Automobiles are essential in everyday life. However, exhaust gas leads to air pollution, and CO₂ in the exhaust gas is a cause of global warming. This large environmental burden is a major problem, and eco-driving and -cars are promoted to reduce this burden. Eco driving refers to driving methods that are environmentally friendly and reduce CO₂ emissions.

Idling-stop systems are a primary example of eco-driving. Idling-stop systems automatically shut off the engine when the vehicle is stationary at a red light or in traffic, thereby reducing fuel consumption and exhaust gas emissions. These systems should significantly contribute to resolving environmental problems. Previously, manually shutting off the engine every time the vehicle stopped was necessary, but almost all vehicles are currently equipped with idling-stop systems that automatically shut off the engine.

Although these systems provide advantages, including reduced environmental burdens and improved fuel efficiencies, the engine is started very frequently, and thus, the starter motor (the motor used to start the engine) is also activated very frequently. The high currents involved when using the starter motor significantly burden the battery, and thus, the performances of traditional batteries generally deteriorate rapidly, resulting in reduced battery lifetimes.

The battery should not last long if it simply discharges the stored electric potential. Vehicles are equipped with an electrical generator denoted an alternator, which charges the battery while the engine is running.

When the vehicle is stopped, the engine is shut off and the battery is forced to discharge without charging via the alternator. Therefore, rapid charging of the battery is necessary while the engine is running.

Electrical power is required to operate air conditioning, audio systems, and lamps. If the vehicle stops repeatedly and the battery discharge is too high, the idling-stop system should deactivate to prevent excessive discharge, i.e., the idling-stop system should not function unless the battery is sufficiently charged while the engine is running.

Therefore, the batteries used in idling-stop systems should exhibit high charge acceptance that enables rapid charging, in addition to high durability to withstand the large load requirements.

Although batteries for use in idling-stop systems require high charge acceptance and durability, these features are challenging to achieve in tandem and involve a trade-off. Considerable effort was required to achieve both simultaneously.

Automotive batteries are traditionally lead-acid batteries, and the types used in vehicles with idling-stop systems include the EFB (enhanced flooded battery) and AGM (absorbent glass mat), which are mainly used in Japan and Europe, respectively.

Comparing these two battery types, EFBs exhibit high charge acceptance but low durability, whereas AGM batteries exhibit poor charge acceptance but high durability.

Because AGM batteries are high-cost, development commenced with the aim of increasing EFB durability to the level of those of AGM batteries.

The low durability of EFBs are due to stratification of the sulfate ions that are generated at the surfaces of their negative electrodes when charging. Stratification is the separation of sulfate ions into areas of high and low concentrations, resulting in different concentrations in the upper and lower sections of the battery. Heavier sulfate ions generally sink to the bottom of the battery. If the battery is insufficiently charged, no gas is generated at the electrode, and the electrolyte is not mixed, allowing the sulfate ions to settle at the bottom.

Upon stratification, the lead sulfate formed during battery discharge crystallizes in the lower section of the battery with high ion concentrations, causing the deterioration in electrode performance. This is denoted “sulfation”, and as it progresses and the crystals accumulate on the electrode, the battery capacity and charging rate decrease. In addition, the charging and discharging reactions are concentrated in the upper section of the battery with low ion concentrations, which causes the electrodes to deteriorate more easily. At the positive electrode, in particular, “flaking” generally occurs, wherein the lead oxide flakes off the electrode.

In AGM batteries, the separator is filled with an electrolyte. As a result, sulfate ions do not easily sink to the bottom, preventing stratification and increasing the battery durability. Therefore, the EFB battery was improved by adding a fiber layer between the negative electrode and separator to prevent sulfate ion stratification. Through the inclusion of a fiber layer, no concentration gradient forms in the electrolyte, making it possible to maintain consistent electrode function from top to bottom.

Conversely, the mutual interactions between the sulfate ions and fiber layer molecules altered the diffusion of the sulfate ions, which could result in poor charge acceptance. The design of the fiber layer began with the aim of identifying materials and structures that enabled sulfate ion diffusion without lowering charge acceptance for use in the fiber layer.

With cooperation from the Hitachi’s research center, dynamic simulations were conducted to investigate the motions of sulfate ions within the fiber layer. The movements of the sulfate ions and their interactions with the fiber layer were investigated by changing multiple parameters, including the thickness and porosity of the fiber layer, to realize a high charge acceptance and durability.

The simulations indicate that the diffusion coefficient of the sulfate ions and the surface tension of the fibers correlate, with a maximum diffusion coefficient at the corresponding surface tension. Therefore, when the surface tension of the fibers is high, the interactions with the sulfate ions are stronger, and as the diffusion coefficient increases, charge acceptance improves. Therefore, adjusting the surface tension of the fiber layer may lead to a large improvement in charge acceptance.

Therefore, adjusting the surface tension of the fiber layer may lead to a large improvement in charge acceptance.

The results are obtained via a simulation, and if this relationship is not confirmed in reality, proceeding with development should be impossible. Hence, evaluating the diffusion coefficient and surface tension of the fiber layer is necessary. With the assistance of the Hitachi’s research center, a method of evaluation was established. The diffusion coefficient was evaluated based on the changes in conductivity when sulfuric acid diffused through the fiber layer. The surface tension was evaluated based on the contact angle of a sulfuric acid droplet on the surface of the fiber layer.

First, prototype fiber layers were fabricated using different parameters, such as thickness and fiber density. As the fiber layer narrowed, the internal resistance acting on the electrolyte decreased, rendering the diffusion of sulfate ions easier, leading to improved charge acceptance. In addition, the surface tension was adjusted by coating the individual fibers in the fiber layer to increase their hydrophilicities. The coated fibers were observed using scanning electron microscopy, which confirmed that the coating remained stable even when submerged in sulfuric acid.

When the diffusion coefficient is measured using fiber layers with different surface tensions, revealing the correlation between the diffusion coefficient and surface tension is possible, along with the maximum diffusion coefficient, as indicated by the simulation results. Thus, a fiber layer with a surface tension that maximizes the diffusion coefficient, which is key in determining charge acceptance, was identified, and the development proceeded.

Thus, a battery was created with a fiber layer between the negative electrode and separator, and the stratification was evaluated by measuring the sulfate ion concentrations in the electrolyte at the top and bottom of the battery. Compared with that of the product prior to modification, the addition of the fiber layer educed the stratification.

In addition, in the durability studies, the lifetime of the battery increased in terms of the number of charge cycles, indicating a considerably increased durability.

Hitachi Chemical Co., Ltd. (currently Energywith Co., Ltd.) launched this novel battery with a high charge acceptance and durability in June 2016 as the “Tuflong G3” battery for use in idling-stop system vehicles. With a product warranty of 38 months, Tuflong G3 shows a battery life of approximately 2.1 times those of existing batteries.Using this battery improves fuel efficiency via its rapid charging while the engine is running and maintains the idling-stop function for an extended duration. The resulting improved fuel efficiency should lead to lower overall CO₂ emissions. Based on the reduction in CO₂ emission per idling-stop system vehicle equipped with a Hitachi Chemical Co., Ltd. (currently Energywith Co., Ltd.) battery and the number of vehicles equipped with such batteries, the technology is estimated to reduce CO₂ emissions by approximately 750 000 metric tons annually.

Owing to the increased use of eco-driving and the prevalence of eco-cars, CO₂ emissions from vehicles declined recently. The number of vehicles with idling-stop systems, which improve fuel efficiency by reducing unnecessary fuel consumption, has increased to the point where obtaining a vehicle not equipped with such a system is challenging. With previous automotive batteries, sufficiently utilizing this function was impossible, but the novel technology should enable increased utilization of the idling-stop function and contribute to reducing CO₂ emissions.

Of the 1.138 billion metric tons of CO₂ emitted in Japan in 2018, vehicles accounted for 15.9%. Under the Paris Agreement signed by over 100 nations in 2015, these nations aim to eliminate greenhouse gas emissions, such as CO₂, by the latter half of the 21st century, and the global trend toward reducing CO₂ emissions should accelerate further. We intend to expand this technology overseas and contribute to reducing global greenhouse gas emissions.