- 3415 - BATTERIES - revolutions of old technologies? Today's lithium batteries commonly use a liquid electrolyte to carry ions between the two electrodes. New technology use cellulose derived from wood as the basis for one of these solid electrolytes, which is paper-thin and can bend and flex to absorb stress as the battery cycles.
--------------------- 3415 - BATTERIES - revolutions of old technologies?
- A shortcoming of the electrolytes used in today's lithium batteries is that they contain volatile liquids that carry a risk of fire if the device short circuits, and can promote the formation of tentacle-like growths called “dendrites” that compromise performance.
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- Solid electrolytes can be made from non-flammable materials, make the device less prone to dendrite formation. One of these new devices relates to the anode, one of the two electrodes, which in today's batteries is made from a mix of graphite and copper.
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- Solid electrolytes are a key stepping stone to making batteries work with an anode made from pure lithium metal instead, which could help break the energy-density bottleneck and enable electric cars and planes to travel much farther without charging.
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- Many of the solid electrolytes developed so far have been made from ceramic materials, which are highly effective at conducting ions but don't stand up so well to stress during charging and discharging owing to their brittle nature.
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- Instead use wood-derived polymer tubes combined with copper to form a solid ion conductor boasting a conductivity similar to ceramics and between 10 and 100 times better than other polymer ion conductors.
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- The addition of copper creates space in between the cellulose polymer chains in the wood for "ion superhighways" to form, enabling the lithium ions to travel with record efficiency. By incorporating copper with one-dimensional cellulose nanofibrils the normally ion-insulating cellulose offers a speedier lithium-ion transport within the polymer chains.
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- Because the material is paper-thin and flexible, it will better tolerate the stresses of battery cycling. It has the electrochemical stability to accommodate a lithium-metal anode and high voltage cathodes, or could act as a binder material that encases ultra-thick cathodes in high-density batteries.
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- One of the ways scientists hope to improve the charging rates of batteries is by using porous structures for the anode, one of its two electrodes. This offers a greater contact area with the liquid electrolyte that transports lithium ions and enables them to diffuse more easily through the material, potentially making for batteries that charge much, much faster.
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- Making the anode out of the porous materials features disorganized and random channels that cause the structures to cave in during charging and the battery to fail. Nickel niobate has a higher volumetric density than the graphite used for today's anodes, which could also lead to commercial batteries that are lighter and more compact.
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- When a battery is cycled, lithium ions travel back and forth between the two electrodes, but not all of them complete the journey all of the time. This causes electrochemically inactive "islands" of lithium to form in between that remain disconnected from the electrodes, with these clumps causing a decline in the device's storage capacity or even causing it to catch fire.
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- There is a way to not just neutralize these damaging clumps of dead lithium, but bring them back to life to boost the performance of the battery. By adding a high-current voltage during recharging spurred this inactive lithium into action, causing it to creep "like a worm" and reconnect with the electrode, increasing the battery's lifespan by 30 percent.
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- This breakthrough could lead to improved designs for fast-charging batteries or rechargeable batteries with greater capacities and lifespans. One of the reasons scientists see so much potential in lithium-metal batteries is because lithium metal has a far higher capacity and energy density than the graphite and copper used for the anodes in today's batteries.
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- A new sandwich-style battery that could overcome some of the stability issues to plague lithium-metal designs so far. These stability issues stem from needle-like protrusions called dendrites that form on the lithium-metal anode during charging, causing the battery's performance to decline, and it to fail or even catch fire.
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- To overcome this swapping the battery's liquid electrolyte for a pair of solid ones, which are layered together in a BLT-style sandwich and work to safely control and contain the dendrites as they form.
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- The sandwich-style battery is able to backfill the gaps created by dendrites. The battery retained 82 percent of its capacity after 10,000 cycles and, most promisingly, demonstrated the kind of current density that could one day enable electric vehicles to charge within 20 minutes.
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- In October, 2021, a solution to the stability issues associated with lithium-metal batteries hinged on using a solid electrolyte rather than a liquid one to carry the charge, with the scientists using cellulose nanofibrils derived from wood as their starting point.
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- These microscopic polymer tubes were combined with copper to form a solid ion conductor, featuring tiny openings in between the polymer chains that acted as "ion superhighways," enabling lithium ions to travel with record efficiency.
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- This meant the material had a conductivity between 10 and 100 times greater than other polymer ion conductors. The researchers also say because the material is paper-thin and flexible, the electrolyte could better tolerate the stresses of battery cycling and withstand the environment of a lithium-metal architecture.
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- Alkali metal-chlorine batteries have been around since the 1970s and offer a high energy density, but the highly reactive chlorine means that they only last for a single use. In August, 2021, scientists at Stanford University came up with a way to stabilize these reactions, and actually allow these types of high-density batteries to be recharged.
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- The solution consisted of a novel electrode material made of porous carbon that sponged up erratic chlorine molecules, and safely converted them back into sodium chloride, their original form prior to discharging. This cycle was able to be repeated up to 200 times in an experimental battery offering around six times the density of today's lithium-ion technology.
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- Scientists have also developed a novel lithium-metal battery that retains its functionality over 600 cycles. The solid electrolyte interphase (SEI), which is a thin film on top of the anode that plays an important gate keeping role by controlling which molecules enter from the electrolyte during cycling.
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- Complex reactions occur around the anode and affect the performance of SEIs in current designs, but scientists found a novel solution in the form of very thin strips of lithium with a width of around 20 microns, far thinner than a human hair. These were used as the basis for an anode with an SEI that interacts more healthily with the electrolyte than anodes with thicker strips that smother important electrochemical reactions.
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- This anode that retained 76 percent of its capacity over a record 600 cycles, with an energy density of 350 Wh/kg. For reference, the best-in-class lithium-ion batteries in use today have a density of 250 to 300 Wh/kg. (watt-hours per kilogram)
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- The battery featured a "semi-solid" electrode made of sodium-potassium alloys, likened by the researchers to the material dentists use to fill cavities in that it was firm, but able to flow and be molded. When this material comes into contact with the solid electrolyte, it has just the right amount of give in it to prevent the type of cracks that would form on a more rigid and brittle electrode material.
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- This self-healing material prevented the formation of damaging dendrites and also allowed for far higher current densities than other solid-state batteries have allowed for 20 times greater charging rates.
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- Who said batteries were old school? Today they are everywhere. Maybe even replacing gas engines in automobiles.
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January 15, 2022 BATTERIES - revolutions of old technologies? 3415
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