In situTEM electrochemistry of anode materials in lithium ion batteries

Abstract

We created the first nanobattery inside a transmission electron microscope (TEM), allowing for real time and atomic scale observations of battery charging and discharging processes. Two types of nanobattery cells, one based on room temperature ionic liquid electrolytes (ILEs) and the other based on all solid components, were created. The former consists of a single nanowire anode, an ILE and a bulk LiCoO₂ cathode; the latter uses Li₂O as a solid electrolyte and metal Li as the anode. Some of the important latest results obtained by using the nanobattery setup are summarized here: (1) upon charging SnO₂ nanowires in an ILE cell with one end of the nanowire contacting the electrolyte, a reaction front propagates progressively along the nanowire, causing the nanowire to swell, elongate, and spiral. The reaction front is a “Medusa zone” containing a high density of mobile dislocations, which continuously nucleate at the moving front and absorbed from behind. This dislocation cloud indicates large in-plane misfit stresses and is a structural precursor to electrochemically driven solid-state amorphization. When the nanowire is immersed in the electrolyte (in a flooding geometry), a multiple-strip-multiple-reaction-front lithiation mechanism operates. (2) Upon charging <112>-oriented Si nanowires, the nanowires swell rather than elongate. We found unexpectedly the highly anisotropic volume expansion in lithiated Si nanowires, resulting in a surprising dumbbell-shaped cross-section, which developed due to plastic flow and necking instability. Driven by progressive charging, the stress concentration at the neck region led to cracking and eventually fracture of the single nanowire into sub-wires. Moreover, the fully lithiated phase was found to be crystalline Li₁₅Si₄, rather than the widely believed Li₂₂Si₅ phase, indicating the maximum capacity of Si being 3579 mA h g⁻¹ at room temperature. (3) Carbon coating not only increases rate performance but also alters the lithiation induced strain of SnO₂ nanowires. The SnO₂ nanowires coated with carbon can be charged about 10 times faster than the non-coated ones. Intriguingly, the radial expansion of the coated nanowires was completely suppressed, resulting in enormously reduced tensile stress at the reaction front, as evidenced by the lack of formation of dislocations. (4) The lithiation process of individual Si nanoparticles was observed in real time in a TEM. A strong size dependent fracture behaviour was discovered, i.e., there exists a critical particle size with a diameter of ∼150 nm, below which the particles neither cracked nor fractured upon lithiation, above which the particles first formed cracks and then fractured due to lithiation induced huge volume expansion. For very large particles with size over 900 nm, electrochemical lithiation induced explosion of Si particles was observed. This strong size-dependent fracture behaviour is attributed to the competition between the stored mechanical energy and the crack propagation energy of the nanoparticles: smaller nanoparticles cannot store enough mechanical energy to drive crack propagation. These results indicate the strong material, size and crystallographic orientation dependent electrochemical behaviour of anode materials, highlighting the powerfulness of in situTEM electrochemistry, which provides not only deep understanding of the fundamental sciences of lithium ion batteries but also critical guidance in developing advanced lithium ion battery for electrical vehicle and backup power for fluctuation energy sources such as wind and solar energy.