Synthesis And Performance Of High Safty Cathode Materials For Li-ion Batteries

Lithium-ion battery has become the primary choice of the green battery in the 21th century due to its advantages such as high operating voltage,no memory effect and environmental friendly.At present,LiCoO₂ is the major cathode material of commercial lithium-ion batteries.However,the main problems of LiCoO₂ cathode material such as its expensive cost,low practical capacity,poor overcharge tolerance and safety performance have hindered the applications of lithium-ion batteries to apply in wide fields.So many attempts have been made to develop other cathode materials with low cost,excellent electrochemical performance and high safety to meet the emerging industries such as electric vehicles.Based on the safety of lithium-ion battery,this work employs several approaches such as ion substitution and surface treatment to improve the performance of layered LiCoO₂,LiNiO₂ and spinel LiMn₂O₄ such as overcharge tolerance,thermal stability and cycling property at high temperature.The conclusions have been summarized as following: 1.Layered LiCoO₂ and pillared layered Li_1-2xCa_xCoO₂ have been prepared by cationic exchange under hydrothermal conditions,and then subsequently calcined in air.The products retain the layeredα-NaFeO₂ type structure.The effects of the Ca²⁺ amount and thermal treatment on their structure,morphology and electrochemical behavior have also been investigated.The pillared layered Li_0.946Ca_0.027CoO₂ calcined at 800℃retainsα-NaFeO₂ structure.Moreover,the pillared layered Li_0.946Ca_0.027CoO₂ has a higher capacity with an initial discharge specific capacity of 177.9 and 215.8 mAh·g^-1 within the potential range of 3.0-4.5 and 3.0-4.7 V(vs.Li⁺/Li), respectively,and has a better overcharge tolerance than LiCoO₂.In addition, the pillared layered Li_0.946Ca_0.027CoO₂ has higher lithium diffusion coefficient than LiCoO₂.These results can be attributable to the pillar effect of Ca²⁺ ions which prevents the local collapses of the structure during the lithium intercalateion/deintercalation process and provides larger space for the movement of lithium ions.2.LiCoO₂ has been modified with MnSiO₄ as a novel coating material by sol-gel method.The effects of the MnSiO₄ amount on their structure, morphology,overcharge tolerance and thermal stability have also been investigated.When MnSiO₄ amount based on LiCoO₂ is 3 wt.%,the MnSiO₄-coated LiCoO₂ has an initial discharge specific capacity of 181.1 and 232.2 mAh·g^-1 within the potential ranges 2.75-4.5 and 2.75-4.7 V(vs.Li⁺/Li), respectively.It has been found that the overcharge tolerance of the coated cathode material is significantly better than that of the pristine LiCoO₂ under the same conditions—the discharge specific capacities of the coated cathode at upper charge cutoff voltages of 4.5 and 4.7 V are as high as 168.7 and 154.3 mAh·g^-1,respectively,after 50 cycles.Moreover,differential scanning calorimetry(DSC) shows that the coated LiCoO₂ had a higher thermal stability than the pristine LiCoO₂.3.LiNi_0.8Co_0.2O₂ cathode material has been synthesized by calcination of Ni-Co hydroxide-carbonate precursor and lithium salt under air atmosphere. The Ni-Co hydroxide-carbonate precursor is prepared by the method involving separate nucleation and aging steps(SNAS),in which rapid mixing and nucleation in a colloid mill is followed by a separate aging process.The precursor prepared by this method has both small crystallite size and a very narrow distribution of crystallites,which is responsible for favoring well-mixing of lithium source material and precursor.These characteristics are more favorable to synthesize LiNi_0.8Co_0.2O₂ cathode material with a narrow particle distribution and a well-developed layered structure,which have a positive effect on discharge specific capacity and cycling performance.In addition,NH₄HCO₃ has lower cost than other precipitating agents,such as KOH or NaOH,and can avoid contamination due to metallic cation. LiNi_0.8Co_0.2O₂ synthesized at optimization condition has a good electrochemical performance with an initial discharge capacity of 188.2 mAh·g^-1 and a reversible capacity of 158.7 mAh·g^-1 at 50th cycles at 2.75～4.5V(vs.Li⁺/Li) charge-discharge voltage range.4.LiNi_0.93Co_0.07O₂ cathode material has been surface modified with Co₃(PO₄)₂ prepared by using ethanol as a solvent.The problem of Li dissolution from the bulk of Ni-based cathode material during coating process has been solved by using ethanol to replace water as solvent.The effects of the different solvents on the structure,morphology and electrochemical behavior of Co₃(PO₄)₂ coated LiNi_0.93Co_0.07O₂ have also been investigated.The Co₃(PO₄)₂ coated LiNi_0.93Co_0.07O₂ has significantly improved machining performance because of preventing the cathode slurry gelation.The Co₃(PO₄)₂ coated LiNi_0.93Co_0.07O₂ has a higher initial discharge specific capacity than the pristine one,moreover,the coated cathode material has a better electrochemical stability,a better high-temperature storage characteristic,a better overcharge tolerance and a higher thermal stability than pristine LiNi_0.93Co_0.07O₂.5.Co-Al mixed metal oxide(CoAl-MMO) as a novel coating material has been used for surface modification of spinel LiMn₂O₄.The mixed metal oxides has been prepared from a Co-Al layered double hydroxide(CoAl-LDH) precursor;this has a uniform distribution of metal cations on atomic level, which affords a mixed metal oxide with a more uniform distribution of cobalt and aluminum than conventional methods.The effects of different calcination temperatures and coating amounts on the structure,morphology and electrochemical cycling performance of CoAl-MMO coated LiMn₂O₄ have been studied,and the mechanism of improving electrochemical performance of CoAl-MMO coated LiMn₂O₄ has also been studied.The CoAl-MMO coated LiMn₂O₄ after heat-treatment at 400℃with Co and Al loadings of 3 wt.%and 0.5 wt.%,respectively,has an initial discharge specific capacity of 105.3 and 104.5 mAh·g^-1 with a discharge specific capacity of 100 and 92.2 mAh·g^-1 after 50 cycles at 25℃and 55℃,respectively.Enhancement of the electrochemical properties can be-attributed to the surface coating of CoAl-MMO,giving rise to blocking the direct contact between the spinel and electrolyte,which inhibits both decomposition of the electrolyte and dissolution of the spinel LiMn₂O₄ into the electrolyte.In addition,LiMn₂O₄ spinel has also been modified with Zn-Al mixed metal oxide(ZnAl-MMO) in an attempt to lower the synthetic cost.ZnAl-MMO coated LiMn₂O₄ also shows a better cycling performance than the pristine LiMn₂O₄.