Does the increase in internal pressure of a lithium-ion battery equal the decrease in capacity?

In lithium-ion batteries, the negative electrode potential is low, which will cause the electrolyte to undergo a reduction reaction on its surface. The resulting decomposition products become what we often call the SEI film. The SEI film is electronically insulated, but it can conduct Li+, so it is good The SEI film can effectively inhibit the decomposition of the electrolyte and increase the cycle life of the lithium-ion battery. However, the SEI film actually cannot completely prevent the decomposition of the electrolyte. First, because the initial SEI film is relatively thin, some electrons can also diffuse to The surface of the SEI film causes the electrolyte to continue to decompose. With the thickening of the SEI film, the difficulty of electron diffusion becomes more and more difficult, so the growth rate of the SEI film will also decrease accordingly. Secondly, volume expansion will occur during the lithium insertion process of the negative electrode and cause the destruction of the SEI film. The fresh electrode surface is exposed, which leads to the continuous decomposition of the electrolyte and the continuous growth of the SEI film. Therefore, the SEI film will continue to occur during the cycle of the lithium ion battery. The growth of the anode leads to an increase in the thickness of the negative electrode, which creates pressure inside the battery. From the above analysis, it is not difficult to see that there is a close relationship between the growth of the SEI film and the increase in the internal pressure of the electrode, and there is also a close relationship between the growth of the SEI film and the reversible capacity decline of the lithium ion battery, so can we How to predict the life decline of lithium-ion batteries by monitoring changes in the internal pressure of lithium-ion batteries? Recently, AJ Louli (first author) and JRDahn (corresponding author) of Dalhousie University in Canada found that there is a close relationship between the continuous pressure of lithium-ion batteries and the reversible capacity loss, which can be measured by measuring the internal pressure of lithium-ion batteries. The change realizes the prediction of the rate of decline in the capacity of lithium-ion batteries. The volume expansion of the Si anode during the cycle is relatively large, and higher requirements are put forward for the stability of the SEI film. Numerous studies have shown that adding a part of FEC to the electrolyte can well improve the cycle performance of the Si-containing anode. Figure A below shows the cycle performance of batteries B and D after adding 1% and 10% FEC, respectively. It can be seen that battery B with 1% FEC starts to dive after 50 cycles, while battery B with 10% FEC After 100 cycles of the battery, there is still no sudden capacity jump. The capacity of battery D with 1% FEC started to dip after 250 cycles, but battery D with 10% FEC did not show significant capacity diving after more than 300 cycles. This is mainly due to the fact that batteries with less FEC added to FEC After the additives are gradually consumed, the decomposition of the electrolyte cannot be effectively prevented, which results in the rapid decomposition of the electrolyte on the surface of the negative electrode and consumes limited active Li. The figure B below shows the pressure changes during the cycle of several batteries. It can be seen that the number of cycles at which the batteries B and D with 1% FEC start to dive at their respective capacities also shows an exponential increase in pressure, while adding 10% The pressure increase of the FEC battery during the cycle is relatively gentle, and there is no sudden rapid increase, which is also very consistent with the cycle performance of several batteries. The prediction and analysis of the decay of lithium-ion batteries usually requires the help of complex models or expensive equipment. AJLouli can quickly predict the reversible capacity of the battery by simply measuring pressure data. The slower the increase in irreversible pressure, the cycle performance of the battery is usually also The better, when the pressure of the battery increases exponentially, it usually means that the capacity of the battery is about to dive.