The basic energy storing units in any EV packs are battery cells. Each cell contains energy storing cathode and anode materials layered with inactive packing (membrane, electrolyte, current collectors and a shell) that helps to extract energy contained in the form of electrons. The theoretical energy density is defined by the specific capacity of the cathode and anode materials used as well as by the cell’s voltage. The practical energy density of the cells is typically much lower than its theoretical energy density but improvements to cell packaging and electrode preparation are increasing the storage capacity continuously.
Lithium-ion (Li-ion) batteries -- the current enabling technology for EVs -- come in a few chemistry flavors that are expected to expand in the near future. Li-ion batteries have the advantage of high energy density, relatively light weight and the ability to retain capacity after hundreds of recharging cycles. While they are the fastest growing segment of the battery industry, their technology is far from mature even after nearly 40 years of development. Most leading suppliers are planning radical changes -- from anode to cathode to electrolyte and format -- over the coming decade in order to eliminate flammability and increase energy density, among other improvements.
The top five battery cell manufacturers (Panasonic, LG Chem, and Samsung SDI, China’s BYD and CATL, and dozens of up-and-coming manufacturers) are all ramping up capital expenditures with a view to tripling capacity by 2020 (to 300 GWh). Current commercial batteries use liquid electrolytes and predominantly graphite (G) or lithium titanium oxide (LTO) anodes in combination with different cathodes: lithium cobalt oxide (LCO), lithium nickel cobalt manganese oxide (NMC), nickel cobalt aluminum oxide (NCA), lithium manganese spinel (LMO), or lithium iron phosphate (LFP). The most immediate advances in battery materials are expected to be nickel-rich cathodes and silicon-doped anodes. The higher theoretical energy density will help lower costs per kWh by as much as 50%. The table below summarizes the chemistry of cells produced by major battery cell manufacturers for different EV makers.
Prices for Li-ion cells have been decreasing at an average rate of 6-8%/year from over $1000/kWh in 2010. The current cost of EV battery cells is a bit speculative, with GM reporting cells for 2016 Bolt EVs provided by LG Chem costing $145/kWh and with Tesla cells being rumored to cost $190/kWh. It is unclear whether current prices reflect real costs, as big battery manufacturers may be offering significant discounts and sacrificing margins for market share in hopes that manufacturing costs will eventually come down with scale.
Due to the cost of raw minerals, the price of Li-ion batteries has a hard lower limit, estimated to be ~$125/kWh at the cell level and $150/kWh at the pack level. It is projected that this price will be reached by 2022 due to improvements in materials, processes and automation, coupled with new cell designs and higher volumes. Some analysts expect this to happen sooner, although increasing demand for raw minerals may keep costs higher than expected.
Innovation in the Li-ion battery space continues on many fronts, including new cathode and anode materials, membranes, electrolytes, electrode processing and battery assembly. Toshiba has recently announced a next-generation lithium-ion battery in their SCiB line planned for commercialization in 2019. This battery features a new anode material (lithium titanium niobium oxide, LTNO) which stores lithium ions more efficiently by using a proprietary method of synthesizing and processing. This new material provides twice the capacity of the standard graphite anode and is able to maintain 90% of its initial capacity after 5,000 charge/discharge cycles at ultra-rapid 10C charging rate.
Extension of cycle life is another characteristic of Li-ion batteries in electric vehicles that continues to improve. A collaboration between Dalhousie University and Tesla Canada Industrial Research has resulted in doubling the cycle life of high voltage NMC cathodes by coating them with a “certain aluminum coating” in order to limit gas generation. The tested cells showed barely any degradation after 1200 cycles at moderate temperature (95% of their original energy capacity) and only slight degradation under harsh conditions. Currently Tesla EV packs use NCA cathode chemistry because of the lower cost, higher energy density and better tolerance to fast charging than NMC cathodes (used by Tesla in Powerpack and Powerwall applications). The advancement in cycle life at high cell voltages with NMC cathodes could increase pack energy density and would help an EV battery last for over 300,000 miles.
Despite all the improvements to Li-ion batteries they still have an inherent ability to short, overheat and catch fire because of minute faults in the manufacturing process (for example, faults in separators that keep apart chemicals in the cells), or because of mishandling during transport. These issues have spurred other improvements in Li-ion technology which include new solid and liquid electrolytes, eliminating or replacing the polyethylene separator with less flammable options such as ceramic-coated or Kevlar membranes. It is projected that swapping liquid electrolytes for solid ones could provide a 15-20% increase in energy storage capacity. Ionic Materials is developing rechargeable solid-state alkaline batteries that use a plastic-like polymer as electrolyte and standard battery chemicals like zinc and manganese dioxide. Because all these components are solid and flexible, such a battery can be molded into different shapes. The idea of deformable electrodes is also explored by 24M, which has developed dough-like, semi-solid cathodes and anodes that can be made into thicker, soft electrodes. This approach decreases the amount of packaging materials, providing up to 25% greater energy density and a 25% lower cost than conventional lithium ion batteries.
The most anticipated competitor to Li-ion technology is an all-solid-state battery with a Li-metal anode combined with solid electrolyte and high voltage cathode. Since these types of batteries are thought to be much safer than common Li-ion cells, weigh less, charge faster and require less packaging and housing infrastructure, it is broadly anticipated that by 2022 lithium-metal anodes will replace graphite.
Earlier this year a team from the University of Texas at Austin reported breakthrough glass electrolytes enabling the use of an alkali-metal anode. Over 1200 charge/discharge cycles, the research team seemed to have solved the dendrite growth problem that comes with fast-charging cells, with a technology that could decrease costs once in mass production. Polyplus battery company, also, has recently released glass-protected Li-metal batteries featuring a solid-state lithium anode laminate integrated into existing battery technology (NMC cathode) which doubles the energy density of current rechargeable batteries.
Several automakers are reportedly working on their version of all-solid-state batteries. In 2015 Bosch acquired Seeo Inc., a California-based startup with exclusive licensing rights to Lawrence Berkeley National Laboratory’s portfolio of patents for lithium polymer battery cells. The new technology is expected to allow the production of 50 kWh battery packs weighing less than 200 kg. VW has a stake in a solid-state focused startup, QuantumScape from Silicon Valley, while BMW is also rumored to be working on a solid-state battery. Toyota’s all-solid batteries are targeted for commercialization in 2020 with an all new electric vehicle release in 2022.
Dyson Electric has announced that it is working on a luxury, “radically different” electric car expected to reach the market in 2020. The car’s electric motor is ready, while two types of batteries, including solid-state batteries, are under development. Dyson has acquired the University of Michigan’s spin-off Sakti3, whose next generation solid-state technology with lithium metal anodes can store almost twice as much energy as traditional rechargeable batteries. With 400 Wh/kg vs. the 265 Wh/kg available from today’s best Li-ion batteries, the new batteries will be safer, will recharge more quickly and will hold a charge longer.
Fisker was working with Nanotech Energy to develop graphene supercapacitor technology for its new car, but recently announced that the EMotion super sedan (400+ mile range, 145 kWh pack, scheduled to hit the markets in 2019) will be using advanced NMC cells provided by LG Chem. According to the company, the graphene supercapacitor technology was not ready and they did not want to miss the expected EV release date. The company continues its research into solid-state supercapacitor cells aiming for "a unique solid state battery pack" that will solve thermal issues, be able to fast-charge and deliver over 500-mile range, per recently filed patents. Fisker’s original EV (Karma), despite its striking looks, was dealt a nearly fatal blow in 2012 with repeated recalls of its A123 Systems batteries. Henrik Fisker says he is "taking lessons learned, which includes make sure the battery works first.”
Many other battery technologies are currently in development (sodium-ion, magnesium-ion and other metal anodes, and sulfur and air/oxygen cathodes, as well as high energy density flow batteries), and may have significant advantages over the most anticipated technologies, but have not made it to the productive end of the “Hype Cycle” yet.
Li-ion technology has had decades of development to refine the manufacturability, choice of electrolyte, and the nanoscale characteristics of electroactive materials, turning it from an intriguing idea into a dominant technology. It is reasonable to expect that a similar effort will be required of any competing technology.
It is interesting to note the projects that have current support from the Department of Energy (DOE) Vehicles Technologies office. Battery500 consortium, whose goal is to “triple the energy density of today's electric cars from 170-200 Wh/kg to 500 Wh/kg”, announced 15 battery seedling projects this July. Of these projects, six are working on improving the Li-metal anode, six on sulfur cathodes, and three on high energy density cathodes, including projects by Mercedes-Benz R&D North America and General Motors, LLC which are proposing advancements for Li-sulfur batteries. These projects are considered to be “the best options to create the most powerful next-generation lithium batteries for electric cars.”
EV innovation relies not only on batteries: electric motors, converters and transmissions, materials and designs can also contribute to extend the range of cars with current battery technologies, as proven by 620 miles range Tesla Roadster 2. Nevertheless, more breakthroughs in battery technologies will be necessary to electrify other modes of transport, such as air and water vehicles.
Contributors are Elena V. Timofeeva, Carlo. U. Segre and John P. Katsoudas.