Exploring the Synergy of SnS2 and Graphite in Composite Anodes: Influence of Graphite Size and Morphology
Authors: Kupka Jana Katharina , Surace Yuri , Cupid Damian Marlon , Flandorfer Hans 
Published: December 2024

Lithium-ion batteries (LIBs) are key components in today’s electric vehicles and energy storage systems and play a fundamental role in enabling various aspects of modern life. At the same time, the continuous demand of LIBs with higher energy density and prolonged cycle life has driven the research into exploring advanced anode materials.

SnS2 has emerged in the past few years as a promising anode material for LIBs for several reasons. It exhibits a high theoretical capacity of 1209 mAh/g, as 4.4 Li ions can be stored per formula unit of SnS2, which can lead to high energy density cells. It also has high electrical conductivity, facilitating electron transport within the anode material.

Additionally, SnS2 is stable in water, enabling water-based processing, which is a necessary step to increase the environmental friendliness of electrode fabrication. However, SnS2 suffers from low first-cycle coulombic efficiency and considerable volume expansion (~300%) during cycling, which causes mechanical stress, loss of electrical contact, continuous SEI cracking and re-formation leading to relatively fast capacity fading.

Graphite, the state-of-the-art anode material, possesses excellent electrical conductivity, and exceptional cycling stability, allowing the anode to maintain its structural integrity over multiple charge-discharge cycles. It also experiences minimal volume expansion (~10%) and contraction during lithiation and de-lithiation, which helps maintain the anode’s structural stability. However, graphite shows a rather low theoretical capacity of only 372 mAh/g.

By combining SnS2 and graphite, the strengths of both materials can be exploited, creating synergies that enhance the overall electrochemical performance. The symbiotic relationship between SnS2 and graphite opens possibilities for overcoming the limitations of the individual components, boosting the development of high-performance anodes.

In this work, we aim to investigate the synergistic effects of SnS2 and graphite in composite anodes by carrying out a systematic study to assess the influence of the following parameters on the electrochemical performance: i) SnS2/graphite mass ratio, namely 50/50 and 90/10, ii) graphite morphology, i.e. flakes and spherical-shaped particles, iii) graphite particle size, i.e. particles with D90= 24µm and D90=6µm. In addition, to also evaluate the impact of preparation methods on electrochemical performance, two distinct approaches are employed: 1) high energy planetary ball-milling to create SnS2/graphite composite powders which are further processed into slurries and coated and 2) manual blending/mechanical mixing, i.e. mixing of SnS2 and graphite directly during slurry preparation for the creation of so-called "blends". Electrode processing was done in water using carboxymethylcellulose (CMC) and styrene-butadiene rubber (SBR) as water-soluble binders. This approach allows the effects of graphite structures, particle sizes, composite ratios, and preparation methods on the electrochemical performance of lithium-ion battery anodes to be quantified.

Pre- and post-mortem analysis techniques (Raman spectroscopy, X-ray diffraction (XRD), and scanning electron microscopy (SEM)) were employed to study the structural evolution and chemical changes within the electrodes, providing crucial insights into the performance differences observed.

Preliminary results indicate that the preparation method does not affect the initial capacity but only the cycling stability. Indeed, both composites and blends with 90/10 and 50/50 ratios show initial capacities of around 520 and 460 mAh/g respectively. However, the composites exhibit better cycling stability compared to their counterparts processed with low-energy mixing. In blends with larger, spherical graphite particles, an 80% state-of-health (SOH) is achieved after 122 cycles, whereas composites made using the same graphite reach 80% SOH after 250 cycles. This pattern persists when comparing blends to composites incorporating smaller graphite particles with a flake-like structure. The composite achieves an 80% SOH at 202 cycles, while the blend only achieves 117 cycles. The SnS2/ graphite ratio also affects the cycling stability with a 50/50 ratio showing higher capacity retention in comparison to the 90/10 ratio for both composites and blends. As a final step, the best performing composite and blend were tested in full-cells versus NMC811 to determine the practical applicability of the developed materials.

Figure 1