The growing environmental concern over carbon dioxide emission has driven the demand for next-generation green vehicles like electric vehicles (EVs) and hybrid electric vehicles (HEVs). This in turn calls for higher capacity and higher energy rechargeable batteries for supporting long-distance driving of EVs. In this research, freestanding flexible Si nanoparticles-carbon nanotubes (SiNPs-CNTs) composite paper anodes for lithium-ion batteries (LIBs) have been prepared by a simple, inexpensive, and scalable approach of ultrasonication and pressure filtration. No conductive additive, binder or metal current collector is used. The composite using multi-walled CNTs (MWNTs) shows electrochemical properties superior to those using single-walled CNTs (SWNTs) or vertically aligned carbon nanotubes (VACNTs). The SiNPs-MWNTs composite (Si-MW) sample achieves first cycle specific discharge and charge capacities of 2298 and 1492 mAh/g, respectively. The first cycle irreversibility is compensated for by stabilized lithium metal powder (SLMP) prelithiation, leading to reduction of initial capacity loss from 806 to 28 mAh/g and increase of initial coulombic efficiency from 65% to 98%. The relationship between different SLMP loadings and cell performance has been established to understand the prelithiation mechanism of SLMP, optimize the construction of Si-based cells, and enable the exploration of novel cathode materials. The positive effect of FEC as electrolyte additive (10%) on the cyclability is verified. Through control of Si/CNT weight ratio, the optimal combination between the high capacity of SiNPs and the high electrical conductivity and structural stabilization ability of MWNTs is found in the case of the Si-MW 3:2 composite, resulting in improved cycling stability and high rate capability. The reversible capacity can be recovered to 1866 mAh/g when the current rate returns to 100 mA/g during cycling at current rate from 100 to 1000 mA/g. After 100 cycles, the electrode retains a reversible capacity of 1170, 850, and 750 mAh/g at the current rate of 100, 280, and 500 mA/g, respectively. FEC-based electrolytes using FEC as the co-solvent (50 wt%) are compared with the one using FEC as the additive. It is found that the EC-free FEC-based electrolyte achieves higher specific capacity and better capacity retention in terms of long-term cycling. After 500 cycles, the capacity retention of the cell using DEC-FEC (1:1) is increased by 88% and 60% compared to the cells using EC-DEC-FEC (45:45:10) and EC-FEC (1:1), respectively. Through SEM-EDX and XPS analyses, a possible reaction route of formation of fluorinated semicarbonates and polyolefins from FEC is proposed. The inferior cell performance related to the EC-containing electrolytes is likely attributed to the formation of excessive polyolefins which do not favor Li ion migration. The strategy of capacity-control cycling is employed to seek extended cycle life. Stable 326 charge-discharge cycles at designated capacity of 506 mAh/g are attained for the Si-MW 1:1 cell. A self-healing phenomenon is observed by studying the specific capacities and charge/discharge end voltage, and proposed as the possible mechanism behind the improved cycling stability. Prolonged cycling of over 500 cycles under capacity control (500 mAh/g) and the interesting pattern of variation in the discharge/charge end voltage are successfully reproduced with different electrode/electrolyte and current conditions. EIS and SEM-EDX analyses suggest that by setting the capacity/voltage limits for charge-discharge cycling, the growth of SEI can be limited. We believe the 3D network of MWNTs forms a continuous conductive pathway within the composite structure, which ensures sufficient electrical conductivity, holds the Si particles together, and alleviates the volume expansion of Si. Moreover, the freestanding feature of our electrode eliminates the non-active mass, giving rise to specific energy enhanced by 27% compared to current graphite-based cells by theoretical calculation.
