An ultrasonic method for lithium-ion battery (LIB) state of charge (SoC) estimation is a promising emerging technology which may largely improve the SoC estimation accuracy. Previously, it was unknown whether the SoC change induced ultrasonic signal change originated from the anode or the cathode, because the thicknesses of cathodes, anodes and separators are much smaller than the ultrasonic wavelength, which makes it impossible to decouple the anodic and cathodic influence. To quantitatively solve the above problem, we have designed a special half-cell architecture with an extra-thick separator (675 μm) to study the reflected ultrasonic signal. The thickened separator would significantly delay the reflection of ultrasonic waves from the counter-electrode (Li), so that the influence of the working electrode (LiFePO₄ or graphite) on the ultrasonic wave can be studied separately. As a result, in the Gr anode, the time of flight (ToF) of the ultrasonic wave decreases with SoC, the changing rate coefficient of which is in the range of −110 to −70 ps μm_{Gr thickness}⁻¹, depending on the compact density. A lower compact density electrode leads to a more significant ultrasonic ToF decrease and intensity increase while in the LFP cathode, the ToF increases with SoC, the changing rate coefficient of which is in the range of 15–43 ps μm_{LFP thickness}⁻¹. The ToF change of the transmitted ultrasound through multilayered LIB matches very well with the sum of the ToF change in each electrode measured with our half-cells.