Supplementary Information

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1 Supplementary Information Formation of stable phosphorus-carbon bond for enhanced performance in black phosphorus nanoparticle-graphite composite battery anodes Jie Sun, 1,6 Guangyuan Zheng, 2 Hyun-Wook Lee, 1 Nian Liu, 3 Haotian Wang, 4 Hongbin Yao, 1 Wensheng Yang, 6 * Yi Cui 1,5 * 1 Department of Materials Science and Engineering, 2 Department of Chemical Engineering, 3 Department of Chemistry, 4 Department of Applied Physics, Stanford University, Stanford, California 94305, USA, 5 Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA, 6 State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing , P.R. China. *Corresponding author: yicui@stanford.edu (Y. Cui), yangws@mail.buct.edu.cn (W. Yang).

2 Part I. Experimental procedures Synthesis of black phosphorus: Red phosphorus was washed with 5 % sodium hydroxide solution and distilled water to remove oxides before processing. Red phosphorus (7 g) and stainless steel balls (9, 20 mm in diameter; 20, 10 mm in diameter; 30, 6 mm in diameter) were put into a stainless steel vessel having a capacity of 0.1 L, which was sealed in an argon-filled glove box and performed with a rotation rate of 400 revolutions per minute (rpm) for 12 h using the high energy mechanical milling (HEMM) technique in a ball mill instrument under a 1.2 MPa Ar atmosphere. This procedure is applied to transform red phosphorus into black phosphorus. Synthesis of black phosphorus-carbon composites: The black phosphorus and ball-milled carbon materials (graphite, graphite oxide, C 60, or carbon black) were well mixed with a P/C molar ratio of 1/3. The mixed powder (5 g), as well as (7, 25 mm in diameter; 30, 6 mm in diameter) stainless steel balls, were sealed in an argon-filled glove box and performed under a 1.2 MPa Ar atmosphere with a rotation rate of 700 rpm for 12 h via HEMM. The corresponding black phosphorus-carbon (graphite, graphite oxide, C60, or carbon black) can be obtained. Synthesis of the mixture of black phosphorus and graphite (BP/G): The black phosphorus (BP) and ball-milled graphite, with the P:C molar ratio of 1:3, was evenly mixed by mechanical stirring 24 h in an argon-filled glove box to generate the BP/G mixture. Synthesis of the mixture of red phosphorus and graphite (red P/G): Red phosphorus

3 and graphite, with the P:C molar ratio of 1:3, was evenly mixed by mechanical stirring 24 h in an argon-filled glove box to generate the red P/G mixture. Electrochemical characterization: Electrochemical performance was examined using coin-type (CR 2032) cells with lithium foil as counter and reference electrodes. The working electrode was fabricated by coating a paste of active materials (BP-G, BP/G, or red P/G) and polyvinylidene fluoride (PVDF) binder (90:10 wt.%) on a copper foil collector. The electrodes were dried at 120 C for 12 h in Ar atmosphere. The electrodes on copper foil were punched with the same 1.0 cm diameter arch punch and weighed. The thickness of electrode without a current collector was mm, and the mass load of active material was mg cm -2. Cells were assembled in a argon-filled UNIlab glove box (MBRAUN, Stratham, NH) maintained at <1 ppm H 2 O with an electrolyte of 1 mol L -1 LiPF 6 in ethylene carbonate ethyl methyl carbonate dimethyl carbonate (EC EMC DMC) (1:1:1, volume ratio) solution and a Celgard 2400 separator. The electrochemical data were collected using a Arbin MSTAT battery test system within the potential range V (vs. Li/Li + ) at various current densities. The specific capacity was calculated based on the weight of BP and BP-G composites, respectively. Electrochemical impedance spectra (EIS) were recorded using IM6e electrochemical workstation (Zahner-Electrik, Gemany), where frequency response analyzer was installed at 25 C in the frequency range from 10 5 to 0.01 Hz.

4 Part II. Characterization Raman spectroscopy Raman spectroscopy was performed on a Renishaw RM2000 confocal Raman spectrometer with a 514 nm excitation laser (laser spot size of 0.5 µm) operated at a low power level (~2 mw) in order to avoid any heating effects. Samples were sealed in transparent glass for Raman measurements. Powder X-ray diffraction (XRD) XRD patterns were recorded on a PANalytical X Pert Diffractometer operated at 45 kv and 40 ma at the wavelength of Cu K α radiation (λ = nm). XRD samples were prepared by pressing powders onto glass substrates. Samples were sealed by polyimide tape for XRD measurements. Scanning electron microscopy (SEM) SEM images were taken using a FEI XL30 Sirion SEM (accelerating voltage 5 kv). Elemental analysis was performed using energy-dispersive X-ray spectroscopy equipped in the SEM. Transmission Electron Microscope (TEM) High-resolution TEM images and energy-filtered TEM maps were performed on a FEI Tecnai G2 F20 TEM installed with a CCD camera operating at an acceleration voltage of 200 kv. Energy-dispersive X-ray spectroscopy (EDS) measurements were collected on a FEI Tecnai G2 F20 TEM operating at an acceleration voltage of 200 kv and equipped with a DX-4 analyzer (EDAX). X-ray photoelectron spectroscopy (XPS)

5 XPS was performed on PHI 5000 VersaProbe, using an Al K α (λ = 0.83 nm, hυ = ev) X-ray source operated at 2 kv and 20 ma. Part III. Supplementary Tables and Figures Calculation of the specific energy of different types of Li-ion batteries The specific energy (E) of these cell is calculated using E= C c C a C c + C a ( V c V a ) Where C indicates the capacity and V indicates the average potential versus Li/Li +. The subscripts c and a represent cathode and anode, respectively. Table S1 shows the values used for calculating each of the type of battery. Table S1. Values used to calculate theoretical specific energy. Anode Cathode Voltage Specific Anode/Cathode Capacity Capacity Difference Energy (mah/g) (mah/g) (V) (Wh/kg) Graphite/LiCoO Graphite/LiFePO Graphite/Mixed layer oxide Silicon/LiCoO Li/S P/Li 2 S Si/Li 2 S

6 Table S2. Physical properties of red P, black P and Si. White P Red P Black P Si graphite Tetrahedro n Amorphous Orthorhombi c Diamond Cubic Hexagonal Crystal type Band gap (ev) Density (g/cm 3 ) Ignition point C 240 C >400 C - - Electrical resistivity (Ωm) 10 9 (at 10 C) 3* (at 20 C) 5*10-2 3*10-3 (perpendicula r to graphene planes) 2.5~5*10-6 (parallel to graphene planes) Electrical conductivit y (S/m) 10-9 (at 10 C) 0.2~ 10-3 (at 3.3* C) 3.3*10 2 ~ 2*10 5 Theoretical capacity

7 Calculation of volume expansion taking place from black P to Li 3 P. volume expansion=4*v cell (Li 3 P)/V cell (black P)=307% Figure S1. The FESEM images of (a) micron-sized red phosphorus and (b) nano-sized black phosphorus.

8 Figure S2. The XRD patterns of red phosphorus and black phosphorus. The amorphous red P (JCPDS ) was converted to orthorhombic black P (JCPDS ) via HEMM technique under high pressure. Figure S3. Raman spectra of red phosphorus and black phosphorus. The Raman spectrum in the region cm 1 contains bands which are indicative of P P bonds, it is significantly different from red P and black P. Features of the spectrum of red P evident can be attributed to P P stretching bands of P 9 and P 7 cages arranged to form pentagonal tubes in paired layers. [1,2] The BP, with orthorhombic (A11) phase, has three active modes of 465, 435 and 362 cm -1, which are assigned to A 2 g, B 2g and A 1 g, respectively. [3,4]

9 Figure S4. FESEM images of (a) micron-sized graphite (b) nano-sized graphite after ball milling (BMG). Figure S5. Raman spectra of graphite and BMG. Comparing with that of graphite, the full width half maximum (FWHM) of D peak and G peak become wider, simultaneity, I(D)/I(G) ratio increases. Those illustrate that the crystallinity of graphite is degraded during HEMM process. The G band maxima (Gmax) of the BMG and graphite are same at 1588 cm -1.

10 Table S3. The calculated length of the different P-C bonds (sp 3, sp 2 in plane, and sp 2 at edge) based on periodic ab initio density-functional theory (DFT) method. Table S4. Analysis of the deconvoluted C1s peaks from XPS and their relative atomic percentage in terms of various carbon materials and the corresponding black phosphorus/carbon composites. Bonding type Sample name sp 2 (C=C/C-C) ev G BP-G GO BP-GO CB BP-CB C 60 BP-C % 56.7 % 67.2 % 57.9 % 80.5 % 66.8 % 100% 95.8 % sp 3 (C-C) ev 8.9 % 32.1 % 1.7 % 5.9 % 19.5 % 29.8 % % sp 3 (C-OH/C-O-C) ev % 16.2 % sp 3 (C=O) ev % 10.4 % sp 3 (O-C=O) P-C ev % 3.8 % % % % %

11 Figure S6. Experimental and fitted XPS spectra for phosphorus 2p from the BP and various black phosphorus/carbon composites (BP-C60, BP-CB, BP-GO and BP-G). a, the observed data and fitted spectra of the BP, BP-C60,BP-CB, BP-GO and BP-G, respectively. b, the comparison of the P2p XPS spectra among BP, BP-C60,BP-CB, BP-GO and BP-G. The spectra was fitted to the 2p 1/2 and 2p 3/2 doublet, which is split by ~0.85 ev with an integrated intensity ratio of 1:2. [5] The vertical blue dashed line is the center of the phosphorus 2p 3/2 peak from P-P bond (129.9 ev), while the vertical pink dash dot line is the center of the phosphorus 2p 3/2 peak from P-C bond (130.4 ev).

12 Figure S7. (a) The charge discharge profiles of graphite electrode at the first two cycle between V with a current density of 0.2 C. (b) The cycling performance of graphite electrode at a current density of 0.2 C. Reference [1] Olego, D. J., Baumann, J. A., Kuck, M. A., Schachter, R. & Michel, C. G. The microscopic structure of bulk amorphous red phosphorus: A Raman scattering investigation. Solid State Commun. 52, (1984). [2] Fasol, G., Cardona, M., Hönle, W. & von Schnering, H. G. Lattice dynamics of Hittorf's phosphorus and identification of structural groups and defects in amorphous red phosphorus. Solid State Commun. 52, (1984). [3] Akahama, Y., Kobayashi, M. & Kawamura, H. Raman study of black phosphorus up to 13 GPa. Solid State Commun. 104, (1997). [4] Sugai, S. Raman and infrared reflection spectroscopy in black phosphorus. Solid State Commun. 53, (1985). [5] Kang, B. & Ceder, G. Battery materials for ultrafast charging and discharging. Nature 458, (2009).