With over a million hybrid vehicles on the roads today, NiMH has proven to be safe, reliable, and more importantly a stable battery chemistry. Nilar began work to develop a bipolar module to improve the volumetric power density and reduce the overall complexity found in battery pack construction. Many companies seen the potential benefits for developing a bipolar NiMH battery but the obstacles associated with this type of chemistry and construction proved for many to be fruitless endeavors. Understanding the problems and engineering solutions were the primary focus for Nilar to develop a working battery that could compete in the ever growing battery market. In addition to solving the issues of a bipolar NiMH battery, determining the ideal markets to approach based on the battery's performance were identified.
One of the major obstacles for developing a bipolar battery using the NiMH battery chemistry is the presence of potassium hydroxide (KOH). KOH is very difficult to seal in an electrochemical environment. KOH is commonly used as the electrolyte in consumer alkaline batteries. Early in the consumer alkaline battery business the sign of white plumes (potassium bicarbonate KHCO3) forming around the seal was the tell sign of KOH coming into contact with the carbon dioxide in the air. The presence of KHCO3 is a physical sign of leakage, and the loss of electrolyte causes premature failure in the form of separator dry-out. Electrolyte in the separator is the medium for ions to cross back and forth between anode and cathode. If ions cannot transport between electrodes then the battery stops working correctly. Once leaking starts, failure is rapid and abrupt. Determining a sealing technique that would prevent loss of electrolyte required understanding the sealing properties employed by cells that reached an oxidation failure mechanism (the ideal end of life failure mechanism).
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