Fuquan Cheng

Monodispersed mesoporous Li4Ti5O12 submicrospheres as anode materials for lithium-ion batteries: morphology and electrochemical performances

Although nanosizing Li4Ti5O12 (LTO) materials is an effective way to improve their rate performances, their low tap density and first cycle coulombic efficiency limit their practical applications. To tackle these problems while preserving the advanced rate performances, monodispersed mesoporous LTO submicrospheres are developed here. These submicrospheres are synthesized via a solvothermal method using TiO2 submicrospheres and LiOH as precursors followed by a mild calcinations. The roles of the solvent used in the solvothermal process and calcination temperature are systematically investigated and optimized. The LTO submicrospheres fabricated by the solvothermal process using a water-ethanol (60 vol%) solvent followed by a calcination process at 600 °C reveal a large sphere size of 660 ± 30 nm with a small primary particle size of 20-100 nm, a large specific surface area of 15.5 m(2) g(-1), an appropriate pore size of 4.5 nm and an ultra-high tap density of 1.62 g cm(-3). Furthermore, they show high crystallinity and no blockage of Li(+) ion transportation pathways. Due to the novel morphology and ideal crystal structure, these submicrospheres exhibit outstanding electrochemical performances. They display a high first cycle coulombic efficiency of 93.5% and a high charge capacity of 179 mA h g(-1) at 0.5 C between 1.0 and 2.5 V (vs. Li/Li(+)), surpassing the theoretical capacity of LTO. Their charge capacity at 10 C is as high as 109 mA h g(-1) with a capacity retention of 97.8% over 100 cycles. Therefore, this LTO material can be a superior and practical candidate for the anodes of high-power lithium-ion batteries.

Li4Ti5O12-based anode materials with low working potentials, high rate capabilities and high cyclability for high-power lithium-ion batteries: a synergistic effect of doping, incorporating a conductive phase and reducing the particle size

Doping, incorporating a conductive phase and reducing the particle size are three strategies for improving the rate capability of Li4Ti5O12 (LTO). Thus, the synergistic employment of these three strategies is expected to more efficiently improve the rate capability. To achieve this goal, Fe2+ doped LTO/multiwall carbon nanotube (MWCNT) composites were prepared by post-mixing MWCNTs with Fe2+ doped LTO particles from a solid-state reaction, while Cr3+ doped LTO/MWCNT composites were fabricated by a facile one-step solid-reaction using MWCNT premixing. Fe2+/Cr3+ doping not only remarkably improves the electronic conductivity and Li+ ion diffusion coefficient in LTO but also lowers its working potential. The carbon existed in the material fabrication processes leads to the reduction of the particle size. The introduction of MWCNTs in the Fe2+/Cr3+ doped LTO/MWCNT composite significantly enhances the electrical conduction between Fe2+/Cr3+ doped LTO particles. As a result of this novel synergistic strategy, performances of Li3.8Fe0.3Ti4.9O12/MWCNT and LiCrTiO4/MWCNT composites are comprehensively improved. The Li3.8Fe0.3Ti4.9O12/MWCNT composite shows a working potential of 8.9 mV lower than that of pristine LTO. At 10 C, its capacity is up to 106 mA h g−1 with an unexpected capacity retention of 117% after 200 cycles in a potential window of 1.0–2.5 V (vs. Li/Li+). The corresponding values for LiCrTiO4/MWCNT composites are 46.2 mV, 120 mA h g−1 and 95.9%. In sharp contrast, the pristine counterpart shows a very disappointing capacity of only 11 mA h g−1 at 10 C. Therefore, the novel Li3.8Fe0.3Ti4.9O12/MWCNT and LiCrTiO4/MWCNT composites possess great potential for applications in high-power lithium-ion batteries.

Monodisperse Li1.2Mn0.6Ni0.2O2 microspheres with enhanced lithium storage capability

Monodisperse spherical Mn0.75Ni0.25(OH)2 precursors built up from plate-like primary particles have been successfully synthesized by the control of pH values during a co-precipitation reaction. The size of spherical particles, namely the secondary particles, is observed to decrease with increasing pH value from 9.0 to 11.0, and is accompanied by a series of shape changes of the primary particles from close-packed plates to well-exposed nanoplates, and then to nanoparticles. Further lithiation of these hydroxide precursors produces the final lithium-rich layered Li1.2Mn0.6Ni0.2O2 cathode materials without destroying the morphology of the precursors. Electrochemical measurements show that the spherical cathode material assembled from well-exposed nanoplates exhibits superior rate capability and good cyclability compared to other electrode materials, which can be attributed to its uniform particle size and the favorable shape which facilitates the diffusion of lithium ions. Through the control of the sample morphologies, we provide a simple and effective way to enhance the lithium storage capability of lithium-rich layered oxide cathode materials for high-performance lithium-ion batteries.

Optimizing the carbon coating on LiFePO4 for improved battery performance

Core–shell structures as LiFePO4@carbon with a continuous and uniform carbon coating were achieved by means of the in situ polymerization of dopamine. Systematic control of the coating layer identified that a 5 nm carbon coating produces the best battery performance. Our results provide conclusive evidence for an optimal carbon coating for polyanion-type cathode materials.

Accurate surface control of core–shell structured LiMn0.5Fe0.5PO4@C for improved battery performance

Manganese-based mixed polyanion cathodes known as LiMn1−xFexPO4 can show much higher energy density as compared to the well-commercialized product of lithium iron phosphate. However, their much lower electronic conductivity has long plagued their further application. Here, by means of a facile solution-based synthesis route, we are able to introduce a uniform and conformal carbon coating layer onto LiMn1−xFexPO4 nanoparticles. The versatility in the synthesis control endows us with the capability of controlling the shell thickness with one nanometer accuracy, offering an effective way to optimize the battery performance through a systematic shell control. Detailed investigation reveals that the carbon nanoshells not only act as good electronic conducting media, but also contribute to the inhibition of the metal (Mn and Fe) dissolution and reduce the exothermic heat released during cycling. The core–shell structured cathode materials show promising potential for their application in lithium ion batteries as revealed by their high charge–discharge capacity, remarkable thermal stability, and excellent cyclability.

Optimizing LiFePO4@C Core–Shell Structures via the 3-Aminophenol–Formaldehyde Polymerization for Improved Battery Performance

Polyanion-type cathode materials are well-known for their low electronic conductivity; accordingly, the addition of conductive carbon in the cathode materials becomes an indispensable step for their application in lithium ion batteries. To maximize the contribution of carbon, a core-shell structure with a full coverage of carbon should be favorable due to an improved electronic contact between different particles. Here, we report the formation of a uniform carbon nanoshell on a typical cathode material, LiFePO4, with the shell thickness precisely defined via the 3-aminophenol-formaldehyde polymerization process. In addition to the higher discharge capacity and the improved rate capability as expected from the carbon nanoshell, we identified that the core-shell configuration could lead to a much safer cathode material as revealed by the obviously reduced iron dissolution, much less heat released during the cycling, and better cyclability at high temperature.

Recent Development in the Rate Performance of Li4Ti5O12

Lithium-ion batteries (LIBs) have become popular electrochemical devices. Due to the unique advantages of LIBs in terms of high operating voltage, high energy density, low self-discharge, and absence of memory effects, their application range, which was primarily restricted to portable electronic devices, is now being extended to high-power applications, such as electric vehicles (EVs) and hybrid electrical vehicles (HEVs). Among various anode materials, Li4Ti5O12 (LTO) is believed to be a promising anode material for high-power LIBs due to its advantages of high working potential and outstanding cyclic stability. However, the rate performance of LTO is limited by its intrinsically low electronic conductivity and poor Li + ion diffusion coefficient. This review highlights the recent progress in improving the rate performance of LTO through doping, compositing, and nanostructuring strategies.