Washington State University (WSU) and Pacific Northwest National Laboratory (PNNL) researchers, with colleagues at Beijing University of Technology and Brookhaven National Laboratory, have created a sodium-ion battery that holds as much energy and works as well as some commercial lithium-ion battery chemistries, making for a potentially viable battery technology out of abundant and cheap materials. A paper on their work is published in the journal, ACS Energy Letters.

Although O3-layered metal oxides are promising cathode materials for high-energy Na-ion batteries, they suffer from fast capacity fade. The WSU-PNNL team developed a high-performance O3-NaNi_0.68Mn_0.22Co_0.10O₂ cathode by suppressing the formation of a rock salt layer at the cathode surface with an advanced electrolyte. The cathode can deliver a high specific capacity of ∼196 mAh g^–1 and demonstrates >80% capacity retention over 1,000 cycles.

NaNi_0.68Mn_0.22Co_0.10O₂–hard carbon full-cells with practical loading (>2.5 mAh cm^–2) and lean electrolyte (∼40 μL) demonstrated ∼82% capacity retention after 450 cycles. A 60 mAh single-layer pouch cell was also fabricated and demonstrated stable performance.

Sodium-ion batteries (SIBs), with the intrinsic advantages of resource abundance and geographic uniformity, are desired alternative battery technology to Li-ion batteries (LIBs) for grid-scale energy storage and transportation applications. To make SIBs comparable to LIBs in electrochemical performance, it is essential to develop advanced cathode materials of high capacity and long cycle life. Among the recent efforts made in this regard, the O3-type layered transition-metal oxides (O3-NaTMOs) with high specific capacity, tunable transition metal composition/secondary particle structure, and high tap density are expected as promising candidate cathodes towards practical applications. However, the nickel rich O3-NaTMOs suffer from irreversible phase transition at high voltage and limited cycle life, similar to their Li analogues, if not even worse.

… Interface stability, particularly the structural and chemical stability, has been known to be essential for battery performance. In the development of LIBs, electrolyte optimization and electrode surface engineering are classic protocols for controlling the formation of solid electrolyte interphase (SEI), cathode electrolyte interphase (CEI) and electrode surface structure/chemistry. However, these strategies have not been systematically studied on SIB cathodes.

Here, we demonstrate a long life O3-type layered NaNi_0.68Mn_0.22Co_0.10O₂ cathode for practical SIBs by controlling the surface phase transition and interphase stability. The O3-NaNMC682210 cathode, by suppressing the propagation of surface rock salt layer with localized high-concentration electrolyte,27 delivers a specific capacity of ~196 mAh g^-1, a specific energy of ~612 Wh kg^-1 (based on the cathode), and >83% capacity retention over 100 cycles between 2-4.2V (vs. Na/Na⁺). The cathode has ~80% capacity retention over 1000 cycles with a specific capacity of ~151 mAh g^-1 between 2-4V.

—Song et al.

As part of the work, the research team created a layered metal oxide cathode and a liquid electrolyte that included extra sodium ions, creating a saltier soup that had a better interaction with their cathode. Their cathode design and electrolyte system allowed for continued movement of sodium ions, preventing inactive surface crystal build-up and allowing for unimpeded electricity generation.

Our research revealed the essential correlation between cathode structure evolution and surface interaction with the electrolyte. These are the best results ever reported for a sodium-ion battery with a layered cathode, showing that this is a viable technology that can be comparable to lithium-ion batteries.

—Yuehe Lin, professor in WSU’s School of Mechanical and Materials Engineering, co-corresponding author

The researchers are now working to better understand the important interaction between their electrolyte and the cathode, so they can work with different materials for improved battery design. They also want to design a battery that doesn’t use cobalt, another relatively expensive and rare metal.

This work paves the way toward practical sodium-ion batteries, and the fundamental insights we gained about the cathode-electrolyte interaction shed light on how we might develop future cobalt-free or low cobalt cathode materials in sodium-ion batteries as well as in other types of battery chemistries. If we can find viable alternatives to both lithium and cobalt, the sodium-ion battery could truly be competitive with lithium-ion batteries.

—Junhua Song, first author

Resources

• Junhua Song, Kuan Wang, Jianming Zheng, Mark H. Engelhard, Biwei Xiao, Enyuan Hu, Zihua Zhu, Chongmin Wang, Manling Sui, Yuehe Lin, David Reed, Vincent L. Sprenkle, Pengfei Yan, and Xiaolin Li (2020) “Controlling Surface Phase Transition and Chemical Reactivity of O3-Layered Metal Oxide Cathodes for High-Performance Na-Ion Batteries” ACS Energy Letters 0, 5 doi: 10.1021/acsenergylett.0c00700