Synthesis Optimization And Modification Of Nickel-rich Ternary Cathode Materials

The spherical and dense LiNi_1-x-yCo_xMn_yO₂ particles work excellently as the cathode material for Li-ion secondary batteries.The key to their preparation lies in synthesizing the spherical and dense Ni_1-x-yCo_xMn_y?OH?₂ precursor via co-precipitation in a mechanically stirred tank reactor.The effect of impeller type on the precursor property was investigated by taking Ni_0.6Co_0.2Mn_0.2?OH?₂ as an example.Four different types of impellers were tested and compared,which were the propeller turbine?PT?,pitched blade turbine?PBT?,flat blade turbine?FBT?,and Rushton turbine?RT?.Only with the PT could the spherical and dense precursor be prepared,whose tap density exceeded 2.0 g/ml.As expected,the LiNi_0.6Co_0.2Mn_0.2O₂ obtained in this case exhibited the best electrochemical property,which delivered a discharge capacity of 182.3 mAh/g at 0.2C rate between 2.7 and 4.3 V.The capacity retention was 97.9%after 200 cycles at 2C rate.The different properties of the precursors resulted from the different flow fields in the reactor.The single-loop flow field excited by the PT caused the nuclei to grow into flat and thick flaky primary particles,which then could assemble tightly to form the dense secondary particles.Additionally,the circulation of secondary particles in the whole reactor further enabled them to grow,consolidate,become spherical and smooth by adsorbing the free nuclei and primary particles.On the other hand,the double-loop flow field generated by the FBT or RT caused the primary particles to be thin and curve,which could only pile up loosely to form the secondary particles.What's worse,the secondary particles were entrapped in the lower loop,having little chance to grow and consolidate by adsorbing the nuclei and primary particles that were mainly formed in the upper loop.Preparing the full concentration gradient FCG-LiNi_1-x-yCo_xMn_yO₂ is an effective modification method to improve the cyclic stability of nickel-rich ternary cathode materials.It maintained the overall transition metal ratio unchanged,but the molar fraction of Ni²⁺decreased,whereas that of Mn²⁺increased from the core to the surface of the particles.Full concentration gradient FCG-Ni_1-x-yCo_xMn_y?OH?₂ is generally synthesized via varying the molar ratio of metal ions in the feeding flow during co-precipitation.Accordingly,pH and[N]?ammonia concentration?should change with the reaction time given that the optimum pH and[N]are different with variations of x or y.Hence,taking LiNi_0.8Co_0.1Mn_0.1O₂ for example,the effect of controlling strategies of pH and[N]on preparing the FCG-precursor and the electrochemical performance of the final cathode materials were systematically explored,and the mechanism behind was revealed.The four Full concentration gradient samples were prepared by four different controlling strategies of pH and[N]:constant pH and constant[N]?FCG-NCM811-I?;varied pH and constant[N]?FCG-NCM811-II?;constant pH and varied[N]?FCG-NCM811-III?;co-varied pH and[N]?FCG-NCM811-IV?.Meanwhile,the homogeneous Li(Ni_0.8Co_0.1Mn_0.1)O₂ was prepared for comparison.The FCG-precursor with co-variation of pH and[N]had the densest and firmest structure,highest tap density(2.115 g mL^-1),and well-ordered layered crystal structure.Additionally,its discharge capacity could reach as high as189.4 mAh g^-1?capacity retention of 96.3%?after 200 cycles at 1C rate,which was9.1%and 15.8%higher than the FCG-NCM811 with constant-pH and-[N]and the Homo-NCM811.The match between pH and[N]was crucial to get the favorable FCG-precursor and-NCM811.