Study On The Electrochemical Properties Of High Nickel Layered LiNi_0.80Co_0.15Al_0.05O₂ Cathode Materials

Lithium-ion batteries(LIBs)are recognized as reliable energy storage and conversion systems for electric vehicles and portable electronics due to their high energy density and excellent cycling performance.The cathode material is one of the important components of LIBs,which determines the operating voltage of the battery in the safe electrochemical window of electrolyte,thus limiting the actual energy density of the battery.The high nickel layered LiNi0.80Co0.15Al0.05O2(NCA)cathode material stands out among many LIBs cathode materials for its excellent reversible capacity,small polarization and good thermal stability.However,NCA cathode materials still face some problems and challenges.Surface side reactions that occur when the active electrode is in contact with an electrolyte can affect the electrochemical performance of the cathode material.First,the active Ni3+is inevitably oxidized to the unstable and inactive Ni4+during the discharge process.Due to the strongoxidation of Ni4+,it accelerates the decomposition of electrolyte,causes surface side reaction between active material and electrolyte,forms non-conductive solid electrolyte interface(SEI)film,affects the transport of Li+in NCA cathode material,and increases electrochemical polarization.Secondly,the inevitable trace water on the surface of the positive particles will react with the solute of the electrolyte(LiPF6)to generate HF,LiF,POF3 and so on.The presence of HF can cause the dissolution of transition metal ions and further deteriorate the surface structure of active materials.In addition,due to the similar ionic radius of Ni2+(69 pm)and Li+(76 pm),Ni2+transformed from Ni3+or Ni4+are easy to exchange positions with adjacent Li+,resulting in cation mixing and formation of NiO-like passivation layer,which will lead to permanent loss of active lithium layer in the lattice.In this paper,NCA cathode material is selected as the research object.In view of the above mentioned problems,in-depth scientific discussion and modification research are carried out,and some effective methods are proposed for the large-scale application of NCA cathode material.NCA cathode material coated with polysiloxane containing ethoxy-functional group(Polysiloxane with ethoxy-functional groups,EPS)was prepared by in-situ hydrolysiscondensation method.The effect of EPS coating amount on the crystal structure and electrochemical properties of NCA cathode material was studied.The results show that EPS coating can effectively improve the cycling stability of NCA cathode material without affecting the crystal structure of NCA.When EPS coating amount is 3 mol%(NCA-EPS3),it shows much better cyclic stability than NCA samples at room temperature(25℃)and high temperature(55℃).At 25℃,the original discharge capacities of NCA and NCA-EPS3 samples are 158 and 159 mAh g-1,respectively.After 150 cycles at 1 C,the capacity retention rate of NCA-EPS3 samples is 96%of the original capacity,which is 25%higher than that of NCA samples under the same conditions.Even at 55℃,the volume retention rate of NCA-EPS3 samples retained 90%after 150 cycles,which was 20%higher than that of untreated NCA samples.The good cyclic stability of NCA is due to EPS as a protective layer,which reduces trace water on the surface of NCA particles during the formation process,inhibits the interface side reaction,and removes harmful substance HF from the electrolyte through nucleophilic substitution reaction.Li4SiO4 coated NCA cathode material with monoclinic structure was prepared by precoating the precursor with ethyl orthosilicate(TEOS)and then calcination with lithium.The effect of Li4SiO4 coated amount on the electrochemical performance of NC A cathode material was studied.The results show that Li4SiO4 coating can effectively improve the rate performance and cycling performance of NCA cathode material,especially the cycling performance of large rate current.The crystal structure of NCA was not changed while electrochemical performance was improved,and the diffusion of lithium ion was promoted.The experimental results show that the initial coulomb efficiency of the modified NCA cathode material is significantly improved because Li4SiO4 is a fast ionic conductor.At the rate of 1 C,the capacity retention rate of NCA samples was 70.6%after 100 cycles.The cyclic retention rate of 1 mol%Li4SiO4 coated NCA samples(NCA-S1)was 86.1%under the same conditions.The cyclic properties of NCA-S1 samples were tested at high rate of 10 C,and th e capacity retention rate was 70.6%,which proves that the NCA cathode material modified with Li4SiO4 has good rate performance at high rate of charge and discharge current.The spinel Li4Mn5O12 coated NCA cathode material was prepared by using potassium permanganate preoxidation precursor and lithium calcination method.The effects of Li4Mn5O12 coated NCA cathode material on crystal structure and electrochemical properties were studied,and the modification mechanism was discussed in detail.The results showed that the initial discharge capacity of the modified sample(LNCA-2)was greatly increased when the modification amount was 2 wt%.The discharge capacity of NCA is 188.4 mAh g1 and 218.2 mAh g-1 at 0.1 C.LNCA-2 samples showed good cycling stability,with a capacity retention rate of 84.1%after 200 cycles at 1 C rate,18%higher than that of NCA samples under the same conditions.The results show that Li4Mn5O12 coated with spinel structure can stabilize the oxygen lattice and effectively inhibit the oxygen activity of NCA layered oxides.The lattice matching consistency of the spinel layered heterostructure ensures the structural stability and improves the electrochemical performance of the composite cathode material during multiple charge and discharge processes.The good thermal stability of fast ionic conductor LiNbO3 coated NCA cathode material was prepared by simple wet chemical method.Through XRD,FESEM,HRTEM,XPS characterization and electrochemical performance test analysis,the positive effect of LiNbO3 coating on the electrochemical performance of high Ni layered NCA cathode material was confirmed.The NCA sample(LN-NCA-2)coated with LiNbO3 modified at 2 wt%showed significant improvement in cycling performance and rate performance,and maintained good cycling stability at high temperature.At 25℃ and 55 ℃,the retention rates of LN-NCA-2 samples after 100 cycles at 1 C rate were 86.4%and 89.2%,respectively,which were much higher than 62.8%and 50.5%of the unmodified NCA samples.Moreover,the discharge specific capacity of LN-NCA-2 sample is as high as 147.4 mAh g-1 at 5 C,which is much higher than other samples.The results show that LiNbO3 coating can inhibit the formation of bad LiOH or Li2CO3 on the surface of NCA particles,and promote the migration of reversible Li+in the long-acting circulation process,thus improving the electrochemical stability.