Aluminium should be an attractive anode material for rechargeable Li-ion batteries for many reasons, such as low cost (~$2,000 ton⁻¹), high theoretical capacity (2,235 mAh g⁻¹ if Li₉Al₄, low potential plateau (~0.19–0.45 V against Li⁺/Li₃), high electrical conductivity and so on. However, despite the historical efforts on developing Al-Li electrodes, the practical performance fell far short of the theoretical promise and many other high-capacity anodes.

Most of the batteries using aluminum films of micron thickness displayed a high initial capacity, but faded rapidly in a few cycles. Fundamentally this is because of two damage mechanisms, both exacerbated by aluminum’s ~100% volume expansion/shrinkage during lithiation/delithiation: (a) the volume changes cause repeated breaking and re-formation of the solid-electrolyte interphase (SEI) film coating the active material, making Coulombic efficiency (CE)≠100% in a cycle and converting cycleable or ‘live’ lithium in the electrodes and electrolyte to ‘dead lithium’ in SEI, and eventually the battery dies out of lithium exhaustion, and (b) the active material (Al-Li) could pulverize or be pushed away during cycling, thus losing electrical contact from the current collector. The above are actually generic problems for all high-capacity anode materials.

—Li et al.

Other researchers, such as Professor Yi Cui at Stanford, have shown that yolk-shell nanoarchitectures can be an effective solution to the problems caused by volume expansion with silicon in the anode. The inert nanoshell facing the electrolyte is covered with the SEI but does not change volume, while the active yolk expands/shrinks in the internal cavity without SEI. (Earlier post.)

The MIT/Tsinghua team reasoned that this general paradigm should also work for an aluminum active core. A key practical question is how to generate a fully closed TiO₂ nanoshell around aluminum nanoparticles with a tunable vacuum interspace in between (Al@TiO₂) in a cost-effective and industrially scalable way. From a materials design point of view, the shell must be thin enough to be a good electron and Li⁺ conductor, but also must be mechanically robust. The shell must also be fully closed, and there must be sufficient void spacing between the yolk and shell to support the volume expansion (again, around 96% for aluminum.

The researchers devised a one-pot synthesis method that is simple, cheap, scalable and uses only Earth-abundant elements (Al, Ti, O, H, C and S)—and therefore can be used for mass production.

The aluminum particles they used, which are about 50 nanometers in diameter, naturally have an oxidized layer of alumina (Al₂O₃, which is not a good electrical conductor. They converted the alumina layer to titania (TiO₂), a better conductor of electrons and lithium ions when it is very thin.

Aluminum powders were placed in sulfuric acid saturated with titanium oxysulfate. When the alumina reacts with sulfuric acid, excess water is released which reacts with titanium oxysulfate to form a solid shell of titanium hydroxide with a thickness of 3 to 4 nanometers. While this solid shell forms nearly instantaneously, if the particles stay in the acid for a few more hours, the aluminum core continuously shrinks to become a 30 nm “yolk” (showing that small ions can get through the shell). This shrinkage also allows the interspace to be tuned. The particles are then treated to get the final aluminum-titania (ATO) yolk-shell particles.

Half-cell testing (with metallic Li-foil) showed that at a rate of 1 C, the Al@TiO₂ yolk-shell nanocomposite exhibited first discharge and charge capacities of 1,237 and 1,360 mAh g⁻¹, respectively, which indicated a first-cycle CE of 90.9%. The specific capacity stabilizes at 1,170 mAh g⁻¹ in later cycles. Capacity decay is only <0.01% per cycle.

Half-cell battery performance of Al@TiO2. (a) Cycling life and the corresponding Coulombic efficiency during 500 cycles. The charge/discharge rate was set at 1 C. (b) Charge/discharge voltage profiles with the 1st, 250th and 500th cycling. (c) Cyclability test at different charge/discharge rates. (d) Delithiation capacity evolution by varying charge/discharge rates ranging from 0.1, 0.5, 1, 2, 5, 10 C and back to 0.1 C. Li et al. Click to enlarge.

At rate 10 C, the Al@TiO2 yolk-shell electrode can still achieve a capacity of 661 mAh g⁻¹ after 500 cycles—twice the capacity of graphite. The team attributed the excellent rate performance to aluminum’s good electrical conductivity—an advantage over silicon as the active material. This high performance persists to 750 cycles, even though faster capacity decay (~0.03% per cycle) appeared after the 500th cycle or so.

The team then fabricated full cells with the anode and LiFePO4 (LFP) cathode with only 35% more lithium relative to the ATO capacity in half-cells. The full cell exhibited a first discharge capacity of 1,123 mAh (g of ATO)⁻¹ at a rate of 1 C from 2.5 to 4.0 V, with a first-cycle CE=79.4%.

(In the first cycle, >20% within the ~50% excess lithium was used to form the initial SEI that covers the large surface area of Al@TiO2 yolk-shells. This is normal and common treatment in all commercial batteries.)

The specific capacity stabilized at ~968 mAh (g of ATO)⁻¹ up to 200 cycles in the full cell. This, the team argued, proves that the TiO₂ shells are robust enough that a great majority of the Al@TiO2 yolk-shell survives, and that the SEI is stable outside of TiO₂.

We contrast ATO with existing anode technologies. Compared with metallic lithium, ATO does not form dendrites at high rate and is less of a safety concern because of air stability. Compared with Si yolk-carbon shell, ATO has ~20% lower capacity below 1 C rate, but provides higher capacity with long cycle life above 1 C. Compared with high-rate Li₄Ti₅O₁₂ anode which has extremely long cycle life, ATO has 8 × gravimetric capacity at 1 C, and much better (lower) voltage. Compared with conventional graphite anode (theoretical capacity 372 mAh g⁻¹) used in current batteries, ATO has similar voltage characteristics, but has 4 × gravimetric capacity at 1 C charge/discharge rate. The fact that ATO achieves 10 C charge/discharge rate with reversible capacity exceeding 650 mAh g⁻¹ even after 500 cycles makes it a high-rate and ultrahigh-capacity anode, at an industrially satisfactory loading of 3 mg cm⁻². Simple and scalable nanostructuring has thus realized the intrinsic potential of this abundant element for battery anode.

—Li et al.

The research team included Sa Li, Yu Cheng Zhao, and Chang An Wang of Tsinghua University in Beijing and Junjie Niu, Kangpyo So, and Chao Wang of MIT. The work was supported by the National Science Foundation and the National Natural Science Foundation of China.

Resources

• Sa Li, Junjie Niu, Yu Cheng Zhao, Kang Pyo So, Chao Wang, Chang An Wang & Ju Li (2015) “High-rate aluminum yolk-shell nanoparticle anode for Li-ion battery with long cycle life and ultrahigh capacity” Nature Communications 6, Article number: 7872 doi: 10.1038/ncomms8872