Influence of Transition-Metal Order on the Reaction Mechanism of LNMO Cathode Spinel: An Operando X-ray Absorption Spectroscopy Study

KOH-Assisted Molten Salt Route to High-Performance LiNi0.5Mn1.5O4 Cathode Materials

A simple and cost-effective route based on a KOH-assisted molten salt method is designed here to synthesize LiNi0.5Mn1.5O4 spinel. Pure-phase LiNi0.5Mn1.5O4 can be successfully prepared using chlorides as raw materials and adding KOH at 700 °C. The structure, morphology, and performance are discussed in detail. The measurements reveal that using KOH-assisted synthesis can optimize the crystal structure of the obtained LiNi0.5Mn1.5O4 samples, resulting in grain refinement while maintaining the predominantly octahedral structure that grows along the (111) crystal plane. This new synthesis pathway provides excellent performance in terms of cycle life. Electrochemical tests show that the KOH-assisted sample exhibits higher initial specific capacities (124.1 mAh·g-1 at 0.2 C and 111.4 mAh·g-1 at 3 C) and superior cycling performances (capacity retention of 85.0% after 200 cycles at 0.2 C and 95.70% after 100 cycles at 3 C). This provides a potential solution for the practical application of high-voltage LiNi0.5Mn1.5O4 lithium-ion batteries.

Understanding and Controlling Structural Defects and Disordering in LiNi0.5Mn1.5O4 Cathodes for Direct Recycling

Despite significant progress in recycling spent lithium-ion batteries (LIBs), nondestructive, direct recycling methods still face untenable discrepancies in multiple cathode chemistries, which primarily originate from a variety of structure stabilities during the recycling process. Through systematic investigation of the microstructure evolution during the relithiation treatment, we observed the inevitably induced defects and Li/Mn disordering in the LiNi0.5Mn1.5O4 cathode, contributing to the sluggish Li+ transport and irreversible capacity loss. Employing a defect engineering approach to achieve twin boundaries and preferred grain orientation, we show the regenerated cathodes demonstrate a substantial enhancement of Li+ diffusion and cycling stability, retaining 97.4% capacity after 100 cycles and 87.96% after 200 cycles at C/3. This work not only elaborates on a systematic investigation of defect inducement and structural restoration mechanism but also provides an effective approach to directly recycle high-voltage spinel-type cathodes, contributing to the sustainability of next-generation LIBs.

Kinetically controlled synthesis of low-strain disordered micro-nano high voltage spinel cathodes with exposed {111} facets

High-voltage LiNi0.5Mn1.5O4 (LNMO) is one of the most promising cathode candidates for rechargeable lithium-ion batteries (LIBs) but suffers from deteriorated cycling stability due to severe interfacial side reactions and manganese dissolution. Herein, a micro-nano porous spherical LNMO cathode was designed for high-performance LIBs. The disordered structure and the preferred exposure of the {111} facets can be controlled by the release of lattice oxygen in the high-temperature calcination process. The unique configuration of this material could enhance the structural stability and play a crucial role in inhibiting manganese dissolution, promoting the rapid transport of Li+, and reducing the volume strain during the charge/discharge process. The designed cathode exhibits a remarkable discharge capacity of 136.7 mA h g-1 at 0.5C, corresponding to an energy density of up to 636.4 W h kg-1, unprecedented cycling stability (capacity retention of 90.6% after 500 cycles) and superior rate capability (78.9% of initial capacity at 10C). The structurally controllable preparation strategy demonstrated in this work provides new insights into the structural design of cathode materials for LIBs.

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.

Engineering Cathode-Electrolyte Interface of High-Voltage Spinel LiNi0.5Mn1.5O4 via Halide Solid-State Electrolyte Coating

The high-voltage spinel LiNi0.5Mn1.5O4 (LNMO) cathode material with high energy density, low cost, and excellent rate capability has grabbed the attention of the field. However, a high-voltage platform at 4.7 V causes severe oxidative side reactions when in contact with the organic electrolyte, leading to poor electrochemical performance. Furthermore, the contact between the liquid electrolyte and LNMO leads to Mn dissolution during cycles. In this work, we applied the sol-gel method to prepare Li3InCl6-coated LNMO (LIC@LNMO) to address the mentioned problems of LNMO. By introducing a protective layer of halide-type solid-state electrolyte on LNMO, we can prevent direct contact between LNMO and electrolyte while maintaining good ionic conductivity. Thus, we could demonstrate that 5 wt % LIC@LNMO exhibited a good cycle performance with a Coulombic efficiency of 99% and a capacity retention of 80% after the 230th cycle at the 230th cycle at 1C at room temperature.

Dynamic Structure Evolution of Extensively Delithiated High Voltage Spinel Li1+xNi0.5Mn1.5O4 x < 1.5

High voltage spinel is one of the most promising next-generation cobalt-free cathode materials for lithium ion battery applications. Besides the typically utilized compositional range of Li_xNi_0.5Mn_1.5O4 0 < x < 1 in the voltage window of 4.90-3.00 V, additional 1.5 mol of Li per formula unit can be introduced into the structure, in an extended voltage range to 1.50 V. Theoretically, this leads to significant increase of the specific energy from 690 to 1190 Wh/kg. However, utilization of the extended potential window leads to rapid capacity fading and voltage polarization that lack a comprehensive explanation. In this work, we conducted potentiostatic entropymetry, operando XRD and neutron diffraction on the ordered stoichiometric spinel Li_xNi_0.5Mn_1.5O₄ within 0 < x < 2.5 in order to understand the dynamic structure evolution and correlate it with the voltage profile. During the two-phase reaction from cubic (x < 1) to tetragonal (x > 1) phase at ∼2.8 V, we identified the evolution of a second tetragonal phase with x > 2. The structural evaluation during the delithiation indicates the formation of an intermediate phase with cubic symmetry at a lithium content of x = 1.5. Evaluation of neutron diffraction data, with maximum entropy method, of the highly lithiated phase Li_xNi_0.5Mn_1.5O₄ with 2 < x < 2.5 strongly suggests that lithium ions are located on octahedral 8a and tetrahedral 4a positions of the distorted tetragonal phase I4₁amd. Consequently, we were able to provide a conclusive explanation for the additional voltage step at 2.10 V, the sloping voltage profile below 1.80 V, and the additional voltage step at ∼3.80 V.

Enhancing the Electrochemical Performance of High Voltage LiNi0.5Mn1.5O4 Cathode Materials by Surface Modification with Li1.3Al0.3Ti1.7(PO4)3/C

A novel method for surface modification of LiNi_0.5Mn_1.5O₄ (LNMO) was proposed, in which a hybrid layer combined by Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP) and carbon (C) composite on LNMO material were connected by lithium iodide. Structure and morphology analyses illustrated that a higher contact area of active substances was achieved by the LATP/C composite layer without changing the original crystal structure of LNMO. XPS analysis proved that I^- promoted the reduction of trace Mn⁴⁺, resulting in a higher ion conductivity. Galvanostatic charge-discharge tests exhibited the capacity of the LNMO with 5% LATP/C improved with 35.83% at 25 °C and 95.77% at 50 °C, respectively, compared with the bare after 100 cycles, implying the modification of high-temperature deterioration. EIS results demonstrated that one order of magnitude of improvement of the lithium-ion diffusion coefficient of LATP/C-LNMO was achieved (3.04 × 10^-11 S cm^-1). In conclusion, the effective low-temperature modification strategy improved the ionic and electronic conductivities of the cathode and suppressed the side reactions of high-temperature treatment.