Preparation And Modification Of Nickel-rich Single-Crystal LiNi_0.83Co_0.07Mn_0.10O₂ Cathodes Material

There is an increasing need for developing green and sustainable energy due to energy crisis and global climate change.However,renewable energy production is limited by nature conditions,leading to a stored requirements to form an uninterrupted supply chain.Lithium-ion batteries（LIBs）are widely used in electronic devices,such as mobile phone and electric vehicle,and have an essential role in several energy storage areas.The energy storage level of LIBs is determined by its energy density,long cycling life and cost.Meanwhile,there is a long-standing consideration about avoiding its environmental footprint in the process of storing renewable energy.Currently,these are depended mainly on the development of cathode.The nickel-rich layer oxide,Li Ni_xCo_yMn_1-x-yO₂（NCM,x≥0.8）,are expected to facilitate the supremacy of LIBs owing to high reversible capacity of exceed 200 m Ah g^-1.However,high nickel combined with high voltage,a general method to push higher specific capacity,results in structure instability and rapidly capacity decay.Origin of the capacity degradation is related to the interface reactions and granule cracks.During deeply Li-ion delithiation states,NCM undergoes a large anisotropic volume change that promote the formation of lithium/oxygen vacancies and intergranular cracks,resulting in surface parasitic reaction and structural collapse.An investigation of strategies was an attempt to develop nickel-rich single crystal cathodes prevent the generation of internal strain caused by the anisotropic shrink and expansion during cycling.However,during high temperature synthesis,increasing Ni content will aggravate the Li/Ni cation disorder accompanied by negative effect for Li-ion diffusion and Li-ion delithiation/lithiation.This negative effect in the Li-ion delithiation states aggravate the irreversible H2-H3 phase transition with abrupt lattice contraction,eventually resulting in the structural degradation.Therefore,the crucial challenge for the researcher is to develop strategies that will improve structural stability at a satisfactory extent in a safe,low-cost cathode with a much higher specific capacity than is practicable with present cathode materials.Here we address these problems related to strategies for high-performance nickel-rich cathode taking single crystal Li Ni_0.83Co_0.07Mn_0.10O₂（SC-NCM）as the base material.During the study,modified process of the representative methods for SC-NCM cathode becomes more systematic,as does the optimization.The corresponding modified mechanism are deeply analyzed for an acceptable LIBs application.Four specific research,ordinally,emerge with the increasing content:1)The anisotropic volume variations associated with the traditional secondary particles result in mechanical instability and intergranular cracks.To address these issues,a novel low-cobalt single-crystal Li Ni_0.83Co_0.07Mn_0.10O₂ cathode material（SC-NCM）is rationally synthesized using Li OH·H₂O and Ni_0.83Co_0.07Mn_0.10（OH）₂ precursors as raw materials.Various raw materials ratio and sintering temperature are designed for synthesizing optimal SC-NCM single crystal particles.Results reveal that SC-NCM synthesized at 800℃using 1.05 ratio of Li OH·H₂O and Ni_0.83Co_0.07Mn_0.10（OH）₂ delivers the best electrochemical performance.It was showed that 0.5-2.0μm SC-NCM is consisted with hexagonalα-Na Fe O₂ type crystal structure with R-3m space group.SC-NCM cathode delivers a reversible specific capacity of 160.0m Ah·g^-1 at a current density of 200 m A·g^-1 with a capacity retention of 82.8%after 100 cycles.While,traditional secondary particle NCM cathode obtains a specific capacity of 144.4 m Ah·g^-1 with capacity retention of 78.7%after 100 cycles.Meanwhile,SC-NCM cathode exhibits a better rate performance than NCM cathode.These results further verify that SC-NCM single crystal particles are not affected by the anisotropic volume change of traditional secondary particles,and have a stable and multifaceted Li⁺transport path.However,modified strategies are needed for SC-NCM cathode that overcomes structural instability at high cut-off voltage.2)Along the ever-increasing attention for nickel-rich single-crystal materials,the degeneration of current single-crystal cathodes on structure instability poses serious nanocracks and decomposition issues.To overcome these problems,a new Al and Sm co-doped SC-NCM single-crystal materials with general formula of Li Ni_0.83Co_0.06Mn_0.10Al_0.005Sm_0.005O₂（SC-NCM-AS）are synthesized by solid phase method and high temperature calcine.Owing to homogeneous Al-O bond introduced into SC-NCM,there is reduced Li/Ni disorder.Meanwhile,Sm-enrich external surface continuously lowers the electrolyte decomposition and rock-salt phase on surface of SC-NCM-AS cathodes.The structural and mechanical stability of SC-NCM-AS cathode originates from the pillar effect of Al-O and Sm-O bonds in transition metal layer,causing reversible phase transition as well as an inhibition of oxygen vacancy and surface residual lithium compounds.Which further suppress the dislocation and nanocracks of SC-NCM-AS during cycling.Consequently,SC-NCM-AS is an alternative cathode material,obtains a discharge capacity of 207.4 m Ah·g^-1 at 1.0 C(1.0 C=200 m A·g^-1)between 2.75 and4.4 V,demonstrating reasonable cycle stability with 86.2%capacity retention over 100 cycles.This modified strategy of single-crystal cathode research could promote the development of nickel-rich cathode with high-performance.3)One expectation is Al and Y co-doped Li Ni_0.83Co_0.07Mn_0.10O₂ single-crystal materials(Li Ni_0.83Co_0.06Mn_0.10Al_0.005Y_0.005O₂,SC-NCM-AY).A class of cathodes exhibit acceptable performance apparently because it has lower Li/Ni disorder,oxygen vacancy and surface residual lithium compounds in lattice plane.Through in situ XRD tracking of phase distribution,we found that inhomogeneous distribution and irreversible phase transition in SC-NCM is suppressed by homogeneous Al and enriched Y co-doping comprising ordered Li/TM layer in Li⁺deintercalation structure.This work verifies that structural/granular and surface-chemistry stability of monocrystal materials may be enabled by constructing stronger metal-O bonds throughout particles,which is positive effect for restraining irreversible lattice dislocation,nanocracks and electrolyte decomposition during cycling.As a result,SC-NCM-AY presents high structural stability and capacity retention of 88.2%after 100 cycles within voltage range of 2.75-4.4 V and at 1.0 C(1.0 C=200 m A·g^-1)current density.This co-doping strategy further present an insight in designing for nickel-rich single-crystal materials with high-performance.4)To improve the structural and mechanical stability of single-crystal Li Ni_0.83Co_0.07Mn_0.10O₂（SC-NCM）cathode material,an in-situ coating strategy was explored for the first time.Concomitant with this unique coating process,three-dimensional network Li Ce_0.9Gd_0.1O₂（LCGO）coating layer as well as Ce and Gd co-doping are integrated in SC-NCM.The LCGO coating layer connects with SC-NCM bulk via forming a lattice-coherent phase at external surface.Owing to this coherent Li⁺ion diffusion channels penetrate straightforwardly from surface to center,remarkably improving Li⁺diffusion rate and reversibility of Li⁺（de）-intercalation.The scalable pillaring effect of Ce and Gd co-doping and a strong oxidation of Ce⁴⁺provides a lower Li/Ni disorder and surface residual lithium compounds.By investigating for structure evolution,we found that LCGO modification enhance the reversible H2-H3 phase transition while inhibiting irreversible oxygen release,interface reaction and formation of inhomogeneous Li⁺concentration with mechanical stress.Subsequently,LCGO modification significantly suppress Ni³⁺→Ni²⁺redox,irreversible structural dislocation and nanocracks,improving mechanical stability during high-voltage cycling.Accordingly,SC-NCM@LCGO2 cathode delivers an initial discharge specific capacity of 219.7 m Ah·g^-1 at 0.1 C(1.0 C=200 m A·g^-1),and 85.1%capacity retention after100 cycles at 1.0 C within a voltage range of 2.7-4.4 V.The in situ LCGO coating process provides a boost strategy for the development of nickel-rich single-crystal cathode materials.