A Facile Process to Make Phosphorus-doped Carbon Xerogel as Anode for Sodium Ion Batteries
11 A Facile Process to Make Phosphorus-doped Carbon Xerogel as Anode for Sodium Ion Batteries
Changyu Deng and Wei Lu Department of Mechanical Engineering, University of Michigan, Ann Arbor, MI 48109, United States Department of Materials Science & Engineering, University of Michigan, Ann Arbor, MI 48109, United States *e-mail: [email protected]
Abstract
Sodium ion batteries become popular due to their low cost. Among possible anode materials of sodium ion batteries, phosphorus has great potential owing to its high theoretical capacity. Previous research that yielded high capacity of phosphorus anode used very expensive materials such as black phosphorus (BP) and phosphorene. To take advantage of the low cost of sodium ion batteries, we proposed a new method to make anode: condensing red phosphorus (RP) on carbon xerogel.
Even with large particle size (~ 50 μm) and high mass loading (2 mg/cm ), the composite cycled at 200 mA/g yielded a capacity of 242 mA/g, or 1993 mAh/g based on phosphorus after subtracting the contribution of carbon. The average degradation rate is only 0.06% in 80 cycles. The average columbic efficiency is as high as 99.2%. Our research provided an innovative approach to synthesis of anodes for sodium ion batteries, which could promote their commercialization.
1. Introduction
Sodium ion batteries (SIBs) have been attracting much attention due to abundant resources of precursors and thus low cost. Phosphorus has great potential to serve as the anode of sodium ion batteries owing to its high theoretical capacity (2596 mAh/g). However, large radius of sodium ions makes it difficult to achieve and maintain high capacity after cycling. Numerous methods have been attempted to increase the capacity of phosphorous anode. [1]
For instance, Sun et al. showed a specific capacity of 2440 mAh/g at current density of 50 mA/g by mixing graphene and phsphorene. [2]
Gao et al. integrated red phosphorus with graphene aerogel to yield a capacity of 1867 mAh/g at current density of 260 mA/g. [3]
Li et al. mixed red P with carbon nanotubes (CNTs) to deliver a reversible capacity of 1675 mAh/g at currenty density of 143 mA/g. [4]
Besides red phosphorus, black phosphorus with higher conductivity is used as anode materials as well and deliver a specific capacity up to 1500-2000 mAh/g. [5,6]
Among these recent discoveries and achievements, anode with high capacity is composed of extremely expensive materials, such as graphene, black phosphorus and phosphorene. These materials have already lost the low expense advantage of sodium ion batteries, impeding SIBs from industrialization and commercialization. The principal strategy for making anode with P is to fabricate a conductive base that can help phosphorus to transfer both electrons and sodium ions. This motivated us to use carbon xerogel to build a carbon network. Carbon xerogel is a carbon skeleton with interconnected porous structure and high surface area. Xerogel is similar to and sometimes regarded as a category of aerogel which has been attractive as electrodes for electrochemical double layer capacitors and electrosorptive processes. [7]
The major difference between xerogel and aerogel is that xerogel is dried at ambient atmosphere, yet aerogel is dried by other advanced and costly methods such as supercritical carbon dioxide. [8]
Carbon xerogel from polymers pyrolysis can be manufactured at a very low cost. For instance, Li et al. [9] reported a polymer xerogel manufacture process by simply dissolving and precipitating polyvinyl chloride (PVC) which allows reusing plastic waste. In this paper, we report a method to condensing red P on carbon xerogel to synthesize anode with low cost and high capacity. We mixed resorcinol with formaldehyde and carbonized the polymer at 1000°C to obtain carbon xerogel. Carbon xerogel is sealed with red phosphorus in vacuum and heated up to 900°C to condense phosphorus vapor on the carbon skeleton. The composite could take advantage of the high conductivity of carbon and the high theoretical capacity of phosphorus. Our method is easy to implement with low cost ready for commercialization to meet the large demands of energy storage.
2. Experimental [10]
Resorcinol and formaldehyde were mixed with a mole ratio of 1:2. Sodium carbonate was added as catalyst and the mole ratio of R-C is 50:1. Deionized water was used to dilute the solution such that the mass of resorcinol was 5% of the whole solution. The initial pH of the solution was set as 6.0 by dilute HNO . The solution was sealed and stirred magnetically for 30 min and kept at 85 °C for one week (no stirring). After gelation, the gel was washed by ethanol for three days with fresh solvent replaced daily. The wet gel was dried in a CVD furnace with flowing Argon gas. We used argon instead of nitrogen to get rid of any influence of nitrogen on the capacity. [11] At a heating rate of 0.5 °C/min, the gel was heated to 65 °C and held there for 5 h; then it was heated to 110 °C and held there for 5 h too. Afterwards, it was heated to 1000 °C at a heating rate of 5 °C/min and held for 4 h. The flow rate of argon was set as constant 200 SCCM. 2.2 Phosphorus condensation 0.05 g carbon xerogel (CX) prepared in the previous step was sealed in a quartz tube with 0.05 g commercial red phosphorus. The tube was heated to 900 °C and held there for 4 hours, where the heating rate was 4 °C/min. Next, the tube was cooled down to 280 °C at a rate of 1 °C/min and held for 24 h to convert white P to red P. After naturally cooled down, the product, denoted as P@CX, was then washed by CS to remove white P formed during condensation. 2.3 Thermogravimetric Analysis Thermogravimetric analysis (TGA) was conducted by a thermal analysis device, TGA 5500 (TA Instruments), to measure the mass ratio of phosphorus in the P@CX composite. Samples were immersed in 25 mL/min nitrogen gas flow. They all stayed at room temperature for 30 min to purge prior to heating. We assumed all phosphorus would evaporate when temperature reached around 400 °C so we heated the samples till 500 °C. The heat rate was 2 °C/min. 2.4 Electrochemical characterization The P@CX composite was granulated into powder by zirconium oxide beads in SpeedMixer. The powder (80 wt.%) was mixed with CMC-Na binder (10 wt.%), super P (10 wt.%) and water to make a homogeneous slurry. The slurry was pasted on 9 um-thick Cu foil (MTI corp.) and vacuum-dried at 60 °C for 4 hours. The electrode sheet whose mass loading was about 2 mg/cm was cut into circles. Next, the electrodes were assembled as 2032-type coin cells. Next, graphite electrodes were assembled to sealed 2032 type coin cells (MTI Corp.) with sodium metal counter and reference electrode with a separator (Celgard 2320) in an argon-filled glove box (MBraun) containing less than 0.1 ppm oxygen and moisture. The electrolyte solution was 1 M sodium perchlorate (NaClO ) dissolved in a mixture (1:1, v/v) of ethylene carbonate (EC, Sigma Aldrich) and dimethyl carbonate (DMC, Sigma Aldrich). Fluoroethylene Carbonate (FEC, 5 wt.%) and Vinylene Carbonate (VC, 1 wt. %) was added to stabilize and reduce the thickness of solid electrolyte interphase (SEI) layers. [6,12] All cells were cycled within voltage range of 0.01~1.5 V by a MACCOR test system.
3. Results
It can be observed from Figure 1 that the mass of carbon xerogel almost remained constant when temperature increased up to 500 °C. The constant mass shows the gel was sufficiently carbonized, at least enough to overcome such temperature. The mass of the P@CX composite increased gradually first and then dropped sharply to 87.96% when the temperature reached around 450 °C. The gradual increase was deemed to be caused by explosive sublimation with recoil effect [13,14] . Considering the mass decrease of carbon skeleton, the mass ratio of phosphorus in the composite is estimated to be 1-87.96%/(1-1.34%)=10.8%.
Figure 1. TGA results for carbon xerogel with and without phosphorus. Both arrows show difference between the stagnation points of two curves and 100% baseline. The capacity of P@CX and CX was measured, shown in Figure 2. Even with large particle size (~
50 μm , observed from Figure 3) and high mass loading (2 mg/cm ), the P@CX composite cycled at 200 mA/g yielded 242 mA/g maximum capacity. The current density is equivalent to 1.85 mA/g calculated based on phosphorus. after subtracting the contribution of carbon (CX, around 30 mAh/g from Figure 2b), the contribution if phosphorus is 1993 mAh/g P . The average degradation rate is 0.06% in 80 cycles. The average columbic efficiency is as high as 99.2%. Figure 2. Capacity of a) P@CX cycled at 200 mA/g (calculated based on P@CX composite) and b) CX cycled at 50 mA/g (calculated based on CX).
100 200 300 400 500889092949698100102
CXP@CX
Cycle number C apa c i t y / m A h g - C ou l o m b i c e ff i c i en cy DischargeChargeCoulombic efficiency
Cycle number C apa c i t y / m A h g - C ou l o m b i c e ff i c i en cy DischargeChargeCoulombic efficiency a) b)
Figure 3. SEM images P@CX a) composite at high magnification and b) electrode (containing carbon black, binder, etc.) and low magnification.
Figure 4. Energy Dispersive X-Ray Spectroscopy (EDS) images. Phosphorus is uniformly doped in macro scale (Figure 4b), but not as uniform as carbon in microscale. This may imply that the carbon skeleton is not fully saturated.
4. Conclusions
High capacity anode is required for sodium ion batteries. We proposed a method to synthesize anode with low cost and high capacity: condensing red P in carbon xerogel. In our product P@CX, the mass ratio of P was measured to be 10.8% by thermogravimetric analysis. The capacity of CX and P@CX was measured, and the reversible capacity was around 30 and 242 mAh/g (calculated based on composite) respectively. The coulombic efficiency of P@CX is as high as 99.2%. Subtracting the contribution of carbon, the capacity of P was estimated to be 1993 mAh/g with columbic efficiency. From the results we can conclude that carbon xerogel could help phosphorus to achieve high capacity. This research shed light on commercial anode for sodium ion batteries. There still remains much room for improvement. Despite high capacity contribution from P, the capacity of the composite P@CX is not high enough due to low phosphorus ratio. Two major strategies can be implemented in future works: optimizing the carbon skeleton and enhancing the phosphorus condensation conditions. For the carbon xerogel, many parameters (e.g., precursor ratios, temperature and time) are able to tune for higher surface area and thus more adsorption sites. The surface area could also be increased by carbon oxide activation. [10]
For the condensation conditions, higher pressure and temperature could be attempted to increase phosphorus doping. One possible challenge is the low tolerable pressure of quartz tube used to seal phosphorus, which would be easier to solved in industry. Our next step includes parameters optimization, higher phosphorus doping pressure and temperature, and CO activation. We will also try to further increase capacity retention by strengthening the carbon skeleton and the P-C bonding. Acknowledgement
This research was supported by SAMSUNG Global Research Outreach (GRO) program. The authors gratefully acknowledge the support.
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