Facile Synthesis and High Rate Capability of Silicon Carbonitride/Boron Nitride Composite with a Sheet-Like Morphology

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SiCN generally suffers from very high first cycle loss of Li (up to 70% in certain cases) 29,31 and has low electrical conductivity that brings down its reversible capacity and rate capability to values similar or worse than commercial graphite after only a few initial cycles. [35][36] Research has shown that some of these weaknesses can be addressed by changing the pyrolysis conditions (such as extended heat treatment), altering the polymeric precursor type (e.g., polysilylcarbodiimide is preferred over polysilazane) or, by introducing a suitable quaternary element, such as boron or aluminum, [40][41][42][43][44][45][46] at molecular scale. In previous work, precursor-derived amorphous Si(B)CN ceramics showed improved electric property (up to 4 orders of magnitude higher conductivity than SiCN) and superior electrochemical performance. 46,47 In addition, composites made of SiCN matrix with filler phases comprised of carbon nanotubes (CNTs), 48 graphite, 49 and carbon nanofiber 50 have been shown to improve electrical conductivity and resistance to mechanical cracking. However, rate capability and cycling stability of Si(B)CN and SiCN-based nanocomposite electrodes remain inferior to polymer-derived SiCO or other nanocomposite electrodes currently under investigation. One reason for this inferiority could be the presence of Si, C, N, O danglings bonds which may form Li irreversible phases resulting in high first cycle loss. For precursor-derived Si(B)CN, it could be the limited amounts of B that is ultimately retained in the pyrolyzed ceramic. Therefore, based on improved electrochemical performance of Si(B)CN ceramics compared to SiCN ceramics, we conclude that introduction of Accepted Manuscript higher quantities of boron as (nano) sheets or (nano) particles in the SiCN network is a logical parallel step toward the improvement of electrochemical performance. Sheet or particle-like morphology is expected to improve overall accessibility for Li ions in the electrode during successive charge/discharge cycles.
We synthesized a SiCN/BN composite by functionalizing hexagonal boron nitride (h-BN, expressed here as BN) sheets with a commercial off-the-shelf polysilazane precursor, followed by pyrolysis at 1000 °C. We observed that preparation of SiCN/BN composite with a sheet-like morphology provided a unique opportunity to tailor the nitrogen bonds with boron and also increased free carbon content in the SiCN matrix, thereby, improving its electrical conductivity by orders of magnitude compared to boron-free SiCN ceramics. These changes in the molecular structure of the final ceramic yielded a material with excellent electrochemical stability even at high current densities. Composites made of SiCN matrix and precursor-derived BN (SiCN/BNF) and powdered boron (SiCN/BP) as filler phase were also prepared in order to extrapolate and highlight the distinctive chemistry that governs high capacity in SiCN/BN.

MATERIALS AND INSTRUMENTATION
Boron nitride (99.9%) was purchased from Sigma Aldrich. Poly (ureamethylvinyl)silazane (commercial name: Ceraset) was obtained from Clariant Corporation. All materials were used as received without further purification.
Scanning electron microscopy (SEM) of the synthesized material was carried out on a Carl Zeiss EVO MA10 system with incident voltage of 5 kV-30 kV. Transmission electron microscopy (TEM) images were digitally acquired by use of a Phillips CM100 operated at 100 kV. Surface chemical composition was studied by X-ray photoelectron spectroscopy (XPS, PHI Quantera Accepted Manuscript SXM) using monochromatic Al Kα X-ray radiation. 11 B MAS NMR spectra were recorded at 11.7 T on a Bruker Avance500 wide-bore spectrometer operating at 160.47 MHz, using a Bruker 4 mm probe and a spinning frequency of the rotor of 12 kHz. Spectra were acquired using a spinecho θ-t-2θ pulse sequence with θ=90° to overcome problems of probe signal. The t delay (83 μs) was synchronized with the spinning frequency and recycle delay of 1s was used. Chemical shifts were referenced to BF 3 (OEt) 2 (δ=0 ppm). The FTIR spectra were collected using Thermo-Nicolet Nexus 870FT-IR spectrometer. FTIR samples were prepared by mixing ~1 wt% of the finely powdered sample with FTIR grade KBr powder. Phase evolution was characterized by using Bruker powder X-ray diffractometer (Madison, WI) operating at room temperature, with CuKα radiation and nickel filter. The pyrolyzed samples were finely crushed with mortar and pestle and laid on the palette for analysis. Thermogravimetric analysis was performed using Shimadzu 50 TGA (Columbia, MD) (limited to 1000° C). Sample weighing, ~2.5 mg, was heated in a platinum pan at a rate of 10 °C min -1 in air flowing at 10 mL min -1 .

PREPARATION OF SiCN/BN, SiCN/BNF, SiCN/BP
SiCN/BN: Chemical modification of Polyureasilazane (commercial name: Ceraset) was performed using commercially obtained BN (99.9%) nanosheets from Sigma Aldrich TM . 1 g of BN (sonicated in propanol and dried) was mixed with 10 mL of polyureasilazane and stirred for 12 h at room temperature. Subsequently the mixture was cross-linked at 250 °C (heating rate 100 °C h − 1 ) for 180 min to obtain a white powder. The cross-linked powder was then pyrolyzed in N 2 atmospheres at 1000 °C in a tube furnace to obtain an amorphous black powder termed as SiCN/BN.

PREPARATION OF PAPER ELECTRODE
10 mL colloidal suspension of GO (graphene oxide) in 1:1 (v/v) water was made by sonication for 10 minutes. GO was synthesized using Hummer's method. 58  and a top casing were placed on top to complete the assembly before crimping. The entire procedure was conducted out in an Ar-filled glovebox.
Electrochemical performance of assembled coin cells was tested using a multichannel BT2000 Arbin TM test unit sweeping 2.5 V to 10 mV versus Li/Li + using the following cycle schedule: (a) Asymmetric mode: Li was inserted at 0.1 A g -1 (with respect to total electrode weight), while the extraction was performed at increasing current densities of 100, 200, 400, 800, 1600 and 2400 mA g -1 for five cycles each, and returning to 100 mA g -1 for the subsequent 10 cycles. (b) Symmetric mode: Later, all the cells were subjected to symmetric cycling at a current density of 1600 mA g -1 for up to 1000 cycles, returning to 100 mA g -1 for the last 20 cycles.

RESULTS AND DISCUSSIONS
The schematic in Figure 1  Further characterization involved X-ray photoelectron spectroscopy (XPS) analysis of SiCN/BN composite as shown in Figure 3(a-b), which revealed distinct peaks from which elemental composition of the final material was confirmed. Survey scans in Supporting Information Figure   S2 of SiCN/BN showed the existence of Si, B, N and C elemental peaks rising from valence energy levels for the respective atoms. Atomic percentage of boron in the composite was 6.8%, which is much higher than other boron-doped SiCN. 40 The peak at approximately 190.5 eV for high resolution B 1s in Figure 3b Figure S6). Specific capacity values were calculated with respect to total weight of the electrode and mass of active material. Figures 4a and 4b show the charge/discharge profile and differential capacity curves of the SiCN/BN electrode's first two cycles. First cycle discharge and charge capacities were observed to be 891 and 517 mAh g -1 with a first cycle loss of 58% at current density of 100 mA g -1 . This capacity value is one of the highest reported for polysilazane-based anode materials. [34][35][36][37][38] For example, first reversible capacity of SiCN/BN hybrid composite is better than SiCN annealed at 1000 °C (456 mAh g -1 ), 36 C-rich SiCN (~263 mAh g -1 ), 37 SiCN-1300 °C (383 mAh g -1 ), 38 B-doped SiCN (~100 mAh g -1 ) 47 and SiCN/graphite (~474 mAh g -1 ) . 49 The differential capacity curve showed first cycle lithiation peaks at 50 mV, 150 mV, a weak plateau at ~700 mV, and a delithiation plateau at ~500 mV. From literature, the peak at 50 mV could be attributed to Li intercalation/adsorption in graphitic carbon 32,34 and the peak at 150 mV corresponds to interaction of Li in nanovoids or interaction with dangling bonds present at Si and C sites in the ceramic. 32,37,47 The plateau at ~700 to 800 mV, present only in the first cycle, originated due to formation of passive solid electrolyte interface (SEI) on graphite.
This plateau was common to all electrodes including 'neat' rGO electrode and data from the literature (see Supporting Information Figure S7 for voltage profiles of BP/rGO, and BN/rGO electrodes). The charge/discharge profile and corresponding differential capacity curves of SiCN/BNF/rGO and SiCN/BP/rGO paper electrodes are shown in Supporting Information Figure   S8. Lithiation peaks were at 50 mV and ~600 mV with a corresponding delithiation peak at 200 mV (similar to BN/rGO and BP/rGO differential capacity curves). From these results we deduce With additional cycling (Figure 4c), SiCN/BN/rGO maintained high capacity at 474 mAh g -1 (96% of initial capacity retained) which is higher than BN/rGO and SiCN/rGO with charge capacities of 67 and 154 mAh g -1 , respectively, after 5 cycles at 100 mA g -1 . Current density was gradually increased to 200, 400, 800, 1600 and 2400 mA g -1 for each five cycles consecutively.
Importantly, SiCN/BN/rGO hybrid composite maintained its reversible capacity of 283 mAh g -1 even at 2400 mA g -1 . This capacity was 52% of the initial capacity. When the cells were cycled back at 100 mA g -1 , all electrodes regained initial charge capacities at 449, 60, and 154 mAh g -1 for SiCN/BN/rGO, BN/rGO, and SiCN/rGO, respectively. However, SiCN/BNF/rGO, SiCN/BP/rGO, and BP/rGO performances, shown in Supporting Information Figure S9, were lower than SiCN/BN/rGO. In order to test cell performance during long-term cycling, cells were cycled at 1600 mA g -1 during charge and discharge half cycles (see Figure 4d). SiCN/BN/rGO demonstrated stable and high charge capacity of ~62 mAh.g -1 than BN/rGO and SiCN/rGO anode at ~25 and 40 mAh g -1 , respectively. All electrodes regained most of initial capacity when they were cycled at 100 mA g -1 after 1000 cycles. SiCN/BN/rGO was the best performing anode with a charge capacity of ~401 mAh g -1 at 100 mA g -1 even after symmetric cycling at 1600 mA g -1 for 1000 cycles. Summary of the electrochemical data and comparison with recent literature (including graphite 65 and rGO paper electrode 66 ) is presented in Table 1. Further, Galvanostatic Intermittent Titration Technique (GITT) was performed to obtain the solid state Li-ion diffusion which is several orders of magnitude higher than that of 'neat' SiCN prepared under similar conditions. Additionally, unlike its individual constituents, the SiCN/BN composite offers high electrochemical activity and stability toward lithium-ions with charge capacity reaching ~517 mAh g -1 at 100 mA g -1 and ~283 mAh g -1 at 2400 mA g -1 with respect to total electrode weight.
This behavior is attributed to the increased amount of -sp 2 carbon in SiCN phase (observed only in presence of BN) that may have exceeded its percolation limit and provided necessary sites for reversible lithium adsorption. Facile synthesis and improved thermal, electrical and Accepted Manuscript