Silicon holds the key to longer battery life – Part One
Lithium ion (Li-ion) technology is chosen today for most rechargeable battery applications, thanks to its excellent performance characteristics. Li-ion cells have a high energy density, and so high cell capacity for a given size and weight. They do not suffer from ‘memory effect’ or particular environmental issues; they have low self discharge rates and give good lifetime between charges. Battery modules based on Li-ion batteries are commonly found in applications from cell phones and digital cameras to portable computers and power tools.
Despite these inherent advantages, laptop users and mobile phone users (that’s all of us) will be only too aware of the limitations of battery storage capacity which result in short operating time between charges and consequently frustrating mobile experiences. Overcoming the present limitations in battery performance requires changes to the chemistry of the anode, cathode and electrolyte to allow better take up and release of the lithium ions. The most promising developments at the moment are coming from those looking into anode performance.
Current commercial Li-ion batteries use carbon as the anode material. Nexeon, based in Oxfordshire, UK, has shown that silicon used in place of carbon for battery anodes could be the key to the next generation of Li-ion batteries. Carbon can deliver only a maximum theoretical capacity at the anode of 372 mAh/g, yet the new range of silicon materials with differing morphologies and capacities have been found 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. Capacities available with various combinations of lithium and silicon are shown in the figure below. These are based on gravimetric capacity, but good volumetric results are also obtained.
So with such a clear capacity advantage why has silicon not been used in place of carbon before? Researchers in the 1970s had shown the capacity benefits of silicon, but had found 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 (see Figure 2). 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, developing technology first worked on in Prof. Mino Green’s laboratory at Imperial College, has been using high aspect ratio silicon as a solution to the problem of dimensional instability. Using special morphology ‘structured’ silicon in the form of micron-dimension pillars (see Figure 3), the inherent fragility of the silicon can be overcome.
To take the technology from laboratory demonstration to commercial readiness, the company has developed an advanced manufacturing approach that involves a room temperature chemical etching process that is fast and inherently low cost. No special grade of silicon is needed and most of the reagents can be recycled. Most importantly, this process is scalable for mass production. Using some of the $25 million raised so far from early stage investors, the company 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, making changing from carbon an easier transition for commercial battery producers.
Having recently moved to Milton Park, Nexeon has installed a fully automated and instrumented pilot plant at Milton Park, capable of producing several kilogrammes of material a day – more than enough for evaluation in customer 18650 cells and equivalent to the production of over a million cells a year.
Nexeon also has the advantage of being able to produce fully functional prototype cells itself, allowing the technical and economic advantages of its materials in battery manufacture to be understood. It also enables the materials to be optimized for maximum performance. 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. 18650 cells are a standard size in commercial production, and this allows comparison with existing commercial performance.
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 3.55 Ah compared with typical carbon-based cell performance on a commercial scale of 2.5-2.7 Ah. The short-term target now is to create a 4.00 Ah cell. Optimization through ongoing development and volume scale effects should allow even higher capacities as manufacturing progresses.
In the next part of this two-part article, Nexeon’s CEO Dr Scott Brown will look at safety issues and the likely impact on the market of higher performance batteries.