Mesoscopic Phase Transition Kinetics in Secondary Particles of Electrode-Active Materials in Lithium-Ion Batteries

Many compounds used as battery storage electrodes undergo large composition changes during use that are accompanied by a first-order phase transition. Most studies of these phase transitions have focused on the unit cell to single-crystallite scale, whereas real battery electrodes are typically composed of mesoscopic assemblies of nanocrystallites, for which phase transformation mechanisms are poorly understood. In this work, a systematic study is conducted of the potentiostatic (constant driving force) kinetics of phase transition in secondary particles of representative intercalation compounds: LiFePO₄, LiMn_1–<i>x</i>Fe_<i>x</i>PO₄, and Li₄Ti₅O₇. Storage kinetics are studied as a function of overpotential, material composition, primary particle size, and temperature. We find that in regimes where phase transformation occurs, the results can be self-consistently explained as nucleation and growth kinetics within the framework of the Johnson–Mehl–Avrami–Kolmogorov model. This implies that despite the common secondary particle topology, the electrochemically driven phase transformations occur by nucleation and growth with little apparent resistance to phase propagation across the grain boundaries. Growth appears to be one-dimensional in nature, consistent with a hybrid growth model in which rapid surface propagation is followed by slower growth into particles

Mesoscopic Phase Transition Kinetics in Secondary Particles of Electrode-Active Materials in Lithium-Ion Batteries

Many compounds used as battery storage electrodes undergo large composition changes during use that are accompanied by a first-order phase transition. Most studies of these phase transitions have focused on the unit cell to single-crystallite scale, whereas real battery electrodes are typically composed of mesoscopic assemblies of nanocrystallites, for which phase transformation mechanisms are poorly understood. In this work, a systematic study is conducted of the potentiostatic (constant driving force) kinetics of phase transition in secondary particles of representative intercalation compounds: LiFePO₄, LiMn_1–<i>x</i>Fe_<i>x</i>PO₄, and Li₄Ti₅O₇. Storage kinetics are studied as a function of overpotential, material composition, primary particle size, and temperature. We find that in regimes where phase transformation occurs, the results can be self-consistently explained as nucleation and growth kinetics within the framework of the Johnson–Mehl–Avrami–Kolmogorov model. This implies that despite the common secondary particle topology, the electrochemically driven phase transformations occur by nucleation and growth with little apparent resistance to phase propagation across the grain boundaries. Growth appears to be one-dimensional in nature, consistent with a hybrid growth model in which rapid surface propagation is followed by slower growth into particles

In Situ Observation of Random Solid Solution Zone in LiFePO₄ Electrode

Nanostructured LiFePO₄ (LFP) electrodes have attracted great interest in the Li-ion battery field. Recently there have been debates on the presence and role of metastable phases during lithiation/delithiation, originating from the apparent high rate capability of LFP batteries despite poor electronic/ionic conductivities of bulk LFP and FePO₄ (FP) phases. Here we report a potentiostatic in situ transmission electron microscopy (TEM) study of LFP electrode kinetics during delithiation. Using in situ high-resolution TEM, a Li-sublattice disordered solid solution zone (SSZ) is observed to form quickly and reach 10–25 nm × 20–40 nm in size, different from the sharp LFP|FP interface observed under other conditions. This 20 nm scale SSZ is quite stable and persists for hundreds of seconds at room temperature during our experiments. In contrast to the nanoscopically sharp LFP|FP interface, the wider SSZ seen here contains no dislocations, so reduced fatigue and enhanced cycle life can be expected along with enhanced rate capability. Our findings suggest that the disordered SSZ could dominate phase transformation behavior at nonequilibrium condition when high current/voltage is applied; for larger particles, the SSZ could still be important as it provides out-of-equilibrium but atomically wide avenues for Li⁺/e^– transport

Improving the Capacity of Sodium Ion Battery Using a Virus-Templated Nanostructured Composite Cathode

In this work we investigated an energy-efficient biotemplated route to synthesize nanostructured FePO₄ for sodium-based batteries. Self-assembled M13 viruses and single wall carbon nanotubes (SWCNTs) have been used as a template to grow amorphous FePO₄ nanoparticles at room temperature (the active composite is denoted as Bio-FePO₄-CNT) to enhance the electronic conductivity of the active material. Preliminary tests demonstrate a discharge capacity as high as 166 mAh/g at C/10 rate, corresponding to composition Na_0.9FePO₄, which along with higher C-rate tests show this material to have the highest capacity and power performance reported for amorphous FePO₄ electrodes to date

Azasilicon-bridged heterocyclic arylamines: syntheses, structures and photophysical properties

The lithium κ1-enamides, Me2NSiMe2CHC(Ph)-{2,6-(R1)2-4-(R2)C6H2}NLi·3THF (R1 = iPr, R2 = H L1; R1 = Et, R2 = H L2; R1 = Me, R2 = H L3; R1 = R2 = Me L4; R1 = Et, R2 = Me L5), in the presence of titanium tetrachloride, undergo intermolecular rearrangement cyclization reactions resulting in 1,3-migration of the silicon groups and the elimination of dimethylamine affording five examples of bis-azasilicon-bridged heterocyclic arylamines, [{2,6-(R1)2-4-(R2)C6H2}N(Ph)CCSiMe2]2 (R1 = iPr, R2 = H D1; R1 = Et, R2 = H D2; R1 = Me, R2 = H D3; R1 = R2 = Me D4; R1 = Et, R2 = Me D5) in good yield, respectively. The molecular structures of D1–D5 show the two fused N–Si–C–C–C rings to be co-planar indicative of extended π-conjugation, while their photophysical properties reveal them to be green/blue emitting with high luminescence quantum yields (ΦF range: 75–99%). Furthermore, the compounds D serve as versatile reactants undergoing ring opening on hydrolysis to afford the saturated 1,4-diimines [{2,6-(R1)2-4-(R2)C6H2}N(Ph)C-CH2]2 (R1 = iPr, R2 = H E1; R1 = Et, R2 = H E2; R1 = Me, R2 = H E3; R1 = R2 = Me E4; R1 = Et, R2 = Me E5). Alternatively, D can be employed in a redox-promoted cascade reaction to afford the conjugated 1,4-diimines, (E)-[{2,6-(R1)2-4-(R2)C6H2}N[double bond, length as m-dash]C(Ph)CH]2 (R1 = iPr, R2 = H F1; R1 = Et, R2 = H F2; R1 = Me, R2 = H F3; R1 = R2 = Me F4; R1 = Et, R2 = Me F5). In addition to D1–D5, E1–E3, E5, F2 and F3 have been the subject of single crystal X-ray diffraction studies

Engineering the Transformation Strain in LiMn_<i>y</i>Fe_1–<i>y</i>PO₄ Olivines for Ultrahigh Rate Battery Cathodes

Alkali ion intercalation compounds used as battery electrodes often exhibit first-order phase transitions during electrochemical cycling, accompanied by significant transformation strains. Despite ∼30 years of research into the behavior of such compounds, the relationship between transformation strain and electrode performance, especially the rate at which working ions (e.g., Li) can be intercalated and deintercalated, is still absent. In this work, we use the LiMn_<i>y</i>Fe_1–<i>y</i>PO₄ system for a systematic study, and measure using operando synchrotron radiation powder X-ray diffraction (SR-PXD) the dynamic strain behavior as a function of the Mn content (<i>y</i>) in powders of ∼50 nm average diameter. The dynamically produced strain deviates significantly from what is expected from the equilibrium phase diagrams and demonstrates metastability but nonetheless spans a wide range from 0 to 8 vol % with <i>y</i>. For the first time, we show that the discharge capacity at high C-rates (20–50C rate) varies in inverse proportion to the transformation strain, implying that engineering electrode materials for reduced strain can be used to maximize the power capability of batteries

Accommodating High Transformation Strains in Battery Electrodes via the Formation of Nanoscale Intermediate Phases: Operando Investigation of Olivine NaFePO₄

Virtually all intercalation compounds exhibit significant changes in unit cell volume as the working ion concentration varies. Na_<i>x</i>FePO₄ (0 < <i>x</i> < 1, NFP) olivine, of interest as a cathode for sodium-ion batteries, is a model for topotactic, high-strain systems as it exhibits one of the largest discontinuous volume changes (∼17% by volume) during its first-order transition between two otherwise isostructural phases. Using synchrotron radiation powder X-ray diffraction (PXD) and pair distribution function (PDF) analysis, we discover a new strain-accommodation mechanism wherein a third, amorphous phase forms to buffer the large lattice mismatch between primary phases. The amorphous phase has short-range order over ∼1nm domains that is characterized by <i>a</i> and <i>b</i> parameters matching one crystalline end-member phase and a <i>c</i> parameter matching the other, but is not detectable by powder diffraction alone. We suggest that this strain-accommodation mechanism may generally apply to systems with large transformation strains