Modification And Electrochemical Performance Of Nickel-rich Cathode Materials For Lithium Ion Batteries

The energy density,price and safety factors of lithium-ion battery seriously affect the development of new energy vehicle market.Given the high specific capacity and low cost,LiNi_xCo_yMn_zO₂(x ? 0.6)cathode has been widely studied recently.However,the material suffers from lithium residue,transition metal(TM)/Li⁺mixing,oxygen evolution reaction(OER),phase transition,undesirable reactions at the electrode/electrolyte interface,dissolution of transition metal,and microcracking,wich severely hinders its commercial application.To solve the issues mentioned above,firstly,the electrode/electrolyte interface of LiNi_0.6Co_0.2Mn_0.2O₂(NCM622)is designed by traditional oxide(ZrO₂)and lithium containing oxide(Li_1.4ZrO_3.4)with high litium ionic conductivity using atomic layer deposition(ALD),and the different mechanisms of these two materials are studied.Secondly,as the higher nickel content in the NCM cathode,the more unstable the structure is,in order to regulate the internal structure of the electrode,LiNi_0.8Co_0.1Mn_0.1O₂(NCM811)is doped with Zr⁴⁺ion of different concerntrations,and the relationship between the structure and the electrochemical performance of NCM811 cathode is systematically studied.The research might provide new ideas and methods for the development of high capacity and high stability NCM cathode materials.Main research results of this paper are as follows:Firstly,the uniform ZrO₂ coating with controllable thickness is coated on the surface of LiNi_0.6Co_0.2Mn_0.2O₂(NCM622)by atomic layer deposition(ALD).The NCM622 cathode coated with ZrO₂ shows higher capacity retention and rate performance,and it is found the optimal thickness of ZrO₂ coating is about 3.2 nm.The deposition of ZrO₂ coating by ALD has many positive effects on NCM622 cathode:(i)As a physical barrier,it can inhibit the undesirable reactions between NCM622 cathode and electrolyte,reduce the formation of solid electrolyte interface(SEI),improve the Li⁺conductivity,reduce the corrosion on electrode materials by HF,thus inhibiting the dissolution of transition metal(ii)As the structural stabilizer of NCM622 cathode,it can reduce the irreversible phase transition and microcracks in the primary particles(iii)As a strong network structure to wrap the NCM622 electrode,the electrode material is firmly fixed on the aluminum current collector,which enhances the mechanical integrity and electrical contact of the electrodeSecondly:The influence of the regulated of electrode/electrolyte interface with the lithium containing oxide(Li_1.4ZrO_3.4)coating on the ionic conductivity is investigated.Due to the limited research on lithium zirconia oxide by atomic layer deposition,we explore the growth characteristics of LixZrOy by atomic layer deposition(ALD)with different deposition temperature and different growth ratio first,the ALD procedure for the stable growth of LixZrOy is determined,and the element composition and surface morphology for the deposited films is also analyzed.By depositing Li_1.4ZrO_3.4 on the surface of the LiNi_0.6Co_0.2Mn_0.2O₂(NCM622)electrode,we find that the Li⁺insertion/de-insertion rates of NCM622 with 20 cycles of atomic layer deposition Li_1.4ZrO_3.4are 3.4 and 2.6 times higher than that of the bare NCM622,respectively.The increase of lithium ion conductivity helps to reduce the polarization and side reactions of the electrode,wich is believed to be the main reason for the electrochemical performance improvement of NCM622 with 20 ALD-cycles of Li_1.4ZrO_3.4 coating.This result provides a new design idea for the modification of nickel rich cathode materials by using lithium containing oxide with high Li⁺conductivity.Thirdly,the internal structure of LiNi_0.8Co_0.1Mn_0.1O₂(NCM811)cathodes is regulated by introducing Zr⁴⁺ion of different concentrations including 0.005 Zr(Li(Ni0.8Co0.iMn0.1)0.995Zr0.005O2),0.01 Zr(Li(Ni0.8Co0.1Mn0.1)0.99Zr0.01O2),0.015 Zr(Li(Ni0.8Co0.1Mn0.1)0.985Zr0.015O2)etc.It is found that the lattice expansion of NCM811 electrode increased with the Zr⁴⁺concentration,which is beneficial to the insertion and de-insertion of Li⁺in the electrode,and to increase the rate performance of NCM811 cathode.However,the higher doping concentration of Zr⁴⁺ions is not always desirable,too much Zr⁴⁺ion doping will affect the capacity of the electrode.Our study found that the sample of 0.01 Zr⁴⁺ion doped NCM811 has the best electrochemical performance.Zr⁴⁺doping is also helpful to relieve the voltage drop of NCM811 cathode.It is because that Zr⁴⁺doping stabilizes the internal structure of NCM811 cathode,accelerates the transport of lithium ions and electrons in the electrode particles,and reduces the cation mixing problems and electrode/electrolyte side reactions in NCM811 electrode.Therefore,the appropriate concentration of Zr⁴⁺ion doping is helpful to stabilize the structure and improve the electrochemical performance of the NCM electrodes.