Lead acid batteries (LAB) continues to be the major energy storage system for variety of applications such as stationary, uninterruptible power supplies, telecommunications, automotive and even in hybrid and electric vehicles. Besides, LABs are of low cost, >98 % recycling, safety, high abundance of the materials. In spite of being randomly optimized its various aspects in the past, lead acid battery researchers are still facing major challenges of sulfation at the both positive and negative plates. Besides, positive electrode grid corrosion, longer curing as well as formation time of the positive plate and also its low energy density because of high atomic weight of lead (Pb) limits its application [1]. Besides, the major issue of negative plate operating under partial-state-of-charge (PSoC) conditions are sulfation and low charge acceptance. PSoC can quickly diminish the overall life of a battery, which results in frequent, costly battery replacements. So the current research in LAB is directed to achieve high energy density, long-cycle life and to reduce sulfation [2-3].

Different forms of carbon (such as graphene, graphite, carbon nanotubes etc.) additives to the negative active mass (NAM) was used to decrease the lead sulfate formation, increase the conductivity of negative active material by forming conductive network with lead sulfate crystals. Beside carbon acts as a capacitor. This capacitive property of carbon is useful in charge-discharge operations conditions at high rate to provide high voltage [4-5].

In this work we present reduced graphene oxide (rGO) - TiO₂ as an ideal additive for advanced LABs operating under HRPSoC conditions. Nano structured TiO₂ grown (decorated) on reduced graphene oxide (rGO-TiO₂) has been successfully synthesized from the solvothermal method. Optimized 0.5 wt. % of rGO-TiO₂ employed as NAM additive to enhance the initial discharge capacity, decreases the hard sulfation, improves the charge acceptance and more utilization of the NAM. Addition of 0.5 wt. % of rGO - TiO₂ to the NAM in composition of rGO: TiO₂ is 1:10 and 1:3 (Hereafter rGO-TiO₂-1:10, 1:3) delivers > 67% and 85 % increase in capacity during the first formation cycle compared to conventional lead-acid cell (LAC). The rGO - TiO₂ additive based cells are formed in one cycle whereas conventional electrode takes about 3 cycles to achieve the full capacity.

0.5 wt. % of rGO - TiO₂ additive LAC delivers 20% more capacity compared to the conventional cell at all C-rates (C/20 to 2C). Presence of rGO in the negative plate to increases the charge acceptance of the cell. Cycling study was performed on 2V/2.1 Ah conventional and 0.5 wt. % of rGO, TiO₂, and rGO-TiO₂ (1:100, 1:10, 1:3) additive cell at 1C rate shows an increase in capacity of about 40% was observed with rGO - TiO₂ additive based cells as compared to conventional cells. This could be due to the reversibility of PbSO₄ to lead. As cycling progresses at the 1C rate, conventional cell has poor capacity retention due to the formation of PbSO₄ at the negative plate and does not convert into the active material (Pb). It leads to the early evolution of hydrogen at the negative plate and rapidly decreases the capacity. Significant amount of RGO in the active material shows the capacitive phenomenon. During high rate charging, charging occurs at electrical double layer of the surface of the rGO. Besides addition of rGO - TiO₂ in NAM could limit the sulfation of negative plates, due to the high porosity of RGO which could acts as an electrolyte reservoirs in the interior of the negative plates. This helps the active sites for the migration of PbSO₄ to Pb during charge and inhibition of PbSO₄ during cycling. Beside, TiO₂ Nano particles occupies the pores of the active mass and also supplies electrolyte to the bulk. The combination of both rGO and TiO₂ in the active mass helps to improve more active material utilization of NAM during 1C rate cycling. HRPSoC cycling increases to 22630 cycles for rGO-TiO₂ additive cell compared to the 7614 cycles of conventional LAC.

Acknowledgements:

Financial support for this project work from Clean Energy Research Initiative (CERI) under Department of Science and Technology (DST), Govt. of India (Project Code: DST/TM/CERI/C141) gratefully acknowledged.

References:

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