Silicon-based anode is the “golden partner” for all-solid-state batteries

Introduction:

The iteration of wireless devices, the popularization of electric vehicles, and the in-depth development of 5G technology have put forward unprecedentedly stringent requirements on the performance of batteries, the core of energy storage. The current commercial liquid lithium-ion batteries, while increasing in energy density, are characterized by increasing safety hazards and bottlenecks, making it difficult to meet the dual demands of high safety and ultra-high energy density for emerging applications. All-solid-state batteries (ASSB) have become the focus of global R&D because of their potential to realize intrinsic safety under high energy density systems, and silicon-based anode is the “golden partner” of ASSB.

Technology routes and opportunities with sulfide electrolytes:

The core of all-solid-state batteries lies in the electrolyte material, which is divided into three main technology routes: oxide, sulfide and polymer. Among them, sulfide solid state electrolyte (SSE) is regarded as the most promising system for practical application due to its highest lithium ion conductivity, excellent mechanical properties and wide operating temperature range.

Challenges and the rise of silicon-based anodes:

However, sulfide SSEs face key challenges: a narrow electrochemical stabilization window; severe interfacial side reactions and the risk of lithium dendrites when matched with lithium metal anodes. This results in sulfide ASSBs being less competitive than mature liquid systems in terms of cathode loading, cycle life and multiplier performance. Therefore, it is imperative to find alternative anode for lithium metal.

Silicon-based anode: The ideal partner for all-solid-state

Silicon-based anode materials have attracted a lot of attention in recent years due to their near-lithium specific capacity (3579 mAh/g, nearly 10 times that of graphite) and suitable lithium-embedded potentials (effectively avoiding lithium dendrites). It is not only a key transition solution before the commercialization of lithium metal, but also the core direction of all-solid-state battery anode research.

• History: Silicon in sulfide ASSB research began in 2009 (Se-Hee Lee team), followed by Japanese and Korean teams from 2014-2018, with an early focus on half-cells, and then a shift to full-cells after 2018, marking the move toward the commercialization of silicon-based anode sulfide all-solid-state batteries (Si-ASSBs).
• Milestone Breakthrough: In 2021, Ying Meng’s team at the University of California achieved 500 long cycles at high currents and an average Coulombic efficiency of 99.95% using a 99.9 wt% micron silicon anode in a sulfide ASSB full battery. This historic breakthrough not only promoted the development of Si-ASSB, but also shocked the whole field of silicon-based anode materials and triggered a global research boom. In the past two years, the related research and reviews have shown explosive growth.

Silicon Anode x Sulfide ASSB: Core Advantages Explained

Combining silicon negative electrodes with sulfide SSE offers significant advantages that hit the nail on the head:

1. Strongly inhibit volume expansion: 

In the solid-solid contact mode, the external pressure applied during cycling causes SSE to exert a reaction force on the expanding silicon particles. This not only limits the magnitude of expansion, but also improves the crack growth resistance of silicon and prevents chalking. Moreover, the modulus of silicon decreases after lithium embedding, and plastic deformation occurs under pressure, which relieves the internal stress concentration of the electrode and maintains the integrity of the electrical contact.

2. Electrode self-healing and structural stabilization: 

Cycling under external pressure, transverse cracks and pores within the electrode tend to heal and form longitudinal cracks (without affecting Li⁺/e-transport). The pressure ensures that the electrodes are in close contact with the fluid collector and avoids detachment.

3. electrochemical fusion and fully active electrode potential: 

The electrochemical fusion effect of silicon during de-embedded lithium allows the particles to gradually fuse into a whole during cycling. The resulting LixSi alloy has a significantly higher electronic conductivity and lithium ion diffusion coefficient. This opens up the possibility of using fully active electrodes (no inert additives), reduces the demanding design of the initial silicon structure, and simplifies the process.

4. Interfacial Compatibility and Process Flexibility: 

When using a fully active anode, the electrode/electrolyte contact interface is limited. This reduces the requirement for binder compatibility with SSE, widens the choice of binder, and makes it easier to adapt to existing silicon anode production lines.

5. High interfacial stability & low lithium depletion: 

The silicon/sulfide SSE interface is relatively stable, with major lithium depletion occurring during the first week when the passivation layer is formed. In subsequent cycles, the non-permeable SSE does not intrude into the porous structure of the silicon, significantly minimizing ongoing interfacial side reactions and lithium loss. The fully active anode design further minimizes interfacial contact area and reactions.

Key factors affecting the electrochemical performance of Si-ASSBs

Despite the promising future, Si-ASSB research is still in its infancy, with many fundamental scientific issues to be solved: failure mechanisms, safeguarding ion/electron conduction paths during cycling, and interfacial contact failure due to volume expansion. The current key factors for performance enhancement are focused on:

1. External pressure (key!) External Pressure (Key!):

External pressure in the range of 0.1-300 MPa is the cornerstone of Si-ASSB operation. It ensures tight solid-solid interface contact, maintains an effective ion/electron conduction network within the electrode, and potentially widens the substable electrochemical window of sulfide SSE. The magnitude and stability of the external pressure is critical to performance.

2. Binders (bottlenecks and opportunities): 

Liquid silicon negative electrode binders (self-healing, high strength, high elasticity, conductive) are well researched, but face challenges in Si-ASSB: commonly used binder solvents (polarity) are not compatible with sulfide SSE. If the negative electrode is not premixed with SSE (to avoid solvent problems), the conductive performance of pure silicon negative electrode at low pressure is a concern. Therefore, the development of SSE-compatible, high-performance specialized binders is a top priority.

3. Conductive agent (trade-off): 

Although it improves electron conduction, the carbon-based conductive agent may catalyze the decomposition of sulfide SSE. Choosing a low surface area conductive agent (e.g., VGCF) is an effective strategy to reduce decomposition.

4. Silicon particle size (balance of performance and cost): 

Nanosilicon (nm-Si) has good stress relief, short ionic paths, and excellent performance in liquid systems. However, in solid-solid contact ASSB composite negative electrodes, nm-Si is prone to agglomeration and uneven dispersion, affecting the utilization rate of active substances. With a more homogeneous electrode morphology, micron silicon (μm-Si) may exhibit superior or comparable cycling performance while significantly reducing cost, making it an important option for practicalization.

5. Structural Design (Allowance for Expansion): 

Properly designed porous or cushioned structures can provide room for volume expansion and improve cycling stability. However, there is a trade-off between porosity and compaction density to avoid over-sacrificing volumetric energy density.

6. Surface modification (potential to be tapped): 

The surface modification of Si anode in liquid system (stabilizing structure, improving conduction, inhibiting SEI) is effective. However, there are fewer studies in sulfide Si-ASSB, which is a potential research direction.

Conclusion: 

The combination of silicon-based anode and sulfide solid-state electrolyte provides a highly promising path to solve the high energy density and high safety challenges of all-solid-state batteries. Although many scientific and engineering challenges still need to be overcome in terms of interfacial stability, cycle life optimization, and scale-up preparation, the breakthroughs in recent years, especially the excellent performance demonstrated by the micrometer silicon system, signify that the Si-ASSB technology is accelerating towards practicalization. Continuing to deepen the understanding and innovation of key influencing factors (e.g., external pressure mechanism, new binder, optimal silicon structure) will be the core driving force to promote the realization of this “golden pair”.