Investigations On Material Preparation And Surface-modification Of NCM811 Cathode For Rechargeble Li-ion Batteries

Lithium-ion batteries have been ubiquitous as one kind of energy storage and conversion device in our daily life,due to their high energy conversion efficiency,long service life and environmental friendliness.LIB has been widely served as power sources in 3C industry(computer,communication and consumer electronics products).However,a series of problems still challenge the practical application of LIBs,such as the low energy density,poor rate capability,safety concerns and so on.To promote the progress of LIBs,the energy density of cathode is one of the main bottlenecks to be overcome.In this context,the LiNi0.8Co0.1Mn0.1O2(NCM811)material that has a theoretical capacity as high as over 270 mA h·g'1 is investigated systematically from material preparation and surface modification perspectives as follows:(1)Material Preparation:Two different synthesis ways,namely the hydroxide and citric acid coprecipitations,are adopted to make the precursors,and then the calcination temperatures are also carefully compared upon the synthesis of NCM811 material.The results reveal that at 900?,the NCM811 material could be synthesized with perfect layered structure,without obvious transition metal-Li antisites.The NCM811 also has the better crystallinity and structural ordering degree using the hydroxide as precursor at 900?.Moreover,the electrochemical tests show that the specific capacity after 95 cycles is 156.3 mA h g-1,and the capacity retention is 66%;while in rate test,the capacity retention is 75.9%at 1C.The NCM811 electrode delivers good capacity retention and rate performance.(2)Structural Characterizations:In-situ XRD and ex-situ STEM measurements show that upon delithiation,the structure of NCM811 changes from the initial 03 phase along with the stretching of lattice parameter of c and shrinkage of a/b to a structure of 01-03 mixed phase,which is in good agreement with the first principles predication.Further atomic-scale electron energy loss spectroscopy(EELS)results show that the charge compensations from the transition metal ions works for different electrochemical processes during the battery charging.The loss of electrons from the lattice Ni/Co ions is resulted from the Li extraction,while the electron loss of Mn and O ions is due to the dissolution of transition metal ions into the electrolyte and the side reactions inside the battery.(3)Surface Modifications:PIL carbide and Ketjen black are employed to modify the surface of NCM811 material by ball milling and low-temperature heat treatments,which are denoted as the NCM811@PIL C and NCM811@Ketjen black,respectively.The successfully surface coating of NCM811 electrode is explored from XRD,SEM and other structural characterizations and the thermogravimetry(TG)curve indicates the ultimate carbon coating amount is around 4%on the NCM811 surface.Electrochemical tests display that with respect to the pristine material of 66%:after 95 cycles the NCM811@PIL C and NCM811@Ketjen black electrodes manifest the higher capacity retention of about 82%and 87%,respectively.The delithiation capacity retention of the three electrodes are 75.9%,85.9%and 84.9%,at 1C rate,respectively.What is more,the practical capacities of NCM811@PIL C and NCM811@Ketjen black materials are about 180 mA h g-1 and 190 mA h g-1,both of which demonstrate the high energy density feature during electrochemical cycles.Overall,the surface modification of NCM811 with Ketjen black exhibits the best electrochemical performance of all the NCM811 samples.