Peiyu Hou

A high energy density Li-rich positive-electrode material with superior performances via a dual chelating agent co-precipitation route

A new Li-rich positive-electrode Li1.13(Ni0.26Co0.09Mn0.52)O2 is successfully achieved via a dual ammonia and oxalate chelating agent co-precipitation route for the first time, which delivers a high volumetric energy density of over 2100 W h L−1, superior cycle life and stable high median-voltage. The dual or multiple chelating agent method gives a new insight towards high energy density for Li-rich materials with outstanding electrochemical performances for advanced lithium-ion batteries.

Understanding the Origin of Enhanced Performances in Core–Shell and Concentration-Gradient Layered Oxide Cathode Materials

Core-shell and concentration-gradient layered oxide cathode materials deliver superior electrochemical properties such as long cycle life and outstanding thermal stability. However, the origin of enhanced performance is not clear and seldom investigated until now. Here, a specific structured layered oxide (LiNi0.5Co0.2Mn0.3O2) consisting of concentration-gradient core, transition layer, and stable outer shell, is designed and achieved from double-shelled precursors to overcome the great challenge by comparison with the normal layered LiNi0.5Co0.2Mn0.3O2. As expected, the specific structured layered oxide displays excellent cycle life and thermal stability. After numerous cycles, the valence state of Ni and Co at normal layered oxide surface tends to a higher oxidation state than that of the specific structured oxide, and the spinel phase is observed on particle surface of normal layered oxide. Also, the deficient spinel/layered mixed phases lead to high surface film and charge-transfer resistance for normal layered oxide, whereas the specific structured one still remains a layered structure. Those results first illustrate the origin of improved electrochemical performance of layered core-shell and concentration-gradient cathode materials for lithium-ion batteries.

A novel core-concentration gradient-shelled LiNi0.5Co0.2Mn0.3O2 as high-performance cathode for lithium-ion batteries

A novel core-concentration gradient-shelled LiNi0.5Co0.2Mn0.3O2 was successfully synthesized for the first time by a simple method from core-shelled precursors [(Ni0.6Co0.2Mn0.2)1/2(Ni0.4Co0.2Mn0.4)1/2](OH)2 that were synthesized via a co-precipitation route. Particle size increase of hydroxide precursors from core to shell, in combination with subsequent investigations of Energy Disperse X-ray Spectrum (EDS) on precursors, supported the formation of a core-shelled structure. To obtain concentration gradient layer between core and shell, a high calcined temperature of 900 °C was selected as high temperature calcination gave rise to diffusion of cations in the core-shelled structure. Thus, the prepared precursor powders were then calcined with stoichiometric ratio lithium carbonate (Li/M = 1.05) at 900 °C in air, which resulted in core-concentration gradient-shelled (CCGS) LiNi0.5Co0.2Mn0.3O2. The compositions of core and shell separately were LiNi0.60Co0.20Mn0.20O2 and LiNi0.44Co0.20Mn0.36O2, between which was a concentration gradient layer. X-ray diffraction (XRD) studies show that the prepared material was indexed to a typical layered structure with a Rm space group. Compared to LiNi0.5Co0.2Mn0.3O2, the CCGS-LiNi0.5Co0.2Mn0.3O2 presented remarkably improved cycling performance and thermal stability, which can be ascribed to LiNi0.44Co0.20Mn0.36O2 shell providing structural and thermal stability.

Carbonate coprecipitation preparation of Li-rich layered oxides using the oxalate anion ligand as high-energy, high-power and durable cathode materials for lithium-ion batteries

Rechargeable lithium-ion batteries (LIBs) present an urgent demand to develop cathode materials that combine high-energy and high-power density with long cycle life. For meeting the demand, dual ligands (ammonia and oxalate anion) by hydroxide coprecipitation have been introduced to prepare spherical precursors for the above desired cathode in our previous study, in which low efficiency, toxic and volatile ammonia is still utilized as one of the ligands and an inert atmosphere is needed due to the high content of Mn. Thus, in this work, the feasibility of using the oxalate anion as a single ligand by carbonate coprecipitation for Li-rich layered oxides is investigated. Consequently, they deliver a high volumetric energy density of about 2000 W h L−1, a high-power density of over 940 W h L−1 at a current density of 1000 mA g−1, and superior cycling stability with a capacity retention of 98.1% after 80 cycles, indicating much better performances than the Li-rich oxides prepared via the ammonia ligand. Also, their performances approach the level for the sample prepared via dual ligands. The enhanced properties are likely ascribed to the smaller primary particles and the possibly suppressed phase transformation from layered to spinel phases due to a large amount of stacking faults, lower cation mixing and higher Mn oxidation state according to SEM, TEM, XRD and XPS experiments. These findings demonstrate that the oxalate anion is a desired ligand to prepare Li-rich layered oxides as high-energy, high-power and durable cathode materials for LIBs.

Li-ion batteries: Phase transition*

Progress in the research on phase transitions during Li+ extraction/insertion processes in typical battery materials is summarized as examples to illustrate the significance of understanding phase transition phenomena in Li-ion batteries. Physical phenomena such as phase transitions (and resultant phase diagrams) are often observed in Li-ion battery research and already play an important role in promoting Li-ion battery technology. For example, the phase transitions during Li+ insertion/extraction are highly relevant to the thermodynamics and kinetics of Li-ion batteries, and even physical characteristics such as specific energy, power density, volume variation, and safety-related properties.

Synthesis and electrochemical characteristics of Li1.2(Ni0.2Mn0.6)x(Co0.4Mn0.4)y(Ni0.4Mn0.4)1−x−yO2 (0 ≤ x + y ≤ 1) cathode materials for lithium ion batteries

According to the tetrahedral phase diagram of LiNiO2–LiCoO2–LiMnO2–Li2MnO3, a series of Li1.2(Ni0.2Mn0.6)x(Co0.4Mn0.4)y(Ni0.4Mn0.4)1−x−yO2 (0 ≤ x + y ≤ 1) have been designed to explore new Li-rich solid solution cathode materials. The effects of Li1.2Ni0.2Mn0.6O2, Li1.2Co0.4Mn0.4O2 and Li1.2Ni0.4Mn0.4O2 content in solid solutions on structure and electrochemical properties are investigated. Micro-sized spherical or ellipsoidal precursors are first prepared via a carbonate co-precipitation route. After calcination with lithium sources, all samples are indexed to a typical layered structure with an Rm space group as detected by X-ray diffraction (XRD). It is found that the introduction of Co can improve the tap density. However, these Co referred samples reveal lower discharge specific capacities and inferior cycle life. For these Co-free materials with high Ni content, for instance Li1.2Ni0.3Mn0.5O2, although low capacity is observed in the initial cycle, a large capacity of above 250 mA h g−1 is achieved after about 10 cycles. Importantly, the activated Li1.2Ni0.3Mn0.5O2 material still delivers a high capacity of over 230 mA h g−1 after 70 cycles, displaying superior cycle stability. These results may be instructive in designing and exploring high performance cathode materials for advanced LIBs.