Mid Sweden University

Silicon-based anodes offer substantially higher theoretical capacities than graphite in lithium-ion batteries, but their practical deployment is hindered by severe volume changes that induce mechanical degradation and unstable interfacial chemistry. While nanoscaling strategies can mitigate these effects, they often suffer from low tapped density, complex synthesis, and limited scalability. Porous silicon microparticles provide a promising alternative by partially accommodating volume expansion while preserving processability and electrode-level integrity. Here, a HF-free urea-assisted etching strategy is employed to generate porous silicon microparticles under mild conditions, leveraging the coupled action of thermally induced structural disruption and chemically driven surface modification. Control experiments confirm that the combined action of these effects is essential to achieve BJH-resolved mesoporosity and increased surface area. The resulting porous silicon exhibits oxygen- and nitrogen-containing surface functionalities. Composite electrodes prepared with nanographite and sodium alginate binder at graphite:silicon:binder ratios of 8:1:1, 7:2:1, and 4.5:4.5:1 demonstrate improved electrochemical behavior. In half-cell testing, electrodes containing 10-20 wt % porous silicon deliver stable redox activity and retain 630-880 mAh g-1 after 100 cycles at 0.1 C, with Coulombic efficiencies of 98.8-99.7%, whereas higher silicon loadings lead to rapid capacity decay. Cycling-resolved impedance and differential-capacity analyses reveal the formation of a thicker yet mechanically resilient interphase that stabilizes charge-transfer kinetics, while rate capability tests show 65-74% capacity retention at 2 C.

Silicon is a leading candidate for next-generation lithium-ion battery anodes thanks to its high theoretical capacity, yet its use is restricted by severe volume expansion and rapid capacity fading. A further challenge is that many approaches to stabilize Si rely on hazardous or complex syntheses.

This thesis presents a green, hydrofluoric acid-free thermochemical route to engineer porous silicon microparticles using urea as an etching agent. The process combines chemical reactions and mechanical stress from urea decomposition, producing mesoporous networks while maintaining crystalline integrity. Under favorable conditions, surface areas up to ~27 m2 g-1 were achieved, along with stabilizing Si–O and Si–N surface species confirmed by structural and chemical analyses.

Porous silicon was then incorporated into graphite composites for lithium-ion battery anodes. Electrodes with 10-20 wt% porous silicon delivered stable specific capacities of 630-880 mAh g-1 after 100 cycles, more than doubling untreated silicon composites and tripling pure graphite, while maintaining coulombic efficiencies above 98%. Higher silicon loadings caused instability, whereas rate tests showed porous silicon retained ~70% of its capacity at 2C.

These results establish urea-assisted porosification as a sustainable path toward practical silicon anodes and highlight the role of porosity in enabling stable, high-capacity batteries. Future work will focus on optimizing porous silicon as a stand-alone active material and performing postmortem analyses to clarify degradation mechanisms and the role of porosity in electrode stability.

Kisel (Si) är ett lovande anodmaterial för nästa generations litiumjonbatterier (LIB) tack vare sin mycket höga teoretiska kapacitet. Den praktiska användningen begränsas dock av kraftiga volymförändringar under cykling, vilket leder till sprickbildning, instabila gränssnitt och snabb kapacitetsförlust. Dessutom bygger många metoder för att framställa poröst kisel på vätefluoridsyra (HF), vilket innebär miljö- och säkerhetsproblem. I denna avhandling presenteras en grön och skalbar termokemisk metod för att framställa porösa kiselmikropartiklar med hjälp av urea-baserad etsning. Processen kombinerar kemisk reaktivitet och mekanisk stress från ureas fasövergångar vid förhöjd temperatur och ger mesoporösa strukturer samtidigt som kristalliniteten bevaras. Under gynnsamma förhållanden uppnåddes ytor på upp till ~27 m2 g-1, och analyser visade stabiliserande Si–O, och Si–N-bindningar vid ytan. Poröst kisel införlivades i grafitkompositer för elektrokemiska tester. Elektroder med 10-20 vikt% kisel uppvisade stabila kapaciteter på 630-880 mAh g-1 efter 100 cykler vid 0,1C, med coulombiska verkningsgrader över 98 %. Detta är mer än dubbelt så mycket som obehandlat kisel och nästan tre gånger så mycket som ren grafit. Högre kiselhalter ledde däremot till försämrad stabilitet. Vid hastighetstester behölls 65-74 % av kapaciteten vid 2C, vilket visar god effekt, och cyklingsprestanda. Arbetet visar att urea-baserad porosifiering är en hållbar metod för att producera funktionella kiselanoder. Framtida studier kommer att fokusera på att optimera poröst kisel som ett fristående aktivt material samt genomföra post mortem-analyser för att förstå degraderingsmekanismer och porositetens betydelse.

Porous silicon (Si) has gained significant interest in various applications due to its high surface area, tunable pore structure, excellent chemical reactivity, biocompatibility, and surface functionalization potential. Traditional methods for synthesizing porous Si often rely on hydrofluoric acid, a hazardous chemical that poses significant environmental and safety risks, limiting its scalability and sustainability. In this study, a green and scalable approach for synthesizing porous Si microparticles through urea-assisted etching is presented and evaluated as a function of temperature and container conditions (crucible vs. autoclave). The urea etching transformed pristine silicon microparticles, with a non-porous structure and a BET surface area of 2.3 m2/g, into porous silicon with surface areas as high as 26.7 m2/g. The highest porosity was achieved at 400 °C, while higher temperatures (600 °C and 800 °C) led to diminished porosity and surface restructuring. Quantitative analysis revealed a maximum etching yield of 17.5 %, etching rate of 14.6 mg/h, and a pore formation efficiency of ∼43 %. The crystalline structure of silicon remained intact across all treatments, with minor surface disorder observed at higher temperatures. The urea-assisted etching produced a temperature-and environment-dependent surface oxidation and nitrogen incorporation. At 220 °C and 400 °C, a thick oxide layer formed, particularly under high-‍pressure conditions, while oxidation was less pronounced at 600 °C and 800 °C, likely due to rapid thermal decomposition limiting sustained gas-solid interactions. Nitrogen incorporation was most significant in Si-220-HP, where multiple nitrogen environments were detected, including Si–N, NH2/NH3+, and NOx species. At higher temperatures, only stable Si–N bonds persisted, while other nitrogen species diminished.