Enhancing humidity resistance of nickel-rich layered cathode materials by low water-soluble CaF₂ coating†

Zijian Zhang ^a, Jiajin Feng ^a, Ran He ^a, Yelong Zhang ^a, Hangmin Zhu ^a, Feng Wu ^a, Qingguang Zeng ^a, Hui Yu ^b, Kwun Nam Hui ^c, Xi Liu *^b and Da Wang *^a
aSchool of Applied Physics and Materials, Wuyi University, Jiangmen, 529020, China. E-mail: dawang@mail.ustc.edu.cn
^bGuangdong-Hong Kong Joint Laboratory for New Textile Materials, College of Textile Science and Engineering, Wuyi University, Jiangmen, 529020, China. E-mail: liuxi@wyu.edu.cn
^cJoint Key Laboratory of the Ministry of Education, Institute of Applied Physics and Materials Engineering, University of Macau, Avenida da Universidade, Taipa, Macao

First published on 27th August 2024

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In the context of rapidly growing commercial demand, nickel-rich layered cathode materials, such as LiNi_0.8Mn_0.1Co_0.1O₂ (NMC811), are widely studied due to their high energy density and low cost.^1–8 Despite these benefits, they encounter significant challenges, such as cation (Li/Ni) mixing, interfacial instability of the electrode materials with electrolyte, and notably Li⁺/H⁺ exchange in humid environments when it comes to practical use.^9,10 This exchange process engenders several adverse effects, including the depletion of active lithium species, reduction of nickel ions, and simultaneous generation of detrimental impurities. It also induces structural transformations within the material, transitioning it from a layered configuration to a rock salt NiO arrangement.^11,12 The coexistence of inert components within the material hinders the transport of lithium ions and electrons, resulting in electrochemical polarization and consequential capacity degradation.^13–15 Additionally, the susceptibility of nickel-rich compounds to moisture renders the utilization of aqueous solutions impractical for cathode fabrication, a method commonly employed in the preparation of graphite anodes. This undoubtedly increases manufacturing costs and raises environmental concerns.¹⁶

To overcome the detrimental effects of moisture on nickel-rich layered cathode materials, encapsulating NMC particles with a hydrophobic organic layer was considered feasible to prevent moisture-induced degradation and has been widely adopted in the past.^17–21 For example, a hydrophobic alkyl surface coating of dihexadecyl phosphate has been found to be highly effective in mitigating the side reactions between the electrode surface and moisture.²² Mu et al. employed a hydrophobic coating of polydimethylsiloxane to form a passive layer on the surface of a nickel-rich electrode, which effectively inhibited the formation and growth of residual lithium compounds, thus enhancing its tolerance to humid environments.²³ Similarly, Xue et al. used a fluoroalkylsilane coating on NMC811 to inhibit the dissolution of transition metal ions in the electrolyte, and its hydrophobic feature has been demonstrated to be effective in preventing the formation of residual lithium compounds even when exposed to humid air for 30 days.²⁴

Recently, it has been brought to our attention that Sun et al. successfully improved the moisture resistance of NMC811 through Zr⁴⁺ doping as opposed to the more commonly employed hydrophobic coating.²⁵ We believe that the mitigated interfacial susceptibility could be attributed to low solubility of the resulting inorganic doped surface layer. To demonstrate this hypothesis, we screened several common low-soluble inorganic compounds that exhibit potential as suitable coatings for reducing the susceptibility of electrode materials, as shown in Table S1 (ESI†). Silver compounds have the lowest water solubility, but the use of silver containing materials raises the cost. Although carbonates come second, the risk of gas generation derived from HF attack of carbonates excludes them from consideration.²⁶ Fluoride coatings have been investigated to improve the electrochemical performance of nickel-rich cathodes.²⁷ Among those fluorides considered, CaF₂, owing to its limited water solubility and robust thermodynamic stability, was selected as a representative example in this study to assess the potential applicability of low-soluble inorganic compounds as protective coatings for mitigating moisture-induced erosion.

Here, we demonstrate that the low water-soluble CaF₂ coating strategy can achieve excellent moisture resistance for high-performance NMC811 cathodes. Compared to traditional hydrophobic coatings, the newly proposed CaF₂ coating strategy enables NMC cathodes to exhibit better performance under elevated temperatures. More importantly, it allows for the aqueous preparation of nickel-rich cathode electrodes without compromising their electrochemical performance.

NMC811 particles were coated with various levels of CaF₂ through hydroxide co-precipitation and gas-phase methods. All samples exhibit a single phase, a well-defined hexagonal lattice structure with the Rm space group, as shown in the X-ray diffraction (XRD) patterns in Fig. 1(a). The (003)/(104) peak intensity ratio of all samples is above 1.2. CaF₂ coating does not alter the original structure of NMC811 and is not detectable due to its low concentration and low temperature calcination. SEM observation (Fig. 1(b) and (c)) reveals a fuzzy surface of 3.0% CaF₂@NMC811 compared with the smooth and clean surface of pristine NMC811, which indicates that the surface morphology of NMC811 has been changed by gas-phase fluorination. Ca and F signals are detected by energy dispersive spectroscopy (EDS) mapping (Fig. S1, ESI†). XPS measurements (Fig. 1(d) and (e)) show clear Ca and F signals, Ca 2p_1/2, 2p_3/2 and F 1s peaks at 351.0, 347.5 and 685.0 eV, respectively, demonstrating that Ca and F exist as CaF₂. Furthermore, the TEM image (Fig. 1(f)) shows a continuous layer with thickness ranging from 3 to 20 nm on the material surface, demonstrating that CaF₂ has been successfully coated on the surface of NMC811 particles.

The electrochemical performance of all samples was tested in coin-type half cells. 3.0% CaF₂@NMC811 shows a slightly lower discharge capacity than that of pristine-NMC811 (Fig. 2(a)). Consistent with previous reports, all the samples with CaF₂ surface modification exhibit better capacity retention compared to the untreated NMC811 (Fig. 2(b)), attributed to the protection of NMC811 by CaF₂.²⁸ Furthermore, 3.0% CaF₂@NMC811 has the best capacity retention of 88.0% after 100 cycles. Moreover, consistent with previous reports,^29,30 CaF₂ coating improves the cycling stability of NMC811 at higher charge voltage and elevated temperature, respectively, and under both conditions (Fig. 2(c), (d) and Fig. S3–S6, ESI†). Fluorides can suppress parasitic side reactions between NMC811 and the electrolyte, thereby enhancing its electrochemical performance, particularly during high-temperature and high-voltage cycling, which has been investigated thoroughly by previous studies.^27–29 Therefore, the examination of CaF₂'s role after cycling falls outside the scope of our present work.

To evaluate the moisture resistance effect of CaF₂ coating for NMC811, the untreated NMC811 and 3.0% CaF₂@NMC811 samples were subjected to storage in a humid environment (RH >85%) for 7 days (Fig. 3(a)). After exposure, pristine-NMC811 particles appeared more adhesive to the glass container's inner wall and bottom, compared to 3.0% CaF₂@NMC811 particles. SEM images further provide insights into these observations (Fig. S7, ESI†). The surface of the pristine-NMC811 particles displays the presence of amorphous phases, indicative of some form of alteration or deterioration. By contrast, the surface of the 3.0% CaF₂@NMC811 sample appears unaltered and intact. Furthermore, Fourier-transform infrared spectroscopy (FTIR) was employed as an effective analytical tool to identify the chemical species formed during the storage period. The pristine-NMC811 sample, after exposure to a humid environment, exhibits more pronounced signals associated with Li₂CO₃ at approximately 1404 cm⁻¹ and 862 cm⁻¹, Fig. S8a (ESI†). These signals are indicative of moisture-induced erosion and stem from the reaction of LiOH, formed on the electrode surface, with CO₂ present in the surrounding air. This reaction ultimately leads to the formation of Li₂CO₃.³¹ Notably, the 3.0% CaF₂@NMC811 sample exhibits unaltered signals associated with carbonates, signifying a reduction in side reactions and highlighting the effective protective role of the CaF₂ coating against moisture-induced erosion.

Both samples after storage were dried at 80 °C overnight to remove absorbed water before being made into an electrode for electrochemical tests. 3.0% CaF₂@NMC811 exhibits the discharge capacity of 191.9 mA h g⁻¹ with CE of 87.0%, while pristine NMC811 only delivers 156.1 mA h g⁻¹ with CE of 61.6% (Fig. S9, ESI†). Moreover, after 350 cycles at 0.5C, 3.0% CaF₂@NMC811 still retains 67.8% of the initial capacity, but pristine NMC811 suffers huge degradation with only 54.1 mA h g⁻¹, 34.7% of the initial capacity (Fig. 3(b)).

In contrast to the utilization of water for the preparation of graphite anodes, the prevailing practice in cathode manufacturing employs N-methyl-2-pyrrolidone (NMP), which has led to heightened production expenses and environmental concerns. Nonetheless, due to the vulnerability of nickel-rich compounds to moisture, the conventional approach of preparing cathodes employing water has proven unfeasible. To comprehensively assess the water resistance characteristics of CaF₂ coated NMC811 and to examine the viability of water-based processes for cathode fabrication, we opted to employ water instead of NMP in the electrode manufacturing process. Electrochemical performance was compared between NMP baseline cells and cells with aqueous-based electrodes for pristine NMC811 and 3.0% CaF₂@NMC811. The voltage profile of the pristine aqueous-NMC811 electrode, as illustrated in Fig. S10a (ESI†), exhibits a distinct “spike voltage” reaching a maximum of 4.03 V at the beginning, then dropping to 3.86 V followed by a gradually elevating plateau. These observations serve as indicative evidence for the presence of a substantial quantity of residual lithium compounds on the electrode surface, which are by-products resulting from the degradation of the electrode surface due to the erosive effects of air and moisture. Conversely, the initial charging plateau of the 3.0% CaF₂@NMC811 aqueous based electrode is 3.76 V, merely 0.13 V higher than 3.63 V observed for the pristine NMC811 electrode prepared with NMP. Notably, the disparity in their voltage profiles diminishes to insignificance after two cycles, Fig. S10b (ESI†), indicating that the coating of the low water-soluble CaF₂ renders NMC811 more resilient to moisture-induced erosion. Fig. 3(c) and Fig. S10 (ESI†) reveal a substantial capacity loss in the case of the pristine-NMC811 electrode prepared in an aqueous medium. This loss can be attributed to the active material's degradation, primarily stemming from structural deterioration resulting from Li⁺/H⁺ exchange. Although the aqueous-based 3.0% CaF₂@NMC811 electrode initially has a slightly lower capacity of 175 mA h g⁻¹ at 0.5C, it exhibits a superior cycling performance after 40 cycles compared to the NMP-based pristine electrode. Collectively, all electrochemical assessments substantiate that CaF₂ coating with low water solubility makes aqueous fabrication of nickel-rich cathodes feasible without compromising its electrochemical performance.

To understand the difference in water susceptibility of both materials, the samples were stirred in deionized water for 15 min and dried before characterization (see the Experimental section for details). The XRD patterns of the samples after water exposure are shown in Fig. 4(a), in which the (003)/(104) peak intensity ratio of the 3.0% CaF₂@NMC811 sample remains at 1.21, considered as structurally ordered and low cation mixing, which suggests that the original NMC811 structure remained intact. The (003)/(104) ratio of pristine NMC811 decreases dramatically to 0.88 after water exposure, indicating that the pristine material, without CaF₂ protection, undergoes a severe Li⁺/H⁺ exchange and the simultaneous reduction of Ni³⁺ to Ni²⁺, prompting the structural rearrangement of the material surface from an ordered layered structure to disordered rock salt, and with the exacerbation of irreversible extraction of Li⁺ from the structure, Li⁺/Ni²⁺ mixing spreads from the surface to the bulk of NMC811. FTIR and X-ray photoelectron spectroscopy (XPS) were employed to gain insight into the surface chemical changes. For the 3.0% CaF₂@NMC811 sample, it is particularly noteworthy that carbonate signals persist in the FTIR spectrum, while they are nearly absent in the pristine NMC811 sample. This observation strongly suggests that water exposure removes some of the Li₂CO₃ from the pristine NMC811, likely due to the solubility of Li₂CO₃ in water. In contrast, for the coated sample (3.0% CaF₂@NMC811), the Li₂CO₃ species remain detectable, Fig. S8b (ESI†), implying that they are shielded or encapsulated by the CaF₂ coating, thus preventing their removal by water. The XPS results for the Ni 2p binding energy spectrum of pristine NMC811 (as shown in Fig. 4(b)) reveal that the Ni³⁺ percentage (main characteristic peaks at 856.1 eV) drops from 87.49% to 69.99% after exposure, indicating a more pronounced Li⁺/Ni²⁺ mixing and a structural deterioration in the electrode material. This is in accordance with the XRD results. In addition, the pH of water was analyzed after stirring. The pH of water increased from the initial 6.6 to 12.3 after rinsing pristine NMC811, while after rinsing 3.0% CaF₂@NMC811, the pH of the solution rose slightly to 7.1. These observations provide further evidence of the robust protective role played by the CaF₂ coating on NMC811 against the influence of water exposure.

Despite the considerable enhancement in water resistance achieved through the application of the CaF₂ coating to NMC811, the wetting angle measurements for all the samples, including commercial CaF₂ and 3.0% CaF₂@NMC811, do not exceed 15.04°, Fig. S14 (ESI†), exhibiting hydrophilic nature. For moisture erosion observed in nickel-rich layered cathode materials, the exchange of Li⁺ and H⁺ ions serves as the primary reason, suggesting that the bonding of Li⁺ within the structure is comparatively feeble; indeed, were this not the case, the electrochemical deintercalation of lithium ions would be unattainable. Conversely, the bonding between Ca²⁺ and F⁻ ions within the coating layer is robust enough, effectively inhibiting H⁺ ions/water molecules from breaking through, thus impeding the occurrence of Li⁺/H⁺ exchange. From a macroscopic perspective, the CaF₂ layer exhibits low water solubility. Therefore, the ability to resist water erosion is not inherently linked to either hydrophobicity or hydrophilicity, but lies in robust chemical bonding within the protective layer, which, on a macroscopic scale, exhibits low water solubility. More discussions are in the ESI.†

We have employed a low water-soluble compound, CaF₂, to protect NMC811, not only improving the electrochemical performance of NMC811 at high cut-off voltage and elevated temperatures but also enhancing NMC811's tolerance in a highly humid environment (RH >85%). Remarkably, the hydrophilic CaF₂ coating, due to its strong Ca–F bonding, is insoluble in water, which makes it effective in enabling NMC811 to withstand moisture and even water exposure. These findings not only open up new avenues for the practical implementation of high-nickel cathodes in advanced high-energy-density lithium-ion batteries, but also offer valuable principles for enhancing the performance of other moisture-sensitive electrode materials in electrochemical energy storage devices.

This work was supported by the National Natural Science Foundation of China (21975187, 22005224), Guangdong Pearl River Talent Program (2019QN01L309), and Guangdong Key Building Discipline Research Capability Enhancement Funds (2021ZDJS093). The research was also supported by Wuyi University (AL2019003, 2018TP031), Wuyi University–Hong Kong/Macau Joint Research Funds (2019WGALH02, 2019WGALH10) and Science Foundation for Young Teachers of Wuyi University (2019Td01).

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3cc05730e

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