Avanti Battery ($8M for aluminum-sulfur battery storing renewable energy to power a single home or small to medium-sized business)

Avanti Battery, an American energy storage tech startup founded in 2021, develops and commercializes a new type of aluminum-sulfur (Al-S) battery that was discovered at MIT. This innovative aluminum-sulfur battery is cheap, has a high capacity, can be rapidly charged, and won’t catch fire. It is designed for small-scale stationary energy storage with a storage capacity of several tens of kilowatt-hours, which is enough to power a single home or small to medium-sized business.

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Challenges: lithium-ion battery

As more and more renewable energy systems, such as solar and wind, are installed worldwide, the demand for cost-effective, large-scale energy storage systems to provide uninterrupted power is increasing at a rapid rate. Lithium-ion batteries are strong candidates for this solution due to their small spatial footprint, mechanical simplicity, and siting flexibility. They have been used to store energy on a grid that integrates intermittent solar and wind power.

The high cost of lithium-ion batteries, however, prevents their widespread use in such applications. Low-cost technologies, such as pumped hydroelectric, are preferred despite their low energy and power densities and geographical limitations. Several dozens of stationary lithium-ion energy storage systems have reported explosions or fires, raising safety concerns about lithium-ion batteries used in stationary energy storage applications.

Therefore, it is necessary to develop low-cost, high capacity, and safe battery technology.

This book Beyond Li-ion Batteries for Grid-Scale Energy Storage discusses existing technologies beyond Li-ion battery storage chemistries that have seen grid-scale deployment, as well as several other promising battery technologies. (see on Amazon)

Avanti Battery Technology

Avanti Battery utilizes aluminum, sulfur, and molten salts to produce aluminum-sulfur batteries with rapid charging, high capacity, and fire resistance. Aluminum, sulfur, and molten salts are earth-abundant, low-cost resources. The capital cost of aluminum-sulfur batteries is only 10 to 15% of that of today’s lithium-ion batteries. Additionally, the volumetric energy density of aluminum-sulfur batteries is comparable to that of lithium-ion batteries.

This book Next-Generation Batteries with Sulfur Cathodes provides a comprehensive review of a modern class of batteries with sulfur cathodes.

(see on Amazon)

Avanti aluminum sulfur battery

Avanti Battery’s technology is different from other aluminum batteries because it uses an elemental-chalcogen positive electrode instead of different low-capacity compound formulations and a molten-salt electrolyte instead of high-polarization ionic liquids at room temperature.

The diagram below depicts the structure of the Avanti aluminum-sulfur battery. Its structure is similar to that of a liquid metal battery developed by Ambri. The aluminum-sulfur battery is composed of an aluminum (Al) negative electrode, an elemental sulfur (S) positive electrode, and a molten electrolyte.

The structure of aluminum-sulfur battery from Avanti Battery.

Aluminum plate is connected to the negative current lead. Sulfur powders are contained in the metal matrix that is welded to the inner wall of cell housing and also serves as a positive current collector.

The electrolyte is composed of non-flammable and cheap salts of sodium chloride (NaCl), potassium chloride (KCl), and alumina chloride (AlCl₃). Due to the formation of large covalent Al_nCl_3n+1^– moieties, the acidic NaCl-KCl-ALCl₃ melt has a significantly lower eutectic point (approximately 93 ºC) than a simple molten-salt system such as LiCl-NaCl-KCl (347 ºC). This low melting point allows aluminum-sulfur batteries to operate at low-temperature (110 ºC) without the use of ionic liquid electrolytes. To avoid the formation of toxic hydrogen sulfide (H₂S) in the battery, extra care must be taken to avoid water contamination in the melt.

The negative electrode, positive electrode, and molten electrolyte are contained within a cell housing. The cell housing has a porous glass fiber separator to separate sulfur powders from the negative electrode. Through a hoe in the cell housing, a negative current lead extends into the cell housing.

To prevent short circuit, the hole in the cell housing is hermetically sealed by a robust aluminum nitride insulator disposed between the negative current lead and the cell housing. The seal also prevents air from entering the cell, which degrades performance.

An empty headspace is formed above the negative electrode.

How Avanti battery works

The battery operates at a low temperature of 110 ºC. It can “self-heat” while cycling and maintain its temperature by proper thermal insulation. The battery would not require an active cooling system, which is absolutely necessary for large lithium-ion batteries.

At this temperature, the electrolyte (melting point about 93 ºC) is molten. The sulfur (melting point: 112.8 ºC) powders and aluminum (melting point: 660.3 ºC) remain solid. Thus, the symbol for the battery is Al(solid)‖NaCl-KCl-AlCl₃ (liquid)‖S(solid).

The acidic NaCl-KCl-AlCl₃ melt in the battery generates catenated Al_nCl_3n+1^– moieties, which are essential for supporting ultrafast aluminum electrodeposition (cell charging) while inhibiting dendrite formation. Higher-order Al_nCl_3n+1^– moieties (such as Al₃Cl₁₀^– and Al₄Cl₁₃^–) have longer and weaker bridging Al-Cl bonds, which break more easily and are energetically more favorable to form Al₂Cl₆ than lower-order AlCl₄^– and Al₂Cl₇^– moieties. The generated Al₂Cl₆ further dissociates into AlCl₄^– and AlCl₂⁺, and the AlCl₂⁺ dissociates into AlCl₄^– and Al³⁺, which is driven by an applied electric field. Coincidentally, the favorable desolvation of Al³⁺ in the chloroaluminate melt engenders resistance to dendrite proliferation.

On charge, the aluminum is plated on the negative electrode and forms compact crystals with well-defined facets and high conformity (10-15 um size) at 10 mA cm^-2 and of connected platelets (5-8 um wide, about 1 um thick) at 50 mA cm^-2. The Al deposits in this NaCl-KCl-AlCl₃ melt electrolyte pose no or minimal risk of puncturing the separator and causing short-circuiting of the battery.

The charging of aluminum-sulfur battery from Avanti Battery.

At high charging rates of 20C and 50C, the aluminum-sulfur battery exhibits a high capacity of 430 and 360 mAh/g, respectively. At extreme charging rates of 100C and 200C, the battery retains a high capacity of 280 and 210 mAh/g, respectively. The battery can sustain hundreds of cycles at high charging rates of 50-100C.

The battery also has a good fast-discharging capacity: 670 and 360 mAh/g at 2D and 20D, respectively. There is a discrepancy between charging and discharging rate capabilities, which is attributed to asymmetry in the Al³⁺ solvation and desolvation. The asymmetry can be attributed to the acidic AlCl₃-rich chloroaluminate melt, which is kinetically more favorable for exsolving AlCl₃ than for dissolving it.

The discharging of aluminum-sulfur battery from Avanti Battery.

During battery discharging, the less favored Al³⁺ solvation (accompanied by AlCl₃ dissolution) occurs at the planar Al negative electrode, which carries a significantly higher local current density than the powdered sulfur electrode with its greater surface area. This indicates that the reaction at the negative electrode is rate limiting during discharging. On charge, Al³⁺ deposition is kinetically favored, so it is easier to achieve a higher rate capability on charge than on discharge.

Advantages of Avanti battery

As shown in the figure below, the projected cell-level energy density of the Al‖NaCl-KCl-AlCl₃‖S battery is 526 Wh l^-1, which is comparable to graphite-NMC622 (LiNi_0.6Mn_0.2Co_0.2O₂) and other lithium-ion batteries.

Comparison of volumetric energy density of batteries.

The capital cost of active components in aluminum-sulfur batteries is less than $10 per kilowatts per hour, which is only 10-15% of that of today’s lithium ion batteries, as shown in the figure below.

Comparison of cost of batteries — Comparison of the cost of batteries.

Avanti Battery Applications

The aluminum-sulfur battery from Avanti Battery would be ideal for installations of about the size required to power a single home or small to medium-sized business, with a storage capacity of several tens of kilowatt-hours.

The smaller size of the aluminum-sulfur batteries would also make them suitable for applications such as electric vehicle charging stations. When electric vehicles become so prevalent on the roads that several cars want to charge up at once, as is the case with gasoline fuel pumps today, we want rapid charging. Therefore, having a battery system like this to store power and then quickly release when needed could eliminate the need to install costly new power lines to serve these chargers.