New electrolyte additive boosts lithium-ion battery lifespan to 95.6% capacity retention

A new electrolyte additive has enabled graphite anodes to retain up to 95.6% of their capacity after 1,000 charge cycles, demonstrating the potential to extend lithium-ion battery life without altering the graphite itself. Researchers at the Japan Advanced Institute of Science and Technology (JAIST) have designed a compound called pentafluorophenyl thiophene imine (FPTI) to improve the protective layer that forms on graphite during the initial few charge cycles. This layer, known as the solid electrolyte interphase (SEI), is critical for battery longevity. It allows lithium ions to move between the electrolyte and the electrode while limiting unwanted chemical reactions. When the SEI becomes unstable, it can consume active lithium and increase resistance, accelerating capacity loss. The researchers added FPTI to conventional lithium-ion battery electrolytes at concentrations of 2 milligrams and 4 milligrams per milliliter. They found that the additive preferentially reacts during early cycles and helps create a more conductive and stable SEI on the graphite surface. At the higher concentration, the SEI resistance dropped from 7.6 ohms in the control group to 2.2 ohms. The charge transfer resistance also decreased from 41.8 ohms to 19.8 ohms, while lithium-ion diffusion increased. The researchers found that different parts of the FPTI molecule contributed to the formation of the protective layer. Compounds from the sulfur and imine were incorporated into the interface, while its fluorinated component helped form a lithium fluoride-rich layer. Professor Matsui explained, "These components working together seem to reduce parasitic reactions and support smooth lithium-ion movement across the electrode-electrolyte interface."

Impact of FPTI on Battery Performance

The benefits became more pronounced during extended cycling. The graphite cell with 2 milligrams per milliliter of FPTI retained 89.4% of its maximum capacity after 1,000 cycles, while the one with 4 milligrams per milliliter retained 95.6%. The control group without the additive retained only 62.7% of its capacity and began showing significant capacity loss after about 350 cycles. The research team then tested whether this approach could be applied to a full cell using an NMC811 cathode and a graphite anode. This revealed a key limitation: adding FPTI directly to the full-cell electrolyte increased the cathode's resistance, impairing performance. As a result, the researchers applied FPTI only to pre-treat the graphite anode. They formed the improved SEI during pre-cycling and then assembled the full cell using a standard electrolyte. This targeted chemical treatment significantly enhanced the full cell's energy density. The full cell with graphite pre-treated with 4 milligrams per milliliter of FPTI achieved about 233 Wh/kg, while the 2 milligrams per milliliter treatment resulted in about 192 Wh/kg, compared to the control group's approximately 130 Wh/kg.

Practicality of Targeted Treatment

This result highlights why battery additives cannot be evaluated solely based on their impact on a single electrode. A chemical that improves the graphite-electrolyte interface may cause issues at the cathode, making targeted treatment more practical than adding the compound to the entire battery. Professor Matsui concluded, "Overall, the introduction of FPTI significantly enhanced the electrochemical performance of the graphite half-cell." The researchers noted that further testing with larger commercial-format cells and a wider range of operating conditions is needed to determine if this strategy can translate into practical batteries. However, this study demonstrates how controlling the graphite surface at the molecular level can extend battery life without replacing widely used anode materials. The research has been published in the journal Energy & Fuels.

Item	Specification
SEI resistance	2.2 ohms
Charge transfer resistance	19.8 ohms
Energy density (4 mg/ml FPTI)	233 Wh/kg
Energy density (2 mg/ml FPTI)	192 Wh/kg
Energy density (control)	130 Wh/kg

How to Extend Battery Life Without Changing Materials

This study demonstrates that by improving the solid electrolyte interphase (SEI) of the graphite anode, it is possible to effectively extend the lifespan of lithium-ion batteries without replacing existing anode materials. The introduction of the FPTI additive not only improves the capacity retention of the battery but also enhances its electrochemical performance. This indicates that targeted chemical treatments can overcome some of the limitations of traditional battery designs, providing new avenues for the development of future lithium-ion batteries, particularly in the context of pursuing higher energy densities and longer lifespans.