Research On Cation Disordered Rock-salt Cathode Materials For Lithium Ion Batteries

The cathode materials have become the bottleneck which restricts the development of lithium-ion batteries due to their lower discharge capacities.In this case,design and exploitation of cathode materials with high discharge capacities and better cycle stabilities turn into the hotspots and difficulties for lithium-ion batteries.Cation disordered rock-salt Lithium-excess oxides have gradually become a research hotspot of cathode materials for lithium-ion batteries because of their high discharge capacities.However,cation disordered rock-salt cathode materials still have some practical drawbacks including poor cycle stabilities and inferior rate capabilities,which may restrict the process of their commercial applications.Up to now,many studies have been carried out to solve the aforesaid issues.Elemental substitutions,heating pretreatments,and adjustments of the working voltage window have been used in succession to improve the electrochemical performances of the cation disordered rock-salt cathode materials,and good results have been achieved.Nevertheless,the fundamental electrochemical properties of this kind of materials（such as thermal,kinetic and ion transport properties）,which have good guiding significances for material modifications,have hardly been reported so far.In addition,elemental substitutions at present almost focus on the anion substitution（such as F^-）,and highly poisonous vanadium ions are usually used to achieve the cation substitutions,which will cause some damage to the environment.Furthermore,surface coating,a common effective modification technique,has never been reported to improve the electrochemical performances of this kind of cathode materials.In the thesis,on the basis of investigating the kinetic and transport properties of the cation disordered rock-salt Li_1.25Nb_0.25Mn_0.5O₂（LNMO）cathode material,modifications including Mo-substitution and surface modification with Al₂O₃by ALD technique were used to improve the electrochemical performances of cation disordered rock-salt Li_1.2Ti_0.4Mn_0.4O₂（LTMO）.First,cation disordered rock-salt LNMO cathode material was prepared by solid state reaction,and the crystal structures and electrochemical properties of LNMO were also investigated.The structure of synthesized LNMO sample belongs to typical cation disordered rock-salt type,and the representative voltage platform of common Li-excess layered oxides located at～4.5 V is absent during the first charge.Besides,the initial coulombic efficiency of the synthesized LNMO is as high as 93.0%,which is much higher than those of usual layered Li-excess oxides cathode materials.The chemical diffusion coefficients of Lithium(D_Li)can be measured through a series of mathematical derivation,which are 1.03×10^-13-1.38×10^-10 cm²·s^-1 and 6.89×10^-13-5.01×10^-11 cm²·s^-1 during the charging process and discharge process,respectively.Meanwhile,electrochemical impedance spectroscopy（EIS）results show that the charge transfer resistances decrease first and then increase both the charging process and the discharging process,which is consistent with the variation trend of polarization over-potential of the LNMO electrode.Next,similar LTMO cathode material was successfully synthesized via the solid state reaction,and coated with different thickness of Al₂O₃ using the atomic layer deposition（ALD）technique.The result shows that Al₂O₃ coating treatment does not change the morphology and crystal structure of LTMO sample,and appropriate thickness of Al₂O₃coating layer can obviously enhance the electrochemical performance of LTMO sample.The discharge capacity of LTMO/24Al₂O₃ is still as high as 266.7 mAh·g^-1 after 15cycles,and the capacity retention is 90.9%.While,the primary LTMO sample can only obtain the discharge capacity of 228.1 mAh·g^-1 after 15 cycles,and the capacity retention is 79.7%.In addition,the rate capability is also improved.The discharge capacity of LTMO/24Al₂O₃ sample is 229.2 mAh·g^-1 at the current density of 500mA·g^-1,while that of the bare LTMO sample is only 93.8 mAh·g^-1 at the same current density.ALD process can alleviate the erosion of electrolyte on LTMO material,restrain the undesirable side reactions between and electrode and electrolyte,and suppress the increasing charge transfer resistance during cycling,which results in significantly improved electrochemical performance of LTMO/24Al₂O₃.At the same time,the thermal stability of the LTMO/24Al₂O₃ was also significantly improved under the protection of Al₂O₃ coating layer.Then,LTMO cathode materials were modified by doping Mo with lower toxicity.The result shows that appropriate content of Mo-doped can obviously improve the electrochemical performance of LTMO sample.The discharge capacity of Li_1.19Ti_0.4Mn_0.39Mo_0.01O₂（Mo01）sample is still as high as 281.5 mAh·g^-1 after 10 cycles,while bare Mo00 sample can only obtain the discharge capacity of 258.3 mAh·g^-1 after10 cycles.It is mentioned that the rate capability of Mo01 sample is also greatly improved.The discharge capacity of Mo01 sample is 177.5 mAh·g^-1 at the current density of 500 mA·g^-1,while the primary Mo00 sample can only obtain the discharge capacity of as low as 12.3 mAh·g^-1.In addition,suppression of rapidly increasing charge transfer resistance and improvement of structural stability result in the better electrochemical performance and higher thermal stability of the Mo01 sample.On the basis of the satisfying experimental results obtained via the two modification schemes mentioned above,the samples synthesized at the optimum Mo subsititution were coated with different thickness of Al₂O₃ by ALD,and the satisfactory experimental results were also obtained.Finally,the electrochemical performances of the sample obtained at optimum condition(Li_1.19Ti_0.4Mn_0.39Mo_0.01O₂/24Al₂O₃,LTMMO/24Al₂O₃)and the bare unmodified LTMO were compared,which shows that the electrochemical performance of LTMMO/24Al₂O₃ sample is significantly improved.The discharge capacity of LTMMO/24Al₂O₃ is still as high as 228.4 mAh·g^-1 after 30 cycles,while the primary LTMO sample can only obtain the discharge capacity of 102 mAh·g^-1 even after 25 cycles.Moreover,the rate capability of LTMMO/24Al₂O₃ sample is much better than that of LTMO sample.The above-mentioned studies deepen the understanding of thermal,kinetic and ion transport properties of cation disordered rock-salt cathode materials.Besides,cation-substitution（Mo-substitution）and surface modification with Al₂O₃ by ALD technique suggested in the thesis can well enhance the electrochemical performances of cation disordered rock-salt cathode materials,which shows that both the cycle stabilities and rate capabilities of the modified cathode materials can be greatly improved.In summary,the researches of this thesis provide references value and significance for the theoretical studies and future commercial applications of cation disordered rock-salt cathode materials.