Na2CoPO4f As High Voltage Positive Materials for Sodium Ion Batteries

Although Li-ion batteries became the most popular power sources for personal digital assistants, rare-metal free or low-cost materials derived from more abundant resources have become increasingly desirable. Especially, studies of positive materials for sodium-ion batteries (NIBs) have been intensely discussed for past 5 years, some of reported candidates such as layered oxides [1,2] delivered promising outcome for productization of NIBs. However, their overall electrochemical performance is still inferior to the Li system and developing new materials becomes challenging due to its fundamental nature such as lower working voltage, heavier atomic weight, and lager ionic radius, leading to low energy density and poor cycle performance. Despite of these demerits, some phosphate materials [3,4] have demonstrated good electrochemical performance with medium discharge capacity of around 100 mAh/g and high redox voltage close to 4.0 V. We, therefore focused on phosphate materials, Na₂CoPO₄F, to explore the potential of high voltage positive active materials to achieve higher energy density for the NIBs. Na₂CoPO₄F was first reported in 2010 in Li system [5] and its performance as active materials was not so attractive because the potential window in the Li system is not satisfactory for operation of this material. In 2014, our group reported its electrochemical performance in the Na system where Na₂CoPO₄F exhibited high working voltage of 4.3 V with relatively large discharge capacity, and however it suffered from impurity and serious performance degradation during few cycles [6]. In this study, we developed synthesis route for single phase of Na₂CoPO₄F while adding citric acid and VGCF to increase conductivity and we studied its electrochemical properties. 

Na₂CoPO₄F products were obtained by a two-step solid synthesis method. Stoichiometric amounts of Co(NO₃)₂-H₂O, NaNO₃ and (NH₄)₂HPO₄ are mixed with critic acid in distilled water to obtain sol-gel precursor mixtures. 20 w/w% of VGCF was added to the precursor and the mixture was heated at 650 ^oC for 10 hours. And then, stoichiometric amount of NaF was mixed to the obtained material, and the mixture was heated again 600 ^oC for 3 hours under Ar flow. The electrode for charge and discharge measurement was made of Na₂CoPO₄F /VGCF composite, acetylene black and polytetrafluoroethylene binder in a weight ratio of 90:5:5 and R2032-type coin cells were assembled with 1 M NaPF₆ / EC:DEC (= 1:1) 2 v/v% FEC as electrolyte and sodium metal as a negative material. In addition, ex-situ XRD measurements for the electrodes at various charge and discharge states on the 1st cycle were carried out in order to examine the structural change accompanying Na⁺extraction/insertion. The ex-situ XRD measurement was done by using an Ar-filled hermetic sample holder.  

The obtained powder product was identified to be a single phase of Na₂CoPO₄F with a space group, Pbcn, by XRD. Na₂CoPO₄F showed the first discharge capacity of ca. 112 mAh/g, with two discharge redox voltages at 4.39 V and 4.46 V, and its average working voltage was 4.3 V with good capacity maintenance in Fig. 1. As shown in Fig. 2, the ex-situ XRD data revealed that diffraction profiles of Na₂CoPO₄F become broad after charge up to 4.8 V and returned to the original profile located at the original 2 theta position after one cycle, suggesting its reversibility of phase transition during cycles. Although the electrochemical profile indicated the two-phase reaction, obvious second phase in the ex-situ data was not observed. However the ex-situ XRD profile of end-members after 4.8 V charge was quite similar to that previously reported in Na₂FePO₄F[6]. For investigating more detailed charge and discharge mechanize of Na₂FePO₄F, we performed in-situ XRD and further measurements. The result will be discussed in the conference.