Single-Particle Performances and Properties of LiFePO4 Nanocrystals for Li-Ion Batteries

DOI: 10.1002/aenm.201601894 their redox potential difference during the delithiation,[2] which primarily originates from the dependence on thermodynamic properties on particle size.[3] The behavior of pressing the active material into thick slices or assembling to full battery can also lead to the coupling of stress with lithium chemical potential during the lithiation or delithiation process and introduce the concentration polarization, electrochemical polarization, and other internal interference within the electrode.[2,4] As a result, the obtained data based on thick battery electrode mainly reflect the properties of the collective particles with multipolarization effects. Recently, we employed 3D printing to fabricate thin LiMn0.21Fe0.79PO4 LIB electrodes, which show impressive electrochemical performance: a capacity of 108.45 mA h g−1 at 100 C and a reversible capacity of 150.21 mA h g−1 at 10 C after 1000 cycles.[5] We then demonstrated that except for the bulk Li-ion diffusion in cathode nanoparticles and particle interfacial reaction, the solution intrinsic diffusion coefficient, efficiency porosity, and electrode thickness could also dominate high rate performance of the cathode. However, the ultrathin electrodes are still composed of assembled LiMn0.21Fe0.79PO4 nanocrystals and cannot reflect the intrinsic properties of a single nanoparticle, which would share deep insight to how to further improve the performance of electrode materials. Thus, to get the intrinsic properties of single nanocrystallites becomes our target in this work. New testing method on this subject should be developed first. Recent progresses about this have been made to study the intercalation mechanism of LiFePO4 nanocrystallites. Chueh et al. imaged ≈450 individual LiFePO4 particles to confirm the particle-by-particle pathway during intercalation.[6] Brunetti et al. used precession electron diffraction to obtain LiFePO4 and FePO4 phase mapping at the scale of a particle and proved the domino-cascade model at the nanoscale level.[7] Li et al. track the migration of Li in LiFePO4 electrodes with single particle sensitivity by using operando fluorescence-yield X-ray microscopy platform.[8] Using transmission electron microscopy and by the new electron forward scattering diffraction technique, Robert et al. unambiguously shows that the small particles delithiate first by the statistical analysis of 64000 LiFePO4 particles.[9] Using It has been recently reported that the solution diffusion, efficiency porosity, and electrode thickness can dominate the high rate performance in the 3D-printed and traditional LiMn0.21Fe0.79PO4 electrodes for Li-ions batteries. Here, the intrinsic properties and performances of the single-particle (SP) of LiFePO4 are investigated by developing the SP electrode and creating the SPmodel, which will share deep insight on how to further improve the performance of the electrode and related materials. The SP electrode is generated by fully scattering and distributing LiFePO4 nanoparticles to contact with the conductive network of carbon nanotube or conductive carbon to demonstrate the sharpest cyclic voltammetry peak and related SP-model is developed, by which it is found that the interfacial rate constant in aqueous electrolyte is one order of magnitude higher, accounting for the excellent rate performance in aqueous electrolyte for LiFePO4. For the first time it has been proposed that the insight of pre-exponential factor of interface kinetic Arrhenius equation is related to desolvation/solvation process. Thus, this much higher interfacial rate constant in aqueous electrolyte shall be attributed to the much larger pre-exponential factor of interface kinetic Arrhenius equation, because the desolvation process is much easier for Li-ions jumping from aqueous electrolyte to the Janus solid–liquid interface of LiFePO4.