(49c) Autocatalytic Reactions and Surface Diffusion Control Phase Separation in LiXFePO4
AIChE Annual Meeting
Sunday, October 28, 2018 - 4:00pm to 4:15pm
We developed synchrotron operando X-ray microscopy to track lithium insertion and migration within individual particles at ~50 nm spatial resolution and ~ 30 sec temporal resolution. We directly map the surface reaction kinetics as a function of the lithium stoichiometry as the battery particles charge and discharge. Our results show that lithium extraction (charge) is auto-catalytic while lithium insertion (discharge) is auto-inhibitory. The auto-catalytic charge amplifies the intrinsic tendency of LiXFePO4 to phase-separate, while auto-inhibitory discharge suppresses phase separation. When auto-inhibitory behavior is sufficiently strong at elevated rates of lithium insertion, separation can be completely suppressed and replaced with a solid solution. This shows how how surface reaction can be used to control a bulk material property.
For auto-catalytic reactions and for insufficiently auto-inhibitory reactions, phase separation can also be suppressed by reducing the ambipolar transport of lithium ion/electron pairs. Using both experiment and phase-field modeling, we show that the effective Damkohler number in this solid electrode is a crucial parameter at determining whether or not a particle phase-separates. When lithium diffusion is faster than lithium insertion, then the particles phase-separate. When lithium reaction is faster than diffusion, then the solid solution is kinetically stabilized because the particle does not have time to phase-separate before it finishes (de)lithiation. Experimental results show that this ambipolar transport is dominated by lithium surface diffusion at the particle/electrolyte interface. Thus, reducing surface diffusion and ambipolar transport provides another route towards controlling phase separation in LiXFePO4.