Enabling Long-term Cycling Stability of Na3V2(PO4)3/C vs. Hard Carbon Full-cells

Abstract

Sodium-ion batteries are becoming an increasingly important complement to lithium-ion batteries. However, while extensive knowledge on the preparation of Li-ion batteries with excellent cycling behavior exists, studies on applicable long-lasting sodium-ion batteries are still limited. Therefore, this study focuses on the cycling stability of batteries composed of Na3V2(PO4)3/C based cathodes and hard carbon anodes. It is shown that full cells with a decent stability are obtained for ethylene carbonate / propylene carbonate electrolyte and the conducting salt NaPF6. With cathode loadings of 1.2 mAh/cm², after cell formation discharge capacities up to 92.6 mAh/g are obtained, and capacity retentions > 90 % over 1000 charge / discharge cycles at 0.5 C / 0.5 C are observed. It is shown that both, the additive fluoroethylene carbonate and traces of water in the cell, negatively affect the overall discharge capacity and cycling stability and should therefore be avoided. Remarkably, the internal resistances of well-balanced and wellbuilt cells did not increase over 1500 cycles and 5 months of testing, which is a very promising result regarding the possible lifespan of the cells. The initial loss of active sodium in hard carbon remains a major problem, which can only be partially reduced by proper balancing.

Influences on Reliable Capacity Measurements of Hard Carbon in Highly Loaded Electrodes

“…The accurate knowledge of the capacity of HC electrodes allows for proper balancing of corresponding cathodes, as show in an additional work on Na 3 V 2 (PO 4 ) 3 /C vs. HC full cells with a decent cycling stability and cathode loadings of 1.2 mAh/cm 2 . [11] Cathodes with specific capacity of ~2 mAh/cm 2 have likewise been developed and are currently being tested in combination with the HC electrode investigated herein.…”

“…These experiments allow, for instance, to determine practical areal capacities and thus results are used to develop suitable electrode balancing (capacity ratio of anode to cathode or N/P) for full-cell systems. [7,10,11] A general challenge of half-cell measurements inserting elemental sodium is its reactivity not only towards oxygen and moisture, but also with different organic solvents used for electrolytes. If any component of the electrolyte is reacting with the sodium counter electrode over time, its surface is not stable during cycling and, thus, the recorded electrochemical behavior is influenced.…”

“…Our presented measuring cell setup has direct and high impact on the development of NIB full-cell systems, as proper capacity readings of cathode and anode are mandatory to set up ideal cell balancing, as shown in a further study. [11] Too high N/P ratio will result in a loss of energy density because the excess hard carbon material is not utilized. Too low N/P ratio will result in sodium plating on the hard carbon due to the excess sodium ions that are released from the cathode during charging and cannot be incorporated by the hard carbon.…”