Porous Boron Nitride as Scaffold for Silicon Composite Anode Material

Aiming for higher specific energies and energy densities, lithium-ion battery (LIB) research explores silicon (Si) as an alternative high-capacity negative electrode/anode material. One of the challenges associated with employing Si materials in LIBs is their strong volume expansion upon lithiation (more than 300%), leading to significant stress in the particles. In the case of micrometre-sized Si particles, they tend to crack or detach from the conductive network of the electrodes. An effective way to overcome these issues is to apply nano-structured Si. However, reducing structure dimensions to the nanoscale results in increased specific surface areas, which promote a larger amount of electrolyte decomposition and subsequent solid electrolyte interface (SEI) formation. This irreversible process consumes electrolyte and diminishes the energy density of a LIB cell by immobilizing lithium.^[1]<br/><br/>Composite materials address this challenge, one possible concept is combining carbon and nano-Si to micrometre-sized particles.^[2] A successful approach to synthesize such composites is infiltrating silicon in porous carbon particles (~65% porosity) by chemical vapour deposition (CVD) using silane. Due to the micrometre particle size, the resulting composite exhibits a moderate surface area (below 25 m² g^-1), significantly limiting SEI formation. The reduced volume expansion of the particles leads to promising electrochemical performance,^[2] but some drawbacks persist. Porous carbon is not temperature stable in ambient atmosphere and may ignite at temperatures above 250 °C.^[3] In addition, exposed carbon surfaces are conductive and promote electrolyte decomposition as well as subsequent Li consumption upon lithiation.^[4]<br/><br/>Because of these drawbacks there is a growing interest in non-conducting and more safe alternatives. Hexagonal boron nitride (h-BN) fulfils the necessary requirements to replace carbon scaffolds in Si composites. It features a comparable low bulk density, and can be fabricated with a similar porosity to the previously employed carbon particles.^[2,5] In addition, hexagonal boron nitride exhibits a high temperature stability of up to 800 °C in oxygen atmosphere^[5], improving safety during material handling and cells in operation. As h-BN is an insulator, residual BN surfaces in contact with the electrolyte are not able to provide electrons for decomposition reactions. Thus, using insulating scaffolds is another factor reducing the undesired SEI formation, in addition to the generally low particle surface area of the Si composite concept.<br/><br/>In this work we present novel silicon-boron nitride composite materials, their fabrication, and their electrochemical performance as a negative electrode/anode material in a LIB. The synthesis of the highly porous h-BN scaffold is carried out in a furnace under nitrogen flow, using boric acid and an amide as precursors. Ammonia atmosphere forms inside the furnace upon heating, allowing the formation of h-BN sheets. By adjusting the processing time and temperature, a highly defective h-BN structure forms, exhibiting porosities of over 65%. The porosity is similar to those achievable for the porous carbon particles.^[2,4] In the next step, pores are filled with silicon using the same CVD-process as reported in literature for carbon composite materials.^[2] Employed in a LIB electrode, the composites show competitive cycle lives of over 500 cycles with 80% capacity retention at a capacity exceeding industry standard graphite by more than a factor of two.<br/><br/><i>References:</i><br/>^[1]Ozanam, F. and M. Rosso,<i>.</i> Materi. Sci. Eng. B, 2016. <b>213</b>: p. 2-11.<br/>^[2]Liu, Y., et al., J. Mater. Chem., 2013. <b>1</b>(45): p. 14075-14079.<br/>^[3]Buettner, L.C., C.A. LeDuc, and T.G. Glover, Ind. Eng. Chem. Res., 2014. <b>53</b>(41): p. 15793-15797.<br/>^[4]David Linden, T.R., <i>Handbook of batteries, 4th ed.</i> 2011, Portland: Ringgold, Inc: Portland. p.26.17-26.25<br/>^[5]Saha, D., et al., ACS Applied Materials & Interfaces, 2017. <b>9</b>(16): p. 14506-14517.