Cuberg is a battery technology company that was acquired by Northvolt in 2021. It operates as Northvolt’s Advanced Technology Center and is focused on developing sustainable, high-quality battery cells and systems. Cuberg has achieved a breakthrough in battery development, putting its lithium metal cells among the highest performing in the world. The company was spun out of Stanford University in 2015 and has created battery cells that deliver increased range and capacity by 60% versus conventional lithium-ion batteries.
Challenges: lithium battery
After several decades of development, the performance lithium-ion (Li-ion) battery is approaching the fundamental limits of the materials that comprise the batteries. However, two major drawbacks of the Li-ion battery technology could limit or hinder their application in the expansion of Li-ion-powered electric vehicles.
First, the energy density of Li-ion batteries is intrinsically limited. It can be challenging to achieve specific energy beyond 300 Wh/kg at the cell level. With today’s Li-ion batteries, electric vehicles would be heavy and require frequent charging. Anode materials that have been used to increase energy density include lithium metal. However, the Li metal anode is incompatible with conventional Li-ion electrolytes based on organic carbonate solvents due to electrolyte decomposition and depletion, resistance growth, and, ultimately, a very short cycle life.
Second, traditional Li-ion electrolytes based on organic carbonate solvents are volatile, which makes the operation of rechargeable Li-ion batteries very dangerous. If heated internally above a threshold temperature due to internal short circuit, overcharge, and/or puncture, they will almost instantly go into thermal runaway, releasing large amounts of heat and energy. The engineering of pack-level safety can easily reduce energy density and specific energy by 30% or more. It also increases the cost and time to the development process of new battery technologies.
Therefore, there is a need to develop stable high energy density batteries based on a metallic anode and a stable, non-flammable electrolyte. Some startups, such as Adden Energy, developed safe and high energy density batteries based on lithium metal anode and solid-state electrolyte.
Cuberg develops high energy density and stable rechargeable batteries with lithium-magnesium (Li-Mg) metal alloy anode and non-flammable ionic liquid electrolyte. Cuberg batteries are fully compatible with current manufacturing technology, allowing for a straightforward easy scaling up of its batteries.
As illustrated in the figure below, Cuberg battery comprises an anode, a cathode, a separator that separates the anodes and the cathodes, and an ionic liquid electrolyte.
The anode is composed of Li-Mg alloy that contains about 10 wt% magnesium. During charging, the metal alloy is electrically deposited in situ on an existing metal alloy anode or a current collector foil from the electrolyte.
The cathode is constructed by coating a mixture of metal oxide active material, polymeric binder and conductive additives onto a current collector foil. Typically, the metal oxide active materials are single crystal LiNi0.8Mn0.1Co0.1O2 (NMC811) particles.
The electrolyte comprises a nonflammable ionic liquid solvent combined with a minority-fraction ether co-solvent, wetting agent additives, dissolved lithium bis(fluorosulfonyl)imide (LiFSI), and dissolved magnesium bis(trifluoromethanesulfonyl)imide (Mg(TFSI)2). The electrolyte permits ion conduction through the separator and in between/within the electrodes.
The separator is typically a porous polymer film, such as polyolefin, that physically and electrically separates the anode cathode while allowing the electrolyte to convey throughout the battery.
Battery working mechanisms
During charging, lithium ions are extracted from the cathode. Along with magnesium ions, they diffuse through the porous separator to the anode’s surface, where they are reduced by externally circulated electrons. Thus, the lithium-magnesium alloy is formed on the surface of anode, as shown in the figure below.
During discharge, lithium-magnesium alloy is oxidized, releasing metal ions into the electrolyte that diffuse to the cathode through the separator. Lithium ions are intercalated within cathode active materials, as shown in the figure below.
The lithium metal batteries have several challenges. During charge, the lithium metal anode tends to form lithium dendrites during charge, which can lead to catastrophic cell failure. In addition, the deposition of lithium metal produces a “mossy” surface morphology with a large surface area, which consumes Li in the electrolyte as new surface electrolyte interphase (SEI) forms on the new surface. Upon subsequent Li stripping, the new surface electrolyte interphase remains, increasing the interfacial resistance with each cycle. This increase in resistance and capacity loss is one of the most common non-catastrophic failure modes of lithium metal batteries.
Moreover, conventional Li-ion electrolytes contain highly flammable solvents and they are not chemically and electrochemically stable against lithium metal anode. Solid electrolytes are typically used in lithium metal batteries. However, the solid-state battery technology is incompatible with the current Li-ion battery manufacturing line and new production techniques need to be developed.
Cuberg developed stable and high energy-density lithium metal batteries that can be produced with the current Li-ion battery production line. The key innovations of Cuberg lie in the lithium-magnesium anode and the ionic liquid electrolyte that are compatible with the metal alloy anode. Such a system yields a smooth and compact anode surface with minimal surface electrolyte interphase formation and highly conductive surface electrolyte interphase layer, which enables high energy density and stable batteries.
1. Lithium-magnesium alloy anode
Unlike the lithium metal anode, the lithium-magnesium alloy anode allows for smooth metal anode deposition, as shown in the figure below.
The overpotential significantly affects the morphology of Li deposition. A larger density of smaller nuclei is produced by deposition at higher overpotentials, resulting in a lower volume-to-surface area ratio. Therefore, it is beneficial for Li deposition to occur at lower overpotentials.
The deposition overpotential of Li depends on the substrate’s composition. It has been observed that the overpotential for Li deposition on lithium-zinc (Li—Zn) alloys is lower than on pure Zn, and it is anticipated that this trend will also hold true for Li—Mg alloys.
Even at high deposition rates, it is known that the electrodeposition of Mg does not produce the high surface area structures characteristic of Li deposition on Li metal anodes. This is due in part to the fact that metallic Mg—Mg bonds are 0.18 eV stronger than metallic Li—Li bonds and in part to the fact that surface diffusion of Mg in Mg metal is faster than that of Li in Li metal. In this regard, it is anticipated that a Li—Mg alloy anode will have higher bond energies and, therefore, faster surface diffusion for newly deposited Li than a pure Li metal anode, and that this faster surface diffusion will similarly smooth out Li dendrites.
Not only does Li-Mg metal alloy permit smooth electrodeposition, but also combining the Li-Mg alloy with the electrolyte may improve surface electrolyte interphase quality.
Common anions of bis(trifluoromethanesulfonyl)imide (“TFSI”) and bis(fluorosulfonyl)imide (“FSI”) from lithium salts in the electrolyte can form LiF-rich surface electrolyte interphase layers. The LiF-rich surface electrolyte interphase with a high Li-ion conductivity is crucial, because Li ions must diffuse through the surface electrolyte interphase in order to transfer between anode and electrolyte, thereby facilitating stable cycling.
Li ion vacancies are the main Li-diffusion carriers in LiF. Doping LiF with Mg ions increases the concentration of Li ion vacancies, which occurs naturally when surface electrolyte interphase forms on top of an Li—Mg alloy anode. Thus the combination of Li—Mg alloy and fluorine-rich anions can produce synergistic effects that exceed the benefits of each alone.
Additionally, the Li-Mg alloy anode has a greater thermal tolerance than a lithium metal anode.
The melting point of lithium metal is only 180.5 ºC, which is well within the range of internal temperatures that occur in a shorted, punctured, overcharged, or otherwise mistreated battery. If a lithium metal anode melts during battery operation, the molten metal can cause massive internal shorting, rapidly generating high temperatures and pressures inside the battery. Alloying lithium with magnesium increases its melting point, which complements the safety improvements provided by the non-flammable ionic liquid electrolyte.
2. Ionic liquid electrolyte
Cuberg developed non-flammable ionic liquid electrolytes that are preferably chemically and electrochemically stable against both the anode and cathode. Additionally, the electrolyte facilitates the formation of a protective solid electrolyte interphase and uniform stripping and deposition of lithium. The composition of the electrolyte is shown in the figure below.
The ionic liquids are the main solvents in the electrolyte. They have bulky organic cations, such as N-methyl-N-butylpyrrolidimium, and tetrabutylammonium, and anions, such as bis(trifluoromethanesulfonyl)imide (TFSI) and bis(fluorosulfonyl)imide (FSI). The low viscosity of these ionic liquids is advantageous for ionic mobility.
The electrolyte also contains a linear ether co-solvent, such as dimethoxyethane, which enhances the solubility of lithium salts and the mobility of lithium ions. The co-solvent is also reductively stable and permits the reversible plating and stripping of lithium metal.
Combining ionic liquids with an ether co-solvent enables a lithium salt concentration over 30 wt%. This high Li-salt concentration can increase the surface concentration of lithium ions at the anode during plating, which increases plating uniformity, decreases the plating overpotential, and decreases electrolyte decomposition.
Mg salts are also contained in the electrolyte. Thus, the Li-Mg anode can be formed in situ by an electrochemical alloying reaction between the existing Li-Mg anode anode and an electrolyte system containing Mg salts. The Mg salts can be magnesium bis(trifluoromethanesulfonyl)imide (Mg(TFSI)2).
Wetting agents in the electrolyte are also crucial. The wetting agents include surfactants (such as Triton X-100) that provide a more compatible interfacial layer between the non-polar separator surface and the moderately polar electrolyte. Wetting agents also include phosphate esters (e.g. PO(OR)3), which improve Li deposition morphology and electrolyte oxidation stability by forming a stable and high quality solid electrolyte interphase on the cathode particle surface. Wetting agents include fluorinated ethers that allow for the formation of a more stable solid electrolyte interphase on both the cathode and anode surface.
Cuberg batteries, unlike solid-state lithium metal batteries, can be manufactured on the existing battery production lines.
Mobile Power Solutions, an independent third party, has evaluated and validated the performance of Cuberg batteries. Testing of 5 Ah pouch batteries confirmed that cycle life can achieve 672 cycles with 80% capacity retention with a C/2 charging rate (2-hour charge). The results indicate that Cuberg has successfully developed the world’s highest-performing and longest-lived lithium metal battery. For the first time, a lithium metal battery surpasses the cycle life of the most recent energy/power batteries developed by leading lithium-ion manufacturers when tested under identical cycling conditions and battery capacities. Cuberg batteries could make electric vehicles become cheaper and more efficient.
Richard Wang is CEO.