Yun Lu

Enhanced Electrochemical Performance of Layered Lithium-Rich Cathode Materials by Constructing Spinel-Structure Skin and Ferric Oxide Islands

Layered lithium-rich cathode materials have been considered as competitive candidates for advanced lithium-ion batteries because they are environmentally benign, high capacity (more than 250 mAh·g-1), and low cost. However, they still suffer from poor rate capability and modest cycling performance. To address these issues, we have proposed and constructed a spinel-structure skin and ferric oxide islands on the surface of layered lithium-rich cathode materials through a facile wet chemical method. During the surface modification, Li ions in the surface area of pristine particles could be partially extracted by H+, along with the depositing process of ferric hydrogen. After calcination, the surface structure transformed to spinel structure, and ferric hydrogen was oxidized to ferric oxide. The as-designed surface structure was verified by EDX, HRTEM, XPS, and CV. The experimental results demonstrated that the rate performance and capacity retentions were significantly enhanced after such surface modification. The modified sample displayed a high discharge capacity of 166 mAh·g-1 at a current density of 1250 mA·g-1 and much more stable capacity retention of 84.0% after 50 cycles at 0.1C rate in contrast to 60.6% for pristine material. Our surface modification strategy, which combines the advantages of spinel structure and chemically inert ferric oxide nanoparticles, has been shown to be effective for realizing the layered lithium-rich cathodes with surface construction of fast ion diffusing capability as well as robust electrolyte corroding durability.

Ni-Rich LiNi0.8Co0.1Mn0.1O2 Oxide Coated by Dual-Conductive Layers as High Performance Cathode Material for Lithium-Ion Batteries

Ni-rich materials are appealing to replace LiCoO2 as cathodes in Li-ion batteries due to their low cost and high capacity. However, there are also some disadvantages for Ni-rich cathode materials such as poor cycling and rate performance, especially under high voltage. Here, we demonstrate the effect of dual-conductive layers composed of Li3PO4 and PPy for layered Ni-rich cathode material. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy show that the coating layers are composed of Li3PO4 and PPy. (NH4)2HPO4 transformed to Li3PO4 after reacting with surface lithium residuals and formed an inhomogeneous coating layer which would remarkably improve the ionic conductivity of the cathode materials and reduce the generation of HF. The PPy layer could form a uniform film which can make up for the Li3PO4 coating defects and enhance the electronic conductivity. The stretchy PPy capsule shell can reduce the generation of internal cracks by resisting the internal pressure as well. Thus, ionic and electronic conductivity, as well as surface structure stability have been enhanced after the modification. The electrochemistry tests show that the modified cathodes exhibited much improved cycling stability and rate capability. The capacity retention of the modified cathode material is 95.1% at 0.1 C after 50 cycles, whereas the bare sample is only 86%, and performs 159.7 mAh/g at 10 C compared with 125.7 mAh/g for the bare. This effective design strategy can be utilized to enhance the cycle stability and rate performance of other layered cathode materials.

Exposing the {010} Planes by Oriented Self-Assembly with Nanosheets To Improve the Electrochemical Performances of Ni-Rich Li[Ni0.8Co0.1Mn0.1]O2 Microspheres

A modified Ni-rich Li[Ni0.8Co0.1Mn0.1]O2 cathode material with exposed {010} planes is successfully synthesized for lithium-ion batteries. The scanning electron microscopy images have demonstrated that by tuning the ammonia concentration during the synthesis of precursors, the primary nanosheets could be successfully stacked along the [001] crystal axis predominantly, self-assembling like multilayers. According to the high-resolution transmission electron microscopy results, such a morphology benefits the growth of the {010} active planes of final layered cathodes during calcination treatment, resulting in the increased area of the exposed {010} active planes, a well-ordered layer structure, and a lower cation mixing disorder. The Li-ion diffusion coefficient has also been improved after the modification based on the results of potentiostatic intermittent titration technique. As a consequence, the modified Li[Ni0.8Co0.1Mn0.1]O2 material exhibits superior initial discharges of 201.6 mA h g-1 at 0.2 C and 185.7 mA h g-1 at 1 C within 2.8-4.3 V (vs Li+/Li), and their capacity retentions after 100 cycles reach 90 and 90.6%, respectively. The capacity at 10 C also increases from 98.3 to 146.5 mA h g-1 after the modification. Our work proposes a novel approach for exposing high-energy {010} active planes of the layered cathode material and again confirms its validity in improving electrochemical properties.

Synergistic Effects of Stabilizing the Surface Structure and Lowering the Interface Resistance in Improving the Low-Temperature Performances of Layered Lithium-Rich Materials

The layered lithium-rich cathode material, Li1.2Ni0.2Mn0.6O2, was successfully synthesized by a sol-gel method followed by coating with different amounts of Li2O-2B2O (LBO, 1, 3, and 5 wt %). The effects of LBO-coating layer on the structure, morphology, and low-temperature (-30 °C) electrochemical properties of these materials are investigated systematically. The morphology, crystal structure, and grain size of the Li-rich layered oxide are not essentially changed after surface modification; according to the TEM results, the Li-B-O coating layer exists as an amorphous layer with a thickness of 5-8 nm when the amount is 3 wt %. Electrochemistry tests reveal that 3 wt % LBO-coated samples present the best electrochemical capability at low temperature. At -20 °C, the 3 wt % LBO-coated sample could retain 45.7% of the initial discharge capacity (131.7/288.0 mAh g-1) of that at 30 °C, while the pristine material could only retain 22.5% (57.5/256.0 mAh g-1). XPS spectra and EIS results reveal that such an enhancement of low-temperature discharge capacity should be attributed to the proper LBO-coating layer, which not only endows the modified materials with more stable surface structure but also lowers the interface resistance of Li+ diffusion through the interface and charge transfer reaction.