2019 Volume 60 Issue 11 Pages 2451-2455
This paper creatively introduces the high-pressure torsion treatment (HPT) technique for the preparation of lithium ion battery electrodes. Through the preparation of a nanocrystalline lithium/graphene composite structure, a high-performance dendrite-free electrode was obtained. Furthermore, for light metal processing via the HPT method, the strain-grain relationship is still applicable under high-pressure torsion and results in excellent electrochemical performance. As a physical metallurgical method for the preparation of nano-metal composite materials, the HPT method has advantages that are not possessed by hydrothermal methods. These include a rapid, low-cost synthesis with minimal by-products from the chemical reaction.
Fig. 4 Characterizing and comparing battery performance (a) Constant-current cycle diagram of coarse-Li and graphene-doped Li symmetrical batteries at a current density of 1 mA cm−2 and peeling/electroplating capacity of 1 mAh cm−2; (b) Rate capability diagram of Coarse-Li and graphene-doped Li|LiFePO4 batteries within a current density of 0.2–20 C (c) Cycling performances of Coarse-Li and graphene-doped Li|LiFePO4 batteries at a current density of 0.2 C.
With increasingly strict requirements for mobile computing devices, high-performance energy-storage devices that can match these demands are also under rapid development.1,2) Lithium has an extremely high specific capacity (3860 mAh g−1), the lowest reduction potential (−3.040 V vs. standard hydrogen electrode), and excellent mechanical flexibility. Given these positive attributes, it is considered to be a highly competitive and promising anode material for the new generation of flexible secondary batteries with high specific energy.3,4) However, during the charging-discharging process, lithium anodes are prone to grow dendrites. These lithium dendrites could puncture the battery’s separator and cause internal short-circuiting due to contact between the anode and cathode; this can possibly result in serious safety issues such as spontaneous combustion or explosion.5) Additionally, the dendritic structure of lithium is loose and can easily peel off to form electrochemical inactivity “dead lithium”. Furthermore, the growth of lithium dendrites enlarges the total surface area of anode. The larger anode surface consumes more electrolyte and then forms a solid electrolyte interfacial film, which results in a lower battery capacity and a shorter cycle life.6–8) Therefore, the problem of lithium dendrite growth seriously retards the practical application of secondary lithium metal batteries with a high specific energy such as lithium-sulfur batteries,9,10) lithium-air batteries11) and solid metal lithium batteries.12)
Towards solving the issue of lithium dendrites, liquid electrolyte modification (additives in the electrolyte solution),13,14) the creation of an interfacial film made of artificial solid electrolyte,15,16) solid/polymer composite electrolyte solutions,17) interfacial modification of the battery separator,18) solid electrolytes19) and micro/nano anode structures with a high specific area20,21) have been proposed. However, these methods rarely consider the metallurgical nature of lithium, namely its grain size and microstructure. At the start of the lithium embedding process that occurs in the battery under working conditions, lithium ions migrate to the surface of the anode, accept electrons, and then begin nucleation. The subsequent deposition of lithium ions tends to preferentially occur on existing grain boundary sites. Since the density of grain boundaries is low and sparsely distributed, the continuously deposited metallic lithium preferentially grows on these nucleation sites and forms lithium dendrites. Therefore, the nucleation stage of the lithium deposition plays a decisive role in the final morphology of the deposited lithium.22,23) Through treatment that causes large plastic deformation of the lithium, nanocrystalline lithium with a high dislocation density was obtained; in addition, a conductive skeleton24,25) was introduced during treatment to reduce the local current density26) and regulate the nucleation and growth of lithium. The above findings indicate that treatment has the potential to result in dendrite-free growth and achieve a pure lithium metal battery that is safe and has a high specific energy.
In this study, the large plastic deformation technique was applied as a pure physical metallurgy method. Using the technique, graphene with three-dimensional electrical conductivity was used as a nanoscale skeleton. The nano-lithium composite structure with a large number of grain boundary sites, induces and regulates the uniform nucleation of lithium; thereby, the growth of lithium dendrites is effectively inhibited. By means of such structural variation, this work demonstrates for the first time nanocrystalline lithium metal batteries with superior electrode electrochemical performance compared with coarse-grained Li metal batteries.
Schematic illustration of the concept and introduction of the experimental procedure.
In the experiment, a high-pressure torsion (HPT) device was used to treat Li metal foil (Goodwill, Beijing) and reduced graphene oxide (Sigma-Aldrich). The thickness of the Li metal foil was 0.05 mm. Prior to the HPT process, the Li metal and graphene were mixed in a mass ratio of 14:1; for this, the graphene powder was uniformly placed between the Li foil layers and pre-rolled. The HPT treatment parameters were 5, 10, and 20 turns. All the treatments were carried out at room temperature and under an argon-protected environment. The torsional pressure was 6 GPa and the rotational speed was 1 r/min. Finally, round specimens with a sample diameter of 1.6 mm and thickness of 0.45 mm were obtained.
2.2 Electrochemical measurementThe above two raw materials were mixed with acetylene black and polytetrafluoroethylene (PTFE) binding agents in a mass ratio of 8:1:1. The paste was pressed into an electrode on a 9 mm × 1.5 mm foam Ni substrate. Electrochemical measurements were carried out in a LiFePO4 cathode model battery with 1 mol·L−1 LiPF6/EC+DMC as the electrolyte and microporous polypropylene film (Celgard-2300) as the battery separator. The assembly of the battery was carried out in an argon-filled glove box. The constant-current charge-discharge measurement was carried out at 30°C using a DC-5 type multi-channel tester with a current density of about 1 mA·cm−2 within the voltage range of 0.05–1.0 V.
First, there is a strong positive correlation between the number of HPT turns and the dimension of the grains. The empirical formula has been confirmed several times previously in alloy systems that are difficult to deform.27,28) In this work, the correlation has been extended to the lithium system and was well verified. Figure 2 illustrates the electron backscattered diffraction (EBSD) results of the composite lithium samples after different applied strains via the HPT process. It can be seen from Fig. 2(a) that the grains with an initial size greater than 100 nm can be considered as coarse grains. However, after undergoing only 5 HPT turns, the average grain size was reduced to less than 100 nm, reaching the standard for nanocrystals in structural materials. Subsequently, we found that after experiencing a large strain (number of turns exceeded > 20), all the grains in the system had a uniform distribution and exhibited an average grain size of 34 nm. The result directly illustrates the outstanding capability of HPT in the refinement of the lithium metal grains.
EBSD characterization results of microstructures in Li metal samples after HPT treatment, (a) Coarse grains (b) HPT N = 5 (c) HPT N = 10 (d) N = 20.
As can be seen from the TEM analysis shown in Fig. 3, lithium and graphene are not just simply mechanically bonded during the high-pressure torsion deformation process. First, under a relatively small strain shown in Fig. 3(b), most of the graphene was dispersed in the grain boundaries of the lithium. As the number of torsions increased, i.e., the input external strain increased, interfacial reactions begin to occur between the graphene and the grain boundaries of the lithium metal. These grain boundary reactions gradually penetrate into the grain’s interior under the action of a high strain rate and entangled dislocations. Meanwhile, the mechanical-alloying process further promotes the refinement of the grains. From this point of view, the HPT technique changes the diffusion coefficient by enhancing the strain input and accelerates interfacial diffusion between lithium and graphene. It must be mentioned that the mechanical deformation nature and lower deformation temperature of the high-pressure torsion, also indirectly limits the production and chemical reactions of intermetallic compounds. This plays a significant role in maintaining the capacity and structural strength of the lithium metal electrodes.
TEM characterization of Li metal specimen after HPT treatment, (a) Coarse grains (b) HPT N = 5 (c) HPT N = 10 (d) HPT N = 20.
The Li dissolution/deposition behavior of the graphene-doped Li electrode was evaluated by assembling a symmetrical battery. Figure 4(a) shows the voltage-time curve of graphene-doped Li and Coarse-Li symmetrical batteries at a current density of 1 mA cm−2 and a peeling/electroplating capacity of 1 mAh cm−2. It can be seen from the figure that the graphene-doped Li electrode has a much smaller voltage hysteresis and exhibits an ultra-long cycle life (517 cycles) without significant fluctuation in the voltage or increase in the overpotential. In contrast, the Coarse-Li electrode exhibits a gradually increasing over-potential in the first 116 cycles. Subsequently, the voltage suddenly decreases and abnormally fluctuates in subsequent cycles, indicating the formation of Li dendrites. The difference in the Li dissolution/deposition behavior between the two types of electrodes can be clearly illustrated by the voltage curves shown in Fig. 4(a). The difference is attributed to the following reasons: 1) the grain boundary of the nanocrystalline composite pre-stores lithium and not all of it can be completely removed during each peeling process. Therefore, each electroplating/peeling process can be performed on the grain boundary site, so the nucleation barrier for Li electroplating/peeling is very small; 2) The nanocrystal/graphene grain boundary composite structure can provide fast transport channels for the lithium ions. As a result, the internal lithium metal could quickly participate in the reaction. In addition, the Li treated HPT has a high structural strength, so it can resist the stress caused by volume change during the electroplating/peeling process.
Characterizing and comparing battery performance (a) Constant-current cycle diagram of coarse-Li and graphene-doped Li symmetrical batteries at a current density of 1 mA cm−2 and peeling/electroplating capacity of 1 mAh cm−2; (b) Rate capability diagram of Coarse-Li and graphene-doped Li|LiFePO4 batteries within a current density of 0.2–20 C (c) Cycling performances of Coarse-Li and graphene-doped Li|LiFePO4 batteries at a current density of 0.2 C.
For exploring the potential applications of graphene-doped Li and coarse-Li anodes in real battery systems, a full battery was assembled by pairing the LiFePO4 cathode and an anode made of the composite material. Compared to batteries using the pure Li anode, the new battery exhibited lower polarizability, significantly improved electrochemical kinetic performance, and a higher capacity. It is worth noting that using the graphene-doped Li anode provides a higher capacity retention, especially for higher current density. For example, at 20°C, a battery with the graphene-doped Li anode maintains a larger capacity of 103.1 mAh g−1 (based on the mass of the LiFePO4 cathode). This is three times as high as that of the Coarse-Li anode at the same temperature (37.2 mAh g−1, see Fig. 4(b)). After 500 cycles at a current density of 0.2 C, it is easy for the graphene-doped Li anode battery to achieve a reversible capacity of 142.1 mAh g−1, exhibiting a high retention of 90.8% of the initial capacity (153.2 mAh g−1). The capacity of the Coarse-Li anode battery decays rapidly from 153.0 mAh g−1 to 103.2 mAh g−1 (Fig. 4(c)). More significantly, the coulombic efficiency of the graphene-doped Li anode battery remains nearly constant during cycling period, while that of the Coarse-Li anode fluctuates greatly. Since these batteries have the same cathode and electrolyte solution, the difference in battery performance is mainly because of the anode’s characteristics. The high rate and cycling performance of the complete battery indicates that the performance of the graphene-doped Li anode can be extended to the complete battery.
The excellent performance of the graphene-doped Li anode results from its light weight, remarkable electrical conductivity, extremely high mechanical strength, and high chemical stability. The large specific surface area with far-reaching open channels, as well as high electrical conductivity provided by the seamless, interconnected graphene effectively dissipates high current density to the ultra-low local current areas; in this manner, the nucleation and growth of Li dendrites is inhibited. After HPT treatment of the composite system, graphene concentrates on the grain boundaries. This provides well-distributed anchoring points for uniform Li deposition and is also beneficial to the Li stripping/electroplating process in the presence of dendrites. It is worth mentioning that after HPT treatment of the composite material, its mechanical strength could reach an excellent 83 MPa, and the elongation at break reached 250% at room temperature (20°C). The results indicate that the combination of fast lithium ion transport channels and extremely high mechanical strength, enhances the electrochemical performance of the composite anode significantly, and is beneficial for application in high-energy Li batteries.
In conclusion, a high-performance lithium composite anode was developed. The high-pressure torsion technique was adopted to distribute graphene on the grain boundaries of the Li metal. This greatly enhanced the mechanical properties of the composite Li metal material by reducing the average grain size. The nano-layered graphene, as a stable and lightweight support for Li, also provided reaction sites and fast reaction channels. By means of fast mass transfer channels and a high conductivity, the large amount of grain boundaries and entangled dislocations caused by the nanocrystal could resolve crucial issues during the charging-discharging period. These include uncontrolled dendrite growth and large volume changes in the Li anode. A full battery fabricated using the composite anode and LiFePO4 exhibited excellent performance in terms of its specific capacity, cycle life, and rate capability. In summary, the study demonstrated a novel nanocrystalline Li composite anode and also provides a new design approach, namely by preparation of the metal electrode via a physical metallurgical method. Differing from traditional hydrothermal chemical synthesis, the top-down design scheme proposed in this work could better address issues like uncontrolled grain size and the high cost of large-scale industrial production.
Regarding author's contributions to this paper, Chenhao Qian and Ziyang He contributed equally. This work was supported by the Fundamental Research Funds from Jiangnan University, China (JUSRP116027, JUSRP51732B).
