Electrochemical lithium storage properties of desert sands

Ionics https://doi.org/10.1007/s11581-017-2381-6

ORIGINAL PAPER

Electrochemical lithium storage properties of desert sands Chu Liang 1 & Caihong Zhou 1 & Zichong Chen 1 & Yongping Gan 1 & Yang Xia 1 & Hui Huang 1 & Xinyong Tao 1 & Jun Zhang 1 & Wenkui Zhang 1 Received: 5 October 2017 / Revised: 26 November 2017 / Accepted: 30 November 2017 # Springer-Verlag GmbH Germany, part of Springer Nature 2017

Abstract SiO2 is one of the most promising lithium storage materials for lithium-ion batteries anodes due to its low cost, good environmental compatibility, low working voltage, and high-specific capacity. In this work, the desert sands, which are rich in SiO2, are investigated as the anode material for lithium-ion batteries. The electrochemical activation, lithium storage capacity, and cycle properties are highly dependent on the particle size distribution of sands. As the average particle sizes of sands gradually decrease, the reversible lithium storage capacity increases from 137 mAh g−1 (several microns) to 492 mAh g−1 (several submicrons). The 72 h-milled sands (average particle size: ~1 μm) deliver a stable lithium storage capacity of ~400 mAh g−1 over 400 cycles with the capacity retention as high as 95%. The reason for the electrochemical activation, lithium storage capacity, and cycle properties of sands associated with their particle size distribution is also discussed. Keywords Lithium-ions battery . Sand . Electrochemical activity . Reversible capacity . Cycle property

Introduction The ever-growing consumption of fossil fuels has serious impact on the sustainable development of both environment and the mankind [1, 2]. More and more attention has been paid to the development of renewable and clean energy technologies including efficient energy storage and conversion, advanced carbon capture and utilization, etc. [3, 4] Lithium-ion batteries, as a new type of energy storage devices, have occupied the largest market share of the small rechargeable battery. It is believed that lithium-ion batteries exhibit a promising future in the power sources for electric vehicles because of its major advantages of high-energy density and friendly environment. Anode material is one of the most important parts of lithium-ion batteries. The selection of the anode materials should follow the principles of low cost, good environmental compatibility, low working voltage, and high-specific capacity. In this respect, SiO2 is regarded as the promising candidate

* Yang Xia nanoshine@zjut.edu.cn * Wenkui Zhang msechem@zjut.edu.cn 1

College of Materials Science and Engineering, Zhejiang University of Technology, Hangzhou 310014, People’s Republic of China

for anode materials of lithium-ion batteries. SiO2 was believed to be electrochemically inactive until the reversible lithium storage properties of nano-SiO2 were firstly reported in 2001 [5]. In the voltage window of 0–1.0 V, the SiO2 nanoparticles (7 nm) delivered a reversible capacity of 400 mAh g−1 [5]. After that, more and more interest has been focusing on the design and synthesis of various nanosized and porous SiO2 materials for electrochemical lithium storage [6, 7]. In comparison with bulk SiO 2, the sample with porous and/or nanosized structure presents significantly improved electrochemical activity and lithium storage properties [8–12]. Unfortunately, the synthesis process of nanosized and porous SiO2 materials from tetraethyl orthosilicate and silicon tetrachloride is usually complicated, high cost, and environmental unfriendly, limiting the large-scale applications. As is well-known, sands are rich in SiO2, which are one of the most abundant raw materials on the earth. Inspired by this concept, we directly use the sands from the KuBuJi desert as the lithium storage materials. The particle sizes of sands are tuned by a facile mechanical milling method since their particle sizes are ~300 μm. The effect of particle sizes on electrochemical properties of sands will be systematically investigated in this work. To the best of our knowledge, it is the first time to report SiO2 from the desert sands as anode materials for the lithium-ion batteries. This green and facile fabrication of the silica anode materials from naturally available materials

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may be of significance for the large-scale applications of SiO2 anodes.

Experimental and characterizations The sands were collected from the KuBuJi desert (Inner Mongolia, China). Two grams of sands and 100 g of milling balls were put into a milling jar (100 ml) in Ar atmosphere (1.0 bar). The jars were milled for 12–108 h on a planetary ball mill (Nanjing, QM-1SP2) at a constant speed of 500 rpm. The post-milled sands were then washed with excess HCl under magnetic stir for 24 h in order to get rid of the metal impurities. The elemental composition was analyzed by X-ray fluoresc e n c e s p e c t r o m e t r y ( X R F, A R L A D VA N T ’ X IntelliPowerTM 4200, Themo Fisher, USA). The X-ray diffraction patterns were obtained by using an X’ Pert Pro diffractometer with Cu Kα radiation (XRD, λ = 1.5418 nm) in the 2θ range of 10–80°. The morphology and microstructure of sands were observed on a scanning electron microscopy (SEM, Hitachi S-4700) equipped with an energy dispersive spectroscopy detector (EDS, EDAX) and a transmission electron microscopy (TEM, Tecnai G2 F30). The particle size distribution of sands was measured on a laser particle size analyzer (Microtrac S3500). The Si–O vibrations of sands were characterized by a Fourier transform infrared spectra (FTIR), which were carried out on an infrared spectrophotometer (Nicolet 6700, Thermo Fisher) in the transmission mode. The electrochemical properties of sands were evaluated by using CR2032 coin-type cell. Sands (60 wt%), carbon black (30 wt%), and carboxymethyl cellulose (CMC, 10 wt%) were mixed and dispersed in deionized water to form a homogeneous slurry. The working electrode was prepared by spreading the slurry onto a copper foil, followed by drying at 120 °C for 12 h under vacuum. The CR2032 cells were assembled in an MBRAUM glovebox by using lithium metal as the counter and reference electrode, Celgard 2400 membrane as the separator, and 1 M solution of LiPF6 in ethylene carbonate, dimethyl carbonate, and diethyl carbonate (1:1:1, by volume) as the electrolyte. The galvanostatic charging and discharging test were carried out on a Neware battery test system (BTS5 V/5 mA) at various current densities. The voltage for galvanostatic charge/discharge ranges from 0.01 to 3.0 V versus Li/ Li+. Cyclic voltammetry (CV) curves were measured at a scanning rate of 0.1 mV s−1 in the voltage range of 0.0– 3.0 V on a CHI650B electrochemical workstation (Chenhua, Shanghai).

Fig. 1 EDS spectrum of the sands from the KuBuJi desert

major elements in the pristine sands. The minor remainder is metallic elements (Na, Mg, Al, K, Ca, and Fe). As shown in Fig. 2, the XRD peaks of sands are consistent with the SiO2 crystallized in a hexagonal structure (PDF# 01-087-2096), indicating that the sands from the KuBuJi desert are rich in SiO2. The contents of Si, O, and metallic impurities in sands were determined by XRF (Table 1). For the pristine sands, the contents of Si, O, and metallic impurities are 27.18, 64.68, and 8.14 at%, respectively. The atomic ratio of O to Si is greater than 2:1, indicating that the impurities in the sands may present in the form of metal oxides and/or metal silicates. The sands were washed with the excess HCl to remove the metal oxides. After washing with HCl, the amount of metallic impurities in sands is obviously decreased as shown in Table 1. The remaining metallic impurities may exist in the form of metal silicates since HCl cannot react with metal silicates at room temperature. Moreover, the weak XRD peaks of metal silicates (such as CaAl2Si2O8, (Na, Ca)Al(Si, Al)3O8, and/or NaAlSi3O8) were detected in the sands (Fig. 2).

Results and discussion The composition of sands was first analyzed qualitatively by EDS (Fig. 1). It can be seen that silicon and oxygen are the

Fig. 2 XRD pattern of the sands from the KuBuJi desert

Ionics Table 1

XRF results of the sands before and after HCl washing Before washing

After washing

wt%

at%

wt%

at%

Si Al

37.33 4.51

27.18 3.42

39.88 3.80

28.79 2.86

Na

2.35

2.09

1.86

1.64

Fe K

2.13 1.45

0.78 0.76

1.03 1.20

0.37 0.62

Mg Ca

0.77 0.86

0.65 0.44

0.44 0.37

0.37 0.19

O (others)

50.60

64.68

51.42

65.16

The sands with various particle sizes were prepared by ball milling. Figure 3 displays the SEM images of the sands milled for 36–108 h. After milling, all the sands exhibit an irregular particle shape. Also, it can be seen that the particle sizes of sands decrease with the milling time. The particle size distribution of sands (Fig. 4) was determined by the laser particle size analyzer. It can be clearly seen that the average particle sizes of sands are associated with the milling time. The average particle sizes decrease from 1.835 μm for the 36 h-milled sand to 446 nm for the 108-h-milled one. Moreover, the distribution of the particle sizes becomes narrow down with the increasing milling time. Fig. 5 shows the XRD patterns of the sands milled for 36–108 h. The XRD peaks of SiO2 can be detected in the sand milled for 36 h. However, the XRD peaks of metal silicates are not observed in the post-milled samples, which may result from the low contents of metal silicates and Fig. 3 SEM images of the sands with different milling treatments. a BM 36 h; b BM 60 h; c BM 72 h, and d BM 108 h

its low crystallinity after milling treatments. Moreover, it is obvious that the intensity of XRD peaks of the SiO2 reduces with the increasing milling time. For the sands milled for 60 and 72 h, there are only two main diffraction peaks around 21° and 26°. When the milling time reaches 108 h, all the obvious characteristic XRD peaks of SiO2 disappear, implying that the crystal SiO2 in sands may be converted into amorphous one. A FTIR test was carried out to determine the effects of the mechanical collision on the Si–O bonds of the sands. The result is shown in Fig. 6. Seven absorption bands (461, 514, 694, 778, 797, 1085, and 1168 cm−1) are observed in the spectrum of the sand milled for 36 h, which match well with the characteristic bands of SiO2 [13]. As the milling time extended from 36 to 108 h, the absorption bands of SiO2 gradually get widened with the increasing milling time. When the milling time reaches 108 h, the absorption bands around 514, 694, and 1168 cm−1 can be hardly observed and two adjacent bands (778 and 797 cm−1) have been broadened to look like one band. Only three absorption bands remain in the spectrum, consistent with the cases of submicron and nanosized SiO2 [14]. The above phenomena indicate that the crystallinity and particle sizes of sands can be remarkably affected by the ball-milling treatments, but the active material of SiO2 has remained unchanged during ball milling. The initial galvanostatic charge/discharge profiles of the sands milled for 36–108 h are plotted in Fig. 7. The 36 hmilled sample delivers an initial discharge capacity of 473 mAh g−1 with a Coulombic efficiency of 28.9%. During charging, no clear voltage plateau is observed in the 36 hmilled sample. Such low capacity indicates the poor electrochemical activity of the sand and low utilization of active

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a

b

c

materials. As the milling time gradually increases, the lithium storage capacities and initial Coulombic efficiencies of the sands are significantly enhanced. While the milling time increases to 60, 72, and 108 h, the initial discharge capacities of the sands are increased to 860, 920, and 1031 mAh g−1, respectively, in which the corresponding Coulomb efficiencies are 38.4, 43.2, and 45.6%. The gradual increase in lithium storage capacities suggests that the electrochemical activity and the reversible lithium storage of the SiO2 in the sands are improved by the ball-milling treatment. The reason can be summarized as follows: (1) The large particles of sands are broken down by the impacts of grinding balls and a lot of lattice defects were produced on the surfaces of sands, which increases the electrochemical activity of SiO2 towards lithium storage; (2) The specific surface area and defects of sands increases with the decreasing particle sizes, resulting in the increase in the utilization of active materials; (3) The decrease in the particle sizes of sands also greatly enhance the ability of diffusion and migration of Li+ during charging and discharging. After ball-milling treatment, the initial Coulomb efficiency of sands is increased from 28.9 to 45.6%. However, the maximum is still less than 50%. These low Coulombic efficiencies of sands result from the irreversible lithium storage reaction of SiO2 and the formation of SEI layers on the electrode surfaces [15, 16]. As shown in Fig. 8, two weak reduction peaks at ~0.75 and 1.08 V, which can be attributed to the formation of SEI layers and irreversible reaction of carbon black with electrolyte [17], are observed in the first-cycle CV curves of the sands milled for 60, 72, and 108 h. The remarkable reduction peak below 0.4 V at the first cycle is the electrochemical reaction of SiO2 with Li+. In the subsequent cycles, the lithiation behaviors are different from the first cycle, in which the reversible lithium storage behaviors of Si can be seen in the second and third cycles (Fig. 8) [18], implying the

d

Fig. 4 Particle size distribution of post-milled sands. a BM 36 h, b BM 60 h, c BM 72 h, and d BM 108 h Fig. 5 XRD patterns of the sands with different milling treatments

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a

Fig. 6 FTIR spectra of the sands milled for different times

irreversible lithium storage reaction of SiO2 in the first cycle. In this case, SiO2 was firstly reduced to lithium silicate, Li2O and Si, and then the Si reacted with Li+ to form Li-Si alloy, in good agreement with the previous reports [19, 20]. Figure 9 shows the discharge-charge cycle properties of post-milled sands at a current density of 100 mA g−1. The cycling stability and reversible capacities of sands are found to depend on milling times because the electrochemical properties of SiO2 are associated with its particle sizes as discussed above. For the sand milled for 36 h, the reversible capacities are less than 230 mAh g−1 during 200 cycles on account of the poor electrochemical activity of SiO2. As the particle sizes of post-milled sands decrease, the reversible capacities gradually increase but the cycle properties deteriorates. The reversible capacity of the 60-h-milled sand increases from 330 mAh g−1 at the first cycle to 348 mAh g−1 at the 200th cycle. As the discharge-charge cycling continues, the sand particles may be cracked or even pulverized in the repeated lithiation/

b

c

Fig. 8 CV curves of the post-milled sands. a BM 60 h, b BM 72 h, c BM 108 h

Fig. 7 The discharge and charge voltage profiles of the post-milled sands at the first cycle

delithiation process. This is the reason for the increase in reversible capacities of the sands milled for 36 and 60 h during cycling. Conversely, the sands milled for 72 and 108 h show a decreasing reversible capacity in the discharge/charge cycling process. The sand milled for 72 h exhibits a stable reversible

Ionics Fig. 9 The discharge-charge cycle properties of post-milled sands at a current density of 100 mA g−1

capacity of ~400 mAh g−1 even over 400 cycles, in which the capacity retention is as high as 95%. The excellent cycling stability of the sands milled for 36, 60, and 72 h can be attributed to the buffering effect of the Li2O and lithium silicate produced in the first lithiation of SiO2 and the impurities of metal silicates and electrochemically inactive SiO2 existing in the sands. The presence of these substances can effectively alleviate the huge volume changes of the active material of Si during repeated discharge and charge. In contrast, the 108h-milled sand delivers a highest capacity of 492 mAh g−1; however, its reversible capacity fades to 319 mAh g−1 at 200th cycle, corresponding to low capacity retention of 65%. The content of electrochemically inactive SiO2 in the sands decreases with the particle sizes of sands, leading to the increase of reversible capacities and the decrease of volume buffering effect for the 108-h-milled sand. This may be responsible for the fact that the reversible capacities gradually increase, but the cycle properties deteriorates with the milling treatments (Fig. 9). Further improvement in electrochemical lithium storage properties is needed for developing the 108-hmilled sands as high-performance anode materials for lithiumions batteries.

Conclusions In summary, a series of desert sands with various particle sizes were prepared by a facile mechanical milling method. SiO2 are the major constituent of sands from the KuBuJi desert. The residual impurities exist in the form of metal silicates. The effect of mechanical milling on the morphology, microstructures, and electrochemical properties of desert sands were investigated by means of XRD, SEM, FTIR, laser particle size analyzer, galvanostatic charging and discharging, and cyclic voltammetry. The electrochemical activation, lithium storage capacity, and cycle properties of sands are found to highly depend on the mechanical

milling times. The reversible lithium storage capacity increases from 137 mAh g−1 for the sands milled for 36 h to 492 mAh g−1 for the sands milled for 108 h. Among them, the integrative performances of the 72-h-milled sand are the best. It delivers a stable lithium storage capacity of ~400 mAh g−1 with the capacity retention as high as 95% after 400 cycles. It is revealed that crystal defects, specific surface area, and impurities (including electrochemical inactive SiO2) in sands are the key factors for determining their electrochemical activation, lithium storage capacity, and cycle properties. This study provides a facile and environmental friendly route to developing one of the most abundant materials of sands as energy storage materials. Funding information This work was funded by the National Natural Science Foundation of China (grant numbers 21403196, 51572240, and 51677170), the Natural Science Foundation of Zhejiang Province (grant numbers LY16E070004 and LY17E020010), and the 2017 National Students’ Innovation and Entrepreneurship Training Program (grant number 201710337003).

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