Formation of an Anti-Core–Shell Structure in Layered Oxide Cathodes for Li-Ion Batteries

The layered → rock-salt phase transformation in the layered dioxide cathodes for Li-ion batteries is believed to result in a “core–shell” structure of the primary particles, in which the core region remains as the layered phase while the surface region undergoes a phase transformation to the rock-salt phase. Using transmission electron microscopy, here we demonstrate the formation of an “anti-core–shell” structure in cycled primary particles with a formula of LiNi_0.80Co_0.15Al_0.05O₂, in which the surface and subsurface regions remain as the layered structure while the rock-salt phase forms as domains in the bulk with a thin layer of the spinel phase between the rock-salt core and the skin of the layered phase. Formation of this anti-core–shell structure is attributed to oxygen loss at the surface that drives the migration of oxygen from the bulk to the surface, thereby resulting in localized areas of significantly reduced oxygen levels in the bulk of the particle, which subsequently undergoes phase transformation to the rock-salt domains. The formation of the anti-core–shell rock-salt domains is responsible for the reduced capacity, discharge voltage, and ionic conductivity in cycled cathodes

Formation of an Anti-Core–Shell Structure in Layered Oxide Cathodes for Li-Ion Batteries

The layered → rock-salt phase transformation in the layered dioxide cathodes for Li-ion batteries is believed to result in a “core–shell” structure of the primary particles, in which the core region remains as the layered phase while the surface region undergoes a phase transformation to the rock-salt phase. Using transmission electron microscopy, here we demonstrate the formation of an “anti-core–shell” structure in cycled primary particles with a formula of LiNi_0.80Co_0.15Al_0.05O₂, in which the surface and subsurface regions remain as the layered structure while the rock-salt phase forms as domains in the bulk with a thin layer of the spinel phase between the rock-salt core and the skin of the layered phase. Formation of this anti-core–shell structure is attributed to oxygen loss at the surface that drives the migration of oxygen from the bulk to the surface, thereby resulting in localized areas of significantly reduced oxygen levels in the bulk of the particle, which subsequently undergoes phase transformation to the rock-salt domains. The formation of the anti-core–shell rock-salt domains is responsible for the reduced capacity, discharge voltage, and ionic conductivity in cycled cathodes

Rock-Salt Growth-Induced (003) Cracking in a Layered Positive Electrode for Li-Ion Batteries

For the first time, (003) cracking is observed and determined to be the major cracking mechanism for the primary particles of Ni-rich layered dioxides as the positive electrode for Li-ion batteries. Using transmission electron microscopy techniques, here we show that the propagation and fracturing of platelet-like rock-salt phase along the (003) plane of the layered oxide are the leading cause for the cracking of primary particles. The fracturing of the rock-salt platelet is induced by the stress discontinuity between the parent layered oxide and the rock-salt phase. The high nickel content is considered to be the key factor for the formation of the rock-salt platelet and thus the (003) cracking. The (003)-type cracking can be a major factor for the structural degradation and associated capacity fade of the layered positive electrode

Structure and Electrochemistry of Vanadium-Modified LiFePO₄

Doping LiFePO₄ with vanadium has proven to enhance electrochemical performance, but the underlying reasons for this improvement are not well understood. To better comprehend the relationships between the electrochemical performance, crystal structure, and surface carbon layer, we prepared vanadium-modified LiFePO₄ by three different methods. The electrochemical performance of each sample was determined via a series of cycling studies, the detailed crystal structures of the doped samples were identified by X-ray diffraction and absorption spectroscopy, and the surface carbon coating was examined by high resolution transmission electron microscopy. In V-modified LiFePO₄ prepared by a modified solid-state reaction, the vanadium is present in an impurity phase at the surface, which improves conductivity but has only a slight improvement in the electrochemical properties. The V-modified LiFePO₄ samples prepared by the conventional solid-state reaction method and a solution method revealed that the vanadium was substituted into the lattice occupying iron sites in the FeO₆ octahedron. This structural modification improves the cycling rate performance by increasing the Li⁺ effective cross-sectional area of the LiO₆ octahedral face and thereby reducing the bottleneck for Li⁺ migration. In addition, analysis of the carbon coating revealed that the material prepared by the solution method forms a uniform carbon coating with a thin, well-ordered interface between the LiFePO₄ and the carbon. The surface properties improve the electronic and ionic conductivities (with respect to the other samples), resulting in a high rate capability (87 mAh g^–1 at 50 C)

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

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

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