Wu Zhongshuai of Dalian Institute of Chemical Technology & Pan Feng of Peking University & Chen Chengmeng EES of Shanxi Institute of Coal Chemical Technology: Gradient Fluorination Strategy of Table Interface Significantly Improve Fast Charge and Long Life Performance of 4.6V High Pressure LiCoO2

research background]
LiCoO2(LCO) with high volumetric energy density is the main cathode material for lithium-ion batteries in consumer electronics. With the increase of consumer demand, higher requirements are put forward for energy density. Therefore, under the premise of meeting safety, environmental protection and long cycle, the new generation of lithium-ion batteries needs to have excellent fast charging characteristics and higher energy density. It is found that increasing the LCO charge cut-off voltage is a direct and effective way to increase the energy density of LCO batteries. For example, 4.6 V-LCO can achieve a discharge specific capacity of about 220mAh g-1, which is significantly improved compared with the 170 mAh g-1 of commercial 4.4 V-LCO. However, the practical application of high-voltage fast charging is hindered by the unstable interface structure and the unfavorable phase transition caused by the superimposed high-flux Li diffusion during the deep delithiation of LCO (≥ 4.6V). In essence, the instability and decay of the interface structure of high-pressure LCO originate from the fact that near-surface lattice oxygen (O2-) undergoes oxygen redox (O2-→On-,n < 2) with deep delithiation to participate in charge compensation. This process not only contributes capacity, but O2-is also easily oxidized by Co4 to peroxide ions (O-) with high mobility and high activity, it will further evolve into O2, further leading to oxygen loss, cobalt dissolution and interfacial side reactions, * ultimately accelerating cell failure. Therefore, the construction of a functional interface structure to inhibit the spread of structural decay triggered by the evolution of high-valent oxygen is the key to achieve high-pressure fast-charge stable long-cycle LCO.
 
job introduction]
Recently, the Wu Zhongshuai team of Dalian Institute of Chemical Physics, Chinese Academy of Sciences, in conjunction with Professor Pan Feng of Peking University and researcher Chen Chengmeng of Shanxi Institute of Coal Chemistry, Chinese Academy of Sciences, proposed for the first time a general positive electrode interface engineering strategy, which realizes fast charging, stable and long cycle at 4.6V by fluorinating 4.45V commercial LCO (recorded as F-LCO) on the surface. The surface fluorination reconstruction mechanism was explored by STEM-EELS, XPS, sXAS and EPR. The resulting electrode has a ~ 1 nm dense ultra-thin LiF coating and a 10~20 nm near-surface gradient fluorinated lattice structure, which provides extraordinary stability for surface lattice oxygen. F-LCO achieved a record capacity retention rate of 92% after 1000 cycles at 3 C, far exceeding the commercial LCO(31%) and the reported 4.6V LCO. In addition, the combination of in-situ electrochemical impedance and DFT calculations theoretically reveals that the anti-bond orbital electron transfer in the Co-F bond greatly inhibits the migration of cobalt when the delithiation is close to 4.6V. The reconstructed high-energy barrier fluorine-rich interface with enhanced charge transfer ability finally prevents the high-valence oxygen species (On-,0 
content expression]
In order to realize the high voltage fast charge LCO battery system, the rational design of the surface interface structure is particularly critical. It not only needs to meet the high oxidation resistance requirements of the positive electrode side, but also needs to meet the requirements of being able to regulate the reversibility of the near-surface lattice oxygen redox and induce the generation of an ideal CEI interface. In view of this, this paper uses commercial 4.45V LCO (recorded as P-LCO) as a model to study the electrode, through its surface wet coating LiPF6 and further 600 degrees C thermal fluorination reconstruction, obtained 4.6V F-LCO, in order to stabilize the high voltage fast charge long cycle. The structure of the designed table interface is LiF @ Co-F & Co-O. The reason for choosing the fluorinated interface reconstruction here is that it has good oxidation resistance and can use the fluorinated interface derived from LiPF6 pyrolysis to induce the formation of a stable fluoride-enriched organic-inorganic hybrid CEI film, and greatly inhibit the dissolution and migration of cobalt when the delithiation is close to 4.6V through the electron transfer of the anti-bond orbital in the Co-F bond, the reconstructed high-energy barrier fluorine-rich interface with enhanced charge transfer ability finally prevents the high-valence oxygen species (On-,0 

The Co-F & Co-O(10-20 nm) of the F-LCO near-surface gradient fluorinated region and the uniform ultra-thin dense LiF coating (~ 1 nm) are confirmed by STEM-EDS coupling as shown in Figures 1a-h. As shown in Figure 1i-j F 1s XPS spectrum, it is further confirmed that the state of surface species is mainly LiF @ Co-F-Co-O structure, and the residual distribution of trace amount of Li3PO4 or LixPOyFz.
 

By cycling the Li | LCO half-cell at 28°C, the author evaluated the electrochemical performance of F-LCO and P-LCO (Figure 2a-c). F-LCO achieved a record capacity retention rate of 92% after 1000 cycles at 3 C, far exceeding the P-LCO(31%). Moreover, F-LCO in the practical graphite | | F-LCO soft-pack full battery of FIG. 2d, there is almost no capacity attenuation at 5C cycle 1100 times, showing excellent practicability. Such excellent stability is due to the enhanced Co/O redox kinetics (Figure 2e) and the enhanced Li + diffusion capability in the high voltage region (Figure 2f) of the fluorinated interface reconstruction. Furthermore, the affinity of the fluorinated interface induces the production of a low interfacial impedance cathode electrolyte interphase (CEI). As shown in Figure 2h, the fluorination mechanism ensures the high voltage, fast charge, stable and long cycle of the lithium cobalt oxide-based battery system.
 

The inducing and optimizing effect of fluorination on interface CEI has been further verified by TOF-SIMS (Figure 3a-e). After 1000 3C long cycles, the F-LCO still maintains a thin and continuous dense ideal CEI structure, while the P-LCO is due to the instability of the interface lattice oxygen structure. Lead to the formation of thicker CEI. Moreover, the phase change reversibility of the fluorinated F-LCO at high voltage is significantly enhanced, which is consistent with the improved Li diffusion ability in the high voltage region. Therefore, after cycling, the F-LCO maintains a good layered structure and the surface ultra-thin LiF is still firmly covered on the electrode surface, verifying the mechanical stability of the fluorinated bonding structure, while the P-LCO has undergone severe interfacial structural degradation, including a large amount of Li insulating phase Co3O4 derived from cobalt dissolution and oxygen loss.
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The mechanisms by which fluorination stabilizes the near-surface lattice and suppresses the loss of active oxygen are elucidated by STEM-EELS and DEMS. First, STEM-EELS and O-K-edge sXAS spectra confirmed the oxygen sublattice defects near the surface of the fluorinated reconfigurable P-LCO (Figs. 4a-e), and significantly increased the oxygen evolution initiation potential F-LCO high valence oxygen evolution to 4.55V(vs. 4.29V/ P-LCO). However, STEM images show that the mechanically stable ultra-thin and dense LiF covering layer on the electrode surface is beneficial to induce the formation of interfacial organic-inorganic hybrid CEI during the initial charging to 4.6V stage (Figure 4h-i). Characterized by in-situ EIS (Figure 4j-k) This can confirm that there is a significant Rct charge transfer effect at the LiF @ Co-F & Co-O fluorinated interface when F-LCO delithiation to around 4.6V, which can optimize and reduce the interface impedance as the cycle progresses until a stable CEI is formed. This is further supported by the following theoretical calculations with underlying mechanisms.

The mechanism of inhibition of Co dissolution and O loss in F-LCO by fluorination interfacial reconstruction was discussed by density functional theory (DFT) calculations. Each O atom in the LCO bulk is coordinated by three Co and three Li atoms (Figure 5a). It is generally believed that the interaction between O and Li atoms is weak because the energy levels of the O 2 p and Li 2s orbitals are severely mismatched. In contrast, the energy levels of O 2 p and Co 3d are close enough to form stable Co-O bonds. According to the coordination field theory, the Co-O interaction causes the energy level to split, forming a1g, t1u and eg bonding orbitals. These orbitals are mainly composed of O 2 p orbitals, a1g *, t1u *, and eg * anti-bonding orbitals mainly from Co 3d orbitals, along the non-bonding orbitals t2g composed of Co 3d orbitals. Consistent with what has been reported, LCO appears to be non-magnetic, and the t2g orbital is completely occupied by 6 electrons from Co3, (Figure 5 B). However, F([He] 2s 22p5) has one more valence electron than O([He] 2s 22p4), while the number of bonding orbitals formed by F and Co atoms remains unchanged (Figure 5a). Therefore, one of the eg * anti-bonding orbitals is occupied alone, and the energy shifts downward with F substitution, resulting in spin polarization in the partial wave density of states (PDOS) (Figure 5c). In the LCO, the Fermi level (EFermi) shifts to a lower energy state as the charge approaches 4.6V. In this case, the bonding orbitals are partially occupied, resulting in weakening of the Co-O bond. However, after F substitution, the Co atom occupies only one eg * anti-bond orbital, and electrons are transferred from the occupied eg * orbital rather than from the eg orbital, resulting in the enhancement of Co-F bonds and making Co dissolution more difficult. In addition, when studying the O loss mechanism, it is believed that the migration of O species radicals in LCO follows the vacancy mechanism. In other words, O species radicals migrate by changing position through O vacancies. By studying the kinetics of this migration Climbing the image nuded elastic band (CI-NEB) method, it is proved that the introduction of F atoms into LCO can increase the energy barrier of oxygen migration from 0.74 eV to 1.09 eV (Figure 5d). Coupled with a dense LiF coating on the electrode surface induces the formation of LiF-rich external CEI, which further effectively suppresses oxygen loss and cobalt dissolution. This reasonably explains that these processes (eg * antibond orbital electron transfer enhanced Co-F bonding and increased O migration barrier) act synergistically to promote stable electrochemical properties of the F-LCO at voltages up to 4.6V.
 
[Conclusion]]
This work proposes a reliable engineering strategy for the gradient fluorination of the positive electrode surface, which effectively inhibits oxygen loss and cobalt dissolution in the 4.6V F-LCO, and achieves excellent 3C fast-charge long-cycle capability. Driven by the surface/near-surface lattice fluorination design, F-LCO not only obtain ultra-thin and dense LiF conformal cladding, but also obtain a near-surface oxygen sublattice reconstructed by Co-F & Co-O gradient fluorination. Therefore, F-LCO at 4.6V, after 1000 C fast charges in the Li | | LCO half-cell, the capacity retention rate reached 92%. Even more impressively, F-LCO exhibit unprecedented high voltage cycling stability in a practical graphite | F-LCO soft package. Theoretical simulations have elucidated the internal mechanism of fluorination reconstruction to stabilize the interface structure and enhance the oxygen redox reversibility, that is, through fluorination reconstruction to enhance the ion transport of the bonding interface and inhibit Co dissolution and O loss. In short, this cathode surface gradient fluorination engineering strategy will provide more opportunities for the development of high-voltage and fast-charging cathode materials to form functional artificial CEI layers, thereby accelerating the rapid development of advanced lithium-ion batteries.
 
Z. H. Bi, Z. L. Yi, L. Z. Zhang, G. R. Wang, A. P. Zhang, S. H. Liao, Q. H. Zhao, W. G. Zhao, X. Y. Shi, M. R. Li, J. X. Mi, S. N. Li, P. Das, Y. Cui, C. M. Chen, F. Pan, and Z-S Wu., Ultrathin dense LiF coverage coupled with a near-surface gradient fluorination lattice enables fast-charging long-life 4.6 V LiCoO2. Energy Environ. Sci., 2024.
https://doi.org/10.1039/D3EE03464J