Anode for Na-ion batteries

An extremely cheap anode made from red phosphorus and carbon xerogel.

This project aims at anode material for sodium-ion batteries with extremely low cost. We report a method to condense red P on carbon xerogel to synthesize anode with low cost, high capacity, and good capacity retention. Even with large particle size (~ 50 μm) and high mass loading (2 mg cm-2), the composite cycled at 100 mA g-1 yielded a capacity of 357 mA g-1 (mass calculated from composite), or 2498 mAh gP-1 based on phosphorus mass after subtracting the contribution of carbon. The average coulombic efficiency is as high as 99.4%. When cycled at 200 mA g-1, it yielded a capacity of 242 mA g-1 or 1723 mAh gP-1, with average degradation rate only 0.06% in 80 cycles.

Background

Sodium ion batteries (SIBs) have been attracting much attention due to abundant resources of precursors and the potential low cost. Phosphorus has a great potential to serve as the anode of sodium ion batteries owing to its high theoretical capacity (2596 mAh g-1). However, the large radius of sodium ion makes it difficult to achieve and maintain high capacity after cycling. Various methods have been attempted to increase the capacity of phosphorous anode. Almost all of succesfully ones are composed of very expensive materials such as black phosphorus or graphene, losing the low-cost advantage of sodium-ion batteries. We want to come up with a synthesis procedure with extremely low cost and comparable performance with state-of-the-art methods.

Methods

We mixed resorcinol with formaldehyde to form resorcinol-formaldehyde gel. After carbonizing the polymer at 1000°C, we obtained carbon xerogel (CX). Carbon xerogel was sealed with red P in vacuum and heated up to 900°C to condense phosphorus vapor on the carbon skeleton to produce P/carbon xerogel composite (P@CX).

Intermediate and synthesized materials. Top row: workflow showing the synthesis of resorcinol-formaldehyde (RF) gel to carbon xerogel (CX) and further to phosphorus/carbon xerogel composite (P@CX). Middle row: illustration of the microstructures. Bottom row: images of the materials.

Results

TGA. We use thermogravimetric analysis (TGA) to measure the mass ratio of phosphorus in the P@CX composite. Assuming that all phosphorus would evaporate when the temperature reached around 400 °C, we heated the samples till 500 °C which was sufficient to cover the range of interest. From a below, we determined mass ratio of P to be 1-87.96%/(1-1.34%)=10.85%.

a, TGA results for carbon xerogel with and without phosphorus. The arrows show that 1.34% of mass is evaporated in CX, while 12.04% of mass is evaporated in P@CX. Here the stationary points are used to calculate the mass loss. b, SEM images of P@CX electrode (containing added carbon black and binder) at low magnification. c, The P@CX composite at high magnification.

SEM. SEM images in b and c above show a porous microstructure. We can observe that the particle sizes are fairly large: some particles are about 50 μm in diameter.

Capacity. As shown in the figure below, the reversible capacity is around 357 mAh g-1 at 100 mA g-1 and 242 mAh g-1 at 200 mA g-1, with coulombic efficiency as high as 99.4% and 99.2% respectively. Subtracting the contribution of carbon, the capacity of P was estimated to be 2498 mAh gP-1 and 1723 mAh gP-1 at current densities of 0.92 A gP-1 and 1.84 A gP-1, respectively. From the results we can conclude that carbon xerogel could help phosphorus to achieve high capacity.

Cycling performance of P@CX cells. Current density and capacity are calculated based on the total mass of the composite. a, Voltage vs. capacity for the first two cycles between 0.01~2 V. b, Reversible capacity of P@CX cycled at different current densities (between 0.01~1.5 V). c, Capacity and coulombic efficiency over 80 cycles, cycled at 100 mA g-1 between 0.01~1.5 V. Activation cycles are not included. d, Capacity and coulombic efficiency over 300 cycles, cycled at 200 mA g-1 between 0.01~1.5 V. Activation cycles are not included.

Video

Presentation recording in 239th ECS meeting. It is recorded before getting all the results; some figures in this page are not in the video.

  1. A Facile Process to Fabricate Phosphorus/Carbon Xerogel Composite as Anode for Sodium Ion Batteries
    Changyu Deng, and Wei Lu
    Journal of The Electrochemical Society 2021
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