Synthesis And Modification Of 5V Spinel LiCoMnO₄ Cathode Material

Spinel Lithium cobalt manganate(LiCo³⁺Mn⁴⁺O₄)oxide is considered as one of high-energy lithium-ion cathode materials with the most potential development due to its stable structure,excellent safety,high working voltage and energy density.However,LiCoMnO₄ is often reported to coexist with a Li₂MnO₃ secondary phase due to oxygen loss during heating,resulting in the capacity density of LiCoMnO₄ far lower than its theoretical capacity(145mAh/g).In this paper,LiCoMnO₄ with high voltage is selected as the research object.Through the exploration of its synthesis process and the study of ion modification,the performance of LiCoMnO₄ is improved in all aspects.In this paper,5V spinel cathode material LiCoMnO₄ was synthesized by high temperature solid-phase method assisted by nano-ball milling,and process parameters,such as ball milling parameters and synthesis(temperature,rising and cooling rate and holding time,etc.),were selected to be optimized.Eventually optimal synthesis process was obtained:the precursor was first heated at 750?for 12h,then annealed at 600?for 24h in tube furnace under O₂ and with heating and cooling rates are respectively 3?/min and 1?/min.The cathode material with high phase purity and low ion mixing was synthesized.As a result,its discharge specific capacity is increased from 87.6 mAh/g to 108.3 mAh/g at 0.1 C.In addition,in order to improve the electrochemical performance of the material,that was modified by Cr-ion.LiCoMn_1-xCr_xO₄(x=0,0.025,0.05 and 0.075)were synthesized.Due to the oxygen affinity of Cr element,Cr-doping inhibits the production of Li₂MnO₃ impurity,but also improves the structural stability of the spinel.Moreover,the ion radius of Cr³⁺(0.615(?))is larger than that of Mn⁴⁺(0.53(?)),which prompts the lithium-ion diffusion and is conducive to the rate performance.Test results showed that the electrochemical performance was the best when x=0.05.The specific capacity of the LiCoMn_0.95Cr_0.05O₄ is 120.8 mAh/g at 0.1 C and 93.3mAh/g at 10 C,and the capacity retention is 90.1%after 100 cycles at 1 C.Finally,to further improve the electrochemical performance of the material,the spinel LiCoMnO_3.9F_0.1 cathode material was prepared.Because the electronegativity of F^-is greater than O^2-,the bonding energy of the Mn-F and Co-F bonds is stronger than that of Mn-O and Co-O,which suppresses the decomposition of LiCoMnO₄ at high temperature.Therefore,the content of Li₂MnO₃ impurity phase in LiCoMnO_3.9F_0.1 is significantly reduced,from 13.75%to 6.84%,and Li/Comixing is also inhibited.The specific discharge capacity of LiCoMnO_3.9F_0.1is 128.1 mAh/g at 0.1C,close to its theoretical capacity.At the same time,the fluorinated LiCoMnO₄ has better structural stability,and a discharge capacity of 115.4 mAh/g after 100cycles with a cycle retention rate of 92.8%at 1 C.what's more,the rate performance has been greatly improved,and the discharge capacity is increased from 38.0 mAh/g to 112.7 mAh/g at10 C compared with LiCoMnO₄.