Study On Modification Of Sodium And Molybdenum Doped LiNiO₂ Cathode Materials

With the development of science and technology,the contradiction between people’s demand for high-quality life and the increasing shortage of energy is becoming more and more serious.In order to achieve the harmonious coexistence of human and nature,green environmental protection for new energy sources was attached great importance by researchers.Among them,the lithium-ion batteries with the advantages of low pollution,high energy density,long cycle life,no memory,high working voltage,have become a research hot spot.At present,commercial high-energy lithium-ion batteries use Li Co O₂or Co-doped forming the ternary cathode materials.But cobalt element has disadvantages such as scarce resources,high toxicity,and great environmental pollution.So it is necessary further to develop higher energy and more environmentally friendly cathode materials.Among them,LiNiO₂and Li Co O₂cathode materials with similar layered structures,and LiNiO₂materials deliver the high theoretical discharge specific capacity,becoming the advantage materials to replace Li Co O₂.However,LiNiO₂cathode material suffers from rapid capacity fade during charge-dischage cycles,which has hindered its application of practical lithium-ion batteries.Based on the above problems,LiNiO₂materials were produced via a sol-gel method,and the effects of the ion-doping on the stabilization of crystal structure and improvement of electrochemical properties of LiNiO₂cathode material were studied.The specific research is given in the follows.Firstly,Li_1-xNa_xNiO₂ cathode material was successfully synthesized by introducing Na⁺ions to occupy the Li⁺site.By measuring the electrochemical properties of different as-synthesized LiNiO₂based materials,it is found that the introduction to Na⁺ions could effectively inhibit the cation mixing of Li⁺and Ni²⁺ions,stabilize the layered structure of the material,significantly improve the cycle stability of the material,and suppressing the discharge voltage decay.In addition,Li_0.97Na_0.03Ni O₂materials exhibit improved rate performance,owing to Na-doped Li_1-xNa_xNi O₂materials reduced electrode polarization and charge transfer impedance,and promoted the Li⁺ion intercalation/de-intercalation,and exhibiting superior capacity retention.The discharge capacity of Li_0.97Na_0.03Ni O₂is 93.5m Ah·g^-1at a 5C rate,and the capacity retention rate is 88.3%after returning to 0.1C.It demonstrates that the better electrochemical reversibility of the Li_0.97Na_0.03Ni O₂electrode during cycling.Secondly,Mo-doped LiNi_1-xMo_xO₂ cathode materials were synthesized by sol-gel method,the structure analysis showed that the appropriate amount of Mo doped Li Ni_1-xMo_xO₂cathode materials can inhibit the harmful structural changes of the material during cycling to stable crystal structure of materials,and enhance the cycle stability of the materials.Li Ni_0.98Mo_0.02O₂materials were measured at the current density 0.5C with a high capacity retention of 87.1%over 100 cycles.Importantly,Li Ni_0.98Mo_0.02O₂samples present a gentle slope over 500 cycles at 1C with the discharge specific capacity of 78.42 m Ah·g^-1.By measuring the electrochemical impedance spectroscopy of different as-synthesized LiNiO₂based materials,it is found that an appropriate amount of Mo doped cathodes materials also reduces the charge transfer impedance and improves the rate performance of the material.Especially,Li Ni_0.98Mo_0.02O₂sample has the best rate performance at the current density 10C,and the discharge specific capacity is still 48.1 m Ah·g^-1.Finally,we have synthesized Na-Mo co-doped layered Li_1-xNa_xNi_1-yMo_yO₂cathode materials by a sol-gel method,and realizing the synergistic effect of a single element.Meanwhile,Na-Mo co-doped Li_1-xNa_xNi_1-yMo_yO₂materials further improved the cycling or rate performance.Meanwhile,the results show that the electrochemical performance of Na-Mo co-doped Li_1-xNa_xNi_1-yMo_yO₂sample is better than the Na or Mo single doped LiNiO₂materials.Among them,Li_0.97Na_0.03Ni_0.98Mo_0.02O₂materials were measured at the current density 1C with a high capacity retention of 87.4%over 150 cycles;In addition,Li_0.98Na_0.02Ni_0.98Mo_0.02O₂and Li_0.97Na_0.03Ni_0.98Mo_0.02O₂samples maintained superior discharge specific capacity,were 90.7 m Ah·g^-1and 90.4 m Ah·g^-1at the current density 10C,and have obvious discharge platform.The results show that the synergistic effect of two-element doping can effectively improve the electrochemical properties of the materials.