Preparation Conditions of Porous Carbon Monolith Support Materials for Air Electrodes and Their Application to Lithium Air Secondary Batteries

                                                                                        Introduction

         Lithium air secondary batteries exhibit higher theoretical energy density than lithium ion batteries and are expected to be used as the next generation of secondary batteries.   However, there remain technical challenges related to their poor cycle properties.

         Since the first report by K. M. Abraham et al. [1], various oxygen reduction/evolution catalysts [1-4] and electrolyte [1, 5] materials have been intensively investigated for air batteries to improve their electrochemical properties, such as the their cyclability.  However, there have been a few studies on support materials for air electrodes, for example, nanoporous gold [6].

         We think that a decrease in the electrical contact of the air electrode during Li₂O₂deposition at discharge and oxygen generation at charge is one of the reasons for the poor cyclability.  The purpose of our research is to improve the cyclability by using a new air electrode structure.  We are focusing on a porous carbon monolith, which has a continuous interconnected network of carbon, as support material for air electrodes.  We have already fabricated porous-carbon-monolith supports to replace the current carbon-powder supports.  Here we report the preparation conditions for porous-carbon-monolith support materials and the performance of air batteries incorporating these supports.

                                                                                        Experimantal

         A porous polyacrylonitrile (PAN) monolith was prepared as the precursor of the porous-carbon-monolith support.  PAN was dispersed in dimethyl sulfoxide (DMSO).   After the solution with a concentration of 94 g/l was dissolved, water was sprayed on the solution.  The porous PAN monolith film was deposited from the solution due to water, which is poor solvent for PAN.  The monolith was washed with methanol and dried in vacuum at room temperature.  Then it was carbonized in Ar atmosphere at 1300 °C to obtain a porous carbon monolith.  In the next step, for activation treatment, the porous carbon monolith was heat treated in CO₂ atmosphere at 900 °C for 1 h [7].  The lithium air secondary battery consisted of the air electrode, a lithium metal sheet, and 1.0-mol/l lithium bis(trifluoromethanesulfonyl)amide (LiTFSA)/propylene carbonate (PC) as the positive electrode, negative electrode, and electrolyte solution, respectively.  The battery preparation is described in detail in our previous paper [4].  Electrochemical measurements were carried out under a galvanostatic condition of 0.05 mA/cm² in an O₂atmosphere.  The discharge and charge capacities were normalized by the weight of the air electrodes.

                                                                                  Results and discussion

         In the SEM image of the porous carbon monolith in Fig. 1, we see the continuous interconnected network of the carbon.  These particles build a 3-D disordered macroporous framework, with sizes in the range of 2-5 mm.  In XRD measurements of the porous carbon monolith, all of the peaks in the XRD patterns corresponded to the ICDD data for carbon (#01-077-7164).  These results indicate that our method can produce porous carbon monoliths.

         Figure 2 shows the pore distributions of the porous carbon monolith measured using mercury intrusion porosimetry.  The porous carbon monolith has mesopores with size of ~10 nm, which correlated with first discharge capacities [8].

         Figure 3 shows the first discharge/charge curves of the air batteries incorporating the as-synthesized and activation-treated porous carbon monolith.  The air batteries, showed higher discharge and charge capacities, with the activation-treated porous carbon monolith.   This is because activation-treated porous carbon monolith has many more active sites than the as-synthesized one.  In particular, the air batteries with the activation-treated porous carbon monolith show the capacity of 12 mAh/g and the average discharge voltage of 2.6 V.  This indicates that the porous carbon monolith can be used as support material for air electrodes. 

         However, compared with Ketjen Black EC600JD (KB) powder [9], the air batteries with the porous carbon monolith show lower discharge capacities.  It seems that KB-powder has many more active sites than the porous carbon monolith because its BET surface area of 1300 m²/g is larger than that of the porous carbon monolith, which is 4.3 m²/g.

                                                                                          References

[1] K. M. Abraham et al., J. Electrochem. Soc., 143 (1996) 1.

[2] T. Ogasawara et al., J. Am. Chem. Soc., 128 (2006) 1390.

[3] A. K. Thapa et al., Electrochem. Solid-State Lett., 13 (2010) A165.

[4] H. Minowa et al., Electrochemistry, 78 (2010) 353.

[5] N.-S. Choi et al., J. Power Sources, 225 (2013) 95.

[6] Z. Peng et al., Science, 375 (2012) 563.

[7] M. Nandi et al., Func. Mater. Lett., 4 (2011) 407.

[8] M. Hayashi et al., Electrochemistry, 5 (2010) 325.

[9] H. Lim et al., Chem. Commun., 48 (2012) 8374.