Until now, the energy storage field has had two options to try to drive down costs—high volume production or entirely new chemistries that may never move from the lab to the commercial floor. 24M says it is presenting a third option.

The lithium-ion battery is a brilliant, enabling technology, but its economics are flawed. It’s prohibitively expensive; it’s cumbersome and inefficient to make; and today’s version is approaching the limits of its cost reductions. 24M has fixed the flaws. We’ve made the world’s favorite battery better, fundamentally changing its cost curve by designing a more elegant and simpler cell and then making the batteries the right way—the way they should have been made from day one.

—Dr. Yet-Ming Chiang

Schematic of a 24M cell, from the patent. Click to enlarge.		24M cell compared to conventional Li-ion cell. Click to enlarge.

The semisolid thick electrode is a material science innovation originating in Dr. Chiang’s lab at MIT. Conventional lithium-ion battery cells have a large fraction of inactive, non-charge carrying materials—supporting metals and plastics—that are layered, one-on-top of the other, within a cell’s casing. Those inactive materials are expensive and wasteful.

With the invention of the semisolid thick electrode, 24M eliminates more than 80% of these inactive materials and increases the active layer thickness over traditional lithium-ion by up to 5x. As described in the patent “Semi-solid electrodes having high rate capability”, published 31 March 2015:

Furthermore, known conventional batteries either have high capacity or high rate capability, but not both. A battery having a first charge capacity at first C-rate, for example, 0.5 C generally has a second lower charge capacity when discharged at a second higher C-rate, for example, 2 C. This is due to the higher energy loss that occurs inside a conventional battery due to the high internal resistance of conventional electrodes (e.g. solid electrodes with binders), and a drop in voltage that causes the battery to reach the low-end voltage cut-off sooner. A thicker electrode generally has a higher internal resistance and therefore a lower rate capability. For example, a lead acid battery does not perform well at 1 C C-rate. They are often rated at a 0.2 C C-rate and even at this low C-rate, they cannot attain 100% capacity. In contrast, Ultracapacitors can be discharged at an extremely high C-rate and still maintain 100% capacity, however, they have a much lower capacity then conventional batteries. Accordingly, a need exists for batteries with thicker electrodes, but without the aforementioned limitations. The resulting batteries with superior performance characteristics, for example, superior rate capability and charge capacity, and also are simpler to manufacture.

Semi-solid electrodes described herein can be made: (i) thicker (e.g., greater than 250 μm-up to 2,000 μm or even greater) due to the reduced tortuosity and higher electronic conductivity of the semi-solid electrode, (ii) with higher loadings of active materials, and (iii) with a simplified manufacturing process utilizing less equipment. These relatively thick semi-solid electrodes decrease the volume, mass and cost contributions of inactive components with respect to active components, thereby enhancing the commercial appeal of batteries made with the semi-solid electrodes. In some embodiments, the semi-solid electrodes described herein are binderless and/or do not use binders that are used in conventional battery manufacturing. Instead, the volume of the electrode normally occupied by binders in conventional electrodes, is now occupied by: 1) electrolyte, which has the effect of decreasing tortuosity and increasing the total salt available for ion diffusion, thereby countering the salt depletion effects typical of thick conventional electrodes when used at high rate, 2) active material, which has the effect of increasing the charge capacity of the battery, or 3) conductive additive, which has the effect of increasing the electronic conductivity of the electrode, thereby countering the high internal impedance of thick conventional electrodes. The reduced tortuosity and a higher electronic conductivity of the semi-solid electrodes described herein, results in superior rate capability and charge capacity of electrochemical cells formed from the semi-solid electrodes. Since the semi-solid electrodes described herein, can be made substantially thicker than conventional electrodes, the ratio of active materials (i.e., the semi-solid cathode and/or anode) to inactive materials (i.e. the current collector and separator) can be much higher in a battery formed from electrochemical cell stacks that include semi-solid electrodes relative to a similar battery formed form electrochemical cell stacks that include conventional electrodes. This substantially increases the overall charge capacity and energy density of a battery that includes the semi-solid electrodes described herein.

—“Semi-Solid Lithium Rechargeable Flow Battery”

As described in the patent, the semi-solid electrodes can be a flowable semi-solid or condensed liquid composition. “Semi-solid” refers to a material that is a mixture of liquid and solid phases—e.g., particle suspension, colloidal suspension, emulsion, gel, or micelle. “Condensed liquid” refers to a liquid that is not merely a solvent, as in the case of an aqueous flow cell semi-solid cathode or anode, but rather is itself redox active. Such a liquid form may also be diluted by or mixed with another, non-redox active liquid that is a diluent or solvent, including mixing with such a diluent to form a lower-melting liquid phase, emulsion or micelles including the ion-storing liquid.

Using thick electrodes, the cell also stores more energy, bettering the performance of the battery as well as its cost. The materials design enables up to 5x the area capacity of standard Li-ion.

Manufacturing. The simplicity of 24M’s new cell design likewise begets a simplified advanced manufacturing process. The traditional method for making lithium-ion batteries takes days, is extremely capital-intensive and must run at high-volume to achieve economies of scale. 24M’s novel approach to manufacturing yields significant improvements:

• Time. Start to finish, 24M’s cell creation takes one-fifth of the time of a conventional battery. Because semisolid lithium-ion doesn’t require binding, drying, solvent recovery or calendaring, it removes entire steps in the manufacturing process.

• Cost. While the removal of manufacturing steps certainly contributes to lower capex, eliminating the need for entire plants makes semisolid lithium-ion ultra-low cost. A 24M factory requires about one-tenth the investment of a conventional plant.

• Flexible and modular. Manufacturers can scale in small steps to match supply to demand, making lithium-ion cost effective even at low-volume production.

• Environmentally friendly. 24M’s solvent-free manufacturing platform creates the most easily recycled lithium-ion cell ever made.

Since its founding in 2010, 24M has raised $50 million in private capital, closing Series A and B rounds, from Charles River Ventures, North Bridge Venture Partners and industrial partners. The company is also the recipient of a $4.5 million grant from the US Department of Energy. 24M’s cells are currently undergoing customer trials with large, global integrators of power systems for the grid. The company now employs more than 50 people and runs a fully automated manufacturing line from its 32,000 square foot facility in Cambridge, Massachusetts.

Resources

• US Patent Nº 8,993,159: Semi-solid electrodes having high rate capability

• Mihai Duduta, Bryan Ho, Vanessa C. Wood, Pimpa Limthongkul, Victor E. Brunini, W. Craig Carter, and Yet-Ming Chiang (2011) “Semi-Solid Lithium Rechargeable Flow Battery” Advanced Energy Materials doi: 10.1002/aenm.201100152