First Atomic-Scale Insight into Degradation in Lithium Iron Phosphate Cathodes by Transmission Electron Microscopy

Abstract

The capacity-voltage fade phenomenon in lithium iron phosphate (LiFePO4) lithium ion battery cathodes is not understood. We provide its first atomic-scale description, employing advanced transmission electron microscopy combined with electroanalysis and first-principles simulations. Cycling causes near-surface (∼30 nm) amorphization of the Olivine crystal structure, with isolated amorphous regions also being present deeper in the bulk crystal. Within this amorphous shell, some of the Fe2+ is transformed into Fe3+. Simulations predict that amorphization significantly impedes ion diffusion in LiFePO4 and even more severely in FePO4. The most significant barrier for ion transfer will be in the partially delithiated state due to the presence of FePO4, resulting in the inability to extract the remaining Li+ and the observed capacity fade. The pyrrole coating suppresses the dissolution of Fe and allows for extended retention of the Olivine structure. It also reduces the level of crossover of iron to the metal anode and stabilizes its solid electrolyte interphase, thus also contributing to the half-cell cycling stability.

Synergy and Symbiosis Analysis of Capacity-Contributing Polypyrrole and Carbon-Coated Lithium Iron Phosphate Nanostructures for High-Performance Cathode Materials

“…(C-LFP) 1– x (PPy) x composites deliver charge/discharge capacities of 176.2/127.2, 114.0/126.8, 206.2/162.3, 212.6/209.1, 163.2/137.7, and 65.2/36.2 mAh g –1 , corresponding to the x varying from 0, 0.07, 0.16, 0.25, and 0.30 to 1.0 at 0.1C, respectively (Figure S13). It is amazing that (C-LFP) 0.75 (PPy) 0.25 shows an ultrahigh discharge capacity of 209.1 mAh g –1 at 0.1C, superior to that of most recently reported results. − This is because the (C-LFP) 0.75 (PPy) 0.25 composite possesses more uniform and smaller particle size as well as larger specific surface area. Besides, PPy components in (C-LFP) 0.75 (PPy) 0.25 expand the crystal plane spacing of the LiFePO 4 crystal.…”

“…Clearly, the synergy effect of PPy and C-LiFePO 4 significantly improves the rate capability of (C-LFP) 0.75 (PPy) 0.25 (Figure c), especially for reversible capacity from 0.1 to 0.5C, which is superior or comparable to those reported for the LiFePO 4 composite cathodes modified with PPy (Table S3). ,,,− …”

Development of a New Crosslinked Highly Conductive Hybrid Electrolyte based on Polyetherdiamine, Diphenylmethane Diisocyanate and Organosilanes for Efficient Lithium‐Metal Battery

“…Two satellite peaks are also observed at 729.6 eV for Fe 2p 3/2 and 716.4 eV for Fe 2p 1/2 . The characteristic peaks suggest the presence of Fe 2+ oxidation state in the cycled LiFePO 4 cathode [61] . The deconvoluted Li 1s spectrum displays three peaks at 54.6, 56.5 and 58.8 eV, corresponding to ROLi, Li 2 O 3 and Li x PO y F z , indicating the formation of these species on the surface of LiFePO 4 [62] .…”

“…The characteristic peaks suggest the presence of Fe 2 + oxidation state in the cycled LiFePO 4 cathode. [61] The deconvoluted Li 1s spectrum displays three peaks at 54.6, 56.5 and 58.8 eV, corresponding to ROLi, Li 2 O 3 and Li x PO y F z , indicating the formation of these species on the surface of LiFePO 4 . [62] The deconvoluted P 2p spectrum shows two peaks at 132.8 eV and 133.6 eV for P 2p 3/2 and P 2p 1/2 , respectively, which divulges 5 + valence state of P. [63] The high resolution O 1s spectrum depicts three deconvoluted peaks around 530.9, 531.9 and 534 eV for PO 4 3À , Li 2 CO 3 and Li x PO y F z , respectively.…”

Atomic Rearrangement and Amorphization Induced by Carbon Dioxide in Two-Dimensional MoO3–x Nanomaterials

“…Amorphization is the transition process from crystal to amorphous phase, which significantly affects the mechanical, , electrical, thermal, and optical performance of materials. , The amorphization phenomena have been found in most matter with various amorphization methods, such as high pressure, − irradiation, − thermal effect, , doping, plastic deformation, , electrochemistry, − crystallization inhibition, − nanoindentation, , and so on. A series of mechanisms have been proposed to help explain and understand amorphization and are summarized as follows: High-pressure-induced amorphization is believed to be a special kind of melting due to the overlap of the pressure-induced amorphization and temperature-induced melting. − Deformation-induced amorphization is considered to be induced by dynamic lattice instability led by shear and stress. − Irradiation-induced amorphization originates from the accumulation of defects and vacancies. − …”