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The limitations of current lithium ion battery technology are becoming increasingly apparent and are set to worsen. However, the use of silicon in place of carbon for battery anodes could be the key, as Scott Brown explains
Our lifestyles would be hard to imagine without batteries. The plethora of electronic devices we carry around, from cell phones, tablets and portable computers to digital cameras, depend on specially designed battery modules. The technology of choice is lithium ion (Li-ion) rechargeable batteries. As well as avoiding the high cost and environmental disposal problems of disposable cells, Li-ion batteries have the best performance characteristics: in particular, the lifetime between charges, the cell capacity and the size and weight of the batteries.
However, the limitations of current products will be readily apparent to anyone whose cell phone has died at a critical moment, or who can get only a fraction of the claimed working life from their laptop while on the move. These problems are set to worsen as electronic products feature increased power and functionality, and sport larger and brighter display screens. Yet current Li-ion battery technology has almost reached its theoretical capacity limit and future performance is capped by the fundamental performance of the materials used.
Nexeon, based in Oxfordshire, has shown that silicon used in place of carbon for battery anodes could be the key to the next generation of Li-ion batteries. By way of comparison, carbon can deliver only a maximum theoretical capacity at the anode of 372 mAh/g, yet Nexeon is developing a range of silicon materials with differing morphologies and capacities which are capable of capacities up to 3600 mAh/g. At the cell level, this translates to an increase in energy capacity of 30—40% in the near term and approximately 200% when improved cathode technology is introduced to harness the full potential of silicon anodes.
In fact, while researchers have been able to show that silicon is a far superior material for use in Li-ion battery anodes, and the capacity benefits of silicon had been demonstrated on both a weight and volume basis, there had been no commercial success realised. The reason for this lies in the fact that silicon suffers from dimensional instability when repeatedly charged and discharged. When charging a Li-ion battery, lithium is inserted into the silicon, causing a large increase in volume, and the process is reversed on discharge. This 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. The typical result is a shortened charge-discharge cycle life for conventional silicon-based anodes.
Nexeon has a solution to the problem of silicon’s dimensional instability, based on the use of high aspect ratio silicon. The company has developed a unique advanced manufacturing technology to produce a special morphology ‘structured’ silicon in the form of micron-dimension pillars. Viewed at high magnification, Nexeon’s silicon material variously resembles a hedgehog, or sometimes a pile of straw! The manufacturing process involves a chemical etching process in which the structured silicon is washed and dried to achieve good porosity. Most importantly, this process is scalable for mass production. Nexeon’s high aspect materials are produced in a room temperature etching process that is fast and inherently low cost. No special grade of silicon is needed and most of the reagents can be recycled.
Battery OEMs readily identify with the opportunity and the scale of potential improvement that silicon offers. They have showed themselves eager to evaluate the approach. Not only does Nexeon’s approach give its customers the opportunity to produce higher performance batteries at lower cost, but also potentially expands the range of applications which rechargeable cells can satisfy.
In the related field of electric vehicles (EVs), advanced battery technology is critical if they are to become fully viable and economic. The EV industry is vigorously searching for superior approaches to improving power to weight ratio, range, and an affordable solution to vehicle power. Major automotive corporations are urgently seeking solutions to accelerate the rate of introduction of electric vehicles. The advantages of lighter Li-ion batteries with more energy and power are highly relevant to low carbon vehicles. Nexeon’s technology will be optimised for the higher rate charge and discharge required for automotive applications (i.e. HEVs, PHEVs and BEVs). Nexeon is part of a consortium recently awarded over $1.5 million by the UK’s Technology Strategy Board to produce a prototype for a plug-in electric vehicles (PHEV) application. The current global size of the Li-ion battery market is estimated at $10 billion, according to independent market analyst Takeshita, with a predicted market of $30 billion by 2017.
So to recap: the benefits of adopting silicon anodes for Li-ion cells include: longer time between charges, greater cell capacity and improvements in size / weight performance. In addition, safety margins should increase, and manufacturing costs should be lower.
However, as many fledgling companies have found, establishing commercial traction requires more than good technology performance, it requires adoption and buy-in from the supply chain. Here again Nexeon seems to have the advantage. A significant factor in encouraging rapid uptake of silicon anode technology is that disruption to existing battery manufacturing lines is minimised by a ‘drop in’ approach, requiring only specific parts of the operation to be changed to allow silicon to be used.
The Nexeon management team is keenly aware of the need to understand and develop the battery manufacturing process as well as the materials, and for this reason, it has invested in new dry rooms and prototype cell production lines to demonstrate that its silicon materials can be readily used in traditional processes and cell designs. Nexeon has raised over $25 million in funding, and has in place a strong multi-disciplinary technical team and a highly experienced senior management team that between them has 150 years of experience in technology development and commercialization.
It has installed a fully automated and instrumented pilot plant at Milton Park, capable of producing more than 1 kg of material a day (enough material for approximately 500 18650 cells). Nexeon has the advantage of being able to produce fully functional prototype cells, and this allows the technical and economic issues in battery manufacture to be understood, and the materials optimized to suit. Currently the plant is used to produce 18650 cylindrical cells and ‘383562’ soft pack cells. A coin-type cell is also produced for testing purposes. Producing standard sized cells also allows comparison with existing commercial types, and in December 2010, Nexeon announced that it had produced cells that exceeded the current best known performance from commercial cells – effectively a new world record for Li-ion batteries. Prototype 18650 cells based on the silicon anode have been made at a capacity of 3.55Ah compared with typical cell performance on a commercial scale of 2.5-2.7Ah. The short-term target now is to create a 4.00Ah cell. These figures are all the more impressive given that further optimization should be possible as manufacturing progresses. The Company hopes full-scale manufacture of its materials will commence in 2013.
Building on work carried out at Imperial College, London by Professor Mino Green’s team in the Department of Electrical and Electronic Engineering, Nexeon has a growing portfolio of patent families relating to Li-ion batteries and some patent families specifically relating to silicon, including patents on high-aspect ratio silicon materials and the use of these materials in lithium ion batteries. Nexeon has developed a number of proprietary processes and equipment for producing silicon anode materials and for making electrodes.
In the last two years, Nexeon has won more than its fair share of accolades, including: two Rushlight awards – including the award for ‘The most significant achievement towards the exploitation and adoption of clean fuels’; a nomination as one of the ‘Top 20 fastest growing Clean & Cool companies in the UK’ and a place on the Clean & Cool Mission to the USA in 2010. It has been given a Global Cleantech/Guardian 100 Company listing and a listing as one of Cleantech Connect’s Top 15 list of ‘Ones to Watch’.
The future looks bright for silicon anodes, and materials are already being sampled to battery OEMs and branded products companies around the world with a view to licensing its technology and supplying its materials in commercial volumes.
Soon you may get that report finished on the train after all.
Dr Scott Brown is CEO of Nexeon
