The Modification Of Li-Rich Mn-Based Li_1.2Ni_0.17Co_0.07Mn_0.56O₂ Cathode Material For Lithium-Ion Batteries

In recent years,the electric vehicle field has rapidly developed,which calls for batteries with a high specific energy.As one of the core materials of lithium-ion batteries,the current commercial cathode materials suffer from low specific capacity which is the key factor limiting the endurance of electric vehicles.By contrast,Li-rich Mn-based cathode materials can achieve a high energy density of more than 900 Wh kg^-1,showing great potential for commercialization.However,this kind of material still has some problems to be solved,such as structural instability and electrochemical performance degradation.In this context,it is of great significance to study the electrochemical modification of Li-rich Mn-based cathode materials.The modification of Li-rich Mn-based cathode materials includes a variety of schemes,and these ideas can be roughly classified into two categories:one is to regulate the morphology and structure of bulk phase crystals;the other is to construct a stable material surface conducive to the electrochemical reaction.Therefore,this paper applied bulk doping and surface modification to modify the basic material Li _1.2Ni_0.17Co_0.07Mn_0.56O₂（LRMO）,and the effects of different modification routes on the structure and electrochemical performance of LRMO were invest igated.On the one hand,Na and Mn elements were selected for bulk co-doping modification of LRMO.Compared with Na or Mn single element do ping,the synergistic modification effect of Na and Mn co-doping was studied.The results show that the doped Na element can increase the interlayer spacing and maintain t he crystal structure.Therefore,Na doping can largely suppress the capacity fading o f LRMO.The Mn doping mainly changes the local bonding mode of atoms,shortens the TM-O bond length,and ameliorates structural stability.Meanwhile,the Mn-doped material exhibits a short discharge plateau around 2.5 V,corresponding to the existence of t he surface spinel structure.These structural changes make the Mn-doped materials have a certain improvement in various electrochemical performance indicators.On this basis,Na/Mn co-doping can achieve a further improvement of electrochemical performance through the synergy of these two elements.For example,the capacity retention rate was improved from 66.3%to 82.4%after 150 cycles at 1 C.On the other hand,the surface coating modification of LRMO was carried out by using NaVO₃.The capacity retention rate of the modified sample increased from 76.8%of LRMO to 81.0%after 150 cycles at 1 C.However,the properties achieved by NaVO₃ coating materials still fall short of the modification target.I n order to better solve the defect problems faced by LRMO and further improve its electrochemical performance,this paper innovatively selects phytic acid for simple surface treatment of LRMO,and successfully builds a spinel transition layer and a Li ₃PO₄ coating layer on the surface of the material.By using XRD,SEM,TEM and other characterization tests,it can be proved that this unique double protective layer design can not only reduce interfacial side reactions,inhibit irreversible phase transition an d improve the stability of the bulk structure but also provide Li⁺transport channels which facilitate the reaction kinetics.Therefore,the electrochemical performance of the phytic acid surface-modified samples is significantly improved,among which the 3 wt%phytic acid treated sample achieves markedly enhanced electrochemical performance.Specifically,the initial Coulombic efficiency of this material is as high as 90.0%,and the discharge specific capacity at the 5 C high rate is as high as 163.6 mAh g^-1.After150 cycles at 1 C,the optimized sample delivers increased capacity retention（from69.6%to 87.8%）and alleviated average discharge voltage（a drop of per cycle 3.01 mV to 1.63 mV）.In conclusion,in this paper,several modification attempts have been made on LRMO from bulk doping modification and surface modification,and the structural stability and electrochemical performance of LRMO are greatly enhanced,which provides some new strategies for the modification design of Li-rich cathode materials.