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Cite This: ACS Appl. Energy Mater. 2019, 2, 217−221
Component-Selective Passivation of Li Residues of Ni-Based Cathode Materials by Chemical Mimicry of Solid Electrolyte Interphase Formation Gene Jaehyoung Yang and Yongseon Kim* Department of Materials Science and Engineering, Inha University, Incheon, 22212, Republic of Korea
ACS Appl. Energy Mater. 2019.2:217-221. Downloaded from pubs.acs.org by UNIV OF WINNIPEG on 01/28/19. For personal use only.
S Supporting Information *
ABSTRACT: A new treatment method is proposed for the Li residues in Ni-based cathode materials for lithium ion batteries. The method passivates the Li residues by converting them into a protective coating layer. The coating layer is controllable as a composite of LiF nanoparticles and a thin film. The selective removal of the most problematic LiOH component is achieved. Thus, the method not only passivates the Li residues but also can flexibly control both the chemistry and morphology of the resulting coating layer, enabling optimized performance from Nibased cathodes. The simple and cost-effective process is readily applicable to commercial production. KEYWORDS: lithium ion battery, cathode, lithium residue, surface passivation, nanocomposite the first strategy of preventing the formation of Li residues is not expected to be effective. The second strategy of removing Li residues by postsynthesis treatments is easily accomplished by washing the active material with water. However, it is reported that Li ions seep from the crystals and react with air, permitting reconstruction of the Li salt layer, when the active material is exposed to ambient atmosphere after the washing process.11,13−15 Restricting the reactivity of Li residues by adding a coating layer is one alternative measure. Numerous coating methods have been proposed with some technologies being commercialized.16−23 However, the problem of Li residues for high Nicontent active materials still remains unsolved and obstructs the development and full-scale commercialization of these active materials.11,16 We judged that the following development guidelines for surface treatment techniques are necessary to effectively and efficiently address the problem of residual Li salts: (1) The Li residues should be converted into a stable coating layer by passivating them, rather than washed off or covered by an additional layer of foreign materials, to prevent battery swelling caused by the Li salts and keep the cathode surface from being exposed to air; (2) LiOH in particular must be removed to prevent moisture adsorption and battery manufacturing problems; (3) the coating layer’s interference with the movement of Li ions should be minimized; for this, the coating layer should be as thin as possible, and it would be helpful if the layer showed partial or complete Li-ion conductivity; and (4) a simple and cost-efficient process
L
i-ion batteries (LIBs) are widely used in mobile electronic devices, and their application is broadening to electric vehicles and electric power storage stations.1,2 Consumer demand requires increased battery capacity for longer device operation times or higher vehicle mileages per charge. The battery capacity is directly related to the capacity of the electrode active materials; therefore, the development of highcapacity active materials is necessary. Currently, Ni-based layer-structured materials, such as LiNixCoyMnzO2 (LNCM) and LiNix CoyAlz O2 (LNCA), are the most pragmatic alternatives for conventional LiCoO2 cathodes.3−5 By replacing Co with Ni in the layer-structured cathode active material, a higher battery capacity is obtained under current commercial charging voltage conditions, and the material cost can be lowered. However, disadvantages including decreased crystal structure stability and increased Li residues on the active material surface accompany the increase of the Ni content.6−9 The Li residues, mainly comprising Li2CO3 and LiOH, are known to cause severe problems in battery performance;5,10,11 their side reactions with electrolytes during battery operation generate gases that cause battery swelling. In addition, LiOH in particular causes not only battery swelling but also uncontrollable polymerization of the polyvinylidene fluoride (PVDF) used as a cathode binding material during the electrode fabrication process.6,7 Therefore, the LiOH content in the active material must be strictly controlled. Two possible solutions may be considered in addressing the problem of residual Li salts: suppression of their formation during the active material synthesis, and their postsynthesis removal. Our previous studies indicated that the increase of Li residues with higher Ni contents is thermodynamically unavoidable in layer-structured active materials.12,13 Therefore, © 2018 American Chemical Society
Received: November 10, 2018 Accepted: December 18, 2018 Published: December 18, 2018 217
DOI: 10.1021/acsaem.8b01939 ACS Appl. Energy Mater. 2019, 2, 217−221
Letter
ACS Applied Energy Materials
Figure 1. Schematic of the experimental procedure for the conversion of residual Li salts to pseudo-SEI (p-SEI) layers on the surface of a LiNi0.85Co0.12Al0.03O2 (LNCA) cathode.
Figure 2. (a−c) SEM images of untreated LNCA, p-SEI1, and p-SEI2 samples, (d) gas generation during the storage test, (e) cycle performance (two cells each), and (f) Nyquist plots from EIS measured after cycling.
F source in this study, is advantageous in being easier to handle than hydrofluoric acid. The designed process may be regarded as a chemical mimicry of the side reactions at the cathode− electrolyte interface and the resulting formation of SEI layers occurring in LIBs, in that the reaction includes the main participants in the side reactions of the cathodic Li residues, organic solvents, and fluorine salts. Therefore, the samples are named LNCA with pseudo-SEI layers p-SEI1 and p-SEI2. The surface images of untreated LNCA, p-SEI1, and p-SEI2 are presented in Figure 2a−c. Unlike the smooth surface of the untreated LNCA, the p-SEI1 sample is apparently covered by a rough-textured layer. This may indicate that a chemical reaction caused by the DMC has occurred on the LNCA surface. The surface of p-SEI2 also shows the roughness, but the surface planes and grain boundaries are more visible, therefore, the layer formed by the reaction with DMC is expected to be thinner in p-SEI2 than that in p-SEI1. Transmission electron microscopy (TEM) images of the
should be designed for commercial use, preferably without additional heat treatment. Here, we report a new surface treatment method to satisfy all these requirements, in which the Li residues are converted to stable surface layers by a simple one-step wet process. The method is designed by mimicking the side reactions occurring at the cathode−electrolyte interface, thus enabling solid electrolyte interphase (SEI)-like layer formation. Our previous studies showed that Li2CO3 or LiOH reacted with the electrolyte solutions of LIBs to release gases, and that these reactions could be promoted by controlling the temperature.7 The surface treatment process of this study relies on this point: the Li residues of the LiNi0.85Co0.12Al0.03O2 (LNCA) active material are reacted with dimethyl carbonate (DMC), one of the solvent components of the electrolyte solution, by heating at 60 °C (Figure 1). The same treatment, but with the addition of NH4FHF, is also performed for the partial conversion of the Li residues into LiF. The NH4FHF powder, which is used as an 218
DOI: 10.1021/acsaem.8b01939 ACS Appl. Energy Mater. 2019, 2, 217−221
Letter
ACS Applied Energy Materials
Figure 3. (a−d) SEM and XRD data of the powders obtained by reacting Li salts with DMC or DMC + NH4FHF solutions: Reaction of (a) Li2CO3 + LiOH·H2O with DMC, (b) Li2CO3 + LiOH·H2O with DMC+NH4FHF, (c) LiOH·H2O with DMC, and (d) LiOH·H2O with DMC + NH4FHF, where the EDS data of a cubic nanoparticle is included. (e) Properties of the Li salts used as the reaction material. (In XRD patterns, ●, Li2CO3; ▲, LiOH·H2O; ◆, LiF).
3a,b). Interestingly, the same results, that is, the formation of Li2CO3 under p-SEI1 conditions and LiF + Li2CO3 under pSEI2, are observed even when only LiOH·H2O is used as the reactant (Figure 3c,d; the trace of LiOH in Figure 3d may be because the reaction is not fully completed). The particle size of the reactants is far larger than that of the product powders (Figure 3e), indicating that the Li2CO3 detected after the reactions is not the original reactant powder but is instead a new product of the chemical reactions. From the analysis, the reactions occurring during the treatments of this study are concluded as follows: (1) Under the p-SEI1 treatment conditions, the Li residues react with DMC, thus forming the Li2CO3 phase. (2) With the p-SEI2 condition, they react with both DMC and NH4FHF, thus forming the composite phase of LiF and Li2CO3. The Li2CO3 film layer becomes far thinner in this condition because much of the Li residues are consumed by the formation of LiF nanocrystals. The LiF particles appear homogeneously distributed over the surface, anchored in the Li2CO3 film layer (Figure 3c, Figure S1 in Supporting Information). This morphological feature may be obtained because both the LiF particle and Li2CO3 film are newly formed simultaneously by chemical reactions with NH4FHF and DMC respectively during the process. The excellent properties of the p-SEI2 sample can be explained by the characteristics of the surface layer, in which LiF nanocrystals are embedded in a very thin Li2CO3 film. Considering the Li2CO3 film first, it is one of the main components of the Li residues and may experience side reactions with the electrolyte in LIBs. However, if it were thoroughly removed, the clean LNCA surface would be directly exposed to air and attacked by moisture and CO2, thus reforming the Li salts on the surface as well as damaging the crystal structure near the surface.11,13 Therefore, it is desirable to retain the minimum necessary amount of Li2CO3 in order to
cross-section also supported this interpretation (Supporting Information). In addition to the film layer, particulates of tens of nanometers in size are observed on the surface of p-SEI2 (Figure 2c). Energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) analyses indicated that they were LiF particles (Supporting Information). Conclusively, it is found that a rough-textured encapsulating coating film is formed by the reaction of surface Li residues with DMC, whereas a composite layer of LiF nanoparticles anchored in a much thinner film layer is obtained by reacting the Li residues with a mixture of DMC and NH4FHF. Figure 2d shows the amount of generated gas when the charged-state cathodes are stored with the electrolyte solution in a sealed pouch, which indicates that the gas evolution is reduced by the surface treatment. In particular, p-SEI2 shows less than half of the gas evolution shown by untreated LNCA, indicating that the battery swelling can be greatly suppressed via the p-SEI2 type surface treatment. The p-SEI2 sample also shows excellent cycling performance (Figure 2e). The electrochemical impedance spectroscopy (EIS) data measured after cycling show that the impedance of p-SEI2 is far lower than those of the others in the entire high- to middle-frequency range. This indicates that the composite coating of p-SEI2 maintains a low interfacial resistance during the cycling, thus providing smooth kinetics of battery operation.24 Direct analysis of the surface layers was difficult because of their very small amounts, so we treated powders of Li2CO3 and LiOH·H2O, the main components of the LNCA cathodic Li residues, using the proposed surface treatments to obtain sufficient amounts of the treatment product components. When a mixture (50:50 in weight) of Li2CO3 and LiOH·H2O is reacted, only the Li2CO3 phase is detected from the p-SEI1 treatment product (i.e., that from reaction with DMC only), while a mixed phase of LiF and Li2CO3 is obtained with the pSEI2 condition (reaction with DMC + NH4FHF) (Figure 219
DOI: 10.1021/acsaem.8b01939 ACS Appl. Energy Mater. 2019, 2, 217−221
Letter
ACS Applied Energy Materials Notes
stabilize the cathode surface and participate in SEI layer formation during the battery operation; whereas the SEI layer generally increases the interfacial resistance, it also stabilizes the interface by preventing continued side reactions.25 When we increased the amount of NH4FHF without using DMC to make only LiF particles without Li2CO3 film, we could obtain similar level of improvement in the gas evolution as p-SEI2 but less improvement in cycle performance. In this work, a very thin layer of Li2CO3 is achieved while removing the LiOH component from the original Li residues, which is beneficial for processing stability of the electrode fabrication (Supporting Information). LiF, another characteristic component of the p-SEI2, may have Li-ion conductivity depending on the inner point defects of the crystal.26,27 It is also reported that the cyclic performance of batteries is improved with increasing LiF proportions in the cathode SEI layer.28 In this work, some Li residues are converted into the electrochemically stable LiF phase by the p-SEI2 treatment condition. Considering the possible ionic conductivity of LiF, the LiF nanocrystals embedded in the Li2CO3 layer may act as migration channels for Li ions. In addition to this effect, the consumption of Li residues for the formation of LiF decreases the amount of Li2CO3, thus permitting minimization of the Li2CO3 film thickness, which may minimize the disruption by the film layer of Li ion flow while protecting the LNCA surface. In summary, a new treatment method is proposed for the Li residues in Ni-based cathode materials. It is found that the Li residues can be converted to Li2CO3 films by reaction with DMC. The LiOH component is selectively removed by this reaction. By introducing NH4FHF to the reaction, LiF nanocrystals can be composited with the Li2CO3 film. The thickness of the Li2CO3 film can be minimized by controlling the amount of Li residues consumed for LiF formation. Thus, the surface Li residues are converted to a coating layer in which LiF nanocrystals are embedded in a very thin Li2CO3 film. The conversion of the Li residues to electrochemically inert LiF stabilizes the cathode surface, whereas the Li2CO3 film prevents the reaction of the cathode material with moisture or CO2 in air. Elimination of LiOH stabilizes the electrode manufacturing process, because LiOH otherwise causes uncontrollable polymerization of the PVDF binder. Because LiF may conduct Li ions, the combination of LiF nanocrystals with a very thin Li2CO3 film favors smooth Li-ion migration in the passivation layer. With these features, greatly improved cyclic and swelling behaviors of the battery are obtained. The method is very simple, cost-effective, and readily applicable to commercial production.
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Education (NRF2016R1D1A1B03933704).
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.8b01939.
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REFERENCES
Experimental details, analysis of the surface, proposed reaction mechanism, and effect of LiOH removal (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Yongseon Kim: 0000-0003-3622-0999 220
DOI: 10.1021/acsaem.8b01939 ACS Appl. Energy Mater. 2019, 2, 217−221
Letter
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DOI: 10.1021/acsaem.8b01939 ACS Appl. Energy Mater. 2019, 2, 217−221
