Suppression of Phase Separation in LiFePO4 Nanoparticles During Battery Discharge

Using a novel electrochemical phase-field model, we question the common belief that LixFePO4 nanoparticles separate into Li-rich and Li-poor phases during battery discharge. For small currents, spinodal decomposition or nucleation leads to moving phase boundaries. Above a critical current density (in the Tafel regime), the spinodal disappears, and particles fill homogeneously, which may explain the superior rate capability and long cycle life of nano-LiFePO4 cathodes.Comment: 27 pages, 8 figure

Size-dependent spinodal and miscibility gaps for intercalation in nano-particles

Using a recently-proposed mathematical model for intercalation dynamics in phase-separating materials [Singh, Ceder, Bazant, Electrochimica Acta 53, 7599 (2008)], we show that the spinodal and miscibility gaps generally shrink as the host particle size decreases to the nano-scale. Our work is motivated by recent experiments on the high-rate Li-ion battery material LiFePO4; this serves as the basis for our examples, but our analysis and conclusions apply to any intercalation material. We describe two general mechanisms for the suppression of phase separation in nano-particles: (i) a classical bulk effect, predicted by the Cahn-Hilliard equation, in which the diffuse phase boundary becomes confined by the particle geometry; and (ii) a novel surface effect, predicted by chemical-potential-dependent reaction kinetics, in which insertion/extraction reactions stabilize composition gradients near surfaces in equilibrium with the local environment. Composition-dependent surface energy and (especially) elastic strain can contribute to these effects but are not required to predict decreased spinodal and miscibility gaps at the nano-scale

Direct observation of active material concentration gradients and crystallinity breakdown in LiFePO4 electrodes during charge/discharge cycling of lithium batteries

The phase changes that occur during discharge of an electrode comprised of LiFePO4, carbon, and PTFE binder have been studied in lithium half cells by using X-ray diffraction measurements in reflection geometry. Differences in the state of charge between the front and the back of LiFePO4 electrodes have been visualized. By modifying the X-ray incident angle the depth of penetration of the X-ray beam into the electrode was altered, allowing for the examination of any concentration gradients that were present within the electrode. At high rates of discharge the electrode side facing the current collector underwent limited lithium insertion while the electrode as a whole underwent greater than 50% of discharge. This behavior is consistent with depletion at high rate of the lithium content of the electrolyte contained in the electrode pores. Increases in the diffraction peak widths indicated a breakdown of crystallinity within the active material during cycling even during the relatively short duration of these experiments, which can also be linked to cycling at high rate

Theory of Chemical Kinetics and Charge Transfer based on Nonequilibrium Thermodynamics

Classical theories of chemical kinetics assume independent reactions in dilute solutions, whose rates are determined by mean concentrations. In condensed matter, strong interactions alter chemical activities and create inhomogeneities that can dramatically affect the reaction rate. The extreme case is that of a reaction coupled to a phase transformation, whose kinetics must depend on the order parameter -- and its gradients, at phase boundaries. This Account presents a general theory of chemical kinetics based on nonequilibrium thermodynamics. The reaction rate is a nonlinear function of the thermodynamic driving force (free energy of reaction) expressed in terms of variational chemical potentials. The Cahn-Hilliard and Allen-Cahn equations are unified and extended via a master equation for non-equilibrium chemical thermodynamics. For electrochemistry, both Marcus and Butler-Volmer kinetics are generalized for concentrated solutions and ionic solids. The theory is applied to intercalation dynamics in the phase separating Li-ion battery material Li$_x$FePO$_4$.Comment: research account, 17 two-column pages, 12 figs, 78 refs - some typos corrected Accounts of Chemical Research (2013

Effects of Elastic Strain Energy on Phase Boundary Morphology in Nanoscale Olivine Particles

Many lithium insertion compounds undergo considerable volume expansion upon Li insertion. One important family of cathode materials, LiMPO 4 (M = Fe, Mn, Ni, Co) olivines, exhibits 6% -11% volume difference between the Li-rich and Li-poor olivine phases. While such a large volume change usually results in fracture in bulk electrodes, defect generation appears to be significantly suppressed in nanoscale electrode particles We have previously developed a phase-field model for predicting phase transition pathways in nanoscale olivine electrodes where M Li is the lithium mobility tensor, with its most significant component along the [010] direction where the Li electrochemical potential in the electrolyt

A bifunctional auxiliary electrode for safe lithium metal batteries

© 2019 The Royal Society of Chemistry.Increasing demands for performance beyond the limit of current lithium ion batteries for higher energy densities have rejuvenated research using lithium metal as an anode. However, commercial implementation has still been hampered due to safety issues. Herein, we introduce a lithium rechargeable battery system with an auxiliary electrode that can detect the potential signs of an internal short-circuit and simultaneously prevent cell failure by inhibiting further dendritic growth of lithium metal. Based on this working principle, we provide guidelines for an auxiliary electrode design and demonstrate that it can act as both a safety sensor and a lithium scavenger. Finally, we show that our in-house designed cell, using a flexible and self-standing auxiliary electrode, can effectively alert the danger of a short circuit in real-time without additional dendrite growth. We expect that this finding will open up unexplored opportunities utilizing various auxiliary electrode chemistries for safe rechargeable lithium metal batteries11sciescopu