High-Voltage Spinel Cathode Materials: Navigating the Structural Evolution for Lithium-Ion Batteries

High-voltage LiNi0.5Mn1.5O4 (LNMO) spinel oxides are highly promising cobalt-free cathode materials to cater to the surging demand for lithium-ion batteries (LIBs). However, commercial application of LNMOs is still challenging despite decades of research. To address the challenge, the understanding of their crystallography and structural evolutions during synthesis and electrochemical operation is critical. This review aims to illustrate and to update the fundamentals of crystallography, phase transition mechanisms, and electrochemical behaviors of LNMOs. First, the research history of LNMO and its development into a LIB cathode material is outlined. Then the structural basics of LNMOs including the classic and updated views of the crystal polymorphism, interconversion between the polymorphs, and structure-composition relationship is reviewed. Afterward, the phase transition mechanisms of LNMOs that connect structural and electrochemical properties are comprehensively discussed from fundamental thermodynamics to operando dynamics at intra- and inter-particle levels. In addition, phase evolutions during overlithiation as well as thermal-/electrochemical-driven phase transformations of LNMOs are also discussed. Finally, recommendations are offered for the further development of LNMOs as well as other complex materials to unlock their full potential for future sustainable and powerful batteries.

Identifying Element-Modulated Reactivity and Stability of the High-Voltage Spinel Cathode Materials via In Situ Time-Resolved X-Ray Diffraction

Designing and identifying a dopant-involved material is quite significant, especially for battery science. LiNi_0.5Mn_1.5O₄, being one of the most appealing candidates for high-potential lithium-ion batteries, has attracted immense attention and been investigated with Al or F dopants for its undesirable inherent structural challenges. Although the excellent performance of Al- or F-doped LiNi_0.5Mn_1.5O₄ has been reported previously, the relationship between dopants, structural variation, and electrochemistry has not been fully identified. Hence, synchronous time-resolved XRD techniques are applied for identifying a guideline of the phase variations in cathodic (Al³⁺)- and anodic (F^-)-substituted LiNi_0.5Mn_1.5O₄, which revealed a three-phase evolution as a function of structural stability. Also, the Al-substituted materials exhibit excellent reactivity and stability, which can be clearly identified via the stable buffer phase existing in high power density or after long cycling due to the improvement in reaction kinetics of phase transition and the lithium-ion diffusion coefficient, just opposite to F doping. This provides a good guideline for identifying an element-modulated mechanism of reactivity or stability of materials science.

Review of Recent Development of In Situ/Operando Characterization Techniques for Lithium Battery Research

The increasing demands of energy storage require the significant improvement of current Li-ion battery electrode materials and the development of advanced electrode materials. Thus, it is necessary to gain an in-depth understanding of the reaction processes, degradation mechanism, and thermal decomposition mechanisms under realistic operation conditions. This understanding can be obtained by in situ/operando characterization techniques, which provide information on the structure evolution, redox mechanism, solid-electrolyte interphase (SEI) formation, side reactions, and Li-ion transport properties under operating conditions. Here, the recent developments in the in situ/operando techniques employed for the investigation of the structural stability, dynamic properties, chemical environment changes, and morphological evolution are described and summarized. The experimental approaches reviewed here include X-ray, electron, neutron, optical, and scanning probes. The experimental methods and operating principles, especially the in situ cell designs, are described in detail. Representative studies of the in situ/operando techniques are summarized, and finally the major current challenges and future opportunities are discussed. Several important battery challenges are likely to benefit from these in situ/operando techniques, including the inhomogeneous reactions of high-energy-density cathodes, the development of safe and reversible Li metal plating, and the development of stable SEI.

Phase transformation mechanism in lithium manganese nickel oxide revealed by single-crystal hard X-ray microscopy

Understanding the reaction pathway and kinetics of solid-state phase transformation is critical in designing advanced electrode materials with better performance and stability. Despite the first-order phase transition with a large lattice mismatch between the involved phases, spinel LiMn_1.5Ni_0.5O₄ is capable of fast rate even at large particle size, presenting an enigma yet to be understood. The present study uses advanced two-dimensional and three-dimensional nano-tomography on a series of well-formed Li_xMn_1.5Ni_0.5O₄ (0≤x≤1) crystals to visualize the mesoscale phase distribution, as a function of Li content at the sub-particle level. Inhomogeneity along with the coexistence of Li-rich and Li-poor phases are broadly observed on partially delithiated crystals, providing direct evidence for a concurrent nucleation and growth process instead of a shrinking-core or a particle-by-particle process. Superior kinetics of (100) facets at the vertices of truncated octahedral particles promote preferential delithiation, whereas the observation of strain-induced cracking suggests mechanical degradation in the material.

As an intercalation electrode material for lithium ion batteries, spinel Li_xMn_1.5Ni_0.5O₄ possesses a metastable nature during the electrochemical operation. Here the authors reveal the phase transformation mechanism by using single-crystal hard X-ray microscopy to detect the local phase distribution.

High-voltage positive electrode materials for lithium-ion batteries

The ever-growing demand for advanced rechargeable lithium-ion batteries in portable electronics and electric vehicles has spurred intensive research efforts on high-voltage positive electrode materials over the past decade.