Evolution of the local structure and electrochemical properties of spinel LiNixMn2−xO4 (0≤x≤0.5)

Conformal Coating of a High-Voltage Spinel to Stabilize LiCoO2 at 4.6 V

The ever-growing demand for portable electronic devices has put forward higher requirements on the energy density of layered LiCoO₂ (LCO). The unstable surface structure and side reactions with electrolytes at high voltages (>4.5 V) however hinder its practical applications. Here, considering the high-voltage stability and three-dimensional lithium-ion transport channel of the high-voltage Li-containing spinel (M = Ni and Co) LiM_xMn_2-xO₄, we design a conformal and integral LiNi_xCo_yMn_2-x-yO₄ spinel coating on the surface of LCO via a sol-gel method. The accurate structure of the coating layer is identified to be a spinel solid solution with gradient element distribution, which compactly covers the LCO particle. The coated LCO exhibits significantly improved cycle performance (86% capacity remained after 100 cycles at 0.5C in 3-4.6 V) and rate performance (150 mAh/g at a high rate of 5C). The characterizations of the electrodes from the bulk to surface suggest that the conformal spinel coating acts as a physical barrier to inhibit the side reactions and stabilize the cathode-electrolyte interface (CEI). In addition, the artificially designed spinel coating layer is well preserved on the surface of LCO after prolonged cycling, preventing the formation of an electrochemically inert Co₃O₄ phase and ensuring fast lithium transport kinetics. This work provides a facile and effective method for solving the surface problems of LCO operated at high voltages.

Improved Electrochemical Performance for High-Voltage Spinel LiNi0.5Mn1.5O4 Modified by Supercritical Fluid Chemical Deposition

Among candidates at the positive electrode of the next generation of Li-ion technology and even beyond post Li-ion technology as all-solid-state batteries, spinel LiNi_0.5Mn_1.5O₄ (LNMO) is one of the favorites. Nevertheless, before its integration into commercial systems, challenges still remain to be tackled, especially the stabilization of interfaces with the electrolyte (liquid or solid) at high voltage. In this work, a simple, fast, and cheap process is used to prepare a homogeneous coating of Al₂O₃ type to modify the surface of the spinel LNMO: the supercritical fluid chemical deposition (SFCD) route. This process is, to the best of our knowledge, used for the first time in the battery field. Significantly improved performance was demonstrated vs those of bare LNMO, especially at high rates and for highly loaded electrodes.

Phonon Structure, Infra-Red and Raman Spectra of Li2MnO3 by First-Principles Calculations

The layer-structured monoclinic Li₂MnO₃ is a key material, mainly due to its role in Li-ion batteries and as a precursor for adsorbent used in lithium recovery from aqueous solutions. In the present work, we used first-principles calculations based on density functional theory (DFT) to study the crystal structure, optical phonon frequencies, infra-red (IR), and Raman active modes and compared the results with experimental data. First, Li₂MnO₃ powder was synthesized by the hydrothermal method and successively characterized by XRD, TEM, FTIR, and Raman spectroscopy. Secondly, by using Local Density Approximation (LDA), we carried out a DFT study of the crystal structure and electronic properties of Li₂MnO₃. Finally, we calculated the vibrational properties using Density Functional Perturbation Theory (DFPT). Our results show that simulated IR and Raman spectra agree well with the observed phonon structure. Additionally, the IR and Raman theoretical spectra show similar features compared to the experimental ones. This research is useful in investigations involving the physicochemical characterization of Li₂MnO₃ material.

Surface LiMn1.4Ni0.5Mo0.1O4 Coating and Bulk Mo Doping of Li-Rich Mn-Based Li1.2Mn0.54Ni0.13Co0.13O2 Cathode with Enhanced Electrochemical Performance for Lithium-Ion Batteries

To improve the initial Coulombic efficiency, cycling stability, and rate performance of the Li-rich Mn-based Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ cathode, the combination of LiMn_1.4Ni_0.5Mo_0.1O₄ coating with Mo doping has been successfully carried out by the sol-gel method and subsequent dip-dry process. This strategy buffers the electrodes from the corrosion of electrolyte and enhances the lattice parameter, which could inhibit the oxygen release and maintain the structural stability, thus improving the cycle stability and rate capability. After LiMn_1.4Ni_0.5Mo_0.1O₄ modification, the initial discharge capacity reaches 272.4 mAh g^-1 with a corresponding initial Coulombic efficiency (ICE) of 84.2% at 0.1C (1C = 250 mAh g^-1), far higher than those (221.5 mAh g^-1 and 68.9%) of the pristine sample. Besides, the capacity retention of the coated sample is enhanced by up to 66.8% after 200 cycles at 0.1C. Especially, the rate capability of the coated sample is 95.2 mAh g^-1 at 5C. XRD, SEM, TEM, XPS, and Raman spectroscopy are adopted to characterize the morphologies and structures of the samples. This coating strategy has been demonstrated to be an effective approach to construct high-performance energy storage devices.

Graphene modulated LiMn1.5Ni0.4Cr0.1O4 spinel cathode for lithium ion battery

We report the development of a rechargeable lithium ion battery with homogeneous mixing of 10% of graphene oxide in active LiMn1.5Ni0.4Cr0.1O4 cathode material for enhanced electrochemical performance. The redox behavior of the cell, which is normally too slow for practical applications, is accelerated with highly conductive graphene. Intimate mixing of the two materials is achieved by a slurry maker using an organic solution for a cathode paste. The fabricated electrode is repeatedly charged/discharge at C/10 and C/2 rates without any significant degradation in the electrochemical capacity. The gravimetric energy density of the composite cathode material exceeds that of the LiMn1.5Ni0.4Cr0.1O4 oxide electrode in lithium-ion batteries, and the addition of graphene in active material is likely to prove advantageous in applications where weight, rather than volume, is a critical factor.

Broken translational and rotational symmetries in LiMn1.5Ni0.5O4 spinel

In condensed matter physics broken symmetries and emergence of quasi-particles are intimately linked to each other. Whenever a symmetry is broken, it leaves its fingerprints, and that may be observed indirectly via its influence on the other quasi-particles. Here, we report the strong signature of broken spin rotational symmetry induced due to long range-ordering of spins in Mn - sublattice of LiMn_1.5Ni_0.5O₄ below T _c ~ 113 K reflected with the marked changes in the lattice vibrations using Raman scattering. In particular, the majority of the observed first-order phonon modes show a sharp shift in frequency in the vicinity of long range magnetic-ordering temperature. Phonons exist in a crystalline system because of broken translational symmetry, therefore any renormalization in the phonon-spectrum could be a good gauge for broken translational symmetry. Anomalous evolution of the few modes associated with stretching of Mn/NiO₆ octahedra in the intermediate temperature range (~60-260 K) marked the broken translational symmetry attributed to the charge ordering. Interestingly same modes also show strong coupling with magnetic degrees of freedom, suggesting that charge-ordering and magnetic transition may be linked to each other.

A Bifunctional Electrolyte Additive for High-Voltage LiNi0.5Mn1.5O4 Positive Electrodes

4-(Trimethylsiloxy)-3-pentene-2-one (TMSPO) is tested as an electrolyte additive to enhance Coulombic efficiency and cycle retention for the Li/LiNi_0.5Mn_1.5O₄ (LNMO) half-cell and graphite/LNMO full-cell. TMSPO carries two functional groups, siloxane (-Si-O-) and carbon-carbon (C═C) double bonds. It is found that the siloxane group reacts with hydrogen fluoride (HF), which is generated by hydrolysis of lithium hexafluorophosphate (LiPF₆) by impure water in the electrolyte solution, to produce 4-hydroxypent-3-ene-2-one (HPO). The as-generated HPO, as well as TMSPO itself, is electrochemically oxidized to form a protective surface film on the LNMO electrode, in which it is inferred that the carbon-carbon (C═C) double bond initiates radical polymerization. The surface film derived from the TMSPO-added electrolyte shows a superior passivating ability to that generated from the pristine (TMSPO-free) electrolyte. The suppression of electrolyte oxidation enabled by the superior passivating ability offers two beneficial features to the half-cells and full-cells: the suppression of both HF generation and deposition of the resistive surface film on LNMO. As a result, the metal dissolution by HF attack on LNMO appears to be smaller by the addition of TMSPO. The cell polarization is also less significant because of the latter beneficial feature. In short, the bifunctional activity of TMSPO (HF scavenger and protective film former) allows an enhanced Coulombic efficiency and cycle retention to the half-cell and full-cell.

Constructing a Heterostructural LiNi0.4Mn1.6O4-δ Material from Concentration-Gradient Framework to Significantly Improve Its Cycling Performance

A heterostructural LiNi_0.35Mn_1.65O_4-δ-LiNi_0.5Mn_1.5O₄ was achieved via constructing the concentration-gradient material with an average composition of LiNi_0.4Mn_1.6O_4-δ. The as-obtained samples were characterized by cross-sectional scanning electron microscopy and energy dispersive X-ray spectrometry, and it was found that the core material LiNi_0.35Mn_1.65O_4-δ is encapsulated completely by a concentration-gradient shell with the outmost layer of LiNi_0.5Mn_1.5O₄. Demonstrated by high resolution transmission electron microscopy and X-ray photoelectron spectroscopy analysis with a low-energy Ar neutral beam etching method, the structurally stable P4₃32 phase on the surface coordinates the high conductive Fd3̅m phase in the bulk without any detectable interface. At room temperature, the heterostructural samples deliver the initial discharge capacity of 145.1 mAhg^-1 at 1 C, close to the theoretical capacity, and the capacity retention after 300 cycles was up to 96.5% of the first discharge capacity. Moreover, it shows excellent rate capability with discharge capacity of 144.3 mAhg^-1 at 10 C and significantly improved cycling stability with capacity retention of 85.51% over 50 cycles between 3.0 and 4.95 V at 55 °C. The as-obtained material with coordination of two crystallographic structure domains is a promising cathode to develop high energy, high power, long lifespan, and low cost lithium-ion batteries.

Lithium chromium pyrophosphate as an insertion material for Li-ion batteries

Lithium chromium pyrophosphate (LiCrP2O7) and carbon-coated LiCrP2O7 (LiCrP2O7/C) were synthesized by solid-state and sol–gel routes, respectively. The materials were characterized by X-ray diffraction (XRD), thermogravimetric analysis (TGA), scanning electron microscopy (SEM) and conductivity measurements. LiCrP2O7 powder has a conductivity of ∼ 10−8 S cm−1, ∼ 104 times smaller than LiCrP2O7/C (∼ 10−4 S cm−1). LiCrP2O7/C is electrochemically active, mainly between 1.8 and 2.2 V versus Li+/Li (Cr3+/Cr2+ redox couple), whereas LiCrP2O7 has limited electrochemical activity. LiCrP2O7/C delivers a reversible specific charge up to ∼ 105 mAh g−1 after 100 cycles, close to the theoretical limit of 115 mAh g−1. Operando XRD experiments show slight peak shifts between 2.2 and 4.8 V versus Li+/Li, and a reversible amorphization between 1.8 and 2.2 V versus Li+/Li, suggesting an insertion reaction mechanism.

A study of room-temperature LixMn1.5Ni0.5O4 solid solutions

Understanding the kinetic implication of solid-solution vs. biphasic reaction pathways is critical for the development of advanced intercalation electrode materials. Yet this has been a long-standing challenge in materials science due to the elusive metastable nature of solid solution phases. The present study reports the synthesis, isolation, and characterization of room-temperature LixMn1.5Ni0.5O4 solid solutions. In situ XRD studies performed on pristine and chemically-delithiated, micron-sized single crystals reveal the thermal behavior of LixMn1.5Ni0.5O4 (0 ≤ x ≤ 1) cathode material consisting of three cubic phases: LiMn1.5Ni0.5O4 (Phase I), Li0.5Mn1.5Ni0.5O4 (Phase II) and Mn1.5Ni0.5O4 (Phase III). A phase diagram capturing the structural changes as functions of both temperature and Li content was established. The work not only demonstrates the possibility of synthesizing alternative electrode materials that are metastable in nature, but also enables in-depth evaluation on the physical, electrochemical and kinetic properties of transient intermediate phases and their role in battery electrode performance.