NANOSTRUCTURED SILICON FOR ANODES OF LITHIUM-ION BATTERIES
There has been an increasing research activity in development of high-capacity, reliable and durable energy sources, in particular Li-ion batteries, for compact electronic devices, electric vehicles and various energy systems. Majorefforts are focused on finding new and improving existing materials for electrodes of the batteries. At that point silicon (Si) is of considerable interest because it is characterized by a much higher charge/discharge capacity as compared to the commonly used carbon materials. Meanwhile, a principal problem of Si anodes in the batteries is their mechanical durability during volume expansion accompanying the lithiation process resulting in cracking of the electrodes. Nanostructuring of the electrodes could overcome this problem.
Further we present experimental results on fabrication and study of nanostructured Si anodes for Li-ion batteries demonstrating appropriate accumulation properties and durability.
The nanostructured Si film after selective removal of Alisshown in Fig. 1. Highly porous skeleton of Si is visible to be formed by connected 20-200 nm grains. Al is completely etched off while the Si crystalline grains left look to be covered with amorphous Si like it was previously observed.
Figure 1. Surface SEM image of Al+Si film after selective etching off Al.
A voltage-time characteristic of the prototype Li-ion cell at charge/discharge cycling is shown in Fig. 2. In general, it confirms conventional operation of Li‑ion cell after the first charge cycle when the initial lithiation of anode takes place.
Figure 2. Typical voltage-time characteristic of the prototype Li-ion cell at cyclic lithiation. The straight lines show the test current variation
Figure 3. Self-discharge dynamics of the prototype Li-ion cell.
Self-discharge dynamics of the prototype Li-ion cell is shown in Fig. 3. Despite the losses (about 35% within an hour) associated with the imperfection of the experimental system and the presence of leaks, the studied prototype Li‑ion cell has demonstrated considerable potential for further improvement.
The nanostructured Si film morphology after 20 charge/discharge cycles is shown in Fig. 4. The test sample has retained the original coral-like structure, but microcracks have appeared therein. The number of the microcracks correlates with the current density used for charging the cells. Nevertheless, microcracks have not led to the fragmentation of the whole structure. The microcracks appearance is associated with the quite hard test conditions owing tohigh current density through the cell.
Figure 4. Surface SEM images of nanostructured Si film after 20 charge/discharge cycles: a) general view b) fragment of the structure.
Rather thick cracked layer can be also seen on the upper surface of the anode. Apparently, it is SEI (solid-electrolyte interphase), which is formed on the anode surface and is the product of the electrolyte reduction. Formation of thick SEI layers on Si nanostructures was observed in other works. They were noted to play a stabilizing role providing an increase of mechanical durability of anode materials.
It is obvious that the coral-like structure is more mechanically strong in comparison to needle or tubular ones because grains in it are interconnected to each other over the whole film thickness.
The coral-like nanostructured Si films fabricated by magnetron sputtering of the composite Al+Si target and selective etching off Al have clearly demonstrated an ability to accumulate Li duringcharging and to maintain a certain level of voltage on a discharge time. Herewith, they have excellent mechanical durability even at high charging current densities which is their undoubted advantage. An extended study of the role of porosity and crystal structure of the grains has to optimize their performance in Li-ion batteries.