Facile fabrication of red phosphorus/TiO₂ composites for lithium ion batteries

Abstract

Red phosphorus (RP) is an attractive anode material with an ultrahigh specific capacity of 2596 mA h g⁻¹. However, its rapid capacity decay attributed to the volume expansion during the lithiation process presents a noteworthy technical challenge. Meanwhile, titanium oxide (TiO₂) is a good candidate for lithium ion batteries owing to its high safety and outstanding stability, but it is restricted by the low capacity of 167 mA h g⁻¹ at room temperature. Inspired by reinforced concrete structures, we fabricate an RP built-in amorphous TiO₂ (A-TiO₂) composite in consideration of achieving complementary effects. Herein, A-TiO₂ could act as “concrete” to prevent RP from escaping the electrode. While RP plays the role of “steel”, which could improve the electrochemical capacity of the composite. As a result, the RP/A-TiO₂ composite demonstrates an enhanced cycling capacity of 369 mA h g⁻¹ over 100 cycles as well as an acceptable rate capacity of 202 mA h g⁻¹ at the current density of 1 A g⁻¹. This designed unique reinforced concrete structure may provide a novel strategy to fabricate high electrochemical performance anodic materials for advanced lithium ion batteries.

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1. Introduction

Rechargeable lithium ion batteries (LIBs) are considered as the most promising power sources for emerging portable electric vehicles and renewable power stations.^1–6 Massive efforts have been made to develop advanced electrode materials with high power density, long cycle stability, and practical reliability.^7–10 TiO₂ is one of the most attractive anode materials because of its high safety and outstanding stability.^11–14 However, the low theory capacity restricts its application. Fortunately, the remarkable stability during the lithium insertion/extraction processes provides a favourable possibility to composite with other modest anode materials.^15–20 In this aspect, elemental phosphorus is a particularly attractive anode material, which can react with 3 Li atoms to form Li₃P compounds, giving a theoretical specific capacity of 2596 mA h g⁻¹.²¹ This ultrahigh capacity makes phosphorus as a good candidate for compositing with TiO₂. Among the three allotropes of phosphorus, white phosphorus is highly toxic and easily oxidized, which is fundamentally unsuitable as an electrode material. While black phosphorus is an alternative anode material in consequence of graphite-like structure and good electrical conductivity.^22,23 Yet to synthesize black phosphorus, extremely high pressure (1.2 Gpa) is usually needed.²⁴ Red phosphorus is a suitable allotrope because it is abundant, safe, and chemically stable.^25,26 Nevertheless, due to its electronic insulation and volume expansion effects, the practical capacity of RP is far from the theoretical value. Recently, Jiang et al. reported that nanosized phosphorus hosted in porous carbon composite, which could deliver a highly reversible capacity.²⁵ However, RP composite with TiO₂ to enhance the electrochemistry capacity has not been reported yet.

Herein, we attempt to design and fabricate RP/A-TiO₂ composites via a reinforced concrete structural strategy. On the one hand, RP would greatly increase the power density of RP/A-TiO₂ composite. On the other hand, A-TiO₂ in rational constructed RP/A-TiO₂ could effectively stabilize the RP phase. In comparison with A-TiO₂ and bare RP, RP/A-TiO₂ composite exhibits the intensified capacity in consequence of this unique reinforced concrete structure.

2. Experimental section

2.1 Preparation of samples

2.1.1 Preparation of nanosized red phosphorus. In typical experiment, 20 g commercial RP (98.5%, purity) was processed by high energy ball-milling for 48 hours. The resulting RP powder was then added in 500 ml PVP (K30) aqueous solution (4 wt%). The mixed suspension was further treated by using ultrasonic for 2 hours. Afterward, in order to obtain the nanosized RP, the suspension (Scheme 1a) was placed in atmosphere for 48 hours.

Scheme 1 The preparation and structure illustration of RP (red color)/A-TiO₂ (green color).

2.1.2 Preparation of RP/A-TiO₂. 100 ml of suspension contained nanosized RP particles (Scheme 1a) was extracted to prepare RP/A-TiO₂ composite (Scheme 1b). Firstly, 2 g isopropyl titanate was uniformly dispersed into 20 ml ethanol. Then isopropyl titanate solution was slowly dripped into the as prepared RP suspension so that the hydrolyzed titanium dioxide would attach on phosphorus (Scheme 1c). After stirring for 10 minutes, the precipitation was centrifuged and washed for several times. Finally, the sample was dried at 80 °C in an oven for 12 hours (product is illustrated in Scheme 1d).

2.2 Characterization

The morphology of the product was observed by scanning electron microscopy (SEM, Hitachi S-4700) and transmission electron microscopy (TEM, FEI, Tecnai G2 F30). Powder X-ray diffraction (XRD) was performed using Rigaku Ultima IV with Cu Kα radiation (λ = 0.15418 nm) in the 2θ range from 10–80°. Nitrogen adsorption–desorption was determined by Brunanuer–Emmett–Teller (BET) tests using a Nova 1000e (Quantachrome Instruments) surface area and pore analyzer. The content of RP in RP/A-TiO₂ composite was tested by a thermo gravimetric analyzer (TGA, Q5000IR) in N₂ atmosphere with the heating rate of 10 °C min⁻¹. Fourier transform-infrared spectroscopy (FT-IR) spectrum was characterized on Nicolet 6700.

2.3 Electrochemical measurements

The electrochemical tests were performed using a coin-type half cell (CR 2025). Active material, acetylene black and polyvinylidene fluoride (PVDF) binder were mixed by the weight ratio of 70 : 15 : 15 in N-methylpyrrolidone (NMP) as the dispersant. The resultant viscous slurry was cast on copper foil and dried at 120 °C under vacuum for 12 hours. Cells were assembled in an argon-filled glovebox with the metallic lithium foil as the counter electrode, 1 M LiPF₆ in ethylene carbonate (EC)-dimethyl carbonate (DMC) (1 : 1 in volume) as the electrolyte, and a polypropyle (PP) microporous film (Cellgard 2300) as the separator. The charge–discharge test was carried out on a Neware battery test system in the voltage range of 0.01–3 V at room temperature. A CHI 660b work-station was applied for cyclic voltammograms (CV) tests in the voltage range of 0–3 V at a scan rate of 0.1 mV s⁻¹.

3. Results and discussion

Fig. 1a shows FT-IR spectrum of isopropyl titanate hydrolyzate. The broad peak at ∼3350 cm⁻¹ is corresponding to the surface-adsorbed water and hydroxyl groups.²⁷ The sharp peak at ∼600 cm⁻¹ confirms to the absorption of Ti–O bond.^28,29 The amorphous structure of A-TiO₂ is supported by broad XRD peaks in Fig. 1b. The peak around 15° in RP/A-TiO₂ pattern is in agreement with amorphous red phosphorus.^21,26 Nitrogen (N₂) adsorption–desorption isotherms BET results are depicted in Fig. 1c. The BET surface area of A-TiO₂ is identified to be 344.7 m² g⁻¹. However, RP/A-TiO₂ sample decreases to 254.2 m² g⁻¹, which can be attributed to the built-in RP. Both isotherms are typical type II with an inflection point around 0.2 relative pressure (p/p_o), representing surface adsorption consequence. TGA was carried out to investigate the content of RP in the RP/A-TiO₂ composite under N₂ atmosphere. As shown in Fig. 1d, A-TiO₂ shows a weight loss of 14.4% up to 450 °C, arising from the coordinated water. There is a sharp weight loss about 96% in RP sample ranging from 410 to 530 °C, which is resulting from the sublimation process. Based on the above results, the weight loss of 15.2% in RP/A-TiO₂ sample is due to both coordinated water and sublimated RP. Thus the content of RP in RP/A-TiO₂ is calculated to be 12.6%.

Fig. 1 (a) FT-IR spectrum of amorphous TiO₂. (b) XRD patterns of A-TiO₂ and RP/A-TiO₂. (c) Nitrogen (N₂) adsorption–desorption isotherms of A-TiO₂ and RP/A-TiO₂. (d) TGA curves of RP/A-TiO₂.

SEM observations were employed to characterize the morphology of the products. A-TiO₂ sample (Fig. 2a and b) consists of self-aggregated nanoparticles. Fig. 2c and d depict irregular particles of bare RP with a particle size ranging from micrometer to nanometer. In the counterpart (Fig. 2e and f), RP/A-TiO₂ composite has a rough surface, which is due to the hydrolyzed TiO₂ particles wrap on the surface of RP. As mentioned in the experimental part, A-TiO₂ nanoparticles are in situ hydrolysed in the preparing process. A part of A-TiO₂ will directly wrap on the surface of RP, while the rest of A-TiO₂ nanoparticles will fill into the interval among the A-TiO₂ coated RP particles since the main component is A-TiO₂ based on the TG results (Fig. 1d). Therefore the A-TiO₂ coated RP particles will act as “steel”, and the dispersed A-TiO₂ nanoparticles will act as “concrete”, achieving a unique reinforced concrete structure. Such reinforced concrete model offers a very stable structure, which has the following merits. Firstly, A-TiO₂ plays the role as “concrete”, which could provide better feasibility and plasticity to accommodate the volume change of RP particles and avoid the direct contact between RP and electrolyte during Li-insertion/extraction reactions. Secondly, RP particles act as “steel”, which will offer high electrochemical capacity and enhance the energy density for the composite. Thus this strategy will improve the structural stability and specific capacity.

Fig. 2 (a and b) SEM images of A-TiO₂; (c and d) SEM images of bare RP; (e and f) SEM images of RP/A-TiO₂.

TEM was utilized to get a deep insight into the detail microstructure. As revealed in Fig. 3a, the primary particle size of A-TiO₂ cluster is identified to be ∼50 nm. In order to further clarify the microstructural differences of three samples, TEM images of bare RP (Fig. 3c) and RP/A-TiO₂ (Fig. 3e) were also supplied. Compared with bare RP, RP/A-TiO₂ reveals plenty of nanosized particles (A-TiO₂) wrap on the surface of the inner nanosized particles (RP), matching well with the proposed reinforced concrete model. Fig. 3b, d and f represent the high-resolution TEM (HRTEM) images and selected area electron diffraction (SAED) patterns (insert in top right corner) of A-TiO₂, bare RP and RP/A-TiO₂ respectively. HRTEM images obviously show that all the three samples only have disordered lattice fringes, suggesting TiO₂, RP and RP/A-TiO₂ are all amorphous. Moreover, SAED patterns also confirm that all the samples have the poor crystallization, which is consistent well with the XRD results.

Fig. 3 (a and b) TEM and HRTEM images of A-TiO₂; (c and d) TEM and HRTEM images of bare RP; (e and f) TEM and HRTEM images of A-TiO₂/RP; the insert pictures in HRTEM are the corresponding SAED images.

To verify the composition of A-TiO₂ and RP/A-TiO₂, scanning transmission electron microscope (STEM) and area-scan elemental mapping images are supplied. Fig. 4a–c present the morphology and elements distribution of A-TiO₂. As seen in Fig. 4a, A-TiO₂ exhibits rough surface, which is similar to the SEM results (Fig. 2f). Taken from red square area of A-TiO₂ in Fig. 4a, the energy-dispersive spectroscopy (EDS) mappings demonstrate titanium element (Fig. 4b) and oxygen (Fig. 4c) element are dispersed homogeneously, implying the chemical composition of A-TiO₂. The detailed microstructure and elements mappings of RP/A-TiO₂ sample are showed in Fig. 4d–g. As seen in Fig. 4d, mapping square is selected in nanoparticles cluster area. Three elements of titanium (Fig. 4e), oxygen (Fig. 4f) and phosphorus (Fig. 4g) have the uniform distribution, suggesting the in situ formed A-TiO₂ is tightly contacted with RP particles. These results also indicate that RP/A-TiO₂ has the unique reinforced concrete structure.

Fig. 4 (a) STEM image of A-TiO₂; elemental mappings of titanium (b) and oxygen (c); (d) STEM image of RP/A-TiO₂; elemental mappings of titanium (e), oxygen (f), and phosphorus (g).

Fig. 5 depicts the electrochemical performance of A-TiO₂ and RP/A-TiO₂. There is a pair of broad anodic/cathodic peaks in the CV curves of A-TiO₂ (Fig. 5a), corresponding to the typical lithium insertion/extraction potential of A-TiO₂.³⁰ Compared with A-TiO₂ sample, the broad redox potential of RP/A-TiO₂ (Fig. 5b) is lower than A-TiO₂. This result can be attributed to the electrode polarization since the RP has a much poorer electronic conductivity.³¹ Moreover, a small anodic peak at ∼2.4 V can be seen in the first two cycles, representing some irreversible process. As shown in Fig. 5c, the first discharge and charge capacities of A-TiO₂ sample are 440 and 180 mA h g⁻¹, respectively. The irreversible capacity loss reaches up to 260 mA h g⁻¹. The sharply dropped capacity loss could be explained by the formation of solid electrolyte interface (SEI) layer³² and the reaction of H₂O/–OH species adsorbed at the surface of sample.³³ Meanwhile, the first discharge and charge capacities of RP/A-TiO₂ sample are 770 and 450 mA h g⁻¹, which has a larger irreversible capacity loss of 320 mA h g⁻¹. This larger irreversible capacity loss compared with A-TiO₂ could be related to the formation of SEI layer in RP component.²⁵

Fig. 5 (a and b) CV curves of A-TiO₂ and RP/A-TiO₂, respectively. (c and d) Discharge–charge profiles of A-TiO₂ and RP/A-TiO₂ with a current density of 100 mA g⁻¹, respectively. (e) Cycling performances of A-TiO₂ and RP/A-TiO₂ with a current density of 100 mA g⁻¹. (f) Rate performance of A-TiO₂ and RP/A-TiO₂ electrodes.

Fig. 5e presents the cycling performance of RP, A-TiO₂, and RP/A-TiO₂ samples. As seen in Fig. 5e, the capacity of bare RP sharply falls from 2222 to 143 mA h g⁻¹ in the initial several cycles, and continually fall to 12 mA h g⁻¹ at 30th cycle. That exhibits an extremely poor electrochemical performance. However, A-TiO₂ and RP/A-TiO₂ electrodes show the excellent cycling stability. The Coulombic efficiency of A-TiO₂ and RP/A-TiO₂ steadily reaches around 99% accompanied by the cycle number increasing. The discharge capacities of bare RP (30th), A-TiO₂ (50th) and RP/A-TiO₂ (100th) are 12, 157 and 369 mA h g⁻¹, corresponding to 21.1, 93.4 and 89.3% capacity retentions of their initial discharge capacities (here we use the 3rd cycle data because the discharge capacity is becoming stable) of 57, 167 and 413 mA h g⁻¹, respectively. It is note that the capacity retention of RP/A-TiO₂ at 50th reaches to 92.3%, which is comparable with the A-TiO₂ result of 93.4%. However, RP/A-TiO₂ shows a much higher capacity retention of 369 mA h g⁻¹ compared with bare RP and A-TiO₂. Fig. 5f displays the rate performance of A-TiO₂ and RP/A-TiO₂ at various rates current densities. The discharge capacities of A-TiO₂ and RP/A-TiO₂ are reserved at 388/177, 336/141, 285/117, and 202/97 mA h g⁻¹ at the current densities of 100, 200, 500 and 1000 mA g⁻¹, respectively. It is noticed that the capacities at higher currents faded rapidly. However, when the current is restored to 100 mA g⁻¹, A-TiO₂ and RP/A-TiO₂ both deliver the reversible capacity of 359 and 174 mA h g⁻¹, respectively. This phenomenon is mainly due to the kinetic-limited effects of the electrochemical reaction in nature, rendering a higher over potential and a lower capacity at a higher current.³⁴ Obviously, RP/A-TiO₂ composite has a high reversible capacity and stable cycling performance that is mainly attributed to the unique with reinforced concrete structure. On the one hand, A-TiO₂ stores lithium by addition-type reaction (xLi⁺ + TiO₂ + xe⁻ → Li_xTiO₂),³⁰ which has a remarkable structural stability. It also can prevent the continuous capacity loss of RP derived from the crack and pulverization. On the other hand, RP has a large capacity in nature, which could provide more reversible capacity for the RP/A-TiO₂ composite.

Since RP is extremely unstable during the Li⁺ insertion/extraction, we do the comparison of fresh and cycled RP/A-TiO₂ electrode via STEM and area-scan elemental mapping tests to clarify the structural stability directly. Fig. 6a and f clearly present the STEM image of fresh RP/A-TiO₂ electrode. Combining with elemental mapping analysis, P, Ti and O (Fig. 6b–d) are well dispersed in the acetylene black (Fig. 6e). For the cycled counterpart, the electrode was charged to 3 V (lithium extraction status) after 100 cycles. Elemental mappings in Fig. 6f–j reveal that the RP is distributed in the electrode as uniform as the fresh one. Fig. 6k illustrates the working principle of RP/A-TiO₂ with reinforced concrete structure during the charge–discharge processes. When the expansion of inner RP occurs, the outer TiO₂ cluster could buffer flexibly, preventing the RP running off. By effectively use the inner RP, 10 wt% RP could supply more than 200 mA h g⁻¹ capacity in the composite, which render the double capacity comparing with A-TiO₂.

Fig. 6 (a) STEM image of RP/A-TiO₂ electrode before cycling; (b–e) area-scan elemental mapping images of red square area in (a). (f) STEM image of RP/A-TiO₂ electrode after cycling; (g–j) area-scan elemental mapping images of red square area in (f). (k) Schematic of lithiation process in bare RP and RP/A-TiO₂.

4. Conclusions

In summary, a novel RP/A-TiO₂ composite with the unique reinforced concrete structure was synthesized by in situ hydrolyzing on RP particles. RP and A-TiO₂ play as the roles of “reinforced” and “concrete” respectively. Due to the synergistic effect of RP and A-TiO₂, the as-prepared RP/A-TiO₂ composite not only offers an enhanced reversible capacity, but also keeps the structural stability, presenting the remarkable electrochemical performance (369 mA h g⁻¹, 100 cycles). Therefore, such a RP/A-TiO₂ composite is a very promising anode material for advanced LIBs.

Acknowledgements

This work was supported by National Natural Science Foundation of China (51201151, 51172205 and 201403196), Natural Science Foundation of Zhejiang Province (LR13E020002 and LY13E020010), Scientific Research Foundation of Zhejiang Provincial Education Department (Y201432424) and New Century Excellent Talents in University (NCET 111079).

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