Preparation And Modification Of High-Voltage Spinel LiNi_0.5Mn_1.5O₄ Cathode Material

Spinel LiNi_0.5Mn_1.5O₄（LNMO）cathode material is an environmentally friendly cathode material for lithium-ion batteries,which has been widely studied because of its high voltage plateau of 4.7 V and energy density of 650 Wh/kg.However,the electrochemical performance of LNMO is severely affected by the dissolution of manganese elements and the erosion of LNMO by HF generated from the decomposition of Li PF₆ in the electrolyte.Especially,these reactions are further accelerated at high temperatures and high-magnification cycles.To improve the performance of LNMO,this paper investigates the synthesis process of LNMO precursors,the sintering process of LNMO cathode materials,the modification of LNMO cation doping and the modification of LNMO surface coating.First of all,the LNMO precursor(Ni_0.25Mn_0.75)CO₃ was successfully prepared by co-precipitation method,and the best precursor synthesis process was obtained after systematic study:the p H of the reaction system was 9.0±0.05;the reaction temperature was 55℃;the stirring speed was1500 r/min;the concentration of the precipitant sodium carbonate was 1.0mol/L;The(Ni_0.25Mn_0.75)CO₃ synthesized by co-precipitation method was prefired at 500℃for 5 h and cooled to room temperature with the furnace,and sphere-like oxide precursors of high purity were obtained with a molar ratio of 1:3 of Ni-Mn elements,a vibrational density of 1.56 g/cm³,and a particle size D₅₀ of 11.289μm.Then,the optimal synthesis process of LNMO cathode material was obtained by high-temperature solid-phase method:sintering temperature was 800℃,sintering time was 12 h,and the ratio of Li/（Ni+Mn）was 0.54.SEM analysis shows that the synthesized grains have good morphology,and the surface has a microporous structure which is conducive to the diffusion of lithium ions,and XRD analysis shows that the products are well crystallized.The electrochemical performance tests showed that the first discharge capacity of LNMO cathode material was 133.49 m Ah/g at0.1 C in the voltage range of 3.0～5.0 V.The capacity retention rate was98.1%after 20 charge/discharge cycles,and the discharge capacity could reach 71.12 m Ah/g at 5 C multiplier,which showed that the material has high discharge capacity and good rate performance.In the next part,Ti⁴⁺doped LNMO cathode material was successfully synthesized by high-temperature solid-phase method,and the EDS characterization results showed that Ti⁴⁺was uniformly doped into LNMO,and the XRD results indicated that Ti⁴⁺doping increased the cell volume of LNMO.The electrochemical performance tests showed that Ti⁴⁺doping was beneficial to improve the cycling performance and rate performance of the material,and the appropriate amount of Ti⁴⁺doping also enhanced the discharge specific capacity of the material;the discharge specific capacity of LNMO-Ti0.1 at 0.1 C was 137.22 m Ah/g at voltages of 3.0～5.0 V,and the discharge specific capacity at 5 C was 74.4%of that at 0.1C in the rate performance test.In the cycling performance test,the capacity retention rate was 95.6%after 100 cycles of 1 C at 25℃and 70.2%after50 cycles of 1 C at 55℃.Finally,the composite cathode material coated with Li Mn PO₄ on the surface of LNMO was successfully synthesized by sol-gel method.TEM characterization showed that a uniform Li Mn PO₄ coating layer was formed on the surface of LNMO;XRD analysis showed that the coated material did not change its spinel-type structure.Electrochemical performance tests show that when the voltage range is 3.0～5.0 V,the first discharge specific capacity of LNMO@LMP-5 at 0.1 C is 126.01 m Ah/g;the discharge specific capacity at 5 C can still reach 109.2 m Ah/g,The capacity retention rate is as high as 85.9%;the discharge specific capacity after 100 cycles at1 C at 25℃is 122.95 m Ah/g,and the capacity retention rate is 97.3%;the discharge specific capacity after 50 cycles at 55℃at 1 C is 111.25 m Ah/g g,the retention rate is 83.1%.The cycling performance and rate capability of the material are greatly improved by Li Mn PO4 coating.