Lithium diffusion in Li4Ti5O12 at high temperatures

Synthesis of the spinel lithium titanate Li₄Ti₅O₁₂ by an alkoxide-free sol-gel method is described. This method yields highly pure and crystalline Li₄Ti₅O₁₂ samples at relatively low temperature (850 °C) and via short thermal treatment (2 h). ⁶Li magic angle spinning nuclear magnetic resonance (MAS NMR) measurements on these samples were carried out at high magnetic field (21.1 T) and over a wide temperature range (295-680 K). The temperature dependence of the chemical shifts and integral intensities of the three ⁶Li resonances demonstrates the migration of lithium ions from the tetrahedral 8a to the octahedral 16c sites and the progressive phase transition from a spinel to a defective NaCl-type structure. This defective structure has an increased number of vacancies at the 8a site, which facilitate lithium diffusion through 16c → 8a → 16c pathways, hence providing an explanation for the reported increase in conductivity at high temperatures. Molecular dynamics simulations of the spinel oxides Li_4+xTi₅O₁₂, with 0 ≤ x ≤ 3, were also performed with a potential shell model in the temperature range 300-700 K. The simulations support the conclusions drawn from the NMR measurements and show a significant timescale separation between lithium diffusion through 8a and 16c sites and that out of the 16d sites.     

Nonflammable electrolyte for 3-V lithium-ion battery with spinel materials LiNi0.5Mn1.5O4 and Li4Ti5O12

The compatibility between dimethyl methylphosphonate (DMMP)-based electrolyte of 1 M LiPF₆/EC + DMC + DMMP (1:1:2 wt.) and spinel materials Li₄Ti₅O₁₂ and LiNi_0.5Mn_1.5O₄ was reviewed, respectively. The cell performance and impedance of 3-V LiNi_0.5Mn_1.5O₄/Li₄Ti₅O₁₂ lithium-ion cell with the DMMP-based nonflammable electrolyte was compared with the baseline electrolyte of 1 M LiPF₆/EC + DMC (1:1 wt.). The nonflammable DMMP-based electrolyte exhibited good compatibility with spinel Li₄Ti₅O₁₂ anode and high-voltage LiNi_0.5Mn_1.5O₄ cathode, and acceptable cycling performance in the LiNi_0.5Mn_1.5O₄/Li₄Ti₅O₁₂ full-cell, except for the higher impedance than that in the baseline electrolyte. All of the results disclosed that the 3 V LiNi_0.5Mn_1.5O₄/Li₄Ti₅O₁₂ lithium-ion battery was a promising choice for the nonflammable DMMP-based electrolyte.     

Effect of capacity matchup in the LiNi0.5Mn1.5O4/Li4Ti5O12 cells

The effect of the capacity matchup between cathode and anode in the LiNi_0.5Mn_1.5O₄/Li₄Ti₅O₁₂ cell system on cycling property, choice of electrolyte, high voltage and overcharge tolerances was investigated by comparing the cells with Li₄Ti₅O₁₂ limiting capacity with the cells with LiNi_0.5Mn_1.5O₄ limiting capacity. The former exhibits better cycling performance and less limitation of electrolyte choice than the latter. Furthermore, the Li₄Ti₅O₁₂-limited cell exhibits better tolerance to high voltage and overcharge than the LiNi_0.5Mn_1.5O₄-limited cell, owing to taking advantage of the extra capacity of Li₄Ti₅O₁₂ below 1 V. It is thus recommended that the LiNi_0.5Mn_1.5O₄/Li₄Ti₅O₁₂ cell whose capacity is limited by Li₄Ti₅O₁₂ anode should be used to extend the application of the state-of-the-art lithium-ion batteries.     

In situ X-ray diffraction studies of mixed LiMn2O4-LiNi1/3Co1/3Mn1/3O2 composite cathode in Li-ion cells during charge-discharge cycling

The structural changes of the composite cathode made by mixing spinel LiMn₂O₄ and layered LiNi_1/3Co_1/3Mn_1/3O₂ in 1:1 wt% in both Li-half and Li-ion cells during charge/discharge are studied by in situ XRD. During the first charge up to ∼5.2 V vs. Li/Li⁺, the in situ XRD spectra for the composite cathode in the Li-half cell track the structural changes of each component. At the early stage of charge, the lithium extraction takes place in the LiNi_1/3Co_1/3Mn_1/3O₂ component only. When the cell voltage reaches at ∼4.0 V vs. Li/Li⁺, lithium extraction from the spinel LiMn₂O₄ component starts and becomes the major contributor for the cell capacity due to the higher rate capability of LiMn₂O₄. When the voltage passed 4.3 V, the major structural changes are from the LiNi_1/3Co_1/3Mn_1/3O₂ component, while the LiMn₂O₄ component is almost unchanged. In the Li-ion cell using a MCMB anode and a composite cathode cycled between 2.5 V and 4.2 V, the structural changes are dominated by the spinel LiMn₂O₄ component, with much less changes in the layered LiNi_1/3Co_1/3Mn_1/3O₂ component, comparing with the Li-half cell results. These results give us valuable information about the structural changes relating to the contributions of each individual component to the cell capacity at certain charge/discharge state, which are helpful in designing and optimizing the composite cathode using spinel- and layered-type materials for Li-ion battery research.     

Physical and electrochemical properties of spherical Li1+x(Ni1/3Co1/3Mn1/3)1−xO2 cathode materials

A (Ni_1/3Co_1/3Mn_1/3)CO₃ precursor with an uniform, spherical morphology was prepared by coprecipitation using a continuously stirred tank reactor method. The as-prepared spherical (Ni_1/3Co_1/3Mn_1/3)CO₃ precursor served to produce dense, spherical Li_1+x(Ni_1/3Co_1/3Mn_1/3)_1−xO₂ (0 ≤ x ≤ 0.15) cathode materials. These Li-rich cathodes were also prepared by a second synthesis route that involved the use of an M₃O₄ (M = Ni_1/3Co_1/3Mn_1/3) spinel compound, itself obtained from the carbonate (Ni_1/3Co_1/3Mn_1/3)CO₃ precursor. In both cases, the final Li_1+x(Ni_1/3Co_1/3Mn_1/3)_1−xO₂ products were highly uniform, having a narrow particle size distribution (10-μm average particle size) as a result of the homogeneity and spherical morphology of the starting mixed-metal carbonate precursor. The rate capability of the Li_1+x(Ni_1/3Co_1/3Mn_1/3)_1−xO₂ electrode materials, which was significantly improved with increased lithium content, was found to be better in the case of the denser materials made from the spinel precursor compound. This result suggests that spherical morphology, high density, and increased lithium content were key factors in enabling the high rate capabilities, and hence the power performances, of the Li-rich Li_1+x(Ni_1/3Co_1/3Mn_1/3)_1−xO₂ cathodes.     

On the LiNi0.2Mn0.2Co0.6O2 positive electrode material

Layered LiNi_0.2Mn_0.2Co_0.6O₂ phase, belonging to a solid solution between LiNi_1/2Mn_1/2O₂ and LiCoO₂ most commercialized cathodes, was prepared via the combustion method at 900 °C for a short time (1 h). Structural, electrochemical and magnetic properties of this material were investigated. Rietveld analysis of the XRD pattern shows this compound as having the α-NaFeO₂ type structure (S.G. R-3m; a = 2.8399(2) ?; c = 14.165(1) ?) with almost none of the well-known Li/Ni cation disorder. SQUID measurements clearly indicate that the studied compound consists of Ni²⁺, Co³⁺ and Mn⁴⁺ ions in the crystal structure. X-ray analysis of the chemically delithiated Li_xNi_0.2Mn_0.2Co_0.6O₂ phases reveals that the rhombohedral symmetry was maintained during Li-extraction, confirmed by the monotonous variation of the potential-composition curve of the Li//Li_xNi_0.2Mn_0.2Co_0.6O₂ cell. LiNi_0.2Mn_0.2Co_0.6O₂ cathode has a discharge capacity of ∼160 mAh g⁻¹ in the voltage range 2.7-4.3 V corresponding to the extraction/insertion of 0.6 lithium ion with very low polarization. It exhibits a stable capacity on cycling and good rate capability in the rate range 0.2-2 C. The almost 2D structure of this cathode material, its good electrochemical performances and its relatively low cost comparing to LiCoO₂, make this material very promising for applications.     

Improvement of electrochemical properties of Li1.1Al0.05Mn1.85O4 achieved by an AlF3 coating

The electrochemical performance of AlF₃-coated Li_1.1Al_0.05Mn_1.85O₄ spinel was investigated. The morphology of the AlF₃-coated Li_1.1Al_0.05Mn_1.85O₄ was observed by SEM and TEM, and the thickness of the coating layer was approximately 10 nm. Capacity retention and rate capability were substantially improved by the AlF₃-coating, as compared to pristine Li_1.1Al_0.05Mn_1.85O₄. Manganese dissolution was also dramatically reduced for the AlF₃-coated Li_1.1Al_0.05Mn_1.85O₄, which may reflect lower impedance for the coated spinel. The thermal stability of the AlF₃-coated Li_1.1Al_0.05Mn_1.85O₄ was improved, exhibiting an exothermic reaction at higher temperature with reduced heat generation, compared to the pristine Li_1.1Al_0.05Mn_1.85O₄.     

Influence of conductive additives and surface fluorination on the charge/discharge behavior of lithium titanate (Li4/3Ti5/3O4)

Effect of conductive additives and surface modification with NF₃ and ClF₃ on the charge/discharge behavior of Li_4/3Ti_5/3O₄ (≈4.6 μm) was investigated using vapor grown carbon fiber (VGCF) and acetylene black (AB). VGCF and mixtures of VGCF and AB increased charge capacities of original Li_4/3Ti_5/3O₄ and those fluorinated with NF₃ by improving the electric contact between Li_4/3Ti_5/3O₄ particles and nickel current collector. Surface fluorination increased meso-pore with diameter of 2 nm and surface area of Li_4/3Ti_5/3O₄, which led to the increase in first charge capacities of Li_4/3Ti_5/3O₄ samples fluorinated by NF₃ at high current densities of 300 and 600 mA g⁻¹. The result shows that NF₃ is the better fluorinating agent for Li_4/3Ti_5/3O₄ than ClF₃.     

Eliminating the irreversible capacity loss of high capacity layered Li[Li0.2Mn0.54Ni0.13Co0.13]O2 cathode by blending with other lithium insertion hosts

The large irreversible capacity loss generally encountered with the high capacity layered oxide solid solutions between layered Li[Li_1/3Mn_2/3]O₂ and LiMO₂ (M = Mn, Ni, and Co) has been reduced by blending layered Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂, which is a solid solution between Li[Li_1/3Mn_2/3]O₂ and Li[Mn_1/3Ni_1/3Co_1/3]O₂, with spinel Li₄Mn₅O₁₂ or LiV₃O₈. The irreversible capacity loss decreases from 68 to 0 mAh g⁻¹ as the Li₄Mn₅O₁₂ content increases to 30 wt.% in the Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂-Li₄Mn₅O₁₂ composite and the LiV₃O₈ content increases to 18 wt.% in the Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂-LiV₃O₈ composite. The decrease in irreversible capacity loss is due to the ability of Li₄Mn₅O₁₂ or LiV₃O₈ to insert the extracted lithium that could not be inserted back into the layered Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂ during the first cycle. The Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂-LiV₃O₈ composite with ∼18 wt.% LiV₃O₈ exhibits a high capacity of ∼280 mAh g⁻¹ with little or no irreversible capacity loss and good cyclability.     

A high energy Li-ion battery based on nanosized LiNi0.5Mn1.5O4 cathode material

The electrochemical performance of a Li-ion battery made from nanometric, highly crystalline LiNi_0.5Mn_1.5O₄ as positive electrode and mesoporous carbon microbeads (MCMBs) as negative electrode was assessed. The best performance was obtained by using a slight excess of spinel (a cathode/anode mole ratio of 1.3) and lithium bis-oxalate borate (LiBOB) instead of LiPF₆ as an electrolyte salt. Higher spinel contents caused the formation of metallic Li in the carbon and the rapid degradation of battery performance as a result. The calculated output energy was 322 Wh kg⁻¹ which is higher than the value reported for the LiMn₂O₄/C cell (250 Wh kg⁻¹).     

Enhanced electrochemical properties of Li(Ni0.4Co0.3Mn0.3)O2 cathode by surface modification using Li3PO4-based materials

The surface of a commercial Li[Ni_0.4Co_0.3Mn_0.3]O₂ cathode is modified using Li₃PO₄-based coating materials. The electrochemical properties of the coated materials are investigated as a function of the pH value of the coating solution and the composition of coating materials. The Li₃PO₄ coating solution with pH 2 is found to be favorable for the formation of stable coating layers having enhanced electrochemical properties. The Li₃PO₄, Li_1.5PO₄, and PO₄ coating layers are formed as amorphous phases. However, the Li_3−xNi_x/2PO₄ coating layers are composed of small particles with a crystalline phase covered with an amorphous phase. Li₃PO₄ and Li_1.5PO₄ coatings considerably enhance the rate capability of the Li[Ni_0.4Co_0.3Mn_0.3]O₂ electrode. In contrast, the Li_3−xNi_x/2PO₄ coating material, which contained Ni, has an inferior rate capability compared to the Li_xPO₄ series (x = 1.5 and 3), although the LiNiPO₄-coated electrode shows a better rate capability than a pristine one. Li₃PO₄-based coating materials are effective at enhancing the cyclic performance of the electrode in the voltage range of 3.0-4.8 V. DSC analysis also confirms the improved thermal stability attained by coating the cathode with Li₃PO₄-based materials.     

Synthesis, structure and electrochemical Li insertion behaviour of Li2Ti6O13 with the Na2Ti6O13 tunnel-structure

Li₂Ti₆O₁₃ has been prepared from Na₂Ti₆O₁₃ by Li ion exchange in molten LiNO₃ at 325 °C. Chemical analysis and powder X-ray diffraction study of the reaction product respectively indicate that total Na/Li exchange takes place and the Ti-O framework of the Na₂Ti₆O₁₃ parent structure is kept under those experimental conditions. Therefore, Li₂Ti₆O₁₃ has been obtained with the mentioned parent structure. An important change is that particle size is decreased significantly which is favoring lithium insertion. Electrochemical study shows that Li₂Ti₆O₁₃ inserts ca. 5 Li per formula unit in the voltage range 1.5-1.0 V vs. Li⁺/Li, yielding a specific discharge capacity of 250 mAh g⁻¹ under equilibrium conditions. Insertion occurs at an average equilibrium voltage of 1.5 V which is observed for oxides and titanates where Ti(IV)/Ti(III) is the active redox couple. However, a capacity loss of ca. 30% is observed due to a phase transformation occurring during the first discharge. After the first redox cycle a high reversible capacity is obtained (ca. 160 mAh g⁻¹ at C/12) and retained upon cycling. Taking into consideration these results, we propose Li₂Ti₆O₁₃ as an interesting material to be further investigated as the anode of lithium ion batteries.     

Structural changes and thermal stability of charged LiNi1/3Co1/3Mn1/3O2 cathode material for Li-ion batteries studied by time-resolved XRD

Structural changes and their relationship with thermal stability of charged Li_0.33Ni_1/3Co_1/3Mn_1/3O₂ cathode samples have been studied using time-resolved X-ray diffraction (TR-XRD) in a wide temperature from 25 to 600 °C with and without the presence of electrolyte in comparison with Li_0.27Ni_0.8Co_0.15Al_0.05O₂ cathodes. Unique phase transition behavior during heating is found for the Li_0.33Ni_1/3Co_1/3Mn_1/3O₂ cathode samples: when no electrolyte is present, the initial layered structure changes first to a LiM₂O₄-type spinel, and then to a M₃O₄-type spinel and remains in this structure up to 600 °C. For the Li_0.33Ni_1/3Co_1/3Mn_1/3O₂ cathode sample with electrolyte, additional phase transition from the M₃O₄-type spinel to the MO-type rock salt phase takes place from about 400 to 441 °C together with the formation of metallic phase at about 460 °C. The major difference between this type of phase transitions and that for Li_0.27Ni_0.8Co_0.15Al_0.05O₂ in the presence of electrolyte is the delayed phase transition from the spinel-type to the rock salt-type phase by stretching the temperature range of spinel phases from about 20 to 140 °C. This unique behavior is considered as the key factor of the better thermal stability of the Li_1−xNi_1/3Co_1/3Mn_1/3O₂ cathode materials.     

Effect of protecting metal oxide (Co3O4) layer on electrochemical properties of spinel Li1.1Mn1.9O4 as a cathode material for lithium battery applications

Metal oxide (Co₃O₄) was coated on spinel Li_1.1Mn_1.9O₄ using glutamic acid. Powder X-ray diffraction pattern of Co₃O₄-coated spinel Li_1.1Mn_1.9O₄ showed that the Co₃O₄ coating medium was not incorporated in the spinel bulk structure. Morphology of the Co₃O₄-coated spinel Li_1.1Mn_1.9O₄ was observed by scanning electron microscopy and transmission electron microscopy. The cycling performance of the Co₃O₄-coated spinel Li_1.1Mn_1.9O₄ was obviously improved, compared to the pristine Li_1.1Mn_1.9O₄ at elevated temperature (55 °C). Improvement of rate capability was also achieved at high C-rates.     

Influence of lithium content on high rate cycleability of layered Li1+xNi0.30Co0.30Mn0.40O2 cathodes for high power lithium-ion batteries

Layered Li_1+xNi_0.30Co_0.30Mn_0.40O₂ (x = 0, 0.05, 0.10, 0.15) materials have been synthesized using citric acid assisted sol-gel method. The materials with excess lithium showed distinct differences in the structure and the charge and discharge characteristics. The rate capability tests were performed and compared on Li_1+xNi_0.30Co_0.30Mn_0.40O₂ (x = 0, 0.05, 0.10, 0.15) cathode materials. Among these materials, Li_1.10Ni_0.30Co_0.30Mn_0.40O₂ cathode demonstrated higher discharge capacity than that of the other cathodes. Upon extended cycling at 1C and 8C, Li_1.10Ni_0.30Co_0.30Mn_0.40O₂ showed better capacity retention when compared to other materials with different lithium content. Li_1.10Ni_0.30Co_0.30Mn_0.40O₂ exhibited 93 and 90% capacity retention where as Li_1.05Ni_0.30Co_0.30Mn_0.40O₂, Li_1.15Ni_0.30Co_0.30Mn_0.40O₂, and Li_1.00Ni_0.30Co_0.30Mn_0.40O₂ exhibited only 84, 71, and 63% (at 1C), and 79, 66 and 40% (at 10C) capacity retention, respectively, after 40 cycles. The enhanced high rate cycleability of Li_1.10Ni_0.30Co_0.30Mn_0.40O₂ cathode is attributed to the improved structural stability due to the formation of appropriate amount of Li₂MnO₃-like domains in the transition metal layer and decreased Li/Ni disorder (i.e., Ni content in the Li layer).     

Li4Ti5O12/Sn composite anodes for lithium-ion batteries: Synthesis and electrochemical performance

Li₄Ti₅O₁₂/tin phase composites are successfully prepared by cellulose-assisted combustion synthesis of Li₄Ti₅O₁₂ matrix and precipitation of the tin phase. The effect of firing temperature on the particulate morphologies, particle size, specific surface area and electrochemical performance of Li₄Ti₅O₁₂/tin oxide composites is systematically investigated by SEM, XRD, TG, BET and charge-discharge characterizations. The grain growth of tin phase is suppressed by forming composite with Li₄Ti₅O₁₂ at a calcination of 500 °C, due to the steric effect of Li₄Ti₅O₁₂ and chemical interaction between Li₄Ti₅O₁₂ and tin oxide. The experimental results indicate that Li₄Ti₅O₁₂/tin phase composite fired at 500 °C has the best electrochemical performance. A capacity of 224 mAh g⁻¹ is maintained after 50 cycles at 100 mA g⁻¹ current density, which is still higher than 195 mAh g⁻¹ for the pure Li₄Ti₅O₁₂ after the same charge/discharge cycles. It suggests Li₄Ti₅O₁₂/tin phase composite may be a potential anode of lithium-ion batteries through optimizing the synthesis process.     

Synthesis and electrochemistry of Li3MnO4: Mn in the +5 oxidation state

Computational and experimental work directed at exploring the electrochemical properties of tetrahedrally coordinated Mn in the +5 oxidation state is presented. Specific capacities of nearly 700 mAh g⁻¹ are predicted for the redox processes of Li_xMnO₄ complexes based on two two-phase reactions. One is topotactic extraction of Li from Li₃MnO₄ to form LiMnO₄ and the second is topotactic insertion of Li into Li₃MnO₄ to form Li₅MnO₄. In the experiments, it is found that the redox behavior of Li₃MnO₄ is complicated by disproportionation of Mn⁵⁺ in solution to form Mn⁴⁺ and Mn⁷⁺ and by other irreversible processes; although an initial capacity of about 275 mAh g⁻¹ in lithium cells was achieved. Strategies based on structural considerations to improve the electrochemical properties of MnO₄ⁿ⁻ complexes are given.     

Structural and transport properties of layered Li1+x(Mn1/3Co1/3Ni1/3)1−xO2 oxides prepared by a soft chemistry method

In this work structural and transport properties of layered Li_1+x(Mn_1/3Co_1/3Ni_1/3)_1−xO₂ oxides (x = 0; 0.03; 0.06) prepared by a “soft chemistry” method are presented. The excessive lithium was found to significantly improve transport properties of the materials, a corresponding linear decrease of the unit cell parameters was observed. The electrical conductivity of Li_1.03(Mn_1/3Co_1/3Ni_1/3)_0.97O₂ composition was high enough to use this material in a form of a pellet, without any additives, in lithium batteries and characterize structural and transport properties of deintercalated Li_1.03−y(Mn_1/3Co_1/3Ni_1/3)_0.97O₂ compounds. For deintercalated samples a linear increase of the lattice parameter c together with a linear decrease of the parameter a with the increasing deintercalation degree occurred, but only up to 0.4-0.5 mol of extracted lithium. Further deintercalation showed a reversal of the trend. Electrical conductivity measurements performed of Li_1.03−y(Mn_1/3Co_1/3Ni_1/3)_0.97O₂ samples (y = 0.1; 0.3; 0.5; 0.6) showed an ongoing improvement, almost two orders of magnitude, in relation to the starting composition. Additionally, OCV measurements, discharge characteristics and lithium diffusion coefficient measurements were performed for Li/Li⁺/Li_1.03−y(Mn_1/3Co_1/3Ni_1/3)_0.97O₂ cells.     

An amorphous LiCo1/3Mn1/3Ni1/3O2 thin film deposited on NASICON-type electrolyte for all-solid-state Li-ion batteries

Amorphous LiCo_1/3Mn_1/3Ni_1/3O₂ thin films were deposited on the NASICON-type Li-ion conducting glass ceramics, Li_1+x+yAl_xTi_2−xSi_yP_3−yO₁₂ (LATSP), by radio frequency (RF) magnetron sputtering below 130 °C. The amorphous films were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The Li/PEO₁₈-Li(CF₃SO₂)₂N/LATSP/LiCo_1/3Mn_1/3Ni_1/3O₂/Au all-solid-state cells were fabricated to investigate the electrochemical performance of the amorphous films. It was found that the low-temperature deposited amorphous cathode film shows a high discharge voltage and a high discharge capacity of around 130 mAh g⁻¹.     

Electrochemistry and safety of Li4Ti5O12 and graphite anodes paired with LiMn2O4 for hybrid electric vehicle Li-ion battery applications

A promising anode material for hybrid electric vehicles (HEVs) is Li₄Ti₅O₁₂ (LTO). LTO intercalates lithium at a voltage of ∼1.5 V relative to lithium metal, and thus this material has a lower energy compared to a graphite anode for a given cathode material. However, LTO has promising safety and cycle life characteristics relative to graphite anodes. Herein, we describe electrochemical and safety characterizations of LTO and graphite anodes paired with LiMn₂O₄ cathodes in pouch cells. The LTO anode outperformed graphite with regards to capacity retention on extended cycling, pulsing impedance, and calendar life and was found to be more stable to thermal abuse from analysis of gases generated at elevated temperatures and calorimetric data. The safety, calendar life, and pulsing performance of LTO make it an attractive alternative to graphite for high power automotive applications, in particular when paired with LiMn₂O₄ cathode materials.