C hollow microspheres derived from nanoporous biomass carbon as anode materials for lithium ion batteries

C hollow microspheres derived from nanoporous biomass carbon as anode materials for lithium ion batteries

Solid State Ionics 344 (2020) 115132 Contents lists available at ScienceDirect Solid State Ionics journal homepage: www.elsevier.com/locate/ssi Li4...

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Solid State Ionics 344 (2020) 115132

Contents lists available at ScienceDirect

Solid State Ionics journal homepage: www.elsevier.com/locate/ssi

Li4Ti5O12-TiO2/C hollow microspheres derived from nanoporous biomass carbon as anode materials for lithium ion batteries

T



Wenhan Zhanga,b, Jinjin Aia, Yike Leia, Yong Lib, Chunyan Laia, , Jingying Xieb a b

Shanghai Key Laboratory of Materials Protection and Advanced Materials in Electric Power, Shanghai University of Electric Power, Shanghai 200090, China State Key Laboratory of Space Power-Sources Technology, Shanghai Institute of Space Power Sources, Shanghai 200245, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Li4Ti5O12-TiO2 Microsphere Biomass carbon Lithium ion battery

A flower-like Li4Ti5O12-TiO2/C hollow microsphere derived from biomass carbon is successfully fabricated and used as anode for lithium ion battery. The results show that the biomass carbons derived from mulberry leaves are uniformly compounded on the Li4Ti5O12-TiO2 microspheres. The Li4Ti5O12-TiO2/C1 microsphere sample shows perfect electrochemical performance due to its smaller grain size and larger specific surface area, it displays a discharge capacity of 174.3 mAh g−1 at 1 C in the first cycle and still maintains above 90% after 1000 cycles, even the current is increased to 40 C, the discharge capacity can still up to 125.8 mAh g−1. Excellent electrochemical performance of Li4Ti5O12-TiO2/C can be attributed to the dual action of biomass carbon and produced Ti3+ which enhances the conductivity of the material, and the biomass carbon also provides a convenient and efficient channel for lithium ion transport.

1. Introduction As a new chemical energy storage device, lithium ion battery has become the most important and advanced battery due to its long cycle life, high energy density and no memory effect [1–3]. At present, carbon materials are the most common used commercial anode materials for lithium-ion batteries. However, because of the phenomenon of “lithium precipitation” and the safety problems caused by volume expansion during charging and discharging of carbon materials, the development of lithium-ion batteries urgently needs some new anode materials that are safe, reliable and long-lived to replace the carbon anode [4–7]. Lithium titanate (Li4Ti5O12), a kind of spinel crystal with Fd3m space group and cubic symmetry, is such a promising anode material for its notable “no volume change” characteristic. During charging and discharging, the volume of unit-cell changes only 0.2%. What's more, lithium titanate has a relatively high potential (1.55 V vs Li/Li+) which can avoid the generation of lithium dendrite. However, due to the lack of electron of the 3d electron layer in Ti atom, Li4Ti5O12 has very poor electrical conductivity (10−13 S cm−1) and low lithium diffusion coefficient (10−9–10−13 cm2 s−1) which limit its electrochemical performances at high charge/discharge rate [8–14]. A lot of research work has been done to improve its performance, for example, adding different high conductive materials into the electrode materials. Polymeric organic compound such as polyaniline [15], polydopamine [16],



polypyrrole [17], and polythiophene [18] are good choices for carbon precursors. In addition, some one-dimensional and two-dimensional carbon materials with unique structure are also used in combination with lithium titanate, such as grapheme [19], carbon fibers [20] and carbon nanotubes [21]. Recent years, researchers have reported and proved that biomass carbon also can be used in lithium-ion batteries. V. Selvamani [22] used fish scales as nitrogen-doped biomass carbon for lithium ion battery anode materials. The electrode has an extra-large specific surface area (1980 m2 g−1) and shows steady state reversible capacities of 480 mAh g−1 at a current density of 75 mA g−1. Sun [23] used grapefruit peels as biomass carbon for lithium ion battery anode materials and this anode material can provide a capacity of 452 mAh g−1 at a current density of 90 mA g−1. Xia [24] used butterfly wings as template to synthesize LTO-TO2 materials with periodic 3-dimensional nanostructures and this material exhibits excellent performance at high rates. Yao [25] used crab shells to prepare biotemplate of hollow carbon nanofibers and the product was used to encapsulate sulfur and silicon to form the electrodes for lithium ion battery. The resulting nanostructured electrode shows a high specific capacity (1230 mAh g−1 sulfur and 3060 mAh g−1 silicon). In addition, many different types of leaves are also used as biomass carbon for electrode materials, such as maple leaves, ginkgo leaves, and green tea leaves. [26–28] All in all, biomass carbon has become very popular for electrode materials. Mulberry leaves are widely produced from most parts of China.

Corresponding author.

https://doi.org/10.1016/j.ssi.2019.115132 Received 31 August 2019; Received in revised form 21 October 2019; Accepted 31 October 2019 0167-2738/ © 2019 Elsevier B.V. All rights reserved.

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Fig. 1. (a) TGA curves of LTO-TO2/C1 and LTO-TO2/C2. (b) XRD patterns of LTO-TO2,LTO-TO2/C1, LTO-TO2/C2 and C. (c) Raman spectra of LTO-TO2,LTO-TO2/ C1 and LTO-TO2/C2. (d) Nitrogen adsorption curve of LTO-TO2,LTO-TO2/C1 and LTO-TO2/C2.

2.2. Synthesis of hollow Li4Ti5O12-TiO2 microspheres

They have been used for sericulture or medicine since ancient times. Due to their medicinal value and edible value, the annual output of mulberry leaves in China is huge. As we know, mulberry leaves contain a large amount of cellulose and cellulose is a kind of good carbon source [26–28]. As an extremely inexpensive and fairly readily available biomass carbon source, it is tried to be used as energy storage materials. Herein, we report a work using mulberry leaves as biomass carbon precursor and compound with Li4Ti5O12-TiO2. The modified material presents a discharge capacity of 174.3 mAh g−1 in the first cycle at 1C and maintains above 90% after 1000 cycles. Additionally, the reversible specific capacities can still reach 125.8 mAh g−1 at 40 C. This work proves that biomass carbon can be used as a good kind of carbon source for the modification of electrode materials.

First, 2 ml tetrabutyltitanate (TBT), 1.0288 g LiOH.H2O and 3.5 ml H2O2 (30%) were added into 70 ml deionized water and stirred for 30 min. Then, the prepared solution was transferred to a 100 ml teflonlined stainless-steel autoclave and reacted at 150 °C for 5 h. After hydrothermal process, the product was rinsed with deionized water for several times and dried in a vacuum oven at 80 °C for 2 h. Finally, the samples were placed in a tube furnace and heated to 550 °C with a heating rate of 3 °C/min for 5 h under air atmosphere. The final product was named as LTO-TO2. 2.3. Synthesis of LTO-TO2/C1 and LTO-TO2/C2 In method I, the precursor was prepared by using one-step hydrothermal. First, the MLC was added to 70 ml deionized water and ultrasonic treated for 1.5 h. Subsequent steps are similar to the appeal method of LTO-TO2, and the final sample was named as LTO-TO2/C1. In method II, the product was obtained by simple mixing and secondary calcination. The obtained LTO-TO2 microspheres were dissolved in deionized water and CTAB (2 mg) was added as surfactant, then the MLC was added and stirred for 2 h. The product was rinsed with deionized water for several times to remove excess CTAB and then dried in a vacuum oven at 80 °C for 12 h. Finally, the sample was placed in a tube furnace for 550 °C for 5 h with a heating rate of 3 °C/min under Ar atmosphere, and it was named as LTO-TO2/C2. The content percentages of MLC powders are 3 wt% in both of two methods.

2. Experiment 2.1. Preparation of biomass carbon Mulberry leaves collected from campus were used as biomass precursor in this work. Typically, the obtained mulberry leaves were rinsed repeatedly with deionized water to remove impurities on the leaves, then it was ground and added to 500 ml of potassium hydroxide solution (10 g KOH) and stirred for 1 h. After stirring, the obtained materials were dried in a convection oven at 80 °C for 12 h. After that, the leaves were pyrolyzed in tube furnace at 800 °C for 2 h with a heating rate of 3 °C/min under nitrogen atmosphere. Finally, the obtained materials were soaked in 30% nitric acid and stirred for 3 h,then washed several times with deionized water and ethanol. The final product was named as MLC.

2.4. Materials characterization The crystal structure of samples was investigated by X-ray 2

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Fig. 2. XPS survey of LTO-TO2/C1 (a) and LTO-TO2/C2(c), XPS spectra of Ti 2p of LTO-TO2/C1 (b) and Ti 2p of LTO-TO2/C2 (d).

Fig. 3. SEM images of LTO-TO2 (a, b), LTO-TO2/C1 (c, d) and LTO-TO2/C2 (e, f).

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Fig. 4. SEM images of LTO-TO2/C1 (a) and LTO-TO2/C2 (e), Elemental mapping of LTO-TO2/C1 (b–d) and LTO-TO2/C2 (f–h).

Fig. 5. Schematic illustration of the formation of LTO-TO2/C1.

spectroscopy (XPS Thermo K-Alpha XPS spectrometer). The morphology of the samples was obtained by scanning electron microscope (SEM JSM7610F) and transmission electron microscope (TEM PHILIPS, FEGCM200). Energy-dispersive X-ray spectroscopy (EDS) mapping was used to investigate the distribution of element.

2.5. Electrochemical test The anode electrodes were formed by a mixture of 80% active material, 10% acetylene black and 10% polyvinylidene fluoride (PVDF). At first, the mixture was dispersed in N-methyl-2-pyrrolidone to form homogeneous slurry. Then, the formed slurry was coated on copper foil and dried in oven at 80 °C for 10 h. After that, the electrodes were pressed into slice (diameter = 14 mm, the mass of active material on each electrode is approximately 1.3–1.6 mg). The Celgard 2400 polyethylene was used as separators and Li metal as the counter electrode to assemble the CR2016-type coin cells in glove box. 1 M LiPF6 dissolved in DMC、EC and EMC (volume ratio of 1:1:1) was used as the electrolyte. Charge-discharge cycling was performed at different rates (1 C = 175 mA g−1) between 1.0 V and 2.5 V at 25 °C with Land CT2001A tester. The CV (0.1 mV s−1, 1–2.5 V) and EIS (10−2 Hz to 105 Hz) test was conducted by CHI660E electrochemical workstation.

Fig. 6. HRTEM image of LTO-TO2/C1.

diffraction (XRD Rigaku D/max 2500 using Cu Kα radiation, 2θ ranges from 10° to 80°). Thermogravimetric analysis (TGA) was carried out from 30 °C to 600 °C in air (NETZSCH STA 449F3, Germany). Raman spectroscopy was carried out using Raman spectroscope (HORIBA Scientific Instruments Division). The specific surface area (SSA) of samples was analyzed by Brunauer–Emmett–Teller (BET, ASAP2460). XPS analysis results were obtained from X-ray photoelectron 4

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Fig. 7. Cycle performances of LTO-TO2, LTO-TO2/C1 and LTO-TO2/C2 over 1–2.5 V (versus Li+/Li) at 1C (a), rate performances of LTO-TO2, LTO-TO2/C1 and LTOTO2/C2 (b), charge–discharge profiles over the voltage range of 1–2.5 V (versus Li+/Li) at 1C of LTO-TO2 (c) and LTO-TO2/C1 (d).

3. Results and discussion

The Raman spectroscopy of LTO-TO2/C1 and LTO-TO2/C2 were analyzed and shown in Fig. 1c. Two different Raman peaks of carbon can be observed in Fig. 1c, the G band appearing at 1590 cm−1 is related to the sp2 graphite structure, while the peak of the D band around 1340 cm−1 is related to defects in the graphite structure [27]. The ratio of D-band to the G-band peak intensity (ID: IG) of the LTO-TO2/C1 and LTO-TO2/C2 is 1.27 and 1.38 respectively, which means that the graphitization degree of LTO-TO2/C1 material obtained by one-step hydrothermal is higher than that of LTO-TO2/C2 material. The nitrogen adsorption-desorption isotherms of samples were obtained by BET test and the results are shown in Fig. 1d. The specific surface area of LTO-TO2 is calculated to be 56.56 m2/g while the LTOTO2/C2 and LTO-TO2/C1 is 81.52 m2/g and 108.34 m2/g respectively. There is no doubt that larger specific surface area can provide larger contact area with the electrolyte and leads to high li-ion flux across the electrolyte/electrode interface [31–33]. XPS analysis was carried out and the results are shown in the Fig. 2. Fig. 2a and c show the full-spectrum scan results of LTO-TO2/C1 and LTO-TO2/C2 and there are three elements Ti, O and C in both of LTOTO2/C1 and LTO-TO2/C2. To further analyze the valence state of the Ti element, a high-resolution spectrum of Ti 2p was carried out and the results are shown in Fig. 2b and d. It can be seen that two samples have obvious peak positions at the binding energies of 463.8 eV and 458.1 eV which correspond to 2p1/2 and 2p3/2 of Ti4+. In addition, a small shoulder peak corresponding to Ti3+ appeared at a binding energy of 457.2 eV, indicating some Ti4+ in the material was reduced to Ti3+, and it further verifies the speculation that the XRD peak was shifted to the left. According to previous reports, the presence of Ti3+ can further enhance the conductivity of Li4Ti5O12 [12].

Carbon contents in LTO-TO2/C1 and LTO-TO2/C2 were evaluated by thermogravimetric analysis and the results are shown in Fig. 1a. The mass loss from room temperature to 100 °C can be attributed to the reduction of water in the samples, and LTO-TO2/C2 contains less water because it was obtained by secondary calcination. The mass loss from 220 °C to 500 °C can be attributed to the reduction of carbon content in the sample. It can be seen that the carbon content in LTO-TO2/C1 is about 2.8%, and the carbon content in LTO-TO2/C2 is about 2.2%. X-ray diffraction (XRD) patterns of samples are shown in Fig. 1b. All the strong diffraction peaks of the three samples can be perfectly indexed to spinel Li4Ti5O12 [29,30], the weak peaks at 25° and 55° correspond to a small amount of TiO2. In addition, a small peak near 21° in LTO-TO2/C1 and LTO-TO2/C2 can be considered as carbon peak. When further enlarge the peak at 43°, it can be seen that the modified materials' peak has a certain left shift phenomenon relative to the pure phase, which may be due to the fact that the carbonization process further provides a small amount powerful reducing inert gas, and some of the Ti4+ was reduced to Ti3+ which has a larger diameter, resulting in a larger overall lattice constant of the material [12,31]. It was further confirmed by the calculation of the cell parameters, the 2θ of the (111) planes of LTO-TO2, LTO-TO2/C1, and LTO-TO2/C2 are 18.33°, 18.22°, and 18.02°, respectively. According to the formula 2dsin θ = nλ and d = a/√h2 + k2 + l2, where d is the interplanar spacing, θ is the diffraction half angle, n is the diffraction order, λ is the wavelength of the copper target, a represents the cell parameter and the calculated values of a are 0.42, 0.426 and 0.431, respectively. Increased cell parameters can further prove the existence of Ti3+. 5

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Fig. 8. (a) CV curves of LTO-TO2 and LTO-TO2/C1 for the 1st cycle at a scanning rate of 0.1 mV s−1 between 1.0 and 2.5 V (vs. Li+/Li), (b) Nyquist plots of C, LTOTO2, LTO-TO2/C1 and LTO-TO2/C2 electrodes before cycling, (c) high rate performances of LTO-TO2/C1, (d) Long cycle performance of LTO-TO2/C1 at 1 C.

with a radius of 500 nm. However, as shown in Fig. 3e the LTO-TO2/C2 sample particles are significantly enlarged with a radius of about 1.5 μm, this may be due to the fact that the secondary calcination causes the particle agglomeration to be more serious. To prove the distribution and composition of the elements, the EDS mapping of the materials was also carried out. Fig. 4a and e show the SEM images of selected regions of LTO-TO2/C1 and LTO-TO2/C2 respectively. Fig. 4b–d and f–h prove the uniform distribution of C, O and Ti on the surface of the material which proves the uniform compounding of MLC. In order to further clarify the effect of biomass carbon, the schematic illustration of the formation of LTO-TO2/C1 is given. As shown in Fig. 5, the LTO-TO2/C1 precursor is formed by a hydrothermal reaction, and LTO-TO2/C1 is formed by subsequent calcination. The carbonization process further provides a small amount powerful reducing inert gas, and some of the Ti4+ was reduced to Ti3+. More importantly, after calcination, the biomass carbon is compounded on the surface of LTOTO2 microspheres, forming a convenient bridge for lithium ion transport and accelerating the transport of lithium ions between the

Table 1 Impedance parameters and calculated DLi+ of the samples.

C LTO-TO2 LTO-TO2/C1 LTO-TO2/C2

Rs(Ω)

Rct(Ω)

Σw (Ω cm2·s−1)

DLi+ (cm2 s−1)

3.64 7.36 5.17 4.37

43.12 81.84 23.74 33.75

6.58 3.04 4.23

1.99 × 10−13 10.14 × 10−13 5.23 × 10−13

Fig. 3 and Fig. S1 show the representative SEM images of the samples. Fig. S1 shows the typical morphology of the MLC and many small holes with diameter of 500–800 nm can be observed.,this confirms that the prepared biomass carbon is nanoporous. What's more, it can be seen from Fig. 3b, d and f that the LTO, LTO-TO2/C1 and LTOTO2/C2 are all hollow spheres composed of dense nanosheets, and the composite of MLC did not change the general morphology of the LTOTO2 material. Fig. 3c shows that the LTO-TO2/C1 sample has a particle radius of about 700 nm which is not much different from the pure phase Table 2 Rate capacities for modified Li4Ti5O12 at various rate from 1 to 40 C (mAh/g). Materials LTO-TO2/C1 LTO-TO/C 0.15Co-LTO LTO-TO LTO-5:5 LTO/TiO2 F-LTO

Methods 3+

Carbon coated Ti Li4Ti5O12-TiO2 N doped Carbon coated Li4Ti5O12-TiO2 3+ Co doped Ti LTO Li4Ti5O12-TiO2 carbon-coated Li4Ti5O12-TiO2 H2O2-assisted Li4Ti5O12-TiO2 F doped Ti3+ LTO

1C

2C

5C

10C

20C

40C

Ref.

174.3 170 172 143 165 157 163.4

165.8 155 164 – 155 142 157.8

159.8 150 155 120 147 129 148.9

153.7 141 152 – 125 117 124.8

147.8 122 138 – 100 102 –

125.8 – – – – – –

This work [40] [41] [42] [43] [44] [12]

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DLi + = R2T 2/2A2n4C2F 4σ 2

microspheres. The high resolution TEM is shown in Fig. 6, a large number of lattice fringes with a pitch of 0.48 nm can be observed which correspond to the (111) plane of the spinel Li4Ti5O12, and a small number of lattice fringes with a pitch of 0.35 nm can also be observed which correspond to the (101) plane of anatase TiO2. This proves that the sample is a mixed phase of Li4Ti5O12 and TiO2, the results are consistent with the XRD patterns well. In addition, some areas without lattice fringes can be found in the lower right corner of the picture which correspond to amorphous carbon, this can further prove the successful compounding of MLC. The electrochemical performances of all samples are shown in Fig. 7 and S2. As shown in Fig. 7a, the LTO-TO2/C1 and LTO-TO2/C2 have better cycle performance and higher discharge capacity than that of LTO-TO2. After 500 cycles, the discharge capacity of LTO reduced from 156.2 mAh g−1 to 137.5 mAh g−1, while the discharge capacities of LTO-TO2/C1 and LTO-TO2/C2 reduced from 174.3 mAh g−1 to 159.2 mAh g−1 and 162.7 mAh g−1 to 147.5 mAh g−1. Excellent performance can be due to the successful compounding of MLC, it reduces the Li+ diffusion pathway and improves the electron conductivity of LTO-TO2 [34]. To further demonstrate their advantages for high-rate LIB applications, the rate performance were tested from 1 to 20 C and the results are shown in Fig. 7b. Obviously, the discharge capacity of LTO-TO2/C1 and LTO-TO2/C2 is higher than that of LTO-TO2 at all current density, in particular, the discharge capacity of LTO-TO2/C1 is 170.2, 165.9, 159.8, 153.7, 147.8 mAh g−1 at 1, 2, 5, 10 and 20 C respectively, which indicates that the LTO-TO2/C1 electrode has the best electrochemical performance. Fig. S2 shows the cycle curve of MLC at 1C. It can be seen that the discharge capacity of the first circle is only 75 mAh g−1, and it drops rapidly to 52 mAh g−1 after one cycle. This further proves the main role of biomass carbon is improving the conductivity of the material and providing a convenient and efficient channel for lithium ion rather than providing additional capacity. Fig. 7c and d show the charge and discharge curves of LTO-TO2 and LTO-TO2/C1 at 1C. The long platform of about 1.55 V corresponds to the lithium insertion/delithiation process of Li4Ti5O12. A small platform observed at about 1.75 V corresponds to the delithiation from LixTiO2 [35], which obviously adds additional capacity at a low current rate. Meanwhile, compared to LTO-TO2, LTO-TO2/C1 exhibits higher charge and discharge capacity, which was attributed to the synergistic effects of the MLC and the LTO-TO2 microsphere [21]. Fig. 8a shows the CV curves of LTO-TO2 and LTO-TO2/C1. The characteristic peaks around 1.55 V (vs. Li/Li+) corresponding to the process of Li+ insert into and extract from the spinel Li4Ti5O12. A pair of characteristic peaks can be observed around 1.75 V (vs. Li/Li+), corresponding to anatase TiO2. What's more, the potential difference (△V) of LTO-TO2/C1 is lower than that of LTO-TO2, suggesting that LTO-TO2/C1 has higher reversibility. The anode and cathode peaks of LTO-TO2/C1 are sharper than that of LTO-TO2 sample, which corresponds to better electrochemical Li+ insertion/deintercalation kinetics. Therefore, it is reasonable to believe that the composite of MLC is beneficial to further improve the electrochemical performance of LTOTO2 material. Fig. 8b shows the EIS curves of all samples. All Nyquist plots consist of a high frequency concave semicircle and a low frequency oblique line. Specifically, the intercept represents the ohmic resistance, the semicircle represents the charge transfer impedance, and the oblique line represents the diffusion of Li+ in the bulk anode [36]. The equivalent circuit diagram of the EIS spectrum is also shown in the inset of Fig. 8b. As shown in Table 1, the Rs, Rct of LTO-TO2/C1 and LTOTO2/C2 are 5.17, 23.74 and 4.37, 33.75, respectively, smaller than that of LTO-TO2 (7.36 and 81.84). This can be attributed to the good conductivity of MLC. The Warburg coefficient σw and Li+ diffusion coefficient DLi+ for all the samples are calculated by the following equations [37,38]:

ZRs = K +

(1)

σω−1/2

(2) −1

−1

R, T represent gas constant (8.314 J·mol ·K ) and Kelvin temperature (298.15 K), A, n, represent the area of electrode and electrons transfer moles, F represent Faraday constant (9.65 × 104C·mol−1) C represent the concentration of Li+ and σ is the line slope of ω−1/2 and Z'. The results are shown in Fig. S3 and Table 1, the DLi+ of LTO-TO2 is 1.99 × 10−13, less than 10.14 × 10−13 of LTO-TO2/C1 and 5.23 × 10−13 of LTO-TO2/C2. The improved Li-ion diffusion coefficient can indicate faster electrochemical reaction kinetics [39]. To further illustrate the excellent electrochemical performance of LTO-TO2/C1, the higher rates performance and longer cycles performance were tested, and the results are shown in Fig. 8c and d. When the current was increased to 40 C, the discharge capacity of LTO-TO2/C1 can still up to 125.8 mAh g−1, which is higher than those published papers (Table 2). Fig. 8d shows the long-cycle performance,after 1000 charge-discharge cycles at 1 C, the discharge capacity of LTO-TO2/C1 can still reach 157.1 mAh/g, and the capacity retention rate is over 90%. 4. Conclusions In summary, the LTO-TO2/C hollow microspheres derived from mulberry leaves were obtained by two different methods. Biomass carbon provides a convenient and efficient channel for lithium ion, and the composite of biomass carbon increases the specific surface area of the material which provides a large contact area with the electrolyte and leads to high li-ion flux across the electrolyte/electrode interface. What's more, the dual role of biomass carbon and Ti3+ greatly enhances the conductivity of the material. The LTO-TO2/C1 has the best performance for its larger specific surface area and smaller particle size and the discharge capacity can reach 174.3 mAh g−1 in the first cycle at 1C and the capacity can still maintain above 90% after 1000 cycles, even the current was increased to 40 C the discharge capacity can still up to 125.8 mAh g−1. This work provides a simple and cost-effective method for battery materials. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by Science and Technology Commission of Shanghai Municipality (16020500800) and Natural Science Foundation of China (51402187). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ssi.2019.115132. References [1] [2] [3] [4] [5] [6] [7] [8] [9]

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