Solid State LIB : Vibration
Solid state batteries (SSB) are very much the center of attention – it’s safer, has higher energy density, and can be the key to solidifying the position of LIBs for vehicle applications. The focus thus far seems to be on simply making SSBs that demonstrates good electrochemical properties at room temperature – BUT with a not so discrete note that reads ‘at laboratory setup’. There hasn’t been much considerations of how the SSBs will react in a moving, vibration, stop-starting, crashing system.
Yes, the SSBs are expected to withstand much harsher abuse conditions especially nail penetration and thermal stability, but what about the day-to-day rigors of just driving along the road?
SSBs replace the conventional liquid electrolyte with solid electrolyte in one form or another (gel-types, ceramic based, etc.) The question is… will they be ok after 10 years of driving?
Will the gel-type electrolyte not experience phase separation (agglomeration of the gel scaffold) to form a clearly phase-separated blob akin to blobs of cheese floating in whey? Even choosing to briefly overlook the issues associated with active material volume changes during charge/discharge… Will the solid electrolyte and active material interphase remain compact and in contact through years of driving over potholes?
Even for LIBs there is limited understanding on the impact of vibration, M-shock on the performance of LIBs with the overwhelming consensus being: it depends.
It depends on if it’s a pouch, cylindrical, or prismatic / what sort of cathode or anode it uses / if we’re considering cell level, module level, or pack level where resonance behavior changes and becomes increasingly difficult to predict.
Based on a short search, it seems like that the cylindrical cell is most impacted owing to its central mandrel structure causing internal damage (1) and that the pouch and prismatic cells are more robust, but all formats experience some degradation in electrochemical performance – ohmic resistance, OCV recovery, capacity of cells, etc. (2). What was surprising was the work by Somerville et al (3) where detailed XPS analyses of 18650 cylindrical cells composed of NMC and graphite were completed after being put through sequences of vibration tests detailed in (4).
The gist of the (3) is as follows:
i) Traces of Lithium Carbide in the electrolyte (while it was only found in the surface film in the control cell) indicating the Li-C was detached from the surface film through vibration
ii) An increase in phosphate concentration in the vibrated cells’ surface film compared to the control cells’ surface film indicating that with the detachment of the initial Li-C surface film, a new electrolyte-depleting surface film was formed. The authors noted that since that in commercial cells, additive like VC are commonly used to artificially create stable SEI layers and because vibration caused these stable films to detach, new less-stable layers formed as evidenced by the increase in phosphate concentrations. (Hence the increase in DC resistance and decrease in capacity).
iii) Higher concentrations of C-O and O-C=O in the vibrated cells’ environments indicative of an increase in surface film reactions.
What is surprising is that SEI layers – which are known to be very stable and extremely… attached to the active material – are susceptible to becoming delaminated due to vibration and causing overall decrease in cell performance. What is even more surprising is that if liquid electrolyte base LIBs are this susceptible to performance degradation… then what will be the impact of SSBs? I’ve conducted a brief search looking for literature… but nothing that explores this facet of the SSBs. So, not much going on to assess the impact of driving on SSBs – this is worrisome.
A small suggestion if a ceramic based SSBs is a must is to consider the concept of brittle to ductile transition (BTD). BTD is a phenomenon where a brittle material (like ceramics) exhibit ductile deformation behavior at nanoscale – an example would be ZnO where it is brittle at hundreds of nm scale but is ductile and deforms like metals would at under 10 nm thicknesses. This may be an interesting idea (or a stupid one) as making sure that the ceramic electrolyte is composed of agglomerated particles that are small enough to exhibit BTD phenomenon, then the SSB system may be able to form surface films that are also in the BTD range and will not deform (i.e. delaminate) as easily due to vibration.
I know I’m stretching the concept here and realize it doesn’t really make sense – the delamination of SEI layers is the result of surface affinity (or lack thereof) between the active material and SEI layer. However, having a ductile ceramic SEI layer will at least allow for some cushioning effect against the volume changes during charge/discharge. After all, it will be this volume changes during charge/discharge that exacerbates the interface mismatch between the active material and SEI layer that ultimately leads to delamination.
References
1) Martin J. Brand et al, 2015, Journal of Power Sources, 288, p.62-69
2) Wenhua Li et al, 2019, IEEE Access, 7, p.112180-112190
3) Limhi Somerville et al, 2017, Energies, 10(6), 741
4) James Michael Hooper et al, 2016, Energies, 9(1), 52
