Improving the Autonomy of the Electric Vehicle Through the Use of Almond Shells

The continuing increase in CO2 emissions is real and concerning. The efficient use of sources of renewable energy and the replacement of internal combustion engines with electric motors in the development of more sustainable vehicles, such as hybrids (HEV), plug-in hybrids (PHEV) and, ultimately, completely electric vehicles (EV), is a key objective for our society. All of these require an on-board source of energy to power the electric motor. From the available options, the most suitable are rechargeable batteries, that is, portable devices capable of delivering stored chemical energy as electrical energy with high conversion efficiency and without producing emissions. Lithium-ion batteries are particularly attractive for this purpose given their high energy-density value, which is why they are currently used in all electric vehicles. However, there are several reasons to suspect that this battery’s future may be uncertain, especially if all petrol and diesel vehicles on the road are to be replaced by their electric counterparts. Among the challenges faced are autonomy, rapid charging, the economic and environmental cost of their production, the availability of their constituent materials and the safety of the vehicle when in use. In terms of performance, lithium-ion batteries cannot compete with the autonomy of conventional vehicles running on fossil fuels. What is more, the current configuration of these batteries does not allow for charging in as little time as it takes to fill the tank on a conventional vehicle. Although the manufacturing costs of batteries decreases with the introduction of enormous factories (gigafactories), the environmental impact of manufacturing is considerable, with several international studies suggesting that it would be incorrect to define EVs as ‘zero-emission’. The cost is directly linked to the need for critical metals to make the battery, such as cobalt, lithium and copper, whose availability for mass-production is questionable. Lastly, there is room for improvement in the safety of the technology, although fire and explosions will pose a threat as long as volatile flammable electrolytes are required for them to function.

One very promising technological alternative is lithium-sulfur (Li-S) batteries, a system which works by way of an electrical process which delivers a theoretical energy density of 2600 Wh/kg, an order of magnitude greater than conventional Li-ion batteries.
 
In addition to this enormous energy advantage, due to which it would be possible to increase autonomy by almost 10 times, Li-S batteries offer several benefits with respect to the drawbacks of Li-ion. For one, the battery’s main component is sulfur, an abundant naturally occurring material which is considered environmentally safe in comparison with the materials used in Li-ion technology. Sulfur can be obtained directly from the ground by mining, although it is mostly produced as a byproduct in petrochemical plants. Therefore, it eliminates the need for critical materials such as cobalt, nickel and copper. The main challenge posed by sulfur is the need for a matrix to provide electrical conductivity and space to react with the lithium. At present, the most widely used material for this purpose is carbon. Most of the carbons studied as a sulfur matrix are derivatives of petrochemical products or synthetic organic compounds with complicated production processes and costly raw materials. In order to reduce the expense, our investigation team develops sustainable carbon materials designed to be used as a sulfur matrix in Li-S batteries. For this purpose, we propose the use of agri-food industry by-products as a source of these carbons, avoiding the use of products which can otherwise be used as food (soya, rice, corn, etc.). For a by-product of this type to be suitable in this study, it must contain a high percentage of carbon and it is for this reason that almond shells have been proposed as the ideal candidate.

The starting point for this study was ground and washed almond shells. The process by which they are turned into carbon consists of a simple pyrolysis, avoiding the presence of air during heating. Converting the shells into carbon directly by calcining at high temperature does not produce the desired type of carbon, which would be a material with a large active surface and high internal porosity, with the smallest possible pores to be able to trap sulfur inside and which can react correctly with lithium during the use and recharging of the battery in the vehicle. To achieve this, an activation stage is employed, during which it is impregnated with phosphoric acid, an inexpensive and widely available compound with no environmental impact. Thanks to this activation it is possible to produce a microporous type of carbon which has the ideal characteristics for use as a matrix for the sulfur in the battery. The energy results of button batteries containing this type of material are very positive in terms of autonomy, stability and the possibility of rapid charging. Studies of simulations of use in electric vehicles have shown a more than 50% increase in vehicle autonomy when using this battery. As a result, this study proves the feasibility of using batteries based on carbon derived from almond shells and opens the way for studies into the possibility of using other nut by-products, such as walnut, hazelnut and pistachio shells, whose chemical characteristics would suggest their suitability.