Ningbo researchers propose mixed-ion Li/Na batteries
12 June 2013
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.
|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