Scientists from the University of Bologna developed new environmentally friendly separators for batteries and super condensers based on alginate – a natural polymer received from brown alga. These separators can replace traditional polyethylene and polypropylene membranes widely used in lithium batteries, which are produced from oil and are practically not degradable in nature.
A separator is a thin porous film between the anode and the cathode of the battery. It is not part of chemical reactions, but has other very important functions – it prevents short circuit and lets the ions pass through providing for the charge motion. Its properties define the stability, capacity and the service life of the appliance. Commercial separators made from olefins perform well enough, but they are not bio-degradable and create difficulties during disposal.
Searching for solutions, the researchers turned to alginate – a natural polysaccharide contained in cell membranes of brown alga. Alginate is not toxic, it is cheap and resistible to warming-up, and when interacting with calcium ions it creates firm gel structures. Two types of membranes were created based on alginate. The first one – from sodium alginate and polyethylene oxide – for lithium batteries with organic electrolytes. The second one – from calcium alginate with added polyethylene oxide reinforced by polyvinylidene fluoride fibers – for water super condensers.
Membranes were received by phase inversion method – the controlled transition of polymer from a solution to a solid state. Different from traditional technologies using toxic solvents, only water and ethanol were applied here. This allowed for precise control of porosity and thickness of the material.
In the course of trials, the membranes demonstrated high thermal stability (disintegration started under the temperatures above 240 °C) and durability sufficient for working in real appliances. Scanning electron microscopy confirmed that sodium membrane has compact structure with circa 100 nanometers pores, while as calcium membrane forms 3D network with big pores providing for electrolyte absorption and high-speed ion migration. Both types of membranes successfully carried the charge without any short circuits. In lithium elements the sodium-alginate-based membrane survived through lengthy cycles of lithium subsidence and dissolution providing for stable performance without degradation of the material. In water super condensers the calcium membrane demonstrated unprecedented life span – over 250 thousand charging-discharging cycles preserving over 90% of initial capacity and minimal drain current. These parameters are typical for high-class pre-production prototypes.
In future, the researchers plan to improve the composition of membranes by increasing ion conductivity and sustainability during long operation. To achieve that, they intend to experiment with proportion of components and pores structure.
