Battery challenges for electric vehicles
Car manufacturers place extraordinary demands on the power density, energy density and safety of the lithium-ion batteries they use, pushing the development of batteries that combine high performance with low costs. As result, over the past four years, research and development on batteries for BEVs and plug-in hybrid electric vehicles funded by the US Department of Energy has led to cost reductions of approximately 50%. During the same period, battery energy density increased by 150%.
The auto industry also demands a large number of cells to make batteries for BEVs, pushing the development of a considerable market. For instance, Tesla Motors, a luxury electric car manufacturer, will deliver 35,000 cars by 2014 and expects to enter the mass market, delivering 500,000 electric vehicles by 2020. Other manufacturers have similar plans to enter the mass markets with BEVs at affordable prices in the next few years.
Panasonic’s lithium-ion (Li-ion) battery division is resurgent thanks to Tesla Motors. In the second quarter of 2013, it made about US$40 million in profits, a turnaround from one year before, when it lost US$20 million. Panasonic went to the top of the list of cell suppliers for electric vehicles thanks to the use of several thousands of its cylindrical 18650-type cells in Tesla Motors’ Model S luxury sedan, an electric vehicle that packs a massive 60 kWh to 85 kWh worth of batteries. About 16,000 Model S units have been sold thus far, but a mere 20,000 Tesla Model S units use three times more battery capacity than 230,000 Toyota’s popular Prius hybrid family.
Accessibility to more, and cheaper, cells is key to ramp up BEV production further and target the mass market, as Tesla intends to. So it caused a stir in the industry when Tesla announced earlier this year, plans to build its own lithium-ion battery factory based in the US. The Teslas’ US$ 5 billion ‘Gigafactory’, as it has been called, is designed to reduce costs of cells for battery electric vehicles much faster than the status quo and, by 2020, produce more lithium ion batteries annually than were produced worldwide in 2013. This means that this factory is designed to churn out cells for BEVs with a combined energy storage capacity of 35 GWh per year. Whether these plans are implemented as announced, or not, it illustrates the extent to which growth in the electric vehicle market and the battery industry are now intertwined.
Despite this progress, the wider market success of BEVs will strongly depend on further improvements in the specific energy (the amount of storable charge per weight) of commercially available batteries. Unless the specific energy of Li-ion cells could be increased above their current maximum of around 250 Wh/kg, the autonomy of BEVs will be much lesser than that of cars with combustion engines.
The anode is the secret
The strategy for increasing the specific energy of batteries, is to store larger amounts of lithium in the anode. At the present, commercial lithium-ion batteries use nanostructured graphite anodes that are capable of storing lithium ions between the graphene layers in a process called lithiation. This type of anode has a theoretical specific capacity of about 370 mAh/g, that is too little to address mass market BEVs challenges. In the other hand, silicon is an attractive material for anodes because it has a theoretical specific capacity of up to 4,200 mAh/g, ten times the theoretical capacity of the state-of-the-art graphite anode. Silicon anodes can be used both in traditional lithium-ion batteries and in more recent Li–O2 and Li–S batteries. But the main challenges associated with silicon anodes is that repeated changes in volume (~ 300%) during battery cycling causes a rapid structural degradation of the anode as well as instability of the solid-electrolyte interphase, that forms at the silicon surface.
To address this problem, researchers have begun to work at the nanoscale engineering of silicon-based anodes in order to accommodate these large volume changes. One example is the hierarchical structured silicon anode proposed by Yi Cui and his colleagues at Stanford University. The design is inspired by the structure of a pomegranate, a fruit considered to have originated in Iran, where single silicon nanoparticles are encapsulated by a conductive carbon layer that leaves enough room for expansion and contraction at lithiation and delithiation. Using a multistep synthesis process, the researchers embedded silicon nanoparticles in a carbon framework with internal void spaces that accommodate the vast volume increases during charging. This carbon framework allows lithium ions and electrons to be transported while keeping the electrolyte — and the formation of the solid–electrolyte interphase — away from the silicon nanoparticles. As a result of this hierarchical arrangement, the solid-electrolyte interphase remains stable and spatially confined. The anode developed by the researchers can maintain a specific capacity of over 1,160 mAh/g after 1,000 lithiation cycles. In addition, the microstructures lower the electrode–electrolyte contact area, resulting in high Coulombic efficiency and volumetric capacity.
Although the anode remains stable even when the real capacity is increased to the level of commercial lithium-ion batteries, the material is yet to be tested in a cell with a commercial cathode. For applications in battery electric vehicles, the anode would also be required to sustain a performance of over 2,000 cycles or more.
Despite the advances that have still to be made, these latest results illustrate the potential of nanoengineering at lithium-ion batteries and narrow the focus of start-up companies and larger manufacturers to silicon anodes. However, even large manufacturers face considerable technological challenges. For instance, Panasonic’s first commercial 18650-type cell with a silicon-based anode, which had been expected in 2013, has already been delayed.
But what will dictate the success of electric vehicles at mass markets is primarily the cost of batteries. Today, the cost of electrode materials already accounts for a significant proportion of the cost of Li-ion batteries. Less accessible base materials and complicated as well as lengthy syntheses will probably make high-capacity anodes based on nanostructured silicon more expensive than the graphitic ones that are being used at present. That is the reason why Tesla Motors is investing five billion dollars in a battery Gigafactory. Cheaper and more efficient cell fabrication processes wont account for cost reduction as much as the economies of scale of the gigaproduction achieved at Teslas’ Gigafactory.
By André Sionek
