Improved Performance Of Electrode Materials For Li-Ion Batteries By Coating Method

Advanced electrode material is the key to improve the performance of lithium ion battery (energy/power density, cycle life, safety, etc.) and broaden the application fields. In this dissertation, coating method was used to modify commercial cathode materials to improve the performance of LiCoO2 under high upper cut-off voltage and LiMn2O4 under high temperature to promote application, besides, new coating structures were designed to develop high-performance MnO- or MnS-based anode materials. The specific works are as follows:1. Co-Al mixed metal oxide coated LiCoO2 and LiAlSiO4 coated LiCoO2 have been prepared to enhance the cyclic performance of commercial LiCoO2 under a high upper cut-off voltage of 4.5 V vs. Li+/Li. Co-Al mixed metal oxide coated LiCoO2 (LiCoO2@CoAl-O) has been prepared by calcining the precursor of Co-Al layered double hydroxide coated LiCoO2 synthesised in the suspension of LiCoO2 by coprecipitation method. LiAlSiO4 coated LiCoO2 (LiCoO2@LiAlSiO4) has been prepared via sol-gel method with a subsequent calcination process. Both LiCoO2@CoAl-O and LiCoO2@LiAlSiO4 show good capacity retention of～90% after 200 cycles within a voltage range of 2.75-4.5 V vs. Li+/Li and at 90 mA·g-1. The greatly improved cyclic performance is closely related to the coating layer. The stable CoAl-O (or LiAlSiO4) coating layer reduces direct contact between LiCoO2 and the electrolyte, improves the stability of the interface, and slows down the increasements of the charge transfer resistance Ret. The differences between LiCoO2@CoAl-O and LiCoO2@LiAlSiO4 are a high initial discharge capacity for LiCoO2@CoAl-O and a low cost for LiCoO2@LiAlSiO4. Furthermore, cylinder-type R14500 full cells (AA) using LiCoO2@CoAl-O as cathode material were assembled to study the cyclic performance under a high upper cut-off voltage of 4.4 V (corresponding to 4.5 V vs. Li+/Li). LiCoO2@CoAl-O in AA shows excellent capacity retention of 86.4% after 400 cycles within a wide voltage range of 2.75-4.4 V and at 165 mA·g-1, which is much superior than the pristine LiCoO2 with capacity retention of 86.4%.2. LiNi0.5Mn1.5O4 coated LiMn2O4 has been prepared to enhance the cyclic performance of commercial LiMn2O4 under a high temperature (55℃). A novel wet chemical method has been successfully developed to prepare LiNi0.5Mn1.5O4 coated LiMn2O4 (LMO@LNMO), in which commercial LiMn2O4 produced by solid state reaction method was used as the starting material and a nitrate precursor containing Li, Ni and Mn was used to form LiNi0.5Mn1.5O4 coating layer. There is no precipitant, chelating agent and washing process needed. LMO@LNMO shows a relatively high initial discharge capacity of-100 mAh·g-1 and greatly improved cyclic performance with a capacity retention of 81.9% after 400 cycles within a voltage range of 3.0-4.3 V vs. Li+/Li and at 100 mA·g-1 at 55℃. The greatly improved cyclic performance is due to the formation of Ni concentration-gradient structure, which reduces the interface resistance between LiMn2O4 and LiNi0.5Mn1.5O4 and provides a high average valence of Mn on the surface to enhance the stability of the interface between LMO@LNMO and electrolyte.3. N-doped C coated MnO has been prepared to obtain good cyclic and rate performance. A novel Chinese lantem-like N-doped C coated MnO (MnO@N-C) has been fabricated via a Chinese lantem-like MnC03 precursor with a dopamine (DA) coating process and a subsequent calcination process under N2 atmosphere. MnO@N-C shows a high initial discharge capacity of 1297.7 mAh·g-1 and a high reversible capacity of 810.2 mAh·g-1 at 0.2 A·g-1, favorable cyclic stability of 640 mAh·g-1 after 400 cycles, and excellent rate capability of～451 mAh·g-1 at 1 A·g-1 and ～285 mAh·g-1 at 4 A·g-1 within the voltage range of 0.01-3.0 V vs Li+/Li. The excellent performance of MnO@N-C can be attributed to its unique Chinese lantern-like structure. The homogeneously embedded MnO nanoparticles within the N-C matrix can realize a high electrochemical utilization of the materials for electrochemical conversion reaction to generate a high specific capacity. The N-C matrix and the voids between the nanoplates can limit strain and thus maintain the structural of the MnO@N-C electrode during the discharge/charge process, consequently enabling improved cyclic performance. The MnO@N-C nanoplates with high specific surface area in the Chinese lantern-like framwork can shorten the path diffusing length of the lithium ions and the N-C matrix can provide efficient electrical integrity to the electrode, which can enhance the rate capability.4. S and N-doped C coated MnS has been prepared to obtain good cyclic and rate performance. A spherical S and N co-doped C coated MnS (s-MnS@SN-C) has been fabricated by calcining a novel precursor dried from a solution containing m-aminobenzenesulfonate anion and Mn2+ after the complete reaction between m-aminobenzenesulfonic acid and MnCO3 in deionized water. The carbon source, nitrogen source and sulfur source needed in MnS@.SN-C are all from m-aminobenzene sulfanilate ion and the synthesis process is mild, simple and safety. s-MnS@SN-C shows favorable cyclic stability with discharge capacity of～380 mAh·g-1 and capacity retention of～100% after 500 cycles at 1.0 A·g-1 within the voltage range of 0.05-3.0 V vs Li+/Li. The excellent performance of s-MnS@SN-C depends on its structure. The homogeneously embedded MnS nanoparticles within the SN-C matrix can realize a high electrochemical utilization of the materials for electrochemical conversion reaction to generate a high specific capacity. The SN-C matrix can anchored Li2S which is the discharge product of MnS; the SN-C matrix can also buffer volume expansion of MnS; the small surface area can reduce the side reaction to decrease the consumption of electrolyte, consequently enabling improved cyclic performance. The mesoporous in s-MnS@.SN-C is good for electrolyte infiltration and the SN-C matrix can provide efficient electrical integrity to the electrode, which can enhance the rate capability.