Electrochemical properties of carbonyl substituted phthalocyanines as electrode materials for lithium-ion batteries

Abstract

Tetra-carboxylic acid lithium phthalocyanine (t-CAL-Pc) and tetra-(di-acyl) substituted phthalocyanine (t-DA-Pc) were prepared to construct a new type of organic electrode material. The electrochemical properties were investigated and the effects of group substitution on the electrochemical properties were analyzed for the first time. The results indicated that after the hydrogen in the carboxyl group was substituted by lithium and a di-acyl group respectively, the cyclic stability performance of phthalocyanines (t-CAL-Pc and t-DA-Pc) was improved and the irreversible capacities were reduced. The capacities can still remain at 169 mA h g⁻¹ after 100 cycles for t-CAL-Pc and 241 mA h g⁻¹ after 200 cycles for t-DA-Pc, and the initial columbic efficiencies were 94% and 97% respectively, which were much higher than that of 70% for tetra-carboxyl phthalocyanine (t-C-Pc).

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In the 21st century, scientists have not given up trying to improve the electrochemical performance of existing batteries and are constantly seeking novel electrode materials.^1,2 In the past two decades, Li-ion batteries have achieved superior comprehensive battery performance. However, their development has slowed recently because of the following shortcomings of traditional inorganic electrode materials (LiCoO₂, LiNiO₂, LiMn₂O₄, LiFePO₄, etc.), such as limited space for capacity improvement, failure to meet the requirements of sustainable development, existence of security risks and high cost.^3,4

Compared with inorganic cathode materials, electroactive organics involving reversible redox reactions⁵ as electrode materials of rechargeable lithium-ion batteries (LIBs) have captured worldwide attention due to their high capacities, molecular controllability, structural diversity, and resource renewability. Recently, a lot of work concerned with organic compounds as cathode materials for lithium ion batteries has been carried out,^6–10 especially some high electrochemical activity of carbonyl functional groups gives a pointer to the design of cathode organic compounds. Some studies on the methods of solving the problem of solubility in the electrolyte^11–13 and the low conductivity of organic cathode materials^14–16 may yet provide essential information. However, for the small molecule of organic carbonyl compounds, there still have problems of low utilization rate of active sites,¹⁷ high solubility in the electrolyte¹⁸ and poor conductivity¹⁹ etc., which limits the performance of Li-ion battery.

By substituting the carbonyl groups into the stable macrocyclic conjugated system is an effective way to improve the cyclic stability and conductivity of organic cathode materials. Phthalocyanine based compounds have highly conjugated π electron system, make it show extraordinary optical, electrical and magnetic properties.²⁰ In our previous work,²¹ the electrochemical performances of tetra and octa carboxyl substituted phthalocyanines are studied. Although carboxyl substituted nickel phthalocyanines possess superior capacities, the irreversible capacity is large and the early decay rate is very fast, because of the presence of hydrogen in the carboxyl group. In this communication, novel molecules of 2 (t-CAL-Pc) and 3 (t-DA-Pc) were prepared adopting the method of substituting proton hydrogen in the carboxyl group of 1 (t-C-Pc) by lithium and a di-acyl groups (Scheme 1), the electrochemical properties was reported for the first time to our knowledge.

Scheme 1 Molecular models of substituted phthalocyanine compounds of 2 (t-CAL-Pc) and 3 (t-DA-Pc).

CH₃OLi (99.9%) was purchased from Klamar Reagent Company Inc, Shanghai. The pyromellitic diimide potassium salt (97%) was purchased from Alfa Aesar (China) Chemical Co. Ltd., Shanghai. The synthetic procedures of 1 (t-C-Pc) were previously described and summarized.²¹ For preparing 2 (t-CAL-Pc), phthalocyanine 1 (t-C-Pc) (4.33 g, 0.005 mol) was added to a 500 mL three-necked flask, 200 mL of methanol containing 1% CH₃OLi was added, and stirred to reflux at 80 °C overnight. The ultimate reaction mixture was precipitated, filtered and washed repeatedly in methanol. The residual product was heated at 120 °C for 24 h, to obtain 6.2 g (yield: 81%) of 2 (t-CAL-Pc) (C₃₆H₁₂N₈NiO₈Li₄). MALDI-TOF: 770.5 (M⁺), 719.4 (M⁺ − COOLi), 668.5 (M⁺ − 2COOLi); ¹H-NMR (DMSO-d₆, 400 Hz): 8.35–8.54 (m, 4H), 7.86–7.18 (m, 8H); elemental analysis (%): calculated C 56.08, H 1.57, N 14.53; found C55.93, H 1.66, N 14.44; IR (KBr): ν = 2961 (m; Ar–H), 2156 (m; C=C), 1550 (s; C=O), 1400 (s; C–O); UV-vis (NMP): λ_max (ε) = 320 (4500), 460 nm (3900), 660 nm (2100), and 702 nm (2300).

For preparing 3 (t-DA-Pc), 1 (t-C-Pc) (4.33 g, 0.005 mol) in 100 mL DMF was added to a 500 mL three-necked flask, stirred to reflux at 80 °C in a nitrogen atmosphere. Then an excess of sulfinyl dichloride (3.57 g, 0.03 mol) was slowly dropped into the mixture and allowed to reflux for 12 h. After the excess sulfinyl dichloride was removed by vacuum distillation, 4.17 g (0.025 mol) of pyromellitic diimide potassium salt was added to the residual mixture, heated to 120 °C, and allowed to reflux for 12 h. The ultimate reaction mixture was poured into 300 mL of dichloromethane, precipitated under stirring, filtered, and repeatedly washed by dichloromethane. The residual product was heated at 120 °C for 12 h, to obtain 4.2 g (yield: 55%) of compound 3 (t-DA-Pc) (C₇₆H₂₀N₁₆NiO₂₀). MALDI-TOF: 1535.7 (M⁺), 1270.6 (M⁺ − 1R), 1005.4 (M⁺ − 2R); ¹H-NMR (DMSO-d₆, 400 Hz): 11.51 (s, 8H), 8.47–8.59 (m, 4H), 7.92–7.28 (m, 8H); elemental analysis (%): calculated C 56.08, H 1.24, N 13.77; found C 55.91, H 1.34, N 13.59. IR (KBr): ν = 3336 (w; N–H), 2961 (m; Ar–H), 2156 (m; C=C), 1605 (w; C=O), 1377 (m; C–O); UV-vis (NMP): λ_max (ε) = 310 (2400), 380 nm (1700), 470 nm (1400), 655 nm (1200), and 700 nm (1300).

Preparation of electrode material and buckle type battery: 2.4 g of carboxylic phthalocyanines (t-CAL-Pc or t-DA-Pc) and 0.09 g iodine were added to a sealed sample bottle, and placed in an oven at 120 °C for 12 h. Then, 3.0 g of a mixture including carboxylic phthalocyanine (doped I₂), acetylene black and graphene (83 : 10 : 2) were ball-milled for 4 h, then 5 mL of NMP dissolved 0.15 g PVDF (5%) was added and ball-milled for another 4 h. The resulting slurry was evenly coated onto a nickel mesh and dried overnight in a vacuum oven at 120 °C. The electrode was cut into circular pieces with a diameter of 1.2 cm for coin cell testing, and the area mass loading of the electrode was approximately 8 mg cm⁻². Li-ion batteries were assembled with lithium metal as the counter electrode, 1 M LiPF₆ in a mixture of ethylene carbonate/diethyl carbonate (EC/DEC, 1 : 1 by volume) as the electrolyte, and Celgard®3501 (Celgard, LLC Corp., USA) as the separator.

Before the electrochemical performance was tested, all the cells were pre-treated by discharging them against lithium from an open-circuit voltage (OCV) (1.9–2.5 V) to 0.3 V at a 0.1 mA current, and finally charging it against lithium to 3.2 V at the same rate. The electrochemical performance was tested using an Arbin battery test station (BT2000, Arbin Instruments, USA). Capacity was calculated on the basis of the mass of carboxylic phthalocyanine. Cyclic voltammograms were recorded at a scan rate of 0.1 mVs⁻¹ between 0.3 and 3.0 V using a Solatron 1260/1287 Electrochemical Interface (Solatron Metrology, UK). Impedance analysis was also assessed using Solatron 1260/1287 Electrochemical Interface kit.

Fig. 1(A and B) showed the UV-vis absorption spectra of iodine doped 2 (t-CAL-Pc) and 3 (t-DA-Pc). After doping with I₂, the Q-band absorption between 600 and 700 nm became wider and weaker, and a new absorption peak appeared at about 470 nm, which is due to the formation of CTC complex with the aggregation structure of two ordered molecular columns phthalocyanin.²¹ Normally, phthalocyanine with its aggregation structure would lead to an obvious change of Q-band absorption,²² i.e. the Q-band with an absorption peak becomes wider and shorter. In addition, the new absorption peak at 470 nm is produced due to the formation of I₃⁻.²³

Fig. 1 UV-vis spectra in NMP solution before and after I₂ doping (A) 2 (t-CAL-Pc), (B) 3 (t-DA-Pc); SEM micrographs of 2 (t-CAL-Pc) (C) powder, (D) after coating on nickel mesh.

Fig. 1(C and D) showed the particle size of 2 (t-CAL-Pc) before and after coating onto the nickel mesh. The size of phthalocyanine particles after doping with I₂ and coating onto the nickel mesh (Fig. 1D) was enhanced compared with that of phthalocyanine powers before I₂ doping (Fig. 1C). After doping I₂, the aggregation particle size of 2 (t-CAL-Pc) powder is larger, and the aggregation particles become more dense (ESI Fig. S1†). This result further proved that the formation of CTC after I₂ doping, formed an aggregated structure of one-dimensional conductor, which enhanced the size of the active phthalocyanine particles. The morphology after I₂ doping changed from small thickness irregular blocks, to larger, thicker planar blocks.

After I₂ doping, the intermolecular distance between the two phthalocyanine rings was shortened, resulting in the enhancement of electronic cloud overlaps: the bandwidth widened and the energy gap narrowed, so as to improve the conductivity significantly.^21,24 This conclusion can be confirmed by EIS spectra in Fig. 2, the charge transfer resistance of 1 (t-C-Pc) (10% CB + 2% KS-6 + 3% I₂) was about 50 Ω, which was lower than that of 1 (t-C-Pc) (doped with 12.5% CB + 2.5% KS-6, 150 Ω) and 1 (t-C-Pc) (undoped conductor, 400 Ω). While in the sample containing an undoped conductor on the copper foil, the charge transfer resistance was about 3000 Ω, which was much larger than that of samples coated on nickel foam. The charge transfer resistances of carbonyl phthalocyanines 1 to 3 were 50 Ω, 75 Ω, and 150 Ω respectively. Taking I₂ doping and foam nickel coating technologies, the electrical conductivity of the phthalocyanine electrode can be effectively improved.

Fig. 2 EIS spectra of carboxylic phthalocyanine electrode doped with different ratios of conducting agent on nickel foam and undoped conductor on copper foil.

The cyclic voltammograms (CV) in Fig. 3a show broad peaks at 1.15 V, 1.25 V, and 1.05 V with small shoulders at 1.60 V, 1.70 V, and 1.50 V during lithiation for phthalocyanines 1 to 3, respectively. The sharp peak at 0.3 V became weaker after each cycle, suggesting that an irreversible reaction was occurred during the first cycle, and resulting in irreversible capacity for 1 (t-C-Pc). However, after the hydrogen in 1 (t-C-Pc) was substituted by lithium and a di-acyl group, the irreversible peak at 0.3 V for 2 (t-CAL-Pc) and 3 (t-DA-Pc) disappeared and the symmetry of CV curves improved. The change of this symmetry in CV curves for 2 (t-CAL-Pc) and 3 (t-DA-Pc) may have implied an improvement in charge/discharge cycling performance.

Fig. 3 Electrochemical properties of phthalocyanines 1 (t-C-Pc), 2 (t-CAL-Pc) and 3 (t-DA-Pc). (a) Cyclic voltammetry curves; (b) charging/discharging cycle curves; (c) charging/discharging efficiency.

Fig. 3(b and c) shows typical charge/discharge curves of phthalocyanines (1–3) coated on nickel foam at a constant current of 0.4 mA. The average discharge plateaus were around 1.3 V, 0.8 V, and 1.0 V for phthalocyanines 1 to 3 respectively. The first discharge and charge capacities were 841 and 586 mA h g⁻¹ for 1 (t-C-Pc) respectively. The initial columbic efficiency of 1 (t-C-Pc) was very low (about 70%). However, the initial discharge capacities were 190 and 274 mA h g⁻¹ and the initial charge capacities were 178 and 266 mA h g⁻¹ with initial columbic efficiencies of 94% and 97% for 2 (t-CAL-Pc) and 3 (t-DA-Pc) respectively. The initial efficiencies were obviously improved for 2 (t-CAL-Pc) and 3 (t-DA-Pc) relative to that of 1 (t-C-Pc). However, the initial capacities of 2 (t-CAL-Pc) and 3 (t-DA-Pc) decreased compared with 1 (t-C-Pc), which is due to the electron donating effect of additional multiple functional groups (Li⁺–O⁻ and pyromellitic diimide) presenting a lower capacity and a lower discharge plateau voltage. This was also why 2 (t-CAL-Pc) and 3 (t-DA-Pc) had lower discharge plateau voltages (0.8 V and 1.0 V) than that of 1 (t-C-Pc) (1.3 V).

On the basis of molecular weights for compound 1 (M_w = 747), 2 (M_w = 770), and 3 (M_w = 1535), and the 4-, 4-, and 20-step-redox processes, the theoretical capacities of the batteries were calculated as approximately 143, 139, and 347 mA h g⁻¹ for compounds 1 to 3, respectively. However, observed capacities were 841, 190, and 274 mA h g⁻¹ for compounds 1 to 3, respectively. The observed capacities of 1 (t-C-Pc) and 2 (t-CAL-Pc) were much higher than their theoretical capacities. Therefore, in addition to the contribution of the carbonyl group to the capacity of lithium intercalation, it was suspected that the redox processes of phthalocyanine parts may also engender energy storage. The feasibility of cathodes containing phthalocyanines (Pc) in secondary lithium cells was studied in our previously reported paper.²¹ It is reported that discharge capacities of more than 17 and 26 electrons per molecule of phthalocyanine requiring the intercalation of the equivalent number of charge compensating lithium cations. Metallophthalocyanine (MPc) derivatives have stacked layer structures and can promote Li⁺-intercalation, which is also one of the contributions to a higher capacity. While for compound 3 (t-DA-Pc), the observed capacity was lower than the theoretical capacity, which was due to the low active site utilisation rate. Although there were more active sites in compound 3 (t-DA-Pc) (20 sites) than in compounds 1 (t-C-Pc) and 2 (t-CAL-Pc) (4 sites), the utilisation efficiency thereof was much lower than that of compounds 1 and 2, because of its larger side chain structure.

It is interesting from Fig. 3 and ESI S2† that for compound 1, the charge/discharge process during the first 20 cycles is the process of compound 1 converting into compound 2, and the charge/discharge process after the 20 cycles is actually the lithiation process of compound 2. This can be convinced by the improved symmetry of the CV curves of compound 1, and the shape of the CV curves is more and more close to that of the compound 2. And it can be also verified from Fig. 3c that the capacity change trend of compound 1 after 20 cycles was almost identical to that of compound 2.

The capacity was able to remain at 169 mA h g⁻¹ after 100 cycles for 2 (t-CAL-Pc), and 241 mA h g⁻¹ after 200 cycles for 3 (t-DA-Pc). The capacity retention rates were 89% for 2 (t-CAL-Pc) (after 100 cycles) and 91% for 3 (t-DA-Pc) (after 200 cycles), respectively. After the proton hydrogen in compound 1 (t-C-Pc) was substituted by either a lithium ion or a bulk pyromellitic diimide group, it not only avoided the decomposition of the electrolyte, but also reduced the solubility of the active substance (see in ESI S3†), so that resulted in improvement of the cycle performance.

In conclusion, the proton hydrogen in phthalocyanine 1 (t-C-Pc) was substituted by a lithium ion and pyromellitic diimide group, to obtain 2 (t-CAL-Pc) and 3 (t-DA-Pc). Through I₂ doping and foam nickel coating technology, the conductivity of phthalocyanines have been improved obviously. The average discharge plateaus of the cells were around 1.3 V, 0.8 V, and 1.0 V for phthalocyanines 1 to 3, with initial capacities of 841, 190, and 274 mA h g⁻¹, respectively. After being substituted by either a lithium ion or pyromellitic diimide group, the symmetry of the CV curves were improved. The initial columbic efficiencies were 94% and 97%, which were much higher than the value of 70% for 1 (t-C-Pc). In addition, the capacities remained at 169 mA h g⁻¹ after 100 cycles for 2 (t-CAL-Pc), and 241 mA h g⁻¹ after 200 cycles for 3 (t-DA-Pc). Obvious improvements were made in the initial columbic efficiency and capacity retention rates. As a new type of electrode material, carbonyl substituted phthalocyanine compounds would give important insights into developing a new generation of organic electrode materials with higher performance.

Acknowledgements

Authors would express their sincere thanks to Science Research Project of Jiangxi Provincial Department of Education (No. GJJ150672), the Science and Technology Project of Jiangxi Province (No. 508144667047, 20141BBE50019) and the National Natural Science Foundation of China (No. 51372104).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra09826f

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