Can new batteries allow electric cars to keep their promises?
The issue of electric range remains one of the largest stumbling blocks between consumers and the decision to switch to an electric vehicle. With a host of new battery technologies racing to solve this challenge, we outline the new developments that aim to banish range anxiety once and for all.
Every few years, a research company or two claims to have a breakthrough battery technology that will put large products like electric cars fully into the mainstream. In the last decade, several have come forward with either new materials or new techniques that will make batteries lighter, stronger and longer lived.
Battery buyers around the world have concerns about the current standard component: lithium. Lithium-based (Li-ion) batteries in all sizes power everything from phones to cars. Lithium is expensive to extract and fragile when worked. The element is abundant but never found as a solitary ore; its reactivity with air and water means it is always embedded in other materials that keep it from deteriorating.
Mining for lithium has an environmental impact, as does recycling and disposal. This is a problem with most mining operations and does not disqualify lithium from future manufacturing considerations, but the industry is constantly searching for better materials.
Liquids and solids
Perhaps the most intense research in battery materials is to find dense solid materials that will catalyse and recharge. If materials research can make the electrolytic connection with solids instead of liquids, the battery game will change forever. Li-ion batteries still hold the top spot in performance charts, as noted in this review of current Li-ion battery technology. Looking at the structure and function of Li-ion and other materials, this report sheds light in the problems facing solid-state battery research.
Using solid-state with Li-ion is one aspect of research. The lithium battery material produces dendrites that eventually short out the cell. Solid electrolytes prevent or delay dendrite formation but the passage of ions through solids is many times slower than in liquids. The result is a battery that cannot recharge quickly and discharge with enough power.
“The ideal solid-state battery replaces the liquid electrolyte and separator with a solid electrolyte that is impenetrable to Li metal dendrites, thereby enabling Li metal as the anode,” the report said. A lithium anode would be ideal in this situation.
New materials combined in unique ways will begin the process of finding suitable solid electrolytes. There are exciting new discoveries and some new uses for old materials now being developed. Let’s look at some materials considered high priorities for solid-state batteries:
Primary goals in finding a solid electrolyte include safety, cost and weight reduction. Some materials offer one or more solutions but have drawbacks. Sulfides fall into this category. Sulfides offer a more solid interface between the anode and electrolyte, but they are less chemically stable than other materials.
Newly developed sulfides are highly conductive as shown in this formulation: “high conductivity of more than 10−3 S cm−1 has been reported in crystalline Na3PSe4*”. This research developed a more conductive sulfide for Li than Na but the results are promising for both. One drawback of sulfides is their tendency toward hydrolysis, or breaking down by exposure to water. Researchers are trying to suppress this as a major part of sulfides research, since it is also a safety issue: sulfides release harmful H2S during hydrolysis.
In early 2018, Ionic Materials announced the development of a solid polymer that conducts ions at room temperature. The Massachusetts company is understandably keeping much of this discovery under wraps, but has expanded facilities and received significant investments from battery and car companies.
Like most battery research, investigators start small and find ways to go larger as the product is tested and approved. The company opened a new research facility in May 2018 and released news that’s as close to a description of the product as we are likely to get: “The special properties of Ionic Materials’ polymer electrolyte allow the use of high-energy materials and support lithium-ion cells with little to no cobalt in their cathodes.”
Solid polymer would be ideal in batteries for several reasons. They will be safer and lighter, per power unit, than liquid-electrolytes. The products coming from this research seem ripe to explode onto the market by 2022. Several companies researching solid state batteries have been bought by big electronics or automotive firms in recent years. Apple, Dyson and Bosch all bought facilities recently.
Like polymers, some breakthroughs have occurred lately in oxide solid-crystal electrolytes. A garnet-type molecule in polycrystalline form is likely to short in charging and discharging, but a single-crystal form appears to be a fast Li-ion conductor that does not short out. In this report, researchers believe they have found a workable option: “We are the first to successfully grow centimetre-sized single crystals of garnet-type by the floating zone method. The single-crystal solid electrolyte exhibits an extremely high lithium-ion conductivity of 10−3 S cm−1 at 298 K. The garnet-type single-crystal electrolyte has an advantageous bulk nature to realize the bulk conductivity without grain boundaries such as in a sintered polycrystalline body,” the report said.
The search for better battery materials might not find a new material but new uses for old materials, even those used as conductors. Magnesium is getting a new look as possibly the “perfect but complex” material for electrolytes. Magnesium has two positive ions while lithium has one. Magnesium can store twice the energy of lithium in the same volume.
Problems with magnesium as a solid electrolyte occur due to the ions’ reluctance to move as energy through the battery. They move well at high temperatures over 400 degrees F, obviously far too hot for battery use. However, one company has a test completed of magnesium ions traveling at room temperature. The prototype is being designed.
In the air
What could be better than having stored, transmittable energy made from the most abundant elements on earth? This question could be answered some day with the advent of air-reactive batteries. First up is aluminium-air. The basis for the reaction with metals as electrolytes is oxygen derived from water. Aluminium is cheap and plentiful. This battery works on fresh water or salt water added through membranes, much like a fish gill extracts oxygen from water.
Energy retention and capacity is very large for its mass, the report said. The potential for aluminium-air reactions electric storage is 40 times that of Li-ion. Aluminium-air cells recently powered test vehicles in Israel up to 135 kilometres in travel distance on a charge.
Zinc-air is another air intake battery type. It is not new and is used in small, button-type batteries for miniature electronic devices. New designs might make zinc viable for larger batteries. A zinc slurry held in a tank can be charged and store electricity with constant flow; when turned off, the reaction stops but the slurry loses little power in the off state.
The zinc slurry battery is complicated and heavy, and only holds half the power of Li-ion currently. It is one of the least likely matrices for new batteries in electric vehicles.
Used as an anode material, polypyrene has shown a lot of promise. It replaces graphite and offers large interstitial spaces for storing energy. It is easily managed and manipulated, this report said. One of the advantages of electrodes containing polypyrene is that scientists are able to influence their properties, such as the porosity. “The material can… be adapted perfectly to the specific application. In contrast, the graphite used at present is a mineral. From a chemical engineering perspective, it cannot be modified,” the report continued.
Finally, we look at the use of silicon, one of the most abundant elements. Research once again brings us back to the Li-ion battery but with another potential breakthrough. As with many of the materials discussed here, silicon has plusses and minuses. “It can bond with 25-times more lithium ions than graphite, the main material used in lithium-ion batteries today,” according to this report, but silicon tends to expand and contract through the recharging process.
A company named Sila is experimenting with various silicon forms at the nano level. Molecules that resist heat, expansion and energy loss will one-day allow silicon to replace graphite, the company says. The more ions it stores as an anode, the more silicon expands. Expansion due to heat is a battery’s biggest structural problem. This company is combating physical expansion by creating spherical particles, among other shapes.
Power collection and transmission will most likely lead the world’s transportation research and investment for the next decade when it comes to fuels. The electric car (and airship) is tied irrevocably to the weight-to-power ratios of all these products.