Divalent Iron Nitridophosphates: A New Class of Cathode Materials for Li-Ion Batteries

Divalent Iron Nitridophosphates: A New Class of Cathode Materials for Li-Ion Batterie

Oxygen-Release-Related Thermal Stability and Decomposition Pathways of Li_<i>x</i>Ni_0.5Mn_1.5O₄ Cathode Materials

The thermal stability of charged cathode materials is one of the critical properties affecting the safety characteristics of lithium-ion batteries. New findings on the thermal-stability and thermal-decomposition pathways related to the oxygen release are discovered for the high-voltage spinel Li_<i>x</i>Ni_0.5Mn_1.5O₄ (LNMO) with ordered (<i>o</i>-) and disordered (<i>d</i>-) structures at the fully delithiated (charged) state using a combination of in situ time-resolved X-ray diffraction (TR-XRD) coupled with mass spectroscopy (MS) and X-ray absorption spectroscopy (XAS) during heating. Both <i>o</i>- and <i>d</i>- Li_<i>x</i>Ni_0.5Mn_1.5O₄, at their fully charged states, start oxygen-releasing structural changes at temperatures below 300 °C, which is in sharp contrast to the good thermal stability of the 4V-spinel Li_<i>x</i>Mn₂O₄ with no oxygen being released up to 375 °C. This is mainly caused by the presence of Ni⁴⁺ in LNMO, which undergoes dramatic reduction during the thermal decomposition. In addition, charged <i>o</i>-LNMO shows better thermal stability than the <i>d</i>-LNMO counterpart, due to the Ni/Mn ordering and smaller amount of the rock-salt impurity phase in <i>o</i>-LNMO. Two newly identified thermal-decomposition pathways from the initial Li_<i>x</i>Ni_0.5Mn_1.5O₄ spinel to the final NiMn₂O₄-type spinel structure with and without the intermediate phases (NiMnO₃ and α-Mn₂O₃) are found to play key roles in thermal stability and oxygen release of LNMO during thermal decomposition

Correlating Structural Changes and Gas Evolution during the Thermal Decomposition of Charged Li_<i>x</i>Ni_0.8Co_0.15Al_0.05O₂ Cathode Materials

In this work, we present results from the application of a new in situ technique that combines time-resolved synchrotron X-ray diffraction and mass spectroscopy. We exploit this approach to provide direct correlation between structural changes and the evolution of gas that occurs during the thermal decomposition of (over)­charged cathode materials used in lithium-ion batteries. Results from charged Li_<i>x</i>Ni_0.8Co_0.15Al_0.05O₂ cathode materials indicate that the evolution of both O₂ and CO₂ gases are strongly related to phase transitions that occur during thermal decomposition, specifically from the layered structure (space group <i>R</i>3̅<i>m</i>) to the disordered spinel structure (<i>Fd</i>3̅<i>m</i>), and finally to the rock-salt structure (<i>Fm</i>3̅<i>m</i>). The state of charge also significantly affects both the structural changes and the evolution of oxygen as the temperature increases: the more extensive the charge, the lower the temperature of the phase transitions and the larger the oxygen release. Ex situ X-ray absorption spectroscopy (XAS) and in situ transmission electron microscopy (TEM) are also utilized to investigate the local structural and valence state changes in Ni and Co ions, and to characterize microscopic morphology changes. The combination of these advanced tools provides a unique approach to study fundamental aspects of the dynamic physical and chemical changes that occur during thermal decomposition of charged cathode materials in a systematic way

Structural Changes and Thermal Stability of Charged LiNi_<i>x</i>Mn_<i>y</i>Co_<i>z</i>O₂ Cathode Materials Studied by Combined <i>In Situ</i> Time-Resolved XRD and Mass Spectroscopy

Thermal stability of charged LiNi_<i>x</i>Mn_<i>y</i>Co_<i>z</i>O₂ (NMC, with <i>x</i> + <i>y</i> + <i>z</i> = 1, <i>x</i>:<i>y</i>:<i>z</i> = 4:3:3 (NMC433), 5:3:2 (NMC532), 6:2:2 (NMC622), and 8:1:1 (NMC811)) cathode materials is systematically studied using combined <i>in situ</i> time-resolved X-ray diffraction and mass spectroscopy (TR-XRD/MS) techniques upon heating up to 600 °C. The TR-XRD/MS results indicate that the content of Ni, Co, and Mn significantly affects both the structural changes and the oxygen release features during heating: the more Ni and less Co and Mn, the lower the onset temperature of the phase transition (i.e., thermal decomposition) and the larger amount of oxygen release. Interestingly, the NMC532 seems to be the optimized composition to maintain a reasonably good thermal stability, comparable to the low-nickel-content materials (e.g., NMC333 and NMC433), while having a high capacity close to the high-nickel-content materials (e.g., NMC811 and NMC622). The origin of the thermal decomposition of NMC cathode materials was elucidated by the changes in the oxidation states of each transition metal (TM) cations (i.e., Ni, Co, and Mn) and their site preferences during thermal decomposition. It is revealed that Mn ions mainly occupy the 3<i>a</i> octahedral sites of a layered structure (<i>R</i>3̅<i>m</i>) but Co ions prefer to migrate to the 8<i>a</i> tetrahedral sites of a spinel structure (<i>Fd</i>3̅<i>m</i>) during the thermal decomposition. Such element-dependent cation migration plays a very important role in the thermal stability of NMC cathode materials. The reasonably good thermal stability and high capacity characteristics of the NMC532 composition is originated from the well-balanced ratio of nickel content to manganese and cobalt contents. This systematic study provides insight into the rational design of NMC-based cathode materials with a desired balance between thermal stability and high energy density

Structural Changes in Reduced Graphene Oxide upon MnO₂ Deposition by the Redox Reaction between Carbon and Permanganate Ions

We explore structural changes of the carbon in MnO₂/reduced graphene oxide (RGO) hybrid materials prepared by the direct redox reaction between carbon and permanganate ions (MnO₄^–) to reach better understanding for the effects of carbon corrosion on carbon loss and its bonding nature during the hybrid material synthesis. In particular, we carried out near-edge X-ray absorption fine structure spectroscopy at the C K-edge (284.2 eV) to show the changes in the electronic structure of RGO. Significantly, the redox reaction between carbon and MnO₄^– causes both quantitative carbon loss and electronic structural changes upon MnO₂ deposition. Such disruptions of carbon bonding have a detrimental effect on the initial electrical properties of the RGO and thus lead to a significant decrease in electrical conductivity. Electrochemical measurements of the MnO₂/reduced graphene oxide hybrid materials using a cavity microelectrode revealed unfavorable electrochemical properties that were mainly due to the poor electrical conductivity of the hybrid materials. The results of this study should serve as a useful guide to rationally approaching the syntheses of metal/RGO and metal oxide/RGO hybrid materials

Sodiation <i>via</i> Heterogeneous Disproportionation in FeF₂ Electrodes for Sodium-Ion Batteries

Sodium-ion batteries utilize various electrode materials derived from lithium batteries. However, the different characteristics inherent in sodium may cause unexpected cell reactions and battery performance. Thus, identifying the reactive discrepancy between sodiation and lithiation is essential for fundamental understanding and practical engineering of battery materials. Here we reveal a heterogeneous sodiation mechanism of iron fluoride (FeF₂) nanoparticle electrodes by combining <i>in situ/ex situ</i> microscopy and spectroscopy techniques. In contrast to direct one-step conversion reaction with lithium, the sodiation of FeF₂ proceeds <i>via</i> a regular conversion on the surface and a disproportionation reaction in the core, generating a composite structure of 1–4 nm ultrafine Fe nanocrystallites (further fused into conductive frameworks) mixed with an unexpected Na₃FeF₆ phase and a NaF phase in the shell. These findings demonstrate a core–shell reaction mode of the sodiation process and shed light on the mechanistic understanding extended to generic electrode materials for both Li- and Na-ion batteries

Role of Surface Structure on Li-Ion Energy Storage Capacity of Two-Dimensional Transition-Metal Carbides

A combination of density functional theory (DFT) calculations and experiments is used to shed light on the relation between surface structure and Li-ion storage capacities of the following functionalized two-dimensional (2D) transition-metal carbides or MXenes: Sc₂C, Ti₂C, Ti₃C₂, V₂C, Cr₂C, and Nb₂C. The Li-ion storage capacities are found to strongly depend on the nature of the surface functional groups, with O groups exhibiting the highest theoretical Li-ion storage capacities. MXene surfaces can be initially covered with OH groups, removable by high-temperature treatment or by reactions in the first lithiation cycle. This was verified by annealing f-Nb₂C and f-Ti₃C₂ at 673 and 773 K in vacuum for 40 h and <i>in situ</i> X-ray adsorption spectroscopy (XAS) and Li capacity measurements for the first lithiation/delithiation cycle of f-Ti₃C₂. The high-temperature removal of water and OH was confirmed using X-ray diffraction and inelastic neutron scattering. The voltage profile and X-ray adsorption near edge structure of f-Ti₃C₂ revealed surface reactions in the first lithiation cycle. Moreover, lithiated oxygen terminated MXenes surfaces are able to adsorb additional Li beyond a monolayer, providing a mechanism to substantially increase capacity, as observed mainly in delaminated MXenes and confirmed by DFT calculations and XAS. The calculated Li diffusion barriers are low, indicative of the measured high-rate performance. We predict the not yet synthesized Cr₂C to possess high Li capacity due to the low activation energy of water formation at high temperature, while the not yet synthesized Sc₂C is predicted to potentially display low Li capacity due to higher reaction barriers for OH removal

Correction to Anomalous Pseudocapacitive Behavior of a Nanostructured, Mixed-Valent Manganese Oxide Film for Electrical Energy Storage

Correction to Anomalous Pseudocapacitive Behavior of a Nanostructured, Mixed-Valent Manganese Oxide Film for Electrical Energy Storag

Structures of Delithiated and Degraded LiFeBO₃, and Their Distinct Changes upon Electrochemical Cycling

Lithium iron borate (LiFeBO₃) has a high theoretical specific capacity (220 mAh/g), which is competitive with leading cathode candidates for next-generation lithium-ion batteries. However, a major factor making it difficult to fully access this capacity is a competing oxidative process that leads to degradation of the LiFeBO₃ structure. The pristine, delithiated, and degraded phases of LiFeBO₃ share a common framework with a cell volume that varies by less than 2%, making it difficult to resolve the nature of the delithiation and degradation mechanisms by conventional X-ray powder diffraction studies. A comprehensive study of the structural evolution of LiFeBO₃ during (de)­lithiation and degradation was therefore carried out using a wide array of bulk and local structural characterization techniques, both in situ and ex situ, with complementary electrochemical studies. Delithiation of LiFeBO₃ starts with the production of Li_<i>t</i>FeBO₃ (<i>t</i> ≈ 0.5) through a two-phase reaction, and the subsequent delithiation of this phase to form Li_{<i>t</i>–<i>x</i>}FeBO₃ (<i>x</i> < 0.5). However, the large overpotential needed to drive the initial two-phase delithiation reaction results in the simultaneous observation of further delithiated solid-solution products of Li_{<i>t</i>–<i>x</i>}FeBO₃ under normal conditions of electrochemical cycling. The degradation of LiFeBO₃ also results in oxidation to produce a Li-deficient phase D-Li_<i>d</i>FeBO₃ (<i>d</i> ≈ 0.5, based on the observed Fe valence of ∼2.5+). However, it is shown through synchrotron X-ray diffraction, neutron diffraction, and high-resolution transmission electron microscopy studies that the degradation process results in an irreversible disordering of Fe onto the Li site, resulting in the formation of a distinct degraded phase, which cannot be electrochemically converted back to LiFeBO₃ at room temperature. The Li-containing degraded phase cannot be fully delithiated, but it can reversibly cycle Li (D-Li_{<i>d</i>+<i>y</i>}FeBO₃) at a thermodynamic potential of ∼1.8 V that is substantially reduced relative to the pristine phase (∼2.8 V)

Ionic Conduction in Cubic Na₃TiP₃O₉N, a Secondary Na-Ion Battery Cathode with Extremely Low Volume Change

It is demonstrated that Na ions are mobile at room temperature in the nitridophosphate compound Na₃TiP₃O₉N, with a diffusion pathway that is calculated to be fully three-dimensional and isotropic. When used as a cathode in Na-ion batteries, Na₃TiP₃O₉N has an average voltage of 2.7 V vs Na⁺/Na and cycles with good reversibility through a mechanism that appears to be a single solid solution process without any intermediate plateaus. X-ray and neutron diffraction studies as well as first-principles calculations indicate that the volume change that occurs on Na-ion removal is only about 0.5%, a remarkably small volume change given the large ionic radius of Na⁺. Rietveld refinements indicate that the Na1 site is selectively depopulated during sodium removal. Furthermore, the refined displacement parameters support theoretical predictions that the lowest energy diffusion pathway incorporates the Na1 and Na3 sites while the Na2 site is relatively inaccessible. The measured room temperature ionic conductivity of Na₃TiP₃O₉N is substantial (4 × 10^–7 S/cm), though both the strong temperature dependence of Na-ion thermal parameters and the observed activation energy of 0.54 eV suggest that much higher ionic conductivities can be achieved with minimal heating. Excellent thermal stability is observed for both pristine Na₃TiP₃O₉N and desodiated Na₂TiP₃O₉N, suggesting that this phase can serve as a safe Na-ion battery electrode. Moreover, it is expected that further optimization of the general cubic framework of Na₃TiP₃O₉N by chemical substitution will result in thermostable solid state electrolytes with isotropic conductivities that can function at temperatures near or just above room temperature