Batteries Look Beyond Lithium

Lithium batteries dominate today’s rechargeable battery market, and while they have been wildly successful, challenges with lithium have spurred research into alternative chemistries that can improve on some of lithium’s downsides and still keep as many of the upsides as possible.

So far, none of the alternative batteries has seen commercial success, but several variants have moved beyond pure research into product development. The process of designing batteries with new chemistries is also evolving, as well, because the tools must keep up with new ideas.

Finding homes for these different batteries requires looking beyond consumer items and vehicles. Renewable energies have inconsistent availability in a system accustomed to meeting changes in demand with changes in generation and routing. New requirements include the ability to store energy when conditions favor renewable generation, releasing it later when generation isn’t possible. That can become one more tool energy managers can wield to ensure that energy is available whenever needed.

Further tweaks to lithium
Lithium is a desirable material based on energy density, but it has its limitations, including slow charging and the risk of overheating and fires. An additional concern relates to the conditions under which lithium is mined in some parts of the world. Other metals, such as cobalt, copper, manganese, and nickel also are employed in the cathodes, and they’re expensive.

“When you look at lithium-ion batteries, you’re talking about nickel, manganese, and cobalt,” said Puneet Sinha, senior director, global head of battery at Siemens Digital Industries Software. “Cobalt is very expensive. Also, a majority of the materials come from countries that have geopolitical challenges.”

Alternative chemistries can alleviate some of the concerns about lithium and address the need for grid storage, but all come with tradeoffs. The U.S. Department of Energy has a goal of being cobalt-free by 2030, which means a new cathode material is necessary. Of interest are lithiated metal oxides and phosphates — particularly lithium iron phosphate (LFP).

“Today, almost 50% of electric vehicles have LFP chemistry,” observed Sinha. “There is a lot of investment in improving LFP chemistry by adding manganese, so it becomes a manganese-iron-phosphate chemistry. The idea is to keep the robustness and longer life of LFP chemistry, but by adding manganese it increases the voltage, thereby adding more capacity.”

Yet another approach is to build a sulfur-based cathode that, when combined with a lithium-metal anode, can provide four times the theoretical energy density of lithium-ion batteries. However, these corrode more easily, and last fewer charging cycles. Conamix is exploring this technology, but it’s not as far along as other lithium variants. Challenges include a so-called shuttle effect that has polysulfides moving back and forth between the electrode, effectively taking small amounts of lithium out of service and contributing to self-discharge. Dendrites, cathode expansion, and excessive heat or failure during charging also remain to be solved.

While the move to metallic anodes is underway, an intermediate step is to move from a graphite-based anode to one that instead uses silicon for higher energy density and faster charging. Sila is one company developing this type of battery. A mix of graphite and silicon may be a good compromise, which is being developed by OneD Battery Sciences and Sionic Energy.

Also, expansion remains an issue for silicon-based anodes. “Silicon expands four times from 0% lithiation to 100%,” noted Sinha. “When you have that much expansion, things start cracking. A lot of companies are trying to mix silicon and carbon so that you have silicon, but not 100%, and create something more like a nanotube or other hollow nanostructure so that it expands internally rather than externally.”

Fig. 1: Battery schematic. Source: Alksub at the English Wikipedia under GNU Free Documentation License

Jumping a periodic row
Those developments represent tweaks to the well-understood lithium chemistry. Other projects explore completely different chemistries, moving away from lithium.

One approach is patterned after lithium, but moves one row down the periodic table to sodium. Sinha noted it’s inherently very safe, and cheaper than a lithium-ion version. Sodium is an abundant element, and batteries based on sodium ions don’t require the same problematic metals used in lithium versions. Here, iron can be employed instead. Iron doesn’t work with lithium, because the small lithium ion can mix with the iron rather than intercalating. But sodium doesn’t mix with iron, solving that issue, although intercalation of sodium is slower than that of lithium.

Charging speed benefits
A highly desirable attribute of any new technology is charging speed. This speed is typically expressed as a “C rate,” and has a baseline such that a 1C charging rate will take a battery from 0% to 100% charged in one hour. A 4C rate would charge four times as quickly, in 15 minutes, and sodium-ion batteries can charge at the 4C rate. A comparison between lithium-ion and sodium-ion batteries gives the energy-density nod to lithium, but power per energy, recharge time, and cycle life improve with sodium.

Table 1: A comparison between lithium-ion and sodium-ion batteries based on select key parameters. Charging rate is expressed as a C rate, where 1C equals full charging in one hour. 4C charges in 15 minutes (four times as fast). Compiled by Bryon Moyer/Semiconductor Engineering based on multiple sources.

One other challenge is weight. “The one issue with a sodium-ion battery is that it’s going to be a lot heavier than a lithium-ion battery,” noted Sinha. But that’s not an issue for some systems, since for most companies the primary application is energy storage. “Because it’s cheap, you don’t have to worry about how heavy it is.”

CATL and Natron are examples of companies developing commercial sodium-ion batteries. CATL, the largest battery supplier in the world, has announced production-grade commercially available sodium-ion batteries.

Leveraging rust
Energy density is especially important when it comes to portable electronics, but not all batteries serve such systems. Grid storage, which is useful for storing energy from less reliable sources such as wind and solar, doesn’t need to be portable once it’s in place. Size and weight then become less important, but costs rise. Sodium provides one opportunity, but other chemistries can, as well.

One example is the iron-air battery. It features an iron anode and an air cathode that contributes oxygen. Effectively, iron moves back and forth between its metallic state and its oxidized state, i.e., rust. When it rusts, iron gives up an electron. It takes it back when discharging. Form Energy is one company developing this technology, and the company says that iron-air batteries cost around $20/kWh. That compares with $200 to $300/kWh for lithium-ion.

Zinc is another metal already present in everyday alkaline batteries. ReVolt is developing zinc-air batteries for vehicles and grid storage. Eos is working on a zinc hybrid battery with a conductive plastic anode and a carbon-felt cathode, with bipolar electrodes. The company intends it for grid storage.

Those are but two of the zinc approaches under evaluation. Other zinc chemistries are in play. In fact, other metals are also under consideration for metal-air batteries, including lithium, sodium, potassium, magnesium, calcium, and aluminum (which brings high energy density but isn’t rechargeable). The following table compares the theoretical gravimetric energy density (also called the specific energy) of different metal-air chemistries (including the weight of the oxygen, which reduces the energy density because it adds weight without adding energy).

Table 2: Energy density (by weight) and open-circuit voltage of different metal-air batteries. The weight includes oxygen. Aluminum-air batteries aren’t rechargeable. Source: Wikipedia

Design tools for batteries improving
Battery design is challenging in that the various chemistries aren’t understood at a fundamental level. Some behaviors lack governing equations. “When we do simulation, we would like to understand the governing equations because we’re good at solving equations,” explained Xiao Hu, senior principal application engineer at Ansys. “Since people don’t know those equations that well, our hands are somewhat tied.”

This makes analysis challenging because much of the data is empirical. Tools can manage, but it’s harder to provide a known-accurate solution when working off measured data and then extrapolating (or interpolating) from that. “If you provide some data, and if you’re okay with some assumptions, we can work with that,” said Hu.

Simulation capabilities are improving, however. “Thermal runaway for batteries was an R&D topic for simulation five years ago,” he noted. “By now, the dust has settled, and it is almost production-ready.” Meanwhile, aging performance remains largely empirical.

Whether simulation relies on equations or empirical data, it can operate at several levels, according to Comsol. Battery modeling can be done with different scopes — microscale, cell scale, and pack scale, Comsol’s Niloofar Kamyab said in a webinar. At the microscale, they study micro-structure. At the cell scale, they study each individual component of the cell, and then put a number of cells together to form modules and packs that eventually will be integrated into a system such as an electric vehicle.

Heterogeneous models break out individual particles, while homogenous models jump a level of abstraction, viewing cell components as uniform. They can be 1D, 2D, or 3D models with the addition of 1D particle diffusion to track the movement of charged particles for evaluating performance and aging. Single-particle models focus exclusively on the particles for understanding electrode kinetics and estimating battery parameters. Finally, high-level equivalent-circuit or lumped models operate at the pack level, allowing determination of the state of charge, which in turn determines the open-circuit voltage.

Designers have more than just chemistry to consider. “Battery engineers are not only interested in electrochemical characterization, but they are also interested in thermal management,” said Kamyab. “Thermal analysis becomes extremely important when it comes to the battery pack, because cell performance has a very strong temperature dependency. When the temperature is high, many degradation mechanisms become more intensified. When you go low in temperature, you’re triggering some of the aging mechanisms. You are after a uniform temperature, because non-uniform temperature means non-uniform degradation.”

Analysis also must focus on any potential for thermal runaway to improve battery safety. “I want to make sure that none of the cells goes under thermal runaway. And even if it does, I have a good estimate of when this is going to happen. And if it happens, I try to minimize the damage by avoiding its propagation,” she added.

Conclusion
Lithium-based batteries will be around for years, but improvements should make the electrolyte less hazardous and improve performance. Lithium batteries have the benefit of incumbency, so incremental improvements may keep them in play for a long time — particularly for mobile devices and vehicles.

Research into new battery chemistries shows no sign of abating, however. The transformation of the energy sector from dominance by fossil fuels to electricity, generated by fossil fuels and other means, provides compelling motivation to improve the consumer experience and build robustness into the energy grid. It’s not yet evident which non-lithium batteries will succeed, because laboratory tests have not yet resulted in commercial viability.

How the industrial infrastructure will adapt to new batteries also remains to be seen. Will local home-based solar and wind generators include batteries at the house that homeowners can control? Will they instead reside in substations under the control of grid operators, or perhaps a combination of the two? How that infrastructure will evolve must play out in tandem with the new chemistries over the coming decade.

Related Reading
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