Research On Synthesis And Modification Of Single Crystalline Lithium Nickel-Cobalt-Manganese Oxide Cathode Materials For Li-ion Batteries

In recent years,due to its advantages of high specific capacity,high voltage plateau and long cycle life,lithium nickel-cobalt-manganese oxide(Li Ni_1-x-yCo_xMn_yO₂)can provide longer driving range and cycle life for electric vehicles,which has become one of the most promising candidates for cathode materials in power battery.Nowadays,in addition to the conventional NCM materials in secondary particle morphology,single crystalline NCM materials are drawing growing attention from both academia and industry.However,some drawbacks are still limiting the further application of single crystalline NCM materials,such as residual lithium,irreversible phase transition,cation mixing,particle cracking,transition metal dissolution.Furthermore,the synthesis of single crystalline materials with large grain size and mono-dispersion is still complex and the mechanism for primary particle growth is not clear yet.In this dissertation,Li Ni_0.5Co_0.2Mn_0.3O₂ and Li Ni_0.65Co_0.07Mn_0.28O₂ are selected as research subjects,and micron-sized single crystalline cathode materials are synthesized via a combined method of co-precipitation and high temperature calcination.The single crystalline materials are modified through crystal facet controlling and surface coating to enhance their electrochemical performance.The primary particle growth is studied via introducing different calcination additives.The effect of grain size of single crystalline material on its physical property and electrochemical performance is analyzed and the grain size is optimized for desirable performance.The main research results are concluded as following:（1）The micron-sized single crystalline Li Ni_0.5Co_0.2Mn_0.3O₂ cathode material is prepared via co-precipitation followed by high temperature calcination method with the assistance of Sr CO₃.The effect of the amount of Sr CO₃ added on the morphology change of materials is studied and it is discovered that Sr CO₃ has a significant influence on promoting the primary particle growth and（104）plane.When 0.5wt%of Sr CO₃ is added into the reaction system,the disk-liked micron-sized single crystalline NCM523material with preferred（104）plane can be obtained,whose primary grain size is around 4μm.The as-prepared sample exhibits excellent electrochemical performance.The testing results indicate that single crystallization can dramatically improve the cycle stability and suppress the cracking initiation and irreversible phase transition.Besides,（104）plane processes the highest stability among all crystal facet due to its lowest surface energy,thus,promoting（104）plane growth can further increase the electrochemical stability of single crystalline material.（2）Several elements,such as Cs₂CO₃,K₂CO₃,Ba CO₃,Sr CO₃,La₂O₃,Na₂CO₃,Ca CO₃ and Mg（OH）₂,are selected based on analyzing the property of Strontium,and then added into the calcination reaction system as additives to study their influence on preparing single crystalline NCM materials.By comparing the SEM images of single crystalline materials prepared by different additives,it is found that the effect of additives on promoting primary particle growth is greater with the increasing ionic radius of additives,therefore,the primary particle grain size of the obtained single crystalline materials grows larger.Especially,single crystalline samples with primary particle grain size over 3μm can be prepared with the assistance of Cs⁺,K⁺,Ba²⁺and Sr²⁺.The XPS etching and DFT calculation indicate that the energy required for the additive cation entering the NCM lattice rises with the increasing ionic radius,which suggests that it is difficult for the large radius ions diffuse into the NCM lattice.In conclusion,we believe that the additives with large ionic radius can promote primary particle growth by some surface effect.（3）Micron-sized single crystalline Li Ni_0.65Co_0.07Mn_0.28O₂ is synthesized by the co-precipitation and high temperature calcination method,and coated by nano-sized Co（OH）₂ to enhance its electrochemical performance.The coated Co（OH）₂ can react with residual lithium compound to lower the residual lithium compound on the particle surface.Therefore,the side reaction between residual lithium compound and electrolyte can be suppressed,what’s more,the electrochemical active reaction products of Co（OH）₂ and residual lithium compound can improve the electrochemical performance.The coating amount is optimized as0.5wt%by comparing the electrochemical performance of samples with different coating amount.Co（OH）₂ coating can improve the electrochemical reversibility,as well as reduce polarization and resistance during cycling.After coated by 0.5wt%Co（OH）₂,the MC-0.5Co sample show integrated single crystalline particles and only spinel phase is found on the particle surface without rock-salt phase generated.Hence,superior electrochemical performance is achieved for Co（OH）₂ coated single crystalline materials.（4）Single crystalline Li Ni_0.65Co_0.07Mn_0.28O₂ materials with different grain size are synthesized via carefully controlling the key experimental parameters.Testing results show that the specific surface area decreases and the tap density and compact density increase with the growing grain size of single crystalline material.The electrochemical tests indicate that the initial coulombic efficiency,specific discharge capacity,capacity retention and rate capability of samples with smaller grain size are superior.Moreover,the particle morphology of small-sized sample is well-preserved without cracking initiated and slight planar gliding is observed,besides,the absence of rock-salt phase on particle surface indicates alleviated irreversible phase transition.In contrast,the large-sized sample show deteriorated structure after cycling.Massive cracking and planar gliding are found,as well as the severe irreversible phase transition,by which thick rock-salt phase is formed on the particle surface.Therefore,by comparing the property of single crystalline material with different grain size,we conclude that the grain size of single crystalline material should be controlled between around 3-5μm to balance the physical property and electrochemical performance.There are 73 figures,17 tables and 295 references in this dissertation.