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/Na_0.22MnO₂ and Na_0.44MnO₂/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/Na_0.22MnO₂ 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 Na_0.22MnO₂ 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 Na_0.22MnO₂ 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/Na_0.22MnO₂ system can enrich Li⁺ in electrolytes by charging, and enrich Na⁺ by discharging.

—Chen et al.

The Ningbo team built two batteries (LiMn₂O₄/Na_0.22MnO₂ and Na_0.44MnO₂/TiP₂O₇). The theoretical capacities of Na_0.44MnO₂, LiMn₂O₄ and TiP₂O₇ are 60 mAh g^-1, 148 mAh g^-1 and 121 mAh g^-1, respectively. The average working voltages of LiMn₂O₄/Na_0.22MnO₂ and Na_0.44MnO₂/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 Na_0.44MnO₂/TiP₂O₇ could thus be superior to CuHCF/AC-PPy and Na_0.44MnO₂/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.

Resources

• 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