  We investigate reconfigurable intelligent surface (RIS)-assisted simultaneous
wireless information and power transfer (SWIPT) Internet of Things (IoT)
networks, where energy-limited IoT devices are overlaid with cellular
information users (IUs). IoT devices are wirelessly powered by a RIS-assisted
massive multiple-input multiple-output (MIMO) base station (BS), which is
simultaneously serving a group of IUs. By leveraging a two-timescale
transmission scheme, precoding at the BS is developed based on the
instantaneous channel state information (CSI), while the passive beamforming at
the RIS is adapted to the slowly-changing statistical CSI. We derive
closed-form expressions for the achievable spectral efficiency of the IUs and
average harvested energy at the IoT devices, taking the channel estimation
errors and pilot contamination into account. Then, a non-convex max-min
fairness optimization problem is formulated subject to the power budget at the
BS and individual quality of service requirements of IUs, where the transmit
power levels at the BS and passive RIS reflection coefficients are jointly
optimized. Our simulation results show that the average harvested energy at the
IoT devices can be improved by $132\%$ with the proposed resource allocation
algorithm. Interestingly, IoT devices benefit from the pilot contamination,
leading to a potential doubling of the harvested energy in certain network
configurations.
