Silicon battery energy potential
A battery plant that uses silicon anodes could transform the battery manufacturing sector. Ledetta Asfa-Wossen explores Nexeon’s Oxford, UK, based pilot facility.
A pilot plant, located in an unassuming part of Oxfordshire, UK, is actively demonstrating silicon anode technology for potential low-cost manufacturing of lithium-ion batteries.
Silicon is a well known contender as a high performance anode material for batteries, yet charge instabilities have often marred its potential. When charging a lithium ion battery, lithium is inserted into the silicon, causing a dramatic increase in volume. On discharge, lithium is extracted from the silicon which returns to a smaller size. Repeated expansion and contraction places great strain on the silicon, causing it to fracture or pulverise. This, in turn, leads to the electrical isolation of silicon fragments and a loss of conductivity in the anode. For this reason, charge-discharge cycle life for conventional silicon-based anodes is typically short.
Ian McDonald, Engineering Director at Nexeon, who has been there since 2007 elaborates, ‘In contrast to carbon, silicon anode materials have a much higher capacity for lithium and as a result are capable of storing almost ten times the gravimetric capacity per gramme. At the carbon anode, you can achieve 372mAh/g, at the silicon anode 3,580mAh/g.’
Nexeon’s founder, Professor Mino Green, discovered early in 2005 that chemical etching of silicon could modify its morphology, creating fibres or pillars of silicon around 100 micron thick. These halt silicon from degrading in a battery. An electrical engineer by profession, his primary inclination was to show how this development could improve battery performance and cycle life.
With a little seedcorn funding of a half a million pounds, a five-man team was formed and Nexeon was conceived to begin developing the silicon anode material in a small testing lab at Imperial College London's business incubator.
By 2009, the team had secured £10m in funding from investors, including Imperial Innovations and Invesco Perpetual, growing to a 30-person strong operation. This enabled the firm to scale up the process and develop a pilot plant to produce the silicon material, fabricate prototype cells, test and demonstrate their capability.
On the line
To optimise the silicon, silver is used to nucleate etching of the silicon powder, followed by hydrofluoric acid as the etchant, to produce pillars of up to 50μm. The pillared particle can then be used as it is.
The plant operating system appears simple in design and operates via a touch panel. At the heart of the system is a reaction vessel where the silver and silicon are added using an automatic feed system. This vessel is used for the nucleation and etching steps, to stir the powder while the reaction is taking place, maximising the pillar:core ratio of the final material.
McDonald illustrates, ‘During the etching process, accumulated silver on the silicon surface is recovered in very high yield. It can be washed off and reused for nucleation’.
The silicon-based material, notes McDonald, has been designed to operate as a low-cost ‘drop-in approach’ for carbon on existing battery production lines. He continues, ‘You may need new formulations – the binder and electrolytes – but the machinery is the same. You just need to replace the carbon with silicon’.
Product quality is ensured using scanning electron microscopy and energy dispersive X-ray spectroscopy techniques to assess particle size distribution and surface area before the anode slurry is coated onto 10μm copper foil, reeled and dried by a dual-zone electrically heated air-dryer.
The prototype cells produced at the plant are the 18650 cylindrical cell and the 383562 soft pack cell, as well as a coin-type test cell. So far, prototype 18650 cells based on the silicon anode have been made with a capacity of 3.55Ah at the facility. McDonald states that, at present, cell performance on a commercial scale is typically 2.5-2.7Ah. The target here is now to create a 4.00Ah cell.
‘Higher capacity opens up the possibility of longer operating times for consumer electronics products and electrical vehicles where a high capacity and power to weight ratio is critical. Batteries made using the silicon anode material would deliver these benefits and allow dramatic increases in vehicle range between charges,’ adds McDonald.
However, cost and weight implications are the main draw for manufacturers, he states, as less material is needed to form the cells.
Currently the plant produces five tonnes of the material a year on average. With a drive to reduce etching process times, McDonald hopes full-scale manufacture will be on the cards for 2013.