Bending to power: mechanically rechargeable battery and mechanical energy harvester

Rechargeable batteries are usually charged by applied voltage. Here we demonstrate that mechanical bending can charge a thin-film lithium ion battery and removing the bending discharges the battery. Bending and relaxation (or bending in the opposite direction) thus realizes mechanically rechargeable battery, or more generally, mechanical energy harvester. 

The fundamental physics is quite simple. If one bends a wet sponge, water molecules would flow from the compressive to tensile side. The same would occur for lithium ions in a thin film lithium ion battery: bending induced mechanical asymmetry generates a chemical potential difference of the lithium ions in the two electrodes, higher in the compressed electrode, lower in the tensed one, driving lithium flow and hence mechanically charging the battery. Removing the bending force after charging would cause lithium flow back, driven by the concentration gradient. 

In 2014, we demonstrated bending induced symmetry-breaking in lithiation kinetics in a GeNW (Nano Letter, 2014). This naturally leads to the electrochemically driven energy harvester (Nature Comm. 2016). 

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