Activation with Li Enables Facile Sodium Storage in Germanium

Germanium is a promising sodium ion battery (NIB, SIB) anode material that is held back by its extremely sluggish kinetics and poor cyclability. We are the first to demonstrate that activation by a single lithiation - delithiation cycle leads to a dramatic improvement in practically achievable capacity, in rate capability, and in cycling stability of Ge nanowires (GeNWs) and a Ge thin film (GeTF). TEM and time-of-flight secondary ion mass spectrometry (TOF-SIMS) analysis shows that without activation, the initially single crystal GeNWs are effectively Na inactive, while the 100 nm amorphous GeTF sodiates only partially. Activation with Li induces amorphization (in GeNWs) reducing the barrier for nucleation of the Na_xGe phase(s). Introducing a dense distribution of nanopores via activation with Li at slower cycling rate, e.g. 0.1C, reduces the Na solid-state diffusion length by lowering the effective Ge thickness, and leads to improvement in cycling performance by buffering the sodiation stresses. The resultant kinetics are promising: Tested at 0.15C (1C = 369 mA/g, i.e. Na:Ge 1:1) for 50 cycles the GeNWs and GeTF maintain a reversible (desodiation) capacity of 346 mAh/g and 418 mAh/g, respectively. They also demonstrate a capacity of 355 and 360 mAh/g at 1C and 284 and 310 mAh/g at 4C. Even at a very high rate of 10C the GeTF delivers 169 mAh/g.