Study On Controllable Preparation And Interface Modification Of Ni-rich Cathode Materials For Lithium Ion Batteries

The state-of-the-art lithium-ion batteries(LIBs)are moving for further enhancement of energy&power density.However,the very limited reversible capacity of the existing cathode materials greatly restricts the improvement for the energy density.To further enhance the reversible capacity while reduce Co utilization,layered Ni-rich material(LiNi0.8Co0.1Mn0.1O2,or NCM811)turns to be an important choice owing to its high specific capacity(200 mAh g-1),lower toxicity,higher working voltage and lower cost.However,as the radius of Ni2+(0.069 nm)is very close to that of Li+(0.076 nm),Li/Ni ion mixing and the irreversible structural rearrangement lead to the formation of a Li-deficient and highly resistive rock-salt phase(NiO),especially at the external layer of the particle.Besides,with Li insertion and extraction,the well-known volume effect produces high internal strain and results in structural deterioration and even crack of the assembled particle.Interfacial side reactions between Ni4+ and the electrolytes is another cause for the serious capacity decline during long-term cycles.In order to solve these problems,this paper firstly starts from the material itself and changes the synthesis process to obtain the materials with optimized structure,so as to improve the intrinsic electrochemical properties of the materials.At the same time,the surface interface properties of high nickel materials(high nickel and high residual lithium)were fully utilized,and the interface was modified by liquid/solid phase method to improve the performance of the materials.1)Hierarchically assembled LiNi0.8Co0.1Mn0.1O2(HLNCM)composed of highly exposed {010} nanobricks was synthesized via a modified co-precipitation method through gradient increase of the ammonia concentration in the reactor.XRD analyses show that the relative intensity of the {101} crystal plane is much stronger than that of typical {001}plane.The porous structure of the HLNCM is also demonstrated by using focused ion beam(FIB)technique.The obtained HLNCM cathode shows superior electrochemical properties with a reversible capacity of 192.8 mAh g-1 at 0.2 C between 2.8-4.3 V(vs.Li/Li+)and the initial coulombic efficiency(CE)of 88.10%.At 20 C discharge rate,it is still able to deliver a capacity of 128.9 mAh g-1.After 100 deep charge-discharge cycles,a capacity retention of 94.4%is obtained.The excessive content of Li salt and the calcination time,which greatly impact the electrochemical performances of the target product,are also systematically optimized2)Anchoring the interfacial nickel cations on single crystal LiNi0.8Co0.1Mn0.1O2(SC-NCM)cathode through controllable electron transfer strategy is realized by applying an ultra-thin PMMA layer.Different with traditional physical coatings,electron transfer from Ni2+ cations to the ester group is confirmed,which effectively anchors the interfacial Ni cations and greatly inhibits the surface Ni dissolution into organic electrolyte.Meanwhile,Li+diffusion on the cathode surface is significantly enhanced.As the result,the surface functionalized single crystal LiNi0.8Co0.1Mn0.1O2 cathode by PMMA layer xhibits excellent electrochemical performances.A reversible capacity of 181.1 mAh g-1 at 1 C rate is obtained with much improved rate capability and cycling stability.Moreover,the surface functionalized single crystal LiNi0.8Co0.1Mn0.1O2 is able to work well at high voltage and high temperature conditions,especially when working with PVDF incorporation.The underlying mechanisms are explained by the decreased Ni dissolution and the stable single crystal LiNi0.8Co0.1Mn0.1O2 surface during electrochemical cycles.3)Through the low temperature solid reaction,the functional interface of Al(Li)BOB on the NCM811 with residual lithium was in situ constructed.The results show that the high temperature and high voltage properties of the modified materials are significantly improved.