The Secret of Building Better Batteries: Importance of SEI in Cell Life

Electric Vehicles are going to rule the roads of our future. Electric vehicles represent the vital technology in decarbonizing road transport, which contributes to over 15% of global energy-related emissions. The global EV market has surged from 4% to 14% in the last three years, driven by factors such as fuel costs, environmental awareness, and improvement of rechargeable battery technology .Powering our portable electronics, electric vehicles, and even grid storage solutions, rechargeable batteries are the backbone of our modern world. Lithium-Ion batteries have long been the primary choice in rechargeable batteries, but now Sodium-Ion batteries are stepping up to offer enhanced convenience and sustainability for both people and the environment. But within these batteries lies a hidden factor, often overlooked yet critical for their performance and lifespan: the Solid Electrolyte Interphase (SEI).

What is the SEI?

Imagine a thin film forming on the surface of a battery’s negative electrode (anode) during its first charge cycle. That’s the SEI, a naturally occurring layer arising from the interaction between the electrolyte (the conductive liquid) and the anode material. While seemingly simple, the SEI plays a complex role in a battery’s health.

The SEI’s Contribution to Longer Life

So, how does the SEI contribute to this extended lifespan? Here’s where its magic lies:

• Selective Protection: The SEI acts as a discerning barrier, permitting the essential flow of sodium ions for battery operation while safeguarding the anode from the deleterious effects of the electrolyte’s harsh chemicals. This selective protection significantly hinders the degradation of the anode material, a primary culprit in reduced battery lifespan.

• Paramount Stability: Consistent battery performance hinges on a stable SEI. It facilitates the unimpeded movement of sodium ions during charging and discharging cycles. Conversely, an unstable SEI, prone to breakdown and reformation, disrupts this flow, leading to diminished battery capacity and compromised energy storage efficiency

• Enhanced Safety: Lithium-ion batteries are susceptible to the perilous phenomenon of dendrite formation. These needle-like structures, growing on the anode, can pierce the separator, a critical component separating the electrodes. This breach can lead to catastrophic consequences like short circuits, fires, and even explosions. The SEI plays a vital role in sodium-ion batteries by mitigating dendrite formation, thereby enhancing overall battery safety.

• Extended Cyclability: A battery’s ability to endure repeated charging and discharging cycles, while maintaining optimal capacity and performance, is termed cyclability. A stable SEI minimizes unwanted side reactions between the electrolyte and the anode. These reactions would otherwise degrade the anode material, leading to reduced cyclability. In contrast, an unstable SEI can deteriorate more rapidly, ultimately limiting the battery’s overall lifespan.

• Improved Rate Capability: Rate capability refers to a battery’s ability to deliver or absorb energy rapidly, a crucial factor for applications like electric vehicles where fast charging is essential. A stable SEI acts as a well-defined pathway for sodium ions, minimizing resistance to their movement and enabling faster charging and discharging. An unstable SEI, however, creates a bottleneck, hindering the battery’s rate capability.

• Minimized Irreversible Capacity Loss: All batteries inevitably experience some capacity loss over time, known as irreversible capacity loss. A stable SEI acts as a guardian, shielding the anode material from excessive side reactions. These reactions consume active materials within the anode, ultimately leading to a decrease in the battery’s overall capacity. An unstable SEI allows these side reactions to occur more readily, accelerating irreversible capacity loss and reducing the battery’s usable lifespan.

What Affects the SEI in Na+ Ion Batteries?

Several factors can influence the formation and stability of the SEI in sodium-ion batteries:

• Electrolyte Composition: The chemical makeup of the electrolyte plays a critical role. Electrolytes with specific additives can promote the formation of a stable and thin SEI layer. Conversely, incompatible electrolytes can lead to the formation of a thick and unstable SEI, hindering performance.

• Anode Material: The type of anode material used in the battery can also affect the SEI. Different anode materials can react differently with the electrolyte, influencing the composition and stability of the SEI layer.

• Operating Conditions: Factors like temperature and voltage during battery operation can also impact the SEI. Extreme temperatures can accelerate unwanted reactions at the electrode/electrolyte interface, leading to an unstable SEI.

The Role of Electrolyte Additives:

Traditional electrolytes in sodium-ion batteries often lead to the formation of thick, unstable SEI layers. These layers hinder sodium ion movement, reducing capacity and accelerating cell degradation. Here’s where electrolyte additives come in:

• Tailoring the SEI: Specific additives can be incorporated into the electrolyte to promote the formation of a thin, stable SEI layer. These additives can react with the electrolyte or the anode surface, influencing the composition and morphology of the SEI film.

• Examples of Additives: Fluoroethylene carbonate (FEC) is a common additive that promotes the formation of a more stable SEI. Other additives like vinyl carbonate (VC) or nitriles can further enhance specific properties of the SEI, such as improving its mechanical strength or reducing its electronic resistance.

The SEI and its Impact on Cell Performance:

The composition of the SEI layer directly affects the performance and lifespan of a sodium-ion battery in an EV:

• Cell Resistance: An unstable, thick SEI can act as a barrier, hindering the movement of sodium ions. This increases the cell’s internal resistance, leading to a voltage drop during charging and discharging. This translates to reduced power output and efficiency for the EV.

• Cell Lifetime: An unstable SEI can continuously react with the electrolyte, leading to its breakdown and the formation of unwanted byproducts. These byproducts can accumulate on the electrode surfaces, further increasing resistance and accelerating cell degradation. This ultimately shortens the battery’s lifespan, a critical factor for EVs requiring long-range and reliable performance.

Optimizing the SEI for Better EV Batteries:

By carefully selecting and combining electrolyte additives, researchers aim to achieve an SEI with the following characteristics:

• Thin and Stable: A thin layer minimizes the barrier effect and allows for efficient sodium ion movement, reducing cell resistance and improving power output. Stability ensures the SEI remains intact over time, preventing further electrolyte breakdown and cell degradation.

• Good Ionic Conductivity: The SEI should allow for the easy passage of sodium ions while acting as a barrier to other unwanted components in the electrolyte. This ensures efficient charging and discharging cycles.

• Mechanically Strong: A robust SEI can prevent the formation of dendrites, needle-like structures that can grow on the anode and pierce the separator, leading to safety hazards.

Consequences of an Unstable SEI

The importance of a stable SEI cannot be overstated. An unstable one can have detrimental consequences for sodium-ion batteries:

• Reduced Capacity: As mentioned earlier, an unstable SEI can hinder the movement of sodium ions, leading to a decrease in the battery’s overall capacity. This translates to a shorter runtime and reduced energy storage potential.

• Fast Degradation: An unstable SEI can continuously react with the electrolyte, causing it to break down and form unwanted byproducts. This accelerates cell degradation, significantly shortening the battery’s lifespan.

• Safety Concerns: An unstable SEI increases the risk of dendrite formation, posing a safety hazard. This is especially crucial for large-scale applications like grid storage, where safety is paramount.

SEI holds the key to durability and safety in Sodium-Ion batteries, as well as enhancing EV performance. Uneverse is dedicated to researching and providing technological support to commercialise and bring these batteries to market. Our team strives to make Sodium-Ion cells readily available, aiming to revolutionise the energy landscape. Are you ready to welcome the future with us?