Silicon has been targeted as an anode material that can deliver higher capacity for lithium ion batteries than current prevailing graphite. One strategy to minimize the detrimental effects of volume expansion during lithiation of silicon is to use a limited amount of silicon in combination with graphite. Even in these cases, however, cell volume expansion is a key concern when considering battery packaging for applications like electric vehicles, where cells are cycled over a wide state-of-charge window and packaged energy density is critical to delivering long range. The first step towards understanding the implications of expansion to cell design and packaging is to characterize the volume expansion of silicon-containing cells during use.

In the current work, in-situ volume and pressure responses of pouch cells were carefully measured with specially designed in-house fixtures (Figure 1). Volume expansion of pouch cells was measured by tracking the buoyancy force with a high precision balance, giving volume change precision to 1 micro liter with a long-term drift of < 10 microliter/day. Cell stack loading pressure was investigated by constraining cells between two fixed plates and a floating plate under a load cell. We found that the volume and pressure fixtures provided very similar patterns during pouch cell charging and discharging. Commercially available NMC/Graphite pouch cells showed symmetric and reversible P/V responses, while in-house cells with silicon-composite anodes showed an asymmetric response and long-term drift associated with continued SEI formation and gas generation.

 [1] L. Lu, X. Han, J. Li, J. Hua, M. Ouyang, J. Power Sources, (226) 2012, pp 272 - 288