This proceedings-perspective paper introduces a new publication format that connects the immediacy of conference reporting with the depth of expert analysis. It summarizes contributions and discussions from the E -MRS 2025 Spring Meeting Symposium K on “ Solid State Ionics: Functional Materials and Devices for Electrochemical Energy Conversion and Storage Applications” , highlighting recent advances and open challenges in the field. Three major research directions are outlined: First, the deliberate control of material defects is shown to govern ionic and electronic transport as well as stability in functional oxides. Second, tailored surface modifications emerge as a powerful tool to tailor oxygen exchange and catalytic activity of mixed conducting electrodes. Third, exsolution of metal particles from oxide hosts is presented as a versatile route to design durable and adaptive catalysts. Together, these developments illustrate how coupling defect control, surface engineering, and exsolution enables next-generation materials for electrochemical energy conversion and storage. • The novel format of this paper bridges conference reports and expert perspectives. • Controlling material defects tunes how ions and electrons move in oxides. • Surface modifications reshape how oxides react with their surrounding atmosphere. • Metal particles that are exsolved from oxides make tougher and smarter catalysts.

The demand for high-energy and high-power energy storage devices motivates the search for electrode materials with both high capacity and fast ion transport. One class of materials that could achieve such performance are oxides containing crystallographic shear (CS) planes. Here, we compare the structural dynamics of tungsten trioxide (WO 3 ) and its oxygen deficient, CS Magnéli phase (WO 2.9 ) during electrochemical insertion of H + and Li + ions using a combined experimental and computational study. We found that WO 3 inserts more H + per formula unit than WO 2.9 yet operando electrochemical atomic force microscopy shows more deformation in WO 2.9 than WO 3 per inserted H + . In contrast, WO 2.9 accommodates ∼0.2 more Li + per formula unit than WO 3 and has higher Li + diffusion and better rate capability. Operando electrochemical X-ray diffraction shows that Li + insertion into WO 2.9 leads to lattice contraction and 5 % volume change up to Li 0.6 WO 2.9 followed by a zero-strain region up to Li 1.4 WO 2.9 . We find that the presence of CS planes, and its effect on octahedral tilting, lead to different outcomes depending on the inserting ion: while octahedral tilting and lack of CS planes promote H + insertion into WO 3 , their absence in WO 2.9 favor Li + insertion. We propose that the presence of CS planes impart structural rigidity, enabling higher capacity, improved rate capability, and enhanced cyclability during Li + insertion but remove favorable bridging oxygen sites for H + insertion. • H + insertion is suppressed in WO 2.9 due to the absence of octahedral tilting and bridging oxygen sites. • Li + insertion is enhanced in WO 2.9 relative to WO 3. • DFT calculations reveal a wider range of energetically favorable Li + insertion sites in WO 2.9 than WO 3 . • Operando EC-AFM reveals that WO 2.9 undergoes greater deformation than WO 3 during H + insertion. • Operando EC-XRD shows fewer phase transitions in WO 2.9 during Li + insertion.