Research On Synthesis And Modiifcaiton Of LiNi_0.4Co_0.2Mn_0.4O₂Cathode Material For High-capacity Litium-ion Battery

The layered LiNixCo1-x-yMnyO2material has become the most promising cathodecandidate material to replace LiCoO2owing to its advantages as high capacity, stable structure,safety and low cost. However, further the cost reduction and improvement of the capacity andthe rate performance have become the key problems to be conquered. In this article, wecentered on the investigation on the LiNi0.4Co0.2Mn0.4O2cathode material, synthesis methodswere investigated systematically, effect of single doping and co-doping with cations andanions, along with the surface coating were analyzed as well. Techniques of TG, SEM, XRD,CV, EIS and electrochemical performance were adopted to analyze the effects of synthesismethods and modification ways.In the present work, layered LiNi0.4Co0.2Mn0.4O2were synthesised via sol-gel method,high presure solid method, and co-precipitate method. It is indicated that the difference in thesynthesis method resulted in the difference in crystal structure, particle morpholigy, sizedistribution and electrochemical performance. Compared with the other two method, samplesobtain by sol-gel method exhibited higher specific capacity, delivering a capacity of182.7mAh/g at the charge rate of0.2C between2.5-4.6V, with a coulomb efficiency of90.4%.While samples synthesized by co-precipitate method showed excellent cycling performance,exhibiting a capacity retaintion rate of92.8%after50cycles at0.2C between2.5-4.6V. Thesynthesis conditions of LiNi0.4Co0.2Mn0.4O2by sol-gel method were thoroughly investigated,and the result proved that sintering at850for20h is most suitable.Then the layered LiNi0.4Co0.2Mn0.4O2was doped with different amount of anion ions Fand Cl. It is indicated that doping did not destroy the crystal structure but stabilized the crystalstructure. For the doped samples, LiNi0.4Co0.2Mn0.4O1.95Cl0.05and LiNi0.4Co0.2Mn0.4O1.97F0.03exhibited the best electrochemical performance, showing an initial capacity of200and194.3respectively and capacity retention of95.5%and96.1%after50cycles at0.2C. At relativelyhigh rates, sample LiNi0.4Co0.2Mn0.4O1.95Cl0.05and LiNi0.4Co0.2Mn0.4O1.97F0.03also exhibitedimproved capacity and cycling performance. CV and EIS tests indicated that doping of F andCl improved the reversibility of the material and probibited the increase of theelectrochemical impedance in the cycling process.Then different amount of Mg and Ti were incorporated into the LiNi0.4Co0.2Mn0.4O2crystals. It is indicated that doping with cations decreased the concentration of the anti-site inthe crystals thus stabilized the crystal structure. Electrochemical investigation indicated thatsamples showed the best performance when the dopant concentration was0.04. Even thoughdoping deteriorate the initial capacity of the material, capacity retention were elevated to94.3%and95.7%after50cycles at0.2C. Co-doping the crystal with Ti and F indicated thatco-doping facilitate the formation of well-organized hexagonal layered structure and smallerspecific surface area. Electrochemical investigations suggested that co-doping improved thecycling performance greatly. For the sample of Li(Ni0.4Co0.2Mn0.4)0.96Ti0.04O1.97F0.03, there wasalmost no capacity loss after50cycles at0.2C between2.5-4.4V or2.5-4.6V and rate capacitywas also improved after doping. Then2wt%FePO4, TiO2and A12O3were coated on the surface of LiNi0.4Co0.2Mn0.4O2.SEM and XRD investigations showed that FePO4, TiO2and A12O3were successfullingcoated onto the surface of the cathode material but did not change the location of thediffraction peaks. The initial capacity of the coated material did not change but cyclingperformance and rate capacity were improved to a great extent. Capacity retention after50cycles at0.2C of the materials after FePO4, TiO2and A12O3coating were93.7%,94.2%and94.3%respectively and93.5%,94.0%and94.2%respectively after50cycles at different ratesbetween0.2-2.5C.