Sequence‐Defined Peptoids with —OH and —COOH Groups As Binders to Reduce Cracks of Si Nanoparticles of Lithium‐Ion Batteries

Abstract Silicone (Si) is one type of anode materials with intriguingly high theoretical capacity. However, the severe volume change associated with the repeated lithiation and delithiation processes hampers the mechanical/electrical integrity of Si anodes and hence reduces the battery's cycle‐life. To address this issue, sequence‐defined peptoids are designed and fabricated with two tailored functional groups, “—OH” and “—COOH”, as cross‐linkable polymeric binders for Si anodes of LIBs. Experimental results show that both the capacity and stability of such peptoids‐bound Si anodes can be significantly improved due to the decreased cracks of Si nanoparticles. Particularly, the 15‐mer peptoid binder in Si anode can result in a much higher reversible capacity (ca. 3110 mAh g−1) after 500 cycles at 1.0 A g−1 compared to other reported binders in literature. According to the density functional theory (DFT) calculations, it is the functional groups presented on the side chains of peptoids that facilitate the formation of Si−O binding efficiency and robustness, and then maintain the integrity of the Si anode. The sequence‐designed polymers can act as a new platform for understanding the interactions between binders and Si anode materials, and promote the realization of high‐performance batteries.

adding 2 mL of 20 % (v/v) 4-Methylpiperidine/N, N-dimethylformamide (DMF), agitatied for 20 minutes, drained, and washed with DMF. All DMF washes consisted of the addition of 1.5 mL of DMF, followed by an agitation for 1.5 minutes (repeated five times). An acylation reaction was then performed on the amino resin by an addition of 1.6 mL of 0.6 M bromoacetic acid in DMF, and followed by 0.35 mL of 50 % (v/v) N, Ndiisopropylcarbodiimide (DIC)/DMF. The mixture was agitated for 30 minutes at room temperature, drained, and washed with DMF for 5 times. Nucleophilic displacement of the bromide with various primary amines was carried out by a 1.6 mL addition of the primary amine monomer as a 0.6 M solution in N-methyl-2-pyrrolidone (NMP), and followed by an agitation for 60 minutes at room temperature. The monomer solution was drained from the resin, and the resin was washed with DMF for 5 times. The acylation and displacement steps were repeated until the peptoid with the desired length was synthesized. Desired peptoids were cleaved from resin by adding 95 % trifluoroacetic acid (TFA)/water, and then dissolving into water and acetonitrile (v/v = 1 : 1) for further HPLC purification.

Purification of Peptoid
Petpoid crudes were purified by a reverse-phase HPLC on a XBridgeTM Prep C18 OBDTM column (10 μm, 19 mm ×100 mm), using a narrow gradient of acetonitrile in H 2 O with 0.1 % TFA over 15 minutes. Purified peptoids were analyzed using Waters ACQUITY reverse-phase UPLC (the corresponding gradient at 0.4 mL/min over 7 minutes at 40°C with a ACQUITY®BEH C18, 1.7 μm, 2.1 mm × 50 mm column) that was connected with a Waters SQD2 mass spectrometry system. The final peptoid product was lyophilized from its solution in a mixture (v/v = 1:1) of water and acetonitrile. The lyophilized peptoid powder was then used for battery test.

Preparation of Si anodes
CMC and P1 were respectively used as binders to prepare nanosized Si-based anodes.
Firstly, Si powder was mixed with carbon black and the binder in the weight ratio of 6 : 2 : 2.
Carbon black was used as a conductive additive. Then the mixture was dissolved in DI water, followed by a vigorous stirring with homogenizer at 1900 rpm for 5 minute to form homogeneous slurry. Finally, the mixed slurry was spread onto a 20 μm thick copper foil and vacuum dried at 150 °C for 12 hours. The coating thickness was ~10 μm.

Electrochemical measurement
The coin half-cells (CR2025) were assembled to test the electrochemical performance of the obtained anodes. Cells were assembled in an Ar-filled glovebox, using 1 M LiPF 6 -EC/DEC /DMC (1 : 1 : 1, v/v/v) as electrolyte, Li foil as the counter electrode and Celgard 2400 as the separator. Wuhan Landian battery cycler (China) was used to measure battery performance through cyclic voltammetry and charge/discharge cycling at various current density values between cut-off voltage of 0.01 and 1.50 V (vs. Li/Li + ). Cyclic voltammetry (CV) was conducted in cells at the scan rate of 0.2 mV•s -1 from 10 mV to 1.5 V at room temperature. Electrochemical impedance spectroscopy (EIS) was measured by applying an oscillating voltage of 5 mV over the frequency ranging from 10 -2 to 10 5 Hz. The CV and EIS measurements were carried out on an IM6e electrochemical workstation (Zahner, Germany).

Density functional theory (DFT) calculations:
All DFT calculations were performed using the Vienna ab initio simulation package (VASP) at a PAW-PBE level. [2,3] Wavefunctions were expanded with plane-waves and the cut off energy was set to be 500 eV. An eight layer 4 × 4 Si-(111) surface (contains 128 Si atoms) with a vacuum layer of about 20 Å was used to simulate the surface of Si anode, and a single layer 6 × 6 graphene was used to simulate the conductive carbon. During the simulation, we fixed the atomic positions of 4-layer Si atoms, while all other atomic positions were fully relaxed until the forces were converged to 0.05 eV/Å. In order to better describe the adsorption interactions between the adsorbed molecules and anode materials, the van der Waals (vdW) corrections of DFT-D3 method with Becke-Jonson damping were included. [4] The adsorption of the -COOH group onto the Si-(111) surface was associated with the bonding of the oxygen atom, which was double bound with the C atom, with the Si atom.
Then, the C=O double bond became single bond, and therefore the C-O bond to the -OH group became weakened, resulting in a dissociation of the H atom from the -OH group, which was then adsorbed on the Si surface. On the other hand, Si-O chemical bond might also be formed between the -OCNCCOgroup and the Si surface. In this case, The C=O double bond was also first weakened with the O atom bound with the Si atom, then the bonding state of the C atom became unsaturated.

Materials Characterizations:
The morphology of nano-Si anodes with CMC, P1 and P2 binders before and after cycling was observed by SEM (Hitachi S-4800, Japan). FTIR measurement was recorded on a TENSOR27 spectrometer (Bruker, Germany) from 400 to 4000 cm -1 at the resolution of 4 cm -1 . The XPS spectra were obtained with ESCALAB250 XPS (Thermo Fisher Scientific, USA) at 2 × 10 -9 mbar, using Al K (1486.6 eV) radiation at 15 keV of anode voltage. Peak fitting of the high-resolution data was carried out using the Xpspeak41 software. X-ray diffraction (XRD) experiments were conducted on a Bruker D8 Advance X-ray diffractometer (Bruker Optics, Ettlingen, Germany) with Cu-Ka radiation, operated at 40 kV and 40 mA.     Transmittance (a.u.) Figure S5. FTIR spectra of pure CMC, P1 and P2.        Schemes Scheme S1. Proposed mechanism of (a) normal binders like PVDF and (b) P1 binder for nano-Si anode.
Scheme S2. Schematic drawing of the proposed mechanism of (a) CMC and (b) P1 binders for nano-Si anodes.