Preparation of Resin Added Graphite Sheet As Current Collector of Lead Acid Batteries
Authors: Hano Yuta , Okano Hiroshi , Inoue Takashi , Hosokawa Toshihiro , Takeda Akiyoshi , Iwai Taichi , Yabutsuka Takeshi , Takai Shigeomi , Yao Takeshi 
Published: December 2020

Introduction

Lead Acid Batteries(LAB) have been widely used for more than 100 years. Conventionally, LAB cannot be recharged after over-discharged, and its performance is greatly declined. We found that the deterioration after deep-discharge is caused by local cell reaction between lead current collector and cathode active material.[1] We also revealed that the decline of the battery performance can be prevented by using gold or graphite material current collector.[2-3] In that case, the LAB can be recharged even after over-discharge. Graphite sheet is practical as current collector, however, it was revealed that raw graphite sheet absorbs sulfuric acid electrolyte, and the properties deteriorates. In this study, to prevent the property degradation, we tried to mix resin materials into raw graphite sheet and investigated the characteristics improvement as a function of type and amount of additive resins.

Experimental

We used four kinds of resin material(polyethylene(PE), polypropylene(PP), ethylene-propylene copolymer(EPM) and polyvinylidene fluoride(PVDF)) and dispersed it by 5, 10, and 15wt% into expanded graphite. Then we made graphite sheet from expanded graphite mixed with resin by a rolling mill. Next, in the heat treatment process, we carried out preheating at 100°C for 10 minutes, and raised the temperature to various appropriate high temperature for each resin, then hold for 10 minutes.
For characterization of resin added graphite sheet, we measured the tensile strength of longitudinal direction and contact angle of pure water. The test piece size was 100×20×0.5mm for tensile strength measurement. We also conducted charge/discharge experiments by using 5%PP or 5%PE added graphite sheet as current collector. The cathode paste was prepared by mixing β-PbO2 powder as active material, acetylene black as conductive additive, and PTFE as binder at the ratio of 80:15:5 in weight. We constructed the cathode by pressing the cathode paste on each current collector. We used 35% H2SO4 as electrolyte and lead plate as the anode.

Results and discussion

We conducted characterization for PP or PE resin dispersed graphite sheet, because it was found that resin was not dispersed uniformly into graphite even after heat treatment at 200℃ for EPM or PVDF. The tensile strength of raw graphite sheet was 4.8 MPa, and graphite sheet with PP was 7.5, 9.7, and 10.8 MPa for the 5, 10, and 15% additive amount, respectively. The Tensile strength of graphite sheet with PE was 6.2, 4.1, and 4.4 MPa for the 5, 10, and 15% respectively. It was confirmed that addition of PP gives higher tensile strength than raw graphite sheet. Graphite sheets with PE showed the highest value at 5%PE addition, but when 10 or 15% was added, the tensile strength was reduced due to inhomogeneous force concentration.
The contact angle of pure water was about 40° for raw graphite sheet, and over 80° for each resin dispersed graphite sheet regardless of kind or amount of resin. This means that dispersion of resin makes water repellency increase for graphite sheet.
Figure 1 shows the weight increase of graphite sheet with resin when immersed in 47% sulfuric acid at around 70°C. It is indicated that graphite sheets containing 5%PP and 15%PE showed no mass increase due to sulfuric acid absorption even after 14 d.
Figure 2 shows the charge-discharge results of LAB using graphite sheet with 5%PP or 5%PE or raw graphite sheet for reference, as the current collector. It is shown that the charge/discharge cycle was restarted after complete discharge to 0 V and 48 h open circuit rest, for all three cases. This result is very much consistent with our previous study that graphite material current collector prevents degradation of cathode active material by local cell reaction control.[4] In the case of raw graphite sheet, cell voltage was fluctuating. For PP or PE cases, the voltage fluctuation is well suppressed. It is thought that this is due to graphite sheet degradation by absorption of sulfuric acid and addition of PP or PE is effective to prevent the degradation.
From these results, especially in the improvement of tensile strength and discharge potential, it is concluded that 5% polypropylene addition to raw graphite sheet is most suitable for use as a cathode current collector of LAB.

References

[1] T.Iwai, M.Murakami, S.Takai, T.Yabutsuka and T.Yao, J. Alloys and Compounds, Vol.780, 85-89(2019).
[2] T.Iwai, D.Kitajima, S.Takai, T.Yabutsuka and T.Yao, J. Electrochem. Soc., Vol.163, Issue 14, A3087-A3090(2016).
[3] T.Yao, H.Okano, T.Iwai, S.Takai, T.Yabutsuka, T.Hosokawa, N.Misaki, K.Kurihara “Positive Electrode for Lead Storage Battery and Lead Storage Battery Using Same” PCT/JP2018/005947.
[4] Y.Nakamura, T.Ohkubo, H.Okano, T.Inoue, T.Hosokawa, A.Takeda, T.Iwai, T.Yabutsuka, S.Takai and T.Yao, 235th ECS Meeting, Z01-2143(2019).

Figure 1