the phases involved in a battery reaction and the formation of the phases in localized areas in the electrode architecture. They provide new insights that could be used to refine the design of these architectures to minimize transport limitations while maximizing energy density.

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 performance of LiFePO4 nanoparticles. These results demonstrate the importance of engineering the active electrode material in next generation electrical energy storage systems, which will achieve theoretical limits of energy density and extended stability. This work establishes soft X-ray ptychographic chemical imaging as an essential tool to build comprehensive relationships between mechanics and chemistry that guide this engineering design.

Chemical degradation at electrode/electrolyte interfaces in high-energy storage devices, such as Li-ion batteries, imposes durability challenges that affect their life and cost. In oxide electrodes, degradation is linked to the presence of redox active transition metals at the surface. Here, we demonstrate a strategy toward the stabilization of interfaces using core–epitaxial shell nanocrystals. The core of the nanocrystal is composed of an electroactive oxide, which is passivated by an ultrathin epitaxial oxide shell enriched in a redox inactive ion. This approach imparts interfacial stability while preserving the high storage capability and fast carrier transport of the material, compared to unmodified versions. The validity of the concept is proved with Li1+xMn2–xO4 nanocrystals with a 1–2 nm Al-rich shell, which showed reduced sensitivity to harsh environments, compared to bare counterparts. The approach is generalizable to any transition-metal-based battery system where electrode–electrolyte interactions must be controlled.

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 electrode exhibited a discharge capacity of 92 mAh/g at C/3 rate. The chemical underpinnings of stable performance were revealed by soft X-ray absorption spectroscopy. First, both manganese and nickel were detected on the graphite electrode surfaces, and their oxidation states were determined as +2. Second, the ultrathin coatings on the anode alone were found to be sufficient to significantly reduce this deleterious process.

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 interfaces. This magnitude was comparable in all the materials at room temperature. In contrast, a significant drop in efficiency was observed in the untreated material when the cycling temperature was raised to 50 °C, but not in the modified materials. The origin of the reduced reactivity of the materials after introducing Mg-based modifications was evaluated by probing the chemical changes at the Ni–O bonds using soft XAS. Taken together, the results of this study revealed that incorporation of Mg stabilizes highly oxidized Ni–O species, which can be related to the better stability toward the electrolyte. They point to a pathway toward the guided design of efficient surface modifications to yield battery electrode materials with increased stability against the electrolyte.

A series of layered oxides within the NaxNiix/2Mn1–x/2O2 (2/3 ≤ x ≤ 1) system were synthesized by classical solid-state methodologies. A study of their long and short-range structure was undertaken by combining X-ray diffraction and NMR spectroscopy. A transition from P2 to O3 stacking was observed at x > 0.8 when samples were made at 900 °C, which was accompanied by disordering of ions in the transition metal layer. The magnetic properties of the materials were consistent with this picture of ordering, with all samples showing antiferromagnetic character. At x = 2/3, competition between a P2 and a P3 structure, with different degrees of transition metal ordering, was found depending on the synthesis temperature. Na/Li exchange led to structures with octahedral or tetrahedral coordination of the alkali metal, and Li/Ni crystallographic exchange in the resulting O3 phases. The transition from alkali metal prismatic coordination to octahedral/tetrahedral coordination involves [TMO6]∞ layer shearing that induces some structural disorder through the formation of stacking faults.

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.