Novel fiber separator boosts lithium metal battery performance, capacity rises by 42%

A novel cellulose separator incorporating bikitaite helps lithium metal batteries cope with rapid cycling while improving the performance of high-nickel NCM90 cathodes. Researchers from South Korea found that the separator can lift capacity at high discharge rates and effectively prevent dendrite formation on lithium metal anodes. Known as CBT, the separator leverages a porous cellulose structure combined with bikitaite, a zeolite mineral. Together they create pathways that allow lithium ions to move more uniformly through the cell. This is critical for lithium metal batteries, which offer higher energy density than conventional lithium-ion cells but still struggle to operate at high discharge rates.

Non-uniform lithium deposition can generate dendrites, while electrolyte degradation reduces performance and raises the risk of short circuits.

CBT separator performance gains

A team led by Professor Sunwoo Lee of Hannam National University found that the modified separator also addresses an issue on the cathode side, where high discharge rates can cause available capacity to plunge sharply. The CBT separator achieves an ionic conductivity of 3.45 × 10⁻³ S cm⁻¹ and a lithium-ion transference number of 0.742. The researchers linked these properties to faster and more uniform ion transport, reducing polarization under demanding charge/discharge conditions. At a 1C discharge rate, cells using the CBT separator and a conventional polyethylene separator both delivered about 197 mAh g⁻¹.

At higher discharge rates, however, the gap widened. At 2C, the CBT separator delivered 187 mAh g⁻¹, compared with 165 mAh g⁻¹ for the conventional separator. At 4C, NCM90 with CBT reached 163 mAh g⁻¹, while the conventional separator produced only 115 mAh g⁻¹. This represents an improvement of roughly 42%, showing that the separator can affect not only anode stability but also cathode performance.

Potential of the modified separator

Professor Lee said: "Our results show that improvements in battery performance depend not only on new cathode and anode materials but can also be achieved through separator engineering. Particularly interesting is that the effect of the modified separator extends to the lithium metal anode, significantly improving the high-rate performance of the NCM90 cathode." The separator also helps control lithium deposition. Real-time observations showed no visible dendrite growth in cells using CBT. Instead, lithium formed a smoother, denser deposit and was stripped more uniformly. The cells retained about 60% of their capacity after 2,500 cycles at 2C/4C.

After 150 cycles at -25 °C (-13°F), the separator still retained about 68.9% of capacity and remained stable at 200 °C (392°F).

The approach avoids redesigning electrodes and could potentially be adapted to existing battery production lines. However, it still needs to be tested in commercial pouch and cylindrical cells. Professor Lee stressed: "Functional separators should not be viewed merely as barriers separating two electrodes, nor used solely to protect the lithium metal anode. By controlling ion transport within the cell, they can become important design elements for realizing next-generation batteries that combine high energy density with high-rate performance." The research has been published in the journal Advanced Functional Materials.