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Lithium-ion batteries have become a cornerstone of modern technology, powering everything from smartphones to electric vehicles (EVs). Yet they come with two major challenges. First, storing lots of energy in a small package can make the battery susceptible to thermal runaway, or the explosive release of energy. And second, actually charging the battery can be inconvenient and time-consuming. Charging faster also degrades the battery faster, creating a tradeoff – do we charge the battery faster and then replace it more often? Or do we charge it slowly, extending the battery’s life but waiting a longer time for it to charge?
Fortunately, there are ways to improve the fast charge cycle life of a battery using a chemically active separator. By absorbing water and neutralizing hydrofluoric acid (HF), this technology keeps the electrolyte healthy and allows the battery to function normally for a longer period of time.
Mechanisms of fast-charging cell degradation
Fast charging accelerates battery degradation through several mechanisms:
- Loss of lithium inventory (LLI): LLI occurs when lithium becomes inactive due to plating or the formation of solid electrolyte interphase (SEI) and cathode electrolyte interphase (CEI). Fast charging exacerbates these issues by increasing the rate of lithium plating and SEI growth.
- Loss of active material (LAM): Extreme volume changes during fast charging can crack the anode or cathode materials, leading to a loss of active material.
- Conductivity loss (CL): CL results from the degradation of conductive materials or disconnection of conductive particles, often caused by binder degradation or physical damage to the cell.
- Loss of electrolyte (LE): High voltages during fast charging can lead to electrolyte oxidation at the cathode and decomposition, further contributing to SEI growth and the loss of lithium and active materials.
These mechanisms reveal three stages of degradation. First comes growth of the SEI layer, followed by loss of active material through cracking and finally lithium plating on the anode.
Experimental results with cellulose-blend separators
Chemically active separators, such as the Dreamweaver (DWIG) separator, are designed to mitigate these degradation mechanisms. This separator consists of cellulose nanofibers and fibrillated para-aramid fibers, which are not present in conventional olefin separators.
During tests, cells with the DWIG separator demonstrated exceptional cycle life in both 25 Ah NMC523-graphite pouch cells and 3.2 Ah NMC811-graphite 18650 cylindrical cells. Testing at 1C-1C showed significantly reduced capacity fade compared to conventional separators. The pouch cells maintained similar capacity until roughly 2,000 cycles, after which they declined sharply while DWIG cells continued performing well. The 18650 cells were designed for C/2 charging, making the 1C-1C cycling more demanding than their design specifications. Results are shown in Figure 1.
Following these initial promising results, 3,000 18650 cells were manufactured with the DWIG separator. These were tested against commercial cells from major manufacturers using various charge-discharge protocols. The DWIG cells demonstrated remarkable performance, achieving more than 1,500 cycles at C/2-C/2 compared to a maximum of 600 cycles for commercial alternatives. At the more demanding 1C-1C cycling, DWIG cells surpassed 1,000 cycles while commercial cells reached only 300 cycles at best. Results are shown in Figures 2 and 3.
Additional high-rate charging tests revealed even more dramatic differences. At 1.5C-C/2, DWIG cells maintained more than 450 cycles, and at the aggressive 2C-C/2 rate, they still achieved more than 340 cycles (see Figure 4). Under these same conditions, commercial cells showed very rapid capacity degradation, confirming that high-rate charging is a primary cause of capacity fade in conventional cells – a limitation mitigated by the DWIG separator technology.
Potential mechanisms for improved performance
The DWIG separator likely extends cycle life through several interconnected mechanisms. For electrolyte protection, it absorbs water and neutralizes HF, interrupting the cycle of HF attacking cathode materials and creating more water, thus preventing cascading electrolyte degradation. The separator also provides anode protection by reducing binder decay in the absence of HF, minimizing anode cracking and delamination and maintaining anode capacity and conductivity.
In addition, the separator prevents lithium plating by reducing SEI growth rate (see Figure 5), maintaining pathways for lithium intercalation and preventing dendrite formation that could lead to internal shorts. The cellulose-based separator’s polar nature allows it to act as a template for deposition of electrolyte degradation products, potentially reducing SEI layer growth on the anode itself.
Yet while the DWIG separator has shown promise in extending the cycle life of lithium-ion batteries, a full understanding of the specific mechanisms requires further studies. Future research will focus on testing the rate capability of aged cells, measuring electrochemical impedance spectroscopy (EIS) and performing autopsies to study features such as wet electrolyte amount, anode cracking, and lithium plating.
Learn more
Chemically active separators, such as the DWIG separator, represent a significant advancement in extending the fast charge cycle life of lithium-ion batteries. By addressing key degradation mechanisms, these separators not only enhance the longevity and performance of batteries but also improve safety.
As research continues, the potential for widespread adoption of these technologies in various applications, from consumer electronics to electric vehicles, will become increasingly viable.
To learn more, contact Soteria Battery Innovation Group.
About the author: Brian Morin is CEO of Soteria Battery Innovation Group.
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