ADVANO’s Approach to the Adoption of Silicon in Lithium-ion Batteries
Silicon is widely regarded as one of the most promising candidates for application as a next-generation anode material in rechargeable lithium-ion batteries. Compared to graphite, the most widely used anode material in commercial lithium-ion batteries, silicon is expected to deliver an order of magnitude higher specific capacity. This is an attractive proposition for its application in many practical energy storage applications. This advantage derives from the nature of how lithium ions interact with silicon (by alloying) in comparison with graphite (by intercalation). The faster diffusion kinetics of lithium alloying with silicon, compared to that of lithium intercalation with graphite — also bring forth the possibility of faster battery cell charging rates with silicon integration. However, these advantages are undermined by several limitations that stem from the fundamental nature of the interactions between lithium and silicon within a battery cell. The extreme volumetric expansion and contraction of silicon, experienced during repeated storage and release of lithium ions during the lifetime of a battery, makes it highly challenging to integrate silicon in practical battery cells. Additionally, the reactivity of elemental silicon with conventional lithium-ion battery electrolytes accelerates the formation of uncontrolled electrochemical side reactions, which are detrimental to long-term battery performance.
A primer on material design for silicon anodes
Several material designs for silicon anodes have been implemented by the industry, some of which can be classified into the following architectures:
- Elemental silicon particles with protective surface coatings. The size distribution of such particles can be either on the micro or nanoscale.
- A hierarchical silicon microparticle, where the particle surface is restructured into high surface area nanoscale features.
- Silicon nanoparticles are deposited inside the pores of a porous carbon matrix. Silicon can be infiltrated within such pores by condensation of silicon-based gaseous precursors (silanes).
- Deposition of nanostructured silicon in the form of nanoparticles or nanowires on a carbon substrate such as graphite or amorphous carbon.
- Continuous or ‘binderless’ nanometrically thin silicon films or forests of silicon nanowires directly deposited on an electrode.
- Nonstoichiometric silicon oxide particles (SiOx)*. These particles may also be designed with silicon nano-domains dispersed within a silicon oxide matrix.
*Note: Silicon oxides and elemental silicon are often erroneously categorized as the same material. Silicon oxides are a fundamentally different chemical compound compared to elemental silicon, and there are marked differences in their respective electrochemical behavior in a battery cell.
In all the above silicon anode material architectures, protective coatings are often necessary to protect the exposed silicon surface against electrochemical side reactions in a battery cell. The presence of surface coatings and combinations with carbons inevitably reduce the total specific capacity of silicon as an anode material. However, the beauty of silicon as a lithium storage material is its extremely high specific capacity (with some practical limits on how much of the actual capacity can be used). But, if the stability of silicon can be increased significantly by trading off some of the theoretically achievable capacity by engineering its material structure, it is a worthwhile compromise for the sake of commercialization.
Let’s go through some design rules for silicon to be transformed into a battery anode material:
Design rule# A: The microstructure of the silicon material should possess the necessary physical characteristics required for integration in battery electrode manufacturing processes.
Design rule# B: The inevitable swelling or volume expansion of the battery cell caused by the interactions of silicon with lithium ions and the electrolyte during cell cycling should be controllable.
Design rule# C: The integrity of the silicon structure in the battery electrode should remain intact after repeated charging and discharging of the cell.
Design rule# D: The silicon should be protected against detrimental reactions in the electrochemical environment of the battery cell which can cause an irreversible loss in battery cell capacity.
ADVANO’s team has developed a material architecture that adopts these design rules, which is manifested in the form of a nanocomposite structure composed of elemental silicon nanoparticles dispersed within a porous and crosslinked carbon matrix. This is identified as REALSi Micro (or REALSi). More broadly, we classify this architecture as a silicon/carbon composite*. Now, to demonstrate how each of the design rules described above applies to the design features of REALSi Micro, we can first take a look at its typical physical characteristics:
*Note: Si/C composites are sometimes erroneously described as ‘silicon carbide’ which is a fundamentally different chemical compound compared to a ‘composite’ of silicon and carbon.
Now let’s see how ADVANO material architecture fairs with the design rules stated above:
Design rule# A: REALSi particle properties allow it to be compatible with typical anode slurry mixing and coating processes, in combination with a wide variety of binder/solvent systems, graphites, and conductive additives.
- The spherical shape and polydisperse PSD of the REALSi particles are designed for enabling ease of integration with graphite with high packing efficiency.
- The specially formulated porous carbon matrix is mechanically robust on a particle level to withstand the forces experienced during electrode calendaring and keep its structure intact as designed.
Design rules #B-D: The silicon nanoparticles inside REALSi particles are well encapsulated within the carbon matrix, and the carbon matrix itself is porous. The porous carbon matrix acts similarly to a sponge to provide sufficient space for the silicon nanoparticles to expand and contract during the cycling of the anode in a battery cell. The majority of the silicon nanoparticles themselves are sufficiently small in size to be able to sustain the repeated stresses of lithiation and delithiation without succumbing to pulverization. The state of dispersion of silicon nanoparticles inside the composite and their interface with the carbon matrix are controlled by our proprietary silicon surface functionalization process.
Integration of silicon into Lithium-ion battery anodes
Now that we understand the diversity of novel material architectures of silicon anodes, the question arises — how do we go about integrating them into Lithium-ion batteries? Is the industry as a whole ready to completely replace graphite with silicon in conventional cells or do we pursue this in a staged approach? We must first consider that the technological development related to graphite as an anode material has undergone significant levels of maturity in terms of almost every aspect of battery cell engineering, including pairing with different cathodes, cell design, optimization of electrochemistry, and optimization of manufacturing processes required to build cells with high reliability and consistency. Having considered this, ADVANO is taking a staged approach to the commercialization of silicon anodes in the immediate future. The gradual replacement of graphite facilitates incremental but significant energy density gains.
But how is this gradual replacement supposed to work? Why can’t we just increase the silicon loading in significantly larger quantities and speed up graphite replacement altogether? Is it possible to add silicon in the anode without making any other modifications to the baseline cell and expect the upgraded cell to hit all performance gains from the get-go? The problem is that despite the apparent simplicity of the design of a lithium-ion battery cell, it is a marvelous synergy of components all working together like clockwork — each component carefully selected and matched with each other to enable the cell to reach the performance targets it was designed for.
The inviolable fact of the matter is that the nature of the interaction of silicon with lithium is fundamentally different from that of graphite (alloying vs. intercalation), resulting in widely different mechanical responses when they are combined with each other inside the same cell. The interaction of silicon with the source of the lithium ions in the cell itself (the components which make up the electrolyte and the additives) results in drastically varying degrees of responses based on differences in silicon surface chemistry and physical structure. This gets even more confounded when these differences in responses change along with the operational window of the battery (voltage range or state of charge, temperature, charge/discharge rates, etc.).
What about silicon-dominant anodes? If the silicon becomes the majority or the only component in the anode, then it earmarks the shift to new cell chemistry. New cell chemistry requires rebuilding the ecosystem from the ground up to reach the complete maturity required for mass commercialization. It is, however, encouraging to see exciting innovations in cell ecosystem development being demonstrated in both liquid electrolyte-based cells and solid-state cells using silicon-dominant anodes.
ADVANO’s approach to supporting silicon adoption
ADVANO is taking a systematic approach to support the adoption of silicon, firstly targeting the application of silicon as a ‘range extender’ for graphite anodes. We take into consideration the fact that the established knowledge of silicon anode performance regimes is not always immediately projectable to any type of silicon anode solution due to the wide diversity of silicon anode solutions currently available. We are actively collaborating with capable cell partners to map out the dependence of the cell ecosystem and cell electrochemistry with respect to silicon loading in graphite anodes, in practically relevant cell formats using commercially relevant cell designs and balanced pairing of cell components. Considering these cells as the ‘test vehicle’, we aim to develop a structured dataset to seek answers to the following questions:
- Within different sets of operational windows, how long can REALSi keep contributing to the energy delivered by these cells until the REALSi material’s contribution becomes negligible?
- With a constant cell design and electrolyte formulation, what is the extent of REALSi stability when these cells are stored under certain environmental conditions?
- What is the relative importance of different cell design parameters and operational parameters for increasing cell energy density gains derived from graphite anodes with respect to the loading of REALSi?
Our intuition toward increasing the loading of silicon in graphite anodes is a balancing act based on our material architecture and the characteristics of graphite. In order to maintain minimum disruption of a graphite anode’s structure, we envision two different approaches for REALSi integration to enhance the cell energy density. As illustrated in the graphic below, we can add REALSi particles rated at a certain specific capacity in a preset volume fraction with graphite to target a certain level of cell energy density. For another step change in energy density, we can add REALSi particles rated at a higher specific capacity with a lower volume fraction. Instead of choosing too low of a volume fraction and risking the generation of localized hotspots of electrochemical activity, we can target a volume fraction of the REALSi particles in such a way that they are appropriately dispersed within the graphite anode with good packing efficiency.
Takeaways
We don’t think it is necessary to get trapped into thinking that one type of material design of silicon anode is superior to another. ADVANO’s takeaway is that these different approaches are diverse ideas for solving interesting challenges with ingeniously conceived solutions to address a similar class of problems. The performance benefits achievable with respect to costs of manufacturing and scalability are not easy to ignore either. However, in order to preferably adopt any of these solutions, systematic experimentation is indeed necessary to understand what accommodations are necessary to be made to extract the optimal benefit of silicon integration in lithium-ion battery cells.
The article is written by Saheem Absar.
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