Layered Li-rich, Co-free, and Mn-based cathode material, Li1.17Ni0.25–xMn0.58MgxO2 (0 ≤ x ≤ 0.05), was successfully synthesized by a coprecipitation method. All prepared samples have typical Li-rich layered structure, and Mg has been doped in the Li1.17Ni0.25Mn0.58O2 material successfully and homogeneously. The morphology and the grain size of all material are not changed by Mg doping. All materials have a estimated size of about 200 nm with a narrow particle size distribution. The electrochemical property results show that Li1.17Ni0.25–xMn0.58MgxO2 (x = 0.01 and 0.02) electrodes exhibit higher rate capability than that of the pristine one. Li1.17Ni0.25–xMn0.58MgxO2 (x = 0.02) indicates the largest reversible capacity of 148.3 mAh g–1 and best cycling stability (capacity retention of 95.1%) after 100 cycles at 2C charge–discharge rate. Li1.17Ni0.25–xMn0.58MgxO2 (x = 0.02) also shows the largest discharge capacity of 149.2 mAh g–1 discharged at 1C rate at elevated temperature (55 °C) after 50 cycles. The improved electrochemical performances may be attributed to the decreased polarization, reduced charge transfer resistance, enhanced the reversibility of Li+ ion insertion/extraction, and increased lithium ion diffusion coefficient. This promising result gives a new understanding for designing the structure and improving the electrochemical performance of Li-rich cathode materials for the next-generation lithium-ion battery with high rate cycling performance.Keywords: Co-free, Li-rich cathodes; cycling performance; density functional theory; fast charge−discharge property; Li-ion battery; magnesium doping;

A facile strategy was developed to prepare Li5Cr7Ti6O25@CeO2 composites as a high-performance anode material. X-ray diffraction (XRD) and Rietveld refinement results show that the CeO2 coating does not alter the structure of Li5Cr7Ti6O25 but increases the lattice parameter. Scanning electron microscopy (SEM) indicates that all samples have similar morphologies with a homogeneous particle distribution in the range of 100–500 nm. Energy-dispersive spectroscopy (EDS) mapping and high-resolution transmission electron microscopy (HRTEM) prove that CeO2 layer successfully formed a coating layer on a surface of Li5Cr7Ti6O25 particles and supplied a good conductive connection between the Li5Cr7Ti6O25 particles. The electrochemical characterization reveals that Li5Cr7Ti6O25@CeO2 (3 wt %) electrode shows the highest reversibility of the insertion and deinsertion behavior of Li ion, the smallest electrochemical polarization, the best lithium-ion mobility among all electrodes, and a better electrochemical activity than the pristine one. Therefore, Li5Cr7Ti6O25@CeO2 (3 wt %) electrode indicates the highest delithiation and lithiation capacities at each rate. At 5 C charge–discharge rate, the pristine Li5Cr7Ti6O25 only delivers an initial delithiation capacity of ∼94.7 mAh g–1, and the delithiation capacity merely achieves 87.4 mAh g–1 even after 100 cycles. However, Li5Cr7Ti6O25@CeO2 (3 wt %) delivers an initial delithiation capacity of 107.5 mAh·g–1, and the delithiation capacity also reaches 100.5 mAh g–1 even after 100 cycles. The cerium dioxide modification is a direct and efficient approach to improve the delithiation and lithiation capacities and cycle property of Li5Cr7Ti6O25 at large current densities.Keywords: anode; CeO2; Li5Cr7Ti6O25; lithium-ion battery; rate performance;

A facile strategy was developed to prepare Li5Cr7Ti6O25@CeO2 composites as a high-performance anode material. X-ray diffraction (XRD) and Rietveld refinement results show that the CeO2 coating does not alter the structure of Li5Cr7Ti6O25 but increases the lattice parameter. Scanning electron microscopy (SEM) indicates that all samples have similar morphologies with a homogeneous particle distribution in the range of 100–500 nm. Energy-dispersive spectroscopy (EDS) mapping and high-resolution transmission electron microscopy (HRTEM) prove that CeO2 layer successfully formed a coating layer on a surface of Li5Cr7Ti6O25 particles and supplied a good conductive connection between the Li5Cr7Ti6O25 particles. The electrochemical characterization reveals that Li5Cr7Ti6O25@CeO2 (3 wt %) electrode shows the highest reversibility of the insertion and deinsertion behavior of Li ion, the smallest electrochemical polarization, the best lithium-ion mobility among all electrodes, and a better electrochemical activity than the pristine one. Therefore, Li5Cr7Ti6O25@CeO2 (3 wt %) electrode indicates the highest delithiation and lithiation capacities at each rate. At 5 C charge–discharge rate, the pristine Li5Cr7Ti6O25 only delivers an initial delithiation capacity of ∼94.7 mAh g–1, and the delithiation capacity merely achieves 87.4 mAh g–1 even after 100 cycles. However, Li5Cr7Ti6O25@CeO2 (3 wt %) delivers an initial delithiation capacity of 107.5 mAh·g–1, and the delithiation capacity also reaches 100.5 mAh g–1 even after 100 cycles. The cerium dioxide modification is a direct and efficient approach to improve the delithiation and lithiation capacities and cycle property of Li5Cr7Ti6O25 at large current densities.Keywords: anode; CeO2; Li5Cr7Ti6O25; lithium-ion battery; rate performance;

Relying on a solvent thermal method, spherical Na2Li2Ti6O14 was synthesized. All samples prepared by this method are hollow and hierarchical structures with the size of about 2–3 μm, which are assembled by many primary nanoparticles (~300 nm). Particle morphology analysis shows that with the increase of temperature, the porosity increases and the hollow structure becomes more obvious. Na2Li2Ti6O14 obtained at 800°C exhibits the best electrochemical performance among all samples. Charge-discharge results show that Na2Li2Ti6O14 prepared at 800°C can delivers a reversible capacity of 220.1, 181.7, 161.6, 144.2, 118.1 and 97.2 mA h g−1 at 50, 140, 280, 560, 1400, 2800 mA g−1. However, Na2Li2Ti6O14-bulk only delivers a reversible capacity of 187, 125.3, 108.3, 88.7, 69.2 and 54.8 mA h g−1 at the same current densities. The high electrochemical performances of the as-prepared materials can be attributed to the distinctive hollow and hierarchical spheres, which could effectively reduce the diffusion distance of Li ions, increase the contact area between electrodes and electrolyte, and buffer the volume changes during Li ion intercalation/deintercalation processes.本文采用溶剂热法合成了球形Na2Li2Ti6O14材料. 所有溶剂热法制备得到的材料均具有中空的分级结构, 并且均由粒径约为300 nm的 初级粒子通过组装形成, 微球的直径大约为2−3 μm. 粒子的形貌分析表明, 随着合成温度的增加, 孔隙率逐渐增加且中空结构更加明显. 在 所有材料中, 800°C合成的Na2Li2Ti6O14具有最好的电化学性能. 充放电测试表明, 在电流密度为50、140、280、560、1400、2800 mA g−1时, 800°C合成的Na2Li2Ti6O14样品的可逆容量分别为220.1、181.7、161.6、144.2、118.1、97.2 mA h g−1. 但是在相同电流密度条件下, 块状的 Na2Li2Ti6O14的可逆容量分别为187、125.3、108.3、88.7、69.2、54.8 mA h g−1. 中空分级结构微球可以有效地减小锂离子的扩散距离、增 加电极与电解液的接触面积、以及缓冲锂离子嵌脱过程中的体积变化, 从而使其具有较高的电化学性能.

Li1.2Mn0.54Ni0.13Co0.13O2 hollow hierarchical microspheres (LNCM-HS) were successfully synthesized by molten salt method used the as-prepared MnO2 microspheres as the precursor and template. The sharp and well-defined reflection peaks suggest a high crystallization degree of the samples, and no impurities were observed. Li1.2Mn0.54Ni0.13Co0.13O2 material obtained is a solid solution consisting of rhombohedral R3-m and monoclinic C2/m group symmetries, which is confirmed by XRD, Raman spectra, and HRTEM. SEM and TEM shows that the hierarchical microspheres of LNCM-HS are composed of primary nano particles with the size of about 50 nm. EDS mapping demonstrates that Ni, Mn, Co, and O elements are evenly distributed without any phase separation in LNCM-HS, and the atomic ratio of Mn, Co, Ni is calculated to be 0.54: 0.13: 0.12, which is quite close to the stoichiometry of 0.5 Li2MnO3·0.5LiMn1/3Co1/3Ni1/3O2. LNCM-HS exhibits excellent rate capacity of 309.9 (0.1C), 280.1 (0.3C), 226.5 (0.75C), 178.3 (1C), 139.3 (3C), and 101.0 mAh g−1 (5C), respectively, whereas LNCM-bulk cathode displays a discharge capacity of 290.1, 230.0, 163.3, 135.4, 92.7, and 60.2 mAh g−1 at the same rates. The improved capacity of LNCM-HS is ascribed to the increased lithium diffusion coefficient and reduced charge transfer resistance. The enhanced electrochemical performances can be attributed to the distinctive hollow microspheres structures, the increase contacting area between electrodes and electrolyte and the buffered volume changes during Li ions intercalation/deintercalation processes.

•A general microreactor strategy has been developed for structure-optimized Li-contained electrode materials.•Ultrafine LiFePO4 quantum dots are first reported through the designed microreactor strategy.•The synthesized G/LFP-QDs@C exhibits ultra-fast, surface-reaction-controlled Li storage behavior.•A combined experimental and DFT calculation study is introduced to reveal the energy storage mechanism of G/LFP-QDs@C.Due to the relatively slow, diffusion-controlled faradaic reaction mechanisms of conventional LiFePO4 (LFP) materials, which is hard to deliver satisfied capacity for high rate applications. In this work, ultrafine LFP quantum dots (LFP-QDs) co-modified by two types of carbonaceous materials - amorphous carbon and graphitized conductive carbon (graphene) have been successfully synthesized through a novel microreactor strategy. Because of the very limited area constructed by the dual-carbon microreactor for the growth of LFP crystal, it's demension was furthest suppressed to a very small level (~ 6.5 nm). Such a designed nano-composite possesses a large specific surface area for charge adsorption and abundant active sites for faradaic reactions, as well as ideal kinetic features for both electron and ion transport, and thus exhibits ultra-fast, surface-reaction-controlled lithium storage behavior, mimicking the pseudocapacitive mechanisms for supercapacitor materials, in terms of extraordinary rate capability (78 mAh g−1 at 200 C) and remarkable cycling stability (~ 99% over 1000 cycles at 20 C). On the other side, due to the quasi-2D structure of the synthesized LFP-QDs composite, which can be used as the basic unit to further fabricate free-standing film, aerogel and fiber electrode without the addition of binder and conductive agent for different practical applications. In addition, to deeper understand its electrochemical behavior, a combined experimental and density functional theoretical (DFT) calculation study is also introduced.Ultrafine LiFePO4 quantum dots (~ 6.5 nm) co-modified by two types of carbonaceous materials - amorphous carbon and graphitized conductive carbon (graphene) have been successfully synthesized through a novel microreactor strategy, which exhibit ultra-fast, surface-reaction-controlled energy storage behavior, mimicking the pseudocapacitive mechanisms for supercapacitor materials, in terms of excellent rate capability and outstanding cycling stability.Download high-res image (230KB)Download full-size image

The thermodynamic stability is a very important quantity for the electrode materials, because it is not only related to the electrochemical performances of the materials but also the safety issue of the cells. To evaluate the thermodynamic stability of LixNi0.5Mn1.5O4 (x = 0, 1), the formation enthalpies from elemental phases and oxides were obtained. The values for LiNi0.5Mn1.5O4 were calculated to be −1341.10 and −141.84 kJ mol−1, while those for Ni0.5Mn1.5O4 were −949.11 and −49.21 kJ mol−1. These values are much more negative than those of LiCoO2 and LiNiO2 compounds, indicating that the thermodynamic stability of LixNi0.5Mn1.5O4 is better than the two classic compounds. To clarify the microscopic origin, the density of states, magnetic moments, and bond orders were systematically investigated. The results showed that the excellent thermodynamic stability of LiNi0.5Mn1.5O4 is attributed to the absence of Jahn-Teller distortions, strong electrostatic interactions of Li–O ionic bond, and strong Ni–O/Mn–O ionic-covalent mixing bonds. After lithium extraction, the disappearance of the pure Li–O bonds leads to an increase of formation enthalpy, indicating a decreasing thermodynamic stability for Ni0.5Mn1.5O4 with respect to LiNi0.5Mn1.5O4.

•The thermodynamic stability of LixTi2O4 and origin for the stability were revealed.•Evolution of the morphologies as functions of chemical environments was clarified.•Surface effects on the electrochemical performance of LiTi2O4 were discussed.The thermodynamic stabilities, surface morphologies, and electronic structures of the LiTi2O4 compound were investigated by the first-principles methods. The formation enthalpies and lattice constants of LixTi2O4 decrease at first and then increase again. This phenomenon is related to the balance between LiO attractions and LiLi repulsions. Population analysis revealed that pure ionic and strong covalent bonds are formed respectively between lithium and oxygen and between titanium and oxygen in LiTi2O4 material. These interactions are very crucial for the thermodynamic stability of the compounds. The surface stability was considered as functions of the chemical potentials, and five terminations, (100)-Ti2O4, (110)-Ti2O4, (210)-Ti2O4, (111)-LiTiO4, and (310)-Ti2O8ones, are dominant in the stability diagram. Our calculation showed that a particle morphology with mono (110) facet can be obtained at Ti- and/or O-moderate conditions, and this morphology will be very helpful for improving the rate performance of the material via reduction of the lithium diffusion distance. Furthermore, partially filled electronic states at the Fermi energy were confirmed for bulk LiTi2O4 and some of the surfaces, and they are responsible for the excellent electronic conductivity of the material. Further calculations showed that the work functions are sensitive to the stoichiometry of the surfaces.

A facile solid-state method to improve the rate performance of Li4Ti5O12 in lithium-ion batteries by LiAlO2 in situ modification is presented in this work. XRD shows that the LiAlO2 modification does not change the spinel structure of Li4Ti5O12 but forms Al-doped Li4Ti5O12–LiAlO2 composites, and little Al doping decreases the lattice parameter of doped Li4Ti5O12. SEM shows that all samples are composed of 1–2 μm primary particles with irregular shapes. Raman spectra reveal that the intensity of these lines for Li4Ti5O12–LiAlO2 composites obviously decreases caused by a modification of the LiAlO2 phase. CV and EIS tests indicate that the doping of Al3+ and the combination with in situ generated LiAlO2 on the surface of Li4Ti5O12 are favorable for reducing the electrode polarization and charge-transfer resistance, and then improve the reversibility and lithium ion diffusion coefficient of Li4Ti5O12, resulting in its relatively higher rate capacity. Charge–discharge tests reveals that Li4Ti5O12–LiAlO2 composite (5 wt %) exhibits the highest rate capacity and cycling stability at various rates, which is capable of large-scale applications, such as electric vehicles and hybrid electric vehicles, requiring high energy, long life and excellent safety.Keywords: Li4Ti5O12; LiAlO2; Lithium-ion battery; Modification; Rate capability

•0.3 Li2MnO3·0.7 LiMn1/3Co1/3Ni1/3O2 shows an excellent fast charge-discharge performance.•Stability origin of Li-rich material from microscopic point were studied.•The strong covalent bond between Mn-3d and O-2p orbit is helpful for stabilizing layer structure.Well-crystallized and high-performance xLi2MnO3·(1-x)LiMn1/3Co1/3Ni1/3O2 cathode materials over a wide compositional range (0.1 ≤ x ≤ 0.5) were prepared by a facile co-precipitation strategy. All samples show small and uniform particle dimensions in the range of 200-500 nm. 0.3Li2MnO3·0.7LiMn1/3Co1/3Ni1/3O2 (LMNC3) delivers the highest discharge capacities of 321.1 mA h g−1 at 0.1C, 305.2 mA h g−1 at 0.2 C, 243.2 mA h g−1 at 0.5C, 205.9 mA h g−1 at 1C, 182.1 mA h g−1 at 2C, and 133.7 mA h g−1 at 5C, respectively. LMNC3 also exhibits an excellent fast charge-discharge performance, and even at a charge-discharge rate of 2C in the 100th cycle its capacity still remains 128.2 mAh g−1. The improved performance of LMNC3 can be ascribed to the improvement of electrochemical reversibility, lithium ion diffusion and ionic conductivity. First-principles calculation based on density function theory reveals that the strong covalent bonds formed between Mn-3d and O-2p orbits and stable Li2MnO3-enriched layer are helpful for stabilizing the LiMn1/3Co1/3Ni1/3O2 structure. The same strategy adopted in this work could be helpful to develop other Li-rich cathodes with long cycle life and high rate performance.

Lithium-ion batteries are considered as one of the most promising power sources for energy storage system for a wide variety of applications such as electric vehicles (EVs) or hybrid electric vehicles (HEVs). The anode material often plays an important role in the determination of the safety and cycling life of lithium-ion batteries. Among all anode materials, spinel Li4Ti5O12 has been considered as one the most promising anode candidates for the next-generation large-scale power lithium-ion batteries used for HEVs or EVs because it has a high potential of around 1.55 V (vs. Li/Li+) during charge and discharge, excellent cycle life due to the negligible volume change, and high thermal stability and safety. In this review, we present an overview of the breakthroughs in the past decade in the synthesis and modification of both the chemistry and morphology of Li4Ti5O12. An insight into the future research and further development of Li4Ti5O12 composites is also discussed.

Li4Ti5O12–Li2TiO3–TiO2 and pristine Li4Ti5O12 electrodes have been synthesized by a solid-state method. The structure and electrochemical performance of these as prepared powders have been characterized by X-ray diffraction (XRD), Raman spectroscopy (RS), scanning electron microscopy (SEM), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and the galvanostatic charge–discharge test. All samples have agglomerate structure with the primary particle size of about 1 µm. Electrochemical tests demonstrate that Li4Ti5O12–Li2TiO3–TiO2 (LLT5) possesses high capacity and excellent rate capability. The improved electrochemical performance of Li4Ti5O12–Li2TiO3–TiO2 (LLT5) is clearly related to the improved electronic conductivity and lithium ion diffusivity. The high cycling performance and wide discharge voltage range, as well as simple synthesis route and low synthesis cost of the Li4Ti5O12–Li2TiO3–TiO2 (LLT5) is expected to show a potential commercial application.

Spinel Li4Ti5−xScxO12(x=0, 0.05, 0.10, 0.15) anode materials were successfully synthesized by a facile solid state reaction. The structure, morphology and electrochemical performance in a broad voltage window were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and charge–discharge tests in sequence. Li4Ti4.95Sc0.05O12 material exhibits excellent cycling stability and rate capability in relevant lithium-ion batteries, which can retain a capacity of 138.6 mAh g−1 after 80 cycles at 2 C charge–discharge rate. This performance is much better than that of pristine Li4Ti5O12 (121 mAh g−1), whose capacity fades seriously. The improved electrochemical performance of Sc3+-doped Li4Ti5O12 can be ascribed to the reduction of charge transfer resistance, increase of electronic conductivity, and fast lithium ion diffusion behavior. Hence, Li4Ti4.95Sc0.05O12 can be considered as an economical and potential candidate as anode material for power lithium-ion batteries.

•Rutile TiO2 modifying improves conductivity and Li migration ability of electrode.•Li4Ti5O12-rutile TiO2 shows a high rate capability in a broad voltage window.•Li4Ti5O12-rutile TiO2 exhibits a good cycle stability even at 12 C rates.Li4Ti5O12 and Li4Ti5O12-rutile TiO2 nanosheet composite were synthesized by a facile solvothermal method with further calcination. The addition of rutile TiO2 does not affect the crystal structure, particle size, morphology of spinel Li4Ti5O12. XRD shows that the molar ratio of Li/Ti has much influence on the chemical composition of the products. TEM indicates that both Li4Ti5O12 and Li4Ti5O12-rutile TiO2 samples are composed of nanoplates with particle size of 50–100 nm. CV and EIS imply that Li4Ti5O12-rutile TiO2 has higher reversible intercalation and deintercalation of Li+, larger lithium-ion diffusion coefficient and smaller charge transfer resistance corresponding to a much higher conductivity than those of Li4Ti5O12 corresponding to the extraction of Li+ ions. Li4Ti5O12-rutile TiO2 material exhibits excellent cycling stability and rate capability in relevant lithium-ion batteries, which can retain a capacity of 120.5 mAh g−1 after 150 cycles at 5 C charge–discharge rate cycled between 1.0 and 2.5 V. This performance is much better than that of pristine Li4Ti5O12 (82.2 mAh g−1), whose capacity fades seriously. Li4Ti5O12-rutile TiO2 also exhibits a good rate performance in a broad voltage window. The capacities of Li4Ti5O12-rutile TiO2 and Li4Ti5O12 charge-discharged at 12 C rates remains at 125.4 and 50.2 mAh g−1 cycled between 0.0 and 2.5 V after 200 cycles, respectively. The enhanced performance of Li4Ti5O12-rutile TiO2 is ascribed to the improved electronic conduction and the reduced polarization resulting from the rutile TiO2 modification together with nanosized structure.

•CNT modification reduces resistance and polarization of Li4Ti5O12.•CNT modification results in fast lithium insertion/extraction kinetics.•Li4Ti5O12–CNTs (4 wt.%) exhibit a good fast charge–discharge performance.Li4Ti5O12 nanosheet and Li4Ti5O12 nanosheet/CNT nanocomposite were prepared in ethyl alcohol medium by a low-cost hydrothermal method. The CNTs are incorporated into the Li4Ti5O12 nanosheet homogenously, and then provide a highly conductive network for electron transportation. Li4Ti5O12 nanosheet/CNT composite with 4 wt.% CNTs exhibits excellent rate capability and superior cycle life between 1 and 2.5 V. Li4Ti5O12 nanosheet/CNTs (4 wt.%) shows superior rate performance with discharge capacities of 137.5 mAh g− 1 at 2 C charge–discharge rate after 90 cycles and 128.3 mAh g− 1 at 3 C charge–discharge rate after 120 cycles. The significantly improved rate capability of the Li4Ti5O12 nanosheet/CNT electrodes can be mainly ascribed to the reduced resistance and polarization and improved lithium diffusion coefficient. The same strategy adopted in this work could be helpful to develop advanced electrochemical energy storage systems with long cycle life and high rate performance.

Well-defined Li4Ti5O12–TiO2 nanosheet and nanotube composites have been synthesized by a solvothermal process. The combination of in situ generated rutile–TiO2 in Li4Ti5O12 nanosheets or nanotubes is favorable for reducing the electrode polarization, and Li4Ti5O12–TiO2 nanocomposites show faster lithium insertion/extraction kinetics than that of pristine Li4Ti5O12 during cycling. Li4Ti5O12–TiO2 electrodes also display lower charge-transfer resistance and higher lithium diffusion coefficients than pristine Li4Ti5O12. Therefore, Li4Ti5O12–TiO2 electrodes display lower charge-transfer resistance and higher lithium diffusion coefficients. This reveals that the in situ TiO2 modification improves the electronic conductivity and electrochemical activity of the electrode in the local environment, resulting in its relatively higher capacity at high charge–discharge rate. Li4Ti5O12–TiO2 nanocomposite with a Li/Ti ratio of 3.8:5 exhibits the lowest charge-transfer resistance and the highest lithium diffusion coefficient among all samples, and it shows a much improved rate capability and specific capacity in comparison with pristine Li4Ti5O12 when charging and discharging at a 10 C rate. The improved high-rate capability, cycling stability, and fast charge–discharge performance of Li4Ti5O12–TiO2 nanocomposites can be ascribed to the improvement of electrochemical reversibility, lithium ion diffusion, and conductivity by in situ TiO2 modification.Keywords: Li4Ti5O12; lithium-ion battery; rapid charge−discharge property; rate capability; TiO2

To elucidate the microscopic origin of the difference behaviors, first-principles calculations were performed to investigate the thermal and mechanical stabilities of LixFePO4 and LixMnPO4. The calculated free energies suggested that LiFePO4 and LiMnPO4 are thermal stable with respect to relevant oxides both in their pristine and fully delithiated states. According to the calculations, it can be identified that the shear deformations are more easier to occur with respect to the volume compressions in LixFePO4 and LixMnPO4, and this phenomenon is related to M–O(I) and M–O(II) bonds. Typically for MnPO4, Li+ extraction from the host structures further weakens the Mn–O(I) bonds by about 33%, and it thus becomes very brittle. The shear anisotropy (AG) of MnPO4 is abnormally large and has already reached 19.05 %, which is about 6 times as large as that of FePO4. Therefore, shear deformations and dislocations occur easily in MnPO4. Moreover, as the Mn–O(I) bonds in MnPO4 are mainly spread within the {101} and {1̅01} crystal planes, the relevant slip systems thus allow the recombination of bonds at the interfaces, leading to the experimentally observed phase transformation. It can be concluded that mechanical reason will play an important role for the poor cycling performance of MnPO4.Keywords: cathode material; lithium iron phosphate; lithium manganese phosphate; mechanical stability; thermal stability;

•LTO-LLTO anode shows super fast charge-discharge performance.•LTO-LLTO shows high lithium diffusion coefficient and good rate capability.•LTO-LLTO (10 wt%) shows a potential advantage for high power battery applications.Well-defined submicron Li4Ti5O12-Li3xLa(2/3)−xTiO3 (LTO-LLTO) materials have been synthesized by a solid-state reaction followed by hydrothermal procedure. Li4Ti5O12-Li3xLa(2/3)−xTiO3 materials show much improved rate capability and specific capacity compared with pristine Li4Ti5O12 when used as anode materials for lithium-ion batteries. The results here give clear evidence of the utility of Li3xLa(2/3)−xTiO3 (LLTO) to improve the kinetics of Li4Ti5O12 toward fast lithium insertion/extraction. LTO-LLTO (10 wt%) shows a high capacity of 101 mAh g−1 even at a charge-discharge rate of 5 C in the 150th cycle, about 150% that of pristine Li4Ti5O12 particles (68 mAh g−1). The improved high-rate capability, cycling stability, fast charge-discharge performance of LTO-LLTO composites can be ascribed to the improvement of electrochemical reversibility, lithium ion diffusion and ionic conductivity by LLTO modifying.

Well-defined LiNi0.5Mn1.5O4 powder has been synthesized by ethylene glycol (EG)-assisted oxalic acid co-precipitation method. The structure and physicochemical properties of this as-prepared powder were investigated by powder X-ray diffraction (XRD), Raman spectra (RS), scanning electron microscopy (SEM), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic charge–discharge test in detail. XRD shows impurity phase of LiyNi1-yO due to the oxygen loss reaction at high temperatures. SEM indicates that LiNi0.5Mn1.5O4 synthesized by EG-assisted method has a uniform and narrow size distribution under 1 μm. RS confirms that both samples have Fd-3 m space group. EIS and CV test exhibit that LiNi0.5Mn1.5O4 electrode synthesized by EG-assisted method has lower potential polarization, smaller charge transfer resistance, higher reversibility and larger lithium ion diffusion coefficient than the one obtained by ammonium hydroxide co-precipitation method. Galvanostatic charge–discharge test reveals that LiNi0.5Mn1.5O4 synthesized by EG-assisted method shows higher discharge capacity than that the one synthesized by ammonium hydroxide co-precipitation method at each rate.

The impact of lithium extraction on the structural stabilities, electronic structures, bonding characteristics, and electrochemical performances of LiFePO4 compound was investigated by first-principles technique. The results demonstrated that the partition scheme of electrons not only affects the calculated atomic charges but also the magnetic properties. In FePO4 and LiFePO4 compounds, all Fe ions take high spin arrangements and have large magnetic moments (MMs), while the MMs of other ions are very small. The magnetisms of LixFePO4 compounds are mainly originated form Fe ions. It was found that the changes in d band electrons of the transition metals do play an important role in determining the voltage of a battery (versus Li/Li+). Furthermore, the variations in d band electrons also provide us a method to control the density of states (DOS) and carrier concentration at the Fermi energy. Our calculations confirmed that the substitution of Fe by Co and Ni ions leads to a voltage increase by about 0.70 V and 1.23 V respectively. According to the bond populations, it can be identified that strong covalent bonds are formed between O and P ions. The P–O bonds are much stronger than Fe–O ones. The partial DOSs further revealed that the covalent bonds in LixFePO4 are derived from the orbital overlaps between O2s,2p and P3s,3p states, and the overlap between Fe3d and O2p states. Such covalent bonds are of particularly importance for the excellent thermodynamic stabilities of the two-ends structures of LixFePO4.

The structural and thermal stability are essential to understand the safety of Li4Ti5O12, but it is not fully understood. Here, the structural and thermal stability were investigated by the density functional theory (DFT) plane-wave pseudopotential technique and experimental method. Sub-micro Li4Ti5O12 particles were synthesized by a solid-state reaction. The calculated results of lattice parameters are highly coincident with the experimental values. XRD and Raman spectra demonstrate the formation of pure phase Li4Ti5O12. There is an amorphous phase and no phase transition when discharged to 0 V, which confirms that there is a certain reversible intercalation processes cycled below 1 V instead of a reduction–decomposition reaction. SEM shows that Li4Ti5O12 powder has a uniform, nearly cubic structural morphology with a narrow size distribution of about 500 nm. The low formation enthalpy (−6061.45 ± 4) indicates that Li4Ti5O12 has a high thermodynamic stability. The superior cycling performance at high rates cycled between 0 and 2.5 shows that Li4Ti5O12 has a very high structural stability. The high thermodynamic stability of Li4Ti5O12 is related to the strong covalent bonding characteristic between Ti and O according to the electron density difference diagram. DSC reveals that PF5 is the main species which damages the SEI layer.Graphical abstractThe covalent bonds formed between Ti-3d and O-2p orbits are of particularly importance to excellent thermodynamical stability of the compounds, and then lead to the high cycling performance.Highlights► The relationship between the thermodynamic stability and microscopic bonding of Li4Ti5O12 is investigated. ► The low formation enthalpy indicates that spinel Li4Ti5O12 has a high thermodynamic stability. ► PF5 is the main species which damages the SEI layer.

Submicron-sized Li4Ti5O12 anode was synthesized through a two-step solid-state reaction. The relationship between the structural properties, electrochemical performance discharged to 0 V and lithium-ion intercalation kinetics was discussed using the experiments as well as the first-principles calculations. Structural analyses reveal that Li4Ti5O12 synthesized by the two-step solid-state method has high phase purity. Scanning electron microscopy (SEM) shows that Li4Ti5O12 has a homogeneous size distribution in the range of 0.4–0.6 μm. The initial discharge capacity of Li4Ti5O12 is 234.6 mA h g−1 at 0.5 C rate between 0 and 2.5 V, and it is close to the theoretical capacity value. The high structure stability of Li4Ti5O12 is related to the strong covalent bonding characteristic between Ti and O according to the first-principles calculation. Electrochemical impedance spectroscopy (EIS) indicates that the charge transfer resistance of the Li4Ti5O12/Li cell evidently decreases with increasing of the temperature, and the apparent activation energies of Li4Ti5O12 electrode on the lithium diffusion process and the charge transfer are calculated to be 19.05 and 22.48 kJ mol−1, respectively.Graphical abstractHighlights► Quantitative analysis of the structural stability for spinel by DFT methods. ► The excellent cycling stability of Li4Ti5O12 from the strong covalent bonding characteristic between Ti and O. ► The cell temperature affects the Li extraction reaction of the Li4Ti5O12.

Nb-doped LiMn1.5Ni0.5O4 materials have been synthesized through a solid-state reaction, and Nb doping achieves some encouraging results. Both crystal domain size and electronic conductivity are influenced by this kind of doping. The lattice parameter of the Nb-doped LiMn1.5Ni0.5O4 samples are slightly larger than that of pure LiMn1.5Ni0.5O4 samples, and Nb doping does not change the basic spinel structure. Even though the material has a particle size of 1–2 μm, the capacity retention is improved remarkably compared to that of the undoped one when charge-discharged at high rates. The LiNi0.525Mn1.425Nb0.05O4 has a discharge capacity of 102.7 mAh g−1 at 1 C charge–discharge rate after 100 cycles. Though all samples exhibit similar initial discharge capacities at various high C rates, the Nb-doped LiMn1.5Ni0.5O4 samples display remarkable cyclabilities. Capacity retention of Nb-doped LiMn1.5Ni0.5O4 is excellent without a significant capacity loss at various high C rates. This is ascribed to a smaller crystallite, a higher conductivity, and a higher lithium diffusion coefficient (DLi) observed in this material. As a result, our microscale Nb-doped LiMn1.5Ni0.5O4 can be used for battery applications that require high power and long life, including HEVs and energy storage devices for renewable energy systems.Highlights► Nb-doped LiNi0.5Mn1.5O4 cathode is first synthesized. ► Nb doping improves the conductivity and reversibility of the LiNi0.5Mn1.5O4. ► Nb-doped LiNi0.5Mn1.5O4 cathodes remarkably exhibit high rate-performance.

A micro-sized particle Li4Ti5−xLaxO12 (0 ≤ x ≤ 0.2) material has been synthesized by a simple solid-state method at air. The obtained Li4Ti5−xLaxO12 materials are Li3xLa2/3−xTiO3 (LLTO)–Li4Ti5O12 (LTO) solid solution, and well crystallized with a particle size in the range of 1–2 μm. The electronic conductivity and lithium diffusion coefficient of La-modified LTO (Li4Ti4.95La0.05O12) are improved because LLTO exhibits a high ionic conductivity during Li+ extraction. Li4Ti4.95La0.05O12 material shows discharge capacities of more than 206 and 197 mAh g−1 after 100 cycles at 1 C and 3 C charge–discharge rates, respectively. Especially, in rate performance, the Li4Ti5−xLaxO12 (x = 0.1, 0.2) samples maintain capacity of about 181 mAh g−1 until 5 C rates after 200 cycles, while the pure LTO sample shows a severe capacity decline at corresponding rate. These results suggest that La modifying is an effective way to improve the chemical stability of the electrode in contact with the electrolyte and improve their cyclability and rate capability during long term cycling. Since high rate performance is an important factor that needs to be considered in fabricating power batteries in industry, the La-modified LTO moves closer to real and large-scale applications.Highlights► The over-discharge performance of La-modified Li4Ti5O12 anode is first reported. ► La modification improves the conductivity and reversibility of the Li4Ti5O12. ► La-modified Li4Ti5O12 anodes remarkably exhibit high rate performance.

A theoretical study of the structural, elastic and electronic properties of spinel LiTi2O4 anode has been performed by density functional theory (DFT) plane-wave pseudopotential method. The independent elastic constants, shear modulus (G), bulk modulus (B), and Young's modulus (E) are evaluated, respectively. The results suggest that cubic LiTi2O4 is mechanically stable. The G/B ratio of 0.584 indicates the ductility of LiTi2O4 is good. The electron density difference of LiTi2O4 shows that the O2p orbits overlap effectively with Ti3d ones, confirming the formations of strong covalent bonds between them, while Li is fully ionized in the lattice. The formation enthalpy for LiTi2O4 is calculated to be −2070.723 ± 1.6 kJ mol−1. The strong covalent bonds between O and Ti atoms are not only responsible for the excellent mechanical stabilities but also very crucial for the thermodynamic stability of LiTi2O4 compound. Furthermore, in Li2Ti2O4 compound, the full occupation of 16(c) sites by Li+ not only leads to a smaller C12 value but also leads to a much larger C44 one. Therefore, the plasticity and ductility of the Li2Ti2O4 become poor in comparison to LiTi2O4, while the thermodynamic stability of Li2Ti2O4 can be further improved after the Li+ intercalation of LiTi2O4.Highlights► The formation enthalpy for LiTi2O4 is calculated for the first time. ► LiTi2O4 is mechanically stable. ► The G/B ratio of 0.584 indicates the ductility of LiTi2O4 is excellent.

Microscale Li4Ti5−xZnxO12 (0 ≤ x ≤ 0.2) particles with high phase purity were synthesized by a simple solid-state reaction. The effect of the zinc doping on the physicochemical properties of Li4Ti5O12 (LTO) was extensively studied by TG-DSC, XRD, Raman spectroscopy, SEM, CV, EIS, and galvanostatic charge–discharge testing. The crystallization of lithium titanate oxide forms at about 750 °C. The lattice parameter of the Zn-doped LTO samples is slightly smaller than that for the pure LTO samples, and zinc doping does not change the basic Li4Ti5O12 structure. Even though the material has a particle size of 1–2 μm, Zn-doped LTO shows very high excellent capacity retention between 0 and 2.5 V. Especially, in rate performance, the Li4Ti4.8Zn0.2O12 sample maintains capacity of about 180 mAh g−1 until 5 C rates after 200 cycles, while the pure LTO sample shows severe capacity decline at corresponding rates. The reason for the high rate performance of Zn-doped LTO anode is ascertained to the diffusion coefficient (DLi) and reversible intercalation and deintercalation of lithium ion. The superior cycling performance and wide discharge voltage range, as well as simple synthesis route and low synthesis cost of the Zn-doped LTO are expected to show a potential commercial application.Graphical abstractZn doping enhances the reversibility of Li4Ti5O12, and then exhibits excellent lithium storage capability and high rate performance.Highlights► The over-discharge performance Li4Ti5−xZnxO12 (0 ≤ x ≤ 0.2) anode is first reported. ► Zn doping improves the conductivity and reversibility of the Li4Ti5O12. ► Li4Ti4.8Zn0.2O12 anode remarkably exhibits high rate performance.

•LiNi0.5Mn1.5O4-CeO2 shows an excellent fast charge-discharge performance.•LiNi0.5Mn1.5O4-CeO2 shows excellent cycling stability.•The introduction of CeO2 is conducive to the improvement of the kinetics property.A new type of microsized porous spherical LiNi0.5Mn1.5O4-CeO2 cathode material composed of aggregated nanosized particles with P4332 space groups was prepared by an ethanol-assisted hydrothermal method. The nanosized particle shortens the Li+-ion diffusion path in the bulk LiNi0.5Mn1.5O4 and then improves the fast charge–discharge performance of this material. Moreover, a thin CeO2 layer with nanometer thickness on the surface of the LiNi0.5Mn1.5O4 particles is helpful for suppressing the interfacial side reactions. Because of these advantages, the LiNi0.5Mn1.5O4-CeO2 materials exhibit excellent electrochemical properties. Compared with the pristine LiNi0.5Mn1.5O4, LiNi0.5Mn1.5O4-CeO2 (3 wt%) exhibits outstanding discharge capacity, cycling stability and rate capability. LiNi0.5Mn1.5O4-CeO2 (3 wt%) delivers discharge capacities of 129.7, 121.2, 118.1, 109.8, and 86.3 mAh g−1 at 0.2, 0.5, 1, 2, and 5 C discharge rates, but the pristine one only delivers discharge capacities of 119.9, 103.7, 91.8, 84.7 and 34.4 mAh g−1 at the corresponding discharge rates. The introduction of CeO2 is a valid approach to enhance the electrochemical property of the LiNi0.5Mn1.5O4 material by forming an excellent electrical contact between CeO2 layer and LiNi0.5Mn1.5O4 surface, leading to an enhanced lithium-ion diffusion coefficient, reduced electrochemical polarization, and improved conductivity.

Lithium-ion batteries are considered as one of the most promising power sources for energy storage system for a wide variety of applications such as electric vehicles (EVs) or hybrid electric vehicles (HEVs). The anode material often plays an important role in the determination of the safety and cycling life of lithium-ion batteries. Among all anode materials, spinel Li4Ti5O12 has been considered as one the most promising anode candidates for the next-generation large-scale power lithium-ion batteries used for HEVs or EVs because it has a high potential of around 1.55 V (vs. Li/Li+) during charge and discharge, excellent cycle life due to the negligible volume change, and high thermal stability and safety. In this review, we present an overview of the breakthroughs in the past decade in the synthesis and modification of both the chemistry and morphology of Li4Ti5O12. An insight into the future research and further development of Li4Ti5O12 composites is also discussed.