Research On Capacity Loss Mechanisms And Modifacations Of Lithium Cobalt Oxides Cathodes

Lithium ion batteries?LIBs?have been widely used in a wide array of consumer electronic products and also gained the attention in the field of electric vehicles and energy storage applications due to their high operating voltage,high energy density and long cycle life.As the first commercialized cathode material for LIBs,lithium cobalt oxides?LCO?is still applied in cell phones,laptops and many other products.However,limited by the specific capacity and cycle life,it is difficult to meet the increasing demands of these products.In this research,the capacity fade mechanism of LCO cathode during cycling at high SOC for LCO/MCMB full cells was studied.The changes of physical and electrochemical properties of the LCO cathode were investigated during the long-term charge/discharge cycling.The capacity fade mechanism of the LCO cathode was analyzed from the aspects of both interface and bulk phase structure degeneration.Base on the mechanism,the LCO materials were modified by surface coating with electronic conductive oxides and Li ion conductive materials to improve the rate and cycle properties at high cut-off voltage.The capacity fade mechanism of LCO cathode in the LCO/MCMB full cells during the long-term charge/discharge cycling was studied.The full cells were disassembled after different cycles.The changes of interface and phase structure properties were investigated by FTIR,XPS,TEM,XRD and electrochemical tests.From the view of interface properties,a solid electrolyte interface?SEI?layer,containing Li2CO₃?RCOOLi?Li F and other lithium salts,has been detected on the surface of LiCoO₂ particles after cycling.And the thickness of SEI on the LCO cathode increased with cycle number,resulting in the increase of the interface impedance.The increased impedance affected the polarization of the full cell and led to the capacity loss.From the view of bulk phase structure properties,the lattice parameter c decreased as a function of cycle number,due to the repeated expansion and shrinkage of c-axis during Li insertion/extraction.The Rietveld refinement results suggested that the cation disorder increases during cycling,which directly resulted in the capacity loss and slowed the Li ion diffusion.Correspondingly,the Li+ diffusion coefficients and rate performance of LiCoO₂ electrode also faded gradually with the increasing cycles.The F-doped tin oxide?FTO?coated LiCoO₂ materials were prepared by sol-gel method.And the influence of heat treatment temperature was studied.The results showed that the coating of FTO did not affect the crystal structure of LCO materials.The coated materials with the heat treatment temperature of 600 oC showed the best cycling stability and rate performance.Another electronic conductive oxide,Al-doped zinc oxide?AZO?,was also applied as a coating layer to modify the LCO materials.It was found that the AZO coating layer on LCO particles was built by nano-sized AZO particles and the electronic conductivity of LCO powder increased by more than one order of magnitude after AZO coating.The electrochemical measurements demonstrated that when the coating concentration was 2 wt.%,the coated sample showed the excellent cycling stability of 98.2% capacity retention after 50 cycles?83.3% of uncoated sample?,and remarkably high rate capability with a reversible capacity of 156.6 mAh?g^-1 at the rate of 8 C?86.6% of the capacities at 0.1 C?.Therefore,after being coated by electronic conductive oxide,the interface stability of LCO at high cut-off voltage improved and the conductive networks on active electrode surface were established,leading to good cycle stability and rate capability.The LCO cathode materials coated with sodium aluminate?NA?were synthesized by means of hydrolyzing and heating technique.It was found that the NA-coating layer acted as a physical barrier,which stabilized the electrode/electrolyte interfaces in batteries and improved the cycling stability.Additionally,part of Na+ in the coating layer deintercalated and provided a two-dimensional ion diffusion channel for lithium ions,resulting in improvement of rate performance for LCO.In order to verify that surface modification by lithium ionic conductors was effective for improving the electrochemical properties of LCO materials,the fast Li+ conducting Li Al Si O4?LAS?was coated on LCO.The research result showed that the strong Si-O bond in LAS increased the stability of the coating layer,resulting in the excellent cycling stability of coated LCO in the voltage of 2.75-4.55 V.Moreover,the superior lithium ion conductivity of LAS coating layer led to high rate property.LCO coated with lithium ion conductive LiAlPO_3.95F_1.05?LAPF?coated on LCO was synthesized employing a freeze drying method.LAPF has both superior lithium ion and electron conductivity.Comparing with insulative AlPO₄ coated LCO,the improvement of cycling stability of LAFP coated LCO was remarkable.It also showed superior rate capacity: at 4 C and 8 C,the discharge capacity were 161.4 mAh?g^-1 and 112.3 mAh?g^-1,respectively.The surface modification by mixed Li ionic-electronic conductor can also be referred for other layered oxide cathode materials.