The EV, eVTOLs, and drones industries are seeking Li-ion batteries capable of faster charge and higher power output. Achieving these capabilities requires rapid movement of lithium ions and electrons within the cell. However, when cells are pushed to fast rates, limitations occur that can limit the power and lifetime of these cells. Historically, these constraints have been attributed to material-related factors, including ionic and electronic transport limitations and degradation of the solid electrolyte interphase (SEI). However, new insights suggest that rate capability is also affected by electrolyte movement, specifically through the process now recognized as Electrolyte Motion induced Salt Inhomogeneity (EMSI).
A Closer Look at EMSI
EMSI was recently defined in a 2024 study published in Energy & Environmental Science. It describes a salt concentration imbalance that emerges from electrolyte movement during charge-discharge cycles. When a battery is charged, the anode experiences volume change (~13% for graphite and silicon up to 300% – 400%). This expansion leads to the expulsion of low salt concentration electrolyte from the anode to the extra space at the ends of the cell can and the center void. During discharge, as the anode contracts, the low-salt concentration electrolyte is absorbed. Over multiple cycles, this phenomena builds on itself and creates localized regions of low salt concentration that cause unwanted lithium plating. This phenomena intensifies with fast charge/discharge and with vertical (positive or negative terminal facing upward) cell orientation.
This process does not even require extreme operating conditions to manifest. Research published in the Journal of The Electrochemical Society has shown that EMSI can occur even at relatively modest charge rates, such as C/4, after approximately 100 – 400 cycles. This has been observed in commercial 18650 cells, which are widely used in both consumer and industrial applications. The problem becomes even more pronounced as the cells get larger, such as 46XX can cells, and large format pouch and prismatic cells, which are increasingly favored in EV applications. The rigid exteriors and large electrode size make these formats more susceptible to EMSI, especially when they incorporate materials like silicon, which exhibit significantly more volume change during cycling.
Strategies and Challenges for Mitigating EMSI
Efforts to mitigate EMSI have largely focused on material and electrolyte design. One approach is to minimize volume changes by using solely graphite for the anode, however this limits the energy density of the cell. Another approach is to enhance the porosity of the electrode. Higher porosity can improve lithium-ion transport, but this can also limit a cell’s energy density. Additionally, the amount and composition of the electrolyte itself plays a crucial role. Reducing the overall amount of electrolyte in the cell can limit the extent of imbalance, and using high salt concentration electrolytes can increase the lower limit of salt concentration, however lower electrolyte amount can increase ionic resistivity and high salt concentration electrolyte is expensive.
The Overlooked Role of the Current Collector
An overlooked cell component that can affect EMSI is the current collector. In most commercial cells, the current collector is a solid sheet of metal, typically copper on the anode side and aluminum on the cathode side. These metal foils serve as hosts to the active materials and electron conductors, but they are impermeable to the electrolyte. Consequently, large-scale ionic transport in the cell is limited to a two-dimensional path.
This geometry contributes to formation of salt concentration gradients and enhances the effects of EMSI. What if the current collector could allow salt ions to pass through? A current collector designed with through-pores for electrolyte transport could enable ion flow along a third axis. This would facilitate a more uniform salt distribution and reduce the effective distance lithium ions need to travel, potentially mitigating EMSI formation. After fast cycling, diffusion length for the salt concentration to equilibrate can be on the order of centimeters with a relaxation time of weeks. With Addionics 3D Current Collectors, the diffusion length is reduced from centimeters to millimeters or less, significantly shortening the diffusion time during relaxation >50x (from weeks to hours or less).
Such an approach would mark a fundamental shift in current collector design, from a purely electronic component to an active participant in ion transport management. Addionics 3D Current Collectors are a promising solution to EMSI. Through third party testing we have observed improved rate capability and cell life time, both of which point towards a mitigation of EMSI effects. The figure below shows the proposed mechanistic differences in electrolyte motion between standard current collectors and Addionics current collectors.
Overcoming EMSI with Addionics
As the lithium-ion battery industry continues its push toward higher rate cells, it must address EMSI. EMSI represents a newly recognized, yet critical barrier to high-rate performance, especially in large-format and silicon-rich cells. Material and electrolyte design offer partial solutions, but a different approach, one that considers the role of current collectors, may be necessary to truly unlock next-generation capabilities.
Addionics has reimagined the current collector as a medium to transport both electrons and ions that could open new avenues for mitigating EMSI, and ultimately enabling faster charging. Continued exploration in this direction may prove essential in meeting the accelerating demands of the energy storage market.
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