Magnetic Resonance Study On The Charge/Discharge Mechanism Of Cathode Materials For Sodium Ion Batteries

With the tremendous development of electric vehicles and stationary energy storage,there is a need for a sustainable supply of energy to meet the future demand of the market.Due to the abundant sodium reserves on the earth and similar chemical properties to lithium-ion batteries,sodium-ion batteries have become the promising candidates for lithium-ion batteries that are currently commercialized.The cathode material is the decisive factor affecting the actual performance of sodium-ion battery.In this paper,Na₃V₂（PO₄）₃and Na_0.63Li_0.21Mn_0.79O₂with high specific capacity and simple preparation methods were selected as the research objects.Magnetic resonance technology including nuclear magnetic resonance（NMR）and electron paramagnetic resonance（EPR）,combined with other characterization methods were used to deeply understand the structural-performance relationship and electrochemical reaction mechanism of these two materials.This paper provides a basis for the comprehensive theoretical and experimental guidance as well as subsequent continuous improvement of the batteries.The main work of this paper is as follows:（1）Na₃V₂（PO₄）₃is a widely studied NASICON-type phosphate cathode material.We synthesized a series of Na₃V₂P_3-xB_xO₁₂（0≤x≤0.5）by sol-gel and followed by solid-state reaction.The reaction mechanism of Na₃V₂（PO₄）₃in the whole potential range of charge/discharge（1.0-3.8 V）is elaborately investigated by the combination of ²³Na/³¹P magic angle spinning NMR（MAS NMR）and cryogenic-temperature EPR for the first time.EPR measurement under 1.8 K manifests the generation of V²⁺with rhombohedral distortion upon the fourth Na⁺intercalation process of Na₃V₂（PO₄）₃.Besides,this study pinpoints the profound impact of polyanion site substitution to the local structural transformation of Na₃V₂（PO₄）₃upon Na⁺（de）intercalation,which corroborates that the boron substitution into phosphorus site can broaden the range of solid-solution reaction,accelerate the structural transition toward V²⁺-containing phase,and refrain the short scale heterogeneity of P and Na nuclei.（2）Irreversible anion redox in sodium layered cathode materials usually leads to structural deformation and poor capacity retention during cycling.The degradation mechanism of high voltage Na_0.63Li_0.21Mn_0.79O₂was revealed by magnetic resonance technology,involving the formation of molecular O₂trapped inside the bulk structure of the charged material.We then propose a facile Cu doping strategy to relieve the irreversible oxygen redox.We synthesized the Na_0.63Li_0.21Mn_0.79O₂and Cu-doped Na_0.73Li_0.21Mn_0.74Cu_0.05O₂by a simple high-temperature solid-state synthesis method.The results of NMR and EPR show that on the basis of comprehensive comparison with Cu-free materials,Na_0.73Li_0.21Mn_0.74Cu_0.05O₂effectively inhibits irreversible O₂loss and Mn-related Jahn-Teller distortion by simultaneously inhibiting oxygen activity and Mn⁴⁺/Mn³⁺redox reactions.