Surface Of LiMn2O4 Research Articles

Dextran Sulfate Sodium as Multifunctional Aqueous Binder Stabilizes Spinel LiMn2O4 for Lithium Ion Batteries

AbstractSpinel LiMn2O4 (LMO) has several beneficial properties for utilization as a cathode material for lithium‐ion batteries, including a high operating voltage, thermal stability, low cost, and eco‐friendliness. However, its application is limited by capacity fading due to structural transformation and manganese dissolution upon cycling. A method is proposed to resolve these two issues, using dextran sulfate sodium (DSS) as an aqueous binder to coat the LMO surface. DSS stabilizes the surface/interface structure of LMO by incorporating Na into the interstitial 16c sites, creating a uniform surface coating layer and promoting the formation of a homogeneous and stable cathode‐electrolyte interphase derived from the sulfate groups in DSS and enriched with LiF/LiSOxF. The aqueous DSS binder effectively suppresses the surface degradation and Mn dissolution of spinel LMO. The LMO electrode based on DSS exhibits superior cycling stability and rate performance compared with those of electrodes using the conventional poly(vinylidene difluoride) binder. These findings demonstrate the potential value of DSS as a multifunctional aqueous binder in stabilizing spinel LMO for lithium‐ion batteries.

Multistage regulation of LiMn2O4 electrode for electrochemical lithium extraction from salt-lake

The sustainable development of future energy relies significantly on the abundant lithium resources present in salt-lake brines. LiMn2O4 (LMO) electrochemical Li+ pump has garnered substantial attention due to its capacity to efficiently extract lithium resources from aqueous solutions with minimal energy consumption. However, the industrial application of LMO electrodes is often limited by Mn dissolving during cycles, as well as the complex and variable brine environment. To address these challenges, this study conducted a multistage regulation of LMO electrodes. Originally, the sol-gel technique was used to prepare CePO4 loading LMO. Due to CePO4 rivet load reforming LMO surface charge, the best-performing 1 wt% CePO4-loaded LMO exhibited a remarkable capacity retention rate of 83.8 % after 30 cycles. Subsequently, a comprehensive hydrophilic modification was applied to the entire electrode. 20%PAA-1CP-LMO//Ag system displayed an admirably release capacity of 24.7 mg·g−1 in Zabuye real brine characterized by an ultra-low Li/Na ratio, with Li/Na ratio in recovery solution reaching as high as 0.62.

Multifunctional AlPO4 reconstructed LiMn2O4 surface for electrochemical lithium extraction from brine

LiMn2O4 (LMO) electrochemical lithium-ion pump has gained widespread attention due to its green, high efficiency, and low energy consumption in selectively extracting lithium from brine. However, collapse of crystal structure and loss of lithium extraction capacity caused by Mn dissolution loss limits its industrialized application. Hence, a multifunctional coating was developed by depositing amorphous AlPO4 on the surface of LMO using sol-gel method. The characterization and electrochemical performance test provided insights into the mechanism of Li+ embedment and de-embedment and revealed that multifunctional AlPO4 can reconstruct the physical and chemical state of LMO surface to improve the interface hydrophilicity, promote the transport of Li+, strengthen cycle stability. Remarkably, after 20 cycles, the capacity retention rate of 0.5AP-LMO reached 93.6% with only 0.147% Mn dissolution loss. The average Li+ release capacity of 0.5AP-LMO//Ag system in simulated brine is 28.77 mg/(g h), which is 90.4% higher than LMO. Encouragingly, even in the more complex Zabuye real brine, 0.5AP-LMO//Ag can still maintain excellent lithium extraction performance. These results indicate that the 0.5AP-LMO//Ag lithium-ion pump shows promising potential as a Li+ selective extraction system.

Transformation of waste battery cathode material LiMn2O4 into efficient ultra-low temperature NH3-SCR catalyst: Proton exchange synergistic vanadium modification

It is essential to develop the catalyst for NH3-SCR with excellent performance at ultra-low temperature (≤150 °C), and resource recycling is another important part of environmental protection. Based on the principle of environmental friendliness, the LiMn2O4, one of the waste battery cathode materials, was successfully modified into a novel high-value catalyst for ultra-low temperature NH3-SCR through hydrogen ion exchange and two-dimensional vanadic oxide modification. The optimized LiMn2O4-0.5V-10H catalyst performed the best balance of NOx conversion and N2 selectivity, with activity reaching 96 % at 150 °C and N2 selectivity exceeding 70 % at ultra-low temperature. Due to the unique three-dimensional network structural characteristics of LiMn2O4 spinel, hydrogen exchange could exchange Li+ from the lattice and increase surface acidity; and a small amount of two-dimensional vanadic oxide loading could appropriately regulate redox ability and increase acidic sites. The in-situ DRIFTS results still showed that the L-H and E-R mechanisms coexisted during the reaction. Moreover, combining first-principles calculations and in-situ DRIFTS, the dual modification of H and V could enhance the adsorption of NH3 on the surface of LiMn2O4 but weaken the adsorption of NO, and promote the decomposition of nitrites while inhibit the formation of surface nitrate species, which was the core reason for the improvement of N2 selectivity. The modification mode in this work was simple and inexpensive, which provided a new idea for the high-value utilization of waste batteries and the design of NOx purification catalyst at ultra-low temperature.

Surface modification of Li3InCl6 provides superior electrochemical performance for LiMn2O4 cathode materials

LiMn2O4 (LMO) is the substance of choice for small and medium-sized energy storage materials in daily life. In this work, Li3InCl6 (LIC) is prepared on the surface of LiMn2O4 by hydrothermal method using InCl3 and LiCl as raw materials. This method stabilizes the LMO crystal structure by uniformly coating the LIC on the LMO surface and effectively maintains the morphology of LMO crystals during the cycling process. SEM and EDS analysis confirm the morphology and homogeneity of the synthesized material LIC on the LMO surface. The prepared material is put into a battery, and the charge-discharge test is carried out at 0.5 C and 1 C. The results show that the LIC surface-modified samples exhibit more than 6% higher cycling performance than the unmodified samples after long cycling.

A DFT + U-D3 Study of the Adsorption of Hydrogen Fluoride and Ethylene Carbonate on the Niobium-Doped (001), (011), and (111) Surfaces of Lithium Manganese Oxide

Cationic doping has been recommended as one of the most effective methods of reducing the number of trivalent manganese (Mn3+) ions that undergo a disproportionation reaction in lithium manganese oxide-based (LiMn2O4) lithium-ion batteries. However, the effect of surface doping on the major LiMn2O4 surfaces and their interactions with the electrolyte components is not yet fully understood. In this work, spin-polarised density functional theory-based calculations [DFT + U-D3 (BJ)] were employed to study the adsorption of the electrolyte components ethylene carbonate (EC) and hydrogen fluoride (HF) onto the Nb-doped major LiMn2O4 (001), (011), and (111) surfaces. During the substitution of niobium for manganese ions in the second surface layers (Nb second ), it was found that the (111) surface stability improves, resulting in an enhanced (111) plane on the morphology. However, replacing the first (Nb first ) as well as both top and sub-surface (Nb both ) layers of Mn atoms in the slabs maintains the same stability trend as in the pure pristine surfaces. Moreover, both adsorbates greatly preferred binding to the surfaces through the Nb instead of Mn atoms, and the largest adsorption energy was calculated for EC on the LiMn2O4 (011) surface doped on the Nb second site and for HF on the LiMn2O4 (111) surface doped on the Nb both site. Furthermore, the EC/HF adsorptions further enhance the stability of the Nb second (111) surface plane. However, minimal charge transfer was calculated for both HF and EC interacting with the pure and Nb-doped surfaces. Our findings are interesting, since exposing the (111) surface promotes the formation of a stable solid electrolyte interface (SEI), significantly reducing Mn dissolution and enhancing the adsorption of EC and HF.

Modification of LiMn2O4 surfaces by controlling the Acid–Base surface chemistry of atomic layer deposition

In this combined theoretical and experimental study, we report the effects of Al precursor selection on the growth chemistry, interfacial structure, and electrochemistry of Al2O3 coatings on spinel LiMn2O4 (LMO) surfaces by atomic layer deposition (ALD). Five Al precursors, exhibiting a range of Lewis acid–base properties, were used to establish trends in the ALD Al2O3 growth chemistry on LMO powders in comparison to redox-inactive planar silicon substrates. We show that the Lewis acid–base properties of the Al precursor ligands can be used to tune both the Mn oxidation states and the Al2O3 coverage on the LMO surface. Density functional theory calculations are used to examine the reaction mechanism of each precursor on LMO surfaces, elucidating a correlation between the Lewis acidity of the ligands and the decomposition thermochemistry. In-depth X-ray photoelectron spectroscopy and in situ Fourier transform infrared spectroscopy measurements support these theoretical predictions and further reveal how different Al precursors modify the atomic and electronic structure near the LMO surface during ALD. While the Mn oxidation state is strongly influenced by the Lewis acidity of the precursor ligand, the surface coverage and thickness of the Al2O3 coating are a more representative descriptor of the electrochemical performance measured in coin cell experiments. We show how the ligand acid–base properties of ALD precursors can be used to rationally tailor atomic layer growth mechanisms, which may enable atomic level control over the structure of functionalized interfaces for various applications related to catalysis, semiconductors, and energy storage.

Boosting the cycling performance of spinel LiMn2O4 by in situ MnBO3 coating

Spinel LiMn2O4 (LMO) is a promising cathode material for lithium-ion batteries (LIBs) due to its low cost, high safety, and high energy density. However, surface dissolution of manganese and irreversible phase transitions reduce its long-term cycling performance. Although surface coating is a promising strategy for improving the structural stability of LMO, the commonly used coating process is tedious and complicated. In this work, a one-step sintering process is developed to prepare LMO materials coated in situ with MnBO3. It is found that the MnBO3 layer formed in this way is uniformly coated on the surface of LMO, and effectively suppresses Mn dissolution and undesirable phase transition during repeated charge/discharge cycles. Furthermore, the MnBO3-coated LMO has improved electrode kinetics, demonstrating significantly improved rate performance compared to the pristine LMO. As a result, the optimized LMO sample exhibits 85.2% capacity retention after 1000 cycles at 1 C and still can deliver 96 mAh g−1, even at 5 C. This work proposes a facile and efficient coating strategy for improving the electrochemical performance of LMO as a cathode material for LIBs.

Atomic-Level Changes during Electrochemical Cycling of Oriented LiMn2O4 Cathodic Thin Films.

Spinel LiMn2O4 is an attractive lithium-ion battery cathode material that undergoes a complex series of structural changes during electrochemical cycling that lead to rapid capacity fading, compromising its long-term performance. To gain insights into this behavior, in this report we analyze changes in epitaxial LiMn2O4 thin films during the first few charge-discharge cycles with atomic resolution and correlate them with changes in the electrochemical properties. Impedance spectroscopy and scanning transmission electron microscopy are used to show that defect-rich LiMn2O4 surfaces contribute greatly to the increased resistivity of the battery after only a single charge. Sequences of {111} stacking faults within the films were also observed upon charging, increasing in number with further cycling. The atomic structures of these stacking faults are reported for the first time, showing that Li deintercalation is accompanied by local oxygen loss and relaxation of Mn atoms onto previously unoccupied sites. The stacking faults have a more compressed structure than the spinel matrix and impede Li-ion migration, which explains the observed increase in thin-film resistivity as the number of cycles increases. These results are used to identify key factors contributing to conductivity degradation and capacity fading in LiMn2O4 cathodes, highlighting the need to develop techniques that minimize defect formation in spinel cathodes to improve cycle performance.

Rational Modification of LiMn2O4 Surfaces By Controlling the Acid-base Surface Chemistry of Atomic Layer Deposition

Next-generation energy storage devices demand superb control over interfaces. Atomic layer deposition (ALD) coatings can suppress undesirable electrochemical reactions between lithium ion battery (LIB) cathode surfaces and the organic liquid electrolyte. However, our limited understanding of ALD cathode coatings restricts our ability to predict which coating/cathode combinations will succeed. In this study, we performed a combined theoretical-experimental study of a model coating/cathode system aimed at elucidating synthesis-structure-property relationships. We selected five ALD Al precursors exhibiting a range of Lewis acid-base properties to establish trends in the ALD Al2O3 chemistry on LiMn2O4. Density functional theory calculations deduced a correlation between the Lewis acidity of the ligands and the ALD thermochemistry. X-ray photoelectron spectroscopy and in situ infrared spectroscopy measurements supported these predictions and further revealed how the Al precursors modify the LiMn2O4 atomic and electronic surface structure. Finally, coin cell electrochemical measurements established the link between materials properties and device performance. We discovered that while the Mn oxidation state is strongly influenced by the Lewis acidity of the precursor ligand, the surface coverage and thickness of the Al2O3 coating are a more representative descriptor of the electrochemical performance measured in coin cell experiments.

Establishment of PPy-derived carbon encapsulated LiMn2O4 film electrode and its performance for efficient Li+ electrosorption

Improving the cycle stability and Li+ electrosorption rate of the LiMn2O4 (LMO) film electrode in the selective Li+ electrosorption process is still a considerable challenge. In this work, a novel polypyrrole (PPy) - derived carbon encapsulated material LiMn2O4@Carbon/Nitrogen-4 (LMO@CN-4) was prepared by the three-dimensional in-situ polymerization and high-temperature annealing of PPy on the LMO surface. The LMO@CN-4 film electrode showed a faster Li+ electrosorption rate, higher lithium-ion selectivity, and higher stability than the LMO film electrode. The maximum capacity of the LMO@CN-4 film electrode for Li+ could reach 34.57 mg/g within 40 min in 0.05 mol/L LiCl solution. Even in the simulated concentrated seawater with a low Li+ concentration (0.17 g/L), it could quickly reach adsorption equilibrium and maintain the high selectivity of Li+. The adsorption capacity was about 37.14 mg/g and the separation coefficient of α (Li+/Mg2+), α (Li+/Na+), and α (Li+/Ca2+) was about 228.25, 2110.64, and 1381.89. The carbon encapsulation served as a conductive layer to enhance charge and ion transport. It also provided a buffer layer to prevent the volume collapse of the crystal and the dissolution of Mn. Therefore, the charge and discharge capacity of the LMO@CN-4 film electrode could be maintained at about 80% after 50 cycles at the 1C (148 mAh/g) rate, which was 20% higher than that of the bare LMO. What’s more, the high cycle stability and Li+ electrosorption rate of the modified film electrode also suggested its potential for lithium electrosorption from concentration seawater.

Boosting ultrafast Li storage kinetics of conductive Nb-doped TiO2 functional layer coated on LiMn2O4

Interface engineering of LiMn2O4 (LMO) is a promising strategy to enhance the lithium storage capability and cycling stability of cathode materials in Li-ion batteries (LIBs). This is because the strategy prevents structural degradation; however, Li storage kinetics remains unsatisfactory, resulting in poor ultrafast cycling performance. Therefore, we fabricated an Nb-doped TiO2 (NTO) functional layer as a conductive passivation layer on the LMO surface by horizontal ultrasonic spray pyrolysis deposition. The NTO functional layer suppressed the volume expansion of LMO and exhibited high electrical and ionic conductivity, which resulted in improved structural stability of LMO (related to cycling stability) and increased electron/ion transfer rate (related to ultrafast cycling performance). In the TiO2 structure, Ti4+ ions were replaced by Nb5+ ions, which possess high electrical conductivity and a wide c-axis as a Li-ion diffusion route. As a result, the NTO-coated LMO cathode material showed an outstanding specific capacity of 112.7 mAh/g with a remarkable capacity retention of 96.2% after 100 cycles at a current density of 1 C and excellent ultrafast cycling capacity and stability of 70.0 mAh/g after 500 cycles at a current density of 10 C.

Improving Electrochemical Properties of LiMn2O4/LiNi0.5Mn1.5O4 Core/Shell Composites

The electrochemical properties of LiMn2O4 surface modified with LiNi0.5Mn1.5O4 at different current densities are presented. The electrode with a maximum percentage of covering (14% of LiNi0.5Mn1.5O4) can endure higher current loads compared with the sample consisting 7% of surface layer. The capacity values for these samples at 45° and 30°C are 22 and 31 mAh∙g-1, respectively. This means that the proposed approach can be an effective tool for obtaining electrode materials for high-rate applications.

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 Li1-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.

Surface Reorganization and Rate Enhancement of Spinel LiMn2O4 (LMO) on First Trimethylaluminum/Water Exposure

Recent work has identified enhanced charge storage capacity in the spinel lithium manganese oxide (LiMn2O4;LMO) lithium ion battery cathode upon a single atomic layer deposition (ALD) cycle comprised of one chemical exposure of trimethylaluminum (TMA) and one exposure of water (H2O). Here, we report further study of the rate capability following one TMA/H2O exposure and identify enhanced rate capability versus pristine LMO. To understand this effect, we experimentally probe the surface composition of LMO with TMA/H2O treatment using X-ray photoelectron spectroscopy (XPS) measurements with inert-transfer. This includes a study of the LMO surface properties following TMA exposure before exposure to H2O. We identify the removal of a surface carbonate layer from LMO upon TMA exposure, and the formation of a Li-rich aluminum oxide surface layer upon subsequent H2O exposure. We also observe a previously undescribed phenomenon of Li ions depleting from the LMO surface upon TMA exposure and returning upon H2O exposure. Ab initio materials modeling is used to complement our measurements and provides fundamental understanding for the effects we observe. The chemical effects we observe are connected with the enhanced rate capability of ALD-coated LMO and are related to a range of emerging studies on carbonate surface layers in battery cathodes, as well as the use of ALD to stabilize battery interfaces. Figure 1

Atomic Resolution Imaging of the Cathode-Electrolyte Interface on a LiMn2O4 Electrode

Understanding the nature and composition of the cathode-electrolyte interface (CEI) layer in lithium-ion cells is a key element for the development of safer and longer lasting batteries. The CEI layer is a passivation layer formed, after electrolyte decomposition, on the surface of a positive electrode in lithium-ion batteries. Analogous to the solid electrolyte interface (SEI) on the anode, it is usually thinner (~5 nm vs 10-100 nm) and composed of poor ion-conductors that cause an additional resistance to the Li+ ions path [1]–[3]. Different surface sensitive techniques have been previously used to study the composition of the CEI layer and a model for its formation has been proposed [4]. However, its structure and composition are not fully understood yet and experimental techniques with sub-nanometer resolution are needed for an accurate characterization of this layer. Atom probe tomography (APT) is probably the best candidate, being a powerful technique for three-dimensional analysis of sub-nanometer structures and compositional variations that can achieve atomic resolution, along with chemical sensitivity for Li [5], [6]. In this work we successfully investigated for the first time the CEI layer formation on the surface of a LiMn2O4 (LMO) electrode using the APT technique. A commercial LMO cathode (from MTI) was calendared for ten days in a cell using standard 1M LiPF6 electrolyte in EC:DMC, and lithium metal as a counter and reference electrode. Three-electrode impedance measurements were performed at the OCV with regular intervals to observe and resolve the growth of the CEI layer on the cathode from the growth of the SEI layer on the lithium metal anode. After disassembling the cell, we prepared the sample for APT analysis and determined with atomic resolution the composition and structure of the CEI. A mixed layered- and mosaic-structure was observed, consisting of two inner Mn-rich layers adsorbed on the LMO surface and an external mosaic structure, mainly composed by LiF, Li2O and Li-carboxylates.

First Principles Studies of Solid-Solid Interfaces between LiMn2O4 and Protective Coatings

Numerous studies have shown that protective surface coatings are effective in reducing the well-documented problem of Mn ion dissolution from the surface of the spinel LiMn2O4 (LMO) Li-ion battery cathode.[1] Although recent computational work has identified many promising coating chemistries based on bulk thermochemical properties,[2,3] there remains much to be understood regarding the interface formed between protective coatings and the LMO electrode and how such interfacial properties may influence electrochemical performance. In this work, first principles density functional theory calculations are performed on models for both oxide and metallic coatings on the LMO surface. First, we elucidate the mechanism for the onset of Al2O3 film growth by atomic layer deposition (ALD) and show that precursor decomposition in early ALD pulses leads to adsorption site blocking and sub-monolayer film growth.[4] During ALD, the trimethylaluminum [Al(CH3)3] precursor loses its -CH3 groups to the LMO surface, where the Al(CH3)x* and CH3* surface adsorbates each behave as Lewis bases (electron donors), where near-surface Mn ions are the Lewis acidic adduct (electron acceptor). These studies are extended to various thermodynamically stable low- and high-index LMO surface models,[5] demonstrating that ALD on LMO is structure-sensitive toward stepped sites, which are more electron accepting as suggested by calculated oxygen vacancy formation energies. Moreover, we find that the thermodynamics of decomposed intermediates are linearly related to the corresponding oxygen vacancy formation energy, or the relative Lewis acidity of near-surface Mn ions, of each given surface facet and termination. We propose that selective ALD leads to partial coatings formed at defect sites which may be particularly vulnerable to Mn dissolution. Next, we model the process of delithiation near the surface of an LMO coated with metallic Au, which effectively mitigates Mn dissolution. Moreover, Au coatings have been shown to improve kinetics and do not strip under the potentials where LMO electrochemistry occurs.[6] DFT calculations show that a Li+-deficient near-surface is thermodynamically favored for Au-coated LMO. We demonstrate through local density of states and chemical bonding analyses that Au hybridizes with LMO upon delithiation, leading to partial oxidation on Au electronic states rather than full localization of holes on Mn ions. The Au coating has a lower work function than LMO, driving electron transfer from Au to LMO. This is also associated with downward bending of the LMO conduction band in response to Fermi level pinning at the LMO/Au interface. Operando X-ray diffraction experiments show that these interfacial effects may translate to a reduction in the amount of Li+ that can intercalate into the bulk of Au-coated LMO. These studies help elucidate the structure-property relationships of protective coatings at the LMO surface. In particular, this work highlights the driving forces for nucleation and growth of protective coatings by ALD, as well as the influence of interfacial electronic structure on intercalation chemistry of coated electrode materials. This research is supported as part of the Center for Electrochemical Energy Science, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

Mechanism for Al2O3 Atomic Layer Deposition on LiMn2O4 from In Situ Measurements and Ab Initio Calculations

Summary Here, we elucidate the mechanism for Al2O3 atomic layer deposition (ALD) on LiMn2O4 (LMO) cathodes for lithium-ion batteries by using in situ and ex situ experimental characterization coupled with density functional theory (DFT) calculations. We demonstrate that not only does Al2O3 coat the LMO, but the Al heteroatom of the trimethylaluminum (TMA) precursor also dopes to interstitial sites on the LMO surface, thereby reducing the oxidation state of near-surface Mn ions. DFT calculations further suggest facile transfer of methyl groups from the TMA precursor to oxygen atoms on the LMO surface, which blocks adsorption sites for subsequent TMA adsorption. These predictions are supported by quartz crystal microbalance experiments demonstrating inhibited growth below ten ALD Al2O3 cycles, suggesting that sub-monolayer coverages of alumina are present on the LMO surface in the early stages of film growth. In comparison with fully conformal films, these sub-monolayer coatings show enhanced electrochemical capacity when cycled in coin cells.

Effect of electrolyte additives on high-temperature cycling performance of spinel LiMn2O4 cathode

Lithium difluoroborate (LiDFOB), lithium bis(oxalato)borate (LiBOB), lithium difluoro(oxalato)borate (LiDFBOP) and lithium difluorophosphate (LiPF2O2) are investigated as electrolyte additives to alleviate the severe cycle capacity fading of spinel LiMn2O4 cathode of lithium-ion batteries, especially at elevated temperatures. Compared with that of the routine electrolyte, the capacity retention is significantly improved at both room temperature and 55 °C by adding LiBOB and LiDFOB as electrolyte additives. Moreover, surface layer formation processes on the LiMn2O4 electrode in the presence of the LiBOB, LiDFOB, LiDFBOP and LiPF2O2 are investigated by photoelectron spectroscopy (XPS) and X-ray diffraction. According to the analysis results, BOB− anions from LiBOB or LiDFOB bond with the dissolved Mn2+ to form an insoluble and stable surface layer on the LiMn2O4 surface, which is beneficial to the suppression of the LiMn2O4 dissolution and electrolyte decomposition, and eventually to the improvement of the cycling performance at elevated temperatures.

Surface Engineering of a LiMn2O4 Electrode Using Nanoscale Polymer Thin Films via Chemical Vapor Deposition Polymerization.

Surface engineering is a critical technique for improving the performance of lithium-ion batteries (LIBs). Here, we introduce a novel vapor-based technique, namely, chemical vapor deposition polymerization, that can engineer nanoscale polymer thin films with controllable thickness and composition on the surface of battery electrodes. This technique enables us to, for the first time, systematically compare the effects of a conducting poly(3,4-ethylenedioxythiophene) (PEDOT) polymer and an insulating poly(divinylbenzene) (PDVB) polymer on the performance of a LiMn2O4 electrode in LIBs. Our results show that conducting PEDOT coatings improve both the rate and the cycling performance of LiMn2O4 electrodes, whereas insulating PDVB coatings have little effect on these performances. The PEDOT coating increases 10 C rate capacity by 83% at 25 °C (from 23 to 42 mA h/g) and by 30% at 50 °C (from 64 to 83 mA h/g). Furthermore, the PEDOT coating extends the high-temperature (50 °C) cycling life of LiMn2O4 by over 60%. A model is developed, which can precisely describe the capacity degradation exhibited by the different types of cells, based on the aging mechanisms of Mn dissolution and solid-electrolyte interphase growth. Results from X-ray photoelectron spectroscopy suggest that chemical or coordination bonds form between Mn in LiMn2O4 and O and S in the PEDOT film. These bonds stabilize the surface of LiMn2O4 and thus improve the cycling performance. In contrast, no bonds form between Mn and the elements in the PDVB film. We further demonstrate that this vapor-based technique can be extended to other cathodes for advanced LIBs.