Improved electrochemical properties of tavorite LiFeSO₄F by surface coating with hydrophilic poly-dopamine via a self-polymerization process

Abstract

Poly-dopamine coated Li_1−xFeSO₄F is prepared via a self-polymerization process. The material shows larger discharge capacities, better rate capability and longer cycle stability than the pristine LiFeSO₄F. The improved electrochemical properties are attributed to the highly hydrophilic and elastic properties of poly-dopamine.

---

The increasing achievements in portable electronics, electric vehicles and smart grids promote scientists worldwide to explore new cathode and anode materials for lithium ion batteries. Iron-based polyanion materials such as LiFePO₄, LiFeBO₃ and Li₂FeSiO₄ could be a promising alternative to the traditional LiCoO₂ cathode due to their low cost and high thermal safety properties. In particular, LiFePO₄ has been used in commercial lithium ion batteries since its low electronic conductivity was successfully overcome by surface carbon coating or cation doping. Tavorite LiFeSO₄F, whose theoretical specific energy is comparable to that of LiFePO₄, is a newly discovered iron-based cathode material.^1–3 The three-dimensional framework of LiFeSO₄F is more favourable for lithium migration compared to the one-dimensional lithium pathway of LiFePO₄. However, as an intrinsic property of polyanion materials, LiFeSO₄F shows very low electronic conductivity (5.2 × 10⁻¹¹ S cm⁻¹) which is two orders of magnitude lower than that of LiFePO₄.^1,4 This seriously hinders the electrochemical performance of LiFeSO₄F especially at high current rates. Various surface coating strategies have been used to improve the electrochemical properties of battery materials.^5–7 However, it must be pointed out that conventional carbon coating is not suitable for LiFeSO₄F due to its low thermodynamically stable temperature. Alternately, conductive polymer coating could be an appropriate resolution since this coating technique could be achieved at mild temperature. Several methods such as oxidant-assisted polymerization and electro-polymerization have been used to prepare polymer coated electrode materials in an aqueous solution.^8–11 Unfortunately, these aqueous coating processes are not suitable for LiFeSO₄F because of its high humidity sensitivity which results in decomposition of the material.

Herein, we report a method that could significantly improve the electrochemical properties of LiFeSO₄F via a non-aqueous polymer coating process. Fig. 1 shows a schematic diagram for the synthesis and design principles of the material. The first step is carried out by preparing tavorite LiFeSO₄F using the tetraethylene glycol (TEG) assisted solvothermal method. Then, dopamine monomers are self-polymerized on the surface of Li_1−xFeSO₄F. The Li_1−xFeSO₄F material was prepared by chemical de-lithiation of the pristine LiFeSO₄F in NO₂BF₄. The material thus obtained is called as PDA@Li_1−xFeSO₄F. Detailed experimental procedures are described in the ESI.† Dopamine is a nature-inspired bio-mimetic material containing several functional groups such as catechol, amine and imine. As an organic semiconductor with limited electronic conductivity, dopamine is not competitive enough to compensate the low electronic conductivity of LiFeSO₄F. But, the robust wet-resistant adhesion and strong elasticity of PDA may be highly beneficial for LiFeSO₄F to adsorb electrolyte efficiently and release its large volume change (ΔV/V = 10.4%) during repeated cycling.¹ We show here that the PDA coated Li_1−xFeSO₄F exhibits much improved electrochemical properties than the pristine LiFeSO₄F. To the best of our knowledge, this is the first report that improving the electrochemical performance of cathode materials by surface coating with highly hydrophilic polymers.

Fig. 1 Schematic diagram for the synthesis and design principles of PDA@Li_1−xFeSO₄F.

Fig. 2a shows the X-ray diffraction (XRD) patterns of the LiFeSO₄F and PDA@Li_1−xFeSO₄F samples. The diffraction pattern of the pristine material fits well with that of tavorite LiFeSO₄F without any detectable impurities. The lattice parameters of the material are calculated as a = 5.1855(1) Å, b = 5.5048(1) Å, c = 7.2296(1) Å, α = 106.517(1)°, β = 107.184(1)°, γ = 87.818(1)°, which are in good agreement with those reported in literatures.^1,2 This batch of material was then chemically de-lithiated by NO₂BF₄ in acetonitrile via the reaction, LiFeSO₄F + xNO₂BF₄ → Li_1−xFeSO₄F + xLiBF₄ + xNO₂↑ (1) in the following step, the oxidative Fe³⁺ ions of Li_1−xFeSO₄F could initiate the polymerization of dopamine, forming the PDA@Li_1−xFeSO₄F target material. XRD shows that the crystal structure of PDA@Li_1−xFeSO₄F is almost identical to that of FeSO₄F which indicates that most Li ions are removed from the LiFeSO₄F lattice.¹² No evidence of PDA is observed because of its small amount and polymeric state in the material.

Fig. 2 (a) X-ray diffraction patterns and (b) Fourier transform infrared spectra of the LiFeSO₄F and PDA@Li_1−xFeSO₄F samples.

Fig. 2b shows the Fourier transform infrared spectra of the LiFeSO₄F and PDA@Li_1−xFeSO₄F samples. Both materials show a series adsorption peaks below 1400 cm⁻¹ which are due to the SO₄ tetrahedral.¹³ PDA@Li_1−xFeSO₄F shows several additive peaks. The one located at 1635 cm⁻¹ corresponds to the C=C vibration of the aromatic ring and the other broad one between 3000 and 3600 cm⁻¹ is attributed to the overlap of the N–H and catechol-OH vibrations.^14,15 The amount of PDA in PDA@Li_1−xFeSO₄F could be determined by thermogravimetric analysis (TGA, ESI, Fig. S1†). Comparing to the TG curve of Li_1−xFeSO₄F, PDA@Li_1−xFeSO₄F shows an extra weight loss of 1.8 wt% between 300 and 500 °C which is attributed to the carbonization of PDA. Therefore, the amount of PDA in the PDA@Li_1−xFeSO₄F sample is calculated as 2.87 wt%.

The morphologies of the samples are studied by scanning electron microscope (ESI, Fig. S2†). The pristine LiFeSO₄F shows polyhedral shape with particle size ranging from 500 to 800 nm. The particle shape and particle size of Li_1−xFeSO₄F are maintained well after PDA coating.

However, the particle surface becomes much rougher comparing to the smooth surface of LiFeSO₄F. Transmission electron microscope shows that the pristine LiFeSO₄F possesses an overwhelmingly clean surface (Fig. 3a). In comparison, the surface of PDA@Li_1−xFeSO₄F shows a uniform amorphous layer with thickness ca. 10 nm which is attributed to PDA (Fig. 3b). Owing to the excellent adhesion ability of the catechol group, dopamine has a strong inclination to spontaneous deposit a conformal PDA film on virtually any substrates.^16–18 X-ray energy dispersive spectroscopy (Fig. 3c) shows that not only the Fe, S, O, F elements but also N are uniformly distributed in the material, suggesting that PDA is evenly coated on the Li_1−xFeSO₄F sample.

Fig. 3 TEM images of (a) LiFeSO₄F and (b) PDA@Li_1−xFeSO₄F; (c) mappings of Fe, S, O, F, N elements of the PDA@Li_1−xFeSO₄F material.

X-ray photoelectron spectroscopy (XPS) was used to study the surface chemistry of the materials (ESI, Fig. S3†). The N 1s peak of PDA@Li_1−xFeSO₄F could be fitted by two components centred at 400.2 and 402.1 eV which are assigned to the C–N and N–H bonds of PDA, respectively.¹⁹ The Fe 2p XPS of LiFeSO₄F shows two peaks at 711.3 and 724.8 eV, corresponding to the 2p_3/2 and 2p_1/2 binding energies of Fe²⁺, respectively.²⁰ As for PDA@Li_1−xFeSO₄F, the above XPS peaks shift to 713.9 and 727.6 eV which are close to those of Fe³⁺ ions.²¹ Mössbauer spectroscopy was further used to analysis the Fe²⁺/Fe³⁺ concentrations and their coordinative environments in the crystal lattice. As shown in Fig. 4, the Mössbauer spectrum of LiFeSO₄F is composed of two sets of well fitted doublets. These doublets have almost equal area indicating that the iron ions equally occupy two crystallographic sites, i.e. Fe(1) and Fe(2). The obtained isomer shift, 1.28 mm s⁻¹, fits well with that of high spin Fe²⁺. The Mössbauer spectrum of PDA@Li_1−xFeSO₄F indicates that about 87 at% of the Fe²⁺ ions are oxidized to Fe³⁺. The oxidation power of Fe³⁺, φ (Fe³⁺/Fe²⁺) = 0.771 V vs. SHE (standard hydrogen electrode), is much higher than that needed for adsorption of dopamine (φ ≈ 0.2 V vs. SHE).²² Consequently, the adsorbed dopamine monomers are self-polymerized to form a PDA film on the Li_1−xFeSO₄F surface. Reasonably, the pristine LiFeSO₄F which do not contain Fe³⁺ ions could not initiate self-polymerization.

Fig. 4 Mössbauer spectroscopy of the LiFeSO₄F and PDA@Li_1−xFeSO₄F samples.

Fig. 5a shows the first charge–discharge curves of the samples evaluated at the 0.1C rate (I = 15 mA g⁻¹). Different from the pristine LiFeSO₄F, the electrochemical reactions of PDA@Li_1−xFeSO₄F starts with a discharge process due to lack of Li in the material. Both materials show a pair of voltage plateaus at about 3.6 V which are attributed to the LiFeSO₄F/FeSO₄F two-phase transition. The voltage gap between the charge/discharge plateaus gets to be smaller with PDA coating, indicating that the PDA film reduces the polarization and ohmic resistance of the battery. During the first charge process, the PDA@Li_1−xFeSO₄F seems to have a small additional potential plateau at ca. 4.4 V. In order to clearly understand this additional potential plateau, we study the CV curves of PDA and PDA@Li_1−xFeSO₄F (ESI, Fig. S4†). The CV curve of PDA@Li_1−xFeSO₄F shows an irreversible oxidation peak at ca. 4.2 V which is correlated with the aforementioned additional potential plateau. This current peak is consistent well with the oxidation peak of PDA. This indicates that the PDA layer of PDA@Li_1−xFeSO₄F is oxidized in the initial electrochemical process. The oxidized PDA layer could not be reduced again in the following electrochemical process which indicates high stability of the surface coating layer. The initial charge/discharge capacities of pristine LiFeSO₄F are 119/101 mA h g⁻¹, resulting in a coulombic efficiency of 84.9%. The irreversible capacity loss could be attributed to the formation of solid electrolyte interface (SEI) film.²³ The formation of SEI film is further confirmed by TEM analysis. It is seen from Fig. 6a that a thick SEI film was formed on the surface of LiFeSO₄F after the first charge. But only a thin surface film is observed for PDA@Li_1−xFeSO₄F (Fig. 6b) which could be attributed to the PDA layer and the SEI film. This indicates that PDA@Li_1−xFeSO₄F has less SEI film than that of LiFeSO₄F. Fig. 5b shows the cycling performances of the samples. The discharge capacity of LiFeSO₄F gradually decreases to 61 mA h g⁻¹ after 100 cycles, corresponding to capacity retention of 60.4%. The fast capacity fading could be attributed to the relatively large volume change of 10.4% between the LiFeSO₄F and FeSO₄F phases resulting in loss of electrical contact after repeated lithium removal and uptake. In comparison, the initial discharge/charge capacities of PDA@Li_1−xFeSO₄F are recorded as 121/114 mA h g⁻¹, resulting in a larger coulombic efficiency of 94.2%. Moreover, the material shows improved cycle stability. A much larger discharge capacity of 97 mA h g⁻¹ is obtained after 100 cycles, corresponding to capacity retention of 85.1%. The rate dependent cycling performances of the materials are displayed in Fig. 5c. At each current rate, PDA@Li_1−xFeSO₄F always shows a larger discharge capacity than that of LiFeSO₄F. For example, LiFeSO₄F delivers a discharge capacity of 14 mA h g⁻¹ at the 1C rate, while PDA@Li_1−xFeSO₄F exhibits a much larger discharge capacity of 56 mA h g⁻¹. When the current rate returns to C/20, the discharge capacity can still return to 114 mA h g⁻¹. The low rate capability of LiFeSO₄F is most likely related to the poor electronic conductivity of the material.

Fig. 5 (a) Initial charge–discharge profiles and (b) cycling performance of LiFeSO₄F and PDA@Li_1−xFeSO₄F at the current rate of C/10; (c) rate dependent cycling performance and (d) Nyquist plots of the samples.

Fig. 6 TEM images of the LiFeSO₄F (a) and PDA@Li_1−xFeSO₄F (b) materials after the first charge; SEM images of the LiFeSO₄F (c and e) and PDA@Li_1−xFeSO₄F (d and f) electrodes after 50 cycles.

Electrochemical impedance spectroscopy was used to study the effects of PDA coating on the electrochemical kinetic properties of LiFeSO₄F. The impedance data were collected at 3.5 V of the first discharge. In the Nyquist plots of the materials (Fig. 5d), the well-defined semicircles in the high-to-medium frequency region are due to the SEI film and the charge transfer process, respectively.²⁴ The slope line in the low frequency region is due to lithium diffusion in the electrode bulk. Based on this, the Nyquist plots can be simulated by the equivalent circuit given in the inset of Fig. 5d. In this equivalent circuit, R_s represents the ohmic resistance of the cell. R_f and C_sl are the resistance and capacitance of the SEI film, respectively. R_ct and C_dl represent the charge transfer resistance and double layer capacitance, respectively. W is the Warburg diffusion parameter. The simulated R_s, R_f and R_ct values are listed in Table 1. It clearly shows that all of the electrochemical kinetic parameters of PDA@Li_1−xFeSO₄F are smaller than those of LiFeSO₄F. Specifically, the smaller R_f of PDA@Li_1−xFeSO₄F is attributed to its thin SEI film as observed by TEM, and its smaller R_ct indicates improved charge transfer kinetics of the electrode after PDA coating.

Table 1 Fitted electrochemical kinetic parameters of the LiFeSO₄F and PDA@Li_1−xFeSO₄F samples

	Rs (Ω)	Rf (Ω)	Rct (Ω)
LiFeSO4F	1.694	107.8	610.5
PDA@Li1−xFeSO4F	2.22	84.36	186.5

---

As an organic semiconductor, the electronic conductivity of PDA is only 10⁻¹³ S cm⁻¹.^1,25 AC impedance spectroscopy (Fig. S5†) shows that the electronic conductivity of PDA@Li_1−xFeSO₄F is 1.55 × 10⁻¹¹ S which is even a little smaller than that of pristine LiFeSO₄F. Therefore, it seems that the improved electrochemical performance of PDA@Li_1−xFeSO₄F is not ascribed to the electronic conductivity of PDA. In spite of this, the strong and robust wet-resistant adhesion of PDA could be highly beneficial for the battery performance since all the battery components are in contact with each other within a liquid environment. Inspired by the wet-resistant adhesion ability of PDA, Choi et al. prepared PDA modified polyethylene separator for lithium ion batteries. After PDA modification, the contact angle of the separator with the electrolyte was significantly decreased, indicating that the surface of the separator becomes highly hydrophilic.²⁶ Moreover, PDA was also used as a binder for the silicon anode,²⁷ as well as a composite additive for SnO₂ to substantially improve the electrochemical properties.²⁸ Fig. 6c–f shows the SEM images of the LiFeSO₄F and PDA@Li_1−xFeSO₄F electrodes after 50 cycles. It is seen that the electrode film of PDA@Li_1−xFeSO₄F is very uniform after charge–discharge cycling. From the cross section image, it is seen that the electrode film of LiFeSO₄F is cracked and tends to be peeling off from the current collector. On the contrary, the electrode film of PDA@Li_1−xFeSO₄F is still compact and sticks on the current collector very well. The improved electrode stability could be attributed to the wet-resistant adhesion property of PDA which improves the compatibility of the electrode. In addition, the intrinsic elasticity of PDA is beneficial in allowing the LiFeSO₄F material to release its partial pressure caused by the large volume change during Li insertion/extraction. Additionally, Zhang and co-workers reported that tavorite LiFeSO₄F is highly sensitive to moisture, which will rapidly decompose to FeSO₄·nH₂O and LiF in a humid environment.²⁹ After PDA coating, the water contamination in the electrolyte could be consumed by this polymer layer due to the hydration reaction of PDA with water,²⁵ thus eliminating the decomposition of LiFeSO₄F. Moreover, the PDA layer isolates the direct contact of the active material and the electrolyte, thus protecting the material from any possible side reactions at the electrode/electrolyte interface resulting in a thin SEI film.

Conclusions

In summary, we have prepared a PDA coated tavorite Li_1−xFeSO₄F cathode material via the self-polymerization process. The PDA coating layer could improve the hydrophilicity of the electrode, reduce the water contamination in the electrolyte, depress the side reactions of the electrode with the electrolyte; and accommodate the volume change of the electrode. Due to these advantages, the PDA coated material shows larger discharge capacities, higher rate capability and longer cycle life than the pristine LiFeSO₄F. This work provides a good example for improving the electrochemical performance of electrode materials by coating with hydrophilic and elastic polymers.

Acknowledgements

This work was supported by Ministry of Science and Technology of China (No. 2015CB251103), National Natural Science Foundation of China (No. 51472104, 51272088, 21201073), Jilin Provincial Science and Technology Department (No. 20150204078GX, 20150204030GX).

Notes and references

1. N. Recham, J. N. Chotard, L. Dupont, C. Delacourt, W. Walker, M. Armand and J. M. Tarascon, Nat. Mater., 2010, 9, 68–74.
2. R. Tripathi, T. N. Ramesh, B. L. Ellis and L. F. Nazar, Angew. Chem., Int. Ed., 2010, 49, 8738–8742.
3. M. Ati, B. C. Melot, J. N. Chotard, G. Rousse, M. Reynaud and J. M. Tarascon, Electrochem. Commun., 2011, 13, 1280–1283.
4. C. Delacourt, L. Laffont, R. Bouchet, C. Wurm, J.-B. Leriche, M. Morcrette, J.-M. Tarascon and C. Masquelier, J. Electrochem. Soc., 2005, 152, A913–A921.
5. X. Li and Y. Xu, Electrochem. Commun., 2007, 9, 2023–2026.
6. S.-X. Liao, C.-H. Shen, Y.-J. Zhong, W.-H. Yan, X.-X. Shi, S.-S. Pei, X. Guo, B.-H. Zhong, X.-L. Wang and H. Liu, RSC Adv., 2014, 4, 56273–56278.
7. Z.-J. Zhang, S.-L. Chou, Q.-F. Gu, H.-K. Liu, H.-J. Li, K. Ozawa and J.-Z. Wang, ACS Appl. Mater. Interfaces, 2014, 6, 22155–22165.
8. P. Zhang, L. Zhang, X. Ren, Q. Yuan, J. Liu and Q. Zhang, Synth. Met., 2011, 161, 1092–1097.
9. W.-J. Li, S.-L. Chou, J.-Z. Wang, J.-L. Wang, Q.-F. Gu, H.-K. Liu and S.-X. Dou, Nano Energy, 2015, 13, 200–207.
10. D. Cintora-Juarez, C. Perez-Vicente, S. Ahmad and J. L. Tirado, Phys. Chem. Chem. Phys., 2014, 16, 20724–20730.
11. D. Lepage, C. Michot, G. Liang, M. Gauthier and S. B. Schougaard, Angew. Chem., Int. Ed., 2011, 50, 6884–6887.
12. C. Delacourt, M. Ati and J. M. Tarascon, J. Electrochem. Soc., 2011, 158, A741–A749.
13. A. V. Radha, J. D. Furman, M. Ati, B. C. Melot, J. M. Tarascon and A. Navrotsky, J. Mater. Chem., 2012, 22, 24446–24452.
14. C. Cao, L. Tan, W. Liu, J. Ma and L. Li, J. Power Sources, 2014, 248, 224–229.
15. F. Yu, S. Chen, Y. Chen, H. Li, L. Yang, Y. Chen and Y. Yin, J. Mol. Struct., 2010, 982, 152–161.
16. Y. Liu, K. Ai and L. Lu, Chem. Rev., 2014, 114, 5057–5115.
17. L. Yang, S. L. Phua, J. K. H. Teo, C. L. Toh, S. K. Lau, J. Ma and X. Lu, ACS Appl. Mater. Interfaces, 2011, 3, 3026–3032.
18. H. Lee, S. M. Dellatore, W. M. Miller and P. B. Messersmith, Science, 2007, 318, 426–430.
19. H. Li, L. Shen, K. Yin, J. Ji, J. Wang, X. Wang and X. Zhang, J. Mater. Chem. A, 2013, 1, 7270–7276.
20. C. V. Ramana, A. Ait-Salah, S. Utsunomiya, J. F. Morhange, A. Mauger, F. Gendron and C. M. Julien, J. Phys. Chem. C, 2006, 111, 1049–1054.
21. R. Dedryvère, M. Maccario, L. Croguennec, F. le Cras, C. Delmas and D. Gonbeau, Chem. Mater., 2008, 20, 7164–7170.
22. U. Chandra, B. E. Kumara Swamy, O. Gilbert and B. S. Sherigara, Electrochim. Acta, 2010, 55, 7166–7174.
23. Z. Guo, D. Zhang, H. Qiu, T. Zhang, Q. Fu, L. Zhang, X. Yan, X. Meng, G. Chen and Y. Wei, ACS Appl. Mater. Interfaces, 2015, 7, 13972–13979.
24. B. Yan, M. Li, X. Li, Z. Bai, L. Dong and D. Li, Electrochim. Acta, 2015, 164, 55–61.
25. M. M. Jastrzebska, H. Isotalo, J. Paloheimo and H. Stubb, J. Biomater. Sci., Polym. Ed., 1996, 7, 577–586.
26. Y. Lee, M.-H. Ryou, M. Seo, J. W. Choi and Y. M. Lee, Electrochim. Acta, 2013, 113, 433–438.
27. C. Fang, Y. Deng, Y. Xie, J. Su and G. Chen, J. Phys. Chem. C, 2015, 119, 1720–1728.
28. L. Wang, D. Wang, Z. Dong, F. Zhang and J. Jin, Small, 2014, 10, 998–1007.
29. L. Zhang, J.-M. Tarascon, M. T. Sougrati, G. Rousse and G. Chen, J. Mater. Chem. A, 2015, 3, 16988–16997.

---

Footnote

† Electronic supplementary information (ESI) available: Experiment details, TG curves, SEM images, XPS spectra. See DOI: 10.1039/c5ra24488a

---