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Ningbo researchers propose mixed-ion Li/Na batteries

Schematics of Li+/Na+ mixed-ion battery. Lithium-intercalation compounds and sodium-intercalation compounds are used for anode and cathode, respectively. During charging (or discharging), the storage (or release) of Li+ takes place at anode, and the release (or storage) of Na+ occurs at cathode. Chen et al. Click to enlarge.

Researchers from the Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, are proposing a novel concept of Li+/Na+ mixed-ion electrolytes for rechargeable batteries for large-scale energy storage. A paper on their work is published in Scientific Reports, the Nature Publishing Group’s open access journal.

Sodium-ion based rechargeable batteries (SIBs, e.g., earlier post) are of interest due to sodium’s abundance, far lower prices, and a greener synthesis while maintaining a similarity in ion-insertion chemistry. However, a number of issues remain before SIBs could become commercially competitive with Li-ion batteries (LIBs). Among these, the Ningbo team notes, is that only a couple of materials have been reported for the deintercalation/intercalation of sodium ions in aqueous media.

Since sodium-intercalated materials suitable for aqueous media are limited, an innovative concept of Li+/Na+ mixed-ion electrolytes is employed to construct rechargeable batteries...In such batteries, one side involves the immigration of Li+ between electrolytes and electrode, and the other one refers to the exchange of Na+ between electrode and electrolytes.

During charging and discharging, the total concentration of Li+ and Na+ is fixed to ensure the charge neutrality of the electrolytes, but the Li+/Na+ ratio is changed. They are unlike traditional “rocking-chair” lithium-ion battery on a basis of the immigration of Li+ between cathode and anode.

Herein, two systems based on Li2SO/NaSO4 mixed electrolytes (LiMn2O4/Na0.22MnO2 and Na0.44MnO2/TiP2O7), which to our best knowledge have never been reported before, are demonstrated. The capacity, operating voltage, and stability of such batteries are dependent on the electrolytes. A LiMn2O4/Na0.22MnO2 system to separate Li+ and Na+ based on the unique mechanism of mixed-ion battery is also validated.

—Chen et al.

A key feature of the mixed-ion battery is the change of the Li+/Na+ ratio during charging and discharging. To experimentally demonstrate such changes, they used a micro electrochemical cell consisting of a Na0.22MnO2 anode and a LiMn2O4 cathode, using Na2SO4 and Li2SO4 solutions.

Before charging, pristine Na2SO4 solution with the Li+/Na+ratio of 1/83 was injected into the cell. After charging, the Li+/Na+ ratio rose to 1/10. This is due to the release of Li+ from a LiMn2O4 cathode, and selective capture of Na+ from electrolytes by a Na0.22MnO2 anode.

Li2SO4 then replaced Na2SO4 for the discharging process. The Li+/Na+ ratio in electrolytes dropped from 144/1 to 13/1 after discharging. The reason, the authors said, is that Li+ in electrolytes was intercalated into cathode, and Na+ is diffused from anode to electrolytes during discharging.

Similar changes are also observed for mixed-ion solutions. The Li+/Na+ ratio increases after charging, and decreases after discharging. Therefore, LiMn2O4/Na0.22MnO2 system can enrich Li+ in electrolytes by charging, and enrich Na+ by discharging.

—Chen et al.

The Ningbo team built two batteries (LiMn2O4/Na0.22MnO2 and Na0.44MnO2/TiP2O7). The theoretical capacities of Na0.44MnO2, LiMn2O4 and TiP2O7 are 60 mAh g-1, 148 mAh g-1 and 121 mAh g-1, respectively. The average working voltages of LiMn2O4/Na0.22MnO2 and Na0.44MnO2/TiP2O7 batteries are 0.6 V and 1.0 V, respectively.

If the theoretical capacities of the materials are used, batteries with specific energy of around 21 Wh kg-1 or 40 Wh kg-1 based on the total weight of active electrode materials could be achieved. The Na0.44MnO2/TiP2O7 could thus be superior to CuHCF/AC-PPy and Na0.44MnO2/active-carbon systems for large-scale energy storage explored elsewhere for large scale energy storage, the authors noted.

In their study, the Ningbo team reported obtaining two batteries with specific energy of 17 Wh kg-1 and 25 Wh kg-1. Achieving the theoretical capacities of these compounds and long cycle life of the systems “still remains a challenge”, they noted.

The Na0.44MnO2/TiP2O7 mixed-ion battery. (a) schematic of the battery; (b) charging-discharging curves at a rate of 0.4 C in 1 M Na2SO4 + 0.25 M Li2SO4; (c) charging-discharging curves at a rate of 0.4 C; (d) cycle life tests of the battery at a rate of 0.4 C in mixed electrolytes. (1C = 60 mAh g-1). Chen et al. Click to enlarge.
Schematic representation of the working principle on the separation of Li+ and Na+ by the mixed-ion battery. Chen et al. Click to enlarge.

Separation. They also found that the Li+/Na+ mixed-ion batteries not could deliver a new energy storage system, but also give a prospective technique for Li+/Na+ separation.

The system is based on the Li+-releasing cathode and Na+-capturing anode. During the first step (charging), the battery releases Li+ and captures Na+ to enrich Li+ in mixed-ion solution. After charging, solution I is replaced by another mixed-ion solution II. Then the battery is discharged to enrich Na+, as Li+ are captured by cathode and Na+ are released from anode.

In the last step, mixed-ion solution II is exchanged with mixed-ion solution I for the next cycle. Throughout repetitive cycles, Li+-enriched and Na1-enriched solutions are finally obtained respectively.

Compared with other purification methods for obtaining lithium from brine lakes and salt pans, the mixed-ion method has two advantages, the Ningbo team proposed.

  • First, it is dual-functional. The Li+/Na+ mixed-ion battery can be used for both purification of lithium and energy storage.

  • Second, this electrochemical method is green and energy efficient. Unlike wet chemical methods, it does not require any chemicals and generates zero waste for disposal.


  • Liang Chen, Qingwen Gu, Xufeng Zhou, Saixi Lee, Yonggao Xia & Zhaoping Liu (2013) New-concept Batteries Based on Aqueous Li+/Na+ Mixed-ion Electrolytes. Scientific Reports 3, Article number: 1946 doi: 10.1038/srep01946


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