Study On Synthesis And Electrochemical Performance Of High Specific Capacity Cathode Materials Based On Bonded Coating And Doping For Lithium-ion Batteries

With the significant consumption of fossil energy and the resulting environmental pollution problems becoming more and more prominent,the development and utilization of green energy have become the consensus of human society.Lithium-ion batteries have received much attention as efficient energy storage and conversion devices,have been widely used in electronic products and large-scale energy storage,and have been extended to the field of new energy vehicles.Meanwhile,people have put forward higher requirements on the driving range and calendar life of new energy vehicles.As we all know,cathode material,as the core component of the battery,determines the energy density,power density,and manufacturing cost of lithium-ion batteries.Therefore,it is urgent to develop cathode materials with high energy density and low cost.In this regard,high nickel ternary（NCM811）and lithium manganese-rich（LM）cathode materials have received high attention due to their high energy density and abundant raw material reserves,and are considered to be strong contenders for next-generation high energy density lithium-ion battery cathode materials.However,NCM811 and LM cathode materials still have issues such as low ion diffusion rate,poor rate performance,and cycle stability,which seriously limit their large-scale application in commercial lithium-ion batteries.Surface coating of NCM811 and LM cathode materials is a common strategy to improve their electrochemical performance,and this approach is effective in improving cycling performance,but most of the recommended surface coatings are not beneficial for lithium-ion transport.On this basis,this work selects a surface modification of the NCM811 cathode by a coating layer with lithium-ion conductivity and bonds with the NCM811 surface to form an interfacial bonding coating that provides a stable surface interfacial environment and also promotes the lithium-ion transport rate.The LM materials were modified by bulk phase doping synergistic surface coating for performance optimization.The relationship between the structure and properties of NCM811 and LM cathode was investigated in-depth using in situ XRD,TEM,XPS,TOF-SIMS,and DFT theoretical calculations,mainly including the following.（1）Li_xNi_1-yFe_yO₂&Ni Fe₂O₄（LNFO&NFO）-coated NCM811 cathode materials were successfully prepared by wet chemical method.The first-coulomb efficiency and cycling performance of NCM811 material were improved after the surface coating of LNFO&NFO.Among them,the capacity retention rate of the NFO-2 sample was 87.43%after 250 cycles at 1C,2.75-4.4V,and the capacity retention rate was 74.32%after 100cycles at a high temperature of 55℃.The characteristics of XRD,SEM,XPS,and TEM show that the LNFO&NFO coating layer is successfully coated on the surface of NCM811,and the LNFO is bonded to the surface of the substrate through TM-O-Ni bonding,which can be stably"pinned"on the surface of NCM811 during the cycling process and inhibit the side reaction between the electrode active material and the electrolyte.The LNFO&NFO coating layer consumes both residual lithium and oxygen vacancies on the surface of NCM811 material during the formation process while suppressing the irreversible phase transition of the material and increasing the diffusion rate of lithium ions on the surface.In addition,in situ XRD and DFT theoretical calculations verify that the NFO&NFO coating layer can improve the Li⁺transport kinetics of NCM811 and reduce the polarization at the electrode interface,thus optimizing the electrochemical performance of NCM811 material.（2）Surface treatment of NCM811 using PH₃ gas to generate an in situ Li_x（TM）_yPO₄（LTMPO）coating layer on the surface of the NCM811 material by the gas-solid phase reaction method.The LTMPO coating layer can reduce the residual lithium and oxygen vacancies on the NCM811 surface during the formation process,and enhance the bonding of the coating layer to the NCM811 surface through the P-O-TM bond,thus stabilizing the fixation on the material surface.The formation mechanism of the LTMPO coating layer on the surface of NCM811 was investigated by using ex-situ XPS and EPR.In situ XRD measurements demonstrated that the LTMPO surface coating leads to a more stable phase transition process and smaller changes in the lattice parameters of the NCM811,which reduces the mechanical stress induced inside the lattice during the Li⁺ion de-incorporation process and stabilizes the crystal structure.SEM and TEM analyses of the cycled electrode materials showed that the LTMPO surface coating can suppress the phase transition of the NCM811 material from the R-3m lamellar phase to the Ni O rock salt phase.The results of DFT theoretical calculations show that the interfacial binding energy of NCM811/LTMPO is-22.59 e V,indicating the existence of a TM-O-P bond between NCM811 and LTMPO coating.After LTMPO coating,the lithium ions on the surface of the NCM811 material can diffuse along with the a,b,and c axes in a three-dimensional path,thus having a lower kinetic energy barrier for Li⁺ion diffusion.At 2.75-4.5V,the capacity retention rate of the NHP-2 sample was 80.36%for 300 cycles of 1C at room temperature and 61.22%for 200 cycles at high temperature,which was higher than that of the uncoated sample at 35.41%and 24.95%.In addition,the discharge specific capacity at 10C current density was 141.78 m Ah g^-1,which was 48.8%higher than that of the uncoated sample.（3）The surface of LM was treated with ammonia and hydrogen sulfide gas,and a in situ Li₂TM（SO₄）₂（LTSO）coating layer was constructed on the LM surface by the gas-solid phase reaction method,and the N body phase doping was achieved simultaneously.GITT measurements showed that the lithium-ion diffusion coefficient of the LM cathode was significantly improved after surface N doping and LTSO coating.XRD refinement indicated that N doping could expand the layer spacing of the lithium layer,thus enhancing the diffusion rate of lithium ions within the lattice.TOF-SIMS,ICP,and SEM were employed to analyze the electrode and electrolyte after cycling,and the results indicated that the LTSO coating could suppress the secondary reaction between the LM cathode and electrolyte,while the N doping could alleviate the lattice mechanical stress induced by the phase transition process and reduce the resulting problems such as accumulation of interfacial inactive materials,increased interfacial impedance,particle crushing,and transition metal dissolution.Therefore,N doping and LTSO coating have a synergistic effect on the improvement of the physical phase structure and interfacial structural stability of LM materials,and this synergistic effect enhances the structural stability as well as the electrochemical properties of LM materials.The modified LS-2 sample exhibited better electrochemical performance with a first-coulomb efficiency of 87.17%and an increase of 81.3 m Ah g^-1 and 73.2m Ah g^-1 at 5C and 10C rate,respectively,compared to the LM sample.The capacity retention after 500 cycles at a 1C rate was 74.68%,while the capacity retention of the unmodified LM sample after 300 cycles was only 27.3%.（4）Se and H₂ were reacted with the surface of the LM material at 500°C to successfully prepare a Se-based integrated coating layer and Se-doped LM material.XRD refinement and XPS analysis revealed that Se was successfully doped into the lattice oxygen sites of LM and a Se&Se O_x integrated layer was coated on the LM surface.GITT,TOF-SIMS,and ICP measurements showed that the content of inactive deposits on the electrode surface of the modified samples after cycling was significantly less than that of the LM electrode,and the Se-based integrated layer could suppress the secondary reactions at the electrode interface.XPS and EPR analysis of the LSE-2electrode with different cycles showed that the Se-based integrated layer on the LM surface could capture the active oxygen release from the LM and the free radicals in the electrolyte,thus inhibiting the intensified decomposition of the electrolyte.DFT theoretical calculations reveal that some of the electron clouds of Se atoms in the lattice overlap with O atoms,and the lattice O"captures"some of the external electrons of the Se atoms when it is oxidized,leading to an increase in the lattice oxygen release energy barrier,thus inhibiting the lattice oxygen release.Calculations of the diffusion kinetics of lithium ions indicate that Se doping decreases the diffusion energy barrier of lithium ions in the LM lattice,thus increasing the transport rate of lithium ions.The LSE-2sample exhibited excellent electrochemical performance with a first-coulomb efficiency of 88.63%and discharge specific capacities of 150.3 m Ah g^-1 and 116.4 m Ah g^-1 at 5C and 10C rates,and capacity retention of 83.51%after 300 cycles at 1C multiplicity.