Improved electrochemical performance of LiMn2O4 cathode material by Ce doping

A nonsolvolytic fluorine/LiNO3-containing electrolyte for stabilizing dynamic interfaces in Li||LiMn2O4 batteries

Mn-based high voltage cathodes, e.g., spinel LiMn2O4, are considered among the most promising materials for cost-effective, next generation energy storage. When paired with a Li metal anode, secondary batteries based on Li||LiMn2O4 in principle offer a straightforward, scalable approach for achieving cost-effective and high energy density storage demanded in applications. In practice, however, such batteries fail to live up to their promise. Rapid capacity fading caused by irreversible Mn dissolution at the cathode coupled with mossy/dendritic Li deposition at the anode limit their useful life. In this study, we report on the design of electrolytes based on a binary blend of two widely available salts, LiNO3 and LiTFSI, in ethylene carbonate (EC), which simultaneously overcome failure modes at both the cathode and anode of Li||LiMn2O4 batteries. The electrolyte design is motivated by a recent finding that compared with their linear counterparts (e.g., dimethyl carbonate), cyclic carbonates like EC dissolve considerably larger amount of LiNO3, which markedly improves anode reversibility. On the other hand, it is known that nonsolvolytic fluorine-containing Li salts like LiTFSI, lowers the electrolyte's susceptibility to solvolysis, which generates HF species responsible for Mn leaching at the cathode. In particular, we report instead that fluorine groups in the TFSI salt, promote formation of a favorable, fluorine-rich interphase on the Li metal anode. Electrochemical measurements show that the electrolytes enable remarkably improved charge-discharge cycling stability (>1000 charge-discharge cycles) of Li||LiMn2O4 batteries. In-depth atomic-resolution electron microscopy and X-ray/synchrotron diffraction experiments reveal the fundamental source of the improvements. The measurements show that crystallographic degradation of Mn-based cathodes (e.g., surface Mn leaching and bulk defect generation) upon cycling in conventional electrolytes is dramatically lowered in the LiNO3 + LiTFSI/EC electrolyte system. It is shown further that the reduction of Mn dissolution not only improves the cathode stability but improves the reversibility of the Li metal anode via a unique re-deposition mechanism in which Li and Mn co-deposit on the anode. Taken together, our findings show that the LiNO3 + LiTFSI/EC electrolyte system holds promise for accelerating progress towards practical Li||LiMn2O4 batteries because it stabilizes the dynamic interfaces required for long-term stability at both the Li anode and the LiMn2O4 cathode.

Simultaneous modification of dual-substitution with CeO2 coating boosting high performance sodium ion batteries

Na3V2(PO4)3 (NVP) is highly valued based on the stable construction among the polyanionic compounds. Nevertheless, the drawback of low intrinsic conductivity has been impeded its further application. In this paper, the internal channels of the crystal structure are extended by the introduction of larger radius Ce3+, which increases the transport rate of Na+. The introduction of Mo6+ replacing the V site leads to a beneficial n-type doping effect and facilitates the transportation of electrons. Besides, CeO2 cladding is introduced to further enhance the electronic conductivity of NVP system. Initially, CeO2 serves as an n-type semiconductor and functions as a conductive additive to significantly enhance the electronic conductivity of the electrode, thereby improving the electrochemical characteristics. Moreover, CeO2 functions as an oxygen buffer, aiding in the maintenance of active metal dispersion during operation and enabling efficient electron transfer between CeO2 and [VO6] octahedra in NVP, thus fostering outstanding electrical connectivity between the oxides. CeO2 cladding can be effectively integrated with the carbon layer to stabilize the NVP system. Comprehensively, the modified Na3V1.79Ce0.07Mo0.07(PO4)3/C@8wt.%CeO2 (CeMo0.07@8wt.%CeO2) composite exhibits excellent rate and cycling properties. It delivers a capacity of 113.4 mAh/g at 1C with a capacity retention rate of 80.3 % after 150 cycles. Even at 10C and 40C, it also submits high capacities of 84.7 mAh/g and 76 mAh/g, respectively. Furthermore, the CHC//CeMo0.07@8wt.%CeO2 asymmetric full cell possesses excellent sodium storage property, indicating its prospective application potentials.

Co-doping studies to enhance the life and electro-chemo-mechanical properties of the LiMn2O4 cathode using multi-scale modeling and neuro-computing techniques

A number of engineered cathode materials with longer life cycles and better electro-chemo-mechanical properties can be obtained by partially replacing some of the elements with other relevant ones without compromising much with the structure. To design such superior cathode materials, in this work, we replace a small number (5% or 10%) of Mn³⁺, with one of the following elements: aluminium, nickel, magnesium, gallium, chromium, and yttrium. Additionally, S^2- and F^- were used to replace some (∼1%) of the O^2- ions (anion) in the crystal. In this work, we have used a combination of Quantum Mechanics (QM), Classical Molecular Dynamics (CMD), Neural Network (NN) and Computational Fluid Dynamics (CFD) modeling. QM has been used to validate the Classical Molecular Dynamics (CMD) simulation results for engineered structures where experimental data are not available. CMD simulations are used to obtain material properties such as lattice expansion, Young's modulus, and diffusion coefficients for un-doped, doped and co-doped structures. NN modeling was used to reduce the computational time to evaluate millions of possible crystal configurations. Finally, the impact of co-doping strategies at the macroscale has been studied using CFD simulations. As a first step, we employed neuro-computing techniques to identify the optimum ionic configuration for all crystal structures, saving ∼88% of the computational time. Next, molecular scale simulations were performed to study the material properties. Molecular dynamics (MD) modeling findings suggest that the relative volume expansion between the fully charged and discharged states of the battery can be reduced by ∼1.9% to ∼2.25%, indicating an improvement in the life of the cathode material by several hundreds of cycles. Findings from both QM and CMD simulations suggest that for these novel engineered materials, electro-chemo-mechanical properties, such as ionic mobility, chemical diffusion coefficient and elasticity, improved. Furthermore, CMD simulations showed that the inter-ionic space between doped metal ions and oxygen is smaller compared to the spacing between Mn³⁺-O^2- in the original LMO spinel, indicating an improvement in the material's structural strength along with the total number of the discharge cycle. Finally, macro scale computational modelling results show that chances of thermal runaway can be reduced significantly for some of the co-doped structures since the intercalation induced maximum stress is lower.

Tracking Electrochemical-Cycle-Induced Surface Structure Evolutions of Cathode Material LiMn2O4 with Improved Operando Raman Spectroscopy

Linking surface structure evolution to the capacity fading of cathode materials has been a problem in lithium ion batteries. Most of the strategies used to solve this problem are focused on the differences between the unaged and aged materials, leading to the loss of intermediate dynamic change information during cycling. Raman spectroscopy is a convenient, nondestructive, and highly sensitive tool for characterizing the surface/near-surface region structure. In this work, we improved an operando Raman system, which is able to record in situ and in real time a series of Raman spectra during charging/discharging cycles and is even able to record very weak Raman peaks without the use of SRES enhancement, which facilitates sample preparation. These series of Raman spectra revealed an inherent correlation between the electrode potential/Li content and the surface structure changes of the as-prepared pure LiMn₂O₄ film, including the biphase reaction, the evolution of the peroxo O-O bond, and the formation of the Mn₃O₄ surface phase. They were the first to show that the number of peroxo O-O bonds was decreased with an increasing number of cycles and that this decrease was accompanied by an increase in the Mn₃O₄ phase. With the help of the data measured by XPS, c-AFM, electrochemical testing equipment, and the calculation based on density functional theory, the causes of the capacity fading of the material are discussed. This work not only showed a direct correlation between the surface structure evolution and the capacity fading of the LiMn₂O₄ but also could provide an alternative operando Raman system that could be widely used for the in situ characterization of battery electrode materials.

A Fast Approach to Obtain Layered Transition-Metal Cathode Material for Rechargeable Batteries

Li-ion batteries as a support for future transportation have the advantages of high storage capacity, a long life cycle, and the fact that they are less dangerous than current battery materials. Li-ion battery components, especially the cathode, are the intercalation places for lithium, which plays an important role in battery performance. This study aims to obtain the LiNixMnyCozO2 (NMC) cathode material using a simple flash coprecipitation method. As precipitation agents and pH regulators, oxalic acid and ammonia are widely available and inexpensive. The composition of the NMC mole ratio was varied, with values of 333, 424, 442, 523, 532, 622, and 811. As a comprehensive study of NMC, lithium transition-metal oxide (LMO, LCO, and LNO) is also provided. The crystal structure, functional groups, morphology, elemental composition and material behavior of the particles were all investigated during the heating process. The galvanostatic charge–discharge analysis was tested with cylindrical cells and using mesocarbon microbeads/graphite as the anode. Cells were tested at 2.7–4.25 V at 0.5 C. Based on the analysis results, NMC with a mole ratio of 622 showed the best characteristicd and electrochemical performance. After 100 cycles, the discharged capacity reaches 153.60 mAh/g with 70.9% capacity retention.

Li2ZrO3-Coated Monocrystalline LiAl0.06Mn1.94O4 Particles as Cathode Materials for Lithium-Ion Batteries

Li₂ZrO₃-coated and Al-doped micro-sized monocrystalline LiMn₂O₄ powder is synthesized through solid-state reaction, and the electrochemical performance is investigated as cathode materials for lithium-ion batteries. It is found that Li₂ZrO₃-coated LiAl_0.06Mn_1.94O₄ delivers a discharge capacity of 110.90 mAhg⁻¹ with 94% capacity retention after 200 cycles at room temperature and a discharge capacity of 104.4 mAhg⁻¹ with a capacity retention of 87.8% after 100 cycles at 55 °C. Moreover, Li₂ZrO₃-coated LiAl_0.06Mn_1.94O₄ could retain 87.5% of its initial capacity at 5C rate. This superior cycling and rate performance can be greatly contributed to the synergistic effect of Al-doping and Li₂ZrO₃-coating.

Density Functional Theory Study of Ethylene Carbonate Adsorption on the (0001) Surface of Aluminum Oxide α-Al2O3

Surface coating is one of the techniques used to improve the electrochemical performance and enhance the resistance against decomposition of cathode materials in lithium-ion batteries. Despite several experimental studies addressing the surface coating of secondary Li-ion batteries using α-Al₂O₃, the reactivity of the material toward the electrolyte components is not yet fully understood. Here, we have employed calculations based on the density functional theory to investigate the adsorption of the organic solvent ethylene carbonate (EC) on the major α-Al₂O₃(0001) surface. During adsorption of a single EC molecule, it was found that it prefers to bind parallel to the surface through its carboxyl oxygen. As the surface coverage (θ) was increased up to a monolayer, we observed larger adsorption energies per EC molecule (E _ads/N _EC) for parallel interactions and a reduction for perpendicular interactions. We also noted that increasing the surface coverage with both parallel and perpendicularly interacting EC molecules led to a decrease of the surface free energies and hence increased stability of the α-Al₂O₃(0001) surface. Despite the larger E _ads/N _EC observed when the molecule was placed parallel to the surface, minimal charge transfer was calculated for single EC interactions and at higher surface coverages. The simulated scanning tunneling microscopy images are also presented for a clean corundum α-Al₂O₃ surface and after adsorption with different coverages of parallel and perpendicularly placed EC molecules.

Turning commercial MnO2 (≥85 wt%) into high-crystallized K+-doped LiMn2O4 cathode with superior structural stability by a low-temperature molten salt method

The obtainment of low-cost, easily prepared and high-powered LiMn₂O₄ is the key to achieve its wide application in various electronic devices. In this work, a mild and easily scaled molten salt method (KCl@LiCl) is utilized to convert commercial MnO₂ to the high-performance LiMn₂O₄. At the same reaction temperature, the molten salt method leads to the formation of K⁺-doped LiMn₂O₄ with higher crystallinity compared to the conventional solid state method, which contributes to the improved inner charge transfer. The Li₃PO₄ protective layer is coated on the surface of K⁺-doped LiMn₂O₄ to elevate the interfacial stability and the Li⁺ transfer on interface. Thus, the optimized electrode shows a higher specific discharge capacity (103/60 mAh g^-1 at 0.02/2 A g^-1) and a longer cyclic life (80 mAh g^-1 after 500 cycles at 0.5 A g^-1) compared to those of LiMn₂O₄ by solid state method (49/2 mAh g^-1 at 0.02/2 A g^-1 and 20 mAh g^-1 after 500 cycles at 0.5 A g^-1).

Understanding the Effect of Al Doping on the Electrochemical Performance Improvement of the LiMn2O4 Cathode Material

It is well known that the electrochemical performance of spinel LiMn₂O₄ can be improved by Al doping. Herein, combining X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, and spherical aberration-corrected scanning transmission electron microscopy (Cs-STEM) with in situ electron-beam (E-beam) irradiation techniques, the influence of Al doping on the structural evolution and stability improvement of the LiMn₂O₄ cathode material is revealed. It is revealed that an appropriate concentration of Al³⁺ ions could dope into the spinel structure to form a more stable LiAl_xMn_2-xO₄ phase framework, which can effectively stabilize the surface and bulk structure by inhibiting the dissolution of Mn ions during cycling. The optimized LiAl_0.05Mn_1.95O₄ sample exhibits a superior capacity retention ratio of 80% after 1000 cycles at 10 C (1 C = 148 mA h g^-1) in the voltage range of 3.0-4.5 V, which possesses an initial discharge capacity of 90.3 mA h g^-1. Compared with the undoped LiMn₂O₄ sample, the Al-doped sample also shows superior rate performance, especially the capacity recovery performance.

Boosting Electrocatalytic Oxygen Evolution: Superhydrophilic/Superaerophobic Hierarchical Nanoneedle/Microflower Arrays of CexCo3-xO4 with Oxygen Vacancies

The oxygen evolution reaction has become the bottleneck of electrochemical water splitting for its sluggish kinetics. Developing high-efficiency and low-cost non-noble-metal oxide electrocatalysts is crucial but challenging for industrial application. Herein, superhydrophilic/superaerophobic hierarchical nanoneedle/microflower arrays of Ce-substituted Co₃O₄ (Ce_xCo_3-xO₄) in situ grown on the nickel foam are successfully constructed. The hierarchical architecture and superhydrophilic/superaerophobic interface can be facilely regulated by controlling the introduction of Ce into Co₃O₄. The unique feature of hierarchical architecture and superhydrophilic/superaerophobic interface is in favor of electrolyte penetration and bubbles release. In addition, the presence of oxygen vacancy and Ce endows the catalyst with enhanced intrinsic activity. Benefiting from these advantages, the optimized Ce_0.12Co_2.88O₄ catalyst shows a superior electrocatalytic performance for the oxygen evolution reaction (OER) with an overpotential of 282 mV at 20 mA cm^-2, and a Tafel slope of 81.4 mV dec^-1. The turnover frequency of 0.0279 s^-1 for Ce_0.12Co_2.88O₄ is 9.3 times larger than that for Co₃O₄ at an overpotential of 350 mV. Moreover, the optimized Ce_0.12Co_2.88O₄ catalyst shows a robust long-term stability in alkaline media.

Surface Modification of Nanocrystalline LiMn2O4 Using Graphene Oxide Flakes

In this work, a facile, wet chemical synthesis was utilized to achieve a series of lithium manganese oxide (LiMn₂O₄, (LMO) with 1–5%wt. graphene oxide (GO) composites. The average crystallite sizes estimated by the Rietveld method of LMO/GO nanocomposites were in the range of 18–27 nm. The electrochemical performance was studied using CR2013 coin-type cell batteries prepared from pristine LMO material and LMO modified with 5%wt. GO. Synthesized materials were tested as positive electrodes for Li-ion batteries in the voltage range between 3.0 and 4.3 V at room temperature. The specific discharge capacity after 100 cycles for LMO and LMO/5%wt. GO were 84 and 83 mAh g⁻¹, respectively. The LMO material modified with 5%wt. of graphene oxide flakes retained more than 91% of its initial specific capacity, as compared with the 86% of pristine LMO material.

Enhancing Lithium Manganese Oxide Electrochemical Behavior by Doping and Surface Modifications

Lithium manganese oxide is regarded as a capable cathode material for lithium-ion batteries, but it suffers from relative low conductivity, manganese dissolution in electrolyte and structural distortion from cubic to tetragonal during elevated temperature tests. This review covers a comprehensive study about the main directions taken into consideration to supress the drawbacks of lithium manganese oxide: structure doping and surface modification by coating. Regarding the doping of LiMn2O4, several perspectives are studied, which include doping with single or multiple cations, only anions and combined doping with cations and anions. Surface modification approach consists in coating with different materials like carbonaceous compounds, oxides, phosphates and solid electrolyte solutions. The modified lithium manganese oxide performs better than pristine samples, showing improved cyclability, better behaviour at high discharge c-rates and elevated temperate and improves lithium ions diffusion coefficient.

Advances in Materials Design for All-Solid-state Batteries: From Bulk to Thin Films

All-solid-state batteries (SSBs) are one of the most fascinating next-generation energy storage systems that can provide improved energy density and safety for a wide range of applications from portable electronics to electric vehicles. The development of SSBs was accelerated by the discovery of new materials and the design of nanostructures. In particular, advances in the growth of thin-film battery materials facilitated the development of all solid-state thin-film batteries (SSTFBs)—expanding their applications to microelectronics such as flexible devices and implantable medical devices. However, critical challenges still remain, such as low ionic conductivity of solid electrolytes, interfacial instability and difficulty in controlling thin-film growth. In this review, we discuss the evolution of electrode and electrolyte materials for lithium-based batteries and their adoption in SSBs and SSTFBs. We highlight novel design strategies of bulk and thin-film materials to solve the issues in lithium-based batteries. We also focus on the important advances in thin-film electrodes, electrolytes and interfacial layers with the aim of providing insight into the future design of batteries. Furthermore, various thin-film fabrication techniques are also covered in this review.

Ethylene carbonate adsorption on the major surfaces of lithium manganese oxide Li1-xMn2O4 spinel (0.000 < x < 0.375): a DFT+U-D3 study

Understanding the surface reactivity of the commercial cathode material LiMn₂O₄ towards the electrolyte is important to improve the cycling performance of secondary lithium-ion batteries and to prevent manganese dissolution. In this work, we have employed spin-polarized density functional theory calculations with on-site Coulomb interactions and long-range dispersion corrections [DFT+U-D3-(BJ)] to investigate the adsorption of the electrolyte component ethylene carbonate (EC) onto the (001), (011) and (111) surfaces of the fully lithiated and partially delithiated Li_1-xMn₂O₄ spinel (0.000 < x < 0.375). The surface interactions were investigated by evaluating the adsorption energies of the EC molecule and the surface free energies. Furthermore, we analyzed the impact of EC adsorption on the Wulff crystal morphologies, the molecular vibrational frequencies and the adsorbate/surface charge transfers. The adsorption energies indicate that the EC molecule strongly adsorbs on the (111) facet, which is attributed to a bidentate binding configuration. We found that EC adsorption enhances the stability of the (111) facet, as shown by the Wulff crystal morphologies. Although a negligible charge transfer was calculated between the spinel surfaces and the EC molecule, a large charge rearrangement takes place within the surfactant upon adsorption. The wavenumbers of the C[double bond, length as m-dash]O stretching mode for the interacting EC molecule are red-shifted with respect to the isolated adsorbate, suggesting that this bond becomes weaker. The surface free energies show that both the fully lithiated and partially delithiated forms of the LiMn₂O₄ surfaces are stabilized by the EC molecule.

Improved electrochemical properties and kinetics of an LiMn2O4-based cathode co-modified via Cu doping with truncated octahedron morphology

Spinel LiMn₂O₄ has been widely investigated as a cathode material for lithium-ion batteries, but it suffers from a limited cycle life due to the dissolution of Mn and structural distortion.

Enhancing the durable performance of LiMn2O4 at high-rate and elevated temperature by nickel-magnesium dual doping

Various nickel and magnesium dual-doped LiNi_xMg_0.08Mn_1.92−xO₄ (x ≤ 0.15) were synthesized via a modified solid-state combustion method. All as-prepared samples show typical spinel phase with a well-defined polyhedron morphology. The Ni-Mg dual-doping obviously decreases the lattice parameter that gives rise to the lattice contraction. Owing to the synergistic merits of metal ions co-doping, the optimized LiNi_0.03Mg_0.08Mn_1.89O₄ delivers high initial capacity of 115.9 and 92.9 mAh·g⁻¹, whilst retains 77.1 and 69.7 mAh·g⁻¹ after 1000 cycles at 1 C and high current rate of 20 C, respectively. Even at 10 C and 55 °C, the LiNi_0.03Mg_0.08Mn_1.89O₄ also has a discharge capacity of 92.2 mAh·g⁻¹ and endures 500 cycles long-term life. Such excellent results are contributed to the fast Li⁺ diffusion and robust structure stability. The anatomical analysis of the 1000 long-cycled LiNi_0.03Mg_0.08Mn_1.89O₄ electrode further demonstrates the stable spinel structure via the mitigation of Jahn-Teller effect. Hence, the Ni-Mg co-doping can be a potential strategy to improve the high-rate capability and long cycle properties of cathode materials.

Improved Electrochemical Properties of LiMn2O4-Based Cathode Material Co-Modified by Mg-Doping and Octahedral Morphology

In this work, the spinel LiMn₂O₄ cathode material was prepared by high-temperature solid-phase method and further optimized by co-modification strategy based on the Mg-doping and octahedral morphology. The octahedral LiMn_1.95Mg_0.05O₄ sample belongs to the spinel cubic structure with the space group of Fd3m, and no other impurities are presented in the XRD patterns. The octahedral LiMn_1.95Mg_0.05O₄ particles show narrow size distribution with regular morphology. When used as cathode material, the obtained LiMn_1.95Mg_0.05O₄ octahedra shows excellent electrochemical properties. This material can exhibit high capacity retention of 96.8% with 100th discharge capacity of 111.6 mAh g⁻¹ at 1.0 C. Moreover, the rate performance and high-temperature cycling stability of LiMn₂O₄ are effectively improved by the co-modification strategy based on Mg-doping and octahedral morphology. These results are mostly given to the fact that the addition of magnesium ions can suppress the Jahn–Teller effect and the octahedral morphology contributes to the Mn dissolution, which can improve the structural stability of LiMn₂O₄.

Nb-doped and Al2O3 + B2O3-coated granular secondary LiMn2O4 particles as cathode materials for lithium-ion batteries

In this work, to improve the cyclability and high-temperature performance of cubic spinel LiMn₂O₄ (LMO) as cathode materials, Nb⁵⁺-doped LiMn₂O₄ powders coated and uncoated with Al₂O₃ and/or B₂O₃ were synthesized via the modified solid-state reaction method. It was found that Nb⁵⁺-doped and B₂O₃ + Al₂O₃-coated LMO powders comprising 5 μm granular agglomerated fine primary particles smaller than 350 nm in diameter exhibited superior electrochemical properties with initial discharge capacity of 101.68 mA h g^-1; we also observed capacity retention of 96.31% after 300 cycles at room temperature (RT) and that of 98% after 50 cycles at 55 °C and 1C rate.