Kinetically controlled formation of uniform FePO4 shells and their potential for use in high-performance sodium ion batteries

Abstract

Amorphous iron phosphates are potential cathode materials for sodium ion batteries. The amorphous FePO 4 matrix is able to insert/extract sodium ions reversibly without apparent structural degradation, resulting in stable performance during the charge/discharge process. However, the extremely low electronic conductivity of FePO 4 itself becomes a formidable obstacle for its application as a high-performance cathode material. Here, by tuning the growth kinetics of FePO 4 in an aqueous solution, we were able to control its formation onto a large variety of substrates, forming uniform core-shell structures. Specifically, the use of multiwalled carbon nanotubes as the core material together with the growth control of FePO 4 produced the core-shell structure of MWCNTs@FePO 4 with a delicately controlled shell thicknesses. We confirmed that such a nanocomposite can act as an effective cathode material by taking advantage of both the highly conductive core and the electrochemically active shell, leading to improved battery performance as revealed by the high discharge capacity and the greatly improved rate capability. We anticipate that our progress in FePO 4 control offers new potential in different research fields, such as materials chemistry, catalysis and energy storage devices.

Approaching the Downsizing Limit of Maricite NaFePO4 toward High‐Performance Cathode for Sodium‐Ion Batteries

“…During the first charge process, low capacities of 21.7 and 74.9 mA h g −1 are obtained when charging to 4.0 and 4.5 V, respectively, in consistent with the common belief that maricite NaFePO 4 possesses poor electrochemical activity due to the lack of free channels for Na + ions' diffusion. While, after charging to 4.7 V and holding at this potential for 12 h, a surprisingly high capacity of 176.3 mA h g −1 is achieved, more excitingly, a reversible capacity of 147.2 mA h g −1 (95.6% of the theoretical capacity) is delivered when discharging to 1.5 V. During subsequent cycles in the potential window of 1.5–4.5 V versus Na + /Na, a satisfactory reversible capacity of ≈145 mA h g −1 can be obtained, and the voltage profiles with smooth slopes are similar to the well‐known behavior of the amorphous FePO 4 in SIBs . The capacity fading (especially in the high‐voltage region) during the initial few cycles is mainly caused by the decomposition of electrolyte and the formation of a solid–electrolyte interface (SEI) film, then, either the charge or discharge curves are almost overlapped up to 300 cycles, implying the good reversibility of NaFePO 4 @C electrode.…”

“…Notably, the initial reversible capacity can attain 124.5 mA h g −1 with the operating voltage of ≈2.25 V; this corresponds to a promising energy density of 168.1 Wh kg −1 based on the total mass of the cathode and anode active materials. The charge/discharge hysteresis is mainly caused by the inherently low electrical conductivity and the amorphous structure of the NaFePO 4 cathode; future improvements such as ion doping to modify the intrinsic conductivity and structure of NaFePO 4 may relieve the hysteresis. Moreover, the reversible capacity still remains at 108.1 mA h g −1 after 200 cycles, affording a high capacity retention of 87% with a high CE of over 99% (Figure b).…”

Precise Surface Engineering of Cathode Materials for Improved Stability of Lithium‐Ion Batteries

“…Therefore, it becomes a continuous effort for researchers to identify suitable precipitating agents which are capable of precipitating metal ions in a step‐by‐step style. In this regard, amino group‐containing species such as hexamethylenetetramine and urea become favorable choice due to their unique capability to slowly release ammonia upon decomposition at an elevated temperature which thereafter change the concentration of OH − to induce a gradual precipitation of metal ions, forming either metal oxides or metal phosphates depending on the different reactants involved in the reaction.…”

“…It is noted that the precipitation process in solution is complicated and different metal ions can exhibit different styles depending on their chemical properties. For example, although the formation of either AlPO 4 or FePO 4 could be easily tuned with the assistance of urea, other coating materials such as Ni 3 (PO 4 ) 2 , Co 3 (PO 4 ) 2 , and Mn 3 (PO 4 ) 2 , however, turned out to be more complicated and be much harder to be controlled toward a heterogeneous growth mode. As schemed in Figure a, a similar growth operation by only using urea for growth control can only produce separated particles of Ni 3 (PO 4 ) 2 (route 3) and the precipitation is also observed to be very fast.…”

A Yolk–Shell‐Structured FePO4 Cathode for High‐Rate and Long‐Cycling Sodium‐Ion Batteries

“…[3][4][5] Benefiting from the high theoretical capacity and excellent redox reversibility, amorphous FePO 4 has been reported as a feasible cathode material for sodium storage. [6,7] However, it suffers from low electronic conductivity and remarkable volumetric change of over 22 % during Na + insertion/extraction processes, [8,9] leading to poor rate capability and unstable cyclic performance. Proper designs in nanoarchitectures and carbon decoration are two valid strategies to resolve the above problems thus to improve the electrochemical properties.…”