Study On Synthesis,Modification And Interfacial Electrochemistry Of Ni-rich LiNi_0.76Mn_0.14Co_0.10O₂ Cathode Materials In Li-ion Batteries

With the increasing demanding for high-energy density Lithium ion batteries as the application of electrical vehicles/hybrid vehicles,Ni-rich NMC has been regarded as the most promising cathode material due to its high specific capacity,high specific energy density and relatively low cost.However,there are also some drawbacks which hinder the wide application of Ni-rich NMC.Firstly,the Ni-rich NMC is sensitive to moisture,the residual active lithiated oxygen species(Li2O,Li2O2)are prone to reacting with the H2O and CO2 once the active oxide exposed in the ambient air,forming LiOH and Li2CO3 on the surface of NMC.Moreover,spontaneous side reaction will also occur on the surface of Ni-rich NMC through the contact with electrolyte,especially the remaining of LiOH and Li2CO3,which will cause continuous consumption of electrolyte and hinder the migration of Li+ on the surface of cathode material,leading to the degradation of surface stability and capacity.Secondly,the large numbers of Ni2+/Ni3+ will convert into Ni4+ while the Ni-rich NMC is in a high state of charge(SOC),which will result in the continuous decomposition of electrolyte,forming a thick layer on the cathode/electxolyte interface(CEI),leading to inhibition of Li+ transfer and capacity decay.Furthermore,the Ni2+(0.69 A)ions easily occupy the Li+(0.76 A)sites in the Li slabs when the large numbers of Li+ are extracted in a high SOC,giving rise to "cation mixing" or "cation disorder",resulting in the degradation of capacity.Furthermore the severe "cation mixing" accompanying with phase transformation,forming LiMn204 and NiO species" which will lead to the O2 release and layered structure collapse of NMC.In addition,deep charge and discharge can also cause a significant change in the unit cell volume of the material,which will lead to cracks between the grain boundaries of unit cells.The cracks in turn will cause electrolyte infiltration and the formation of a new CEI layer,exacerbating the anisotropy among the broken particles,and leading to severe crack and fragmentation of secondary particles in long-term cycling.In addition,the high voltage operation also can induce the unit cell viriation and result in the formation cracks as the H2/H3 phase transformation.In order to address the above issues,exploring the suitable electrolytes/additives and doping or coating inactive materials to stabilize the bulk and surface as well as the cathode/electrolyte are of great importance important significance.The LiNi0.76Mn0.14Co0.10O2(NMC76)cathode material was prepared by calcining precursor of Ni0.76Mn0.14Co0.10(OH)2 and a stoichiometric amount of LiOH·H2O at 500? for 10 h,followed by further calcination at 750? for 20 h.Based on NMC76,Borate-containing additive and dual-salt electrolyte which can withstand the high charging voltage,effective coating and doping strategies as well as combing doping were systematically investigated to improve the electrode/electrolyte interface stability and long-term cycling performance.The long-term cycling performance,rate capability,and voltage stability of lithium(Li)metal batteries with LiNi0.76Mn0.14Co0.10O2(NMC76)cathodes is greatly enhanced by lithium bis(oxalato)borate(LiBOB)additive in the LiPF6-based electrolyte.With 2%LiBOB in the electrolyte,a Li?NMC76 cell is able to achieve a high capacity retention of 96.8%after 200 cycles at C/3 rate(1C = 200 mA g-1),the significantly enhanced electrochemical performance can be ascribed to the good stabilization of both the NMC76-cathode/electrolyte interface and Li-metal-anode/electrolyte interfaces as the decomposition of LiBOB on the surface of electrode.Electrochemical impedance spectroscopy(EIS),Scanning electron microscopy(SEM),x-ray photoelectron spectroscopy(XPS),and scanning transmission electron microscopy(STEM)results can confirm that LiBOB-containing electrolyte not only facilitates the formation of a more compact solid electrolyte interface on the Li metal surface to prevent the deep corrosion of Li bulk anode,it also forms a enhanced cathode electrolyte interface layer,which efficiently prevents the corrosion of the cathode interface and mitigates the formation of disordered rock-salt phase after cycling.To further improve the stability of cathode and anode as well as the cycling performance under high charging/discharging current rate,we report for the first time that LMBs based on an Lithium metal anode(LMA)and a Ni-rich layered cathode,LiNi0.76Mn0.14Co0.10O2(NMC76)is able to deliver an excellent cycling performance in the voltage range of 2.7?4.5 V using an electrolyte consisting of 0.6 M LiTFSI + 0.4 M LiBOB + 0.05 M LiPF6 dissolved in a solution of ethylene carbonate and ethyl methyl carbonate(EC:EMC,4:6 by weight),called E-optimized for simplicity.With this electrolyte,Li?NMC76 cells can retain more than 80%of their capacity after 1000 cycles at 2C rate for both charge and discharge,which is much better than that obtained using the conventional electrolyte of 1M LiPF6/EC-EMC(Called E-baseline)(20.8%retention after 300 cycles).To the best of our knowledge,the high energy density and excellent cycling performance demonstrated in this work are among the best ever reported for LMBs.Furthermore,combining the E-optimized with NMC622 and NMC811,the Li?NMC622 cell delivers the capacity retention of 86.9%after 500 cycles under 2C/2C charge/discharge rates,while the Li?NMC811 cell exhibits the capacity retention of 83.5%after 500 cycles at the same test condition.Therefore,the Li?NMC batteries combining the high-energy-density,Ni-rich NMC cathode and the highly stable electrolyte E-optimized will be a promising energy storage system.EIS,SEM,XPS,and STEM were adopted to explore the structural/interfacial degradation of the LMA and Ni-rich NMC76 cathode to gain deep insight into the fundamental mechanism for the enhanced performance.The remarkably enhanced cycling performance can be ascribed to two qualities:(1)the enhanced CEI layer can protect the Ni-rich NMC76 cathode from electrolyte corrosion and suppress the structural transformation from layered to disordered rock-salt(NiO)phase;(2)the robust and conductive SEI layer generated on the Li surface prevents deep corrosion of Li metal and improves the stability of the LMA.To enhance the structural stability of NMC76 bulk materials,A1 doping strategies were employed through annealing the mixture of NMC76 precursor and Al,and the EDS-mapping confirms that A1 go into the inner of the NMC76 particles.EIS evolutions and cycling performances indicate that 1%mol Al species can significantly reduce the electrode/electrolyte interface polarization,thus improve the long-term cycling performance and high temperature cycling stability.The cell using 1%mol Al doped NMC76(1%Al-NMC76)cathode can maintain 87.7%capacity retention after 250 cycles under 0.33C current density,compared with the 80.1%while using pristine NMC76 cathode.More importantly,the capacity retention can reach up to 79.2%under 0.2C/5C charging/discharging current density after 500 cycles,rather that 3.4%while using pristine NMC76.XPS and STEM results EIS further confirm that the Al species alleviate the oxidation decomposition of electrolyte on the surface of Al-NMC76,result in the formation of more dense and thinner CEI layer with less relative amount of LiF.In addition,the A1 species can suppress the formation of Li+/Ni2+ cation mixxing and disordered NiO,leading to the improvement of electrode/electrode interfaces stability and long-term cycling capability.Combining with atomic layer deposition(ALD)coating and additive can greatly improve the cycling stability and interface stability.The NMC76 was coated with a thin layer of ZrO2 by ALD method and annealed(Called Zr-NMC76)under 500?.Andthe cell using the Zr02 coated cathode shows enhanced capacity retention,which was increased to 91%from 85.1%.While the cell using Zr-NMC76 and moderate LDFOB as additive,the capacity retention can reach up to 98.1%after 200 cycles under 0.33C current density.And the capacity retention can maintain higher than 90.8%under 0.2C/5C charging/discharging current density after 500 cycles.In addition,the EIS indicates that the cell combining with the Zr-NMC76 and LDFOB delivers super electrode/electrolyte interface stability,the value of charge transfer impedance just show a very small enhancement even after 200 cycles.The HRTEM was employed to characterize the raw ZrO2-NMC76 cathode,confirming that ZrO2 goes into the inner space of the NMC76 particles.And XPS result confirms that Borate-containing CEI layer formed on the surface of cycled Zr-NMC76 cathode in electrolyte with LDFOB additive,the enhanced CEI layer can improve the interface stability and protect the Ni-rich NMC76 cathode from electrolyte corrosion as well suppress the structural transformation from layered to disordered rock-salt(NiO)phase.So we can safely conclude that synergic effect between Zr4+ doping and LDFOB additive significantly enhance the cell's long-term cycling stability and interface stability.