Silicon holds the key to longer battery life – Part Two
In part one of this two-part article, we noted the dominance of Lithium ion (Li-ion) battery technology, but recognized its present limitations. We discussed how the use of silicon in place of carbon as an anode material could result in Li-ion cells with much higher energy density and cell capacity. In this second part, we consider the production process and safety issues before reviewing the possible impact of silicon anode technology on market applications.
To overcome the dimensional instability which has limited the use of silicon anodes in the past, Nexeon has taken a discovery made at Professor Mino Green’s laboratory at Imperial College, London: that creating particular morphologies of silicon can result in a far greater resilience to the physical stresses of repeated charging and discharging. The particular morphology which has been found to be most effective involves creating a silicon structure where the silicon particle has the appearance of needles on a hedgehog (see Fig 1).
An advantage of the approach is that no special grade of silicon is required (standard metallurgical grade is used); the innovative steps being in the production process itself. Development of this process is therefore critical to demonstrating the new material’s potential, and an end-to-end facility for processing silicon right through to assembly of prototype cells is an important asset. In Nexeon’s case, it has established a state-of-the-art pilot facility capable of producing several kilogrammes of material a day – a volume equivalent to the production of over a million ‘18650’ laptop cells a year!
The production process
The production process begins with chemical etching of the silicon: the focus of the process is a reaction vessel where the silicon and other reagents are automatically added, and in which the nucleation and etching steps occur. The result is the formation of ‘pillared particles’. These are then mixed with chemicals such as binders, and the resulting anode slurry is coated onto 10 µm copper foil, and reeled and dried by a dual-zone electrically heated dryer for subsequent lamination (see Fig. 2). Of course, the precise nature of these processes are closely guarded secrets, forming part of the intellectual property companies such as Nexeon are building.
Figure 2
What is clear is that certain parameters have to be closely monitored to ensure the performance of the materials; these include particle size distribution, surface area, electrical properties and overall morphology, as well as production measures such as yield. Use of scanning electron microscopy and energy dispersive X-ray spectroscopy techniques ensure product quality at this stage.
A key requirement of such a novel technology is that it should be scalable for mass production, and Nexeon has an advantage here: all the materials are produced at room temperature and pressure in processes that are fast and inherently low cost. As a bonus, most of the reagents can be recycled. The equipment design allows battery manufacturers to continue using much of their existing production lines i.e. a ‘drop in’ approach.
Safety
Any battery can represent a safety hazard and Li-ion is no exception. Well publicized instances of laptop battery fires have ensured that safety is a high profile issue. In fact, the use of silicon in place of carbon as anode material acts to enhance the inherent safety margin as the silicon has a greater capacity to store lithium. This is helpful in ensuring thermal runaway cannot occur through severe overcharging due to a failure in a power supply, or from internal short circuits due to puncturing for example.
Take a tour of Nexeon’s Oxfordshire facility and you may notice what appears to be a concrete shell festooned with all manner of test equipment and scrutinized by CCTV monitors. This is the safety testing zone where cells can be subject to destructive testing including shorting, overcharging, crushing, vibration, setting on fire and various other forms of abuse. International test protocols such as UN T and UL-1642 are followed, with voltage, current and temperature being monitored over several hours as test conditions change.
Not all cell testing is so extreme of course, and cell quality testing is an inherent part of pilot plant production as it would be in full scale commercial production. Cell capacity in mAh per gram and voltage are plotted to give the primary cell characteristics for the main cell types: 18650 cylindrical cells as used in laptop battery packs, ‘soft pack’ cells of the type used in mobile phones, as well as other specialized types.
Markets
With capacities up to 3600 mAh/g available from silicon anode materials, there are clear advantages to be gained in many market sectors. By allowing longer life between charges, operating advantages will be welcomed eagerly by users of smart phones, computers, tablets, cameras and all manner of other consumer gadgets. Not only are we increasingly reliant on these devices, but the ever-wider range of functions they perform leads inevitably to greater demands on the source of power: the rechargeable battery pack.
If 2011 is the year of the electric car, then the limitations of present battery technology have come into sharper focus, especially regarding the range which vehicles can achieve between charges. Higher power and lower battery weight are clear requirements for hybrid and all-electric cars and bikes. In a recent RAC report , the greatest concern for would-be adopters of electric vehicles was the distance they could travel on a single charge.
In yet another application, the growth of renewable energy technologies requires greater storage capacity for the energy generated, and the incumbent technology has reached its theoretical capacity limit.
Estimates of the predicted growth of rechargeable batteries in key markets are impressive; Takeshita Research for example predict a growth from $10 billion to
$60 billion by 2020. Pressure on disposal of one-time use batteries will ensure the demand for rechargeable alternatives for the foreseeable future.
In Nexeon’s case, materials evaluation is well under way with major global partners in the consumer electronics and automotive sectors, and the Company partners with leading industrial companies such as Axeon and Ricardo, while maintaining links with academic groups such as the University of St Andrews and Imperial College. Users of the latest technology could expect an increase in cell capacity of 30 – 40% predicted in the near term, and approximately 200% when improved cathode technology is introduced to harness the full potential of silicon anodes.
In the first part of this two-part article, Nexeon’s CEO Dr Scott Brown discussed how silicon can be used in place of carbon for battery anodes to boost the lifetime performance of the the next generation of Li-ion batteries.
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