Forming multilayered solid electrolyte interphase structures on high capacity electrodes
Provided are novel methods of fabricating electrochemical cells containing high capacity active materials that form multilayered solid electrolyte interphase (SEI) structures on the active material surface during cell fabrication. Combining multiple different SEI layers on one surface can substantially improve cell performance by providing each layer with different properties. For example, an outer layer having a high electronic resistance may be combined with an inner layer having a high ionic permeability. To form such multilayered SEI structures, formation may involve changing electrolyte composition, functionalizing surfaces, and/or varying formation conditions. For example, formation may start with a boron containing electrolyte. This initial electrolyte is then replaced with an electrolyte that does not contain boron and instead may contain fluorine additives. In certain embodiments, cell's temperature is changed during formation to initiate different chemical reactions during SEI formation. Variations in multilayered SEI structures may be also achieved by varying current rates.
1. An electrode of an electrochemical cell, the electrode comprising:
an electrochemically active structure comprising a high capacity active material; and
a multilayered SEI structure formed on a surface of the electrochemically active structure, the multilayered SEI structure comprising:
an inner SEI layer over the electrochemically active structure; and
an outer SEI layer over the inner SEI layer,
wherein the inner SEI layer has a different composition and/or morphology than the outer SEI layer, wherein the inner SEI layer has a higher ionic conductivity than the outer SEI layer, and wherein the inner SEI layer is at least about twice as thick as the outer SEI layer.
2. The electrode of claim 1 , wherein the inner SEI layer has a thickness of between about 5 nanometers and 50 nanometers.
3. An electrochemical cell comprising:
a negative electrode comprising an electrochemically active structure comprising a high capacity active material and a multilayered SEI structure formed on a surface of the electrochemically active structure during formation of the electrochemical cell, the multilayered SEI structure comprising:
an inner SEI layer formed over the electrochemically active structure; and
an outer SEI layer formed over the inner SEI layer,
wherein the inner SEI layer has a different composition and/or morphology than the outer SEI layer, wherein the inner SEI layer has a higher ionic conductivity than the outer SEI layer, and wherein the inner SEI layer is at least about twice as thick as the outer SEI layer.
4. The electrode of claim 3 , wherein the outer SEI layer comprises one or more boron-based salts.
5. The electrode of claim 3 , wherein the one or more boron-based salts comprise lithium bis(oxalato)borate (LiBOB).
6. The electrode of claim 1 , wherein the outer SEI layer comprises one or more ethylene carbonate reduction products.
7. The electrode of claim 1 , wherein the inner SEI layer comprises one or more poly-vinylene carbonates.
8. The electrode of claim 1 , wherein the porosity of the inner SEI layer is greater than a porosity of the outer SEI layer.
9. The electrode of claim 1 , wherein the inner SEI layer has a porosity of between about 30% and 60%.
10. The electrode of claim 1 , wherein the high capacity active material is selected from the group consisting of silicon, tin, and germanium.
11. The electrode of claim 1 , wherein the electrochemically active structure is a nanostructure.
12. The electrode of claim 1 , further comprising an intermediate SEI layer positioned in between the inner SEI layer and the outer SEI, wherein the intermediate SEI layer has a different composition or morphology than the outer SEI layer and a different composition or morphology than the inner SEI layer.
13. An electrochemical cell comprising:
a negative electrode comprising an electrochemically active structure comprising a high capacity active material and a multilayered SEI structure formed on a surface of the electrochemically active structure during formation of the electrochemical cell, the multilayered SEI structure comprising:
an inner SEI layer formed over the electrochemically active structure; and
an outer SEI layer formed over the inner SEI layer,
wherein the inner SEI layer has a different composition and/or morphology than the outer SEI layer.