The Modification Of Key Materials For High-Voltage LiNi_0.5Mn_1.5O₄ Lithium-Ion Batteries

Lithium-ion batteries have been widely used in mobile communication tools,computers and portable electronic devices such as digital cameras because of its advantages of no pollution to the environment,wide operating temperature range,high specific energy,long service life,low self-discharge rate and no memory effect.It has attracted great attention from researchers at home and abroad.In addition,with a series of problems increasingly serious such as global warming,energy shortage,and environmental pollution,the electric vehicles,hybrid vehicles,energy storage and conversion are receiving increasing attention.Therefore,the application of lithium-ion batteries as a new type of power battery in the field of new energy electric vehicles is an inevitable development trend.With the implementation of China’s“13th Five-Year Plan”and the promotion of new energy vehicles,the lower specific energy of traditional cathode materials can not fully meet the current range of electric vehicles in terms of cruising range,and develop lithium with higher energy density has become the trend of the times.Spinel Li Ni_0.5Mn_1.5O₄（LNMO）has a three-dimensional lithium ion diffusion channel with a high voltage platform of～4.7 V,a theoretical capacity of 146.7 m Ah/g,high energy density and power density,and is considered to be most promising and attractive cathode materials in batteries.However,LNMO-based high-voltage full-cell batteries constructed with carbon materials such as graphite as the negative electrode have not been successfully commercialized.The reason why LNMO material has not been commercialized is that the capacity attenuates too fast in the full-cell system.The main reason for this is that there is no electrolyte system compatible with it,resulting in serious capacity degradation of the whole battery system.This phenomenon is more obvious,especially in high temperature environment.Therefore,studying the high voltage LNMO full-cell system has far-reaching practical significance.This thesis mainly discusses the surface coating of the positive LNMO,the optimization of the negative mesophase carbon microspheres（MCMB）and the electrolyte additives.The main research contents are as follows:（1）Surface optimization of LNMO cathode material with LLTO as coating material.The effects of different synthesis temperatures and coating amounts of LLTO on the electrochemical properties of LNMO cathode materials are investigated.The pristine cathode material or coated with LLTO is used as the positive electrode,and the Li is used as the counter electrode to form the half-cells for electrochemical test.The room-temperature test of LNMO/Li half-cells show that the Pristine LNMO have a discharge specific capacity of 129.8 m Ah/g at 0.3C（1C=146.7 m Ah）for the first cycling,but capacity decay occurred in the cycle to～270,and the capacity retention rate of 300cycles is only 90.21%.However,when the LLTO coating content is 3wt%and the synthesis temperature is 650°C,the room temperature test of LNMO@LLTO3-650/Li half-cell shows that the first discharge specific capacity of LNMO@LLTO3-650 composite is as high as 134.4 m Ah/g,the capacity retention rate of 300 cycles is as high as 95.65%.When cycling at 55°C,the pristine LNMO is difficult to operate normally.However,LNMO@LLTO3-650 composites still operate relatively smoothly under the same condition.In summary,LNMO@LLTO3-650 has the best electrochemical performance.（2）Study on the effect of LiNO₃（LNO）of the electrochemical performance of high voltageLNMO/MCMB full-cells.Firstly,the XPS analysis of the MCMB/Li half-cells pole pieces after cycling show that the introduction of LNO can improve the SEI film composition,effectively inhibit the interface side reaction of the electrolyte on the surface of the electrode material,and reduce the decomposition of the electrolyte.Therefore,LNO plays an important role in ensuring the stable performance of the electrochemical properties of the material.In addition,the LNMO/MCMB full-cells research show that the introduction of LNO on the surface of graphite anode material can greatly improve the first coulombic efficiency of the battery.When the LNO introduction amount is 4 wt%,the first charge-discharge efficiency of LNMO/MCMB-4LNO sample is 70.68%.and the 1C first discharge specific capacity is 121.1 m Ah/g;While the pristine LNMO/MCMB sample has a first charge-discharge efficiency of only 63.7%,and the first discharge specific capacity at 1C is 104.9 m Ah/g.The capacity retention rates of Pristine LNMO/MCMB,LNMO/MCMB-0.5LNO,LNMO/MCMB-1LNO,LNMO/MCMB-2LNO,LNMO/MCMB-4LNO,and LNMO/MCMB-8LNO at 1C discharge rate are 85.64%,86.60%,88.98%,90.43%,97.32%,75.4%respectively at 50 cycles,of which LNMO/MCMB-4LNO has the best capacity retention rate.（3）Exploring the effects of electrolyte functional additives on the electrochemical performanceof LNMO/Li half-cells and LNMO/Graphite full-cells.In order to study and reveal the effects of the interfacial reaction occurring on the surface of the electrode materials on the LNMO-based high voltage full-cells,this experiment introduces the functional additives triphosphite（TSP）and1,3-propanediol cyclic sulfate（PS）into the electrolyte.By exploring the amount of introduction of different additive,it is found that the side reaction of the electrolyte at the solid/liquid interface is effectively suppressed.In the LNMO/Li half-cells,when the TSP introduction amount is 0.5wt%,the first charge-discharge efficiency is 90.39%,and the first discharge specific capacity is132.8 m Ah/g;While the pristine electrolyte’s charge-discharge efficiency is only 70.96%,and its first discharge specific capacity is 131.6 m Ah/g.In the LNMO/Graphite full-cells,when the introduced amount of binary functional additive（TSP+PS）is 1 wt%,the first discharge specific capacity is 122.1 m Ah/g,and the capacity retention rate is 82.8%after 200 cycles at 1C;But the first discharge specific capacity is 95.8 m Ah/g of ptistine electrolyte at 1C,and it experienced severe capacity decay,with a capacity retention of only 15.1%after 200 cycles.Moreover,when the binary functional additive is 1 wt%,the rate performance and the AC impedance indicate that the PE-TSP+PS-1 sample has the best kinetic performance.It is found by S_2p spectra that after cycling,dense ROSO₂Li and lithium inorganic substance Li₂SO₄ which promoted lithium ion diffusion through the SEI layer are formed on the surface of the positive electrode,and the presence of ROSO₂Li is also detected at the negative electrode.It is found by Mn_2p spectra and Ni_2pspectra that the binary functional additive assisted the formation of a stable SEI film in the positive and negative electrodes,which reduced the dissolution of Mn and Ni metal cations.The above analysis shows that the LNMO/Li half-cell has an optimal electrochemical performance when the TSP introduction amount is 0.5 wt%and the full-cell has an optimal electrochemical performance when the binary functional additives（TSP+PS）are introduced in an amount of 1 wt%.