  Spin-orbitronic devices can integrate memory and logic by exploiting
spin-charge interconversion (SCI), which is optimized by design and materials
selection. In these devices, such as the magnetoelectric spin-orbit (MESO)
logic, interfaces are crucial elements as they can prohibit or promote spin
flow in a device as well as possess spin-orbit coupling resulting in
interfacial SCI. Here, we study the origin of SCI in a Py/Cu/W lateral spin
valve and quantify its efficiency. An exhaustive characterization of the
interface between Cu and W electrodes uncovers the presence of an oxidized
layer (WO$_x$). We determine that the SCI occurs at the Cu/WO$_x$ interface
with a temperature-independent interfacial spin-loss conductance of $G_{||}
\approx$ 20 $\times$ 10$^{13} \Omega^{-1}m^{-2}$ and an interfacial spin-charge
conductivity $\sigma_{SC}=-$1610 $\Omega^{-1}cm^{-1}$ at 10 K ($-$830
$\Omega^{-1}cm^{-1}$ at 300 K). This corresponds to an efficiency given by the
inverse Edelstein length $\lambda_{IEE}=-$0.76 nm at 10 K ($-$0.4 nm at 300 K),
which is remarkably larger than in metal/metal and metal/oxide interfaces and
bulk heavy metals. The large SCI efficiency at such an oxidized interface is a
promising candidate for the magnetic readout in MESO logic devices.
