SES (formerly known as SolidEnergy Systems) is a battery company that specializes in Li-Metal batteries. It was founded in 2012 and has strong capabilities in material, cell, module, and AI-powered safety. SES is headquartered in Boston and has operations there as well. The company aims to develop indigenous battery technologies and synthesize everything from mines to materials to batteries. SES differentiates itself from other battery technology producers with its innovative platforms for monitoring and creating products.
Challenges: lithium battery
Commercial lithium-ion batteries typically consist of a metal oxide based cathode, a graphite based anode, and a non-aqueous electrolyte. They have a specific energy of around 250 Wh/kg and energy density of about 600 Wh/L. However, the current lithium-ion technology cannot satisfy the increasing energy density demands of the future.
Lithium metal is an attractive anode material for rechargeable batteries. It offers the highest theoretical specific capacity of 3860 Ah/kg (vs. 370 mAh/g for graphite) and the lowest negative electrochemical potential (−3.04 V vs. SHE) of all metals. Substituting the graphite anode in lithium-ion batteries with metallic lithium can potentially enhance the overall energy density of the battery above 1,000 Wh/L. However, the production of large-sized lithium metal batteries for electric vehicles remains difficult.
SES develops and manufactures large hybrid lithium metal batteries that can meet with the demands of practical applications that require high high capacity and energy density batteries, owing to the unique design of hybrid lithium metal battery and the simple production lines that can produce batteries on a large scale.
The structure of hybrid lithium metal battery
The structure of the SES’s lithium metal battery is depicted in the figure below. The battery comprises an anode, a cathode, a separator that separates the anodes and the cathodes, and an electrolyte.
The anode is composed of a layer of ultrathin lithium metal foil attached to the copper foil current collector through an ultrathin layer of conductive carbon binding agent (around 1 μm).
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 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. A functional coating composed of aluminum oxide nanoparticles and polymer binders is typically coated on the surface of the separator facing the lithium metal foil.
The electrolyte contains highly concentrated lithium bis(fluorosulfonyl)imide (LiFSI) dissolved in longer-side chain glymes solvents and fluorinated glyme or fluorinated ether diluents.
The battery working mechanism
During charging, lithium ions are extracted from the cathode. They diffuse through the porous separator to the anode’s surface, where they are reduced by externally circulated electrons. Thus, the lithium metal is formed on the surface of anode, as shown in the figure below.
During discharge, lithium 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.
1. High concentration solvent-in-salt electrolyte
In conventional electrolytes with lithium salt concentration of about 1 M, the solvent molecules undergo reduction at the surface of the lithium-metal anode, resulting in the formation of a solid-electrolyte interphase (SEI) layer. High-salt-concentration electrolytes, particularly those containing 2 M of bis(fluorosulfonyl)imide (LiFSI) dissolved in glyme-based solvents, can improve the charge-discharge cycle-life performance in lithium metal batteries, thanks to the association of most solvent molecules with lithium ions. In the absence of free solvent molecules, a more compact and stable solid-electrolyte interphase (SEI) is formed on the lithium metal anode’s surface, which leads to higher lithium plating/stripping coulombic efficiency and enhanced cycle life performance of the battery.
However, high-salt-concentration electrolytes have several disadvantages.
- Low conductivity, high viscosity, and poor wetting of electrodes and separator result in lower charge-discharge rates (C-rates).
- A high salt content increases production cost. Diluting high-salt-concentration electrolytes with excess solvent creates more free-solvent molecules that react with lithium-metal anode, thereby decreasing the battery’s coulombic efficiency and cycle life.
- Offsetting the high viscosity by using low-boiling-point DME (1,2-dimethoxyethane or monoglyme or ethylene glycol dimethyl ether) as the solvent typically results in significant gas generation in cells exposed to high ambient temperature.
SES created high-salt-concentration (LHSC) electrolytes based on the combination of longer-side chain glymes and diluents of a fluorinated glyme or fluorinated ether, as depicted in the figure below.
There are several benefits of adding fluorinated diluents.
- The fluorinated diluents are typically miscible with the longer-sidechain glyme-based solvent. Due to their low polarity, they have much lower LiFSI solubility than glyme-based solvents. Thus, the longer-sidechain glyme solvent and the lithium ions will remain associated with one another even after the addition of diluent, preserving the benefits of a high-salt-concentration electrolyte.
- The addition of the fluorinated diluent reduces the viscosity and improves the conductivity and wetting properties of the LHSC electrolyte without increasing the amount of free solvent molecules in the electrolyte. In turn, this allows the use of higher charge-discharge rates without compromising the battery’s high coulombic efficiency and cycle life.
- The fluorinated diluent also helps form a more compact and stable solid-electrolyte interphase (SEI) on the lithium metal surface of the anode, in a manner analogous to the lithium salt’s fluorinated anion.
- The fluorinated diluent allows the use of the more stable longer-sidechain glyme-based solvents, such as DEE (1,2-Diethoxyethane or Ethylene glycol diethyl ether). The use of the more-stable longer-sidechain glyme-based solvents prevents or decreases gas generated in cells exposed to high ambient temperatures. Having more glyme-based solvent lowers the volumetric cost of producing the LHSC electrolyte.
In sum, by using the larger molecules of the longer-sidechain glyme-based solvent and the fluorinated diluent in the electrolyte of the lithium metal battery, less gas is produced at higher temperatures, cycle-life performance is improved, higher coulombic efficiency are possible, and faster charging-discharging rates are possible.
Typically, the salt concentration of the high-salt-concentration electrolyte is between 2 and 3 M. The ratio of solvent to diluent ranges from 60:40 to 80:20. Other salts, such as alkali-metal-based salts, alkali-earth-metal-based salts, perfluorinated sulfonimide salts based on sodium or magnesium, and other salts commonly used in lithium-ion batteries, such as LiPF6, LiAsF6, LiBF4, LiBOB, Li-triflate, may also be present in the electrolyte.
2. Anode production
Lithium foil is laminated to both sides of a copper foil (current collector) to form the conventional lithium-copper-lithium (Li/Cu/Li) anode structure for typical lithium metal batteries. In a conventional approach of producing Li/Cu/Li anodes, as depicted in the figure below, the width of a copper-foil web (WCu) is typically greater than the width of the lithium foil (WLi).
The Li/Cu/Li laminated structure has a region of bare copper along one edge of the copper-foil web. The bare copper region is provided for forming electrical tabs that enable electrical contact to the Li/Cu/Li anodes that are stamped out from the webbed anode precursor when the Li/Cu/Li anodes are assembled into a stacked “jellyroll” of an electrochemical cell. Without the electrical tabs, it would be difficult to connect multiple layers of the Li/Cu/Li anodes in the stacked jellyroll to an external electrical lead in the final electrochemical cell. This conventional approach restricts the width of the lithium-foil web to about 120 mm when using the desired ultrathin lithium foil, thereby limiting the size of the Li/Cu/Li anodes.
This limitation hinders the development of lithium-metal batteries for practical applications, such as electric vehicles, that require high-capacity batteries. For electric vehicles, lithium-metal batteries with a capacity greater than 100 Ah are required. Comparatively, a 53 mm × 45 mm anode forms a 4 Ah cell while a 550 mm × 107 mm anode will form a 100 Ah cell for the same number of stack layers. Additionally, Since larger cells require less packaging, the gravimetric and volumetric energy densities (Wh/Kg & Wh/L, respectively) of these batteries will be greater. For example, in the preceding comparison of 4 Ah versus 100 Ah cells, the gravimetric energy density will increase from about 400 Wh/kg to about 410 Wh/kg.
SES developed a method for the production of larger Li/Cu/Li anodes. As depicted in the figure below, the lithium foil laminated on both sides of the current collector has a length (LLi) in a direction parallel to the direction of extension of the tab from the active-material region. The length may be any suitable length. The width of lithium foil can correspond to the maximum width of copper foil at a given thickness that can be formed using conventional lithium foil-forming techniques. For example, the current maximum width for lithium foil is about 120 mm, with a thickness of about 50 μm or less.
A mass production of Li/Cu/Li anodes based on roll-to-roll (R2R) processing system is depicted in the figure below. The R2R system includes providing a current-collector web, coating-application equipment, a lamination region where the individual lithium foil sheet is engaged with and laminated to the current-collector web, and a roller press that presses the foil sheets into firm engagement with the current-collector-web ribbon to form the anode-active-material patches.
The system optionally includes an alignment system that utilizes the conductive-coating patches to precisely align the individual lithium foil sheets with corresponding respective ones of the conductive-coating patches. The system also includes anode-forming equipment for suitable automated punching, sheering, or other cutting tool, that may be configured to form one or more of the anodes at a time. The formed anodes may now be ready for use in the next step of making one or more electrochemical devices using the anodes so formed.
3. Hybrid lithium metal battery production
The conventional stacking process for making stacked jellyrolls for lithium-metal batteries involves alternatingly adding individual cathode sheets and anode sheets and moving one, the other, or both of the growing stacked jellyroll and roll back and forth so that the separator web wraps around one lateral side of each of the cathode and anode sheets and becomes sandwiched between pairs of the cathode sheets and anode sheets as the stacking continues. Due to the zigzag shape of the continuous separator web in the finished stacked jellyroll, this process is often referred to as a “zigzag stacking process ” or a “Z-fold stacking process” as shown in the following video.
The conventional Z-fold stacking is extremely challenging to handle and process the ultra thin and fragile lithium metal anode. It often requires specialized components and the limitation of the speed at which the machinery can operate. This causes the machinery to take a considerable amount of time to complete the stacking process for each stacked jellyroll it produces.
SES developed a direct-stacking method that can be used to make a directly stacked jellyroll, as shown in the following video. The direct-stacking method involves alternatingly adding only cathode sheet and anode-subassembly sheet to the growing stacked jellyroll. Stacking only two types of sheets with one another greatly simplifies the process of making stacked jellyrolls for use in lithium-metal batteries. This process is realized by using the anode-subassembly sheets.
The machinery for the direct-stacking method can be considerably simpler than the machinery for the Z-folding process of the conventional stacking method. This is due to the fact that the machinery for the direct-stacking method does not include a separator as a separate and distinct component in the stacking process. Therefore, separator-handling components and actuators or other components/features for moving the stacked jellyroll and/or separator roll are unnecessary. In addition, machinery for the direct-stacking method does not directly handle a lithium-metal anode and therefore does not need to be specially designed to deal with the anode’s fragility.
The cathode sheet for the direct-stacking process, as depicted in the figure below, includes an aluminum foil layer as a positive current collector substrate. The foil layer is coated on both sides with a slurry containing a high-nickel NMC811, a polymer binder (such as polyvinylidene difluoride (PVDF)), and a conductive carbon.
The anode-subassembly sheets for the direct-stacking process, as depicted in the figure below, includes a copper foil as negative current collector substrate. Both sides of the copper foil are electronically attached to the lithium-metal foils using the conductive carbon paste. The lithium metals on both sides of the copper foil are sandwiched between two separators layers coated with lithium metal protective layers facing to the lithium metal foils.
The figure below depicts the anode-subassembly sheet fabrication process. The separator layers and the lithium-metal layer are paid-out from corresponding rolls. Prior to pressure lamination via a pair of pinch rollers, both separator layers are coated with a functional coating for using suitable coating applicators, such as spray applicators. When the separator layer is pressure laminated to the lithium-metal layer, the separator layer and then the functional coating come into contact with the lithium-metal layer.
The functional coating is made using a slurry containing nano-sized aluminum oxide (Al2O3, particle size 50 nm) and one or more polymer binders, such as poly(vinylidene fluoride-co-hexafluoropropene) (PVDF-HFP). The slurry is dried before pressure lamination.
The functional coating can protect the lithium metal anode and inhibit the growth of lithium dendrite within the battery. Because lithium metal and its oxides are not easily wetted with liquids having surface tension in excess of 25 dynes/cm, it is difficult to directly apply a functional coating that is beneficial for the lithium-metal layers.
SES targets the automotive industry. SES developed Apollo™, a 107 Ah Li-Metal battery that is the largest in the world and a breakthrough for the automotive industry. This is also the world’s first 100 plus Ah Li-Metal battery ever demonstrated.
The Apollo Li-Metal cell can deliver 107 Ah, weighs only 0.982 kg, and has an energy density of 417 Wh/kg and 935 Wh/L. Apollo also demonstrated similarly high capacity and energy density when tested at C/10 (10- hour discharge), C/3 (3-hour discharge), and 1C (1-hour discharge) at room temperature.
SES announced the Shanghai Giga factory that will be a 300,000 square-foot facility located in Shanghai, capable of producing 1 GWh of Li-Metal batteries annually. To leverage the supply chain and engineering and manufacturing efficiency, SES launched Shanghai Giga in November 2021, completed Shanghai Giga Phase I (0.2 GWh) and achieved ready-to-use (RTU) in March 2022, and expect to complete Phase II (1 GWh) and achieve RTU in 2023. SES also started SES Korea in January 2022 and expects to complete and achieve RTU in 2023.
SES has raised a total of $600.1M in funding over 10 rounds, including a Grant round, a Series A round, a Series B round, two Venture rounds, two Series C rounds, a Series D round, a Corporate round, and a Post-IPO Equity. Their latest funding was raised on Feb 4, 2022 from a Post-IPO Equity round.
SES is registered under the ticker NYSE:SES.
SES is funded by 28 investors, including Vertex Ventures, General Motors Ventures, Vertex Ventures China, Applied Ventures, Temasek Holdings, SAIC Motor, General Motors, Hyundai Motor Company, SAIC, SK Holdings, Khosla Ventures, MassVentures, Massachusetts Clean Energy Center, Franklin Templeton Investments, ITOCHU Corporation, Beringea, SAIC Venture Capital, LG Technology Ventures, Koch Strategic Platforms, Honda Motor, Geely, Kia Motors, Fidelity Canada, SK, Foxconn, Dyson, Tianqi Lithium, and Applied Materials. Honda Motor and SAIC Motor are the most recent investors.
Qichao Hu is Founder.
Qichao Hu is CEO.
SES Board Member and Advisor
Anand Kamannavar is a board observer.