NaTiSi2O6/C复合材料用于锂离子电池负极材料

刘昆，刘瑶，朱海峰，董晓丽，王永刚，王丛笑，夏永姚

复旦大学化学系，新能源研究院，上海市分子催化和功能材料重点实验室，上海 200433

1 Introduction

As a clean energy source, lithium-ion batteries have gained widespread usage in various electronic devices and electric vehicles. Traditional anode material graphite is widely used because of its low cost and high coulombic efficiency, but it suffers from low lithium-ion intercalation potential that is close to 0 V (vsLi+/Li), making it prone to the formation of lithium dendrite, bringing on the safety risk of a short circuit1. Alloying material silicon has a high specific capacity of about 4200 mAh·g-1but it suffers from huge volume expansion up to 300%during charging and discharging process2,3. Transition metal oxides, such as Fe2O34-7, Fe3O48-10, CoO11-13and NiO14-16,undergo conversion reaction17when they are used as anode materials. They feature in high capacity, but unfortunately, they suffer from great volume change and electrochemical polarization just like silicon.

Titanium-based anode materials, such as TiO218-23,Li4Ti5O1224-31, Li2Ti3O732,33, Li2Ti6O1334,35etc. have been extensively studied owing to their low cost, facile synthesis and environmental friendliness, among which stands out the “zerostrain” anode material Li4Ti5O12, with a tiny volume change ofca0.2% during the lithiation and delithiation process. However,it exhibits a low specific capacity (175 mAh·g-1) along with a high working potential (1.55 VvsLi+/Li)36, which reduces its output voltage and specific energy when assembled in full battery.

For a better electrochemical performance, silicon element is introduced into titanium-based materials. Titanosilicates are a kind of low-cost, non-toxic, environmentally-friendly porous material, which grants them the feature of being used as catalyst,zeolites or molecular sieves37-39. Besides that, the inherent three-dimension channels within their structure40-42are a great advantage for ion transport, which makes it possible for them to be used as electrode materials. Moreover, the network consisting of silicon-oxygen tetrahedra builds a sturdy framework, which is of benefit to lithium intercalation as well as the utilization of Ti(IV)/Ti(III) and Ti(III)/Ti(II) redox couple43. For example, in 2017, Liuet al.44reported lithium storage in Li2TiSiO5, a promising anode material which delivers a high reversible capacity of 308 mAh·g-1with a low working potential of 0.28 V(vsLi+/Li). By introducing silicon element into titanates, the working potential of titanates is lowered, which is good for an anode material. This working potential is higher than that of graphite and lower than that of Li4Ti5O12, which brings it great advantages. It avoids the lithium deposition potential, reducing the risk of lithium dendrite formation; at the same time, it raises the output voltage when assembled in a full battery compared with Li4Ti5O12. Other reported titanosilicates that were used as anode for lithium ion batteries include sitinakite Na2Ti2O3SiO4·2.76H2O (Milneet al.45), narsarsukite Na2TiOSi4O10(Liuet al.46and Chaupetnaiket al.47), and Na2TiSiO5(Heat al.48). Here we introduce a novel anode material, NaTiSi2O6. In 2002, Isobeet al.49reported successful preparation of it under ambient pressure rather than high pressure, which was helpful to study this pyroxene material. To the author’s knowledge, this is the first time that NaTiSi2O6 is reported as an anode material for lithium ion batteries.

2 Experimental

2.1 Synthesis

The reagents used in the experiment are listed as follows:tetrabutyl orthotitanate (CP, ≥ 98.0%), tetraethyl orthosilicate(AR), absolute ethanol (AR, ≥ 99.7%), acetic acid (AR, ≥99.5%), sodium acetate anhydrous (AR, ≥ 99.0%), D-(+)-glucose (AR), sodium carbonate (AR, ≥ 99.8%), silicon dioxide(AR), titanium dioxide (CP, ≥ 98.0 %), titanium powder (2N, ≥99.0%), all bought from Sinopharm Chemical Reagent(Shanghai, China). All these were used without further purification.

NaTiSi2O6/C composite (hereinafter referred to as NTSO/C)was prepared by sol-gel method, followed by a solid sintering procedure. In a typical process, 10 mmol tetrabutyl orthotitanate and 20 mmol tetraethyl orthosilicate were dissolved in 25 mL absolute ethanol, followed by adding 10 mL acetic acid. The mixture was homogenized by a magnetic stirrer. Meanwhile, 10 mmol sodium acetate anhydrous and 10 mmol glucose were weighed and dissolved in 10 mL deionized water. After complete solvation, the water solution was added dropwise into the alcohol solution. The mixture was kept under magnetic stirring until it started to turn turbid. Then the magnetic stirrer was taken out and the mixture was allowed to stand still for full hydrolysis. Next, the obtained white gel was dried by a rotatory evaporator. Finally, the resulting powder was calcinated at 1050 °C in argon atmosphere for 4 h. For comparison, pure NaTiSi2O6without carbon (hereinafter referred to as NTSO) was synthesized according to the literature previously reported49with some minor modification: stoichiometric amounts of Na2CO3, SiO2and TiO2(to form narsarsukite Na2TiSi4O11) with 20% excess of Na2CO3 were mixed thoroughly and ball-milled by a ball-milling machine (Fritsch, Germany) under 300 r·min-1for 6 h. Then the mixture was pelleted and sintered at 800 °C in air for 4 h in a muffle furnace. The obtained white solid(narsarsukite Na2TiSi4O11) was crushed and mixed with titanium metal powder and titanium dioxide powder, and then put into the ball-milling machine again for 6 h ball-milling process. After that the mixture was pelleted and heated at 1050 °C under argon atmosphere for 4 h in a tube furnace. The resulting dark green solid was crushed and milled for further tests.

2.2 Characterization

X-ray diffraction (XRD) tests for the prepared materials were performed on a Bruker D8 Advance X-ray diffractometer(Germany), utilizing Copper Kα beamline with 40 kV tube voltage and 40 mA tube current, where 2θangle ranged from 10°to 90° with each step of 0.02° and a dwell time of 2 s for each sampling point. The crystal morphology images were collected by a field emission scanning electron microscope (FESEM,Hitachi S4800, Japan) and a high-resolution transmission electron microscope (HRTEM, Tecnai, G2 F20 S-Twin,America). The nitrogen sorption isotherm was obtained by Quantachrome Autosorb (USA) at liquid nitrogen temperature(-196 °C). The specific surface area was calculated according to the Brunauer-Emmett-Teller (BET) formula. The carbon content in as-prepared composite was determined by thermogravimetric analysis, conducted on NETZSCH TG 209F1 Libra instrument(Germany).

2.3 Electrochemical tests

The obtained products were crushed in an agate mortar, mixed thoroughly with conductive Super P and binder polyvinylidene difluoride (PVdF, dissolved in 8% (w) 1-methyl-2-pyrrolidone solution) in a mass ratio of 8 : 1 : 1 and then cast onto a copper foil with active material mass loading of about 1-2 mg·cm-2.After dried at 80 °C overnight in a vacuum oven, the copper foil was cut into small round disks with a diameter of 12 mm. CR-2016-type half batteries were assembled using the as-prepared electrode disks, electrolyte (1 mol·L-1LiPF6dissolved in mixed solvent of ethylene carbonate : diethyl carbonate : dimethyl carbonate = 1 : 1 : 1 by weight), Celgard 2300 separator and lithium disk as counter electrode in an argon-filled glovebox.Galvanostatic charging and discharging (GCD) test was performed on LAND CT2001A (Land Co., Ltd., Wuhan, China)at room temperature within the potential range of 3-0.01 V (vsLi+/Li). Cyclic voltammetry (CV) tests were performed on Biologic EC-Lab electrochemical workstation (France). Finally,in situX-ray diffraction (XRD) test was conducted in order to determine its reaction mechanism during charge and discharge process.

3 Results and discussion

3.1 Characterization and morphology

XRD test results show that the pure NaTiSi2O6phase can be obtained by sintering the mixture of narsarsukite Na2TiSi4O11, Ti and TiO2, while the NTSO/C sample prepared by glucose carbonization contains minor impurity phases of TiO1.51 and Ti3O5. The obtained XRD patterns were refinedviaRietveld method using GSAS software50,51and the refined crystal parameters are shown in Table 1. The refined plot and the NaTiSi2O6crystal structure model are shown in Fig. 1. The refined result shows that NaTiSi2O6is isostructural to jadeite NaAlSi2O6, where TiO6 octahedra share their edges to form onedimensional chains, and SiO4tetrahedra share their corner oxygen atoms to link with themselves and TiO6octahedra. The sodium atoms lie in the space between them. It belongs to monoclinic crystal system with a space group ofC2/c, and the cell parameters fora,b,candβare 0.97217(2) nm, 0.89017(2)nm, 0.53131(1) nm and 106.7616(16)°, respectively, with a cell volume of 0.44026(2) nm3. According to the Rietveld Refinement result, in the as-prepared NTSO/C sample the phase fraction of NaTiSi2O6is 97.66%, while the fraction for TiO1.51and Ti3O5is 1.04% and 1.30%, respectively, suggesting a predominant phase of NaTiSi2O6.

The scanning electron microscopy image of NTSO, and scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) as well as energy dispersive X-ray spectroscopy (EDS) mapping images of NTSO/C are shown in Fig. 2. From Fig. 2a it can be observed that the particles of NTSO prepared by solid sintering are relatively large, up to several micrometers. On the contrary, as is shown in Fig. 2b,c, the secondary particles forming NTSO/C composite are about 500to 600 nm in size, which are stacked by primary particles in size of less than 100 nm. In Fig. 2d, TEM image shows that NaTiSi2O6particles are surrounded by amorphous carbon, which enhances its conductivity. The carbon comes from the pyrolysis and carbonization of glucose during the heating process. In the synthesis of sol-gel, the addition of acetic acid retards the hydrolysis of titanate and silicate, which provides sufficient time for the reactants mixing thoroughly and ensures the formation of a homogeneous gel. During the sintering process, the glucose decomposes as the temperature increases. The pyrolyzed carbon and released gas hinder the reactant particles from growing up,such that the resultant particles can maintain a relatively small size. Adequate amount of glucose ensures that there is enough carbon produced, so as to cover all the NaTiSi2O6 particles.Adequate heating time guarantees that all the glucose carbonized. However, the carbonization reaction of glucose was a complicated process and the pyrolysis gas was released in a disordered way, so the carbon coating layer was not so homogeneous. This may also account for the minor impurity phases appearing in NTSO/C, since the randomly released pyrolysis gas destroyed the uniform distribution of reactant concentration. In HRTEM image shown in Fig. 2e, an interplanar spacing of 0.254 nm can be measured, which matches well with the (-112) crystal plane of NaTiSi2O6. Furthermore, from the EDS mappings listed in Fig. 2g-k, it can be seen that in NTSO/C particles, the elements Na, Ti, Si, O and C are almost evenly distributed. The carbon content was measured by thermogravimetric analysis, and the test result indicates that carbon is about 10.7% of the total mass, as is shown in Fig. 3.Enough carbon provides sufficient conducting media, making the insulating silicate material a possible electrode material. To determine the specific surface area, the nitrogen sorption curve was measured, and the result is shown in Fig. 4. The specific surface area calculated by the Brunauer-Emmett-Teller (BET)formula was 132 m2·g-1. On one hand, the small particle size leads to high specific surface area, which shortens the diffusion path of lithium ions and benefits the contact of electrode material and electrolyte. On the other hand, this relatively high specific surface area may lead to larger solid electrolyte interphase (SEI)film, and relatively low initial coulombic efficiency accordingly.

Fig. 1 (a) The X-ray diffraction (XRD) plot of precursor Na2TiSi4O11; (b) Rietveld Refinement result of NaTiSi2O6 prepared by sintering Na2TiSi4O11, Ti and TiO2; (c) Rietveld refinement result of NaTiSi2O6 prepared by sol-gel method; (d) crystal structure for monoclinic NaTiSi2O6 viewing along b axis, showing TiO6 octahedra in dark yellow and SiO4 tetrahedra in bluish green.

Fig. 2 (a) The scanning electron microscopy (SEM) image for NTSO without carbon prepared by solid sintering; (b, c) SEM image of different magnifying scale for NTSO/C prepared by sol-gel method; (d, e) Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) image for NTSO/C; (f) SEM image of NTSO/C where energy dispersive X-ray spectroscopy (EDS) mappings were analysed; (g-k) EDS mappings showing element distribution of Na, Ti, Si, O and C.

3.2 Electrochemical performance

Fig. 3 Thermogravimetric analysis result of the as-prepared NTSO/C composite.

Fig. 4 Nitrogen sorption isotherm of the prepared material at -196 °C.

The galvanostatic charging and discharging curves and electrochemical performance of NTSO and NTSO/C composite are shown in Fig. 5. From Fig. 5a it can be seen that there was no obvious plateau exhibited during all the discharging and charging process for both materials. The average charging potential is 1.2-1.3 V (vsLi+/Li), slightly lower than that of Li4Ti5O12. By downsizing the particles and mixing with carbon,the electrochemical performance of NTSO was clearly enhanced. As is shown in Fig. 5b, the specific discharge capacities in the first cycle at 0.2 A·g-1of NTSO and NTSO/C are 152.7 and 503.8 mAh·g-1, respectively; and the specific charge capacities are 68.6 and 235.2 mAh·g-1, respectively.Although the specific capacity increased a lot, the initial coulombic efficiency remained less than 50%. This was probably due to the formation of solid electrolyte interphase(SEI) layer as well as other irreversible reactions. For NTSO/C composite, after 100 cycles the specific charge capacity was 196.7 mAh·g-1, corresponding to a capacity retention rate of 83.6% compared with that of first cycle. Fig. 5c shows the cycling performance of NTSO/C at 0.1 and 0.5 A·g-1. Herein,for cycling test at 0.5 A·g-1, first two cycles were activated under a smaller current density of 0.05 A·g-1. The specific charge capacities for 1st and 100th cycle under 0.1 A·g-1were 266.6 and 224.1 mAh·g-1, corresponding to a capacity retention rate of 84.1%. As for cycling test under 0.5 A·g-1, specific charge capacity of 201.8 mAh·g-1was exhibited at the third cycle,which decreased to 170.5 mAh·g-1after 100 cycles,corresponding to a capacity retention rate of 84.5%. All these capacity values were near or above the theoretical specific capacity of Li4Ti5O12 (175 mAh·g-1), which can accommodate three lithium ions per formula unit. Fig. 5d shows the rate performance, where current density ranged from 0.05 to 1 A·g-1and then back to 0.05 A·g-1again. It shows a reversible specific capacity ofca250 mAh·g-1at 0.05 A·g-1, which is equal to 2 lithium-ion intercalations per formula unit. Even if current density increases to 1 A·g-1, it exhibits a reversible capacity ofca125 mAh·g-1. As is summarized in Table 2, it can be observed that the as-prepared NaTiSi2O6/C composite shows good electrochemical performance among those titanosilicate materials ever reported.

Fig. 5 (a) The galvanostatic charging and discharging (GCD) curve of pure NTSO and NTSO/C from 1st cycle to 100th cycle at 0.2 A·g-1;(b) Cycling performance of pure NTSO and NTSO/C at 0.2 A·g-1; (c) Cycling performance of NTSO/C at 0.1 and 0.5 A·g-1.For cycling test at 0.5 A·g-1, first two cycles were activated under 0.05 A·g-1; (d) Rate performance of NTSO/C.

Table 2 List of ever-reported titanosilicate anode materials for lithium ion batteries.

To measure the diffusion coefficient of lithium ions in NTSO/C, galvanostatic intermittent titration technique (GITT)was performed and the calculated result of diffusion coefficient for lithium ion is shown in Fig. 6. Typically, the fabricated cell using lithium metal as a counter electrode alternated between discharging at a current density of 0.05 A·g-1for 20 min and standing still for 2 h, till the potential reached 0.01 V; then, the cell alternated between charging at the same current and standing still, for the same time, till the potential reached 3 V again. After such operation the diffusion coefficient of lithium ions can be measured according to the following equation52:

where ΔEsis the steady-state potential difference before and after current impulse;τis the lasting time of current impulse,i.e.20 min in this case;mB,VM,MB andSare the mass, the molar volume, the molar mass and the area of the active material,respectively. If the derivative of potential versus square root of impulse time satisfies linear relationship, then the formula above can be simplified as follows:

Fig. 6 (a) Potential versus time plot for galvanostatic intermittent titration technique (GITT) test after two cycles;(b) the calculated result of diffusion coefficient of lithium ion.

where ΔEtis the difference of instant potential at the beginning and end of current impulse. From Fig. 6, it can be seen that the diffusion coefficient of lithium ions ranges from 10-10to 3 ×10-12cm2·s-1. This is comparative to graphite, nano-Si,Li3V2(PO4)3and Li3V(MoO4)3previously reported53-56and exceeds that of LiFePO457. This may account for its superior rate performance.

To determine the mechanism of charging and discharging reaction of NaTiSi2O6, cyclic voltammetry (CV) test andin situX-ray diffraction (XRD) test were conducted. CV test result is shown in Fig. 7, where the scan rate was set at 0.3 mV·s-1, with a potential range between 3 V and 0.01 V. It can be seen that in first cycle of scanning, there is a large cathodic current peak at 0.67 V, which may account for the solid electrolyte interphase(SEI) layer formation. The minor cathodic peak at 1.40 V may result from other irreversible reactions. The anodic current peak at 0.24, 1.00 V and cathodic peak at 0.83 V repeatedly occurred in the next scanning cycles, which are in the similar position compared with that of Ti2O3as previously reported58. This means that the behavior of Ti in NaTiSi2O6is the same as in Ti2O3, which undergoes redox reaction during lithium intercalation and deintercalation while the silicates remain unchanged, stabilizing the molecule framework.

In situXRD test was conducted by successively collecting XRD data of the material over a beryllium disk that was placed inside anin situcell mold while the cell was under discharging and charging. The beryllium disk served as a sealed conductive window that X-ray could pass through. The mass loading of asprepared NaTiSi2O6material was about 7.5 mg, and the current density was set at 0.1 A·g-1. The result is shown in Fig. 8, from which it can be observed that the peaks at 29.8°, 30.5°, 30.8°,35.0°, 35.3° and 35.9°, corresponding to (-221), (310), (-311),(-131), (-112) and (221) planes respectively, all remain unchanged during the cycling process, and there are no new peaks generated. This result suggests that NaTiSi2O6undergoes intercalation reaction during cycling process, and the crystal structure of NaTiSi2O6remains stable during the insertion and extraction of lithium ions. The result is also confirmed by selected area electron diffraction (SAED) tests for the pristine sample and the fabricated electrode disk that was discharged to 0.01 V, which is shown in Fig. 9. It can be clearly seen that the diffraction circles matched well with corresponding lattice planes of NTSO, indicating that the crystal structure of NTSO remained nearly unchanged during the whole cycling process.Based on the specific capacity it delivers, the cycling reaction can be described as the following equation:

Fig. 7 Cyclic voltammetry test of NTSO with a scan rate of 0.3 mV·s-1.

Fig. 8 In situ XRD plot of NTSO material during charging and discharging, where the peaks marked as spades correspond to NTSO,and peaks marked as club correspond to BeO.

Finally, full coin cells were assembled using NaTiSi2O6/C as anode and LiFePO4as cathode and the result is shown in Fig. 10.The test voltage ranged from 3.8 to 0.1 V, with a current density of 0.2 A·g-1. The current density and specific capacity are calculated based on the mass of NaTiSi2O6/C composite material. It delivered a specific discharge capacity of 225.7 mAh·g-1in the first cycle, which dropped to 89.1 mAh·g-1after 150 cycles, corresponding to 39.4% retention rate. The calculated specific energy varied between 72 Wh·kg-1for the first cycle and 26 Wh·kg-1for the 150th cycle based on the total mass loading on both current collectors. The drastic capacity and energy fading, as well as the relatively low initial coulombic efficiency, remains a problem to be solved. Approaches such as modifying its morphology, or homogenizing the carbon coating layer might be helpful to improve its performance.

Fig. 9 Selected Area Electron Diffraction (SAED) pattern of NTSO material before discharging (a) and after discharging to 0.01 V.

Fig. 10 (a) GCD curve of assembled full cell at 0.2 A·g-1 using LiFePO4 as cathode material; (b) cycling performance of the full cell at 0.2 A·g-1.

4 Conclusions

We have successfully synthesized NaTiSi2O6/C composite material and studied its electrochemical performance in lithium ion batteries. It reacts with lithium through intercalation reaction. By downsizing the titanosilicate particles and mixing with carbon, the electrochemical performance was clearly enhanced. It shows a reversible capacity of 224 mAh·g-1at 0.1 A·g-1, which corresponds to two lithium ions insertion per formula unit. It is of low cost, environmentally friendly, and easy to synthesize. However, the relatively low initial coulombic efficiency still remains to be a problem, and that is what further work may be done to solve.