The long-term consistency of lithium-ion battery cells remains a major challenge in the design of lithium-ion battery packs. Consistency here refers not only to traditional parameters like capacity and voltage but also to factors such as the rate of capacity loss, internal resistance degradation, and temperature distribution across the pack. Ideally, batteries from the same batch should have identical electrochemical performance, but manufacturing variations lead to inconsistencies between individual cells.
Battery packs typically consist of hundreds or even thousands of single cells connected in series and parallel. These inconsistencies significantly affect the overall performance and lifespan of the battery pack. Issues like inconsistent Coulomb efficiency, self-discharge rates, and internal resistance growth can drastically reduce the cycle life of the entire system, even if individual cells perform well. Studies show that while a single cell may last over 1000 cycles, the pack’s life could be less than 200 cycles due to these discrepancies.
To address this, balancing equipment is essential for large-scale battery systems. Traditional methods rely on electronic devices to balance voltages between cells, but this approach varies widely in effectiveness. Recently, Alexander U. Schmid and his team at the University of Stuttgart introduced a novel method using NiMH (Nickel-Metal Hydride) and Ni-Zn (Nickel-Zinc) batteries to achieve electrochemical equilibrium in lithium-ion packs.
Lithium-ion batteries are vulnerable to overcharging, which can cause electrolyte decomposition and lithium plating. In contrast, NiMH batteries handle overcharging more effectively. During overcharge, water in the electrolyte decomposes into oxygen and hydrogen at the electrodes. A catalyst then recombines these gases back into water, creating a closed-loop system. This makes NiMH batteries highly resistant to overcharging, especially at low charge rates.
Using this principle, Schmid integrated NiMH and Ni-Zn batteries with lithium-ion cells to create an automatic balancing system. The NiMH/Ni-Zn batteries act as current bypasses during charging, diverting excess current away from lithium-ion cells. This eliminates the need for complex voltage monitoring and electronic balancing circuits, simplifying the system and improving reliability.
In experiments, LiFePO4 (LFP) and Li4Ti5O12 (LTO) materials were used because they exhibit rapid voltage increases when fully charged. At this point, the NiMH/Ni-Zn batteries take over, allowing current to flow through them instead of the lithium-ion cells. This prevents overcharging and ensures all cells are evenly charged.
The setup involves connecting NiMH/Ni-Zn batteries in parallel with lithium-ion cells. When a low-capacity cell reaches its voltage threshold, the NiMH/Ni-Zn batteries shunt the current, protecting the lithium-ion cells. This process was demonstrated with various configurations, including LFP/LTO, LMO/LTO, and LFP/C batteries paired with NiMH or Ni-Zn units.
Several experiments showed that this method effectively balances the battery pack without requiring active monitoring. For example, in one test, two LFP/C-2NiZn modules were connected in series. After one charge-discharge cycle, the capacity difference between the two modules reduced by 8%, demonstrating the effectiveness of the electrochemical balancing technique.
This approach offers several advantages: it reduces system complexity, enhances reliability, and allows for automatic balancing. It also avoids overheating issues by controlling the charging current in steps. High-power NiMH/Ni-Zn batteries are necessary to maintain performance during discharge.
Overall, Schmid's research presents a promising alternative to traditional balancing methods. By leveraging the inherent properties of NiMH and Ni-Zn batteries, this technique provides a simple, efficient, and reliable way to improve the consistency and longevity of lithium-ion battery packs.
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