Enhancing the Cycle Life of Solid-State Batteries by Restraining the Volume Change of Silicon Anodes
Authors: Poetke Stephanie , Cangaz Sahin , Hippauf Felix , Haufe Stefan , Dörfler Susanne , Althues Holger , Kaskel Stefan 
Published: September 2023

The high gravimetric and volumetric capacity of silicon makes it an attractive anode material for lithium-ion solid-state batteries (SSBs). [1,2] However, silicon suffers from high volume changes during cycling and requires high stack pressures to stabilize the interfaces. [3-5] Herein, we present different approaches to minimize the breathing of silicon-based anodes leading to stabilized cycling.
Silicon carbon void structures (Si-C) obtain a void between the silicon nanoparticle and the surrounding carbon matrix to compensate the volume changes already within the anode structure. As a result, excellent cycling performance in solid-state cells with areal loadings as high as 7.4 mAh cm-2 can be demonstrated. Thereby, Si-C composite electrodes show higher lithiation capacities, better rate stability and higher capacity retentions than pristine silicon nanoparticles (SiNPs), which rapidly degrade due to the immense mechanical stress upon charging and discharging. Hence, the volume changes of the SiNPs are well compensated by the carbon matrix, which also stabilizes the entire electrode. In full cells with nickel-rich NCM (LiNi0.9Co0.05Mn0.05O2, 210 mAh g-1) as cathode, higher initial discharge capacities and coulombic efficiencies (72.7 % vs. 31.0 %) can be achieved compared to the liquid system. The solid electrolyte (Li6PS5Cl, 3 mS cm-1) does not penetrate the whole carbon matrix of the Si-C particles resulting in less side reactions. Consequently, prelithiation of the Si-C anodes is not required in SSBs. By applying either a low (1.1) or rather high n/p ratio (2.0) capacity retentions of up to 87.7 % after 50 cycles can be reached. [6]
Especially for industrial fabrication of SSB anodes other procedures than the complex multi-step synthesis of e.g. Si-C composites are needed. [7] Consequently, we evaluated low-cost silicon microparticles (µm-Si) as partially lithiated electrode material (800 mAh g-1) in SSB half- and full cells. By reducing the utilized fraction of silicon, the breathing during cycling is reduced from 300 % to 66 %, which reduces the armorphization of the active material. In addition, the grain boundaries of silicon are connected by a matrix of solid electrolyte and carbon additive, which drastically reduces the need for a high stack pressure. After limiting the charge cut-off potential of NCM|SE|µm-Si full cells, significant increased capacity retentions from 32 % to 71 % after 50 cycles can be reached. In addition, similar performance compared to the Si-C electrodes can be demonstrated making it to an auspicious alternative. [8]
Overall, the herein presented silicon materials achieved decent electrochemical performance without active pressure control on the cells being beneficial for electric vehicle and other applications. Hence, both Si-C and µm-Si particles are promising concepts for stable, high-capacity SSB anodes.
Literature:
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