Surface Modification And Research On Low Temperature And High Voltage Energy Storage Characteristics Of Lithium Cobalt Oxide

The poor low-temperature performance of lithium-ion batteries is an important problem restricting their application in high altitude or high latitude areas in winter,relevant data show that the effective mileage of electric vehicles in northern winter will be reduced by more than half.The low temperature performance of lithium-ion batteries is closely related to cathode materials,so it is of great significance to develop cathode materials with excellent low temperature performance.Lithium cobalt oxide（Li Co O₂）cathode possesses the advantages of good electronic conductivity and lithium diffusion coefficient,but it still faces many problems in cycling at high voltage and low temperature,such as the formation of uneven and unstable cathode solid electrolyte interface（CEI）,bulk phase structure destruction and poor compatibility between electrode and electrolyte.In this paper,Li Co O₂ with superior performance at high voltage and low temperature was successfully developed by surface doping and coating modification strategies.Relevant research results are as follows:1.Uniform amorphous Zr₃（PO₄）₄ coated Li Co O₂ with good stability at high voltage and low temperatureAt low temperature,the interface wettability between electrolyte and electrode material becomes worse,which greatly increases electrode polarization and seriously affects electrochemical performance.In addition,the surface of the high-voltage cathode is prone to side reactions with the electrolyte,leading to the excessive growth of CEI and structural damage.Based on the above questions,we achieve an amorphous and uniform Zr₃（PO₄）₄ coating with a thickness of 5 nm on Li Co O₂ surface by a novel and facile soft-chemistry method.The amorphous Zr₃（PO₄）₄ layer can effectively reduce the direct contact between electrode and electrolyte and participate in the formation of the CEI with high electrochemical stability,good wettability and low interface resistance.At-25°C and 4.6 V,the modified Li Co O₂shows a capacity of179.2 m Ah g^-1 at 0.2C and an excellent cyclability with 91%capacity retention after300 cycles（1C）.As a comparison,bare Li Co O₂ shows only 161.6 m Ah g^-1 and 1%capacity retention under the same circumstances.This research shows that constructing a highly stable amorphous Zr₃（PO₄）₄ hetero-interfacial layer on the surface of the electrode material can improve the wettability of electrode and electrolyte at low temperature and inhibit the interface side reaction,and improve the electrochemical performance of Li Co O₂ at low temperature and high voltage.2.Shallow surface Zr-doped and superwetting surface modified Li Co O₂with superior performance at high voltage and low temperatureIn order to further improve the specific capacity and rate performance of high-voltage Li Co O₂ at low temperature,a co-modification strategy of surface doping and coating was adopted.First,the doped high coordination Zr element in the shallow surface of Li Co O₂can anchor the lattice oxygen,suppress the release of the lattice oxygen and increase the interplanar spacing to promote the rapid transport of lithium-ion.In addition,island Li₂Zr（PO₄）₂ nanoparticles with high ionic conductivity(??10^-4m S cm^-1)are constructed on the surface of Li Co O₂ to increase the surface roughness achieving a super-wettability to the electrolyte and participating in the formation of CEI layer with strong stability and low interfacial resistance.At-25°C and 4.6 V,the modified Li Co O₂shows an ultrahigh capacity of 191.3 m Ah g^-1 at 0.2C,even at the high rate of 5C,it can still retain 131.2 m Ah g^-1.In addition,the modified sample has good cycling stability with 94%capacity retention after 100 cycles（1C）as well as a high average coulomb efficiency of 99.9%.In the voltage range of 2.0–4.4 V,the assembled full cell maintains capacity of 94%after 100 cycles at 1C.