Nickel metal hydride (NiMH) batteries generally have a high recycling rate because they are made of materials that do not include heavy metals and are mostly recyclable. Their reputation as the most environment friendly among the existing types of rechargeable batteries is expected to be further reinforced with a recent study from Stockholm University in Sweden. According to the study, the electrode material derived from an improved recycling process for NiMH batteries can provide better performance than the pristine counterpart.
NiMH batteries were developed as an improvement to nickel-cadmium (NiCd) batteries. NiMH and NiCd batteries are similar in most respects with the major exception of the negative electrode. For this component, NiMH batteries substitute the toxic and rare cadmium with a hydrogen-absorbing alloy called metal hydride. Compared with NiCd batteries, NiMH batteries are a better option for the same price because they can offer greater storage capacity and less memory effect in addition to being less toxic. An AA-size NiMH cell can have a maximum capacity of 2,900mAh, which is higher than what is attainable for NiCd and alkaline cells of the same size. Also, further design improvements led to the introduction of the low self-discharge (LSD) NiMH battery in 2005. In contrast to normal NiMH batteries, fully charged LDS NiMH batteries can retain much of their charge for one to two years as they have a higher leakage resistance.
NiMH batteries are especially in a strong position vis-à-vis other battery technologies when it comes to safety. If left unused for a long time, alkaline batteries will start to leak their corrosive and hazardous electrolyte. The improper use or handling of lithium-ion (Li-ion) batteries may cause a spontaneous combustion and then a fire. NiMH batteries do not have these safety risks and are easier to recycle due to the absence of heavy metals in their composition.
On the other hand, NiMH batteries are at a disadvantage when competing against Li-ion batteries in the consumer electronics market because they have a lower energy density and are heavier in weight. Nevertheless, NiMH batteries are a popular choice for powering items such as electric toothbrushes and RC toys. They are also used in some models of hybrid vehicles. All in all, NiMH batteries have remained a major form of energy storage in our everyday lives.
The latest innovation that researchers at Stockholm University have brought to the development of NiMH batteries is related to the aspect of recycling (or upcycling). Their project, which was carried out in partnership with Swedish battery maker Nilar, aims to improve the whole recycling process for this type of battery. To this end, they have devised a new method that does away with the expensive steps such as re-melting and mainly adopts mechanical treatments including washing and separation. Specifically, ultrasonic bath and ball milling are used to remove the corrosion from the electrode of the used batteries and separate the same material for reuse later. Their process is described in an article published in the scientific journal Molecules.
Dag Noréus, who is a professor at the Department of Materials and Environmental Chemistry in Stockholm University and a member of the research team, said that the new method can recover up to 95% of the materials in a NiHM battery. Another advantage is that the recovered materials can be directly inputted into the manufacturing of new batteries.
Noréus pointed out that rechargeable batteries all contain materials that are either rare or have to undergo complicated and costly processing. Furthermore, newly made rechargeable batteries have to be “activated” by experiencing a few charge and discharge cycles before they leave the factory. The recycling method that Noréus’s team has conceived yields battery materials that have already completed processing and activation. This, in turn, saves time and money for the battery manufacturers.
The study from Stockholm University finds that the economic benefits of recycling NiMH batteries increases significantly through the new and simplified method. Lab tests also show that NiMH batteries made of materials recovered from this process not only cost less but also perform better than the counterparts made of pristine materials. Apparently, the properties of the electrode alloy are enhanced after being first used in new batteries and then gone through the improved recycling process. However, the study does not get into much detail about the gains in performance and the cost reduction.
(News source: TechNews. Photo credit: Shutterstock.)