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Substitution of liquid electrolyte with solid‐state electrolytes (SSEs) has emerged as a very urgent and challenging research area of rechargeable batteries. NASICON (Na3Zr2Si2PO12) is one of the most potential SSEs for Na‐ion batteries due to its high ionic conductivity and low thermal expansion. It is proven that the ionic conductivity of NASICON can be improved to 10−3 S cm−1 by Sc‐doping, of which the mechanism, however, has not been fully understood. Herein, a series of Na3+xScxZr2−xSi2PO12 (0 ≤x ≤ 0.5) SSEs are prepared. To gain a deep insight into the ion transportation mechanism, synchrotron‐based X‐ray absorption spectroscopy (XAS) is employed to characterize the electronic structure, and solid‐state nuclear magnetic resonance (SS‐NMR) is used to analyze the dynamics. In this study, Sc is successfully doped into Na3Zr2Si2PO12 to substitute Zr atoms. The redistribution of sodium ions at certain specific sites is proven to be critical for sodium ion movement. For x≤ 0.3, the promotion of sodium ion movement is attributed to sodium ion concentration increase at the Na2 sites and decrease at the Na1 and Na3 sites. For x> 0.3, the inhibition of sodium ion movement is due to the phase change from monoclinic to rhombohedral and an increasing impurity content. [ABSTRACT FROM AUTHOR]