Modification And Electrochemical Properties Of High Performance Cathode Materials For Lithium Ion Batteries

With the development of society,the current commercial lithium ion batteries are gradually unable to meet the rigorous requirements for high energy density,high cycle stability and high safety.Therefore,the development of new cathode materials for lithium ion batteries has become a research hotspot.Li-rich cathode materials are considered to be one of the most promising cathode materials because of its high specific capacity（>250 m Ah/g）,wide voltage window and low cost.However,it still suffers from large initial irreversible capacity loss,serious capacity and voltage degradation with cycling and poor high-rate capability.Nevertheless,it is still facing a great challenge to prepare Li-rich cathode materials with high specific capacity,high rate capability and reliable structural stability.On that account,in this paper we focus on the preparation of Li-rich manganese-based cathode materials based on carbonate co-precipitation method,and carry out their surface coating and modification.The influence of different coating amount and modification process on the morphology and structure of cathode materials was explored,and the electrochemical properties were investigated.The main contents are as follows:1.The Mn_0.48Ni_0.24Co_0.08（CO₃）_0.8 carbonate precursor was syntiesized by the carbonate co-precipitation method.In the water-ethylene glycol（H₂O-EG）system,NaH₂PO₄ was used to etch the surface of the carbonate precursor to form a coating layer of transition metal phosphates.Through the synchronous lithiation strategy,Li_1.2Mn_0.48Ni_0.24Co_0.08O₂ coated with transition metal lithium phosphates was prepared.The experimental results show that through a facile phosphate coating and synchronous lithiation strategy,the hetero-epitaxial nanostructure was engineered on the interface of Li-rich cathode materials,which makes the cathode mtaerials have excellent cycle stability and electrochemical performaces at high current densities.The modified samples achieve a low capacity fading of 6.7%after 140 cycles at 0.5 C,and large specific capacity of 209.4 m Ah/g after being cycled for 100 cycles at 1 C.And even at high current density of 10 C,the modified materials still have a specific capacity of 91.1 m Ah/g.The remarkable performance can be attributed to the fact that the epitaxial heterostructure provodes neatly arranged Li⁺ion pathways,combined with the fast ion conducting layer coated on the surface to improve the Li⁺ion conductivity effectively.At the same time,the coating layer on the surface can avoid the direct contact between the cathode materials and the electrolyte,and inhibit side reactions.Moreover,the oxygen vacancies generated from high-temperature calcination can effectively improve the reversibility of oxygen during the charge and discharge process,and suppress the initial irreversible capacity loss.Furthermore,the lithiated phosphates coating could suppress the residual alkali formation during the powder storage and cell fabrication process.2.Different amounts of NaH₂PO₂ were used to treat lithium-rich Li_1.2Ni_0.13Co_0.13Mn_0.54O₂cathode materials.The experimental results show that the material treated by 4 wt%NaH₂PO₂ exhibits excellent cycle stability with 14.7%of capacity fading after 150 cycles at 0.5 C,and high coulombic efficiency.The enhanced electrochemical performance is attributed to the following aspects.On one hand,NaH₂PO₂ pyrolysis produces reducing PH₃ gas which reacts with the active oxygen in the surface layer of the lithium-rich cathode materials to yield oxygen vacancy,thus effectively reducing the irreversible capacity loss in the first charge process.On the other hand,NaH₂PO₂ pyrolysis treatment induces Na⁺ion doping,which can not only effectively stabilize the lattice structure and inhibit phase transition,but also reduce the electrochemical impedance and accelerate Li⁺ion diffusion.In addition,the formed Na₄P₂O₇ coating can protect the electrode material form the electrolyte,and reduce the dissolution of the transition metals,thereby improving the cycle stability.