Enabling high-performance lithium metal batteries through nanostructure engineering

Abstract

The lithium metal battery (LMB) is considered one of the most promising high-energy battery systems for the growing electric vehicles market owing to the high specific capacity and low electrochemical potential of metallic Li. Nevertheless, various problems, including dendritic deposition, unstable solid-electrolyte interphase (SEI), universal cathode crossover, excessive Li usage, and unstable cathodes, not only affect the electrochemical performance of LMBs but also cause severe safety hazards, posing obstacles to the commercialization of LMBs. This thesis addresses the above problems based on nanostructure engineering approaches to obtain stable high-energy-density LMBs for practical applications. First, a free-standing interlayer (CoZn-YSIL) was fabricated based on the well-designed yolk-shell structure (CoZn-YSs) with N-rich yolk and Co, Zn, N-rich shell. The lithiophilic sites (Co, Zn, N) can decrease the Li+ nucleation barrier, enabling efficient Li stripping/plating process in various electrolytes, leading to a dendrites-free deposition and prolonged cycling performance. The yolk-shell structure can promote the selective deposition of Li metal to alleviate volume change. As a result, Li||Cu cell with CoZn-YSIL stably cycled for 200 cycles with a high CE of 99.2 %. With the protection of CoZn-YSIL, the Li deposition can be regulated even under a large current density of 10.0 mA cm-2. When coupling with high-loading LiFePO4 cathode (12.4 mg cm-2), the LMB with CoZn-YSIL exhibited a high capacity retention rate of 92 % after 100 cycles. The stable interface also plays an important role to prohibit cathode-released species (i.e., transition metal ions, polysulfides, cathode-released O2) from corroding LMAs. To alleviate universal cathode crossovers, the CoZn-YSIL was set in LMBs with various high-­energy-density cathodes, such as Ni-rich layered oxides, Li- and Mn-rich cathode, and sulfur cathode. The Co, Zn, N active sites show strong binding ability towards universal cathode-released species during long-term cycling. As a result, with the protection of the CoZn-YSIL, the Li||NMC cell delivered a high capacity retention rate of 87.3% for 230 cycles, and the Li-S cell presented a high areal capacity of 5.32 mAh cm-2 after 100 cycles. In order to construct stable LMAs for anode-less LMBs, a near-surface Li+ irrigation strategy was proposed by coating a PPy layer on Cu foil surface via oCVD technique. The PPy layer can create a near-surface Li+-rich region (NSLR) to promote the uniform Li nucleation and deposition at the interregional space between Cu and PPy, as well as induce the formation of robust N-rich SEI. As expected, a uniform and dense deposition of Li metal on Cu@PPy was achieved. The cycling life of Li anode (1× Li excess) with Cu@PPy collector was prolonged to 3000 h with a small overpotential of 8 mV at 0.5 mA cm-2/5 mAh cm-2. In order to construct a stable sulfur cathode to match the LMAs for the practical LMBs, the core-shell carbon spheres (N-C@CoZnN-C) with high binding and catalytic ability towards LiPSs were designed as S host. The core-shell carbon structure can encapsulate S to improve the electric conductivity. The porous property can enhance ion transportation under high S loading. The rich active sites can anchor and catalyze the LiPSs to improve the reaction kinetics. The N-C@CoZnN-C/S cathode exhibited a high initial specific capacity of 1343.03 mAh g-1 and outstanding capacity retention rate of 94.3% after 100 cycles. With an increased S loading of 3 mg cm-2, this cathode can still deliver a high initial specific capacity of 1188.45 mAh g-1.