(49c) Autocatalytic Reactions and Surface Diffusion Control Phase Separation in LiXFePO4

Many battery electrodes phase-separate upon lithium insertion and removal. Phase separation within electrodes arises when lithium ion/electron am bipolar pairs prefers to aggregate near other lithium ion/electron ambipolar pairs, often creating coexisting domains with different lithium stoichiometries within a single battery particle. Phase separation lead to chemo-mechanical stresses that may reduce cycleability. Li_XFePO₄ (0<X<1) is a model electrode that phase-separates into Li-rich and Li-poor domains under equilibrium. Under non-equilibrium conditions of lithium insertion and removal, we show the crucial role that surface reaction and surface diffusion play in phase separation kinetics, and how these parameters can be used to altogether suppress phase separation.

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 Li_XFePO₄ 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 Li_XFePO_4.