Harnessing the third sodium of the Na3V2(PO4)2F3 (or NVPF) positive electrode to increase the energy density of sodium-ion batteries could be the key to make them competitive towards lithium-ion batteries. A RS2E team (Collège de France/LRCS) managed for the first time to trigger the activity of the third Na electrochemically, by oxidizing NVPF at high potential.
This way, full sodium-ion cells based on the NVPF phase as positive electrode and carbon as negative electrode (Na3V2(PO4)2F3/C ) show a 10-20% increase in the overall energy density. It also provides safety advantages such as the possibility to discharge and store the cells at 0 volt without damaging their performances.
Beyond lithium-ion batteries
Sodium-ion batteries are quickly seen as possible alternatives to lithium-ions batteries for stationary storage. Higher abundance of sodium could provide a cheaper solution for the scarcity of lithium that increases the cost of lithium ion battery production.
Among the studied electrode systems for sodium ion batteries, Na3V2(PO4)2F3/C caught the attention for its cycling stability and power rate capabilities over the Na-based layered oxide NaxMO2/C. Unfortunately, the specific energy of NVPF is lower (500 Wh.kg-1) than the well-established lithium ion battery electrode LiCoO2 (600 Wh.kg-1). The present study partially fill the gap by increasing the specific energy of NVPF up to 563 Wh kg-1.
Figure 1 : The energy density of NVPF-2.0, NVPF-2.25, NVPF-2.5, NVPF-2.75 and NVPF-3.0 samples
(based on product of the discharge capacity and its voltage when the NVPF/Na cells were discharged to 1.0 V)
Disordered “NVPF” phase
By triggering the activity of the third Na, the researchers led by Prof. Jean-Marie Tarascon, Professor at college de France and director of RS2E network, hoped to increase the Na+ insertion and disinsertion capacity of the electrode material.
Their works show the removal of 3rd sodium from NVPF can be achieved by oxidizing NVPF up to 4.8 V (vs Na+/Na0). While oxidising to 4.8 V, they also observed the formation of a disordered “NVPF” phase of tetragonal symmetry which on subsequent cycles reversibly uptake 3 sodium ions. The deformation on first charging process is irreversible because the new phase remains disordered on cycling while its average vanadium oxidation state varies from 3 to 4.5.
The structure of the disordered phase and the vanadium oxidation state modifications were followed by various in situ and ex situ characterisations such as NMR, XRD and XAS.
The researchers demonstrated that the onset of a low insertion plateau can be used to design Na-ion cells whose performances will not be affected even while discharging the cell to 0 volt. This finding will lead to a safer transport and safer storage of Na-ion batteries.
Guochun Yan, Sathiya Mariyappan, Gwenaelle Rousse, Quentin Jacquet, Michael Deschamps, Renald David, Boris Mirvaux, John William Freeland & Jean-Marie Tarascon
Nature Communications: DOI : 10.1038/s41467-019-08359-y
Contact : Jean-Marie Tarascon, email@example.com
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