Mechanical and Electrochemical Behavior of Silicon Anode for Li-ion Batteries on Soft Substrate

Abstract

Nowadays, there is an increasing demand for full bendable and wearable electronic devices. To meet this need, soft substrate is a promising candidate for those next-generation electronics. For miniaturization and high integration of current device, high-capacity electrode materials are necessary for operation of those devices. Among many active materials for anode electrode, alloying types (Si, Sn, Ge, As, Al, etc) are widely known due to their high gravimetric and volumetric capacity than any other electrodes. However, these alloying types of anode materials undergo large volume expasions, inducing huge internal stress. This intrinsic stress lead to mechanical degradations of the electrodes and decaying the drastic capacity fading. When the electrodes were subjected to mechanical deformation, such as bending, twisting, and stretching, the electrodes shows fatal failures. Therefore, relieving these intrinsic and extrinsic stress become very important to improve the performance of alloying type anodes for the bendable Li-ion batteries. In this study, we have introduced novel designs of Si anode to overcome the two main limitations of Si: largest volume expasion and brittle nature. This is because, if the drawbacks of Si can be solved, any electrode can be employed. From the fundamental understandings of Si thin film anode during charge and discharge process, we first observed certain mechanical failures (buckle, delamination) of the thin-film-type Si anode on soft substrate, which resulted in rapid capacity decay. Post analysis of the buckles led us to estimate the interface toughness of lithiated Si, which were 6.52±0.14 J/m2 from circular buckles and 5.89±0.27 J/m2 from telephone cord buckles. To improve the adhesion strength between Si and Cu, as well as to relieve the intrinsic and extrinsic stress, we introduced a nano-hairy structure on a polyimide (PI) substrate and successfully developed a robust Si anode electrode for PI-substrate-based Li-ion batteries. Through in-situ SEM observation, we elucidated the working mechanism of the nanostructured amorphous Si anode. A direct lithiation process revealed that, the stress due to Si volume expansion was released via the inter-spacing between the nano-hairy Si anodes and by the compliable nature of the polymer nanowire. Subsequently, using a coin-cell test, the nano-hairy Si anode exhibited a much longer cycle life and higher capacity (1573 mAh/g at 100th cycle). In the C-rate test, outstanding response and high capacity recovery (when returning to the initial current value) from a high rate charge/discharge test was obtained to compare with a Si thin-film electrode. Solid-electrolyte-interphase (SEI) was found to form over the nano-hairy Si, and to release the internal stress, cracks appeared in the SEI with very uniform crack length below the critical delamination length, thereby, further delaminations were prevented. For flexible and bendable applications, we also evaluated mechanical resistance to cyclic sliding for a current collectoor and cyclic bending for a full-pouch battery. Cu on nano-hairy PI showed extremely low change (<10%) in electrical resistance up to 500,000 cycles, whereas Cu on pristine PI showed over 300% change. With a full-pouch battery, we succefully turned on the back light unit and maintained 3.7 V under 3000 bending cycles on a 12.7 mm-bending radius. Even, excellent C-rate was also observed under the bent state. For further study of electrochemical and electrical properties, we introduced O2 gas, and successfully obtained very thin and long nano-hairy PI. Using this structure, we were able to improve the electrochemical performance, higher capacity and excellent rate capability due to the larger surface area. In cyclic bending fatigue test for Cu current collector, O2-nano-hairy Cu showed lower change in electrical resistance than both CF4-nano-hariry and pristine PI under 4% strain condition. We also introduced graphene oxide (GO) dispersed conductive polymer composites to replace the Cu current collector. From the results of electrochemical test, Si on nano-hairy conductive PI showed the typical capacity of Si in the lower C-rate region. These results can suggest the potential of fabric or polymer (with low conductivity) based Li-ion battery system to understand the limit of the charge or discharge current. This study demonstrated the direct integration of Si anode on a polymer substrate for bendable Li-ion battery and attempted to address all of the challenges of Si by designing a core-shell Cu/a-Si nano hairs. One aspect that stands out is the ability to use bendable polymer substrate to integrate any active electrodes. We believe that our study would widen the choice of the materials and open up the option of using polymer substrate-based bendable batteries.