The Preparation, Improvement And Investigation Of The High-capacity Li-rich Cathode Material For The Lithium Ion Battery

High capacity is the main developing trend of the lithium ion battery. However, the cathodematerial of the commercial lithium ion battery now is mostly LiCoO2，which displays thedrawback of the low capacity of only140mAh·g-1, the toxicity and the high price. So to developthe new cathode material is the focus of the lithium ion battery research.The layered li-rich cathode material Li[NixLi1/3-2x/3Mn2/3-x/3]O2has attracted a wide attention inthe field of the lithium ion battery cathode material for its high capacity, the new charge/dischargemechanism and low cost. With the introduce of the structure-stable unit Li2MnO3, when firstcharged above4.5V, this charging curves of material displays L shape and the primary dischargingcapacity is above200mAh·g-1. Based on the material, the main contents of this thesis are asfollows:(1) The li-rich cathode materials were prepared by sol-gel. Firstly, we investigated theimpact of prepared temperature on the performance of the Li[Ni0.2Li0.2Mn0.6]O2. With theoptimized prepared temperature of850℃, the material showed a primary discharging capacity of254mAh·g-1with a coulombic efficient of76.1%and the capacity remained141.6mAh·g-1after16cycles. At850℃, we prepared a series of li-rich cathode materials Li[NixLi1/3-2x/3Mn2/3-x/3]O2(x=0.1-0.5).With the increasing x, the primary charging and discharging capacity of thesematerials both decreased with a increasing coulombic efficient. We also investigated the Co3+, V5+ion doping in Li[Ni0.2Li0.2Mn0.6]O2and found that the performance of the capacity and the cycle ofthe Co-doped Li[Ni0.2Li0.2Mn0.6]O2were improved and the primary coulombic efficient of V-doped Li[Ni0.2Li0.2Mn0.6]O2was improved, but the structure stability and the cycle performancewere deteriorated(2) The lithium ion diffusion coefficient of the li-rich material Li[Ni0.2Li0.2Mn0.6]O2wasdetermined by the CV and CITT method. The apparent lithium ion diffusion coefficientdetermined by the CV method was6.04×10-12cm2s-1and the lithium ion diffusion coefficient bythe CITT ranged from3.13×10-12to1.22×10-10cm2s-1. The trend of lithium ion diffusioncoefficient with the charging voltage was that lithium ion diffusion coefficient increased with thevoltage between3.8V and4.0V and reached the max1.22×10-10cm2s-1at4.0V, and when thevoltage beyond4.3V，lithium ion diffusion coefficient decreased rapidly.(3) Time domain transients to current step of multiple current densities are calculated toevaluate the polarization and fractional contribution of Li[Ni0.2Li0.2Mn0.6]O2at differentdischarging rates based on the equivalent circuit determined from the electrochemical impedancespectrum. The calculated data match the experimental profiles well. The results tell us that the totalpolarization of the battery can be divided into the ohmic resistance(R0), interfacial transportationimpedance(R1–Q1parallel), charge-transfer impedance(R2–Q2parallel) and lithium ion diffusionimpedance (Q3). The individual contribution to the total polarization is successfully differentiated within the discharging process. The total polarization can be classified reasonably as the parallelR1–Q1dominating Stage I and the parallel R2–Q2and Q3jointly controlling Stage II, respectively.The polarization of the charge transfer reaction, lithium transportation across the SEI film andsolid-state lithium diffusion process must be equally paid attention to and minimized by theoptimized preparation method and cell design.