3.1 | DIRECTLY USING LITHIUM ALLOYS TO REPLACE METALLIC LITHIUM AS ANODE

The lithium-free alloy anodes, such as Sn-Sb, Sn-Co, Ni-Sn alloy, etc., without pre-stored lithium, the overall energy density is limited by the low-capacity lithium metal oxide cathodes, while the pure lithium metal anode face its high reactivity and uncontrolled dendrite growth[39]. Li-containing alloy anodes inheriting the desirable properties of alloy anodes and pure Li metal anodes. Lithium alloy anodes for rechargeable ambient temperature lithium batteries have been studied since the early 1970[40, 41]. During the past 40 years, great deals of literatures have been reported using the lithium-containing alloys as the anode materials for lithium ion batteries[18, 30, 32-34, 38, 42-44]. They can effectively reduce Li nucleation overpotential and decrease interfacial resistance, guiding the formation and growth of non-dendritic Li[25]. Among these lithium alloys anode, they can mainly divided into two categories: binary lithium alloys and ternary lithium alloys. And different lithium alloys anodes have their own advantages and disadvantages.
For example, Li-Si alloy anodes exhibit multiple attractive properties: i) fully lithiated Lix Si alloy has a sufficiently low potential of around 10 mV versus Li/Li+ to prelithiate all types of anodes including graphite, Si, Ge and Sn[45]; ii) due to the super-high capacity of Si (4200 mA·h·g−1), Lix Si alloy anode could also illustrate high specific capacity even a small percentage of pre-storing lithium, i.e., Li4.4Si shows a capacity of 2000 mA·h·g−1 [45]. Most of the Li-Si electrodes were obtained by electrochemical lithiation of Si-based electrodes[46-48], which is very difficult to employ for practical application[47]. But recently, there have been reported other two methods to synthesis the Li-Si alloy: one is pressing plus heat-treatment process as shown in Figure 1a[47], another one is ball milling method (Figure 1b) at argon atmosphere[49-51].
Except the Lix Si alloy, the lithium alloy anodes of other IVA group elements has also been widely reported, such as Li-Sn and Li-Ge alloy anodes[48, 52-58]. Similar as Lix Si alloy, the Li-Sn and Li-Ge alloy anodes also exhibit relative high specific capacity[48, 57]. While the Li-Sn alloy anode also shows its unique merits, including the fast interdiffusion of Li in Sn and the < 500 mV separation between Li-Sn alloy formation and Li plating. Archer’s group reported a Li-Sn hybrid battery anodes created by depositing an electrochemically active Sn on a reactive Li metal electrode by a facile ion-exchange chemistry as shown in Figure 1c, leading to very high exchange currents and stable long-term performance[59]. The Li-Sn anodes were shown to be stable at 3 mA·cm−2and 3 mA·h·cm−2. While in contrast to Si, Ge has the benefit of forming a minimal amount of native oxide in its outermost layer and the diffusivity of lithium in Ge is 400 times greater than that of lithium in Si at room temperature, but as the high cost of Ge, the Li-Ge alloy anodes have not gained much attention[34].
The Li-Al alloy anode showed higher stability in the air, carbonate-based electrolyte and the electrolyte with LiNO3 additive[60, 61]. Also, Al alloying with Li exhibits much smaller volume change (≈96%) compared with other alloy anodes, such as Li-Si (320%) and Li-Sn (260%) alloys anode[62]. In addition, Li diffusion coefficient in Li-Al alloy (6.0×10-10cm2·s−1) exceeds that in bulk Li metal (5.69×10-11cm2·s−1)[63]. LiB alloy is widely used as anode in thermal battery, which can be regarded as free metal lithium metal filled in the fibrillar network framework of Li/B compound (Li7B6)[64, 65], such a porous structure can increase the specific surface area and adjust Li ions even distribution. As the discharge potential of Li7B6 is over 0.4 V (vs. Li/Li+), thus when Li-B alloy used in metal lithium battery, its free metal lithium participates in electrochemical reaction preferentially[64]. Additionally, Li7B6 has a good conductivity (1.43×103−1·cm−1) and a high Li ion diffusion rate comparative to metallic lithium[64]. Thus, in 2013, Yang’s group firstly investigated Li-B alloy as anode for lithium/sulfur battery[64]. It is because of the above advantages, Li-B alloy has better behaviors in restraining the formation of dendritic lithium, reducing the interface impedance of electrode and improving the cycle performance of the battery. For Li-In alloy electrode, Archer’s group found the interfacial of the resultant Li-In alloy electrode was significant lower than that of the pristine Li metal, which allowed Li ions diffused along the surface to form uniform deposit on the hybrid electrode[66]. As a result of the enhanced interfacial ion transport mechanism, compact and uniform electrodeposition for the Li-In alloy anode at long time scales has been realized. And the Li-In hybrid anodes to full cells employing high-loading commercial cathodes (LTO and nickel manganese cobalt oxide) showed that the electrodes can be cycled stably for over 250 cycles with close to 90% capacity retention. Recently, Adelhelm investigated the different In/Li ratio on the performances of Li-In anode[67]. The right In/Li ratio, i.e. 1.27:1, enabled stable lithium insertion/deinsertion in symmetrical cells for at least 100 cycles; while too much lithium in the electrode leaded to a drop in redox potential combined with a rapid build-up of interface resistance.
Compared with the group IVA and group IIIA lithium alloys, even as early as 1970s, the Li3Sb and Li3Bi alloy anodes have also been investigated in metal lithium batteries[68, 69], group VA lithium alloys directly used as anodes in metal lithium batteries are not too many[43, 70-72]. That mainly because the higher toxicity of some VA elements, such as Sb and As, and the smaller gravimetric capacity of Bi[34]. In addition, the synthesis conditions of VA lithium alloys are higher demanding, taking the Li-Sb alloy, except complex prelithiation with Sb, another method is prepared by electrolysis of the molten LiCl-KCl eutectic mixture with a liquid antimony cathode at high-temperature[71]. In contrast, the Bi/Sb-based nanocomposites and Bi/Sb-based intermetallics could be easily and large-scale production, as well as demonstrated good electrochemical performances when used as anode materials for LIBs[34]. Therefore, during the past 40 years, there has been no significant development of the group VA lithium alloys as the anodes in metal lithium batteries.
Beside these three group elements, some other elements such as Na[73, 74], Mg[75-80], Zn[81, 82], In[66, 67], Ag[83, 84], Au[84], etc., can alloy with Li as well. And recently, these lithium-alloys used as the anode materials in lithium metal batteries have gained increasing attentions.
Li-Na alloy can supply Li+ on stripping and thus ensure the electrostatic shield effect of Li+ [85]. And Li-Na alloy would not sacrifice the specific capacity of the anode because Li and Na metals exhibit similar reaction activities as well as electrolyte compatibility of Li+ and Na+ [85]. However, developing a Li-Na alloy anode might be difficult because of volume expansion[73, 74, 85], which causes SEI damage, large internal resistance and low Coulombic efficiency. Recently, Zhang’s group reported a Li-Na alloy anode used in Li-O2batteries[85]. By optimizing the Na/Li value of the alloy, a dendrite-suppressed, oxidation-resistant and crack-free Li-Na alloy anode could be obtained[85], thus realizing an alloy anode with a long cycle life.
The Li-Mg alloy is advantageous because of the generally lower reactivity of Li (or relatively low Li activity), the large solid solution range, the mechanical integrity of Mg framework and a relatively large diffusion coefficient of Li in Mg (∼10−7 cm2·s−1 for the Li-Mg alloy produced by vapor deposition)[75, 77-79, 86]. Mg alloying can increase lithium utilization, when no external pressure is applied while pure lithium metal is superior for setups that allow stack pressures in the MPa range[75]. And appropriate amount Mg, i.e, 10 at%, introducing into Li metal anode can also effectively prevent contact loss[75]. Due to these various advantages, recently, Gao’s group reported Li-Mg alloy as an anode for Li-S batteries[76]. Compared to the metallic Li anode, the Li-Mg alloy showed remarkable improvement on stability at the surface and in the bulk during cycling as shown in Figure 1d and 1e. And they also found after Li stripping, a conducting Li-poor Li-Mg alloy matrix was formed, facilitating subsequent plating and diffusion of Li ions.