  We present a physics-based model that accurately predicts the performance of
Medtronic's implantable medical device battery lithium/carbon monofluoride
(CF$_x$) - silver vanadium oxide (SVO) under both low-rate background
monitoring and high-rate pulsing currents. The distinct properties of multiple
active materials are reflected by parameterizing their thermodynamics,
kinetics, and mass transport properties separately. Diffusion limitations of
Li$^+$ in SVO are used to explain cell voltage transient behavior during pulse
and post-pulse relaxation. We also introduce change in cathode electronic
conductivity, Li metal anode surface morphology, and film resistance buildup to
capture evolution of cell internal resistance throughout multi-year electrical
tests. We share our insights on how the Li$^+$ redistribution process between
active materials can restore pulse capability of the hybrid electrode, allow
CF$_x$ to indirectly contribute to capacity release during pulsing, and affect
the operation protocols and design principles of batteries with other hybrid
electrodes. We also discuss additional complexities in porous electrode model
parameterization and electrochemical characterization techniques due to
parallel reactions and solid diffusion pathways across active materials. We
hope our models implemented in the Hybrid Multiphase Porous Electrode Theory
(Hybrid-MPET) framework can complement future experimental research and
accelerate development of multi-active material electrodes with targeted
performance.
