| The growing demand for renewable energy storage is driving the active exploration of low-cost,environmentally friendly,safe and high-energy density Li-and Na-ion batteries.The recent boom in areas such as electric vehicles and large-scale energy storage has opened up new opportunities for the development of rechargeable ion batteries.As the important cathode materials for rechargeable ion batteries(lithium and sodium),layered oxides have undergone a development process from LiCoO2 to ternary(NMCs)and then to lithium-rich cathode materials with anionic redox.The layered structure as its intrinsic characteristic has been accompanied by this type of material.In this paper,two materials,LiNi0.8Mn0.1Co0.1O2(NMC811)and Na0.6Li0.2Mn0.8O2(NLMO),are selected as representatives of ternary and anionic redox layered materials.And the evolution of their layered structures during electrochemical processes and their influence on electrochemical properties are systematically investigated in combination with experiments and first-principles calculations,revealing that how the coupled interlayer interactions to stabilize the structure and promote electrochemical reversibility.Despite the high energy density of Ni-Rich layered-oxide electrodes,their realworld implementation in batteries is hindered by the substantial voltage decay on cycling,which mainly originates from bulk and surface structural degradation.In the first part of this paper,the voltage decay mechanism of Ni-Rich layered cathode(NMC811)is revealed through in-situ observation of cationic disorder phenomena.Viewed along the[110]and[110]directions by STEM,we demonstrate that transition metal migration causes drastic fluctuations in the interlayer spacing and Ni-O bond length,but has little effect on the atomic sites in ab plane.DFT calculations show that fluctuations in Ni-O bond length will trigger voltage decay by raising the energy level of antibonding(3dz2-2p)*orbital.Shorter Ni-O bonds cause a broadening of energy band to increase the voltage slope of the cell,reducing the available lithium capacity in the stable voltage range of the electrolyte,and leading to capacity decay.This chapter reveals the effect of interlayer coupling interactions caused by cation disorder on the thermodynamic equilibrium voltage,which is beneficial for the design and optimization of high-voltage electrode materials.Anionic redox cathode materials have high capacity and voltage due to the additional lattice oxygen redox(>4 V vs.Li+/Li or Na+/Na).However,the irreversibility of structural transformations or lattice oxygen losses during lattice oxygen redox(LOR)related processes leads to their poor cycling stability,including voltage hysteresis and voltage decay,which affects the widespread use of such materials.In the second part of this paper,the key role of topological protection in improving LOR reversibility is identified by comparing two closely related intercalated cathodes,namely P2-and P3-NLMO.P3-NLMO cathode exhibits good LOR reversibility in sodium half-cells,and provides high capacities of～240 mAh g-1 and excellent capacity retention in Li half-cells.Experimental and theoretical calculations show that the topological features of the-α-γ-stacks provide topological protection for the cyclic reversibility of LOR in P3-NLMO,but not in P2-NLMO.The topological order RT=[1 3 5…2q+1]is used to elucidate the origin of the protected topological states in P3-NLMO structure,distinguishing it from the traditional phase definitions(O-or P-type).The topological order,an interlayer coupling interaction revealed in this chapter,helps explore layered oxide structures compatible with reversible LOR,and facilitates the development of high-energy,low-cost,environmentally sustainable,and safe cathode materials. |
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