Microstructure of LiCoO₂ with and without “AlPO₄” Nanoparticle Coating:  Combined STEM and XPS Studies

Abstract

AlPO 4 "-coated LiCoO 2 was shown to exhibit markedly improved capacity retention relative to bare LiCoO 2 upon cycling to 4.7 V. Scanning and transmission electron microscopy imaging showed that the coating thickness of "AlPO 4 "-coated LiCoO 2 varied from ∼10 to ∼100 nm. Energy-dispersive X-ray mapping revealed that the coating was not single-phase "AlPO 4 ", rather consisting of P-rich thick regions (∼100 nm) and Al-rich thin regions (∼10 nm). Detailed X-ray photoelectron spectroscopy (XPS) studies of the "AlPO 4 "-coated LiCoO 2 in comparison to bare LiCoO 2 and various reference compounds such as Li 2 CO 3 , Li 3 PO 4 , and AlPO 4 indicate that (1) AlPO 4 is absent on the surface; (2) the surface consisted of Li 3 PO 4 and heavily Al substituted LiAl y Co 1-y O 2 , which may result from AlPO 4 nanoparticles reacting with bare LiCoO 2 during the coating heat treatment at 700°C; and (3) the amount of surface Li 2 CO 3 is markedly reduced in the coated sample relative to the bare LiCoO 2 . The existence of Li 3 PO 4 in "AlPO 4 "-coated LiCoO 2 was confirmed with X-ray powder diffraction. The coating microstructure of "AlPO 4 "-coated LiCoO 2 is proposed, and the mechanisms of enhancement in the cycling and thermal characteristics by particle surface microstructure are discussed in detail.

XPS Investigation of the Electrolyte Induced Stabilization of LiCoO2and “AlPO4”-Coated LiCoO2Composite Electrodes

“…27,28 The lattice oxygen at ≈529.5 eV, the surface oxygen species O-C = O * and Li 2 CO 3 collectively at ≈532.1 eV, the O * P(OR) 3 and O * -C = O species at ≈533.5 eV and Li x PF y O z at ≈534.6 eV are identified by the dashed lines (Figure 3).…”

“…However, this benefit can be temporary as recent work indicates that the Li-Al-Co-O surface region is consumed during cycling. 27 The "AlPO 4 "-coating on LiCoO 2 can induce the formation of Al-rich regions (LiAl y Co 1-y O 2 ) and P-rich regions (Li + conducting Li 3 PO 4 ), 28 and that Co-and Alcontaining oxyfluoride species and species like PF x (OH) y developed on the surface during cycling can lower the rate of Co dissolution and deposition of Co-containing species on the negative electrode and further degradation of LiPF 6 . 27 For a more detailed discussion of the * Electrochemical Society Active Member.…”

“…However, this benefit can be temporary as recent work indicates that the Li-Al-Co-O surface region is consumed during cycling. 27 The "AlPO 4 "-coating on LiCoO 2 can induce the formation of Al-rich regions (LiAl y Co 1-y O 2 ) and P-rich regions (Li + conducting Li 3 PO 4 ), 28 and that Co-and Alcontaining oxyfluoride species and species like PF x (OH) y developed on the surface during cycling can lower the rate of Co dissolution and deposition of Co-containing species on the negative electrode and further degradation of LiPF 6 . 27 For a more detailed discussion of the metal fluoride formation mechanism, the reader is directed to Figure 12 and the corresponding discussion in the report by Lu et al 27 In this paper, we examine the influence of the fluorine source on the formation of metal fluorides and the efficacy of the surface coating by relating cycling performance to the surface chemistries of bare and "AlPO 4 "-coated LiCoO 2 electrodes cycled in 1 M LiPF 6 or LiClO 4 in EC:DMC (1:1) to 4.6 V. For a more detailed discussion of the metal fluoride formation mechanism, the reader is directed to Figure 12 and the corresponding discussion on page 4421 (second column) of Lu et al 27 The LiClO 4 salt was used to help determine if the formation of the metal fluorides was a product of the interaction between the active material with the LiPF 6 electrolyte and/or the PVDF binder.…”

Aluminum-based materials for advanced battery systems

“…Cho et al [86][87][88][89] pointed out that the cycling and thermal stability of cathode materials can be enhanced by surface modification with AlPO4· (PO4) 3− polyanions and Al 3+ with high electronegativity, which resist the side reaction with the electrolyte, and oxides with (PO4) 3− bonding are thermally stable, improving the cycling performance [88,90]. Besides, AlPO4 coating acts as a protective layer, reducing the surface exposure of cathode in the electrolyte, thus remitting metal dissolution and reducing oxygen generation [87,89,[91][92][93]. The application of AlPO4 coating involves various kinds of cathode materials, including LiCoO2, LiNi0.8Co0.1Mn0.1O2, LiMn1.5Ni0.5O4 (Fig.…”

Enhanced Overdischarge Stability of LiCoO2by a Solution Deposited AlPO4Coating

“…[21][22][23][24][25] In particular several studies have shown that AlPO 4 coatings of LiCoO 2 particles stabilizes the LiCoO 2 against charge to high potentials >4.2 V vs. Li/Li + . [26][27][28][29][30][31][32][33][34] Such stabilizing effects may extend to over-insertion of lithium into the cathode and thus an AlPO 4 coating on the surface of LiCoO 2 could have a dual utility of enhancing z E-mail: brian.landi@rit.edu overdischarge and overcharge tolerance of advanced lithium ion cells with added reversible lithium.In the present work, a solution deposited coating of AlPO 4 onto LiCoO 2 particles is tested for its utility in improving the tolerance of LiCoO 2 to over-insertion of lithium at potentials <3.0 V vs. Li/Li + . Half-cell testing in coin cells is used to study the effect of an AlPO 4 coating to stabilize a LiCoO 2 cathode discharge performance against a 5% (of the 140 mAh/g nominal capacity of LiCoO 2 ) over-insertion of lithium by fixed resistive load.…”