Extensive demands fr develping the lithium (Li) in battery with high energy density have pursued alternative ande materials because f the limited capacity f graphite. Amng prmising candidates t replace graphite, silicn (Si) is ne f the best materials wing t its high theretical capacity f 3579 mA h g^-1 at rm temperature^{[1, 2]}. Hwever, dramatic capacity fades are bserved during repeated charging-discharging cycles as a result f large vlume change and pulverizatin in Si by allying and de-allying with Li ins^{[3, 4, 5, 6, 7, 8]}. In rder t vercme the year in Si electrdes, many researchers have suggested emplying additinal carbn structures that can prhibit the vlume expansin r pulverizatin. The carbn layers als increase the electrical cnductivity f silicn electrdes remarkably^{[9, 10, 11, 12, 13]}. Hwever, emplying additinal carbnaceus materials is nt suitable t achieve gd battery perfrmance due t their lw Li-in cnductivity.

One pssible slutin t imprve Li-in cnductivity wuld be utilizing nitrgen (N) by making silicn-carbn-nitrgen (SiCN) cmpsite, as reprted by Yamane et al. ^[14]. Riedel and cwrkers als reprted gd electrchemical perfrmance by using carbn-rich SiCN cmpsite made frm varius Si-cntaining plymers such as plyphenylvinylsilazane r plysilxanes^{[15, 16, 17, 18, 19, 20]}. The SiCN materials derived frm plysilazane precursrs pssess amrphus 3D netwrk structures, which are sure t give dimensinal stability during lithiatin/delithiatin prcess^[21]. The slid SiCN netwrk makes its structure mre stable during charge-discharge t attain gd cycle stability. Nevertheless, the exact mechanism f Li-in lading in the materials has nt been clearly elucidated due t the difficulty in characterizing amrphus inrganic materials. There have been reprts claiming that the lithiatin sites wuld be the mixed bnd cnfiguratin f the ceramics^[22], the free nancarbn clusters embedded in the netwrk^{[20, 23]}, and the free dangling bnds f Si^[24]. In additin, the s-called ‘ cnversin electrde’ interpretatin has als been presented, claiming that ally electrde wuld be frmed frm cnverted precursrs. Fr example, Suzuki and cwrkers used silicn nitride thin film made by a pulsed laser depsitin^[25]. n the first charging fr the SiN electrde, metallic Si was generated with frming Li₂N and acted as a reversible Li-in lading sites n the fllwing charging-discharging prcesses. As a result, characterizatins and perfrmance f the prepared electrde varied accrding t slight variatins f initial materials and prcessing cnditins. Fr example, Si₃N₄ has been usually regarded inactive r very lw in the activity t lithium^{[26, 27]}. Hwever, the silicn nitride thin film f Suzuki and cwrkers shwed clear charge-discharge behavir^[25]. There has als been discrepancy in the effect f nitrgen t xygen (N/) rati. In the reprt f Ahn and Raj^[22], high capacities f 600 mA h g^-1 can nly be fund fr materials with an N/ rati belw 1. Hwever, Su et al. reprted that ~456 mA h g^-1 f discharge capacity was achieved frm SiCN with N/ rati f 5.6^[28]. These discrepancies indicate that additinal studies t understand the structure-prperty relatinship f SiCN are required t design effective plymer-derived SiCN.

In this wrk, we prepared nvel SiCN cmpsites by pyrlyzing silsesquiazane (SSQZ) plymer. The SSQZ wuld be a gd precursr t frm SiCN cmpsites wing t its phenyl substituents n the Si center, which affects the physical state f SSQZ t be slid by high glass transitin temperature f higher than 200 ° C. Because the structure f the final inrganic material culd als be affected by mdifying pyrlysis cnditins, we further varied the temperature frm 700 t 1300 ° C.

As shwn in Fig. 1(a), SSQZ plymer was synthesized by a sequential reactin f ammnlysis and cndensatin f phenyltrichlrsilanes. The FT-IR spectrum f SSQZ exhibits the bnding grups f N-H, C-H, phenyl, Si-phenyl and Si-N. The SiCN cmpsites were prepared by pyrlysis under argn atmsphere at 700 ° C (SiCN-700), 1000 ° C (SiCN-1000), and 1300 ° C (SiCN-1300). Table 1 exhibits the results f elemental analysis fr SSQZ and the SiCN cmpsites. SSQZ plymer has 38.2 wt% Si, 42.2 wt% C and 11.9 wt% N. Althugh all preparatin prcesses were perfrmed under Ar cnditin, we cannt avid xygen cntaminatin frm air. Fr all the pyrlyzed samples (SiCN-700, SiCN-1000 and SiCN-1300), the chemical cmpsitins shwed similar trends f decreasing carbn cntent (32.6, 30.6 and 29.5 wt%) with the pyrlysis temperature. With decreasing carbn cntent, Si cntent in SiCN-1300 increased t 56.7 wt% frm thse f SiCN-700 (54.1 wt%) and SiCN-1000 (54.9 wt%). Interestingly, SiCN-1000 has the highest N/ rati f 5.35 amng the SiCN cmpsites due t the high cntent f N and lw cntent f . This wuld be beneficial fr SiCN-1000 t have better battery perfrmance than thers, as mentined in literature^{[22, 26]}.

Fig. 1.

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	Fig. 1. (a) verall reactin steps t synthesize SSQZ plymer, its FT-IR spectrum, and SEM image f SiCN cmpsite made frm pyrlysis f SSQZ; (b) X-ray diffractin patterns; and (c) Raman spectra f SiCN cmpsites pyrlyzed at 700, 1000 and 1300 ° C.

Table 1.

Table 1.

Table 1. Element analysis f SSQZ plymer and SiCN cmpsites after pyrlyzing the SSQZ plymer Sample Si (wt%) C (wt%) N (wt%) (wt%) H (wt%) N/ SSQZ 33.2 42.2 11.9 7.7 5 1.55 SiCN-700 54.1 32.6 8.4 2.2 2.7 3.82 SiCN-1000 54.9 30.6 10.7 2.0 1.8 5.35 SiCN-1300 56.7 29.5 9.8 2.4 1.6 4.08

Table 1. Element analysis f SSQZ plymer and SiCN cmpsites after pyrlyzing the SSQZ plymer

The SEM image f Fig. 1(a) shws that the SiCN cmpsites made frm pyrlysis f SSQZ are particle-like with apprximately 500 nm diameter. Fig. 1(b) shws X-ray diffractin patterns f the SSQZ-derived SiCNs. In all X-ray diffractin patterns f SiCNs, n crystalline peaks are bserved, which indicates the SiCN cmpsites are cmpsed f amrphus structure (Fig. 1(b)). Furthermre, ne weak but distinct brad signal fr all samples was nticed at arund 43° . This is related t carbn segregatins within the SiCN micrstructure, which are riginated frm arbitrary diffractin related t graphene sheets, a characteristic fr amrphus and disrdered carbns^[30]. Hwever, n signal at 26.5° , crrespnding t the interlayer distance f graphite, was present, indicating that n nticeable amunt f graphitic carbns was prepared^[31]. The carbn structure in the SiCN cmpsites was examined using Raman spectrscpy as shwn in Fig. 1(c). SiCN-700 des nt shw any peaks, meaning that n nticeable free carbnaceus micrstructure was develped at 700 ° C. n the ther hand, tw peaks at 1315 and 1585 cm^-1 that crrespnd t the G and D band f graphite fr SiCN-1000 and SiCN-1300 suggest that carbn phase can be evlved abve 1000 ° C. The relatively strng signals fr bth G and D bands further imply that the carbn nanclusters are disrdered^[32]. The Raman spectra can be used t estimate the size f the carbn clusters (L_aLa) accrding t the frmula reprted by Ferrari and Rbertsn^[33]: , where I_D and I_G are the integrated intensities f the D and G bands, respectively, and λ is the laser line wavelength. C' is a cefficient dependent n the excitatin wavelength f laser. The carbn phase was mre rdered than that in SiCN-1300, giving lwer I_D/I_G value (0.92) than that fr SiCN-1300 (0.99). As a result, SiCN-1000 cmpsite has smaller carbn dmain (L_a=1.29 nmLa=1.29 nm) than SiCN-1300 (L_a=1.34 nmLa=1.34 nm). Such difference f carbn structures depending n the pyrlysis temperatures will affect the battery perfrmance f the SiCN cmpsites.

The half cells cntaining the SiCN cmpsites were cycled with a galvanstatic charge/discharge prcess t examine their battery perfrmance. The initial charge-discharge curves tested at a current rate f 100 mA g^-1 are pltted in Fig. 2(a). The first discharge capacity f the SiCN cmpsites was in an rder f SiCN-1000 (860 mA h g^-1) > SiCN-1300 (463 mA h g^-1) > SiCN-700 (85 mA h g^-1), indicating 700 ° C is nt enugh t frm slid SiCN cmpsite. n the ther hand, bth SiCN-1000 and SiCN-1300 culd react with Li ins well, resulting in the frmatin f definite SiCN cmpsites. The capacities f all samples during successive charge prcess drpped t much lwer values f 314 mA h g^-1 (SiCN-1000) > 210 mA h g^-1 (SiCN-1300) > 45 mA h g^-1 (SiCN-700), resulting in the first Culmbic efficiencies (η = C_discharge/C_charge × 100) f 37% (SiCN-1000), 20% (SiCN-1300) and 40% (SiCN-700), respectively. Such lw efficiencies f SiCNs wuld be attributed t a strng irreversible lithium capture in the disrdered sft carbn phase r xygen in the cmpsites. Amng them, sft carbns have been reprted t display a high irreversible capacity, which can be made by heat treatment at a temperature f arund 1000 ° C[34, 35]. Mrever, the silicn xide (Si_x) is knwn t underg reactins with Li ins irreversibly t frm elemental silicn and r lithium xides ^[8]. All SiCN cmpsites have excellent battery cycle life as seen in Fig. 2(b). The discharge capacities are almst cnstant up t 100 cycles with the variable values f 312 mA h g^-1 (SiCN-1000) > 183 mA h g^-1 (SiCN-1300) > 54 mA h g^-1 (SiCN-700). SiCN-1000 has the highest reversible capacity with gd battery retentin due t a prper N/ rati f 5.35, cmpared t 3.82 (SiCN-700) and 4.08 (SiCN-1300). Furthermre, the Culmbic efficiency in Fig. 2(c) als reaches ~100% fr SiCN-1000 and SiCN-1300 during 100 cycles, althugh SiCN-700 shws a much lwer Culmbic efficiency f abut 90%. The rate capabilities tested by variable current rates frm 100 mA g^-1 t 1000 mA g^-1 are shwn in Fig. 2(d). With increasing the current rate, the capacity f SiCN-1000 drpped t 208, 146, 92 mA h g^-1 at the current rate f 200, 500, and 1000 mA g^-1, respectively. Hwever, its riginal capacity culd be regained t 370 mA h g^-1 when the current rate decreased back t 100 mA g^-1, meaning that the SiCN cmpsites pssess gd capacity recvery feature.

Fig. 2.

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	Fig. 2. (a) Initial lithiatin/delithiatin curves f SiCN cmpsites, (b) battery cycle life and (c) Culmbic efficiency f SiCN cmpsites during 100 cycles, and (d) the rate capability f SiCN cmpsites tested frm 100 t 1000 mA mg-1.

In rder t understand the electrchemical behavir f ur SiCN cmpsites in battery cells, cyclic vltammetry and electrchemical impedance spectrscpy were carried ut as shwn in Fig. 3. The cyclic vltammgrams (CVs) f Fig. 3(a) were taken at a sweep rate f 0.01 mV s^-1 in the ptential range between 0 and 1.5 V vs. Li/Li⁺ fr the SiCNs. Interestingly, three kinds f samples shw different electrchemical behavir against Li ins. SiCN-700 des nt display any evlved peaks, indicating that lithium-active SiCN structure is nt fully frmed at a lw temperature f 700 ° C. Hwever, the peak current density f SiCN-1000 largely increased, which indicates mre lithiatin/delithiatin events tk place due t the better develped sft carbn phase. The current density f SiCN-1300 decreased, which can be attributed t the frmatin f the hard carbns at a high temperature f 1300 ° C. Such CV behavir is well matched with the previus results f battery perfrmance. Nyquist plts were carried ut t study bth mass diffusin transfer characterized by the slpe at lw frequency and charge transprt resistance (R_ct) calculated frm the diameter f the semicircle in Fig. 3(b). Amng the SiCN cmpsites, SiCN-700 has the prest mass transfer and the largest charge transprt resistance. This impedance results culd lead t pr Li in activity in CVs and battery perfrmance. SiCN-1000 pssesses much better mass transfer feature and less R_ct value than SiCN-700. Interestingly, SiCN-1300 has a reduced R_ct value with same mass transfer feature, cmpared t SiCN-1000. This wuld be cntributed by the frmatin f the cnductive hard carbn during the high temperature as well as less Li in active area. In rder t check the Li in cnductivity f the SSQZ-riented SiCN cmpsites, we cmpsed a three electrde cell as previusly described in the literature^{[36, 37]}. LiC₂ and Li metal chip were used as a wrking electrde and a cunter electrde, respectively. ur SiCN cmpsites were laded n a cpper film used as a reference electrde. All electrdes were sandwiched tgether within tw stainless steel jackets, and separatrs ensure aviding the cntact f each electrde directly. Because Li ins diffused frm the wrking electrde t the reference electrde, we can measure the transprt features f Li ins f ur SiCN by the semicircle diameters in Nyquist plts. As shwn in Fig. 3(c), the semicircle diameter is smaller than that f Si NPs, indicating that the SiCN cmpsite has better Li in cnductivity than pure Si nanparticles (NPs). Frm this result, we cnfirmed that silicn cmpsites cmpsing f carbn and nitrgen culd elevate the accessibility f Li ins, cmpared t pure Si NPs.

Fig. 3.

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	Fig. 3. (a) Cyclic vltammgrams and (b) Nyquist plts f SiCN cmpsites pyrlyzed at 700, 1000 and 1300 ° C; and (c) Nyquist plts f SiCN-1000 and Si nanparticles characterized by hmemade electrde cells.

Fr identifying the effect f nitrgen in the SiCN cmpsites, we fabricated SiC cmpsite by pyrlysis f PMPS that des nt cntain N-H functinal grups as shwn in the inset f Fig. 4(a). The initial charge/discharge curves f Fig. 4(a) exhibit that the SiCN cmpsite wns much higher charge/discharge capacity (314 mA h g^-1) than SiC cmpsite (ca. 250 mA h g^-1). Furthermre, the SiCN cmpsite wns higher reversible capacity and gd battery cycle life/Culmbic efficiency fr 100 cycles as seen in Fig. 4(b). Such imprvement f battery perfrmance fr the SiCN shuld be ascribed t the inclusin f nitrgen in the cmpsite.

Fig. 4.

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	Fig. 4. Cmparisn f SiCN pyrlyzed frm SSQZ and SiC pyrlyzed frm ply(methylphenyl silane): (a) initial lithiatin/delithiatin curves and (b) cycle life fr 100 cycles.

In cnclusin, we successfully fabricated SiCN cmpsites pyrlyzed frm SSQZ plymer as Li-in battery andes. The SiCN made frm the prper temperature f 1000 ° C shwed gd electrchemical prperties such as high Li in activity r cnductivity, as well as less charge transprt resistance, resulting in utstanding battery cycle life with the highest charge/discharge capacity. It shuld be attributed t the prper N/ rati and the frmatin f sft carbn within the SiCN cmpsite. ur nvel SiCN cmpsites culd be beneficial t preparing stable silicn electrdes by cntrlling carbn, nitrgen and xygen fr rechargeable batteries.