Synthesis And Modification Of Li-rich Manganese-based Layered Oxide Cathode Materials

Since the state-of-art energy density of the Li-ion batteries can't meet the demands of the portable devices and electric vehicles,it is urgently needed to develop new Li-ion batteries with higher energy density.The energy density mainly depends on the specific gravimetric capacity and operating voltage.In this regard,the Li-rich manganese-based layered oxides are considered as the most attractive cathode materials for the next generation Li-ion batteries because of their high specific capacities and low cost.However,they face some intrinsic drawbacks,such as high irreversible initial capacities,inferior rate performance,voltage decay and low pellet density.In this paper,new synthesized and modified methods are introduced to overcome the aforementioned drawbacks.1.Synthesis and electrochemical performance of micro-sized Li-rich layered cathode material for Li-ion batteriesMicron Li-rich Li_1.172Ni_0.135Co_0.135Mn_0.539O₂ oxides with high pellet density?3.18 g/cm3?and good electrochemical performance were synthesized through a new chemical lithiation with micro-sized spinel-precursors and Na₂S₂O₈ surface treatment.The micron sample can deliver 260 m Ah/g at a charge/discharge rate of 0.1 C.Benefited from its larger particles and lower specific surface area,the obtained micron Li-rich layered material manifests its good cycling performance.After 70 cycles at 0.1 C,it can still deliver a capacity of 201 m Ah/g with retention of 77.3 %,much higher than 184 m Ah/g with retention of 74.3 % of the nano-sized sample.Moreover,the micron Li-rich layered material also exhibits good rate performance with a capacity of 174 m Ah/g at 2 C benefited from the 3D Li⁺ insertion/extraction channel of its surface spinel component.2.Two step chemical-lithiated synthesis of Li_1.14Ni_0.136Co_0.136Mn_0.544O₂ for Li-ions batteriesThe precursors with different structure were prepared by sintering the(Ni_1/6Co_1/6Mn_4/6)CO₃ with various amounts of Li₂CO₃.Afterward,the additional Li₂CO₃ were mixed with the obtained precursors and annealed to synthesize Li_1.172Ni_0.135Co_0.135Mn_0.539O₂ with different spacing.It is shown that the spinel-precursor is beneficial to the rate performance of the Li-rich layered materials and their spacing increase as the amount of lithium in the sinel-precursor enhances.Among all the samples,the Li-rich layered oxide prepared by firstly introducing Li₀.5(Ni_1/6Co_1/6Mn_4/6)O₂ manifests its largest spacing,lowest resistor and best rate performance.3.Structure and electrochemistry of B doped Li(Li_0.2Ni_0.13Co_0.13Mn_0.54)_1-xB_xO₂ as cathode materials for Lithium-ion batteriesMigration of transition metal?TM?ions to tetrahedral sites plays a crucial role on structural transformation and electrochemical behaviors for Li-rich layered oxides.Thus,small B³⁺ doped in the tetrahedral interstice is employed to block the migration channel of TM ions and stabilize crystal structure of Li-rich layered oxides based on reported electrochemical mechanisms.Benefited from their good structural stability,the low boron-doped Li-rich layered materials exhibit excellent cycling performance and voltage stability.After 51 cycles at 0.2 C,the 1 mol.% boron incorporated sample can still deliver 211 m Ah/g with capacity retention of 89.9 %,much higher than 177 m Ah/g with retention of 79.2 % of the undoped sample.Meanwhile,the declined voltage per cycle decrease from 3.6885 mV to 2.7530 mV after 2 mol.% boron doping for the Li-rich layered oxides.XRD patterns after extensive cycling also verify the inhibiting of the structural transformation towards tetragonal I4₁ which is the transition phase from layered to spinel by incorporated boron.