Silicon and germanium nanowire based lithiophilic current collectors for lithium metal batteries

This work focuses on the development of current collectors using silicon (Si) and germanium (Ge) nanowires (NWs) as lithiophilic materials for Lithium metal batteries (LMBs).

With increasing demand of developing long milage electric vehicles, the need to develop high energy density rechargeable batteries has increased as well. With the current state-of-the-art Lithium ion batteries (LIBs) using graphite anode, the energy density is limited due to the low specific capacity of graphite (372 mAh g-1 ) anode. Therefore, much interest has been shown during the last decade to develop alternate anode materials to replace graphite. Li metal with its high theoretical specific capacity (3860 mAh g-1), low reduction potential (-3.04 V vs. standard hydrogen electrode (SHE)) and low density (0.53 g cm-3 ) has emerged as a viable anode to develop high energy density LMBs. However, issues such as dendrite formation, inactive Li and low columbic efficiency during cycling has plagued the use of pristine Li metal as anode for LMBs. Therefore, strategies such as electrolyte development and current collector design are being investigated to overcome the challenge of Li dendrite formation. In this work, current collector design has been widely investigated with an aim to limit the amount of Li in the final LMB to increase energy density of the LMB full cells.

Chapter 3 describes the use of Si, Ge and SiGe alloy NWs as lithiophilic host for LMBs. Here, the Li metal was thermally infused onto the carbon paper – nanowire (CP-NW) substrates and directly used as Li host against LFP, NMC, and S cathodes. The effect of each NW composition was comprehensively studied using symmetric cell electrochemical testing, post-cycling microscopy characterization as well as authenticated by density functional theory (DFT) calculations. It was revealed that the incorporation of Ge in SiGe alloy increased the binding energy of the NWs with Li, with pure Ge demonstrating the highest binding energy. The benefits of incorporating Ge as lithiophilic host for Li anode resulted in a high capacity retention of 90 % after 200 cycles at 0.5 C, when paired with high areal capacity NMC cathode. The high lithiophilicity of Ge as compared to Si was further utilized in Chapter 4 by designing carbon cloth (CC) based interlayers where Ge was grown on one side of the CC cloth and pasted on the pristine Li metal with Ge side facing Li. This induced ‘bottom-up’ Li infilling as compared to lithiophobic CC where Li plated on the top (side facing separator). This directional Li plating on the Li-Ge interface resulted in no dendrite formation even at high current density of 2 mA cm-2 with stable symmetric cell performance up to 2500 h at 2 mAh cm-2 areal capacity. The post-cycling microscopy analysis revealed a smooth interface with X-ray diffraction also suggesting the formation of lithiophilic Li15Ge4 phase at the Li/Ge interface. The GeCC interlayer based Li anode when paired with NMC cathode resulted in high capacity retention of 80 % after 400 cycles with an average C.E of 99.7 %. Binder-free Ge NWs coating on Cu current collector (Cu-Ge) reported in Chapter 5, were designed with an aim to further lower the amount of Li in the full cells. Cu as current collector was deliberately chosen to develop current collectors which could be industrially acceptable as well. With the formation of lithiophilic Li15Ge4 phase, uniform Li deposition was achieved on the lithiated NW while larger dendrites formed on the pristine Cu current collector. This resulted in achieving low nucleation overpotential of 10 mV in Cu-Ge current collector which was 4 times lower than that of pristine Cu foil. Overall Cu-Ge current collector showed better C.E performance at high current densities and plating capacities. Further, N/P ratios (negative to positive electrode) were controlled and assembled with LFP, NMC and S cathode systems. Due to the lithiophilic nature and uniform Li-ion flux provided by Ge NWs, the Cu-Ge@Li/NMC full cell delivered a capacity retention and average C.E of 86.5 % and 99.2 % over 100 cycles, respectively. Corresponding Cu@Li/NMC full cell couldn’t achieve stable cycling due to formation of dendrites and inactive Li during cycling. Overall, the Cu-Ge@Li anode (paired with NMC cathode) achieved a massive anode mass reduction of 68.3 % as compared to a standard commercial graphite anode (paired with NMC).

The final research chapter (Chapter 6) involves the exploration of anode materials for Na?ion batteries (NIBs). In this chapter, Si, Ge and SixGe1-x NWs were tested as potential anodes in NIBs. With crystalline phases of Si, Ge and SiGe alloy NWs couldn’t activate in NIB, a pre-amorphization step increased the Na-ion diffusivity which resulted in their activation. Among all the compositions tested, a-Si0.5Ge0.5 resulted in the best capacity retention of 75 % while the lowest capacity retention of 3 % was shown by a-Si after 100 cycles. Post-cycling microscopy analysis revealed cycled a-Si0.5Ge0.5 NWs adhering well to the stainless steel (SS) substrate as compared to the cycled a-Ge NWs. This study highlighted the importance of alloying Si and Ge in NW morphology and its subsequent pre-amorphization to activate Na-ion diffusion and the effect of Si incorporation to stabilize electrochemical performance.