Towards High Performances Of Lithium-rich Layered Oxides:Structure Regulation And Electrochemical Properties

Due to the high specific capacity exceeding 250 mAh g^-1 and low cost,Lithium-rich manganese-based layered oxides?LLOs?has been termed as the most attractive cathode materials for next generation high energy density Li-ion batteries.However,they still suffer from drawbacks,among which the voltage decay has been considered as the greatest obstacles that must be addressed prior to full scale utilization.This problem is now believed to be an intrinsic nature that correlates with a gradual structure transformation.To address it,the bulk structure must be modified.Herein,we tuned the structure of lithium-rich layered oxides from both atomic-level?foreign ion doping?and phase-level?constructing multiple phases structure?,and successfully obtained obvious performance improvements.Beyond that,the driving force of structure evolution?as well as voltage decay?was also investigated from novel thermodynamic perspectives.Details are listed below:?1?Towards mitigation of voltage decay for Li-rich layered oxides by a small amount of Al substitution for CoAl was introduced into Li_1.14(Ni_0.136Co_0.136Mn_0.544)O₂ to attain stabilized frameworks via a simple sol-gel method.The Rietveld refined X-ray diffraction and transmission electron microscope results proved the substitution of Al for Co as well as its influence mainly on the rhombohedral components.The Al doped cathode exhibited a superior cycling performance,with capacity retention of 94.7%after 100 cycles.Most importantly,the voltage decay was found significantly alleviated through the obstructed formation of spinel-like phases,which were confirmed by the ex-situ XRD and further estimated by Raman spectra.These results suggest a notable stabilization effects on host framework in Li-rich oxides through a simple manipulation on rhombohedral phase.?2?New insight into the driving force of voltage decay in Li-rich layered oxidesDoping foreign elements may be an effective method to eliminate the voltage decay in Li-rich layered oxides.However,the poor understanding of the origin for this issue limited the development of further reliable solutions.In this paper,We used pristine Li_1.2(Mn_0.54Ni_0.13Co_0.13)O₂ and inert ion doped counterparts to investigate correlations between the thermal behaviors and the voltage instability.Systematic thermal analyses and electrochemical results revealed that the intrinsic structure transformation during the cyclic process was potentially driven by the excess energy generated?in the term as heat flow?,since their evolution trends synchronized with each other.The improvements from incorporating foreign ions may derive from the alleviated generation of excess free energy and consequent mitigated structure transformation.These findings shed light on unraveling the intrinsic nature.?3?Constructing near-equilibrium layered/spinel structure in Li-rich layered oxides via ion-exchange procedureAfter prolonged cycling,a layered/spinel mixed structure of lithium-rich layered oxides could in-situ form from the initial layered ordering.Since close to the end point,this structure was thermodynamic near-equilibrium,and would bear little?or only slight?further rearrangement,exhibiting negligible voltage decay in following cycles.Herein,we realized similar structure to mitigate the voltage decay via ex-situ ion exchange procedure with following annealing process.Through simple manipulation on the ratio of[H⁺]_aq/[Li⁺]_s,a certain amount of Li and Oxygen vacancies were created,and the consequent component ratio of layered/spinel as well as their distributions were controllably adjusted.The results showed that the optimal structure reached the target near-equilibrium state?shown by DSC tests?,with the formation of surface reconstruction layer and nanoscale spinel domain embedded in the main layered lattice.Most importantly,the optimal modified PE05A could significantly improve the voltage stability with negligible cost of reversible capacity,a high reversible capacity of 272.2mAh g^-1(2.0-4.8 V,25 mA g^-1)could be delivered,with initial coulombic efficiency near 100%;and the drop-rate of average voltage were only 0.75 mV/cycle?1.72mV/cycle for PE00A?even working for 200 cycles(2.0-4.6 V,50 m A g^-1).This finding will benefit the commercialization of these materials.