An Organic Coprecipitation Route to Synthesize High Voltage LiNi_0.5Mn_1.5O₄

High-voltage cathode material LiNi_0.5Mn_1.5O₄ has been prepared with a novel organic coprecipitation route. The as-prepared sample was compared with samples produced through traditional solid state method and hydroxide coprecipitation method. The morphology was observed by scanning electron microscopy, and the spinel structures were characterized by X-ray diffraction and Fourier transform infrared spectroscopy. Besides the ordered/disordered distribution of Ni/Mn on octahedral sites, the confusion between Li and transition metal is pointed out to be another important factor responsible for the corresponding performance, which is worthy further investigation. Galvanostatic cycles, cyclic voltammetry, and electrochemical impedance spectroscopy are employed to characterize the electrochemical properties. The organic coprecipitation route produced sample shows superior rate capability and stable structure during cycling

²H and ²⁷Al Solid-State NMR Study of the Local Environments in Al-Doped 2‑Line Ferrihydrite, Goethite, and Lepidocrocite

Although substitution of aluminum into iron oxides and oxyhydroxides has been extensively studied, it is difficult to obtain accurate incorporation levels. Assessing the distribution of dopants within these materials has proven especially challenging because bulk analytical techniques cannot typically determine whether dopants are substituted directly into the bulk iron oxide or oxyhydroxide phase or if they form separate, minor phase impurities. These differences have important implications for the chemistry of these iron-containing materials, which are ubiquitous in the environment. In this work, ²⁷Al and ²H NMR experiments are performed on series of Al-substituted goethite, lepidocrocite, and 2-line ferrihydrite in order to develop an NMR method to track Al substitution. The extent of Al substitution into the structural frameworks of each compound is quantified by comparing quantitative ²⁷Al MAS NMR results with those from elemental analysis. Magnetic measurements are performed for the goethite series to compare with NMR measurements. Static ²⁷Al spin–echo mapping experiments are used to probe the local environments around the Al substituents, providing clear evidence that they are incorporated into the bulk iron phases. Predictions of the ²H and ²⁷Al NMR hyperfine contact shifts in Al-doped goethite and lepidocrocite, obtained from a combined first-principles and empirical magnetic scaling approach, give further insight into the distribution of the dopants within these phases

Single-Phase Lithiation and Delithiation of Simferite Compounds Li(Mg,Mn,Fe)PO₄

Understanding the phase transformation behavior of electrode materials for lithium ion batteries is critical in determining the electrode kinetics and battery performance. Here, we demonstrate the lithiation/delithiation mechanism and electrochemical behavior of the simferite compound, LiMg_0.5Fe_0.3Mn_0.2PO₄. In contrast to the equilibrium two-phase nature of LiFePO₄, LiMg_0.5Fe_0.3Mn_0.2PO₄ undergoes a one-phase reaction mechanism as confirmed by ex situ X-ray diffraction at different states of delithiation and electrochemical measurements. The equilibrium voltage measurement by galvanostatic intermittent titration technique shows a continuous change in voltage at Mn³⁺/Mn²⁺ redox couple with addition of Mg²⁺ in LiMn_0.4Fe_0.6PO₄ olivine structure. There is, however, no significant change in the Fe³⁺/Fe²⁺ redox potential

Li₃Mo₄P₅O₂₄: A Two-Electron Cathode for Lithium-Ion Batteries with Three-Dimensional Diffusion Pathways

The structure of the novel compound Li₃Mo₄P₅O₂₄ has been solved from single crystal X-ray diffraction data. The Mo cations in Li₃Mo₄P₅O₂₄ are present in four distinct types of MoO₆ octahedra, each of which has one open vertex at the corner participating in a MoO double bond and whose other five corners are shared with PO₄ tetrahedra. On the basis of a bond valence sum difference map (BVS-DM) analysis, this framework is predicted to support the facile diffusion of Li⁺ ions, a hypothesis that is confirmed by electrochemical testing data, which show that Li₃Mo₄P₅O₂₄ can be utilized as a rechargeable battery cathode material. It is found that Li can both be removed from and inserted into Li₃Mo₄P₅O₂₄. The involvement of multiple redox processes occurring at the same Mo site is reflected in electrochemical plateaus around 3.8 V associated with the Mo⁶⁺/Mo⁵⁺ redox couple and 2.2 V associated with the Mo⁵⁺/Mo⁴⁺ redox couple. The two-electron redox properties of Mo cations in this structure lead to a theoretical capacity of 198 mAh/g. When cycled between 2.0 and 4.3 V versus Li⁺/Li, an initial capacity of 113 mAh/g is observed with 80% of this capacity retained over the first 20 cycles. This compound therefore represents a rare example of a solid state cathode able to support two-electron redox capacity and provides important general insights about pathways for designing next-generation cathodes with enhanced specific capacities

Composition-Structure Relationships in the Li-Ion Battery Electrode Material LiNi_0.5Mn_1.5O₄

A study of the correlations between the stoichiometry, secondary phases, and transition metal ordering of LiNi_0.5Mn_1.5O₄ was undertaken by characterizing samples synthesized at different temperatures. Insight into the composition of the samples was obtained by electron microscopy, neutron diffraction, and X-ray absorption spectroscopy. In turn, analysis of cationic ordering was performed by combining neutron diffraction with Li MAS NMR spectroscopy. Under the conditions chosen for the synthesis, all samples systematically showed an excess of Mn, which was compensated by the formation of a secondary rock-salt phase and not via the creation of oxygen vacancies. Local deviations from the ideal 3:1 Mn:Ni ordering were found, even for samples that show the superlattice ordering by diffraction, with different disordered schemes also being possible. The magnetic behavior of the samples was correlated with the deviations from this ideal ordering arrangement. The in-depth crystal-chemical knowledge generated was employed to evaluate the influence of these parameters on the electrochemical behavior of the materials

Structure Stabilization by Mixed Anions in Oxyfluoride Cathodes for High-Energy Lithium Batteries

Mixed-anion oxyfluorides (<i>i.e.</i>, FeO_<i>x</i>F_2–<i>x</i>) are an appealing alternative to pure fluorides as high-capacity cathodes in lithium batteries, with enhanced cyclability <i>via</i> oxygen substitution. However, it is still unclear how the mixed anions impact the local phase transformation and structural stability of oxyfluorides during cycling due to the complexity of electrochemical reactions, involving both lithium intercalation and conversion. Herein, we investigated the local chemical and structural ordering in FeO_0.7F_1.3 at length scales spanning from single particles to the bulk electrode, <i>via</i> a combination of electron spectrum-imaging, magnetization, electrochemistry, and synchrotron X-ray measurements. The FeO_0.7F_1.3 nanoparticles retain a FeF₂-like rutile structure but chemically heterogeneous, with an F-rich core covered by thin O-rich shell. Upon lithiation the O-rich rutile phase is transformed into Li–Fe–O(−F) rocksalt that has high lattice coherency with converted metallic Fe, a feature that may facilitate the local electronic and ionic transport. The O-rich rocksalt is highly stable over lithiation/delithiation and thus advantageous to maintain the integrity of the particle, and due to its predominant distribution on the surface, it is expected to prevent the catalytic interaction of Fe with electrolyte. Our findings of the structural origin of cycling stability in oxyfluorides may provide insights into developing viable high-energy electrodes for lithium batteries

What Happens to LiMnPO₄ upon Chemical Delithiation?

Olivine MnPO₄ is the delithiated phase of the lithium-ion-battery cathode (positive electrode) material LiMnPO₄, which is formed at the end of charge. This phase is metastable under ambient conditions and can only be produced by delithiation of LiMnPO₄. We have revealed the manganese dissolution phenomenon during chemical delithiation of LiMnPO₄, which causes amorphization of olivine MnPO₄. The properties of crystalline MnPO₄ obtained from carbon-coated LiMnPO₄ and of the amorphous product resulting from delithiation of pure LiMnPO₄ were studied and compared. The phosphorus-rich amorphous phases in the latter are considered to be MnHP₂O₇ and MnH₂P₂O₇ from NMR, X-ray absorption spectroscopy, and X-ray photoelectron spectroscopy analysis. The thermal stability of MnPO₄ is significantly higher under high vacuum than at ambient condition, which is shown to be related to surface water removal

Electrochemical Performance of Nanosized Disordered LiVOPO₄

ε-LiVOPO₄ is a promising multielectron cathode material for Li-ion batteries that can accommodate two electrons per vanadium, leading to higher energy densities. However, poor electronic conductivity and low lithium ion diffusivity currently result in low rate capability and poor cycle life. To enhance the electrochemical performance of ε-LiVOPO₄, in this work, we optimized its solid-state synthesis route using in situ synchrotron X-ray diffraction and applied a combination of high-energy ball-milling with electronically and ionically conductive coatings aiming to improve bulk and surface Li diffusion. We show that high-energy ball-milling, while reducing the particle size also introduces structural disorder, as evidenced by ⁷Li and ³¹P NMR and X-ray absorption spectroscopy. We also show that a combination of electronically and ionically conductive coatings helps to utilize close to theoretical capacity for ε-LiVOPO₄ at C/50 (1 C = 153 mA h g^–1) and to enhance rate performance and capacity retention. The optimized ε-LiVOPO₄/Li₃VO₄/acetylene black composite yields the high cycling capacity of 250 mA h g^–1 at C/5 for over 70 cycles