A review of recent developments in the surface modification of LiMn2O4 as cathode material of power lithium-ion battery

LiMn₂O₄ (LMO) is a very attractive choice as cathode material for power lithium-ion batteries due to its economical and environmental advantages. However, LiMn₂O₄ in the 4-V region suffers from a poor cycling behavior. Recent research results confirm that modification by coating is an important method to achieve improved electrochemical performance of LMO, and the latest progress was reviewed in the paper. The surface treatment of LMO by coating oxides and nonoxide systems could decrease the surface area to retard the side reactions between the electrode and electrolyte and to further diminish the Mn dissolution during cycling test. At present, LiMn₂O₄ is the mainstreaming cathode material of power lithium-ion battery, and, especially the modified LMO, is the trend of development of power lithium-ion battery cathode material in the long term.     

Ionic liquid-based electrolyte with dual-functional LiDFOB additive toward high-performance LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript> batteries

Manganese oxide-based cathodes are one of the most promising lithium-ion battery (LIB) cathode materials due to their cost-effectiveness, high discharge voltage plateau (above 4.0 V vs. Li/Li⁺), superior rate capability, and environmental benignity. However, these batteries using conventional LiPF₆-based electrolytes suffer from Mn dissolution and poor cyclic capability at elevated temperature. In this paper, the ionic liquid (IL)-based electrolytes, consisting of 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfon)imidate (PYR_1,4-TFSI), propylene carbonate (PC), lithium bis(trifluoromethanesulfon)imide (LiTFSI), and lithium oxalyldifluoroborate (LiDFOB) additive, were explored for improving the high temperature performance of the LiMn₂O₄ batteries. It was demonstrated that LiTFSI-ILs/PC electrolyte associated with LiDFOB addition possessed less Mn dissolution and Al corrosion at the elevated temperature in LiMn₂O₄/Li batteries. Cyclic voltammetry and electrochemical impedance spectroscopy implied that this kind of electrolyte also contributed to the formation of a highly stable solid electrolyte interface (SEI), which was in accordance with the polarization measurement and the Li deposition morphology of the symmetric lithium metal cell, thus beneficial for improving the cycling performance of the LiMn₂O₄ batteries at the elevated temperature. Cyclic voltammetry and electrochemical impedance spectroscopy implied that the cells using this kind of electrolyte exhibited better interfacial stability, which was further verified by the polarization measurement and the Li deposition morphology of the symmetric lithium metal cell, thus beneficial for improving the cycling performance of the LiMn₂O₄ batteries at the elevated temperature. These unique characteristics would endow this kind of electrolyte a very promising candidate for the manganese oxide-based batteries.     

Effect of heptamethyldisilazane on the electrochemical performance of LiMn2O4/Li

The effect of heptamethyldisilazane as an electrolyte stabilizer on the cycling performance of a LiMn₂O₄/Li cell at different rates at 30 °C and the storage performance at 60 °C is investigated systematically based on conductivity test, linear sweep voltage, electrochemical impedance spectroscopy, scanning electron microscopy, X-ray diffraction, and charge–discharge measurements. The results show that heptamethyldisilazane added into the LiPF₆-based electrolyte can increase the stability of the original electrolyte; coulomb efficiency, the initial discharge capacity, and cycling performance at different rates in a sense, meanwhile, improve the storage performance at elevated temperature, although the C-rate performance of the cell is a little worse than that without heptamethyldisilazane in the electrolyte. When the LiMn₂O₄/Li cell with heptamethyldisilazane in the LiPF₆-based electrolyte stored at 60 °C for a week cycles 300 times, the capacity retention is up to 91.18 %, which is much higher than that (87.18 %) without the additive in the electrolyte. This is mainly due to the lower solid electrolyte interface resistance (R _f) in the cell, followed by the better morphology and structure of the cathode after storage at 60 °C for a week compared with the LiMn₂O₄/Li cell without heptamethyldisilazane.     

Enhancement of the electrochemical properties of LiMn2O4 by glycolic acid-assisted sol-gel method

LiMn₂O₄ spinel cathode was synthesized by the sol–gel method by using glycolic acid as a chelating agent. The sample exhibited a pure cubic spinel structure without any impurities in the X-ray diffraction (XRD) patterns. The result of the electrochemical performances on the sample compared to those of electrodes based on LiMn₂O₄ spinel synthesized by solid state. LiMn₂O₄ synthesized by glycolic acid-assisted sol–gel method improves the cycling stability of electrode. The capacity retention of sol–gel-synthesized LiMn₂O₄ was about 90• after 100 cycles between 3.0 and 4.4 V at room temperature. The electrochemical performance of the LiMn₂O₄ (sol–gel) and LiMn₂O₄ (solid state) were investigated under 40• between 3.0 and 4.4 V. XRD results of the cathode material after 50 cycles at 40• revealed that LiMn₂O₄ (sol–gel) could effectively suppress the LiMn₂O₄ dissolving of into electrolyte and resulted in a better stability.     

Recent progress in the electrolytes for improving the cycling stability of LiNi<Subscript>0.5</Subscript>Mn<Subscript>1.5</Subscript>O<Subscript>4</Subscript> high-voltage cathode

High-voltage spinel LiNi_0.5Mn_1.5O₄ has been considered one of the most promising cathode materials for lithium-ion power batteries used in electrical vehicles (EVs) or hybrid electrical vehicles (HEVs) because the high voltage plateau at around 4.7 V makes its energy density (658 Wh kg^?1) 30 and 25 % higher than that of conventional pristine spinel LiMn₂O₄ (440 Wh kg^?1) or olivine LiFePO₄ (500 Wh kg^?1) materials, respectively. Unfortunately, LiNi_0.5Mn_1.5O₄-based batteries with LiPF₆-based carbonate electrolytes always suffer from severe capacity deterioration and poor thermostability because of the oxidization of organic carbonate solvents and decomposition of LiPF₆, especially at elevated temperatures and water-containing environment. The major goal of this review is to highlight the recent advancements in the development of advanced electrolytes for improving the cycling stability and rate capacity of LiNi_0.5Mn_1.5O₄-based batteries. Finally, an insight into the future research and further development of advanced electrolytes for LiNi_0.5Mn_1.5O₄-based batteries is discussed.     

Phenomenon of enhanced diffusion of lithium-ion in LiMn2O4 induced by electrochemical cycling

Lithium-ion diffusion in insertion-host materials is of significant interest because of its importance in improving the power density of lithium-ion batteries. In this study, the dependence of the chemical diffusion coefficient (D) of lithium-ion in spinel LiMn₂O₄ cathode material on electrochemical cycling has been investigated by the capacity intermittent titration technique (CITT). Results show that there are two minimum peaks in the curves of D∼E respectively at ∼3.95 and ∼4.12 V in the voltage range from 3.85 to 4.30 V. The curves of D∼E at different cycles show an interesting phenomenon that the values of D tend to increase with the cycling numbers. This phenomenon indicates an enhanced diffusion of lithium-ion in LiMn₂O₄ cathode material induced by the electrochemical cycling.     

Dinitrile compound containing ethylene oxide moiety with enhanced solubility of lithium salts as electrolyte solvent for high-voltage lithium-ion batteries

A dinitrile compound containing ethylene oxide moiety (4,7-dioxa-1,10-decanedinitrile, NEON) is synthesized as an electrolyte solvent for high-voltage lithium-ion batteries. The introduction of ethylene oxide moiety into the conventional aprotic aliphatic dinitrile compounds improves the solubility of lithium hexafluorophosphate (LiPF₆) used commercially in the lithium-ion battery industry. The electrochemical performances of the NEON-based electrolyte (0.8 M LiPF₆?+?0.2 M lithium oxalyldifluoroborate in NEON:EC:DEC, v:v:v?=?1:1:1) are evaluated in graphite/Li, LiCoO₂/Li, and LiCoO₂/graphite cells. Half-cell tests show that the electrolyte exhibits significantly improved compatibility with graphite by the addition of vinylene carbonate and lithium oxalyldifluoroborate and excellent cycling stability with a capacity retention of 97 % after 50 cycles at a cutoff voltage of 4.4 V in LiCoO₂/Li cell. A comparative experiment in LiCoO₂/graphite full cells shows that the electrolyte (NEON:EC:DEC, v:v:v?=?1:1:1) exhibits improved cycling stability at 4.4 V compared with the electrolyte without NEON (EC:DEC, v:v?=?1:1), demonstrating that NEON has a great potential as an electrolyte solvent for the high-voltage application in lithium-ion batteries.     

Significance of Mg doped LiMn2O4 spinels as attractive 4 V cathode materials for use in lithium batteries

LiMn₂O₄ spinel is one of the most promising cathode materials for lithium-ion batteries because of its cheapness and eco-friendliness. Due to Jahn-Teller distortion, the capacity fades, however, upon repeated cycling. Attempts are being made to improve the cycle life of the spinel by substitution of manganese with other cations. In this paper we report the effect of partial substitution of manganese by Mg²⁺ ions in the LiMn₂O₄ phase. LiMg_yMn_2−yO₄ (y=0 – 0.3) has been synthesized by a thermal method and characterized using XRD, TG/DTA and FTIR. The electrochemical performance is correlated with the dopant concentration.     

Investigation of the synergetic effects of LiBF<Subscript>4</Subscript> and LiODFB as wide-temperature electrolyte salts in lithium-ion batteries

Herein, we present the use of lithium tetrafluoroborate (LiBF₄) as an electrolyte salt for wide-temperature electrolytes in lithium-ion batteries. The research focused on the application of blend salts to exhibit their synergistic effect especially in a wide temperature range. In the study, LiCoO₂ was employed as the cathode material; LiBF₄ and lithium difluoro(oxalate)borate (LiODFB) were added to an electrolyte consisting of ethylene carbonate (EC), propylene carbonate (PC), and ethyl methyl carbonate (EMC). The electrochemical performance of the resulting electrolyte was evaluated through various analytical techniques. Analysis of the electrical conductivity showed the relationship among solution conductivity, the electrolyte composition, and temperature. Cyclic voltammetry (CV), charge-discharge cycling, and AC impedance measurements were used to investigate the capacity and cycling stability of the LiCoO₂ cathode in different electrolyte systems and at different temperatures. Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were applied to analyze the surface properties of the LiCoO₂ cathode after cycling. The results indicated that the addition of a small amount of LiODFB into the LiBF₄-based electrolyte system (LiBF₄/LiODFB of 8:2) may enhance the electrochemical performance of the LiCoO₂ cell over a relatively wide temperature range and improve the cyclability of the LiCoO₂ cell at 60 °C.     

Design of ionic liquid-based hybrid electrolytes with additive for lithium insertion in graphite effectively and their effects on interfacial properties

To address the challenge of the IL-based electrolyte cannot be effectively intercalated in graphite anode, and especially the urgent needs for the compatibility between high performance and high security, the IL-based hybrid electrolyte systems with ethylene carbonate/propylene carbonate (EC/PC) as a co-solvent and vinylene carbonate (VC) as an additive were designed. The high dielectric constant of EC/PC significantly increased the ionic conductivity and lithium ion migration of the electrolyte system. Meanwhile, the presence of VC can form SEI preventing EC and PYR₁₄⁺ reductive decomposition on the electrode interface, and at the same moment, the SEI promotes effective Li cation insertion into the graphene interlayer. The Li/C half-cells showed high reversible capacity, cycling efficiency, and good cycle stability with the IL-based hybrid electrolyte. It is worth to highlight the better performance, in terms of the excellent thermal stability and high safety. Thus, the IL-based hybrid electrolyte combined with good electrochemical performance holds substantial promise for lithium-ion battery, and should have broad application prospects in the high energy density, especially high-security requirements, of the new lithium-ion battery.     

Recent developments in the doping of LiNi<Subscript>0.5</Subscript>Mn<Subscript>1.5</Subscript>O<Subscript>4</Subscript> cathode material for 5 V lithium-ion batteries

LiMn₂O₄ has been considered a promising cathode material for lithium-ion batteries in electric vehicles. However, there are still a number of problems of severe capacity fading before any materials modifications. Among all doped LiMn₂O₄, spinel LiNi_0.5Mn_1.5O₄ material is seen as a potential cathode material for use in electric vehicles and energy storage systems in the future because of its high working potential (4.7 V), high energy density (the energy density of LiNi_0.5Mn_1.5O₄ is 20% higher than that of LiCoO₂), acceptable stability, and good cycling performance. In the presented paper, the structure and electrochemical performance of doped LiNi_0.5Mn_1.5O₄ are reviewed. The rate capability, rate performance and cyclic life of various doped LiNi_0.5Mn_1.5O₄ materials are described. This review also focuses on the present status of doped LiNi_0.5Mn_1.5O₄, then on its near future developments.     

Preparation, characterization and electrochemical performance of γ-MnO2 and LiMn2O4 as cathodes for lithium batteries

The spinel LiMn₂O₄ is a very promising cathode material with economical and environmental advantages. LiMn₂O₄ materials have been synthesized by solid state method using γ-MnO₂ as manganese source, and Li₂CO₃ or LiNO₃ as Li sources. γ-MnO₂ is a commercial battery grade electrolytic manganese dioxide (TOSOH-Hellas GH-S) and LiMn₂O₄ samples were synthesized at a calcinations temperature up to 800 °C. γ-MnO₂ and LiMn₂O₄ samples were characterized by X-ray diffraction, thermal and electrochemical measurements. X-ray powder diffraction of as prepared LiMn₂O₄ showed a well-defined highly pure spinel single phase. The electrochemical performance of LiMn₂O₄ and its starting material γ-MnO₂ was evaluated through cyclic voltammetry, galvanostatic (constant current charge-discharge cycling) The electrochemical properties in terms of cycle performance were also discussed. γ-MnO₂ showed fairly high initial capacity of about 200 mAhg⁻¹ but poor cycle performance. LiMn₂O₄ samples showed fairly low initial capacity but good cycle performance.     

Nanocrystalline and long cycling LiMn2O4 cathode material derived by a solution combustion method for lithium ion batteries

A nanostructured LiMn₂O₄ spinel phase is used as a cathode for 4 V lithium batteries and is prepared by solution combustion synthesis using urea as a fuel. Lithium-manganese oxides have received more increasing attention in recent years as high-capacity intercalation cathodes for rechargeable lithium-ion batteries. Nanostructured electrodes have been shown to enhance the cell cyclability. For optimum synthesis, the spinel LiMn₂O₄ showed that the optimal heat treatment protocol was a 10 h calcination at 700 °C, which sustained 229 cycles between 3.0 and 4.3 V at a charge-discharge rate of 0.1 °C before reaching an 80% charge retention cut-off value. X-ray diffraction and electron diffraction pattern investigations demonstrate that all the LiMn₂O₄ products are a spinel phase crystal. TEM micrographs show the prepared products were highly crystalline with an average particle size of 20-50 nm. Cyclic voltammetry shows the absence of phase transitions in the samples ensures negligible strain, resulting in a longer cycle life. This work shows the feasibility of the solution combustion method for obtaining manganese oxides with nano-architecture and high cyclability, and suggests it is a promising method for providing small diffusion pathways that improve lithium-ion intercalation kinetics and minimize surface distortions during cycling.     

Nanopowders of spinel-type electrode materials for Li-ion batteries

Nanomaterials are becoming important for use in Li-ion battery electrodes as these can deliver increased capacity and improved power performance. Our work is focused on Mg-doped high-voltage spinel materials, such as LiNi_0.5Mn_1.5O₄, in order to improve its stability. LiMg_δNi_0.5−δMn_1.5O₄ with δ = 0.05, having the cubic spinel structure (P4₃32) were made via four different synthesis routes – a solid-state route, a sol–gel method, a xerogel route and an auto ignition method.The powders were investigated with SEM and TEM analysis. XRD was used to determine the crystallographic structure. Electrochemical tests were performed in CR2320 coin cells built with 1 M LiPF₆ in EC/EMC/DMC 1:2:2 as electrolyte and metallic Li as negative electrode – cells were measured with a MACCOR cycler.LiMg_0.05Ni_0.45Mn_1.5O₄ made via the sol–gel and xerogel routes revealed agglomerated nanoparticles with sizes ranging from 10 to 200 nm, whereas the auto ignition method gives particle sizes between 10 and 50 nm. Although agglomerated, often residual LiMn₂O₄ is observed, with increasing concentration going from solid-state, sol–gel, xerogel to auto ignition.Hence, thanks to these different synthesis routes, we are able to obtain particle sizes reaching from 10 to 200 nm, with a narrow particle size distribution. The electrochemical tests of the xerogel particles showed promising results. The auto ignition method show also promising results, however, the impurity phase needs to be suppressed significantly. The sol–gel method, the xerogel route and the auto ignition method show increased capacity retention at high power rates compared to the solid state method.     

Synthesis of sea urchin-like LiMn2O4 hollow macrospheres via in situ conversion for rechargeable lithium-ion batteries

Among several materials (transition metal oxide) under development for use as a cathode in lithium-ion batteries, cubic spinel LiMn₂O₄ is one of the most promising cathode materials. In this study, the sea urchin-like LiMn₂O₄ hollow macrospheres were synthesized by using sea urchin-like α-MnO₂ precursors through solid-state in situ self-sacrificing conversion route. The as-prepared LiMn₂O₄ was assembled by many single-crystalline “thorns” of ca.10–20 nm in diameter and ca. 400–500 nm in length. Galvanostatic battery testing showed that sea urchin-like LiMn₂O₄ had an initial discharge capacity of 126.8 mAh/g at the rate of 0.2 C in the potential range between 3.0 and 4.5 V. More than 96.67 % of the initial discharge capacity was maintained for over 50 cycles. The improved electrochemical properties were attributed to the reduced particle size and enhanced electrical contacts by the materials. This particular sea urchin-like structured composite conceptually provides a new strategy for designing electrodes in energy storage applications.     

Lithium polyacrylate-coated LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript> cathode materials with excellent performance for lithium ion batteries

A novel facile approach to coat LiMn₂O₄ by lithium polyacrylate (PAALi) is demonstrated. The PAALi-coated LiMn₂O₄ (LMO@2%PAALi) and LiMn₂O₄ (LMO) are characterized by charge–discharge tests, X-ray diffraction (XRD), PAALi dissolving experiment, transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), thermogravimetry (TG), and inductively coupled plasma optical emission spectrometer (ICP-OES). XRD and FTIR analyses indicate that there are no clear differences between LMO@2%PAALi and LMO. PAALi dissolving experiment indicates that PAALi is indissolvable in LiPF₆-EC/DMC/EMC electrolyte. TEM results reveal that LiMn₂O₄ particles are coated by PAALi. ICP-OES results indicate that this stable PAALi coating can prevent the Mn ions dissolving from active LiMn₂O₄ materials and then the stability of LiMn₂O₄ crystals in electrolyte are greatly enhanced. These unique features ensure that LMO@2%PAALi possesses much better rate performance, higher discharge capacity, better cycling performance, and lower charge transfer resistance over LMO. The discharge capacity of LMO@2%PAALi at 0.2 C reaches up to 127.2 mAh g^?1 at room temperature.     

Glassy carbon - A promising substrate material for pulsed laser deposition of thin Li1+xMn2O4−δ electrodes

The spinel LiMn₂O₄ is a promising candidate for future battery applications. If used as a positive electrode in a battery, the charging capacity of such a battery element is limited by the formation of a solid electrolyte interphase like layer between the electrolyte and the spinel. To study the electrolyte-electrode interaction during electrochemical cycling, spinel thin films are deposited as model electrodes on glassy carbon substrates by pulsed laser ablation. The obtained polycrystalline oxide thin films show a well defined surface morphology and are electrochemical active. Adhesion of these thin films on glassy carbon is in general poor, but can be improved considerably by a surface pretreatment or adding a thin metallic coating to the substrate prior deposition. The best adhesion is obtained for films deposited on argon plasma pretreated as well as Pt coated glassy carbon substrates. During the electrochemical characterization of Li_1.06Mn₂O_3.8 thin film electrodes, no additional reactions of the substrate are observed independent of the used electrolyte. The best cycle stability is achieved for films on Pt coated glassy carbon substrates.     

Effect of monocationic ionic liquids as electrolyte additives on the electrochemical and thermal properties of Li-ion batteries

The conventional electrolyte system has been compared with the ionic liquid (IL) additive containing electrolyte system at room temperature as well as elevated temperature. In this work, two types of monocationic ILs such as 1-butyl-3-methylpyrrolidinium hexafluorophosphate (Pyr IL) and 1-ethyl-3-methylimidazolium hexafluorophosphate (IMI IL) are added as an additive at two different weight ratios in 1.15 M LiPF₆ (EC/EMC = 3/7 v/v) electrolyte solution, the structural, electrochemical and thermal characteristics of LiNi_0.80Co_0.15Al_0.05O₂ (NCA)/carbon full-cell in different electrolyte formulations have been reconnoitered. X-ray diffraction (XRD) studies have proved that IL as an electrolyte additive does not alter the structural stability of cathode materials after cycling. Under room temperature, Pyr IL additives at 1 wt% and 3 wt% deliver better cycleability than others, with the retention ratios of 93.62% and 92.8%, respectively. At elevated temperature, only 1 wt% Pyr IL additive is giving stable capacity retention ratio of 80.74%. Ionic conductivity and self-extinguishing time (SET) values are increasing with respect to the amount of additive added to the electrolyte. Thermal studies reveal that 3 wt% Pyr IL is favorable regarding the safety of the battery as it shows shifting of peak to higher temperature of 272.10 °C. Among the IL additives evaluated in this study, addition of 1 wt% Pyr IL is the most desirable additive for achieving the best cycling performance as well as thermal behavior of Li-ion batteries.     

Investigation on the storage performance of LiMn2O4 at elevated temperature with the mixture of electrolyte stabilizer

In order to investigate the effect of different electrolytes of LiPF₆-based and LiPF₆-based with the mixed additives of ethanolamine and heptamethyldisilazane on the storage performance of LiMn₂O₄, the commercial LiMn₂O₄ are added into these different electrolytes for storing deliberately at 60 °C in air for 4 h. The results show that the electrolyte with additives can prevent LiMn₂O₄ from being eroded by HF to a certain extent, and improve the storage performance of the material. The initial discharge capacities are 97.7 and 88.4 mAh g^?1 at 0.1 and 1?C, respectively, which are much higher than that 84.4 and 63.6 mAh?g^?1 of LiMn₂O₄ stored in the electrolyte without additives. Moreover, the former LiMn₂O₄ retains 89.1 % of its initial discharge capacity at 1?C after 150 cycles, while this is not up to 84 % for the latter.     

Novel approach to preparation of LiMn2O4 core/LiNixMn2−xO4 shell composite

Combining two methods, coating and doping, to modify spinel LiMn₂O₄, is a novel approach we used to synthesize active material. First we coated the LiMn₂O₄ particles with the nickel oxide particles by means of homogenous precipitation, and then the nickel oxide-coated LiMn₂O₄ was calcined at 750 °C to form a LiNi_xMn_2−xO₄ shell on the surface of spinel LiMn₂O₄ particles. Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), cyclic voltammetry (CV) and charge-discharge test were performed to characterize the spinel LiMn₂O₄ before and after modification. The experimental results indicated that a spinel LiMn₂O₄ core is surrounded by a LiNi_xMn_2−xO₄ shell. The resulting composite showed excellent electrochemical cycling performance with an average fading rate of 0.014% per cycle. This improved cycle stability is greatly attributed to the suppression of Jahn-Teller distortion on the surface of spinel LiMn₂O₄ particles during cycling.