Studies On High-Capacity Layered Oxide Cathode Materials For Sodium-Ion Batteries With Oxygen Redox

Recently Na-ion batteries?NIBs?have attracted growing attention,particularly for large-scale energy-storage systems.NIBs have the same working principle as Li-ion batteries?LIBs?,called rocking chair batteries,but predicted to be of lower cost than LIBs due to the abundant reserves and wide distribution of sodium resources as well as utilization of low-cost Al foil as current collector for both cathode and anode.It is difficult to make accurate comparison between NIBs and LIBs even if LIBs have been commercialized for decades.At the present stage,to further reduce the cost per Ah of NIBs is a major challenge for its practical application.Anionic redox reaction?ARR?in lithium-and sodium-ion batteries is under hot discussion,mainly regarding oxygen anion participation principle and reversible extent of redox.Here,a P3-type Na_0.6[Li_0.2Mn_0.8]O₂ cathode material with reversible capacity from pure ARR was studied.The interlayer O-O distance?peroxo-like O-O dimer,2.506?3???,associated with oxidization of oxygen anions,was directly detected by using a neutron total scattering technique.For P3-type Na_0.6[Li_0.2Mn_0.8]O₂ with relatively weak Mn-O covalent bonding,crystal structure factors might play an even more important role in stabilizing the oxidized species,as both Li and Mn ions are immobile in the structure and so that the irreversible phase transpiration and oxygen evolution were inhibited.Layered metal oxides,as cathodes for Na-ion batteries?NIBs?,have attracted widespread attention because of synthesis easiness,high specific capacity,and high energy density.However,most reported layered oxides suffer from the complex phase transitions owing to a large amount of Na deintercalation.Here we designed a P2-type Na_0.72[Li_0.24Mn_0.76]O₂,exhibiting an exceptionally high initial charge capacity of 210mAh/g?correspongding to 0.72 Na?based on a pure ARR.Surprisingly,holonomic P2structure can be maintained with minimal volume change?1.35%?upon complete removal of Na⁺.This is due to the reduced Coulombic repulsion associated with ARR and consequent suppression of the phase transition as observed in other P2 materials.Here we the first time propose that ARR has the functionality of stabilizing the structure,in addition to the role in increasing its already known capacity.This would pave the way for the further improvement of high-energy-density NIBs.On the basis of Na_x[Li,Mn]O₂,two P2-type materials were designed by increasing the Na⁺content against the original material and introducing the transition metal redox Ni²⁺/Ni³⁺/Ni⁴⁺and Cu²⁺/Cu³⁺.There are two materials,P2-Na_0.9[Li_0.2Ni_0.15Mn_0.65]O₂and P2-Na_0.83[Li_0.17Cu_0.17Mn_0.66]O₂,which have both cation and anion redox processes.In addition to a certain degree of improvement in cycle performance and rate performance,it was found that the two materials can maintain P2 structure in the voltage range of 1.5-4.5 V.For P2-Na_0.83[Li_0.17Cu_0.17Mn_0.66]O₂,the volumetric strain of the structure during the ARR is much lower than it in the Cu²⁺/Cu³⁺redox process.Further,by comparison with materials that are not doped with Li⁺,it is Li⁺doping can achieve higher utilization of transition metal redox couples?close to 100%?in a narrower voltage range,which can be used as an effective strategy for further reduction of material cost.