Effects Of Different Conductive Additives On The Performance Of Li-Rich Mn-Based Layered Oxide Electrodes

With the rapid development of electric vehicles,the requirements for lithium ion batteries with high energy density,long cycle life and low cost,are increasing.Li-rich Mn-based layered oxide(LMRO)materials,xLi2MnO3·(1-x)LiMO2(M=Ni,Mn,Co etc.,0<x<1),with high specific capacity and low raw material cost,are considered to be most potential candidates for next generation high-energy cathode materials for liquid lithium ion batteries.During the first charging process,the activation of Li2MnO3 will give rise to a large amount of lithium vacancies and migration of transition metal ions,resulting in the formation of a large number of two-phase interfaces in the bulk phase of the material,which hinder the diffusion of Li+.So,LMRO material has poor kinetic properties.In addition,LMRO particles tend to be spherical in order to increase the compaction density of the electrode,and the materials has a high manganese content and a low cobalt content,which make the LMRO electrodes have poor electronic conductivity.The low ion conductivity and electronic conductivity result in large polarization and poor performance at high current density for LMRO electrodes in practical applications.In this paper,under the premise of high loading density and high active material ratio,the influence of conductive additives with different morphological features,including conductive carbon black(Super P),vapor phase carbon fibers(VGCF)and multi-walled carbon nanotubes(MCNTs),on the performance of the LMRO electrodes has been studied in order to reduce the polarization and improve the high current performance of the electrodes.The feasibility of using solid ion conductor Li 1.5Al0.5Ge1.5(PO4)3(LAGP)as ion conductive additive has been discussed as well.The main contents and results of this paper are as follows:(1)The effect of zero-dimensional conductive additive carbon black(Super P)on the polarization and performance of LMRO electrode was investigated.The experimental results showed that charge transfer resistance(Rct)of the fresh electrodes firstly increased and then decreased with the increase of Super P content and that Rct of the electrode with 5wt%Super P was smallest.When the addition amount of Super P was 5wt%,the electrode showed most excellent electrochemical performance,and the specific discharge capacity at 3C was 67.7 mAh g-1,and the capacity retention after 200 cycles was 68.6%,increasing by 43.0%and 14.4%compared with the electrodes with 2wt%and 6wt%Super P respectively.During the charge and discharge cycles,the electrode with 5wt%Super P showed the smallest change in Rct,IR drop at the beginning of discharge stage and the difference value between average charge and discharge voltage,consistent with the cycle performance of the electrode.(2)The effect of one-dimensional conductive additive vapor grown carbon fibers(VGCF)with low specific surface area on the polarization and performance of LMRO electrode was investigated.As the VGCF content increased in the range from 2wt%to 6wt%,surface resistance of the electrodes was decreased and the increasing of electrochemical impedance and difference value between average charge and discharge voltage during charge/discharge cycles was suppressed.The electrode with 6wt%VGCF showed the best rate and cycle performance that specific discharge capacity at 3C was 73.7 mAh g-1 and that the capacity retention after 200 cycles was 71.0%.(3)The effect of one-dimensional conductive additive multi-walled carbon nanotubes(MCNTs)with high specific surface area on the polarization and performance of LMRO electrode was investigated.Cycle stability of the electrodes firstly improved and then decreased with the increase of MCNTs content.When the content of MCNTs in the electrode is 3wt%,the electrode showed the slower increasing of polarization and better electrochemical performance,and specific discharge capacity at 2C was 101.0 mAh· g-1 and that the capacity retention after 100 cycles was 60.3%.(4)The inorganic solid Li-ion conductor LAGP was mixed with LMRO material by mechanical grinding.The impedances of LMRO electrodes were reduced,the change in difference value between average charge and discharge voltage during charge/discharge cycles was limited,the increasing of polarization was released and the electrochemical performance was improved.The LMRO electrode with 5wt%LAGP showed the smallest impedance,the largest apparent Li+diffusion coefficient,the highest coulombic efficiency,the slowest increasing of polarization.Meanwhile,specific discharge capacity at 5C and capacity retention after 200 cycles of the electrode with 5wt%LAGP were increased by 37.3 mAh g-1 and 21.1%respectively,compared with the electrode without LAGP.