The Advantages of Lithium-Metal Anodes
Rechargeable lithium-metal batteries have been the subject of intense research for decades, and today they seem closer than ever to reaching the marketplace. But why are lithium-metal anodes such a sought-after goal in battery science? Here, we’ll try to shed some light on the excitement around lithium metal.
Many elements can be used in rechargeable batteries. So, what is it about lithium that makes it so attractive for electric vehicles?
- It’s light and small
Lithium is the lightest metal on the periodic table and can store a lot of energy relative to its mass. Lithium is part of a group of elements known as alkali metals, which have several properties in common – they are all soft, have low melting points and are highly reactive. Because lithium atoms are less massive than atoms of other elements, it is an excellent material for use cases where weight and size matter, such as consumer electronics or electric vehicles. Compare this with sodium (Na), the next element in the alkali metals group: sodium is approximately three times more massive than lithium. Because of this, sodium-based batteries will always be at a significant disadvantage when it comes to energy density.
- It’s generous with electrons
Alkali metals readily give up electrons, and lithium has the lowest reduction potential in the group. This means that lithium-ion batteries have a relatively high voltage compared to other types of batteries, and higher voltage translates to storing more energy.
- There’s lots of it
Lithium is abundant: there is enough lithium in the Earth’s crust to make 100 million electric vehicles per year for the next billion years. It’s roughly as common as chlorine, which is found in everyday table salt (sodium chloride). Like chlorine, lithium is typically found in salts all over the world, in clay, hot springs, and even seawater. Lithium is rarely found in high concentrations, so collecting lithium can be tricky, but better and more environmentally friendly methods are being developed and deployed. Also, like many other metals, lithium can be recycled and reused to make new batteries.
Why lithium-metal anodes?
When it comes to choosing a material for a lithium-based battery anode, there are various options. The most important characteristics of an anode material are its capacity and voltage, which together dictate its overall energy storage capability. When it comes to anodes for EV batteries, there are three leading contenders: graphite, silicon and lithium metal. Lithium metal is the winner in energy density, but each has its challenges.
Graphite is by far the most common anode material in conventional lithium-ion batteries. Graphite anodes were initially introduced to stop the formation of lithium dendrites, the root-like structures of pure lithium metal that can grow and destroy a battery from the inside. The molecular structure of graphite provides natural gaps for lithium to nestle into when the battery is charged, in a process known as intercalation, while also keeping them far enough apart that they can’t come together and begin to form dendrites.
However, graphite has a relatively low capacity, so it can’t pack many lithium ions into a given space. This means graphite-based anodes offer poor energy density, resulting in a limited driving range in EVs. There is also a limit to how fast lithium can intercalate into graphite, a critical bottleneck that constrains fast vehicle charging speeds.
Graphite also reacts with liquid electrolytes in conventional lithium-ion batteries. This reaction creates a stable solid-electrolyte interphase (SEI) layer, which protects the bulk of the graphite from contacting the electrolyte. Capacity loss from side reactions, while still an issue, is less of a problem with graphite than other materials like silicon. This relative stability is what makes graphite so popular as an anode host, despite its other drawbacks. However, these side reactions still consume lithium, reducing battery capacity and creating side products that make it harder for lithium to move in and out of the anode. These two effects reduce both the power the battery can deliver and the energy it can store, meaning slower charging, slower acceleration, and lower driving range.
Silicon has been discussed as a candidate to replace graphite in anodes. In theory, silicon can store roughly 10 times as much lithium as graphite. However, lithium does not intercalate neatly into silicon as it does with graphite; the silicon actually bonds chemically with the lithium, creating an entirely new molecular structure. As a result, silicon swells dramatically during charge and contracts during discharge, which causes the silicon to crack and pulverize over many cycles.
This cracking process means that the SEI layer is continually destroyed and reformed over many cycles, increasing the rate at which side reactions consume the lithium in the battery. This causes the battery to lose capacity and increases cell resistance, and therefore reduces its voltage. This means batteries based on silicon anodes tend to lose energy storage capability rapidly.
There are a few workarounds for this problem, but all have tradeoffs. The silicon may have lithium added to offset the capacity losses, known as pre-lithiation, but this adds to cost and makes manufacturing more difficult. The batteries can be kept under enormous pressure to prevent the silicon from cracking, but the mass of the pressure apparatus offsets much of the benefit the silicon provides in the first place. Silicon nanostructures can be engineered to overcome these challenges, but these are more complex and expensive than simple graphite, and lower the theoretical capacity advantage of using silicon in the first place. Lastly, silicon pays a voltage penalty that reduces cell energy. Due to these drawbacks, silicon is usually mixed with graphite, and typically only in small amounts.
- Lithium metal
Lithium metal can be an ideal anode material for lithium-based batteries for several reasons.
- A lithium-metal anode offers the highest gravimetric energy density (the amount of energy that can be stored per unit of mass) possible.
- Charge rates can be substantially improved by allowing lithium to be deposited directly on the anode.
- With the right electrolyte, a battery with anode-free architecture using only pure lithium metal can be designed, saving on materials and manufacturing costs and improving energy density.
Why aren’t all anodes made of lithium metal?
Pure lithium metal tends to form dendrites, which can diminish a battery’s safety and service life. Many electrolyte materials, such as liquids, solid polymers, and sulfides, have not been shown to prevent dendrites.
Lithium metal is also highly reactive, which poses a problem for sulfides, liquids, and solid polymers. This reactivity consumes lithium, and so the battery must have extra lithium added in the form of a thin lithium foil to function. Unfortunately, lithium foil introduces a tradeoff between volumetric energy density (the amount of energy stored in a given volume) and cost. Very thin lithium foils are expensive to produce and difficult to handle, and thicker lithium foils eliminate the volumetric energy density advantage that motivates the switch from graphite.
Lithium foils also make the dendrite problem harder to solve. Manufactured foil always has slight impurities or variations in the material itself, which intensifies lithium plating at specific points, and catalyzes the growth of dendrites.
QuantumScape’s battery technology is designed to address the fundamental challenges of lithium metal. Our ceramic solid-electrolyte separator has demonstrated the ability to resist dendrites at rates of power relevant for EVs. Our separator offers very good stability with lithium metal, which reduces lithium consumption to side reactions and contributes to excellent Coulombic efficiency. This stability means extra lithium isn’t required, and cells can be manufactured without an anode. Instead, as the cell is charged, lithium is drawn from the cathode through the ceramic solid-electrolyte separator and plates on the anode as pure lithium metal. This process ensures that the anode is free of impurities and eliminates the material and manufacturing costs required to produce and integrate lithium foil into the cell.
Instead of adding expensive extra anode materials, QuantumScape’s technology makes better batteries by simplifying the cell design. Eliminating the need for graphite, silicon, or lithium foil offers a pathway for QuantumScape’s technology to increase energy density and improve vehicle range while simultaneously enabling 15-minute fast charging. We believe our solid-state lithium-metal battery technology represents the most promising pathway to next-generation battery performance.