The technique assesses the macroscopic height change of electrodes upon electrochemical (de)lithiation. This allows us to verify the hypothesis of a nanoconfinement-induced cointercalation mechanism in HTO-PA. Electrochemical (de)lithiation of HTO and HTO-PA electrodes over five consecutive cycles at a rate of 50 mA g⁻¹ and the associated dilatometric responses are shown in Fig. . The significant dilatometric height change in HTO-PA is in line with previously identified cointercalation phenomena by ECD, such as the sodium-diglyme system in graphite, though smaller in absolute magnitude. This can be explained by the already pre-expanded structure of pillared HTO-PA that can more readily accommodate (partially) solvated ions. It underlines that nanoconfinement-design of host materials can address the issue of excessive volumetric change of cointercalation reactions that was identified, for example, in the graphite case. With strong indications of cointercalation phenomena in HTO-PA given from the perspective of the host material response, we now employ electrochemical quartz crystal microbalance (EQCM) measurements to directly probe the nature of the (co)intercalating species in HTO-PA. The method makes use of the linear relation between the change of resonance frequency of a gold-coated quartz sensor and its mass according to the Sauerbrey equation. The linearity between frequency and mass change is only valid for ideally rigid coatings on the sensor, which is why we employ a thin film coating of active material and binder by dropcasting. The dissipation of the crystal at different overtones was monitored in air, prior to cycling, to ensure the rigidity of the film. To verify the rigidity of the coating, values of changes in dissipation ΔD should be minimal and the frequency shift Δf should be independent of overtone n (Fig. ). We verify the rigidity of coatings in air (Fig. ) as well as during cycling in the electrochemical cell and in contact with organic electrolyte (Fig. ), where the condition of ΔD/Δf < 4*10 -7 Hz -1 is met indicating validity of the Sauerbrey equation electrochemical operation. This now allows us to investigate the mass change of the HTO-PA electrode during electrochemical intercalation via cyclic voltammetry at 0.5 mV/s (Fig. ). Plotting of the electrode mass change against the stored charge then allows for (semi-)quantitative analysis of the intercalant. Three distinct lithiation regimes (1-3) are detected (Fig. ). Initial lithiation occurs in the desolvated state (Mw,1=12 g/mol), coinciding with the initial shrinking detected by ECD. Subsequently, a drastic increase in the intercalant mass is detected (Mw,2=148 g/mol), indicating Li + intercalation together with ca. 1.5 solvent molecules. Finally, the mass is decreased when the electrode is almost fully reduced, i.e., the interlayer space is almost completely filled (Mw,3=68 g/mol). The similar mass of both solvents (EC and DMC) prevents us from directly identifying the involved solvent type. During delithiation, the reverse trend is observed. However, a significant hysteresis of the HTO-PA mass change of 25.6 µg cm -2 (i.e., the mass not returning to return to its initial value) indicates that either some solvent remains confined in the interlayer space after Li + deintercalation, or the accumulation of solvent molecules at the interface of the electrode . Overall, the semiquantitative EQCM investigation demonstrates a cointercalation mechanism of partially solvated Li + in HTO-PA (4 solvent molecules assumed for full solvation of Li + in EC/DMC mixture ). For a complete quantification of the chemical nature of the intercalant (precise solvation number and solvent type), further spectroscopic analysis will be required which is out of the scope of the present work.