Abstract
Lithium titanate and titanium dioxide are two best-known high-performance electrodes that can cycle around 10,000 times in aprotic lithium ion electrolytes. Here we show there exists more lithium titanate hydrates with superfast and stable cycling. That is, water promotes structural diversity and nanostructuring of compounds, but does not necessarily degrade electrochemical cycling stability or performance in aprotic electrolytes. As a lithium ion battery anode, our multi-phase lithium titanate hydrates show a specific capacity of about 130 mA h g−1 at ~35 C (fully charged within ~100 s) and sustain more than 10,000 cycles with capacity fade of only 0.001% per cycle. In situ synchrotron diffraction reveals no 2-phase transformations, but a single solid-solution behavior during battery cycling. So instead of just a nanostructured intermediate to be calcined, lithium titanate hydrates can be the desirable final destination.
Highlights
Lithium titanate and titanium dioxide are two best-known high-performance electrodes that can cycle around 10,000 times in aprotic lithium ion electrolytes
Once we realize that not all water is bad, and some water can do good, it might be possible to synthesize many more electrode materials with superior power density and ultralong cycle life in the Li2O–TiO2–H2O composition space, that beat what are available on the Li2O–TiO2 binary axis, by taking advantages of the structural diversity of hydrated crystals (2D layered), and dehydration induced phase transformation and nanostructure refinement
The materials design philosophy we have established is the following: (i) water may not be bad actor for RT electrochemical cycling in aprotic electrolyte, if they are trapped in the lattice and not free; (ii) as water promotes structural diversity in hydrothermal or sol-gel process, it could be used to tailor the initial structures such as 2D sheets or nanotubes, which greatly improves the ion diffusivity; (iii) the dehydration process thereafter should be carefully optimized: first, all the “free” water should be driven out; but as it is not integral part of the lattice, this usually does not cause drastic phase change
Summary
Lithium titanate and titanium dioxide are two best-known high-performance electrodes that can cycle around 10,000 times in aprotic lithium ion electrolytes. Water promotes structural diversity and nanostructuring of compounds, but does not necessarily degrade electrochemical cycling stability or performance in aprotic electrolytes. Because water is considered “harmful” in high-voltage window aprotic electrolytes (free water can be highly reactive to LiPF6, lithium metal anode and lithium alkyl carbonates)[7], most researchers calcine the nanostructured LTHs to completely remove all water by raising temperature to above 500 °C8. This can cause an unwanted side effect of coarsening and aggregation of the structure. The trapped water can promote structural diversity and nanostructuring that could be highly beneficial for battery performance in aprotic electrolytes
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