Electrochemical Atomic Force Microscopy of Black Phosphorus Composite Anodes: Electrode Destabilization and Degradation Mechanisms in Alkali-Ion Batteries

Despite their higher capacity compared to common intercalation- and conversion-type anodes, black phosphorus (BP) based anodes suffer from significant capacity fading attributed to the large volume expansion (∼300%) during lithiation. Downsizing BP into nanosheets has been proposed to mitigate this issue, and various methods, particularly mechanical mixing with graphitic materials (BP-C), have been explored to enhance electrochemical performance. However, the understanding of BP-C hybridization is hindered by the lack of studies focusing on fundamental degradation mechanisms within operational battery environments. Here we address this challenge by employing electrochemical atomic force microscopy (EC-AFM) to study the morphological and mechanical evolution of BP-C composite anodes during lithiation. The results reveal that BP-C binding interactions alone are insufficient to withstand the structural reorganization of BP during its alloying reaction with lithium. Furthermore, the study emphasizes the critical role of the solid electrolyte interphase (SEI) and BP-C interface evolution in determining the long-term performance of these composites, shedding light on the disparity in final electrode morphologies between binder-inclusive and binder-free BP-C composites. These findings provide crucial insights into the challenges associated with BP-based anodes and underscore the need for a deeper understanding of the dynamic behavior within operating cells for the development of stable and high-performance battery materials.


XPS: Characterizing the Chemical and Structural Composition of the SEI/BP-C Interface
that could be assigned to inorganic species such as Li2CO3, ROCO2Li and organic species, such as polycarbonates, generated by the reduction of solvents in SEI. [1]An additional extremely low binding energy of 283.2 eV found was ascribed to C in the lithiated Lix-C electrode. [2]e O 1s spectra presented in Figure S14b (i-iii), are more complicated to deconvolute, due to the presence of many overlapping bonding states expected in all samples from adventitious carbon-oxygen environments in addition to those arising from other bonding environments Normalised Intensity within the sample.In the Pristine sample Figure S14b (i), a minimum of two peaks were fitted, the O1s peak centred at ~534 eV, were assigned to P-O bonding environments, as has been reported in other studies. [3]For the cycled electrodes, new peaks presented themselves, and a minimum of 6 peaks were assigned to known SEI species that have been widely reported in literature which are reported in The Li 1s XPS peak, again is difficult to deconvolute and a minimum of 3 peaks have been defined at 52.3 eV, 54.0 eV and 55.8 eV.According to literature, the peak at 52.3 eV is assigned to Li2O. [4]The broad peak at 54.0 eV likely has overlapping contributions from Li2CO3, [4] LixP, [3] and LixC, [5] in the case of the electrode cycled above alloying, 0.6 V, whereas LixP was not detected for the electrode cycled below alloying potential, 0.01 V, due to the absence of this peak in the P 2p spectra (Figure 5).Finally, the peak at 55.8 eV likely has overlapping contributions from LiF, LixPFy and/or LixPFyOz.6][7][8]

Figure S1 :
Figure S1: Electrical impedance spectroscopy study of binder inclusive BP-C(C45) electrodes collected at different cell potentials during the 1 st (a) dicharge and (b) charge.(c) Fitted resistance values of the SEI film, charge transfer processes, and sum of total resistances (RTotal = RBulk + Rct + RSEI).Preparation of Binder Free BP-C(HOPG) electrodes by Liquid Phase Exfoliated BP.

Figure S2 :
Figure S2: Schematic diagram showing the formation of liquid phase exfoliated BP (LPE BP), followed by deposition and drying onto HOPG to form the binder free BP-C(HOPG) electrodes.

Figure S4 :
Figure S4: Optical microscopy images of the BF BP-C(HOPG) electrodes in the EC-AFM cell, immersed in 1M LiPF6 and at OCV. Scale bars are shown at the bottom left of each image.

Figure
Figure S5: (a) EC-AFM Image of the surface of the BF BP-C(HOPG) anode in the electrochemical cell with 1M LiPF6 EC/DEC electrolyte at OCV, captured across a 15 x 15 µm 2 area.The z-scale bar is shown to the right of image.(b) Height profiles extracted from line 1 and 2 marked in (a), with the direction of the arrow indicating the direction.

Figure S6 :
Figure S6: CV comparison of BP-C and graphite cells vs. Li metal (a) binder free BP-C(HOPG) electrode collected in the EC-AFM cell (b) binder inclusive BP-C(graphite) electrode collected in a coin cell (c) bare HOPG electrode collected in the EC-AFM cell (d) binder inclusive BP-C(graphite) electrode collected in a coin cell.(a & c) were collected at 0.5 mV s -1 ; (b & d) were collected at 0.1 mV s -1 .

Figure S7 :
Figure S7: EC-AFM Images of the surface of the BP-C(HOPG) electrode in the electrochemical cell with 1M LiPF6 EC/DEC electrolyte at OCV, inset shows the scale of the image.(a) at OCV (b-f) captured under electrochemical control in between 3 ─ 0.01 V vs. Li/Li + at 0.5 mV s -1 .All EC-AFM images captured across 15 x 15 µm area.All z-height scale bars to the right of each row.

Figure S8 :
Figure S8: EC-AFM images of the surface of the BP-C(HOPG) electrode vs. Li metal in the electrochemical cell with 1M LiPF6 EC/DEC electrolyte.(a) At OCV (b-f) under electrochemical control between 3 ─ 0.01 V vs.Li/Li + at 0.5 mV s -1 .All EC-AFM images were captured across 6 x 6 µm 2 area, with their corresponding z-height scale reported to the right of the respective image.

Figure S9 :
Figure S9: EC-AFM imaging with DMT modulus maps of a binder free BP-C(HOPG) electrode collected whilst under electrochemical control in the range 3 ─ 0.01 V vs. Li/Li + at 0.5 mV s -1 in 1M LiPF6 EC/DEC electrolyte.This Figure accompanies Figure 4. (a) AFM height images (b) AFM height images with adjusted height scale to highlight the topography of the HOPG step edges and associated SEI, and (c) DMT modulus maps.The voltage vs. Li/Li + of each image series is as follows: (i) OCV, (ii) 2.0-1.0V during discharge, (iii) 1.0-0.01V during discharge, (iv) 0.01-1.0V during charge, (v) 1.0-2.0V during charge, (vi) 2.0-3.0V during charge.All EC-AFM images were captured across 20 x 20 µm 2 area, and voltages plotted vs. Li/Li + .The black arrow corresponds to the AFM scan direction.All maps are reported with the z/modulus scalebar to the right of the respective image, and x-y scalebar inset in (ai) and (bi).

Figure S11 :
Figure S11: EC-AFM morphology (i) and DMT modulus (ii) maps of the binder free BP-C(HOPG) electrode (a) at OCV, across 18.67 x 11.95 µm area, inset shows the scale bar, (b-d) EC-AFM height and modulus maps of the magnified BP (6.45 x 4.25.µmarea) whilst under electrochemical control, taken across voltage range specified to the left of each image.All images were captured in 1M LiPF6, voltages plotted vs. Li/Li + , and all map scale bars are shown to the right of each image.

Figure S12 :
Figure S12: (a) Operando EC-AFM morphology map of the binder free BP-C(HOPG) anode, in 1M LiPF6 EC/DEC, captured during the cathodic sweep 2.0 -1.0 V. Inset shows the scale bar of the image, and z-scale height shown to the right.(b) Plot of the height, width and DMT modulus line scan across wrinkle 1, and wrinkle 2 (c), marked by the red arrows in (a).

Figure
Figure S14 shows the C 1s, O 1s and Li 1s spectra for the composite BF BP-C(HOPG) b

Table S1 :
Reported high resolutions XPS spectral binding energies for common LIB SEI components and BP related components.