Large Scale Preparation of Freestanding Porous Silicon Anodes and Their Implementation in Lithium Ion Batteries

Abstract

Lithium ion batteries are one of the most promising secondary battery technologies with high gravimetrical and volumetric energy densities, enabling the path towards a green and environmentally friendly future. To further improve the energy density of the battery, much effort is spent in the development of new electrode materials, like silicon and sulfur, to replace the materials with limited specific capacity and critical environmental impact that are currently used. Silicon, with a specific capacity higher than pure lithium (4200 mAh/g vs. 3862 mAh/g [1]), is one of the most promising anode materials being cheap, earth-abundant and less dangerous compared to pure lithium metal anodes. Silicon offers the chance to replace state of the art graphite anodes, which have low specific capacity (372 mAh/g [1]). However, rechargeable silicon anodes still suffer from the intrinsic problems of silicon during cycling, making it unsuitable for battery applications. The huge volume expansion of 400% between the charged and discharged state lead to a pulverization of the silicon anode and consequently poor cycling stability. Since the 1990s many approaches like Si nanoparticles, nanowires or nanotubes, Si thin films or compounds with different metals, metal oxides or carbon lead to an improvement in cycling stability but so far with limited application in real batteries. [1] Here, we present a scalable top down approach where a defined porous micro-structure is etched into silicon wafers. The pores allow enough space in order to cope for the large volume expansion of Si during charging and discharging of the battery. In previous studies, it was already shown that Si microwire anodes could be cycled with 3150 mAh/g for hundreds of cycles without significant capacity. [2][3] Porous silicon can also be fabricated as thick thin films (>30 µm), possessing a defined and adjustable porosity, for use as electrodes with high areal capacity for lithium ion batteries. This generation of silicon anodes has the advantage that it can be processed with cheap silicon material in much larger scale. By the electrochemical top-down approach, the surface of a silicon wafer is porosified and transferred onto copper, which is used as current collector. The use of chemical and electrochemical copper deposition techniques, create a direct ohmic contact without the need of additional, inactive material that only adds to the weight of the anode. This study shows the performance of these porous silicon anodes. By using these anodes, the performance was shown to be almost constant from the 5th to the 100th cycle, with a capacity of 3150 mAh/g and columbic efficiencies around 99%.This novel electrode design is able to counteract the volume expansion problems, and already lead to promising results in the battery performance. To further increase the energy density of lithium ion batteries, cathode materials, like sulfur, are of great interest. Since sulfur is unlithiated, contrary to NMC, NCA and other cathode materials, lithium needs to be incorporated either in the silicon anode or in the cathode. With silicon present as a film, simple prelithiation strategies can be applied, according to Cui et al.[4]. By simply short-circuiting silicon and lithium in presence of an electrolyte, a silicon anode can be prelithiated. Varying time, concentration and other electrochemical and chemical parameters, it is possible to define a certain state of charge of the silicon anode. First results of this simple prelithation strategy demonstrate high capacities and high columbic efficiencies of the silicon anodes, already possible from the first charging cycle, offering the potential to be combined with lithium-free cathodes. [1] X. Zuo, J. Zhu, P. Müller-Buschbaum and Y. Cheng, "Silicon based lithium-ion battery anodes: A chronicle perspective review", 2019. [2] S. Hansen, E. Quiroga-González, J. Carstensen, R. Adelung and H. Föll, "Size-dependent physicochemical and mechanical interactions in battery paste anodes of Si-microwires revealed by Fast-Fourier-Transform Impedance Spectroscopy", Journal of Power Sources, vol. 349, pp. 1-10, 2017. [3] S. Hansen, E. Quiroga-González, J. Carstensen and H. Föll, "Size-dependent cyclic voltammetry study of silicon microwire anodes for lithium ion batteries", 2019. [4] N. Liu, L. Hu, M. McDowell, A. Jackson and Y. Cui, "Prelithiated Silicon Nanowires as an Anode for Lithium Ion Batteries", ACS Nano, vol. 5, no. 8, pp. 6487-6493, 2011.