The rise of renewable solar and wind power is demanding sustainable storage technologies using components that are inexpensive, Earth-abundant and environmental friendly. There have been exciting advances in the performance of SIBs utilizing layered transition-metal oxides and polyanions, which suggest that the sodium compounds can be promising compared with their lithium analogues. The layered metal oxides offer high energy densities but are limited by cycling stability and air sensitivity, whereas polyanions show rather stable cycle life but only deliver a moderate energy density. Thus, further research is required to find better sodium host materials. In addition, material sustainability is also a critical factor when considering the total economic and environmental benefits for grid-scale energy storage applications.

Organic compounds that can be obtained from natural biomass with minimum energy consumption are an attractive low-cost and sustainable choice for battery electrode materials, provided a high energy density and long cycling stability can be obtained. … Later work turned to disodium rhodizonate (Na₂C₆O₆), and demonstrated excellent cycle stability in SIB. Although it seemed intuitive to expect a similar four-electron redox reaction in Na₂C₆O₆, previous studies have experienced challenges in repeatedly utilizing four electrons. Rather, there was a substantial capacity loss after the first cycle, and the following reversible capacity was much lower than its theoretical capacity of 501 mAh g⁻¹. Despite its crucial importance, the origin of this discrepancy remains unknown.

Here, we reveal the origin of the limited electrochemical performance of Na₂C₆O₆ and provide an effective path to achieve reversible four-sodium storage.

—Lee et al.

In the Stanford battery, a sodium ion binds to a compound known as myo-inositol—a household product found in baby formula and derived from rice bran or from a liquid byproduct of the process used to mill corn. Crucial to the idea of lowering the cost of battery materials, myo-inositol is an abundant organic compound familiar to industry.

The sodium salt makes up the cathode; the anode is made up of phosphorous. For this prototype, postdoctoral scholar Min Ah Lee and the Stanford team improved how sodium and myo-inositol enable the electron flow, significantly boosting the performance of this sodium-ion battery over previous attempts.

They found that the irreversible phase transformation of Na₂C₆O₆ during cycling is the origin of the deteriorating redox activity of Na₂C₆O₆. They identified active-particle size and electrolyte conditions as key factors to decrease the activation barrier of the phase transformation during desodiation.

The researchers focused mainly on the favorable cost-performance comparisons between their sodium-ion battery and lithium. In the future they’ll have to look at volumetric energy density—how big must a sodium ion battery be to store the same energy as a lithium ion system.

In addition, the team optimized their battery’s charge-recharge cycle. To better understand the atomic-level forces at play during this process, postdoctoral scholar Jihyun Hong and graduate student Kipil Lim worked with Chueh and Michael Toney, a scientist with the SLAC National Accelerator Laboratory. They studied precisely how the sodium ions attach and detach from the cathode, an insight that helped improve their overall battery design and performance.

The Stanford researchers believe their Nature Energy paper demonstrates that sodium-based batteries can be cost-effective alternatives to lithium-based batteries. Having already optimized the cathode and charging cycle, the researchers plan to focus next on tweaking the anode of their sodium-ion battery.

This is already a good design, but we are confident that it can be improved by further optimizing the phosphorus anode.

—Yi Cui

Other members of the team included Stanford researchers Jeffrey Lopez, Yongming Sun and Dawei Feng. The work was funded by the U.S. Department of Energy’s Advanced Battery Materials Research (BMR) Program. X-ray measurements were carried out at the Stanford Synchrotron Radiation Laboratory (SSRL), a national user facility operated by Stanford University on behalf of the U.S. Department of Energy, Office of Basic Energy Sciences.

Resources

• Minah Lee, Jihyun Hong, Jeffrey Lopez, Yongming Sun, Dawei Feng, Kipil Lim, William C. Chueh, Michael F. Toney, Yi Cui & Zhenan Bao (2017) “High-performance sodium–organic battery by realizing four-sodium storage in disodium rhodizonate” Nature Energy doi: 10.1038/s41560-017-0014-y