Illustration of the silicon safety switch. (a) The switch before charging. The silicon pieces are in their original, non-expanded form, while the two parts of the switch are connected, allowing a closed electrical circuit to charge the battery. (b) As the battery charges, the silicon pieces expand until at the critical cell voltage, Vc, the battery is switched off by the separation caused by the silicon expansion. (c) The battery cell used to test the SESS, which is located inside the cell and connects the anode to the power source through a copper terminal (i). An additional copper terminal, (ii), is connected directly to the anode and used to monitor its connection status to the power source (resistance measurement). The battery’s cathode is connected to the positive side of the power source with an aluminum terminal (iii).Researchers from the University of California, Los Angeles have developed an intrinsic safety switch for lithium-ion batteries that turns what is usually a problem into a solution [Borenstein et al., Materials Today Energy 10 (2018) 89-97].
Almost every portable electronic device, electric vehicle, and grid energy storage system relies on lithium-ion batteries but they suffer from a failure mechanism called over-charging, which can lead to catastrophic fire or explosion. Over-charging occurs when a battery is charged at too low a potential, allowing dendritic growth to take place at the anode. If the dendrites create an internal short circuit to the cathode, thermal runaway generates large amounts of heat that can ignite the flammable electrolyte.
“To prevent such a process, standard battery management systems are designed to monitor the physical properties of the system, such as temperature and voltage, and activate a secondary reaction to stop further charging,” explain Richard B. Kaner, who led the research, and first author Arie Borenstein.
However, these regulatory systems initiate shutdown only once danger levels are detected. By this time, fatal reactions may already be underway making thermal runaway inevitable. Moreover, as these systems do not monitor a single device or cell, response time can be slow and entire battery packs are shut down in the event of a problem.
“We have developed a silicon safety switch that protects lithium-ion batteries from over-potential… by chemically reacting to the same potentials that cause dangerous over-charging,” say Kaner and Borenstein.
The silicon expansion safety switch (SESS) uses the extreme volume expansion that occurs when lithium ions permeate into silicon during charging and discharging (lithiation). Usually lithiation-driven volume expansion is considered a disadvantage, because it degrades electrode materials but the SESS uses the effect to disconnect the anode from its power source and prevent dangerous over-charging. The switch, which is located inside the cell as part of the anode, consists of two separate pieces of silicon linked at a contact point. An electrical current runs through the silicon pieces via two terminals, powering the anode. When the cell is fully charged, the expansion in volume of the silicon physically separates the terminals and switches off the current to the anode, preventing overcharging. Simply changing the dimensions of the silicon sets the ‘switch-off’ potential.
“Instead of permanently disabling the battery, thanks to the special SESS mechanism, the battery can return to standard and safe use after emergency shutdown,” points out Kaner.
The researchers believe the idea has real commercial benefits and could be more reliable than existing internal safety measures for preventing catastrophic overcharging in real time. The device could be added as a supplementary safety measure to battery management systems without sacrificing performance.