Alternative Chemistries to Li-ion for E-mobility

Next-generation Batteries

Li-ion Batteries and their Role in E-mobility

The automobile industry worldwide is rapidly steering away from internal combustion engines, thereby increasing the prominence of Electric Vehicles (EVs). Electric vehicles display the potential to reduce greenhouse gas emissions and decrease the dependence of the automotive sector on oil. In the near future, EVs including Hybrid Electric Vehicles (HEVs), Plug-in Hybrid Electric Vehicles (PHEVs), and Pure Battery Electric Vehicles (BEVs) will dominate the clean vehicle market.

By 2020, it is expected that more than half of the new vehicle sales will likely be of EV models. The key and enabling technology for this revolutionary change is the battery. EV batteries are quite different from those used in consumer electronic devices, such as laptops and cell phones. EV batteries are required to handle high power (up to a hundred kW) and high energy capacity (up to tens of kWh) within a limited space and weight and at an affordable price. Extensive research and investments have been done in advanced battery technologies that are suitable for EVs. The two current major battery technologies used in EVs are Nickel Metal Hydride (NiMH) and Lithium-ion (Li-ion). At present, Li-ion batteries are representing the most used technology in electric vehicles, particularly in PHEVs and BEVs, owing to their high energy density and increased power per mass battery unit. These advantages of Li-ion batteries are allowing the development of some types of batteries with reduced weight and dimensions at competitive prices.

Significant advancements in lithium-ion battery technology have been a game-changer in the automobile sector. Cheaper, more-effective lithium-ions are currently taking over the battery market. By 2025, it is estimated that the rechargeable battery market will grow considerably, reaching more than $100 billion, while the market share of lithium-ion will be more than 70%. The key surge in this demand for Li-ion batteries has been due to recent developments in automotive segments. Estimates suggest that the global EV production will increase from 3.2 million units in 2017 to 13-18 million units by 2025, ultimately reaching 26-36 million units by 2030. There are several drivers that impact the extent and speed of global EV adoption, including government regulations and targets, future battery costs, the availability of EV charging and servicing infrastructure, OEM’s automotive platform choices, and consumer preferences.

Although the Li-ion battery market is fairly established with a robust supply chain, there are some concerns associated with raw materials that make up the current Li-ion battery, and thus, there is a substantial market appeal to battery chemistries beyond Li-ion. Any alternative chemistry that can help overcome limitations associated with the current Li-ion technology can potentially revolutionize the e-mobility industry.

Typical Battery Material Composition for Li-ion Batteries

Each lithium-ion cell contains 4 major parts: Cathode, Anode, Electrolyte, and Separator.

A lithium-ion battery is a rechargeable battery in which lithium-ions move between the anode and cathode, creating electricity flow useful for electronic applications. In the discharge cycle, lithium in the anode (carbon material, usually graphite) is ionized and emitted to the electrolyte. Lithium-ions move through a porous plastic separator and are inserted into atomic-sized holes in the cathode (lithium metal oxide). At the same time, electrons are released from the anode. This becomes the electric current traveling to an external electric circuit.

When charging, lithium-ions move from the cathode to the anode through the separator. Since this is a reversible chemical reaction, the battery can be easily recharged. While the anode and electrolytes are pretty straightforward as far as lithium-ion technology goes, it is the cathode where most developments are being made. Lithium is not the only metal that goes into the cathode – other metals such as cobalt, manganese, aluminum, and nickel are also used in different formulations. Mentioned below are four cathode chemistries, the metal proportions (excluding lithium), and an example of what they are used for, such as Lithium-Nickel-Cobalt-Aluminum (NCA), Lithium-Nickel-Manganese-Cobalt (NMC), Lithium-Manganese-Spinel (LMO), Lithium Titanate (LTO), and Lithium-Iron Phosphate (LPF). While manganese and aluminum are important for lithium-ion cathodes, there are cheaper metals with significant markets. Lithium, graphite, and cobalt have much smaller and less-established markets, along with supply concerns that remain unanswered.

Lithium: Countries in the “Lithium Triangle” host a whopping 75% of the world’s lithium resources: Argentina, Chile, and Bolivia.

Graphite: 65% of flake graphite is mined in China. With poor environmental and labor practices, China’s graphite industry has been under scrutiny, and some mines have ceased operations.

Nickel: Price swings of nickel can impact battery makers. In 2014, Indonesia banned exports of nickel, which caused the price to soar nearly to 50%.

Cobalt: 65% of all cobalt production comes from the DRC, a country that is extremely politically unstable with deeply-rooted corruption.

While all sorts of supply questions exist for these energy metals, the demand situation is much more straightforward. Consumers are demanding more batteries, and each battery is made up of raw materials, such as cobalt, graphite, and lithium. Owing to this, pertinent researchers are resorting to alternative chemistries that would deliver performance comparable to that of the existing Li-ion batteries. In addition, carmakers across the globe are exploring new battery technology. This, in turn, is leading to more research into lithium-ion alternatives and encouraging further cost reductions for the dominant technology.

The drive to find alternatives to lithium-ion concoction is bound to continue as the world prepares for a fossil-fuel-free future.

Potential Alternative Chemistries to Li-ion

Sourcing raw materials for lithium-ion batteries will be critical for the energy mix. However, there are several other battery technologies that could help in solving the most pressing energy issues. The following are a few of the promising chemistries that can substitute Li-ion batteries for mobility needs in the near future.

Magnesium-ion Batteries

Magnesium-ion batteries could serve as an alternative to lithium-ion batteries in electric cars and grid storage. Such batteries would use a cathode and an electrolyte similar to that of lithium-ion. However, the anode would be critically different. A typical Mg-ion battery would not make use of graphite, or any sort of intercalation anode, and would directly use magnesium metal.

On paper, magnesium-ion offers a tremendous potential energy boost over lithium-ion—possibly as much as two-to-one. In theory, such capabilities would enable automakers to use batteries that are half the size, while offering the same power. However, such advancements face several technical challenges and are still far from the prototype stage.

Lithium-sulfur Batteries

Research is being conducted to examine the possibility of replacing the graphite in a conventional lithium-ion anode with lithium metal and using sulfur for the cathode, instead of an expensive inorganic solid (such as cobalt). That combination would result in vastly lower costs, as sulfur is said to be literally “cheaper than dirt.”

In the past, lithium-sulfur has been plagued by low-cycle life and problems associated with the use of flooded electrolytes. Scientists are working on practical solutions to address such issues.

Nickel-zinc Batteries

Nickel-zinc batteries are cost-effective, safe, non-toxic, eco-friendly batteries that could compete with Li-ion batteries for energy storage. However, the main barrier for commercialization has been the low cycle life of nickel-zinc batteries.

Chinese researchers from the Dalian University of Technology have developed a breakthrough in-situ cutting technique to improve the performance of Ni-Zn batteries by solving the issue of Zn electrode dissolution and suppressing the formation of dendrites. The team developed a novel graphene-ZnO hybrid electrode with the in-situ cutting technique, which can cut graphene directly into short nanoribbons. Strong interatomic interactions anchor Zn atoms onto graphene surfaces. This approach thoroughly fixes the issues of Zn electrode dissolution, dendrite formation, and performance.

With ongoing research and approaches undertaken by companies, these batteries show immense potential for widespread commercial applications of EVs and energy storage.

Silicon-based Batteries

Li-ion batteries have traditionally used graphite anodes; however, researchers and companies are presently focusing on silicon anodes. Si-dominant anodes can bind Li-ion 25 times more than graphite ions. However, these batteries suffer from low electrical conductivity, a slow-diffusion rate, and large volumetric fluctuations during lithiation. These limitations result in Si pulverization and instability of the Solid Electrolyte Interphase (SEI).

Two primary strategies have been used to circumvent these challenges: nanotechnology and carbon coating. In the former method, various nanosized Si anodes are used, which have a high surface area as well as improved cycle life and rate stability as compared to bulk Si anodes. They can withstand lithiation and delithiation without cracking. Carbon coating uses a combination of nanosized Si with different forms of carbon materials for the generation of high-performance Si/C nanocomposite anodes. Recently, doped carbon with heteroatoms as coating agents has attracted a lot of interest. The heteroatom-doped Si-C electrodes bind Li-ions more strongly than carbon atoms, leading to an excellent electrochemical performance with stable electrical conductivity.

Si-based batteries have generated a lot of commercial interest due to their low cost and enhanced capabilities for cars and smartphones. The competition is fierce, with many start-ups, including Sila Nanotechnologies, Enovix, Angstron Materials, and Enevate, planning to commercialize Si-dominant Li-ion batteries.

Sodium Metal batteries

Stanford researchers released a paper claiming that their sodium batteries could compete with lithium-ion batteries. The Stanford battery uses sodium—a cheaper, more abundant material than lithium—and is still in the development stage.

The cathode of this battery is made up of sodium, and the anode is made from phosphorus, with the addition of a compound known as Myo-inositol, which can be derived from rice bran or corn. According to researchers, this chemical combination yields efficiency rates comparable to that of lithium-ion batteries at a lower cost. The main advantage of the sodium battery lies in the fact that sodium is much more abundant than lithium, and it costs $150 per ton versus $15,000 for lithium. This, along with the performance optimization efforts of researchers, has turned their battery into a cost-effective alternative to Li-ion batteries. However, the new battery development has been fairly slow against the backdrop of the projected electric car market size, and so far, no innovation has proved to be as economical as lithium-ion.

Aluminum-Air and Aluminum-ion Batteries

Aluminum-ion and Lithium-ion batteries are very similar, except that the former has an aluminum anode. Aluminum-ion batteries provide increased safety and faster charging time at a lower cost than lithium-ion batteries; however, there are still issues with cyclability and life span. Stanford University is a leading developer of aluminum-ion batteries that incorporate a graphite cathode. The research holds the potential for making cheap, ultra-fast charging, and flexible batteries, with thousands of charge cycles, in addition to being a safe, non-flammable option with a high charge storage capacity.

Aluminum-air flow batteries for EVs outperform the existing lithium-ion batteries in terms of higher energy density, lower cost, longer cycle life, and higher safety. Aluminum-air flow batteries are primary cells, which means that they cannot be recharged via conventional means. In EVs, they produce electricity by replacing the aluminum plate and electrolyte. Considering the actual energy density of gasoline and aluminum of the same weight, aluminum is superior.

Solid-state Batteries

Arguably the greatest potential for a breakthrough in battery technology lies in the development of solid-state batteries. Unlike lithium-ion batteries, solid-state lithium batteries have no liquid electrolyte and offer much higher energy density, about twice that of lithium-ion batteries. Solid-state batteries have solid elements, providing several advantages: less fire-related safety issues, extended lifetime, decreased need for expensive cooling systems, and operable in an extended temperature range. However, their biggest advantage lies in the manufacturing process, which is similar to the existing industrial processes. With economies of scale in production, electric cars would be cheaper to make and buy.

Outlook for the Future

Researchers are addressing issues that prevent different alternative technologies from reaching commercialization, including poor cycle life, low power, low efficiencies, and issues with safety. Lowering the cost and improving the performance of batteries for PEVs require improving every part of the battery, from underlying chemistry to packaging.

All alternative chemistries examined in this article are not commercialized as of yet, but they are just a major breakthrough away from toppling Li-ion batteries as the market leader for battery production for electric vehicles as well as for stationary energy storage. The most promising alternative in the immediate future seems to be silicon-based batteries. Silicon-based batteries, although cost-effective, still have some inherent limitations that prevent them from replacing the current Li-ion batteries for electronics and mobility applications. The growing interest in silicon-based batteries is evident from the research and development from a host of different start-ups and established players, who are actively trying to overcome limitations associated with these batteries. Major advancements in the field of nanotechnology and thin-film coating can help overcome inherent limitations with silicon-based batteries, and thus, provide a viable alternative to Li-ion batteries. Moreover, silicon-based batteries not only promise similar performance to Li-ion batteries at a much lower cost but also empower battery manufacturers to move away from using graphite in batteries, which has a host of supply issues associated with it.

Presently, intense competition among various battery chemistries has propelled the development of powertrain technologies and mobility business models. Although it is fairly evident that electric vehicles are going to be the way of the future, it remains to be seen which technologies and chemistries would drive to that future.