In Operando X-ray Studies of High-Performance Lithium-Ion Storage in Keplerate-Type Polyoxometalate Anodes

Polyoxometalates (POMs) have emerged as potential anode materials for lithium-ion batteries (LIBs) owing to their ability to transfer multiple electrons. Although POM anode materials exhibit notable results in LIBs, their energy-storage mechanisms have not been well-investigated. Here, we utilize various in operando and ex situ techniques to verify the charge-storage mechanisms of a Keplerate-type POM Na₂K₂₃{[(Mo^VI)Mo^VI₅O₂₁(H₂O)₃(KSO₄)]₁₂ [(V^IVO)₃₀(H₂O)₂₀(SO₄)_0.5]}·ca200H₂O ({MoV}) anode in LIBs. The {MoV} anode provides a high reversible capacity of up to ∼1300 mA h g^-1 without capacity fading for up to 100 cycles. The lithium-ion storage mechanism was studied systematically through in operando synchrotron X-ray absorption near-edge structure, ex situ X-ray diffraction, ex situ extended X-ray absorption fine structure, ex situ transmission electron microscopy, in operando synchrotron transmission X-ray microscopy, and in operando Raman spectroscopy. Based on the abovementioned results, we propose that the open hollow-ball structure of the {MoV} molecular cluster serves as an electron/ion sponge that can store a large number of lithium ions and electrons reversibly via multiple and reversible redox reactions (Mo⁶⁺ ↔ Mo¹⁺ and V⁵⁺/V⁴⁺↔ V¹⁺) with fast lithium diffusion kinetics (D_Li: 10^-9-10^-10 cm² s^-1). No obvious volumetric expansion of the microsized {MoV} particle is observed during the lithiation/delithiation process, which leads to high cycling stability. This study provides comprehensive analytical methods for understanding the lithium-ion storage mechanism of such complicated POMs, which is important for further studies of POM electrodes in energy-storage applications.