Perspective on Li-containing alloys for high-safety and high-energy density batteries

The introduction of Li-containing alloys seems to be a fundamental tool to resolve the safety hazards caused by lithium dendrites. But there are still many unanswered questions, for instance, the protective mechanisms for different alloys are not clearly understood[86]; the Li-containing alloy materials can effectively improve the state of first-layer Li deposition, however, the substrate will revert to pure Li as deposition continues[54]; the Li-containing alloy anodes still face the great volume change and serious side reaction during the striping/plating process, etc. Therefore, these scientific problems are still urgently awaiting our solutions.
Simply using lithium metal alloy anode/lithium alloy artificial SEI film or using lithium alloy to modify the current collector, separator and electrolytes cannot fully solve the challenges faced by lithium anode. Only by combining multiple strategies can the problems caused by lithium anode be better or completely solved. Here we proposed several suggestions (as shown in Figure 11) for the future research of lithium-containing alloys employed in Li metal batteries:
i) As the binary Li-containing alloy either has the high reaction activity (i.e Li-Na), or the great volume change (i.e. Li-Si, Li-Sn), or the low energy density (i.e. Li-Bi, Li-Zn), or the high cost (i.e. Li-Ag, Li-Cu) each kind of shortcoming, it can take more consideration into ternary/multicomponent lithium alloys, which can make up for each other through multiple components. But the presence of additional metal that are not directly involved in electrochemical reaction results in additional weight and volume and thus cause the specific energy density reducing compared to using the pure lithium or binary lithium alloy anodes[129]. Another disadvantage is that there still exist a substantial change in specific volume upon charging and discharging alloy electrode reactants, can lead to loss of electrical contact, and thus capacity loss[129]. In addition, the complexity and cost of preparing ternary/ multicomponent lithium alloys also need to be considered
ii) Considering the substantial volume change exactly exists in lithium-containing alloys anodes, constructing nanostructures to host lithium alloy or preparing lithium alloy-based composites is also a good choice[130, 131]. By encapsulating the lithium alloy into special nanostructures, i.e. 3D graphene[132], CNT[131], etc., or using polymers coating[25], the volume change of lithium alloy has been effectively eliminated, because of these nanostructured host or extra components, Li-containing alloy anodes will be more stable in the organic electrolyte and the lithium dendrite would also be effectively inhibited.
iii) Constructing artificial protection/SEI layer on the lithium alloy anode, i.e., recently Won II Cho et al., reported a Li-Al anode protected by a Langmuir-Blodgett artificial SEI composed of MoS2[133]. Such a MoS2 artificial SEI layer exhibited a combination of a high Li binding energy, molecular smoothness, and low barrier to Li adatom diffusion, which favors efficient binding of Li and transport away from the electrode/electrolyte interface as well as favors stable and reversible Li migration of the MoS2 coating Li-Al anode. As a result, the MoS2 coating Li-Al alloy anode exhibited high reversibility stable Li migration during recharge of the cells compared to the Li-Al alloy anode without the MoS2 coating.
iv) Developing the suitable electrolytes and separators for lithium-containing alloys anodes. Modification of electrolyte or separators with lithium alloy will limit its application in lithium metal batteries, as electrolyte components have significantly influences on the electrochemical performances of electrodes, no matter anode or cathode. For example, the absence of volatile or flammable compounds is expected to make solid electrolytes safer than their liquid counterparts at elevated temperatures[11]. Additives, can decompose, polymerize or adsorb on the Li surface, modifying the physico-chemical properties of the SEI and therefore regulating the current distribution during Li deposition[19, 23, 134]. Solvents, i.e, the ionic liquid, shows an exciting role in improving the low temperature performances of batteries[135]. Lithium salts could not only benefit to stabilize the spontaneous solid electrolyte interphase (SEI) films, but also control the nucleation and growth of metallic lithium, thus enhancing the stabilities of lithium anodes during the stripping and plating processes[21, 136].
While separators serving as a physical barrier between electrodes, traditional polyolefin-based separators easily suffer a “shut-down” problem when penetrate through lithium dendrites or exposed to overheating and/or overcharge[11]. A suitable separator for lithium-containing alloys anodes should have good thermal stability and function to suppress the lithium dendrites. Additionally, a suitable separator may also have the function to inhibit the polysulfides shuttling for Li-S batteries and for Li-air batteries. Therefore, a better strategy is combing the lithium alloys anodes with various functional electrolytes and separators rather than use it to modify the electrolyte or separators.