Understanding the structure-property relationships and structural evolution under external stimuli is critical for developing high-performance lead-free antiferroelectric energy storage ceramics, while the underlying physics mechanism remains ambiguous. Here, the structural evolution, phonon behavior, structural heterogeneity, and optical transitions in $(1-x)\left(0.7{\mathrm{AgNbO}}_3\text{−}0.3{\mathrm{NaNbO}}_3\right)\text{−}x{\mathrm{Sr}}_{0.7}{\mathrm{Bi}}_{0.2}{\mathrm{TiO}}_3$ (AN-NN-SBT) energy storage ceramics through advanced spectroscopy techniques have been studied. Doping with ${\mathrm{Sr}}_{0.7}{\mathrm{Bi}}_{0.2}{\mathrm{TiO}}_3$ (SBT) induces lattice distortion via ionic radius mismatch, driving a phase transition from antiferroelectric to paraelectric states. The optical band gap ($E_g$) is broadened and the breakdown strength is enhanced, owing to the concomitant formation of Ti-O bonds, vacancy creation, and orbital hybridization. Furthermore, temperature-dependent Raman analyses reveal reversible phase transitions ($M\text{−}O\text{−}T$) governed by phonon merging, abnormal frequency shift, and soft mode suppression. Thermal modulation induces nonlinear $E_g$ narrowing, attributed to synergistic effects of lattice thermal expansion and electron-phonon interactions. Importantly, the phase coexistence and structure evolution process with temperature were discovered by in situ Raman mapping directly. A composition-critical temperature phase diagram was established, demonstrating that SBT doping reduces the structural transition temperatures and enables the $M_3/O$ phase boundary at room temperature in the AN-NN-SBT20 system, which in turn yields optimal energy storage performance. These findings elucidate the interplay between lattice symmetry, electronic properties, and energy storage performance, providing a strategic framework for designing high-performance lead-free energy storage ceramics.

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• Phase diagrams
• Phase transitions
• Ferroelectrics
• Ellipsometry
• Raman spectroscopy