Prof. Li Baohua's research team, along with other contributing researchers, investigate “ionic shuttle relay”, a synergistic exploitation to produce high-voltage quasi-solid-state lithium metal batteries

Lithium (Li)-based batteries, particularly Li-ion batteries, have dominated the market of portable energy storage devices for decades. However, the specific energy of Li-ion batteries is approaching their theoretical limit (300 Wh kg^-1), making it difficult to satisfy the requirement for long-distance driving with a single charging of electric vehicles. To further increase the energy density of Li-based batteries, the upgrading of electrode and electrolyte materials is urgently desired.

For anode materials, Li metal has been regarded as the ideal candidate due to its specific capacity (3860 mAh g^-1) and the lowest redox potential (-3.04 V versus standard hydrogen electrode). However, its practical application has been severely hampered by uncontrollable Li dendrite growth during cycling. It is well-recognized that the highly reactive Li metal is prone to react with the electrolytes and form an unstable passivated solid electrolyte interphase (SEI) layer on the surface. On the cathode side, layered transition metal oxides, e.g., nickel-rich oxides and Li-rich oxides, are desirable for high-energy Li-based batteries considering their combined merits in specific capacity, working potential, and cycling performance. However, for the intercalation-based Li-ion batteries, only the Li ions in the electrolyte participate in the electrochemical reactions based on a “rocking-chair” mechanism, while no extra capacity contribution is made by the anions in electrolytes. Therefore, unlocking the additional potential of anions in the electrolyte is a promising approach to further enhance battery energy density.

Recently, dual-ion batteries (DIBs) based on graphitic cathode materials have attracted extensive attention, in which anions reversibly intercalate into/deintercalated from graphite interlayers at cell voltage > 4.5 V during charge/discharge processes. The operating voltage of these DIBs generally is about 5 V vs. Li/Li⁺, which is favorable for energy density improvement. However, such a high intercalation voltage of graphite leads to severe oxidative decomposition of the electrolytes, and tends to construct a high-resistance cathode electrolyte interphase (CEI) on the cathode surface. This seriously impedes anion insertion, resulting in inferior reversibility and poor cycling stability. Furthermore, the co-intercalation of the solvent molecules into graphite cathode causes an exfoliation of graphite layers and the subsequent irreversible loss of active materials during cycling. As for the electrolytes, the flammable solvents (e.g., organic carbonates and ethers) widely applied in Li-based batteries trigger safety concerns including fire, explosion, and leakage of toxic electrolyte components. All these drawbacks have brought great challenges for the development of high-energy Li-based batteries.

To solve these problems, Prof. Baohua Li and Dr. Dong Zhou from Tsinghua Shenzhen International Graduate School, Prof. Guoxiu Wang from the University of Technology Sydney and Prof. Michel Armand from the CIC ENERGIGUNE Institute in Spain have collaborated to report the combination of a heteroatom-based gel polymer electrolyte (HGPE) with a hybrid cathode comprising of a Li-rich oxide active material and graphite conductive agent to produce a high-energy “shuttle-relay” Li metal battery, where additional capacity is generated from the electrolyte’s anion shuttling at high voltages. The HGPE, prepared via in situ co-polymerization of diethyl allyl phosphate (DAP) monomer and pentaerythritol tetraacrylate (PETEA) cross-linker in the presence of an all-fluorinated electrolyte. This HGPE exhibited high safety (i.e., non-flammability and non-leakage), high ionic conductivity (1.99 mS cm^-1at 25 ^oC), wide electrochemical window (up to 5.5 V vs. Li/Li⁺), and compatibility with both Li metal anode (a Li deposition/stripping Coulombic efficiency of 99.7%) and graphite cathode (93% capacity retention after 1000 cycles). On this basis, we developed a “shuttle-relay” Li metal battery (SRLMB) consisting of a hybrid cathode with Li-rich oxide (LRO) as active material and KS6 graphite as conductive agent and the HGPE as electrolyte. During the charge process, a reversible insertion of PF₆^- anions into the KS6 graphite occurs after the stripping of Li ions from the LRO, in which anions contributes 8.2 % (i.e., 3.2 Wh L^-1) extra energy density of the cell. The as-developed SRLMB exhibited high capacity and cycling stability, ascribed to the stable electrode|HGPE interfaces.

Figure 1 The design of HGPE

As shown in Figure 1, the HGPE enables a characteristic mechanism (i.e., the shuttle-relay), which synergistically exploits the LRO’s rocking-chair and the graphite’s dual-ion mechanisms. Li ions are stripped from the LRO cathode in the voltage range of 2.0-4.8 V, followed by an insertion of the PF₆ anions into the conductive graphite at 4.8-5.0 V. In the HGPE, the synergistic effect of liquid-state electrolyte components constructs robust SEI/CEI to improve the electrode|electrolyte compatibility; meanwhile, the polymer matrix efficiently retards the migration of Li⁺/PF₆^- ions to SEI/CEI surface defects through strong interaction, thereby favoring a uniform Li⁺/PF₆^- ion flux to promote uniform Li insertion and anions intercalation into graphite.

Figure 2 Lithium plating/stripping behavior in various electrolyte formulations

Benefitting from the tuning of the Li⁺-fluorocarbonate solvent sheath by the novel diluent 1,1,2,3,3,3,3-hexafluoropropyl-2,2,2-trifluoroethyl ether (HTE), the Li|HGPE|Li cell delivered a stable voltage hysteresis of ≈100 mV with no oscillation throughout a 1400 h cycling (the general decrease in overpotential in the initial cycles is related to the activation of Li anode with a pristine oxide layer on the surface), indicating a dendrite-free Li deposition when using HGPE. In the Li|Cu cell employing the HGPE, for comparison, the plating Li showed a compact morphology as aggregated large particles, and the plated thickness (≈10.7 μm) was very close to the theoretical value. Such a dense Li deposition with a smaller surface/volume ratio effectively minimizes the parasitic reaction between metallic Li and electrolyte, and thus enables the high CE_avg of Li|HGPE|Cu cells.

Figure 3 Electrochemical energy storage performance of the Li||KS6 graphite batteries

The long-term cycling performance of Li||KS6 graphite DIBs employs various electrolytes at 1 C. The Li||KS6 graphite cell using the HGPE demonstrated a high initial discharge capacity of 89.8 mAh g^-1 with a capacity retention of ≈93% after 1000 cycles, and the Coulombic efficiency was maintained at ≈98.9% except for the activation process in the first 10 cycles. The above results were further corroborated by the small interfacial resistances (R_sei and R_ct) of the HGPE-based cells, and the interfacial resistance changes were much smaller than in the cells using other electrolytes during cycling. This cycling stability is mainly because the HGPE effectively suppresses solvent co-intercalation and protects the structural integrity of graphite, thus allowing a highly reversible and durable insertion/extraction of anions into/from the KS6 graphite.

Figure 4 Electrochemical energy storage performance of the SRLMBs

SRLMBs have been further developed by applying KS6 graphite as conductive agent in the cathode of the LRO|HGPE|Li cells. During the charging of LRO|HGPE|Li and Li|HGPE|LRO/graphite cells, a sloping potential below 4.5 V corresponded to Li ion extraction from LRO cathode. For the hybrid LRO/KS6 graphite cathode an extra plateau at 4.9 V appears, which is ascribed to a “relay” intercalation step of PF₆^- into the graphite. KS6 contributed ≈ 6.2 % of the areal capacity and ≈ 8.2 % of the energy density. Single-layer SRLMB pouch cells with 50 μm-thick Li foil as anodes were assembled to further evaluate the battery performance under abuse conditions. The Li|HGPE|LRO/graphite pouch cell not only showed adequate cycling performance, but also exhibited flexibility (i.e., consistently powering a red light-emitting diode (LED) under flatted, bent, or even clustered states. Whereas the cell using traditional liquid electrolyte loses power supply ability in bent or clustered states. This verifies that the electrode|HGPE interfaces can maintain tight adhesion under significant shape deformations. Moreover, when aging the fully charged cells at 130 ^oC, owing to the high thermal stability of fluorinated solvents and leakage-free property of the gel, the shape and open circuit voltage of Li||LRO/graphite pouch cell with HGPE did undergo not change at the 130 ^oC test. All these enable a highly safe operation of SRLMBs in practical applications. In this work, we demonstrated that the “shuttle-relay” concept utilizing such graphitic carbon component in the cathode can provide additional capacity, which increases the energy density of existing Li batteries

This work was recently published in Nature Communication (IF: 14.919). The first author of the paper is Junru Wu, a graduate student at Tsinghua Shenzhen International Graduate School.

The research team’s paper can be viewed on: https://www.nature.com/articles/s41467-021-26073-6