Effect of sulfolane and lithium bis(oxalato)borate-based electrolytes on the performance of spinel LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript> cathodes at 55 °C

To seek a promising candidate electrolyte at elevated temperature for lithium manganese oxide (LiMn₂O₄)/Li cells, the electrochemical performance of 0.7 mol L^?1 LiBOB (lithium bis(oxalate)borate)-SL (sulfolane)/DEC (diethyl carbonate) (1:1, in volume) electrolyte was studied at 55 °C. The Mn dissolution in electrolyte was analyzed by inductively coupled plasma (ICP) analysis. AC impedance measurement and scanning electron microscopy (SEM) analysis were used to analyze the formation of the surface film on the LiMn₂O₄ electrode. The results demonstrate that the LiBOB-SL/DEC electrolyte can slow down the dissolution and erosion of Mn ions, and decrease the interface impedance. Moreover, the LiBOB-SL/DEC electrolyte could obviously improve the capacity retention, the operating voltage (4.05 V), and the rate performance of LiMn₂O₄/Li cells.     

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.     

An approach to improve the electrochemical performance of LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript> at high temperature

In order to overcome the severe capacity decay of LiMn₂O₄ at high temperature, TiN is used as an active materials additive in this paper. The XRD and XPS test results indicate that the TiN can effectively prevent Mn from dissolving in electrolyte; galvanostatic charge-discharge test shows that LiMn₂O₄ electrode with TiN exhibits remarkably improved capacity retention at high temperature with capacity of 105.1 mAh g^?1 at 1 C in the first cycle at 55 °C and the capacity maintains 88.9% retention after 150 cycles. And the electrochemical impedance spectroscopy result demonstrates TiN’s effectiveness in easing the increase of charge-transfer resistance during cycling. Therefore, we can conclude that TiN, as an addictive, made obvious contribution to the greatly improved electrochemical cycling performance of LiMn₂O₄.     

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.     

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.     

Storage performance with different charged state of manganese spinel battery

The power battery was manufactured with the commercial LiMn₂O₄ and graphite, and its storage performances with different charged state were studied. Structure, morphology, and surface-state change of the LiMn₂O₄ before and after storage were observed by XRD, SEM, XPS, CV, and AC technique, respectively. The electrochemical performances of LiMn₂O₄ battery were tested. The result shows that the capacity recovery of LiMn₂O₄ stored at discharge state is best (99.2%). While that of full-charged state is worst (93.6%). The cyclic performance of LiMn₂O₄ battery after storage is improved. The cyclic performance of LiMn₂O₄ stored at full-charged state is best (capacity retention ratio of 89.8% after 200?cycles), while that of before storage is 83.0%. The crystal of the spinel was destroyed after storage, and the intensity of breakage is increased with charge state increasing. The amount of soluble Mn and Li-ion migration resistance (R _f) are increased with charge state increasing, and the oxygen loss is detected.     

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.     

LiPF6-EC-MPC electrolyte for LiMn2O4 cathode in lithium-ion battery

Methyl propyl carbonate (MPC) is a promising single solvent for lithium-ion battery without addition of ethylene carbonate (EC), but it is unstable upon cycling because of exposure to the spinel LiMn₂O₄ cathode. Thus, we attempted to add EC to MPC in order to form LiPF₆-EC-MPC electrolyte; the effects of solvent ratio and salt concentration on the cycling performance of LiMn₂O₄ cathode were also investigated. The experiments were characterized by conductivity measurements, charge-discharge at a constant current density and voltage–capacity curves at low temperature. To further enhance our understanding of the performance improvement of LiMn₂O₄/Li cells, the electrochemical characterization techniques (such as, LSV, EIS) were performed on these cells. The results show that the ionic conductivity of the electrolyte and the cycling performance of the spinel LiMn₂O₄ cathode have been dramatically enhanced. From the point of view of operation at low temperature (− 20 °C), 1 M LiPF₆ EC/MPC (1/3) electrolyte is highly recommended for spinel LiMn₂O₄ cathode in lithium-ion battery.     

Surface modification of LiMn2O4 thin films at elevated temperature

In order to improve the cycle stability of spinel LiMn₂O₄ electrode at elevated temperature, the LiCoO₂-coated and Co-doped LiMn₂O₄ film were prepared by an electrostatic spray deposition (ESD) technique. LiCoO₂-coated LiMn₂O₄ film shows excellent cycling stability at 55 °C compared to pristine and Co-doped LiMn₂O₄ films. The samples were studied by X-ray diffraction, scanning electron microscopy, Auger electron spectroscopy, cyclic voltammetry and electrochemical impedance spectroscopy. The excellent performance of LiCoO₂-coated LiMn₂O₄ film can be explained by suppression of Mn dissolution. On the other hand, the LiCoO₂-layer on the LiMn₂O₄ surface allows a homogenous Li⁺ insertion/extraction during electrochemical cycles and improves its structure stability.     

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.     

One-step solid-state synthesis of nanosized LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript> cathode material with power properties

A simple one-step solid state reaction way of preparing nanosized LiMn₂O₄ powders with high-rate properties is investigated. Oxalic acid is used as a functional material to lose volatile gases during the process of calcining in order to control the morphology and change the particle size of materials. The results of X-ray diffraction and scanning electron microscopy show that particle size of materials decreases with the increase of the oxalic acid content. The electrochemical test results indicate that optimal LiMn₂O₄ particles (S_0.5) is synthesized when the molar ratios of oxalic acid and total Mn source are 0.5:1. It also manifests that LiMn₂O₄ sample with middle size has the optimal electrochemical performance among five samples instead of the smallest LiMn₂O₄ sample. The obtained sample S_0.5 with middle size exhibits a high initial discharge capacity of 125.8 mAh g^?1 at 0.2C and 91.4% capacity retention over 100 cycles at 0.5C, superior to any one of other samples. In addition, when cycling at the high rate of 10C, the optimal S_0.5 in this work could still reach a discharge capacity of 80.8 mAh g^?1. This observation can be addressed to the fact that the middle size particles balance the contradictory of diffusion length in solid phase and particle agglomeration, which leads to perfect contacts with the conductive additive, considerable apparent Li-ion diffusion rate, and the optimal performance of S_0.5.     

Improvement of storage performance of LiMn2O4/graphite battery with AlF3-coated LiMn2O4

LiMn₂O₄/graphite batteries using AlF₃-coated LiMn₂O₄ have been fabricated and their electrochemical performance including discharge capacity and cyclic and storage performances have been tested and compared with pristine LiMn₂O₄/graphite batteries. The LiMn₂O₄/graphite battery with AlF₃-coated LiMn₂O₄ shows better capacity (108.5 mAhg^?1), cyclic performance (capacity retention of 92.7 % after 70 cycles), and capacity recovery ratio (98.6 %) than the pristine LiMn₂O₄ battery. X-ray diffraction patterns shows that the spinel structure of AlF₃-coated LiMn₂O₄ can be controlled better than that of pristine LiMn₂O₄ after storage. The improvement in electrochemical performance of the AlF₃-coated LiMn₂O₄/graphite battery is due to the fact that AlF₃ acts as a stabilizer and can protect the oxide structure from damaging during storage, leading to a smaller resistance and polarization after storage.     

Effects of binder content on manganese dissolution and electrochemical performances of spinel lithium manganese oxide cathodes for lithium ion batteries

In this study, the effects of the polyvinylidene fluoride (PVdF) binder on the Mn dissolution behavior and electrochemical performances of LiMn₂O₄ (LMO) electrodes are investigated. It is found that increasing the PVdF content (3, 5, 7, and 10 wt.%) leads to reduced Mn dissolution, and thus superior cycle performance at elevated temperature (60 °C). This can be ascribed to increased binder coverage on the LMO surface, as evidenced by X-ray photoelectron spectroscopy measurements, which acts a role as a passivation layer for Mn dissolution. The rate capability of the LMO electrode is hardly deteriorated as the PVdF content increases, despite the increasing surface coverage. Electrochemical impedance measurements reveal that the LMO electrode with higher binder loading exhibits lower electrode impedance, which is suggested to be due to enhanced electronic passage through the composite LMO electrode.     

Ultrathin surface coatings to enhance cycling stability of LiMn2O4 cathode in lithium-ion batteries

Spinel LiMn₂O₄ suffers from severe dissolution when used as a cathode material in rechargeable Li-ion batteries. To enhance the cycling stability of LiMn₂O₄, we use the atomic layer deposition (ALD) method to deposit ultrathin and highly conformal Al₂O₃ coatings (as thin as 0.6–1.2 nm) onto LiMn₂O₄ cathodes with precise thickness control at atomic scale. Both bare and ALD-coated cathodes are cycled at a specific current of 300 mA g^?1 (2.5 C) in a potential range of 3.4–4.5 V (vs. Li/Li⁺). All ALD-coated cathodes exhibit significantly improved cycleability compared to bare cathodes. Particularly, the cathode coated with six Al₂O₃ ALD layers (0.9 nm thick) shows the best cycling performance, delivering an initial capacity of 101.5 mA h?g^?1 and a final capacity of 96.5 mA h?g^?1 after 100 cycles, while bare cathode delivers an initial capacity of 100.6 mA h?g^?1 and a final capacity of only 78.6 mA h?g^?1. Such enhanced electrochemical performances of ALD-coated cathodes are ascribed to the high-quality ALD oxide coatings that are highly conformal, dense, and complete, and thus protect active material from severe dissolution into electrolytes. Besides, cycling performances of coated cathodes can be easily optimized by accurately tuning coating thickness via varying ALD growth cycles.     

Impacts of lithium tetrafluoroborate and lithium difluoro(oxalate)borate as additives on the storage life of Li-ion battery at elevated temperature

The impacts of boron-based Li salt additives including lithium tetrafluoroborate (LiBF₄) and lithium difluoro(oxalate)borate (LDFOB) on the storage life of Li-ion battery at elevated temperature are investigated. Adding 1 wt% additives in the electrolyte significantly affects the storage life of the LiNi_0.8Co_0.15Al_0.05O₂/graphite full cell at 55 °C. The anode solid electrolyte interphase (SEI), preventing the loss of Li⁺ and e^? in anode, is the key factor affecting the storage life. The formation and aging of SEI on the graphite anode with and without additives are investigated. It is found that the SEI formed with the addition of LiBF₄ is thick and loose due to LiF crystals produced by the decomposition of LiBF₄ and the SEI cannot prevent the Li⁺ and e^? loss in anode and the decomposition of the electrolyte solvent, resulting in shorter storage life of the battery. On the contrary, the SEI formed with the addition of LDFOB is thick and compact due to formation of the lithium oxalate in the SEI, produced by the decomposition of LDFOB. The SEI efficiently inhibits decomposition of the electrolyte solvent on anode and makes a longer storage life of the battery.     

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.     

Thin aluminum film improving the cycle performance of positive electrode of lithium ion battery

A novel method was investigated to improve the cycle performance of the spinel LiMn₂O₄. It is widely different from the traditional way of modifying LiMn₂O₄ particle with compounds or metals. In our study, instead of coating LiMn₂O₄ particle itself with compounds or metals, first we covered the current collector with the mixture of LiMn₂O₄ particle, conductive agents and binders, and then deposited an aluminum film onto it by vacuum evaporation technique. Both of the pristine electrode and the modified one were characterized by scanning electron microscope (SEM), X-ray diffraction (XRD), and charge-discharge tests. The results of SEM and XRD demonstrate that the aluminum film was formed successfully onto the positive electrode. And the charge-discharge tests show that the capacity retention of pristine electrode and modified one are 63.7% and 93.5% at C/2 rate in the voltage range of 3.5-4.3 V after 200 cycles, respectively. The modified electrode also shows better rate capability in comparison with the pristine one. The improved cycling stability is attributed to the minimizing of Mn dissolution into electrolyte solution and the good electronic conductivity of deposited aluminum film.     

Synthesis of Co-coated lithium manganese oxide and its characterization as cathode for lithium ion battery

Co-coated LiMn₂O₄ was synthesized by electroless plating. The phase identification, surface morphology, and electrochemical properties of the synthesized powders were studied by X-ray diffraction, scanning electron microscopy, electrochemical impedance spectroscopy, and galvanostatic charge–discharge experiments, respectively. The result shows that Co-coated LiMn₂O₄ particle has a coarse surface with a lot of holes. The specific capacity of Co-coated LiMn₂O₄ is 118 mAh g⁻¹, which is a bit less than 123 mAh g⁻¹ for the uncoated LiMn₂O₄. The capacity retention of Co-coated LiMn₂O₄ is 11% higher than the uncoated LiMn₂O₄ when the electrode is cycled at room temperature for 20 times. When cycled at the temperature of 55 °C, the capacity retention of Co-coated LiMn₂O₄ becomes 15% higher than the uncoated one.     

表面效应对锂离子电池正极材料LiMn2O4性能的影响

应用基于自旋极化和广义梯度近似(generalized gradient approximation,GGA)的密度泛函理论计算,研究了锂离子电池正极材料LiMn₂O₄ (001)表面原子和电子结构.发现表面和亚表面附近的原子在垂直于(001)面的方向上具有非常大的弛豫,这对LiMn₂O₄材料在锂离子电池中应用时发现的表面Mn的溶解现象有很大关联.由于表面效应,在LiMn₂O₄ (001) 表面只有三价Mn³⁺离子存在,而这些三价锰离子非常活跃,在该材料电极/电解液界面很容易发生歧化反应,从而加速了Mn的溶解.其他计算结果也和实验观察相符合. 关键词： 锂二次电池 表面弛豫 从头算     

Elevated electrochemical property of LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript> originated from nano-sized Mn<Subscript>3</Subscript>O<Subscript>4</Subscript>

By employment of nano-sized pre-prepared Mn₃O₄ as precursor, LiMn₂O₄ particles have been successfully prepared by facile solid state method and sol-gel route, respectively. And the reaction mechanism of the used precursors of Mn₃O₄ is studied. The structure, morphology, and element distribution of the as-synthesized LiMn₂O₄ samples are characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). Compared with LiMn₂O₄ synthesized by facile solid state method (SS-LMO), LiMn₂O₄ synthesized by modified sol-gel route (SG-LMO) possesses higher crystallinity, smaller average particle size (~175 nm), higher lithium chemical diffusion coefficient (1.17 × 10^?11 cm² s^?1), as well as superior electrochemical performance. For example, the cell based on SG-LMO can deliver a capacity of 85.5 mAh g^?1 at a high rate of 5 °C, and manifests 88.3% capacity retention after 100 cycles at 0.5 °C when cycling at 45 °C. The good electrochemical performance of the cell based on SG-LMO is ascribed mainly to its small particle size, high degree of dispersion, and uniform element distribution in bulk material. In addition, the lower polarization potential accelerates Li⁺ ion migration, and the lower atom location confused degree maintains integrity of crystal structure, both of which can effectively improve the rate capability and cyclability of SG-LMO.