Study On Preparation And Synergistic Modification Of Lithium-rich Manganese-based Cathode Materials

With the development of electric vehicles,the higher demands are raised over energy density,rate and cycle capabilities of lithium ion batteries.Exploiting high-performance cathode material is the key to solve the current problem.Among reported cathode materials,lithium-rich manganese-based cathode(Li1.2Mn0.54Co0.13Ni0.13O2)(LMO)is considered as one of the most promising cathode materials for the next generation lithium ion batteries owing to the properties of high operating potential,high specific capacity,environmentally friendly and low cost.However,the low initial coulombic efficiency and voltage/capacity decay during the cycle process,seriously hinder the practical applications.Furthermore,the rate and cycle capabilities of LMO are still unsatisfactory for the moment.There are two main causes for these problems,the first is the structural properties of LMO.During the charging process,with the formation of Li and O vacancy,the system energy of LMO is increased,which induces a unstable thermodynamic state of LMO,resulting in the migration of transition metal elements which finally leads to the undesired phase transformation.The above process occurs continuously in the cycle process,leading to the constant attenuation of the voltage and capacity.The second is the side reactions between LMO and the electrolyte.Transition metal ions and oxygen ions with high activity can react with the electrolyte,resulting in surface erosion and the formation of electrochemical inert by-products,which decreases the electrochemical performance of the materials.In view of this,firstly,we compared the electrochemical performance of LMO,prepared by different methods.With selecting proper raw materials,we propose a synergistic modification strategy and achieve improvement of the electrochemical performance of LMO by stabilizing the structure of LMO and suppressing the side reactions.The research contents of this thesis are concluded as follows:(1)Sol-gel and co-precipitation methods were used to prepare LMO materials firstly,we compared their morphology,the degree of cation mixing,and the specific surface area.Results show that the materials synthesized by sol-gel method(LMO-SG)are nano-sized,while the materials synthesized by co-precipitation method(LMO-CP)are micro-sized.The degree of cation mixing of LMO-SG is higher than that of LMO-CP.Furthermore,the specific surface area of LMO-SG is much higher than that of LMO-CP.After comparison,LMO-SG exhibits higher discharge specific capacity(0.1 C,269.9 mAh/g),higher initial coulomb efficiency(81.97%)and excellent rate performance(5 C,142.7 mAh/g).However,LMO-SG also suffers lower capacity retention(0.5 C,92.8%after 100 cycles)and serious voltage decay.The voltage decayed by 0.32 V after 100 cycles at 0.5C.LMO-CP shows an activation process during the cycle process,displaying a low initial coulombic efficiency(69.57%),poor rate performance(5 C,117.5 mAh/g),but good structural stability.Specifically,the capacity retention rate is 95.58%after 100 cycles at 0.5 C,with a voltage decay of 0.2V.After wards,the materials are detected by XRD measurements after cycles,the results show that the degree of cation mixing of LMO-CP is still higher than that of LMO-SG,indicating a better electrochemical stability.Which should be due to the large specific surface area and short lithium-ion conduction path of the nano-sized LMO materials,ensuring the sufficient contact between the particles and the electrolyte.There for the higher capacity and excellent rate performance can be realized.In contrast,the micro-sized materials,show a low specific surface area and a long Li-ion conduction path,while the inner part of materials are well protected,thus exhibiting good cycle stability.(2)The method of cation doping(Na+)and surface coating(Li2-xNaxSiO3)is used to suppress the voltage fade of LMO cathode.The synergistic modification is realized by an in-situ hydrolysis and high temperature calcination process.Na+doped into the lithium layer exhibits a pinning effect,which stabilizes the structure of LMO,the coating layer Li2-xNaxSiO3 protects the surface of LMO.The electrochemical comparisons among pristine LMO,Li2SiO3 coated LMO and Li1.5Na0.5SiO3 modified LMO are carried out.Results show that Li1.5Na0.5SiO3 modified LMO shows the degree of cation mixing and the obviously increased layer spacing of lithium ions.The 1wt%Li1.5Na0.5SiO3 modified LMO exhibits an excellent electrochemical performance.The voltage drop after 100 cycles at 0.5 C is merely 0.198 V,which is 52%of the voltage decay of the sample coated with 1wt%Li2SiO3,and 29%of the unmodified LMO.Further more the specific discharge capacity(0.1 C 280.9 mAh/g)the rate performance(1 C,200.6 mAh/g)and the structure stability are significantly improved.(3)Synergistic anion doping(S2-)and surface(Cu9S5)coating method is carried out to modify LMO materials.Taking full advantage of the decomposition of CuS at a high temperature.The sulfur doping and Cu9S5 coating on LMO can be achieved simultaneously.S2-doping could efficiently release the structural stress,and adjust the electronic structure of transition metal ions.Cu9S5 displays the same layered structure and space group(R3m)with that of LMO,furthermore,Cu9S5 also shows the excellent electronic conductivity.After modification,the electrochemical performance of LMO is significantly improved.The 1%CS@LMOS materials exhibit the highest initial coulombic efficiency(84.07%),excellent rate performance(5 C,151 mAh/g)and the highest capacity retention(92.76%,at 1 C after 100cycles),furthermore,the apparent lithium-ion diffusion rate of 1%CS@LMOS materials is 1.2 times of 0%CS@LMO,during the charging process and 1.17 times during the discharging process.(4)The method of polyanion doping and surface spinel coating is used to modify LMO.To solve the issues like falling off of the coating layer,mismatch of crystallinity and nonuniform coating,the Lewis acid material bis(catechol)diborate was selected due to its electrophilic and reducing characteristics.The high-temperature calcination process during modification induces the in-situ formation of B3+doped LiMn2O4 spinel structure on the surface of lithium-rich manganese-based materials and BO33-doped structure on the LMO surface(SLLMO).After modification,the initial coulombic efficiency is significantly improved(from 79.9%of the LMO material to 85.6%of the SLLMO materials).The cycle stability is significantly improved,and the capacity retention of the SLLMO sample is 97%after 100 cycles at 0.5 C and 72.5%after 400 cycles at 1 C,which is much higher than that of LMO materials(88%and 57.8%)under the same conditions;The lithium ion diffusion rate is also increased.The apparent lithium-ion diffusion rate of SLLMO is 1.55 times and 2.19 times of the unmodified material during charging and discharging process,respectively.After 400 cycles at 1 C,the ionic diffusion rate of SLLMO is still 1.6 times of the unmodified LMO cathode.