Boron-Doped Carbon Nano-/Microballs from Orthoboric Acid-Starch: Preparation, Characterization, and Lithium Ion Storage Properties

A boron-doped carbon nano-/microballs (BC) was successfully obtained via a two-step procedure including hydrothermal reaction (180C) and carbonization (800C) with cheap starch and H 3 BO 3 as the carbon and boron source. As a new kind of boron-doped carbon, BC contained 2.03 at% B-content and presented the morphology as almost perfect nano-/microballs with different sizes ranging from 500 nm to 5 μm. Besides that, due to the electron deficient boron, BC was explored as anode material and presented good lithium storage performance. At a current density of 0.2 C, the first reversible specific discharge capacity of BC electrode reached as high as 964.2mAh g–1 and kept at 699mAh g–1 till the 11th cycle. BC also exhibited good cycle ability with a specific capacity of 356mAh g–1 after 79 cycles at a current density of 0.5 C. This work proved to be an effective approach for boron-doped carbon nanostructures which has potential usage for lithium storage material.


Results and Discussion
2.1.Synthesis.The boron-doped carbon was synthesized via a two-step procedure [48][49][50].The most important procedure was the first step, of which the high temperature, high pressure, and ionic water got three purposes: (a) to promote the swelling and reshaping of microcrystal bundle of starch with deionized water; (b) to improve the penetration and absorbance of HBO 3 into the microcrystal bundle of starch; (c) to speed up the bonding of HBO 3 with -OH groups of starch.It has been reported that the morphology of starch powder tended to be nano-/microspheres [49,50] with sizes in a certain range.A black powder was finally obtained as boron-doped carbon (BC) representing a successful carbonation procedure in the second step.

Morphology.
To find out the detailed morphology of BC, FE-SEM was applied.Figure 1 both wider compared to the smallest graphitic spacing (002) 0.34 nm [51] and 0.21 nm [52], hinting the more crystalline defects of this new boron-doped carbon compared to graphite.

Raman Spectrum.
As depicted in Figure 2(b), the Raman spectrum exhibited two distinct peaks at D banded ca.1336 cm −1 and G banded ca.1588 cm −1 representing graphitic and disordered sp 2 -carbon atoms of BC.The more intensive G bands marked the typical graphitic lattice vibration [53,54].The less intensive D bands represented the defect lattice vibration [55,56].The intensity ratio of D-band versus Gband ( D / G ) value was calculated to be 0.985 indicating the structural and intrinsic defects and amorphous disorder [57][58][59][60] for BC.

X-Ray Photoelectron Spectroscopy (XPS). XPS was mea-
sured to analyze the elemental species and their corresponding atom percentage in the obtained boron-doped carbon.As presented in Figure 3(a), BC mainly contained C, B, and O dopants with three characteristic peaks at ∼284 eV, ∼192 eV, and ∼532 eV corresponding to C 1s, B 1s, and O 1s, respectively.For BC, the total contents of C, B, and O elements were 84.06 at%, 2.03 at%, and 13.91 at%, respectively.
In Figure 3(b), the C 1s spectrum of the BC could be deconvoluted into several individual peaks.The top peak at ca. 284.4 eV was most likely ascribed to the sp 2 -C of C=C double bonds [15][16][17][18]27].However, the C 1s of B-C-X could not be observed due to the low percentage.According to the fact of boron-doping ratio, the second peak at 285.5 eV was partially ascribed to the C-B bond which was buried in the second peak with other bond species.Partial of the second peak and the last one was defined as the signal of C-O/C=O bonds [16,17].These C of BC took corresponding ratio as 56.94 at%, 21.29 at%, and 5.2 at%, respectively.In Figure 3(c), after analyzing the high resolution spectra of BC sample in the range of 180-196 eV.The high resolution B 1s peak at 191.6 eV was evidence for the existence of B species [39,46].The splitted two peaks at 191.2 eV and 192.6 eV took 0.58 at% and 1.55 at%, respectively, belonging to B 1s of B-C 3 , B-C 2 O with different bonding species [18].In Figure 3(d), the O 1s spectrum of BC was also detected and divided to be two peaks at 531.9 eV and 533.6 eV covering 2.60 at% and 11.84 at% and matched with the O 1s from O=C double bond and O-C single bond, respectively.Neither for O 1s was observed from B-O bond due to the low percentage.4(a) presents the first three voltage curves of BC electrode measured by cyclic voltammogram (CV) at room temperature from 0.005 V to 3.0 V with a 0.5 mV s −1 scan speed.The discharge curve of 1st cycle has not covered the 2nd and 3rd.The initial obvious sharp peaks at 0.01-0.7 V were most likely induced by the solid electrolyte interphase (SEI) layer [61][62][63][64][65]. Figure 4(b) represented the rate cycle curve at several increased current densities from 0.2 C, 0.5 C, 1 C, 2 C, and 5 to 10 C. At 0.2 C, there was a great decrease from the initial irreversible discharge capacity (2059 mAh g -1 ) to the initial reversible charge capacity (1030 mAh g -1 ) which was most likely induced by the SEI reaction.However, at the 1st reversible cycle, the specific discharge capacity of BC electrode reached as high as 964 mAh g -1 and stabilized at 699 mAh g -1 by the end of the 11th cycle which were nearly two times higher than the theoretical top value of graphite electrode (372 mAh g -1 ) with the LiC 6 mechanism [63][64][65].At the highest current density of 10 C, the capacity kept at ∼116.8 mAh g -1 till the 57th cycle [66][67][68][69][70].When the current density was adjusted back to 0.2 C at the 70th cycle, the discharge capacity recovered to 490 mAh g -1 for BC electrode.electrode possessed stable cycle stability with the corresponding capacity as high as 356 mAh g -1 after 79 cycles at 0.5 C. The 1st irreversible discharge capacity of BC reached 1829 mAh g -1 .Beginning with the 37th cycle, the discharge capacity stabilized between 386 and 357 mAh g -1 till the 79th cycle.However, the columbic efficiencies are almost over 90% from the 2nd cycle to the 79th cycle.As presented in Figure 5, The nano-/microstructures of BC particles determined the charge-transfer process of lithium ion insertion/extraction reaction [67,68].According the reported literature, the calculated value as 138 Ω for BC electrode was regarded as a composite resistance value determining the charge transfer of Li + ions insertion/extraction [69,70].The composite resistance value contained the Li + ions migration through the SEI film and charge-transfer resistance.As illustrated above, the carbonaceous body of BC played a role as the conductive channels [20,23,71] for electron transportation.The enlarged electrode/electrolyte interface of BC could promote the rapid absorption and release of Li + ions with fast charge-transfer process.Meanwhile, the transport distances of Li + ions were shortened on the carbon framework.

Conclusions
In summary, a novel boron-doped carbon has been obtained by a two-step approach hydrothermal reaction and carbonization treatment and explored as anode materials for Li + ion battery.The ratio of boron-doping reached as high as 2.03 at%.The morphology presented as perfect nano-/microballs ranging from 500 nm to 5 m.At a current density of 0.5 C, BC electrode exhibited good cycle ability with a discharge capacity of 356 mAh g -1 till 79 cycles.We gave a facile approach to reach a boron-doped carbon.Further investigation to much higher ratio of borondoping is undergoing for more highly performed Li + ion battery.

Experimental Section
4.1.Materials.Potato starch was purchased from supermarket and H 3 BO 3 , and ZnCl 2 [72][73][74] were purchased from Sigma-Aldrich Co., Ltd.Other reagents and solvents were purchased from Energy Co., Ltd.All solvents were used without further purification.

Synthesis of BC Nano-/Microballs
. There were mainly two steps for the synthetic routine.In the first step, the mixture of HBO 3 , starch and deionized water was treated with hydrothermal reaction under a high temperature circumstance at 180 ∘ C. In the second step, the starch particles loaded with HBO 3 were grinded with overdose ZnCl 2 to isolate the starch particles and avoid the conglutination and then carbonized at 800 ∘ C for 2 h under Ar atmosphere.The detailed procedure was carried out as 10 g potato starch, 7.14 g H 3 BO 3 , and 180 ml deionized water was added to hydrothermal reactor and heated at 180 ∘ C for 24 h.After cooling to room temperature, brown yellow powder was obtained by vacuum filtration, washed by deionized water, and dried at vacuum oven at 120 ∘ C overnight.The powder was grinded with ZnCl 2 (1 : 4, weight ratio) for 15 min and divided to be three parts which were calcined at 800 ∘ C for 2 h with a heat ascending rate of 5 ∘ C min -1 , respectively.The obtained three kinds of black powder were washed by HCl (6 mol L -1 ) and deionized water till pH = 7.0 giving target boron-doped carbon materials entitled BC.The particles were scanned by FE-SEM and confirmed to be nano-/microballs.4.3.Methods.Field emission scanning electron microscopy (Hitachi S-4800, Tokyo, Japan) was used to observe the micromorphology of particles.Bruker D8 X-ray diffractometer with Cu K Radiation ( = 1.5405Å) was used to measure the X-ray diffraction (XRD) patterns of aggregate sample.WITec alpha 300M+ micro-Raman confocal microscopy was used to test collect the Raman spectra of as-prepared sample.Thermo Scientific ESCALAB 250XI system with a monochromatic Al K X-ray source was used to carry out the XPS measurements of elemental data.

Electrochemical Tests.
The electrochemistry property of BC was tested with button cells.Pure lithium was used as the counterelectrode and reference electrode.The working electrodes were fabricated by mixing the mixture of BC and polyvinylidene fluoride (PVDF) (90 wt% : 10 wt%) in N-methyl-2-pyrrolidone (NMP).The obtained mixture was coated onto Al sheet and dried for 12 h in a vacuum oven.The electrolyte was a 1.0 mol L -1 LiPF 6 in Et 2 CO 3 /Me 2 CO 3 .The electrodes were assembled into button cells in an Arfilled glove box (moisture/oxygen < 0.1 ppm).The galvanostatic tests of the button cells were measured with a NEWARE battery-testing system.The alternative current (AC) impedance was carried out on a CHI 760D electrochemical workstation (CH Instruments, Inc.).

Figure 1 :
Figure 1: FE-SEM images of BC with different magnificence.
(a) presented with structures as imperfect round nano-/microballs spreading in the range of 500 nm-5 m with magnificent picture as shown in Figure 1(b).The low intensity meant the weak graphitization for BC.Calculated with Bragg's Law 2 sin  =  ( = 1.5405Å), 002 peak was 0.40 nm (22.3 ∘ ) and 2.08 nm (43.3 ∘ )