Synthesis and electrochemical properties of cation‐disordered rock‐salt x Li ₃ NbO ₄ ·(1 − x ) NiO compounds for Li‐ion batteries

Abstract

In this study, Li-excess cation-disordered rock-salt cathode materials, xLi 3 NbO 4 Á(1 − x) NiO (LNNO, 0.55 ≤ x ≤ 0.7), are synthesized to explore the effect of composition on the structure and electrochemical performance using a sol-gel method for the first time. X-ray diffraction results combined with Rietveld refinements demonstrate that pure cation-disordered rock-salt structure can be obtained at x ≥ 0.6. The electron microscopy results of samples demonstrate the irregular particles with size of 1 to 10 μm and rock-salt structure is further confirmed. The samples with x = 0.6, 0.65, and 0.7, respectively, exhibit the much higher reversible capacity than their theoretical Ni 2+ /Ni 3+ capacity attributing to anionic redox reaction. The LNNO6 sample deliver the first discharge capacity 155.35 mAh/g and 82.9% capacity retention after 40 cycles. In addition, X-ray photoelectron spectroscopy analyses and dV/dQ measurements are utilized to investigate the restricted electrochemical behaviors and performances which are constrained by the competition between Ni 2+ /Ni 3+ and O 2− /O −. K E Y W O R D S cation-disordered, electrochemical performance, lithium ion battery, Li 3 NbO 4-based rock-salt structure 1 | INTRODUCTION With the advantages of low pollution, high energy density, and long cycle life, lithium-ion batteries have become the most popular rechargeable batteries in portable electronics and vehicles. 1-5 To solve the crisis of energy resource and environmental pollution, some promising materials, such as transition metal (TM) oxides, metal-organic frameworks (MOFs), and Si-based materials are designed for the areas of lithium batteries, catalysis, supercapacitor, and optical devices. 6-16 Therefore, aiming at the application of Li-ion batteries for advanced energy powers, researchers around the world have paid a lot of efforts to develop cathode materials, especially the ordered TM oxides, such as LiMO 2 and Li 1 + x M 1 − x O 2 (M = Mn, Ni, Co, etc.). 17-19 It was generally accepted in the past several decades that wellordered structure between Li and the TM sublattice is good for Li diffusion and then improve the electrochemical performance effectively. However, most of layered cathode materials suffer from instability and oxygen release in the charging process as the result of cations mixing and phase change. Recently, cation-disordered materials have attracted more attention because of unique lithium migration

Research Progress in Lithium‐Excess Disordered Rock‐Salt Oxides Cathode

“…Oxygen oxidation was first proved by the reversible O 2− /O − redox chemical and the peroxide speciation observed through a series of in situ tests including in situ Raman spectrum. [ 45 ] However, although the increase of Li content provides more extra capacities from reversible oxygen oxidation, [ 41 ] there are still existing issues of (i) overpotential caused by unstable and slow kinetics of O 2− /O − redox [ 42 ] and (ii) the generation of irreversible O 2 and side reactions between oxidized oxygen species and electrolyte during the first charge. The former issue can be improved by cationic substitution or pre‐cycling treatment, while the latter can be resolved by fluorination and surface coating.…”

“…Incomplete oxidation of Ni is due to the strong overlap between Ni 3d and O 2p redox states or the overlap between Li-O-Li and Ni 2+ /Ni 4+ bands (Figure 2c). [41,42,45] It was analyzed that a decrease in lattice parameters by cationic substitution can reduce the overlap to enhance Ni 2+ /Ni 4+ . [29,30] Li 3 NbO 4 was introduced to LiNi 0.5 Ti 0.5 O 2 to achieve smaller lattice parameters than the same lithium content Li-Ni-Ti-O compounds, [43] whose capacity is shown in Figure 2d,e.…”

“…Incomplete oxidation of Ni is due to the strong overlap between Ni 3d and O 2p redox states or the overlap between Li–O–Li and Ni 2+ /Ni 4+ bands (Figure 2c). [ 41,42,45 ] It was analyzed that a decrease in lattice parameters by cationic substitution can reduce the overlap to enhance Ni 2+ /Ni 4+ . [ 29,30 ]…”

Realising higher capacity and stability for disordered rocksalt oxyfluoride cathode materials for Li ion batteries

“…13 Almost all of these DRXs contain a type of TMs such as Ni, Mn and V with an unlled outer shell electron orbital, and another type of d 0 elements that are TM elements with a lled outer shell electron orbital such as Ti 4+ , Nb 5+ , Ru 5+ , Mo 6+ . [14][15][16][17][18][19] The TMs with an unlled outer shell electron orbital are redox centres that contribute to capacities, whereas the d 0 elements do not contribute to the overall capacity, but play a crucial role in stabilising the crystal structures to accommodate distorted lattice sites due to their smaller sizes and higher charges. DRX materials present exible compositional space, their stoichiometry is not limited to the explored compositions so far.…”